Environmental stress cracking (ESC) has long been an issue for medical devices. One of the leading causes of plastics failures, it typically begins with small cracks in the material and can eventually lead to full-blown disintegration. And while materials scientists have made great strides in countering the problem, today’s fast-evolving healthcare environments leave little room for complacency.

“Environmental stress cracking is always an issue,” says James Runt, professor of polymer science at Pennsylvania State University. “If you put a non-biological material, be it a polyurethane or anything else, into a biological system, the body will see that material as a foreign object, so there is the possibility of oxidative degradation. The material needs to be very stable in that environment.”

The phenomenon was first described in 1959, when researcher JB Howard called it “failure in surface-initiated brittle fraction of a polyethylene specimen or part under polyaxial stress in contact with a medium in the absence of which fracture does not occur under the same conditions of stress”. In simpler terms, ESC is the product of two conditions – mechanical stress, alongside exposure to an aggressive chemical.

Prior to Howard’s research, polyethylene (the most commonly used plastic) was thought to be inert to all liquids. Unfortunately, this is not the case. After a litany of failures, mostly involving chemicals like hydrofluoric acid and methanol, manufacturers turned their attentions to making polymers more resilient.

One of the early movers in this field was Bell Labs, which had been asked to investigate a mysterious cracking in Western Electric’s cables. The company not only determined the cause (namely, the lubricating soaps used during installation) – it also created the first test for environmental stress cracking resistance (ESCR).

While this improved matters somewhat, it did not consign ESC to the past. Decades after the problem was identified, there was little consensus surrounding the appropriate way to test for ESCR, alongside a lack of understanding about the molecular mechanisms at stake. As new materials were introduced for new applications, full ESCR remained elusive.

Take the thermoplastic polyether urethanes (PEU) introduced in the late 1970s. Used for insulation in cardiac pacing leads, these materials benefitted from a combination of mechanical strength and biocompatibility. Unfortunately, by
the early 1980s, some PEU-insulated leads had failed, showing that, despite their robustness overall, they were subject
to biodegradation.

Causes of cracking

Even today, ESC poses challenges. In 2013, 27 medical equipment manufacturers were interviewed at the National Teaching Institute & Critical Care Exposition in Boston. 13 of them reported having experienced cracking problems with polymer coatings, with five of those describing the problem as severe.

Because there are two factors at stake – the mechanical stress and the chemical agent – materials scientists need a very good awareness of exactly how their material will be used. After all, a device might function perfectly when exposed to one chemical, and easily crack in the presence of another.

“Environmental stress cracking is always something you have to be aware of and test for in advance,” says Runt. “Yes, you can do in-vitro testing, but the proof of the pudding is what happens when you do animal testing and, more importantly, human testing further on. It’s potentially a huge issue, but it’s gotten better over the last decade or so, as new materials and materials strategies have developed.”

ESC occurs when the chemical enters into the molecular structure of the plastic. While the molecules themselves are not affected, the chemical may interfere with the forces binding the molecules together. This causes the polymer chains to disentangle, which initiates a microscopic crack. As the crack spreads throughout the polymer structure, the material sustains a brittle, fast-developing fracture.

It is easy to see why medical devices might be susceptible. For starters, there are those – like cardiac-lead insulation materials – that are implanted within the body. Here, the polymer faces a complex range of stresses, biochemical and biomechanical in nature. It can be hard to gauge how the material will perform once in contact with blood and bodily tissues.

Then there are non-implantable devices, which, while simpler to test, certainly aren’t immune from ESC. At some point during their life cycle, they will likely be exposed to abrasive chemicals, most commonly in the form of disinfectant.

“One has to assess the stability of that material to that disinfectant, or its operation in a particular fluid,” says Runt. “Does it swell, does it take up the fluid in any way, does it react chemically and does that affect the performance of the material? This is the sort of thing people in the polymers community are really concerned about. You need to make sure the cleaning procedure is not going to chemically destabilise the material.”

This form of ESC has come to prominence in recent years, as hospitals grow more vigilant about their cleaning procedures. Healthcare-associated infections (HAIs) pose an enormous challenge for healthcare providers, with around one in 25 hospital patients acquiring an HAI on any given day. This means no hospital can afford to have poorly disinfected equipment, and facilities are cleaning their devices with more powerful chemicals than ever before.

As the CDC recommends: “Medical equipment surfaces… can become contaminated with infectious agents and contribute to the spread of HAIs… Ensure that at a minimum, noncritical patient-care devices are disinfected… after use on each patient.”

