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Smaller Than Dust, Voltage Like Triple a Battery: New Biosupercapacitor Provides Energy for Biomedical Applications
Researchers have developed a nano-biosupercapacitor that can be integrated with sensors to detect pH changes in vivo.

Bigger is not always better, especially in today’s age of miniaturisation. While modern technology has propelled us forward to invent tools unlike anything our ancestors have seen before, most of these grand gadgets are no bigger than the size of our palm. This trend in technology downsizing has led us to the small, thin, and light-weight electronics we have today.

In the field of biomedical sensor technology, miniaturised microelectronics are progressing rapidly, with microelectronic robots and intramuscular implants at the forefront. Unfortunately, being small also comes with its own setbacks as powering tiny electronics requires an equally small but efficient energy source. To date, scientists are still struggling to develop small, implantable energy storage systems due to the strict criteria of size, flexibility, and biocompatibility among many others.

Fulfilling all essential requirements, a breakthrough study led by Professor Dr. Oliver G. Schmidt at Chemnitz University of Technology has reported the development of the world’s smallest microsupercapacitors that can power a tiny pH sensor in artificial blood vessels. This energy storage system is expected to advance next-generation intravascular implants and microrobotic systems by enabling them to operate even in hard-to-reach spaces deep inside the human body. With this biosupercapacitor, clinicians could potentially make early predictions of tumour growth through real-time detection of blood pH as well.

“It is extremely encouraging to see how new, extremely flexible, and adaptive microelectronics is making it into the miniaturaised world of biological systems,” said research group leader Prof. Dr. Schmidt, who was extremely pleased with the success of their research.

As a rule, small energy storage devices in the sub-millimetre range called nano-biosupercapacitors use corrosive electrolytes and quickly discharge themselves in the event of defects and contamination. Therefore, they are usually unsuited for biomedical uses. However, the team’s novel biosupercapacitor has been designed to solve these issues. They can fully operate in body fluids and be used for medical studies. The biosupercapacitors can also compensate for the usual self-discharge through bio-electrochemical reactions of the body. By relying on the natural reactions of the body like redox enzymatic reactions, the performance of the device can also be increased by 40 per cent.

In terms of size, the newly developed tubular nano-biosupercapacitor surpasses existing technologies as well. It is 3,000 times smaller than the smallest energy storage device used today for in vivo applications (3 mm3). With a volume of 0.001 mm3, the energy storage device occupies less space than a grain of dust. But contrary to its minuscule size, it can deliver up to 1.6 V of supply voltage to power microelectronic sensors in blood systems. This power level is roughly equivalent to the voltage standard of AAA battery, though the actual current flow on the smallest scales would be lower.

Besides its biocompatibility and compact size, Prof. Dr. Schmidt and colleagues also engineered their supercapacitor using origami structure technology to make it robust and flexible. They placed the components of the nano-biosupercapacitor on a wafer-thin surface under high mechanical tension and then detached the material layers in a controlled manner. In doing so, the strain energy is released and the layers wind themselves into compact 3D devices with high accuracy and yield up to 95 per cent. Upon testing the storage device in saline, blood plasma, and blood electrolytes, their tool was sufficiently successful. In blood, the nano-biosupercapacitor showed excellent lifetime, holding up to 70 per cent of its initial capacity even after 16 hours.

Because the supercapacitor is intended for biomedical use, the team sought to test the performance of their technology in a physiologically relevant environment. In this study, they chose to experiment in an artificial blood vessel system. The team used microfluidic channels to mimic blood vessels and studied the device’s behaviour under different flow and pressure conditions. In particular, they installed an electronic pH sensor in the nano-biosupercapacitor to analyse its performance.

They integrated the pH-sensitive nano-biosupercapacitor into a ring oscillator made from a 5-micrometre thin-film transistor technology. The ring oscillator was then formed into a tubular 3D geometry to create an ultra-compact system that combines both energy storage and sensor. By modelling their device in this fashion, the device can also be protected against deformations caused by pulsating blood or muscle contractions. Upon testing the device in artificial blood vessels, it was shown that the nano-biosupercapacitor could efficiently and stably operate the complex fully integrated pH-sensing system in the blood even when subjected to different conditions.

By offering “the first potential solution to one of the biggest challenges – tiny integrated energy storage devices that enable the sufficient operation of multifunctional microsystems,” as said by Dr. Vineeth Kumar, one of the members on the research team, their biosupercapacitor is expected to enable future innovations and strengthen the pipeline for embeddable technologies. Although the device was only tested with a pH sensor, it is believed the nano-superbiocapacitor could potentially be adapted to power a wide range of implantable microelectronics to aid monitoring, diagnostics, and medication.

Source: Lee et al. (2021). Nano-biosupercapacitors enable autarkic sensor operation in blood. Nature Communications, 12, 4967.

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