Neurodegenerative diseases are increasingly becoming a global health problem and occur when nerve cells, or neurons, begin to deteriorate. Various conditions are associated with this degeneration, such as Alzheimer’s disease, which affects 5 million Americans, Parkinson’s disease affecting 400,000, and multiple sclerosis, affecting 30,000. The total number of Americans affected annually by neurodegenerative diseases totals 55 million.
Though concerted efforts are being made to understand and treat the symptoms of these diseases, there is currently no method for regenerating or replacing the tissue lost during the disease course.
With the help of bio-inks, scientists are developing methods to grow cells that may be useful for replacing brain cells damaged by disease or injury. Bio-inks are materials that can be used for, among other purposes, to recapitulate the extracellular environment, acting as scaffolds to support cell division, adhesion, and differentiation. This controlled environment can be precisely regulated to enable cells to grow in the manner necessary for therapy.
Induced pluripotent stem cells (iPSCs) are a type of cells in humans that can differentiate into all other cell types. In the laboratory, these cells are typically grown in 2D (monolayer) culture. Through technological advances, it has become possible to form 3D structures composed of iPSCs; the cells can even be obtained from the patient’s own body, preventing any rejection issues typically associated with implantation.
Researchers in Australia used a bio-ink based on sugar polymers known as polysaccharides to form a matrix. They also added iPSCs, growth factors, and other molecules needed for the cells to grow. Specific molecules were added to induce the cells to differentiate into neuron cells as well as other cells needed to support neurons. These cells had all three layers present in normal cells, including mesoderm (middle layer), ectoderm (outer layer), and endoderm (inner layer). Neural tissue was formed when the cells grew together. The conditions used more faithfully represent that physiological environment of human tissue growth compared to 2D cell culture. Other groups grew cells that even contained voltage-gated ion channels in the cell membranes, which are important proteins in neurons for transporting charged particles into/out of cells; these channels help transmit the signals that comprise the basis of neuronal function. Many other neuron-specific proteins have been successfully detected in 3D printed neurons as well.
While these results are preliminary and many more studies are needed to make the tissues applicable for human transplantation, this is the most promising progress that has been made for treating neurodegenerative diseases to date.
It is also possible to use 3D printed neural tissues for studying other aspects of the diseases, such as to understand their pathogeneses and develop and test potential drugs. Using 3D bioprinted neurons would reduce ethical concerns related to animal and human testing by ensuring that the developed drugs being tested were closer to achieving therapeutic outcomes before evaluating them in whole, living organisms.
Other research groups created a brain-like structure using 3D printing techniques. In one month’s time, they were able to grow a structure that possessed some of the characteristics of the human brain, complete with “mini organs,” which survived for up to 10 months. However, because the scientists had not developed a method for growing blood vessels in this assembly, its duration was limited. As the structure grew larger, the cells on the inside were not supplied with sufficient oxygen; however, these results demonstrate that we are getting ever-closer to the reality of using 3D printing techniques to grow very complex human tissues and organs.
There are still many challenges on the road to developing neurons that are applicable in patients with neurodegenerative diseases, such as making the processes more high-throughput. Additionally, effectively using CAD to increase resolution would enable scientists to combine tissue types, such as printing a vasculature for supplying blood to the tissues. However, these exciting advances in tissue engineering techniques using 3D printing show high potential for both cell therapeutic and laboratory testing purposes.