Design of a multistable finger prosthesis with programmable metamaterials

Shape change, as well as the ability to hold specific deformation, is a main requirement for finger prostheses. Currently, these functionalities are realized by screwed hinges with latching elements. To avoid the assembly process and increase the sustainability a non-assembly, 3D printable prosthesis built from one material would be beneficial. Programmable materials, consisting of structured monomaterials and overtaking several functions simultaneously, can be used to tackle this challenge. We present the design of a programmable material that combines a bending with a multistable structure. Through this combination, the material can reach a bending angle of 90° with stable shapes at 30° steps. The shape changes are activated by applying mechanical force and can be hold without energy consumption. We restricted ourselves on hyperelastic flexible base materials (thermoplastic copolyester, thermoplastic polyurethane) that can be processed by additive manufacturing (fused filament fabrication, selective laser sintering) to achieve the necessary deformations. The bending structure and multistable part were designed separately using parametric finite element studies. Finally, a hierarchically structured (non-assembly) demonstrator was constructed and produced by filament printing, showing the desired behavior with sixteen stable shapes. With this, we show the potential and challenges to use programmable materials in orthopedics and related fields.