Eight years ago, Stephen and Amelie Trice learned that their three-year-old son Troupe needed a new heart. After many months of perplexing symptoms—small size, low energy, enlarged liver—Troupe was diagnosed with restrictive cardiomyopathy, an extremely rare and deadly condition in which the heart is unable to relax between heartbeats.
There is no cure and very few treatments. Despite his doctors’ attempts to manage his condition with medicine, Troupe’s health deteriorated. Afraid of losing their son, the Trices sought the expertise of a cardiologist at Children’s Hospital in Atlanta. The doctor sat the worried parents down and over two hours explained that Troupe needed a heart transplant. Amelie recalls, “With a diagnosis of certain death, our only option was to cross our fingers that we were going to get more time with a new heart.” The Trices went back home to Alabama, explained what they had learned as best they could to Troupe, and waited. Two months later, the couple were out with friends when they got the call.
“Across the table [Stephen] yelled to me, ‘Amelie, they got a heart for Troupe!’ I didn’t say goodbye or pay a tab. I literally picked up my purse, walked out of the restaurant, and got into the car.” The next morning, Troupe was in surgery. Amelie, a registered nurse, knew that soon the surgeons would be cracking open their toddler’s chest. Another thought weighed on her mind, too. “As I walked to the waiting room unable to stop the tears and the feeling of my knees buckling, I knew that somewhere a mother and a family was grieving for their baby.”
Four weeks later, Troupe’s heart was still not working as it should. Again, he was dying. Quickly, doctors found a second organ and performed an emergency surgery. This time the transplant took. Still, Amelie remains haunted. “I live with a complex ball of guilt and grief,” Amelie says. “I benefited from someone else’s pain.”
At Troupe’s tenth birthday celebration, while he skated with his friends, chatted up his family, and scarfed pizza, the heart of another child pumped blood into his lungs, through his body, and back again. But the Trices’ could not relax. The family had learned that again they would need the generosity of another despairing family—their youngest son Henry would also need a new heart. Thankfully, last year, Henry received his match.
According to Stephen, when the Trice family began their relationship with the United Network for Organ Sharing, “the average wait for a child’s heart was six months. The current average wait for a child of Henry’s size is one year.” In the United States, and worldwide, we face an organ shortage. Today, over 123,000 people in the U.S. are waiting for lifesaving organs, according to the network. Many patients will never receive a match.
“Only about 14,000 people in the entire United States are suitable for organ donation.”
Right now, only a human body can grow a complex human organ. Unlike relatively simple tissues such as bones and skin, to give a heart, the donor must be declared brain dead and the family must consent. According to Howard Nathan, the president and CEO of Gift of Life, the nonprofit group appointed by the U.S. government to match transplant donors with those in need, 2.4 million people die in the United States every year. About half die in acute care hospitals where organs can be obtained. Of that roughly 1.2 million people, only 1% are suitable for organ donation. “That’s it,” he says. “Only about 14,000 people in the entire United States are suitable for organ donation. 8,200 people last year were donors.”
Faced with those numbers, scientists and doctors have begun to ask, what would it take to give a family like the Trices a pair of new hearts without guilt or delay? Currently, tissue engineering can provide simple, thin tissues including cartilage, skin, and bone. Larger, more complex organs like the liver and heart present much more formidable challenges. They are structurally intricate, contain many different cell types, and require an extensive network of blood vessels.
Some researchers are working to create useable organs within animals. Much of this work has focused on genetically engineering animals such as pigs to grow human-compatible organs. But while researchers have succeeded in engineering pig organs to express human cell-membrane proteins, over time, additional incompatibilities have emerged. Some which were implanted in non-human primates suffered from immune attacks. Others failed due to incompatibilities with blood clotting proteins, which can cause blockages in some places and uncontrolled bleeding in others.
The fact remains that today, if someone needs a complex organ, a human is required to provide it. But in the near future, perhaps, families may not have to live with conflicting emotions of guilt, grief, and joy. Maybe all that will be required are a knowledgeable team of dedicated people, a slurry of cells, and a printer.
