Borrowing from microfabrication techniques used in the semiconductor industry, MIT and Harvard Medical School (HMS) engineers have developed a simple and inexpensive way to create three-dimensional brain tissues in a lab dish.
The new technique yields tissue constructs that closely mimic the cellular composition of those in the living brain, allowing scientists to study how neurons form connections and to predict how cells from individual patients might respond to different drugs. The work also paves the way for developing bioengineered implants to replace damaged tissue for organ systems, according to the researchers.
Brain tissue includes many types of neurons, including inhibitory and excitatory neurons, as well as supportive cells such as glial cells. All of these cells occur at specific ratios and in specific locations. To mimic this architectural complexity in their engineered tissues, the researchers embedded a mixture of brain cells taken from the primary cortex of rats into sheets of hydrogel. They also included components of the extracellular matrix, which provides structural support and helps regulate cell behavior.
Those sheets were then stacked in layers, which can be sealed together using light to crosslink hydrogels. By covering layers of gels with plastic photomasks of varying shapes, the researchers could control how much of the gel was exposed to light, thus controlling the 3-D shape of the multilayer tissue construct. This type of photolithography is also used to build integrated circuits onto semiconductors — a process that requires a photomask aligner machine, which costs tens of thousands of dollars. However, the team developed a much less expensive way to assemble tissues using masks made from sheets of plastic, similar to overhead transparencies, held in place with alignment pins.
The tissue cubes can be made with a precision of 10 microns, comparable to the size of a single cell body. At the other end of the spectrum, the researchers are aiming to create a cubic millimeter of brain tissue with 100,000 cells and 900 million connections. The new system is the first that includes all of the necessary features for building useful 3-D tissues: It is inexpensive, precise, and allows complex patterns to be generated, says Metin Sitti, a professor of mechanical engineering at Carnegie Mellon University. “Many people could easily use this method for creating heterogeneous, complex gel structures,” says Sitti, who was not part of the research team.