Hybrid direct ink writing/embedded three-dimensional printing of smart hinge from shape memory polymer

Abstract

Three-dimensional (3D) printing technology has been widely developed and utilized to make diverse parts from shape memory polymers (SMPs). However, it is still challenging to fabricate functional devices with both complex geometries and embedded circuits using current 3D printing strategies. The objective of this study is to propose a hybrid direct ink writing (DIW)/embedded 3D printing (e-3DP) method, in which direct ink writing is used to print a self-supporting ink into a complex 3D structure at an uncured state while e-3DP is then applied to deposit a conductive ink within the structure. After crosslinking, the device at a cured state with embedded circuits can be formed in one step. This hybrid 3D printing method can only be realized by using ink materials with required rheological properties. Thus, how to develop the self-supporting ink is mainly discussed. Herein, a shape memory photocurable resin, aliphatic urethane diacrylate (AUD), and its chain builder, tert-butyl acrylate (tBA), are selected as the main components of the polymer precursor and fumed silica is used to tune the rheology for printing purposes. It is found that the tBA/AUD ratio affects the rheological properties, dry-out process, filament geometries, and self-supporting capability significantly. For demonstration, a smart hinge with an embedded circuit as strain sensor is successfully printed, which can program and recover its shape by controlling the ambient temperatures, validating the effectiveness of the proposed method for creating functional devices.