Orthodontic appliances having extra soft tooth gripping layer

Abstract

A multi-layer orthodontic appliance has an extra soft or rubbery first layer for contacting the patient's teeth. The first layer is supported by a harder second sheet. The second harder sheet provides a repositioning force to move the patient's teeth according to a treatment plan. The extra soft first layer grips the teeth, and in preferred embodiments, is characterized by a contact angle sufficient to create a non-adhesive hydrophobic bond with the teeth. The appliance is operable to grip and move the patient's teeth according to a treatment plan without the need of attachments, or in the alternative, significantly reduces their prominences or the number of attachments required to treat a case, and without permitting water into the interface between the appliance and the teeth.

Description

ORTHODONTIC APPLIANCES HAVING EXTRA SOFT TOOTH GRIPPING LAYERCROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This claims priority to application no. 63 / 675,170, filed July 24, 2024, and entitled “ORTHODONTIC APPLIANCES HAVING EXTRA SOFT TOOTH GRIPPING LAYER”, the entirety of which is herein incorporated by reference for all purposes.FIELD OF THE INVENTION

[0002] The subject matter of the present disclosure relates generally to the field of orthodontic devices. More particularly, the present disclosure relates to user removable orthodontic devices.BACKGROUND

[0003] An objective of orthodontics is to move a patient's teeth to positions where function and / or aesthetics are optimized. Traditionally, appliances such as braces are applied to a patient's teeth by a treating practitioner and the set of braces exerts continual force on the teeth and gradually urges them toward their intended positions. Over time and with a series of clinical visits and reactive adjustments to the braces by the practitioner, the appliances to move the teeth toward their final destination.

[0004] More recently, alternatives to conventional orthodontic treatment with traditional affixed appliances (e.g., braces) have become available. For example, systems including a series of molded plastic aligners have become commercially available from AlignTechnology, Inc., San Jose, Calif, under the trade name Invisalign® System. The Invisalign® System is described in numerous patents and patent applications assigned to Align Technology, Inc. including, for example in U.S. Pat. Nos. 6,450,807, and 5,975,893.

[0005] The Invisalign® System typically includes designing and fabricating multiple aligners to be worn by the patient before the aligners are administered to the patient and used to reposition the teeth (e.g., at the outset of treatment). Often, designing and planning a customized treatment for a patient makes use of computer-based 3-dimensional planning / design tools. The design of the aligners relies on computer modeling of the patient's teeth in a scries of planned successive tooth arrangements, and the individual aligners arc designed to be worn over the teeth, such that each aligner exerts force on the teeth andelastically repositions the teeth to each of the planned tooth arrangements.

[0006] Arguably, such aligners are less noticeable than traditional braces because typically aligners are constructed from a transparent material, however, many believe that aligners are easily noticeable due to the glossy sheen of the transparent material. Like traditional braces, aligners are required to be worn nearly constantly (20-22 hours a day), with breaks allowed for eating and cleaning teeth. Only small breaks are allowed because aligners do not have enough flexibility to account for teeth drifting out of alignment, which is based on physical and material characteristics of the aligner. Increasing the working tolerance to account for higher drift requires increasing the working elasticity of an aligner, i.e., the amount an aligner can stretch to mount to teeth without causing permanent deformation, but a highly elastic aligner typically will not provide enough force to move teeth required for orthodontic treatment.

[0007] Additionally, it is not uncommon for a treatment case to veer “off-track”. By “offtrack” it is meant that the prescribed aligner no longer fits the patient dentition. This phenomenon can arise due to a number of reasons the most common being the patient failing to wear the device as prescribed. When a treatment case goes “off-track”, the typical course of action is to restart the treatment from the beginning. This, of course, is inconvenient and undesirable for the lost time and cost.

[0008] Additionally, specific teeth may need more movement than others for a treatment. In such instances, with reference to FIG. 1A, the orthodontist may opt to apply aligner nubs 4, also known as attachments, to specific teeth 6 or directly onto clear aligners 8. The nubs are small (typically protruding about 0.75 to 1.25 mm from the surface of the tooth) transparent or tooth-colored handles and provide enhanced grip. Because aligners tend to slide off teeth easily, the use of attachments provide a better grip, moving teeth more efficiently during treatment. Nubs also allow for more complex manipulations. Attachments enable more intricate adjustments, such as rotations or vertical height changes.

[0009] Aligner nubs, however, can present a number of problems including, for example: intrusion forces and attachment dislodgment, comfort and speech issues, and attachment alignment.

[0010] A novel aligner that overcomes the above-mentioned challenges is therefore desirable.SUMMARY OF THE INVENTION

[0011] Embodiments of the invention relate to orthodontic appliances, systems, and methods of use and manufacturing as described herein.

[0012] In embodiments, an orthodontic appliance for repositioning a patient's teeth comprises: a first sheet to contact the teeth formed of an extra soft thermoplastic elastomer; and a second sheet arranged over the first sheet, wherein the second sheet does not contact the teeth, and wherein the second sheet comprises a hard thermoplastic operable to continuously apply the repositioning force to move the patient’s teeth according to the treatment plan.

[0013] In embodiments, the first layer of the first sheet is characterized by a first hardness less than Shore A 60, optionally, less than Shore A 40.

[0014] In embodiments, the first layer of the first sheet has a contact angle greater than 90 degrees, wherein the contact angle and first hardness of the first layer of the first sheet collectively operate to hydrophobically bond the first layer of the first sheet to the patient’s teeth such that (a) saliva from the patient is excluded or prohibited from an interface between the teeth and the appliance after the appliance is installed on the patient’ s teeth, and (b) attachments are not required in order to grip and reposition the patient’ s teeth according to the treatment plan, or if attachments arc required, the number or size of the attachments is reduced.

[0015] In embodiments, the first layer of the first sheet has a thickness in a range from 0.002 - 0.015 in.

[0016] In embodiments, the first layer of the first sheet comprises a first hardness of less than 40A, and optionally, a contact angle greater than 100 degrees.

[0017] In embodiments, the first layer of the first sheet has a first water absorption of less than or equal to 0.05%.

[0018] In embodiments, the first layer of the first sheet comprises thermoplastic polyurethane, a thermoplastic elastomer, or a block copolymer elastomer.

[0019] In embodiments, the first layer of the first sheet comprises a foam, optionally a polyurethane foam.

[0020] In embodiments, the second sheet comprises a co-polyester, a thermoplastic polyurethane, a polypropylene and polyethylene copolymer, a cyclic block copolymer, a polyethylene terephthalate, a polybutylene terephthalate, a polyethersulfone, a polytrimethylene terephthalate, a homopolymer, Silicone rubber, ethylene vinyl acetate, styrene block copolymer, polyolefin elastomers, polyvinyl chloride, or a combination thereof.

[0021] In embodiments, the hydrophobic bonding is adhesive free.

[0022] In embodiments, the second sheet has a second hardness at least 2 times (2x) greater than the first hardness, and in some embodiments, at least 5 times (5x) greater than the first hardness.

[0023] In embodiments, the first sheet comprises a second scaffolding layer that does not contact the teeth and provides support to the extra soft first layer.

[0024] In embodiments, the second scaffolding layer of the first sheet has a third hardness equal to or greater than the second hardness.

[0025] In embodiments, the appliance further comprises a first chamber defined between a top first surface of the first sheet and a bottom second surface of the second sheet, and a first perimeter seal formed between a portion of the top first surface of the first sheet and the bottom second surface of the second sheet.

[0026] In embodiments, the first perimeter seal is a formed by a sealing technique selected from the group comprising: heat; solvent; and adhesive.

[0027] In embodiments, the appliance is characterized by a cross sectional shape of a torus, annulus, ring, or donut.

[0028] In embodiments, the orthodontic appliance is made by forming a stack of sheets, and arranging the stack over a dental model, and thermoforming the stack over the dental model.

[0029] In embodiments, the stack comprises a sacrificial layer between a first sheet and the second sheet.

[0030] In embodiments, the sacrificial layer is a liquid, and optionally water.

[0031] In embodiments, the first sheet consists solely of the first layer formed of the extra soft thermoplastic elastomer.

[0032] In embodiments, an orthodontic appliance for repositioning a patient's teeth comprises: a first sheet to contact the teeth, wherein the first sheet comprises a first layer formed of an extra soft thermoplastic elastomer; and a second sheet arranged over the first sheet, wherein the second sheet does not contact the teeth, and wherein the second sheet comprises a hard thermoplastic operable to continuously apply the repositioning force to move the patient’ s teeth according to the treatment plan, and wherein the first layer of the first sheet has a first hardness value and the second sheet has a second hardness value such that the appliance generates a non-adhesive gripping force greater or equal to 1 Ib-f without use ofany tooth attachments, and in some embodiments, wherein the appliance generates a nonadhesive gripping force greater or equal to 1.4 Ib-f.

[0033] In embodiments, an orthodontic appliance for repositioning a patient's teeth comprises: a first sheet to contact the teeth, wherein the first sheet comprises a first layer formed of an extra soft thermoplastic elastomer; and a second sheet arranged over the first sheet, wherein the second sheet does not contact the teeth, and wherein the second sheet comprises a hard thermoplastic operable to continuously apply the repositioning force to move the patient’ s teeth according to the treatment plan, and wherein the first layer of the first sheet has a first hardness value and the second sheet has a second hardness value such that the appliance generates a non-adhesive gripping force greater or equal to 2 Ib-f with use of tooth attachments as small as 0.25 mm prominence.

[0034] In embodiments, a method of making an orthodontic appliance comprises: providing a hydrophobic extra soft or rubbery thermoplastic polyurethane characterized by a contact angle greater than 90 and a shore A hardness equal to or less than 60; providing a second layer having a hardness at least two times greater than the hardness of the first layer; forming stacked layers by stacking a second layer on top of the first layer; thermoforming the stacked layers over a dental model to simultaneously form a shaped first shell and a shaped second shell bonded to the first shell along a first perimeter.

