Those with failing kidneys can probably look forward to plugging themselves in to dialysis machines up to three times. The equipment removes fluid waste from blood that would otherwise poison the individual. While the equipment is a life saver, those who use it report being exhausted and depressed after sessions. The United Network for Organ Sharing reports that over 90,000 people are on the waiting list for kidney transplants, and at least two of them die each day.

Those on the list and their families might find hope in the efforts of a few researchers who think they will be able to print a functioning human kidney using a combination of rapid-prototyping technology and recent medical developments.

Rapid-prototyping machines are most at home in mechanical engineering and design firms where several varieties take in CAD files and generate physical models using a range of materials.

But the same machines can be tuned for tasks such as building tissue and organs from cells, layer upon layer. Another equipment application hints using absorbable materials to build temporary bones around which real bone will grow. And in the OR, RP is building parts to patch those wounded in battle and mangled by accidents.

Printing a kidney

Researchers at the Medical University of South Carolina have been putting the building blocks in place to “print” working and transplantable human kidneys. This is not the first research team to try printing organs and tissue. About 10 years ago the forecast was that tissue-engineered organs would be available today. But they aren't. “We have skin, cartilage, and a little for the spine, but there is no great demand for them,” says Biologist and Director of Bioprinting at the university, Vladimir Mironov. He acknowledges a few hurdles and that the goal will take the combined effort of biologists, computer technicians, and chemical and mechanical engineers, to name a few. But it's technically feasible, he insists.

“I see two applications of rapid prototyping in tissue engineering in the near term: printing 3D soft tissue and synthetic scaffolds,” says Mironov. “These tissues would be 3D and vascularized, but small, that is, complex and human but not whole organs. Such tissues would be useful in drug-discovery trials if its predictive power as an assay is better than animal tissue. Other applications include toxicity testing, environmental factors, and as a research tool for pathological anatomy.”

The term printing is used because the RP machines here are essentially inkjet printers adapted to spitting cells instead of ink. The printers Mironov uses to place cells needed little modification other than changing the ink cartridge. His experiments have shown that thermal printing units can place cells without damaging them. “Two other independent groups have verified that this type of thermal ink jetting works without damaging cells.”

He says his team will learn to build kidneys naturally, but faster. “A human matures in 18 years, but kidneys must print in hours, no more than two hours is reasonable. It's really tissue construction. After putting cells and growth factors in place, they must be processed in a bioreactor, a device that fuses them into a unit,” he says.

The first big step in building an organ requires a plan — a detailed CAD model of it. “I've asked anatomists for a digital kidney model with the x, y, and z position of every cell. But they don't have one, because, they say, no one has ever asked for one. Still, there are sophisticated, noninvasive ways to capture such medical information. MRIs, for example, have resolutions of about 2 mm, enough to see the large branching of blood vessels. But such maps are still just gross anatomy.”

Printing an organ will require a model on the microscopic level. “Everything is in place for such a model,” says Mironov. For instance, noninvasive clinical bioimaging can show interior structures. Reconstructions can be based on histological sections. And construction can use math modeling involving principles of optimal branching. You must bind all three approaches. For instance, take a gross-anatomical 3D model of a person and slice it into many thin cross sections. Then reconstruct them in a computer. This is relatively easy for bone. “But for soft tissues, whatever we print must tolerate compaction, contraction, and reshaping. To print a 10-cm long kidney, for example, we'll first print a 20-cm version and compact it,” says Mironov.

So far Mironov has jetted or printed only generic lab cells. “These are genetically tagged with a green color. When damaged, such as when a membrane ruptures, they loose the green marker. You need no additional chemistry to tell if they are alive. Just look at them. Dead cells are not green. Of course, long term will require cells for specific organs,” he says.

Current standard procedure is to jet cells into a biogel that cradles them for experiments. Over a few years, the gel has been modified into a family of materials that are thermodynamically stable — they do not reliquefy. The gels also have low initial viscosities suitable for tissue perfusion or injection. Depending upon the formulation, the gels can be resorbable (biodegradable), or nonresorbable.

