Inducing macroporosity in hydrogels using hydrogen peroxide as a blowing agent†
A new gas blowing method to induce a macroporous structure in pH-responsive hydrogel materials with basic functional groups is reported by a new technique that generates oxygen bubbles via hydrogen peroxide decomposition to template the polymer. This overcomes pH limitations associated with the more traditional approach of using a carbon dioxide gas blowing agent. This new approach is shown to effectively induce a macroporous structure which overcomes the diffusional limitation of bulk hydrogels and, as a result, dramatically increases swelling rates. The hydrogel comprises an aliphatic diamine, Jeffamine®, cross-linked with polyethylene glycol diglycidyl ether (PEGDGE) in a single simple polymerisation step, generating a polymeric network with pendant basic groups. This cross-linking reaction requires protonation of the amine groups, which precludes it from being compatible with the carbon dioxide gas blowing method as this requires a low pH for the decomposition of carbonate. To overcome this, the production of oxygen bubbles in situ via a catalytic decomposition of hydrogen peroxide on silver nanoparticles is used to induce a macroporous structure in the hydrogel. This method was shown to successfully induce a macroporous structure whereby interconnected pores from sub-micron up to 0.5 mm diameter are created within the hydrogel. This new method of inducing macroporosity is described here in terms of the hydrogel polymerisation conditions and the nature of the porosity is characterised in terms of its ability to overcome the diffusional limitations of bulk hydrogels. Finally, rapid, reversible pulsatile pH-sensing is demonstrated using these hydrogels.