One of the problems of designing neural implants is their production, which generally involves the use of metal and other rigid materials. In the long run, these materials can cause inflammation and damage the soft tissue of one of our most vulnerable organs: the brain. MIT engineers are working on the development of soft and flexible neural implants that can adapt to the brain’s contours and monitor activity over longer periods, without irritating surrounding tissue. These flexible electronic devices could be a softer alternative to existing metal-based electrodes designed to monitor brain activity. This technology could also be useful in brain implants that stimulate neural regions to relieve the symptoms of epilepsy, Parkinson’s disease, and psychiatric disorders.
The research team led by Xuanhe Zhao, professor of mechanical engineering and of civil and environmental engineering, has developed a way to print 3D neural implants and other soft devices, flexible as rubber. The researchers have already printed several soft electronic devices, including a small rubbery electrode, which they implanted into a mouse brain. While the mouse moved freely in a controlled environment, the neural probe was able to capture the activity of a single neuron. Monitoring this activity can provide scientists a high-resolution picture of brain activity and can help customize therapies and long-term neural implants for a variety of neurological disorders. The results of the study were published on March 30 in the journal Nature Communications.
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From the liquid polymeric mix to the viscous hydrogel
Conductive polymers are a class of materials that scientists have investigated with interest in recent years for their unique combination of flexibility and electrical conductivity. In the new study, the team reported modifying poly (3,4-ethylenedioxythiophene) polystyrene sulfonate, or PEDOT:PSS, a conductive polymer typically found in the form of a dark blue liquid. The liquid is a mixture of water and nanofibers of PEDOT: PSS. This substance obtains its conductivity from the nanofibers which, when they come in contact, act as a sort of tunnel through which any electrical charge can flow.
The problem is that this polymer in liquid form cannot be used as ink in a 3D printer. Therefore, scientists looked for a way to thicken it while maintaining the intrinsic electrical conductivity of the material. First, they freeze-dried it, removing the liquid and leaving only the nanofiber matrix. Without liquid in which they are immersed, the nanofibers become fragile and tend to split. Then, the researchers created a mix of nanofibers with an aqueous solution and an organic solvent previously developed to form a hydrogel, i.e. a water-based rubbery material incorporated with the nanofibers. They made this hydrogel with various concentrations of nanofibers and found that a range between 5% to 8% by weight of nanofibers produced a material with a consistency like toothpaste, electrically conductive and suitable for acting as ink into a 3D printer.
The possibility to detect the activity of a single neuron
The researchers inserted the new conductive polymer into a 3D printed and discovered they could produce complex models that remained stable and electrically conductive. So, they printed a small, rubbery electrode, the size of which is comparable to a piece of confetti. The electrode consists of a flexible and transparent polymeric layer, over which they printed the conducting polymer in thin parallel lines that converge at a point measuring about 10 microns wide, small enough to pick up electrical signals from a single neuron. The electrode was then implanted in the brain of a mouse, proving its ability of capturing electrical signals from a single neuron.
Traditionally, electrodes are rigid metal wires, and once there are vibrations, these metal electrodes could damage tissue. We’ve shown now that you could insert a gel probe instead of a needle.
~Xuanhe Zhao professor of mechanical engineering and of civil and environmental engineering at MIT
Such soft hydrogel-based electrodes may even be more sensitive than conventional metal electrodes. That is because most metal electrodes conduct electricity in the form of electrons, whereas neurons in the brain produce electrical signals in the form of ions. Any ionic current produced by the brain must be converted into an electrical signal that can be recorded by a metal electrode, a conversion that can lead to the loss of part of the signal during translation. In addition, ions can only interact with a metal electrode on its surface, which can limit the concentration of ions that the electrode can detect at any time. On the contrary, the soft electrode is made up of electron conduction nanofibers embedded in a hydrogel, a water-based material through which ions can flow freely.
“The beauty of a conducting polymer hydrogel is, on top of its soft mechanical properties, it is made of hydrogel, which is ionically conductive, and also a porous sponge of nanofibers, which the ions can flow in and out of,” Baoyang Lu says, associate professor at Jiangxi Science and Technology Normal University and co-author of the article. “Because the electrode’s whole volume is active, its sensitivity is enhanced.”
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The new 3D printing technique would improve the production of several neurological devices
In addition to the neural probe, the team also fabricated a multielectrode array, a small plastic square as big as a Post-it, printed with very thin electrodes, over which the researchers also printed a round plastic well. Neuroscientists generally fill the wells of these arrays with cultured neurons to study their activity through the signals that are detected by the device’s underlying electrodes. The group has shown it can replicate the complex models of these arrays using 3D printing with polymeric hydrogel compared to traditional lithographic techniques, which involve accurate engraving of predefined models or masks on metals such as gold, a process that can take days to complete a single device.
“We make the same geometry and resolution of this device using 3D printing, in less than an hour,” says Hyunwoo Yuk, PhD student at MIT and author of the article. “This process may replace or supplement lithography techniques, as a simpler and cheaper way to make a variety of neurological devices, on-demand.”