Holschuh, Brad

Abstract On-body compressive therapy is an essential treatment for a variety of health conditions. Traditional compression methods like pneumatics and knit stockings offer quality compression, but present issues with user experience (donning/doffing challenges or bulk/mass issues). This paper covers two studies related to active compression stockings, which utilize knitted shape memory alloy (SMA) wire to produce dynamic compression forces on the body. When these SMA knit panels are heated with an at-home heating pad, the knit contracts and creates dynamic compression. The first study characterizes the forces produced on the body by a new iteration of an SMA knit stocking. The second study compares two versions of the SMA knit garment with a commercially available knit compression garment. The results show that SMA knit compression garments are able to produce medical grade compression on the body, but that sizing and material properties can greatly impact the forces produced. In a comparison of different compression stockings, the user experience of the SMA compression garment was preferred over the alternative garments. Overall, the potential of SMA knit compression is further highlighted, with a clear need for further development to ensure consistent medical grade compression.

Abstract Advancements in the design of knitted Shape Memory Alloy (SMA) actuators have produced highly functional and easy-to-wear advanced textiles with variable fit/stiffness properties. Annealing SMA at high temperatures can alter their thermomechanical properties; however, heat treatments of SMA knitted actuators have yet to be systematically studied. This study investigates the relationship between annealing temperatures and the resultant SMA knit thermomechanical behavior (maximum force and actuator transformation temperature). Twelve SMA knitted samples were manufactured using Dynalloy Flexinol® 70°C (0.012″). Nine samples were annealed at three different temperatures (350°C, 450°C, 550°C), with the remaining samples acting as controls. The blocked force mechanical performance was characterized by varying the thermal load across various applied structural strains. The samples were thermally cycled between 20°C and 90°C at each strain level. The results indicate that samples annealed while stretched along the knit-course direction enabled large-force actuation at smaller displacements than the control samples. Annealed samples showed higher peak force than control samples as annealing temperature increased until the 450°C condition; however, the 550° samples showed a dip in force. The actuator transformation temperatures showed reduced Austenite start (As) temperature compared to the control sample, enabling actuation at a lower temperature while also reducing the Martensite Finish (Mf) temperatures. This work allows for greater tuning of knitted SMA actuators to meet the needs of researchers and product developers.

AbstractNew medical compression technologies that are simultaneously low‐profile, facile to don, and dynamic—applying medical compression only when needed—can expand the use of wearable compression, increase patient compliance, and lead to better medical outcomes. Dynamic and conformal wearable compression devices are presented that can be donned in a low‐stiffness state and transition into a high‐stiffness and, consequently, high‐compression state, on‐demand. These devices are enabled by active textiles developed from custom NiTi filaments that remain inactive at room temperature and accomplish actuation proximal to the human body surface. Further, these compression devices exploit NiTi material hysteresis to sustain a high‐compression state post‐heating and upon equilibrium with the body surface temperature for thermally‐comfortable, on‐body performance. Two case study examples—1) a consumer medical compression device and 2) a custom astronaut compression device—demonstrate the generalizability and flexibility of the engineering and design methods to develop a range of dynamic, tunable, and conformal compression devices with different goals and requirements. Further, this work demonstrates a roadmap for developing wearable systems that can accommodate a range of users without sacrificing system performance. This research opens doors for new NiTi‐based medical and consumer applications that interface with the body surface.

Medical compression garments are used to treat lymphatic disorders or conditions of poor venous return (e.g. orthostatic intolerance). Conventional compression garments, namely elastic compression sleeves and inflatable compression systems, may aid in relief of these conditions but are also limited in usability. Fixed levels of compression in elastic materials may induce challenges in donning/doffing, complicating patient compliance [1]. Compression levels in inflatable systems are customizable, but these garments are also bulky and require a tethered inflation source [2]. We are interested in developing easy-to-don/-doff compression garments using shape memory alloy (SMA) coil actuators that contract with heat from an applied current which can be wrapped around the body to apply compressive forces. More specifically, we use a spring coil formation as described by Holschuh et al. [3]. Further developed in the Wearable Technology Lab at the University of Minnesota, our current SMA compression garment (SMA-CG) design improves upon the work of Duvall et al. [4] presented in previous year’s (2017) Design of Medical Devices Conference, representing a year’s evolution in active compression garment design that integrate actuators made with SMA.

Current compression garments are often made from a spandex-type elastic material with static levels of compression and can become uncomfortable and difficult to don/doff [1]. This limits their usability, especially for unhealthy or aging populations. The only current alternative to elastic compression stockings are inflatable compression sleeves that are controllable, but highly immobile and must be tethered to an inflation source [2]. Neither design offers a solution that is simultaneously low profile, mobile, and controllable. Here we present the design and development of compression garments with embedded shape-changing materials that can produce controllable compression without the need for a bulky inflation system. This active materials approach enables dynamic control over the degree, timing and location of compression, and allows for graded, synchronized, pulsed, and peristaltic compression patterns, which provide the medical benefit of moving fluid in the body [2]. Such a design combines the best features of both elastic and inflatable compression garments: a slim, low-profile form factor that is easy to don/doff and provides dynamic control. Shape memory alloy (SMA) coil actuators, as described by Holschuh et al., [3] have the ability to apply compressive forces to the body when paired with passive textiles and wrapped circumferentially around the body. These actuators are engineered to contract when heated, creating controllable forces and displacements that are modulated through an applied current. SMA compression garments (SMA-CG) have important applications, from consumer uses to clinical interventions, including: augmenting venous return for conditions of orthostatic intolerance (e.g., postural orthostatic tachycardia syndrome (POTS)); cardiac rehabilitation in heart failure patients; lymphedema venous insufficiency; reducing deep vein thrombosis (DVT) risk; sports performance; and countermeasures for flight or space flight. While the potential uses for this technology are broad, the basic design is similar across many conditions. Key research areas include: 1) identifying and addressing design considerations relevant to prototype development of SMA-CG; 2) determining the compression thresholds needed to dynamically oppose orthostatic changes; and 3) evaluating the effectiveness of the prototypes for augmented venous return by synchronizing compression during cardiac diastole. Here, we focus on the first question: design of SMA-CG prototypes.