Flexible, four-electrode conductivity cell for biologging applications

Global ocean circulation, governed by the salinity of seawater, is a key contributor in supporting marine life and in regulating climate. Biologging has enabled researchers to record in-situ ocean parameters from free-ranging animals, as they swim through their environment. Current salinity sensors are bulky, expensive, highly intrusive and also susceptible to corrosion and biofouling. We present a four-electrode conductivity cell based on laser-induced graphene (LIG) on a polyimide substrate for salinity measurements. The flexible, lightweight and cost-efficient sensors operate under various bending conditions with an accuracy of ±0.5 psu. The sensors offer a linear response to salinity, as well as a high sensitivity of 0.85 mS/psu, and they operate over a wide range of frequencies (10 kHz–100 kHz). These characteristics considerably relax the requirements for the circuit of the data logger. A four electrodes configuration reduces the dependency on the electrical double layer, since the electrodes used to drive a current are different from the electrodes measuring the voltage drop. The sensors' deployment in the Red Sea has revealed its capability to withstand the harsh seawater environment. The mechanical flexibility and low thickness and weight of the conductivity cell allow for a less-intrusive attachment of this sensor to marine animals, while the versatile fabrication process enables customization of the sensor to a wide range of applications.