CYROLITE ®with balanced optical transmission and mechanical robustness offers chemical resistance to IPA, lipids, blood, disinfectants, and oncology drugs, necessary for safe BPA-free, infusion therapy applications such as filter housings.

A previous article discussed acrylic-based medical copolymers that are designed through extensive R&D to carefully balance properties and performance for healthcare applications. 1 More importantly, it emphasized the key role of polymer mechanical behavior in governing the chemical resistance of CYROLITE ®and delivered an understanding of the chemical attack, specifically environmental stress cracking. Here in Part 2, the key parameters for evaluating chemical resistance, understanding industry-wide testing discrepancies, and identifying opportunities for a unified and regulated approach for material evaluation are presented. A case study on chemical resistance testing of CYROLITE acrylic-based medical copolymers is presented following these key considerations against various chemical agents.2

Fig. 1 - Typical tensile stress-strain curve of plastic material showing ductile and brittle failure behaviors. ASTM-D543-14, Section 12 and ASTM D638-14 recommend ESCR testing using tensile testing.

Evaluating Environmental Stress Crack Resistance (ESCR) in Medical-Grade Polymers

Leading resin manufacturers offer products designed for ESCR applications in medical devices. They support performance metrics to quantify chemical resistance and cross reference material endurance with chemical agents.2-6 These compatibility matrices are insightful in understanding their material specific performance but could be highly subjective when screening through industry-wide choices. Foremost, the test methods to quantify compatibility against a chemical agent show disconnect and widespread practices across suppliers. Therefore, the medical device industry should combine input from resin producers, medical device manufacturers, and the application environment to define a regulated approach for measuring ESCR and corroborating material comparisons. As a result, the different resin solutions available for a given application would be assessed through the same metrics and ranked for performance/resistance against these chemicals. Key aspects to consider for universal harmonization and evaluating ESCR are discussed below.

Preconditioning. Foremost, polymer mechanical response to applied mechanical stress or strain depends on its chemical structure and morphology during polymer processing. Test specimens should be annealed to minimize the effect of molded-in stress (i.e., part design/geometry and external factors that are application dependent). Annealing and preconditioning are practices to establish comparable testing conditions and to eliminate specimen history (storage, processing). It is also critical to bring the material into equilibrium by establishing controlled temperature and humidity conditions before testing. Depending on the polymer, crystalline or amorphous, controlled conditioning ensures reproducibility and repeatability of analysis. Polymer-specific standards should be followed for conditioning before testing. For example, ASTM D-4066 stipulates the need for testing to be carried out on dry as-molded specimens for hygroscopic materials such as nylon. ASTM-D 618-13 specifies preconditioning the test bars for >40h, at 23 ± 2 °C, 50 ± 5% relative humidity before strain exposure.7

Fig. 2 - ASTM tensile type 1 exposed to three different strains at 0.5 percent, 1.0 percent, and 1.5 percent. The strain is determined primarily by the thickness of the tensile bar type, T, and the radius of curvature of the strain jig, R, as shown in the expression.

Measuring Mechanical Properties. Mechanical strength of medical plastics can be expressed using tensile, compressive, flexural, impact, fatigue, weathering, and other similar metrics, depending on the targeted application. For ESCR evaluation of medical plastics, the mechanical test type studied is varied, e.g., tensile or impact or flexural, etc.3-6 However, the property retention upon chemical agent exposure to establish compatibility is an industry-wide practice. The stress-strain curves as part of tensile testing provide the ductile or brittle response of the material along with modulus and elongation at break properties (see Figure 2). For tensile testing, ASTM-D618 lists tensile bar dimensions and tolerances for standard tensile specimens with regard to size and geometry. Moreover, the test parameters such as extensometer capacity and crosshead speed for testing require rigorous attention specific to a material tested. 8ASTM D543-14 provides guidelines for chemical exposure under strained environments and evaluating tensile property retention.9

Fig. 3 - Scenario to evaluate mechanical response of material upon strained exposure to a chemical agent. Compatibility assessment showing the stress-strain curves at variable strains (0.5 percent, 1.0 percent, and 1.5 percent) for duration and environment of exposure. The curves in different colors denote stress strain behavior for five test specimens tested from the same sample/product.

Percent Strain Loading and Chemical Agents for Exposure Parameters. ESCR testing measures the tensile property retention of material upon simultaneous exposure to a controlled strain and a chemical agent. The test assembly is designed to mimic the mechanical loads in use and simulate the internal stresses the material undergoes in the healthcare environment. Figure 3 shows control strain jigs at 0.5 percent, 1 percent, or 1.5 percent strain, with tensile bar held in place and exposed to chemical agent. Notice the increase in curvature as the strain is increased from 0.5 to 1.5 percent. Depending on the application, chemical exposure to numerous chemical agents is tested for the duration of exposure (typically ranging from 5 to 24 hours), to evaluate material response in a controlled environment.

