People requiring rhythmic delivery of drugs, notably Gonadotropin Releasing Hormone (GnRH), currently rely on catheter-based subcutaneous or intravenous delivery systems with a host of inconveniences and dangers. A hydrogel-based delivery system has been demonstrated which may replace the current therapy with a simple, implantable device.

This system consists of a reservoir covered by a hydrogel membrane which undergoes a phase transition (and consequent permeability change) at particular pH values. The reservoir contains the therapeutic agent along with the enzymes glucose oxidase and catalase. In its swollen state, the membrane is quite permeable, allowing external glucose to diffuse into the reservoir where it is enzymatically converted to gluconic acid. The subsequent drop in pH causes the hydrogel to collapse and dramatically decrease its permeability. Glucose can no longer enter the reservoir (and therapeutic agent can no longer leave) until the chemical signal relaxes (e.g. faster-diffusing protons exit the system) and the membrane returns to its permeable state. The cycle then repeats.

This system has been demonstrated and refined on the lab-bench scale (~80 mL total volume) under a variety of conditions and parameter values, but many challenges remain before a viable implantation is achieved. Among these challenges are the miniaturization and biocompatibilization of the bench-scale system. To miniaturize the device, fabrication techniques adapted from the semiconductor industry have been used to develop new reservoirs and membrane mounts. The main challenge in biocompatibilization is protecting the stimuli-sensitive hydrogel from immunoproteins without hindering the passive transport of glucose or hormone. A composite nanoporous membrane is being developed to be used as a protective covering for this purpose.