Textination Newsline

• Small, lightweight, stretchable, cost-efficient thermoelectric devices signify a breakthrough in sustainable energy development and waste heat recovery.
• Next-gen flexible energy harvesting systems will owe their efficiency to the integration of graphene nanotubes. They offer easy processability, stable thermoelectric performance, flexibility, and robust mechanical properties.
• Nanocomposites have high market potential in manufacturing generators for medical and smart wearables, vehicles sensors, and efficient building management.

Around half of the world’s useful energy is wasted as heat due to the limited efficiency of energy conversion devices. For example, one-third of a vehicle’s energy dissipates as waste heat in exhaust gases. At the same time, vehicles contain more and more electronic devices requiring electrical energy. As another example, lightweight wearable sensors for health and environmental monitoring are also becoming increasingly demanding. The potential to convert waste heat or solar energy into useful electrical power has emerged as an opportunity for more sustainable energy management. Convenient thermoelectric generators (TEGs) currently have only low effectiveness and a relatively large size and weight. Based on expensive or corrosion-vulnerable materials, they are rigid and often contain toxic elements.
 
Recently developed, easy-to-process, self-supporting and flexible nonwoven nanocomposite sheets demonstrate excellent thermoelectric properties combined with good mechanical robustness. A recent paper in ACS Applied Nano Materials described how researches combined a thermoplastic polyurethane (TPU) with TUBALLTM graphene nanotubes to fabricate a nanocomposite material capable of harvesting electrical energy from sources of waste heat.

Thanks to their high aspect ratio and specific surface area, graphene nanotubes provide TPU with electrical conductivity, making it possible to achieve high thermoelectrical performance while maintaining or improving mechanical properties. “Stiffness, strength, and tensile toughness were improved by 7, 25, and 250 times compared to buckypapers, respectively. Nanocomposite sheet shows low electrical resistivity of 7.5*10-3 Ohm×cm, high Young’s modulus of 1.8 GPa, failure strength of 80 MPa, and elongation at break of 41%,” said Dr. Beate Krause, Group Leader, Leibniz-Institut für Polymerforschung Dresden e. V.

Graphene nanotubes, being a fundamentally new material, provide an opportunity to replace current TEG materials with more environmentally friendly ones. The sensors powered by such thermoelectric generators could act as a “smart skin” for vehicles and buildings, providing sensoring capabilities to monitor performance and prevent potential issues before they lead to breakdowns, ensuring optimal operational efficiency. In aircraft, no-wire nanocomposites could serve as stand-alone sensors for monitoring deicing systems, eliminating the need for an extensive network of electrical cables. The high flexibility, strength, and reliability of graphene nanotube-enabled thermoelectric materials also extend their applications into the realm of smart wearable and medical devices.

What comes to mind when you think about “wearable technology?” In 2023, likely a lot, at a time when smartwatches and rings measure heart rates, track exercise and even receive text messages. Your mind might even drift to that “ugly” light-up sweater or costume you saw last Halloween or holiday season.

At the Wilson College of Textiles, though, researchers are hard at work optimizing a truly new-age form of wearable technology that can be proven useful in a wide range of settings, from fashion and sports to augmented reality, the military and medicine.

Currently in its final stages, this grant-funded project could help protect users in critical situations, such as soldiers on the battlefield and patients in hospitals, while simultaneously pushing the boundaries of what textiles research can accomplish.

“The goals set for this research are quite novel to any other literature that exists on wearable connectors” says Shourya Dhatri Lingampally, Wilson College of Textiles graduate student and research assistant involved in the project alongside Wilson College Associate Professor Minyoung Suh.

Ongoing since the fall of 2021, Suh and Lingampally’s work focuses on textile-integrated wearable connectors, a unique, high-tech sort of “bridge” between flexible textiles and external electronic devices. At its essence, the project aims to improve these connectors’ Technology Readiness Level — a key rating used by NASA and the Department of Defense used to assess a particular technology’s maturity.

To do this, Lingampally and her colleagues’ research examines problems that have, in the past, affected the performance of wearable devices.

Sure, these advances may benefit fashion, leading to eccentric shirts, jackets, or accessories — “to light up or change its color based on the wearer’s biometric data,” Lingampally offers — the research has roots in a much deeper mission.

Potential benefits to military, medicine and beyond
The project is funded through more than $200,000 in grant money from Advanced Functional Fabrics of America (AFFOA), a United States Manufacturing Innovation Institute (MII) located in Cambridge, Massachusetts. The mission of AFFOA is to support domestic manufacturing capability to support new technical textile products, such as textile-based wearable technologies.

A key purpose of the research centers around improving the functionality of wearable monitoring devices with which soldiers are sometimes outfitted to monitor the health and safety of their troops remotely.

