Interfacial Reinforcement of a 3D-Printable Double Network Granular Hydrogel

Load-bearing soft materials that can be processed into 3D shapes gain increasing importance in the biomedical field. Granular hydrogels have the potential to be used as 3D-printable artificial tissues and their local composition can be tuned over micrometer-length scales. However, the development of mechanically strong granular hydrogels that can be used for load-bearing applications remains challenging. To overcome this challenge, a secondary hydrogel network that covalently crosslinks adjacent microgels can be introduced.^[1] However, the resulting double network granular hydrogel is relatively brittle because of the weak interparticle interactions and the lack of energy dissipation mechanisms.<br/>In this work, two types of oppositely charged polyelectrolyte microgels, one made of the negatively charged polyacrylic acid (PAA), the other made of the positively charged poly(3-Acrylamidopropyl)trimethylammonium chloride (PATC) are used to produce a double network granular hydrogel (DNGH). Due to the electrostatic interactions between the oppositely charged PAA and PATC microgels, the hydrogel exhibits values for the Young’s modulus and fracture energy that are similar to those of cartilage^[2,3] and skeletal muscles.^[4] We experimentally investigate the contribution of the interparticle reinforcement to the overall fracture energy of the hydrogel. Based on these studies, we propose an empirical model that takes into account the damage zone size, contact area, and adhesion energy. We demonstrate that this model, albeit being simple, can predict the mechanical properties of DNGHs with a reasonably high accuracy. Thanks to the interfacial reinforcement between PAA and PATC microgels, free-standing structures can be 3D-printed with high printing resolutions, providing new possibilities for designing strong and tough soft materials with versatile processability.<br/><br/><b>References</b><br/>[1] M. Hirsch, A. Charlet, E. Amstad, <i>Adv. Funct. Mater.</i> <b>2021</b>, <i>31</i>, 2005929.<br/>[2] M. V. Chin-Purcell, J. L. Lewis, <i>Journal of Biomechanical Engineering</i> <b>1996</b>, <i>118</i>, 545.<br/>[3] U. G. K. Wegst, M. F. Ashby, <i>Philosophical Magazine</i> <b>2004</b>, <i>84</i>, 2167.<br/>[4] D. Taylor, N. O’Mara, E. Ryan, M. Takaza, C. Simms, <i>Journal of the Mechanical Behavior of Biomedical Materials</i> <b>2012</b>, <i>6</i>, 139.