Safety, performance of medical devices

Safety, performance of medical devices

Ventricle assist devices are developed to improve the lives of patients with heart failure and numerical simulation is used throughout the design process.

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Developing a medical device to assist or completely replace heart function is complex, and the design process involves immense challenges – from supplying power to the device to ensuring it does not interfere with normal biological functioning. St. Jude Medical researchers use Comsol Multiphysics simulation software to engineer left ventricular assist devices (LVADs) in an ongoing effort to improve the outlook and quality of life of patients with heart failure. In a patient with a poorly functioning left ventricle, an LVAD (Figure 1) can provide mechanical circulatory support – circulating the entire human blood stream to support life.

Design for biocompatibility

LVAD design must take many factors into consideration. The device must be:

  • Small enough to connect to the heart
  • Made of compatible materials
  • Geometrically suitable to reside in the body without rejection

Fluid dynamics, thermal management, and power supply must also be considered. As multiple interacting physical effects must be accounted for throughout development, multiphysics simulation is vital to the design process.

Figure 1: A left ventricular assist device (LVAD) pump is responsible for circulating oxygen-rich blood throughout the body. Image courtesy of St. Jude Medical.

Freddy Hansen, senior research and development engineer at St. Jude Medical, uses his expertise in physics and mathematical modeling to characterize the system before experimental studies.

“I use Comsol Multiphysics much like people used pocket calculators in the past. Some models are not too complicated. I can build one in a couple of hours and run it and get an answer. Others are quite sophisticated, and include computer aided design (CAD) models with a lot of detail. I’ll work with some complex models for months before I’ve taken all of the information I want from them,” Hansen says.

Figure 2: (a) The centrifugal pump of a left ventricular assist device (LVAD).

Hansen began using Comsol Multiphysics software in 2011, and has created upward of 230 models that address design challenges pertaining to the unique physics of artificial pumping devices.

With each generation of LVAD, improvements contribute to enhanced safety and quality of life. Research and development efforts at St. Jude Medical center on improving biocompatibility, hemocompatibility, and immunocompatibility, so the device does not illicit an adverse immune response or interfere with other bodily systems. Numerical simulation is key to incorporating these considerations into the final design.

Geometry and size of the device play an important role in its overall effectiveness. To implant the LVAD, the surgeon connects one end of the LVAD to the left ventricle and the other end to the ascending aorta. A smaller, less cumbersome device is less likely to interfere with neighboring organs or tissue. Simulation allows for the evaluation of changes in size or geometry before implementation in a physical prototype.

Figure 2: (b) Visualization of the magnetically levitated rotor, which eliminates the need for ball bearings and other components with geometries that might promote clotting. Here, the magnitude and direction of the magnetic field in the rotor, and the magnitude of the magnetic field in the stator are shown.

Blood clotting in spaces around LVAD centrifugal pumps was a major design challenge. To address this, engineers developed a magnetically levitated rotor, eliminating the need for ball bearings and other components with geometries that might promote clotting. Hansen used Comsol Multiphysics’ Rotating Machinery technology to model the magnetically levitated rotor and turbulent fluid flow.

In the stator, 12 coils drive a permanent magnet in the pump rotor. The coils exert torque on the rotor and provide active control of the position of the rotor axis. The vertical position (levitation) of the rotor is accomplished by magnetic field line tension and does not need active control. The rotor receives blood axially and redirects it radially, into the volute, or fluid collector (Figure 2a, 2b, 2c). Some of the blood backflows around the outer edge of the rotor and flows back into the rotor inlet, resulting in a constant washing of the blood, eliminating places where blood can stagnate and clot.

Another significant advance was the development of a pump with pulsatile flow rather than continuous flow, more closely mimicking a functioning heart. The pulsatile flow aids in the washing of the blood to prevent blood clots and is believed to have a positive physiological effect on blood vessels throughout the body.

Wireless power
Figure 2: (c) 3D computational flow dynamics (CFD) simulation depicting fluid velocity within the pump chamber.

Current LVADs require biocompatible cables to transfer power from external batteries in a controller outside the body to the pump. But what if the cable could be eliminated?

Hansen explored transferring power by way of magnetic resonance coupling, which occurs when two objects having almost the same resonance frequency transfer energy to each other through their oscillating magnetic fields. In this way, power can be transferred from a power source to another device, even through tissue.

A Fully Implantable LVAD System (FILVAS) would decrease infection risk and improve patient quality of life by eliminating cable management when showering.

To assess the feasibility of wireless power transfer to an LVAD and determine how power could be delivered between reasonably sized coils, Hansen coupled the simulation to a 3D magnetic field model and a 0D electrical circuit model to determine operating efficiency and power loss, as well as optimal circuit design and component values. He also used Comsol to evaluate different materials for components, such as the wires of the transformer coils, and to consider misalignment of a coil; the motions of coils due to patient walking, running, and other activities; and the presence of nearby magnetic or metallic objects. Hansen also used Comsol to ensure that body temperature and biological systems will not be affected by the implant.

The wireless power transfer system induces small currents in the body tissue near the coils (Figure 3). Hansen modeled the heat generated in the tissue as a result of the induced currents, combined this with models of heat generated inside the implant – in magnetic wires, electronics, and batteries – and then used the thermal conductivity coefficient that was determined from the simulated Cleveland Clinic study (see sidebar at bottom), to determine the temperature in body tissue near the implant.

Component strength
Figure 3: Fully implantable LVAD system early concept. Magnetic field and heat transfer model showing power density [W/m3].

Hansen also used numerical simulation to develop the external components of an LVAD. External LVAD controllers must be able to withstand the wear and tear of life, as well as the occasional dropping of the controller to the floor. To ensure that the controller and its crucial, life-saving batteries will continue to function even if the patient tosses it around, Hansen simulated a steel ball dropping on the controller to assess its resilience.

Hansen compared the amount of mechanical energy necessary to deform the device with the known amount of kinetic energy in the steel ball at the moment of impact to determine if the controller is sufficiently resilient (Figure 4). He also checked edges and surfaces of the deformed structural shell and frame for twisting that would imply that the controller would break. The analysis proved that the controller would continue to provide life-sustaining power to the LVAD even after a substantial impact.

Enhanced future treatment
Figure 4: Simulation of a steel ball impacting a LVAD controller to evaluate resiliency of the controller (top). Visualization of displacement along the vertical axis (bottom).

In designing devices to assist and replace the function of the heart, numerical simulation has proven to be essential. Hansen combines experimental characterization and mathematical modeling to thoroughly understand the physics of ventricle assist devices, and improve biocompatibility and overall patient experience.

The latest innovations to mechanical pumping systems – including a smaller device size, a more hemocompatible pump, introduction of pulsatile flow, and now the possibility of wireless power transfer – hold much promise for better treatment in the future.

Comsol
www.comsol.com

St. Jude Medical
www.sjm.com

Editor’s note: Thoratec, now part of St. Jude Medical, brought left ventricular assist devices (LVADs) to a wide market in 2010, after years of clinical trials.Abbott Laboratories completed its acquisition of St. Jude Medical in January 2017.

*Reference: C. R. Davies et al., “Adaptation of Tissue to a Chronic Heat Load,” ASATO Journal 40(3), p. M514-M517, 1994. https://goo.gl/tu2j5V

This article appeared in the March 2017 issue of Today's Medical Developments magazine.