Sterilization is a universally accepted technique in the healthcare and pharmaceutical industries for the complete removal of microorganisms or any form of life that contributes to contamination. In many cases, pathogens or microbials can linger on washed surfaces. So when personal health is on the line, there is zero tolerance for potential contaminants.

Pharmaceutical products are manufactured in accordance with the highest standards. Of the many requirements is sterilization. Specialized equipment is designed and manufactured for the medical sterilization process, and almost all global health regulatory bodies require a manufacturing facility to have certified procedures and sterilization techniques.

To make the pharmaceutical product contamination free, each source needs to be destroyed or removed. This includes every material or item that comes in contact with sensitive products, such as the containers for medications and vaccines, diagnostic and surgical tools, apparel and personal protective equipment and various components used for diagnostic and laboratory processes, such as vials, vessels, tubing, gaskets and valves.

Methods of sterilization

There are many methods for medical sterilization, the selection of which depends upon factors like the ability of a product or material to bear temperature, or if a particular technology provides enough. The most common methods of sterilization are by heat — either moist heat or dry heat.

Moist heat technology

In this process, moist heat in the form of steam is used to kill microorganisms residing on the surfaces exposed to the steam. The high temperature steam eliminates the proteins of the microorganism, upon which the microorganisms subsist. This process is completed in an autoclave, a type of pressure vessel.

The steam in an autoclave is pressurized, allowing the autoclave to kill more organisms in less cycle time and at a lower steam temperature (approximately 100° C). This increases the efficiency of the autoclave, as it requires less thermal energy to be effective. Total cycle time is dependent on things like materials to be sterilized and autoclave performance.

Different components and instruments are used to achieve and monitor the autoclave sterilization process, including the following:

• Temperature sensor
• Pressure sensor
• Data logger, for recording the corresponding values of temperature and other process variables
• Gauges or displays, for quickly determining pressure at the chamber and jacket
• Vacuum pump, for creating vacuum inside the chamber
• Main controller, such as programmable logic controller
• A user interface or human machine interface for parameter setting and readouts

Autoclave cycle

Step 1: Pre-conditioning

In the pre-conditioning stage, the air is removed from the autoclave chamber after items have been loaded into the chamber. If not removed, then there can be small air pockets inside the chamber. Air pockets inhibit the steam from uniformly filling the volume of the chamber, potentially allowing microorganisms to persist.

There are two methods to remove the air. The first requires a vacuum pump to mechanically remove atmospheric gases from the chamber. The second method is by flushing steam. As the steam goes into the chamber, it pushes the air outside the chamber through the drain valve.

Step 2: Heating

In this stage, steam is filled inside the chamber to raise the temperature up to the desired value. In addition to the temperature, steam pressure is also maintained, and, for safety and object integrity reasons, should not exceed desired values.

Step 3: Sterilization

A timer for the sterilization process is started once the autoclave reaches the specified temperature. Cycle time is determined by materials and objects to be sterilized, as well as parameters like thermal loading of the objects, and ambient conditions such as temperature and elevation.

Step 4: Post-conditioning

The autoclave needs time to return the chamber and contents to ambient temperature, pressure and moisture. From this point, the sterilized materials are carefully handled by healthcare professionals to prevent post-processing contamination.

Dry heat technology

Sterilization by dry heat involves high temperature air circulated inside a closed chamber. Temperatures up to 180° C are likely — considerably higher than an autoclave. Oxidation of microorganisms occurs due to high temperatures, nullifying its ability to contaminate. This process occurs in a device known as a dry heat sterilizer (DHS).

This type of sterilization technique is suitable for the materials that can withstand higher temperatures, but could be compromised by the extreme humidity of the autoclave. They are commonly used for the sterilization of glass containers so that they are free from any harmful substances before product filling.

Some key components or systems of a dry heat sterilizer include the following:

• Main controller, such as programmable logic controller
• A user interface or human machine interface for parameter setting and readouts
• A high speed blower creates air circulation inside the sterilizer, and may include a variable frequency drive to control blower airflow
• A heating element heats air as the blower passes air over it
• A HEPA filter, which filters air drawn into the dry heat sterilizer, but must also withstand high temperatures
• Temperature sensor

DHS cycle

Step 1: Pre-heating

The high speed blower forces air over the heating elements and into the chamber.

Step 2: Sterilization

A timer for the process ensures that objects to be sterilized have spent enough time at a high enough temperature to kill all microrganisms.

Step 3: Post-conditioning

The DHS needs time to return to ambient temperature before the items in the chamber can be transferred to sterile storage.

Sterilization measurement

Whether by autoclave or DHS, it is important to monitor the sterilization cycle. Myriad factors reduce the effectiveness of the sterilization technology — machine malfunction, item arrangement in the chamber, wrong cycle parameters — and thereby threaten the lives of the patients downstream.

Sterilization effectiveness is commonly measured by the following:

Chemical indicators

Samples of chemical indicators are arranged throughout the sterilization chamber, and when exposed to a specified temperature, they change color. If the color change is not visible, it indicates that temperature distribution throughout the chamber was uneven. Chemical indicators are more common with autoclaves.

Biological indicators

These indicators contain some sort of live microorganism that is inserted in the chamber and exposed during the sterilization process. After the sterlization process, they are incubated under favorable conditions to check the status of microorganisms. In case the microorganisms are still live, then the test failed, and it is assumed the process failed to eliminate contaminants on the healthcare items as well.

Physical checks

The sterilization process can also be monitored with temperature or pressure probes placed inside the chamber at critical places. These show the actual process variable value and helps in detecting any non-conformity of process variable. Commonly, temperature sensors are placed at the position of the main controller and some are placed at different locations in a chamber. After a predefined interval of time, the readings are recorded and analyzed, which helps healthcare professionals determine the efficacy of the sterilization process.

About the author

Muhammad Asim Nlazi is a freelance author with a bachelor's of science in electronics engineering and 12 years of experience in the pharmaceutical industry. He also has experience in manufacturing, quality control, electronics engineering and project management, as well as equipment maintenance, inspection, troubleshooting, calibration and validation.