Clothing provide near-field connectivity for skin-like sensors

Clothing with completely fabric-based near-field-response patterns establish wireless power and data connectivity between multiple distant points around the body to create a network of battery-free sensors.

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Skin provides an important window into human health. To interface with it, researchers are making wearable devices more skin-like: flexible, soft, and imperceptible to the user. Such skin-like sensors are rapidly becoming more advanced as they acquire the ability to measure temperature, electrical activity, sweat composition, and other important physiological signals. In many clinical applications, however, it is essential to obtain measurements not just at a single location, but from many points on the body. To accomplish this, we require a collection of sensors, sometimes called a body sensor network, that can monitor the whole body and send the data to the cloud. 

Creating such a network requires a wireless technology to connect many sensors to a single hub device, like a smartphone. Ideally, the technology could not only enable the hub to communicate with the sensors, but also wirelessly power them so that the sensors do not need batteries, which need to be regularly replaced. Such a technology exists today in the form of near-field communication (NFC), which is widely used in smartcard and mobile payment systems. The limitation, however, is that the sensor must be “near” – at most a few centimeters from the smartphone. This range is not enough to construct a sensor network that covers a major portion of the body.

Our work addresses this problem by modifying clothing – a wearable device that already covers much of the body. The idea was to take advantage of conductive threads originally developed for use as wires in electronic textiles, but use them instead to engineer the wireless response of clothing. Accomplishing this turns out to be a remarkably straightforward application of Faraday's law of induction. All that is required are two connected loops of wire, in which current is induced by the time-varying magnetic field and vice versa. This simple pattern acts as a relay for the magnetic field, enabling the reader to connect to a distant sensor as long as both are in close proximity to clothing. To test this idea, our first experiment simply wrapped a long piece of copper into a 1-m long relay with 5-turn coils at both ends. We were very surprised to find that the relay immediately established a connection between a commercial NFC device and temperature sensor without requiring any modification of the NFC hardware. 

Figure 1. Magnetic field generated by a reader interconnected to sensor by a near-field relay. Photograph of a smartphone wirelessly powering a sensor node over a relay (40 cm length).

Although conceptually straightforward, converting this idea into to a working sensor network required a multi-disciplinary effort that spanned electromagnetic engineering, materials science, textile fabrication, and embedded systems. Thanks to a talented group of engineers and a collaboration with expert physiologists, we were able to build a prototype of a battery-free sensor network that could monitor temperature and gait during active exercise, all using a smartphone or small custom hub. 

An important feature of our prototype was the use of commercially available conductive threads and computer-controlled embroidery, which allows ordinary clothing to become near-field-enabled at very low cost. Crucially, the result is a piece of clothing that is still entirely fabric-based, and can be washed as normal. There are no sophisticated electronics inside the clothing, and no connectors are needed for it to work. We think that the idea of wirelessly-enabled clothing sidesteps many of the problems currently hindering electronic textiles from becoming mainstream.

Figure 2. Photograph of wireless powering through the near-field-enabled clothing.

Looking back, I find it surprising how a physical law elucidated nearly 200 years ago could be applied to address a critical challenge in modern wearable technologies. To me, this is a reminder of how we need to stand on the shoulders of giants make progress. Although much work is still needed to clinically validate our prototype for diagnostic or therapeutic use, we think that the conceptually simplicity and robustness of the near-field-enabled clothing make it particularly useful in real life. We envision that such clothing will facilitate the connection of skin-like devices into sensor networks for healthcare, athletic, and military applications. 

This work was published in Nature Communications (link).

Rongzhou Lin

Research fellow, National University of Singapore