Material Requirements for Plastics Used in Medical Devices
Vinny R. Sastri, in Plastics in Medical Devices (Third Edition), 2022
4.2.1 Steam Sterilization
Steam sterilization (also called autoclaving) is conducted in an autoclave which is a container that can withstand high pressure and temperature.11 The autoclave is filled with the products and devices that need to be sterilized. The product or device comes into direct contact with steam at high temperature and pressure for a specified period of time. Pressure serves as a means to obtain the high temperatures necessary to quickly kill microorganisms. Moist heat destroys microorganisms by the irreversible coagulation and denaturation of enzymes and structural proteins. After the required time has passed, the steam is released and the sterilized objects are removed. The entire batch processing cycle can take from 10 to 60 min. The four parameters associated with steam sterilization are steam, pressure, temperature, and time. Steam sterilization is inexpensive and has a high sporicidal effect with short application times. Steam sterilization is typically conducted at the hospital or clinical for reusable devices. Metal surgical instruments and glass products can withstand several sterilization cycles and can thus be reused for procedures several times. Plastics on the other hand may only be able to withstand anywhere from 1 to 2 cycles to serval hundred depending upon the material’s hydrolytic stability and temperature resistance.
Steam sterilization is generally carried out at temperatures between 121°C (250°F) and 134°C (273°F), under 15–30 psi (1.0–2.0 bar) pressure, between 10 and 60 min, depending upon the material and the type of organism to be inactivated. Table 4.3 gives typical steam sterilization conditions. The lower the temperature, the longer the exposure time required for sterilization. Reusable devices are exposed to several sterilization cycles as they are sterilized after each use. Materials used in such devices must be able to withstand the number of cycles specified for the device and still maintain performance, safety, and effectiveness.
Temperature (°C) | Sterilization Time (min) for 1 Cycle |
---|---|
132–134 | 3–10 |
121 | 8–30 |
115 | 35–45 |
111 | 80–180 |
It is important to remove all the air from the autoclave before introducing steam because air is heavier than steam and will reduce the steam concentration (and hence the effectiveness) of the sterilization. High-speed steam sterilization is conducted at higher temperatures (134°C/273°F) and shorter cycle times (between 3 and 10 min). High temperatures, along with moisture, will kill microorganisms. High-pressure steam first condenses when it comes in contact with the part while continuing to heat it. Appropriate time/temperature cycles are developed based on the type and the amount of load in the chamber to ensure complete sterilization and destruction of microorganisms. Steam should penetrate and reach all surfaces of the product for proper sterilization efficacy. Poor cleaning, improper moisture, impermeable packaging, or overpacking the autoclave chamber can reduce the effectiveness of steam sterilization.
The critical factors in ensuring the reliability of steam sterilization are (1) the right temperature and time, and (2) the complete replacement of air with steam (i.e., no entrapment of air). The steam cycle is monitored by mechanical, chemical, and biological monitors. Steam sterilizers usually are monitored by measuring and controlling the temperature, the time at the target temperature, and the pressure using chemical indicators. The use of appropriate biological indicators at locations throughout the autoclave is considered as the best indicator of sterilization. The effectiveness of steam sterilization is monitored with a biological indicator containing spores of Geobacillus stearothermophilus (formerly Bacillus stearothermophilus). More recently, parametric release methods have been used to evaluate the sterility of devices.12
Plastic materials that have a higher softening temperature than the sterilization temperature must be used when considering steam sterilization (Table 4.4). Plastics with lower softening points than the steam sterilization temperatures will warp and deform. Hydrolytic stability is another important consideration. Materials that have high heat distortion temperatures [like polycarbonate (PC), polyesters, and polyamides] might be prone to hydrolysis. Steam sterilization might not be the best option for such materials. Polymers like PCs have high heat distortion temperatures but fair hydrolytic stability. Thus they can be steam sterilized for 1–2 cycles only.
Polymer | HDT (at 0.46 MPa) | Steam at 121°C | Dry Heat at 135°C | Hydrolytic Stability |
---|---|---|---|---|
Polyolefins | ||||
HDPE | 80–120 | Fair | Poor | Good |
LDPE | 60–80 | Poor | Poor | Good |
UMHPE | 60–80 | Poor | Poor | Good |
PP* | 100–120 | Good | Fair | Good |
PP copolymers | 85–105 | Good | Fair | Good |
COC | 170 | Good | Good | Good |
PVC | ||||
PVC plasticized | 60–80 | Poor | Poor | Good |
PVC unplasticized | 90–115 | Good | Good | Good |
Polystyrene/Styrenics | ||||
Polystyrene | 70–90 | Poor | Poor | Good |
ABS | 80–95 | Poor | Poor | Good |
SAN | 95–105 | Poor | Poor | Good |
Acrylics | 75–100 | Poor | Poor | Fair |
Polycarbonates | 135–140 | Fair | Fair | Fair |
Polyurethanes | 50–130 | Poor | Poor | Poor |
Acetals | 145–160 | Good | Fair | Good |
Polyamides | ||||
Nylon 6, Nylon 66 | 170–220 | Fair | Fair | Poor |
Aromatic | 250–300 | Good | Good | Good |
Nylon 12, 10, 6/12 | 70–150 | Poor | Poor | Fair |
Polyesters | ||||
PET/PBT | 75–140 | Fair | Fair | Poor |
Copolyesters | 60–80 | Poor | Poor | Poor |
High-temperature thermoplastics | ||||
Polysulfones | 170–215 | Good | Good | Good |
PPS | 195–215 | Good | Good | Good |
LCP | 200–300 | Good | Good | Good |
PEI | 200–210 | Good | Good | Fair |
PEEK | 160 | Good | Good | Good |
Fluoropolymers | ||||
PTFE | 75–130 | Fair | Fair | Good |
FEP | 70 | Good | Good | Good |
ECTFE/ETFE | 115 | Good | Good | Good |
PVF/PVF2 | 140–150 | Good | Good | Good |
Biopolymers | 25–80 | Poor | Poor | Poor |
Elastomers | 20–40 | Poor | Poor | Fair |
Thermosets | 150–300 | Good | Good | Good |
- a
- Refer to Appendix for acronyms.
Sometimes products that have a higher softening temperature than the autoclaving temperature can warp or distort due to the release of molded-in stress.13 Molded-in stress is caused by the rapid cooling or improper design of the part. Heating the part relieves the molded-in stress, causing differential stress and hence deformation. Where autoclaving is to be used, the effect of multiple sterilization cycles needs to be considered to prevent cumulative effects of the treatment on the plastic. If the devices are to be packaged before autoclaving, then the packaging material and packaging method needs to be chosen carefully. The suitability of a package for autoclaving will depend on the material, the size of the package, the wall thickness of the package, and the contents. Autoclaving is used significantly in hospitals for the sterilization of multiple-use articles. It is not the predominant method in the commercial sterilization of medical devices because of the difficulties involved with autoclaving packaged products.
Most plastics will survive 1–5 cycles of steam sterilization. For those reusable devices that need up to 100 sterilization cycles, polysulfones, polyether sulfones, polyetherimides, polyether ether ketone (PEEK), and liquid crystal polymers (LCPs) are generally used. For applications that require more than 100 cycles, polyphenylsulfones, PEEK, and LCPs can be used. Polyphenylene sulfones can be used for up to 1000 cycles of steam sterilization.
The standard that governs the requirements for steam sterilization is ISO 17665-1.14