Material design for an artificial extracellular matrix: Cell entrapment in poly (N-isopropylacrylamide) (PNIPAM)-grafted gelatin hydrogel

Abstract

As a thermoresponsive extracellular matrix, PNIPAM-derivatized gelatin (PNIPAM-gelatin) was synthesized by an iniferter-based graft polymerization of NIPAM on side chains of gelatin (molecular weight, ca. 9.5 ×10⁴ g/mol). The degree of grafting was 22.6 groups per molecule, and the estimated molecular weight of PNIPAM was ca. 1.2×10⁴ g/mol. The phase transition of dissolution/precipitation of PNIPAM-gelatin occurred at around 35°C. At concentrations above 15 w/v% over about 35°C, the solution was converted to hydrogel. The mechanical strength of the produced hydrogel increased with the concentration of PNIPAM-gelatin. The apparent elastic modulus of the hydrogel at a concentration of 20 w/v% was 1.2×10⁴Pa, which is nearly equal to that of collagen gel prepared at 0.15w/v%. When a culture medium containing the PNIPAM-gelatin (concentration, 20 w/v%) and bovine smooth muscle cells was incubated at 37°C, the cells were entrapped into a hydrogel. The entrapped cells apparently died in hydrogel with a thickness of 1 mm. However, the use of thinner hydrogel (thickness, 0.1 mm) or comixing with a small amount of PNIPAM-derivatized hyaluronic acid (PNIPAM-HA), even at 1 mm thickness, appeared to increase the survival of entrapped cells.