Medical Compression Capabilities of Contractile SMA Knitted Actuator Fabrics

Medical compression is widely prescribed by physicians for a range of patient pathologies, including orthostatic intolerance (OI), orthostatic hypotension (OH), postural orthostatic tachycardia syndrome (POTS), deep vein thrombosis (DVT), lymphedema, and some forms of anxiety. Compression stockings are traditionally engineered with elastane yarns or pneumatic chambers. The shape memory behavior of certain shape memory alloys (SMA) offers an opportunity to reinvent medical compression therapy and enable low-profile, dynamic compression stockings designed as a low-power or no-power system, specifically if body heat is the designed stimuli. This work translates concept to device design by characterizing contractile SMA knitted actuator fabrics to determine the range of uniaxial actuation forces currently accomplished by this fabric technology. Through displacement-controlled, thermomechanical testing, a condition that simulates limb compression by fabric, we determined contractile SMA knitted actuators fabrics reach actuation forces up to 17 N. Forces are translated into pressures by pairing industry-standard quality and testing specifications for medical compression hosiery (RAL-GZ 387/1) with the Civilian American and European Surface Anthropometry Resource (CAESAR), an internationally recognized anthropometric database with population body girths. The analysis concludes that contractile SMA knitted actuator fabrics can apply 8-30 mmHg to the human body, depending on limb radii (i.e. 8 mmHg at mean hip circumference, CAESAR men; 30 mmHg at minimum leg circumference, CAESAR women). Furthermore, contractile forces scale through fabric thickness, implying a 2-layer fabric system could apply up to 15-65 mmHg. These findings offer exciting evidence to further develop contractile SMA knitted actuator fabrics for clinical purposes.