The potential for environmental stress cracking is a common concern when plastics are used in medical devices. According to materials expert Jeffrey Jansen of The Madison Group, environmental stress cracking (ESC) is considered a leading cause of plastic part failure. “It is the premature embrittlement and subsequent cracking of a plastic due to the simultaneous and synergistic action of stress and contact with a chemical agent. Because of the frequency and severity of ESC failure, it has been nicknamed ‘the plastic killer,’” says Jansen.1
Treatment in the intensive care unit and operating room frequently involves the use of lipids and other aggressive substances, including anesthetics like Propofol. Lipids and other aggressive solutions can initiate microcracks in the polymers used for the manufacture of disposable medical components such as luer connectors and stopcocks. These cracks can compromise their mechanical integrity. Luer connectors and traditional stopcocks may also crack from overtightening, especially when exposed to lipids. This, in turn, can lead to serious patient injury and can also jeopardize medical personnel. For this reason, current protocols in hospitals call for exchanging lipid sets after 24 hours of use.
Lipid-induced cracks and leakages are considered underreported adverse events or, they are mistakenly attributed to other circumstances. “As we have talked with nurses, we learned that very few report spills, leaks, and so on, to their managers,” says Christopher Friese, RN, PhD, an assistant professor at the University of Michigan School of Nursing in Ann Arbor, MI. “This reduces the ability for managers to troubleshoot current practice to improve safety. I get the sense nurses are embarrassed when spills occur,” he adds. “Just as we have focused on a blame-free culture in patient safety, it’s time to do the same for employee safety.”2
An additional important change hospitals are making is in the equipment they use. More and more hospitals are switching from outdated traditional equipment to safer, more modern products. Currently, hospitals limit the use of certain devices to 24 hours due to the eroding effects of lipids on frequently used raw materials. As they consider new equipment, they are looking for devices made from more resilient materials that will enable using a device for more than 24 hours. As healthcare demands increasingly safer and more effective devices, more reliable disposables are a must. Addressing issues such as lipid-induced cracking is an essential step toward reaching this objective.
Resin manufacturers have also been exploring options for the development of more robust resins that can better handle lipids and other aggressive solutions commonly administered, mainly through the IV route. Eastman Tritan™ copolyester, a new generation copolyester, has been shown in benchtop testing to be resistant to a large spectrum of medical fluids such as oncology drugs, lipids, drug-carrier solvents, and nutrition. Tritan minimizes the risk of cracking and thus increases patient safety.
According to Eastman, “Medical device engineers must therefore carefully consider a material’s chemical resistance to the carrier solvent when the device is expected to come in contact with oncology drugs. From a practical standpoint, testing carrier solvents may potentially reduce the cost and safety concerns of testing actual oncology drugs.”3 Figure 1 shows the results of tests conducted by Eastman comparing Tritan to a lipid-resistant polycarbonate.
Building in Safety
Elcam Medical develops disposable medical devices for specialized flow control needs, including for IV therapy and anesthesia. Ensuring patient and caregiver safety is paramount. To combat environmental stress cracking — in particular lipid-induced cracking — in these critical products, the company has incorporated Tritan into a new line of stopcocks. The four-way Elcam Medical SafeT™ stopcocks are designed specifically for IV use. They have a standard bore and a vented protector for the right end port. The SafeT stopcocks have been tested by Elcam Medical and proven to be resistant to lipids and isopropyl alcohol (IPA). An abstract and summary of this study is described below.4
Stopcocks that were manufactured with a body made of Tritan MX731 were naturally aged for one year, and then exposed to three cycles of gamma radiation (~80 kGy) and two cycles of ethylene oxide (EtO) sterilization. The stopcocks were then chained together by connecting stopcock to stopcock, through male-to-female luer connection, with a tightening torque of 20 oz-in. Side female luer ports were plugged with a cap (tightening torque of 20 oz-in).
The stopcocks were divided into two groups. One group was filled with Lipofundin MCT/LCT 20% (by B|Braun), an emulsion for infusion, and the second group was filled with 2-Propanol (IPA, by Biolab Ltd.). Using a sample size of 32 stopcocks, all of the stopcocks were tested after 48 and 96 hours for leaks by introducing the stopcocks to 15 minutes of 1.8 meter head while the Lipofundin and IPA solutions were still inside the stopcocks. It is important to note that the stopcocks involved in this test were subjected to treatments that represent severe conditions such as exposure to a concentrated lipid solution (20 percent) and three cycles of gamma radiation, which is known to have a degrading effect on polymers.
Results and Conclusions
None of the stopcocks leaked after 48 or 96 hours, even under strenuous testing conditions. It can be concluded that under reasonable industrial conditions and clinical use, e.g., storage at room temperature (~25 °C), exposure to one cycle of either gamma or EtO sterilization, and clinical use by authorized medical staff, the SafeT stopcock can be used for up to 96 hours even with administration of lipid-based solutions (see Figure 2).
The Tritan-based stopcocks appear to have a good chemical resistance that minimizes the risk of crack-induced leakages. Tritan is expected to prove especially reliable for use during administration of lipids. Reduced cracks and leaks allow for secured delivery of medications contained safely within the tubing. Furthermore, less cracking, as is expected with the SafeT stopcocks, can reduce subsequent potential risks of air embolism, bacteria ingress, and contamination. Hence, medical device OEMs can confidently incorporate them into their products for increased treatment safety.
This article was written by Eldad Ohayon, Stopcock and Manifold Product Manager at Elcam Medical, Hackensack, NJ. For more information, Click Here .
- “Plastic Failure Through Environmental Stress Cracking,” Plastic Engineering, November/December 2015.
- “Unintentional Exposure to Chemo High Among Nurses,” Medscape, Aug 26, 2011.
- “Chemical Compatibility with Hospital Disinfectants and Oncology Drugs,” MBS-962, Eastman, Kingsport, TN.
- Document #071716, Elcam Medical, Hackensack, NJ.