Self-Healing Polymer Network with High Strength, Tunable Properties, and Biocompatibility

Nature has designed and optimized materials to possess a range of properties and functions. Here, we introduced a molecular design strategy to impart customizable functionality and varying mechanical properties into gels; mimicking nature's range of tunable materials. We demonstrate a gel that is not only tough but also exhibits self-healing, is easily controllable, and the final materials have a broad range of mechanical properties. To develop these materials, we first prepared a methacrylic acid (MAAc) and poly(ethylene glycol) methyl ether methacrylate (OEGMA) random copolymer: poly(MAAc-co-OEGMA). The network's deliberate inter- and intramolecular hydrogen bondings were modified through some of the acid sites being postfunctionalized with benzaldehyde (BA) and cross-linked with diamine-terminated poly(dimethylsiloxane) (PDMS) to form dynamic imine bonds. Due to the low glass transition temperature of the PDMS cross-linker, the chain mobility can be enhanced, enabling rapid self-healing (>98% within seconds), in addition to improving the stretchability (tensile strain) from a few % to almost 500%. The prepared polymers and gels were well characterized through various techniques including Fourier transform infrared spectroscopy (FTIR), H-1 NMR, and size-exclusion chromatography (SEC) analysis. Mechanical testing and dynamic mechanical analysis (DMA) revealed interesting insights into the broad-range (Young's modulus: 100 kPa to >300 MPa) and tunable mechanical properties, including the tensile strength (from 12 to 0.1 MPa) and strain (up to 500%) as well as the storage (0.1 to 60 MPa) and loss (1 to 40 MPa) moduli of the dynamic self-healing gel. Interestingly, the tensile strength decreasing with increasing cross-link density. Lastly, the biocompatibility of the gels was investigated, with an initial study of both human bone and skin cells indicating increased biocompatibility with gels that had been cross-linked with PDMS.