Thermoreversible hydrogel for in situ generation and release of HepG2 spheroids.

Organ printing is an alternative to the classic scaffold-based tissue engineering approach in which functional living macrotissues and organ constructs are fabricated by assembly of the building blocks: microtissue spheroids. However, the method for scalable fabrication of cell spheroids does not exist yet. We propose here that it may be a suitable one to generate cell spheroids in thermoreversible hydrogel scaffold, followed by liquefying the scaffold and releasing the generated spheroids. We show that concentrated poly(N-isopropylacrylamide-co-acrylic acid) microgel dispersions solidify upon heating and liquefy upon cooling. A hysteresis in the cooling process was observed and explained by the slow kinetics of the dissolution of the aggregated polymer chains in the cooling process due to additional intra- and interchain interactions. Hep G2 cells are seeded by simple mixing the cells with the microgel dispersions at room temperature. Cell/scaffold constructs form in situ when heated to 37 °C. The cells proliferate and form multicellular spheroids. When brought back to room temperature, the hydrogel scaffolds liquefy, thus, releasing the generated cell spheroids. The released spheroids can attach on the cell culture plate, disassemble, and spread on the substrate, confirming the cell viability. The whole process is carried out under mild conditions and does not involve any toxic additives, which may introduce injury to the cells or DNA. It is scalable and may meet the need for large scale fabrication of cell spheroids for organ printing.