Walking through our local mall, my son’s attention was drawn to the new Microsoft store with its flashy XboxOne in the corner. As he moved in to get a closer look, his eyes registered something else, a device that would turn out to capture his attention and curiosity even more than the Xbox for the next half-hour—a small-scale 3-D printer connected to a laptop computer.
This particular printer was pushing out small keychains with the word “Microsoft” emblazed across the top. Although a fairly basic demonstration of the capabilities of 3-D printing, the object and the process by which it was being created in front of our eyes amazed my son.
When I mentioned to him that arrangements of cells, and even organs, could possibly be printed in a similar fashion in the future, he cast a skeptical look at me, asking first what was the purpose of that and then, even if you wanted to, how it would be possible to print something as small and delicate as a cell?
Seeing is believing
The process I was referring to is called 3-D bioprinting, and it is a growing field of technology development. In fact, interesting bioprinting applications and technical advances are being reported with increasing frequency as it becomes clear that many cell types possess the strength and resilience to be “printed.” But to what end?
One of the most surprising recent examples of bioprinting was published last December in the journal Biofabrication. In this case, a team of scientists at the John van Geest Centre for Brain Repair at the University of Cambridge showed for the first time that cells derived from the mature central nervous system could be printed in specific patterns.
Here, the chosen cells were the ganglion and glial cells from the adult rat retina. Using a piezoelectric inkjet printer that ejects cells through an opening less than a millimeter in diameter, lead researchers Keith Martin and Barbara Lorber were able to spot their cells of interest into specific patterns.
“It was a major challenge to ensure successful ejection of intact cells in each droplet,” notes Martin. It turns out that printing conditions need to be modified or optimized for each different cell type. To optimize the spotting of ganglion and glial cells, Martin and Lorber actually watched their cells flowing out of the printer tip using a high-speed camera and recording equipment. Martin says that it was the information gleaned from those videos, along with careful modification of the viscosity of the medium containing the cells and adjustments to the nozzle characteristics, that led to the team’s success.
With the ganglion and glial cells in place, the next step is to look at the other key cellular components of the retina—including the light-sensitive photoreceptor cells, the rods and the cones. “We expect retinal pigment epithelial cells to be relatively straightforward, but photoreceptors are delicate cells, and we will need to ensure that outer and inner segments remain intact after printing,” explains Martin. Still, printing retina components in this fashion could allow Martin and his colleagues to perform novel cell biology experiments by placing different cell types together in specific locations and assessing their interactions.
More ‘print’ on stem cells
Just before Martin and Lorber’s work on patterning retinal cells was published, a team of researchers from Scotland’s Heriot-Watt University optimized another 3-D bioprinting technique, this time for human embryonic stem cells (hESCs), powerful cells that researchers can coax into differentiating into a wide variety of other cell types. The problem with bioprinting hESCs is that these cells are thought to be more fragile than other cell types; therefore, it was uncertain whether it was even possible to effectively print hESCs. But the attraction of possibly being able to specifically pattern hESCs using a 3-D printer for basic research and regenerative medicine applications was too enticing. To this end, Wenmiao Shu and colleagues developed a valve-based printer with dual nozzles that was able to form hESC spheroid aggregates. Most critically though, the impact on the hESC cells could be minimized by using a large nozzle diameter. This produced, for the first time, effective patterning of hESCs without any modification to their phenotype.
Now that they have demonstrated how to pattern hESCs, Shu and his colleagues suggest that hESC printing could lead to the generation of 3-D structures that can more accurately model human tissues. Modeling of human tissues for drug testing, basic research experiments, and even regenerative medicine remains the next and, many agree, most important step in the evolution of bioprinting. But going from patterning hESC or mature retinal cells on surfaces to being able to generate a full, functional tissue or a complete retina could take a long time—although maybe not as long as you might think.
More than just cells?
After cells have been patterned in specific arrangements, what is the next step? How complex can the layering of cells upon cells eventually get? While a big jump from printing cells, there is an expanding community of researchers and technology companies tinkering with the printing of tissues or even whole organs—and they are starting to generate some impressive data.
Organovo is a small company founded in 2007, now located in San Diego, California. Although it has only 39 employees, Organovo hopes to change the lives of the great number of patients currently waiting for organ transplants. In fact, the company is hoping to make a big splash during 2014—by bioprinting a full liver.
The early data look promising. Starting with hepatocytes, according to releases from the company, designs have been created with the shapes and cellular structures typically found in native liver tissues. This hepatic architecture was maintained over time, and the printed tissue even developed microvasculature and intracellular junctions characteristic of hepatocytes. Biochemical data showed that the printed liver tissues even produced liver-specific enzymes. On January 29, 2014, Organovo released another statement saying that it had shipped this 3-D liver tissue to key scientists for evaluation and recommendations; further information should be available later this year.
As the developments in bioprinting of liver tissue seem to be moving along, other tissues might not be far behind, especially given the range of cells that can now be printed. Martin points out that he knows of no cell type that cannot be printed at the moment
While not made of cells, that plastic Microsoft keychain still sits on my son’s desk. He has shown it to the rest of the family and a couple friends. I do have to wonder if he will put his keys or anything else on it. Will he make use of it? Cool is one thing, but practical is another. In the end, it is clear that same question of utility exists for 3-D bioprinting. Fortunately, this year should provide our first answers.