Brain Implants Monitor Neural Activity in Tadpoles Throughout Embryonic Development

Credit: Photo by Wolfgang Hasselmann on Unsplash

Bioengineering researchers at the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS) have developed a soft, thin, stretchable bioelectronic device that can be implanted into a tadpole embryo’s neural plate—the early-stage, flat structure that folds to become the 3D brain and spinal cord. By integrating their stretchable device into the neural plate, the researchers showed they could stably and continuously monitor brain activity during each embryonic stage.

They demonstrated that the device could integrate seamlessly into the brain as it develops and record electrical activity from single brain cells with millisecond precision, and with no impact on normal tadpole embryo development or behavior.

These “cyborg tadpoles” could provide new insights into brain development, and help to understand, or even treat diseases that manifest in early development. “Autism, bipolar disorder, schizophrenia—these all could happen at early developmental stages,” said research lead Jia Liu, PhD, Assistant Professor of Bioengineering at SEAS. “There is just no ability currently to measure neural activity during early neural development. Our technology will really enable an uncharted area.”

Liu is senior and corresponding author of the team’s published paper in Nature, titled “Brain implantation of soft bioelectronics via embryonic development.”

In vertebrate embryos, folding and expansion of the neural plate into the neural tube, which is the precursor to the brain and spinal cord, involves complex morphological changes over millisecond time scales, Liu explained. “Vertebrate brains are complex 3D structures that originate from a two-dimensional (2D) single-cell layer in the embryo,” the authors further noted. During vertebrate development, the neural plate, a 2D, single-cell, ectoderm-derived layer on the embryo’s surface, folds to form the neural tube and, with continued expansion and folding, morphs into the 3D brain and other parts of the nervous system. And as the team noted, “These large morphological changes have previously posed a challenge for implantable bioelectronics to reliably track neural activity throughout brain development.”

In the past, scientists have used patch-clamp or metal electrodes inserted into mature brains to record electrical activity of single neurons with high resolution. Further breakthroughs in tissue-like microelectronics from Liu’s previous work have made single-cell brain recording even less invasive.

Yet, as the researchers pointed out, in fully developed, mature brains, neurons connect with each other at the nanometer scale. “Regardless of how small and soft bioelectronics are designed, implantation into mature brains inevitably causes acute damage.”

Liu added, “If we can fully leverage the natural development process, we will have the ability to implant a lot of sensors across the 3D brain noninvasively, and at the same time, monitor how brain activity gradually evolves over time. No one has ever done this before.”

The team’s newly reported research builds on a multi-year effort to create soft, flexible, non-invasive bioelectronics for brains, which have the consistency of tofu. In previous studies the team embedded electrode arrays into petri dishes of stem cells. The thin electrodes stretched and folded with growing tissue and created cyborg heart and brain organoids. “We previously demonstrated that stretchable electronics could be integrated into developing tissue in vitro via normal developmental processes, the authors stated. “The endogenous forces from tissue growth unfold and distribute the bioelectronics throughout the 3D structure, allowing stable long-term interfacing.”

Though the organoid studies were successful, integrating nanoelectronics into amphibian embryos posed new challenges, according to Liu. “It turns out tadpole embryos are much softer than human stem cell-derived tissue,” he said. “We ultimately had to change everything, including developing new electronic materials.”

The researchers made a new type of implant out of fluorinated elastomers, which are as soft as biological tissue but can be engineered into highly resilient electronic components that can withstand nanofabrication processes and house multiple sensors for recording brain activity.

In their newly reported study, Liu et al. used the perfluoropolyether-dimethacrylate (PFPE-DMA) fluorinated elastomer as an encapsulation layer for the functional stretchable mesh devices. They initially tested the tissue-level-soft, submicrometre-thick mesh microelectrode array in Xenopus laevis (frog) embryos.

The device was non-invasively implanted into the embryo’s neural plate, wherein, as the neural tube forms into a 3D structure, the stretchable array integrated fully with neural networks throughout the brain, allowing the scientists to carry out brain-wide electrophysiological measurements during development. “As the neural plate undergoes 2D-to-3D reorganization during organogenesis, endogenous forces naturally distribute and integrate the mesh across the 3D volume of the neural tube and brain, creating a ‘cyborg’ embryo,” the team wrote. Immunostaining, fluorescence imaging, gene expression analysis and behavioral testing indicated that the implanted devices did not significantly disrupt neural development, stress response or behavior maturation in Xenopus tadpoles.

“The device enables continuous, brain-wide neural recording at millisecond resolution, capturing the emergence, synchronization and propagation of neural dynamics throughout embryonic brain development,” the investigators reported. “The implanted device provided stable, millisecond-resolution neural recordings throughout organogenesis, offering a method for soft electronics implantation in 3D developing tissues.”

The team separately tested stretchable, high-density mesh electrode arrays for recording neural development in axolotl embryos, both before and after injury. Axolotls are a well-established model for studying development and regeneration, as the organisms can regenerate after injury, including their nervous systems.

Separate tests, the researchers reported, have in addition indicated that the devices’ mechanical properties are compatible with mouse embryos and neonatal rats, with the potential for studying in vitro embryonic culture or in utero implantations. “Future combination of this system with virtual-reality platforms could provide a powerful tool for investigating behavior- and sensory-specific brain activity during development,” they concluded.

Perfluoropolyether-dimethacrylate fluorinated elastomer is intellectual property protected by Harvard’s Office of Technology Development, which has licensed the technology to the start-up company Axoft for further development. Liu co-founded Axoft in 2021, and the company is focused on the development of scalable, soft bioelectronics for brain-machine interface applications.

“The key innovation lies in the extreme softness of our materials, which not only improves biocompatibility but also enhances device robustness and scalability,” Liu explained to GEN. “This allows us to significantly increase the number of electrodes per brain probe.”  Current commercial brain probe technologies may typically support only 8–16 electrodes per probe, and so “achieving high-channel-count recordings in humans thus requires implanting hundreds of probes—an invasive and complex procedure,” Liu continued.

Axoft’s soft electronics in contrast enable the fabrication of brain probes with hundreds of electrodes on a single device, Liu further commented to GEN. “This drastically reduces the number of required implantations, making the approach more scalable and clinically viable. Furthermore, the softness of the materials supports repeated implantation and withdrawal without compromising structural integrity—an essential feature for long-term and reconfigurable clinical use.”