Building On-Skin Devices from Self-Assembled Micropyramids

Wearable devices are becoming ever more popular, especially in health and medical monitoring applications, and the list of wearable technology that can provide accurate sensing capabilities continues to grow. One subset of wearable devices that are showing great potential in healthcare, behaviour monitoring, individual protection, self-powered electronics, and human-machine interaction applications is on-skin devices.

On-skin devices are a class of wearable tech composed of functional patches and cloths that can be attached to human skin and be used to detect different signals (physiological and action signals), help to prevent injury to the body through continuous monitoring, and convert energy into electricity (as an energy harvesting device, for example, a nanogenerator).

The on-skin device field has been evolving over the last few years, but more research into these devices is still required if comfortable long-term use with minimal sensory interference is to be achieved. It’s thought that many on-skin devices could also detect action and touch in natural states, with a degree of touch sensation, for technologically complex applications—such as in machine learning applications for learning a craftsperson’s skills or for restoring limb function.

Given the versatility and range of applications that are open to on-skin devices, a lot of research has gone into making different on-skin devices that avoid bringing discomfort to the wearer while maintaining minimal interference in the normal sense of touch. With the range of devices on offer, new fabrication methods are coming to the fore, and researchers have now looked at creating on-skin devices using electrospinning methods.

Looking Towards Gas Permeable Films for On-Skin Devices

A range of materials have been trialled and used to build the patches/cloths that are applied to the skin, but research in this area continues to expand to find the most comfortable materials that don’t limit the electronic performance or the motion ability of the wearer. It’s been proposed recently that ultra-thin and ultra-light gas permeable films could be an excellent choice for bringing all these wearable performance elements together.

For making these films, polymer electrospinning methods are being investigated. However, many electrospinning methods have a random spinning deposition process, and many of the on-skin devices created to date through these methods have functional surfaces that are a geometrically flat plane. For on-skin devices, flat planes have been found to not be the best choice of geometry for these thin films, as it often results in decreased optical, thermal, mechanical, and electrical properties—all of which are key for optimal performance.

However, it has been discovered that 3D microstructure arrays that gradient geometries could offer more in the way of performance. Even though these gas-permeable films are still ultra-thin, a range of microstructure-level gradient geometries are currently being looked at, including micropyramids, microcones, microdomes, and microprism arrays. 

It’s thought that these surface-gradient films could offer more promise of flat plane films in on-skin devices because the inherent features of gradient space-filling, gradient stress distribution, and gradient refractive index are beneficial properties that could help to better regulate force, heat, light, and electricity within the device and offer better macro-level properties.

However, existing fabrication and processing technologies—including photolithography and 3D printing—are not suitable technologies for the simultaneous realisation of high gas permeability, ultralow thickness, ultralight weight, and gradient geometry in a device. This means that on-skin devices would undergo a trade-off of one or more of these critical features, so new approaches are being sought that simultaneously realise these different criteria in the hope of creating more efficient and comfortable on-skin devices.

Building On-skin Devices from Electrospun Micropyramid Arrays

Researchers have now created electrospun micropyramid arrays (EMPAs) with a unique, ultrathin, ultralight, gas-permeable structure. The on-skin devices were created through a self-assembly electrospinning approach using wet heterostructured electrified jets. By using a series of electrified jets, the team was able to create on-skin devices with structurally designed EMPAs using different materials, and this realised an on-skin device with high performance across many areas. 

The EMPA material was used to build a number of on-skin devices that showed high levels of performance. On the one hand, the EMPAs were built into a thin (47 microns thick) radiative cooling fabric that showed a temperature drop of around 4 °C under a solar intensity of 1 kW m-2, a near-infrared (vis-NIR) reflectivity of 97.9% and a midinfrared (MIR) emissivity of 76.3%.

Another on-skin device built with the EMPAs was a piezocapacitive-triboelectric hybrid sensor. The sensor was used to detect an ultra-weak fingertip pulse to determine a health diagnosis while monitoring the natural finger manipulation of the user. The EMPA sensing device had a high sensitivity of 19 kPa-1, a very low detection limit of 0.05 Pa, an ultrafast response of less than 0.8 ms, and could be used over a wide frequency range.

The final on-skin device created was an EMPA nanogenerator that had both triboelectric and piezoelectric outputs (of around 105 µC m-2), allowing biomechanical energy to be harvested effectively.

In the devices created in this study, the flexible design of the EMPA structures allowed for the optical, thermal, mechanical, and electrical properties to be applied to radiative cooling, pressure sensing, and bioenergy harvesting applications. Overall, the flexibility offered with self-assembling EMPA materials means that there are a number of on-skin devices that can be created for a range of applications—such as in individual healthcare and human-machine interaction applications—that are both comfortable and show a good performance.

Reference:

Pan L. et al., Versatile self-assembled electrospun micropyramid arrays for high-performance onskin devices with minimal sensory interference, Nature Communications, 13, (2022), 5839