‘Cyborg Tadpoles’ With Super Soft Neural Implants Shine Light on Early Brain Development

Early brain development is a biological black box. While scientists have devised multiple ways to record electrical signals in adult brains, these techniques don’t work for embryos.

A team at Harvard has now managed to peek into the box—at least when it comes to amphibians and rodents. They developed an electrical array using a flexible, tofu-like material that seamlessly embeds into the early developing brain. As the brain grows, the implant stretches and shifts, continuously recording individual neurons without harming the embryo.

“There is just no ability currently to measure neural activity during early neural development. Our technology will really enable an uncharted area,” said study author Jia Liu in a press release.

The mesh array not only records brain activity, it can also stimulate nerve regeneration in axolotl embryos with electrical zaps. An adorable amphibian known for its ability to regrow tissues, axolotl research could inspire ideas for how we might heal damaged nerves, such as those in spinal cord injury.

Amphibians and rodents have much smaller brains than us. Due to obvious ethical concerns, the team didn’t try the device in human embryos. But they did use it to capture single neuron activity in brain organoids. These “mini-brains” are derived from human cells and loosely mimic developing brains. Their study could help pin down genes or other molecular changes specific to neurodevelopmental disorders. “Autism, bipolar disorder, schizophrenia—these all could happen at early developmental stages,” said Liu.

Probing the Brain

Recording electrical chatter from the developing brain allows scientists to understand how neurons self-assemble into a mighty computing machine capable of learning and cognition. But capturing these short sparks of activity throughout the brain is difficult.

Current technologies mostly focus on mature brains. Functional magnetic resonance imaging, for example, is used to scan the entire brain as it computes specific tasks. This doesn’t require surgery and can help scientists stitch together brain-wide activity maps. But the approach lacks resolution and is laggy.

Molecular imaging is another way to record brain activity. Here, animals such as zebrafish are genetically engineered to grow neurons that light up under the microscope when activated. These provide real-time insight into each individual neuron’s activity. But the method only works for translucent animals.

Neural implants are the newest kid on the block. These microelectrode arrays are directly implanted into brain tissue and can capture electrical signals from large populations of neurons with millisecond precision. With the help of AI, such implants have already restored speech and movement and untangled neural networks for memory and cognition in people.

They’re also unsuitable for developing brains.

“The brain is very soft, like a piece of tofu. Traditional electronics are very rigid, when you put them into the brain, any movement of the electronics can cut the brain at the micrometer scale,” Liu told Nature. Over time, the devices cause scarring which degrades the signals.

The problem is acute during development, as the brain dramatically changes shape and size. Rigid probes can’t continuously monitor single neurons as the brain grows and could damage the nascent organ.

Opening the Box

Picture the brain and a walnut-shaped structure etched with grooves likely comes to mind. But the organ begins life as a flat single-cell layer in the embryo.

Called the neural plate, this layer of cells lines the embryo’s surface before eventually folding into a tube-like shape. As brain cells expand and migrate, they generate tissues that eventually fold into the brain’s final 3D structure. This dimensional transition makes it impossible to monitor single neurons with rigid probes. But stretchable electronics may do the job.

In 2015, Liu and colleagues developed an ultra-flexible probe that could integrate into adult rodent brains and human brain organoids. The mesh-like implant had a stiffness similar to brain tissue and minimized scarring. The team used a material called fluorinated elastomers, which is stretchy like gum but has the toughness of Teflon—and is 10,000 times softer than conventional flexible implants made of plastic-like materials. Implants made of the material captured single-neuron activity in mice for months and were relatively easy to manufacture.

Because of the probe’s stretchiness, the team wondered if it could also monitor developing embryonic brains as they folded up from 2D to 3D. They picked tadpoles as a test case because the embryos grow fast and are easy to monitor.

The first try failed. “It turns out tadpole embryos are much softer than human stem cell-derived tissue,” said Liu. “We ultimately had to change everything, including developing new electronic materials.”

The team came up with a new meshy material that can be embedded with electrodes and is less than a micrometer thick. They then fabricated a “holding” device to support tadpole embryos and gently placed the mesh onto the tadpoles’ neural plates during early brain formation.

“You need a very stable hand” for the procedure, said Liu.

The tadpoles’ developing brains treated the mesh as another layer of their own biology as they folded themselves into 3D structures, essentially stretching the device across their brains. The implant reliably captured neural activity throughout development on millisecond scales across multiple brain regions. The cyborg tadpoles grew into healthy frogs, which acted normally in behavioral tests and showed no signs of brain damage or stress.

The implant picked up different brain-activity dynamics as the tadpoles developed. Early brain cells synchronized into patterns of slow activity as the neural plate folded into a tube. But as the brain matured and developed different regions, each of these established its own unique electrical fingerprint with faster neural activity.

By observing these dynamics, scientists can potentially decipher how the brain wires itself into such a powerful computing machine and detect when things go awry.

Rebuilding Connections

The human nervous system has limited regenerative capabilities. Axolotls, not so much. A type of salamander, these cartoonish-looking creatures can rebuild nearly any part of their bodies, including their nerves. How this happens is still mysterious, but if we can discover their secret, we might use it to develop treatments for spinal cord injuries or nerve diseases.

In one test, the team implanted the recording mesh in an axolotl tadpole with a damaged tail. The critter’s brain activity spiked during regeneration. When they added carefully timed zaps from external electrodes mimicking post-injury neural patterns, the regeneration sped up, suggesting brain activity could play a role in tissue regeneration (at least in some species).

“We found that the brain activity goes back to its early [embryo] development stage, so this is maybe a unique reason why this creature has this regeneration ability,” said Liu. 

The team is giving the technology to other researchers to further probe life’s beginnings, especially in mammals such as rodents. “Preliminary tests confirmed that the devices’ mechanical properties are compatible with mouse embryos and neonatal rats,” they wrote.

Liu is clear the method isn’t ready for implantation in human embryos. Using it in frogs, axolotls, and human brain organoids is already yielding insights into brain development. But ultimately, his team hopes to help people with neurodevelopmental conditions.

“We have this foundation of stretchable electronics that could be directly translated to the neonatal or developing brain,” said Liu.