Can noninvasive glucose monitoring ever become a reality?

Andrea Tura
PhD
Metabolic Unit
ISIB CNR
Padua
Italy

Pathologies such as type 1 and type 2 diabetes require frequent self-monitoring of glycaemia. The traditional technique for this task is based on the use of a stick, requiring a little drop of blood. This means that skin laceration is necessary every time the test has to be performed. In some diabetic subjects, testing has to be carried out several times a day, with obvious great discomfort. To prevent this problem, several studies have been developed in the past two decades to establish a noninvasive technique for glycaemia evaluation, in other words a technique that would not require extraction of blood. However, despite the great interest generated and the numerous studies carried out in the field, noninvasive glucose monitoring is still not a reality. We present here a review of the most relevant technologies for noninvasive glucose monitoring currently under investigation.

Noninvasive technologies
Near-infrared spectroscopy  Near-infrared (NIR) spectroscopy is based on focusing a beam of light in the 750–2,500 nm spectrum on the body.(1) NIR spectroscopy allows glucose measurement in tissues in the range of 1–100 mm in depth, with a decrease in penetration depth for increasing wavelength ­values. The light focused on the body is partially absorbed and scattered, due to its interaction with the chemical components within the tissue. Changes in glucose concentration can influence the absorption coefficient of a tissue through changes of absorption corresponding to water displacement or changes in its intrinsic absorption. These changes also affect the intensity of light scattered by the tissue, which constitutes the scattering coefficient. This coefficient is a function of the density of scattering centres in the tissue observation volume, the mean diameter of scattering centres, their refractive index and the refractive index of the surrounding fluid. In the case of cutaneous tissue, connective tissue fibres are the scattering centres. Erythrocytes are the scattering centres for blood.

However, the absorption coefficient of glucose in the NIR band is low, and is much smaller than that of water by virtue of the large disparity in their respective concentrations. Thus, in the NIR the weak glucose spectral bands only overlap with the stronger bands of water, but also of haemoglobin, proteins and fats. Regarding the scattering coefficient, the effect of a solute (such as glucose) on the refractive index of a medium is nonspecific; hence, it is common to other soluble analytes. Furthermore, physical and chemical parameters such as variation in blood pressure, body temperature, skin hydration, triglyceride and albumin concentrations may interfere with glucose measurement.(2) Errors can also occur due to environmental variations such as changes in temperature, humidity, carbon dioxide and atmospheric pressure. Changes in glucose by themselves can introduce other confounding factors; for instance, it has been proven that hyperglycaemia, as well as hyperinsulineamia (often connected to the former in obese patients), can induce vasodilatation, which results in increased perfusion.(3,4) NIR light transmission, or reflectance, has been studied through an ear lobe, finger web and finger cuticle, skin of the forearm, lip mucosa, oral mucosa, tongue, nasal septum, cheek and arm.

Polarisation changes  This technique is based on the phenomenon that occurs when polarised light transverses a solution containing optically active solutes (such as chiral molecules [ie,glucose]). The light, in fact, rotates its polarisation plane by a certain angle, which is related to the concentration of the optically active solutes.(5) One advantage of this technique is that it can make use of visible light, which is easily available. However, this technique is sensitive to the scattering properties of the ­investigated tissue, since scattering depolarises the light. As a consequence, skin cannot be investigated by polarimetry, since it shows high scattering due in particular to the stratum corneum.(2) Moreover, the specificity of this technique is poor, since several optically active compounds are present in human fluids containing glucose, such as ascorbate and albumin. The preferential site for this technique is the eye, and, more specifically, the aqueous humour beneath the cornea.(6) Cornea has in fact low scattering properties, since it does not have any stratum corneum. However, eye movements and corneal rotations are sources of errors.(7) The corneal birefringence, due to its collagen structure, is another source of error. Moreover, a time delay between glucose concentration in aqueous humour and blood has been observed, which also has to be taken into account.(8)

Ultrasound technology  The most used technology based on ultrasound is photoacoustic ­spectroscopy, which is based on the use of a laser light for the excitation of a fluid and consequent acoustic response.(2) The fluid is excited by a short laser pulse (from pico- to nanoseconds), with a wavelength that is absorbed by a particular molecular species in the fluid. Light absorption causes microscopic localised heating in the medium, which generates an ultrasound pressure wave that is detectable by a microphone. In clear media (ie, optically thin), the photoacoustic signal is a function of the laser light energy, the volume thermal expansion coefficient, the speed of sound in the fluid, the specific heat and the light absorption coefficient.(5) In these media the signal is relatively unaffected by scattering. One variation in photoacoustic spectroscopy is based on combining it with ultrasounds emission and detection.(5)

