Micro-Robots with Fast and Reversible Actuation via Two-Photon Polymerization

The additive manufacturing and its application in micro-robotics have demonstrated the high potential for the fabrication of tiny smart devices.¹ These microscale tools can be used in surgeries to manipulate objects with precision and control while reducing damage and making patient recovery faster. The key element for the movements and actuation of such microsystems is stimuli-responsive material.<br/><br/>In this work, we present functional microgrippers fabricated by direct laser writing via two-photon polymerization (2PP). This technique allows the creation of complex 3D structures on a microscale by scanning a polymer hydrogel with the femtosecond laser, building the structure layer by layer.² By tuning the fabrication parameters such as slicing and hatching as well as laser power and scanning speed it is possible to adjust the mechanical and chemical properties for the desired application.<br/><br/>The actuation of the microgripper is controlled by pH-responsive polymer materials which react to the pH changes of the environment. Their shape-morphing feature is inspired by the artificial muscles which can contract and expand in response to pH changes.³ The microgripper is powered by this type of responsive polymers that can move its arms to reversibly close and open. The actuation takes a few seconds due to the minuature scale of these devices. By mimicking the blood vessels, we can implement such microgrippers as valves and use them in the pH sorter microsystem. The characterization of such microstructures was complemented by optical microscopy, atomic force microscopy (AFM), and scanning electron microscopy (SEM). We showcased the effect of structure design, hydrogel composition and writing parameters on the performance of microgrippers.<br/><br/>These types of micro-robots open new opportunities for minimally invasive surgeries, sensing and photonics. ⁴^,⁵^,⁶ We can incorporate multiple functionalities into a single microdevice by adapting the design and adjusting fabrication parameters. It offers a high degree of flexibility and presents a wide range of potential applications for 2PP-fabricated micro-robots.<br/><br/><b>References:</b><br/>(1) Ye, M.; Zhou, Y.; Zhao, H.; Wang, Z.; Nelson, B. J.; Wang, X. A Review of Soft Microrobots: Material, Fabrication, and Actuation. <i>Advanced Intelligent Systems </i><b>2023</b>, <i>n/a</i> (n/a), 2300311. DOI: https://doi.org/10.1002/aisy.202300311.<br/>(2) O'Halloran, S.; Pandit, A.; Heise, A.; Kellett, A. Two-Photon Polymerization: Fundamentals, Materials, and Chemical Modification Strategies. <i>Advanced Science </i><b>2023</b>, <i>10</i> (7), 2204072. DOI: https://doi.org/10.1002/advs.202204072.<br/>(3) Ma, Z.-C.; Zhang, Y.-L.; Han, B.; Hu, X.-Y.; Li, C.-H.; Chen, Q.-D.; Sun, H.-B. Femtosecond laser programmed artificial musculoskeletal systems. <i>Nature Communications </i><b>2020</b>, <i>11</i> (1), 4536. DOI: 10.1038/s41467-020-18117-0.<br/>(4) Kaufman, G.; Jimenez, J.; Bradshaw, A.; Radecka, A.; Gallegos, M.; Kaehr, B.; Golecki, H. A Stiff-Soft Composite Fabrication Strategy for Fiber Optic Tethered Microtools. <i>Advanced Materials Technologies </i><b>2023</b>, <i>8</i> (12), 2202034. DOI: https://doi.org/10.1002/admt.202202034.<br/>(5) Zhang, Y.; Wang, J.; Yu, H.; Zheng, J.; Zhao, X.; Guo, H.; Qiu, Y.; Wang, X.; Liu, L.; Li, W. J. A chemotactic microrobot with integrated iridescent surface for optical-tracking. <i>Chemical Engineering Journal </i><b>2023</b>, <i>472</i>, 144222. DOI: https://doi.org/10.1016/j.cej.2023.144222.<br/>(6) Liu, B.; Dong, B.; Xin, C.; Chen, C.; Zhang, L.; Wang, D.; Hu, Y.; Li, J.; Zhang, L.; Wu, D.; et al. 4D Direct Laser Writing of Submerged Structural Colors at the Microscale. <i>Small </i><b>2023</b>, <i>19</i> (2), 2204630. DOI: https://doi.org/10.1002/smll.202204630.