Thermoresponsive Artificial Extracellular Matrix for Tissue Engineering: Hyaluronic Acid Bioconjugated with Poly(N-isopropylacrylamide) Grafts

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Abstract
Thermoresponsive hyaluronans (HAs) were prepared by graft polymerization of N-isopropylacrylamide (NIPAM) on HA (number-averaged molecular weight, Mn, ca. 1.5 × 105 and 5.0 × 105) using dithiocarbamate which is a kind of iniferter (initiator, transfer agent and terminator). The degree of dithiocarbamylation (DD) as an iniferter ranged from 0.4 to 11.4% per disaccharide unit of HA. The estimated Mn of the grafted polyNIPAM (PNIPAM) ranged from approximately 5.0 × 103 to 8.4 × 104. The PNIPAM-grafted HAs (PNIPAM-HAs) were water-soluble at room temperature, while they precipitated at temperatures above approximately 34 °C in water. The temperature at the onset of precipitation (lower critical solution temperature: LCST) was independent of parameters of molecular architecture such as Mn of HA, degree of grafting of PNIPAM, and Mn of PNIPAM. Equilibrium transmittance of the aqueous solution above LCST decreased with an increase in both degree of grafting and Mn of PNIPAM. At physiological temperature, the PNIPAM-HA film cast from a cold solution was very wettable with water. A markedly reduced adhesion of endothelial cells to the film was observed, indicating that the PNIPAM-HA film may serve as a non-cell-adhesive matrix. Scanning electron microscopic observation appeared to differentiate supramolecular structures between rapidly freeze-dried PNIPAM-HA and nongrafted HA:PNIPAM-HA exhibited a nonuniform fibrous network, whereas the morphology of which is markedly different from that of a nongrafted HA gel exhibited a mixture of sharp needle- and platelike structures.

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