Impact of bioactive molecule inclusion in lyophilized silk scaffolds varies between in vivo and in vitro assessments

Biomaterials can influence the coordinated efforts required to achieve tissue rehabilitation. Sponge-like silk fibroin scaffolds that include bioactive molecules have been shown to influence tissue repair. However, the mechanisms by which scaffold formulations elicit desired in vivo responses is unclear. Here, acellular silk scaffolds consisting of type I collagen, heparin, and/or vascular endothelial growth factor (VEGF) were used to investigate material fabrication and composition parameters that drive scaffold degradation, cell infiltration, and adipose tissue deposition in vivo. In subcutaneous implants, scaffold degradation was assessed, and results show that the percentage of cells infiltrating the scaffold increased when scaffold formulations contained bioactive molecules. To gain further insight, calculated in vitro enzymatic degradation rates increased with higher enzyme concentrations and theoretical cleavage sites. However, the addition of type I collagen and heparin to the scaffold at relevant concentrations did not change degradation rates, compared to silk alone. These in vitro results are contrary to observations in vivo, where bioactive molecules influence local protein deposition, immune cell infiltration rates, and vascularization. Thus, quantitative in vitro and in vivo evaluations aid in determining the mechanisms by which biomaterials influence tissue repair and support intentional biomaterial design for clinical applications. ### Competing Interest Statement The authors have declared no competing interest.