The Living Artificial Muscle: Design and Development of a Light Switchable Biohybrid Gel

During the last decade, there has been a surge in scientific breakthroughs related to hydrogel-based active systems with electrical, thermal, or pH-sensing capabilities [1], [2]. However, these hydrogels are subject to direct contact with stimuli, drastic environmental changes, or exhibit low biocompatibility. Light-responsive systems offer the advantage of remote activation, wavelength selectivity, and localized actuation.[3] Previously, azo-moieties have been used due to their photo-isomerizable properties, yet harnessing the molecular folding to create tough, photo-responsive hydrogels with appreciable actuation strain is still a significant challenge.[4]<br/>Herein present the design and facile fabrication process of a hierarchical material based on embedded, physically crosslinked azo-nanoclusters within a polyurethane scaffold. Due to its viscous nature, the material is suitable for casting or 3D printing. Furthermore, their surface can be functionalized to host living cells, resulting in a light-responsive active hydrogel, which exhibits mechanical properties, like those of native biological tissues, and high biocompatibility.<br/>Its toughness allows the gel to be strained up 400% and yields the biologically relevant range of 10-17% contractile strain upon light-switching. During the switchable ON state (λ = 405 ± 5 nm) these photochromic nanoclusters fold, changing the local dielectric environment. The isomerization increases the overall material’s hydrophobicity and leads to a decreased water retention capacity, contracting up to 17%. This process is fully reversible on-demand by irradiation, the OFF state (λ = 525 ± 5 nm), where the gels scavenge water from the environment and recover by swelling to its original dimensions.<br/>We have demonstrated the application of this remotely controlled system within the frame of biohybrid soft robotics, where the hierarchical cooperation between the inert and living components contributed to its behavior as a self-reinforcing material. This reversible contraction process has been used to evaluate mechanotransduction cues within a skeletal myoblast cell line (C2C12). The hydrogel protects the hosted cells from blue light irradiation, triggering localized contraction and inducing cellular fusion to form myotubes.<br/>The system evolves with every ON-OFF cycle, beginning as a cell-laden scaffold that strengthens as the individual cells become muscle.<br/>[1] M. C. Koetting, J. T. Peters, S. D. Steichen, and N. A. Peppas, “Stimulus-responsive hydrogels: Theory, modern advances, and applications,” <i>Materials Science and Engineering R: Reports</i>, vol. 93. Elsevier, pp. 1–49, 01-Jul-2015.<br/>[2] G. Gerlach, M. Guenther, and T. Hartling, “Hydrogel-Based Chemical and Biochemical Sensors - A Review and Tutorial Paper,” <i>IEEE Sensors Journal</i>, vol. 21, no. 11. Institute of Electrical and Electronics Engineers Inc., pp. 12798–12807, 01-Jun-2021.<br/>[3] M. P. M. Dicker, “Light and Chemistry Applied to the Control of Smart Materials and Structures,” University of Bristol, 2016.<br/>[4] Y. Hu <i>et al.</i>, “Photoactuators for Direct Optical-to-Mechanical Energy Conversion: From Nanocomponent Assembly to Macroscopic Deformation,” <i>Adv. Mater.</i>, vol. 28, no. 47, pp. 10548–10556, Dec. 2016.