The team’s first task was to test whether their alginate hydrogels could successfully conform to living tissues. After experimenting with different types of hydrogels, they settled on a version that most closely matched the mechanical properties of brain and heart tissue. They then placed their hydrogel onto a fake “brain” made from gelatin-like agarose, and compared its performance to that of a plastic material and an elastic material.
The alginate hydrogel had double the amount of contact with the underlying mock brain compared to the other materials, and was even able to get down into some of the brain’s many deep grooves. When they left the materials on the mock brains for two weeks, the elastic material had substantially moved from its original location and immediately sprang back into its original shape when removed from the underlying mock tissue. In contrast, the alginate hydrogel stayed in position the whole time and retained its brain-like shape after removal.
Going with the flow
Now that the team had a material that could flex and flow around tissues, they had to invent an electrode that could do the same thing. The vast majority of electrodes are made of metal because metals are highly electrically conductive — but also very stiff and inflexible.
After many experiments and late nights in the lab, the team identified a combination of graphene flakes and carbon nanotubes as their top candidate. “Part of the advantage of these materials is their long and narrow shape. It’s a bit like throwing a box of uncooked spaghetti on the floor — because the noodles are all long and thin, they’re likely to cross each other at multiple points. If you throw something shorter and rounder on the floor, like rice, many of the grains won’t touch at all,” said Tringides.
When these spaghetti-like materials were embedded into the alginate hydrogels, they crisscrossed their way through the gel to create porous, conductive pathways through which electricity could travel. These flexible electrodes could be bent more than 180 degrees and tied into knots without breaking, making them a perfect partner for the viscoelastic alginate hydrogel.
To put it all together, the team surrounded their new conductive electrode with an insulating layer of a self-healing silicone polymer called PDMS, which was then sandwiched between two layers of the alginate hydrogel. The resulting device was highly flexible, and could be stretched up to 10 times its length without breaking or tearing. When living brain cells such as astrocytes and neurons were grown on the devices, the cells displayed no damage or other negative effects, suggesting that the device could be safely used on living tissues.
An alternative array for safer surgeries
The team then tested their new viscoelastic electrode array in real-world conditions by attaching it to a mouse heart. The device stayed in place on the tissue as it moved, and remained intact over tens of thousands of muscle contractions. The researchers then scaled up, attaching their device to a rat brain, a rat heart, and a cow heart, all of which experienced no damage and no slipping of the device, even when bent more than 180 degrees. In contrast, a commercial electrode array did not stay in contact with the cow heart when bent more than 90 degrees.
Finally, the viscoelastic electrode array was successfully used to both stimulate nerves and record electrical activity in vivo. When the device was attached to a living mouse’s hind leg, the researchers successfully stimulated different muscles to contract by varying which of several electrodes delivered the stimulation. They then attached their device to a mouse’s heart and a rat’s brain during surgeries. The electrical activity of the heart and the brain were successfully recorded by the device, which was bent to attach to hard-to-reach areas and caused no injury to the animals during use.
“The viscoelasticity of this device marks a new direction in medical devices, which are typically designed to be purely elastic,” said corresponding author Dave Mooney, who is a Wyss Core Faculty member and leader of the Institute’s Immuno-Materials platform. “By taking the opposite approach, we can interface with the body’s tissues much more closely, allowing a more functional interface without damaging the tissue.” Mooney is also the Robert P. Pinkas Family Professor of Bioengineering at SEAS.