Biomedical Engineering Reference
In-Depth Information
surgical equipments and intravenous fluids. However, it is not the predominant
method in the commercial sterilization of medical devices due to the difficulties in
autoclaving-packaged products. Also, heat sterilization is applicable only if dam-
age by heat and/or moisture is not a problem. The high temperature, humidity, and
pressure used during the steam sterilization process can lead to hydrolysis, soften-
ing, or degradation of many biopolymers. Hence, it may not be suitable for the
sterilization of many polymers.
6.3.5.2 Sterilization by Gases
The most widely used gas in medical applications to sterilize heat sensitive items is
ethylene oxide (EtO). The EtO sterilization procedure involves:
1. Creating a vacuum in the sterilization vessel;
2. Injecting EtO at a desired concentration (typically around 600-1,200
mg/L);
3. Maintaining the device at the desired conditions (typically 30-50°C and
40% to 90% humidity for 2 to 8 hours);
4. Evacuating the chamber several times to remove residual EtO.
Furthermore, aeration is usually required after removal from the chamber, with
aeration time ranging from 2 hours to 2 weeks, depending on the device and pack-
aging. EtO has advantages such as low processing temperatures, high penetration,
and compatibility with a wide range of materials. The main disadvantage of EtO
relates to its flammability, explosive nature, residual toxicity (EtO and its secondary
products, i.e., ethylene chlorohydrin and ethylene glycol may remain in the medical
devices after sterilization), and suspected carcinogenicity of the gas and residuals in
the product and manufacturing environment. To reduce its toxic effects and flam-
mability, EtO is usually mixed with inert gases, such as fluorinated hydrocarbons
and carbon dioxide. Furthermore, to prevent residual toxicity, after the steriliza-
tion by EtO it is essential to allow the residual gas to dissipate from the material
to acceptable levels prior to its usage. Methods for rapidly removing absorbed EtO
are based on exposing the materials to repeated air washings, exposing the load to
forced air circulation in a closed chamber, or placing the exposed material in well-
ventilated areas for long periods. In order to overcome some of these drawbacks,
more strict regulations have been established concerning EtO sterilization, such as
the American National Standard ANSI/AAMI ST27-1988 (guidelines for industrial
ethylene oxide sterilization of medical devices). Gas plasmas are increasingly being
employed to sterilize the surfaces of medical components or devices. This method
uses less-toxic materials than does EtO processing and can be more cost-effective
than irradiation.
6.3.5.3 Sterilization by Irradiation
Radiation sterilization utilizes ionizing radiation generated by gamma rays (see
Chapter 8 for details on radiation), an electron beam, X-rays, or ultraviolet rays
(220-280-nm range) to sterilize medical devices. Gamma irradiation is the most
