Metamorphic hybrid compliant mechanisms

Abstract

The disclosed technology herein relates to metamorphic flexure bearings. The metamorphic flexure bearing may include a first body flexibly coupled to a second body via a flexure bearing. The metamorphic flexure bearing may include the second body coupled to a third body via a retention mechanism. In a first mode, the second body is anchored to the third body, and the first body is configured to move relative to the second body. In a second mode, the second body is configured to move relative to the third body.

Description

Docket #: 206030-0333-00WGMETAMORPHIC HYBRID COMPLIANT MECHANISMSCROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority to US Provisional Patent Application No. 63 / 696,647, filed on September 19, 2024, incorporated herein by reference in its entirety.STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH AND DEVELOPMENT

[0002] This invention was made with government support under DP2HD111538 awarded by the National Institutes of Health. The government has certain rights in the invention.BACKGROUND

[0003] Mechanical motion has conventionally been guided by surfaces that constrain that motion. Conventional mechanical bearings (i.e., sliding and rolling bearings) enable components to be conveyed over long distances, and state-of-the-art conventional mechanical bearings do so with substantially reduced friction compared to their historical counterparts. This reduction in friction results in higher efficiency and lifetime of the components used in transportation, manufacturing, and household devices. And yet, all bearing surfaces, because the bearings move by rolling, rubbing, or sliding, inevitably have some amount of friction, limiting the efficiency of the motion and necessitating maintenance and repair as the mechanisms wear.

[0004] Flexure bearings offer a frictionless solution to motion control by guiding motion without the use of sliding or rolling contacts. This motion guidance is achieved via compliant flexures that instead bend to convey their components while guiding their motion in a designable way. Because of this designability, flexure bearings can (similarly to conventional bearings) be used for various types of motion, such as linear (e.g., double parallelogram linear bearings) or rotary motion (e.g., cross pivots), or various combinations of such. By employing compliant flexures, flexure bearings enable highly repeatable motion with low wear, and when optimized for a particular application, they can be designed to outlast the systems in which they are installed. Further, their low friction provides for highly efficient, quiet motion that can also be more easily controlled at a higher bandwidth. While other bearing types, such as magnetic or fluid bearings, also guide motion without the use of surface contact, these bearings are more complex,Docket # 206030-0333-00WG expensive, and difficult to maintain, while also suffering from inefficiencies, such as eddy current or viscous friction losses.

[0005] Unfortunately, flexure bearings are also well-known to have a limited range of motion. As many applications require extended ranges of motion, the applications are thus precluded from the use of flexure bearings and unable to gain the full benefits of using compliant mechanisms. Hence, mechanical designs would benefit from a bearing design that combines elements of both flexure and conventional bearings. Conventional hybrid bearings may combine flexural elements with sliding, rolling, or interlocking elements, however these hybrid bearings do not provide an extended range of motion. Thus, there is a need in the art for a bearing that provides the advantages of a flexure bearing over a typical range of motion while delivering a larger range when needed. The present invention satisfies this need.SUMMARY

[0006] The present disclosure relates generally to implants and bearings, and more particularly, to metamorphic flexure bearings for extended range of motion.

[0007] In one or more aspects, the disclosed technology relates to metamorphic flexure bearings. The metamorphic flexure bearing may include a first body flexibly coupled to a second body via a flexure bearing. The metamorphic flexure bearing may include the second body coupled to a third body via a retention mechanism. In a first mode, the second body is anchored to the third body, and the first body is configured to move relative to the second body. In a second mode, the second body is configured to move relative to the third body.

[0008] In one or more aspects, the disclosed technology relates to metamorphic flexure bearing implant. The implant may include a bearing housed within a cover and operably coupled to a first stem and a second stem. The bearing may include a first body flexibly coupled to a second body via a flexure bearing. The bearing may include the second body coupled to a third body via a retention mechanism. In a first mode, the second body is anchored to the third body, and the first body is configured to rotate towards the second body. In a second mode, the second body is configured to rotate towards the third body.

[0009] A variety of additional aspects will be set forth in the description that follows. The aspects can relate to individual features and to combination of features. It is to be understood that both the foregoing general description and the following detailed description are exemplary andDocket #: 206030-0333-00WG explanatory only and are not restrictive of the broad inventive concepts upon which the embodiments disclosed herein are based.BRIEF DESCRIPTION OF THE DRAWINGS

[0010] The following drawings are illustrative of particular embodiments of the present disclosure and therefore do not limit the scope of the present disclosure. The drawings are not to scale and are intended for use in conjunction with the explanations in the following detailed description.

[0011] FIGs. 1 A-1C illustrate diagrams of linear-motion metamorphic flexure bearing that uses linear-force retention, constant-force retention, and retention handoff.

[0012] FIG. 2 illustrates an example metamorphic flexure bearing with selectable and tunable retention mechanisms.

[0013] FIG. 3A illustrates a force profde of the metamorphic flexure bearing with linear-force retention.

[0014] FIG. 3B is a table illustrating the stage and ground attachment and detachment extensions and forces from linear-force retention force profiles of FIG. 3 A.

[0015] FIG. 4A illustrates a force profile of the metamorphic flexure bearing with constant-force retention.

[0016] FIG. 4B is a table illustrating the stage and ground attachment and detachment extensions and forces from constant-force retention force profiles of FIG. 4A.

[0017] FIG. 5 A illustrates a force profile of the metamorphic flexure bearing using retention handoff.

[0018] FIG. 5B illustrates an alternative sweep zoom insets.

[0019] FIG. 6 illustrates a table describing stage and ground attachment and detachment extension and force values for both force profile sweeps of FIG. 5 A.

[0020] FIG. 7A illustrates an example bidirectional metamorphic flexure bearing with linear- force retention.

[0021] FIG. 7B illustrates an example bidirectional metamorphic flexure bearing with constantforce retention.

[0022] FIG. 7C illustrates an example bidirectional metamorphic flexure bearing using retention handoff.Docket # 206030-0333-00WG

[0023] FIG. 8 illustrates a diagram depicting the principles of a magnetic catch assembly.

[0024] FIGs. 9A and 9B illustrate a transformation of n degrees of compliant bearing freedom into m degrees of conventional bearing freedom, using the principle of serial-chaining.

[0025] FIG. 10 illustrates bearing stiffness gradient with linear-force retention.

[0026] FIG. 11 illustrates bearing stiffness gradient with constant-force retention.

[0027] FIGs. 12A-12C illustrate an example rotary metamorphic flexure bearing transitioning between a flexure-bearing mode and a conventional-bearing mode.

[0028] FIGs. 13-16 illustrate other examples of rotary metamorphic flexure bearings.

[0029] FIGs. 17A-17J illustrate various examples of cross-axis flexural pivot blade rearchitecture for increased range of motion.

[0030] FIGs. 18A and 18B illustrate rotary metamorphic flexure bearing design principles.

[0031] FIG. 19 illustrates an exploded view of an example rotary metamorphic flexure bearing.

[0032] FIG. 20 illustrates a quasistatic analysis of a retention handoff mechanism featuring magnetic pairs.

[0033] FIGs. 21 A and 21B illustrate a dynamic analysis of a retention handoff mechanism featuring magnetic pairs.

[0034] FIG. 22 illustrates a dynamic analysis of the constant stiffness retention mechanism featuring a torsional spring.

[0035] FIGs. 23A-23B illustrate a dynamic analysis of the constant stiffness retention mechanism featuring a torsional spring.

[0036] FIGs. 24A-24B illustrate components of the example rotary metamorphic flexure bearing with a retention handoff transition mechanism.

[0037] FIGs. 25A-25B illustrate components of the example rotary metamorphic flexure bearing with a constant stiffness transition mechanism.

[0038] FIGs. 26A-26F illustrates an example implant with a range-extending bearing.

[0039] FIGs. 27A-27B illustrates preliminary finite element analysis and benchtop testing results.

[0040] FIGs. 28A-28D illustrate large animal test results.DETAILED DESCRIPTIONDocket #: 206030-0333-00WG

[0041] The following discussion omits or only briefly describes conventional features of implant and bearing devices that are apparent to those skilled in the art. It is noted that various embodiments are described in detail with reference to the drawings, in which like reference numerals represent like parts and assemblies throughout the several views. Reference to various embodiments does not limit the scope of the claims attached hereto. Additionally, any examples set forth in this specification are intended to be non-limiting and merely set forth some of the many possible embodiments for the appended claims. Further, particular features described herein can be used in combination with other described features in each of the various possible combinations and permutations.

[0042] Unless otherwise specifically defined herein, all terms are to be given their broadest reasonable interpretation including meanings implied from the specification as well as meanings understood by those skilled in the art and / or as defined in dictionaries, treatises, etc. It is noted that, as used in the specification and the appended claims, the singular forms “a,” “an” and “the” include plural referents unless otherwise specified, and that the terms “includes” and / or “including,” when used in this specification, specify the presence of stated features, elements, and / or components, but do not preclude the presence or addition of one or more other features, steps, operations, elements, components, and / or groups thereof.

[0043] Relative terms such as “horizontal,” “vertical,” “up,” “down,” “top,” and “bottom” as well as derivatives thereof (e.g., “horizontally,” “downwardly,” “upwardly,” etc.) should be construed to refer to the orientation as then described or as shown in the drawing figure under discussion. These relative terms are for convenience of description and normally are not intended to require a particular orientation. Terms including “inwardly” versus “outwardly,” “longitudinal” versus “lateral” and the like are to be interpreted relative to one another or relative to an axis of elongation, or an axis or center of rotation, as appropriate. Terms concerning attachments, coupling and the like, such as “connected” and “interconnected,” refer to a relationship wherein structures are secured or attached to one another either directly or indirectly through intervening structures, as well as both movable or rigid attachments or relationships, unless expressly described otherwise. The term “operatively or operably connected” is such an attachment, coupling or connection that allows the pertinent structures to operate as intended by virtue of that relationship.Docket # 206030-0333-00WG

[0044] Reference throughout the specification to “one embodiment”, “an embodiment” or “some embodiments” means that a particular feature, structure, or characteristic described in connection with an embodiment is included in at least one embodiment of the subject matter disclosed. Thus, the appearance of the phrases “in one embodiment”, “in an embodiment” or “in some embodiments” in various places throughout the specification is not necessarily referring to the same embodiment. Further, the particular features, structures or characteristics of “one embodiment”, “an embodiment” or “some embodiments” may be combined in any suitable manner with each other to form additional embodiments of such combinations. It is intended that embodiments of the disclosed subject matter cover modifications and variations thereof. Terms such as “first,” “second,” “third,” etc., merely identify one of a number of portions, components, steps, operations, functions, and / or points of reference as disclosed herein, and likewise do not necessarily limit embodiments of the present disclosure to any particular configuration or orientation.

[0045] Moreover, throughout this disclosure, various aspects of the invention can be presented in a range format. It should be understood that the description in range format is merely for convenience and brevity and should not be construed as an inflexible limitation on the scope of the invention. Accordingly, the description of a range should be considered to have specifically disclosed all the possible subranges as well as individual numerical values within that range. For example, description of a range such as from 1 to 6 should be considered to have specifically disclosed subranges such as from 1 to 3, from 1 to 4, from 1 to 5, from 2 to 4, from 2 to 6, from 3 to 6, etc., as well as individual numbers within that range, for example, 1, 2, 2.7, 3, 4, 5, 5.3, 6, and any whole and partial increments there between. This applies regardless of the breadth of the range. As used herein, the term “about” in reference to a measurable value, such as an amount, a temporal duration, and the like, is meant to encompass the specified value and / or variations of plus or minus 20%, plus or minus 10%, plus or minus 5%, plus or minus 1%, and plus or minus 0.1% of the specified value, as such variations are appropriate.

[0046] The terms “proximal,” “distal,” “anterior,” “posterior,” “medial,” “lateral,” “superior,” and “inferior” are defined by their standard usage indicating a directional term of reference. For example, “proximal” refers to a position that is situated nearer to the center of a body or point of attachment, while “distal” refers to a position that is situated away from the center of the body or point of attachment. In another example, “anterior” refers to the front of a body or structure,Docket # 206030-0333-00WG while “posterior” refers to the rear of a body or structure. In another example, “medial” refers to the direction towards the midline of a body or structure, and “lateral” refers to the direction away from the midline of a body or structure. In some examples, “lateral” or “laterally” may refer to any sideways direction. In another example, “superior” refers to the top of a body or structure, while “inferior” refers to the bottom of a body or structure. It should be understood, however, that the directional term of reference may be interpreted within the context of a specific body or structure, such that a directional term referring to a location in the context of the reference body or structure may remain consistent as the orientation of the body or structure changes.

[0047] The terms “patient,” “subject,” “individual,” and the like are used interchangeably herein, and refer to any animal amenable to the systems, devices, and methods described herein. The patient, subject or individual may be a mammal, for example, a human. The term “compliant mechanism” refers to a mechanism configured to produce motion and force transmission through the elastic deformation of its flexible components (also known as "flexures"), substitutes of rigid- body linkages or joints. When subjected to an external load, the flexures are capable of bending, twisting, or deforming elastically in precisely tuned ways to produce the desired motion. These slender elements may be designed with specified mechanical properties, such as stiffness and damping, for various degrees of freedom.

[0048] The term “metamorphic flexure bearing” may refer to a bearing that mediates movement via a flexure bearing during standard operation, and automatically transitions to the use of a conventional bearing as needed for further range of motion. The term “stage” may refer to a primary moving component, the position of which always corresponds directly to the bearing’s position. The term “intermediate body” may refer to a component that moves when range extension is needed. The term “grounded body” may refer to a fixed component of the bearing (also referred to, simply, as ‘ground’).

[0049] The term “mode” may refer to current operational configuration of the bearing, defined by which of the internal bearings is active. The term “flexure-bearing mode” may refer to a mode in which all movement directly corresponds to flexure-bearing movement. The term “conventional-bearing mode” may refer to a mode in which all movement directly corresponds to conventional-bearing movement. The term “transition” may refer to a brief period during which the mechanism is between flexure-bearing and conventional-bearing mode. The term “stage attachment” may refer to an attachment of stage to the intermediate body (normalDocket # 206030-0333-00WG lengthening transition start). The term “ground detachment” may refer to a detachment of intermediate body from ground (normal lengthening transition end). The term “ground reattachment” may refer to an attachment of intermediate body to ground (normal shortening transition end). The term “stage detachment” may refer to a detachment of stage from the intermediate body (normal shortening transition end. The term “combined mode” may refer to a mode where neither the flexure bearing nor conventional bearing are prevented from moving, resulting from ground detachment without stage attachment or stage detachment without ground reattachment.

[0050] The term “retention mechanism” may refer to a mechanism that exerts force as needed to hold (i.e., retain) two components together as needed. The term “continuous retention” may refer to a retention mechanism strategy that exerts negative force on the intermediate body in both flexure-bearing and conventional-bearing modes (e g., linear-force retention, constant-force retention). The term “linear-force retention” may refer to a retention mechanism strategy that exerts a negative force on the intermediate body in flexure-bearing mode that grows linearly throughout conventional-bearing mode. The term “constant-force retention” may refer to a retention mechanism strategy that exerts a constant negative force on the intermediate body in flexure-bearing mode that is maintained throughout conventional-bearing mode. The term “gravitational retention” may refer to retention utilizing gravitational force on the intermediate body.

[0051] The term “retention handoff’ may refer to a retention mechanism strategy in which two retention mechanisms are used - a ground catch to hold the intermediate body against the grounded body in flexure-bearing mode, and a stage catch to hold the intermediate body against the stage in conventional-bearing mode. The term “ground catch” may refer to a retention mechanism that exerts force between the intermediate and grounded bodies to maintain contact between the bodies. The term “stage catch” may refer to a retention mechanism that exerts force between the intermediate body and the stage to maintain contact between the body and stage. The term “conditional retention” may refer to a retention mechanism setup in which the intermediate body is anchored to the grounded body during flexure-bearing mode but not anchored to the stage during conventional-bearing mode (i.e., use of a ground catch but no stage catch). The term “ground-stage catch” may refer to the retention mechanism that exerts force between the secondary and ground stages to maintain contact between the stages. The termDocket # 206030-0333-00WG“primary-stage catch” may refer to a retention mechanism that exerts force between the primary and secondary stages to maintain contact between the stages.

