An auxetic insole design with reverse graded-stiffness to relieve detrimental tissue stresses under bony prominence of calcaneus in diabetic foot

To effectively mitigate detrimental tissue stresses of the diabetic foot for preventing ulceration, contemporary strategies frequently utilize pressure-relief insoles. In this study, we have developed an innovative enhanced pressure-relief insole that integrate auxetic structures with a reverse graded-stiffness property. We introduce a novel modification to the insole internal structure, exhibiting untraditional regional stiffness from the center to the periphery. We utilize a validated finite element (FE) heel model of a diabetic patient to evaluate the effectiveness of the insole, computing internal stress of the heel (peak stresses, total stress concentration exposure, pressure on the fat pad, and tensile stress on the skin) and insole deformation. In addition, we conduct in-vitro uniaxial compression and in-vivo biomechanical experiments to assess its effects in static and gait. The FE results showed a significant reduction in internal stress within high-risk ulcer areas of the heel, with peak internal stresses reduced to 232.9 kPa (without insole: 374.6 kPa), and notable changes in the deformation across the insole's coronal plane. Additionally, uniaxial tensile tests demonstrated optimal energy dissipation at 28.76%. During gait, the auxetic insole resulted in a 19.72% reduction in peak pressure and 15.37% reduction in peak pressures-time integral compared to the conventional insole. A novel insole with auxetic structure and reverse graded-stiffness appear to better relieve the internal loads, gait-related pressure as well as enhanced energy dissipation for the plantar soft tissue under bony prominence of calcaneus of human foot. This research also holds substantial promise for optimizing other pressure-relief orthotic devices.