Wearable electronic devices and biosensors are used for health monitoring but it has been difficult to find convenient power sources for them. Scientists have now developed and tested a wearable biofuel cell array that generates electric power from the lactate in the wearer's sweat, opening doors to electronic health monitoring powered by nothing but bodily fluids.
Wearable biosensors are generally meant to be worn directly on the skin in order to measure specific biosignals and by sending measurements wirelessly to smartphones or computers, keep track of the user's health. Although materials scientists have developed many types of flexible circuits and electrodes for wearable devices, it has been challenging to find an appropriate power source for wearable biosensors. Traditional button batteries, like those used in wristwatches, are too thick and bulky, whereas thinner batteries would pose capacity and safety issues.
The scientists are exploring efficient ways of using sweat as the sole source of power for wearable electronics. A biofuel cell array uses a chemical in sweat — lactate — to generate enough power to drive a biosensor and wireless communications devices for a short time.
The cell array looks like a paper bandage that can be worn, for example, on the arm or forearm. It essentially consists of a water-repellent paper substrate onto which multiple biofuel cells are laid out in series and in parallel; the number of cells depends on the output voltage and power required. In each cell, electrochemical reactions between lactate and an enzyme present in the electrodes produce an electric current that flows to a general current collector made from a conducting carbon paste.
The entire device can be fabricated via screenprinting, a technique generally suitable for cost-effective mass production. This was possible via the careful selection of materials and a novel layout. Whereas similar previous cells used silver wires as conducting paths, the present biofuel cells employ porous carbon ink. Another advantage is the way in which lactate is delivered to the cells. Paper layers are used to collect sweat and transport it to all cells simultaneously through the capillary effect — the same effect by which water quickly travels through a napkin when it comes into contact with a water puddle.
In tests, the paper-based biofuel cells generated 3.66 V and an output power of 4.3 mW. To demonstrate their applicability for wearable biosensors and general electronic devices, the team fabricated a self-driven lactate biosensor that could not only power itself using lactate and measure the lactate concentration in sweat, but also communicate the measured values in real time to a smartphone via a low-power Bluetooth device.
Lactate is an important biomarker that reflects the intensity of physical exercise in real time, which is relevant in the training of athletes and rehabilitation patients. The biofuel cell arrays can also power other types of wearable electronics. The team was able to drive a commercially available activity meter for 1.5 hours using one drop of artificial sweat and the biofuel cells.