An MD&DI August 1997 Column
Selecting the proper resin for an application can depend on the sterilization method to be used on the device.
Medical devices that come in contact with the body or bodily fluids must be sterilized before use. Which sterilization method is selected is important because thermoplastics generally react differently to various forms of sterilization. The physical alterations and color changes, for example, that occur in thermoplastics after gamma and EtO sterilization have been documented in several studies conducted by The Dow Chemical Co.14
This article examines how the three most popular forms of sterilization--EtO, gamma, and E-beam--affect the physical and visual properties of rigid thermoplastics. The analysis draws on a recent study that measured the strength and optical qualities of common resins before and after sterilization. The study focused on four different materials: general-purpose polystyrene (GPPS), acrylonitrile-butadiene-styrene (ABS), polycarbonate (PC), and engineering thermoplastic polyurethane (ETPU).
The resins listed in Table I were molded into standard ASTM Type 1 tensile bars (16.5 x 1.27 x 0.32 cm) and impact disks (5.08 cm diam x 0.32 cm thick). All resins were injection molded on the same machine using the manufacturer's recommended molding conditions.
Table I. Thermoplastics tested in this study.
Each resin was exposed to nine different sterilization conditions, including an unexposed control (see Table II). A normal sterilization condition was represented by 25 kGy of radiation for gamma and E-beam and one cycle for EtO; an extreme case of multiple exposures was represented by 100 kGy of radiation and three cycles of EtO. Two EtO systems were tested: the first used a carrier gas, HCFC-124, while the second used 100% EtO with no carrier gas.
Table II. Sterilization conditions in this study.
The samples were preconditioned at 80º110ºF, 4080% relative humidity, for 1224 hours before EtO sterilization. Each sterilization cycle began with a prevacuum of 24 in.Hg at 130ºF and relative humidity of 50% for 3045 minutes. The gas concentrations were maintained at approximately 600 mg/L for 3.5 hours of exposure. After primary processing, a vacuum of 81.3 kPa was achieved twice to complete the sterilization cycle. The samples were then aerated at 120ºF for 1218 hours to reduce the residual EtO.
All samples were subjected to the following tests both before and after exposure: tensile properties (ASTM D 638), notched Izod impact (ASTM D 256), and instrumented dart impact (ASTM D 3763). In addition to the physical tests, the transparent resins were tested for yellowness index (ASTM D 1925), and the opaque resins were tested for color changes using a Macbeth 1500-series colorimeter (ASTM D 2244).
Two studies by M. Sturdevant formed the boundaries for this work.1,2 Her long-term study demonstrated that most of the physical changes in thermoplastics caused by sterilization occur within the first 2 months after sterilization.2 The samples in this study were therefore tested within 1 week of sterilization to see the immediate property changes and 8 weeks after exposure to obtain long-term property effects.
Sturdevant's initial paper showed that thermoplastic resins exposed to gamma radiation discolored to varying degrees and that this discoloration decreased with time.1 This reduction in discoloration can be accelerated in certain resins by exposing them to fluorescent light, a process known as photobleaching. To study this phenomenon for gamma and E-beam, some of the resin samples were stored in the presence of fluorescent light, while others were stored in darkness. The samples were tested every 2 weeks to monitor visual changes.
Model depicting E-beam sterilization site.Rendering courtesy of Titan Scan Systems (Denver, Co)
Tables IIIVIII list the selected physical properties of the control samples and the properties after exposure to gamma, E-beam, and EtO sterilization. The results of the tensile tests are listed under "Tensile Yield" and "Elongation"; the results of the instrumented dart impact test, which uses a dart to puncture the sample, are given in the last column labeled "Peak Energy." Average standard deviations are also listed, and any exceptions to the averages are noted in parentheses. Figures 15 show the color changes observed in the plastics after gamma sterilization.
Table III. Selected physical properties of Styron 666D GPPS before and after sterilization.
Table IV. Selected physical properties of Magnum 2642 ABS before and after sterilization. Exceptions to averages are noted in parentheses.
General-Purpose Polystyrene. Polystyrene is commonly used for in vitro diagnostic products such as petri dishes, tissue culture components, flasks, multiwell trays, pipettes, and roller bottles. GPPS has good stability when exposed to high-energy radiation. The tested physical properties of GPPS did not significantly change after exposure to any dose of either form of radiation (see Table III). As shown in Figure 1, GPPS is also color stable; after 25 kGy of gamma irradiation, no significant discoloration is seen. At 100 kGy, some yellowness is observed immediately following sterilization, but this discoloration fades within a week. Similar color changes are seen after E-beam sterilization.
Figure 1. Color stability of general-purpose polystyrene after gamma sterilization.
The tested EtO sterilization systems did not significantly affect the tested physical properties of GPPS; however, upon visual inspection, the exposed samples had some surface crazing, seen as shiny, silver streaks. In extreme conditions, crazing can contribute to a decrease in physical properties. Ethylene oxide is a known aggressive chemical agent to styrenic resins, and while GPPS can be used with EtO sterilization, care should be taken to minimize its exposure.
Surface crazing following GPPS exposure to 100% EtO sterilization.
