Scientists have demonstrated the first 3D printer able to produce living and structurally stable bone, cartilage or skeletal muscle tissue big enough to be used for human transplants.

Key points: Existing bioprinters are unable to produce living tissue large enough and strong enough to be clinically useful

Existing bioprinters are unable to produce living tissue large enough and strong enough to be clinically useful The new system prints living cells onto a biodegradable structure, with microchannels for nutrients and oxygen

The new system prints living cells onto a biodegradable structure, with microchannels for nutrients and oxygen Researchers have successfully implanted living body parts into mice

Inorganic bone replacements are already being printed out of bone cement or titanium in addition to printed prosthetic body parts, but printing of functioning, full-size tissues faces two critical problems related to size.

"The resulting constructs are often structurally unstable and too fragile for surgical implantation," a media release said.

"Moreover, because they lack blood vessels, their size is constrained by the diffusion limit for nutrients and oxygen, which is around 200 micrometers — too small to make most human tissues and organs."

The diffusion limit refers to how far molecules will move through living tissues once absorbed, so cells further than 200 micrometers (0.2 millimetres), from a blood vessel or other source of nutrients will starve and die.

The new printer, known as the integrated tissue-organ printer or ITOP, is described in a paper published this week in the journal Nature Biotechnology by researchers from the Wake Forest Institute for Regenerative Medicine in North Carolina.

ITOP tackles the problem of structural integrity by printing a biodegradable plastic structure onto which living cells are applied using a water-based gel ink.

The problem of keeping the cells oxygenated and fed was solved by building microchannels into the structural plastic so nutrients and oxygen can reach all cells.

Tissue printed this way can be kept alive long enough to be implanted into a patient and for the patient's body to grow necessary blood vessels as the structure dissolves.

ITOP uses multiple different nozzles customised for the different components, avoiding the problems of individual printing systems, which all have difficulty building stable large structures, the paper says.

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Using this method, the team says it has printed a human-sized ear, a section of muscle tissue and a human-sized jaw bone fragment using human, rabbit, rat and mouse cells.

ITOP also enables replacement tissues to be precisely customised to patients before surgery by using imaging data.

"To demonstrate construction of a human-sized bone structure, we fabricated a mandible fragment in a size and shape similar to what would be needed for facial reconstruction after traumatic injury," the paper says.

"Mandible bone defects have an arbitrary shape. We used data from a CT scan of a human mandible defect ... to produce a CAD model of the defect shape, with dimensions of 3.6 cm × 3.0 cm × 1.6 cm."

Each part was implanted in rats or mice, where blood vessels and nerves grew through them in weeks to months.

Trials are ongoing and researchers are tracking the long-term progress of the implanted tissues.

Further research will move towards using clinical-grade human cells derived from patients, and a wider variety of cell types to replicate a wider range of native tissues.