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Injectable Gel Electronics Allow Researchers To Create Cyborg Leeches

A transparent gel sits atop a gold and black circuitboard.
The injectable gel being tested on a microfabricated circuit. Credit: Thor Balkhed
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Biology and electronics have become more closely married than ever in a new study. The research, published by a team of Swedish scientists, uses a specialized gel that, when injected into an organism, forms ultrasoft conducting electrodes. The team hopes that the findings could set a path towards fully integrated electronic circuits within the body, which could advance diagnostics and medicine.

Professor Magnus Berggren at the Laboratory for Organic Electronics (LOE) at Linköping University, who was the study’s senior author, commented, “For several decades, we have tried to create electronics that mimic biology. Now we let biology create the electronics for us.”

The road to bio-friendly electronics

The journey Berggren hinted at began in response to a central problem of bioelectronics – biological tissue is soft, whereas conventional semiconductors are hard. Implanting such materials into the body is a fraught process. Their efficacy is invariably curtailed by either the formation of scar tissue around the implant or the breakdown of the electrode itself in a fundamentally incompatible environment.

In response, materials researchers have for years pursued the goal of soft electronics that are a better match with the body’s structures. These devices have found some success when integrating with the exterior of the body’s tissues, such as the surface of the brain. But the polymer substrate they are printed on makes it difficult for them to explore below this shallow level.

Even more involved bioelectronics have been created in response. Some were introduced into the body as liquids that then formed into polymers in response to a chemical or electrical energy stimulation. But these reactions posed a risk to the body itself. Others utilized advances in genetic engineering to express proteins in the body that form polymers near the biological target. But these genetic intrusions make such technology unsuitable for use in humans.

Enter the Swedish team’s solution: an injectable gel that is polymerized in the body in response to benign enzymatic reactions.

Help electronics gel with the body

The polymer is introduced as a gel containing biocompatible molecules based on a monomer with the catchy name 2,5-bis(2,3-dihydrothieno[3,4-b][1,4]dioxin-5-yl)thiophene (ETE to its friends). These molecules are polymerized in some model organisms used in research, like the tiny freshwater animal, Hydra vulgaris. In Hydra, this process is controlled by peroxidase enzymes present in the animal’s body.

“Contact with the body’s substances changes the structure of the gel and makes it electrically conductive, which it isn’t before injection. Depending on the tissue, we can also adjust the composition of the gel to get the electrical process going,” says Xenofon Strakosas, a researcher at LOE and Lund University and one of the study's main authors.

But this pro-polymerizing environment isn’t present in many other animals. To overcome this, Berggren’s team created a cocktail that combines a modified form of ETE, called ETE-COONa, with oxidizing enzymes that react with chemicals already in the tissue to induce polymerization.

Robofish and cyborg leeches

With this chemical creation complete, the team were then free to test out their system in several biologically useful organisms. In albino zebrafish, the team were able to successfully create polymers in the fin, heart and brain. The polymers appear as blue structures within the animals’ tissues. The team further showed off their technology by creating polymers built around the nervous systems of medicinal leeches. 

Neither the leeches nor the zebrafish were harmed by the polymers’ creation and were not affected by their presence.

PhD student Hanne Biesmans, one of the study’s main authors, concluded, “Our results open up for completely new ways of thinking about biology and electronics. We still have a range of problems to solve, but this study is a good starting point for future research.”

Reference: Strakosas X, Biesmans H, Abrahamsson T, et al. Metabolite-induced in vivo fabrication of substrate-free organic bioelectronics. Science. 2023;379(6634):795–802. doi: 10.1126/science.adc9998