The company Apple recently introduced the eighth series of its smartwatch — the Apple Watch — a product line that was first released back in 2015. The device can measure temperature and heart rate, monitor blood oxygen levels and take electrocardiograms. At this point, such wearable devices are a mainstream — if expensive — technology. But the devices are made of conventional hard, rigid electronic materials. Within the research community, the potential of soft, flexible electronic materials is a key focus. And this has led to a range of increasingly sophisticated wearable devices that take a variety of different forms, as highlighted by recent work in Nature Electronics.

Photograph of the conformable sensor interface developed by Dagdeviren and colleagues. The interface can be repeatedly attached and detached from different commercial face masks. Credit: Canan Dagdeviren, Massachusetts Institute of Technology.

Devices that can form an intimate contact with the skin are of particular use in health monitoring and can, for instance, provide molecular insights into the body via sweat sensing. Such devices usually need to be connected to external systems for power and to display information. In the last issue of Nature Electronics, Joseph Wang and colleagues at the University of California San Diego and Samsung Display Company reported a wearable sweat sensor that integrates electrochemical sensors, a stretchable battery and an electrochromic display1. The device, which is fabricated via screen printing of stretchable inks onto a flexible substrate, can display the concentration of various electrolytes or metabolites in sweat without any connection to external systems.

An important component of many on-skin — as well as implantable — devices is an elastic conductor. Ideally, the conductor should be thin and breathable, and capable of forming seamless contact with a dynamic and undulating surface, while maintaining consistent electrical properties. But fabricating such materials is challenging. In this issue of Nature Electronics, Kenjiro Fukuda, Xiaodong Chen, Takao Someya and colleagues report a gas-permeable elastic conductor that is only 1.3 μm thick. The material is composed of a gold layer on a polydimethylsiloxane film. The gold has a controlled morphology of microcracks, and the resulting conductor can be stretched and deformed while maintaining stable conductivity.

The researchers — who are based at various institutes in Japan, Singapore and China — combine the conductor with a 22-nm-thick adhesive polymer layer to create on-skin electrodes that are water-resistant, but breathable, and can continuously record electrocardiogram signals. They also use them to create 3-μm-thick sensors that can detect small mechanical forces such as an arterial pulse wave. (See also the News & Views article on the work from Jihong Min, Yu Song and Wei Gao at the California Institute of Technology.)

On-skin devices are probably the most common type of next-generation wearable currently being explored, but they are not the only one. In the last issue of Nature Electronics, for instance, we featured work from Bin Zhou, Xiaogang Liu and colleagues at the National University of Singapore and Tsinghua University on an interactive mouthguard2. In this approach, phosphors that are sensitive to mechanical stimulus, and emit different colours when compressed, are arranged in an array of contact pads in a flexible mouthguard. The optical signals generated by the wearer are detected by integrated optical fibre sensors, and with the help of machine learning algorithms the device can accurately recognize complex bite patterns. These capabilities mean that the mouthguards can be used to operate different electronic devices including computers and smartphones.

Alternatively, and in an Article elsewhere in this issue, Canan Dagdeviren and colleagues at the Massachusetts Institute of Technology report a conformable sensor interface that can be attached to the inside of a face mask. The interface uses a flexible printed circuit board to connect components and incorporates a gecko-inspired thin adhesive layer that allows it to be repeatedly attached and detached from different commercial face masks. The resulting system can monitor breathing patterns, skin temperature, physical activity, coughing and the fit of the mask itself.

Owing to the COVID-19 pandemic, the wearing of low-cost face masks has become commonplace for many. Integrating sensors into such items — and thus providing access to exhaled breath and the various potential health markers it contains — could be a valuable approach to monitoring personal and public health. As Firat Güder and colleagues at Imperial College London and Ca’Foscari University of Venice discuss in the accompanying News & Views article, such technology is only at an early stage of development. But they also note, “In the future, optical, electrochemical and chemiresistive transducers could be integrated into face masks to detect various molecules (expelled in the form of droplets), pathogens (such as bacteria or viruses), and gaseous and volatile biomarkers in exhaled breath.”