Patent Application
20240011199
The present disclosure relates to shape memory alloy microfilament yarns.
Textiles that incorporate multifunctional materials may provide enhanced properties to devices, articles, or systems formed from the textiles. These enhanced textiles may be used in a variety of applications, including medical devices, wearable electronics, energy absorption applications, and thermal insulation. As one example, textiles may incorporate thermoelectric, piezoelectric, or triboelectric materials to provide energy harvesting functionality for wearable electronics, such as sensors, attached to the textiles. For textiles that incorporate relatively simple, solid material structures, such as wires and rods, properties of the materials, such as flexural rigidity, may be limited to bulk properties of the material alone or as incorporated into the textile.
The present disclosure describes coiled shape memory yarns that provide enhanced and tailorable properties to textiles, actuators, and other materials and devices that incorporate the coiled shape memory yarns.
Shape memory yarns described herein include twisted microfilaments made from a shape memory alloy, such as nitinol, that may provide superelastic or shape memory properties. Mechanical properties of the shape memory yarns, such as structural stiffness, plateau strength, and cyclical consistency, can be customized by configuring various structural parameters of the shape memory yarns, such as an amount of twist, a density, and/or a packing efficiency. The shape memory yarns are formed into coils that provide a high degree of actuation or elasticity along an axis of the coiled shape memory yarn, and may have relatively low porosity, low rigidity, and/or low change of volume compared to shape memory coils formed from solid structures. Coiled shape memory yarns may provide further tailorability of a superelastic or shape memory response of a system or device incorporating the coiled shape memory yarns through various coil parameters, such as coil pitch or density, or torque balancing, such as heat treating or plying the coiled shape memory yarns.
The coiled shape memory yarns can be incorporated into textiles and actuators to provide superelastic functionality, such as energy absorption and/or passive strain recovery, or shape memory functionality, such as active strain recovery and actuation, in a variety of applications. As one example, an actuator that incorporates coiled shape memory yarns may provide a relatively high actuation force or displacement while maintaining relatively small volumetric changes and being responsive to relatively low actuation temperatures. As another example, a textile that incorporates coiled shape memory yarns may provide high strain recovery with low structural rigidity and porosity.
In one example, a shape memory coil includes a coiled shape memory yarn having a coil direction around a coil axis. The coiled shape memory yarn includes a plurality of microfilaments having a twist direction around a yarn axis. The plurality of microfilaments includes a shape memory alloy.
In one example, a method for manufacturing a shape memory coil includes coiling a shape memory yarn to form a coiled shape memory yarn that has a coil direction around a coil axis. The coiled shape memory yarn includes a plurality of microfilaments having a twist direction around a yarn axis. The plurality of microfilaments includes a shape memory alloy.
In one example, a textile includes a plurality of interlocked tows. At least a portion of the plurality of interlocked tows includes a plurality of shape memory yarn structures. Each shape memory yarn structure includes one or more shape memory yarns. Each shape memory yarn includes a plurality of microfilaments having a twist direction around a yarn axis. The plurality of microfilaments includes a shape memory alloy.
In one example, a device includes one or more shape memory yarn structures and a current source coupled to the one or more shape memory yarn structures. Each shape memory yarn structure includes one or more shape memory yarns. Each shape memory yarn includes a plurality of microfilaments having a twist direction around a yarn axis. The plurality of microfilaments includes a shape memory alloy. The shape memory alloy is configured to undergo a phase transformation in response to heating above a transformation temperature. The current source is configured to send an actuation signal to the one or more shape memory yarn structures to heat the one or more shape memory yarn structures above the transformation temperature of the shape memory alloy.
The details of one or more examples of the disclosure are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the disclosure will be apparent from the description and drawings, and from the claims.
Systems and devices described herein utilize shape memory yarns to provide enhanced and tailorable properties to articles, devices, or systems that incorporate the shape memory yarns, such as textiles or actuators. Shape memory yarns may be used in a variety of technical fields including, but not limited to: medical devices, such as sutures, grafts, stents, tissue engineering scaffolds, grippers, body navigation aids, drug delivery aids, and compression garments; clothing, such as athletic garments, shoes, insoles, and sports equipment; household devices, such as filters; defense applications, such as impact resistant or energy absorbing wearables; aerospace applications, such as active skin topologies; and the like.
In some examples, shape memory yarns described herein may be incorporated into passive or active energy absorption textiles.
Textile 12 includes a plurality of interlocked structures, such as woven tows, knitted loops, or other interlocked structures. At least a portion of the plurality of interlocked structures include a plurality of shape memory yarn structures 14A and 14B (collectively, “shape memory yarn structures 14”). For example, the plurality of shape memory yarns structures 14 may be mixed with other types of fibers to provide a desired elastic or shape memory effect combined with other textile properties. Each shape memory yarn structure 14 includes one or more shape memory yarns. For example, shape memory yarn structure may include a single yarn, or may include a multiple ply yarn. Each shape memory yarn includes a plurality of microfilaments that include a shape memory alloy. The shape memory alloy may be configured to provide enhanced elasticity and/or actuation in one or more directions.
In some instances, textile 12 may be configured to provide different properties along different axes of textile 12. For example, the plurality of shape memory yarn structures 14 includes a first plurality of shape memory yarn structures 14A oriented in a first direction and a second plurality of shape memory yarn structures 14B oriented in a second direction, different from the first direction. The first and second pluralities of shape memory yarn structures 14A and 14B may be configured with different structural parameters and/or at different densities (e.g., proportions with other, non SMA fibers), such that textile 12 may have different properties in the first and second directions. As one example, first plurality of shape memory yarn structures 14A may have a first elasticity and second plurality of shape memory yarn structures 14B may have a second elasticity, different from the first elasticity, such as for textiles that resist force from a particular direction. As another example, first plurality of shape memory yarn structures 14A may be configured to produce a first actuation force in response to a transformation temperature transition and second plurality of shape memory yarn structures 14B may be configured to produce a second actuation force in response to a transformation temperature transition, different from the first actuation force, such as for textiles that provide force primarily to a particular direction.
In some instances, shape memory yarn structures 14 may incorporate coiled shape memory yarns. As will be described further below, textile 12 that incorporates coiled shape memory yarns may provide high strain recovery with low structural rigidity and porosity. For example, coiled shape memory yarns may be relatively close packed, such that textiles formed from coiled shape memory yarns may have a relatively low porosity. As another example, coiled shape memory yarns may be relatively flexible compared to solid shape memory alloy rods or coils, such that textiles formed from coiled shape memory yarns may have high drapability and conformability to surfaces, such as body parts.
Various structural parameters of textile 12 may provide enhanced properties of textile 12 over individual yarn structures 14. In some examples, textile 12 may have a higher structural strain compared to equivalent shape memory yarn structures 14, as illustrated in
Various structural parameters of textile 12 may be configured to provide particular bulk properties of textile 12. For example, tows, loops, or other shapes of yarn structures 14 may be configured in a manner that enables reconfiguration of load-bearing yarn structures 14 and load sharing of yarn structures 14, such that a force profile (e.g., a generated force over a structural strain) may be tailored for a particular textile 12. In some examples, textile 12 may be configured to have a particular force profile. For example, textile 12 having woven tows may be configured to generate large forces at relatively low structural strains. As another example, textile 12 having knitted loops may be configured to generate large forces at relatively high structural strains, such as due to reorientation of yarn structures to mitigate the strains and stresses applied directly to yarn structures 14. In some examples, textile 12 may include portions having different force profiles, such as a first portion that includes a first textile pattern configured to generate a force at a relatively low structural strain and a second textile pattern configured to generate a force at a relatively high structural strain.
In some examples, shape memory yarns described herein may be incorporated into actuators.
Device 20 includes a current source 24 coupled to the one or more shape memory yarn structures 28. Current source 24 is configured to send an actuation signal to the one or more shape memory yarn structures 28 to heat the one or more shape memory yarn structures 28 above a transformation temperature of the shape memory alloy. In response to being heated above the transformation temperature, the one or more shape memory yarn structures 28 may transform from a first state to a second state, with a corresponding change in dimensions, to create the actuation force. In response to cooling below a transformation temperature, the one or more shape memory yarn structures 28 may transform from the second state back to the first state.
In some instances, shape memory yarn structures 28 may include coiled shape memory yarns. As will be described further below, actuator 22 that incorporates coiled shape memory yarns may provide a relatively high actuation force and/or displacement while maintaining relatively small volumetric changes and being responsive to relatively low actuation temperatures. As such, volume-constrained and temperature-constrained fields, such as artificial muscles, may incorporate coiled shape memory yarns to provide actuation that is compatible with human tissues.
