Sensor element for detecting stretching during the movement of a body part of a living being

Description

[0001] The invention relates to a sensor element according to claim 1, which serves to detect strains during the movement of a body part of a living being, i.e. for biomechanical applications.

[0002] Measuring the expansion and contraction of the body surface of living beings, especially humans and other mammals, is a current challenge. This stretching allows conclusions to be drawn about limb movement, joint condition, muscle activity, and the stretching of ligaments and fascia. For example, it is conceivable to record movement patterns in different body parts or to monitor respiratory activity by tracking chest expansion. Technologies for measuring this body element or skin stretching offer a range of interesting applications with high commercial potential. Examples include: Wearables, control of games (apps), feedback for therapy (start / progress / end), human-machine interaction / HRC, fitness monitor, control of the technical execution of sports exercises, research / developments on the musculoskeletal system, recording of respiration in everyday life, evaluation of therapy success in the treatment of scars through elasticity measurements, sensor patches, sensor insoles, sensor gloves, life science products.

[0003] Optical measuring systems or applicable strain sensors are generally suitable for measuring strain on the human body. The latter category includes, in particular, resistive and capacitive sensors. Their advantages lie in the use of simple evaluation electronics with high flexibility and elasticity. Within these two sensor classes, there are numerous technological solutions with varying properties. Acceleration and position sensors are also used for recording body movements.

[0004] Optical measurement systems are primarily designed for stationary use. They also require the direct visibility of markers applied to the skin. Therefore, everyday situations or mobile measurements are not feasible with optical measurement systems.

[0005] Using accelerometers requires regular calibration, which necessitates stopping the movement. These interruptions disrupt natural movement patterns and make continuous motion tracking impossible.

[0006] Simple strain gauges and stick sensors, as resistive sensors based on constantan or Pt / W wires, are only suitable for small strains. A disadvantage of these sensors is their low yield strength. Due to their low elastic elongation, they cannot be used for repeated strains with amplitudes greater than 0.5%. After just a few cycles, these sensors fail at large amplitudes. Therefore, strain gauges are only suitable for use on skin in a few specific locations.

[0007] Highly elastic resistive strain sensors based on nanomaterials typically consist of a thin film formed with nanoscale structures, combined with a flexible substrate material. Changes in the microstructure lead to a change in electrical resistance upon strain. Several internal phenomena often superimpose to cause this change in electrical resistance. Key factors include changes in geometry, crack propagation within the thin films, changes in electrical contact resistances between microelements, and tunneling effects. Such resistive sensors generally exhibit comparatively higher extensibility and sensitivity, but they display nonlinear behavior and hysteresis.

[0008] Capacitive sensors consist of a highly compliant dielectric layer sandwiched between two stretchable electrodes. Changes in the distance between the two electrodes due to strain cause a change in the electrical capacitance, which is a measurable quantity. This class of elastic strain sensors exhibits excellent stretchability, linearity, and hysteresis properties. However, their sensitivity is very low.

[0009] CNT, carbon black particle, graphene, silver nanowire, and nanoparticle networks are used as conductive micro- or nanomaterials for elastic strain sensors. Silicone-based elastomers, rubber, and thermoplastic elastomers are predominantly used as dielectric substrates. Carbon-based networks, and these nano- and microparticle networks in general, exhibit quite high temperature sensitivity.

[0010] A disadvantage of the approaches mentioned is the frequently occurring strong coupling of the sensor signal under strain and applied pressure. Pressure is introduced into the sensor, for example, by tight or stretched clothing or by external contact. Due to this coupling, the effects of strain and pressure are indistinguishable. Furthermore, solutions for the assembly and interconnection technology of these sensors are currently inadequate. No compact sensor system comprising a sensor, energy storage device, and electronics for data processing and communication has yet been implemented that is robust and reliable enough for long-term monitoring. A mature and marketable technical solution should therefore address these aspects.

[0011] Copper-based conductors have a low yield strength, harden through deformation, and break after a certain number of reciprocal deformations.

[0012] WO 03 / 012384 A2 discloses a method and a device for measuring loads with shape memory alloys.

[0013] WO 2017 / 020111 A1 concerns clothing with pull sensors.

