Can typing on a computer keyboard generate power? Yes, if you can harness the bioelectrical energy at your fingertips! Joseph Wang, University of California San Diego, La Jolla, USA, and colleagues have created a device that converts the chemical and mechanical energy from the surface of our fingertips into electric energy. They used an integrated biofuel cell and piezoelectric energy generator to power useful wearable electronics, such as electrolyte sensors or drug detectors.

Body Power

Wearable electronics include autonomous environmental or biomedical devices, for example, for monitoring biological functions. However, all electronics need power, and supplying constant power to miniaturized devices is still an unsolved problem. Energy sources for these devices can be, e.g., integrated batteries, supercapacitors, solar cells, or external devices connected by wires. However, batteries must be recharged or exchanged, photovoltaics depend on sunlight, and wiring is often impractical or cumbersome.

However, there is an as-yet untapped source of energy: that produced by every living organism in the form of heat, chemical, and mechanical energy. For example, sweat carries lactate, which oxidizes to pyruvate in an exergonic reaction. In lactate-based biofuel cells, the enzyme lactate dehydrogenase, which is attached to the anode, oxidizes lactate in the presence of oxygen, while the cathode performs the oxygen reduction reaction. The electrons generated during the oxidation reaction can then enter an electric circuit.

Unfortunately, to produce sweat, an individual usually has to exercise. To separate energy production from physical effort, the team devised a biofuel cell that also works passively. This lactate-based cell was optimized for a body region where sweat production is constant and high, even when sleeping—the fingertip.

At Your Fingertips

The fingertip biofuel cell is flat, flexible, and the size of a small adhesive bandage. The electrodes were made from a flexible, porous carbon nanotube foam. Two strips of the foam were loaded with the enzyme and served as bioanodes. Another strip was coated with platinum and was placed between the two anodes. The strips were also coated with a hydrogel carrying the electrolyte and enabling the diffusion of the lactate produced by the fingertip, and current collectors were attached below.

Simply touching the hydrogel allowed the biofuel cell to run. The researchers determined an average power output of 20–30 μW per finger, depending on the user. Individuals have different sweat and lactate production, but the team reported the device worked for every person they tested.

More Pressure

The researchers also noted that increasing the pressure increased the power output, and thus, the energy gain. One hour of desktop work with typing and mouse clicking produced almost 30 mJ, they said, and 10 hours of sleeping (with the device wrapped around a finger and encased in a plastic film to stop the hydrogel from drying out) resulted in almost 400 mJ of energy harvested, enough to power an electronic wristwatch for 24 hours.

Since pressing harder provided more energy, the researchers took their harnessing of the body’s power a step further. They added a mechanical energy converter: a piezoelectric energy generator placed under the biofuel cell. It contains a lead–zirconate–titanate chip and converts a forced deformation into electric current. Both integrated generators (fuel cell and piezoelectric chip) were connected to a capacitor the same size as the electrodes.

Pressing the device for four minutes charged the capacitor with 100 μF. The energy generators worked synergistically, the team noted, as using one generator alone took two to five times as long to reach the same level. The researchers also built a double integrated system, with two back-to-back integrated devices, which worked by pinching it between two fingers.

 

 

Biomonitoring Potential

To demonstrate their device in a real-life setting, the researchers connected the capacitor to a microcontroller unit governing a simple electrochromic display. The microcontroller was also connected to potentiometric sensors based on printed electrodes. They detected the concentration of sodium ions or vitamin C in the body, in the latter case by measuring the electrocatalytic oxidation of the vitamin.

The system worked completely autonomously and received its current supply just by being pinched between the index finger and thumb. A realistic on-body setting was also demonstrated. Just by holding the strips, the team was able to follow the readout of the accumulation and degradation of excess vitamin C in the sweat of a person that had taken a vitamin pill shortly beforehand.

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• A passive perspiration biofuel cell: High energy return on investment,
  Lu Yin, Jong-Min Moon, Juliane R. Sempionatto, Muyang Lin, Mengzhu Cao, Alexander Trifonov, Fangyu Zhang, Zhiyuan Lou, Jae-Min Jeong, Sang-Jin Lee, Sheng Xu, and Joseph Wang,
  Joule 2021, 5, 1–17.
  https://doi.org/10.1016/j.joule.2021.06.004