Shape memory coil 40 may exhibit shape memory behavior, such that shape memory coil 40 may be configured to transform between a martensitic state and an austenitic state in response to a change in temperature. For example, shape memory coil 40, shape memory yarn 42, and/or microfilaments of shape memory yarn 42 may be loaded and heat treated at a temperature at or below a martensitic transformation temperature of the shape memory alloy. As a result of this heat treatment, shape memory coil 40 may be configured to transform from the martensitic phase to the austenitic phase when heated above an austenite finish temperature. The change in lattice structure from the phase transformation enables recovery of plastic and elastic strains to a memorized austenitic shape, as set by the heat treatment. This transformation may create an actuation force along a coil axis 56.
Shape memory coil 40 may exhibit superelastic behavior, such that shape memory coil 40 be configured to transform between a martensitic state and an austenitic state in response to mechanical stress. For example, any of shape memory coil 40, a device or textile formed from shape memory coils 40, shape memory yarn 42, and/or microfilaments of shape memory yarn 42 may be loaded and heat treated at a temperature at or above an austenitic transformation temperature of the shape memory alloy. As a result of this heat treatment, shape memory coil 40 may be configured to transform from the austenitic phase to a stress-induced martensitic phase upon loading, and hysterically recover back to the austenitic phase upon unloading. This transformation may enable shape memory coil 40 to have a recoverable form upon unloading.
Coiled portion 44 of shape memory yarn 42 defines coil axis 56, along which shape memory coil 40 may expand or contract. While shown as being straight, coil axis 56 can have a variety of shapes, such as a curved or multi-directional shape. Coiled portion 44 of shape memory yarn 42 also defines a coil direction 58 around coil axis 56. Coil direction 58 may represent a rotational direction of coiled shape memory yarn 42 around coil axis 56. For example, in
Each coil in coiled portion 44 may be separated by a coil spacing 54 that defines a coil pitch. In examples in which shape memory coil 40 is tightly packed, as shown in
Shape memory yarn 42 has a yarn axis 50 that follows a center of shape memory yarn 42 through shape memory coil 40. Within coiled portion 44, yarn axis 50 forms an acute coil angle 51 with coil axis 56. Shape memory yarn 42 has a diameter 48 that represents a radial length of shape memory yarn 42 perpendicular to yarn axis 50. While shown from a side view as being straight, yarn axis 50 may be curved along coil portion 44 and straight along straight portion 46.
The plurality of microfilaments 60 are formed from a shape memory alloy. A shape memory alloy may include any alloy configured to undergo a reversible phase transformation in response to a change in temperature or mechanical stress. In some examples, the shape memory alloy includes at least one of a nickel-titanium alloy, such as nitinol, or a copper-zinc-aluminum alloy.
Each microfilament 60 defines a microfilament axis 62 that forms a twist angle 64 with yarn axis 50 and a twist direction 66A around yarn axis 50. The plurality of microfilaments 60 may be characterized by an amount of twist, such as a number of turns per centimeter (TPCM). A geometry of shape memory yarn 42 may be characterized by a yarn count (or linear density), a yarn twist, and a packing factor, that together influence twist angle 64. Yarn count may represent an amount of active material within shape memory yarn 42, and may be related to a density of the plurality of microfilaments 60. Yarn twist may represent a degree of twist in shape memory yarn 42, and may be related to an amount of twist of individual shape memory yarns 42 and a number of plies of shape memory yarn 42 (not shown). Packing factor may represent a specific volume of the plurality of microfilaments 60 to a specific volume of shape memory yarn 42, and may be related to a size of the plurality of microfilaments 60.
In some examples, various structural parameters of shape memory yarn 42, such as yarn diameter 48, twist angle 64, a density of microfilaments 60, or a number of plies (not shown in
Shape memory coil 40 may be configured to have a homochiral or heterochiral configuration, or a mixture thereof. In the example of
In some examples, various heat treatment parameters of shape memory yarn 42 and/or shape memory coil 40 may be configured to provide various properties to shape memory yarn 42 and/or shape memory coil 40. As one example, as a heat treatment temperature of shape memory yarn 42 and/or shape memory coil 40 increases, a stiffness and/or plateau strength of shape memory yarn 42 and/or shape memory coil 40 may increase. As another example, heat treatment of the plurality of microfilament 60 prior to forming shape memory yarn 42 and/or shape memory coil 40 may result in residual fiber stresses after twisting and/or coiling, while heat treatment of the shape memory yarn 42 and/or shape memory coil 40 may result in relieved fiber stresses after twisting and/or coiling.
In some examples, shape memory coil 40 may be configured to have particular damping properties. For example, superelastic shape memory yarns 42 may exhibit a relatively large hysteresis through the forward and reverse transformations between austenitic and martensitic phases. The hysteresis, which is mechanical energy that is absorbed and dissipated as heat, can be leveraged in damping applications. In some examples, shape memory coils 40 may have a relatively high specific damping capacity (SDC), defined as the ratio of dissipated energy (hysteresis area) to stored energy (total area under loading curve), compared to shape memory yarns 42. This damping behavior may be tailored by controlling an oscillation amplitude strain, pre-strain, yarn/coil thickness, yarn/coil pattern, and yarn/coil configuration of the shape memory coils 40.
Shape memory yarn 42 may be formed into a variety of textiles. In some examples, various parameters of the textiles may be selected to produce a textile having particular bulk properties. For example, a force profile (e.g., generated force across structural strain) may be produced by a textile based on a shape of shape memory yarn 42 and interaction of shape memory yarn 42 with adjacent shape memory yarns in the textile.
In some examples, shape memory coil 40 may be formed into a textile of interlocked tows having a relatively stable orientation between shape memory coils 40, such as woven tows.
In some examples, shape memory coils 40 may be formed into a textile of interlocked tows having a reconfigurable orientation between shape memory coils 40, such as knitted loops.
The method of
The method of
In some examples, the method of
In some examples, shape memory yarns 42 may be further process to form shape memory coils 40. The method of
Referring back to
NiTi Microfilament Yarns
Creation of micrometer-sized (e.g., <10 μm) NiTi filaments and their integration into yarn and textile structures may improve the tailorability of the mechanical performance of the NiTi material for diverse applications. Improvements may include additional and/or more easily controllable geometric parameters—yarn count, surface twist angle, manufacturing strains—that have been experimentally demonstrated to have impacts on NiTi mechanical performance such as a tunable structural stiffness, plateau strength, and cyclical consistency. Since flexural rigidity is important for textile manufacturing, the significant reduction in NiTi microfilament spun yarn flexural rigidity allows it to travel through traditional textile manufacturing equipment smoothly without the need for specialty monofilament feeders, thereby eliminating limitations seen in monofilament textile processing. Additionally, the inherent flexibility of a spun yarn may increase a wearability, tailorability, manufacturability, and lifetime performance of wearable devices. Textiles knitted from NiTi microfilament spun yarns exhibited both the superelastic and shape memory effects from a recovery of strains induced in the yarn and textile manufacturing processes.
Superelastic Mechanical Testing
For testing on an Instron, a sample was cut to a length of ≈100 mm and placed in pneumatic grips at pressures between 60 and 80 pounds per square inch (psi) at room temperature (23° C.). An environmental chamber was heated to 80° C. to heat the sample to austenite and remove residual strains from handling. At this point, any yarn slack was removed, and onset of tension was found at a 0.2 N applied load. A length of the yarn at this point was recorded as the austenite free length and was considered as the original length measurement for structural strain calculations. The sample was then displaced in tension at a constant rate at isothermal conditions while force was measured. The sample was then unloaded at the same constant displacement rate to back to a tension of 0.2 N. For shakedown testing, a similar loading and unloading path was cycled. For temperature-dependent testing, after unloading, the environmental chamber was then set to a new temperature while the sample was held at a constant force.
Shape Memory Mechanical Testing
Shape memory mechanical testing was performed on a TA Instruments RSA-G2 dynamic mechanical analyzer (DMA). A similar procedure to superelastic yarns was completed to define the austenite free length in the DMA. A sample was cut to a length of ≈25 mm, placed in an environmental chamber, and heated to 80° C. Any yarn slack was removed, and an onset of tension was found at 10 grams (0.1 N) applied load. The length was then recorded and considered as the original length measurement for structural strain calculations.
Force generation is an important metric calculated from results from force blocking testing. For force blocking testing, the sample was loaded to and held at 2% structural strain while the environmental temperature was cooled to −40° C., soaked for 4 minutes, heated back to 80° C., and soaked there for 4 minutes. This cycle was repeated for a second time and upon completion, the sample was loaded to 4% structural strain. The same procedure was repeated at 6% structural strain and 8% structural strain.
