Hydrogel based insoles for diabetic feet

Abstract

This research considered an extensive study of double network poly (acrylic acid) and single network polyacrylamide hydrogel as an insole material for the diabetic foot by testing on compression strength, stress relaxation, compression fatigue, shear stress, and shock absorption properties. The previous studies on hydrogel fatigue were mainly focused on fatigue fracture in tension and crack propagation. This study focuses on the mechanical behaviour of hydrogels under diabetic foot-specific loading conditions. The expected testing conditions include minimum 20 000 fatigue cycles under maximum compression stress of 250 kPa at a 200 mm/ min strain rate. The hydrogel synthesis and testing started with double network poly (acrylic acid) hydrogel. The developed double network poly (acrylic acid) hydrogel displayed fatigue properties up to 3000 loading cycles at maximum stress of 390±30 kPa. Further, maximum average shear stress and shear modulus of 80 kPa and 140 kPa respectively were observed at 84% strain before fracture. Developed Single network polyacrylamide hydrogel displayed good fatigue properties up to 13,000 loading cycles at maximum stress of 520±50 kPa and 200 mm/min crosshead speed. When the maximum stress condition was reduced to 350±50 kPa, the maximum number of loading-unloading cycles was increased up to 20 000 indicating a single network polyacrylamide hydrogel capable of withstanding more than 20 000 cycles at 250 kPa. Hydrogels showed superior recoverable and viscoelastic properties when compared with available insole materials. The developed finite element model was validated with pressure insole test data and used to investigate the pressure distribution properties and to optimize the thickness suitable for insole applications The additional properties of a hydrogel such as high thermal capacity and structural similarity to soft tissues are seen as added advantages when compared to other insole materials to prevent re-ulceration.