A gel cocktail uses the body’s sugars to grow electrodes on live fish

For the first time, researchers have harnessed the body’s own chemistry to “grow” electrodes inside the tissues of living fish, straddling the boundary between biology and machines.

The technique uses the body’s sugar to infuse the gel into a flexible electrode without damaging tissue, experiments show. Zebrafish grown with these electrodes in their brains, hearts and tail fins showed no signs of ill effects, and when tested on leech they successfully stimulated the nerve, the researchers report on 24 Dec. Science.

Sometimes, these electrodes can be useful for applications of studying how biological systems work to improve the human machine. They can also be used in “bioelectronic” medicine, such as brain stimulation therapy for depression, Parkinson’s disease, and other conditions (SN: 2/10/19).

Soft electronics tend to bridge the gap between soft, curvy biology and electronic hardware. But these electronics typically still have to carry some parts that can be prone to cracks and other issues, and these devices inevitably bring about damage to the tissues.

“All the machines we’ve made, even if we’ve made them flexible, to make them softer, there will still be a scar. It’s like a knife sticking into the organ,” says Magnus Berggren, a materials scientist at Linköping University in Sweden. That scarring and inflammation can degrade the electrode over time.

Previous efforts have incorporated electronics into the fibers to soften the defects. One approach uses electrical or chemical signals to make the transformation from chemical soup to electrodes, but these zaps also cause damage. A 2020 study conducted around this issue, using cells in worms genetically modified to engineer an enzyme to do the job, but the new method achieves its results without genetic alteration.

Berggren and his colleagues’ electrodes instead of natural energy have a source of energy already present in the body: sugars. The cocktail gel contains molecules called oxidases that react with sugar — glucose or lactate — to produce hydrogen peroxide. What then activates another ingredient in the cocktail, an enzyme called hydrogen peroxidase, is the catalyst needed to transform the gel into a conductive electrode.

“It leverages an elegant approach to chemistry to overcome many technical challenges,” says biomedical engineer Christopher Bettinger of Carnegie Mellon University in Pittsburgh, who was not involved in the study.

To test the technique, the researchers injected the transparent cocktail into the brains, hearts and tails of zebrafish. The gel turns blue when it becomes conductive, giving a view of the bed’s success.

“It’s a beautiful thing you can see: the tail of zebrafish changes color, and we know that blue means a conductive polymer,” materials scientist Xenofon Strakosas, also of Linköping University. “The first time I saw it, I thought, ‘Wow, I really need it!’

The fish appeared to suffer no ill effects, and the researchers saw no evidence of tissue damage. In partially dissected leeches, the team showed that delivering current to the nerve through a soft electrode could induce muscle contraction. Finally, devices of this type could be connected to various wireless technologies in development.

A long time for the implementation of the implant remains to be determined. “The demonstrations are magnificent,” he said better. “What remains to be seen is the stability of the electrode.” Over time, substances in the body react with the material of the electrode, deforming or even producing toxic substances.

The team still needs to work out how the electrodes can precisely move the nerves, says chemical engineer Zhenan Bao of Stanford University, who was not involved in the work. She and her colleagues have developed a way to “grow” electrical components using genetic modifications. Stimulating the prospect of contracting where treatment is needed, while preventing leaks from running to unnecessary areas, will be important, he said.

In a new study, the relative abundance of different sugars in different tissues determines exactly where the electrodes form. But in the future, the main ingredient can be made of elements that attach to specific parts of the biology to make the effort much more precise, says Berggren. “We’re doing experiments right now where we’re trying to bind these materials directly into individual cells.” Notes Strakos: “There are some applications where precision is really important; where we have tried to place it. ”