[0035] hr embodiments, the method further comprises minimizing or prohibiting the first shell from bonding to the second shell within the perimeter.

[0036] In embodiments, the prohibiting is performed by applying a sacrificial layer between the first layer and second layer prior to the step of thermoforming.

[0037] In embodiments, the sacrificial layer is a liquid, and optionally water.

[0038] In embodiments, the method further comprises creating a plurality of holes in at least one of the first layer, and the second layer prior to the thermoforming step.

[0039] Objects and Advantages

[0040] In embodiments of the invention, objects and advantages include one or more of the following:

[0041] More intimate contact between the inner surface of the aligner and the dentition.

[0042] More retention between the aligner and the dentition.

[0043] Eliminating need for aligner attachments, and if attachments are needed, then their prominences or height would be significantly less.

[0044] Eliminate water / saliva between the teeth and the aligner.

[0045] Eliminate the need for more than two layers in the aligner.

[0046] The description, objects and advantages of the present invention will become apparent from the detailed description to follow, together with the accompanying drawings.BRIEF DESCRIPTION OF THE DRAWINGS

[0047] For a better understanding of at least certain embodiments, reference will be made to the following Detailed Description, which is to be read in conjunction with the accompanying drawings.

[0048] FIG. 1A is a perspective view of a conventional dental arch and aligner having attachments or engagers.

[0049] FIG. IB is a perspective view of a jaw and an orthodontic appliance, according to some embodiments of the invention.

[0050] FIG. 2 is an exploded view of an orthodontic appliance, according to some embodiments of the invention.

[0051] FIG. 3A is a connection schematic for an orthodontic appliance, according to some embodiments of the invention.

[0052] FIG. 3B is a detail view of a connection schematic for an orthodontic appliance, according to some embodiments of the invention.

[0053] FIG. 4 is a perspective view of a process for molding an orthodontic appliance, according to some embodiments of the invention.

[0054] FIGS. 5-7 illustrate various orthodontic appliances, each of which includes a ledge, according to embodiments of the invention.

[0055] FIG. 8 is an upper perspective view of another orthodontic appliance in accordance with an embodiment of the invention.

[0056] FIG. 9 is a cross sectional view of the appliance shown in FIG. 8 taken along 9-9.

[0057] FIG. 10 is an illustration of the force action on the appliance shown in FIG. 9.

[0058] FIGS. 11-13 are various views of a dental model for forming orthodontic appliances, each of which includes a trench, according to embodiments of the invention.

[0059] FIGS. 14-16 are labial, buccal, and lingual views, respectively, of a lower dental model for forming orthodontic appliances, each of which includes a bevel, according toembodiments of the invention.

[0060] FIG. 17 is photo of the dental model shown in FIGS. 14-16 and an orthodontic appliance made from the dental model, when viewed from the bottom, according to embodiments of the invention.

[0061] FIG. 18 show cross sectional views of dental models according to embodiments of the invention.

[0062] FIG. 19 is a plot of a flexural modulus of various appliances in accordance with embodiments of the invention.

[0063] FIG. 20A is a top perspective view of an orthodontic appliance used as a control during testing.

[0064] FIG. 20B is an illustration of tooth motions for testing.

[0065] FIGS. 21-23 are photos of test set up and testing.

[0066] FIG. 24A is a plot of activation distance for a control appliance and a test appliance in accordance with embodiments of the invention.

[0067] FIG. 24B is a plot of an unloading curve for a control appliance (CTR) and test formulations in accordance with embodiments of the invention.

[0068] FIG. 25A is a plot of a flexural stress / strain curve of a control sheet and a test sheet in accordance with embodiments of the invention.

[0069] FIG. 25B is a plot of a loading curve for a control appliance (CTR) and test samples in accordance with embodiments of the invention.

[0070] FIG. 26 is an illustration of a sheet arrangement for an aligner in accordance with embodiments of the invention.

[0071] FIGS. 27A-28B are photos of a typodont before and after installation of a conventional aligner, respectively.

[0072] FIGS. 28A-28B are photos of a typodont before and after installation of an aligner in accordance with an embodiment of the invention, respectively.

[0073] FIGS. 29A-29B are photos of the installed conventional and inventive aligners after testing, respectively.

[0074] FIGS. 30-31 are photos of a typodonts with different sized attachments used in testing.

[0075] FIG. 32 is a photo of the testing setup for measuring restraining forces of an aligner on a typodont.

[0076] FIG. 33 is a set of tabular data arising from measuring the bonding forces of different aligners on various typodonts.

[0077] The figures depict various embodiments of the present invention for purposes of illustration only, wherein the figures use like reference numerals to identify like elements. One skilled in the art will readily recognize from the following discussion that alternative embodiments of the structures and methods illustrated in the figures may be employed without departing from the principles of the invention described herein.DETAILED DESCRIPTION

[0078] Embodiments are disclosed that relate to orthodontic appliances constructed from multiple shells for the purpose of maximizing working elasticity. By “working elasticity” is it meant the capability of an orthodontic appliance to elastically deform to attach to an initial location of the teeth. This flexibility can allow an orthodontic appliance to obtain a greater range of initial tooth arranging (i.e., flexing) positions that differ from the appliance's target tooth arranging (i.e., resting) position. Possible benefits include greater break time (e.g., 8-12 hours) between required wear periods and greater latitude for patient non-adherence to required wear-times, and hence increased efficacy.

[0079] Before the present invention is described in greater detail, it is to be understood that this invention is not limited to particular embodiments described, as such can, of course, vary. It is also to be understood that the terminology used herein is for the purpose of describing particular embodiments only, and is not intended to be limiting, since the scope of the present invention will be limited only by the appended claims.

[0080] Where a range of values is provided, it is understood that each intervening value, to the tenth of the unit of the lower limit unless the context clearly dictates otherwise, between the upper and lower limit of that range and any other stated or intervening value in that stated range, is encompassed within the invention. The upper and lower limits of these smaller ranges can independently be included in the smaller ranges and are also encompassed within the invention, subject to any specifically excluded limit in the stated range. Where the stated range includes one or both of the limits, ranges excluding either or both of those included limits are also included in the invention.

[0081] Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which thisinvention belongs. Although any methods and materials similar or equivalent to those described herein can also be used in the practice or testing of the present invention, representative illustrative methods and materials are now described.

[0082] It is noted that, as used herein and in the appended claims, the singular forms “a”, “an”, and “the” include plural referents unless the context clearly dictates otherwise. It is further noted that the claims may be drafted to exclude any optional element. As such, this statement is intended to serve as antecedent basis for use of such exclusive terminology as “solely,” “only” and the like in connection with the recitation of claim elements or use of a “negative” limitation.

[0083] As will be apparent to those of skill in the art upon reading this disclosure, each of the individual embodiments described and illustrated herein has discrete components and features which can be readily separated from or combined with the features of any of the other several embodiments without departing from the scope or spirit of the present invention. Any recited method can be carried out in the order of events recited or in any other order that is logically possible.

[0084] All existing subject matter mentioned herein (e.g., publications, patents, patent applications and hardware) is incorporated by reference herein in its entirety except insofar as the subject matter may conflict with that of the present invention (in which case what is present herein shall prevail). US Application No. 17 / 871,903, filed July 23, 2022, and entitled “ELASTIC ORTHODONTIC APPLIANCES, SYSTEMS, AND METHODS FOR USE”; and US Publication No. 20210153979, filed January 13, 2021, and entitled “ELASTIC ORTHODONTIC APPLIANCES, SYSTEMS, AND METHODS FOR USE”, are incorporated herein by reference in their entirety for all purposes.

[0085] FIG. IB provides an appropriate starting point in a detailed discussion of various embodiments of the present invention with respect to tooth repositioning appliances designed to apply repositioning forces to teeth. An orthodontic appliance 10 can be worn by a patient in order to achieve an incremental repositioning of individual teeth in the jaw 12. The orthodontic appliance 10 can include a shell having teeth-receiving cavities that receive and resiliently reposition the teeth. In some embodiments, a polymeric appliance can be formed from a sheet of suitable layers of polymeric material. An appliance can fit over all teeth present in an upper or lower jaw, or less than all of the teeth.

[0086] In some embodiments, only certain teeth received by an appliance will berepositioned by the appliance while other teeth can provide a base or anchor region for holding the appliance in place as it applies force against the tooth or teeth targeted for repositioning. In some cases, many or most, and even all, of the teeth will be repositioned at some point during treatment. Teeth that are moved can also serve as a base or anchor for holding the appliance as it is worn by the patient. Typically, no wires or other means will be provided for holding an appliance in place over the teeth. In some cases, however, it may be desirable or necessary to provide individual anchors on teeth with corresponding receptacles or apertures in the appliance so that the appliance can apply a selected force on the tooth. Basic methods for determining an orthodontic treatment plan using a series of incremented appliances as well as instructions for molding orthodontic appliances, are described in U.S. Pat. Nos. 6,450,807, and 5,975,893, which are incorporated by reference herein, but only to an extent that those patents do not contradict the newer teachings disclosed herein.

[0087] An appliance can be designed and / or provided as part of a set of a plurality of appliances. In such an embodiment, each appliance may be configured so a tooth-receiving cavity has a geometry corresponding to an intermediate or final tooth arrangement intended for the appliance. The patient's teeth can be progressively repositioned from an initial tooth arrangement to a target tooth arrangement by placing a series of incremental position adjustment appliances over the patient's teeth. A target tooth arrangement can be a planned final tooth arrangement selected for the patient's teeth at the end of all planned orthodontic treatment. Alternatively, a target arrangement can be one of many intermediate arrangements for the patient's teeth during the course of orthodontic treatment. As such, it is understood that a target tooth arrangement can be any planned resulting arrangement for the patient's teeth that follows one or more incremental repositioning stages. Likewise, an initial tooth arrangement can be any initial arrangement for the patient's teeth that is followed by one or more incremental repositioning stages.

