Porous Polypropylene Membrane Based pH Sensing for Skin Monitoring

An iridium oxide-based electrical pH sensor that is suitable to be embedded in an electronic bandage for skin monitoring has been developed. The electrical pH sensor does not entail high-temperature fabrication processes thus is suitable to be built on polypropylene micro membrane (PPMM), a paper-like substrate, which is inert, gas-permeable, and biomechanically-compatible to tissue. The PPMM was metalized by an electroless gold-coating process and iridium oxide nanoparticles were electrodeposited on the porous membranes. The reference electrode was made by screen printing Ag/AgCl paste on the substrate. pH responses of the IrO2 PPMM against a commercial reference electrode or the planar Ag/AgCl reference electrode were examined. A super-Nernstian sensitivity of −66.8 mV/pH was achieved with the PPMM-based sensor in a pH range from pH 2 to 13. The electrodes also produced similar responses in smaller pH ranges of pH 5 to 8 and around pH 7. Output potential characterization, such as cyclic voltammetry, hysteresis, response time, potential drift, deviation, fluctuation, and potential stability, showed repeatable and stable pH responses in physiologically relevant pH ranges. Interference factors such as salt concentration, viscosity and temperature have also been investigated. The results show that the calibration procedures should consider these factors specific to targeted applications. The planar pH-sensitive electrodes show reliable performance in a bandage configuration designed and packaged for wound monitoring. The accuracy assessment in a Clarke error grid and the result of sensing pH induced by uric acid showed the feasibility of bandage applications. The electrical and biocompatible electrodes embedded in breathable porous bandages can be integrated with portable electronics to be used as wearables for wireless tissue monitoring.