Hardware-in-Loop Heart Simulator Takes Top Test System Honors

A team of researchers that built a simulated heart walked off with three design awards—including the cup for best in show—at this year’s National Instruments' Week in Austin, Texas.

David Keeling and colleagues from the University of Leeds in England's Intelligent Ventricular Assist Device (iVAD) design team are developing an implantable mesh “jacket” that will wrap a failing heart and rhythmically contract to help it beat.

Horizontal bands in the jacket are attached to the spindles of miniature DC motors that wind and unwind to tighten and relax the bands. The heart-circling bands must be tightened in a precise sequence that coordinates with the organ’s varying natural rhythms.

The behavior of the mesh, bands, and actuators is complex, though: system backlash and non-linear friction on the bands are just two of the factors that make it hard to model. A hardware-in-loop test system with a physical cardiac proxy was necessary.

The usual modeling approaches require either building a full artificial circulatory system or to using animal test subjects. The Leeds team rejected both approaches. Instead, they opted to build a metal heart to reproduce the mechanical characteristics of a beating heart and model the resulting blood flow (based on the displacement of the metal ventricles) in silico.

Keeling and company’s artificial heart is a spring cage hooked up to two linear actuators—one on the right side and one on the left—to flex the "heart” walls and mimic the alternating strokes of the right and left ventricles.  Pressure sensors arrayed around the mechanical organ measure the pressure exerted by the iVAD bands and feed the data into a field programmable gate array controller that checks the blood-flow model and directs the linear actuators to modify the shape of the heart accordingly.

The engineers model the blood flow as an electrical network. Each of the six blood-holding compartments in the model is characterized by pressure (resistance), vessel compliance (capacitance), and flow inertia (inductance). Modifying each of these parameters in each compartment allows researchers to simulate a wide range of conditions to mimic both disease and good health.

Cardiac assistance devices (like the one implanted in former Vice President Dick Cheney while he awaited a transplant) supplement a weak heart; they don’t replace it, as a total artificial heart does. Many designs of both types, though, put the prosthetic in contact with the patient’s blood supply. This can lead to inflammatory and clotting problems. They also require that lead lines (for electrical or fluid power) protrude through the skin, which can be a source of infection. The iVAD does not contact the blood, and its miniature DC motors can be powered non-invasively by induction through the skin.

The Leeds laboratory received Application of the Year honors, along with trophies for Best Life Sciences Application and a Humanitarian Award at National Instruments’ 7 August Graphical System Design Achievement Awards dinner.

Photos: University of Leeds

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