Photocurable and elastic polyurethane based on polyether glycol with adjustable hardness for 3D printing customized flatfoot orthosis

Orthopedic insoles is the most commonly used nonsurgical treatment method for the flatfoot. Polyurethane (PU) plays a crucial role in the manufacturing of orthopedic insoles due to its high wear resistance and elastic recovery. However, preparing orthopedic insoles with adjustable hardness, high-accuracy, and matches the plantar morphology is challenging. Herein, a liquid crystal display (LCD) three-dimensional (3D) printer was used to prepare the customized arch-support insoles based on photo-curable and elastic polyurethane acrylate (PUA) composite resins. Two kinds of photo-curable polyurethanes (DL1000-PUA and DL2000-PUA) were successfully synthesized, and a series of fast-photocuring polyurethane acrylate (PUA) composite resins for photo-polymerization 3D printing were developed. The effects of different acrylate monomers on the Shore hardness, viscosity, and mechanical properties of the PUA composite resins were evaluated. The PUA-3-1 composite resin exhibited low viscosity, optimal hardness, and mechanical properties. A deviation analysis was conducted to assess the accuracy of printed insole. Furthermore, the stress conditions of the PUA composite resin and ethylene vinyl acetate (EVA) under the weight load of healthy adults were compared by finite element analysis (FEA) simulation. The results demonstrated that the stress of the PUA composite resin and EVA were 0.152 MPa and 0.285 MPa, and displacement were 0.051 mm and 3.449 mm, respectively. These results indicate that 3D-printed arch-support insole based on photocurable PUA composite resin are high-accuracy, and can reduce plantar pressure and prevent insoles premature deformation, which show great potential in the physiotherapeutic intervention for foot disorders.