April_AMP_Digital

A D V A N C E D M A T E R I A L S & P R O C E S S E S | A P R I L 2 0 2 0 6 5 NOVEL APPLICATIONS Based on the recent advancements in comprehending the SMA knitted textile mechanisms, first applications have been proposed and realized to utilize the large actuation con- tractions and force-generation of lightweight and low-profile SMA textiles. Dynamic self-fitting and compression to unique topog- raphies, e.g., the human body, can be accomplished through integrated wearable design with SMA knitted textiles for soft wearable robotics, continuous health monitoring, or haptic feedback applications. Self-fitting is hereby defined as the transition from a large, oversized sleeve to a topography that fully-conforms to the body without an external power supply. Figure 2b presents a self-fitting sleeve for the lower limb in its oversized state. New Ni-rich SMA material compositions and post-processing procedures developed by Fort Wayne Metals enable the skin-temperature induced phase transformation from the relaxed to the actuated SMA knitted textile state [9] . Figure 2c exemplifies the operation of the self-fitting garment. The SMA knitted textile is donned in its oversized martensitic state (1) and pulled over the limb (2). Upon release of the don- ning forces, the garment relaxes on the human body (3). The garment temperature increases to the wearer’s skin tempera- ture and the SMA knitted textile actuates and conforms to the human body (4). Through textile pattern design, local actua- tionmagnitudes andmodes can be employed to fully conform to highly concave and convex parts of the body, e.g., the knee. Consequently, uniform fitting and compressive pressures can be accomplished using SMA knitted textiles. ON THE HORIZON As extensive fundamental research and proof-of-con- cept application designs render this technology attractive for wearables, efforts to further improve SMA knitted textiles are continuously underway. New processing capabilities of Fort Wayne Metals enable the production of ultra-fine NiTi fibers with average diameters of 3-15 μm. Functionalized yarns can be spun from ultra-fine NiTi fibers and present an alternative to monofilament SMA textiles with potential for improved wearability, heat transfer, manufacturability, and lifetime performance. The NiTi yarn is spun with defined twists per inch, which affects the NiTi yarn performance. Figure 3a compares NiTi monofilaments with a single and a plied yarn as obtained in SEM microscopy. Yarns with low twist values act similar to monofilaments in parallel, whereas highly twisted yarns have a larger variation of the single fibers’ stress-strain state due to the complex superpo- sition of bending, torsional, and tensile strains which provides additional parameters for the wearable textile design. When knitted actuators are made of functionalized SMA yarns, they maintain their active capabilities while resembling, in aesthet- ic and haptic experience, the properties of traditional knitted textiles. Further investigations of more complex knit patterns and torsional control have shown first successes in creating ac- tive auxetic and shearing textile properties [10] . SMA knitted textiles capture a valuable actuator space as they provide large actuation strains and specific actuation stresses ( σ ρ -1 ) due to excellent lightweight properties (Fig. 3b). Further improvements of the knitted loop geometry and ther- mal post-processing, enabledbymicroscaleanalysis andphys- ics-based modeling, will widen the ranges of actuation strains and specific actuation stresses. Besides actuator applications, SMA knitted textiles can enable integrated sensing and ener- gy harvesting/absorption based on established research for other SMA geometries. Additionally, the development of yarns and novel knit patterns will improve wearability, actuation fre- (a) (b) Fig. 3— (a) SMAmicrofilaments can be spun into single- andmulti-plied yarns, whichwhen knit, resemble the aesthetic and haptic experience of traditional textiles while retaining active capabilities. (b) Shape memory alloy knitted textiles occupy a valuable actuator design space with large actuation strains and specific actuation stresses due to their lightweight properties. Comparative actuator performance values extracted from literature [11] . 1 1 FEATURE