The miniaturization of electronic devices has made tremendous strides over the past few decades. Today, after smartphones and the multitude of wireless options, there is a special type of device whose development is steadily advancing: wearable biosensors. These tiny devices are generally designed to be worn directly on the skin to measure specific biosignals. Data from them are sent wirelessly to smartphones or computers to track the user’s health.
The problem is that such devices need to be constantly charged, and finding a suitable power source for wearable biosensors is difficult. Traditional batteries used in wristwatches and pocket calculators are too thick and bulky. Thinner batteries pose problems with capacity and even safety. But what if humans themselves were the power supplies of wearable devices?
Now, a team of researchers, led by Associate Professor Isao Shitanda from Tokyo University of Science, Japan, has presented a novel design for a biofuel cell array that generates electricity from the lactate in human sweat – to power biosensors and wireless devices for a short period of time.
The new array of biofuel cells looks like a paper bandage that can be worn, for example, on an arm or forearm. Basically, it consists of a water-repellent paper base on which multiple biofuel cells are laid out in series and in parallel. The number of cells depends on the output voltage and the power required. In each of these, electrochemical reactions between lactate – a chemical found in sweat – and an enzyme present in the electrodes produce an electrical current, which flows to a general current collector made from a conducting carbon paste.
This is not the first lactate-based biofuel cell, but several key differences set the new design apart from existing biofuel cells. In particular, the entire device can be fabricated via screen printing, a technique generally suitable for cost-effective mass production.
Paper layers are used to collect sweat and transport it to all cells simultaneously through the capillary effect – the same process that happens when you dip a paper napkin in water, where it then spreads to the entire surface.
Tests carried out by the researchers showed that the paper-based biofuel cells could generate a voltage of 3.66 V and an output power of 4.3 mW. According to the team, this power is significantly higher than that of previously reported lactate biofuel cells.
“We managed to drive a commercially available activity meter for 1.5 hours using one drop of artificial sweat and our biofuel cells,” explains Dr. Shitanda, “and we expect they should be capable of powering all sorts of devices, such as smartwatches and other commonplace portable gadgets.”
The concept still needs to be perfected, but in the long term, it could open doors to electronic health monitoring powered by nothing but bodily fluids.