While companies such as ISTO Technologies and Integra LifeSciences work on growing viable, implantable human cartilage, Darryl D’Lima, M.D., Ph.D., has managed to print cartilage from nothing less than an old inkjet printer. The doctor, who heads the orthopedic research laboratory at Scripps Clinic in San Diego, Calif., was profiled in the New York Times on Aug. 18 for his groundbreaking work.
D’Lima has succeeded in creating bioartificial cow cartilage by modifying an old inkjet printer to put down layer upon layer of a gel that contains mesenchymal, or progenitor, stem cells. He has also printed cartilage in tissue removed from patients who have undergone knee replacement surgery.
Mainstream media has been abuzz with interest in 3-D printers in recent months, especially after the Cube 3-D printer became commercially available through the office supply store Staples. Similarly, there has been a rise in interest in the medical space for 3-D printing. The technique is often used by contract manufacturers to make complex devices by laying down layers of raw material; however, there is to date no such machine on the market that can create living tissue as D’Lima was able to do in his lab. D’Lima said that his goal is to have “bioprinters” in operating rooms everywhere to assemble living tissue as needed.
Just last year, University of Manchester researcher Brian Derby said in the journal Science that “Nobody who has any credibility claims they can print organs, or believes in their heart of hearts that that will happen in the next 20 years.”
But innovation has to start somewhere. Sand Diego-based Organovo Holdings Inc., for instance, which has developed a bioprinter, is currently making strips of liver tissue about 20 cells thick that it claims can be used to test drugs under development.
A lab at the Hannover Medical School in Germany is one of several experimenting with 3-D printing of skin cells; another German lab has printed sheets of heart cells that might some day be used as patches to help repair damage from heart attacks. A researcher at the University of Texas at El Paso, Thomas Boland, Ph.D., has developed a method to print fat tissue that may someday be used to create small implants for women who have had breast lumpectomies. Boland has also done much of the basic research on bioprinting technologies. “I think it is the future for regenerative medicine,” he said.
D’Lima acknowledges that his dream of a cartilage printer—perhaps a printhead attached to a robotic arm for precise positioning—is years away. But he thinks the project has more chance of becoming reality than some others.
“Printing a whole heart or a whole bladder is glamorous and exciting,” he said. “But cartilage might be the low-hanging fruit to get 3-D printing into the clinic.”
One reason, he said, is that cartilage is in some ways simpler than other tissues. Cells called chondrocytes sit in a matrix of fibrous collagens and other compounds secreted by the cells. As cells go, chondrocytes are relatively low maintenance—they do not need much nourishment, which simplifies the printing process.
In trying to find a printer for their man-made cartilage, D’Lima and his research team went through several printer models until they found one that would fit the bill. It needed a print nozzle wide enough for cells to squeeze through, which newer printers don’t have as they print in such high resolutions. They eventually used a Hewlett-Packard Deskjet 500 from the 1990s, and dug up some old, discontinued ink cartridges fro a supplier in China.
Their idea was to replace the ink in the cartridges with their cartilage-making mixture, which consisted of a liquid called PEG-DMA and the chondrocytes. But even that created a problem: The cells would settle out of the liquid and clog the printhead. So the researchers had to devise a way to keep the mixture stirred up.
The mixture also has to be liquid to be printed, but once printed it must become a gel, or the end product would just be a watery mess. PEG-DMA becomes a gel under ultraviolet light, so the solution was to keep the print area constantly exposed to ultraviolet light to harden each drop as it was printed.
D’Lima told the New York Times that he and his group are investigating other materials for their gel. While PEG-DMA is biocompatible and approved for use by the U.S. Food and Drug Administrationn (FDA), it may eventually cause inflammation if left in the human body. So they are looking for substances that could degrade over time, to be replaced by the matrix produced by the chondrocytes. The printed material could be formulated to degrade at the same rate as the natural matrix is produced.
There are plenty of other challenges as well, D’Lima said, including a basic one—how to get the right kinds of cells, and enough of them, for the printer. It would not make much sense to use a patient’s own limited number of cartilage cells from elsewhere in the body, so his lab is investigating the use of stem cells, precursor cells that can become chondrocytes.
“The advantage of stem cells is that it would mean a virtually unlimited supply,” D’Lima said.
D’Lima’s team is also investigating other technologies that might be used in combination with bioprinting, including electrospinning, a method of creating the fibers in the matrix, and nanomagnetism, a way to orient the fibers. His lab takes a multidisciplinary approach — he even attends Siggraph, the large annual computer graphics convention, to get ideas.
“They’re like 10 years ahead of medical technology,” he said.
Meanwhile, the lab has upgraded its printing technology. The Deskjet is still around, but it has not been used in more than a year. It has been supplanted by a much more sophisticated device from Hewlett-Packard—essentially a programmable printhead that allows the researchers to adjust drop size and other characteristics to optimize the printing process.
While most of the scientific challenges have been met, according to D’Lima, his research’s biggest hurdle will be regulatory, namely proving to the FDA that printed cartilage can be safe.
“I think in terms of getting it to work, we are cautiously optimistic,” he said.
