Systems that can track a person’s vital signs are of paramount importance for medicine and health care. Vital signs include heart rate, blood pressure, respiration rate and breathing rate. Tracking systems that are both wearable and wireless are desirable because they are minimally intrusive1,2 and could allow institutionalized care to be replaced with care at home for the elderly3. Writing in Nature Electronics, Hui and Kan4 report such a system, which could find applications in both home care and clinical facilities.
A variety of technologies are currently used to monitor vital signs. They include electrodes, stethoscopes, strain gauges and ultrasound devices, each of which has drawbacks in terms of comfort and convenience. For instance, electrodes (such as electrocardiograms) and stethoscopes require direct skin contact, which can cause discomfort and restrict a person’s movement. Strain gauges monitor blood pressure and respiration using belts or cuffs, which can disrupt everyday activities. Finally, ultrasound devices are portable, but are often rather cumbersome.
An alternative approach is to use radio-frequency backscattering, in which radio waves are reflected off the body and then detected5. A clear advantage of this method is that it does not require direct skin contact. The radio-frequency technologies that are currently used can monitor breathing, but struggle to detect small mechanical vibrations inside the body, such as a person’s heartbeat or pulse6. Although heart rate can be extracted using careful filtering methods, measuring blood pressure and monitoring many people simultaneously has not been possible.
Hui and Kan’s health-care system combines radio-frequency backscattering with a technique that they call near-field coherent sensing (NCS). In NCS, radio waves are directed into the body’s tissues. This produces stronger backscattered signals from internal organs than signals obtained using conventional methods, in which the waves are reflected mainly off the body’s surface6,7. Furthermore, because the wavelength of a propagating wave is shorter inside the body than outside it, NCS is more sensitive to the body’s mechanical motion than previous approaches.
To distinguish between backscattered signals associated with the various vital signs, it helps to know whether a particular signal originates from inside or outside the body. In the NCS method, the signals corresponding to internal motion are therefore combined (multiplexed), as are those associated with motion on the body’s surface. Multiplexing is useful because it allows a group of independent signals to share a communication channel without interference.
Hui and Kan implemented their NCS technique using a device known as a passive radio-frequency identification tag, which is equipped with a unique digital identification code8 (Fig. 1). The authors’ tag comprises an antenna and sensors, and is powered by the electromagnetic energy from an external device called a reader. When the tag is activated, the antenna transmits radio waves on and into the body, and the backscattered signals are picked up by the sensors. This information is then sent to the reader to allow data on the body’s vital signs to be retrieved. The authors show that their tag can be embroidered directly into clothing. Even when the material was laundered several times, the integrated tag retained its geometry and functionality.
Another advantage of the NCS method is that the downlink (the communication link from the reader to the tag) and the uplink (the link from the tag to the reader) can be separated from each other. The authors’ health-care system can therefore detect signals more effectively than currently used methods6,7. The tag is a passive backscatterer (it communicates by reflecting wireless signals), which means that it can use existing wireless communication technology. The digital information that is transferred between the reader and the tag can include the tag identification code and additional information from the on-tag sensors.
Hui and Kan point out that, to maximize the distance between the tag and the reader across which measurements can be made, and to reduce the impact of the person’s movement, an active radio-frequency identification tag could instead be used. These tags are battery-powered and continuously emit radio waves, which means that they do not need to receive electromagnetic energy from a reader. However, active tags are generally much more expensive than passive tags.
Hui and Kan’s system can monitor many people simultaneously, and could lead to the automation of vital-sign monitoring in care facilities. However, the system will need a reader device that is compact, portable and easily integrated into such environments. This is one of the main challenges that must be overcome before the system can be used in a real-world setting. Future work will also need to test the reliability of such wearable, textile-based devices, and to demonstrate that they are effective in a clinical study.
Wireless health technologies will become increasingly important in the future, particularly given the world’s ageing population. Hui and Kan’s work provides a platform on which to create contactless, wearable devices for various health-care and health-promotion applications.
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