A new device in the biomedical world aims to help restore blood flow in ailing patients. The device twists and compresses in synch with heart muscle contractions to help a failing heart pump blood again. This is a significant advancement that can help prevent heart failure which is a multi-billion-dollar problem affecting 41 million people worldwide.
The cardiac device fits around the heart. Image courtesy of Science Translational Medicine.
Several engineering groups are investigating methods of treating heart failures or even building an artificial heart. 3D-printed heart valves, bionic heart patches, and the light-activated robotic ray are only a few of these attempts. While each of these endeavors is marked as a milestone in the advancement of technology, a commercially feasible solution still remains out of reach.
On the other hand, a current and common approach to improving cardiac circulation is employing a ventricular assist device (AVD). A VAD bypasses a failing heart by removing the blood from the heart and pumping it into the aorta or pulmonary artery.
Despite being common, there are several drawbacks to this approach. Firstly, in a VAD, the blood is in contact with artificial objects such as tubes, pumps, and so forth. This makes health complications like clotting and stroke quite possible unless the patient takes blood thinners. Although blood thinners prevent clotting, they require constant monitoring and may lead to bleeding problems. Secondly, a VAD disrupts the natural motion of heart muscles.
A VAD bypasses the heart to circulate the blood and treat heart failure. Image courtesy of Madhero88 (own work) [CC BY-SA 3.0]
To arrive at a better solution, a team of researchers from Harvard University and Boston Children's Hospital looked for an assistive device which didn’t contact blood at all. They considered a device that could hug the heart and help it pump the blood again. It was obvious that conventional mechanical devices didn’t suit this application because they are rigid and exhibit a tendency to heat up and, consequently, cannot be placed in contact with the heart muscles.
Researchers thought that soft robotics, which is conformable and capable of complex motions, may be used to mimic the outer cardiac muscle layers.
The New Solution
The main challenge in designing the new cardiac device was imitating the twist and compress which occurs when a heart beats. This complicated motion is similar to wringing water out of a wet towel. In other words, when the heart beats, the top and bottom muscles of the heart twist in opposite directions.
To mimic this motion, researchers designed a soft robotic cardiac device with two sets of air-powered pneumatic actuators. As shown in the following image, they employed a group of concentric ring-shaped actuators which could compress the muscle and a group of helical actuators which could twist the cardiac device. Activating these two actuators simultaneously, researchers achieved a device which could mimic the complex contractions of heart muscles.
Use of two sets of actuators allowed the cardiac device to mimic heart motions. Image courtesy of Science Translational Medicine.
Interestingly, the new cardiac device is customizable and can contract one side of the heart without affecting the other side. This is important as chronic heart failure often doesn't affect the entire organ but only a specific section.
Ellen Roche, the lead author of the research paper and former Harvard doctoral candidate, goes even one step beyond this and envisions that perhaps the assistance level of the device can be adjusted as the patient’s condition evolves.
For the cardiac device, the researchers employed McKibben Muscles, which is one of the most widely used types of pneumatic artificial muscles (PAM), and built both ring-shaped and helical muscles as shown in the previous image. They examined several designs to find the best material and manufacturing method which could give a conformal device.
But how exactly does a PAM work?
Pneumatic Artificial Muscles (PAM)
PAMs are a type of pneumatic actuators which contract on inflation. This is in contrast to bellows which extend upon inflation.
The core of a PAM is a flexible membrane which transfers the mechanical power to the load by bulging outward or squeezing depending on the air pressure inside. The following image shows how an arbitrary PAM operates. The device is fixed at one end and has a mass hanging from the other. In this figure, the pressure is increased from an initial value of zero to P2. As the pressure increases, the device bulges and its length decreases. Consequently, the mass is pulled upward.
The operation of a PAM. Image courtesy of Vanderbilt University EECS.
As the above experiment shows, the force and motion generated by these devices are linear and unidirectional. To produce a bidirectional motion, an antagonistic set-up as shown in the following image is required.
The antagonistic set-up to generate a bidirectional motion. Image courtesy of Vanderbilt University EECS.
Despite being lightweight, PAMs are capable of transferring relatively large amounts of energy.
Animal Trials and the Challenges
The team examined the device using both a synthetic heart and pig cadavers. They also drugged live pigs to cause heart failure and then used the invention to restore the animal’s blood flow. The device could successfully conform to the surface of the animal’s heart and synchronize with the contractions. Although these tests prove the feasibility of the idea, there is a long way to go before moving the robot into human trials.
Unfortunately, the experiment showed that the friction between the device and heart leads to inflammation on the tissue surface. To reduce the inflammation, researchers developed a hydrogel which was applied as a protective layer between the heart and the device. However, long-term animal studies are still required to investigate the potential chronic complications of the new method. Researchers elaborate that, to have a portable system, the patient would need to carry a compressed air supply to power the pneumatic actuators.
The invention shows that a safe interaction between soft tissue and soft robotics is quite possible. This opens the door for several other future applications where a similar device can deliver mechanotherapy both inside and outside of the body.
The details of this study are published in Science Translational Medicine.
Featured image used courtesy of Ellen Roche.