Active-Contracting Fabrics for Wearable Compression Applications

Abstract

Compression garments are worn articles of clothing commonly used in medicine, astronautics, sports, and ready-to-wear fashion to provide a wide range of on-body pressures for purposes such as pressure therapy, enhanced mobility/performance, anatomical anchoring, or simple aesthetics. Current compression garment technologies are limited to pneumatic garment and undersized garments, each with their own challenges. Inflatable garments are dynamic, meaning pressures can be varied in quantity, duration, and location; however, these garments inhibit mobility and have a large mass, two factors which limit their use to stationary situations. Undersized garments are designed with stretch fabrics and/or cinching mechanisms, such as straps or lacing, to increase fabric tension around the body. Undersized compression garments are preferred when pressure is desired during daily and/or mobile activities because they are low-profile, do not inhibit mobility, and do not require a power source; however, the trade-off is lack of dynamic functionality. Active-contracting fabrics are an emerging area of research that could advance the capabilities of compression garment design by contracting on command. Shape memory alloys (SMA), for example, are active materials with shape memory properties and can be engineered to remember prescribed forms through an annealing process. Traditional weft knit architectures can be engineered to produce contraction if individual yarns incorporate SMA wire, causing large, dynamic displacements and contractive forces across the fabric surface that can be turned on/off or achieve various compression levels. This research addresses the compression garment technology gap by evaluating active-contracting fabrics, specifically contractile SMA knitted actuator fabrics, for on-body compression applications. Because the effectiveness of compression garments is dependent on the relationship between the garment and the body (i.e. fit), 50% of this work seeks to define the dimensions, range, and variability of the human body, while the remaining 50% seeks to quantify the performance of active-contracting fabrics. Specifically, this work (1) defines lower body static anthropometric variability through the development of a sizing system, (2) defines lower body dynamic anthropometric variability between different working positions to determine average dimensional fluctuation, (3) develops self-fitting, self-stiffening garments using active-contracting fabrics to address individual and population fit challenges, and (4) characterizes the actuation force of active-contracting fabrics for on-body compression applications. The outcomes of this manuscript are the following. (1) Lower body dimensional variability is vast. To accommodate 95% of the consumer population with a fitted, non-compliant lower leg garment would require hundreds of size categories, an approach that is not appropriate for the ready-to-wear market in the absence of mass-customization. (2) Lower body dimensional variability due to posture is minimal at the ankle and calf (max, 8%); however, changing dimensions at the knee and thigh (max 13% and 17%, respectively) may require special design consideration. (3) Active-contracting fabrics can be used in the design of self-fitting, self-stiffening garments that can accomplish actuation contraction up to 40%, a range that is suitable to reduce hundreds of sizes to a handful of sizes for a fitted, non-compliant, lower leg garment. (4) When displacement is fixed, or blocked, active-contracting fabrics can reach fabric tensions between 43 and 359 N/m. When wrapped around the body, these tensions translate to pressures ranging from 15-65 mmHg, depending on body radii. Additionally, these results revealed that increased SMA wire diameter and increased strain produce higher actuation forces. The results of this research endeavor conclude that active-contracting fabrics could be used in two distinct ways to accomplish the design of active-contracting compression garments. Self-fitting, self-stiffening fabrics could be actuated first to pull the garment close to the body and apply necessary strain to high-force compression fabrics. Once strained to their optimal actuation length, the high-force compression fabrics could apply the desired pressures. The results is a controllable, low-profile compression garment design that will be developed in future work.