Researchers used a new technology to three-dimensionally print brain tissue, and its cells developed into functional neurons that can communicate with each other within weeks. They say this method could be used to study healthy and unhealthy brains, test drugs, or simply observe how the brain develops.
Creating an organ that is as close to the real thing as possible is critical for research exploring disease pathology and testing new drugs. The brain faces unique challenges, including that neurons grown in the laboratory must form functional connections and brain tissue needs to support complex and delicate structures.
Researchers at the University of Wisconsin-Madison (UW-Madison) have successfully used 3D printing technology to print brain tissue that grows and functions like a normal brain.
"This could be a very powerful model to help us understand how human brain cells and parts of the brain communicate. It could change the way we look at stem cell biology, neuroscience, and the pathogenesis of many neurological and psychiatric diseases," said Zhang Suchun, the corresponding author of the study.
The researchers' goal was to build layered neural tissue that allows neural progenitor cells (NPCs) to mature and form connections (synapses) within and between layers, while keeping the structure intact. They chose a fibrin hydrogel composed mainly of fibrinogen and thrombin as a "bioink", a biomaterial for tissue printing, because of its biocompatibility with nerve cells. Both fibrinogen and thrombin play a role in the clotting process.
The high viscosity of the fibrin gel makes it difficult to print, so the researchers mixed it with a hyaluronic acid hydrogel. Higher numbers of NPCs placed into the mix survived and matured, while adding another type of hydrogen made their bioink softer than those previously used.
Rather than using traditional vertical stack-up 3D printing methods, which require printing thick layers of tough bioink, the researchers created patterned tissue by horizontally printing a thin layer or strip of cell-infused gel next to another thin layer or strip of cell-infused gel. To prevent mixing of the printed ribbons, the researchers used thrombin as a cross-linker immediately after the mixture was deposited.
While the printed cells stayed within their designated layers, within two to five weeks after printing, the neurons formed functional synaptic connections within and between layers.
"The tissue still has enough structure to hold it together, but it's soft enough to allow neurons to grow with each other and start conversations," Zhang said. "Our tissue is kept relatively thin, which makes it easy for the neurons to get enough oxygen and nutrients from the growth medium."
Researchers are trying to print brain tissue using different combinations of cells in bioinks.
"We printed the cerebral cortex and striatum, and what we found was amazing," Zhang said. "Even though we printed different cells that belong to different parts of the brain, they were still able to talk to each other in a very specific and specific way."
The researchers say their method allows for precise control over the type and arrangement of cells, something not possible with organ tissue and other printing methods. And the printing technology doesn't require special equipment or culture methods to keep the tissue healthy, meaning it can be used by many labs.
"Our lab is very special because we are able to produce almost any type of neuron at any time, and then we can put them together almost at any time and in any way to have a defined system for studying how human brain networks work," Zhang said.
The researchers plan to improve the bioink and equipment to achieve specific cell orientation in printed tissue.
"Right now, our printer is a desktop commercial printer," said Yuanwei Yan, lead author of the study. "We can make some specialized improvements that help us print specific types of brain tissue on demand."
The researchers say the printed brain tissue could be used to study cell-to-cell signaling in Down syndrome, interactions between healthy tissue and tissue affected by Alzheimer's, test new drug candidates, or simply observe brain development.
The research was published in the journal Cell Stem Cells.