If we can't sort this problem out, we could be stuck with lab-based solutions we can't use

Larry Hench, Imperial College, London

Now Dr Vacanti, a transplant surgeon at Massachusetts General Hospital in Boston and Jeffrey Borenstein, micro-engineering expert at the nearby Draper Lab are looking at growing more complex tissues.

Scientists had believed it was not possible to grow anything more complex than simple tissues, such as thin sections of knee cartilage and skin because of problems growing the blood vessel networks.

To do this, they use a "frame" made of a biodegradable plastic.

This is immersed in a solution of the patient's cells, then in a nutrient solution.

As the cells multiply and link together, the frame dissolves, leaving a piece of cartilage which survives because oxygen and nutrients from surrounding fluids feed into the cells.

But in thicker tissues, the nutrients are unable to get past the first few cell layers and need an internal blood supply to survive.

The technique developed by the researchers involves copying the blood vessel network of a real liver and using 3D computer modelling and machining to mimic its construction.

To copy the structure, they injected a liquid plastic into the blood vessels of a liver, wait for it to solidify and dissolve the liver tissue, leaving a solid cast of the organ's blood vessels.

They can then take "measure up" and feed data into a computer to create a 3D model of a liver's blood supply.

3D

This model is then "sliced up" on the computer into horizontal layers, which can be used to make a silicon mould.

Biodegradable plastic is then poured into the mould to make enough slices that can be sandwiched together using pressure and heat, to create a scaffold for a whole liver.

The scaffold then has to be injected with at least seven types of cells that make up the solid part of the liver.

A solution of endothelial cells, which normally line blood vessels, then has to be pumped into the empty channels in the frame where they stick to the walls.

These can then be grown in a nutrient to form a network of blood vessels within the scaffold which again dissolves over a few months.

If each step of this process works, it would result in a functioning liver.

But blood vessel networks grown this way have so far been tested in rats, with no leakage or obstruction of the blood flow.

Linda Griffith, a tissue engineer at MIT, said one of the main problems of the idea would be getting all the right cell types to grow in the right places.

She told New Scientist magazine, which sets out the research: "In just a gram of liver, you have around 100 million cells and it'd be very hard to position each and every one.

Larry Hench, a tissue engineer at Imperial College, London added: "Currently, there's no way of keeping these implants sterile, and no one is really looking at that."

"If we can't sort this problem out, we could be stuck with lab-based solutions we can't use."

Nigel Hughes, chief executive of the British Liver Trust, told BBC News Online: "We know that the demand for livers is going to outstrip supply.

"We hope to see all of this biotech science come to fruition."