Often some medical procedures such as monitoring or use of electrical impulses (such as brain surgery or epilepsy mapping) require the use of metal electrodes. Unfortunately, the metallic nature, together with some plastics in their conformation, are rigid and inflexible, which contrasts with the nature of soft and malleable body tissues.
This difference limits the places where electrode arrays can be used successfully, as well as taking into account the large amount of electrical current between an electrode and its target. Faced with these limitations and inspired by the properties of human tissue, scientists from the Harvard Wyss Institute and the John A. Paulson School of Engineering and Applied Sciences have created flexible, metal-free electrodes.
These electrodes are perfectly adjusted to the shapes of the tissue that requires them, from the deep folds of the brain to the fibrous nerves of the heart. This advancement allows for lower and more precise use of electrical impulses, as well as their use in hard-to-reach areas of the body, and minimizes the risk of damage to delicate tissues.
Alginate hydrogels were developed in the laboratories of the Wyss Institute, whose applications include surgical adhesives and single-cell encapsulation thanks to their viscoelasticity. At the same time, it was observed that this characteristic could be used on the tissues by being able to adapt to the necessary shape.
Given her expertise in neural engineering, researcher Christina Tringides decided to try to create viscoelastic electrodes that could match those in the brain for safer and more efficient neuroelectric monitoring. Standard electrodes are made of conductive metal matrices contained within a thin plastic film and are up to a million times stiffer than the brain.
First, the team tested different alginate hydrogel mixtures that could be seamlessly adapted to living tissues. After experimenting with various hydrogel mixtures, they opted for the version that managed to adapt to the mechanical properties of brain and heart tissue. Subsequently, the hydrogel was tested on a fake “brain” made of gelatin-like agarose and its performance compared with conventional electrodes.
The advantage of alginate hydrogel electrodes over conventional electrodes was concluded, since the former had twice the contact with the simulated brain compared to conventional ones. They were also tested in a prolonged way for two weeks and when they were removed, the conventional electrode returned to its original shape.
In contrast, the alginate hydrogel remained in position the entire time and retained its brain shape after extraction. “Our hydrogel-based electrodes take the shape of whatever tissue they are placed on and open the door to creating personalized and less invasive medical devices,” said Tringides.
Having a flexible matrix, the hydrogel could not have metallic materials in its composition due to the limitation of flexibility. Consequently, the researchers developed a combination of graphene flakes and carbon nanotubes as their main candidate. Through this combination of materials, they created porous and electrically conductive pathways that were sufficiently flexible and without breaking or tearing.
Currently, the team continues to develop their devices and is working on in vivo animal models with an eye toward making them available for use during medical procedures such as brain tumor removal surgery. They also hope that this new technology will allow electrical recording and stimulation to be performed on parts of the body that are currently inaccessible to commercially available devices.