The areas of flexible and wearable electronics are two areas seeing a significant amount of growth and are expanding into many industries, with some applications adopting both flexible and wearable devices. In many cases, wearable electronics—especially new devices—will be flexible to conform to the user. Regardless of the application, these two—three if you include those that are both flexible and wearable—have come about in their current form because of advances in nanotechnology. If it weren’t for advancements in efficient fabrication methods, then many flexible and wearable devices would not be possible because materials would not be available to perform the required functions.
Any material must exhibit a number of properties to be used within wearable and flexible electronics. The most obvious is that they must be both thin and highly flexible. If they are not thin, they will not bend efficiently and will be susceptible to stress fractures. However, some thin materials are not flexible, and these are not useful either. Although they can be used for some wearable devices, many of today’s devices need to conform to the user in one way or another. Overall, electronics, which are flexible, must be strong and resistant to fracturing under various bending and torsional stresses.
Two other properties are highly beneficial but application-dependent. The two properties in question are a high electrical conductivity—and subsequently a high charge carrier mobility—and a high optical transparency. For many sensor and monitoring applications, a high electrical conductivity is needed when the materials change from a stimulus in the local environment, it is detected by the change in the conductivity across the sensing material(s) in the device—which is the nanomaterial in flexible/wearable sensors. On the other hand, optical transparency is more relevant for other applications where light needs to pass through the device or a portion of the device. A flexible screen is an example of this type of application.
Not many materials exhibit all these properties. Thankfully, nanotechnology’s emergence and advancement yielded many materials that possess most, if not all, these properties. No other defined field of materials—from organic molecules to solid-state inorganic complexes and micro-manufacturing—can produce materials with properties that are so well-aligned to the needs of these devices, which is why nanotechnology has been instrumental in the commercial realization of these devices. There is a growing movement of producing flexible electronics using flexible organic molecules, namely polymeric materials, but they are falling behind nanomaterials in terms of efficiency. Despite this, it is an area that is slowly growing. This is mainly because most of these organic electronics are printable. But the area of flexible printed electronics is a subject for another day.
Another reason why nanotechnology has been at the forefront of developing these electronics is because many nanomaterials are tunable, their properties are tunable, and the fabrication process is tunable. In other words, the nanomaterials’ localized structure can be changed and tailored throughout their synthesis, or they can be doped and functionalized after formation. All these factors alter the properties of the nanomaterial to meet the specific requirements of the application. This tunable nature has made nanomaterials a versatile building block for many different flexible and wearable electronic systems.
The incorporation of nanomaterials has not been without its challenges, though. Carbon nanotubes (CNTs) were one of the first nanomaterials to be trialed, but there were some issues with dispersing and aligning CNTs. Since then, CNTs issues have been ironed out, but the industry has moved on to using other nanomaterials, namely different 2D materials. Although 2D hexagonal boron nitride and transition metal dichalcogenides (TMDCs) materials have been used in flexible electronics, graphene has shown the most promise and been widely exploited. There are many reasons for this, but the short answer is that graphene can meet every single property demand in flexible and wearable electronics:
Other key materials to watch out for in this space include nanowires and quantum dots. Silver nanowires have electrical conductivity and resistivity properties that hardly change when flexed, making them ideal for flexible electronics. Quantum dots exhibit excellent and bright fluorescence, with relative ease, for displays. They have the potential to integrate into flexible and wearable electronics and bring unique properties.
Flexible electronics have found a use commercially—or are at least in the pipeline—in two main areas. These are flexible solar cells and flexible touch screens. In terms of flexible screens found commercially, it is Organic Light-Emitting Diodes (OLEDs) that currently lead the way (a thin film of organic molecules on top of another material). Nonetheless, companies are starting to look at the possibility of incorporating quantum dots into OLED devices. Additionally, multiple flexible screens that use graphene are now available where graphene and polymer layers are stacked on top of each other. The first commercially available fully foldable smartphones and laptops that use graphene could emerge soon.
In terms of solar cells that employ nanomaterials, their efficiencies are growing. Although their efficiencies aren’t as high as some other solar cells, they can be made flexible using nanomaterials. Therefore, they can conform to the geometry of a building, enabling them to capture more photons from the sun. So, their energy conversion efficiency might not be as high, but they can capture more photons that can be converted into electricity. Additionally, another growing area is to formulate nanomaterials into an ink form, where they can be used to fabricate printable solar cells.
One of the key areas that use the principles of wearable and flexible electronics is in medical sensors for diagnostics, health monitoring, and exercise monitoring. Flexible electronics use nanomaterials to conform to the shape of the skin and act as a sensor. In some cases, they can be left on the patient and monitored remotely through Internet of Things (IoT) technology.
Each application uses the different properties of the nanomaterial for use in a tailored approach. As fabrication methods advance even further, more flexible and wearable electronic devices are expected to reach the market.
Overall, improvements in fabricating and tailoring properties in nanomaterials helped the fields of flexible and wearable electronics grow. As it stands, many applications in the academic world are being trialed. Given the current rate of nanomaterial advancements, it shouldn’t be long until these advancements are further realized into more commercial flexible and wearable electronic devices.
Liam Critchley is a writer, journalist and communicator who specializes in chemistry and nanotechnology and how fundamental principles at the molecular level can be applied to many different application areas. Liam is perhaps best known for his informative approach and explaining complex scientific topics to both scientists and non-scientists. Liam has over 350 articles published across various scientific areas and industries that crossover with both chemistry and nanotechnology.
Liam is Senior Science Communications Officer at the Nanotechnology Industries Association (NIA) in Europe and has spent the past few years writing for companies, associations and media websites around the globe. Before becoming a writer, Liam completed master’s degrees in chemistry with nanotechnology and chemical engineering.
Aside from writing, Liam is also an advisory board member for the National Graphene Association (NGA) in the U.S., the global organization Nanotechnology World Network (NWN), and a Board of Trustees member for GlamSci–A UK-based science Charity. Liam is also a member of the British Society for Nanomedicine (BSNM) and the International Association of Advanced Materials (IAAM), as well as a peer-reviewer for multiple academic journals.
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