Actuation contraction is an important metric calculated from results from free displacement test. A similar methodology was performed for free displacement testing as force block testing described above. The sample was loaded to 5 N and held while the environmental temperature was cooled to −40° C., soaked for 4 min, heated back to 80° C. and soaked for 4 min. This cycle was repeated twice and at 10, 15, and 20 N forces. For both experimental methodologies, the temperature ramps were programmed at 15° C. min−1.
Microfilament Manufacturing
An accumulative drawing/rolling and bonding technique was used to create NiTi filaments with diameters in the micrometer range. The microfilaments were produced by drawing NiTi inside a sacrificial matrix by a conventional wire drawing process.
Digital scanning calorimetry (DSC) of pre-thermally processed microfilament bundles displayed unstable transformations (multiple austenitic transformation peaks and broad transformation ranges of 24° C.) and extreme transformation temperatures (martensite start temperature, MS=−53.9° C.; martensite finish temperature, MF=undefined). An in-line heat treatment apparatus for thermal processing of the NiTi bundles was manufactured to stabilize transformations and increase the martensite finish temperature. Bundles of microfilaments were heated in air at 500° C. for 120 seconds under 50 grams (0.5 N) of tension as determined by a design of experiments on the 1519-5 bundles. Post-thermally processed microfilament bundles demonstrated stabilized transformation temperatures (austenite finish, AF=47.2° C.; austenite start, AS=40.5° C.; martensite start, MS=−20.2° C., martensite finish MF=−29.8° C.) that were compatible with the temperature ranges of the environmental chamber for mechanical characterization.
The in-line heat treatment did not compromise the mechanical strength of the NiTi microfilaments. A 400-10 NiTi bundle was given a small producer's twist of 0.352 turns per centimeter (TPCM) to hold the structure together and mechanically tested at 80° C., above AF, to determine mechanical properties in the austenitic condition. For mechanical testing on the yarns, the active cross-sectional area was used to define a normalized force, which can be compared to stress in monofilaments. The filament bundles had an active cross-sectional area of 3.14×10−2 mm2, which was used to calculate the linear stiffness of the filament as 73.6 GPa before a stress-induced martensite (SIM) transformation occurred at a normalized force of 646.5 MPa (20.3 N). The filaments were able to achieve a loading plateau strain of 9.04% before restiffening occurred at a normalized force of 697.5 MPa (21.9 N). The filament bundle active cross-sectional area is approximately equivalent to the cross-sectional area of a 0.203 mm diameter monofilament wire. For benchmark comparisons, isothermal displacement-controlled testing of a 0.203 mm diameter wire of the same NiTi composition and thermal processing parameters was completed. The stiffness of the wire was 73.8 GPa before a SIM transformation at 519.1 MPa (16.8 N). The wire was able to achieve a loading plateau strain of 7.36% before restiffening at 522.2 MPa (16.9 N). The difference in strength between the filaments and benchmark wire may be explained by a difference in melts, processing history, the surface-to-volume ratio, and fewer defects in the microfilaments.
Yarn and Textile Manufacturing
The NiTi microfilaments were then processed through a hierarchical procedure from the straight bundles described in
When twisted, the individual NiTi microfilaments adapted successfully to this traditional yarn manufacturing method, and the microfilaments reconfigured into helixes.
Yarns with the same geometry can create stable torque balanced yarns or energetic torque imbalanced yarns. The insertion of twist imposes a mixture of bending and torsional strains into the microfilaments. As a structure, this results in a torque imbalance that causes the yarn to untwist if both ends are not fixed. With both ends constrained, any slack would result in a kinking or pig tailing of the yarn. Torque balanced yarns can be obtained using two approaches. In one approach, thermal processing of the yarns after spinning shape-sets the filaments in their twisted structures and removes any residual stresses from the imposed manufacturing strains. These shape-set, or in this case, twist-set, yarns maintain structure upon slack.
Alternatively, in another approach, the single yarns could be plied together by taking two or more torque imbalanced single yarns and twisting them together in the opposite direction of the initial twist.
Knitting is a manufacturing process in which long filaments or yarns are manipulated into interlacing loops.
Superelastic Performance of NiTi Microfilament Yarns
The impact of a variety of material, structural, and experimental properties of yarns on superelastic mechanical performance was investigated through a series of isothermal experiments, as illustrated in
Experiments were performed through a descending sweep of temperatures from above AF to below MF, as found from DSC results.
As the temperature decreases to 40° C., below AF but well above MS, the NiTi yarns exhibited a traditional NiTi thermal-mechanical dependency and thermally driven strain recoverability. During loading, the linear stiffness decreased to 25.4 GPa, the SIM transformation onset occurred at a lowered normalized force of 229.9 MPa (6.85 N), and restiffening occurred at an earlier structural strain of 6.69%. Although the yarns were initially austenitic and at a temperature above MS, the decreased temperature permitted earlier SIM transformation by decreasing the amount of energy needed to initiate the solid-state transformation. Upon unloading from 9.89% structural strain, the yarns exhibited limited unloading recoverability, only 3.90% structural strain, but upon heating above AF, thermally driven recoverable strains of 5.20% were reached, demonstrating the shape memory potential.
As the temperature decreases to 0° C., well below AF, but right before MS, the yarns displayed increased mechanical deviations from austenitic behavior. The material experienced a SIM transformation at the onset of loading, decreasing the initial stiffness to 14.7 GPa, as well as the normalized plateau force to 117.8 MPa (3.51 N) and restiffening strain to 6.12% structural strain. Upon unloading from 9.90% structural strain, the yarn recovered 2.95% strain, before an additional thermally driven recovery of 5.90% from heating above AF. Finally, at a temperature of −40° C., below MF, the material demonstrated fully martensitic behavior with a martensitic stiffness of 6.57 GPa, before detwinning occurred at a force of 59.1 MPa (1.76 N). Upon unloading from 9.90%, unloading strains of 2.70% structural strain were recovered. The yarns exhibited traditional NiTi monofilament behavior—superelastic recovery, tunable thermal-mechanical dependent stiffnesses, and recoverable residual strains that allow NiTi to be used in a variety of applications.
The influence of the twist on mechanical performance was evident from isothermal strain-controlled results. Under small structural strains of less than 1%, yarns exhibited an initial realignment in which the yarn radially contracted, the filaments began interacting, and the entire yarn began to deform linearly, which resulted in a very low and continuously changing stiffness region.
Once linearly deforming, the effective yarn stiffness was inversely dependent on the surface twist angle of the yarn, which demonstrated the increased tailorability of yarn structures, as shown in
In traditional austenitic NiTi behavior, there is a transition from a linear stiffness to a horizontal force plateau. In isothermally loaded monofilament NiTi wire, the transition is abrupt as the material uniformly undergoes a solid-state phase transformation from austenite to detwinned martensite. Within a yarn structure, this transition occurs over a range of structural strains that is heavily influenced by the amount of twist in the yarn, as shown in
To add strength in monofilament wire applications, engineers typically choose larger wire diameters; however, this added strength comes with a significant increase in flexural rigidity. In the twisted yarn form, strength is increased by increasing the amount of active material in the yarn through plying, which adds significant strength to the yarn structure at a small increase to the flexural rigidity.
Mechanically, there are many qualitative and quantitative differences between torque balanced yarns and torque imbalanced yarns as a result of the impact of thermal processing on the structure and filament strains within the NiTi yarn.
Consistent mechanical performance is important in many NiTi applications. For the NiTi material system, material shake-down is an understood behavior, while structural shakedown is recognized to be prevalent in highly frictional structures, but not thoroughly explored. The mechanical behavior of NiTi yarns differs significantly between the first and all subsequent loading cycles.
To differentiate material shakedown from structural shakedown, shakedown testing was conducted on the benchmark 0.203 mm wire. The difference between the yarn and wire shakedown testing was most evident between a large change in behavior between the first and second loading cycle in the yarn compared with the wire. This suggests that, upon first loading, the filaments in the yarn structure were undergoing a structural settling on top of material settling. All subsequent loading cycles of the NiTi yarn were similar but did exhibit minor decreases in plateau onset strength, unrecoverable deformations, and increases in plateau stiffness. However, with each subsequent cycle, these differences decreased, suggesting a consistent hysteretic loop and filament configuration may be attainable by further cycling of the NiTi yarn structure. It is important to note that the shakedown phenomenon described herein can also be seen in the temperature dependence test between the first cycle at 80° C. to subsequent cycles, as shown in
Shape Memory Effect in NiTi Microfilament Yarns
To demonstrate the shape memory ability of the NiTi microfilament yarns, free displacement (isoforce) and force blocking (isostrain) testing was performed on a 400-10 3.51 TPCM torque balanced single yarn and a 400-10 7.03 TPCM torque balanced single yarn. Shape memory mechanical testing was performed on a TA Instruments RSA-G2 dynamic mechanical analyzer (DMA) with an attached environmental chamber. During testing, displacement-controlled portions were executed at global displacement rates between δL−1=+5×10−5 s−1 and δL−1=+4×10−4 s−1, while isostrain/isoforce temperature ramps were performed at 15° C. min−1. At the end of each temperature ramp, the sample was soaked for 240 s to ensure the completion of the solid-state transformation.