Similar devices allow doctors and other medical personnel to remotely monitor the health of patients even while away from the bedside.

Though such technology has existed for years, it’s too often required running wires and an overall logistically-unfriendly design. That could soon change.

“We have consolidated the electronic components into a small snap or buckle, making the circuits less obtrusive to the wearer,” Lingampally says, explaining the team’s innovations, which include 3D printing the connector prototypes using stereolithography technology.

“We are trying to optimize the design parameters in order to enhance the electrical and mechanical performance of these connectors,” she adds.

To accomplish their goals, the group collaborated with NC State Department of Electrical and Computer Engineering Assistant Research Professor James Dieffenderfer. The team routed a variety of electrical connections and interconnects like conductive thread, epoxy and solder through textile materials equipped with rigid electronic devices.

They also tested the components for compatibility with standard digital device connections like USB 2.0 and I2C.

Ultimately, Lingampally hopes their work will make wearable technology not only easier and more comfortable to use, but available at a lower price, too.

“I would like to see them scaled, to be mass manufactured, so they can be cost efficient for any industry to use,” she explains.

In a bigger-picture sense, though, her team’s work is reinforcing the far-reaching boundaries of what smart textile research can accomplish; a purpose that stretches far beyond fashion or comfort.

Pushing the boundaries of textiles research
Suh and Lingampally’s work is just the latest breakthrough research originating from the Wilson College of Textiles that’s aimed at solving critical problems in the textile industry and beyond.

“The constant advancements in technology and materials present immense potential for the textile industry to drive positive change across a range of fields from fashion to healthcare and beyond,” Lingampally, a graduate student in the M.S. Textiles program, says, noting the encouragement she feels in her program to pursue innovation and creativity in selecting and advancing her research.

Additionally, in the fiber and polymer science doctoral program, which Suh does research with, candidates focus their research on a seemingly endless array of STEM topics, ranging from forensics to medical textiles, nanotechnology and, indeed, smart wearable technology (just to name a few).

In this case, Suh says, the research lent itself to “unexpected challenges” that required intriguing adaptations “at every corner.” But, ultimately, it led to breakthroughs not previously seen in the wearable technology industry, attracting interest from other researchers outside the university, and private companies, too.

“This project was quite exploratory by nature as there hasn’t been any prior research aiming to the same objectives,” Suh says.

Meanwhile, the team has completed durability and reliability testing on its textile-integrated wearable connectors. Eventually, the group would like to increase the sample size for testing to strengthen and validate the findings. The team also hopes to evaluate new, innovative interconnective techniques, as well as other 3D printing techniques and materials as they work to further advance wearable technologies.

Embroidering power-generating yarns onto fabric allowed researchers to embed a self-powered, numerical touch-pad and movement sensors into clothing. The technique offers a low-cost, scalable potential method for making wearable devices.

“Our technique uses embroidery, which is pretty simple – you can stitch our yarns directly on the fabric,” said the study’s lead author Rong Yin, assistant professor of textile engineering, chemistry and science at North Carolina State University. “During fabric production, you don’t need to consider anything about the wearable devices. You can integrate the power-generating yarns after the clothing item has been made.”

In the study published in Nano Energy, researchers tested multiple designs for power-generating yarns. To make them durable enough to withstand the tension and bending of the embroidery stitching process, they ultimately used five commercially available copper wires, which had a thin polyurethane coating, together. Then, they stitched them onto cotton fabric with another material called PTFE.

“This is a low-cost method for making wearable electronics using commercially available products,” Yin said. “The electrical properties of our prototypes were comparable to other designs that relied on the same power generation mechanism.”

The researchers relied on a method of generating electricity called the “triboelectric effect,” which involves harnessing electrons exchanged by two different materials, like static electricity. They found the PTFE fabric had the best performance in terms of voltage and current when in contact with the polyurethane-coated copper wires, as compared to other types of fabric that they tested, including cotton and silk. They also tested coating the embroidery samples in plasma to increase the effect.

“In our design, you have two layers – one is your conductive, polyurethane-coated copper wires, and the other is PTFE, and they have a gap between them,” Yin said. “When the two non-conductive materials come into contact with each other, one material will lose some electrons, and some will get some electrons. When you link them together, there will be a current.”
Researchers tested their yarns as motion sensors by embroidering them with the PTFE fabric on denim. They placed the embroidery patches on the palm, under the arm, at the elbow and at the knee to track electrical signals generated as a person moves. They also attached fabric with their embroidery on the insole of a shoe to test its use as a pedometer, finding their electrical signals varied depending on whether the person was walking, running or jumping.

