Controlled Double‐Sensitivity of Microgels Applied to Electronically Adjustable Chemostats

Stimuli-sensitive hydrogels change their volume significantly in response to small alterations of certain environmental conditions. The combination of both sensor and actuator properties probably allows the simplest realization of an active device for automatic concentration regulation of chemical substances. Because of its functionality, this apparatus is, ultimately, a chemical comparator or chemostat. Automatic flow control depending on ion and solvent concentrations in aqueous solutions has been demonstrated for the control of pH and the concentration of various alcohols. Of particular interest are chemostats able to regulate biochemical substances, because case-sensitive drug-release systems or devices substituting body functions such as that of a pancreas could be realized. However, the lack of adjustability of the operating conditions in order to regulate flow, for example, a critical concentration defined by the used hydrogel, inhibits the broad practical use of chemostats. To achieve electronic adjustment of the device’s operating conditions we use controlled double-sensitivity (CDS) of hydrogels. By controlling the temperature of a thermosensitive hydrogel, the phase-transition concentration is precisely adjustable to the required value. The chemostat consists of a silicon-based upper and bottom plate and a circuit card for the PC-controlled operation (see Fig. 1a and b). The upper plate carries a platinum temperature sensor and contains a hydrogel chamber (600 lm × 600 lm × 350 lm) that is covered by a perforated membrane. The bottom plate has similar construction with an integrated platinum heating element. Polymeric microgels based on poly(N-isopropylacrylamide) (PNIPAAm) have been inserted as a freeze-dried powder (average particle diameter 100 lm) into the hydrogel chamber. The temperature of the hydrogel chamber can be controlled by an integrated heating element and a temperature sensor.