In recent years, we've seen an uptick in research on electronic skin, or "e-skin." This skin-like, sensor-embedded material has shown up in Artificial Ionic Skin (AISkin), a stretchable sensor and antenna material ("BodyNet"), and even a jellyfish-inspired electronic gel with self-healing properties.

E-skin is worn over human skin and can be used to garner information about bodily conditions, such as heart rate, blood pressure, and other biometrics. According to Jun Chang Yang et. al's overview of electronic skin, e-skin is primarily used in skin‐attachable electronics, robotics, and prosthetics.

Various use cases for e-skin. Image used courtesy of Jun Chang Yang et. al

Recently, a research team from Caltech has found a way to work around one particular design constraint of e-skin—power—using an unlikely source of energy: human sweat.

The Obstacles—And Solution—of E-Skin Power

At the start of their project, the Caltech researchers sought a sustainable power source for e-skin—one that could also wirelessly transmit biometric information.

But the researchers were up against a few issues. For one, battery-free e-skin often communicates through near-field communication (NFC), but that requires impractically close proximity between the wearer and the interrogation device. Energy harvesting from the wearer’s physical motions is one option for power, but this method doesn't usually garner enough power. They also found that batteries, which are the predominant power source of wireless e-skin systems, aren't suited for long-term, continuous use.

Wei Gao, an assistant professor in the Andrew and Peggy Cherng Department of Medical Engineering, and his team at Caltech claim they have a solution to these common e-skin limitations: a perspiration-powered electronic skin (PPES).

Their research, published in Science Robotics, explains how lactate can be a power source for e-skin. Lactate is an energy-rich compound that is an intermediary in the complex chemical pathways in humans and other creatures that turn food into energy. Tiny amounts of lactate leak out of the body in the form of sweat, and in this implementation of artificial skin, it can serve as a biofuel.

Perspiration-powered electronic skin, which can wirelessly transmit data via Bluetooth to a user's mobile device. Image used courtesy of Science Robotics

The lactate is converted into electricity with a nanotube-based biofuel cell (BFS) that can generate a surprising amount of power from sweat—as much as several milliwatts per square centimeter of the human skin surface.

Nanotube-based biofuel cell generates sufficient power for PPES to communicate via reliable BLE. Image used courtesy of Science Robotics

This energy not only powers the e-skin’s sensors but can also enable Bluetooth low energy (BLE) communications between the e-skin and a remote monitoring station.

Enough Lactate-Derived Power for Bluetooth Communication

Dr. Gao explains why Bluetooth is the right connectivity protocol for this particular technology: "Bluetooth communication consumes higher power but is a more attractive approach with extended connectivity for practical medical and robotic applications."

The lactate biofuel cell (BFC) is made of carbon nanotubes, which are suffused with a catalyst of platinum and cobalt. This, in turn, holds an enzyme that breaks down the lactate, generating electricity.

The e-skin is fashioned out of rubber and can be embedded with sensors to monitor, for example, body temperature, heart rate, or blood sugar levels. The present invention monitors glucose, urea, NH4+, and pH. The BFC generates a steady electrical output that allows the results to be transmitted in real-time via BLE.

Schematic image of the stretchable BFC-biosensor patch, including its packaging with medical tape. Image (modified) used courtesy of Science Robotics

Dr. Gao envisions this system as a platform. "In addition to being a wearable biosensor, this can be a human-machine interface," he explains. "The vital signs and molecular information collected using this platform could be used to design and optimize next-generation prosthetics."

A New Era in Wearable Electronics?

This research indicates that e-skin may no longer be hampered by dependence on unreliable power sources, like human motion, limited by the impracticalities of NFC, or by the encumbered by short-lived batteries. The biological fuel cell may open up new possibilities in wearables of all descriptions.



If you’re an engineer working on wearable devices, what new applications can you imagine based on the development of biological fuel cells? Share your thoughts in the comments below.