[0014] A system for evaluating and displaying the movement of an athlete is described in US 2012 / 0316798 A1.

[0015] It is therefore an object of the invention to provide possibilities for the permanent detection of strains that occur during movement of body parts of a living being, taking into account relatively large strains of more than 5%, preferably more than 8% with sufficient measurement accuracy, and influencing the respective living being only minimally, if at all, during the movement sequences to be observed.

[0016] According to the invention, this problem is solved with a sensor element having the features of claim 1. Advantageous embodiments and further developments of the invention can be realized with features specified in dependent claims.

[0017] The sensor element according to the invention can be attached to the respective body part. The sensor element comprises at least one wire- or strip-shaped electrical conductor, which can be connected to an electrical voltage source and to a device designed to determine the electrical current, voltage, and / or resistance flowing through the conductor. The electrical conductor is made of a shape-memory metal, in particular a shape-memory alloy, which will be referred to below as FMI. The electrical conductor is connected to an elastically deformable support element.

[0018] The electrical conductor can be provided with an electrically insulating coating on its surface, allowing it to adhere to the skin of the respective body part. This enables the at least one electrical conductor to be applied to the skin surface of a given body part and held in place by the adhesive effect. The coating prevents interference from moisture on the skin surface. The coating material can be selected to allow for manual removal and, if necessary, reuse of the sensor element. The adhesive effect can be achieved through the adhesive properties of the coating material and / or through a suitable surface texture of the coating, for example, using suction cups or a gecko-like structure.

[0019] The at least one electrical conductor can be implanted under the skin. Electrical contact elements present at the ends of the conductor, or connectable there, should extend at least to the skin surface. Inductive transmission of electrical energy and strain measurements taken by the sensor element is also possible. For this purpose, a suitable antenna or dipole can also be implanted, and an additional external antenna can be used for energy and data transmission.

[0020] The electrical conductor can be bent into a sinusoidal, meandering, zigzag, semicircular pattern, or a combination of these shapes and attached to the respective body part. The curved shape can be achieved through thermal treatment with targeted deformation. Alternatively, it can be cut from, for example, a sheet in the desired curved shape.

[0021] A pseudoelastic flexible metal (FGM) can function permanently as a conductor. Furthermore, it offers the possibility of an additional ordering function. A shape can be imprinted into the FGM through heat treatment. When the pressure is released, the pseudoelastic material always tends to return to this shape. An electrical conductor could thus curl itself up.

[0022] The electrical conductor should advantageously be bent so that its two ends are spaced apart and directly adjacent to each other for connection to electrical contact elements. This allows for simple and straightforward electrical contact with the electrical voltage source and the device designed to determine the electric current, voltage, and / or resistance flowing through the conductor.

[0023] The support element can be made of an elastically deformable material that is either bonded to the electrical conductor or in which the electrical conductor is at least partially embedded. Alternatively, it can be made of a textile structure to which the electrical conductor is, for example, positively connected. The electrical conductor can be woven into the textile structure or attached by means of a stitched connection. A textile structure made of elastically deformable fibers is advantageous. The elasticity of these fibers can be influenced by the fiber material, the type of fiber, the number of fibers, and the strength of the textile composite.

[0024] An elastically deformable material used to form a support element can, when applying the invention, additionally exhibit anisotropic mechanical properties. For example, an elastically deformable material can undergo longitudinal elongation with minimal transverse contraction. Longitudinal elongation can be a preferred measuring direction for a sensor element.

[0025] The carrier element can advantageously be designed as a strap or a ring, allowing it to completely encircle a body part when worn on a living being. This enables a sensor element to be easily attached to a body part by simply slipping it on and used to detect stretching. This is easily possible, for example, on the forehead, chest, arms, or legs. The length can be individually adjusted to the specific body part using a suitable strap closure or ring diameter / circumference. Alternatively, a carrier element can also be designed as a garment, such as a stocking or glove.

