Four-dimensional direct laser writing of reconfigurable compound micromachines

Reconfigurable micromachines that are highly conscious of changing environments have significant potential for use in biomedical applications, such as minimally invasive surgery, cell manipulation, and tissue engineering. Current nanofabrication approaches with sophisticated designs appear to enhance the controllability of shape transformations, such as bending, folding, and twisting, while minimizing the response time. However, the construction of three-dimensional (3D) structures at a small scale with a high shape-morphing freedom poses challenges because of the lack of applicable materials and effective fabrication techniques. Here, we develop an advanced four-dimensional microprinting strategy for constructing 3D-to-3D shape-morphing micromachines in a single-material-single-step mode. Using direct laser writing, heterogeneous stimulus-responsive hydrogels can be distributed spatially into arbitrary 3D shapes with sub-micrometer features. The material crosslinking densities, stiffnesses, and swelling/shrinking degrees can be modulated by programming the exposure dosage of femtosecond laser pulses and characterized to predict the shape-morphing behaviors via finite-element methods. With our proposed approach, complex 3D reconfigurable compound micro-machines with mechanical advantages, which exhibit an excellent deformation-amplifying effectiveness, can be constructed to achieve a rapid, precise, and reversible 3D-to-3D shape transformation in response to multiple external stimuli, and they emerge as promising smart and multifunctional micromachine candidates for various engineering applications.