A type of fabric typically used for hiking gear could potentially lead to wearable electronics that successfully cool both the device and the wearer’s skin.

The fabric has heat-conducting properties on par with stainless steel, researchers report.

As smartwatches become more powerful, they will generate more heat. To prevent burns or rashes, what if a material touching the skin could feel as cool as metal, but also be flexible enough to be worn on the wrist?

The hiking gear material is made of ultra-high molecular weight polyethylene fibers, which are sold commercially under the brand name Dyneema. These polymer-based fabrics are marketed for their high strength, durability, and abrasion resistance. They are often used to create body armor, specialty sports gear, ropes, and nets.

Heat transfer researchers recently investigated other uses for the fabric, namely as a cooling interface between human skin and wearable electronics. Their research is published in Scientific Reports.

Fabric’s cool flexibility

“This fabric has great flexibility and thermal properties. If you stitch it differently, weave it differently, or start blending the polymers with different materials, you could tailor the fabric’s properties to different applications,” says Justin Weibel, a research associate professor in the School of Mechanical Engineering at Purdue University.

If a material has a high thermal conductivity, that means heat dissipates through the material more easily. There are many heat-dissipation methods for fabrics, from the simple (moisture-wicking); to the intricate (conventional fabrics with heat-conducting strands woven in); to the very complex (liquid-cooled garments astronauts wear).

“Your next smartwatch or virtual reality headset could be more powerful than your current smartphone, so we need to dissipate heat away from the electronic components to keep the wearer comfortable,” says Aaditya Candadai, who recently completed his PhD at Purdue doing research on this project. “These polymer fabrics have amazing thermal properties that can keep these devices cooler and avoid low-degree skin burns.”

The team discovered these properties by benchmarking Dyneema against conventional cotton fabrics, as well as polyethylene sheets in rigid non-woven form. They obtained several different commercially manufactured fabric samples and even wove their own samples from raw Dyneema fibers.

The researchers put the samples into a small vacuum chamber, with a metal wire laid across the surface as a heat source.

Using an infrared microscope, they could generate detailed data about how much heat was being conducted through the fabric’s surface, and in which direction. They found that the Dyneema fabric has 20-30 times higher thermal conductivity than other fabrics, comparable with steel.

‘Sweet spot’

The team also tested the fabric’s flexibility, which is important for wearable electronics.

“There’s a balance; we don’t want to make thermally conductive materials that are so stiff, people won’t be comfortable wearing them,” Candadai says. “These polymer fabrics are in that sweet spot of having good conductivity and good flexibility.”

The fabric naturally has these properties with no additional circuity or other equipment, but the researchers also have plans to test how weaving in different materials affects the fabric.

“We could integrate other types of fibers—carbon fibers, metal fibers—to achieve different combinations of properties,” says Amy Marconnet, an associate professor of mechanical engineering.

The team’s research was performed within Purdue’s Cooling Technologies Research Center, a graduated National Science Foundation Industry/University Cooperative Research Center with support from industry leaders in thermal materials and electronics.

Source: Purdue University

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Researchers have created ultrathin, stretchable electronic material that is gas permeable, allowing the material to “breathe.”

They designed the material specifically for use in biomedical or wearable technologies, since the gas permeability allows sweat and volatile organic compounds to evaporate away from the skin, making it more comfortable for users—especially for long-term wear.

“The gas permeability is the big advance over earlier stretchable electronics,” says Yong Zhu, a professor of mechanical and aerospace engineering at North Carolina State University and co-corresponding author of a paper on the work in ACS Nano. “But the method we used for creating the material is also important because it’s a simple process that would be easy to scale up.”

Specifically, the researchers used a technique called the breath figure method to create a stretchable polymer film featuring an even distribution of holes. The film is coated by dipping it in a solution that contains silver nanowires. The researchers then heat-press the material to seal the nanowires in place.

“The resulting film shows an excellent combination of electric conductivity, optical transmittance, and water-vapor permeability,” Zhu says. “And because the silver nanowires are embedded just below the surface of the polymer, the material also exhibits excellent stability in the presence of sweat and after long-term wear.”

“The end result is extremely thin—only a few micrometers thick,” says coauthor Shanshan Yao, a former postdoctoral researcher who is now on faculty at Stony Brook University. “This allows for better contact with the skin, giving the electronics a better signal-to-noise ratio.

“And gas permeability of wearable electronics is important for more than just comfort,” Yao says. “If a wearable device is not gas permeable, it can also cause skin irritation.”

