One unsung energy-harvesting phenomenon that has recently attracted more research in the wearables industry is triboelectricity, or small-scale electricity generated through contact and motion between materials—in essence, friction.
As a self-powered solution, triboelectric nanogenerators (TENGs), have become an increasingly attractive solution to embedded designers in the IoT space because they convert mechanical energy into electricity.
A high-level depiction of the different applications of nanogenerators. Image used courtesy of Indra et al. and MDPI
As an example of a recent use case, university researchers from Alfred State developed TENGs for masks and general wearables using magnetic actuators. How else are TENGs showing promise for future energy-harvesting technologies?
What Are Triboelectric Nanogenerators (TENGs)?
Back in 2012, researchers from Georgia Tech and Xiamen University, China, discovered TENGs and created a triboelectric generator (TEG). This device hinges on the principle of electrostatic induction, converting various mechanical energy to electricity. TENGs can harvest energy for electrification from walking, vibration, human motion, wind, rotating tires, and flowing water.
A schematic of a TEG. Image used courtesy of Fan et al.
Types of Friction: Contact Separation and Sliding
TENGs have two main modes of operation: contact separation and sliding.
The contact-separation mode involves two parts of the TENG coming in contact with the other and separating. These parts are made of different materials that exchange electric charges, leaving them with different electric potentials. This process results in current flow between the electrodes attached at the back of each of the parts.
The sliding mode involves the two materials sliding over each other to produce charges on the surfaces. Like contact-separation, these different electric potentials cause current to flow between the electrodes at the back of the two materials.
The two main modes of TENGs: a) vertical contact separation and b) lateral sliding. Image used courtesy of Zheng et al.
The performance of TENGs depends on the material used to develop them; different materials have different triboelectric charges. The superposition principle of electric potential implies that output voltage and current are affected by the density of triboelectric charges.
Materials used to develop TENGs must produce triboelectric charges easily and have different triboelectric polarisation. TENGs are often created from are polytetrafluoroethylene (PTFE), polyamide, polyvinylidene fluoride (PVDF), and silk materials.
Practical Applications of Triboelectricity
Because of TENGs’ triboelectric and energy-harvesting benefits, researchers have been putting this technology to the test in a number of embedded applications—particularly those involving self-powered sensors.
Triboelectricity can be used to create self-powered sensors known as triboelectric sensors (TES). Back in 2014, Chinese researchers teamed up with researchers from the Georgia Institute of Technology to develop a “self-powered, ultrasensitive, flexible tactile sensor based on contact electrification.”
The architecture of the TES was sandwiched in several layers: at the top was a layer of fluorinated ethylene propylene (FEP) modified by polymer nanowires. Next came a three-layer structure—a layer of polyethylene terephthalate (PET) and two transparent indium tin oxide (ITO) layers. Finally, the bottom layer was made of a nylon film.
A diagram of a) the layers of the TES sensor, b) the polymer nanowires, and c) the fabricated TES sensor. Image used courtesy of Zhu et al.
The TES generated a corresponding 35 V when 20 mN of force was applied. Researchers also found that the output voltage could offset a siren alarm if the TES integrated with a signal processing circuit (which was, in fact, one experiment demonstrated in the study).
TENGs are also a point of interest in biometric health monitoring. This year, some of the same researchers from Guangxi University, the Chinese Academy of Sciences, and the Georgia Institute of Technology developed a smart wearable sensor (SWS) for health monitoring using TENGs.
Their SWS consists of PTFE nanowire film and an iron (Fe) ball molded into an acrylic ring with copper electrodes, which can be placed on clothing. The Fe ball’s movement within the PTFE layer causes the electrodes underneath the PTFE to produce an uneven charge distribution.
Thus, the electron transfer between electrodes is necessary to balance the local potential distribution and generate a current corresponding to the Fe ball’s motion.
A diagram of a) the layers of the SWS and b) the physical SWS. Image used courtesy of Li et al. and MDPI
The research team studied their sensor in medical scenarios, such as a fall-down alarm system and sleep monitoring. Used in tandem with other components, the SWS was able to send and analyze biometric information in these scenarios.
Though this is just one of many forms of TENG, the question remains of whether TENGs and triboelectric technologies will move beyond the realm of research.
The Opportunities and Challenges of TENGs
Energy-harvesting technologies like solar cells, piezoelectric nanogenerators, and thermoelectric cells are on the rise—and it seems that TENGs may be yet another technology to join the pack. TENG proponents claim these devices can be paired with other power generators or energy harvesters for a hybrid system, enhancing output performance and stability.
TENGs feature high energy-conversion efficiency, easy fabrication, low cost, and high power density. Nevertheless, TENGs have their share of technical challenges.
For instance, TENGs are not suited for high-performance applications. They are also prone to wear and tear, although materials like graphene, carbon nanotubes, and nano-Ag ink may improve performance and durability if incorporated into future TENG designs. TENGs also need adequate packaging that can protect them from environmental factors.
As future studies unveil remedies to these triboelectric challenges, it’s possible that TENGs could become a common linchpin in future self-powered embedded systems.
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