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The device, which has reportedly been successfully tested, generates electric power from the lactate – an organic molecule produced by most tissues in the human body – in the wearer’s sweat. Their biofuel cell, say the scientists, opens the doors to electronic health monitoring powered by nothing but bodily fluids.

While 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 are too thick and bulky, whereas thinner batteries would pose capacity and even safety issues.

The new biofuel cell array, however, looks like a paper bandage that can be worn, for example, on the arm or forearm and generates enough power to drive a biosensor and wireless communication devices for a short time, say the scientists. It essentially consists of a water-repellent paper substrate onto which multiple biofuel cells are laid out in series and in parallel, with the number of cells depending 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. Compared to previously developed lactate-based biofuel cells, the entire new device can be fabricated via screen printing – a technique generally suitable for cost-effective mass production. This was possible, say the researchers, via the careful selection of materials and an ingenious layout.

For example, where similar previous cells used silver wires as conducting paths, the new 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.

These advantages, say the scientists, make the biofuel cell arrays exhibit an unprecedented ability to deliver power to electronic circuits.

“In our experiments,” says Associate Professor Isao Shitanda, who led the team of scientists that developed the device, “our paper-based biofuel cells could generate a voltage of 3.66 V and an output power of 4.3 mW. To the best of our knowledge, this power is significantly higher than that of previously reported lactate biofuel cells.”

To demonstrate their applicability for wearable biosensors and general electronic devices, the researchers 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.

While lactate itself 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 proposed biofuel cell arrays can power not only wearable lactate biosensors, but also other types of wearable electronics.

“We managed to drive a commercially available activity meter for 1.5 hours using one drop of artificial sweat and our biofuel cells,” says Dr. Shitanda, “and we expect they should be capable of powering all sorts of devices, such as smart watches and other commonplace portable gadgets.”

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