In free displacement testing, an ability of the yarn to produce contractions at a prescribed force, called the actuation contraction, is experimentally assessed. In actuating systems that integrate artificial muscles, the actuation contraction is critical for designers to achieve desired motions under application-level loads. At 5.0 N applied force, the 7.03 TPCM torque balanced single yarn was able to cool to a longer martensitic length than the 3.51 TPCM torque balanced single yarn, and upon heating, was able to recover more of deformations; however, more unrecoverable deformations remained as a result of internal friction in the yarn.
While the maximum average contraction of the 3.51 TPCM was larger than that of the 7.03 TPCM yarn, the influence of twist enabled greater actuation displacements at lower forces, providing a design tradeoff in the shape memory performance. Despite maximum average actuation contractions occurring at different forces, both yarns produced average maximum specific works under a 10.0 N applied load where the 3.51 TPCM yarn produced 2338 J kg−1 of specific work compared to the 7.03 TPCM yarn which produced 1480 J kg−1 of specific work. The higher twisted yarn had more consistent actuation contractions across a larger range of forces, as observed by the 21.9% change in actuation contraction in the 7.03 TPCM yarn from 5.0 to 10.0 N, compared to the abrupt 84.2% change in actuation contraction in the 3.51 TPCM yarn from 5.0 to 10.0 N.
Conversely, in force blocking testing, the generated force of the yarn at a prescribed structural strain is the metric of interest. In compression garment applications, understanding the forces generated at a specific strain is the basis for accurately designing a garment capable of producing pressures that offer medical benefits to the wearer. The 3.51 TPCM torque balanced single yarn saw elevated forces in austenite when compared to the 7.03 TPCM torque balanced singe yarn as a result of increased stiffness and SIM transformational strength.
Textile Performance
Knit manufacturing involves a new set of design parameters that impact the final textile form, including needle spacing, yarn feed tension, and knit pattern. The needle spacing determines the course width while the tension system, which controls the length of material in one loop, determines the wale height. The knit pattern, which is a collection of knit and purl loops, influences the mechanical behavior of the passive and active textiles. Heat treatment at this stage of manufacturing can also tune the material and structural behavior. To demonstrate the hierarchical process from start to finish, three NiTi bundles were given a twist of 4.37 TPCM before being plied together at a twist of 1.46 TPCM. A torque balanced three-ply NiTi yarn was knitted into a garter pattern textile, which has alternating rows of purl and knit loops, on a Taitexma TH-860 hobbyist knitting machine with a needle spacing of 4.5 mm and a tension setting of 8.
For NiTi monofilament wire, the knitting process induces bending deformations that promote large forces, unprecedented actuation contractions, and variable stiffness behavior. NiTi textiles composed of microfilaments were able to demonstrate similar shape memory, as illustrated in
For a benchmark comparison, the three-ply garter knit was stretched and constrained at room temperature, 23° C., above MF and below AS, and then released.
NiTi Microfilament Coiled Yarns
This work presents the novel creation of NiTi microfilament over-twisted coiled yarns and outlines the ideal applied stress at twist insertion to ensure successful coil propagation. Uniaxial testing and empirical analysis quantified the global actuation, force generation, cycled consistency, thermomechanical dependence, and superelastic performance of NiTi over-twisted coiled yarns. NiTi microfilament over-twisted coiled yarns offer exciting new compact solutions for energy absorbing and actuating applications in wearable, medical, and robotic industries.
Experimental Methodology
Both superelastic and shape memory mechanical testing was performed on a TA Instruments RSA-G2 dynamic mechanical analyzer (DMA) equipped with an environmental chamber for controlling temperature. To begin all testing, the chamber is heated to 80° C. to heat the sample to austenite and remove and residual strains from handling. At this point, any slack is removed, and the onset of tension is found at 10 grams applied load. The length at this point is recorded as the austenite free length and is considered as the original length measurement for structural strain calculations. Superelastic mechanical performance was investigated through a series of isothermal, displacement-controlled experiments which used global displacement rates between δ L−1=±5×10−5 s−1 and δL−1=±4×10−4 s−1. The slow displacement rates were necessary to dissipate heat during phase transitions to maintain isothermal conditions during testing.
Shape memory effect was investigated through two different testing methodologies, free displacement (isoforce), and force blocking (isostrain), such as described in Shape Memory Effect in NiTi Microfilament Yarns above. For free displacement testing, the sample was loaded to 2 N and held while the environmental temperature was cooled to −50° C. at a rate of 30° C./min, soaked for four minutes, heated back to 80° C. at the same rate, and soaked for four minutes. This cycle was repeated twice and at 4N, 6N, and 8N forces. The actuation contraction was the key metric calculated from the results from free displacement test and is defined as the difference in the martensitic length at the end of a cool ramp (lmar) and the austenitic length at the end of the subsequent heating ramp (laus), divided by the martensitic length. For each force, there were two actuation contractions derived, and the average was used analytically. A similar methodology was performed for force blocking testing. The sample was loaded to 10% structural strain and held there while the environmental temperature is cooled to −50° C., soaked for four minutes, heated back to 80° C., and soaked there for four minutes. This cycle was repeated for a second time, and upon completion, the sample was loaded to 20% structural strain. The same procedure was performed at 30% structural strain. Force generation was the key metric calculated from the results from force blocking testing and is defined as the difference between the martensitic force (Fmar) at the end of a cooling ramp, and the austenitic force (Faus) at the end of the subsequent heating ramp. For each strain, there were two force generations and the average was used analytically.
Microfilament Manufacturing
As described above, the creation of NiTi over-twisted yarns was enabled by the implementation of an accumulative drawing/rolling and bonding technique to create NiTi filaments with diameters in the micron-range. The microfilaments are produced by drawing inside a sacrificial matrix by a conventional wire drawing process. The filaments are organized in clusters of individually nested filaments. The matrix material is chemically removed, revealing the NiTi microfilaments. SEM imaging of a single filament reveals drawing lines as well as occasional residual iron particles.
The bundle dimensions are dictated by the number of filaments and diameter of the individual filaments. In these examples, bundles containing filaments with diameters of 10 μm in 400, 800, 1200, or 1600 increments were used. The active cross-sectional area is a reference area based on the summation of individual microfilament cross-sectional areas. For instance, a bundle of 400 filaments with diameters of 10 μm (400-10) has a total active cross-sectional area (Ay) of 3.02×10−2 mm2. The NiTi alloy used in this study has a chemical composition of 56 wt. % nickel with 300 ppm oxygen, 310 ppm carbon, and balanced titanium. The NiTi alloy was a nickel-rich material, with an ingot AF of 68° C.
Digital scanning calorimetry (DSC) of pre-thermally processed microfilament bundles unstable transformations (multiple austenitic transformation peaks and broad transformation ranges of 24° C.) and extreme transformation temperatures (martensite start temperature, MS=−53.9° C.; martensite finish temperature, MF=undefined). A custom heat treatment apparatus for thermal processing of the NiTi over-twisted coiled yarns was manufactured to stabilize transformations and increase the martensite finish temperature. Over-twisted coiled yarns were heated in air at 550° C. for 11 minutes as determined by a design of experiments on the 1519-5 bundles. Post-thermally processed microfilament bundles demonstrated stabilized transformation temperatures (austenite finish, AF=52.5° C.; austenite start, AS=40.8° C.; martensite start, MS=−10.3° C., martensite finish MF=−23.2° C.) that were compatible with the temperature ranges of the environmental chamber for mechanical characterization.
Over-Twisted Coiled Yarn Manufacturing
Over-twisted coiled yarns may be manufactured using industrial manufacturing equipment.
Various coiled forms may be created by controlling an amount of stress to NiTi bundles.
Thermal processing is required to shape set the NiTi filaments and remove the residual torsional stresses that remain in the coiled yarn structure after manufacturing. During thermal processing, the coiled yarn was fixed in place after manufacturing and heat treated in a Lindberg/Blue M™ boxfurnace at 550° C. for 11 minutes. As well as torsional stability, post-thermally processed coiled yarns demonstrated stabilized transformations that were compatible with the temperature ranges of the environmental chamber for mechanical characterization.
