Host responses to microgel-based biomaterial interfaces
Bridges, Amanda Walls
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Although medical devices and biomaterial implants are used clinically in a variety of applications, the process of implanting them damages local tissue and initiates a localized non-specific inflammatory response that is detrimental to device performance. Extensive research efforts have focused on developing material surface treatments and systems to deliver anti-inflammatory agents to abrogate such biomaterial-mediated inflammation, yet long-term use of these traditional materials in vivo is limited due to continued inflammation and fibrous encapsulation. This work aims to address these limitations by developing a versatile implant coating with non-fouling properties using a system based on hydrogel microparticles (i.e. microgels). The overall objective of this project was to evaluate host responses to these microgel coatings. Microgel particles were synthesized from poly(N-isopropyl acrylamide) cross-linked with poly(ethylene glycol)-diacrylate and were successfully deposited onto polymeric substrates using a simple and reproducible spin coating technique. We determined that microgel-coated samples adsorbed significantly lower levels of human fibrinogen than controls. Further characterization using an in vitro culture system demonstrated that microgel coatings significantly reduced the adhesion and spreading of murine macrophages and primary human blood-derived monocytes compared to controls. Materials were then evaluated for early cellular responses following implantation in the intraperitoneal cavity of mice to model acute inflammation. Analyses of explanted biomaterials using immunofluorescence staining techniques revealed that microgel-coated samples significantly reduced the density of surface-adherent cells. Additional analysis using flow cytometry revealed that microgel-coated samples exhibited significantly lower levels of pro-inflammatory cytokines in adherent leukocytes compared to controls, indicating that these coatings modulate cellular pro-inflammatory activities. Finally, we implanted samples subcutaneously in rats to determine the efficacy of microgel coatings at longer time points using an established model of chronic inflammation. Explants were processed histologically and stained for various markers. Importantly, staining demonstrated that the microgel coatings significantly reduced fibrous capsule thickness, the capsules appeared less compact and structurally ordered than controls, and also contained significantly fewer cells. Collectively, these results demonstrate that microgel particles can be applied as polymeric coatings to modulate inflammation and achieve more desirable host responses in vivo, with the potential to extend implant lifetime.