Development and Characterization of Modular Elastic Switches for Sensing and Control of Active Compression Garments

Astronauts frequently suffer from orthostatic intolerance (OI) when returning to earth. Conventional wearable interventions for treating or preventing OI exhibit limited controllability (in elastic stockings) or limited mobility (in pneumatically inflatable garments). A new promising method to replace inflatable and elastic stockings for OI treatment is to implement dynamically controllable, conformal OI garments using integrated active materials such as shape memory alloys (SMAs). These garments constrict when thermally (or electrically) stimulated, resulting in a compressive force on the body when worn. This investigation builds on previous work in active compression garment development, introducing a novel feedback control system to provide constant garment tension without the need for precise, real-time pressure sensing or power control. This is accomplished using in-line tension switch mechanisms—switches that break the local actuator control circuit above a prescribed circumferential tension (which we define as the “critical tension”)—enabling passive feedback control of garment tension/pressure during use. A study was conducted to compare the functional performance (critical tension, hysteresis, reliability) of three switch architectures (referred to in this study as copper plate, spring, and reed switches). Critical tension was measured over multiple loading/displacement cycles (50 cycles at 5s per cycle, and 100 cycles at 35s per cycle), and three prototypes of each architecture were manufactured and tested. Two architectures—the copper plate and spring switch samples—showed promise in their performance (as measured by the reliability and repeatability of the measured critical tension over repeated loading cycles), though the switch behavior varied significantly between architectures and between samples. This approach to passively managing SMA-based contractile forces holds promise for any system that requires active tension control, including OI garments, as well as for advanced compression systems such as Mechanical Counter-Pressure (MCP) spacesuits.