Keeping cool with wearable textiles

Dr. Neil Canter, Contributing Editor | TLT Tech Beat February 2018

Researchers have developed a composite textile that can be woven into fabrics.

 

KEY CONCEPTS
• Personal cooling technologies are now commercially available but do not exhibit adequate thermal conductivity. 
• A new composite fiber based on boron nitride and polyvinyl alcohol exhibits good thermal conductivity when woven into a fabric. 
• Composite fibers are efficiently prepared using 3D printing.
TECHNOLOGY IS ADVANCING TO ENABLE INDIVIDUALS to better use their bodies as power sources. In a previous TLT article, researchers developed a wearable device that takes advantage of the temperature difference between two objects to generate power (1). This process is known as thermoelectric generation (TEG) and, in this case, takes advantage of the difference between the temperature of the human body and the ambient environment. 

The maximum amount of power generated was 20 microwatts per square meter when the TEG device is placed on the upper arm and the individual using it is walking at a rate of 1.1 meters per second.

One problem that individuals deal with in hot climates or during the summer is dissipating this body heat. We rely on air conditioning to control temperature, but the cost and the increased emissions make it worthwhile to see what can be done for the individual.

The result has been the development of personal cooling technologies. Liangbing Hu, associate professor in the Department of Materials Science and Engineering & Energy Research Center at the University of Maryland in College Park, Md., says, “Personal cooling technologies provide thermal comfort to the individual by directing local heat to the thermal-regulated environment.”

For the textile industry, personal cooling technologies offer an approach for simultaneously providing thermal comfort and reducing energy consumption for the individual. Hu says, “This function can enable textiles or garments to have a new function and develop enhanced value.”

Personal cooling technologies are commercially available and include moisture management textiles, air-cooled textiles, cold pack textile phase change materials and liquid cooling textiles. But these technologies have limitations.

Hu says, “Moisture management textiles are the most common thermal-management textiles on the market, but its thermal-management mechanism can only be triggered when the microclimate between human skin and fabric is at a high humidity level. The other technologies have limitations such as inconvenience due to the bulky size of the cold pack, massive consumption of power and high cost. These technologies are mainly utilized to reduce the risk of heat-related injuries to the human body and not suitable for general applications.”

A key challenge that needs to be overcome is to find a material that exhibits improved thermal conductivity. Such a material in the form of a composite fiber has now been prepared.

BORON NITRIDE NANOSHEETS
Hu and his colleagues developed a new type of textile fiber based on a composite of boron nitride (BN) nanosheets and polyvinyl alcohol (PVA) that can be woven into a fabric. He says, “BN has traditionally been considered as an effective material in thermal management applications due to its high thermal conductivity, yet this material also is an electrical insulator. The BN nanosheets exhibit high in-plane thermal conductivity and are ideal filler materials for fabrication of a thermal conductive composite.”

The processing of the BN/PVA composite fiber is shown in Figure 2. Initially, BN and PVA were dispersed in dimethyl sulfoxide prior to 3D printing. Hu says, “PVA has a great interfacial compatibility with BN nanosheets that helps with dispersion in solution.”


Figure 2. A new type of textile fiber based on a composite of boron nitride (BN) and polyvinyl alcohol (PVA) that shows superior thermal conductivity is processed in the manner shown. (Figure courtesy of the University of Maryland.)

The next step was preparing the fibers through 3D printing. Hu says, “3D printing, as an efficient additive manufacturing technique, can fast and accurately fabricate an arbitrary and complicated structure. This technique is not only scalable but efficiently fabricates the BN/PVA composite fibers and also can promote the development of new 3D printed textile structures.”

The composite fibers are then drawn at a temperature of 200 C. Hu says, “This process leads to a composite fiber that displays a combination of high mechanical strength (355 megapscals) and favorable heat dispersion. Due to the improved thermal transport property imparted by the thermally conductive and highly aligned BN nanosheets, a better cooling effect (55% improvement over commercial cotton fiber) can be achieved.”

In evaluating the performance of the BN/PVA composite fiber, the researchers used a laser-infrared camera and laser input power to determine the temperature distribution of the fiber bundles and fabrics and to evaluate thermal conductivity. Hu says, “BN/PVA was evaluated versus pure PVA and cotton fabrics. We found that the BN/PVA composite fabric displayed the lowest maximum temperature at laser power inputs of 0.047, 0.079 and 0.096 Watts. The BN/PVA composite fabric also demonstrated the highest thermal conductivity of the three fibers tested.”

For the future, the researchers will further examine the thermal conductivity of BN/PVA. Hu says, “We plan to use a heated mannequin that is similar to an actual human body to further study the thermal properties of the composite fabric. Newer textile structures will be designed and prepared based on 3D printing technology. We also plan to evaluate other fiber candidates with promising physical properties for thermal management applications.”

Additional information on this work can be found in a recent article (2) or by contacting Hu at binghu@umd.edu. 

REFERENCES
1. Canter, N. (2016), “Thermoelectric generation using body heat,” TLT, 72 (12), pp. 10-11.
2. Gao, T., Yang Z., Chen, C., Li, Y., Fu, K., Dai, J., Hitz, E., Xie, H., Liu, B., Song, J., Yang, B. and Hu, L. (2017), “Three-Dimensional Printed Thermal Regulation Textiles,” ACS Nano, 11 (11), pp. 11513-11520.

Neil Canter heads his own consulting company, Chemical Solutions, in Willow Grove, Pa. Ideas for Tech Beat can be submitted to him at neilcanter@comcast.net.