If materials are not appropriately selected for the healthcare environment, the frequent application of cleaning chemicals can cause device enclosures to crack prematurely.

The upshot, unfortunately, is a greater risk of ESC, since not all plastics were designed to withstand this level of chemical exposure. While the scale of the problem is difficult to get a handle on, there have been various recall notices issued over the years associating device failures with disinfectant. In 2009, for instance, a surgical light was recalled because “cracks may form around the screw connections”. This cracking was “significantly influenced by the use of certain disinfectants containing alcohol.”

Joint efforts

In 2016 the thermoplastics company SABIC collaborated with PDI, a supplier of infection-prevention products, to test how well their materials withstood exposure to PDI’s Super Sani-Cloth wipes. Several of its product technologies (including its LEXAN EXL polycarbonate resin and XYLEX PC/polyester blend resin) were shown to deliver improved compatibility.

“By guiding manufacturers towards plastics that are better suited for the specific disinfecting requirements of each medical device, our study benefits both medical device manufacturers and healthcare providers,” says Cheryl Moran, senior director of portfolio management, PDI Infection Prevention. “[This ultimately benefits] the patient, who can be protected from potential adverse events resulting from damaged or improperly disinfected equipment. Continuing our collaboration with SABIC and medical equipment manufacturers will enable even further insights as additional technologies emerge.”

This study, one of the first of its kind to be undertaken, underscores the importance of joint endeavours. It is difficult, after all, to design an optimal medical device if you are not quite sure what environmental stressors will be applied.

“Combatting HAIs is greatly important for hospitals, but if materials are not appropriately selected for the healthcare environment, the frequent application of cleaning chemicals can cause device enclosures to crack prematurely, which can lead to increased maintenance costs for healthcare providers,” said Cathleen Hess, healthcare business leader for SABIC.

Equally critical are collaborations between industry and academia. Runt’s own work, while not solely medical in nature, has led him to consult for various biomedical companies, such as St Jude Medical (now Abbott). The company provides artificial heart valves, cardiac pacemakers and other implantable products for heart-valve disease, many of which run the risk of ESC.

“They use polyurethanes in many applications, so we used our polyurethanes expertise to bring more insight into the materials that they were developing,” he says. “Roughly a decade ago, polyurethanes were developed with so-called ‘soft segments’ containing silicone. This was a significant development when it comes to creating biostable materials for use in the human body, and I believe St Jude Medical is using one of these materials in its current generation of pacemaker leads.”

The material in question, Optim insulation (OPT), was introduced to the market in 2006. It combines the strength and robustness of polyether urethanes with the biochemical stability of silicone, and can therefore be used safely in human patients.

“It has been shown clinically to be very stable against oxidative degradation, which gets right to the heart of environmental stress cracking,” says Runt. “If you have chemical degradation beginning at the surface, or anywhere in your device, you’re going to lose your mechanical properties and then environmental stress cracking becomes a huge issue.”

OPT is not the only material that can be used for this purpose. In fact, today’s manufacturers employ a range of approaches to give their materials long-term stability, no matter what the conditions of use. In Runt’s view, the last ten years have spelled significant advances in the field.  All this said, certain aspects of ESC remain poorly understood. Runt feels that while the chemical side of the equation has been thoroughly researched, the mechanical side has been neglected – a troubling oversight, given that fatigue is responsible for 90% of all structural failures. This is particularly important in a biological environment, where a heart pump, for instance, might be required to flex millions, or even billions, of times.

“Something that is not often discussed in the literature is ‘how do these materials respond to fatigue loading?’” he says. “It’s well known that cumulative damage in a material can lead to progressive failures, so if you’re creating a device where there’s going to be large-scale repetitive cycling, even at not that high a strain, understanding the long-term fatigue characteristics is really important.”

As scientists learn more about ESC – and, more importantly, how to avoid it – medical devices will likely become increasingly resilient. In the meantime, however, it seems clear that this is an issue for the whole industry. Solving the remaining problems will require input from materials scientists, medical devices companies and chemicals manufacturers alike, particularly as new polymer applications present themselves.

“As time has gone on, there’s been more and more literature developed on the sensitivity or stability of polyurethanes against environmental stress cracking, and we’ve learnt a lot more about what chemistries to use,”
says Runt. “Is [ESC] important? Yes. Is it an issue? Yes. Have people completely mitigated the issue? No. But have
we gotten better at understanding it, and improving designs and materials characteristics? Yes.”