[0052] The term “force profile” may refer to the force required to quasi-statically actuate (slowly move) or steadily hold a metamorphic flexure bearing at a given position in its range of motion. The term “home position” may refer to a location of the stage (relative to the grounded body) when the intermediate body is anchored to the grounded body and the flexure bearing is at its zero-force point. The term “home side” may refer to a position towards the side of the bearing that has the ground-side grip-fixture. The term “far side” may refer to a position towards the side of the bearing that has the stage-side stage grip fixture.

[0053] The term “load capacity” may refer to the ability of the flexure bearing to support loads outside its degrees of freedom without sustaining damage. The term “origin” may refer to the location of the primary stage (relative to the ground stage) when the secondary stage is anchored to the ground stage and the flexure bearing is at its zero-force point.

[0054] Conventional flexure bearings provide precise, low-maintenance operation but have a limited range of motion compared to conventional bearings. Flexure bearing design requires a trade-off between the bearing’s range of motion and its load capacity. For instance, increasing the range of motion requires decreasing the stiffness of the flexures. However, the stiffness of the flexures is also correlated with load capacity for a given design volume. Moreover, the concept of force isolation protects a flexure bearing from overloading via a parallel rigid mechanism within given range of motion, but it does not change the range of motion of the bearing or maintain the flexure bearing’s advantages while in its overloaded state. Thus, conventional bearings do not provide the advantages of a flexure bearing, while allowing a larger range of motion when needed.

[0055] To address these needs, the disclosure described herein provides a metamorphic flexure bearing that retains the advantages of precision, low wear, and low hysteresis over its limited flexure-bearing range, but provides an extended range of motion as needed. For instance, the metamorphic flexure bearing functions as a flexure bearing over a small but predominant range of motion and can fully transition to a conventional bearing for intermittent extended-range use. This extended range of motion is achieved via a position-activated transition to a conventional sliding or rolling bearing. Importantly, the bearing typically operates in mutually exclusive modes: a flexure-bearing mode and a conventional-bearing mode. In its flexure-bearing mode,Docket #: 206030-0333-00WG the bearing retains all the benefits of a flexure bearing and can be optimized to support a typical loading profile over that range. In its conventional-bearing mode, the bearing acquires a large range of motion while temporarily inheriting the disadvantages of a conventional bearing. Thus, for mechanisms that typically only require a small range of motion (e.g., automotive steering wheels, aviation control yokes, throttle controls), but occasionally require an extended range (e.g., tight turns, high acceleration, hard braking), the metamorphic flexure bearing adds all the benefits of flexure bearings over the small range, high-frequency portions of the mechanism’s operation, without restricting its motion over the occasionally required extended range. Embodiments of the metamorphic flexure bearing are described below with reference to the Figures. It is noted that examples of the metamorphic flexure bearing are described as being utilized in compliant joint reconstruction, such as compliant knee replacement; however, it should be understood that the metamorphic flexure bearing may be used in other devices and systems, such as, but not limited to, steering systems, biomedical implants, precision manufacturing systems, healthcare robotic systems, and the like.

[0056] FIGs. 1A-1C illustrate diagrams of linear-motion metamorphic flexure bearing 100 that uses linear-force retention, constant-force retention, and retention handoff. FIG. 1A illustrates the linear-motion metamorphic flexure bearing 100 using linear-force retention. FIG. IB illustrates the linear-motion metamorphic flexure bearing 100 using constant-force retention. FIG. 1C illustrates the linear-motion metamorphic flexure bearing 100 using retention handoff. The metamorphic flexure bearing 100 has at least three rigid bodies: a stage 102, an intermediate body 104, and a grounded body 106. The stage 102 is connected to the intermediate body 104 via a flexure bearing 108, and the intermediate body 104 is connected to the grounded body 106 via a conventional bearing (e.g., illustrated as a sliding contact 112).

[0057] The bearing 100 has at least two distinct modes of operation: flexure-bearing mode (i.e., default mode) and conventional-bearing mode (i.e., intermittent mode). In default mode, the intermediate body 104 is anchored to the grounded body 106 by a retention mechanism 110 (e.g., a compression spring) so that only the stage 102 is able to move. In this mode, the range of motion limitations of the stage 102 are enforced by internal hard stops at the end of its range within the intermediate body 104. When external force F on the stage 102 moves the stage 102 to the end of its range of motion and overcomes the preload of the retention mechanism 110, the bearing 100 transitions into conventional-bearing mode. In conventional -bearing mode, theDocket #: 206030-0333-00WG intermediate body 104 moves relative to the grounded body 106 by rolling or sliding. Of note is that the force used to actuate or maintain the bearing position is only applied externally to the stage 102, and any forces on the intermediate body 104 are solely internal forces. Movement of the intermediate body 104 is mechanically programmed to occur automatically at the positiondependent transition to conventional-bearing mode, as is re-anchoring of the intermediate body 104 to the grounded body 106 upon re-entering flexure-bearing mode.

[0058] The first row of FIGs. 1A, IB, and 1C illustrates the bearing 100 in flexure-bearing mode. The second row of FIGs. 1A, IB, and 1C illustrates the bearing 100 after transitioning into the intermittent conventional -bearing mode. The third row of FIGs. 1A, IB, and 1C illustrate the respective theoretical force profiles corresponding to each retention mechanism. The force profiles illustrate the force in the flexure-bearing range (FBR) and the force in the conventional- bearing range (CBR) required to hold the bearing 100 at any given position. The fixed grounded body 106 has a sliding surface 112 contact to the intermediate body 104. The intermediate body 104 is connected to the stage 102 through a linear-motion parallelogram flexure bearing 108. In flexure-bearing mode, the preload is maintained by the retention mechanism 110. FIG. 1A illustrates the intermediate-to-grounded-body retention mechanism 110 as being a preloaded compression spring. FIG. IB illustrates the intermediate-to-grounded-body retention mechanism 110 as being a constant-force spring. FIG. 1C illustrates the intermediate-to-grounded-body retention mechanism 110 as being a magnetic catch. In the conventional -bearing mode, in the spring-based retention cases (i.e., when the retention mechanism 110 is a compression spring or a constant-force spring as illustrated in FIGs. 1 A and IB), the contact between the intermediate body 104 and the stage 102 is maintained by the intermediate-to-grounded-body retention force, which is balanced by the external force on the stage 102. In the retention handoff case as illustrated in Fig. 1C, the contact between the intermediate body 104 and the stage 102 is maintained by a second magnetic catch. For each force profile plot, the range of motion increases while the required force on the stage 102 for a given conventional -bearing mode position decreases.

[0059] While any device that couples force to motion may feasibly be used to automate the position-dependent transition between flexure-bearing and conventional-bearing modes, the disclosure describes three retention mechanism strategies as provided herein. Each retention mechanism has a different profile of the force used to actuate the bearing over its mode transitionDocket # 206030-0333-00WG and conventional-bearing range of motion. For all metamorphic flexure bearings, the force is a piecewise function including the flexure-bearing force over the flexure-bearing range and the retention mechanism force exerted over the mode transition and conventional-bearing range, and in some cases, any hysteretic frictional force due to the conventional bearing. The force profile over the flexure-bearing range is independent of the retention mechanism type except when the retention mechanism exerts a force near the end of that range. One important design consideration includes the amount of force required to overcome the retention mechanism and enter conventional bearing mode. For the bearing to stay in flexure-bearing mode until its transition point, the preload provided by the retention mechanism is tuned to be at least as high as the maximum force of the flexure bearing; otherwise, the bearing enters a combined mode before reaching the end of the flexure bearing’s range of motion. When in this combined mode, both the flexure and conventional bearings are active (and the intermediate body is underconstrained), reducing the range over which the bearing possesses the benefits of solely acting as a flexure bearing.

[0060] The first retention mechanism 110 illustrated in FIG. 1 A may be a single preloaded compression spring. The preloaded compression spring may have a stiffness k and initial compression x exhibiting a force F=kx. Thus, when designing a metamorphic flexure bearing with a compression spring retention mechanism, design considerations may include the appropriate spring stiffness k for the retention mechanism and the necessary initial compression x. To maintain the bearing 100 in flexure-bearing mode throughout its full flexure-bearing range of motion, these parameters may be chosen to ensure that the preload force F is above the force required to move the flexure bearing to its range of motion limit.

[0061] To transition into conventional bearing mode, the external force on the stage 102 must exceed the preload of the intermediate body 104 against the grounded body 106. After the bearing 100 transitions to conventional-bearing mode, further external force on the stage 102 is opposed by the reaction force from the compression spring, which presses the intermediate body 104 against the stage 102 and, consequently, holds the flexures 108 of the stage 102 at the end of their compliant range of motion. Thus, with this retention strategy, the same retention mechanism 110 anchors the intermediate body 104 to the grounded body 106 during flexurebearing mode and holds the intermediate body 104 to the stage 102 during conventional -bearing mode, in both cases by exerting an internal force between the grounded body 106 and theDocket # 206030-0333-00WG intermediate body 104. When considering conventional-bearing mode range extension with a compression spring retention mechanism, another design consideration includes the fully- compressed length of the compression spring relative to its initial preloaded length, as the bearing 100 can only move as far as the compression spring can deform.

[0062] Linear-force retention requires progressively more force to hold the bearing’s position the farther the bearing is actuated. Some applications may benefit from a force profile that is constant throughout their range extension. For such designs, the retention mechanism 110 may be a constant-force (c-f) spring that provides the retention force, as illustrated in FIG. IB. The constant-force spring may be a pre-stressed spiral-wound spring, also known as negator spring. To select an appropriate c-f spring, the c-f spring should have a force Fcabove the maximum force of the flexure bearing to maintain the bearing in flexure-bearing mode throughout its full flexure-bearing range of motion.

[0063] As with linear-force retention, to transition into conventional bearing mode with constant force retention, the external force on the stage 102 must exceed the preload of the intermediate body 104 against the grounded body 106. However, after the bearing 100 transitions to conventional -bearing mode, all that is needed to maintain the position of the stage 102 at any position throughout the full range of the conventional-bearing mode range of motion is the constant force Fcto match the c-f spring. As this external force is matched by the c-f spring, the intermediate body 104 is held against the stage 102, maintaining the flexures 108 of the stage 102 at the end of their compliant range of motion. Thus, the retention needed for both modes is provided by a single retention mechanism, by means of an internal force between the grounded body 106 and the intermediate body 104.

[0064] The range of motion of the metamorphic flexure bearing 100 with the retention mechanism 110 being the c-f spring may be limited by the maximum extension length of the c-f spring (i.e., the point at which one and a half turns of the spring remain on its mounting shaft, in the case of a negator spring). This limit may be accounted for by selecting a c-f spring with a desired fully-extended length and may be enforced by a hard stop to protect the c-f spring.

[0065] In one or more cases, some applications may require a particularly far-reaching extended range of motion, or a very low force profile in conventional-bearing mode. Such applications may benefit from transferring (or “handing off’) the intermediate body 104 retention from the grounded body 106 to the stage 102 at transition (i.e., the retention handoff). This allows theDocket #: 206030-0333-00WG design to break free of all range-of-motion limitations imposed by single ground-to-intermediate- body retention mechanisms. In retention handoff, one retention mechanism 110a anchors the intermediate body 104 to the grounded body 106 during flexure-bearing mode, and a separate retention mechanism 110b anchors the intermediate body 104 to the stage 102 during conventional-bearing mode. During transitions between the modes, the engaged retention mechanism transfers retention of the intermediate body 104 to the previously disengaged retention mechanism before itself disengaging. Thus, only one of the two retention mechanisms 110a, 110b is active at once except at the momentary periods when the bearing 100 transitions from flexure-bearing to conventional-bearing mode or vice versa.

[0066] In an example, the bearing 100 may implement retention handoff using magnetic catches as the two retention mechanisms 110a, 110b, as illustrated in FIG. 1C. The magnetic catch topology may, for example, include two steel plates separated by one or more parallel magnets, with the magnets all oriented in the same direction from one steel plate to the other. An attractive force from this magnetic catch topology occurs when a third steel plate - the armature - is brought close to the two steel plates, and the armature completes a high-permeability path for the magnetic flux. As with the other retention mechanisms, the ground-to-intermediate-body magnetic catch (the “ground catch”) must have a holding force that is greater than the maximum force of the flexure bearing 108. This ensures that the intermediate body 104 stays anchored to the grounded body 106 during flexure-bearing mode.

[0067] As in the cases of continuous retention (e.g., via applying a linear-force or constantforce), to transition into conventional bearing mode with magnetic retention, the external force on the stage 102 must likewise exceed the contact force of the ground catch. Unlike with continuous retention, however, no force is required to maintain the position of the stage 102 after full transition to conventional bearing mode. Specifically, because retention handoff allows the ground catch to restrict its position-dependent force to within a small region, the mechanism requires no forces to maintain any given position in conventional-bearing mode beyond a short distance.

[0068] To enable this zero-force operation, due to the lack of continuous retention force between the intermediate body 104 and the grounded body 106, an intermediate-body -to-stage retention mechanism (the “stage catch”) is required to hold the stage 102 at the end of its flexure-bearing range of motion during conventional bearing mode. This stage catch must also have a holdingDocket # 206030-0333-00WG force greater than the maximum force of the flexure bearing 108, so that the intermediate body 104 stays anchored to the stage 102.

[0069] When the bearing 100 utilizes retention handoff to transition into conventional -bearing mode, its extended range of motion is no longer limited by the characteristics of the retention mechanism, but solely by the length of the conventional bearing, thereby allowing a substantially larger range extension in a more compact form factor.

[0070] FIG. 2 illustrates an example metamorphic flexure bearing 200 with selectable and tunable retention mechanisms.

[0071] The bearing 200 may be a linear-motion metamorphic flexure bearing with a cylindrical form factor. The bearing 200 may be, for example, about 81 mm in diameter and about 238 mm long. The bearing 200 may include grip fixtures on each side of the bearing 200. For ease of design and consistency of testing, the bearing 200 incorporates all three retention mechanisms (i.e., a preloaded compression spring, a constant-force spring, and a magnetic catch) in a manner that would make the retention mechanisms selectable and tunable.

[0072] To maximize the flexure-bearing range of motion, the lengths of the flexures 208 are maximized by inverting the bearing relative to the designs of the bearing 100 illustrated in FIGs. 1A-1C. Specifically, the conventional bearing 204 is positioned on the inside of the bearing 200, and the flexure bearing 202 is positioned on the outside of the bearing 200, while maintaining all functional relationships between the components. The flexure bearing 202 may be, for example, a linear-motion flexure bearing with two pairs of curved blade flexures 208. The linear-motion flexure bearing may be similar to the parallelogram linear-motion flexure bearing illustrated in FIGs. 1A-1C, but in a cylindrical form factor. The conventional bearing 204 may be, for example, a linear-motion plain bearing composed of two square-profile plain bearings. To test different retention mechanisms, the bearing 200 includes a compression spring 206, constantforce springs 210, and magnetic catches 212.

