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 3 TOWARD INTEGRATEDWEARABLE DEVICES: SHAPEMEMORY ALLOY TEXTILES Shape memory alloy textiles provide complex deformation and force profiles for novel wearable applications. Kevin Eschen* and Julianna Abel* University of Minnesota, Minneapolis M arket projections for medical wearables and wear- able technologies predict compound annual growth rates of over 25% and growth to a global market size of US$82 billion until 2023 [1-2] . While current wearable devices primarily rely on the attachment of sensors, actuators, or en- ergy harvesters to traditional, passive textiles, the next gen- eration of wearable devices will introduce fully integrated wearable capabilities provided by the filament and textile ar- chitecture itself [3] . The multifunctional filament, e.g., shape memory alloys (SMAs), electroactive polymers, optoelectronic fibers and piezoelectrics, as well as the textile architecture, e.g., knit, braid, or weave, provide a variety of design approaches and variables for multifunctional textiles to accomplish desired integrated wearable properties. Among the many options for material and textile architecture combinations, the intrinsic multifunctional capabilities of SMA knitted textiles are highly promising for wearable and medical wearable technologies. SMA knitted textiles excel at traditional mechanical perfor- mance metrics, are biocompatible, and provide a wide range of thermo-mechanical operationmodes. *Member of ASM International SMA TEXTILE PRINCIPLES The knitted textile architecture amplifies and directs the NiTi-based SMA material behavior to accomplish three-di- mensional deformations, large and distributed forces and dis- placements, and variable stiffness in a spatial, flexible fabric [4] . An SMA monofilament fiber or yarns spun from SMA microfil- aments [5] (Fig. 1a) are knitted into an interlocking network of loops that forma textilewithadefinednumber of rows/courses and columns/wales (Fig. 1b). The direction in which loops are pulled through their adjacent loop, from behind or the front, is the defining feature for the creation of knit patterns. Loops that interlace frombehind are called purl loops, whereas those that interlace from the front are called knit loops. Different tex- tile deformations—rolling, contraction, or corrugation—can be thermally induced by designing the knit pattern as the NiTi material partially transforms between its martensitic and aus- tenitic phases. As the contractile SMA knitted actuator produces linear contractions and macroscopically planar deformations, and because of its intriguing performance combining large actua- tion forces and displacements, it can be exploited as a gateway Fig. 1 — (a) The active fiber of SMA textiles can be a monofilament, a single-ply yarn spun from microfilaments, and multi-ply yarns. (b) The contractile knitted textile consists of interlacing courses of knit and purl loops. The loop geometry is defined by the loop enclosed area (A l ) and the wire diameter (d). (c) Force-knit length profiles in the actuated (T > A f ) and relaxed (T < M f ) states. The actuation contractions are defined as the engineering strain definition between the two profiles. Large knit indices produce higher actuation contractions. (a) (b) (c) 9 FEATURE