Stimuli-responsive gels that exhibit swelling changes in response to environmental changes such as pH and temperature have many future opportunities as suitable materials for mimicking biomolecules and designing smart systems in the biochemical and biomedical fields. We have prepared novel antigen-responsive gels that have molecular recognition functions, based on the strategy that specific interactions such as antigen-antibody bindings can be used as stimuli-responsive cross-linking points. This paper focuses on the effect of semi-interpenetrating polymer network (semi-IPN) in the gels on their antigen-responsive swelling behavior. Furthermore, we investigated the controlled permeation of model drugs through the antigen-responsive gels.

Two types of gels having antigen-antibody bindings as cross-linking points (semi-IPN gel and entrapment gel) were prepared as follows: The antibody having polymerizable groups was copolymerized with acrylamide (AAm) using redox initiators to synthesize the poly(acrylamide)(PAAm)-grafted antibody. Antigen-antibody semi-IPN gels were prepared by the copolymerization of the antigen having polymerizable groups, AAm and N, N'-methylenebisacrylamide in the presence of the PAAm-grafted antibody. Antigen-antibody entrapment gels without a semi-IPN structure were also synthesized by using a native antibody in place of the PAAm-grafted antibody.

The antigen-antibody semi-IPN gel exhibited a reversible change in swelling behavior in response to stepwise changes in the antigen concentration, but the gel without a semi-IPN structure did not. We investigated the permeation of model drugs through the antigen-antibody semi-IPN and entrapment gels in a buffer solution containing the antigen. A drug was permeated through the antigen-antibody semi-IPN gel in the presence of an antigen but the drug permeation was depressed in its absence. Furthermore, the antigen-antibody semi-IPN gel enabled more sensitive ON-OFF control of the drug permeation than the antigen-antibody entrapment gel. Thus, the pulsatile permeation of a model drug in response to the antigen concentration can be achieved by using the antigen-antibody semi-IPN gels.