[0088] The orthodontic appliances can be generated all at the same stage or in sets or batches, e.g., at the beginning of a stage of the treatment, and the patient wears each appliance until the pressure of each appliance on the teeth can no longer be felt or has resulted in the maximum amount of expressed tooth movement for that given stage. A plurality of different appliances (e.g., set) can be designed and even fabricated prior to the patient wearing any appliance of the plurality. After wearing an appliance for an appropriate period of time, the patient replaces the current appliance with the next appliance in the series until no moreappliances remain. The orthodontic appliances are generally not affixed to the teeth and the patient may place and replace the appliances at any time during the procedure (e.g., patientremovable appliances).

[0089] The final orthodontic appliance or several appliances in the series may have a geometry or geometries selected to overcorrect the tooth arrangement, i.e., have a geometry which would (if fully achieved) move individual teeth beyond the tooth arrangement which has been selected as the “final.” Such over-correction may be desirable in order to offset potential relapse after the repositioning method has been terminated, i.e., to permit movement of individual teeth back toward their pre-corrected positions. Over-correction may also be beneficial to speed the rate of correction, i.e., by having an appliance with a geometry that is positioned beyond a desired intermediate or final position, the individual teeth will be shifted toward the position at a greater rate. In such cases, the use of an appliance can be terminated before the teeth reach the positions defined by the appliance.

[0090] FIG. 2 shows an exploded view of an example of the orthodontic appliance 10. The orthodontic appliance 10 can include a first shell 14 having a teeth engaging surface and an opposite upper surface. The orthodontic appliance 10 can also include a second shell 16 having a lower-shell engaging surface and an opposite upper surface that is exposed to the mouth. Optionally, one or more additional shells 18 can be located between the first shell 14 and the second shell 16. In some embodiments, the more shells that are used, the greater the working elasticity of the orthodontic appliance 10, assuming use of the same material for each shell.

[0091] While the orthodontic appliance 10 is shown in an exploded view for the purpose of better understanding, the shells of the orthodontic appliance 10 are intended to be mechanically engaged with one another in a stack. “Mechanically engaged” is defined herein as the substantially non-affixed or varyingly affixed engagement between one or more shells to approximate the strength of a single shell appliance of approximately the same thickness as the stacked shells.

[0092] Mechanical engagement can be obtained by stacking the shells while having the lower-shell engaging surface of the second shell largely conforming to the upper surface of the first shell. In some embodiments, shells can be stacked loosely, i.e., without a compressive or an interference fit between shells or such that an upturned stack of shells selfdisassembles, before being made substantially non-affixed or varyingly affixed. The shells aresubstantially non-affixed (or varyingly affixed) because a substantial amount of surface areas between the shells are not bonded or otherwise made inseparable through some process, with the remaining surfaces being affixed. In some embodiments, substantially non-affixed or varyingly affixed shells have less than 1-2%, 1-5%, 1-10%, 1-20%, 1-40%, 1-60%, or 1-80% of the combined contacting surfaces of the shells affixed. The area of non-fixation can be limited according to the needs of the appliance, hence, in some embodiments, a majority the surface areas of the appliance are affixed, while the remaining part is non-affixed because only the latter requires high working elasticity.[00931 In some embodiments, the lack of substantial fixation between shells provides greater working elasticity to the orthodontic appliance 10 because the teeth-engaging shell can flex more due to being thinner while the outer shells are allowed to flex in multiple directions away from the teeth-engaging shell. In some embodiments, this can result in partial mechanical disengagement between some of the engaging surfaces of the shells, however the disengagement is not enough to significantly impair flexural modulus of the device required for aligning the teeth to the target position.

[0094] FIG. 3A shows a schematic for affixing the shells of the orthodontic appliance 10 at discrete locations. Each encircled “X” represents a possible point of fixation between the shells. Alternatively, as shown by the dashed line, the edges of each shell can serve as a continuous or non-continuous area of fixation. Generally, the more fixation provided, the less working elasticity the orthodontic appliance 10 will have. Points of fixation can be determined based on the amount of working elasticity required, which teeth are being moved, and which teeth are serving as anchors. Alternatively, the shells can be uniformly and weakly bonded with a highly elastic material of low cohesive strength that allows for a large amount of stretching and / or shearing. Such embodiments are substantially non-affixed or varyingly affixed because the working flexibility of such an orthodontic appliance are maintained due to the properties of the weak bond.

[0095] In some embodiments, shells of the orthodontic appliance 10 can be non-identical such that surface areas of one shell is greater or less than another shell. Accordingly, in some embodiments, edges, which are defined by the top and bottom surfaces of each shell, of such shells can be separated by gaps (e.g., 0.20-3.0 mm), as depicted by FIG. 3B, which shows an example with three shells 14, 16, 18 and three edges 14a, 16a, 18a. In some embodiments, referring to the arrangement shown at FIG. 2, bottom-most shell 14 can have the greatestsurface area, resulting in edge 14a being at the bottom most position, shown, with shells 18 and 16 respectively having smaller surfaces areas such that edge 16a is at the top-most position. In such embodiments, the shells 14, 16, 18 are stacked such that steps formed by edges 14a, 16a, 18a face outward, away from the teeth. In some embodiments, referring to the arrangement shown at FIG. 2, top-most shell 16 can have the greatest surface area, resulting in edge 16a being at the bottom most position, shown, with shells 18 and 14 respectively having smaller surfaces areas such that edge 14a is at the top-most position. In such embodiments, the shells 14, 16, 18 are stacked such that inward facing steps formed by edges 14a, 16a, 18a face inward, i.e., towards the teeth.

[0096] Providing one or more of such gaps can be used to tune flexural modulus of the orthodontic appliance 10 and also result in less tongue irritation to the patient that can occur due to material thickness where edges are bonded at the same location. To alleviate irritation, gaps can be placed in areas that face inwards towards the mouth, resulting in stepped edges (e.g., edges 14a, 16a, 18a) facing the tongue, or the tooth-engaging shell can have a smaller surface area than shells stacked thereon, resulting in interior, tooth-facing steps and a single shell edge (e.g., edge 16a) that can contact the tongue. In some embodiments, the bottommost, tooth-engaging shell, can have a greater or lesser total surface area than a second shell stacked thereon, which can result in at least a portion of the edge of the second shell being separated from the edge of the tooth-engaging shell. In some embodiments, only portions of the edges that face towards the mouth have such a gap, and in other embodiments, a uniform or non-uniform gap can exist between the entirety of edges. In some embodiments, the orthodontic appliance 10 can include shells, each having different surface areas.

[0097] The shells can have thicknesses ranging from 0.001 - 0.040 inches, and in embodiments, from 0.001 - 0.015 inches thick, and can be constructed from a polyester, a copolyester, a polycarbonate, a thermoplastic polyurethane, a polypropylene, a polyethylene, a polypropylene and polyethylene copolymer, an acrylic, a cyclic block copolymer, a polyetheretherketone, a polyamide, a polyethylene terephthalate, a polybutylene terephthalate, a polyetherimide, a polyethersulfone, a polytrimethylene terephthalate or a combination thereof. In some embodiments, shells are coated with lubricous materials or provided with surface treatments to decrease friction between the shells. In some embodiments, interior portions of the shells are treated with hydrophobic coatings to prevent liquid intrusion into the shells. In some embodiments, shells of relatively more flexibility canbe used in conjunction with stiffer shells. Flexible shells can be constructed from hydrogels, styrenic block copolymers (SBC), silicone rubbers, elastomeric alloys, thermoplastic elastomers (TPE), thermoplastic vulcanizate (TPV) elastomers, polyurethane elastomers, block copolymer elastomers, polyolefin blend elastomers, thermoplastic co-polyester elastomers, thermoplastic polyamide elastomers, or a combination thereof. Flexible shells may also provide the benefit of a gasket to prevent liquid intrusion between the shells.

[0098] FIG. 4 depicts an example of a basic process 30 for forming an orthodontic appliance. As shown, a material 32 can be formed into an orthodontic appliance 36. The material 32 can be of one layer to form a single shell or multiple non-affixed layers of material to form multiple shells at once. In this example process, the tooth positioning appliance 36 can be produced with the use of a physical tooth model, or mold, 34. The tooth positioning appliance 36 can be produced by heating the thermoformable material 32 and then vacuum or pressure forming the material over the teeth in the physical tooth model 34. The tooth positioning appliance 36 is a direct representation of the physical tooth model. In some embodiments, material 32 is dimensioned (e.g., 120 mm and / or 125 mm diameter circle) for ready processing on a commercially available forming device (e.g., Erkoform®, Erkoform- 3dmotion®, Biostar®, Ministar S®, Drufomat Scan®, Drufosmart®, Essix® SelectVac®). Guidelines for operating such forming devices can be found at Scheu Dental Technology, Biostar Operating Manual, DE / GB / FR / IT / ES / 1.000 / 06 / 19 G REF PM 0113.01; Scheu Dental Technology, Application booklet for the pressure moulding technique, GB 2.000 / 07 / 19 G REF 0111.02; Erkodent, Thermoforming, S15-3106-48; Erkodent, Erkoform 3D, 61-8002-2; Erkodent, Erkoform-3D+Instructions, BA-Erkoform-3d+-anl-EN-04-04-2019, which are incorporated by reference herein.

[0099] After formation, shells can be affixed to one another according to the desired working elasticity required for the patient. Methods of fixation include chemical bonding, localized melting, fasteners, and / or localized physical deformation to key the shells together. Before or after fixation takes place, excess material from the sheet can be trimmed to form a final tooth positioning appliance that can be used for orthodontic treatment of a patient. The edges of the shells can be sealed with a flexible material such as silicone to prevent liquid intrusion.

