No abstract

Bundles of threads extruded from type I collagen have been researched extensively as scaffolds to promote the repair and regeneration of torn tendons and ligaments. The success of these scaffolds has been limited by insufficient tissue ingrowth from the wound margin, which may be inhibited by the chemical or physical crosslinking treatment used to increase the mechanical properties and decrease the degradation rate of these scaffolds. Recently, self-assembled collagen threads extruded from solutions of type I collagen molecules were shown to possess ultimate tensile strengths and structural properties comparable to native tendon fibers; however the tissue response to these threads has yet to be determined. The goal of this study was to investigate the effects of various crosslinking techniques on the mechanical properties as well as the in vitro rate of new tissue ingrowth on these threads. Our findings indicate that the physical crosslinking techniques, dehydrothermal (DHT) or ultraviolet light (UV), most significantly improve the mechanical strengths of the threads, but most significantly decrease the rate of cell migration. In contrast, carbodiimide (EDC) crosslinking achieved sub-optimal strength generation, but demonstrated improved cell migration rates. Future studies will investigate the design of threads with surface biochemistries that maximize tissue ingrowth while maintaining the mechanical stability of the scaffold.

In this study, we report on the development of discrete fibrin microthreads as well as novel scaffolds composed of arrays of fibrin threads. These scaffolds exhibit mechanical properties that are significantly greater than fibrin gels and cellular responses suggesting that the materials are conducive to the development of organized, aligned tissues. Fibrin microthreads were produced by coextruding solutions of 70 mg/mL fibrinogen and 6 U/mL thrombin through small diameter polyethylene tubing. Uncrosslinked fibrin microthreads averaged 55-65 microm in hydrated diameter and achieved ultimate tensile strengths approaching 4.5 MPa. The strengths and stiffnesses of the microthreads were approximately twofold greater when the materials were treated with exposure to ultraviolet (UV) light. Although UV crosslinking attenuated fibroblast proliferation, uncrosslinked fibrin microthreads supported fibroblast attachment, proliferation, and alignment, suggesting that they represent a viable biomaterial for the aligned regeneration of tissues. Because of the physiologic roles of fibrin matrices in the early phase of wound healing, we anticipate that these fibrin-based microthreads will direct the spatially and temporally complex processes of cell-mediated tissue ingrowth and regeneration.

Collagen threads with mechanical properties and fibrillar substructure similar to native tissue have been synthesized for the repair of injured tendon and ligament. While these scaffolding materials have demonstrated the potential for inducing tissue regeneration, one limitation has been an insufficient rate of tissue ingrowth for complete regeneration. We hypothesize that the structural hierarchy and biochemical cues on the surfaces of these threads will enhance the rate of cell migration and ultimately the rate of new tissue ingrowth. We developed an in vitro assay to measure the effects of various collagen sources and crosslinking on the rate of fibroblast migration on the surfaces of collagen threads. Threads were suspended from elevated platforms and seeded with fibroblast-populated collagen lattices. Cell migration rates ranging from 0.75 to 1.25 mm/day were measured as the fibroblasts left the lattices and migrated onto various thread types. Threads self-assembled from type I collagen were found to have migration rates similar to native tendon threads while crosslinking by severe dehydration decreased the rate. This novel in vitro model system allows examination of cell migration from a wound margin onto biomaterials to determine the effects of various cell types, matrix materials, and surface biochemistries on cell-matrix interactions. Ultimately, this assay will allow us to identify design parameters that will be most effective for enhancing the rate of tissue ingrowth on fiber-based collagen scaffolds for soft tissue regeneration.