[0073] As illustrated in FIG. 2, the top left image shows the physical bearing 200, and the center image shows a cross-section view of the bearing 200. In the cross-section, the mechanical grounded body includes a square linear shaft 214 with two end caps 216 held on by a bolt 218 running through the shaft 214. The intermediate body 220 rides on the shaft 214 via two square linear plain bearings (i.e., comprising the conventional bearing portion 204 of the bearing 200). The intermediate body 220 is connected to the stage 222 through a linear-motion flexure bearingDocket #: 206030-0333-00WG202. The bearing 202 may include flexures 208 and atachment rings (omited in detailed view for ease of viewing). The retention mechanisms 206, 210, 212 hold the intermediate body 220 to the grounded body’s home side 224 (e.g., left side). The bearing 200 includes retention mechanisms 206, 210, 212 for brevity. One retention mechanism was used at a time during testing. To tune the retention force of the compression spring 206, the bearing 200 includes springs of different lengths, and the initial compression is fine-tuned using in-line washers. To tune the retention force of the constant-force spring 210, a subset of the constant-force springs are attached to hooks on the intermediate body 220. To tune the retention force of the magnetic catch 212, the number of parallel, identically-aligned magnets 228 between the steel plates were altered, and the magnetic catch on the stage-side 230 was tuned in the same manner. Grip fixtures 226 on each end facilitate attachment of the bearing 200 to a universal testing machine for force testing.

[0074] The metamorphic flexure bearing 200 transitioned between flexure-bearing mode and conventional bearing mode appropriately with all retention mechanisms tested. Specifically, the bearing 200 automatically transitioned from flexure-bearing mode to and from a conventional- bearing mode at the flexure-bearing range of motion limit in all cases where the retention force was sufficient. A compliant mechanism force profile from approximately -10 to +10 mm (i.e., flexure-bearing mode) that was repeatable across all retention mechanism types 206, 210, and 212 and retention force levels, as illustrated in Figs. 3A, 4A, and 5A, with a stiffness proportional to its distance from its zero-force point. The bearing 200 was swept through five linear-force, six constant-force, five ground-catch, and five stage-catch preload levels. Each plot illustrated in FIGs. 3A, 4A, and 5A display the external force required between the ground and stage grip fixtures to quasi-statically hold the bearing at a given extension over the swept preload levels. Range limits at -10 mm and above +45 mm are illustrated by dramatically-increasing stiffness. The quasi-static forces of the bearing 200 in the different configurations differed only in the force required for mode transitions and in the forces required during conventional bearing mode (at +10 mm and above).

[0075] Hysteresis is observable in the force profiles during conventional-bearing mode due to the friction from the plain bearing. Some hysteresis is also present in the flexure bearing of the bearing 200 due to the use of fused filament fabrication in producing the flexures 208.Docket # 206030-0333-00WG

[0076] With respect to the linear-force retention force profile as illustrated in FIG. 3A, in all five test cases that used linear-force retention, linear force growth throughout the conventional bearing mode was observed. One test with each of three compression springs of different lengths was performed, and then two additional tests with the shortest spring varying its initial compression and preload using one and then two washers in series. The preloads are labeled for each curve on the force plot in FIG. 3A. The transitions and range limits are labeled on the stiffness plot in FIG. 3 A. For the five linear-force retention cases, the measured preloads were 2.1, 4.9, 6.4, 9.7, and 14.5 N, with transitions occurring at approximately 9.1 mm and completing in approximately 0.2 mm in all cases except the 2.1-N preload case, in which case the transition started at 7.2 mm and finished at 9.6 mm. The difference between these transition cases is illustrated in FIG. 3A by the lack of a force step (i.e., no stiffness impulse) for the 2.1-N preload case, as well as a momentary drop in stiffness during the transition to 0.348 N / mm (e.g., as illustrated in the figure inset of the stiffness plot in Fig. 3 A). This 2.1-N preload was below the maximum flexure bearing force of 3.6 N. The spring rates (e.g., 0.660, 0.662, 0.676, 0.575, 0.492 N / mm) for the five test cases did not substantially differ from their datasheet values. The range limits (varying according to spring initial and maximum compression lengths) were 54.0, 49.7, 48.3, 49.0, and 45.9 mm, with forces at those range limits of 33.5, 32.1, 33.6, 32.8, and 33.1 N, respectively.

[0077] As illustrated in FIG. 3A, the force profile of the metamorphic flexure bearing 200 is shown over its full range when using a compression spring for retention. A legend at right in FIG. 3A shows the compression springs used with vertical bars marking their resting lengths and arrows in corresponding colors indicating the initial compression of the spring during flexure bearing mode, as inserted into the mechanism. Stiffness is also shown with a shared extension axis, in which stiffness data smoothed to remove noise. There is a force step at approximately +9. 1 mm in all curves except the curve corresponding to the lowest preload level. Images illustrate in FIG. 3A from the Instron test (e.g., 4.9 N preload case) correspond with the extension axis and include a dotted line (corresponding to its force profile curve) indicating the equilibrium point. All shortening data are plotted in FIG. 3 A with an alpha value of 0.5 to distinguish them from the lengthening data.

[0078] FIG. 3B is a table illustrating the stage and ground attachment and detachment extensions and forces from linear-force retention force profiles of FIG. 3 A. The extension and force valuesDocket #: 206030-0333-00WG at the stage attachment, ground detachment, ground reattachment, and stage detachment are shown, with colors corresponding to the plot colors used in Figure 3A. Forces are provided in bold for ease of comparison with one another, and the preload forces are italicized for ease of reference. The highlighted values with colors corresponding to plot colors were averaged for plot labeling. The values highlighted in gray show the flexure bearing range and maximum flexure bearing force. The unhighlighted values correspond to transition completion in the subthresholdpreload case. In this case, a combined mode is entered, so the ground detachment occurs before the stage attachment, and the stage detachment occurs before the ground reattachment. This combined mode ends at an extension (e.g., 9.3mm and 9.8mm) beyond the flexure bearing range but at approximately the maximum flexure bearing force.

[0079] With respect to the constant-force retention force profile as illustrated in FIG. 4A, in each of the six test cases that used constant-force retention, the force remained substantially constant throughout its conventional bearing mode. These test cases included retention mechanisms composed of two to twelve constant-force springs in increments of two. The preloads are labeled for each curve on the force plot, and the transitions and range limits are labeled on the stiffness plot, as illustrated in FIG. 4A. For the six constant-force retention cases, the measured preloads were 2.8, 4.6, 6.5, 8.3, 10.1, and 11.9 N, with transitions occurring at 9.1 mm for all cases except the 2.8-N preload case, in which case the transition started at 8.3 mm and never fully completed the transition into conventional-bearing mode. As in the linear-force retention case, the lack of a force step (and corresponding lack of a stiffness impulse) sets the 2.8-N constant-force case apart from the higher-force cases. This 2.8-N preload was below the maximum flexure bearing force of 3.6 N. The effect of the far-side hard stop was observed at approximately 69.7 mm in all cases, with forces at those range limits of 3.6, 5.4, 7.5, 9.5, 11.6, 13.9 N, respectively, with force increases localized near the beginning of the conventional bearing range, in which there are relatively small differences between these forces and the preload forces.

[0080] The force profile of the metamorphic flexure bearing 200 is shown over its full range when using constant-force springs for retention, as illustrated in FIG. 4A. The legend in the bottom right corner of FIG. 4A shows the number of constant-force springs used in each case. Stiffness is also shown with a shared extension axis, in which the data is smoothed to remove noise. The force step at approximately +9.1 mm is present in all curves except the curve corresponding to the lowest preload level. Due to the constant-force nature of this retentionDocket #: 206030-0333-00WG mechanism, the lowest constant-force spring never fully enters convention al -bearing mode but stays in a combined mode after transition (i.e., the stage 222 does not contact the intermediate body 220). Images illustrated in FIG. 4A from the Instron test (6.5-N preload case) correspond with the extension axis and include a dotted line which corresponds to its force profile curve and indicates the equilibrium point. All shortening data are plotted in FIG. 4A with an alpha value of 0.5 to distinguish them from the lengthening data.

[0081] FIG. 4B is a table illustrating the stage and ground attachment and detachment extensions and forces from constant-force retention force profiles of FIG. 4A. The extension and force values at the stage attachment, ground detachment, ground reattachment, and stage detachment are shown, with colors corresponding to the plot colors used in Fig. 4A. Forces are provided in bold for ease of comparison with one another, and the preload forces are italicized for ease of reference. The highlighted values with colors corresponding to plot colors were averaged for plot labeling. The values highlighted in gray show the flexure bearing range and maximum flexure bearing force. Note that the stage attachment and stage detachment are intentionally left blank; in the case of constant-force retention, bearings with subthreshold preloads do not complete the transition. Instead, a combined mode is entered into at ground detachment, and the bearing remains in this combined mode until ground reattachment.

[0082] With respect to the retention handoff force profile as illustrated in FIG. 5A, in all nine test cases that used retention handoff, it was observed that a force remained substantially constant throughout the conventional bearing mode and was nearly identical across all test cases (e g., see the intermediate body weight zoom inset of the bottom plot in Fig. 5A). This force offset was due to gravitational force on the intermediate body 220 that assisted all retention mechanisms, and the force would not have been present if the bearing 200 had been oriented horizontally. Rather, this force arose from the positive vertical orientation in which the bearing 200 was placed in the universal testing machine and was also present (though not obviously so) in the continuous retention mechanism test cases above.

[0083] With respect to gravitational retention, the mass of the intermediate body is an important consideration in retention mechanism design. More intriguing, however, is that gravitational force can itself be used as a continuous (specifically, constant-force) retention mechanism. When doing so, the use of a larger intermediate body mass will allow for greater negative acceleration due to the greater proportion of the force being used to accelerate the mass as opposed to keepingDocket #: 206030-0333-00WG the flexure bearing extended. However, because the mass must always increase to increase the gravitational retention force, the maximum negative acceleration of an intermediate body using gravitational force as a retention mechanism is always limited to the standard gravity of whichever celestial body on which the bearing is operated (e.g., 9.8 m / s2on earth.

[0084] The nine test cases included five cases sweeping from two to ten magnets in the groundcatch in increments of two while holding the stage-catch force constant, and vice versa. Groundcatch and stage-catch preloads - corresponding to ground detachment and stage detachment, respectively - are labeled for each curve of each sweep in FIG. 5A. The stage and ground attachment and separation locations and forces are given in the table illustrated in FIG. 6.

[0085] Returning to FIG. 5A, the force profile of the metamorphic flexure bearing 200 is shown over its full range when using magnetic catches for retention. The number of magnets used in the ground catch and stage catch for each case is shown on the right, with the colors of the swept- force catch corresponding to the colors of each curve and with an asterisk indicating data that is shown in both plots in FIG. 5A. The plot above the images from the test shows the extension and return curves for a sweep of ground-catch contact forces at a constant stage-catch contact force, and the plot below the images shows the extension and return curves for a sweep of stage-catch contact forces at a constant ground-catch contact force. Insets show detail of the swept preload values at the ground and stage detachments. As noted above, gravitational retention assisted the system in the particular mounted orientation used. Images from the four-magnet-ground-catch six-magnetic-stage-catch case Instron test correspond with the extension axis and include a dotted line indicating the equilibrium point. The constant-detachment-force data curves are plotted with an alpha value of 0.5 to distinguish them from the detachment-force swept curves. The direction of travel is indicated in the zoom insets with gray arrows. The Table illustrated in FIG. 6 provides the transition data corresponding with this plot illustrated in FIG. 5A. FIG. 5B illustrates alternative zoomed in insets showing changes in attachment forces. FIG. 5B illustrates a zoomed-in view of the attachment forces as the corresponding preloads are being swept. The top plot in FIG. 5B corresponds to the transition on the shortening curves. The bottom plot in FIG. 5B corresponds to the lengthening curves. For consistency with FIG. 5A, curves of interest for FIG. 5B are shown with an alpha value of 0.5.

[0086] The extension and force values at the stage attachment, ground detachment, ground reattachment, and stage detachment are shown, with colors corresponding to the plot colors usedDocket #: 206030-0333-00WG in Fig. 5, and with boxes around values corresponding to the plot zoom insets of Fig. 5. Forces are provided in bold for ease of comparison with one another, and the preload forces are also italicized for ease of reference. Note that the preload forces correspond to detachments and that the attachment forces are substantially lower in magnitude than detachment forces.

[0087] When approaching the transition from flexure-bearing to conventional-bearing mode, the stage-side magnetic catch (“stage catch”) began to exert an attractive force between the stage 222 and the intermediate body 220. This internal force reduced the external force required to extend the flexure bearing 202, typically even requiring an opposing (negative) force just before attachment, as illustrated by the extension curve insets in FIG. 5. After attachment of the stage catch, the force then grew rapidly toward detachment of the ground-side magnetic catch (“ground catch”).

[0088] During detachment of the ground catch as illustrated by the extension curve insets in FIG.5, the force increased to the full magnetic-catch contact force. The force was greater than the maximum flexure bearing force plus the intermediate body weight in all cases tested. After the force increased, the force then dropped to the intermediate body weight upon transition into conventional-bearing mode.

[0089] During re-attachment of the ground catch as illustrated by the return curve insets in FIG. 5, the catch attached with less required external force between the ground and stage than was needed during detachment. The force then momentarily returned to zero as the intermediate body weight was again borne by the grounded body. After momentarily returning to zero, the force again building (downward) towards detachment from the stage catch.

[0090] During detachment of the stage catch as illustrated by the return curve insets in FIG. 5, as with the ground catch, the catch required more force to detach than it required during attachment.

[0091] It was typically found that disengaging a magnetic catch required more travel and force than engaging it. Stronger magnetic catch forces amplified this effect, and it was particularly evident in the stage catch, which was mounted to a less rigid component (the far side of the intermediate-body) than the grounded body.

[0092] Over a selected range of motion, the bearing 200 operates as a compliant mechanism (i.e., in flexure-bearing mode). Beyond this initial limited range, the metamorphic bearing 200 automatically transitions to a conventional mechanism to achieve an extended range (i.e., in conventional-bearing mode). By defaulting to its flexure-bearing mode over a selected range, thisDocket #: 206030-0333-00WG bearing 200 prioritizes the advantages of a flexure bearing up until an extended range is required, at which point it temporarily inherits all the advantages and disadvantages of a conventional bearing.

[0093] An important characteristic of the metamorphic flexure bearing is how it handles bearing loads - that is, loads exerted in directions other than the bearing’s degrees of freedom. With this architecture, bearing loads borne by the conventional bearing may also be borne by the flexure bearing and vice versa. In other words, all bearing loads are borne in series across the two internal bearings. In conventional-bearing mode, the flexure bearing sustains bearing loads at its maximum extension. Thus, the flexure bearing is designed to match the highest load capacity of the bearing, but it need only sustain these loads over its selected range of motion. Bearing loads borne by a stage-catch retention mechanism reduce the bearing load seen by the flexure bearing. The bearing 200 described herein thus makes it possible for some typically low-range applications, for which flexure bearings were not previously suitable, to use flexure bearings with tuned range limits without compromising full extension of the bearing. The features provided by the bearing 200 enables these applications, which were previously unable to use compliant mechanisms, to now claim the advantages of a flexure bearing - high repeatability, low wear, and low friction - over a selected range.

[0094] The metamorphic flexure bearing was implemented and characterized using three example retention mechanisms (i.e., linear-force retention, constant-force retention, and retention handoff). The distinct advantages and disadvantages of each mechanism in the bearing 200 was illustrated

[0095] With respect to the ranges of motion provided by the bearing 200, the bearing 200 has a flexure-bearing range of motion of approximately -10 mm to +10 mm and a range extension of 35 mm or more, depending on the retention mechanism and retention mechanism parameters.

[0096] In the case of the linear-force retention mechanism, which demonstrated a moderate length range extension, the range extension was limited by abutment against the fully compressed spring. While this range extension may be made substantially longer by using a considerably longer compression spring, the further range extension would come at the cost of needing to extend the length of the mechanism itself by additional unused length to accommodate the longer fully-compressed spring length. In addition, longer compression springsDocket # 206030-0333-00WG of a given diameter are less stable against lateral buckling, making it more difficult to find as off- the-shelf components for a longer-range extension with linear-force retention.