[0100] One or a series of physical tooth models, such as the model described above, may be used in the generation of elastic repositioning appliances for orthodontic treatment. Similarto the process above, each of the appliances can be generated by thermoforming a multilayer polymeric material over a mold of a desired tooth arrangement to form a dental appliance. The tooth positioning appliance of the desired tooth arrangement generally conforms to a patient's teeth but is slightly out of alignment with the initial tooth configuration. Placement of the elastic positioner over the teeth applies controlled forces in specific locations to gradually move the teeth into the desired configuration. Repetition of this process with successive appliances comprising new configurations eventually moves the teeth through a series of intermediate configurations to a final desired configuration. rooiou FIG. 5 is an orthodontic appliancelOO in accordance with another embodiment of the present invention comprising a first top shell 110 and a second lower shell 120 to contact the teeth. The lower shell 120 includes a larger surface area than the top shell 110 and extends beyond the edge of the top shell forming a ledge 130. The ledge 130 is shown increasing from the anterior to the posterior of the appliance.

[0102] FIG. 6 is an orthodontic appliance 200 in accordance with another embodiment of the present invention comprising a top shell 210 and lower shell 220. The lower shell 220 has a larger surface area than the top shell 210 and extends beyond the edge of the top shell forming a ledge 230. The ledge 230 is relatively constant from the anterior to posterior of the appliance.

[0103] FIG. 7 is an orthodontic appliance 300 in accordance with another embodiment of the present invention comprising a top shell 310 and lower shell 320. The lower shell 320 includes a larger surface area than the top shell 310 and extends beyond the edge of the top shell forming a ledge 330. Unlike that shown in FIG. 5, described above, the ledge 330 is shown decreasing from the anterior to the posterior of the appliance. The inventors have found creating a larger edge towards the anterior position of the appliance reduces the likelihood of the appliance inadvertently folding as the appliance is placed and removed from the patient’s mouth.

[0104] In embodiments, the appliances 100, 200, and 300 are assembled by mechanical attachment. Preferably, the first lower shell has a larger extended ledge (e.g., 130, 230, 330) and is mechanically roughened by e.g., a sander. Then, the second top shell is thermoformed onto the first shell. The roughened surface increases adhesion between the shells.

[0105] FIG. 8 is an orthodontic appliance 500 in accordance with another embodiment of the present invention comprising a top shell 510 and lower shell 520. Unlike the figuresshown above, the top shell 510 and lower shell 520 have approximately the same surface areas and form a flush edge. The appliance 500 thus lacks a prominent ledge as shown in FIGS. 5-7.

[0106] FIG. 9 is a cross sectional view of the appliance 500 shown in FIG. 8 taken along line 9-9. As shown, a narrow flexibility enhancing chamber 530 is defined between the top and lower shells 510, 520.

[0107] FIG. 10 schematically illustrates the cross section of FIG. 9 in an unwound-like configuration to facilitate understanding of the invention. The volume or size of the chamber between the shells may vary and be based on the performance needs. The larger the spacing or the distance (1) between the layers or shells - the greater the anatomical fitness range the device will possess. In embodiments, the chamber is characterized by a characteristic distance dimension (1) ranging from as little as a thin liquid film (e.g., single molecule thickness), to 100 microns, and more preferably from 5 to 75 microns, and in embodiments, between 10 and 50 microns. Additionally, the chamber in the embodiments shown in FIGS. 9-10 has an elongate tapering cross sectional shape with closed / sealed ends, namely, an elongate elliptical shape where the major axis is much larger than the minor axis or stated another way, the cross-sectional ratio of the major to minor axis is large and, in embodiments, is greater than 100 / 1, and in embodiments between 100 / 1 and 500 / 1 and in one embodiment ranges from 150 / 1 to 250 / 1.

[0108] With reference to FIG. 10, and without intending to be bound to theory, the flexural modulus inherent to the top shell 510 imposes a constant force (F) on the lower shell 520 while chamber 530 provides room for the lower shell 520 to be displaced by tooth force (T) in the event the teeth of the patient veer off-track or are otherwise out of the anticipated target position. This anatomical-fit adaptability is not available in a typical orthodontic aligner because the dentition and the aligner must fit for teeth to be moved and when the teeth are off-track, the typical aligner cannot register the teeth - off-track teeth will not fit the conventional aligner.

[0109] It is also to be understood that although the spacing or chamber 530 between the shells 510, 520 is shown in FIG. 10 being oval, continuous and gently tapering, the chamber may have other shapes. In embodiments, the chamber can have a uniform height, and be continuous from one end to the other end. Additionally, in embodiments, the chamber has regions of varying height as well as tack-less contact points in which there is no separationdistance between the shells. Additionally, and optionally in combination with the contact points, the chamber may feature shell fixation points as described above. Indeed, the chamber may be programmed to have a wide range of configurations to aid in limiting or controlling shell motion.

[0110] FIG. 11 shows an orthodontic appliance 600 and corresponding dental model 620 for forming the appliance in accordance with another embodiment of the present invention. In this variation of the invention, the appliance 600 comprises a trench 610. The trench 610 is shown extending along the gingival margin or cementoenamel junction (CEJ) for the entire perimeter of appliance. The trench has a scallop, ribbon, wave, or sinusoidal-like shape.

[0111] The appliance 600 may be formed by stacking two thin plastic sheets. The upper sheet and lower sheet may be made of the materials described herein such as, for example, polyurethane, polyamide, polycarbonate, or polyester. The unbonded stack of sheets is placed over the tooth model / mold 620. The dental model may be made by casting or more preferably, a 3D printed or SLA model. The plastic stack and model 620 are then sealed in a pressure chamber, where pressure and heat are applied to thermoform the plastic to the model. When the process is finished, the multi-layer plastic “negative” is removed from the 3D printed “positive” model. The multi-layer plastic “negative” is trimmed so excess plastic is removed and the edges are polished. The appliance 600 is complete once it has been cleaned and dried.

[0112] Optionally, a sacrificial layer (not shown) may be disposed between the sheets prior to the thermoforming step. The sacrificial layer serves to maintain an open space between the shells which, as described herein, provides elasticity to the appliance. Exemplary materials for the sacrificial layer include water or distilled water. The water vaporizes during the thermoforming step, leaving a small space between the upper and lower shells. As described herein, apertures or vents may be added to the sheets to allow the vapor to escape during the thermoforming step.

[0113] Notably, in the variation shown in FIG. 11, the presence of the trench 630 in the mold serves to increase adhesion between the plastic layers during the thermoforming process. The sharp inverse transition from the bottom of the tooth crown or gingival margin in the mold 620 to the valley of the trench 630 promotes the upper sheet to grip the lower sheet along the trench. The sheets tend to interlock along the trench during the thermoforming step, even without use of chemical bonding agents. Although in otherembodiments it is to be understood that chemical bonding may be applied in lieu of, or in addition to, the mechanic al / structural mold features recited herein.

[0114] With reference to FIGS. 12-13, which are perspective and cross-sectional views of the dental model 620 shown in FIG. 11, respectively, an exterior trench 680 is shown extending along the exterior gingival margin of the mold. The depth (d) and width (w) of the trench may vary. In embodiments, the depth (d) of the trench 680 ranges from 0.35 to 1 mm and the width (w) of the trench ranges from 0.5 to 2 mm.

[0115] It is also to be understood that although a trench is a suitable feature for facilitating bonding the sheets to one another during the thermoforming step, this structural interlocking feature may vary widely. For example, and with reference to FIGS. 14-16, another dental model 700 for creating an orthodontic appliance is shown in accordance with an embodiment of the present invention. The dental model 700 is shown having an inward- directed bevel 710 just below the gingival margin or cementoenamel junction (CEJ). The bevel is shown on both the front and rear sides of the appliance 700. The bevels 710, 712 are shown extending along the entire perimeter of the dental model. Preferably, the distance from the gingival line or CEJ to the beginning of the bevel (sec, c.g., the CEJ offset shown in FIG. 15) ranges from 0.5 - 5 mm, and is set according to prescribed treatments or wear. In embodiments directed to the customary 22 hour-wear cycles, the CEJ offset ranges from 1-3 mm, and in other embodiments directed to only 8-12-hour-wear cycles, the CEJ offset ranges from 2-4 mm, or at least 4 mm.

[0116] With reference to FIG. 17, an appliance 730 made from the dental model 700 is shown. The appliance 730 may be fabricated by stacking at least two sheets as described above onto one another, and thermoforming the stacked sheets on the dental model 700. The “positive” model bevels 710, 712 impart bevels 732, 734 onto the appliance 730 which serve to interlock the shells together during the thermoforming step.

[0117] It is to be understood that these interlocking features (e.g., 630, 680, 710, 712) may vary in shape. For example, and with reference to FIG. 18, the angle (a) of the bevel or fold may vary. In embodiments, angle (a) ranges from 10 to 80 degrees, and more preferably from 20 to 60 degrees, and most preferably from 35 to 55 degrees. The depth (d) of the bevel may be similar to the depth of the trench described above.

[0118] Additionally, the thermoforming or interlocking features need not be directed inward. In embodiments, the features bow or protrude outward from the mold such as, forexample, button or rib 722 which is shown protruding outward from the gum region of the model 720.

[0119] Additionally, the bevel, fold, trench, rib, trim path, etc. may be continuous or intermittent. Although a continuous bevel is preferable to provide more bonding surface area, and to prevent moisture from seeping into the space between the shells, embodiments of the invention provide for discrete tabs or intermittent structures to create mechanical interlocking. Examples of such intermittent features include buttons, tabs, holes, detents, indents, recesses, etc. These intermittent features may be arranged along or in the vicinity of the gingival margin region on the mold. Indeed, the invention is intended to include a wide variety of features to facilitate thermoforming the shells to one another except where limited by any appended claims.EXAMPLES

[0120] EXAMPLES V 1 , V2, and V3

[0121] Appliances as shown in FIGS. 5-7 were prepared according to the following:

[0122] The lower shell (or shell to contact the teeth) was thermoformed over a dental model from a 0.015” inch polymer sheet of Isoplast® 2530 ETPU, manufactured by Lubrizol Corporation, (Wickliffe, Ohio).

