**4. Conclusions**

In this study, we succeeded in developing a composite hydrogel material sensitive to a water/light/heat environment with a 532 nm femtosecond laser TPP. Compared to the mainstream optics/electron beam mask-projected stereolithography, the proposed two-photon polymerization held several advantages. An ultrafine feature size was obtained by staking the nanoscale voxel of the TPP system. The conventional macroscopic signal-triggered patterns or structures were miniaturized to a three-dimensional micron/nanoscale. The nonlinear characteristics of the fabrication processes still offered a sub-micron writing resolution, which is of great interest to micron-robotics, nano-drivers, and wearable sensors. Meanwhile, the stimuli-responsive photoresist contained no metal or alloy to improve biocompatibility. The controllable behaviors of the micro/nanostructures were being fatigue-free, environment-inspired, and quickly responsive, promising broad applications in micron actuators, sensors, micro-robotics, and biomimetic fields.

**Supplementary Materials:** The following supporting information can be downloaded at: https: //www.mdpi.com/article/10.3390/mi13010032/s1, Figure S1: the fluorescence, phase, and merged images of our fabricated hydrogels loading with cells, Figure S2: the summarized cell viability of asprepared hydrogels in Figure S1, Figure S3: Molecular structure of 2-hydroxy-2-methylpropiophenone used as photon initiator, Figure S4: dimension measurement on the responsive hydrogels before and after saturated swelling using the Nanomeasurer software 1.2, Figure S5: the optical intensity distribution of laser voxel at varied focusing positions, Figure S6: height measurement using advanced laser confocal microscopy, Figure S7: mechanics test platform used for determining the mechanical properties of our hydrogels, Figure S8: compressive and tensile test of micro-probe penetrating or pulling out of as-prepared square hydrogel, Video S1: 532nm laser beam scanning in photoresist, Video S2: Swelled microlenses by dropping water, Video S3: Single swelled microlens, Video S4: pH responsiveness, Video S5: Shrinkage of hydrogel, Video S6: Leg moving of a spider-shaped hydrogel by laser focus, Video S7: No obvious motion when laser scans spider in air, Video S8: Tail swelling of a tadpole-shaped hydrogel, Video S9: Tail shrinkage of a tadpole-shaped hydrogel.

**Author Contributions:** Conceptualization, Y.T.; methodology, C.L. and C.D.; validation, J.L. and S.M; investigation, W.Z. and L.Z.; resources, Z.D. and Z.T.; writing—original draft preparation, Y.X. and Y.T.; writing—review and editing, X.W. and Y.R. Validation: S.M.; Writing-original draft preparation: Y.X. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was financially supported by the National Key R&D Program of China (SQ2018YFB110138), the National Science Youth Fund of China (61805094), the National Natural Science Foundation of China (61774067), the National Science Foundation (CMMI 1265122), and the Fundamental Research Funds for the Central Universities (HUST:2018KFYXKJC027), China Postdoctoral Science Foundation (2017M622417).

**Data Availability Statement:** Data is contained within the article or Supplementary Materials.

**Acknowledgments:** The authors gratefully acknowledge Xiong Wei from Huazhong University of Science and Technology and Lu Yongfeng from University of Nebraska-Lincoln for providing the optical system and constructive instructions.

**Conflicts of Interest:** The authors declare no conflict of interest.