Acrylonitrile-Butadiene-Styrene. ABS is typically an opaque resin, available in high- and low-gloss versions, offering easy processing and good impact resistance. ABS is used in surgical staplers, IV connectors, and various medical housings. The resin tested for this study was a natural low-gloss ABS. After gamma or E-beam sterilization, the Izod impact of low-gloss ABS decreases (see Table IV). Izod impact strength, which measures notch sensitivity, decreases by approximately 20% after 25-kGy cycles and 30% after 100-kGy cycles. Instrumented dart impact testing shows minimal reduction after gamma or E-beam sterilization. The reduction of impact strength in ABS resins is attributed to radiation cross-linking of the butadiene rubber phase, which reduces its ductility.1,2
ABS parts show discoloration after radiation sterilization (see Figure 2). After two weeks, the tested ABS samples exposed to 25 kGy of gamma irradiation return to their original color, while the 100-kGy samples retain a visible yellow hue. When the low-gloss ABS samples are exposed to fluorescent light, the reduction in color is accelerated.
Figure 2. Color stability of natural ABS after gamma sterilization. "Light" samples were stored beneath fluorescent lights, and "dark" samples were stored in darkness.
Figure 3. Color stability of polycarbonate after gamma sterilization. "Light" samples were stored beneath fluorescent lights, and "dark" samples were stored in darkness.
Figure 4. Color stability of polycarbonate after gamma sterilization. "Light" samples were stored beneath fluorescent lights, and "dark" samples were stored in darkness.
Figure 5. Color stability of engineering thermoplastic polyurethane after gamma sterilization.
According to the test results, the ABS samples retain their impact properties and tensile yield after EtO exposure. There is a decrease in tensile elongation, but the significance of the change is masked by large standard deviations. The ABS samples exposed to repeated EtO cycles show minor surface attack.
Polycarbonate. Polycarbonate is available in transparent and opaque grades and is generally characterized by dimensional stability, toughness, and heat resistance. PC is used in cardiotomy reservoirs, hemodialyzers, surgical equipment, and safety syringes. Two PC resins were tested, and the data are shown in Tables V and VI. Calibre MegaRad 2081 (PC-2081) is commonly used for medical devices intended for high-energy sterilization, while Calibre 2061 (PC-2061) is used when EtO is the preferred method. The first resin was tested only for gamma and E-beam radiation sterilization, the second only for EtO.
Table V. Selected physical properties of Calibre 2061 PC before and after sterilization.
As Table VI indicates, the tested physical properties of PC-2081 are not significantly affected by radiation sterilization, and this material can easily withstand up to 100 kGy of gamma or E-beam irradiation.
Table VI. Selected physical properties of Calibre 2081 PC before and after sterilization.
After exposure to high-energy radiation, the transparent PC samples, which had an initial purple tint, became smoke gray. When these materials are sterilized, the color and radiation dosage must be coordinated, or the resulting resin will either be purple (too much tint for the irradiation level) or yellow (too little tint for the irradiation level). No visible color differences were found between the samples exposed to gamma and those exposed to E-beam. As shown in Figures 3 and 4, after gamma sterilization, polycarbonate resins are very sensitive to photo-bleaching; within one week, fluorescent light greatly increases the color reversal of the samples.
Results of gamma sterilization on polycarbonate. Similar changes are seen after E-beam sterilization.
The tested physical properties of PC-2061 are not significantly changed after exposure to either type of EtO sterilization system.
Engineering Thermoplastic Polyurethane. ETPU is used in drug-delivery systems, surgical instruments, and catheters. The material offers good chemical resistance and toughness and is available in transparent, opaque, and glass-filled grades. The two resins tested for this work are transparent resins. Isoplast 2531 was only tested for gamma and E-beam sterilization; Isoplast 2530 was only tested for EtO.
The tested physical properties of ETPU resins are relatively unaffected by high-energy sterilization (see Tables VII and VIII). ETPU does, however, show a large amount of color change after gamma or E-beam sterilization. Figure 5 shows the yellowness index of ETPU after gamma sterilization. The yellowness decreases with time, but significant discoloration remains. Overall, ETPU is not significantly affected by the tested EtO gases.
Table VII. Selected physical properties of Isoplast 2530 ETPU before and after sterilization. Exceptions to averages are noted in parentheses.
Table VIII. Selected physical properties of Isoplast 2531 ETPU before and after sterilization.
After high-energy sterilization, all the resins tested in this study showed some discoloration. Both gamma and E-beam techniques produced similar effects. After exposure to 25-kGy radiation, GPPS and ABS returned to their original colors after a period of time. The notched Izod impact for ABS decreased after gamma and E-beam sterilization. Repeated EtO cycles using 100% EtO or EtO and HCFC-124 as the carrier gas had minimal effects on the physical properties of the tested materials.
1. Sturdevant M, "Sterilization Compatibility of Rigid Thermoplastic Materials," SPE/SPI Medical Regional Technical Conference Proceedings, October 1988.
2. Sturdevant M, "Long-Term Effects of Ethylene Oxide and Gamma Radiation on the Properties of Rigid Thermoplastic Materials," Society of Plastics Engineers Annual Technical Conference Proceedings, May 1990.
3. Hermanson N, "The Physical and Visual Property Changes in Thermoplastic Resins after Exposure to High Energy Sterilization--Gamma Versus Electron Beam," Society of Plastics Engineers Annual Technical Conference Proceedings, May 1993.
4. Hermanson N, "Effects of Alternate Carriers of Ethylene Oxide Sterilant on Thermoplastics," Society of Plastics Engineers Medical Regional Technical Conference Proceedings, October 1991.
Nancy J. Hermanson, Lisa Navarrete, and Pat Crittenden work in technical service and development for Dow Plastics, a business group of The Dow Chemical Co. (Midland, MI).