In fast-moving blood, it can lead to haemolysis: red blood cells rupturing.
For a liver temporarily separated from a human host, that’s a serious issue. If left unchecked, it would need entirely new blood every few hours. Coussios and his colleagues overcame the issue in their Metra device, a first-of-its kind machine that preserves livers between extraction and transplantation into a new owner.
Designed to replace cold storage, the device mimics the human body with flowing blood, a constant temperature of 37°C and changing pressure. Unlike when in cold storage, the liver keeps functioning, producing bile and “sweating” other substances.
Combining electro-mechanical systems, sterile fluid circuit design, firmware coding and a rugged design, the device is made for single-button operation during transportation. Pre-assembled perfusion circuits are designed for ease of connection and disposal, while sensors monitor fluid flow and pressure, blood sugar, carbon dioxide and oxygen levels, feeding information to the autonomous control system.
The device runs on battery power and has a self-regulating oxygen supply – no need for gas canisters – and the multi-tasking software can compensate for failure of individual parts.
To tackle potentially destructive shear stress as blood moves through the machine, the team carefully selected centrifugal pumps to minimise damage. They also redesigned the circuit after running simulations to identify areas of high stress.
A recent study published in Nature analysed 220 transplants that used both cold and warm techniques. The OrganOx method had a 50% lower rate of discarded organs, a 50% decrease in injuries associated with transplantation, and 54% longer preservation times – up to 24 hours, and an average of 12 hours.
Separated from a body for that long, livers need a support mimicking their usual home. To compensate, the team designed a flexible yet rigid “silicon hammock”, allowing organs to function and remain intact.
The machine is probably the most complex medical device designed and built in the UK, said Coussios, who won the 2017 Silver Medal from the Royal Academy of Engineering. “There is almost no engineering system that is not in that device somewhere,” he said.
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