So far, all tissue printing machines rely on such gels. “The completed tissue must be viscoelastic, solid, and cohesive, otherwise the tissue will be dissociated. Chemical engineers are helping us find a fluid that will let cells move into close units and bond together. Current gels are hydronic acids from Glenn Prestwich at the University of Utah in Salt Lake City. The gel is photosensitive, ph sensitive, and thermo sensitive. It's actually a large group of stimuli sensitive or stimuli-responsive hydrogels, meaning that after some stimuli it be comes solid,” says Mironov.

And then there is the issue of blood vessels. “Most tissue must have a vascular system. Organ anatomy shows inter-organ branches starting with large vessels that divide into smaller ones. An organ must have large diameter blood vessels to attach to the person and small capillaries to bring oxygen and nutrition to cells. This is the intraorgan vascular tree. It needs two sections: one to bring in blood and one to take it away,” he says.

Nature will help a little with the capillaries. Some of these grow naturally, but only about 1 mm/week. So growing capillaries throughout a 5 cm organ would take a comparatively long period. Experiments show that as soon as the diameter of ball of tissue cells exceeds 2 mm, center cells die because dense packing limits diffusion.

Collagen, a protein in connective tissue, is another challenge. It's needed to hold a kidney together. “It's part of the problem of accelerating tissue maturation. One way to handle it is to put cells in close contact to increase compaction. When cells are close and cannot drift, they naturally produce collagen, and then fibers, both structural substances,” he says.

Putting all this together will take a rapid-prototyping machine with a bioreactor. This device tells when the kidney is done and ready for transplantation. The “doneness” might be checked by ultra sound or noninvasive imaging, analyzing the fluid in it, or a special assay with oxygen.

All the building blocks are in place to print a kidney, says Mironov. He predicts the fine tuning will take about five more years. After that, Mironov suggests it might be possible to print arms and legs, which would be good news to many veterans and those mangled in accidents. But that, he cautions, is still in the offing.

RP in the OR

Not all medical rapid prototyping is still in the lab. Some is in the OR. Walter Reed Army Medical Center surgeon Erge Edgu-Fry and colleague Stephen Rouse, for example, say RP is widely used to make cranial implants. It works like this: CT machines generate sequential slices of the damaged area. Edgu-Fry says CT scan slices about 1.2-mm apart generate enough geometric information. A surgeon uses this data and software from Materialise Inc., Ann Arbor, Mich. (materialise.com) to generate an STL file of the damaged bone. The software improves the images by removing tissue of low interest, often the soft tissue.

In the case of a soldier wounded in Iraq, the surgeon modeled the soldier's skull which was missing a piece, courtesy of a road-side bomb. The surgeon then generated a precise virtual patch for the hole and an STL file for it, which was sent to an RP machine.

Rapid prototyping machines produce organic shapes more easily, cleanly, and conveniently than other manufacturing processes. The challenge in the process, says Edgu-Fry and Rouse, is finding biocompatible materials with the needed physical characteristics.

Building the case for kidneys

Why make kidneys when hearts or livers might be more in demand? “The heart is difficult to print because of its electrical or conductive nature,” says Vladimir Mironov, professor of medicine at the University of South Carolina. “And livers can be regenerated with stem cells. But there is a long and growing waiting list for kidney transplants. To make matters worse, the number of kidney donors is not increasing.”

The United Network for Organ Sharing reports there are about 92,000 people waiting for a donor kidney. “About two of those die each day. And the lucky ones at the front of the line can look forward to an operation that costs about $30,000. What's more, Medicare says it spends 6% of its funds, about $250,000 per patient, on kidney ailments,” says Mironov. Potentially, there is over a $2 billion dollar kidney market.

Bioprinting Congress slated for October

The third World Bioprinting Congress will be held October 23 to 26 in Honolulu. Biologist and Director of Bioprinting at the University of South Carolina Vladimir Mironov will lead the congress with a goal of creating an academy and a journal titled Bio Assembly.