The over-twisted coiled yarn may form a basis for more complex one-dimensional structures, such as NiTi artificial muscle architectures. Additional OTC yarn structures were manufactured by manipulating an OTC yarn after manufacturing but before thermal processing and shape setting. The different artificial muscle architectures illustrated in
Manufacturing Study
A manufacturing study was performed to identify ideal tensions for different bundle configurations to ensure successful coil propagation and offer insight into coil manufacturing mechanics.
During the manufacturing study, the sample length, inserted rotations, and coil initiation were recorded for each sample. To normalize across sample sizes, the sample length was normalized into a retraction. The retraction is defined as a difference between length of zero-twist sample (l_o) and twisted sample (l), divided by an original length of the zero-twist sample. The inserted rotations were normalized into the twist per unit length by dividing the rotations by the zero-twist length. During the twist angle progression stage (pre-coil) of coiled yarn manufacturing, imaging of the yarn can be used to estimate the geometry of the filaments in the yarn. In an ideal yarn theory, when a bundle of filaments is given twist, the filaments reconfigure in helical pathways each sharing the same pitch height defined as the inverse of inserted twist. Each helix has a constant radius (r) from the yarn axis and helical angle (θ) defined as the acute angle between the yarn axis and filament axis. The helical angle ranges from 0 at the yarn core, to the surface twist angle at the outside of the yarn which can be estimated from the yarn retraction (R).
Over-Twisted Coiled Yarn Characterization
After manufacturing, the over-twisted coiled yarns made from different bundle configurations were geometrically characterized in a scanning electron microscope (SEM) to identify the coil diameter (Dc), coil angle (αc), and yarn diameter (Dy), such as coil diameter 52, coil angle 51, and yarn diameter 48 of
Superelastic Performance of NiTi Over-Twisted Coiled Yarns
Preliminary superelastic testing was interested in determining structural strain capabilities as well as safe operating ranges of the over-twisted coiled yarn structure. An 800-10 over-twisted coiled yarn was mechanically tested to increasing structural strains in 5% increments at 80° C., above AF.
In many NiTi applications, the thermomechanical coupling acts as the mechanism for actuation and force generation. To determine the extent of thermomechanical coupling in NiTi over-twisted coiled yarns, isothermal testing was performed at 80° C., above AF and −50° C., below MF, as found from DSC results illustrated in
In traditional austenitic NiTi behavior, there is a transition from a linear stiffness to a horizontal force plateau. In isothermally loaded monofilament NiTi wire, the transition from linear loading to a constant force plateau is abrupt as the material uniformly undergoes a solid-state phase transformation from austenite to stress induced martensite (SIM). Within a coiled yarn structure, this transition occurs over a range of structural strains that is due to more diversity in filament stress states (extension, bending, and torsion) and complexity among filament pathways. For this reason, individual filaments will reach the material strain to cause a SIM transformation at different structural strains. In this case, this austenitic OTC yarn exhibits A→Md transformation from approximately 3.0% to 12.0% structural strain. During unloading, the OTC yarn exhibits a reverse transformation (Md→A) back to austenite, resulting in superelastic recovery of 18.45% structural strain and 1.55% of unrecoverable structural strain. Superelastic SMA monofilaments exhibit large hysteresis between the austenitic loading and unloading curves. The potential of these coils as energy absorbers is highlighted in this area between the austenitic loading and unloading curve (84.2 N·Structural Strain) as mechanical energy is absorbed and dissipated as heat during material transformation to stress induced martensite. This area represents mechanical energy that is absorbed and dissipated as heat during the forward and reverse SIM transformations. Hysteresis also occurred in the complex OTC yarn structure, resulting in 1670 kJ m−3 of dissipated energy per unit volume. The dissipated energy offers insight into the multifunctional capabilities of over-twisted coiled yarns as compact energy absorbers and elastocaloric coolers that are not observable in other material systems.
At an experimental temperature of −40° C. (Martensite,
Consistent mechanical performance is important in many NiTi applications. For the NiTi material system, material shakedown is an understood behavior, while structural shakedown is recognized to be prevalent in highly frictional structures, but not thoroughly explored. The mechanical behavior of NiTi coiled yarns differs significantly between the first and all subsequent loading cycles. Upon first loading of the 800-10 coiled yarn, the onset and end forces of SIM transformation was estimated to be 3.48N and 8.70N, respectively. After unloading from 20% structural strain, there remained a 1.55% permanent strain after superelastic recovery. Upon reloading to the same strain, the linear stiffness decreased from 1.15 N·Structural Strain−1 to 0.95 N Structural Strain−1, while the onset and end of SIM transformation was estimated to be 3.11N and 6.55N, a large decrease from the first cycle. Upon superelastic recovery, the structure demonstrated a similar unloading path to the first cycle; however, the decrease in SIM transformation from the first to second cycle resulted in a reduction in the hysteresis loop—the mechanical branching between loading and unloading from a reversal in transformation direction. Once fully unloaded, there remained a decreased permanent strain of 0.95% structural strain compared to the first cycle.
While some of the differences between the first and subsequent cycles can be attributed to material shakedown, the difference can also be attributed to the micro filaments being pulled out of their original configurations into positions that, upon unloading, are not reversible. The filaments then remain in a higher deformed and stressed state, and upon reloading, will detwin at lower forces. Additionally, due to their complex pathways, some microfilaments may be plastically deforming at large structural strains. If enough microfilaments are plastically damaged, their individual superelastic recoveries may be limited and the global response of the coiled yarn may exhibit large unrecoverable deformations.
Shape Memory Effect Actuation Performance of NiTi Over-Twisted Coiled Yarns
For more dynamic insight into the shape memory potential of over-twisted coiled yarns, free displacement and force blocking testing was performed. In free displacement testing, the impact of internal friction within a system can be assessed by studying the recoverability of the sample during thermal cycling, such as by quantifying the contraction actuation potential of the OTC yarn at various constant applied forces.
Shape Memory Effect Damping Performance of NiTi Over-Twisted Coiled Yarns
Superelastic SMA exhibits a large hysteresis through the forward and reverse A ⇄Md transformations. The hysteresis, which is mechanical energy that is absorbed and dissipated as heat, can be leveraged in damping applications[49-54]. The specific damping capacity (SDC), defined as the ratio of dissipated energy (hysteresis area) to stored energy (total area under loading curve), is commonly used to compare damping capabilities across SMA structures, such as illustrated in
Oscillatory tests were performed on the coiled yarns on an RSA-G2 DMA. The coiled yarns were fixed within the environmental chamber, heated to austenite, and the austenite free length was found at a light tension of 10 grams. The coiled yarn was strained to a pre-strain and held. Dynamic strain loading was applied to the yarn at a preset oscillation frequency and oscillation amplitude strain for 100 cycles. For the oscillation frequency sweep, the oscillation amplitude strain was a constant 2.5%. Conversely, the constant frequency for the oscillation amplitude sweep was 1 Hz. For calculation of the SDC, the last cycle (n=100) was analyzed to determine the hysteresis area and area under the loading curve. Additional frequencies or amplitudes are applied at this pre-strain before strain loading to the next incremental pre-strain.
Coiled Yarn Textile Manufacturing
As standalone structures, OTC yarns offer large actuation contractions and generated forces for artificial muscle applications. Additionally, they are 1D structures that are integral for textile manufacturing. Woven and knitted textiles were manufactured to demonstrate the potential of OTC yarn textiles for actuating applications. For the woven garment, eight 230 mm long OTC yarns consisting of 800-10 microfilament bundles were manufactured.
Coiled Yarn Textile Performance
The OTC yarn textile actuation performance was encouraging for various artificial muscle applications, especially medical compression garments. Ideal compression garments must undergo large structural strains to stretch over areas of the body, exhibit a low force response in the inactivated state for ease of use, and provide large, scalable generated forces during activation to apply medical-grade pressures to the user.