Bioprinting in 3D
Around the time that Troupe Trice was undergoing surgery to receive his new heart, Dr. Thomas Boland at Clemson University secured a promising sounding patent: “Ink-jet Printing of Viable Cells.”
Boland’s patent relies on 3D printers, which can build complex shapes from little more than blueprints and the right type of material, including cells. An ink-jet printer that spits out three-dimensional objects works much like the familiar two-dimensional versions that sit on our desks—in fact, many started their “lives” printing paper. Like the 2D version, a 3D printer precisely deposits material drop by drop. But instead of feeding a sheet of paper past the print heads, a 3D ink-jet printer ratchets its platform down a notch and then deposits another layer atop the last. The “ink” can be virtually anything that can flow through a print-head and later harden—plastic, wood pulp, proteins, human cells. For 3D bioprinting, scientists fill the ink cartridges with cells and proteins instead of plastic.
There’s also another promising 3D bioprinting technology, called stereolithography, which uses laser light to harden portions of a soup of polymers into a solid frame or “scaffold.” When the machine is done, the excess liquid drains away to expose detailed supports. Researchers then bathe these scaffolds in cells and nutrients and wait for the cells to grow and remodel the scaffold to their liking.
Like many 3D printing techniques, 3D stereolithography was originally designed for more traditional manufacturing. There, the usual polymer materials are efficient, but unsuited to printing organs. “No matter how convenient the 3D printing technology may be, if there are toxicity aspects remaining, it is essentially useless for real life applications,” says Sungho Jin, a professor of materials science at the University of California, San Diego.
Roger Narayan, a biomedical researcher at North Carolina State University, set out to make them biocompatible, so he substituted riboflavin—or vitamin B12—for the toxic polymer hardening substances.
In one set of experiments, he found that the structures were less toxic than glass, meaning the mixture would be suitable for implantable medical devices or possibly as scaffold for fine-featured organs. “What is unique about this technique,” Narayan says, “is that it allows you to do 3D printing at submicron dimensions. You begin to be able to make structures that can interact with cellular or sub-cellular components.”
That high resolution allows researchers to consider 3D printing tissue types that were too detailed for earlier methods. “Many tissues exhibit complex small-scale geometries that are difficult for engineering to mimic using conventional scaffold techniques,” says Mohan Edirisinghe, a biomechanical engineer at University College London. “Complex scaffolds for many types of artificial tissues, including the kidney, the liver, and blood vessels, can be created using this approach.”
Narayan’s technique and its kin are still early in their development, but there is one company that’s 3D printing tissues today. San Diego-based Organovo is using 3D tissue printing to create 3D printed “organs on a chip.” Rather than spending time and money testing drugs on animals, organs on a chip will allow researchers to test immediately on human cells. Cells can come from a research cell line, a patient, or other sources such as stem cells. “These tissues will take the form of both normal health or diseased by design,” says Michael Renard, executive vice president of commercial operations. Organovo’s has released a 3D-printed liver tissue, and the company has partnered with the NIH to create tests for human eye tissue as well.
Organovo liver tissues, while an impressive 20–30 cell layers thick, are not fully-functional organs, but they are a step toward that goal. Several challenges remain, Renard says, including “securing the proper cell types in the quantities needed to achieve the desired level of function and the integration of a vascular network necessary to maintain a healthy condition.” These hurdles are shared by virtually anyone hoping to create complex human organs.
Robert Langer, a professor of chemical engineering at MIT, points out several other oft-forgotten limitations of 3D bioprinting. “Right now, we can’t make any organ or tissue to function properly in humans other than skin. 3D printing hasn’t even been used successfully for that. Making an organ also requires blood vessels, nerves, and cells to behave properly. 3D printing can’t do that.”
Still, the desire to print these organs drives much of today’s 3D bioprinting research. Laboratories have successfully recreated human ears, noses, skull bones, jaw bones, tracheas, ears, noses, skin sections, bladders, arteries, and fat. Rather than cutting squares of healthy skin from a patient to help cover an injury, a 3D printer could print skin perfectly shaped to fit. Patients marred by disfigurement may soon be able to customize a nose on a computer.