[0097] The constant-force retention mechanism provided a full 60 mm of range extension, but this did not fully demonstrate the potential for range extension with a constant-force (c-f) spring. The c-f springs tested in the bearing 200 are capable of a 457 mm maximum extension, allowing the design of a substantially longer conventional-bearing shaft to take advantage of this full length. It is also worth noting that c-f springs have a very limited lifetime. For example, the c-f springs tested in the bearing 200 are rated for just 25,000 cycles. This potentially negates the longevity benefits of a flexure bearing if the flexure bearing is being optimized for low fatigue, but the application requires more than just occasional use of the conventional-bearing-mode range.

[0098] While the demonstration of the retention handoff mechanism was limited by the 70-mm hard stop in the bearing 200, in practice the bearing 200 need not see a limit on its range of motion under retention handoff. For instance, using retention handoff with a trackless wheeled mechanism (e.g., a cart on a horizontal surface) in a metamorphic flexure bearing may provide an infinite range extension.

[0099] With respect to force profiles, the variety of possible retention mechanisms with which the metamorphic flexure bearing can be constructed allows the force applied by the bearing 200 to be tuned, and consequently, the stiffness, of the bearing across its full range of motion. For instance, if a constant-stiffness flexure bearing and a linear-force retention mechanism of the same stiffness are used in the bearing 200, with the preload equal to the maximum flexure bearing force, the bearing 200 will have a constant-stiffness profile over its full range of motion. The bearing 200 may behave similarly to a constant-stiffness spring and the force profile may provide no indication of when the transition occurs. If, on the other hand, an indication of transitions is desired, a large force step or change in stiffness can be designed into the mechanism. The various curves shown in Figs. 3 and 4 demonstrate this designability and suggest the possibilities of many other force profiles.

[0100] Creating characteristically different profiles, however, requires using different retention mechanisms. For instance, of the retention mechanisms explored herein, the retention handoff strategy is the only explored mechanism that can provide zero-force position maintenance once fully in its conventional bearing mode, mimicking the operation of a typical conventionalDocket #: 206030-0333-00WG bearing. For instance, there is a point far enough from the ground-side magnetic catch where the magnetic force fades, the machine’s retention mechanism effects disappear, and all that remains is the stage moving through space as a conventional bearing. It should be noted, however, that the conventional bearing exhibits hysteresis due to friction, an unavoidable limitation of the conventional bearing, which introduces a small amount of force opposing any movement of the bearing while in conventional-bearing mode.

[0101] With respect to combined-mode operation and maximum negative acceleration, the retention mechanism may be tuned so that its force is greater than the maximum flexure bearing force. However, when this retention force requirement is violated, range is reappropriated from the flexure-bearing mode. For instance, suppose the retention force between the grounded and intermediate bodies is below the maximum flexure bearing force. In this configuration, the intermediate body will disconnect from the grounded body before the stage has contacted the intermediate body, causing the bearing to enter a combined (flexure-bearing and conventional- bearing) mode. The amount of range allocated to this mode should typically be minimized, because any range allocated to the combined mode reappropriates range from the flexure-bearing mode.

[0102] Different retention mechanisms interact with combined-mode operation in distinct ways. In the case of linear-force retention, the combined mode is transient and (quasi-statically) repeatable, because the retention mechanism force grows with the range and will eventually exceed the maximum flexure bearing force. In the case of constant-force retention, any combined mode entered into at transition is persistent, because the retention force (being constant) will never grow to exceed the maximum flexure-bearing force. When using retention handoff, if the ground catch has insufficient force, it will likewise enter a combined mode with an underconstrained intermediate body instead of fully transitioning to conventional-bearing mode. This transition into combined mode under incomplete retention handoff will also result in oscillation of the intermediate body mass due to a jump discontinuity in its quasi-static resting position. Further yet, in the retention handoff case, quasi-static transition from combined mode back to flexure-bearing mode only occurs when the stage fully returns to its home position (unless it has first transitioned into conventional-bearing mode by fully extending to the far-side grounded- body hard stop), increasing the amount of distance traveled in conventional-bearing mode.Docket #: 206030-0333-00WG

[0103] Though entering a combined mode at transition is generally undesirable, there may be instances where a combined mode is advantageous. For example, because a combined mode under linear-force retention is transient and repeatable, it guarantees a continuous monotonically- increasing force profde. This could enable the position of the stage to be sensed via the force on the bearing, allowing the bearing to be smoothly positioned-controlled via force control. It should be noted, however, that when in combined mode, the bearing exhibits reduced stiffness due to the series arrangement of the flexure bearing and retention spring. In the subthreshold preload case that was tested, a reduction to approximately half stiffness was observed during the combined mode, due to the flexure bearing and retention springs being approximately matched in stiffness, making their effective series-combined spring constant one half their individual stiffnesses.

[0104] For designs where a combined mode is acceptable, a conditional retention strategy can be implemented wherein only a single force-detachable retention mechanism - the ground catch - is used, resulting in the bearing having only flexure-bearing and combined modes. The disadvantages of such a strategy are ringing of the intermediate body mass at transition and increased conventional-bearing travel before transitioning back to flexure-bearing mode. However, the conditional retention strategy has the advantage of needing only a single catch while still exhibiting unlimited range extension (similar to that provided by the retention handoff strategy), which might be a sufficient advantage in some designs to warrant its implementation. Where relevant, the disadvantage of intermediate body ringing in conditional retention can be mitigated by submerging the bearing in an aqueous, viscous, or pneumatic environment to dampen the ringing, and the disadvantage of a shifted flexure-bearing-mode re-entry transition setpoint is minimized if the bearing typically crosses past its home position after flexure-bearing range re-entry or if it very infrequently enters conventional-bearing mode

[0105] Venturing beyond quasi-static analysis reveals yet another way for the bearing to enter combined mode. Acceleration of the intermediate body mass requires additional force, which affects the preload between the intermediate body and the stage. While positive acceleration of the stage (relative to the grounded body) increases the contact force between the intermediate body and the stage, negative acceleration (decreasing positive velocity or increasing negative velocity) decreases this contact force. Thus, if the force required for negative acceleration of the stage mass exceeds the quasi-static contact force (the difference between the maximum flexureDocket #: 206030-0333-00WG bearing force and the retention force between the intermediate body and the stage), the retention force will be insufficient to accelerate the intermediate body, resulting in the stage disconnecting and the bearing entering combined mode. In the linear-force retention case, the maximum negative acceleration that the bearing can have without entering combined mode grows with increasing positive position of the bearing. In the constant-force retention case, the maximum negative acceleration that the bearing can have without entering combined mode is constant. In either case, however, the bearing will automatically return to its conventional-bearing mode if the bearing is in its conventional-mode region and the stage ceases to exceed this maximum negative acceleration threshold, because during an acceleration-induced combined mode, the intermediate body is being accelerated at its maximum acceleration, so it will eventually catch up to the stage. On the other hand, when using a retention handoff strategy, any combined mode due to acceleration is again persistent, just as in the case of quasi-static combined-mode entry. Notably, however, in the case of retention handoff, the acceleration threshold can be made different from the quasi-static transition by tuning the stage catch force to be different from the ground-catch force. Further, while the tests conducted herein discuss only inertial forces and assumed ideal conventional bearings and ideal retention mechanisms here, in all these cases, friction and tolerances should also be accounted for in maximum acceleration calculations. Importantly, the maximum acceleration of the stage relative to a fixed ground is a factor to consider in the design of a metamorphic flexure bearing.

[0106] With respect to orientation and momentum considerations, while the bearing need not be in any particular orientation for its operation, if the intermediate body is of non-negligible mass, the weight of the intermediate body will detectably alter the force required to maintain the bearing’s position during conventional-bearing mode when the bearing is in any non-horizontal position in an inertial reference frame (e g., on earth). In the vertical position in which the bearing was mounted, when taring the load, the weight of the stage weight was compensated for. However, upon transition to conventional -bearing mode, the intermediate-body weight was added to the weight of the stage and was thus borne by the load sensor, resulting in a corresponding positive increase in the force reading. If the bearing was mounted upside down (i.e., anchoring the stage side to the lower grip and actuating the ground side using the upper grip), when taring the load, the force from the combined grounded and intermediate bodies would have been compensated for. Upon transition to conventional-bearing mode, theDocket #: 206030-0333-00WG intermediate-body weight would have then been removed from the grounded body and thus no longer borne by the load sensor, resulting in a decrease in the force reading corresponding to the weight of the intermediate body. Therefore, the effect of weight on the required force to maintain the bearing position, and particularly its effect on preload levels, should be considered when the intermediate body mass is non-negligible. This effect is related to the discussion above of the simultaneous acceleration of the stage and ground (considering vertical orientation as resulting in gravitational acceleration). Thus, in considering both the acceleration and orientation of the bearing, the intermediate body should generally be designed to be as low mass as possible while still keeping it a rigid body.

[0107] The mass of the stage may also be an important factor for some applications when considering system dynamics. In particular, momentum could be used to smooth out transitions in the retention handoff case. For instance, when quickly transitioning from flexure-bearing to conventional-bearing mode, the momentum of the stage would be transferred to the combined intermediate body and stage, and this momentum could be sufficient to overcome the groundside retention mechanism force without any additional force on the stage. In the opposite direction (when quickly transitioning from conventional-bearing to flexure-bearing mode), the momentum of the stage could also be sufficient to overcome the force of the intermediate-body retention mechanism.

[0108] With respect to magnetic catch asymmetry, whenever a magnetic retention mechanism is attached to a compliant component, the compliant component will need to be stretched to the full contact force of the magnetic retention mechanism before detachment occurs, making detachment occur at full force but at a further distance away from the retention mechanism than would be expected with fully-rigid components. Conversely, upon approaching contact, the compliant component will not begin to stretch until the magnetic retention mechanism is close to contacting, and hence, the attachment will occur at a lower force and at a shorter distance from the contact point than the force and distance required for detachment. FIG. 8 illustrates a diagram depicting the principles of a magnetic catch assembly. FIG. 8 visually highlights the phenomenon resulting in asymmetry between the magnetic catch attachment and separation forces and locations. A compliant component 802 hangs from a mechanically-grounded hook 804 and has a magnet 806 attached to its lower end. The magnet 806 is approached by a ferromagnetic steel plate 808 affixed to the top of a rigid component 810. On approach, theDocket # 206030-0333-00WG magnet 806 only attaches to the steel 808 when it draws close, which slightly stretches the compliant component 802, and the rigid component 810 experiences a force equal only to the compliant component’s stretch times its stiffness. On retreat, because the magnet 806 and steel 808 are already attached, the full contact force is exerted internally between the two elements as a preload, and the compliant component 802 stretches until it exerts a force sufficient to overcome this preload. Thus, when one (or both) of the components containing magnetic catch elements are compliant, they experience a reduced attachment force at a slight offset on approach and a full detachment force at a larger offset on retreat.

[0109] With respect to the Magnetic Catch Asymmetry model, let the force between the steel plate and the magnet be governed by F = elided), where d is the distance between the magnet’s center and the steel plate’s center and a and c are non-zero positive constants. Further, let the compliant component have a constant stiffness with a spring constant of k. The force required for the magnet to extend the spring and attach to the steel, then, is F=k*x, where x is the position of the steel relative to the ideal attachment point (see Fig. 8 for an illustration of this model). For the force between the steel plate and the magnet to match the force required to attach the magnet to the steel, then the steel plate must be moved to or closer than the attachment point .^attachment—cl(k*d^d). Thus, there is some non-zero distance d between the steel and magnet on approach where the force becomes sufficient to extend the spring and the magnet rapidly crosses that distance to attach to the steel.

[0110] On retreat, however, the force between the magnet and the steel is a constant, Fmax, so the magnet and steel separate at Xseparation Fmax I k.

[0111] Note that with infinite stiffness k, the actual attachment and separation points are the ideal attachment and separation points x = 0, and that both the actual attachment and separation locations grow with decreasing stiffness. For a non-zero positive k, it can also be shown that ^atachment increases with increasing c (increasing magnetic catch strength increases attachment distance) and that ^attachment decreases with increasing a (localizing the magnetic catch force [so that the force occurs closer to contact] reduces attachment distance).

[0112] This example illustrates that compliance in rigid components alters attachment and separation points and forces for catches. Because the various possible types of retention mechanisms used for retention handoff will have different force profiles than a magnetic catch, the profile of each should be considered when considering the implications of rigid componentDocket # 206030-0333-00WG compliance. Further, rigid component compliance can have effects on continuous retention mechanisms as well that should be considered in design.

[0113] Because the “rigid” components were composed of plastic, all these components had some non-negligible compliance to them. The effects of this compliance can be seen in the differences between the detachment and attachment forces and locations of the magnetic catches for both the ground catch and stage catch, and is more clearly observed when the catches are tuned to have higher contact forces. This rigid body compliance is also responsible for the smooth transitions observed with the continuous retention mechanisms, albeit with different stiffnesses for each, determined by the force path through the bearing and by differing application locations of the internal retention-mechanism forces. Notably, the difference between the detachment and attachment forces and positions is substantially larger for the stage catch, where the catch was included in the mechanism in a location with substantially higher flexion (i.e., far from the center axis of the bearing and mounted on an unsupported arm as illustrated in FIG. 2, than the location of the grounded body (i.e., close to the center axis of the bearing.

[0114] The embodiment provided herein describe three possible retention mechanisms here, but it is noted that other embodiments are contemplated that can maintain the preload of the intermediate body to the ground and stage in flexure-bearing and conventional -bearing mode, respectively. For example, linear-force retention can be performed with an extension spring instead of a compression spring, or different continuous retention mechanisms can be used (including mechanisms not involving springs) to provide custom force profdes. By choosing a vertical orientation and a high intermediate body mass, even gravitational force can be used as a retention mechanism.

[0115] A compression spring is described herein as implementing linear force retention, but an extension spring could similarly be used (e.g., the c-f spring being replaced with a preloaded extension coil spring). In either case, a spring-like element such as a pneumatic cylinder could be used instead, or a different spring geometry could be used. Further, the same compliant mechanism topology used for the flexure bearing could also be used for the retention mechanism, though the benefits of doing so are not immediately apparent, noting that the metamorphic flexure bearing’s advantage lies in its range of motion extension, and thus any spring used for a retention mechanism ought to have a larger range of motion than its corresponding flexure bearing, a requirement that is easier to attain using a compliant mechanismDocket # 206030-0333-00WG with more degrees of freedom which are then constrained by the conventional bearing. Of course, the force profiles need not be limited to linear or constant force but can vary in position as needed for a given design. Further, active retention, such as a motorized winch, could provide continuous retention with a force that varies through not only space but time as well.

[0116] For operation of the bearing without a fixed ground, in conventional -bearing mode, if the stage is kept fixed and the grounded body is accelerated, acceleration of the intermediate body mass is no longer a factor and only friction must be accounted for. Similarly, if both the grounded body and stage are accelerated relative to the global frame, it is the true acceleration of the intermediate body relative to the global frame that matters. For instance, if both the ground and the stage are negatively accelerated, even if the metamorphic flexure bearing does not change length, this scenario can also switch the bearing into combined mode. Further, when in flexure-bearing mode, “grounded” body negative acceleration can result in a transition to a combined mode if the ground-catch force is insufficient to accelerate the intermediate body mass.

[0117] A variety of other force profiles are also possible. It is noted that the slight increase in force observed in the implementation of constant-force retention agrees with the literature, which describes negator springs as approaching their constant force asymptotically. This can be mitigated by using one of many other possible truly-constant-force mechanisms. Moreover, while retention handoff can be implemented by a variety of magnetic catch topologies for a variety of force profiles, retention handoff need not use magnetic elements; it can be enabled by other catches, such as touch fasteners or roller catches. Further, for some applications, there may only be a need for a single, ground-side retention mechanism.