There is a combination of reasons for the extended strain range of the woven textile compared with a single OTC yarn. First, the OTC yarns are in parallel, sharing the total applied load across each OTC yarn. It is assumed that due to manufacturing inconsistencies, the load is not split evenly across each OTC yarn. It is theorized that the load is applied across a subset of the OTC yarns. When those heavily loaded OTC yarns begin to weaken, either from the A→Md transformation or partial de-coiling, the load is transferred to adjacent OTC yarns that were previously unloaded. This form of load sharing allows the entire structure to be strained to large structural strains, beyond that of a single OTC yarn. It also explains why the austenitic forces of the woven textile, as shown in
Additionally, during donning of a compression garment, it is desirable for the compression garment to provide the least amount of resistance to make the experience easier on the user. For an SMA-based compression garment, donning would occur in the less stiff martensitic phase. Encouragingly, both textiles demonstrate a low force response in the martensitic state, such as illustrated in
The generated force profiles of the two textiles are qualitatively different. The woven textile exhibits immediate large, generated forces at relatively low structural strains, whereas the generated forces of the textile do not significantly increase until 70.0% structural strain. During early loading of the knitted textile, reorientation (sliding) of the OTC yarns mitigates the strains and stresses applied directly to the OTC yarn. Without any strains, the OTC yarns exhibit limited strain recovery during the austenitic transformation resulting in low generated forces. The two profiles could be used to tailor behavior within segments of a medical compression garment to apply pressure to the user under a small donning pre-strain, or a large pre-strain from donning over a large part of the body. The textiles offer an exciting combination of tailorable, scalable, and ideal force generation solutions for actuating applications driven by moderate thermal stimulus.
Multifunctional SMA cloth-like textiles may be used for both active and passive applications. Unlike previous SMA monofilament textiles, the microfilament yarn-based systems exhibit strong wearability and tunable performance. The garter textile designed for active compression wearables exhibits a low-force martensitic response for donning, a large structural strain ranges for stretching over large parts of the body, and generated forces for beneficial therapeutic compression. The generated forces can be scaled for specific applications through an increase in the amount of active material without sacrificing manufacturing abilities. Additionally, the altered deformation modes and fundamental understanding of SMA microfilament yarn mechanics promotes predictable behavior in the textile architecture.
To demonstrate the damping multifunctionality of SMA, a 3D spacer fabric was manufactured for integration within prosthetic attachment systems. The 3D spacer fabric absorbs and dissipates mechanical energy through a combination of inter-yarn friction and SIM transformation mechanisms. The damping ability of the fabric is dependent on the pre-strain and oscillation amplitude, which control the amount of SIM transformation. The performance of the spacer fabric can be tuned for specific user activities through changes to the spacer thickness, spacer yarn pattern, and spacer yarn configuration without sacrificing manufacturing. The work presented showcases the improved wearability and tunability of SMA multifunctional textiles through the inclusion of SMA microfilament yarns and establishes the foundation for optimization of SMA textiles for application-specific performance.
Functionalized SMA Microfilament Fabrics
Multifunctional SMA cloth-like textiles may be used for both active and passive applications. Unlike previous SMA monofilament textiles, microfilament yarn-based systems exhibit strong wearability and tunable performance. As will be illustrated below, such SMA multifunctional textiles may have improved wearability and tunability through the inclusion of SMA microfilament yarns and may be improved or optimized for application-specific performance.
As one example textile, a garter textile designed for active compression wearables may exhibit a low-force martensitic response for donning, a large structural strain ranges for stretching over large parts of the body, and generated forces for beneficial therapeutic compression. The generated forces can be scaled for specific applications through an increase in the amount of active material without sacrificing manufacturing abilities. Additionally, the altered deformation modes and fundamental understanding of SMA microfilament yarn mechanics promotes predictable behavior in the textile architecture.
As another example textile, a 3D spacer fabric for integration within prosthetic attachment systems may exhibit damping multifunctionality. The 3D spacer fabric may absorb and dissipate mechanical energy through a combination of inter-yam friction and SIM transformation mechanisms. The damping ability of the fabric may be dependent on the pre-strain and oscillation amplitude, which control the amount of SIM transformation. The performance of the spacer fabric can be tuned for specific user activities through changes to the spacer thickness, spacer yarn pattern, and spacer yarn configuration without sacrificing manufacturing.
One example textile described herein includes a highly tunable and scalable SMA microfilament yarn integrated garment that exhibits a traditional clothing-like aesthetic, a low force inactivated state, and generated pressures through the SME. For example, on-body compression garments may be used for a wide range of applications, from everyday wearables to performance-enhancing athletic clothing to therapeutic garments. Compression is used in conjunction with wearables such as fitness trackers to ensure a good connection between sensors and users. Athletic garments use targeted compression to promote recovery, prevent injuries, and improve athletic performance. In medical devices, compression may be used to control excessive swelling in lymphedema patients and promote blood flow to prevent deep vein thrombosis (DVT) and orthostatic hypotension (OH).
Current on-body compression technologies typically use passive systems with elastic materials that can undergo large structural strains to stretch over a portion of the body and elastically recover applying pressure to the user. However, elastic materials and structures, which follow Hooke's law, provide continuously increasing pressure with added structural strain making it difficult to achieve specific user pressures across different anthropometric makeups. Additionally, these garments are significantly stretched during donning, requiring large forces from the user, of which, pose a problem for elderly and disabled users. To overcome the forces needed for donning, some current compression technologies utilize an activated pneumatic system. These pneumatic-based devices are easy to don and can provide significant pressures when activated but are large, bulky, loud, and prevent user mobility.
Through the shape memory effect (SME), SMA integrated textiles can undergo large, activated contractions, providing significant on-body pressures to a user. Additionally, the inactivated, less stiff martensitic state provides a low force response ideal for the user experience during donning. However, current SMA compression garments composed of large monofilaments limit their wearability, manufacturability, and performance tunability. SMA microfilament yarns have been demonstrated to maintain strong SME behavior in a highly tunable, flexible architecture ideal for dense knitting manufacturing processes.
Another example textile described herein includes a 3D SMA integrated spacer textile that combines the high stiffness, large recoverability, and hysteretic damping of superelastic SMA in a lightweight textile architecture designed for mechanical energy absorption that may be optimized to mitigate interfacial pressure points afflicting prosthetic users. For example, there remain many difficulties creating properly fitting prosthetics, plaguing lower leg amputees and inhibiting a successful rehabilitation. Most of the pain and discomfort stem from interfacial pressure points because of the varying stresses from daily activities and bodily volume fluctuations. Current technologies emphasize the customized fit of the prosthetic socket system to mitigate pressure points, but these systems require constant fit adjustments and have trouble accounting for bodily volume fluctuations.
As a multifunctional material system, shape memory alloys have unique damping and energy-absorbing properties that make them a natural solution in prosthetic attachments. For example, the constant stress plateau during austenite to detwinned martensitic transformation can be leveraged to provide consistent pressures to a user's unique body contour and over large range of displacement fluctuations. Additionally, superelastic SMA behavior is a hysteretic process where mechanical energy is dissipated as heat—a fundamental characteristic for damping. Leveraging the transformation of energy as a damper in a prosthetic attachment could lessen the pressures felt by the user during strenuous activities. However, prosthetic attachments require lightweight solutions and as a metal alloy, solid SMA structures are typically heavy and dense. For integration in prosthetic attachments, a lightweight yet strong SMA architecture is needed.
Spacer textile architectures provide lightweight solutions to integrate materials in a 3D energy-absorbing structure. Spacer textiles consist of 2D knitted surfaces that are joined by a perpendicular spacer yarn. Spacer textiles geometrically leverage the reorientation and complex loading (bending, shearing, torsion) of the spacer yarns in compression to absorb large amounts of mechanical energy. Spacer yarns typically consist of highly elastic polymers that can undergo deformations and linearly recover —springing the textile back into its original shape. However, these materials are ideally used in low-force applications as higher forces will cause permanent densification and deformations of the textile. Additionally, the thickness of the spacer fabric is limited by strains put on the spacer yarn to avoid permanent deformations during compression.
The SMA microfilament yarns were manufactured at a local spinning mill on industrial ring spinners capable of controlling the delivery speed, spindle angular velocity, and inserted twist. Two yarn variants were manufactured: a 400 filament bundle with 5 μm diameters with a twist of 10.53 TPCM used in the spacer fabric, and a 400 filament bundle with 10 μm diameters with a twist of 5.27 TPCM used in the compression garment. For both yarns, heat treatment was necessary for stabilizing transformations, shifting transformation temperatures, and twist setting the austenitic memorized shape. The yarns were thermally processed under the same conditions (11 minutes at 550° C.) after twist insertion (Thermal Processing). The processed yarns were manufactured into either a 2D garter textile for actuating applications or a 3D spacer textile for energy absorbing applications.