[0026] The support element is designed to limit the maximum elongation of the electrical conductor. This prevents damage to the conductor in case of excessive elongation. For this purpose, the support element incorporates reinforcing elements that limit the maximum possible displacement in the respective axial direction during movement of the body part. These reinforcing elements are subjected to tensile force and prevent further elongation once the maximum permissible displacement of the electrical conductor has been reached. The reinforcing elements thus act as a type of anchor. They can be attached to, or embedded within, an elastically deformable polymer support element. In the case of a support element formed with a textile structure, corresponding fibers or threads can be woven into or attached to the textile structure.Such fibers or threads should have a greater strength than those used in the manufacture of the textile structure. They can, for example, be metal fibers, but the electrical insulation from the electrical conductor should be taken into account.

[0027] An elastic support element can have form-fitting and / or material-locking connecting elements that can be reversibly applied to and removed from the surface of prefabricated textile structures.

[0028] The electrical conductor can advantageously be attached to a body part in such a way that its maximum measurement sensitivity is aligned in an axial direction that deviates by at least 10° and at most 80° from the direction of strain of the respective body part to be determined during a movement being monitored. This allows for an amplification effect by taking into account greater strains than the electrical conductor would actually permit, since it does not need to be stretched to its maximum extent as required by the actual strain amplitude. This prevents damage to the electrical conductor and increases its service life. Strains greater than 10% can also be taken into account without problems. The angle should preferably deviate between 10° and 30° from the actual direction of strain occurring during the movement being monitored to ensure good measurement sensitivity and the electrical conductor's resistance to breakage.

[0029] It is possible to adapt the sensor element to different body parts with varying maximum stretch. The elastic support structure can be self-adhesive for direct fixation to the skin or designed as a garment or wrap-around shape. Once pulled over body parts (torso, arms, legs, feet, hands), the wrap-around shape can stay in place due to its inherent elasticity.

[0030] An offset or mirrored arrangement of the patterns of a correspondingly bent electrical conductor can be used to guide the elongated FGL material back and forth and to create two closely adjacent contact points.

[0031] It is possible to fix and locally stiffen the elongated FGL material at one or more reversal points along the pattern where an electrical conductor changes direction.

[0032] Cross-stiffening can also be used on the elastic support element to reduce lateral contraction or to utilize an increased width of the skin-adhesive surface of this support structure running on both sides of the sensor structure.

[0033] In particular, temperature compensation can be achieved by forming and electrically connecting several electrical conductors on a sensor element as a half / full bridge, whereby the electrical conductors should be aligned at different orientation angles to each other.

[0034] A sensor element can also have multiple electrical conductors that can monitor different movements independently of each other.

[0035] A textile structure as a carrier element can also be designed with reversibly adhesive material on the surface (hook and loop fastener, mushroom fastener, permanent adhesive tape, etc.) for repeated adhesion to the skin or to textile structures (bandages, orthoses, clothing, dressings, hook and loop pads, etc.).

[0036] A sensor element can also be designed as a self-stabilizing sensor, so that no sensor drift can occur during the measurement process. For this purpose, the FGL material used undergoes a suitable thermo-mechanical pretreatment or training. As a result, the phase transition occurs at a lower force and the occurrence of plastic deformation is minimized.

[0037] By using a sensor element according to the invention to detect skin stretching, the expansion of the rib cage as a result of breathing, and the movement of joints, the skin stretching can be calibrated to defined joint movements.

[0038] In particular, the device designed to determine the electric current, voltage, and / or resistance flowing through the conductor, and preferably also the voltage source, should be designed as a portable miniature electronic unit for signal acquisition and wireless signal transmission. It should be reusable. It can be attached to the sensor carrier structure near the sensor by gluing or attaching with hook-and-loop fasteners and should have such a low weight and small form factor, while remaining mechanically stable or flexible, that natural movements are possible without hindrance and the electronics are not damaged.

[0039] The electrical assembly and connection technology should ensure a simple and reliable connection between the electrical conductor and the electronics as well as the electrical voltage source. This can be achieved, for example, by plugging in flat connectors or by magnetically fixing contact connections (sensor-side) to contact pins (electronics-side), whereby the sensor-side connector parts or contact pads should be material-bonded to the elongated FGL material that forms the at least one electrical conductor.

[0040] The electrical insulation of the surface of an electrical conductor, the joining connections and all conductive components of the sensor-side plug-in parts or contact connections not required for the detachable contact can be achieved by a suitable coating.

