*Article* **Four-Dimensional Stimuli-Responsive Hydrogels Micro-Structured via Femtosecond Laser Additive Manufacturing**

**Yufeng Tao 1,2,\* , Chengchangfeng Lu <sup>3</sup> , Chunsan Deng <sup>2</sup> , Jing Long <sup>2</sup> , Yunpeng Ren <sup>1</sup> , Zijie Dai 1,\*, Zhaopeng Tong <sup>1</sup> , Xuejiao Wang <sup>1</sup> , Shuai Meng <sup>1</sup> , Wenguang Zhang <sup>2</sup> , Yinuo Xu <sup>2</sup> and Linlin Zhou <sup>2</sup>**


**Abstract:** Rapid fabricating and harnessing stimuli-responsive behaviors of microscale bio-compatible hydrogels are of great interest to the emerging micro-mechanics, drug delivery, artificial scaffolds, nano-robotics, and lab chips. Herein, we demonstrate a novel femtosecond laser additive manufacturing process with smart materials for soft interactive hydrogel micro-machines. Bio-compatible hyaluronic acid methacryloyl was polymerized with hydrophilic diacrylate into an absorbent hydrogel matrix under a tight topological control through a 532 nm green femtosecond laser beam. The proposed hetero-scanning strategy modifies the hierarchical polymeric degrees inside the hydrogel matrix, leading to a controllable surface tension mismatch. Strikingly, these programmable stimuli-responsive matrices mechanized hydrogels into robotic applications at the micro/nanoscale (<300 <sup>×</sup> <sup>300</sup> <sup>×</sup> <sup>100</sup> <sup>µ</sup>m<sup>3</sup> ). Reverse high-freedom shape mutations of diversified microstructures were created from simple initial shapes and identified without evident fatigue. We further confirmed the biocompatibility, cell adhesion, and tunable mechanics of the as-prepared hydrogels. Benefiting from the high-efficiency two-photon polymerization (TPP), nanometer feature size (<200 nm), and flexible digitalized modeling technique, many more micro/nanoscale hydrogel robots or machines have become obtainable in respect of future interdisciplinary applications.

**Keywords:** femtosecond laser; additive manufacturing; hyaluronic acid methacryloyl; polyethylene glycol diacrylate; stimuli-responsiveness
