3D-Printed Ready-To-Use Variable-Stiffness Structures

Various physical phenomena have been used to develop variable-stiffness (VS) materials and structures, but all have involved some elements of manual fabrication, which limits their applications. In contrast, here we show fully three-dimensional (3D) printed monolithic VS structures that can be used directly from the printer. Thermally responsive polylactic acid (PLA) and conductive graphene PLA (GPLA) are combined in-printer to deliver a new framework for the temperature-controlled 3D VS structures. The embedded GPLA acts as a heating element, and both it and the surrounding PLA can be transitioned from rigid to soft using simple Joule heating because of the glass transition behavior of PLA. The mechanical and electrical properties of printed composite VS structures are studied. The great potential of this technology is demonstrated in a prototype variable-stiffness orthotic for foot drop.