HIGHLIGHTS
• In an effort to develop a fabric that is suitable for producing electricity, it must also be appealing to wear. 
• Friction and chemistry have been found to play important roles in optimizing triboelectric power generation from these fabrics. 
• Amphiphiles were coated onto diverse tribopair combinations of fibers that can simulate contact between textiles and skin.
• Two amphiphiles that are amides produced a better feel on polydimethylsiloxane (PMDS) than pristine PDMS due to reduced frictional change. 
In trying to find ways to generate electricity efficiently without producing emissions, researchers are working to convert motion that individuals do such as running and jumping into power through a process known as triboelectrification. This phenomenon involves the transfer of charges between an electronic device and human skin. 

In a previous TLT article,¹ work was reported on developing a stretchable battery that was 3D printed onto a wearable fabric such as a T-shirt through a process known as screen printing. A zinc-based battery was mixed with a novel binder prepared from a polyolefin block copolymer that is effective because it combines elasticity with hardness. Experiments proved that stretching the printable battery to an elongation of 1,300% did not lead to a reduction in performance.

But to truly develop a technology that can utilize human motion to produce electricity, an approach needs to be developed that takes into consideration adhesion and friction. Lilian Hsiao, associate professor of chemical and biomolecular engineering at North Carolina State University in Raleigh, N.C., says, “Triboelectrification is an energy-generating technique that occurs when two surfaces rub against each other. It is a way to harvest kinetic energy from small amounts of motion that produces static electricity through adhesion and detachment of solid surfaces. The challenge is to find a fabric that will interface with the skin to facilitate charge transfer. In past attempts to effect electricity generation, high energy density remains the biggest challenge.”

Hsiao also points out that a fabric suitable for producing electricity must also be appealing to wear. She says, “Individuals want to experience a pleasant sensation in wearing fabrics, which means that the materials used should exhibit low kinetic friction coefficients, but these types of materials generally do not produce much triboelectricity, so we have to find additives that can balance both aspects.”

The objective of this study is to combine efficient triboelectrification with a pleasant experience in wearing clothing. To find a suitable option, Hsiao and her colleagues evaluated a group of additives known as amphiphiles that can be applied to the polymer systems used in preparing wearable fabrics. 

Hsiao says, “Amphiphiles are molecules that exhibit both water soluble (hydrophilic) and water rejecting (hydrophobic) properties. The appeal of these molecules is that they are compatible with most polymers and tend to migrate to the surface of these materials where they can self-assemble into robust, slippery multilayers that minimize contact between solid surfaces and reduce friction. The potential exists for tuning these amphiphiles to maximize energy-harvesting while maximizing comfort. One of the underlying scientific challenges is to understand why certain amphiphiles reduce friction more than others, and we came up with a model that accounts for the compressibility of the substrate and the formation of sheet-like structures of the molecules that migrate to the surface.”

Hsiao and her colleagues initiated a study to evaluate specific types of amphiphiles that differ slightly in their chemical composition. They also then evaluated a few additional amphiphiles that exhibit different electron-withdrawing properties compared to a material, polydimethylsiloxane (PDMS), that has similar elasticity as human skin. The results show that both friction and chemistry play important roles in optimizing triboelectric power generation from these fabrics. 

Slip additives
The researchers utilized two slip additives, erucamide and behenamide, as small amphiphiles that were coated on diverse tribopair combinations that can simulate contact between textiles and skin. Tribopairs included in this study were polypropylene-polypropylene (PP-PP), PP-silicon wafers, PP-low-density polyethylene (LDPE), PP-PDMS, PDMS-PMDS and polyurethane-PP nonwovens. 

Hsiao says, “We decided to work with the two amides because they are known to migrate to the surface of most materials and self-assemble into slippery interfacial multilayers due to something called π-π stacking, which comes from double bonds in the carbon backbone of the molecule. The big difference between erucamide and behenamide is that the former has a double bond capable of producing ordered mesoscale stacks that slide across each other easily, thus reducing friction to a lower level more effectively than molecules without this type of structure.”

Optical imaging showed that the substrates were completely coated with the additive layer being approximately 3.8 microns thick. Pristine and coated tribopairs were evaluated for boundary friction through the use of a stress-controlled triborheometer. 

Erucamide coated tribopairs demonstrated 80%-90% lower friction than pristine fabrics in most cases. Behenamide coated tribopairs also exhibited lower friction. The researchers found different fiction reduction with different tribopairs indicating that dissipation of friction is dependent upon the substrate elasticity, and the pressure exerted on the amphiphile molecules that have migrated to the surface. 

Individuals then evaluated the different pairs of pristine and coated PDMS substrates using a flexible wearable triboelectric device that consisted of a conformal backing, and a thin copper film as a collecting electrode (see Figure 2). Erucamide coated PDMS displayed a better feel probably due to reduced frictional change compared to the pristine PDMS, whereas individuals had difficulty in distinguishing between erucamide and behenamide coated fabrics. 


Figure 2. A patch containing a wearable triboelectric device, consisting of a conformal backing, and a thin copper film, was used to evaluate how much electricity can be generated while an individual is in motion and to assess how the device feels during use. Figure courtesy of Pedro Henrique Wink Reis (North Carolina State University).

The triboelectric output of PDMS coated with two amphiphiles was lower than seen with the pristine material. The researchers then coated PDMS again with stearic acid and perfluorooctylamine. Hsiao says, “Both stearic acid and perfluorooctylamine exhibit stronger electron-withdrawing properties that increase polarization with PDMS leading to a higher triboelectric output.”

For the future, Hsiao indicates that the fabric needs to be modified to increase triboelectric generation. She says, “We used only one layer in this study, but there needs to be an approach to work with porous PDMS which will increase the area for facilitating triboelectric generation between a textile and human skin. Of course, we are looking for molecules that are good for the environment, and for humans. A better approach needs to be made to collect the triboelectric charge. One possibility is to make each fiber an electrode so that the static electricity can be collected in a battery. We are also evaluating the ability of our current system to collect energy by having one of our colleagues embed a porous tribopair material inside a shoe so it is directly touching the foot. This individual will then go through various motions such as repetitive jumping to assess the magnitude of triboelectric production. We think this technology could soon reach a point where people can power their personal electronic devices simply by moving around.”

Additional information can be found in a recent article² or by contacting Hsiao at lilian_hsiao@ncst.edu.
REFERENCES
1. Canter, N. (2017), “Printing batteries,” TLT, 73 (8), pp. 10-11. Available at www.stle.org/files/TLTArchives/2017/08_August/Tech_Beat_I.aspx.
2. Jani, P., Yadav, K., Derkaloustian, M., Koerner, H., Dhong, C., Khan, S. and Hsiao, L., (2025), “Compressing slippery surface-assembled amphiphiles for tunable haptic energy harvesters,” Science Advances, 11 (3), eadr4088.