[0123] The molded sheet was trimmed and its perimeter or ledge was mechanically roughened by a rotary tool with sanding capability.

[0124] A second 0.015” inch polymer sheet of Eastar® 6763 PETG, manufactured by Eastman Chemical Company, was then thermoformed onto the first shell where the edge of the second shell contacts the roughened ledge of the first shell. The roughened ledge serves to increase adhesion during thermoforming. The sheet(s) were then trimmed as shown in the figures. Each of the manufactured appliances included a chamber as described herein for mitigating lower shell displacement, allowing flexibility, and maintaining the desired flexural modulus or force on the tooth contacting portion of the appliance.

[0125] EXAMPLE V4

[0126] Appliance 400 as shown in FIG. 8 was prepared according to the following:

[0127] Thermoforming. A lower shell was thermoformed over a dental model from a 0.015” inch polymer sheets of Isoplast® 2530 ETPU.

[0128] Dipping. The perimeter edge of the lower ETPU shell (shell to contact the teeth)was dipped in ethanol to form an adhesive or sticky area.

[0129] The lower shell was flipped upside down, and its convex portion was placed in a masking solution (e.g., distilled water) up to but not beyond the desired adhesion area to prohibit bonding across this convex area. The area of non-adhesion is controlled by the area dipped into the masking solution. The area of non-adhesion corresponds to the boundary of the sealed chamber (530) between the shells. In embodiments of the invention, the sealed chamber is created by masking or dipping a predetermined area of the 1stshell in a masking solution.

[0130] Thermoforming second shell. An upper shell of 0.015” inch polymer sheet of Eastar® 6763 PETG was thermoformed over the ethanol-treated lower shell.

[0131] Trimming. The lower and upper shells were trimmed as shown until the edges were substantially flush.

[0132] Inventors observed the edge of the second shell chemically bonded to the edge of first shell. The shells formed a continuous (gapless) bond along the perimeter. The shells cooperated with one another as described above to provide the open chamber serving to mitigate off-track patient cases.

[0133] Mechanical Test Analysis

[0134] With reference to the table below and the plot shown in FIG. 19, the flexural modulus of the appliance made according to example VI is compared to that made according to example V4.

[0135] The table shows the mean values of the flexural modulus are within 3% of one another. However, the data also shows that the range of the flexural moduli of the V4 appliance is tighter than that of VI . Particularly, the range of V4 is 123 MPa and the range of VI is 304 MPa. The V4 aligner thus appears to be at least as good or better than the V 1 aligner in the sense that the mean is within 3% and the range is tighter, and based on this data.

[0136] Benefits

[0137] The chemical-enhanced assembly method described in connection with V4 appears to have some benefits / adv antages over the purely mechanical assembly method described in connection with V 1 without adversely impacting the intrinsic properties of the aligner including, without limitation:

[0138] Greater interlayer adhesivity, strength and consistency in interlayer adhesion because the chemical bonding technique may be applied with better precision than the bonding technique based solely during mechanical engagement.

[0139] Fewer chamber leaks using the chemical bond, thus reducing the need to clean saliva trapped in the aligner chamber.

[0140] Greater likelihood the chamber is well defined and uninterrupted, gapless, and unbonded.

[0141] Greater anatomical fit range, i.e., can adapt to a wider range of teeth configuration and off-track cases.

[0142] Greater ability to accommodate a greater number of attachment locations.

[0143] Improved clinical effectiveness to move teeth to the prescribed position / alignment.

[0144] These and other benefits arise from the configurations and manufacturing techniques described herein.

[0145] EXAMPLE 5

[0146] A test appliance was formed in accordance with embodiments of the invention for comparison in functionality to a conventional dental appliance.

[0147] The test appliance(s) were prepared according to the following:

[0148] A 0.015” inch polymer sheet of Isoplast® 2530 ETPU was arranged on top of a 0.015” inch polymer sheet of Eastar® 6763 PETG to form an unbonded composite stack.

[0149] Thermoforming. The stack was then thermoformed over a dental model as shown in FIGS. 11-13, and described above.

[0150] Trimming. The edges, about 1 mm below the CEJ, of the appliance were trimmed,polished and cleaned.

[0151] A conventional orthodontic appliance 930 as shown in FIG. 20A was made by thermoforming an off-the-shelf multilayered aligner sheet (Zendura FLX Clear Aligner Materials, 0.030” thickness, comprising polyurethane laminated to polyethylene terephthalate glycol manufactured by Zendura, Fremont, California) on the basic dental mold 34 as shown, e.g., in FIG. 4. Hereinafter, the conventional appliance is referred to as the standard or control appliance.

[0152] Both the test and control appliances were tested using the same fixtures and test methodology. The testing conducted was used to determine teeth correction forces resulting from different aligner planning stage movements. Tooth forces were measured when the designated aligners were placed on the stereolithography (SLA) models (namely, the dental models) which had either one or three stages of correcting movement from the base line state.

[0153] The objective of the testing was to determine forces that a one (1) or three (3) stage aligner can impart on a tooth in a simulated clinical setting. The forces applied by the aligner were to cause motions as shown in Figure 20B. Testing involved seven tooth types: Upper Right Central Incisor, Upper Right Canine, Upper Right Lateral Incisor, Upper Left Lateral Incisor, Lower Left Lateral Incisor, Lower Left Canine, and lower Left 1st Molar.

[0154] All testing was conducted in room temperature air.

[0155] The tooth used for force measurements was cut from the SLA model to be used with the appropriate aligners. Prior to cutting the tooth from the SLA model, one or more imprints (e.g., 910 shown in FIG. 21) of the area around the tooth to be tested, were formed using low melt temperature plastic. The plastic model could then be used to position the tooth in its original position. The SLA models (e.g., 900 shown in FIG. 21) had been previously bonded to steel plates (e.g., 902 shown in FIG. 21) ensuring alignment and providing a firm method of attachment to the testing fixtures. The tooth which had been removed was attached to the testing fixture which allowed measurement of the load when an aligner was placed on the SLA model. An example of an SLA model 900, steel plate 902, plastic model 910, lateral incisor tooth 920 are shown in FIG. 21.

[0156] With reference to FIG. 22, the initial base line tooth position was established using the plastic imprint model 910 formed prior to cutting the tooth 920 from the SLA model 900. Once the tooth was properly positioned using the imprint model 910, such that no initial load was indicated on the load cell, the imprint model 910 was removed and the aligner wasinstalled and a force measurement reading taken.

[0157] Figure 23 shows the aligner 930 installed on the SLA. The aligner 930 was removed and reinstalled two additional times to assess consistency. After the three installations of the aligner, the plastic imprint model was reinstalled to recheck final tooth alignment. If alignment was not maintained the SLA model and tooth were re-aligned and the test repeated.

[0158] Multiple appliances were created, each one having a different activation for a particular tooth. Three (3) appliances were made for each activation point and we tested at eight (8) activation points including one activation point for zeroing the transducer. By activation it is meant the amount of tooth movement (namely, distance) per aligner stage.

[0159] Left / Right translation tests were conducted with the tooth in question attached to the load cell allowing tension and compression readout during the range of motion specified. For translation tests, the tooth was mounted such that the centerline of the applied load passed through the center of the portion of the tooth that was covered by the aligner to mitigate bending loads.

[0160] The results arc shown in FIG. 24A.

[0161] FIG. 24A is a chart comparing force (gram-force) versus appliance activation (mm).

[0162] A theoretical optimal activation curve 1110 shows a constant, relatively low, force of about 70 (gf) applied to the teeth for activation points ranging from 0 to 0.6 mm.

[0163] The test appliance 1120 evidences a gradual increase in force from 100 to 140 (gf) between activation points ranging from 0 to 0.6 mm.

[0164] The control appliance 1130 evidences a relatively steep increase in force from 250 to 1225 (gf) between activation ranging 0 to 0.6 mm.

[0165] The data suggests the test device has substantial benefit in maintaining a lower and even force to the teeth during aligner stages and treatment. (1) The clinically desired force (about 70-100 gf) to move teeth appears to continue, even after the tooth is translated. This acceptable force continues to the final destination or desired tooth target location (namely, at activation point 0.0 mm). (2) The maximum force applied by the test aligner to the tooth remains within the optimal clinical force ranges and does not exceed 200 gf. Excessive force causes root resorption and tooth movement. (3) The test aligner data evidences that overcorrection to assure a tooth reaches the prescribed location shall not causepain or discomfort due to excessive force.

[0166] Without intending to be bound to theory, the space between the upper and lower shells allow one shell to flex and slide relative to the other, providing the above-described benefits.

[0167] FIG. 24B is another plot of an unloading curve for a control appliance (CTR) and test formulations in accordance with embodiments of the invention.

[0168] The control sample is a composite laminate commercial material called Zendura FLX. It is a tri-layer, equally divided in thickness, of polyurethane on both sides, and a PETG in the middle. It is a laminate composite, a closed or solid cross-section.

[0169] Sample A is a dual shell material. Each shell is polyurethane. The shells are arranged to have an open or torus shaped cross-section as described herein.

[0170] Sample B is a dual shell material. One shell is a polyurethane, and the other shell is a PETG. The shells are also arranged to have an open or torus shaped cross-section as described herein.

