Biomaterial Design for Functional Tissue Regeneration in Pro-Fibrotic Injury Environments

Biomaterials, such as hydrogels, are promising tools in regenerative medicine to restore damaged tissues. However, their widespread clinical adoption has been impeded by an immune-mediated process called the foreign body reaction (FBR). Upon implantation, immune cells surround the biomaterial and form a dense, fibrotic capsule to isolate it from native tissue, resulting in poor tissue integration and regeneration. The challenges presented by the FBR are further compounded in pro-fibrotic injury environments, such as those created by severe trauma. The healing process after sustaining an injury is highly regulated, starting with the recruitment of immune cells to clear debris, followed by fibroblast deposition of extracellular matrix (ECM) to restore tissue integrity, and ending with the formation of new tissue. However, when this process is disrupted by prolonged inflammation, the tissue becomes fibrotic. The formation of scar tissue hinders regeneration and is an especially important obstacle to regeneration in tissues with dynamic environments due to the upregulation of fibroblasts by mechanical stimulation. Alterations to tissue stiffness and structure caused by fibrosis can significantly impair the overall function of these tissues, leading to a diminished quality of life for the patient.

Microporous Annealed Particle (MAP) hydrogel is a new type of biomaterial with great potential for clinical translation. It is composed of an injectable slurry of hydrogel microspheres that become covalently bonded, or “annealed”, in situ to form a porous scaffold in the injury. MAP hydrogel enables rapid cellular infiltration and significantly diminishes the FBR through porosity-mediated immunomodulation. In this dissertation, we present the use of MAP scaffolds in two pro-fibrotic clinical applications: volumetric muscle loss (VML) and posterior glottic stenosis (PGS).

Volumetric muscle loss is caused by severe traumatic injuries to skeletal muscle and is characterized by the irreversible loss of contractile tissue and permanent functional deficits. VML injuries cannot be healed by endogenous mechanisms and are exceptionally difficult to treat in the clinic due to the excessive upregulation of the inflammatory response, which leads to fibrosis, denervation of muscle fibers, and impaired regeneration. These injuries lead to long- term disability, and current strategies (e.g., muscle grafts) have proven ineffective. Using a rodent model of VML in the tibialis anterior, we investigated MAP hydrogel as a therapeutic to improve muscle regeneration in VML injuries, specifically focusing on the effects of cell-scale porosity compared to bulk (i.e., nanoporous) hydrogel scaffolds. In addition, we studied the effects of degradation and the inclusion of glycosaminoglycans in our scaffold on the formation of muscle fibers and blood vessels.

Posterior glottic stenosis is the result of scarring in the airway that may be caused by infection, trauma, surgical excision, or pressure ulcers from prolonged intubation. This fibrotic pathology leads to tethering of the vocal cords, resulting in clinical dyspnea and, in extreme cases, a potentially fatal lack of ventilation. PGS is a serious complication that requires surgical intervention to remove the scar tissue, as well as systemic anti-fibrotic medication and antibiotics. There is no effective clinical treatment to prevent the primary occurrence or recurrence of PGS due to the harsh mechanical environment created by daily activities (e.g., coughing, sneezing, swallowing) and limited surgical access. Therefore, PGS patients endure a decreased quality of life. To date, biomaterial-based interventions have been unable to integrate with the tissue or have been too invasive to translate to the clinic. Using a leporine model of PGS, we investigated MAP hydrogel as an injectable bandage for PGS-prone airway injuries with the goal of preventing fibrosis. We studied various material strategies, such as charge- charge interactions and catecholamine-mediated adhesion, to improve tissue integration and implant retention.

Overall, the work presented in this dissertation supports the potential use of MAP hydrogel as a treatment strategy for clinical applications that are very difficult to treat due to dysregulated immune responses to injury that lead to fibrosis. Our results highlight the advantages of MAP hydrogel as a biomaterial platform with independently tunable geometric, physical, and chemical properties. In addition, its injectable nature makes MAP hydrogel an excellent candidate for clinical translation.