[Special Feature: Society Changed by Smartphones] Naoji Matsuhisa: Next-Generation Wearable Information Terminals Realized by Softness

Wristwatch-type and ring-type wearable devices, represented by the Apple Watch, are becoming increasingly popular. In 2021, the number of smartwatch users worldwide exceeded 100 million, and the era where wearable devices are as commonplace as smartphones is just around the corner. By attaching them to the wrist or finger, it becomes possible to monitor healthcare information such as electrocardiograms, blood oxygen levels, and activity levels over long periods, or to access smartphone information (emails, news, weather, etc.).

In particular, healthcare functions enable the discovery of medical conditions that cannot be found through regular checkups via long-term monitoring, as well as high-precision automatic diagnosis combined with artificial intelligence (AI). Because they can be used for follow-up observations of diseases and the prevention of pre-symptomatic illnesses without increasing the burden on medical professionals, they are expected to be extremely useful for health management in modern people. Their importance is increasing even more in today's world, where medical resources are easily strained due to the COVID-19 pandemic and the increase in the elderly population.

The biggest challenge in realizing healthcare through wearable devices is wearing comfort. Currently available wearable devices are physically hard, so they inevitably feel uncomfortable when worn on a soft body. To begin with, many people do not even want to wear a wristwatch due to the discomfort of wearing it, and infants or elderly people with dementia may perceive objects attached to the body surface as foreign matter and try to remove them. Furthermore, while the use of wearable devices is expected for managing the training of athletes, they will ultimately not be worn if they hinder natural movement. Additionally, because they must be made small from the perspective of wearing comfort, the size of the display and the contact area with the skin—which determines the quality of the biological signals that can be obtained—become small. There was also the issue that the types and quality of biological signals that could be measured were limited because the attachment locations were restricted to the wrist or fingers.

Therefore, there are high expectations for the realization of wearable devices (Figure 1) that are soft and stretchable like our skin, making them comfortable to wear even if they are large, to the point that one forgets they are wearing them.

Soft electronic devices are not only comfortable to wear but also show high conformability even to the minute irregularities of the skin. This allows for higher quality signals obtained by sensors and makes them less likely to shift during the wearer's movements, reducing noise. Furthermore, if this device allows the entire back of the hand to be utilized as an information terminal, sufficient information can be presented and operated, eliminating the need for users to carry a smartphone as well.

The key to realizing such stretchable devices is the development of electronic materials that are soft and can expand and contract. Many conventional electronic materials, represented by silicon, are hard and brittle. Although smartphones equipped with flexible (bendable) displays have recently become commercially available, they were insufficient considering that skin stretches about 1.3 times its original length and joints stretch more than twice.

While stretchable rubber was a representative example of something that does not conduct electricity, stretchable conductive materials that exhibit conductivity comparable to conventional electronic materials have recently been developed one after another. The author has also worked on the development of printable stretchable conductor materials with the world's highest conductivity, among other projects *1 (Figure 2). By using electronic nanomaterials such as conductive polymers, carbon nanotubes, and metal nanomaterials, materials have been developed that lose almost no conductivity even when stretched to more than four times their original length. Among these, stretchable conductive inks that exhibit high electrical properties at low cost have already progressed to commercialization by various material manufacturers around the world. By combining various stretchable conductive materials, stretchable strain/temperature sensors, batteries, and more have been realized.

Recently, high-performance stretchable semiconductor materials have also been developed. Stretchable optical sensors, displays, and integrated circuits created by combining these with stretchable conductors have been reported. Although these are still in the developmental stage compared to non-stretchable semiconductor devices, the elements necessary to realize wearable devices like the one shown in Figure 1 are steadily coming together.

The author is also working on the development of electronic devices using stretchable semiconductors. One of the major problems with stretchable semiconductor devices was their low driving frequency (slow operation speed). By advancing various material developments, the author succeeded in the world's first driving of a stretchable semiconductor device at 13.56 MHz (megahertz) *2. The frequency of 13.56 MHz is a major target for driving frequencies and is also the frequency of electromagnetic waves used for wireless power transfer. It is also used for communication in transportation IC cards such as Suica and ICOCA.

By integrating the developed stretchable high-frequency device with stretchable antennas, sensors, and displays, it was demonstrated that a system equipped with stretchable sensors and displays can be driven by wireless power transfer (Figure 3). While it is very difficult to attach power supply cables to a device pasted on flexible skin, this technology will make it possible to easily supply power to next-generation wearable devices using antennas embedded in clothes or desks, for example.

In addition to softness, research to enable the measurement of biological signals that were difficult to detect with conventional wearable devices is also active, and devices that detect biomarkers in sweat have been realized. For example, the detection of glucose, which is an indicator of diabetes, and cortisol, which is an indicator of stress levels, has been achieved. Conventionally, sensing these biomarkers required blood collection and large-scale detection equipment.

Also, when considering wearing wearable devices for long periods, it is necessary to consider issues such as skin becoming stuffy with sweat or the device peeling off. It has been confirmed that by creating small holes for sweat to pass through or increasing water vapor permeability, skin inflammation does not occur even when worn continuously for long periods exceeding one week.

The wearable devices introduced in this article, which are soft and adhere closely to the skin, can be described as devices that can expand our bodily functions just by being pasted on. Beyond applications in healthcare and as information terminals, they are also expected to be used as interfaces for augmented reality and virtual reality (AR/VR), which are attracting attention in contexts such as the metaverse, and as electronic artificial skin for robots.

*1 N. Matsuhisa, et al., "Printable elastic conductors by in situ formation of silver nanoparticles from silver flakes" Nature Materials 8, 834-840 (2017).

*2 N. Matsuhisa, S. Niu, et al. "High-frequency and intrinsically stretchable polymer diodes" Nature 600, 246-252 (2021).