[0118] Embodiments are contemplated in which the bearing utilizes bidirectional range extension. For instance, although extending the range of a flexure bearing in a single direction can benefit many applications, many other applications require bidirectional range extension (e.g., automotive steering wheels, aviation control yokes, control valves, etc.). The embodiments described herein relate to range extension beyond one side of the flexure bearing’s range of motion, with a hard stop at the other end of its range of motion that denies further movement of the intermediate body in that direction. This hard stop is one way to anchor the intermediate body during flexure-bearing mode, but it also intrinsically prevents bidirectional range extension via the intermediate body. However, it is noted that intermediate body anchoring can beDocket # 206030-0333-00WG maintained in flexure-bearing mode while enabling bidirectional motion via conventional- bearing mode.

[0119] Just as a flexure bearing with two internal hard stops can be extended in one direction using a conventional bearing, a conventional bearing with two internal hard stops that extends in one direction can be extended in the opposite direction using another conventional bearing. As illustrated in FIG. 7A, the original grounded body 106 may now act as a second intermediate body that is anchored to a new grounded body 702 via an additional retention mechanism 110 (e g., in this case, a compression spring). When sufficient force is exerted in the direction previously limited by the hard stop, the force on the hard stop will overcome the preload between the second intermediate body 106 and the new grounded body 702, allowing movement in the new direction. Of course, this strategy could be implemented with various retention mechanisms, though different considerations may be required for each (e g., the use of pulleys to mediate transitions via gravitational force retention).

[0120] Retention handoff differs from continuous retention in that it requires not one but two retention mechanisms that can each attach and detach as needed for transition of the bearing between modes.

[0121] Magnetic retention is used herein. Magnets are advantageous because they do not require direct contact, and depending on the type of magnetic catch design used, the rigid bodies can even be part of the magnetic circuit. For instance, in the case of the standard magnetic catch configuration employed herein, the magnet(s) and pair of steel plates could be embedded in a plastic rigid body that is attracted to a steel rigid body. These advantages extend to other magnetic retention strategies. For instance, the retention can be magnet-to-metal (without steel plates) or magnet-to-magnet, and the magnets used can be a single magnet, alternating magnets, a Halbach array, a configuration that repels before it attracts, and so on. Further, all magnetic retention strategies can be tuned (both in maximum force and in the force-position profile) by changing the number or strength of magnets, by tuning the distance (“air” gap) between the magnetic components at contact, or by modifying other aspects of the catch’s geometry. All magnetic retention strategies also share the disadvantages described above of having differences in the force levels and locations of attachment and detachment when the rigid bodies exhibit some compliance. Further, magnetic retention may have a heightened potential for noise generation versus the other strategies we demonstrated here, which is especially difficult toDocket # 206030-0333-00WG control if they are attached to rigid components exhibiting some compliance. For noise reduction in general, at the cost of greater complexity, compliant or viscoelastic bumpers (possibly with stress relaxation under steady state force) may be considered to cushion the force impact or impulse. While the magnetic catches described herein have force profiles with the force concentrated as close to the catch contact as possible, it is a valuable area of further investigation to determine how different force profiles (e.g., larger magnets with a larger air gap at contact for a more linear force profile) affect the noise and controllability of the bearing.

[0122] Of course, retention handoff need not include the use of magnets. Spring-loaded mechanisms, such as roller catches, ball detents, and snap buttons similarly provide a holding force that resists any movement up to a particular preload, though with a different force profile, which first resists and then assists movement in the process of engaging and which again first resists and then assists movement in the process of disengaging. Touch fasteners, such as Velcro hook-and-loop fasteners or the stem-and-cap fasteners used in 3M Command picture-hanging strips, provide yet another different force profile, requiring only force toward the fastener when engaging and force away from the fastener when disengaging but may provide less repeatable forces than roller or magnetic catches would. And many other possible mechanisms could be implemented as needed for a given design, such as low-tack pressure-sensitive adhesion (e.g., Post-It Note adhesive), suction cups, surface combinations with high static but low kinetic friction (whether naturally occurring or mediated by the inclusion of a lateral magnetic catch), or active mechanisms such as electrostatic or electromagnetic mechanisms, pin locks, or mechanical jaws that can clutch the intermediate body to the grounded body at any desired location. Where needed for a given design, one retention mechanism can even hand off at transition to a retention mechanism of a fully distinct modality.

[0123] A second conventional bearing can be used to achieve range extension at both ends of the flexure bearing range of motion. The previously-mechanically -grounded stage may become a second intermediate body nested within an outer mechanically-grounded stage. A second preloaded spring serves as a retention mechanism to hold the second intermediate body against the opposite side of the new grounded body.

[0124] In some cases, however, the disadvantages of using a second conventional bearing (e.g., increased part count, increased volume, increased weight) may preclude implementing bidirectional range extension via a second conventional bearing. For such cases, a spring-loadedDocket # 206030-0333-00WG ball or roller detent may provide the required bidirectional preload while only using a single conventional bearing. Because ball detents are pressed by a preloaded compression spring into a groove with two opposing sloped surfaces, two evenly balanced preload forces prevent movement in either range extension direction. To transition a bidirectional metamorphic flexure bearing with ball detents into conventional-bearing mode in either direction, an external force must unbalance these two forces and then overcome the relevant component of the remaining preload on the opposing surface. Hence, a ball detent can ensure that the bearing operates in mutually exclusive modes.

[0125] FIG. 7B illustrates an example bidirectional metamorphic flexure bearing with constantforce retention. FIG. 7C illustrates an example bidirectional metamorphic flexure bearing using retention handoff. In the retention handoff case, the sliding-stage retention mechanisms need to have a higher retention force than the primary-stage retention mechanisms corresponding to the opposite direction so that the mechanism will successfully transition to flexure-bearing mode (detaching the primary-stage retention mechanism) instead of transitioning to conventional- bearing mode in the opposite direction.

[0126] Embodiments are contemplated that utilize rotary metamorphic flexure bearings. The metamorphic flexure bearing may be utilized beyond a linear-motion flexure bearing that transforms into a conventional linear-motion bearing. Rather, the embodiments of the bearing described herein allows a bearing to transition from any set of n degrees of freedom of compliance to any other set of m degrees of freedom of sliding or rolling, all while only exerting force between the grounded body and the stage, as illustrated in FIGs. 9A and 9B. FIGs. 9A and 9B illustrate a transformation of n degrees of compliant bearing freedom into m degrees of conventional bearing freedom, using the principle of serial -chaining. By chaining stages within one another, a transformation of 1 DOF compliant bearing into 2 DOF conventional bearing is achieved, as illustrated in FIG. 9A. Conversely, a transformation of 2DOF into 2DOF is achieved, as illustrated in FIG. 9B. Embodiments provided herein enable a single-degree-of- freedom linear-motion flexure bearing with an intermediate body docked to a ground catch that can transition when needed to two degrees of freedom using conventional linear bearings. More simply, though, the embodiments allow for rotary metamorphic flexure bearings. For instance, by nesting a cross-axis flexural pivot inside a conventional bearing and implementing one of the retention mechanisms discussed above, a rotary flexure bearing can transform into aDocket # 206030-0333-00WG conventional rotary bearing at the end of its range of motion, enabling rotary flexure bearings to be used in applications intermittently requiring substantially higher rotations.

[0127] It is noted that bearings are used in various applications, from precision manufacturing to automotive and aerospace transportation to medical devices. Bearings that wear in such applications can be costly, dangerous, and time-consuming. Metamorphic flexure bearings provide, for applications that typically have a small range of motion but occasionally need range extension, a solution to bearing wear that has the potential to substantially improve the lifetime of such mechanisms. For instance, when using a constant-force retention mechanism, the bearing provided herein enables automotive vibration dampening via flexure-based shock absorbers that do not bottom out due to constant-force range extension beyond the range of the flexure bearing. Similarly, the constant-force retention mechanism could improve human safety as a force-based mechanical clutch for human-robot interaction, where large robots transition into a conventional- bearing mode when a force threshold is exceeded to avoid human injury.

[0128] Metamorphic flexure bearings can also go beyond simply reduction in wear, making some designs possible as flexure bearings that were never before possible as flexure bearings. For instance, a rotary metamorphic flexure bearing could be used to design a prosthetic knee that could withstand all the biomechanical loads of the human body over a most common flexurebearing range of motion (e.g., walking), then allow a user to occasionally utilize an extended range of motion (e.g., sit, squat, or kneel) via a conventional bearing. In the realm of manufacturing, this class of bearing could instead be used for high-precision applications such as pick-and-place machines or 3D printers, where the high-precision of the bearing in flexurebearing mode relative to itself is an important parameter, but then conventional-bearing mode can be used for gross manipulation or to clear the build space between builds. Moreover, the metamorphic flexure bearing concept could be used to design new failure modes for bearings - a bearing that typically operates in conventional-bearing mode could fail into a small-range-of- motion compliant mode so that it can remain in service with a reduced range of motion, or a bearing that typically operates in flexure-bearing mode could fail into a conventional-bearing mode if it is overloaded in a direction that is not protected against by force isolation.

[0129] The embodiments provided herein provides new research directions on what can be done with mechanisms that transform their mechanical properties over their range. For instance, the design space of sequential deformation and motion is a vast playground for mechanism design,Docket # 206030-0333-00WG and metamorphic flexure bearings may offer an improvement over sequential metamaterials that are range-limited and would thus benefit from incorporating conventional bearings. In one instance, these bearings are a new foray into kinesthetic force feedback to human operators. For example, an accelerator pedal with force feedback in the form of a metamorphic flexure bearing with linear-force retention and a large force step could be used to provide feedback on the redline of a vehicle’s engine (e.g., 7,000 RPM) or as the threshold for engaging the vehicle’s kinetic engine recovery system. As such, it is contemplated that the embodiments provided herein will enable numerous applications of the metamorphic flexure bearing and a variety of new technologies.

[0130] The materials and methods of the components discussed herein are as follows. For the conventional bearing, the intermediate body was printed on a Markforged Mark Two in Onyx filament (micro carbon fiber filled nylon), then press fit and bolted in the two square-profile plain bearings (igus QJRMP-01-10) end-to-end within the intermediate body (see the crosssection of Fig. 2). The press-fit linear-motion bearings were slid onto a 150-mm-long, 7.5-mm- wide square profile shaft (Igus AWMQ-10), which served as the main chassis for the grounded body. A 165-mm-long bolt (McMaster-Carr [McM-C] 90044A262) was inserted through the base shaft to secure both ends of the grounded body (also printed on the Mark Two) onto the shaft, tightening the bolt (only lightly, to minimize shaft deflection) into a square nut (McM-C 94785A411) that was press fitted into in the home side (left side of figure) of the grounded body.

[0131] For the flexure bearing, the flexures along with their rigid bars and attachment rings were formed as a single continuous component. The flexure component was printed on an UltiMaker S5 in UltiMaker Tough PLA filament (poly[lactic acid] mixed with acrylic polymer, with a Young’s modulus of 2.8 GPa and a tensile stress at yield of 45 MPa), using UltiMaker Breakaway (polylactic acid mixed with thermoplastic polyurethane) as a support material. The flexures were 600 pm thick and were printed in four 150 pm layers in the X-Y plane. The far- side of the intermediate body (constituting the flexure bearing far-side internal hard stop) was printed on a Bambu Lab X1C in PLA-CF (carbon fiber reinforced polylactic acid) and the stage extension was printed with the same printer and material as the flexure component. Using thin M3 nuts (McM-C 93935A320) and M3 bolts of various lengths (e.g., McM-C 92290A111), the flexure component was fixed to the home side of the intermediate body, then the far side of theDocket # 206030-0333-00WG intermediate body was fixed to the far side of the main body of the intermediate body, and finally the stage extension was attached to the far side of the stage.

[0132] With respect to the linear-force retention design, three compression springs (McM-C 9657K419, 9657K432, 9657K449) of varying lengths were used to demonstrate the linear-force retention concept. These springs were composed of zinc-plated music-wire-steel and had identical inner and outer diameters (13 mm ID, 15.2 mm OD) but varied in their resting and maximum compression lengths (resting / minimum lengths: 63.5 / 13.5 mm, 76.2 / 15.7 mm, and 88.9 / 18.0 mm, respectively) and spring rates (0.665 N / mm, 0.543 N / mm, and 0.473 N / mm, respectively).

[0133] To insert or swap the compression spring, the stage extension, the far side of the intermediate body, and the mechanism-length bolt and far-side grounded-body hard stop were removed. The new compression spring was slid onto the conventional-bearing shaft and all removed components were replaced. The nominal initial compression was designed to have a 61.45 mm length. As needed, 2.58-mm-thickness washers were added in series with the compression spring to tune the initial preload of the spring. Swapping the compression spring for springs of different lengths and spring rates and tuning the initial compression of the springs provided various initial preload force levels.

[0134] With respect to constant-force retention design, to demonstrate the constant-force-spring retention concept, twelve constant-force (c-f) springs were incorporated into the home side of the grounded body. Each c-f spring (McM-C 9293K122) maintained a constant force of 1.02 N and was sized (13.5 mm ID, 15.7 mm OD, 6.35 mm width) to hold itself with some compressive force around a ball bearing (igus B605B3E, 5 mm ID, 14 mm OD, 5 mm width) mounted to one of twelve bolt slots in the grounded body via an M5 threaded rod (McM-C 93805A286) and two thin hex nuts (McM-C 90710A037). To mount each c-f spring onto a ball bearing, the c-f spring was extended to near full extension to render its inner ring compliant, then the inner ring was seated on the outside of the bearing.

[0135] To tune the force of the constant-force retention, the number of parallel c-f springs required to achieve a given force were attached. To avoid unnecessarily adding an internal torque to the bearing, the twelve c-f springs in radially opposite pairs, giving a total of six different force levels to test (2, 4, 6, 8, 10, and 12 springs), were attached.Docket # 206030-0333-00WG

[0136] With respect to retention handoff design, custom-designed tunable magnetic catches were provided to demonstrate the retention handoff concept using steel magnetic-flux-directing components extracted from standard off-the-shelf magnetic latches (Everbilt 9235997). The assembly of each catch began by placing one steel plate on a work surface. Using a 3D-printed support structure (printed on the Mark Two) with a pattern of low-tol erance slots for magnets, a colored bar magnet was used as a manual pick-and-place tool to insert a variable number of rectangular permanent magnets (3 mm long, 2 mm wide, 1 mm thick, grade N50, nickel-plated, and magnetized through their thickness) onto the first steel plate, north-poles up. To tune the size of the magnetic-catch force, only as many magnets as needed were placed to provide the desired force. To make the force tuning scale roughly linearly with the number of magnets, the catch was designed to accommodate magnets in parallel (as opposed to stacking them in series). After placing the magnets, a magnetic viewing film was used to ensure that all magnets were oriented in the same direction by visually checking that the magnetic field strength above the magnets was convex. Then, maintaining the 3D-printed support structure in place, the second steel plate was slid onto the north-pole side of the magnets. The support structure was designed to be thinner than the magnets, so once the catch was assembled, all magnets had contact with both steel plates. A clothespin-style retainer was clipped over the back of the magnetic catch, pressing the steel plates as far forward inside the support structure as possible, ensuring that the steel plates would extend out of the catch a repeatable distance. Finally, a magnetic catch was inserted into the grounded body, and a magnetic catch was inserted into the intermediate body. To prevent movement of the catches relative to their respective rigid bodies, the catches were fixed into place using M3 set screws through the bodies of their respective rigid bodies. The assembly process was repeated each time as needed to tune the force. The steel armatures were permanently affixed at their corresponding locations using 3 / 8-inch #4 zinc Phillips flat-head wood screws, which were tightened until their heads did not protrude further than the extension of the 1.55-mm-thick steel plates from the catch. Though originally designed with two slots for each of the catches, the catches used only one slot for each to increase the repeatability of the transitions.