Evidently, a material is likely to show different response against variable strain, chemical agents tested, and to the extent of exposure. More importantly these are critical variables in context to the application, whether it is short-term use as in the case of disposables, or long-term medical devices as in equipment housings. The current data reports don’t shed much insight on the selection of strain percent or the chemicals in context of the application and do not address the medical device manufacturer’s unmet needs.

Chemical Exposure Method and Duration of Exposure. ASTM D543-14 lists the wet patch method for ESCR testing against various chemical agents. The wet patch method allows for exposing the strained tensile bar to the chemical agent periodically — for example, every 30 minutes for a maximum duration of the test exposure. Resin manufacturers are discussing three approaches for exposure: periodic wipe method, wet patch exposure by periodic saturation, and continuous immersion by creating an isolated environment. Method comparisons are enlightening, revealing the potential variability in mechanical responses and property retention.

A periodic wiping test is reported to be more aggressive than continuous immersion for polycarbonate (PC) and PC blends and linked to the added stressors from the concentration build of high-boiling chemicals in the formulations.4 However, this does not guarantee the same mechanical behavior for all materials. Relevance needs to be established on the chemical exposure method with guidelines laid out for the dimensions of the patch, frequency of chemical agent reapplication, the setup in a controlled environment, and duration of exposure — all with respect to the chemical agent tested. Lack of a standardized methodology leads to inconsistency in data comparisons between resin manufacturers.

Fig. 4 - Compatibility assessment to include visual evaluation across the test specimens and proposed ranking system.

Evaluation of Test Results and Tensile Property Retention.The goal is to gauge representative behavior of the polymer under investigation. Property retention can be expressed in terms of one or a combination of tensile modulus, strength, elongation at yield, and break elongation characteristics. Undoubtedly, all of these present a unique piece of information about the mechanical characteristics of the material to the medical device manufacturer.

Table 1a. Tensile property retention for CYROLITE ®Med 2 after exposure to IPA/water (70 percent) for 5 hours at strain rates of 0.5, 1.0, and 1.5 percent. Property retention: >90% (green), 80–90% (yellow), <80% (red).
Table 1b. Tensile property retention for CYROLITE ®Med 2 after intralipid for 24 hours at strain rates of 0.5, 1.0, and 1.5 percent. Property retention: >90% (green), 80–90% (yellow), <80% (red).

A unanimous parameter(s) (modulus, strength, elongation at yield, and break elongation) should be published for ranking the compatibility matrix against chemicals tested in line with the geared application. Tensile strength and elongation at yield present closer relevance to materials deformation behavior under mechanical loads in use. 7Elongation at break measures the materials failure stress but often is misleading and shows the most scatter across the tested pool of tensile bars as shown in Figure 4. In addition, visual observations linked to onset and development of crazing or stress whitening of material during exposure should be recorded (see Tables 1a and 1b). Figure 4 and Tables 1a and 1b present the twofold scenario expressing ESCR output.

Case Study: Environmental Stress Crack Resistance (ESCR) Performance Testing

For CYROLITE Med 2, ASTM type 1 tensile bars molded from the material were annealed for 4 hours at 64 °C. The test bars were preconditioned for at least >40h, at 23 ± 2 °C, 50 ± 5% relative humidity prior to strain exposure (ASTM-D 618). Note: ESCR Upon Exposure to (70 percent) IPA/Water (ASTM D543-14) and Mechanical Property Changes (ASTM D638, Section 12).

The tensile bars were mounted on control strain jigs at 0.5, 1.0, and 1.5 percent respectively (see Figure 3). A wet patch saturated in 70 percent IPA was applied every 30 minutes for 5 hours. Tensile testing per ASTM D638 was performed on all specimens (5 replicates) and the percent change in tensile properties of each material reported for each reagent reported. Tensile property retention was measured using an extensometer at 10 percent based on 50 mm gage length at a cross head speed of 2 in./min. Tensile strength and modulus were reported to three significant figures with standard deviation reported to two significant figures. Elongation at yield (percent) and elongation at break (percent) were reported to two significant figures (ASTM D6436).

Table 2. ESCR for CYROLITE ®Med 2 after 1.5 percent strain and exposure to 10 different disinfectants for 24 h showing the tensile property retention (percent). Note: For formaldehyde and peracetic acid, CYROLITE ®Med 2 retained ≥90 percent property retention at 1 percent strain. Property retention: >90% (green), 80–90% (yellow), <80% (red).

Interestingly, the material shows tensile property retention ≥95 percent, specifically tensile strength, modulus, and elongation at yield, after exposure to IPA for 5 hours at all strain levels (see Table 2). Certainly, the high performance is a direct function of the composition and the careful formulation of components that present a compatibilized blend. It is essential to understand that the preconditioning enables sufficient segmental motion during annealing to potentially eliminate any residual molding stresses and offer a homogenous part with representative properties, hence yielding a ductile, toughened, and IPA-resistant material.

