4D printing of trigger-free shape-memory hydrogels towards self-adaptive substrates for bioelectronics.

Shape-changing smart substrates are revolutionizing implantable bioelectronics by offering minimal invasiveness and self-adaptive geometric conformity. However, precise manipulation of shapeshifting, which is critical for the successful implantation, are hindered by triggering accessibility and cumbersome customization. Here we introduce a digital 4D printing technique for fabricating customized shape memory hydrogel substrates enabling trigger-free shapeshifting with predefined onset periods. By controlling spatio-temporal light exposure, pixelated polymer sheets with heterogeneous networks are obtained, yielding intricate 3D original shapes after monomer removal. Meanwhile, spatially controlled onset time of the programmable shape recovery is introduced, attributed to local variation of polymeric network structure and the corresponding thermally-induced phase separation. This approach allows the complex 3D objects to execute autonomous multi-stage shape recovery through a single-step digital exposure process, which is not offered by existing techniques. In addition, the material exhibits negligible inflammation proven by in-vivo testing of male mice experiments. Demonstrated with self-winding bioelectronics, this method advances time-dependent adaptability of implantable devices and expands the functional scope of bioelectronics.