[0137] For each of the three retention mechanisms, the quasi-static force profile of the metamorphic flexure bearing were characterized using a universal testing machine (Instron 5966 Dual Column Table Frame). The bearing was mounted vertically into the machine using manualDocket # 206030-0333-00WG wedge-action grips (Instron 2716-015), with the ground-side grip fixture anchored to the lower grip and the stage-side grip fixture actuated by the upper grip. The force required by the upper grip to actuate the bearing was monitored by a load cell installed in series with the upper grip (Instron 2530-500N). Before each test, the load was tared, and the crosshead was manually raised until it reached a tensile force sufficient to indicate that the far-side hard stop had been reached. Then, the extension value of that hard stop was noted, and the crosshead was returned to its origin. The Instron was programmed to shorten the metamorphic flexure bearing until it reached a compressive force of 15 N (indicating the home-side hard stop), lengthen the mechanism to its maximum (far-side hard-stop) extension value, then return the bearing to zero extension, all at a slow speed (10 mm / min) to ensure quasi-static measurements.

[0138] To analyze the data, the first and last two values of collected force data were removed, and the force data remained unfiltered. For the continuous retention mechanism cases, three- point centered finite-difference stencils were used to compute the first and second derivatives (stiffness and stiffness gradient, respectively) of the force data with respect to extension. The stiffness was filtered using a 21 -point-window centered moving average and the stiffness gradient remained unfiltered to maintain accurate transition location calculations. The stiffness filter window length was selected as a trade-off between data clarity and accurate preservation of subthreshold-preload transition locations and stiffnesses.

[0139] For the standard continuous retention mechanism cases, the stage attachment and ground detachment were defined as occurring at the stiffness gradient values that were first to exceed 300 N / mm2 and last to occur below -300 N / mm2, respectively, between an extension of 8.5 and 9.5 mm during lengthening (see Figs. 10 and 11 for plots of the stiffness gradients in the continuous retention cases). Similarly, the ground reattachment and stage detachment were defined as occurring at the stiffness gradient values that were first to drop below -300 N / mm2 and last to remain above 300 N / mm2, respectively, between an extension of 9.5- and 8.5-mm during shortening. Each preload was calculated as the average between the force at ground detachment and the force at ground reattachment. Finally, the flexure-bearing range was calculated as the average between the extension at stage attachment and the extension at stage detachment, and the corresponding forces were averaged at these extensions to calculate the maximum flexure-bearing force. For all curves, range limits were calculated as the firstDocket # 206030-0333-00WG extension value encountered in conventional mode to have a corresponding fdtered stiffness value above 1 N / mm.

[0140] For the subthreshold-preload continuous mechanism cases, the ground detachment was calculated as occurring at the maximum smoothed stiffness between extension values of 6 and 8 mm in the linear-force case and between 8 and 9 mm in the constant-force case. For the linear- force retention mechanism, all spring stiffnesses was calculated from the unfiltered force curves from extension values of 15 to 40 mm. The stage attachment (and detachment) was calculated for the linear-force-retention case as the first smoothed stiffness during lengthening (and last smoothed stiffness during shortening) to exceed its corresponding spring stiffness. The minimum smoothed stiffness between the ground detachment and stage attachment were used to report the combined-mode stiffness, and the minimum smooth stiffness between the stage detachment and ground reattachment was averaged. The preload was calculated for each subthreshold-preload case as the average between the force at ground detachment and the force at ground reattachment. The flexure bearing range was calculated as the average of the extension and ground detachment and the extension at ground reattachment.

[0141] For the magnetic retention handoff cases, the stage attachment was defined as occurring at the minimum force between 7.5- and 8.5-mm during lengthening and the ground detachment was defined as occurring at the maximum force between 8- and 10-mm during lengthening. The ground reattachment was defined as occurring at the maximum force between 9 and 8 mm during shortening and the stage detachment was defined as occurring at the minimum force between 8.5- and 6.5-mm during shortening. The weight of the intermediate body was calculated as the average of all curves from 45 to 65 mm, including both the lengthening and shortening phases.

[0142] It is noted that the embodiments of the metamorphic flexure bearing provided herein may be utilized as a rotary bearing. The rotary metamorphic flexure bearing may be used in devices, such as but not limited to, medical implants (e.g., orthopedic implants), and the like. FIGs. 12A- 12C illustrate an example rotary metamorphic flexure bearing 1200 transitioning between a flexure-bearing mode and a conventional -bearing mode. FIG. 12A illustrates the bearing 1200 in a flexure-bearing mode. FIG. 12B illustrates the bearing 1200 in transition. FIG. 12C illustrates the bearing 1200 in a conventional -bearing mode.

[0143] The bearing 1200 includes a stage 1202, an intermediate body 1204, and a grounded body 1206. The bearing 1200 may, for example, use a cross pivot nested within a conventional slideDocket # 206030-0333-00WG bearing using a magnet and a ferromagnetic material for each retention mechanism. The stage 1202 is connected to the intermediate body 1204 via flexure bearing 1208, and the intermediate body 1204 is connected to the grounded body 1206 via a retention mechanism, such as magnetic catches. The intermediate body 1204 may include a first magnet 1210a configured to interface with a plate 1212 on the grounded body 1206. The intermediate body 1204 may include a second magnet 1210b configured to interface with a plate 1214 on the stage 1202. To transition between the flexure-bearing mode and the convention-bearing mode, the bearing 1200 implements the principles of the magnetic catch assembly described in FIG. 8. For example, during flexurebearing mode, the first magnet 1210a of the intermediate body 1204 may be engaged with the plate 1212 as the stage 1202 rotates via the flexure bearings 1208. As the stage 1202 rotates in direction A, as illustrated in FIG. 12B, at a certain distance, the plate 1214 of the stage 1202 engages with the second magnet 1210b. As the stage 1202 rotates even farther in direction A, at a certain point, the bearing 1200 transitions into the conventional -bearing mode, as illustrated in FIG. 12C, in which the first magnet 1210a disengages from the plate 1210 of the grounded body 1206, allowing the intermediate body 1204 to rotate in direction B.

[0144] FIGs. 13-16 illustrate other examples of rotary metamorphic flexure bearings. As illustrated in FIG. 13, the rotary metamorphic flexure bearing 1300 uses a cross pivot nested within a conventional slide bearing using a standard magnetic catch mechanism. As illustrated in FIG. 14, the rotary metamorphic flexure bearing 1400 uses a cross pivot in series with a conventional ball bearing. As illustrated in FIG. 15, the rotary metamorphic flexure bearing 1500 uses a rotary flexure 1508 in series within a conventional ball bearing 1502. The rotary metamorphic flexure bearing 1500 includes hold stops 1506 using magnets 1504; however, it is noted that other retention mechanisms may be used. The rotary metamorphic flexure bearing 1500 provides for the ground-to-secondary-stage interaction in a separate plane. Moreover, as illustrated in FIG. 16, the rotary metamorphic flexure bearing 1600 uses a rotary flexure 1608 in series within a conventional ball bearing 1602. The bearing 1600 is planar, but this results in less range of motion. The rotary metamorphic flexure bearing 1600 includes hold stops 1606 using magnets 1604. The bearing 1600 may be combined with out-of-plane interactions to avoid the reduced range of motion due to the both sides of the secondary stage interacting with the ground. Examples of retention mechanisms that can be used in a rotary metamorphic flexure bearing areDocket #: 206030-0333-00WG a spiral wound torsion spring for constant-torque retention and an extension coil spring for torque that varies with angle.

[0145] FIGs. 17A-17J illustrate various examples of cross-axis flexural pivot blade rearchitecture for increased range of motion. Cross-axis flexural pivots (x-pivots) provide a circular range of motion. The cross-axis flexural pivot blade 1700 illustrated in FIG. 17A includes the blades formed in an x-pivot, such that each blade curves outward towards the edges. The pivot blade 1700 may handle greater tensile loads. The pivot blade 1700 may be a symmetric cross pivot. FIGs. 17B-17J illustrate rearchitecting the blades in which each blade is either split into multiple blades, branches into multiple blades, or is some combination of split, branching, connecting, and so on.

[0146] FIG. 17B illustrates a cross-axis flexural pivot blade 1702 having multiple blades placed in parallel to replace each single blade in the x-pivot. The single blade may be replaced by two or more blades in parallel to one another. FIG. 17C illustrates a cross-axis flexural pivot blade 1704 in which the blade is split from the middle outwards, continually dividing into more branches. The blade branches from the inside to the outside into multiple blades. FIG. 17C illustrates the blade splitting from the inside out, continually dividing into two additional blades until connecting with the outside of the flexure. Note that only the top is drawn, but its mirror image also extends down and connects to the bottom of the flexure. FIG. 17D illustrates a cross-axis flexural pivot blade 1706 in which the blade is shown splitting from the inside out, continually dividing into three additional blades until connecting with the outside of the flexure. Note that only the top is drawn, but its mirror image also extends down and connects to the bottom of the flexure. FIG. 17E illustrates a cross-axis flexural pivot blade 1708 in which the blade is shown splitting from the inside out, with the outside blades repeatedly dividing until connecting with the outside of the flexure. Note that only the top is drawn, but its mirror image also extends down and connects to the bottom of the flexure.

[0147] Each blade may be rearchitected such that it is itself composed of one or more x-pivots. For example, as illustrated in FIG. 17F, each blade of the x-pivot may include two x-pivots. The cross-axis flexural pivot blade 1710 illustrated in FIG. 17F is a two-stage nested x-pivot. The cross-axis flexural pivot blade 1710 has the advantage of having overall length of blades that is longer than that of the standard x-pivot, which benefits in flexibility from longer blade lengths.Docket #: 206030-0333-00WGThe cross-axis flexural pivot blade 1710 is a compliant mechanism that can handle combinations of larger loads (forces and torques) and displacements (rotation of the mechanism).

[0148] Noting, however, that the primary advantage of cross-axis flexural pivot blade 1710 is a longer blade length, the cross-axis flexural pivot blade 1710 provides for new strategies for further lengthening the blade length. For instance, increasing the number of stages allows lengthening of the total parts of the mechanism which bend. Further, by orienting the blades in each axial slice of the mechanism similarly (though not necessarily identically oriented), the blades can be made even longer by passing through one another. The rigid elements between the blades (i.e., the bridges) must be connected to one another to allow the blades to work together as sub-x-pivots in each virtual blade (each rearchitectured blade). These connections through the middle of the bridges are shown in FIGs. 17F-17J.

[0149] FIG. 17G illustrates a cross-axis flexural pivot blade 1712 having a three-stage nested x- pivot. The three-stage nested x-pivot demonstrates the advantage in total blade length of having multiple stages, and of the further blade length that can be achieved by orienting the blades similarly within a slice of the compliant mechanism. Connections between the bridges (e.g., center bridge connectors) are shown in FIG. 17H.

[0150] Centralized bridge connectors, however, have the disadvantage of high stresses when loading or bending the mechanism. Instead of connecting the bridges in the centers, FIG. 171 illustrates a cross-axis flexural pivot blade 1716 in which bridge connectors connect the bridges (i.e., the rigid elements) on the ends. These connections, which are themselves rigid elements, are formed to not interfere with the operation of the mechanism and can replace or supplement the center connections.

[0011] In two-stage nested x-pivot, the bridge edge connectors do not interfere with one another. However, the cross-axis flexural pivot blade 1718 illustrated in FIG. 17J includes the bridge edge connectors in a more-than-two-stage mechanism so that they do not interfere with one another. As seen in FIG. 17J, by constructing the bridge edge connectors in L-shaped (or symmetric-to-L- shaped) configurations that fit into one another, more than two stages can be used in a way that the bridge edge connectors do not interfere with one another. For more than three stages, the center stages could be constructed similar to the shape of skew tetrominoes.Docket # 206030-0333-00WG

[0152] It is noted that any of the designs illustrated in FIGs. 17A-17J can, of course, be used in a joint replacement: for example, in a knee replacement to form a compliant knee replacement (CKR).

[0153] The rotary metamorphic flexure bearings include one or more concepts from the metamorphic flexure bearing and cross-axis flexural pivot blade embodiments as described herein. The rotary metamorphic flexure bearings may include transition mechanisms, such as constant force and retention handoff. The rotary metamorphic flexure bearings may achieve motion performance requirements while also assuming the low-maintenance benefits of the metamorphic flexure bearing.

[0154] The rotary metamorphic flexure bearing (RMFB) 1800 includes three primary stages: the flexure bearing stage, the traditional bearing stage, and a grounded stage, as illustrated in FIGs. 18A and 18B. FIG. 18A illustrates the RMFB 1800 including a transition mechanism, such as a retention handoff featuring magnetic pairs. FIG. 18B illustrates the RMFB 1800 including a transition mechanism, such as a constant stiffness retention featuring a spring. The RMFB 1800 may operate in a same or similar manner as that described for bearing 1200 in FIGs. 12A-12C.

[0155] The RMFB is driven by both bearing selection and the tunable transition point, enabling optimal use of each bearing’s strengths, and ensuring robust functionality. The RMFB longevity is enhanced through maximum flexure bearing engagement during the primary operational phase. Displacement beyond the flexure bearing’s functional range initiates a temporary transition to the traditional bearing, facilitating extended motion through the traditional bearing’s operational capacity. However, the RMFB also inherits the drawback of friction-based wear during this operation. Maintaining the transition as temporary and isolated is essential for consistent and predictable behavior. Based on the RMFB design, some configurations may result in simultaneous engagement of both bearings in a “combined mode”, leading to unstable behavior. To prevent this, a third component is introduced, referred to a “retention mechanism”. The retention mechanism applies restive forces which guide transition between modes, helping to maintain stable operation. The retention mechanism can take on various forms, each uniquely, but predictably, influencing the behavior of the RMFB. The RFMB may use retention mechanisms, such as but not limited to, retention handoff and constant force retention.

[0156] Retention handoff mechanisms enforce transition without introducing sustained changes to the force profiles of the bearings. FIG. 18A illustrates the RFMB 1800 using magnetic pairs asDocket #: 206030-0333-00WG the handoff retention mechanism. The flexure stage (configuration A, Fig. 18A) serves as the primary operational mode, while the traditional bearing stage and the grounded stage are initially coupled. The retention handoff mechanism incorporates two discrete sets of magnetic pairs 1802, 1804, the first set 1804 remains disengaged during flexure bearing mode, while the second set 1802 is engaged to magnetically constrain the traditional bearing stage relative to ground. Throughout the flexure bearing angular range, the RFMB 1800 exhibits pure flexure bearing behavior. Upon entering the magnetic transition zone, the force profile experiences a perturbation due to engagement of the first magnetic pair 1804 and disengagement of the second 1802. After the transition, the RMFB 1800 exhibits exclusively traditional bearing behavior, with the flexure bearing magnetically constrained. Thus, the magnetic pair alters the behavior of the RMFB 1800 solely within the transition zone, preserving the discrete flexure bearing and traditional bearing modes behaviors outside of this region. Upon reentry into the flexure bearing angular range, the RMFB 1800 automatically reverts to flexure bearing mode.

[0157] Constant stiffness retention mechanisms 1810 provided in RMFB 1808 enforce transition with a sustained resistive force applied during its secondary mode, as illustrated in FIG. 18B.The constant stiffness retention mechanism 1810 may be, for example, a spring. Throughout the flexure bearing angular range, the RFMB 1808 exhibits pure flexure bearing behavior while the traditional bearing is constrained by the resistive spring torque. Upon entering the spring transition zone, the resistive torque is exceeded, decoupling the traditional bearing from ground. Simultaneously, the flexure bearing is mechanically constrained via a hard stop. Following the transition, the RFMB 1808 exhibits coupled behavior governed by both the traditional bearing and the torsional spring. In contrast to the retention handoff mechanism illustrated in FIG. 19A, which imparts a localized force within the transition zone, the constant stiffness retention mechanism imposes a continuous resistive torque throughout the full extent of the secondary mode. This results in sustained mechanical impedance rather than a discrete mode-shift impulse.