[0171] For the control sample (CTRL), due to the effects of ( 1 ) the exponential relationship between stress and strain (2) the dentition having moved, at 0.15mm from the destination of zero, the control sample has become passive. That is, no force is available. For sample A and sample B, however, being independent from Hook’s law resulting from the torus cross-section, force continues to be available to nearly the zero point of activation or the destination point of the dentition. This property is referred to as “hyper elasticity”.

[0172] EXAMPLE 6

[0173] We performed tests on our test aligner sheet and a conventional aligner sheet according to a modified ASTM-D790, in which we performed testing at a temperature of 37 degrees C and 90-95% humidity.

[0174] The test sheet was made by stacking a polyurethane sheet on a polyester sheet, and adding a thin layer of silicone oil between the plastic sheets. Two ends of the sheets were heat sealed, creating a thin oil layer. The oil layer was added to prevent the sheets from contacting each other, and to mimic the chamber (e.g., space 530 of FIG. 10) described above between the opposing aligner shells.

[0175] For the control sheet, we used the Zendura™ FLX Aligner Materials describedabove.

[0176] Our results are shown in FIG. 25A.

[0177] As anticipated, the control sheet has a steep elastic region 1010 along the flexural stress / strain curve until it reaches its yield point, which is shown occurring at about 5% strain, at 1530 gram-force, after which inelastic deformation occurs.

[0178] This has the implication that an orthodontic appliance made from this material, after 5% strain, would no longer apply force to urge the tooth to the desired destination. Such an appliance would no longer be clinically effective and would need to be replaced. It is also apparent from the plot that excessive force is required to reach 5% strain. This level of force is outside the optimal range desired for orthodontic tooth movement.

[0179] Flexural stress / strain curve 1020 represents the data arising from the test sheets in accordance with embodiments of the invention and as described above. The test sheets’ elastic region is initially step until the strain reaches about 5%, 100 gram-force, after which hyper-elasticity continues at a gradual rate until 20% strain, and ultimately until the yield point 1022, 25% strain, 200 gram-force.

[0180] This has the implication that up to 25% strain, and notably between 5% and 25% strain, (and more notably between 5% and 20%), an appliance made from the test sheets can still apply force to teeth that have been adjusted / moved. Such an appliance would continue to have a clinical benefit as long as it remains within the hyper-elastic region of the stress strain curve and up to 25%.

[0181] What is also notable is that the test sheets’ curve 1020 has a much lower force than that shown in the control sheet curve 1010. For example, the force measured in the elastic region 1020 ranges from 0 to 200 gram-force and 150 gram-force is not reached until 20% strain. As stated herein, avoiding excessive force is desirable.

[0182] Without intending to be bound to theory, the anticipated space between the upper and lower shells allow one shell to flex and slide relative to the other, providing the abovedescribed benefits. We submit these flexural stress / strain data evidence this phenomenon.

[0183] FIG. 25B is a plot of loading curves for a control appliance (CTR), test sample A, and test sample B in accordance with embodiments of the invention.

[0184] The control sample is a composite laminate commercial material called Zendura FLX. It is a tri-layer, equally divided in thickness, of polyurethane on both sides, and a PETG in the middle. It is a laminate composite, a closed or solid cross-section.

[0185] Sample A is a dual shell material. Each shell is polyurethane. The shells are arranged to have an open or torus shaped cross-section as described herein.

[0186] Sample B is a dual shell material. One shell is a polyurethane, and the other shell is a PETG. The shells are also arranged to have an open or torus shaped cross-section as described herein.

[0187] The stress / strain behaviors of the various materials shown in FIG. 25B are similar to that shown in FIG. 25 A. That is, the effect of Hooke’s law on the control sample is demonstrated by the stress being exponentially dependent on the strain. For samples A and B, however, up to around 20% and 25% respectively, the Hooke’s law effect is not observed. The stress and strain are independent up to about 25%.

[0188] FIG. 26 illustrates a sheet arrangement for forming another aligner in accordance with embodiments of the invention.

[0189] In embodiments of the invention, an aligner comprises two polymer sheets where sheet 1 contacts the teeth and sheet 2 is formed over sheet 1 and does not contact the teeth (the ‘free-side’).

[0190] In embodiments, sheet 1 comprises a 2-laycr laminate composite construction where side A (the teeth contacting side) is a soft elastomer (preferably extra soft and optionally, foam) and side B (the free side) is a harder homopolymer (e.g., a copolyester such as Eastar 6763, or a TPU such as Isoplast 2530). Without intending to be bound to theory, inventors have found the use of a soft elastomer on the teeth-contacting side A of sheet 1 serves to create a better retention between the aligner and the teeth. As described herein, the soft elastomer in contact with the teeth tends to grip the teeth as side B of sheet 1 provides the necessary scaffolding to support side A. Sheet 2 provides the elasticity to continuously apply force on sheet 1 in order to move the teeth according to the treatment plan. Consequently, the specific teeth that would normally require nubs or engagements for treatment by the aligner can be treated without applying the nubs or engagements because the soft side A of sheet 1 is able to conform around and grip the teeth in order to create the desired movement.

[0191] Sheet 2 can be a homopolymer or co-polymer, and include a hardness and elasticity similar to that of side B of sheet 1 or as described above in connection with the sheets described in FIG. 1-10. Exemplary compositions for sheet 2 are, without limitation, copolyesters such as Eastar 6763, or a TPUs such as Isoplast 2530.

[0192] Optionally, the sheets in the aligner are arranged to have a gap therebetween, andsometimes torus-shaped cross section similar to the Axoplast aligner described above with reference to FIGS. 9-10.

[0193] Hardness

[0194] The hardness of the various sheets can vary.

[0195] In preferred embodiments, the teeth-contacting surface, namely, sheet 1, side A, is extra soft such as a gel shoe insole or rubber band. For embodiments, an exemplary Shore A hardness range to characterize the softness of sheet 1, side A is less than 60, more preferably less than 50, and typically ranges from 10 to 59, preferably from 20 to 50, and in preferred embodiments, from 30 to 59 or 40 to 50. As described herein, surprisingly, the extra soft quality facilitates the tooth-contacting layer (namely, sheet 1, side A) of the aligner to grip onto the teeth without the use of engagers.

[0196] An exemplary range for the hardness of sheet 1, side B, is 20 to 100 Shore D hardness.

[0197] The softness of sheet 2 can be similar to that of the sheet 1, side B, discussed above.

[0198] In embodiments, the hardness of the hard layer (namely, side B of sheet 1 or sheet 2) is at least 2X harder than the soft layer of sheet 1, side A.

[0199] Contact angle

[0200] Inventors have discovered adjusting another property serves to bolster gripping and bonding between the teeth and the teeth-contacting layer of the aligner. In embodiments, the material of the aligner to contact the teeth is selected or controlled for contact angle. By controlling the contact angle, a hydrophobic zone can be formed to exclude saliva from the interfaces between the teeth surfaces and the aligner surface. When this happens a “hydrophobic surface-to-surface adhesion” happens.

[0201] In embodiments, the material surface is controlled to ensure there is a minimum level of hydrophobicity such that a hydrophobic bonding is created between sheet 1 , layer A, and the teeth. In embodiments, the surface of sheet 1, layer A, has a contact angle range greater or equal to 90 degrees, and in some embodiments, greater or equal to 100 degrees. An exemplary material having a contact angle within the above-mentioned range is polyvinylidene fluoride, poly n-butyl methacrylate, poly dimethyl siloxane, polyurethane elastomers, fluorinated polymers.

[0202] Additionally, in some embodiments, the contact angle is modified by applying acoating to the layer A of sheet 1 in order to increase the contact angle to greater than 90. Examples of dip coating include, without limitation: Parylene Coatings such as Aculon, Lotus Effect® Coatings, Cytonix FluoroPei, NeverWet®, and AquaShield. Examples of spray coatings include, without limitation, sprays such as NeverWet by Rust-Oleum, LiquiGlide, Tegaderm by 3M, and SC Johnson Professional® Shout®.

[0203] Water absorption

[0204] Inventors have also discovered adjusting the water absorption can bolster gripping and adhesion between the teeth and the teeth-contacting layer of the aligner. In embodiments, the material of the aligner to contact the teeth is selected or controlled for water absorption. Particularly, in embodiments, the material surface is controlled to ensure there is a minimum level of water absorption such that a hydrophobic adhesion is enhanced between sheet 1, layer A, and the teeth. In embodiments, the material of sheet 1, layer A, has a water absorption of 0.05% or less.

[0205] Forming

[0206] In embodiments, sheet 1 is thermoformed over a 3D printed dentition model (the “dentition”). When thermoforming, the softer side (the A side) of sheet 1 needs to be oriented in way that would be touching the dentition of the 3D printed dental model. Next, sheet 2 is thermoformed over sheet 1. In embodiments, sheet 1 and sheet 2 are connected or adhered together using wide range of methods including, for example, solvent bonding, adhesive, and thermal bonding. Thermal bonds can include, without limitation, laser, heat (e.g., hot tip) and ultrasound (e.g., ultrasonic welding).

[0207] In embodiments, as described above in connection with FIG. 10, sheet 1 and sheet 2 are affixed to one another such that a narrow air gap is present therebetween. The air gap or space (and in embodiments has a torus cross-section) provides the aligner with hyper elastic mechanical properties. This hyper elastic property of embodiments of the invention is reflected in the graph shown in FIG. 25A where the “test” aligner 1020 diverges from the “control” aligner 1010 between zero to the yield point 1012. Indeed, the test aligner 1020 provides continued force all the way up to about 25% strain, well beyond the 5% strain level of the control aligner.

[0208] The dimensions of the sheets may vary. In embodiments, Sheet 1, side A ranges in thickness from 0.002” - 0.010”.

[0209] In embodiments, sheet 1, side B has a thickness in the same range as sheet 1, sideA.

[0210] The thickness of sheet 2 may also vary. In embodiments, the thickness of sheet 2 is selected such that total thicknesses of sheet 1 plus sheet 2 ranges from 0.020” - 0.040”.

