Nanotechnology tackles problems with noninvasive glucose monitoring

Traversing all scientific disciplines its nanoscale nature means that new functionalities and properties of materials can be created, which contribute to the development of innovative products and applications. Nanomedicine extends from the medical applications of nanomaterials to nanoelectronic biosensors, and there are even possible future applications of molecular nanotechnology. Indeed, nanoparticles’ use in medicine is leaping ahead. For example, both organic and inorganic artificial nanoscale materials may be introduced into the human body and assigned diagnostic functions^1,2.

Introduction to diabetes

Diabetes is one of the most prevalent diseases in the world. According to the International Diabetes Federation (IDF), in 2015 415 million people suffered from the illness worldwide, which equates to one in eleven adults³ . And numbers continue to climb; in 2040 642 million people are anticipated to be diabetic. An important aspect of good diabetic control and successful management of the disease is the control of glucose levels, and self-monitoring blood glucose is the established method for assessing glycaemic control. Patients typically self-monitor four to six times a day in order to adapt their insulin dose to their food intake and levels of physical activity, as well as to correct for non-physiological glycaemic excursions. Since 2003 the method of continuous glucose monitoring has been available, but it is not widely established. Although it was designed to significantly reduce the necessary quantity of blood required, and lead to simpler device handling, the underlying process can still be painful. Consequently, patients desire a minimal or non-invasive measuring method.

In principle, non-invasive glucose monitoring is possible. The basis for this is the interaction of glucose molecules with applied energy (radiation, heat and electromagnetic fields, among others). A number of physical principles can be used; for example, light absorption, light scattering, polarisation of light, fluorescence, Raman scattering, and photoacoustic and impedance measurement. No sample material is required, since the applied energy field constitutes directly the measurement probe in a volume of tissue. Scientific studies have already been carried out for each of these methods^4-12. However, for the measurement of glucose in a certain tissue volume, a specific interaction of the glucose molecules with the applied energy needs to take place. This is difficult because the concentration of glucose in the human body is relatively low with levels in the parts-per thousand range. Therefore, glucose signals are weak in comparison to other endogenous substances, such as water or albumin, which prevail in much higher concentrations.

The effect of signals that do not originate from glucose (‘noise’) must be compensated and separated from the true glucose signal by use of complex mathematical algorithms. A further critical consideration is that the radiated energy (e.g., a beam of light) hits a fairly complex structure when it penetrates the skin. Because of the strong anisotropy of skin and tissue, the measurement signal differs considerably, depending, for example, on the penetration depth, making it difficult to accurately measure glucose. It is therefore difficult to obtain the required accuracy for the measurement of glucose by non-invasive methods.

Nano hope

The question is: can nanotechnology provide the opportunity to develop measurement systems with the required accuracy? This complex but exciting field applies to the development and production of devices and structures with dimensions <100 nanometers and/or uses characteristic effects and phenomena in this size range (quantum effects). It opens up completely new possibilities for the development of glucose sensors and, therefore, dramatically improving the quality of diabetes patients’ lives.

In nanostructures characteristic effects take place, which are not found in bulk materials. The ratio of surface area to volume is very large. This leads to a number of physical and chemical results such as interfacial phenomena, altered reactivity, charge carrier effects and quantum mechanical changes^13,14 as well as enhanced optical properties (for example quantum dot fluorescence). In miniaturised glucose sensors, such nanoscale properties can have several advantages, including higher surface areas (yielding larger currents and faster responses) and improved catalytic activities. This should result in an improved sensitivity of the glucose sensor, a lower signal-to-noise ratio and a higher selectivity of the measurement.

Nanofabrication techniques can generate glucose sensors with very small dimensions (for example by laser ablation, chemical vapour deposition (CVD) or arc discharge (nanotubes, fullerene, etc.))¹⁵. Such small sensors can be easily implanted or would be injectable as ‘glucose measurement tattoos’^16,17. Additionally, they could potentially avoid the foreign body response of the immune system due to their small size and, consequently, have longer lifetimes. Finally, micro- and nanoelectronic technologies offer the possibility of cost-effective mass production. As the costs of such sensors depend massively on their widespread usage, diabetes therapy might be revolutionised if low-cost glucose sensors based on nanotechnology become available.

Examples of experimental glucose sensors

Micro- and nanoelectronics offer new solutions for glucose sensors with a high signal-to-noise ratio. Nanotubes have been well studied, especially carbon nanotubes. These can be classified as single-walled nanotubes or multi-walled ones. Typically, the diameter of a nanotube is less than one nanometer, while the tube wall can be up to 0.3 nanometers thick (Figure 1; page 00). Such molecular assemblies of carbon atoms have been presented in literature^18,19. During the production process at very high temperatures, specific substances can be introduced into these nanotubes. These react with glucose (for example: glucose oxidase à bound to a fluorescent dye).

In such a way, the nanotube functions as a light amplifier: the fluorescence signal can be measured and reflects the glucose concentration. It is envisioned that such nanotubes are wrapped in a dialysis fiber and transplanted under the skin. For the glucose measurement, the skin area is irradiated with a laser and the glucose concentration measured via the fluorescent light induced. The thinner the nanotubes are, the larger the energy band gap and the greater the energy absorption and the fluorescence signal²⁰. On the basis of single wall-nanotubes, enzyme-based optical glucose sensors were investigated^21,22. In combination with enzymes for the catalysis of glucose reactions (e.g., immobilised glucose oxidase; GOx), the decisive advantage of such nanostructures is the very large surface area and the efficient electron transfer from enzyme to electrode²³. Sensors of this type have a high sensitivity, linear detection range and linearity for detecting glucose. Furthermore, the carbon nanotube (CNT) fiber-based glucose biosensor was shown to be stable for up to 70 days²³.

