Date Published: May 9, 2019
Publisher: Public Library of Science
Author(s): Elizabeth M. Boazak, Vaughn K. Greene, Debra T. Auguste, Erin Lavik.
The use of hydrogels in load bearing applications is often limited by insufficient toughness. 2-Hydroxyethyl methacrylate (HEMA) based hydrogels are appealing for translational work, as they are affordable and the use of HEMA is FDA approved. Furthermore, HEMA is photopolymerizable, providing spatiotemporal control over mechanical properties. We evaluated the ability of vinyl methacrylate (VM), allyl methacrylate (AM), and 3-(Acryloyloxy)-2-hydroxypropyl methacrylate (AHPM) to tune hydrogel toughness and Young’s modulus. The crosslinkers were selected due to their heterobifunctionality (vinyl and methacrylate) and similar size and structure to EGDMA, which was shown previously to increase toughness as compared to longer crosslinkers. Vinyl methacrylate incorporation into HEMA hydrogels gave rise to hydrogels with Young’s moduli spanning ranges for ligament to cartilage, with a peak toughness of 519 ± 70 kJ/m3 under physiological conditions. We report toughness (work of extension) as a function of modulus and equilibrium water content for all formulations. The hydrogels exhibited 80%-100% cell viability, which suggests they could be used in tissue engineering applications.
Hydrogels, crosslinked polymer networks that absorb water, are widely used for biomedical devices, drug delivery, implants, and tissue engineering . Hydrogel mechanical properties are modified principally by varying macromer molecular weight, and macromer, initiatior, and crosslinker concentrations. The cross-linking density of the gel network is proportional to the gel’s elastic modulus and inversely proportional to its swelling . High degrees of swelling are often desired for improved transport within the hydrogel. However, swelling reduces not only material stiffness, but also toughness. Inherently low stiffness and toughness has limited the use of hydrogels in load bearing applications . Crosslinking density is also related to mesh (or pore) size, which impacts cell differentiation, viability, and migration [4–6]. Previous reports have optimized hydrogel mechanics for specific gene regulation . As such, the development of tough hydrogels with tunable moduli and swelling properties remains a challenge.
We evaluated HEMA hydrogels crosslinked with heterobifunctional crosslinkers alone or at different ratios in an effort to achieve hydrogels with tunable Young’s moduli and toughness. Vinyl methacrylate (VM) and EGDMA/VM co-crosslinked HEMA-based hydrogels were cytocompatible, had equilibrium water content ranging from 15–54%, and exhibited mechanical properties that may be useful in load bearing applications. These hydrogels may be photopolymerized to achieve Young’s moduli under physiological conditions that encompass the full range of values reported for human cartilage.