Trapeze Artistry of Biomimetic Hydrogels
We demonstrate a class of smart hydrogels that exhibit unique biomimicking functions: thermoresponsive volume phase transitions similar to sea cucumbers, self-organization into core-shell hollow structures similar to coconuts, shape memory and self-healing as exhibited by living organisms, and metal ion-mediated cementing, similar to marine mussels.
We begin with a mechanistic analysis of stimuli responsive polymers through Lattice Fluid Hydrogen Bonding (LFHB) model that rigorously accounts for not only hydrogen bonding interactions but also the temperature dependent dispersion interactions between polymer and solvent. We show that a subtle balance of the hydrophilic and hydrophobic interactions is necessary to induce discontinuous transition.
Moving forward, we show that among various acryloyl amino acid monomers, the gels made from acrylol-6-amino caproic acid (A6ACA) had an optimal balance of hydrophilic and hydrophobic interactions, endowing them with unique “life-mimicking” functions. These gels showed reversible macroscopic self-organization leading to reversible shape transitions as also most remarkable rapid (few seconds) self-healing.
But how do these A6ACA based gels do the trapeze artistry with a delicate balance of hydrophilic and hydrophobic interactions? We show using atomistic molecular simulations and spectroscopic measurements that healing was caused by hydrogen bonding between carboxyl end groups of A6ACA alkyl side chains across the interface of two pieces of gels, which required that the side chains stretch out across the interface and bond to each other. Simulations show that shorter side chains could not reach each other for effective binding, whereas longer side chains collapsed onto themselves due to their higher hydrophobicity.
Our discovery of the ability of these A6ACA gels to do the aforementioned trapeze artistry has led to several breakthrough applications. A6ACA functionalized biomedical implants have been shown to promote biomineralization with native bone tissue-like functions, leading to novel synthetic bone grafts and therapeutic interventions to assist bone regeneration and healing. The pH responsive self-healing property of A6ACA gels has been harnessed to develop devices that can be used to plug tissue damage in gastro-intestinal tract. The amphiphilic nature of A6ACA polymers has been proposed for plugging porosity of underwater geological strata to enable enhanced oil recovery. And there are more.
We take these intelligent gels to a next level by creating novel switching biomimetic hydrogels with enzyme like activity (gelzymes), whose hydrolytic activity could be rapidly, precisely and reversibly triggered on / off by UV light and pH. Such gelzymes offer additional features: greater tailorability; complete reversibility; and stability in hostile environments.
The aforementioned applications are just illustrative of the huge potential of this fascinating class of materials, their best is yet to come!