Textile batteries for wearable devices
Washable capacitors may improve the usability of wearables and lead to entirely fabric smart devices. Ceri Jones finds out how it works.
Flexible, ultra-thin batteries are necessary to advance high-performance wearable technologies. Current wearables, such as health trackers, rely on rigid elements like lithium batteries, but these are cumbersome and cannot be worn continuously due to moisture ingress from sweat or cleaning.
Two-dimensional materials applied as conductive ink could bridge the gap, enabling the manufacture of textile-based energy storage devices that are light, breathable and washable. A team at Cambridge University, UK, led by Professor Felice Torrisi, has demonstrated the effectiveness of a fabric capacitor that could be used to make clothing for smart tracking applications, such as health monitoring.
The core of it
Torrisi’s team layered electrically conductive and dielectric insulating materials, in this case graphene and boron nitride fabrics. The functionality is obtained from the uniform coating of positively charged polyester textiles with negatively charged graphene and boron nitride inks. The inks are applied in a process similar to dyeing to result in a superhydrophobic material, while the strong electrostatic charge interaction between the ink and the fabric causes improved adhesion of the functional coating and 2D materials.
‘We used graphite exfoliated in solution and created what we call an ink or a dispersion of graphene in liquid, in this case a low-boiling ethanol or water, and we used this electrostatic, ionic interaction to ensure graphene sticks well to the fabric and withstands washing,’ Torrisi said.
‘In parallel, we developed a hexagonal boron nitride (h-BN) fabric showing dielectric properties. h-BN is a large energy band gap semiconductor, so normally withstands a relatively large electric field and you can use it to store electrical charges. Hence, we sandwiched this h-BN fabric between the two fabric electrodes.’
The resulting sandwich undergoes hot press annealing to bind the layers and create the fabric capacitor, which can withstand a tenth of a volt per millimetre and stores ~25pF/cm2 – enough to power a small biosensor.
Samples went through 20 washing cycles with detergent at 60°C, showing only a 5% change in capacitance, which Torrisi deemed negligible.
In the system
Taking this storage demonstrator forward, the team is working on the next stage to connect energy storage and generator elements to biosensors.
‘Coupling the charge storage device to other textile components is a challenge. In this case, it wouldn’t be a single device or component but a textile electronic system. You could have different devices, e.g., one that generates energy, another that stores it, and the two need to talk together after several washing cycles.’
‘We are working towards an integrated system that combines charge generation and storage, with piezoelectric or thermoelectric materials. Depending on the strain and bending applied to these materials, they generate an electric charge,’ Torrisi explained. ‘The charge could be generated from a piezo actuator producing electricity from the body movement, or a thermoelectric device using body heat. This is stored into the textile capacitor and released when required.’
At this stage, the conductive graphene ink is far cheaper than the equivalent using gold or silver, with a square metre of silver woven fibres representing several thousand pounds and graphene around £100.
‘My future vision would be to have a fabric-based display or a small integrated lab-on-chip that makes calculations that can monitor data and give you an output,’ Torrisi added. ‘Say you have different sensors in a matrix or array and they can monitor different components, such as biomarkers… some circuits can give you an output to tell you if there is an increase and triggers a response.’
A battery of options
A similar concept devised in Hong Kong won three awards at the International Exhibition of Inventions, held in Geneva in April. A Hong Kong Polytechnic University research team, led by Professor Zheng Zijian, created a textile lithium battery specifically to target the continuous surge in wearables, which he said is forecast to grow by more than 20% a year, reaching US$100bln by 2024.
The battery, at less than 0.5mm-thick, has an energy density of 450wh/L and a bending radius of less than 1mm, compared with a conventional lithium battery’s 25mm.
‘As all wearable electronics will require wearable energy supply, our novel technology in fabricating textile lithium battery offers a promising solution to a wide array of next-generation applications, ranging from healthcare, infotainment, sports, aerospace, fashion, Internet of Things to any sensing or tracking uses that may even exceed our imagination of today,’ Zijian said.
Read more in the paper, Wearable solid-state capacitors based on two-dimensional material all-textile heterostructures, published in Nanoscale here: rsc.li/2EV8q6Z.