A Self-Powered Glucose Biosensor Operated Underwater to Monitor Physiological Status of Free-Swimming Fish

Abstract

The changes in blood glucose levels are a key indicator of fish health conditions and are closely correlated to their stress levels. Here, we developed a self-powered glucose biosensor (SPGB) consisting of a needle-type enzymatic biofuel cell (N-EFC), which was operated underwater and connected to a charge pump integrated circuit (IC) and a light emitting diode (LED) as the indicator. The N-EFC consisted of a needle bioanode, which was inserted into the caudal area of a living fish (Tilapia) to access biofuels, and a gas-diffusion biocathode sealed in an airtight bag. The N-EFC was immersed entirely in the water and connected to a charge pump IC with a capacitor, which enabled charging and discharging of the bioelectricity generated from the N-EFC to blink an LED. Using a smartphone, the glucose concentration can be determined by observing the LED blinking frequencies that are linearly proportional to the blood glucose concentration within a detection range of 10-180 mg/dL. We have successfully demonstrated the feasibility of the SPGB used to continuously monitor the physiological status of free-swimming fish treated with cold shock in real time. The power generated by a free-swimming fish with an N-EFC inserted into its caudal area, swimming in a fish tank with a water temperature (T-w) of 25 degrees C, exhibited an open circuit voltage of 0.41 V and a maximum power density of 6.3 W/cm(2) at 0.25 V with a current density of 25 A/cm(2). By gradually decreasing T-w from 25 degrees C to 15 degrees C, the power generation increased to a maximum power density of 8.6 W/cm(2) at 0.27 V with a current density of 31 A/cm(2). The blood glucose levels of the free-swimming fish at 25 degrees C and 15 degrees C determined by the blinking frequencies were 44 mg/dL and 98 mg/dL, respectively. Our proposed SPGB provides an effective power-free method for stress visualization and evaluation of fish health by monitoring a blinking LED through a smartphone.