Spatiotemporal Control of Tendon Healing through Modular, Injectable Hydrogel Composites

Abstract

Biomaterials with the ability to regulate microenvironmental cues present a promising strategy for enhancing tendon repair, addressing issues such as aberrant scar formation, chronic pain, diminished function, and the risk of re-injury in adults. The underlying mechanisms of the natural repair process in tendons, particularly the maladaptive scar formation, pose a significant challenge to achieving full tendon regeneration. While adult tendons typically heal through scar formation, neonatal mouse tendons exhibit superior regenerative capacity. Emulating this regenerative response using biomaterials for precise control over cell recruitment, differentiation, and matrix remodeling after injury could pave the way for improved tendon regeneration in adults.

Among the promising biomaterials, fiber-reinforced hydrogel composites stand out for their ability to introduce fibrous topographical cues to the wound microenvironment. These composites can be localized to internal wound defects through minimally invasive administration. Composed of polymer chains crosslinked into a solid bulk by protease-responsive segments, these materials offer tunable mechanical properties and modular inclusion of biochemical moieties. Moreover, their scalability in polymer and peptide synthesis makes them attractive from a manufacturing perspective.

The thesis aims to enhance the therapeutic potential of synthetic hydrogels by employing a fibrous, composite material approach and temporal release of biomolecules. A synthetic, fiber-reinforced hydrogel composite was developed to investigate whether the recruitment of tendon progenitor cells could be improved through combined mechanical, topographical, and microparticle-delivered soluble cues. The study demonstrated enhanced recruitment of murine tendon progenitor cells into synthetic hydrogels, driven by fibrous topographical cues and microgel-delivered platelet-derived growth factor-BB.

The research further explored critical microenvironmental determinants of tenogenesis using 2D and 3D engineered culture platforms. TGF-β3 and Rho/Rho-kinase inhibition increased Scleraxis expression in murine tendon progenitor cells, with aligned fibrous topography showing unique pro-tenogenic effects in 2D cultures. In 3D settings, fiber alignment did not increase Scleraxis expression but influenced the deposition and organization of type I collagen, defining mechanical anisotropy and tissue function.

Additionally, the thesis investigated the influence of fibrous topography on the tenogenic vs. fibrochondrogenic fate switch. Cell-adhesive fibrous topography biased tendon progenitor cells toward a tenogenic fate, while Rac1 inhibition and cyclic strain favored a fibrochondrogenic phenotype. In an in situ model using transected murine Achilles tendons, fibers primarily affected tendon progenitor cell recruitment with minimal impact on fibrochondrogenic differentiation.

The overall contribution of this dissertation lies in the development of an injectable biomaterial that combines physical and soluble cues to temporally orchestrate crucial phases of tendon healing. This includes the recruitment of tendon progenitor cell populations and their differentiation toward tenogenesis, leading to the synthesis of de novo extracellular matrix with appropriate composition and organization. The research provides valuable insights into microenvironmental cues governing tenogenesis, essential for advancing biomaterial therapeutics targeting connective tissue regeneration.