Approximately 18 people die every day waiting for an organ transplant. But that may change someday sooner than you think -- thanks to 3D printing.

Advances in the 3D printing of human tissue have moved fast enough that San Diego-based bio-printing company Organovo now expects to unveil the world's first printed organ -- a human liver -- next year.

Like other forms of 3D printing, bio-printing lays down layer after layer of material -- in this case, live cells -- to form a solid physical entity -- in this case, human tissue. The major stumbling block in creating tissue continues to be manufacturing the vascular system needed to provide it with life-sustaining oxygen and nutrients.

Living cells may literally die before the tissue gets off the printer table.

Organovo, however, said it has overcome that vascular issue to a degree. "We have achieved thicknesses of greater than 500 microns, and have maintained liver tissue in a fully functional state with native phenotypic behavior for at least 40 days," said Mike Renard, Organovo's executive vice president of commercial operations.

A micron is one-millionth of a meter. To better understand the scale Renard is describing, think of it this way: A sheet of printer paper is 100 microns thick. So the tissue Organovo has printed is the thickness of five sheets of paper stacked on top of each other.

Liver tissue printed in a petri dish. (Image: Organovo)

Printing hepatocytes -- the cells that make up most liver tissue -- isn't enough, however. There are multiple types of cells with different functions in tissue that must be combined to create a living human organ.

Organovo's researchers were able to bring together fibroblasts and endothelial cells, which perform the function of developing tiny vascular networks, allowing the company to achieve thick tissue with good cell viability, Renard said.

The liver tissue model that Organovo plans to release next year is for research use only and will be used in the laboratory for medical studies and drug research. That's important in its own right: Developing a new drug costs, on average, $1.2 billion and takes 12 years.

Organovo has as yet not released any information on possible future implantable organs. Any such initiative would have to undergo rigorous government review before being approved for clinical purposes.

Still, the creation of a viable liver is a watershed moment for the bio-printing industry and medicine because it proves 3D printed tissue can be kept alive long enough to test the effects of drugs on it or implant it in a human body where it can further develop.

"It is too early to speculate on the breadth of applications that tissue engineering will ultimately deliver or on the efficacy that will be achieved," Renard said.

That question, Renard said, can only be answered through continued successful tissue development and the completion of clinical trials, followed by a review by the Food and Drug Administration (FDA) -- a process that can take three to 10 years.

To spur on the development of bio-printed organs, the Methuselah Foundation, a Springfield, Va.-based not-for-profit that supports regenerative medicine research, this month announced a $1 million prize for the first organization to print a fully functioning liver.