Botanical-Inspired 4D Printing of Hydrogel at the Microscale

Botanical systems have evolved the intriguing ability to respond to diverse stimuli due to long-term survival competition. Mimicking these dynamic behaviors has greatly advanced the developments in wide fields ranging from soft robotics, precision sensors to drug delivery and biomedical devices. However, realization of stimuli-responsive components at the microscale with high response speed still remains a significant challenge. Herein, the miniature biomimetic 4D printing of pH-responsive hydrogel is reported in spatiotemporal domain by femtosecond laser direct writing. The dimension of the printed architectures is at the microscale (<10² µm) and the response speed is reduced down to subsecond level (<500 ms). Shape transformation with multiple degrees of freedom is accomplished by taking advantage of pH-triggered expansion, contraction, and torsion. Biomimetic complex shape-morphing is enabled by adopting flexible scanning strategies. In addition, application of this 4D-printed micro-architecture in selective micro-object trapping and releasing is demonstrated, showcasing its possibilities in micromanipulation, single-cell analysis, and drug delivery.