Wearable vital sign monitoring

Monitoring a patient’s vital signs and other physiological parameters is a standard part of medical care, but, increasingly, health- and fitness-minded individuals are looking for ways to easily keep tabs on these measurements.

Enter the biometric watch.

In a pair of papers from The Optical Society’s (OSA) open-access Biomedical Optics Express, groups of researchers from the Netherlands and Israel describe wearable devices that use changing patterns of scattered light to monitor biometrics. One tracks glucose concentration and dehydration levels, the other monitors pulse.

The glucose sensor is the first wearable device that can measure glucose concentration directly but noninvasively, the authors say. In addition, while other wearable devices have been made to monitor pulse, the authors claim their new design would be less sensitive to errors when the wearer is in motion, such as while walking or playing sports

Both of the watches described in the papers make use of the so-called speckle effect, the grainy interference patterns that are produced on images when laser light reflects from an uneven surface or scatters from an opaque material.

When the light-scattering material is moving – say, in the case of blood flowing through the circulatory system – “the speckle pattern changes with changes in the flow,” explains biomedical engineer Mahsa Nemati, a graduate student in the Optics Research Group at the Delft University of Technology in the Netherlands and the lead author of the Biomedical Optics Express paper on monitoring pulse. Those light variations are a valuable source of information, she says.
 

‘Holy Grail’ of diagnostics

In the first paper, bioengineer Zeev Zalevsky, of Israel’s Bar-Ilan University, and his colleagues describe a new wearable biometric system that uses the speckle effect to monitor glucose concentration in the bloodstream and the wearer’s relative hydration level.

“Glucose is the Holy Grail of the world of biomedical diagnostics, and dehydration is a very useful parameter in the field of wellness, which is one of our main commercial aims,” Zalevsky says.

The watch-like device uses a laser to generate a wave front of light that illuminates a patch of skin on the wrist near an artery. A camera measures changes over time in the light that is backscattered off the skin. Unlike other chemicals present in the blood, glucose exhibits a so-called Faraday Effect. In the presence of an external magnetic field, generated by a magnet attached to the device, the glucose molecule alters the polarization of the wave front and influences the resulting speckle patterns. Analyzing these changing patterns provides a direct measurement of the glucose concentration. Because one of the main signs of mild to moderate dehydration is muscle weakness, which will alter the strength of the signals, the same device can also be used to indicate the relative dehydration level of the user as it changes over time.
 

Zalevsky and his colleagues are now working to reduce the margin of error in the device’s readings.

“Around 96% of our in vivo measurements were within a range of 15% deviation from the readout of a medical reference glucometer device,” Zalevsky notes. “The main factor for errors now is the stability of our device on the wrist of the user. We are currently investing efforts in deriving proper calibration and motion cancellation procedures that will allow us to reduce this sensitivity.”

Zalevsky says this is the first step toward non-invasive, continuous, in vivo measurement of glucose that is based on sensing an effect that is directly related to glucose concentration. The team expects a commercial version of the device to reach the market within two to three years.
 

Pulse tracker

In the second Biomedical Optics Express paper, Nemati and colleagues at Delft and at Phillips Research developed a method that could be used to monitor pulse non-invasively with a sensor that isn’t thrown off by the wearer’s movement.

Using simulated heartbeats generated in milk and measurements performed on the finger of a volunteer, they found that speckle changes can be used to accurately measure flow pulsations – that is, the heart rate – even when the light source used to create the speckle pattern is also moving, as would be the case with a wearable biometric sensor. The researchers found that just a couple of pixels from the image were sufficient to extract the pulse rate.

“This paper shows for the first time that a speckle pattern generated from a flowing liquid can give us the pulsation properties of the flow in spite of motion-induced artifacts,” Nemati states. “Sophisticated optics are not necessary to implement this, so the costs for devices can be kept low. Another advantage is that the devices can be non-contact or far from the sample.”

The team is currently working with companies to integrate their motion-friendly pulse-monitoring technique into existing sensors, for potential use clinically as well as in sports, Nemati adds.

For more details, review the research papers

• “Improved noncontact optical sensor for detection of glucose concentration and indication of dehydration level,” http://bit.ly/T1pZaR
• “Dynamic light scattering from pulsatile flow in the presence of induced motion artifacts,” http://bit.ly/1nKrsNT

The Optical Society
www.osa.org

Delft University of Technology
www.tudelft.nl/en

Bar-Ilan University
www1.biu.ac.il/indexE.php