Volume-mediated capacitive transducer and wireless sensor with integrated hydrogel for chemical sensing

A new kind of chemical sensor is presented that has potential to be adapted as a continuous wireless implantable glucose sensor. Such a sensor could greatly improve the treatment of diabetes. The scope of this proof-of-concept study includes materials investigations, device design, fabrication and testing, development of experimental procedures and analysis methods, and identification of issues for further study.; A hydrogel was combined with capacitive pressure sensing methods to produce a chemically-sensitive transducer, referred to here as a HydroCap. Micromachining techniques were used to fabricate capacitive transducers having low parasitic capacitance and resistance. NiTiCu shape memory alloy films were used to make robust diaphragms with high compliance, to measure the large volume changes of hydrogel swelling. The transducers were loaded with nonionic uncrosslinked PHEMA hydrogel that swelled in the presence of small cations, for testing with calcium nitrate solutions. The transducers were built into resonant LC circuits, producing passive sensors that did not require onboard power. A readout system was developed that coupled inductively with the sensors, to transfer power and locate the electrical resonance frequency. Using the sensor-reader system, calcium nitrate concentrations in{09}test bath were measured wirelessly at low power.; Sensors immersed in a test bath were readable wirelessly at distances up to 3 mm. That range should be easy to improve upon with system optimization. By design, the transducers operated in the low pF capacitance range, and the sensors operated in the low MHz frequency range. Sensor performance parameters were predictably and easily varied by changing the capacitor geometry and/or the inductance of the LC circuit. Calcium nitrate concentrations were measured up to 0.5 molar. Transducer capacitance increased by 41% over the concentration range of 0-0.2 molar. Measurement resolution of the wireless sensor system ranged from 2-60 millimolar. The results showed that passive wireless chemical sensors based on the HydroCap concept are feasible. Comparison with performance standards indicates that the HydroCap sensor method has potential to meet requirements for implantable biosensors. The success of a HydroCap sensor will ultimately depend on the reliability of the hydrogel component.