Passive, wireless transduction of electrochemical impedance across thin-film microfabricated coils using reflected impedance

A new method of wirelessly transducing electrochemical impedance without integrated circuits or discrete electrical components was developed and characterized. The resonant frequency and impedance magnitude at resonance of a planar inductive coil is affected by the load on a secondary coil terminating in sensing electrodes exposed to solution (reflected impedance), allowing the transduction of the high-frequency electrochemical impedance between the two electrodes. Biocompatible, flexible secondary coils with sensing electrodes made from gold and Parylene C were microfabricated and the reflected impedance in response to phosphate-buffered saline solutions of varying concentrations was characterized. Both the resonant frequency and impedance at resonance were highly sensitive to changes in solution conductivity at the secondary electrodes, and the effects of vertical separation, lateral misalignment, and temperature changes were also characterized. Two applications of reflected impedance in biomedical sensors for hydrocephalus shunts and glucose sensing are discussed.