Collagen is the primary structural element in extracellular matrices. In the form of fibers it acts to transmit forces, dissipate energy, and prevent premature mechanical failure in normal tissues. Deformation of collagen fibers involves molecular stretching and slippage, fibrillar slippage, and, ultimately, defibrillation. Our laboratory has developed a process for self-assembly of macroscopic collagen fibers that have structures and mechanical properties similar to rat tail tendon fibers. The purpose of this study is to determine the effects of subfibrillar orientation and decorin incorporation on the mechanical properties of collagen fibers. Self-assembled collagen fibers were stretched 0-50% before cross-linking and then characterized by microscopy and mechanical testing. Results of these studies indicate that fibrillar orientation, packing, and ultimate tensile strength can be increased by stretching. In addition, it is shown that decorin incorporation increases ultimate tensile strength of uncross-linked fibers. Based on the observed results it is hypothesized that decorin facilitates fibrillar slippage during deformation and thereby improves the tensile properties of collagen fibers.

Skeletal muscle injuries typically result from traumatic incidents such as combat injuries where soft-tissue extremity injuries are present in one of four cases. Further, about 4.5 million reconstructive surgical procedures are performed annually as a result of car accidents, cancer ablation, or cosmetic procedures. These combat- and trauma-induced skeletal muscle injuries are characterized by volumetric muscle loss (VML), which significantly reduces the functionality of the injured muscle. While skeletal muscle has an innate repair mechanism, it is unable to compensate for VML injuries because large amounts of tissue including connective tissue and basement membrane are removed or destroyed. This results in in a significant need to develop off-the-shelf biomimetic scaffolds to direct skeletal muscle regeneration. Here, the structure and organization of native skeletal muscle tissue is described in order to reveal clear design parameters that are necessary for scaffolds to mimic in order to successfully regenerate muscular tissue. We review the literature with respect to the materials and methodologies used to develop scaffolds for skeletal muscle tissue regeneration as well as the limitations of these materials. We further discuss the variety of cell sources and different injury models to provide some context for the multiple approaches used to evaluate these scaffold materials. Recent findings are highlighted to address the state of the field and directions are outlined for future strategies, both in scaffold design and in the use of different injury models to evaluate these materials, for regenerating functional skeletal muscle.

Large-scale musculoskeletal wounds, such as those seen in trauma injuries, present poor functional healing prognoses. In severe trauma, when the native tissue architecture is destroyed or lost, the regenerative capacity of skeletal muscle is diminished by scar formation. Here we demonstrate that a scaffold system composed of fibrin microthreads can provide an efficient delivery system for cell-based therapies and improve regeneration of a large defect in the tibialis anterior of the mouse. Cell-loaded fibrin microthread bundles implanted into a skeletal muscle resection reduced the overall fibroplasia-associated deposition of collagen in the wound bed and promoted in-growth of new muscle tissue. When fibrin microthreads were seeded with adult human cells, implanted cells contributed to the nascent host tissue architecture by forming skeletal muscle fibers, connective tissue, and PAX7-positive cells. Stable engraftment was observed at 10 weeks postimplant and was accompanied by reduced levels of collagen deposition. Taken together, these data support the design and development of a platform for microthread-based delivery of autologous cells that, when coupled to an in vitro cellular reprogramming process, has the potential to improve healing outcomes in large skeletal muscle wounds.

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.

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