Exposure to Disinfectants

This test was conducted using Wet Patch Method (ASTM D543 - 14). Standard type 1 ASTM tensile testing specimens were mounted to strain jigs designed to apply a predetermined amount of strain onto the specimens. A saturated cotton patch (such as sterile gauze) was draped over them such that test specimen is in direct contact with the disinfectant reagent being tested. The length of exposure was 24 hours at room temperature. Three sets of measurement data were taken; a set of specimens (five replicates) under no strain and no reagent applied, strain and no reagent, and strain and each disinfectant reagent.

Table 3. Tensile property retention for CYROLITE ®Med 2 after 1.0 percent strain and exposure to 10 oncology drugs for 24 hours. Property retention: >90% (green), 80–90% (yellow), <80% (red).

Table 3 shows the impact of disinfectant exposure on CYROLITE Med 2 at 1.5 percent strain for 24 hours. The data shows excellent chemical resistance to 6 of 10 disinfectants tested, with tensile property retention ≥95 percent, and no visual signs of crazing or stress cracking. For the remaining four agents, the bars show ≥75 percent tensile property retention. The elongation at yield and break had ≥90 percent retention for all 10 disinfectants tested, indicating that the ductility of the polymer is maintained after chemical exposure and induced stress.

Exposure to Chemotherapy Drugs in Carrier Solvents

ASTM type 1 tensile bars were annealed at material-specific VICAT softening temperature for ~4 hours. These were mounted on jigs designed to induce a 1 percent strain and held for 24 hours. Five test bars were tested per chemotherapy drug in their carrier solvent for 13 different drugs by wet patch method.

Figure 8 shows the impact of chemotherapy drugs in their carrier solvents for CYROLITE Med 2. CYROLITE Med 2 shows excellent tensile property retention, ≥95 percent after exposure to chemotherapy drugs while strained at 1 percent for 24 hours. It is imperative to understand the impact of the carrier solvents these drugs are administered in; they can potentially pose a more aggressive interaction with the polymer material than the therapeutic drug component and lead to brittle catastrophic failures.

Conclusion

Ideally, the industry would benefit from both objective and standardized metrics to provide an overview of different product offerings with their respective pros and cons and performance thresholds. However, the feasible implication of these standardized metrics would require industry-wide harmonization, collaborative partnership, and a defined framework for assessing the material offerings industry-wide.

This article has presented the key considerations for evaluating ESCR, thereby mitigating the aforementioned inconsistencies in testing and highlighting the need to meet customer needs. Unifying the testing methods in line with the proposed key considerations would present the medical device manufacturer with an objective selection process. Moreover, the standardized approach would synergize the therapeutic measures in healthcare with the protection offered through advanced material solutions from evolving harmful disease-causing pathogens.

References

  • Bajaj, P., Wright, K. D., Bernhard, K., Heyl, D., Biagini, M., Sneeringer, A., Zadrozny, M., CYROLITE ®Impact modified acrylic copolymers Part I – Understanding chemical resistance, Medical Design Briefs, May 2021.
  • Bajaj, P., Wright, K. D., Bernhard, K., Heyl D., “Chemical and Environmental Stress-Crack Resistance of PMMA-Based Compounds in the Medical Environment,” Kunststoffe International, 2019: 3, 12 – 14.
  • Nowatzki, P. J., “ Chemical Resistance testing of Polycarbonates and blends with Hospital Disinfectants and Cleaners,” Antec 2020.
  • Chemical compatibility with hospital disinfectants and oncology drugs, Eastman.
  • Chemical Resistance Performance Testing for Healthcare Materials, SABIC ASTM D618 – 13 Standard Practice for Conditioning Plastics for Testing.
  • Röhm CRYOLITE.
  • ASTM D638 – 14 Standard Test Method for Tensile Properties of Plastics.
  • ASTM D543 – 14 Standard Practices for Evaluating the Resistance of Plastics to Chemical Reagents.
  • ASTM D6436 – 14 Standard Guide for Reporting Properties for Plastics and Thermoplastic Elastomers.

This article was written by Pooja Bajaj, PhD, Product Development Manager – BU Molding Compounds, Medical; Kierra D. Wright, PhD, Product Development Manager – BU Molding Compounds, Medical; Kay Bernhard, Director of Global Technical Competency Center – Medical; Dirk Heyl, Technical Marketing Manager – Medical; Maurice Biagini, Global Business Director – Medical; Andrew Sneeringer, Technical Marketing Specialist – Medical; and Michael Zadrozny, Strategic Account Manager – Medical, Roehm America LLC, Parsippany, NJ. For more information, visit here .


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This article first appeared in the June, 2021 issue of Medical Design Briefs Magazine.

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