On the bookshelf in his office, NanoEngineering professor Shaochen Chen keeps a shoebox full of impossible toys — plastic trinkets that look ordinary enough at first glance, but which couldn’t have been made without a technology known as “3-D printing.”
Chen’s toys are brightly colored and polished to a shine. In his shoebox, there’s a spiny set of sea legs, a trio of concentric wiffle balls and a cube made completely out of interlocking gears. Like ordinary toys, they’re made of plastic, and some of the newer models even feel factory made.
But these are not ordinary toys. They were not injection-molded, as most toys are, but sliced up virtually and printed with thousands of layers of plastic “ink.” They were not made in a factory but in a refrigerator-sized machine in the Structural and Materials Engineering building at the UCSD Jacobs School of Engineering. And if some of their creators are to be believed, they are the future of everything — manufacturing, design and even personalized medicine.
“There are at least six or seven different types of 3-D printing out there now, each more sophisticated than the last,” Nathan Delson, director of UCSD’s Mechanical Engineering Design Center and Chen’s colleague, said. “Maybe someday in the future, you’ll be able to say, ‘Siri, print me a steak.’ Though the truth is the food will never taste good as fresh food — it’s a possibility, and there could be some application for it.”
Chen is too busy making biomaterials — living, breathing bits of human tissue encased in fragile polymer networks — to worry about steak. Companies like San Diego’s own Organovo have used 3-D printing for years to make human tissues for experiments and therapies. But they’ve never made a full organ, partly because they use a nozzle-based approach that splatters cells directly onto a surface that lets cells grow.
“You can think of it as an inkjet printer,” Chen said. “And there are real disadvantages to spraying a ball of cells through a nozzle. What my group does is similar in concept, though we use different machines.”
Normally, a shape is designed on a computer and sliced into thousands of paper-thin sheets. Then the printer recreates the shape by spraying some form of 3-D ink — plastic, metal or even living human cells, as in the Organovo process — onto a gelatinous foundation. Over a period of eight to 10 hours, the cellular paint accumulates into tissue in layers that are mere fractions of a millimeter thick. But for some applications, like the delicate webbing of capillaries that supply blood to the body’s organs, even paper-thin is too thick.
So Chen and his colleagues decided to ditch the old inkjets and upgrade to laser printing — and since the technology didn’t exist, they invented it.
“The big difference between our work and that of others is we abandoned the nozzle-based approach in favor of light polymerization,” Chen said. “We were, and still are, the leading group in that regard. We use materials that polymerize — solidify — upon exposure to light. You start with a big bath of aqueous solution that contains cells, growth factors and polymers. Then you focus a laser beam at a certain point, and the polymers harden into a solid with the living cells inside.”
The process is repeated at different points and focal distances until the desired tissue shape is replicated, forming a meshwork of cells that is fully alive, rather than a substrate with cells growing on top of it.
“Capturing cells inside the gel is a big step, because cells almost never choose to go inside scaffolds when they’re spray printed on,” Chen said. “Living inside is hard. Cells are just like us; they need to eat, breathe and survive, and it’s more challenging to do that on the inside of a scaffold, even if that’s what happens naturally.”
Chen demonstrated by pulling out a green laser pointer from his front pocket and shining it on the wall.
“See this?” Chen said. “This beam is a millimeter wide. If you put a lens on it, you can make it a thousand times narrower. And if you put a second lens, you can make it a thousand times narrower than that. When you print with lasers, you can craft things on the nanoscale. Suddenly, new opportunities open up.”
One of those opportunities is creating seemingly impossible structures that don’t exist in nature and merging them with cells. He and his colleagues have used a clever application of simple geometry to create a material with paradoxical property: It gets thicker when it’s stretched and thinner when it’s compressed.
“It’s what’s known as a negative Poisson ratio,” Chen said. “Our unofficial motto is, ‘As long as you can think of it, you can print it.’ We can’t be certain, but it could one day be used as a cardiac patch. Traditional heart patches tear and wrinkle, but a patch with a negative Poisson ratio could happily expand in multiple directions and far outlast what we have today.”
The dream at the moment, Chen says, is to develop functional organs using 3-D printing. Researchers at the Wake Forest Institute for Regenerative Medicine recently made an artificial bladder using the technology, but that’s been the extent of it. Kidneys, livers, hearts — the organs that most often need replacing — are still beyond our reach. Chen said these organs need to be made using 3-D printing and a patient’s own stem cells for more successful transplants.
“We are constantly heading in the direction of personalized medicine,” Chen said. “There’s a lot of interest from government, from healthcare — and I think it would be very useful to be able to print fully immune-compatible organs from scratch one day.”
When asked how many years it will be until doctors could print kidneys and hearts, Chen said he wasn’t sure.
“I don’t want to put a number of years on it,” he said. “But we are headed in that direction and, with these blood vessels, are closer than ever before.”
