Design and Control of Reduced Power Actuation for Active-Contracting Orthostatic Intolerance Garments

Active-contracting fabrics are an emerging innovation that could revolutionize aerospace compression garment technology, notably orthostatic intolerance garments (OIG), by contracting on demand. Prior research has found that active-contracting fabrics, specifically weft knit garter fabric architectures constructed with shape memory alloy (SMA) filaments, can apply 2-54 mmHg on the body (single-layer construction) or 4-104 mmHg (double layer construction), depending on body radius. Prior garment prototyping and performance validation efforts have been conducted with commercially available Flexinol® wire with an actuation finish temperature of 90°C, a temperature that is not appropriate proximal to the human body. While other chemistries of SMA having lower actuation temperatures used for medical devices inside the human body (Tcore ≈ 37°C) are commonly available, SMA has not been optimized for actuation control against the human skin (TS ≈ 31°C). This research characterizes and validates a novel SMA material designed by Fort Wayne Metals specifically for actuation adjacent to the surface of the body. Through experimental temperature-force-displacement testing on both Dynalloy Flexinol® and Fort Wayne Metals straight SMA wire and SMA knitted actuator configurations, we present data that suggests (1) performance differences between low-temperature, nickel-rich SMA (Fort Wayne Metals) and high-temperature, titanium-rich SMA (Dynalloy Flexinol®) are negated by certain SMA knitted actuator structures, and (2) certain SMA knitted actuator configurations increase in force upon cool down, offering new concepts for SMA system actuation/control that minimize power consumption and waste heat. This manuscript presents experimental evidence for a future OIG that is donned in an oversized and compliant state, heated momentarily above ambient skin temperature to initiate actuation, and remain fully 'activated' once the actuation is complete upon equilibration with skin temperature. The result is an OIG that requires low-operating power that could be doffed through zipper releases and placed in a sub-zero chamber to return the structure to the 'off' state for reuse. Nomenclature Af = austenite finish temperature As = austenite start temperature í µí±‘ = wire diameter FA = force above the austenite finish temperature FM = force below the martensite finish temperature FPS = force at pseudo-skin temperature FS = force at skin temperature í µí±– í µí±˜ = knit index Mf = martensite finish temperature Ms = martensite start temperature OIG = orthostatic intolerance garment SMA = shape memory alloy TA = temperature above austenite finish temperature Tamb = ambient temperature Tcore = core body temperature TM = temperature below martensite finish temperature TPS = pseudo-skin temperature TS = skin temperature