Smartphone compass used to measure glucose

Researchers at the US National Institute of Standards and Technology (NIST) have used an ordinary smartphone’s built-in magnetometer, or compass, to measure concentrations of glucose — a key marker for diabetes. Described in the journal Nature Communications, their method also has the potential to measure other biomedical properties as well as environmental toxins.

In their proof-of-concept study, NIST researchers Gary Zabow and Mark Ferris clamped to a smartphone a tiny well containing the solution to be tested and a strip of hydrogel — a porous material that swells when immersed in water. The researchers embedded tiny magnetic particles within the hydrogel, which they had engineered to react either to the presence of glucose or to pH levels by expanding or contracting. Changing pH levels can be associated with a variety of biological disorders.

As the hydrogels enlarged or shrank, they moved the magnetic particles closer to or further from the phone’s magnetometer, which detected the corresponding changes in the strength of the magnetic field. Employing this strategy, the researchers measured glucose concentrations as small as a few millionths of a mole (the scientific unit for a certain number of atoms or molecules in a substance). Although such high sensitivity is not required for at-home monitoring of glucose levels using a drop of blood, it might in the future enable routine testing for glucose in saliva, which contains a much smaller concentration of the sugar.

‘Smart’ hydrogels like the ones the NIST team employed are inexpensive and relatively easy to fabricate, Ferris said, and can be tailored to react to a host of different compounds that medical researchers may want to measure. In their experiments, he and Zabow stacked single layers of two different hydrogels, each of which contracted and expanded at different rates in response to pH or glucose. These bilayers amplified the motion of the hydrogels, making it easier for the magnetometer to track changes in magnetic field strength.

Because the technique does not require any electronics or power source beyond that of the phone, nor call for any special processing of the sample, it offers an inexpensive way to conduct testing — even in locations with relatively few resources. Future efforts to improve the accuracy of such measurements might allow detection of DNA strands, specific proteins and histamines — compounds involved in the body’s immune response — at concentrations as low as a few tens of nanomoles (billionths of a mole). At present, histamine measurement requires a 24-hour urine collection and a sophisticated laboratory analysis.

“An at-home test using a cellphone magnetometer sensitive to nanomolar concentrations would allow measurements to be done with much less hassle,” Ferris said. More generally, enhanced sensitivity would be essential when only a small amount of a substance is available for testing in extremely dilute quantities, Zabow added.

Similarly, the team’s study suggests that a phone magnetometer can measure pH levels with the same sensitivity as a thousand-dollar benchtop meter but at a fraction of the cost. A home-brewer or a baker could use the magnetometer to quickly test the pH of various liquids to perfect their craft, and an environmental scientist could measure the pH of groundwater samples onsite with higher accuracy than a litmus test strip could provide.

In order to make the phone measurements a commercial success, engineers will need to develop a method to mass produce the hydrogel test strips and ensure that they have a long shelf life, Zabow said. The hydrogel strips should also be designed to react more quickly to environmental cues in order to speed up measurements, he added.

Image caption: Photograph of a prototype sensor platform attached to a phone. Image has been cropped from the original and appears courtesy of the study authors under CC BY 4.0.