The scientist who claimed in 2015 that the first full-body transplant would happen within two years, is claiming that he has made a breakthrough in developing the surgical procedure. Doctor Sergio Canavero has been working on the procedure for years and has now claimed that doctors have “successfully” transplanted a human head onto a cadaver body.

His most recent “success” involves the transplant of a full cadaver body to a head from a different cadaver body. But neither body was living when the transplant was done, the Telegraph reported. “It’s important that people stop thinking this is impossible. This is absolutely possible and we’re working towards it,” Canavero told New Scientist last year.

Canavero thinks that the extensive surgery could offer people who are paralyzed a new life with a working body, but other doctors are skeptical of the procedure and worry about the recovery any living being will have to survive afterwards. Canavero is now saying that doctors are ready to conduct his procedure on the first living human, after a successful transplant between corpses, according to the Independent.

The procedure involves severing the head and the spinal cord to then attach a new one by fusing the spinal cord and reconnecting the tissues, nerves and muscles in the neck and spine. When it was performed on cadaver bodies the procedure supposedly took 18 hours to complete, the Telegraph reported. Canavero also told the Telegraph that the even though the two bodies aren’t alive, the doctors were able to determine that the connection had been successful through electrical stimulation of the nerves.

Now Canavero and the teams he’s been working with to complete the transplant think that the operation could be successful on a live person. The doctors completed a similar procedure on a monkey in 2016. Photos of the procedure show a monkey’s head stitched onto a body. The procedure was successful for establishing blood flow and not causing any brain injury but the doctors did not attempt to connect the spinal cord, the Telegraph reported. But the monkey reportedly never regained consciousness, according to The Guardian. The paralyzed monkey was only kept alive for less than a day. So in all, the tissues were connected, but the monkey was paralyzed, not conscious and didn’t live long.

Completing a full body transplant involves more than simply completing the procedure and the patient pulling through. A review of the history of head transplantation on the National Center for Biotechnology Information website says, “Besides the technical challenges that remain, there are important ethical issues to consider, such as exploitation of vulnerable patients and informed consent.”