Thanks to advances in semiconductor and packaging processes, integrated circuits (ICs) are now widely used in various devices, including medical devices. A special challenge in medical applications is the need to keep products sterile-free of harmful pollution, such as fungi, bacteria, viruses and spores. Although there is a large amount of literature on sterilization methods and equipment, there is little reference to the effects of sterilization methods on electronic devices. This application note compares common sterilization methods and discusses their applicability to objects containing electronic devices.
1 Physical law
There are a variety of physical sterilization methods, the most effective of which is to combine heat, humidity, and pressure in a device called a high-pressure steam sterilizer.
Autoclave sterilization method
The thermal sterilization of medical equipment has been used in ancient Rome. The presence of steam significantly accelerates heat penetration (steam sterilization). The high-pressure steam sterilizer, invented in 1879, combines heat, humidity and high pressure.
Working principle : The high-pressure steam sterilizer is a container similar to a pressure cooker. Place the object to be sterilized and seal it. Next, high-temperature steam is charged under high pressure to replace air. Damp heat kills microorganisms through the irreversible coagulation and denaturation of enzymes and structural proteins. The time and temperature to achieve this goal depend on the pressure and the type of microorganism being killed. After the necessary time has passed, steam is released and the sterilized object is taken out. The entire cycle lasts 15 to 60 minutes (batch processing).
Question : High-pressure steam sterilizer sterilization is suitable for objects that can withstand moisture, high pressure (1 to 3.5 atmospheres above the environment) and high temperature (+ 121 ° C to + 148 ° C) Typical examples are surgical instruments. Semiconductor devices can usually withstand temperatures up to + 125 ° C. However, the high temperature of the embedded battery will greatly shorten the life. Memory devices using floating gate technology, such as EEPROM, are very sensitive to high temperatures. However, if the data retention capacity is specified at + 125 ° C for 10 years, data integrity should not be compromised. Otherwise, the memory data may be accidentally refreshed (overwritten) to restore full charge on the floating gate. This applies to laser trimming EEPROM. Since the type of fine-tuning is often not given in the product data, it is necessary to contact the manufacturer for detailed information.
2 Chemical method
There are many chemical methods available for sterilization in the medical field. This section discusses several common methods. Chemical and physical methods can be used in combination.
2.1 Ethylene oxide (ETO) sterilization
Ethylene oxide (ETO) was first reported in 1859 and occupied an important position in the industrial field as early as the early 20th century. The ETO sterilization method for preserving perfumes was patented in 1938. Since there are few alternatives for sterilizing medical devices that are sensitive to heat and moisture, the use of ETO has continued to evolve.
Working principle : ETO sterilizer is a container that can hold the object to be sterilized. The basic ETO sterilization cycle consists of 5 steps (steam evacuation, gas injection, diffusion, evacuation, and air flushing), which takes about 2 1/2 hours, excluding the ventilation time (ETO exhaust). Mechanical ventilation takes 8 to 12 hours at +50 to + 60 ° C; passive ventilation is also possible, but it may take 7 days. After completing the ventilation, remove the sterilized object (batch processing). ETO chemically reacts with amino acids, proteins and DNA to prevent microbial reproduction.
Question : ETO sterilization is suitable for objects that cannot withstand the high temperatures and steam necessary for steam (high-pressure steam sterilizer) sterilization. Since the temperature conditions are + 30 ° to + 60 ° C, ETO sterilization is very suitable for medical devices containing embedded electronics. However, the embedded battery may not accept vacuum. In addition, this method has a disadvantage: ETO is a highly flammable, petroleum-based gas and carcinogen.
2.2 Chlorine dioxide (CD) gas sterilization
Chlorine dioxide (CD) was discovered in 1811 or 1814 (both listed in both years) and is widely used as a bleaching agent in the paper industry. In 1988, EPA registered it as a fungicide. This opens the door for applications in the medical field.