While some 3D printed body parts, such as skull and facial implants, have shown success in human patients and have received FDA approval, others are still working their way from the laboratory into animal testing. Few tissues are ready for implantation. Size remains a problem, Renard says. Current technologies struggle to build tiny structures such as human retinas, capillaries, and ear cochlea, as well as large, complex organs like livers and lungs. If you need fine detail, it takes so long to build an organ that the cells die. If a researcher quickly prints a large organ from huge blobs of cells, she cannot engineer enough detail to provide blood vessels for the cells to survive, for example.
In many ways, the challenges of 3D organ printing mirror those of organ transplantation. As Nathan from Gift of Life points out, “When taken from a deceased person, tissues can be collected 24 hours after the heart stops, but organs need a blood supply.” Without vasculature, cells do not receive nutrients and oxygen or excrete wastes beyond a thickness of a few layers. “One of the greatest challenges is ensuring that printed structures have an adequate supply of nutrients and oxygen until they can integrate with the body,” says Dr. Anthony Atala, the director of the Wake Forest Institute for Regenerative Medicine.
Researchers have been scrambling to find ways to fix this plumbing problem. Strategies include layering cells on pre-made channels, leaving holes in tissues for blood vessels to grow, and adding chemicals that encourage blood vessel formation.
Tissues are somewhat easier than organs. Cartilage, for example, does not need as much blood as other tissues, which is partly why Faiz Bhora, chief of thoracic surgery at Mount Sinai Roosevelt and Mount Sinai St. Luke’s Hospitals in New York City, is working to 3D print a working trachea from stem cells. Bhora begins by using a gel that is hardened with UV light to create the scaffold. He then seeds the scaffold with stem cells and adds signaling compounds which instruct the cells to turn into cartilage. As a test, the team implanted a printed trachea into a baby pig. It is now a healthy adult. Bhora hopes to gain FDA approval of the first 3D bioprinted artificial trachea within three years.
At Cornell University, biomedical engineering professor Jonathan Butcher is working to build a functional human heart valve. His team starts with a goopy liquid the consistency of toothpaste that contains stem cells, hydrogels, and nutrients, and then they harden the liquid to the perfect stiffness using light. As he’s perfected the system, Butcher has reduced the time required to build a heart valve from 14 hours—long enough for all the cells to expire—to under an hour. “One hour is more than enough. The cells don’t really care about that,” he says.
Rather than placing each cell in the valve exactly where it needs to go and telling each cell what it needs to do, Butcher is betting that if he puts the stem cells in the right environment, they will figure it out. As a part of the process, the cells are trained and tested in a machine that simulates human blood flow before they are implanted in an animal or human. “You have to figure out how to make this thing so that right when you implant it, it is working,” he says. “We are pirating the natural engineering.”
An Agonizing Wait
Thirty years ago, 3D printing was just an experimental technology capable of producing rudimentary objects. Today, researchers are cranking out organ scaffolds that are orders of magnitude more complex. While that progress has been remarkable, we still have a long way to go. We’re nowhere near growing a little heart for a child like Trice or Henry, a liver for a beloved parent, or a kidney for a tiny infant, Atala says. “Based on our current work, we expect segments of bone, muscle and cartilage to be available within five to ten years. More complex structures, such as kidneys and livers, will obviously take much longer.”
That day cannot come soon enough for those who are waiting for organs, but for now they must hope for a human match. “I try not to worry about whether we will get one in time, and just continue on day to day,” says Amelie Trice. Still, the promise of the technology makes her hopeful.
“My greatest wish is someone will grow a new healthy heart, customized to my children’s immune systems and bodies, so we would never need a donor or medications that have so many side effects, and they could live long and happy lives naturally.”
Image credits: Gordon Museum/Wellcome Images (CC BY-NC-ND), Courtesy Oganovo, Mark Riccio