Mussel-inspired tough and stiff injectable gels from inter-linked microgels

Injectable gels that restore the mechanical properties of damaged tissue can provide minimally-invasive solutions to chronic pain without surgery. Despite their potential, injectable gels have not reached the high mechanical performance of their non-injectable gel cousins. Cartilage injury is a major cause of disability worldwide with an economic burden in the US estimated as US$200B in 2020. Annually, 1.6 M patients undergo surgical procedures to repair tendons, ligaments and cartilage. In previous EPSRC-funded research we discovered a method for inter-linking sub-microscopic sponge-like polymer particles (microgels, MGs) under physiological conditions to form injectable gels termed doubly crosslinked microgels (DXMGs). Those DXMGs successfully augmented the mechanical properties of compressed intervertebral discs and are being taken to the clinic. In this study we aim to establish a new class of injectable DXMGs designed for the repair of load-bearing tissues that experience tension. These new DXMGs are designed to have toughness (tearing resistance) and stiffness (modulus) values that match those of non-injectable high-performance synthetic gels and also cartilage. Our new DXMGs will combine a loose permanently bonded network with a tight reversible (dynamic) network. The chemical groups used for the latter are inspired by mussel holdfasts. This study aims to investigate the principles underpinning the design of tough and stiff injectable DXMGs and also to obtain proof-of-concept data to enable their future development for cartilage, tendon, muscle and ligament repair.