Orthogonally Crosslinked Hydrogels to Improve Mechanical Strength and Broaden Applications of Thermo-Responsive Hydrogels

Injectable hydrogels are highly attractive due to their minimally invasive administration to the body. Our group has previously used polyurethane thermogels as well as its component micelles in tissue engineering and drug delivery applications [1-3]. However, conventional injectable hydrogels normally possess poor mechanical properties, and deform and break under repeated loading [4,5]. To improve the mechanical strength and broaden the application of the polyurethane thermogels, we have utilized two different strategies leveraging upon orthogonal crosslinks.<br/>Firstly, we prepared thermogelling polyurethane diacrylate (EPC-DA) hydrogels which are injectable and can be orthogonally photocrosslinked into fatigue-resistant implants. The thermogelation process occurs due to hierarchical physical self-assembly from micelles into hydrogels, and hence thermogels can serve as a dynamic source of polymeric micelles for further photocrosslinking [6].The hybrid-crosslinked poly(PEG/PPG/PCL urethane) diacrylate (EPC-DA) hydrogel has a dissipative micellar network and a secondary covalent network typical to strong hydrogels. The mechanical properties can be tuned by changing photocrosslinking conditions, and the hybrid-crosslinked EPC-DA hydrogels exhibited high stability and sustained release properties. In contrast to common injectable hydrogels, EPC-DA hydrogels exhibited excellent anti-fatigue properties with &gt;90% recovery during cyclic compression tests, and showed shape stability after application of force and immersion in aqueous buffer for 21 days. The EPC-DA hydrogel formed a shape stable hydrogel depot in an ex vivo porcine skin model, with establishment of a temporary soft gel before in situ fixing by UV crosslinking.<br/>Secondly, we covalently conjugated sodium alginate onto thermoresponsive polyurethanes, leveraging upon the ability of alginate to gelate with cations. We prepared hybrid polymers (EPC-Alg) that are responsive to both temperature and Ca²⁺, and can form orthogonally crosslinked hydrogels which are non-toxic to cells. Higher alginate fractions increased the hydrophilicity and Ca²⁺ response of the EPC-Alg hydrogel, enabling tunable modulation of the gel stiffness and gelation temperature. The hydrogels could also encapsulate cell spheroids with high cell viability, demonstrating its feasibility towards 3D cell encapsulation in cell-based biomedical applications such as cell encapsulation and cell therapy.<br/>In summary, we have demonstrated two differing strategies to strengthen thermo-responsive hydrogels for application as an anti-fatigue implant and for cell encapsulation.<br/><br/>References<br/>1. Zhao, X.; Seah, I.; <u>Xue, K</u>.; Wong, W.; Tan, Q. S. W.; ...; Su, X.; Loh, X. J., Antiangiogenic Nanomicelles for the Topical Delivery of Aflibercept to Treat Retinal Neovascular Disease. Advanced Materials 2021, n/a (n/a), 2108360.<br/>2. <u>Xue, K</u>.; Liu, Z.; Jiang, L.; Kai, D.; Li, Z.; Su, X.; Loh, X. J., A new highly transparent injectable PHA-based thermogelling vitreous substitute. Biomaterials Science 2020, 8 (3), 926-936.<br/>3. B.H. Parikh, Z. Liu, P. Blakeley, Q. Lin, M. Singh, J.Y. Ong, K.H. Ho, J.W. Lai, H. Bogireddi, K.C. Tran, J.Y.C. Lim, <u>K. Xue</u>, A. Al-Mubaarak,…, X.J. Loh, X. Su, A bio-functional polymer that prevents retinal scarring through modulation of NRF2 signalling pathway, Nature Communications 13(1) (2022) 2796.<br/>4. Zhang Yu, S.; Khademhosseini, A., Advances in engineering hydrogels. Science 2017, 356 (6337), eaaf3627.<br/>5. <u>Xue, K</u>.; Wang, X.; Yong, P. W.; Young, D. J.; Wu, Y.-L.; Li, Z.; Loh, X. J., Hydrogels as Emerging Materials for Translational Biomedicine. Advanced Therapeutics 2019, 2 (1), 1800088.<br/>6. Zhang, K., <u>K. Xue*</u>, and X.J. Loh, Thermo-Responsive Hydrogels: From Recent Progress to Biomedical Applications. Gels, 2021. 7(3): p. 77. *- co-corresponding