Soft robotic sleeve compresses and twists to restore heart function
For decades, ventricle assist devices (VADs), which help circulate blood using a pump and tubes, have been the standard tool to mechanically help a failing heart. But because VADs physically come in contact with blood, they can cause dangerous blood clots and must be used in conjunction with anticoagulants (blood thinners) that may have serious side effects. Skirting this issue, researchers have now devised a soft robotic sleeve that can help a failing heart pump blood by compressing and twisting it with a series of individually controlled actuators. The pneumatic device, described recently in Science Translational Medicine, was able to restore a pigs’ cardiac output to near-full capacity after experiencing acute heart failure.
The main achievement of the study, says co-author Frank Pigula, is being able to demonstrate that future mechanical circulatory support systems need not follow the designs of the current technologies. “We can start thinking about parallel or alternative approaches to the same problem [of heart failure],” says Pigula, a cardiac surgeon and researcher with Boston Children’s Hospital.
Each year, an estimated 5.7 million patients in United States experience heart failure, a medical condition in which the heart does not pump enough blood to fulfill the body’s needs. Only people with severe heart failure are considered for heart transplantation, but donor organs are not widely available. To assist the failing heart, doctors implant VADs, sometimes permanently. These devices have gone through multiple generations and are becoming increasingly sophisticated, but all work similarly by drawing blood out of one or both ventricles of the heart and into the aorta (main artery that supplies oxygenated blood to the body) or pulmonary artery (artery that carries deoxygenated blood to the lungs). No matter the design, VADs carry a risk of blood clots—which can lead to crippling and even deadly strokes—and must be used along with anticoagulants, which carry a risk of increased bleeding. Still, blood clots and stroke may occur in up to 20% of people with VADs, despite blood thinning medications.
Instead of manipulating blood, another approach to help with heart failure has to do with mechanically compressing the heart—a concept that dates back to the 1930s, according to Dennis Trumble, a Carnegie Mellon University biomedical engineer whose work focuses on cardiac assist devices. Various attempts have been made over the years to produce a device such as the short-lived Anstadt Cup, which compressed the heart using inflatable diaphragms. In the late 1990s, researchers created a more sophisticated device that uses soft pneumatic sleeves. “This device, however, was never developed beyond the testing of a simple preclinical device built as a proof-of-concept prototype,” says Trumble, who was not involved in the current study. “None of the devices really gained any traction,” Pigula adds.
Pigula decided to revisit the idea of developing a cardiac compression device that does not contact blood. After digging into the scientific literature, he came across a type of pneumatic artificial muscle called the McKibben air muscle, which was invented in the 1950s for orthotics and is made up of a pneumatic line, inner tubing, and an outer mesh. When pressurized, the strong, lightweight air muscles axially contract quickly and effectively. Pigula brought the idea to Conor Walsh, a soft-robotics engineer at Harvard University. The two ended up working on developing a soft robotics cardiac assist platform that incorporates McKibben actuators.
The resulting device has gone through multiple iterations and its design takes inspiration from the muscle layers of the heart, which simultaneously undergo twisting and compressive motions. For the study, the researchers investigated two different designs for their cardiac compression device. In one design, they developed pneumatic artificial muscles (PAMs) using a silicone-casting process—that is, they molded a silicone tubing, prepared an outer braided mesh, bonded the mesh to the tubing, inserted an air supply line into one end, and then sealed the ends. Using a multi-component reconfigurable three-dimensional printed mold, they then cast a cup-shaped silicone sleeve that fits around the heart; actuators were embedded into the matrix in a compressing layer and a twisting layer. The second design used PAMs created with a thermoplastic-forming process, which involved creating thermoplastic urethane balloon halves that were sealed together and trimmed, bonding an airline to the balloon neck and curing it with UV light, and then bonding a braided mesh over the bladder with a UV adhesive. They created a sleeve that can wrap around the heart by using a two-dimensional laminate process that selectively bonded thin silicone sheets between compressing and twisting actuator layers.
Both sleeves are attached to the heart with an FDA-approved suction device; to reduce friction and minimize inflammation, the researchers used a nontoxic, biocompatible hydrogel between the device and the heart. The implant is tethered to an external pump, which can activate individual PAMS with air pressure, causing them to compress and twist the heart in synchrony with native heart motion. Pigula, Walsh, and their colleagues tested the device on six pigs. They induced acute heart failure in the pigs, which reduced their cardiac output to 45%. When the cardiac assist device was turned on, however, it brought the pigs’ cardiac output back up to 97%. “I think it demonstrates that there is value and potential in this concept,” Pigula says. “The gold standard or the money shot is being able to reestablish baseline cardiac output.”
Michael Moreno, a biomedical engineer at Texas A&M University, likes the device, adding that it follows a same philosophy and has various similarities with a device he once worked on and is continuing to be developed by CorInnova, Inc. “I personally hypothesized that re-kinesis therapy—cardiac-assist that is applied in a manner that restores normal, healthy cardiac motion—could lead to cardiac rehabilitation and recovery in some patients,” he says. “It would be good to see other innovations along these lines, to see patients of heart failure provided a therapy directed at recovery of lost cardiac function.”
Carnegie’s Trumble says the soft robotic sleeve has a sophisticated control mechanism and torsional component that makes its movement more “heart-like” and sets it apart from previous devices. “Overall, I think this approach to circulatory support holds significant promise as it avoids direct contact with the bloodstream, which, as the authors mention, is a major cause of embolic complications in current cardiac assist devices,” he says. But, he adds, the device in its current form still has notable limitations, such as percutaneous driveline that could be prone to infections and a tethered air compressor that raises quality-of-life issues.
Pigula says that the tethering issue is something the team is currently working on, as well as finding different ways to attach the soft robotic sleeve to the heart. Moving on, he would like to test the device on longer time frames in large animal models (probably sheep or cows), making sure it is suitable for chronic implantation.
Read the abstract in Science Translational Medicine.