Aerosol-jet-printed potentiometric pH sensor for sweat measurements in smart patches

Manufacturing technology of ion-selective electrodes (ISEs) for pH measurements is presented. Plasticized polyurethane membranes with tridodecylamine as a pH-selective ionophore were used as receptor layer, whereas electrodes printed with graphene nanoplatelets paste served as transducers. For preliminary experiments, sensors with screen-printed transducers and pH-selective membranes deposited manually or by direct-ink writing, were employed. However, the use of aerosol-jet printing (AJP) technique for the production of transducer as well as deposition of pH-selective polymeric membrane allowed substantial miniaturization of the sensors, leading to low-cost, automated fabrication of millimeter-scale ISEs. The pH sensors were printed on thermoplastic polyurethane (TPU) or polyethylene terephthalate (PET) substrate, the issues of compatibility of membrane and substrate materials were addressed. The average membrane thickness for the ISEs was 225.2 ± 8.0 μm with an additional 20 μm average thickness of other underlying printed layers. The planar dimensions of ISEs were 300 μm (width) by 2 mm, presenting an opportunity for even further miniaturization. Sensors fully printed with the AJP technique yielded a potentiometric response of −53.48 ± 4.26 mV/pH (N = 69) for PET substrate and − 46.71 ± 10.23 mV/pH (N = 66) for TPU substrate. Presented results are important for developing a fully operational electronic tattoo suitable for large-scale manufacturing.