Power/Performance Bits: June 23

Capturing waste heat
Researchers at Wuhan University and University of California Los Angeles developed a hydrogel that can both cool down electronics and convert the waste heat into electricity.

The thermogalvanic hydrogel consists of a polyacrylamide framework infused with water and specific ions. When they heated the hydrogel, two of the ions (ferricyanide and ferrocyanide) transferred electrons between electrodes, generating electricity. Meanwhile, water inside the hydrogel evaporated, cooling it. After use, the hydrogel regenerated itself by absorbing water from the surrounding air.

To demonstrate the new material, the researchers attached it at 2mm thickness to a cell phone battery during fast discharging. Some of the waste heat was converted into 5 μW of electricity, and the temperature of the battery decreased by 68 F. The reduced working temperature ensured safe operation of the battery, and the electricity harvested was sufficient for monitoring the battery or controlling the cooling system.

Sweat-powered sensors
Researchers at the California Institute of Technology developed an electronic skin that can monitor body information and runs on biofuel cells powered by sweat, without the need for batteries.

The e-skin is made from soft, flexible rubber and can be embedded with sensors that monitor information like heart rate, body temperature, levels of blood sugar and metabolic byproducts that are indicators of health.

“One of the major challenges with these kinds of wearable devices is on the power side,” said Wei Gao, assistant professor in the department of Medical Engineering at Caltech. “Many people are using batteries, but that’s not very sustainable. Some people have tried using solar cells or harvesting the power of human motion, but we wanted to know, ‘Can we get sufficient energy from sweat to power the wearables?’ and the answer is yes.”

Gao explained that human sweat contains very high levels of the chemical lactate, a compound generated as a by-product of normal metabolic processes, especially by muscles during exercise. The fuel cells built into the e-skin absorb that lactate and combine it with oxygen from the atmosphere, generating water and pyruvate, another by-product of metabolism. As they operate, the biofuel cells generate enough electricity to power sensors and a Bluetooth device, allowing the e-skin to transmit readings from its sensors wirelessly.

The sweat-powered electronic skin. (Credit: Caltech)

“While near-field communication is a common approach for many battery-free e-skin systems, it could be only used for power transfer and data readout over a very short distance,” Gao said. “Bluetooth communication consumes higher power but is a more attractive approach with extended connectivity for practical medical and robotic applications.”

Additionally, the device needed to last a long time with high power intensity with minimal degradation. The biofuel cells are made from carbon nanotubes impregnated with a platinum/cobalt catalyst and composite mesh holding an enzyme that breaks down lactate. They can generate continuous, stable power output up to several milliwatts per square centimeter over multiple days in human sweat.

Gao said the plan is to develop a variety of sensors that can be embedded in the e-skin so it can be used for multiple purposes. “We want this system to be a platform. In addition to being a wearable biosensor, this can be a human-machine interface. The vital signs and molecular information collected using this platform could be used to design and optimize next-generation prosthetics. “

Sensor-laden shirts
Researchers at Massachusetts Institute of Technology created wearable sensors that can be incorporated into stretchy, tight-fitting clothing to collect body information. The sensor-embedded clothing is machine washable and can monitor vital signs such as temperature, respiration, and heart rate.

“We can have any commercially available electronic parts or custom lab-made electronics embedded within the textiles that we wear every day, creating conformable garments,” said Canan Dagdeviren, assistant professor of Media Arts and Sciences at MIT.

The electronic sensors consist of long, flexible strips that are encased in epoxy and then woven into narrow channels in the fabric. These channels have small openings that allow the sensors to be exposed to the skin. For this study, the researchers designed a prototype shirt with 30 temperature sensors and an accelerometer that can measure the wearer’s movement, heart rate, and breathing rate. The garment can then transmit this data wirelessly to a smartphone.

“In our case, the textile is not electrically functional. It’s just a passive element of our garment so that you can wear the devices comfortably and conformably during your daily activities,” Dagdeviren said. “Our main goal was to measure the physical activity of the body in terms of temperature, respiration, acceleration, all from the same body part, without requiring any fixture or any tape.”

The researchers chose a polyester blend for its moisture-wicking properties and its ability to conform to the skin. It resembles compression shirts worn during exercise. “From the outside it looks like a normal T-shirt, but from the inside, you can see the electronic parts which are touching your skin,” Dagdeviren said. “It compresses on your body, and the active parts of the sensors are exposed to the skin.”

The researchers tested their prototype shirts as wearers exercised at the gym, allowing them to monitor changes in temperature, heart rate, and breathing rate. Because the sensors cover a large surface area of the body, the researchers can observe temperature changes in different parts of the body, and how those changes correlate with each other.

The team has been exploring mass production of the material used and plans to begin developing other types of garments, such as pants. They are working on incorporating additional sensors for monitoring blood oxygen levels and other indicators of health and believe the sensors could inform telemedicine, enabling doctors to monitor patients without an office visit or video call.