The use of nanostructured materials in glucose sensors, compared with currently-used GOx sensors of the first generation, is shown in Figure 2 (page 00)²⁴. As with ordinary glucose sensors based on GOx, it is possible to improve electron transfer between the enzyme and the electrode by using an electrochemical mediator. This is reduced by the GOx reaction and transfers its electrons to the electrode. In this sense, an approach is to modify the nanotubes with an electrochemical mediator such as ferrocene²⁵ or ferricyanide²⁶. A further improvement in sensor performance is made possible by the combination of CNTs with various nanoparticles such as noble metals (silver, gold, platinum) or silica, titanium dioxide and others (Figure 2, right; page 00). This combination of nanomaterials improves the catalytic activity and the sensitivity of measurement.

Further attractive options for glucose monitoring devices are biocompatible polymeric nanosensors implanted under the skin (like a tattoo). The fluorescence properties of such sensors change in response to changes in the glucose concentration in the interstitial fluid. Such changes can be read using optical interrogation through the skin after light excitation with a laser beam. Such sensors can be based on polymeric nanosensors incorporating boronic acid derivatives to recognise glucose. Nanospheres based on N-isopropylacrylamide containing a covalently bound phenyl-boronic acid derivative, as well as two attached fluorophores, have been synthesised²⁷. In case of a low glucose concentration, the nanospheres are small and hold the fluorophores close together. The consequence is an efficient resonance energy transfer. When there are higher glucose concentrations, the glucose binding to the boronic acid is reduced and subsequently the polymer swells, which increases the average distance between fluorophores. This therefore decreases resonance energy transfer, which increases the donor fluorescence and reduces the acceptor fluorescence. Special fluoresphores can be CNT, which demonstrate a glucose-controlled aggregation onto concanavalin A. Since the aggregates have different fluorescences than free CNTs, detection of glucose is possible through measuring the CNT fluorescence change²⁸.

An interesting nanostructure is that of quantum dots, usually made of semiconductor material (e.g., CdSe, InGaAs or GaInP/InP). Quantum dots can be prepared by molecular beam epitaxy or lithographical processes in semiconductor layer systems (nanolayers with a few atomic layers), for example, with an electron beam and following a dry etching procedure. A quantum dot is small enough to exhibit quantum mechanical properties. Typically, it contains about 10,000 atoms. Charge carriers (electrons, holes) in the dot are so far limited in their mobility in all three spatial directions that their energy no longer continues, and only discrete values may be obtained. Quantum dots thus behave like atoms, but their shape, size or the number of electrons may be influenced. These electronic and optical properties can be tailored¹⁵, enabling production of quantum dots that have favourable optical properties for use in different sensors.

Since this sensor itself does not interact with glucose molecules directly, an attachment, for example of GOx to a quantum dot, is necessary. In this case the luminescence of the quantum dots can be quenched by hydrogen peroxide. In the presence of glucose the enzyme generates hydrogen peroxide, which quenches the quantum dots, providing an optical signal change proportional to glucose concentrations. For enhancement of glucose oxidase activity the assembling of a complex with quantum dots of different compounds, such as cadmium telluride, is possible²⁹.

Conclusions

The ongoing miniaturisation of semiconductor integrated circuits has led to nano-technological solutions, which allow for completely new approaches to glucose sensing. At the existing technology level, it is already possible to produce glucose sensors as integrated components, such as integrated circuits. It is now a case of deciding which of the structures can be effectively produced in large quantities and could thus be inexpensive nanotechnologies, which once resolved is likely to enable the development of glucose sensors that offer a number of advantages over known CGM systems. Such sensors could be injectable under the skin and provide novel physical effects created in new nanomaterials, fabricated by specialised nanotechnologies.

References

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Disclosures

Andreas Thomas is Head of Science in the Diabetes Division of Medtronic, Germany, a manufacturer and distributor of insulin pumps and glucose sensors; Araceli Ramírez is co-worker in Laboratorio de Nanotrónica, Benemerita Universidad Autonoma de Puebla, Mexico with no commercial interest in medical devices; and Alfred Zehe is Head of Laboratorio de Nanotrónica, Benemerita Universidad Autonoma de Puebla, Mexico with no commercial interest in medical devices.

Biographies

Andreas Thomas studied Physics until 1984 at the Technical University Dresden, Germany. He completed his dissertation in solid state physics in 1987 and a postdoctoral thesis in 1992. Afterwards, he worked in different fields of diabetes technology and therapeutics. He is currently Scientific Manager at Medtronic GmbH, Department of Diabetes, in Germany. Furthermore, he is interested in the development of insulin pump therapy, continuous glucose monitoring and the artificial pancreas. He works closely with his former teacher at the University of Dresden, Prof. Alfred Zehe, who heads a laboratory of nanotechnology at the University of Puebla in Mexico. He is a member of the Advisory Board in the German working group of Diabetes and Technology and was Chief Editor of the German Journal Diabetes and Technology. His address for correspondence is: Dr. Andreas Thomas, Earl-Bakken-Platz 1, 40670 Meerbusch, Germany. His email address is: [email protected].

Dr. A. Zehe is a Full Professor of Physics and Director of the Nanotechnology-Center at the Autonomous University of Puebla in Mexico. Over many years, his academic work at universities in Germany involved the progress in solid-state physics, semiconductor materials and microelectronics. His actual research fields in Mexico turn around nanomaterials and nanoelectronics in applications of life sciences and nano-medicine.

Dr. A. Ramírez teaches chemistry at the Autonomous University of Puebla in Mexico and is the leading scientist of nano-microscopy in the Laboratorio de Nanotrónica. As a collaborator of Dr. Zehe’s research group, her actual research fields involve nanomaterials and nanodevices for applications in life sciences and nano-medicine.