[0158] Retention mechanism design includes the resistive forces be sufficiently large to preclude unintended transition into combined mode. Under quasi-static conditions, the retention mechanism must generate retention forces exceeding the stiffness of the bearing prior to reaching the mechanically programmed transition point. The retention forces are quantified herein by prescribing a controlled trajectory through the full range of motion at a constant velocity to isolate the response. Under dynamic conditions the retention mechanism must additionallyDocket # 206030-0333-00WG counter inertial loads arising from device acceleration and momentum. The influence of these dynamic effects was characterized by applying a frequency-swept position chirp within flexure bearing mode and identifying the onset of combined mode.

[0159] FIG. 19 illustrates an exploded view of an example RMFB 1900. The RMFB 1900 includes one or more of the same or similar features as RMFB 1800, RMFB 1808, bearing 200, and bearing 1200. For characterization and testing, the RMFB 1900 includes a cylindrical form factor with end fixtures which interface with a motorized testbench setup. The RMFB 1900 features a flexure bearing 1902 and a conventional bearing 1904 which tradeoff engagement aided by a retention mechanism. There are two retention mechanism options included in RMFB 1900, a retention handoff (magnetic pairs 1908 and 1910) and a constant stiffness (torsional spring 1906). Although both mechanism types are illustrated, only one is used at a time during testing. To maintain testing consistency, a single prototype was developed that accommodates both retention mechanism types, with a modular design enabling interchangeability between mechanisms. For the flexure bearing 1902, a pivot bearing was selected with cylindrical form factor and three flexure blades. The flexure bearing 1902 exhibits constant stiffness and is selflimiting via hard stops with an angular range of ±30 degrees. For the traditional bearing 1904, a ball bearing was selected, which exhibits negligible torsional stiffness and an infinite angular range, allowing decoupling of the traditional bearing 1904 and retention mechanism behaviors and enabling isolated analysis of retention dynamics. Retention handoff is achieved using two magnetic pairs 1908 and 1910, located at the end of range for the flexure bearing 1902 to facilitate transition between bearings. The magnetic pairs 1908 and 1910 can be removed from their notches and the constant force retention mechanism, a torsional spring 1906, is instead inserted into the RMFB 1900. Independent of the retention mechanism tested, the RMFB 1900 maintains the same form factor.

[0160] The engagement of the magnetic retention handoff is monitored using a magnetometer positioned adjacent to a magnet of the first pair. With this placement, magnetometer readings reach a maximum when the first pair engaged, due to increased local magnetic field strength. These readings also provide insight into the behavior of the second magnet pair as an unexpected drop in magnetic field reading prior to expected engagement of the first pair indicates premature disengagement of the second pair. This signals the onset of combined mode where relative motion is occurring between both magnet pairs, indicating that their behaviors are no longerDocket # 206030-0333-00WG decoupled. The ability to sense the onset of combined mode is vital for dynamic testing, as it enables identification of inertial effects and mode coupling that may not be evident under quasistatic conditions.

[0161] In the experimental design, quasi-static force profiles for both retention mechanisms were characterized using a testbench actuated by an EC-i Maxon motor (596077). The RMFB 1900 was mounted vertically with the ground-side bolted to the optical board and the stage-side grip clamped into the motorized setup. Real-time torque data was captured using a micro torque sensor (ATO-TQS-S04). A planetary gearset (EG23-G10-D10) was integrated to provide gear reduction and minimize backlash transmitted from the system to the motor. Magnetic field strength was concurrently measured with a triple-axis magnetometer (MLX90393). Quasi-static force characterization was performed under a constant velocity of 5 deg / sec over a trajectory of 2-300 degrees, controlled via a Teensy microcontroller.

[0162] Dynamic characterization was performed with the same testbench, applying a position chirp centered at a 15-degree setpoint and a 5-degree amplitude. This operating range avoids the transition zone near 30 degrees, isolating dynamic effects from those of the retention mechanism engagement. The chirp frequency was increased linearly until either reaching a maximum of 15 Hz or until manually terminated due to sustained unstable behavior.

[0163] The position chirp is modeled after a sine wave of form: y(t) = Asin(o)t + < >)

[0164] Momentum is related to the velocity and mass as described by: p = mv

[0165] To characterize momentum impacts we integrate the position sine wave form to find velocity: v(t) = Aa)cos( )t -I- )

[0166] Therefore, to increase momentum effects, a position chirp was introduced with linearly increasing frequency.

[0167] The results of the retention handoff are as follows. Under quasi-static analysis, the magnetic retention handoff mechanism exhibited three distinct zones of behavior: flexure bearing zone, magnetic transition zone, and traditional bearing zone. The isolated flexure bearing zone occurs in the first 20 degrees of motion, during which the RMFB 1900 exhibits constantDocket #: 206030-0333-00WG stiffness consistent with that of the isolated flexure bearing. The transition zone occurs from 20 to 40 degrees of the bearing’s range. The physical engagement of the magnetic pair occurs at 26.6 degrees and is confirmed through observation of the magnetometer reading, as illustrated in FIG. 20. FIG. 20 illustrates Quasistatic analysis of retention handoff mechanism featuring magnetic pairs. As illustrated in FIG. 20, the torque profile of the RMFB 1900 is shown over its full range. The magnetometer reading over the full range is shown, indicating engagement of the first magnetic pair signaling the transition out of flexure bearing mode. FIG. 20 also illustrates images from the 3D model that correspond with the position axis and include a red dial and shading to indicate progression within each device mode.

[0168] Following the transition zone, the traditional bearing mode persists for the remainder of the motion, during which the device exhibits negligible stiffness consistent with the traditional bearing. .The results are highly repeatable with negligible hysteresis observable in the quasistatic analysis. The forward motion of 2-300 degrees and the reversed motion from 300-2 degrees are illustrated in FIG. 20. The reversed motion is barely perceivable in FIG. 20 due to the highly consistent behavior of the mechanism. The retention force of the magnets was sufficient in preventing combined mode during the quasi-static scenario.

[0169] Dynamic analysis of retention handoff mechanism featuring magnetic pairs is illustrated in FIGs. 21A and 21B. FIG. 21A illustrates the position chirp across time and the magnetic field reading. FIG. 2 IB illustrates Quantified and normalized magnetic field deviation. In dynamic analysis, the magnetic field reading’s frequency linearly increased in conjunction with the position chirp frequency until a critical value was reached. After this point, unstable behavior was exhibited, consistent with the predicted behaviors indicative of combined mode, as illustrated in FIG. 21 A. The divergence in expected behavior is quantified by plotting magnetic field deviation from expected, as illustrated in FIG. 21B.

[0170] The results of constant stiffness are as follows. Under quasi-static analysis, the constant stiffness retention mechanism exhibited two distinct zones of behavior, flexure bearing zone and traditional bearing zone. The transition occurs at an isolated moment and occurs at the designed 30 degrees. The flexure bearing zone occurs up to 30 degrees and exhibits the expected stiffness consistent with that of the flexure bearing. The traditional bearing zone occurs from 30 degrees until the end of motion and exhibits constant stiffness matching expected stiffness of the constant force torsion spring.Docket #: 206030-0333-00WG

[0171] FIG. 22 illustrates the dynamic analysis of the constant stiffness retention mechanism featuring a torsional spring. FIG. 22 illustrates the torque profile of the RMFB 1900 over its full range. FIG. 22 illustrates the magnetometer reading over the full range, with a maximum reading signaling the transition out of flexure bearing mode. FIG. 22 also illustrates images from the 3D model correspond with the position axis and include a red dial and shading to indicate progression within each device mode. As illustrated in FIG. 22, the constant stiffness spring exhibits hysteresis where the reverse motion deviates from the forward motion. In contrast, the flexure bearing zone shows no observable hysteresis, as evident by overlapping of forward and reverse motion paths. This contrast further illustrates the coupling between spring and traditional bearing behaviors, and the effective decoupling of the flexure bearing.

[0172] FIGs. 23 A and 23B illustrate a dynamic analysis of the constant stiffness retention mechanism featuring a torsional spring. FIG. 23A illustrates the position chirp across time and the magnetic field reading. FIG. 23B illustrates a quantified and normalized magnetic field deviation. As illustrated in FIG. 23 A, during dynamic analysis of the constant stiffness retention mechanism, the magnetic field reading linearly increases until the termination frequency of 15 Hz is reached. This trend confirms that the first stage maintains stable behavior and the device never enters combined mode. As illustrated in FIG. 23B, deviation from expected magnetic field profile was quantified, providing empirical validation. In the magnetic field profile, there was an observed increase in magnetic field magnitude, though no instability occurs. This deviation is likely the result of inertial effects causing the first magnetic pair to approach more closely than predicted but it does not trigger engagement. This behavior indicates that inertial effects still influence the system, but the retention force in the current design was sufficient to prevent combined mode.

[0173] Fundamentally, the RMFB primarily operates in a flexure bearing mode, exhibiting precise wear-free motion. When extended range is required, the RMFB transitions into an isolated traditional bearing mode, temporarily exchanging extended performance for inherited disadvantages of the traditional bearing. The RMFB autonomously transitions back into the primary flexure mode with aid of a retention mechanism. Designing the RMFB involves selection of a flexure bearing, a traditional bearing, and retention mechanism that will exhibit desirable characteristics to meet performance requirements.Docket #: 206030-0333-00WG

[0174] The magnetic pair represents a retention handoff mechanism which enables transition while decoupling bearing behavior during extended motion. However, this decoupling introduces a distinct transition zone characterized by significant torque disturbances caused by each magnetic pair interaction. In the example mechanism, the magnetic engagement occurred at 26.6 degrees, earlier than the mechanically programmed 30-degree setpoint. This premature engagement is caused by the magnetic attraction forces inherent to magnetic handoff and highlights a design consideration specific to this mechanism type. Alternative mechanisms, such as a mechanical latch, can achieve retention handoff without early engagement, but have their own disadvantages such as reduced tunability, wear of the component, hysteresis, and vibration sensitivity. Depending on performance requirements, an appropriate mechanism should be designed.

[0175] The torsional spring mechanism represents a constant stiffness retention mechanism with which restorative forces are coupled with the traditional bearing behavior. The selected spring exhibited a much lower stiffness than that of the x-pivot, resulting in a perceivable transition between bearing modes. However, if the stiffness of the retention mechanism is matched to that of the compliant mechanism, this transition can be effectively seamless. In this case, the system would experience no discernible shift between bearing modes, improving interface continuity. A key drawback of this retention type is the coupling between retention mechanism and secondary bearing behaviors, as evidenced by hysteresis in this case. While the ball bearing alone features negligible hysteresis, the traditional bearing zone demonstrates hysteresis due to the influence of the torsional spring. Additionally, torsional springs typically have a shorter operational life than ball bearings, driving the overall device longevity, emphasizing inheritance of such undesirable characteristics.

[0176] Dynamic analysis revealed a critical design consideration for RMFB systems: dependent on retention mechanism design, both bearings may be simultaneously engaged, resulting in unpredictable and undesirable behavior. This study demonstrated that dynamic momentum could induce such combined mode engagement, as shown in the magnetic retention handoff analysis. However, it also illustrates that with the understanding of momentum impacts garnered from this study, an appropriate retention mechanism design can fully prevent this condition. In the case of the constant-torsional spring, the spring’s restorative torque exceeded the maximum momentum impulse, preventing combined mode engagement. Consequently, RMFB designs must accountDocket # 206030-0333-00WG for peak dynamic conditions and either select or tune a retention mechanism capable of resisting combined engagement or incorporate physical constraints, such as hard stops or mechanical catches, to enforce mode separation.

[0177] With the principles and dynamic understanding acquired from this study, a rotary metamorphic flexure bearing can be designed for a variety of applications. The integrated system offers performance capabilities that exceed those of its individual components, enabling enhanced functionality beyond the limits of traditional or flexure bearings alone.

[0178] FIGs. 24A-24B illustrate components of the example RMFB with a retention handoff transition mechanism featuring magnetic pairs. FIG. 24A illustrates the disassembled components of the RMFB with the retention handoff transition mechanism. FIG. 24B illustrates the assembled components of the RMFB with the retention handoff transition mechanism. FIGs. 25A-25B illustrate components of the example rotary metamorphic flexure bearing with a constant stiffness transition mechanism featuring the torsional spring. FIG. 25A illustrates the disassembled components of the RMFB with the constant stiffness transition mechanism. FIG. 24B illustrates the assembled components of the RMFB with the constant stiffness transition mechanism.

[0179] The flexure bearing selected was a single-ended Free-Flex Pivot (5032-800) from Riverhawk which achieves ±30 degrees of rotation, a torsional spring rate of 0.1175 Ib-degree, and load bearing capacities of 60 lb in compression or 236 lb in tension. The traditional bearing selected was a xiros® deep groove ball bearing xirodur® Bl 80 from Igus. In the RMFB, the flexure bearing was constrained to ±15 degrees of rotation and the traditional bearing was constrained to 270 degrees, equating a maximum of 300 degrees of rotation. The magnetic coupled pair of the retention handoff mechanism was made up of neodymium stepped block magnets (SB443-IN), with a surface field reading of 5409 Gauss and a magnet-to-magnet pull force of 3.421b (15.2N). The constant force torsion spring for the constant force retention mechanism was a rotor spring with maximum torque of 2.58 in-lbs.

[0180] It is noted that embodiments of the RMFB and metamorphic flexure bearing may be provided in a medical implant. For example, the embodiments described herein may be provided as a compliant endoprosthesis (implant 2500) for revision total knee arthroplasty. FIG. 26 illustrates the implant 2500 with a range-extending bearing. The implant 2500 may be used to treat end-stage arthritis by relying on bending of flexible elements provided in the implant 2500.Docket #: 206030-0333-00WG

[0181] Total knee arthroplasty (TKA) is a common and highly successful procedure for treatment of knee arthritis in Service Members and the civilian population. However, implants often do not last the remaining lifetime of the patient, leading to an estimated 120,000 revision TKA (rTKA) procedures in the United States each year. This is particularly challenging in the military, where high-impact activity leads to symptomatic arthritis at much younger ages. In revision procedures, the primary TKA (pTKA) implant is replaced with a larger, more constrained rTKA system. These rTKA systems are much less successful than pTKA implants, with 10-year failure rates as high as 30%. One of the primary causes of failure in rTKA is aseptic loosening, caused by i) high shear stress at the bone-implant interface, and ii) osteolysis from particulate wear. These problems are exacerbated in rTKA implants that have more constraint than the biological knee, because they transmit loads through the polyethylene bearings and implant frame directly to the bone-implant interface. There is a critical need for TKA systems that provide sufficient constraint to stabilize the reconstructed knee over the lifetime of the patient, without creating the wear problems and interface stresses that lead to elevated failure rates.

[0182] The implant 2500 may increase the longevity of joint arthroplasty by replacing conventional articular bearings with compliant mechanisms that guide motion without rubbing, rolling, or sliding. The implant 2500 may be, for example, a Compliant Knee Endoprosthesis with a range-extending compliant bearing to meet the knee’s unique biomechanical requirements. The implant 2500 provides a fundamental re-imagining of the joint architecture to move during high-cycle-count activities by bending of compliant elements, rather than rolling, rubbing, or sliding of articulating components. This shift in paradigm may dramatically reduce particulate wear in the articular bearing. The implant 2500 may be inherently stable but not completely rigid, providing the constraint necessary to prevent instability in the revision setting without creating excessive stress at the bone-implant interface. In light of these benefits, the implant 2500 may substantially extend the functional lifespan of revision arthroplasty beyond what is possible with existing bearing technology.

[0183] Knee osteoarthritis (OA) in patients too young to be good candidates for knee replacement is a significant unsolved problem among Service Members, with increasing incidence rates. Among the largest challenges is that symptomatic knee OA tends to appear at much younger ages in Service Members than in the civilian population. The incidence rate inDocket # 206030-0333-00WOService Members aged 35-39 years is 0.35%, nearly identical to the incidence rate of OA in non- obese male civilians aged 55-64 years (0.37% per year), which is also the median age group at onset of symptomatic knee OA in the civilian population. In other words, the risk of OA in Service Members reaches the same levels as in civilians at the median OA onset age, a full 20 years earlier. This disparity is likely due in part to repetitive high-impact activities and potential knee trauma; as one example, the incidence of ACL-injury in the military is 4-10 times higher than in the general civilian population. Given that people who have tom their ACL are 7 times more likely to need TKA at a young age, and 50% of Service Members develop post-traumatic OA within 15 years of ACL reconstruction it is not surprising that the elevated risks of knee injury lead directly to earlier onset of symptomatic knee OA.

