![]() Heating the part relieves the molded-in stress, causing differential stress and hence deformation. 13 Molded-in stress is caused by the rapid cooling or improper design of the part. Sometimes products that have a higher softening temperature than the autoclaving temperature can warp or distort due to the release of molded-in stress. More recently, parametric release methods have been used to evaluate the sterility of devices. ![]() The effectiveness of steam sterilization is monitored with a biological indicator containing spores of Geobacillus stearothermophilus (formerly Bacillus stearothermophilus). The use of appropriate biological indicators at locations throughout the autoclave is considered as the best indicator of sterilization. Steam sterilizers usually are monitored by measuring and controlling the temperature, the time at the target temperature, and the pressure using chemical indicators. The steam cycle is monitored by mechanical, chemical, and biological monitors. 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). Poor cleaning, improper moisture, impermeable packaging, or overpacking the autoclave chamber can reduce the effectiveness of steam sterilization. Steam should penetrate and reach all surfaces of the product for proper sterilization efficacy. 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. High-pressure steam first condenses when it comes in contact with the part while continuing to heat it. High temperatures, along with moisture, will kill microorganisms. High-speed steam sterilization is conducted at higher temperatures (134☌/273☏) and shorter cycle times (between 3 and 10 min). ![]() 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. 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. Metal surgical instruments and glass products can withstand several sterilization cycles and can thus be reused for procedures several times. Steam sterilization is typically conducted at the hospital or clinical for reusable devices. Steam sterilization is inexpensive and has a high sporicidal effect with short application times. The four parameters associated with steam sterilization are steam, pressure, temperature, and time. ![]() The entire batch processing cycle can take from 10 to 60 min. After the required time has passed, the steam is released and the sterilized objects are removed. Moist heat destroys microorganisms by the irreversible coagulation and denaturation of enzymes and structural proteins. Pressure serves as a means to obtain the high temperatures necessary to quickly kill microorganisms. The product or device comes into direct contact with steam at high temperature and pressure for a specified period of time. 11 The autoclave is filled with the products and devices that need to be sterilized. Steam sterilization (also called autoclaving) is conducted in an autoclave which is a container that can withstand high pressure and temperature. Sastri, in Plastics in Medical Devices (Third Edition), 2022 4.2.1 Steam Sterilization Material Requirements for Plastics Used in Medical Devices ![]()
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