The biofuel cell array essentially consists of a water-repellent paper substrate onto which multiple biofuel cells are laid out in series and in parallel. (Credit: Tokyo University of Science)

Wearable electronic devices and biosensors are great tools for health monitoring, but it has been difficult to find convenient power sources for them. Now, a group of scientists from Japan has successfully 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.

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 wrist watches and pocket calculators, are too thick and bulky, whereas thinner batteries would pose capacity and even safety issues. But what if we were the power sources of wearable devices ourselves?

The researchers present a novel design for a biofuel cell array that uses lactate, a chemical in sweat, to generate enough power to drive a biosensor and wireless communication devices for a short time. Their new biofuel 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, which flows to a general current collector made from a conducting carbon paste.

This is not the first lactate-based biofuel cell, but some key differences make this novel design stand out from existing lactate-based biofuel cells. One is the fact that the entire device can be fabricated via screen printing, a technique generally suitable for cost-effective mass production. This was possible via the careful selection of materials and an ingenious layout. For example, 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.

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