Experimental characterization of the actuating garter textiles assessed the compression potential through force block testing. Force block testing was performed on an Instron 3365 within an environmental chamber. Force block testing is designed to mimic the user experience of an active compression garment. Donning of the garment was assumed to be performed in the flexible martensitic state. Stretching in the lower martensitic stiffness corresponds to a low force response during donning, allowing a user to deform the garment over a section of the body easily. In force block testing, this is captured by an initial strain-controlled loading from unstressed twinned martensite. After donning, the garment will be thermally activated (passively from body heat or through a controlled input) to the stiffer austenitic state. Upon transformation to the stiffer austenitic state, the garment will undergo an increase in force from recovery stress generation (RSG). The difference in force between the donned martensitic state and the activated austenitic state represents the generated forces felt by the user. In force block testing, this is characterized by iso-strain thermal cycles to measure the generated forces. The garment is incrementally strained and thermally cycled to characterize the force-generating abilities for different stretches and areas of the body (structural strains). On-body pressure (P) is calculated using Laplace's Law, which is derived from pressure vessel theory and can be described as the ratio of generated force per unit width (Fw) and the radius of the cylinder (body part).
Energy absorption and damping potential of the 3D spacer textile are characterized through a combination of isothermal quasistatic isothermal compression and dynamic oscillatory tests. The quasistatic isothermal compression tests were performed on a DMA RSA-G2 within an environmental chamber at temperatures either greater than Af or below Mf. The typical compression behavior of a spacer fabric is split up into three distinct stages. The first is a linearly sloped region, which corresponds to compression and light buckling of the spacer yarns. The second stage corresponds to a nearly constant stress plateau, which is a result of additional buckling, rotating, and shearing of the spacer yarn. This is followed by a steep increase in stress from the densification of the fabric, which restricts any further movement of the spacer yarn. Utilizing the constant stress plateau is critical for prosthetic attachments applications where consistent reaction forces are ideally applied to the body over a wide range of displacements. Isothermal compression tests are performed to characterize the compression behavior and identify the staged behavior of SMA microfilament spacer textiles.
Oscillatory testing captured the dynamic response to varying loads. Oscillatory tests were performed on a DMA RSA-G2 by compressing the textile to a pre-strain and oscillating displacements at a controlled oscillation amplitude and frequency. The corresponding force-strain data was analyzed for mapping the force response theoretically felt by the user and measuring the mechanical energy that is dissipated as heat from internal friction and martensitic transformation. A common way to quantify SMA's damping effectiveness across diverse architectures is through the specific damping coefficient (SDC), defined as the ratio of dissipated energy (hysteresis area) to stored energy (total area under loading curve).
Actuating Performance of Cloth-Like SMA Garter Textile
Without optimization, the cloth-like garter textile exhibited behaviors ideal for active compression wearables and expanded tunability for optimized performance compared to previous monofilament-based systems. In monofilament-based SMA knitted textiles, the force response to structural strain is dependent on the high bending stiffness and inter-monofilament frictional behavior. Upon loading, an initial reorientation of the monofilaments through interfacial sliding occurs before interlocking due to the build-up of stick frictional forces. While sliding enables large structural straining of the knitted textiles, force generation is limited in this stage. In the absence of sliding, the recovery of stresses from bending deformations from the interlocked loop geometries drives an increased force generation during thermal heating. With additional loading, the recovery of stresses is not enough to overcome the strained loop geometry, resulting in a decrease in force generation.
Additionally, the large reduction in bending stiffness in the microfilament yarns drastically alters the dominating underlying mechanisms used to generate forces during RSG. Instead of leveraging mainly loop geometry bending deformations in the interlocked state (stick frictional forces), the SMA microfilament yarns leverage a combination of microfilament helical deformations within the yarn (tension, compression, bending, torsion) from direct yarn axial straining during the interlocked stage. The highly compliant yarn structure enables tight loop geometries in the interlocked state, which results in the microfilament helical deformations from axial loading dominating RSG behavior. This loading of microfilament yarns is well understood through previous experimental and modeling works, permitting tunable predictive behavior of SMA microfilament yarns and textiles. The cloth-like, easily tunable, and scalable SMA yarn knitted structure exhibits significant improvements over monofilament SMA textiles, advancing the functionalization of this technology for on-body compression.
Because martensitic reorientation is a one-way phenomenon, the martensitic experimental curve (curve II) is treated as a passive material system, offering significant insight into the impact of the superelastic effect. For example, the mechanical energy absorbed and dissipated is quantified by the area (hysteresis) between the loading and unloading curves. In austenite, the spacer fabric exhibited a large hysteresis between the loading and unloading curve, resulting in 740.2
of dissipated energy per unit volume. The energy dissipated in austenite is a combination of reversible SIM transformation of the SMA material and friction within the yarn and spacer fabric structures. To isolate the impact from SIM transformation, a comparison with the compressional loading in martensite was performed. The spacer fabric martensitic curve exhibited a hysteresis corresponding to 494.4
of dissipated energy per unit volume. This can predominately be attributed to the inter-filament and inter-yarn friction within the fabric system.
The inner loop hysteresis can also be analyzed through damping efficiency metrics. The specific damping capacity (SDC), defined as the ratio of dissipated energy (hysteresis area) to stored energy (total area under loading curve), is commonly used to compare damping capabilities across SMA structures. Compared with monofilament SMA, which possesses a superelastic loading SDC of 23%, the SMA spacer fabric exhibits an increased SDC of 59.5%, which indicates a high damping potential by the SMA within this architecture.
Additionally, the impact of superelastic NiTi can be observed in the strain recoverability of the spacer fabric in austenite compared with martensite. In austenite, the spacer fabric recovered 89.3% of the applied strain, compared with 75.7% in the martensitic curve shown in
The SMA spacer fabric exhibited known SMA damping behavior highly dependent on the pre-strain and amplitude strain. In general, the NiTi Spacer fabric exhibited increased SMA damping performance (>20.0%) compared with commercial NiTi wire (typical SDCs in the 5%-10% range). The best damping performances (SDC-25%) occurred under a light pre-strain (10-20%) and high oscillation amplitude strain (7.5-10%). Under these conditions, the oscillatory loading range stays consistently in Stage I and II behavior, where it was initially identified that SIM transformation might be occurring. These results suggest optimization of the spacer fabric comes from close coordination between initial fabric behavior (filament engagements and reorientation) and loading of the spacer yarn (SIM transformation).
The goal of integrating SMA microfilament yarns within a spacer fabric was to leverage both material and structural behaviors to improve spacer fabric performance. While the spacer fabric presented in this work does not exhibit an ideal constant-force plateau, it successfully leverages SIM transformation within a 3D textile to improve strain recoverability and energy dissipation over passive material systems (martensite). The spacer fabric exhibits damping efficiencies dependent on pre-strain and strain amplitude that can be tailored for specific applications such as prosthetic attachments. Future iterations may optimize the relationship between filament reorientation and SIM transformation through alterations to the yarn configuration, spacer fabric thickness, and spacer yarn pattern.
Energy Absorbing Textiles
3D spacer fabrics with superelastic materials can provide constant force profiles, enhanced damping frequency ranges, and large energy dissipations for prosthetic attachments, helmet technology, and impact resistance fortifications. Usually, researchers manufacture multifunctional textiles with monofilament and yarns but recently, over-twisted coiled structures have demonstrated improved actuation contractions, force generations, and strain recovery. For energy absorption, the NiTi over-twisted coiled yarns were structured within a 3D spacer textile structure, and experimentally investigated quasi-statically for strain recovery and energy absorption through hysteresis, as well as dynamically for mapping damping performance dependent behavior.
Textiles describes herein may be manufactured from NiTi microfilament over-twisted coiled yarns for medical device applications such as medical compression garments, as well as energy absorbers for defense, impact, and damping applications. Over-twisted coiled yarn structures are created on a custom-built manufacturing system by inserting twist into a bundle of NiTi microfilaments until the torsional imbalance imparted on the yarn causes coils in the direction of the yarn axis to form. The coiled yarns are thermally processed to shift transformation temperatures and shape set the final coiled structure.
To demonstrate the superelastic and energy absorbing potential, a textile in the form of a 3D spacer fabric was manufactured. A 3D spacer fabric was manufactured with energy absorbing applications in mind such as impact absorption, and vibrational damping. 3D spacer fabrics consist of two outer knitted layers connected and distanced by a spacer yarn, which gives the fabric it's 3D form. The 3D spacer fabric leverages both traditional structural effects as well as the unique energy absorbing behaviors of superelastic NiTi. Isothermal compression tests were performed to quantify energy absorption and understand coiled yarn spacer fabric kinematics, while dynamic oscillatory tests were performed to map relationships between pre-strain and frequency to loss modulus and tan delta. The NiTi coiled yarn spacer fabric exhibited potential as a high force energy absorber and future optimizations are discussed to improve performance.