Lastly, they tested their yarns in a textile-based numeric keypad on the arm, which they made by embroidering numbers on a piece of cotton fabric, and attaching them to a piece of PTFE fabric. Depending on the number that the person pushed on the keypad, they saw different electrical signals generated for each number.

“You can embroider our yarns onto clothes, and when you move, it generates an electrical signal, and those signals can be used as a sensor,” Yin said. “When we put the embroidery in a shoe, if you are running, it generates a higher voltage than if you were just walking. When we stitched numbers onto fabric, and press them, it generates a different voltage for each number. It could be used as an interface.”

Since textile products will inevitably be washed, they tested the durability of their embroidery design in a series of washing and rubbing tests. After hand washing and rinsing the embroidery with detergent, and drying it in an oven, they found no difference or a slight increase in voltage. For the prototype coated in plasma, they found weakened but still superior performance compared with the original sample. After an abrasion test, they found that there was no significant change in electrical output performance of their designs after 10,000 rubbing cycles.

In future work, they plan to integrate their sensors with other devices to add more functions.
“The next step is to integrate these sensors into a wearable system,” Yin said.

The study, “Flexible, durable and washable triboelectric yarn and embroidery for self-powered sensing and human-machine interaction,” was published online in Nano Energy. Co-authors included Yu Chen, Erdong Chen, Zihao Wang, Yali Ling, Rosie Fisher, Mengjiao Li, Jacob Hart, Weilei Mu, Wei Gao, Xiaoming Tao and Bao Yang. Funding was provided by North Carolina State University through the NC State Faculty Research & Professional Development Fund and the NC State Summer REU program.

 

Flexible electronics have enabled the design of sensors, actuators, microfluidics and electronics on flexible, conformal and/or stretchable sublayers for wearable, implantable or ingestible applications. However, these devices have very different mechanical and biological properties when compared to human tissue and thus cannot be integrated with the human body.

A team of researchers at Texas A&M University has developed a new class of biomaterial inks that mimic native characteristics of highly conductive human tissue, much like skin, which are essential for the ink to be used in 3D printing.

This biomaterial ink leverages a new class of 2D nanomaterials known as molybdenum disulfide (MoS2). The thin-layered structure of MoS2 contains defect centers to make it chemically active and, combined with modified gelatin to obtain a flexible hydrogel, comparable to the structure of Jell-O.

“The impact of this work is far-reaching in 3D printing,” said Dr. Akhilesh Gaharwar, associate professor in the Department of Biomedical Engineering and Presidential Impact Fellow. “This newly designed hydrogel ink is highly biocompatible and electrically conductive, paving the way for the next generation of wearable and implantable bioelectronics.”¹ 

The ink has shear-thinning properties that decrease in viscosity as force increases, so it is solid inside the tube but flows more like a liquid when squeezed, similar to ketchup or toothpaste. The team incorporated these electrically conductive nanomaterials within a modified gelatin to make a hydrogel ink with characteristics that are essential for designing ink conducive to 3D printing.

“These 3D-printed devices are extremely elastomeric and can be compressed, bent or twisted without breaking,” said Kaivalya Deo, graduate student in the biomedical engineering department and lead author of the paper. “In addition, these devices are electronically active, enabling them to monitor dynamic human motion and paving the way for continuous motion monitoring.”

In order to 3D print the ink, researchers in the Gaharwar Laboratory designed a cost-effective, open-source, multi-head 3D bioprinter that is fully functional and customizable, running on open-source tools and freeware. This also allows any researcher to build 3D bioprinters tailored to fit their own research needs.

The electrically conductive 3D-printed hydrogel ink can create complex 3D circuits and is not limited to planar designs, allowing researchers to make customizable bioelectronics tailored to patient-specific requirements.

In utilizing these 3D printers, Deo was able to print electrically active and stretchable electronic devices. These devices demonstrate extraordinary strain-sensing capabilities and can be used for engineering customizable monitoring systems. This also opens up new possibilities for designing stretchable sensors with integrated microelectronic components.

One of the potential applications of the new ink is in 3D printing electronic tattoos for patients with Parkinson’s disease. Researchers envision that this printed e-tattoo can monitor a patient’s movement, including tremors.

This project is in collaboration with Dr. Anthony Guiseppi-Elie, vice president of academic affairs and workforce development at Tri-County Technical College in South Carolina, and Dr. Limei Tian, assistant professor of biomedical engineering at Texas A&M.
This study was funded by the National Institute of Biomedical Imaging and Bioengineering, the National Institute of Neurological Disorders and Stroke and the Texas A&M University President’s Excellence Fund. A provisional patent on this technology has been filed in association with the Texas A&M Engineering Experiment Station.

¹ This study was published in ACS Nano.