Bionic heart for testing prosthetic valves, other cardiac devices

The device made of heart tissue and a robotic pumping system beats like the real thing.

Prosthetic valves are intended to impersonate a real, healthy heart valve in assisting in circulating blood through the body. In any case, a significant number of them have issues, for example, leakage around the valve. Engineers attempting to improve these designs must test them repeatedly, first in simple benchtop simulators, at that point in animal subjects, before reaching human trails- a complicated and expensive procedure.

Now, a team of engineers at MIT and elsewhere have come up with a more realistic model of bionic heart for testing out artificial valves and other cardiac devices.

The device is a real biological heart whose tough muscle tissue has been replaced with a soft robotic matric of artificial heart muscles, looking like bubble wrap. The orientation of the artificial muscles mimics the pattern of the heart’s natural muscle fibers so that when the specialists remotely blow up the bubbles, they act together to squeeze and twist the inner heart, like the way real, whole heartbeats and pumps blood.

Scientists dubbed their model as a ‘biorobotic hybrid heart,’ that could represent what happens in a real heart.

Scientists quickly thought about wrapping a whole, explanted heart in a soft robotic sleeve, but soon realized that the heart’s external muscle tissue, the myocardium, immediately solidified when expelled from the body. Any robotic contraction by the sleeve would neglect to translate sufficiently to the heart within.

A close-up of a synthetic matrix of soft robotic actuators that can be wrapped around a heart ventricle and inflated to squeeze and twist the heart in the same way a real heart pumps blood. Image courtesy of the researchers.
A close-up of a synthetic matrix of soft robotic actuators that can be wrapped around a heart ventricle and inflated to squeeze and twist the heart in the same way a real heart pumps blood. Image courtesy of the researchers.

Rather than looking for different ways to design a soft robotic matrix to replace the heart’s natural muscle tissue, in both material and function. They decided to try out their thought first on the heart’s left ventricle, one of four chambers in the heart, which pumps blood to the rest of the body, while the right ventricle uses less power to pump blood to the lungs.

The heart pumps blood typically by squeezing and twisting, a complex combination of motions that is a result of the alignment of muscle fibers along the outer myocardium that covers each of the heart’s ventricles. Based on this, scientists decided to fabricate a matrix of artificial muscles resembling inflatable bubbles, aligned in the orientations of the natural cardiac muscle. But copying these patterns by studying a ventricle’s three-dimensional geometry proved extremely challenging.

They eventually came across the helical ventricular myocardial band theory, the idea that cardiac muscle is essentially a large helical band that wraps around each of the heart’s ventricles.

Instead of trying to copy the left ventricle’s muscle fiber orientation from a 3D perspective, the team decided to remove the ventricle’s outer muscle tissue and unwrap it to form a long, flat band — a geometry that should be far easier to recreate. In this case, they used the cardiac tissue from an explanted pig heart.

Then by using diffusion tensor imaging, scientists mapped the microscopic fiber orientations of a left ventricle’s unfurled, two-dimensional muscle band. They then fabricated a matrix of artificial muscle fibers made from thin air tubes, each connected to a series of inflatable pockets, or bubbles, the orientation of which they patterned after the imaged muscle fibers.

The soft matrix composed of two layers of silicone, along with a water-soluble layer between them. This prevents the layers from sticking — also, two layers of laser-cut paper, which ensures that the bubbles inflate in a specific orientation.

The researchers also developed a new type of bioadhesive to glue the bubble wrap to the ventricle’s real, intracardiac tissue. Dubbed as TissueSil, the adhesive was made by functionalizing silicone in a chemical cross-linking process, to bond with components in heart tissue.

The result was a viscous liquid that the researchers brushed onto the soft robotic matrix. They also brushed the glue onto a new explanted pig heart that had its left ventricle removed, but its endocardial structures preserved. When they wrapped the artificial muscle matrix around this tissue, the two bonded tightly.

Finally, the researchers placed the entire hybrid heart in a mold that they had previously cast of the original, whole heart, and filled the mold with silicone to encase the hybrid heart in a uniform covering — a step that produced a form similar to a real heart and ensured that the robotic bubble wrap fits snugly around the real ventricle.

Ellen Roche, assistant professor of mechanical engineering at MIT said, “That way, you don’t lose transmission of motion from the synthetic muscle to the biological tissue.”

Ultimately, the researchers hope to use the bionic heart as a realistic environment to help designers test cardiac devices such as prosthetic heart valves.

Co-lead author Chris Nguyen at MGH said, “Imagine that a patient before cardiac device implantation could have their heart scanned, and then clinicians could tune the device to perform optimally in the patient well before the surgery. Also, with further tissue engineering, we could potentially see the biorobotic hybrid heart be used as an artificial heart — a very needed potential solution given the global heart failure epidemic where millions of people are at the mercy of a competitive heart transplant list.”

Roche and her colleagues have published their results in the journal Science Robotics. Her co-authors are lead author and MIT graduate student Clara Park, along with Yiling Fan, Gregor Hager, Hyunwoo Yuk, Manisha Singh, Allison Rojas, and Xuanhe Zhao at MIT, along with collaborators from Nanyang Technology University, the Royal College of Surgeons in Dublin, Boston’s Children’s Hospital, Harvard Medical School, and Massachusetts General Hospital.

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