The role of matrix stiffness and viscosity on lipid phenotype and fat lineage potential.

Autologous fat transfer is a common procedure that patients undergo to rejuvenate large soft tissue defects. However, these surgeries are complicated by limited tissue sources, donor-site morbidity, and necrosis. While the biofabrication of fat tissue can serve as a clinical option for reconstructive surgery, the influence of matrix mechanics, specifically stiffness and viscosity, on adipogenesis requires further elucidation. Additionally, the effects of these mechanical parameters on metabolic and thermogenic fat potential have yet to be investigated. Gelatin methacryloyl (GelMA) polymers with varying degrees of methacrylation (DoM) were fabricated to create matrices with different stiffnesses and viscosities. Human adipose-derived mesenchymal stem cells were then encapsulated in mechanically tunable GelMA and underwent adipogenesis to investigate the effects of matrix mechanics on lipid phenotype and fat potential. Mechanical testing confirmed that GelMA stiffness was regulated by DoM and weight composition, while viscosity was determined by the latter. Further work revealed that while lipid phenotype became more enriched as matrix stiffness and viscosity declined, the potential towards metabolic and thermogenic fat appeared to be more viscous-dependent rather than stiffness-dependent. Additionally, FABP4 and UCP1 gene expression exhibited viscous-dependent behavior despite comparable levels of PPARg. However, despite the superior role of viscosity, lipid quantity and mitochondrial abundance demonstrated stiffness-dependent behavior. Overall, this work revealed that matrix viscosity played a more superior role than stiffness in driving adipogenesis and distinguishing between metabolic and thermogenic fat potential. Ultimately, this differentiation in fat production is important for engineering ideal adipose tissue for large soft tissue defects.