Material Marvels: Synthetic rubber and the artificial heart

Materials World magazine
,
1 Oct 2017

James Perkins investigates the surprising connection between yoga pants and artificial hearts. 

As Joseph Shivers worked on his segmented polyurethane recipe in the 1950s, his vision for the material – later to be known as Spandex, or Lycra – was intended for application in the textile industry as a replacement for natural rubber.

The elastic fibre that came out of the DuPont Laboratory in Virginia, USA, indeed created an apparel – and a sexual – revolution, and it also went on to be the material that made permanent artificial hearts possible.

As the Spandex-loving Madonna burst onto the scene with her seminal album ‘Like a Virgin’ in 1984, researchers from Penn State University, USA, were close to gaining US Food and Drug Administration (FDA) approval to implant their artificial heart into the first human patients. And it was a segmented polymer, directly descended from DuPont’s Lycra, that allowed the Penn State heart, otherwise known as the Pierce-Donarchy Ventricular Assist Device, to become such a breakthrough in healthcare.

William S. Pierce and James H. Donarchy received US patent #4,222,127 on 16 September 1980 and FDA approval was granted the same year, after around a decade of development. 

Anthony Mandia became the first recipient of the Penn State heart on 18 October 1985 at the Hershey Medical Centre. He survived on the device for 11 days until a donor heart was found, but died on 14 November from complications linked to an infection along his chest incision. 

Since then, there have been thousands of recipients of not only the Penn State heart, but many other alternatives, which share a dependence on segmented polyurethane.

Curve-hugging, life-saving

As the name suggests, segmented polyurethane comprises well-defined segments of hard and soft polymers linked along a macromolecular backbone that work together to give the material its signature elasticity and toughness. 

The soft component, a macro glycol, has a low melting temperature, while the hard section, a diisocyanate combined with a chain extender. The
soft segments are described as elastomeric springs, while the hard segments are short rigid units, which provide strength.

It was the perfect material to line the inside of the plastic Penn State heart, in order to protect the delicate blood that it pumps and overcome the problems seen in previous man-made human hearts.

The first total artificial human hearts

In 1968, a relatively primitive-looking device called the Liotta-Cooley Artificial Heart became the first artificial heart to be inserted into a human. It kept Haskell Karp alive for three days as he waited for a human heart replacement. Sadly, he died two days after receiving his heart. 

The implantation of the Liotta-Cooley device also set off one of the greatest feuds in medical history, but that is another story. It was only used once more, in 1974, because, as a bridge to transplant, it was superseded by the Jarvik-7. 

The Jarvik-7 is considered the first permanent pneumatic total artificial heart. It was used in a human for the first time in 1983, when surgeons at the University of Utah, USA, used it to save the life of Seattle dentist Dr Barney Clark, who survived for 112 days on the machine.

Clark was reportedly in a confused state for much of his time on the artificial heart and he asked to be left to die several times. All 100 patients that received the heart before the FDA issued a moratorium on its use reportedly suffered from severe discomfort.

Just like many of the people who received the Jarvik-7 heart, Clarke suffered from strokes due to blood clotting caused by the seams within the artificial heart and crevices on the surface of the polyvinyl chloride contained within.

This is where the Penn State heart and its use of segmented polyurethane was such a breakthrough. The device had no seams, and the polymer that lined the inside of the plastic housing was exceptionally smooth, strong and tough, and had incredible elastic properties.

According to the American Society of Mechanical Engineers (ASME), the pump pioneered the application of fluid mechanics principles to blood pump development, and the use of segmented polyurethane was vital to the machine.

The Penn State heart, which was made entirely of plastic, was the first ‘extremely smooth, surgically implantable, seam-free pulsatile blood pump to receive widespread clinical use,’ according to the ASME. 

Another critical factor that set the Penn State Heart apart from the Jarvik-7 was that it was able to automatically adjust the flow of blood as the patient undertook various activities such as sitting up, lying down and walking. The Jarvik-7 had to be adjusted to allow the patient to sit up in bed. 

Again, the strength and elasticity of the segmented polyurethane was key to this improved feature of the heart. So what made this material so important for
the successful deployment of the first permanent artificial hearts?

Early research showed promise

Even in 1968, segmented polyurethane had been identified as having potential in biomedical devices in the paper, Segmented Polyurethane: a polyether polymer. An initial evaluation for biomedical applications, John Boretos and William Pierce, from the National Heart Institute, USA, described how the elastic fibre that was developed at DuPont was a direct predecessor for the biomedical material, and that DuPont was working with scientists on developing the material for biomedical applications at that time.

The material had ‘exceptional resistance to oils, hydrolysis, oxidation, and thermal degradation’, they wrote. In addition, it was able to recover after stretching and had resistance to flex fatigue.The researchers tested medical tubing made of the material, which needed to be thin to minimise motor loading. Elastomers with low moduli of elasticity had failed because the pressure caused enlargement at the outlet ends – a high modulus of elasticity was required at low strains. 

In testing on calves, there were no failures or changes in tensile properties recorded in 22 implants over nine days, whereas silicone rubber failed after only two days and suffered a 50% loss in stress at 100% elongation. 

Pierce and Boretos, who noted that there were four teams from the USA working with the material on biomedical applications in 1968, recommended that a comprehensive study should be conducted to determine the long-term stability of segmented polyurethane when implanted under the skin.

It goes without saying that other researchers took them up on their offer. SynCardia, which can directly trace its roots back to the Jarvik-7, is today the leading manufacturer of total artificial hearts, and segmented polyurethane remains a vital component. 

SynCardia describes segmented polyurethane as a ‘critical’ material for production of its artificial hearts, and acquired the formula, reactor and manufacturing equipment needed to produce the material in 2011.

‘Fatigue resistance, strength and biocompatibility make the segmented polyurethane solution ideally suited for the blood contacting and flexing components of the SynCardia Heart and other medical devices. For over thirty years it has been used for the housings, diaphragms and connectors of the Total Artificial Heart’, as noted in a company press release.

In addition to SynCardia, the Dutch medical multinational, Koninklijke DSM, also manufactures the BioSpan segmented polyether polyurethane for biomedical applications, including artificial hearts, touting the ‘extraordinary flex life’ that allows it to withstand millions of flex cycles. 

The improvements made to the manufacture of segmented polymers have helped increase the performance of artificial hearts. In 1990, the longest surviving patient lived with a Penn State Heart for a total of 390 days.

At that time, Penn State supported 55 patients at the Milton S. Hershey Medical Centre, and there were a further 200 patients using the heart worldwide, 35% of whom were able to survive because of the device.

Today, instead of having to remain in hospital, the patients can go about their daily lives, and can live at home, with the heart plugged into a battery pack, or straight into the wall.

As of April 2017, Nurullah Balik, 61, is the longest surviving patient worldwide with a total artificial heart. He has been living at home with the SynCardia heart for more than 1,700 days as he waits for a donor heart.

‘I am doing really well,’ Balik, who lives with his wife in the town of Yatagan, Turkey, is reported as saying in a SynCardia press release. ‘I can take care of myself without any limitations. I wear the driver on my shoulder. I go to the bazaar, the shopping mall... My apartment is on the fourth floor, no elevators. I go up and down every day without any problem,’ he says.

Despite the advances in materials science, it is likely that total artificial hearts such as those made by SynCardia will remain bridges to transplant, even if they last for many years. 

The next breakthrough will be the construction of synthetic organs using organic materials, and researchers have already created heart tissue in the lab. But until then, segmented polymers will continue to play a vital role in giving heart disease sufferers a better chance at life. It’s just one more thing to admire about the marvel that is Spandex.