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He is a world-renowned surgeon who has been described both as a daring pioneer and as a cowboy who takes dangerous risks with his patients.

Dr. Paolo Macchiarini of the Karolinska Institute in Stockholm, Sweden, is pushing the boundaries of the emerging field of regenerative medicine, which involves using a patient's own cells to rebuild tissues and organs. Eventually scientists hope to get to the point where any replacement body part or organ you need would simply be manufactured in a lab, man-made, just for you. This could eliminate the need for donor organs, which are in short supply all over the globe.

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But for now, there is only one surgeon in the world who is doing transplants in humans with artificially grown organs. Patients come to the controversial surgeon because he is literally their only hope.

Take Julia Tuulik, a Russian dancer whose trachea was destroyed after a car accident.

“They offered for me this one chance,” Tuulik told Meredith Vieira for NBC News’ “A Leap of Faith: A Meredith Vieira Special,” airing Friday at 8 p.m. ET/7 p.m. CT. “And I haven’t other chance in my life.”

Or little Hannah Warren. Born without a trachea and unable to breathe on her own, she had spent her entire life in the hospital, kept alive only by a tube. No child with her disorder has ever lived past the age of six, and Dr. Macchiarini's artificial trachea was her only hope.

Or Ciaran Finn-Lynch, an Irish boy who was born with a windpipe less than a tenth of an inch wide. At age 10, he was the first child in the world to get a transplant made from a donor organ and his own stem cells.

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Many of Macchiarini's patients have been given only months or a few years to live, left with no options or any hope.

“You see a patient and this patient has no other alternatives,” Macchiarini said. “And he will die very, very soon. As a human and as a doctor are we allowed to say no? I don’t think so.”

Though it might sound like science fiction, scientists around the world are actively experimenting with this promising science. Recent accomplishments include Wake Forest Baptist Medical Center's announcement that four teenage girls with a rare genetic disorder were implanted with lab-grown vaginas, and at the University of Basel in Switzerland, scientists regrew the nose tissue of older people whose noses had been partially lost to skin cancer.

But while scientists are eagerly working toward being able to grow vital organs like hearts, lungs and kidneys in the lab, it will be years before they are ready to attempt transplanting those in humans.

"As a human and as a doctor are we allowed to say no? I don’t think so."

But Dr. Macchiarini has already taken the science out of the lab. He first made headlines six years ago, in 2008, when he transplanted the world's first lab-made windpipe. It was constructed from a donor trachea that had been stripped of its original cells, leaving it as a skeleton upon which a new trachea could be built with the patient’s own stem cells. The groundbreaking method would allow Macchiarini to bypass two of the major problems associated with donated organs: the risk of rejection and the need to take powerful anti-rejection drugs.

By 2011, the Italian surgeon had moved on to plastic as a scaffold, rather than a donated trachea. The first recipient would be Andermariam Beyene, a 36-year-old engineer from Eritrea.

Macchiarini’s team began by collecting stem cells from Beyene’s bone marrow. Those cells were mixed with special growth factors and then poured onto a scaffold made from plastic — in fact, the very same plastic that is used to make soda bottles — which had been made to mimic the shape of a real windpipe.

In just a matter of days, the scaffold began to transform into an actual functioning windpipe.

Macchiarini described the magical sounding process like this: “It’s like if you roast a chicken. It’s the same thing. You fill this box with fluid that includes cells. And then this chicken scaffold just is submerged in this fluid and the cells penetrate inside.”

Eight patients have now received his completely artificial, bio-engineered tracheas, but because the surgery is still highly experimental and unproven, critics worry that he is putting his patients at risk and taking the science out of the lab prematurely.

Skeptics have questioned whether he is using his desperate — and highly vulnerable — patients as human guinea pigs.

“I do believe he’s in the gray zone,” Dr. Joseph Vacanti, surgeon- in-chief at the Massachusetts General Hospital for Children, told Vieira in September of 2013.

Not all of Macchiarini's patients have survived, but supporters argue that this is how surgery advances.

"I do believe he’s in the gray zone."

“Take a look at any major turn in surgery,” said Dr. Rick Pearl, pediatric surgeon-in-chief at Children's Hospital of Illinois in Peoria. “It never started out working, did it?

“Tom Starzl, when he started doing liver transplants, the first seven, eight, nine patients all died. Everybody said he was nuts, OK? Christian Barnard, when he started doing heart transplants, everyone threw rocks at him. This is how we’re going to treat diseases in the future and this is the start of it.”

One of Macchiarini's most promising success stories is Claudia Castillo, a Spanish mother who is doing so well six years after her transplant that an increasing number of Macchiarini's colleagues are beginning to see him in a new light.

“I believe, for the field, we are now at the end of the beginning,” Vacanti said. “And so, he may feel alone, but he is not alone. He’s part of the group that’s making fantasy real.”