10-06-2020 | By Liam Critchey
The fabrication of flexible, yet strong, nanomaterials has ushered in a new age of electronic devices. Flexible and wearable electronic devices are some of the most talked about technology developments today and could play a major role in the healthcare and fitness sectors for continuous and routine monitoring applications. There is a big drive to create a range of wearable electronic devices with an aim of improving the real-time monitoring capabilities across these industries so that the needs of the user can be better adhered to.
Wearable electronics have really come to the fore thanks to nanomaterials. The inherent thinness and flexibility, high tensile strength, and electronic conductivity of a range of nanomaterials (both compositionally and structurally) have enabled many small, real-time and sensitive monitoring devices to be developed. There are two main ways in which nanomaterials are used to create wearable devices. The first is through devices which attach to the skin and are a stand-alone device, and the second is through sensors and monitoring devices being integrated into textiles that a person can wear. These are called electronic textiles, otherwise known as e-textiles.
As it stands, the thinness, electrical conductivity properties, and high degree of flexibility of graphene has meant that it has become one of the most widely researched nanomaterials for wearable electronic applications. It has even been made into a number of different prototype devices, including both stand alone skin sensors and complete e-textile garments.
While this is one of the most talked about areas, there are also a few different inorganic nanomaterials that are being trialled for flexible electronics. The reason that there are more organic nanomaterials in use is because they are softer and more flexible, whereas inorganic nanomaterials tend to be more rigid and hard. However, in some nanoforms―particularly nanofibres and when they are ultra-thin layers―inorganic nanomaterials can be flexible. Because inorganic materials are naturally strong, when they are made flexible, their inherent tensile strength enables them to be flexed without breaking, much like the graphene sheets which are commonly used.
One of the more common inorganic nanomaterials being trialled for wearable electronics is silver nanowires, but a range of semiconducting fibres are also starting to be created. Researchers from the USA, China and Hong Kong have now looked at creating semiconducting metal oxide nanofibres for e-textile applications.
Metal oxide materials, especially bulk metal oxides, have been exploited for a number of high-tech applications, including displays and conventional sensors. However, the properties of bulk semiconductors are too rigid for wearable electronics and they tend to fracture when flexed. So, the other way of exploiting the beneficial properties for wearable applications is to use them in a nanoform, and because nanofibres offer the best way of achieving flexibility/stretchability, it was seen as the best structural approach to take.
The researchers created a number of different metal oxide nanofibres and used these to fabricate different flexible wearable devices that would also be usable within an e-textile garment. The different metal oxide nanofibres were created using a blow spinning method as this generated nanofibres with a very high aspect ratio (length to width ratio) making them more flexible and feasible for wearable electronic devices. The different nanofibres created by this method were indium-tin oxide (ITO), indium-gallium-zinc oxide (IGZO), copper and copper oxide. These nanofibres were fabricated into 3D nanofibre networks, which were subsequently deposited on an organic polymer-based substrate and used to create different devices.
The metal oxide nanofibres were fabricated into a range of wearable electronic devices. The IGZO nanofibres were used to create both thin-film transistors, as well as gas sensors for monitoring the local concentration of nitrogen dioxide gas. The IGZO nanofibres were also utilised as resistors to measure any localised changes in strain, temperature ultra-violet (UV) light and exhaled breath vapours.
While the IGZO nanofibres are the most versatile, the other nanofibres were also able to be transformed into useful devices that could be used in wearable electronic devices. The copper oxide nanofibres were found to create efficient pressure sensors, whereas the ITO nanofibres made excellent motion readers. It was also found that a combination of the different nanofibres could be used to create multifunctional sensing platforms, something which is ideal for wearable electronics as it allows the monitoring of a number of factors simultaneously.
The devices created are stand-alone components that can be used in a number of ways. First off, each of the different components can be integrated into wearable devices that are attached to monitor certain parameters. Second, because they are made of fibres themselves, they can easily be integrated into larger e-textiles. This makes them a very versatile set of monitoring devices because they can be used to sense a number of local stimuli. The integration of such fibre-based devices could open the scope of what is possible with e-textiles.
Perhaps one of the most interesting aspects is the multifunctional sensing platform. The researchers created an example of this by monolithically integrating IGZO, ITO and copper oxide fibres to create a resistor platform. The integration of all these fibres was used to create a patch which could be attached onto the skin, a so-called e-skin device, for monitoring a range of parameters of the wearer, including their temperature, humidity, body movement, and respiratory functions.
Overall, the ability to create a range of fibre-based devices is a clever move as it allows a relatively seamless integration of the devices into e-textiles and other wearable electronic devices. Moreover, the range of monitoring capabilities is good because a number of different fibres have been created which have been used to create different types of monitoring devices, including a stand-alone e-skin device that can measure and obtain a range of information about the wearer. Given that there is a lot of interest in the e-textile market and on-skin devices, such developments showcased here could present an opportunity for expanding the scope of the wearable device sector.
Facchetti A. et al, Flexible and stretchable metal oxide nanofiber networks for multimodal and monolithically integrated wearable electronics, Nature Communications, 11, (2020), 2405.