Thermoreversible and Injectable ABC Polypeptoid Hydrogels: Controlling the Hydrogel Properties through Molecular Design

Abstract

A series of ABC triblock copolypeptoids [i.e., poly(<i>N</i>-allyl glycine)-<i>b</i>-poly(<i>N</i>-methyl glycine)-<i>b</i>-poly(<i>N</i>-decyl glycine) (AMD)] with well-defined structure and varying composition have been synthesized by sequential primary amine-initiated ring-opening polymerization of the corresponding N-substituted <i>N</i>-carboxyanhydride monomers (Al-NCA, Me-NCA, and De-NCA). The ABC block copolypeptoids undergo sol-to-gel transitions with increasing temperature in water and biological media at low concentrations (2.5-10 wt %). The sol-gel transition is rapid and fully reversible with a narrow transition window, evidenced by the rheological measurements. The gelation temperature (<i>T</i>_gel) and mechanical stiffness of the hydrogels are highly tunable: <i>T</i>_gel in the 26.2-60.0 °C range, the storage modulus (<i>G</i>') and Young's modulus (<i>E</i>) in the 0.2-780 Pa and 0.5-2346 Pa range, respectively, at the physiological temperature (37 °C) can be readily accessed by controlling the block copolypeptoid composition and the polymer solution concentration. The hydrogel is injectable through a 24 gauge syringe needle and maintains their shape upon in contact with surfaces or water baths that are kept above the sol-gel transition temperature. The hydrogels exhibit minimal cytotoxicity toward human adipose derived stem cells (hASCs), evidenced from both alamarBlue and PicoGreen assays. Furthermore, quantitative PCR analysis revealed significant up-regulation of the <i>Col2a1</i> gene and down-regulation of <i>ANGPT1</i> gene, suggesting that the hydrogel exhibit biological activity in inducing chondrogenesis of hASCs. It was also demonstrated that the hydrogel can be used to quantitatively encapsulate water-soluble enzymes (e.g., horseradish peroxidase) by manipulating the sol-gel transition. The enzymatic activity of HRP remain unperturbed after encapsulation at 37 °C for up to 7 d, suggesting that the hydrogel does not adversely affect the enzyme structure and thereby the enzymatic activity. These results suggest that the polypeptoid hydrogel a promising synthetic platform for tissue engineering or protein storage applications.