World’s smallest supercapacitor powers medical sensor

Technology News |
By Nick Flaherty

Researchers in Germany have developed the world’s smallest power source for medical and sensor designs.

A prototype supercapacitor measuring 0.001mm3 (1 nanolitre) has been developed by an international research team led by Prof. Dr. Oliver Schmidt, Professorship of Materials Systems for Nanoelectronics at Chemnitz University of Technology,head of the Centre for Materials, Architectures and Integration of Nanomembranes (MAIN) at Chemnitz University of Technology and director at the Leibniz Institute for Solid State and Materials Research (IFW) Dresden. The Leibniz Institute of Polymer Research Dresden (IPF) was also involved in the study.

The microsupercapacitor is being used in artificial blood vessels and can be used as an energy source for a tiny sensor system to measure pH. For example, real-time detection of blood pH can help predict early tumour growing.

Currently, the smallest such energy storage devices are larger than 3 mm3. The protype nano-biosupercapacitor (nBSC) is smaller than a grain of dust but delivers up to 1.6 V supply voltage for microelectronic sensors.

This energy can be used for a sensor system in the blood, for example. The power level also is roughly equivalent to the voltage of a standard AAA battery, although the actual current flow on these smallest scales is significantly lower. The flexible tubular geometry of the nano-biosupercapacitor provides efficient self-protection against deformations caused by pulsating blood or muscle contraction. At full capacity, the nBSC can operate a complex fully integrated sensor system for measuring the pH value in blood.

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“It is extremely encouraging to see how new, extremely flexible, and adaptive microelectronics is making it into the miniaturized world of biological systems,” said Prof Schmidt.

The fabrication of the samples and the investigation of the biosupercapacitor were largely carried out at the MAIN Research Centre at Chemnitz.

“The architecture of our nano-bio supercapacitors offers the first potential solution to one of the biggest challenges – tiny integrated energy storage devices that enable the self-sufficient operation of multifunctional microsystems,” said Dr. Vineeth Kumar, researcher in Prof. Schmidt’s team and a research associate at the MAIN research centre.

Nano-supercapacitors (nBSC) are difficult to produce for medial applications as they do not use biocompatible materials but corrosive electrolytes that quickly discharge themselves in the event of defects and contamination. Both aspects make them unsuitable for biomedical applications in the body.

Instead biosupercapacitors fully biocompatible so that they can be used in body fluids such as blood and can be used for further medical studies. These can also compensate for self-discharge through bio-electrochemical reactions. In doing so, they even benefit from the body’s own reactions.

The origami structure of the nBSC involves placing the materials required for the nBSC components on a wafer-thin surface under high mechanical tension. When the material layers are subsequently detached from the surface in a controlled manner, the strain energy is released and the layers wind themselves into compact 3D devices with high accuracy and yield of 95 per cent. The nano-biosupercapacitors produced in this way were tested in three solutions called electrolytes: Saline, blood plasma, and blood. In all three electrolytes, energy storage was sufficiently successful, albeit with varying efficiency. In blood, the nano-biosupercapacitor showed excellent lifetime, holding up to 70 per cent of its initial capacity even after 16 hours. A proton exchange separator (PES) was used to suppress the rapid self-discharge.

In order to maintain natural body functions in different situations, the flow characteristics of the blood and the pressure in the vessels are under constant change. Blood flow pulsates and varies according to vessel diameter and blood pressure. Any implantable system within the circulatory system must withstand these physiological conditions while maintaining stable performance.

The team studied the performance of the nBSC in microfluidic channels with diameters of 120 to 150 µm (0.12 to 0.15 mm) to mimic blood vessels of different sizes. In these channels, the researchers simulated and tested the behaviour of their energy storage devices under different flow and pressure conditions. They found that the nano-biosupercapacitors can provide their power well and stably under physiologically relevant conditions.

The hydrogen potential (pH) of blood is subject to fluctuations. Continuous measurement of the pH can thus help in the early detection of tumors, for example. For this purpose, the researchers developed a pH sensor that is supplied with energy by the nano-biosupercapacitor.

The 5 µm thin film transistor (TFT) technology previously established in Prof. Oliver Schmidt’s research team could be used to develop a ring oscillator with exceptional mechanical flexibility, operating at low power (nW to µW) and high frequencies (up to 100MHz).

For the current project, the team used a nBSC based ring oscillator. The team integrated a pH-sensitive BSC into the ring oscillator so that there is a change in output frequency depending on the pH of the electrolyte. This pH-sensitive ring oscillator was also formed into a tubular 3D geometry using an origami technique, creating a fully integrated and ultra-compact system of energy storage and sensor. 

The hollow inner core of this micro sensor system serves as a channel for the blood plasma. In addition, three nBSCs connected in series with the sensor enable particularly efficient and self-sufficient pH measurement.

These properties open up a wide range of possible applications, for example in diagnostics and medication.

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