The Beat Goes On
Wed, 09/10/2008 - 8:32am
The Project: Develop a more sophisticated approach to the design and development process beyond the build-and-test method for a novel cardiac pump device.
The Solution: Use FEA software to test a variety of different variables that need to be examined for a successful design of the device.
By Lynn Manning
Lynn Manning is a science and technology writer based in Providence, RI. She can be reached at 401-272-1510 or firstname.lastname@example.org.
Treatment for HF can range from drugs to defibrillators to internal heart pumps, with transplant as the final option. No single therapy works for everyone, and side effects and mechanical issues can arise for the implanted pump devices. Dr. William Peters, a cardiothoracic surgeon and research fellow at Auckland City Hospital in New Zealand, thinks there has to be a better way.
“I’ve always had a strong interest in devices to support the failing heart,” he says (he has also invented a commercially-successful minimally-invasive bypass system). “Because of concerns about existing technologies, I was looking for a device that would not involve contact with the blood.” Common implanted blood-contacting devices such as left-ventricular assist devices (LVADs), while lifesavers for people awaiting transplants, require that the patient remain on blood thinners (which themselves can be a stroke risk) to prevent clots. Reliability has also been an issue with some heart-assist device designs.
Pump Works From Outside Heart
Peters patented his pump idea and formed a company, Sunshine Heart, to develop and test the device—initially on the bench and then in sheep. But once animal trials were successful, when the balloon was ready to be scaled up to a human model, the company decided that they needed a more sophisticated approach to the design and development process than the empirical, build-and-test approach they’d been using. The goal was not only to reduce lead time, but to provide a level of confidence that long-term performance would satisfy product requirements established by physicians for an acceptable medical device.
FEA Optimizes Fatigue Performance“The average human heart rate of 80 beats a minute equates to 42 million inflation cycles a year,” says Scott Miller, manager of mechanical engineering at Sunshine Heart. “The accumulated stress, especially on a polymer, was the design challenge—and C-Pulse is essentially a permanent implant. To ensure that our physical design solution was optimized to give us the long term fatigue performance required, we decided to look at it from a computational perspective using finite element analysis (FEA).”
Miller and his product development team worked with Matrix Applied Computing Ltd. for technical engineering software services. Matrix used Abaqus/Standard software from SIMULIA, the Dassault Systèmes brand for realistic simulation, to model the behavior of the C-Pulse cuff and balloon interacting with the aorta.
“The FEA analysis was an iterative process that required some very unique approaches because of the way our device worked, the materials we were using, and how the device is actually assembled,” says Miller. The balloon had to be easy to manipulate during implant surgery; conform to the shape of the aorta; have the strength and flexibility to “snap through” from concave to convex and back again repeatedly; compress the artery; and perform reliably from initial inflation through years of use—all within a very limited space. The goal of the FEA modeling was to accurately represent the real-world behavior of the device in order to guide design decisions and optimize the C-Pulse’s performance through every stage of this process.
Element and Materials Choices Are Critical
To determine what kinds of elements (the geometric shapes mathematically representing physical units that make up an FEA mesh) to use for modeling the artery, cuff, and balloon, Matrix created a series of test models. Quadrilateral shell elements turned out to be acceptable for the bulk of the parametric design studies (including determining the all-important optimum thickness of the balloon). But for modeling surface strains affecting the balloon in the fillet radius region (a critically important area where failures of the very earliest designs had occurred), hexahedron solid brick elements were chosen for more precise results using substructuring techniques with results from the shell model driving the solid element analysis (Figure 4).
The material modeling portion of the analysis was constrained by physiology and anatomy studies that had already been conducted. “We were given pre-existing data for the biocompatible material (a polymer approved for medical device applications) from which the device would be manufactured,” says Campbell. “The Ogden hyperelastic material model in Abaqus provided an excellent fit with the experimental data.” The Ogden model is often used to model rubber-like materials, such as polymers, and biological materials.
Modeling the ‘Snap Through’ Function
Matrix ran its simulations as quarter, not full, models, using the assumption of symmetry to cut down on processing time and aid solution convergence. “There were some approximations with the quarter model since an aorta is not a straight pipe, but has some curvature,” Campbell says. “However, for the purpose of optimizing the design, the lack of true quarter symmetry was thought to have a minimal effect on the ultimate design parameters. This approach also let us perform a large number of parametric runs in a reasonable amount of time.”
The ultimate goal of the FEA analysis was to arrive at a device shape which had the least variation of strain amplitude and the maximum mean compressive strain during an operational cycle. Says Campbell, “It was a project with interesting physics and the final model we came up with has performed very well in the test environment (Figure 5).”
FEA Provides Final Design Solution
And the durability of the C-Pulse design is being borne out by ongoing testing, Miller notes. “We have been running devices day and night literally for years now: the test machine requires regular maintenance because the C-Pulse keeps wearing the test unit out.”
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