Spacer Fabric Manufacturing
Coiled yarns were manufactured as described in
Spacer Fabric Testing and Characterization
The energy absorption potential of the spacer fabric was investigated through slow, displacement controlled isothermal compression tests as well as dynamic testing performed on a TA Instruments RSA-G2 dynamic mechanical analyzer (DMA). Displacement controlled compression tests were performed at 80° C., above Af, at a global displacement rate of δL−1=±4×10−4 s−1 with 25 mm diameter compression plates corresponding to a testing area, Acomp, of 4.908×10−4 m2. Displacements and forces are converted to structural strain and pressure to quantify common spacer fabric and engineering metrics. In spacer fabric technologies, it's common to analyze the loading curve for the shape and amount of energy absorbed per unit volume (W), defined as the area under the pressure-strain loading curve. However, analysis of the unloading behavior is needed to understand the NiTi material impact within the spacer fabric structure. The dissipated energy per unit volume is defined as the area between the loading and unloading curve and offers insight into the forward and reverse martensitic transformations occurring in the NiTi coiled yarns during loading and unloading.
Lastly, dynamic mechanical testing was performed to isolate the vibration damping potential of NiTi coiled yarns. The damping performance of NiTi structures is known to be dependent on the prestrain, vibration amplitude, and vibration frequency. Dynamic testing consisted of loading the NiTi coiled yarn sample to a fixed prestrain, before cycling through an oscillation strain with a corresponding amplitude and frequency. Oscillatory testing is used to quantify tan delta (tan δ), where delta (δ) is the phase lag between input strain and resulting stress. The tan delta, also known as the loss tangent, is commonly used as a key metric to analyzing damping performance. Tan delta is commonly used as an efficiency metric but does not offer much insight into the amount of energy that is being absorbed and dissipated; instead, the loss modulus represents the amount of energy absorbed and dissipated as heat.
Spacer Fabric Compression Behavior
Typical compression behavior of a spacer fabric is split up in four distinct stages. The first is a lower sloped linear region where the loose outer layers being engaged. This is followed by a higher linearly sloped region which corresponds to compression and light buckling of the spacer yarns. The third stage corresponds to a nearly constant stress plateau which is a result of additional buckling, rotating, and shearing of the spacer yarn. This is followed by a steep increase in stress from the densification of the fabric which restricts any further movement of the spacer yarn.
The NiTi coiled yarn spacer fabric did not exhibit typical spacer fabric compression behavior; however, it did still exhibit three distinct stages of compression behavior.
Analysis of the unloading behavior and comparison with the martensitic loading/unloading offers insight into the NiTi superelastic impact within the spacer fabric.
One potential goal of integrating NiTi coiled yarns within a spacer fabric is to leverage both material and structural stress plateaus to improve spacer fabric performance. While the spacer fabric of the present example is dominated more by structural effects, other examples may leverage more of the active material behavior by increasing the spacer fabric thickness, and altering the spacer yarn pattern to induce more buckling, shearing, and rotation of the spacer yarn. Leveraging both a material and structural stress plateau could improve spacer fabric energy absorbing performance.
Spacer Fabric Dynamic Behavior
While quasi-static isothermal compression tests can give researchers a glimpse into the energy absorbing potential of a material or structure, dynamic oscillatory tests are more commonly used to characterize the damping performance. Six oscillatory tests with increasing oscillation frequencies and constant amplitudes are performed at five structural pre-strains to map the damping capability of the NiTi coiled yarn spacer fabric. It was found that the damping abilities were strongly correlated to the structural pre-strain applied to the spacer fabric, as illustrated in
To investigate the actual amount of energy dissipated by the SMA response, the loss modulus was calculated for each oscillatory test.
The following are some examples described herein.
Example 1: A shape memory coil includes a coiled shape memory yarn having a coil direction around a coil axis, wherein the coiled shape memory yarn comprises a plurality of microfilaments having a twist direction around a yarn axis, and wherein the plurality of microfilaments comprises a shape memory alloy.
Example 2: The shape memory coil of example 1, wherein the coil direction of the yarn and the twist direction of the plurality of microfilaments are different.
Example 3: The shape memory coil of example 1 or 2, wherein the coil direction of the yarn and the twist direction of the plurality of microfilaments are the same.
Example 4: The shape memory coil of any of examples 1 to 3, wherein the plurality of microfilaments includes between about 10 and about 1000 microfilaments.
Example 5: The shape memory coil of any of examples 1 to 4, wherein the shape memory alloy comprises at least one of a nickel-titanium alloy or a copper-zinc-aluminum alloy.
Example 6: The shape memory coil of any of examples 1 to 5, wherein the plurality of microfilaments has an average diameter less than about 10 micrometers.
Example 7: A method for manufacturing a shape memory coil includes coiling a shape memory yarn to form a coiled shape memory yarn that has a coil direction around a coil axis, wherein the coiled shape memory yarn comprises a plurality of microfilaments having a twist direction around a yarn axis, and wherein the plurality of microfilaments comprises a shape memory alloy.
Example 8: The method of example 7, wherein the plurality of microfilaments has an average diameter less than about 10 micrometers.
Example 9: The method of example 7 or 8, further comprising twisting the plurality of microfilaments to define the twist direction.
Example 10: The method of any of examples 7 to 9, wherein the shape memory yarn is coiled around a mandrel to define the coil direction, and wherein the coil direction of the yarn and the twist direction of the plurality of microfilaments are opposite.
Example 11: The method of example 10, further comprising heat treating the yarn to torque balance the yarn.
Example 12: The method of any of examples 7 to 11, wherein the yarn is coiled to create a torsional imbalance to define the coil direction, and wherein the coil direction of the yarn and the twist direction of the plurality of microfilaments are the same.
Example 13: The method of example 12, further comprising plying the shape memory yarn with one or more shape memory yarns to torque balance the shape memory yarn.
Example 14: A textile includes a plurality of interlocked tows, wherein at least a portion of the plurality of interlocked tows comprises a plurality of shape memory yarn structures, wherein each shape memory yarn structure comprises one or more shape memory yarns, wherein each shape memory yarn comprises a plurality of microfilaments having a twist direction around a yarn axis, and wherein the plurality of microfilaments comprises a shape memory alloy.
Example 15: The textile of example 14, wherein the plurality of microfilaments has an average diameter less than about 10 micrometers.
Example 16: The textile of example 14 or 15, wherein the plurality of shape memory yarn structures comprises: a first plurality of shape memory yarn structures oriented in a first direction; and a second plurality of shape memory yarn structures oriented in a second direction, different from the first direction.
Example 17: The textile of example 16, wherein the first plurality of shape memory yarn structures has a first elasticity, and wherein the second plurality of shape memory yarn structures has a second elasticity, different from the first elasticity.
Example 18: The textile of example 16, wherein the first plurality of shape memory yarn structures is configured to produce a first actuation force in response to a transformation temperature transition, and wherein the second plurality of shape memory yarn structures is configured to produce a second actuation force in response to a transformation temperature transition, different from the first actuation force.
Example 19: The textile of any of examples 14 to 18, wherein the plurality of shape memory yarn structures defines a plurality of shape memory coils, and wherein each shape memory yarn structure has a coil direction.
Example 20: The textile of any of examples 14 to 19, wherein each shape memory yarn structure comprises a plurality of yarns plied into the respective shape memory yarn structure.
Example 21: The textile of any of examples 14 to 20, wherein the plurality of shape memory yarn structures comprises at least one of woven tows or knitted loops.
Example 22: A system includes one or more shape memory yarn structures, wherein each shape memory yarn structure comprises one or more shape memory yarns, wherein each shape memory yarn comprises a plurality of microfilaments having a twist direction around a yarn axis, wherein the plurality of microfilaments comprises a shape memory alloy, and wherein the shape memory alloy is configured to undergo a phase transformation in response to heating above a transformation temperature; and a current source coupled to the one or more shape memory yarn structures, wherein the current source is configured to send an actuation signal to the one or more shape memory yarn structures to heat the one or more shape memory yarn structures above the transformation temperature of the shape memory alloy.
Example 23: The system of example 22, wherein the plurality of microfilaments has an average diameter less than about 10 micrometers.
Example 24: The system of example 22 or 23, wherein the one or more of shape memory yarn structures defines one or more shape memory coils, and wherein each shape memory yarn structure has a coil direction.
Various examples of the disclosure have been described. Any combination of the described systems, operations, or functions is contemplated. These and other examples are within the scope of the following claims.
This application is a PCT application claiming priority to U.S. Provisional Patent Application No. 63/077,350, filed Sep. 11, 2020, the entire contents of which is incorporated herein by reference.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/US2021/071423 | 9/10/2021 | WO |
| Number | Date | Country | |
|---|---|---|---|
| 63077350 | Sep 2020 | US |
