A government-funded lab in North Carolina had already grown tissue for 30 human organs, including kidneys, hearts, bladders, urethras and even vaginas. Now scientists have added another vital organ to the list: the penis.

The Wake Forest Institute for Regenerative Medicine in Winston-Salem has successfully engineered the first six lab-grown human penises, the scientists revealed over the weekend — and they're just waiting for FDA approval to move forward with what they call "in-man" testing.

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"Our target is to get the organs into patients with injuries or congenital abnormalities," Institute director Professor Anthony Atala told the Guardian.

The first beneficiaries would be men with erectile dysfunction, most cases of which have to do with scar tissue in the penis, Atala's team says; these would be partial penis replacements, using cells from the patient's own penis.

But in the long run, the scientists expect the same technology to help in transgender surgery, as well as replace penises for men with birth defects, and soldiers with genital injuries. Atala's work is funded by the the U.S. Armed Forces Institute of Regenerative Medicine.

The breakthrough technique involves constructing a collagen "scaffold" out of a donor penis. The scaffold is washed with a special solution that removes the donor's DNA, then injected with cells from the patient's penis — which can be retrieved internally even if no organ remains on the surface.

Previous experiments with grafting a donor penis directly on to the body have ended badly; the patient's body tends to reject the organ if it doesn't have cells with its own DNA.

Atala, who first successfully engineered rabbit penises back in 2008, expects "in-man" trials to begin in the next five years. In the meantime, the engineered penises are being put through their paces in the lab — courtesy of machines that squash, twist and stretch the organs, and pump them full of fluid to test their erections.