(674d) Engineering Tunable Growth Factor Release Rates and Degradation Rates from Silk-Extracellular Matrix Scaffolds

Traumatic soft tissue injuries result in permanent loss of skeletal muscle mass known as volumetric muscle loss (VML). This causes a major clinical challenge for military and civilian medicine. Biomaterials can be used to provide the appropriate structural and mechanical properties as well as the release of bioactive and chemotactic signals to address this issue. Tissue engineering has made it possible to integrate biomaterials into a wound site; however there is not a clear understanding of what growth factor release rates and biomaterial degradation kinetics are clinically relevant.

The long-term goal of our work is to engineer natural biomaterials that, upon implantation, modulate in vivo tissue behavior and collective cell function for soft tissue repair and rehabilitation. One avenue we are employing focuses on strategies to modulate composite silk scaffold composition and formulation to alter growth factor delivery kinetics in conjunction with biomaterial degradation in vivo. To start, composite silk sponges were formed via addition of collagen, heparin, and/or a growth factor (VEGF, bFGF, IGF-1) pre- or post- silk fabrication at varying concentrations. Growth factor activity levels were first analyzed using an ELISA assay demonstrating the effects of sponge scaffold composition and growth factor concentration on release rates and effective diffusivities. These ELISA results and growth factor activity quantification studies were confirmed by evaluating cell response to the released growth factors in a transwell assay. These parameters can then be used to guide scaffold design and optimization. For example, using unseeded silk scaffolds with solubilized or insolubilized key factors (Vascular Endothelial Growth Factor, Heparin) implanted in Sprague Dawley rats increased vascularization and cell migration into the scaffold, while limiting adipose tissue formation over 8 weeks. Results demonstrate that varying the composition of the composite sponge-like scaffold as well as the formulation method for addition of the bioactive components qualitatively alters the types and rate of cell infiltration, the rate of scaffold degradation, and the mass transfer of proteins to the implant area. Ongoing work aims to expand upon these qualitative results to use image analysis methods to quantify the percent cell infiltration, the overall scaffold area, the sponge void space, and the types of cells present within the scaffold after 1, 2, 4, and 8 weeks post implantation. This work enables predictive scale-up of the system for future studies in a model rat system investigating skeletal muscle rehabilitation following VML.