The Road To A Permanent Synthetic Heart

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Heart disease is the leading cause of death worldwide. It is a complex and highly challenging medical condition that can significantly decrease quality of life, and also affects the progression of other diseases such as diabetes and kidney failure.

Patients with late stage heart failure may be able to receive a heart transplant, in which the heart from a deceased donor is used to replace their diseased heart. Unfortunately, the demand for new hearts greatly outweighs the number of hearts available for transplantation. This is why we need synthetic hearts that are able to permanently replace a diseased heart. This technology is still being developed, but it is hoped by many that such devices will one day become available.

The first artificial heart transplant took place in 1969. The device, developed by Texas Heart Institute founder Dr. Domingo Liotta, was implanted in a 47-year-old patient with severe heart failure, allowing them to live for nearly three days until a human heart became available. Current synthetic hearts are made from titanium and/or plastic, which can be made from biocompatible materials, meaning that it is chemically inert and is less likely to be rejected by the body’s immune system. This is an advantage over a real human heart transplant, in which the recipient’s immune system must be suppressed in order to avoid transplant rejection. Unfortunately, this is where the advantages of a synthetic heart end.

Artificial hearts are larger and sometimes heavier than a human heart. They also require a power source located outside the body that must be recharged periodically. Furthermore, an artificial heart will eventually fail and require replacement. Having an artificial heart also carries additional health risks. For example, synthetic heart valves increase the risk of blood clots forming, which can then become dislodged and can block arteries supplying the brain to cause stroke.

At the time of writing, only one fully artificial heart is available on the market. Made by US-based SynCardia, this heart has two artificial ventricles, each containing a balloon like membrane that divides it into two chambers: one for blood, and one for air. The air chambers are inflated and deflated by pulses of air from a battery powered external device carried by the patient. As the air-containing chambers in each ventricle are inflated, blood is ejected from the other ventricular chambers.

Recently, another fully artificial heart by French company Carmat was granted regulatory approval in Europe after years of setbacks. This heart is designed to more closely mimic a real human heart. Both the valves and the inner membranes within the heart’s ventricles are made from bovine heart tissue. It also contains pressure sensors that allow cardiac output to be adjusted in response to increased demand, such as when the patient is exercising. However, the device remains a stop-gap for a real human heart transplant and not a permanent replacement.

Current artificial hearts are inferior to human hearts because the materials from which they are made cannot accurately mimic human cardiac muscle. The contraction of cardiac muscle fibres within a real heart is influenced by a wide range of stimuli including blood pressure, hormonal signals and electrical signals from the brain and sinoatrial node – the heart’s natural pacemaker. Cardiac tissue also has the ability to repair itself and continue to operate indefinitely in the absence of major damage, whereas artificial hearts eventually need to be replaced.

For artificial hearts to offer a permanent solution that is not vastly inferior to heart transplantation, we may need to make hearts out of synthesised heart tissue. Some progress has already been made in this regard. In 2019, researchers were successful in 3D printing vascularised human heart tissue and small heart-like structures using induced pluripotent stem cells from patients.

However, simply making cardiac tissue is not enough – cardiac cells must also be capable of coordinating with each other to contract in unison. For this ‘artificial’ heart tissue to actually behave like real tissue, a critical density of cells is required. This problem was overcome in 2020, when researchers found a way to 3D print a tiny heart with sufficient cell density to allow cells to beat together.

Despite this promising progress, researchers still estimate that it will take at least 10 years for this approach to become viable for use in humans. It is still very exciting to think that in just a decade, hospitals could potentially be printing fully functional human hearts for transplantation.