The heart valve was fabricated by a team involving engineers from Harvard University in Massachusetts, using a method they called ‘focused rotary jet spinning’.
Described as “a cotton-candy machine with a hair dryer behind it” in a research announcement, the technique was used to build valves that were implanted into two sheep.
“The two big advantages of our method are speed and spatial fidelity,” said bioengineer Michael Peters from Harvard, one of the first authors of a study on the work. “We can create really small fibres – on the nanoscale – that mimic the extracellular matrix that heart valve cells are used to living and growing inside, and we can spin full valves in a matter of minutes, in contrast to currently available technologies that can take weeks or months to make.”
Pulmonary heart valves are made up of three partially overlapping ‘leaflets’ that open and close with every heartbeat. They are responsible for controlling one-way blood flow through the heart – with every beat, they open fully to allow blood to flow forwards, and then close fully to prevent blood from flowing backwards.
To make the valves, the researchers used air jets to direct liquid polymer onto a valve-shaped frame, resulting in a ‘seamless meshwork’ of tiny fibres. The valves are designed to be temporary and regenerative, providing a porous scaffold for cells to infiltrate, build upon, and eventually replace as the polymer biodegrades.
“Cells operate at the nanometre scale, and 3D printing can't reach down to that level, but focused rotary jet spinning can put nanometre-scale spatial cues in there, so that when cells crawl up into that scaffold, they ‘feel’ like they're in a heart valve, not a synthetic scaffold,” said senior author and bioengineer Kit Parker of Harvard University. “There's a certain trickery that's involved.”
The team tested the valves’ strength, elasticity, and ability to repeatedly open and close using a pulse duplicator, a machine that simulates the heartbeat.
“A normal heart valve functions for billions of cycles throughout one's life, so they’re constantly being pulled and stretched and stimulated,” said Peters. “They need to be very elastic and retain their shape despite these mechanical stimuli, and they also have to be strong enough to withstand the back pressures from blood trying to flow backwards.”
They also grew heart cells on the valves to test for biocompatibility, and to see how well cells could infiltrate the scaffolds. “Valves are in direct contact with blood, so we need to check that the material doesn't cause any thrombosis or obstruction of the blood vessels,” said biophysicist Sarah Motta, the study’s other first author, who works at Harvard University and the University of Zurich.
Long term in vivo studies are needed to test the valves’ endurance, the researchers said, but they effectively controlled blood flow for an hour. Sheep are a good animal model because the physical forces inside sheep and human hearts are similar, the team said. Sheep hearts also represent an extreme environment for heart valves due to their accelerated calcium metabolism, which presents an increased risk of developing calcium deposits, a common complication for heart valve recipients.
Surgeons implanted the valves into two sheep and monitored their position and function using ultrasound for one hour. Both valves implanted successfully and were immediately functional, but one sheep’s valve dislodged after a few minutes. The researchers think this happened because it was the incorrect size for the animal.
In the second sheep, the valve showed good functionality for an hour, and post-mortem analysis indicated that there were no complications in terms of tears or thrombus formation, and that cells had already begun to infiltrate and adhere to the valve.
Next, the team plan to test the valves’ performance over a longer duration and in more sheep. “We want to see how well our valves function over the scale of weeks to months, and how effectively and quickly the sheep's cells and tissues are actually remodelling the scaffold,” said Peters.
The work was published in Matter.
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