New solution for energy transfer to heart pumps alleviates infections

Approximately 1 in 2 ventricular assist device wearers are diagnosed with an infection. The reason is the thickness of the power cable. Researchers at ETH Zurich have now developed a solution to alleviate this problem.

For many patients waiting for a donor heart, the only way to live a decent life is to use a pump attached directly to their heart. This pump requires about as much power as a television, which it draws from an external battery via a seven-millimeter-thick cable. The system is practical and reliable, but it has a big flaw: despite medical treatment, the cable exit point from the abdomen can be pierced by bacteria.

Andreas Kourouklis, a researcher and engineer at ETH Zurich, is working to ensure that this problem soon becomes a thing of the past. With the support of Professor Edoardo Mazza from ETH Zurich and doctors from the German Heart Center in Berlin, Kourouklis developed a new cable system for heart pumps that does not cause infections. The results are published in the journal Advances in biomaterials.

This is particularly important given that wireless power transmission methods remain unavailable to patients for the foreseeable future. Kourouklis received a Pioneer Fellowship from ETH Zurich to advance his technology.

Thin wires with craters instead of thick cable

“The thick cable used in existing ventricular assist systems creates an open wound that does not heal and seriously compromises patients’ quality of life,” says Kourouklis. Scar tissue with limited blood supply forms around the exit point. This not only impairs the skin’s ability to heal itself, but also increases the risk of infection.

Since the outer layers of skin are injured and weakly attached to the flat surface of the thick cable, they push downward. As a result, bacteria can move from the surface of the skin to deeper tissue layers, often causing patients to battle infections and be rehospitalized.

Researchers at ETH Zurich have developed technology to remedy the situation. Instead of powering the heart pump through a thick cable much stiffer than human skin, they use several thin, flexible wires with a rough, irregular surface. Kourouklis and his team liken their approach to the way human hair passes through the skin without causing infections. “More flexible threads whose surface is full of microscopic craters help the skin heal,” says Kourouklis.

The reason is that the outermost layers of the skin adhere better to these threads and do not push inward. New tissue forms more quickly and the skin is more likely to remain intact and provides a barrier against bacterial infections.

Water drops create tiny craters

To create craters on the surface of cables, a team of engineers led by Kourouklis and Mazza developed a new process to create very small irregular patterns on non-flat surfaces, which was not possible before.

This method, currently patented at ETH Zurich, consists of covering the flexible cables with a thin layer of silicone and cooling them to –20°C. The surface of the cables thus becomes malleable. They are then placed in a condensation chamber, where small water droplets are pressed into the liquid layer of silicone, creating microscopic craters. “We can control the position of the craters on the cables by adjusting the humidity and temperature in the condensation chamber,” explains Kourouklis.

The challenge here is that the craters can be neither too big nor too small. If they are too large, bacteria can settle there and the risk of infection increases; if they are too small, the skin does not adhere to them and grows inward, in which case the risk of infection also increases. A classic optimization problem, which Kourouklis and his team tackle using computational and experimental methods in tissue biomechanics and biomaterials.

First tests confirm lower risk of infection

Kourouklis and his colleagues performed initial tests on skin cell cultures before implanting the old, thick cables and their new cable system into a sheep. The results make the ETH Zurich researcher optimistic: while thick cables with a flat surface caused severe inflammation, thin, flexible cables only showed mild inflammatory reactions. No sheep suffered permanent injury during testing.

Most importantly, unlike thick cables, sheepskin integrates better with new cables and virtually does not push inward. As a result, thin cables with craters did not cause infections in animals.

Kourouklis is currently working with medical device engineers and cardiac surgeons to improve the cable system. Its goal is to commercialize the technology as quickly as possible. But before it can be used on heart patients, a series of tests on skin models, animals and possibly humans will be necessary.