Hospitals might soon be able to keep tabs on the health of dozens of people from afar. Engineers have come up with a new touch-free method for monitoring heart rate, blood pressure, and breathing of several people at the same time.
A small RFID tag placed near the body—say, in a shirt pocket or wrist cuff—is all the system needs in order to read these vital signs. In addition to being more comfortable for patients, the technology could make it easier, faster, and cheaper for hospitals and care facilities to monitor the health of residents.
Continuously monitoring vital signs can be a nuisance. It requires electrodes stuck to skin amid a tangle of wires, or tight arm cuffs and sitting still. Wearable sensors like wristbands, soft wireless patches, and temporary tattoos can gather this information, but those might not often be a good option for the elderly, infants, or people with medical conditions.
Measuring vital signs without any skin contact is already possible with radio waves. For tracking respiration, the idea is to bounce RF beams off someone’s body and measure the periodic changes in the reflected signal caused by the person’s breathing. But measuring heartbeat is more challenging. What’s more, says Edwin Kan, a professor of electrical and computer engineering at Cornell University, this method is “very vulnerable to movement in room. Another person moving on the side causes a lot of interference.”
So Kan and his graduate student Xiaonan Hui have devised a method that relies on near-field coupling. The near-field is the region of the electromagnetic field right around an RFID antenna, a distance of up to one wavelength, or 35 centimeters, away.
The system requires a small antenna and RFID tag to be placed near the chest and a wrist; they can be up to 10 centimeters away from the body. An RFID reader is placed two meters away.
The antenna directs part of the electromagnetic energy from the RFID reader into body tissue using near-field coupling, and reflects part of it back to the receiver. The receiver reads both the amplitude and phase of the reflected signal. The phase is sensitive to the distance between the antenna and the receiver. So external motions like pulse and breathing, which move the tag up and down, change the phase. Heartbeats, meanwhile, change the amplitude of what the antenna reflects back to the detector. And by measuring the slight time lag between heartbeat and pulse, the system also measures blood pressure.
A clever addition allows the system to monitor multiple tags at a time. The researchers incorporated a code-division multiple access (CDMA) module in the RFID tag chip. CDMA is the wireless communication protocol that allows several signals to be mixed and transmitted over the same channel. In this case, every RFID tag sends a signal with a unique code, allowing the reader to detect tens of tags. The engineers’ simulations show that they could detect up to 200 tags, which could a single system monitor the vital signs of dozens of people in a room.
Such multiplexing wasn’t possible with previous RFID-based vital sign-monitoring systems because they suffer from low signal-to-noise ratio, Kan says. But the near-field antenna coupling generates a clean enough signal.
In their Nature Electronics paper, the researchers show an example of a tiny RFID chip and an antenna that can be embroidered into fabric. The components are robust and can withstand washing, so it could be built right into clothes and bedsheets.