The miniaturization of microelectronic sensor technology, microelectronic robots, and intravascular implants is progressing rapidly. However, it also poses major challenges for research. One of the biggest is the development of tiny but efficient energy storage devices that enable the operation of autonomously working microsystems — in more and more small areas of the human body, for example. In addition, these energy storage devices must be biocompatible if they are to be used in the body at all.

A newly developed prototype combines these essential properties. The breakthrough was achieved by an international research team led by Prof. Dr. Oliver G. Schmidt, professor of materials systems for nanoelectronics at Chemnitz University of Technology. The Leibniz Institute of Polymer Research Dresden (IPF) was also a cooperation partner.

An array of 90 tubular nanobiosupercapacitors (nBSCs) on the fingertip enable autarkic operation of sensors in blood. (Credit: Research Group Prof. Dr. Oliver G. Schmidt)

The supercapacitors already function in (artificial) blood vessels and can be used as an energy source for a tiny sensor system to measure pH.

This storage system opens up possibilities for the developement of intravascular implants and microrobotic systems for next-generation biomedicine that could operate in hard-to-reach small spaces deep inside the human body. For example, real-time detection of blood pH could help predict early tumor growing. The investigation was largely carried out at the Research Center MAIN at Chemnitz University of Technology.

Smaller Than a Speck of Dust

So-called biosupercapacitors (BSCs), however, are fully biocompatible, which means that they can be used in body fluids such as blood and can be used for further medical studies. Biosupercapacitors can compensate for self-discharge behavior through bio-electrochemical reactions. In doing so, they even benefit from the body’s own reactions. The flexible tubular geometry provides efficient self-protection against deformations caused by pulsating blood or muscle contraction. At full capacity, it can operate a complex, fully integrated sensor system for measuring the pH value in blood.

Flexible, Robust, Tiny

Origami structure technology involves placing the materials required for the nanobiosupercapacitors (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 (95 percent).

The nBSCs were tested in three electrolytes: saline, blood plasma, and blood. In all three electrolytes, energy storage was sufficiently successful, albeit with varying efficiency. 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. Any implantable system within the circulatory system must withstand these physiological conditions while maintaining stable performance.

The team, therefore, studied the performance in microfluidic channels with diameters of 120–150 μm (0.12–0.15 mm) to mimic blood vessels of different sizes. They found that the nBSCs can provide their power well and stably under physiologically relevant conditions.

Prof. Dr. Oliver G. Schmidt is a pioneer in the field of microrobotics and micromotors. (Credit: Jacob Müller)

Self-Contained Sensor

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 nBSC. The 5-μm thin film transistor technology previously established by 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 100 MHz).

The team used an 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 the Swiss-roll origami technique, creating a fully integrated and ultra-compact system of energy storage and sensor.

The researchers report on their research in a recent issue of Nature Communications.

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