A possible approach is the use of an ultrasound transducer to locate a bolus of blood in a ­vessel and then illuminate it with the laser pulse at a glucose absorption wavelength. The ultrasound transducer also detects the generated photoacoustic signal. Another approach is detecting the ultrasound signal reflected from a blood vessel before and after photoacoustic excitation. Glucose level is then determined from the difference in reflected ultrasound intensity. A third approach is in exciting a blood bolus via photoacoustic effect: the change in the dimensions and speed of the excited bolus causes a Doppler shift in an ultrasound directed towards the blood vessel; glucose level is determined from the magnitude and the delay of the Doppler-shifted ultrasound peak. However, these techniques are sensitive to chemical interferences from some biological compounds and to physical interferences from ­temperature and pressure changes. Moreover, when the laser light transverses a dense media, its contribution to the photoacoustic signal is due not only to the absorption coefficient but also to the scattering­ coefficient, thus possibly resulting in a confounding factor if not taken into account. A technological disadvantage is that the instrumentation is still custom made, expensive, and sensitive to ­environmental parameters. A possible body site for measurement is the eye, and especially the eye sclera. Other sites are fingers and forearms, with contribution in glucose determination by blood ­vessels, by skin and tissues, or by both.(9)

Reverse iontophoresis  Reverse iontophoresis is based on the flow of a low electrical current through the skin, between an anode and cathode positioned on the skin surface. An electric potential is applied between the anode and cathode, thus causing the migration of sodium and chloride ions from beneath the skin towards the cathode and anode, respectively.(10) In particular, it is sodium ion migration that mainly generates the current.(11) Uncharged molecules such as glucose are carried along with the ions by convective flow (electro-osmosis). This flow causes interstitial glucose to be transported across the skin and being collected at the cathode, where a traditional glucose sensor is placed to get direct glucose concentration measurements. Over the typical range of iontophoretic current densities (<0.5 mA/cm2), glucose extraction is approximately in linear relation with the density and duration of iontophoretic ­current. The main drawback of this technique is that it tends to cause skin irritation, although the ­problem may be limited by shortening the time interval of the electrical potential application. On the other hand, a minimum duration is required to get a sufficient amount of glucose for measurement.

Furthermore, this approach cannot be used if the subject is sweating significantly. There is also ­discussion as to whether this technology can detect rapid changes in blood glucose. Some authors consider this technique as minimally invasive. In fact, the glucose is extracted from the skin through the iontophoretic current application and directly measured. The measurement can be obtained with a watch-like device placed on the wrist. To the best of our knowledge, the only one noninvasive glucose monitoring device currently available on the market (at least in Europe and the USA), the GlucoWatch G2 Biographer, is based on reverse iontophoresis. Unfortunately, it is claimed that the device cannot replace self-testing with traditional glucose meters, which limits its usefulness.

Impedance spectroscopy  The impedance of one tissue can be measured by a current flow of known intensity through it. If the experiment is repeated with alternating currents at different wavelengths, the impedance (dielectric) spectrum is determined. The dielectric spectrum is measured in the frequency range of 100Hz–100 MHz.(12) Variations in plasma glucose concentration induce a decrease in sodium ion concentration and an increase in potassium ion concentration in red blood cells. These variations cause changes in the red blood cells membrane potential, which can be estimated by determining the permittivity and conductivity of the cell membrane through the dielectric spectrum.(13,14) However, some ­problems remain to be clarified, such as the effect of body water content and of dehydration.(12) Moreover, some diseases affecting the cell membranes can also have an influence that needs to be evaluated.(5) The best-known study was performed with a watch-like device positioned on the wrist.(15) In fact, a wristwatch device based on this technique, Pendra (see Figure 1), also reached the market a couple of years ago, but it was withdrawn shortly thereafter and the manufacturer filed for bankrupcy.(16)

[[HHE07_fig1_C39]]

Current studies on the impedance spectroscopy technique
Despite the withdrawal of Pendra, impedance spectroscopy seems to be a very promising approach due to its simplicity from a technological point of view. In fact, many studies are currently being carried out based on this approach. For example, a new project is based on an improvement of the Pendra device.(17) We have recently started some preliminary tests on the impedance spectroscopy applicability for possible noninvasive glucose monitoring. In particular, we are investigating a frequency range lower than that selected in other studies.

For instance, the Pendra device worked in the MHz region, whereas we are investigating the kHz region. In some in vitro experiments on bovine blood we found that the impedance of samples at ­different glucose concentrations in the physiological range shows some differences, although at the moment such differences seem modest (in the order of a few ohms). Some results are reported in Figure 2.

[[HHE07_fig2_C40]]

Conclusion
The problem of noninvasive glucose monitoring is currently not solved, and further efforts are still necessary to reach the goal of having a reliable and inexpensive device for the benefit of the diabetic patient. The impedance spectroscopy technique may be a promising approach to this purpose.

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