[0041] For example, a kinesiology tape can be used as an elastic carrier material with a 25 µm thick sensor wire laid in a zigzag pattern and fixed with sewing threads to form a sensor element. The kinesiology tape forms the carrier element and the sensor wire forms the electrical conductor.

[0042] A silicone film can also be used as an elastic carrier material for a carrier element with a 25 µm thick sensor wire as an electrical conductor, which is bent in a sinusoidal pattern and fixed in / on the silicone by means of gluing or material bonding.

[0043] A skin-tight elastic top garment with a sensor wire, as an electrical conductor, oriented transversely to the body's longitudinal axis along the chest or repeatedly oriented at a translation angle transversely to the body's longitudinal axis, can also form a carrier element of a sensor element, making it possible to detect the breathing of a living being.

[0044] A textile sensor sock with an attached and textile-technically fixed sensor wire as an electrical conductor, laid in a zigzag pattern, can also form a sensor element with which, in particular, the deformation of the foot during movement can be detected.

[0045] Sensor strips made of textile carrier material with a zigzag pattern of the bent electrical conductor can be used with a Velcro surface on a sensor element with this carrier material, which can be attached to the defined sensor position of a bandage, an orthosis or to Velcro adhesive pads that have previously been applied to the skin.

[0046] The novel sensor structure and the resulting biomechanical applications essentially enable strain measurements with high strain (>10%) and cycle counts. Strains of the body surface or muscles can thus be recorded repeatedly, reliably, and over extended periods. Strain measurement is also possible with simple means and at low cost. The sensor element can be manufactured in a small size (about the size of a matchbox or smaller).

[0047] The special advantages are: Detection of strains >10%, adjustability of the maximum achievable strain through variable transmission, high sensitivity compared to conventional resistive sensor materials (e.g. constantan), low hysteresis, high linearity, low pressure sensitivity, long-term stability of the measured values.

[0048] The aforementioned effects and advantages reduce the effort and costs of strain measurements compared to the state of the art.

[0049] Standard and well-known FGLs for electrical conductors can be used.

[0050] Pseudoelastic FGLs exhibit a high elastic elongation of up to 15%. This high elongation is made possible by a reversible phase transformation of the material.

[0051] The described phase transformation is also associated with a significant change in electrical resistance. This can be used to measure strain. FGLs can therefore be used as sensors for monitoring strain. Large strains and derived quantities (e.g., forces) can be detected. FGL sensors can be applied to surfaces or integrated into the interior of a structure.

Claims

1. A sensor element for detecting stretching during the movement of a body part of a living being, which can be attached to the respective body part, wherein the sensor element is formed with at least one wire or strip-shaped electrical conductor which can be connected to an electrical voltage source and to a device which is designed to determine the electrical current flowing through the conductor, the electrical voltage and / or the electrical resistance, and the electrical conductor is formed from a shape-memory metal and can be attached to the surface of the respective body part with an elastically deformable carrier element, characterized in that the carrier element is designed as an element limiting the maximum stretch of the electrical conductor, wherein reinforcing elements are disposed on the carrier element which limit the maximum possible elongation occurring in the respective axial direction of movement of a body part.

2. The sensor element according to claim 1, characterized in that the electrical conductor can be attached to the respective body part in a sinusoidal, meandering, zigzag or semicircular shape or in a combination of these shapes.

3. The sensor element according to one of the preceding claims, characterized in that the electrical conductor is bent in such a way that its two front ends are arranged at a distance from and directly adjacent to one another for connection to electrical contact elements.

4. The sensor element according to one of the preceding claims, characterized in that the carrier element is designed with an elastically deformable material that is materially bonded to the electrical conductor or in which the electrical conductor is at least partially embedded, or the carrier element is designed with a textile structure to which the electrical conductor is positively connected.

5. The sensor element according to one of the preceding claims, characterized in that the carrier element is designed in the form of a belt, a ring or an article of clothing, so that it fully encloses a part of the body when worn on a living being.

6. The sensor element according to one of the preceding claims, characterized in that the elastic carrier element has form-fitting and / or material-fitting connecting elements and can be reversibly applied to and detached from the surface of ready-made textile fabrics.