To demonstrate the material’s potential for use in wearable electronics, the researchers developed and tested prototypes for two representative applications.

The first prototype consisted of skin-mountable, dry electrodes for use as electrophysiologic sensors. These have multiple potential applications, such as measuring electrocardiography (ECG) and electromyography (EMG) signals.

“These sensors were able to record signals with excellent quality, on par with commercially available electrodes,” Zhu says.

The second prototype demonstrated textile-integrated touch sensing for human-machine interfaces. The authors used a wearable textile sleeve integrated with the porous electrodes to play computer games such as Tetris.

“If we want to develop wearable sensors or user interfaces that can be worn for a significant period of time, we need gas-permeable electronic materials,” Zhu says. “So this is a significant step forward.”

Additional coauthors are from Nanjing University of Posts and Telecommunications and NC State. The National Science Foundation supported the work.

Source: NC State

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Wearable sensor detects what your sweat is saying

Wearable skin sensors that detect what’s in your sweat could one day replace invasive procedures like blood draws and provide real-time updates on dehydration, fatigue, and other health problems.

Researchers used the sensors to monitor the sweat rate, and the electrolytes and metabolites in sweat, from volunteers who were exercising, and others who were experiencing chemically induced perspiration.

“The goal of the project is not just to make the sensors but start to do many subject studies and see what sweat tells us—I always say ‘decoding’ sweat composition,” says senior author Ali Javey, a professor of electrical engineering and computer science at the University of California, Berkeley.

“For that we need sensors that are reliable, reproducible, and that we can fabricate to scale so that we can put multiple sensors in different spots of the body and put them on many subjects,” says Javey, who is also a faculty scientist at Lawrence Berkeley National Laboratory.

High volume, low cost

As reported in Science Advances, scientists have developed a “roll-to-roll” processing technique that can quickly print the sensors onto a sheet of plastic like words on a newspaper.

The new sensors contain a spiraling microscopic tube, or microfluidic, that wicks sweat from the skin. By tracking how fast the sweat moves through the microfluidic, the sensors can report how much a person is sweating, or their sweat rate.

The microfluidics are also outfitted with chemical sensors that can detect concentrations of electrolytes like potassium and sodium, and metabolites like glucose.

Javey and his team worked with researchers at the VTT Technical Research Center of Finland to develop a way to quickly manufacture the sensor patches in a roll-to-roll processing technique similar to screen printing.

“Roll-to-roll processing enables high-volume production of disposable patches at low cost,” says Jussi Hiltunen of VTT. “Academic groups gain significant benefit from roll-to-roll technology when the number of test devices is not limiting the research. Additionally, up-scaled fabrication demonstrates the potential to apply the sweat-sensing concept in practical applications.”

Real-time health

To better understand what sweat can say about the real-time health of the human body, the researchers first placed the sweat sensors on different spots on volunteers’ bodies—including the forehead, forearm, underarm, and upper back—and measured sweat rates and the sodium and potassium levels in their sweat while they rode on an exercise bike.

They found that local sweat rate could indicate the body’s overall liquid loss during exercise, meaning that tracking sweat rate might be a way to give athletes a heads up when they may be pushing themselves too hard.

“Traditionally what people have done is they would collect sweat from the body for a certain amount of time and then analyze it,” says Hnin Yin Yin Nyein, a graduate student in materials science and engineering at UC Berkeley and one of the paper’s lead authors.

“So you couldn’t really see the dynamic changes very well with good resolution. Using these wearable devices we can now continuously collect data from different parts of the body, for example to understand how the local sweat loss can estimate whole-body fluid loss.”

Researchers also used the sensors to compare sweat glucose levels and blood glucose levels in healthy and diabetic patients, finding that a single sweat glucose measurement cannot necessarily indicate a person’s blood glucose level.

“There’s been a lot of hope that non-invasive sweat tests could replace blood-based measurements for diagnosing and monitoring diabetes, but we’ve shown that there isn’t a simple, universal correlation between sweat and blood glucose levels,” says Mallika Bariya, a graduate student in materials science and engineering and the paper’s other lead author.

“This is important for the community to know, so that going forward we focus on investigating individualized or multi-parameter correlations.”

Additional coauthors are from the VTT Technical Research Center of Finland and UC Berkeley. NSF Nanomanufacturing Systems for Mobile Computing and Mobile Energy Technologies; the Berkeley Sensor and Actuator Center; and the Bakar fellowship funded the work.

Source: UC Berkeley

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