Yesterday’s report that we may see Apple Watch blood glucose monitoring when the Series 7 is released this year has generated both excitement and skepticism.

Excitement because it could provide a far easier method of monitoring blood sugar levels than current devices, which require a pin-prick blood sample. Skepticism because non-invasive measurement has been a goal for a great many years, with very limited success to date. The closest we’ve got so far is a rice-grain sized sensor embedded into the skin that can then be read without any further puncturing of the skin…

Traditional blood glucose monitors require the patient to use a pin-prick device to draw a drop of blood, and transfer this to a test strip, which is then read by a machine. The process isn’t ideal: It’s very slightly painful, somewhat fiddly, and requires a steady supply of test strips.

For this reason, many companies have been trying to develop non-invasive monitors – that is, ones that don’t require a drop of blood.

One form of non-invasive measurement in development uses the thin flap of skin between thumb and forefinger.

The glucose levels are extracted by a non-invasive technique which transmits low-power radio waves through a section of the human body, such as the area between the thumb and forefinger. These areas have adequate blood supply and are thin enough for the waves to pass through the tissue. These signals are then received by a sensor on the opposite side of the GlucoWise device, where the data about the characteristics of the blood within the flesh are collected and analyzed.

But, as the explanation says, that works only because the skin there is very thin. How could an Apple Watch measure blood glucose through the wrist?

The answer may lay in an approach described in Nature in the summer of last year. A skin tag – a little like an RFID one – is taped to the skin, and then energized by a reader embedded in the Apple Watch.

This paper reports a highly sensitive, non-invasive sensor for real-time glucose monitoring from interstitial fluid. The structure is comprised of a chip-less tag sensor which may be taped over the patient’s skin and a reader, that can be embedded in a smartwatch.

The tag sensor is energized through the established electromagnetic coupling between the tag and the reader and its frequency response is reflected on the spectrum of the reader in the same manner. The tag sensor consumes zero power as there is no requirement for any active readout or communication circuitry on the tag side […]

The sensing element itself is just a metallic trace which could be simply taped on the patient’s skin and is replaceable at extremely low cost […]

When measuring changes in glucose concentrations within saline replicating interstitial fluid, the sensor was able to detect glucose with an accuracy of ~1 mM/l over a physiological range of glucose concentrations with 38 kHz of the resonance frequency shift. This high sensitivity is attained as a result of the proposed new design and extended field concentration on the tag.

It works by measuring the shift in radio frequency. This shift is proportional to the relative percentages of water and glucose in the blood.

This frequency is selected because there is a considerable difference between water, as the main material in interstitial fluid, and saturated glucose solution permittivity while their loss factors are still small, and therefore measuring at this frequency will result in a significant frequency shift and hence the device sensitivity.

We illustrated yesterday how the results might be displayed on the Watch.

There are also some infrared approaches in development, which could potentially work with existing Apple Watch models, but the above approach seems more promising in terms of accuracy.

All of this remains highly speculative, however. The method would need to be evaluated and approved for use, and it appears this technology is at a very early stage in that process. While I do think we’re likely to see this in the Apple Watch at some point, the timing seems tight for a Series 7 release.

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