How it works : The CD sterilizer is a container that can hold the object to be sterilized. The basic CD sterilization cycle includes 5 steps (moisture pretreatment, conditioning, generation and supply of chlorine dioxide gas, diffusion, and ventilation), and requires approximately 2 1/2 hours, including the ventilation time (discharge of CD). After completing the ventilation, remove the sterilized object (batch processing). Chlorine dioxide (ClO2) as an oxidant reacts with several cellular components, including the cell membranes of microorganisms. CD "steals" electrons (oxidation) from the subject, breaking its molecular bonds, causing organic cells to rupture and die. As CD changes the protein in the structure of microorganisms, the enzyme function is destroyed, resulting in the rapid death of bacteria. The power of CD is attributed to the simultaneous oxidative erosion of many proteins, so it can prevent the cell from becoming resistant. In addition, due to the low activity of chlorine dioxide, its bacteriostatic effect can be maintained for a long time in the presence of organic matter.
Question : CD sterilization is suitable for objects that cannot withstand the high temperatures and steam necessary for steam (high-pressure steam sterilizer) sterilization. Since the temperature conditions are + 15 ° to + 40 ° C, CD sterilization is very suitable for medical devices containing embedded electronics. CD gas is non-flammable and non-carcinogenic among the concentrated polymers used in this method. It does not require high concentration to achieve the effect of killing spores.
2.3 Sterilization with hydrogen peroxide
Hydrogen peroxide was first isolated in 1818. It has a long history of use in the pharmaceutical industry and is a common alternative to ethylene oxide (ETO). There are two ways to use hydrogen peroxide: a) vaporized hydrogen peroxide sterilization and b) hydrogen peroxide ion sterilization.
2.3.1 Vaporized hydrogen peroxide (VHP) sterilization
Working principle: First put the object to be sterilized in the VHP sterilizer. The basic VHP sterilization cycle consists of 3 steps (including vacuum conditioning, H2O2 injection and ventilation), which takes about 1 1/2 hours, including the ventilation time (draining out H2O2). After completing the ventilation, remove the sterilized object (batch processing). The exact mechanism of action of HPV is yet to be fully understood, and may vary depending on the microorganism. H2O2 generates active oxygen particles, such as hydroxyl groups, which cause oxidative stress and attack multiple targets, including nucleic acids, enzymes, cell wall proteins and lipids.
Question: VHP sterilization is suitable for high-temperature environments and steam-treated objects that cannot withstand steam (high-pressure steam sterilizer) sterilization. Since the temperature conditions are + 25 ° to + 50 ° C, VHP sterilization is very suitable for medical devices containing embedded electronics. However, the embedded battery may not accept vacuum. The penetration capacity of VHP is not as good as that of ETO, and the US FDA has not approved this method for medical device sterilization in health care institutions.
2.3.2 Hydrogen peroxide ion sterilization
How it works: This method is a combination of chemical and physical methods. First place the object to be sterilized in a hydrogen peroxide ion sterilizer. The basic hydrogen peroxide ion sterilization cycle includes 4 steps (vacuum generation, H2O2 implantation, diffusion, and ion discharge), which takes about 1 to 3 hours. No ventilation is required. After the cycle is completed, remove the sterilized objects (batch processing). Hydrogen peroxide ion sterilization mainly uses the combination of hydrogen peroxide gas and the generation of free radicals (hydroxyl and peroxy radicals) in the ionic phase of the cycle to kill microorganisms.
Question: Hydrogen peroxide ion sterilization is suitable for high temperature environments and steam treatment objects that cannot withstand steam (high pressure steam sterilizer) sterilization. The vacuum required is not as deep as VHP sterilization. Although the process temperature of + 40 ° C to + 65 ° C is very suitable, the 13.56MHz RF energy in the ion discharge phase reaches the range of 200W to 400W, which will affect the embedded electronics. Hydrogen peroxide ion sterilization should not be used for objects containing semiconductors.
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