Cartilage tissues perform critical roles in load bearing and distribution as well as motion. Repair of these tissues, which are hydrated and avascular, has remained limited and relies extensively on grafting procedures. Synthetic replacements, such as artificial intervertebral discs (IVDs) and focal resurfacing devices, have emerged but suffer from mechanical mismatch and inadequate hydration. As an alternative, we propose multi-network hydrogels as synthetic grafting materials. An intriguing method to increase the modulus and strength of multi-network hydrogels is through secondary interactions (e.g. electrostatic or hydrophobic) among – and between – networks. Previously, we have shown the ability of poly(2-acrylamido-2-methylpropane sulfonic acid)/ poly(N-isopropylacrylamide-co-acrylamide) (PAMPS/P(NIPAAm-co-AAm)) double network (DN) hydrogels to achieve an articular cartilage-like modulus and strength while maintaining a high water content, wherein hydrophobic interactions of PNIPAAm (forming physical crosslinks) increased the modulus. While mimetic of articular cartilage other cartilaginous tissues have a higher modulus (e.g. IVDs: 3+ MPa, or tracheal cartilage: 2+ MPa). To fabricate a material with these properties, this work investigates physical internetwork crosslinks to increase the modulus without inducing brittleness or decreasing water content. A poly((3-acrylamidopropyl)trimethylammonium chloride (PAPTAC) third network was added to PAMPS/P(NIPAAm-co-AAm) hydrogels forming triple network (TN) hydrogels. This cationic third network allowed internetwork crosslinks to form with the anionic first network. Leveraging these crosslinks, PAMPS/P(NIPAAm-co-AAm)/PAPTAC TN hydrogels achieved an unprecedented modulus (~3 MPa, compressive & tensile), strength (~25 MPa), and water content (~80 %). Accordingly, these unique properties expand the utility of hydrogels and accentuate their ability to serve as synthetic cartilage replacements throughout the human body.

Ultra-stiff and strong triple network hydrogels for cartilage repair

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ACS Fall 2022

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