Cells and Biomaterials as Building Blocks for Tissue Engineering and Regenerative Medicine

Biomaterials are an integral component of tissue engineering and regenerative medicine. The selection and biophysical properties oof the materials are instrumental in guiding the behavior of transplanted or invading host cells. I will highlight our recent work in two areas to create building blocks for tissue engineering: 1) manufacturing cellular spheroids with instructive cues to guide cell fate; and 2) production of instructive microgel scaffolds to dictate macrophage phenotype, instruct cell differentiation, and promote tissue formation.<br/><br/>Our group has developed key advances in generating and guiding cell spheroids as building blocks for tissue formation. The bioactivity and function of spheroids is dictated by the selected cell population(s) (both homotypic and heterotypic), culture conditions such as local oxygen tension or inflammatory stimuli, and the association of spheroids with instructive biomaterials. Our group engineered a decellularized extracellular matrix (ECM) secreted by bone marrow-derived mesenchymal stromal cells (MSCs). After characterizing its composition, we demonstrated that MSC spheroids loaded with cell-secreted ECM exhibited markedly increased osteogenic potential and survival in harsh conditions. We have iterated on this ECM to retain endogenous growth factors more effectively. We examined the composition and bioactivity of ECM produced by MSCs differentiated from induced pluripotent stem cells as an alternative cell population that addresses shortcomings of MSCs. Finally, we identified key biophysical properties of hydrogels that regulate cell migration, proangiogenic potential, and tissue formation by MSC spheroids.<br/><br/>Tunable scaffolds that direct cell fate are desirable in many tissue engineering applications such as wound healing, organoid systems, and drug delivery. As our understanding of tissue complexity increases, so does the demand for heterogeneous biomaterials. Microgels are an emerging tool that fulfill this role given their modularity, injectability, and range of fabrication techniques. Unlike bulk hydrogels, microparticle-based scaffolds possess tunable void space which inherently exists between the particles. Such void space permits cells to migrate and proliferate readily without first remodeling their surrounding environment. While the effects of pore size on cell penetration and proliferation have been studied in bulk hydrogels, it has yet to be thoroughly investigated in a microgel platform.<br/><br/>We employed the ability to readily alter porosity in photoanneable microgel scaffolds to influence macrophage polarization, alter cell spreading and aggregation, and influence cell function. We generated microgels with compressive moduli ranging from ~ 10-80 kPa, which demonstrates their ability to match a range of tissue stiffnesses. The compressive modulus and bioactivity of the microgels remained constant after cryopreservation at -20°C, which facilitates high-throughput production, storage, off-the-shelf availability, and expands their potential for clinical translation. Void volume was a key stimulus to instruct macrophage polarization toward a pre-regenerative M2 phenotype.<br/><br/>In another example, we created bilayer constructs of microgels with distinct compressive moduli and pro-osteogenic and chondrogenic peptides. Using light to anneal the gels, we produced constructs that did not delaminate, as is common for many bilayer materials. When cultured in mixed media, we observed upregulation of osteogenic and chondrogenic markers that were primarily restricted to the designated layer. The ease of loading cells, cryopreservability, and short annealing time make this a promising platform for translation to the clinic.