Power/Performance Bits: Jan. 19

Electronic skin for health tracking
Researchers at the University of Colorado Boulder developed a stretchy electronic ‘skin’ that can perform the tasks of wearable fitness devices such as tracking body temperature, heart rate, and movement patterns.

“Smart watches are functionally nice, but they’re always a big chunk of metal on a band,” said Wei Zhang, a professor in the Department of Chemistry at CU Boulder. “If we want a truly wearable device, ideally it will be a thin film that can comfortably fit onto your body.”

To make the electronic skin, screen printing is used to create a network of liquid metal wires, which are sandwiched between two thin films made out of the highly flexible and self-healing material polyimine.

The team said the resulting device is a little thicker than a Band-Aid and can be applied to skin with heat. It can also stretch by 60% in any direction without disrupting the electronics inside. “It’s really stretchy, which enables a lot of possibilities that weren’t an option before,” said Jianliang Xiao, an associate professor in the Department of Mechanical Engineering at CU Boulder.

A user fits an “electronic skin” device onto the wrist. (Credit: Chuanqian Shi / CU Boulder)

The device is reconfigurable and can be made to fit anywhere on a body. “If you want to wear this like a watch, you can put it around your wrist,” said Xiao. “If you want to wear this like a necklace, you can put it on your neck.”

The electronic skin is self-healing, as well. If it is sliced, pinching the broken areas together for several minutes allows the bonds holding the polyimine together to reform. “Those bonds help to form a network across the cut. They then begin to grow together,” Zhang said. “It’s similar to skin healing, but we’re talking about covalent chemical bonds here.”

It is also easily recyclable. By placing a device in a solution, the polyimine will depolymerize, separating into its component molecules, while the electronic components sink to the bottom. Both the electronics and the stretchy material can then be reused.

There is more work to be done before it is commercializable, however. Currently, the devices still need to be hooked up to an external source of power to work. “We haven’t realized all of these complex functions yet,” Xiao said. “But we are marching toward that device function.”

Super stretchy e-skin
In a different take on electronic skin, researchers at King Abdullah University of Science and Technology (KAUST) and University of California Los Angeles (UCLA) developed an e-skin device with extreme stretchability, capable of deforming up to 28 times its original size while incorporating sensor capabilities.

“The ideal e-skin will mimic the many natural functions of human skin, such as sensing temperature and touch, accurately and in real time,” said Yichen Cai, a postdoctoral fellow at KAUST. “The landscape of skin electronics keeps shifting at a spectacular pace. The emergence of 2D sensors has accelerated efforts to integrate these atomically thin, mechanically strong materials into functional, durable artificial skins.”

Key for the team was creating a good connection between the layers that make up the device, to enable higher sensitivity while maintaining durability and flexibility. Their e-skin is comprised of a hydrogel reinforced with silica nanoparticles as a strong and stretchy substrate and a 2D titanium carbide MXene as the sensing layer, bound together with highly conductive nanowires.

“Hydrogels are more than 70 percent water, making them very compatible with human skin tissues,” said Jie Shen, a postdoctoral fellow at KAUST. By prestretching the hydrogel in all directions, applying a layer of nanowires, and then carefully controlling its release, the researchers created conductive pathways to the sensor layer that remained intact even when the material was stretched to 28 times its original size.

The team said that the prototype e-skin could sense objects from 20 centimeters away, respond to stimuli in less than one tenth of a second, and when used as a pressure sensor, could distinguish handwriting written upon it. It continued to work well after 5,000 deformations, recovering in about a quarter of a second each time. “It is a striking achievement for an e-skin to maintain toughness after repeated use,” said Shen, “which mimics the elasticity and rapid recovery of human skin.”

“One remaining obstacle to the widespread use of e-skins lies in scaling up of high-resolution sensors,” added Vincent Tung, group leader at KAUST, “however, laser-assisted additive manufacturing offers new promise.”

Energy-harvesting nylon
Researchers from the University of Bath, Max Planck Institute for Polymer Research, and University of Coimbra created piezoelectric nylon fibers that could be woven into clothing to harvest energy from everyday movements.

Nylon is piezoelectric material, which generates a charge when deformed. If incorporated into a garment, movements like swinging arms would be enough to distort the fabric.

“There’s growing demand for smart, electronic textiles, but finding cheap and readily available fibers of electronic materials that are suitable for modern-day garments is a challenge for the textile industry,” said Kamal Asadi, a professor from the Department of Physics at Bath. “Piezoelectric materials make good candidates for energy harvesting from mechanical vibrations, such as body motion, but most of these materials are ceramic and contain lead, which is toxic and makes their integration in wearable electronics or clothes challenging.”

However, nylon needs to be in a particular crystal form to become piezoelectric, and handling nylon in a way that preserves its piezoelectric properties is a challenge. The established method for creating these nylon crystals is to melt, rapidly cool and then stretch the nylon. However, this process results in thick slabs (known as ‘films’) that are piezoelectric but not suited to clothing. The nylon would need to be stretched to a thread to be of woven into garments, or to a thin film to be used in wearable electronics.

Instead, the researchers dissolved the nylon powder in an acid solvent mixed with acetone rather than by melting it. Finally, the acid is extracted from the final product. “The acetone bonds very strongly to the acid molecules, so when the acetone is evaporated from nylon solution, it takes the acid with it. What you’re left with is nylon in its piezoelectric crystalline phase. The next step is to turn nylon into yarns and then integrate it into fabrics,” said Asadi.

The researchers envision using garments made of such fabric connected to a battery that would charge throughout the wearing and could connect to a phone or power wearable sensors.