You’ve likely read or seen video reports of various university research projects which use fabric—usually fashioned into a shirt—as an energy-harvesting arrangement. Some of these use fabrics which have been treated to generate small amounts of power via an enhanced triboelectric effect and wearer frictional movement, others have been modified to functions as thermoelectric generators (TEGs) based on body heat, and a few even try to fabricate solar cells on the material to catch ambient light.
Once again, it’s the concept of “something for (almost) nothing” with respect to energy harvesting (or scavenging) which is the lure and headline-grabber. It all sounds so attractive and enticing, and makes a lot of sense, at least in theory.
Of course, “in theory” is one thing and “in practice” is often another. Although some of these developments have been heralded in press releases along the lines of “energy harvesting from your own body to charge your smartphone” or similar, the reality is different. As far as I have been able to find out through some basic searches, none of these have been converted into standard consumer products, although some are being used for specialized sensor systems such as for athletes.
There’s a lot more to a garment than just its fabric. In the case of energy harvesting, there are connections, storage (battery or supercap), some power-management electronics, normal use and abuse, wash cycles, and more.
Recent development
However, there’s the complement of using fabrics and shirts for energy harvesting, and that’s using them for energy capture. A recent project from Drexel University, University of Pennsylvania, and Accenture Labs team has devised in a process for using MXene ink to print a textile energy grid that can be charged wirelessly at 140 kilohertz
What’s “MXene” ink? It’s a nanomaterial substance which Drexel has been involved with for quite a while and with which they have considerable experience. Their work centers on the process and viability of building a small-scale power “grid” by printing it on nonwoven cotton textiles with an ink composed of MXene. MXenes were created at Drexel and are simultaneously highly conductive yet durable enough to withstand the folding, stretching and washing that clothing endures.
[More formally, MXenes are a family of two-dimensional (2D) carbides or nitrides with the formula Mn+1XnTx where n = 1, 2, 3, or 4; M is an early transition metal, X is either carbon and/or nitrogen, and T is a surface termination bonded to the M element (e.g., OH, O, F, or Cl).]
The team’s textile grid was printed on a lightweight, flexible, cotton substrate the size of a small patch. It includes a printed resonator coil, which they called an MX-coil, that can convert impinging RF energy via induction and use it to charge a series of three textile supercapacitors (also previously developed by Drexel and Accenture Labs) that can store energy and use it to power electronic devices.
For various reasons, they chose to use direct-ink-write printing (DIW) for prototyping and development of the MX-coils. They had to perform a rheological analysis as there are several different shear rates during DIW printing, with a high shear through the writing needle but low shear as a deposited material on the substrate. As a shear-thinning material, MXene ink has low viscosity at high shear rates, allowing it to flow easily through the needle, but high viscosity at low shear, meaning that it retains its printed geometry without spreading on the substrate, Figure 1.
Figure 1 Rheological data on a MXene ink. a) Shear rate ramp, b) amplitude sweep, and c) frequency sweep. d) Schematic depicting a direct-ink-write that is used to print wireless charging MXene coils. e) A photograph of a 5×5-cm MX-coil. Light micrographs and nanoCT images of prints on f) hydrophobic and g) hydrophilic woven cotton, showing superior deposition onto hydrophilic cotton. Source: Drexel University, University of Pennsylvania, and Accenture Labs
They evaluated several different woven-cotton substrates for the best print quality. Initial coil designs were modeled in MATLAB using a conductivity of 20,000 siemens/cm and a thickness of 10 µm as assumptions for the MXene trace. The coils were modeled using a140 kHz transmit frequency, which is within the range of the Qi standard.
While this modeling provided a solid design framework, many of the optimizing parameters had to be altered to accommodate the engineering challenges due to practical limitations, primarily based on the surface roughness of the textile surface. To test the effectiveness of MX-coils, they isolated several parameters such as shape, number of turns, and trace pitch to find an optimum combination.
Results
They fabricated an MX-coil with a 1200-µm pitch, 10 turns, and 40 passes (resistance = 80 Ω) to analyze how effectively MXcoils can charge MXene-textile supercapacitors capable of powering on-body electronics and transmitting data via Bluetooth, Figure 2.
Figure 2 a) Schematic of MXene-textile supercapacitor that is being powered by the MX-coil. b) Schematic of testing setup. c) Curves of MX-coil charge and 2 mA discharge. d) Discharging time at 2 mA as a function of MX-coil charging time. e) MX-coil charging current and MXene-textile supercapacitor voltage. f) Powering an Artemis Nano microcontroller for BLE broadcast with MXene-textile supercapacitor charged with MX-coil. Source: Drexel University, University of Pennsylvania, and Accenture Labs
A DC power supply was used to feed 10 V into the transmitter-coil circuitry where power was transferred wirelessly to the MX-coil. AC power was then diverted through a battery-management system that rectifies the signal and limits the voltage going into the supercapacitor. The charging is collected on a potentiostat, voltage data is collected by measuring the voltage at the supercapacitor terminals, and current data is collected in series between the LTC3331 power-management board and the supercapacitor.
The assessed power transfer at up to 10% efficiency, resulting in 100 milliwatts of power directly applied to textiles. They also used it to charge a MXene-textile supercapacitor, introducing the idea of an on-garment energy grid fully made of MXene. Additionally, they showed that MXcoils are capable of directly powering MXene-based surface electromyographic (sEMG) sensors with wireless live data transmission, using an Artemis Nano microcontroller for BLE broadcast.
The project also demonstrated some of the unique challenges faced by flexible, fabric-based charging schemes. They saw significant degradation of the cell over time; however, they could “reconstitute” the cell by squishing it under approximately 10 kilograms for several hours. This led them to speculate that they were losing contact between the carbon foil tabs and the MXene-textile electrodes rather than observing a breakdown of the electrodes themselves.
The work is detailed in their paper “MXene-enabled textile-based energy grid utilizing wireless charging” published in Material Today. The paper has the usual discussion, but also has exposition of the fabrication techniques, test arrangement, production and test equipment used, and more in a supplement at the end.
What’s your view on wireless charging of fabric and clothes? Is it a discovery waiting to hit some inflection point, or are the practicalities of fabrication, longevity, and usefulness too daunting? Will it be limited to being an academic research project or will it find a role in some specialty application niches?
Bill Schweber is an EE who has written three textbooks, hundreds of technical articles, opinion columns, and product features.
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