[0211] In embodiments, sheet 1 and sheet 2 are laminated together into a composite. Consequently, in embodiments, the thermal forming process over the dentition is a one step process.

[0212] Composition

[0213] The composition of Sheet 1, Side A may vary.

[0214] In embodiments, sheet 1, side A, is a soft, preferably hydrophobic and non-porous material. In embodiments, various materials are screened for the properties listed above including without limitation proper hardness / softness, contact angle, and water absorption.

[0215] Examples of classes of materials are, without limitation, thermoplastic polymers, elastomers, rubbers, and foams. The following are non-limiting exemplary compositions for sheet 1, side A: Styrenic Block Copolymers (TPE-S), Thermoplastic Polyolefins (TPO), Thermoplastic Vulcanisates (TPV), Thermoplastic Polyurethanes (TPU), Thermoplastic Copolycstcrs (TPE-E), Mclt-Proccssablc Rubber (MPR), Thermoplastic Polycthcr Block Amides (TPE-A), Ethylene Vinyl Acetate (EVA), and Polyethylene (PE): Low-density polyethylene (LDPE). Sheet 1, side A, can also be a silicone polymer.

[0216] Examples of commercially available TPUs include without limitation: Isothane TPU; BASF Elastollan® Series: Elastollan® Soft Series (This series includes TPUs with a durometer range starting from as low as 50 Shore A); Lubrizol Estane® Series (Estane® Soft Series: Includes TPUs that can go below 60 Shore A, specifically designed for applications requiring very soft materials); Covestro Desmopan® Series (Desmopan® Soft: Offers a range of soft TPUs, including options with a hardness lower than 60 Shore A); Huntsman Irogran® Series (Irogran® Soft Grades: Includes soft TPU grades with a durometer as low as 55 Shore A); and Mitsubishi Chemical Group (TPUevo™: Offers a range of soft TPUs, including options with a hardness lower than 60 Shore A).

[0217] In embodiments, and without intending to be bound to theory, the inventors state the above compositions, after screening or otherwise adjusting for contact angle, softness, and water absorption, for sheet 1, layer A, tend to cause the teeth and the aligner to stick together by way of hydrophobic interactions, and prevent saliva from building up between the aligner and the teeth.

[0218] The composition for Sheet 1, Side B may also vary. Sheet 1, side B, can be a homopolymer and exemplary compositions are, without limitation, polyurethane (e.g., Lubrizol Isoplast 2530, BASF Elastollan, Covestro Desmopan, Huntsman Irogran); polyethylene terephthalate glycol, namely, PET-G (e.g., Eastman Eastar 6763, Eastman Tritan, SK Chemicals Skygreen, Plaskolite (formerly Bayer MaterialScience) Vivak, LUXA Polymers Luxacomp PETG; Polycarbonate; Polypropylene; and copolyester.

[0219] The composition for Sheet 2 may also vary. Exemplary compositions for sheet 2 include those described above in connection with sheet 1, side B.

[0220] Example 7

[0221] A multi-layer aligner was made including: (a) a first sheet (sheet 1) formed by laminating or coextruding layers A and B where layer A is Eplexflex 770A, a polyether based TPU having an elastomer 70A shore hardness, and layer B was Eastar 6763, PET glycol; and (b) a single layer second sheet (sheet 2) was connected along the edges to sheet 1 such that a space (also referred to herein as a chamber) was formed between sheet 1 and sheet 2, and the sheets were arranged together to create the torus cross section aligner as described herein. Sheet 2 was a PET glycol, Eastar 6763.

[0222] Example 8

[0223] Layers 1 and 2 were laminated or coextruded to create a dual layer aligner, where layer 1 was Eplexflex 770A, a polyether based TPU, and layer 2 was Eastar 6763, a PET glycol. Collectively, the two layers formed a single solid cross section sheet with no space therebetween.

[0224] The dual layer sheet was then thermoformed over a typodont into the desired aligner shape.

[0225] TEST 1

[0226] A test device in accordance with the embodiment of the invention was compared to a conventional device for bonding and the presence of water between the teeth and the aligner.

[0227] Manufacture. With reference to FIGS. 27A-27B, the conventional device or standard industry aligner was made by thermal forming the Zendura FLX (as described above) 810 over a typodont 820.

[0228] With reference to FIGS. 28A-28B, the test device was made by thermoforming a composite dual layer material 830 over a typodont 840. The composite dual layer was madeas described above in Example 8 where the side contacting the dentition was an extra soft layer (having a TPU Shore A 40) and the side away from the dentition was PETG, with a Shore A 95.

[0229] Procedure. A solution of green food dye and tap water was prepared at 50 / 50 concentration. Drops of the die solution were added to the inside of the conventional aligner and test aligner. Each aligner was installed onto its corresponding typodont with an equal amount of pressure. Paper towels were used to wipe off excess die on the exterior surfaces of the aligners and the typodonts.

[0230] Results. With reference to FIGS. 29 A and 29B, photos are shown of the conventional aligner 810 and test aligner 830, respectively, installed on the corresponding typodonts 820, 840 following the green die application. A substantial and evenly distributed amount of green die (indicated by the stippled pattern) is shown through the conventional aligner 810 along the entire frontal region of the typodont 820. In contrast, the test aligner 830 of FIG. 29B appears white and the green die is generally absent from the entire frontal region of the typodont 840. The presence of the green color is indicative of the water present between the typodont 820 and the aligner device 810. The lack of the green color in the test sample is indicative of blocking or prohibiting the liquid from forming for the reasons discussed herein, including the extra soft and hydrophobic-type bonding created by the test device 830’ and the typodont 840.

[0231] In contrast, the conventional device 810 permits, perhaps even wicks, liquid into the frontal region and space between the conventional aligner 810 and the typodont 820 surfaces. Applicant submits this testing is indicative of the augmented non-adhesive type bonding between the aligners and the teeth generated by the aligners made in accordance with embodiments of the present invention.

[0232] TEST 2

[0233] Manufacture. A conventional device or standard industry aligner was made by thermal forming the Zendura FLX (as described above) over a typodont. A test device was made by thermoforming a composite dual layer material over a typodont. The composite dual layer was made as described above in Example 8 where the side contacting the dentition was an extra soft layer (having a TPU Shore A 40) and the side away from the dentition was PETG, with a Shore A 95.

[0234] Various typodonts were prepared. A first typodont was provided with noattachments (not shown). With reference to FIG.30, a second design was prepared by installing a 0.25 mm attachment 922 onto a tooth 924 of a typodont. With reference to FIG.31, a third design was prepared by installing a 1 mm attachment 942 onto a tooth 944 of a typodont.

[0235] Procedure. For each design, 5 replica aligners were tested by installing an aligner onto the typodont and measuring the bonding force (Ib-f) to remove the typodont from the aligner. A photo of the setup is shown in FIG. 32. Dental floss fibers were placed at various strategic interproximal locations around the arch. The fibers were attached to tensile tester. A tensile tester (Instron) was used to pull the aligner off the typodont. The forces required to pull the aligner off the typodont were recorded.

[0236] Results.

[0237] With reference to the table shown in FIG. 33, the mean value of the bonding forces of the test device to the typodont without attachments was 1.4 Ib-f versus 0.6 for the industry standard. This is a 133% difference. The test aligner measured much greater bonding to the typodont.

[0238] With reference again to the table shown in FIG. 33, the mean value of the bonding forces of the industry standard device to the typodont with 1 mm attachments was 1.8 Ib-f. The mean value of the bonding forces of the test device to the typodont with 0.25 mm attachments was 2.0 Ib-f. This is a 10% difference. The test aligner measured 10% more bonding force yet the size of the attachments used in the test device was 75% smaller that the size of the attachments in the industry standard device.

[0239] This testing shows (a) the test design in accordance with embodiments of the invention generates substantially greater adhesive forces between the aligner and the teeth than the industry standard tested in the experiment in the case without use of attachments, and (b) in the case with use of attachments, the attachments needed can be much smaller or less prominent to achieve the equivalent or greater bonding forces. Inventors submit the increase in bonding forces arises due to the extra soft tooth-contacting layer and generating strong hydrophobic non-adhesive bonding forces.

[0240] Example 9

[0241] Fours layers were laminated or coextruded to form a single solid cross-sectional sheet, where layer 1 was Eplexllex 770A, a polyether based TPU, layer 2 was Eastar 6763, a PET glycol, layer 3 was Isoplast 2530 and layer 4 was Eastar 6763.

[0242] While the above examples are intended to be illustrative of various embodiments of the invention, it is to be understood that the various layers can be mixed in a wide range of combinations to achieve the advantages and benefits described herein including, without limitation, intimately non-adhesively bonding the extra soft first layer of sheet 1 to the dentition or teeth.

[0243] ALTERNATIVE EMBODIMENTS

[0244] It is to be understood the invention may vary widely and is intended to include any one feature or combination of features described above except where the invention is explicitly limited in any appended claims.

[0245] For example, the roughening, trimming, and dipping may be performed in any order or sequence or combination except where such steps are exclusive to one another.

[0246] Additionally, in embodiments, with or without the mechanical interlock features described above, the trimming or cutting step is performed with a hot tip or laser tool.

[0247] Inventors have discovered the cutting with a hot tip or laser tool also has the capability of sealing. In embodiments, the edges of the stacked shells are trimmed and sealed simultaneously using the hot tip or laser tool. When a hot tip or laser tool is used to trim and seal the edges, polishing is optional or can be omitted. In a sense, the hot tip or laser trimming performs two, and optionally three steps in one. That is, trimming, sealing, and polishing are collectively performed by the hot tip or laser tool. An example of a suitable laser tool to trim, seal and polish the shells is the LAC - Laser Aligner Cutter, manufactured by Dental Axes LLC (West Palm Beach, Florida).

