3D printable polyelectrolyte complex-integrated interpenetrating network hydrogels with customisable mechanical strength and pH-responsiveness Vaishali Pruthi 1 , Valerian Hirschberg 1 , Patrick Théato 1,2 1 Institute for Chemical Technology and Polymer Chemistry (ITCP), Karlsruhe Institute of Technology, Germany, 2 Soft Matter Synthesis Laboratory, Institute for Biological Interfaces III, Germany 3D-printed intelligent hydrogel materials are envisioned to infuse 3D materials with life. As a channel for creating devices rooted in functional materials, the potential of these smart hydrogels is immense. 1 Recently, the quest to 3D print interpenetrating network (IPN) hydrogels, especially for groundbreaking applications in biology and medicine, is gaining momentum. Among the extensive range of polymeric-based matrices, IPNs stand out due to their impressive stability and mechanical strength, primarily attributable to molecular reinforcement due to the interwoven networks of diverse polymers. 2-4 Our present work represents a notable breakthrough in this dynamic field. Herein, we designed a unique approach for 3D printing IPN hydrogels, ingeniously integrating the polyelectrolyte complex (PEC) of Hyaluronic acid (HA) and Chitosan (CS) with a photo-crosslinkable P(OEGMA- co -EGDEMA) polymer. Firstly, we strategically protected the carboxyl group of HA using a photo-labile ortho-nitrobenzyl group, ensuring the prevention of premature PEC formation in 3D ink preparation. Later, the magic unfolds under UV illumination using Digital Light Processing (DLP), where the photo-deprotection of the carboxyl group of HA and the photopolymerization of OEGMA crosslinked hydrogel occur simultaneously. Further, we successfully fabricated and optimized the photopolymer, semi-IPNs and IPNs for 3D printing. Using characterization techniques including NMR, IR, UV/Vis, TGA, DSC, SEM, compression and shear testing, we delved deep into the structural, morphological, and rheological attributes of the synthesized hydrogels. Additionally, we examined the swelling behaviour of these hydrogels in comparison and assessed their response to pH alterations. The composition of the 3D ink was then fine-tuned for further improvement, enabling the production of PEC-integrated IPN hydrogels with a spectrum of mechanical strength and elastic modulus ranging from 1 to 10 kPa. The resultant hydrogels boast remarkable flexibility, impressive compressive strength, and can endure substantial strain without breaking. Besides, they also showcase pH responsiveness and exceptional thermal stability. In essence, our approach is a symphony of nature and innovation, leveraging natural polyelectrolytes to refine 3D ink in a manner that is efficient, cost-effective, and reliable. This method could mark a pivotal advancement in the realm of 3D materials, unlocking numerous prospective applications from biomimetics and soft robotics to tissue engineering and drug-delivery devices. References 1. S. E. Bakarich, R. Gorkin, R. Gately, S. Naficy, M. in het Panhuis and G. M. Spinks, Additive Manufacturing , 2017, 14 , 24-30. 2. K. Bootsma, M. M. Fitzgerald, B. Free, E. Dimbath, J. Conjerti, G. Reese, D. Konkolewicz, J. A. Berberich and J. L. Sparks, J Mech Behav Biomed Mater , 2017, 70 , 84-94. 3. S. Suri and C. E. Schmidt, Acta Biomater , 2009, 5 , 2385-2397. 4. Y. Liu, Y. H. Hsu, A. P. Huang and S. H. Hsu, ACS Appl Mater Interfaces , 2020, 12 , 40108-40120.
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