Disposable Hydrogel Activated Circular Interdigitated Sensor for Monitoring Biological Fluid pH

Significant fluctuations in body fluid pH can indicate the onset of infection or a chronic condition. In this paper, a hydrogel-activated circular discrete interdigitated capacitive (D-IDC) biosensor is introduced for monitoring the acidity of biological fluids. The pH-sensitive transducer is a thin biocompatible chitosan/polyvinyl alcohol hydrogel disk positioned at the center of a multi-terminal D-IDC comprised of numerous distinct circular conductive electrodes covering the active sensing area. The D-IDC is printed on Nylon-66 filter paper that has been coated with a thin layer of polydimethylsiloxane (PDMS). The circular electrode pattern is printed on the PDMS surface using nontoxic graphene/carboxymethyl cellulose (G-CMC) aqueous suspensions as the electrically conductive ink. A thin polymer passivation layer is then spin coated on the surface of the G-CMC conductive electrodes to prevent an electrical short with the liquid filled hydrogel transducer. The pH level of the fluid causes the centrally located hydrogel transducer to swell/de-swell over the circular electrodes producing an incremental change in the measured capacitance. A prototype comprised of seven electrode pairs is fabricated and experimentally tested under controlled conditions using phosphate buffer solutions (PBS) with varying pH. The operational and analytical parameters of the hydrogel-activated biosensor are evaluated in terms of measured capacitance. The prototype sensor demonstrates acceptable repeatability (SD < 0.4%) and reproducibility (SD < 5.8%). Sensor resolution can be modified by reducing the physical size of the electrode micro-traces used in the D-IDC design and increasing the number electrode pairs.