[0248] Additionally, in alternative embodiments, two sheets can be stacked together and thermoformed simultaneously using a mold versus a serial type of thermoforming process. Optionally, one or more vents are added to one or more of the sheets prior to thermoforming. The vents allow air to escape during the thermoforming step, creating a more continuous edge seal. In embodiments, the vents are arranged near the edge of the sheet. In embodiments, the vents are arranged 5-30 mm, and more preferably 10-20 mm from the perimeter of the sheet. The vent diameter may also vary. In embodiments, the diameter of vent ranges from 1-10 mm, more preferably 3-6 mm.

[0249] Additionally, in embodiments, the chamber spacing or space comprises a thin layer or liquid, gel, oil, or release agent.

[0250] In embodiments, the chamber spacing or height is controlled by application of aliquid or gel such as, for example, water. A method includes adjusting how much water is present in order to control the quantity of water vapor generated to control the chamber distance. For example, a thin layer of water may be applied between the shells, and when two thermoforming steps are desired, just prior to the second thermoforming step to keep the shells from sticking during the second thermoforming step. Preferably, the application of these agents is on top of the first shell.

[0251] Additionally, in embodiments, a method of assembly includes the application of a thin layer silicone, PTFE, a mold release agent, or dried non-ionic detergent. Additional categories of agents to prevent stick and control the chamber height distance include high boiling compounds such as mineral oil, silicone oil and natural oils such as com, olive, and canola oil.

[0252] Additionally, in embodiments, the chamber or spacing between the shells varies, and in some embodiments, the inner surfaces of the shells touch one another at one or more locations throughout the chamber. At these so-called ‘contact points’ in the chamber, there is no separation distance between the shells. In such embodiments of the invention, the shells do not stick to one another in these regions. Thus, in embodiments, whether (or not) there is a space between the shells, the opposing shells are able to move and flex relative to one another in response to the forces imparted onto the shells arising from errant or off-track teeth and the inherent flex / stiffness programmed into the shells, described above.

[0253] Additionally, in embodiments, the space or chamber between the shells carries an active agent or ingredient. In embodiments, the space between the shells carries an ingredient formulation for controlled release of the active ingredient. For example, a thick gel may contain an active ingredient that is slowly released over time. Examples of orthodontic appliances for delivery of an active agent are described in copending provisional patent application no. 63 / 287,023, filed December 7, 2021, entitled “ORTHODONTIC DEVICES FOR DELIVERY OF AN ACTIVE AGENT” incorporated herein by reference for all purposes.

[0254] Additionally, and although reference was made throughout the disclosure to a lower shell to contact the teeth corresponding generally to an orthodontic appliance for treating the teeth in the lower jaw, the invention is equally applicable and relevant to treating the teeth in the upper jaw in which case the upper-most shell would be the shell to contact the teeth, and the lower shell would be the shell exposed to the mouth. Indeed, embodiments ofthe invention are intended to cover orthodontic appliances to move and treat the teeth in the upper or lower jaws as the case may be.

[0255] Throughout the foregoing description, and for the purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of the described techniques. It will be apparent, however, to one skilled in the art that these techniques can be practiced without some of these specific details. Although various embodiments that incorporate these teachings have been shown and described in detail, those skilled in the art could readily devise many other varied embodiments or mechanisms to incorporate these techniques. Also, embodiments can include various operations as set forth above, fewer operations, or more operations; or operations in an order. Accordingly, the scope and spirit of the invention should be judged in terms of the claims, which follow as well as the legal equivalents thereof.

Claims

CLAIMS1. An orthodontic appliance for repositioning a patient's teeth, the appliance having a plurality of teeth-receiving cavities shaped to apply a repositioning force to the patient's teeth according to a treatment plan when installed on the patient’s teeth, the appliance comprising: a first sheet to contact the teeth, wherein the first sheet comprises a first layer formed of an extra soft thermoplastic elastomer; and a second sheet arranged over the first sheet, wherein the second sheet does not contact the teeth, and wherein the second sheet comprises a hard thermoplastic operable to continuously apply the repositioning force to move the patient’s teeth according to the treatment plan, and wherein the first layer of the first sheet is characterized by a first hardness less than Shore A 60.

2. The appliance of claim 1, wherein the first layer of the first sheet has a contact angle greater than 90 degrees, wherein the contact angle and first hardness of the first layer of the first sheet collectively operate to hydrophobically bond the first layer of the first sheet to the patient’s teeth such that (a) saliva from the patient is excluded or prohibited from an interface between the teeth and the appliance after the appliance is installed on the patient’ s teeth, and (b) attachments are not required in order to grip and reposition the patient’s teeth according to the treatment plan, or if attachments are required, the number or size of the attachments is reduced.

3. The appliance of claim 1, wherein the first layer of the first sheet has a thickness in a range from 0.002 - 0.015 in.

4. The appliance of claim 1, wherein the first layer of the first sheet comprises a first hardness of less than 40A, and optionally, a contact angle greater than 100 degrees.

5. The appliance of claim 1, wherein the first layer of the first sheet has a first water absorption of less than or equal to 0.05%.

6. The appliance of claim 1, wherein the first layer of the first sheet comprises thermoplastic polyurethane, a thermoplastic elastomer, or a block copolymer elastomer.

7. The appliance of claim 1, wherein the first layer of the first sheet comprises a foam, optionally a polyurethane foam.

8. The appliance of claim 1, wherein the second sheet comprises a co-polyester, a thermoplastic polyurethane, a polypropylene and polyethylene copolymer, a cyclic block copolymer, a polyethylene terephthalate, a polybutylene terephthalate, a polyethersulfone, a polytrimethylene terephthalate, a homopolymer, Silicone rubber, ethylene vinyl acetate, styrene block copolymer, polyolefin elastomers, polyvinyl chloride, or a combination thereof.

9. The appliance of claim 1, wherein the hydrophobic bonding is adhesive free.

10. The appliance of claim 1, wherein the second sheet has a second hardness at least 2 times (2x) greater than the first hardness.

11. The appliance of claim 10, wherein the first sheet comprises a second scaffolding layer that does not contact the teeth and provides support to the extra soft first layer.

12. The appliance of claim 11, wherein the second scaffolding layer of the first sheet has a third hardness equal to or greater than the second hardness.

13. The appliance of claim 1, further comprising a first chamber defined between a top first surface of the first sheet and a bottom second surface of the second sheet, and a first perimeter seal formed between a portion of the top first surface of the first sheet and the bottom second surface of the second sheet.

14. The orthodontic appliance as recited in claim 13, wherein the first perimeter seal is a formed by a sealing technique selected from the group comprising: heat; solvent; and adhesive.

15. The orthodontic appliance as recited in claim 13, wherein the appliance is characterized by a cross sectional shape of a torus, annulus, ring, or donut.

16. The orthodontic appliance as recited in claim 1, made by forming a stack of sheets, and arranging the stack over a dental model, and thermoforming the stack over the dental model.

17. The orthodontic appliance as recited in claim 16, wherein the stack comprises a sacrificial layer between a first sheet and the second sheet.

18. The orthodontic appliance as recited in claim 17, wherein the sacrificial layer is a liquid, and optionally water.

19. The orthodontic appliance of claim 1, wherein the first sheet solely includes the first layer formed of the extra soft thermoplastic elastomer.

20. An orthodontic appliance for repositioning a patient's teeth, the appliance having a plurality of teeth-receiving cavities shaped to apply a repositioning force to the patient's teeth according to a treatment plan when installed on the patient’s teeth, the appliance comprising: a first sheet to contact the teeth, wherein the first sheet comprises a first layer formed of an extra soft thermoplastic elastomer; and a second sheet arranged over the first sheet, wherein the second sheet does not contact the teeth, and wherein the second sheet comprises a hard thermoplastic operable to continuously apply the repositioning force to move the patient’s teeth according to the treatment plan, and wherein the first layer of the first sheet has a first hardness value and the second sheet has a second hardness value such that the appliance generates a non-adhesive gripping force greater or equal to 1 Ib-f without use of any tooth attachments, and in some embodiments, wherein the appliance generates a non-adhesive gripping force greater or equal to 1.4 Ib-f.

21. An orthodontic appliance for repositioning a patient's teeth, the appliance having a plurality of teeth-receiving cavities shaped to apply a repositioning force to the patient's teethaccording to a treatment plan when installed on the patient’s teeth, the appliance comprising: a first sheet to contact the teeth, wherein the first sheet comprises a first layer formed of an extra soft thermoplastic elastomer; and a second sheet arranged over the first sheet, wherein the second sheet does not contact the teeth, and wherein the second sheet comprises a hard thermoplastic operable to continuously apply the repositioning force to move the patient’ s teeth according to the treatment plan, and wherein the first layer of the first sheet has a first hardness value and the second sheet has a second hardness value such that the appliance generates a non-adhesive gripping force greater or equal to 2 Ib-f with use of attachments as small as 0.25 mm prominence.

22. A method of making an orthodontic appliance comprising: providing a hydrophobic extra soft or rubbery thermoplastic polyurethane characterized by a contact angle greater than 90 and a shore A hardness equal to or less than 60; providing a second layer having a hardness at least two times greater than the hardness of the first layer; forming stacked layers by stacking a second layer on top of the first layer; thermoforming the stacked layers over a dental model to simultaneously form a shaped first shell and a shaped second shell bonded to the first shell along a first perimeter.

23. The method of claim 22, further comprising minimizing or prohibiting the first shell from bonding to the second shell within the perimeter.

24. The method of claim 23, wherein the prohibiting is performed by applying a sacrificial layer between the first layer and second layer prior to the step of thermoforming.

25. The method of claim 24, wherein the sacrificial layer is a liquid, and optionally water.

26. The method of claim 22, further comprising creating a plurality of holes in at least one of the first layer, and the second layer prior to the thermoforming step.

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