Bernhard Jank, MD, Ott Laboratory, Massachusetts General Hospital Center for Regenerative Medicine

Mind-controlled robotic limbs for amputees are growing more sophisticated all the time -- but a challenger has appeared. Human limb transplant surgery using the patient's own biological material could one day be an actuality.

The proof of concept, published this week in the journal Biomaterials, is the limb of a rat, grown in a lab by researchers at Massachusetts General Hospital, with functioning vascular (veins) and muscle tissue.

"The composite nature of our limbs makes building a functional biological replacement particularly challenging. Limbs contain muscles, bone, cartilage, blood vessels, tendons, ligaments and nerves -- each of which has to be rebuilt and requires a specific supporting structure called the matrix," said Harald Ott, MD, of the MGH Department of Surgery and the Center for Regenerative Medicine, senior author of the paper.

"We have shown that we can maintain the matrix of all of these tissues in their natural relationships to each other, that we can culture the entire construct over prolonged periods of time, and that we can repopulate the vascular system and musculature."

Muscles and veins have been grown in a lab from stem cells, and the veins have even been transplanted into a patient, created from his own cells. Using the patient's own genetic material reduces the risk of rejection of the transplant and removes the need for life-long immunosuppressant drugs. The problem with growing an entire limb, as Dr Ott pointed out, is that a limb is much more complex and contains more than a single tissue type.

This was solved using a technique already in use for lab-grown organs: stripping a donor organ of its cells to create a neutral matrix. A matrix is the tissue in which more specialised cells are embedded, and a stripped, neutral matrix can be populated with cells from the patient. This technique has been used to create kidneys, livers, hearts and lungs (although only in animal models for research purposes). This research marks the first time a limb -- which is significantly more complex -- has been grown.

Bernhard Jank, MD, Ott Laboratory, Massachusetts General Hospital Center for Regenerative Medicine

To create the limb, the team first took forelimbs from deceased rat, soaking them in a detergent solution for a week to strip them of all cellular material, preserving the primary vascular and nerve matrix. This remaining material was to form the structural basis for new cellular material.

While the donor limb was being stripped, vascular and muscle cells from a second rat were being grown in a culture.

To create the new limb, the stripped limb matrix was placed into a container for biological chemical processes known as a bioreactor. Vascular cells were injected into the limb's main artery, while muscle progenitor cells were injected into the matrix sheaths that define muscle position. Progenitor cells can form one or more kinds of cells, and are sort of mid point between stem cells and fully differentiated cells. The bioreactor was then filled with a nutrient solution to grow the cells. After five days, electrical stimulation was applied to promote muscle formation.

After two weeks, the limb was removed from the bioreactor, and the team found that vascular and muscle cells were populating the limb appropriately. When stimulation was applied to the muscles, it was revealed that they contracted at 80 percent of the strength of the muscles in newborn animals. The vascular cells, transplanted into living animals, functioned normally.

The bio-engineered hand has a clear advantage over a robotic prosthetic, as the patient would not have to train their brain how to control it. It would also be free from the limitations of a robotic limb, such as the inability to gauge pressure or heat.

Although clinical trials are a long way off, and the challenges of transplanting limbs and having them integrate smoothly with the host body are significant, the moderate success of human hand transplant surgery indicates that the technique has potential.

"In clinical limb transplantation, nerves do grow back into the graft, enabling both motion and sensation, and we have learned that this process is largely guided by the nerve matrix within the graft," Dr Ott said.

"We hope in future work to show that the same will apply to bio-artificial grafts. Additional next steps will be replicating our success in muscle regeneration with human cells and expanding that to other tissue types, such as bone, cartilage and connective tissue."