Researchers at the Massachusetts Institute of Technology (MIT) have developed a bionic ‘heart’ – a hybrid made of heart tissue and a robotic pumping system – that offers a more realistic model 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 matrix of artificial heart muscles, resembling bubble wrap.
The orientation of the artificial muscles mimics the pattern of the heart’s natural muscle fibres, in such a way that when the researchers remotely inflate the bubbles, they act together to squeeze and twist the inner heart, similar to the way a real, whole heart beats and pumps blood.
With this new design, which they call a ‘biorobotic hybrid heart’, the team at MIT envision that device designers and engineers could iterate and fine-tune designs more quickly by testing on the biohybrid heart, significantly reducing the cost of cardiac device development.
“Regulatory testing of cardiac devices requires many fatigue tests and animal tests,” said Ellen Roche, assistant professor of mechanical engineering at MIT. “[The new device] could realistically represent what happens in a real heart, to reduce the amount of animal testing or iterate the design more quickly.”
According to research, as the population of older people is expected to increase in the coming decade, rates of heart disease in the US are likely to increase too. Also, the demand for prosthetic heart valves and other cardiac devices – a market that is valued at more than $5bn (£3.9bn) to date – is predicted to rise by almost 13 per cent in the next six years.
Prosthetic valves are designed to mimic a real, healthy heart valve in helping to circulate blood through the body. However, many of them have issues such as leakage around the valve, and engineers working to improve these designs must test them repeatedly, first in simple benchtop simulators, then in animal subjects, before reaching human trials – an arduous and expensive process.
Before coming to MIT, Roche worked briefly in the biomedical industry, helping to test cardiac devices on artificial heart models in a laboratory. “At the time I didn’t feel any of these benchtop set-ups were representative of both the anatomy and the physiological biomechanics of the heart,” she recalled. “There was an unmet need in terms of device testing.”
In separate research as part of her doctoral work at Harvard University, Roche developed a soft, robotic, implantable sleeve, designed to wrap around a whole, live heart, to help it pump blood in patients suffering from heart failure.
At MIT, she and graduate student Clara Park explored the possibilities of combining the two research avenues, to develop a hybrid heart: a heart that is made partly of chemically preserved, explanted heart tissue and partly of soft artificial actuators that help the heart pump blood.
The team proposed that such a model should be a more realistic and more durable environment in which to test cardiac devices. This compares to models that are either entirely artificial but do not capture the heart’s complex anatomy or are made from a real explanted heart, requiring highly controlled conditions to keep the tissue alive.
During the development of the bionic heart, the team briefly considered wrapping a whole, explanted heart in a soft robotic sleeve, in a similar way to Roche’s previous work. However, they found the heart’s outer muscle tissue, the myocardium, quickly stiffened when removed from the body – therefore any robotic contraction by the sleeve would fail to translate sufficiently to the heart within.
As an alternative, the team looked for ways to design a soft robotic matrix to replace the heart’s natural muscle tissue, in both material and function. So, they decided to test out their own idea 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 force to pump blood to the lungs.
“The left ventricle is the harder one to recreate given its higher operating pressures, and we like to start with the hard challenges,” Roche said.
The heart normally pumps blood by squeezing and twisting, a complex combination of motions that is a result of the alignment of muscle fibres along the outer myocardium that covers each of the heart’s ventricles.
The team planned to fabricate a matrix of artificial muscles resembling inflatable bubbles, aligned in the orientations of the natural cardiac muscle. However, copying these patterns by studying a ventricle’s three-dimensional geometry proved to be a challenge.
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. This theory is still a subject of debate by some researchers, but Roche and her colleagues took it as inspiration for their design.
Rather than attempting to copy the left ventricle’s muscle fibre 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.
The researchers used diffusion tensor imaging, an advanced technique that typically tracks how water flows through white matter in the brain, to map the microscopic fibre orientations of a left ventricle’s unfurled, two-dimensional muscle band.
Afterwards, they fabricated a matrix of artificial muscle fibres 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 fibres. The soft matrix consists of two layers of silicone, with a water-soluble layer between them to prevent the layers from sticking, as well as two layers of laser-cut paper, which ensures that the bubbles inflate in a specific orientation.
The team at MIT also developed a new type of bioadhesive to glue the bubble wrap to the ventricle’s real, intracardiac tissue. While adhesives exist for bonding biological tissues to each other, and for materials like silicone to bond, the team found that few soft adhesives do an adequate job of glueing together biological tissue with synthetic materials, silicone in particular.
To create this bioadhesive, Roche collaborated with Xuanhe Zhao, an associate professor of mechanical engineering at MIT who specialises in developing hydrogel-based adhesives. The new adhesive, named TissueSil, was made by functionalising silicone in a chemical cross-linking process, which bonded with components in heart tissue.
This process resulted in 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. And when they wrapped the artificial muscle matrix around this tissue, the two bonded tightly.
Finally, the team placed the entire hybrid heart in a mould that they had previously cast of the original, whole heart, and filled the mould 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. “That way, you don’t lose transmission of motion from the synthetic muscle to the biological tissue,” Roche added.
When the team pumped air into the bubble wrap at frequencies resembling a naturally beating heart and imaged the bionic heart’s response, it contracted in a manner similar to the way a real heart moves to pump blood through the body.
Ultimately, the researchers hope to use the bionic heart as a realistic environment to help designers test cardiac devices such as prosthetic heart valves.
“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,” said Chris Nguyen at Massachusetts General Hospital (MGH) and the Martinos Center for Biomedical Imaging. “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.”
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