[0184] Primary TKA is a highly successful surgical intervention that effectively eliminates the symptoms of knee OA and carries a relatively low risk of complication over the first 10-20 years. However, TKA’s limited lifespan poses challenges for the treatment of symptomatic OA in young patients. Cumulative lifetime risk of revision increases dramatically as age at pTKA procedure decreases and can be as high as 35% for a pTKA performed on a 50-year-old patient. The implant systems used in revision are much less successful than pTKA implants, with 10-year failure rates of rTKA as high as 30%. When rTKA implants fail, the prognosis is bleak: patients face increasingly invasive revision operations with decreasing chances of success, and possible endpoints that include knee fusion or even above-knee amputation. This forces the large number of under-50-year-old Service Members with symptomatic arthritis to either endure sometimes- debilitating joint pain until they age or accept a substantial risk of needing an rTKA procedure with a high rate of failure. This effect is compounded by the increased physical demands of Military Service among younger Service Members, leading to significantly higher separation from Service rates among Service Members under the age of 45 at the time of pTKA (31% medical separation rate) compared to those 45 or older (20%).

[0185] In light of these challenges, there is a critical and urgent need for a treatment path that addresses the needs of young Service Members with either symptomatic knee OA or existing pTKA implants. Decreasing the failure rates and increasing the lifespan of rTKA implants would have immediate impact on these Service Members, by reducing the cost of revision and eliminating the need to wait until the patient has aged to pursue pTKA. It is worth noting that we also hope to eventually enable pTKA implants to last a lifetime; however, because our approachDocket # 206030-0333-00WG requires removal of bone, it is better suited for initial application and validation in the rTKA setting.

[0186] Current pTKA implants fail due to infection, peri-prosthetic fracture, aseptic loosening, and instability. There are two primary causes of aseptic loosening: i) debris from wear of the synthetic articular surface infdtrates the joint tissues and causes inflammation that leads to bone resorption, and ii) the knee’s internal rotation leads to high shear stress at the bone-implant interface. Aseptic loosening is a particular concern for Service Members due to elevated demands on the knee; for instance, 7 of the 9 failures reported were the result of aseptic loosening. Aseptic loosening is an inevitable byproduct of the current TKA design, which relies on independent components that articulate (roll or slide) along each other under load. Although advanced wear-resistant materials have been developed to mitigate this problem (e.g., ultra-high molecular weight polyethylene, or UHWPE), wear-driven loosening remains a key challenge. When pTKA implants fail, patients typically pursue revision procedures that involve removal of bone and placement of larger, more constrained rTKA implants. Constrained implants are used where necessary to ensure stability of the post-revision joint. However, this constraint often increases the shear stresses at the bone-implant interface and causes cross-shear that increases wear in the polyethylene articular bearings, leading tolO-year failure rates in rTKA as high as 30%, approximately 7.5 times greater than pTKA.

[0187] FIGs. 26A-26F illustrates an example implant 2500 with a range-extending bearing 2506. The implant 2500 may switch from a compliant mode into a sliding mode for occasional tasks that require larger ranges of motion. FIG. 26A is an isometric side view of the implant 2500. FIG. 26B is a cross-sectional view of the implant 2500. FIGs. 26C-26E illustrate the implant 2500 transitioning from a compliant mode to a range-extending sliding mode. It is noted that the implant 2500 may transition from the compliant mode to the sliding mode in a same or similar manner as illustrated in FIGs. 12A-12C, which relate to the rotary metamorphic flexure bearing 1200 transitioning between a flexure-bearing mode and a conventional -bearing mode. FIG. 26F is an enlarged cross-sectional view of the implant 2500 illustrated in FIGs. 26C-26E.

[0188] The implant 2500 may be a compliant-mechanism-based rTKA implant. The implant 2500 may decrease the likelihood of aseptic loosening. The implant 2500 may be an inverted cross-axis flexural pivot (x-pivot), which features thin flexure “blades” that cross at the mechanism’s midpoint. Any moment about the mechanism’s primary axis causes the blades toDocket #: 206030-0333-00WG deform, thereby rotating the joint. The inverted x-pivot mechanism is unique in its ability to support large angular deformations and large axial forces. The pivot and implant 2500 may be formed from implant-grade titanium (Ti-6A1-4V, or “Ti64”), which is known for its biocompatibility and excellent fatigue properties.

[0189] One potential hurdle to military adoption of a compliant-mechanism-based implant is the large range of motion (RoM) needed for high-impact military Service. Standard compliant mechanisms in high-cycle environments are typically only able to operate over a limited RoM without compromising fatigue life. As such, the implant 2500 meets the biomechanical requirements of a functional knee by incorporating a novel range-extending bearing architecture that acts in series with the central x-pivot mechanism. During high-cycle count activities that occur within the x-pivot’ s infinite-fatigue RoM (e.g., walking), there is no rolling or sliding whatsoever within the implant 2500, and thus no wear or particulate debris. In the less-frequent settings where a larger RoM is needed, such as squatting or sit-to-stand, the implant 2500 transitions seamlessly via magnetic catches 2514 to a conventional bearing, with the same basic function as a standard rTKA implant. To achieve this, the magnetic catch 2514 holds the conventional bearing in a locked position (i.e. it cannot articulate) until the implant 2500 is moved to the limits of the x-pivot’ s range, at which point the x-pivot locks magnetically against the hardstop 2512 and the conventional bearing is released to articulate freely, as illustrated in FIGs. 26C-26E. Because the implant 2500 only switches into “sliding mode” during sporadic activities that require large RoM, the implant 2500 may eliminate at least 90% of particulate debris from typical use, with no compromise to functional RoM.

[0190] The implant 2500 attaches proximally to the femur via a conventional femoral stem 2516. Distally, the implant 2500 is anchored to the tibia via a compliant stem 2508 that accommodates internal / external rotation of the tibia. The implant 2500 may also include polyethylene “hardstop” features, which are essential to its function. These hardstops 2512 interlock mechanically to prevent overloading of the flexures 2502 under certain out-of-plane and compressive loads, which protects the implant 2500 during sporadic non-gait activities (e.g., jumping, tripping, etc.). Lateral covers 2504 protect the surrounding soft tissue from direct contact with the blades or hardstops.

[0191] Reconstruction of joints with compliant mechanisms is not simply an incremental improvement on conventional arthroplasty, in which joints are merely resurfaced; instead, this isDocket #: 206030-0333-00WG a re-imagining of the fundamental joint structure. Conventional joints, whether synthetic or biological, rely on articulating surfaces to move freely in certain directions while constraining motion in others. In contrast, compliant mechanisms include flexible elements that deform under load to produce desired motion. Compliant mechanisms are inherently stable without being rigidly over-constrained, which is crucial for stability of the post-rTKA joint. Compliant mechanisms are also frictionless and therefore generate negligible wear, reducing the risk of aseptic loosening. The inherent stability of the x-pivot design of implant 2500 allows the implant 2500 to stabilize the joint without constraining the knee’s natural motion and thus transmitting high loads to the bone-implant interface, particularly when used with the compliant tibial stem 2508. There is also no rubbing or sliding in our implant during walking gait, which dramatically reduces the risk of particulate-driven aseptic loosening. The novel extended-range bearing system provides these benefits without limiting the knee’s RoM. Together, these embodiments allow the implant 2500 to fully restore knee function potentially over decades.

[0192] The modeling and benchtop experimental data show initial feasibility of extending the compliant implant 2500 to the knee joint, including both the central x-pivot mechanism and the range-extending bearing. Using finite element analysis (FEA), it is shown that an un-optimized version of the mechanism can withstand the axial loads and angular deformations required during walking without engaging the hardstops, while remaining below the Ti64’s fatigue limit at 108 cycles, as illustrated in FIG. 27A. This corresponds to a conservative 5,000 steps per day for 91 years before failing from fatigue. It is also shown through benchtop testing that FEA is predictive of deformation under combined axial loading and angular displacement, and that the implant 2500 has low rotational stiffness and does not fail under gait-relevant loads, as illustrated in FIG. 27B. The example linear compliant range-extending bearing 200, as illustrated in FIG. 2, shows that switching between modes is smooth, predictable, and robust.

[0193] Large-animal experiments were conducted in two goats to evaluate soft-tissue response to the x-pivot geometry. Goats were selected as a model based on similarity to human bone, and history of use in prosthetic osseointegration literature for evaluation of bone-implant and skinimplant integration and longevity. A goat-sized implant was 3D printed in Ti64, and a surgical technique developed that was as similar as possible to the human technique, while ensuring sufficient osseous fixation distally and proximally. Given concerns for soft tissue erosion, custom porous high-density polyethylene covers were used to encourage soft tissue integration.Docket #: 206030-0333-00WGThe skin envelope healed well, and the implant was in place for approximately 20 weeks without complication, as illustrated in FIGs. 28 A and 28B. Gross post-mortem evaluation revealed diffuse fibrosis within the implant, both within the blade space and between the hardstops. However, the scar tissue was hypercompliant, and deformed readily with the implant as it moved, as illustrated in FIGs. 28C and 28D, such that range of motion was not restricted. No non-fibrotic tissues (muscle, tendon, or nerve) were found within the implant. Histology showed good ingrowth within the implant covers, and no erosion of the skin.

[0194] The focus in this proposed initial study is on mechanical performance of the device. In anticipation of other potential concerns regarding clinical viability, the synopsis of considerations are as follows.

[0195] With respect to scarring, the large-animal studies provide evidence that scar tissue within the device is hyper-compliant and does not impede motion, as illustrated in FIG. 28D. In humans, constrained total knee replacements with metal-on-polyethylene hardstops rarely see scar tissue impede motion.

[0196] With respect to infection, infection is always of concern with implants. To avoid major infection issues, the implant includes the covers to allow fluid flow and eliminate dead space. Further, the implant uses established biocompatible materials.

[0197] With respect to the robustness of bone interface, there is high confidence that bone ingrowth into the outside of the stems will provide robust fixation. The outer surface of both stems is no different from current rTKA stems, and the stem is not compliant in a way that would at all affect bone ingrowth (or cementation).

[0198] With respect to material selection. The implant body is made of Grade 5 Titanium (Ti64), which has near-infinite fatigue life in body fluid if stress is kept below 480 MPa, and a long history in orthopedic implants.

[0199] With respect to implant failure, modularity of the implant allows the central mechanism to be removed and replaced without disturbing the bony interfaces, allowing any failed components to be replaced without removing bone.

[0200] The implants described herein may be optimize for a central compliant knee mechanism for high-cycle-count activities. A cross-axis flexural pivot with variable-thickness blades can support the RoM and loads of the knee during walking, with peak stresses below the material fatigue stress and a relatively low stiffness (peak unloaded reaction torque below 3 Nm). The fullDocket #: 206030-0333-00WG knee implant may have a compliant stem to accommodate internal / external tibial rotation, as described herein. The implants described herein may also allow for a range-extending compliant bearing for low-cycle-count, high-load activities. Switching of the knee into sliding mode at only the ends of the x-pivot range of motion can be achieved with a relatively low switching torque. The implants described herein may be optimized the range-extending bearing to ensure repeatable mode switching that is as smooth as possible.

[0201] Beyond the biological concerns addressed herein, one potential technical challenge is that the x-pivot may be overly sensitive to out-of-plane loads (e.g., varus / valgus moment). To prevent overloading, the implant 2500 may incorporate physical metal-on-UHMWPE “hardstops” to protect the central x-pivot from overloading. Another unlikely possibility is that the x-pivot may not support the full range of deformation required for walking, potentially due to non-linear stress effects at large deformations. To support a full range of deformation in such cases, the x- pivot of the implant 2500 may be optimized for the stance phase of walking. The implant 2500 may then rely on the range-extending bearing for swing phase, during which the joint loads are much lower. If the magnetic mode-switching system feels overly “sticky” or “dicky”, the implant 2500 may utilize a spring-based mode-switching approach.

[0202] The implant 2500 described herein provides for treatment of end-stage arthritis that has the potential to prevent multiple revision operations, increase implant lifespan, and ultimately allow for joint replacement to be safely performed in younger patients.

[0203] The various embodiments described above are provided by way of illustration only and should not be construed to limit the claims attached hereto. Those skilled in the art will readily recognize various modifications and changes that may be made without following the example embodiments and applications illustrated and described herein, and without departing from the spirit and scope of the following claims.

Claims

Docket # 206030-0333-00WGCLAIMSWhat is claimed is:

1. A metamorphic flexure bearing device, comprising: a first body flexibly coupled to a second body via a flexure bearing; and the second body coupled to a third body via a retention mechanism, wherein in a first mode, the second body is anchored to the third body, and the first body is configured to move relative to the second body, and wherein in a second mode, the second body is configured to move relative to the third body.

2. The device of claim 1, wherein the first body comprises a stage, and the third body comprises a grounded body.

3. The device of claim 1, wherein the first mode comprises a default position of the device.

4. The device of claim 1, wherein the device is configured to transition from the first mode to the second mode when the first body moves to an end of a range of motion of the first body and overcomes a preload of the retention mechanism.

5. The device of claim 4, wherein the first body moves to the end of the range of motion by contacting a hard stop of the device.

6. The device of claim 5, wherein the second body comprises the hard stop.

7. The device of claim 1, wherein the device is configured to transition from the second mode to the first mode when the second body is re-anchored to the third body.Docket # 206030-0333-00WG8. The device of claim 1 , wherein the retention mechanism comprises a compression spring, and wherein in the first mode, the compression spring is uncompressed, and wherein in the second mode, the compression spring is compressed.

9. The device of claim 1, wherein the retention mechanism comprises a constant-force spring or a magnetic catch.

10. The device of claim 1, wherein in the first mode, the second body is anchored to the third body via the retention mechanism, and in the second mode, the second body is anchored to the first body via another retention mechanism.

11. The device of claim 10, wherein the retention mechanism and the other retention mechanism each comprise a magnetic catch.

12. The device of claim 1, wherein the first mode, the first body is configured to linearly move relative to the second body, and in the second mode, the second body is configured to linearly move relative to the third body.

13. The device of claim 1, wherein the first mode, the first body is configured to move relative to the second body by rotating towards a portion of the second body, and in the second mode, the second body is configured to move relative to the third body by rotating away from a portion of the third body.

14. The device of claim 1, wherein the flexure bearing comprises a cross-axis flexural pivot configured to move in a circular range of motion.

15. A metamorphic flexure bearing implant, comprising: a bearing housed within a cover and operably coupled to a first stem and a second stem, wherein the bearing comprises: a first body flexibly coupled to a second body via a flexure bearing; and the second body coupled to a third body via a retention mechanism,Docket #: 206030-0333-00WG wherein in a first mode, the second body is anchored to the third body, and the first body is configured to rotate towards the second body, and wherein in a second mode, the second body is configured to rotate towards the third body.

16. The implant of claim 15, wherein in the first mode, the second body is anchored to the third body via the retention mechanism, wherein in the second mode, the second body is anchored to the first body via another retention mechanism, and wherein the retention mechanism and the other retention mechanism each comprise a magnetic catch.

17. The implant of claim 15, wherein the flexure bearing comprises a cross-axis flexural pivot configured to move in a circular range of motion.

18. The implant of claim 15, wherein the first body moves to the end of the range of motion by contacting a hard stop of the second body.

19. The implant of claim 15, wherein the bearing is configured to internally or externally rotate about an object coupled to the second stem.

20. The implant of claim 15, comprises a compliant endoprosthesis for revision total knee arthroplasty.

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