Top 6 Ways To Measure Humidity For Medical Device Sterilization
What is considered sterile for sterilization validation?
Under the strictest definition of sterility, an item or product is sterile when there is a complete absence of viable microorganisms (bacteria, yeasts, viruses, and molds). Regulatory-level sterility is defined by acceptance criteria based on calculated contamination probability. An acceptable level of contamination risk for most items is the probability of contamination for one in a million products. However, sterility criteria may be more stringent or lax depending upon the intended use of the medical device or product. Sterilization validations, what is relative humidity, humidity hygrometers, and how to find accurate hygrometers will be covered in this article.
What are sterilization validations, and why are they important?
Since the sterility of a medical device or product is based on acceptance criteria, the process that a product or device undergoes to become sterile must be validated to prove that sterility acceptance criteria are consistently met. Sterility can be assured only with a validated sterilization process under current good manufacturing practices (cGMP). Sterility cannot be demonstrated by reliance on periodic sterility testing of final products alone. Thus, sterilization validations are tests that accumulate data about a sterilization process and statistically prove that the sterilization process can consistently sterilize medical devices or products under “worst-case scenario” conditions.
How are sterilization validations performed?
Knowledge of sterilization technologies, instrumentation, and equipment is needed to control and verify sterilization process metrics appropriately. Representative biological indicators are selected and placed at strategic locations to cover “worst-case” microbial scenarios. Sterilization validations require multiple formally documented stages. General principles for validation programs (cleaning validations and the like) are applicable to all sterilization processes. Each sterilization mode (e.g., dry heat sterilization, wet heat sterilization, gaseous sterilization, etc.) has individual criteria that must be met. The first stage of sterilization validation is the process development stage. In the process development stage, operating parameters and controls used for the sterilization process are investigated and selected. The next stage is the installation qualification stage, which ensures that equipment controls and instrumentation are installed and calibrated appropriately. As part of the installation qualification, systems to regulate steam, water, and air should be verified and documented. The third sterilization validation stage is the operational qualification stage. Operational qualification makes sure that installed equipment functions within the set sterilization process parameters. After the operation of the equipment is verified, the performance qualification stage begins. Performance qualifications assess the sterilization of materials, items, and biological indicators that pass through the sterilization process under validation. Performance qualifications measure sterilization cycle controls and the effectiveness of the sterilization cycle in overcoming worst-case biological challenges. The fifth and final stage of sterilization validation is the routine process control stage. This final stage ensures that sterilization processes are continuously monitored and controlled to maintain the efficacy of product sterilization.
Why is humidity detection important for medical devices, drugs, and other medical products?
Humidity can negatively affect medical devices during sterilization, product shipping, and product storage. For this reason, humidity monitoring is critical to creating and maintaining product sterility over time. Unlike with temperature monitoring, humidity effects are delayed by product packaging and require more time for their impact to be observed. Indeed, most sterile product packaging is designed to block moisture ingress but not stop or mitigate temperature changes.
What is relative humidity?
Humidity monitoring devices (hygrometers) measure relative humidity. The relative humidity is dependent on temperature and is the ratio of water vapor pressure in air to the air’s saturated water vapor pressure (at the given temperature).
Top 6 Relative Humidity Measurement Devices For Sterilization & Controlled Storage
Several devices exist for measuring relative humidity.
#1: Sling Psychrometers
Sling psychrometers are not used to monitor sterilization. However, they are an example of the simplest hygrometer possible. Sling psychrometers utilize the temperature difference between an ordinary thermometer and an identical thermometer with a wet cloth wick over its bulb. The temperature difference is taken after the thermometers are spun at the end of a chain. The spinning process allows water evaporation from the wick to cool the wet-bulb thermometer. A correlation table matches the temperature difference between the wet and dry thermometers to the relative humidity value.
#2: Hair Hygrometers
A hair hygrometer uses synthetic or human hair to measure relative humidity. Hair length increases as a function of relative humidity. The hair length change is used to move an indicator or strain gauge. Advantageously, a hair hygrometer are accurate hygrometers and measure humidity accurately within +/- 3%. However, hair hydrometers have a slow response time and are inaccurate with significant changes in humidity (very high or extremely low relative humidity levels).
#3: Infrared Hygrometers
Infrared (IR) hygrometers compare the absorption of two diverse IR wavelengths through the air to identify the air’s relative humidity level. Out of the two IR wavelengths, one wavelength will be absorbed by water vapor, while the other will not. Advantageously, IR hygrometers are accurate hygrometers and effectively measure the relative humidity in all volumes of air (gigantic or tiny). Further, these hygrometers accurately measure rapid changes in humidity. As a result, IR hygrometers are often used as sensors in sterilization equipment and are often integrated with electronic data-handling systems.
#4: Dew Point Hygrometers
A cold mirror is utilized to identify the air’s dew point in this hygrometer. Water vapor in the air starts to condense at dew point temperature. The dew point is also the temperature at which the relative humidity is 100%. In other words, the dew point is the temperature at which air is saturated with water vapor. The relative humidity can be calculated from the dew point and the ambient temperature. A majority of hygrometers use the dew point as a calibration standard.
#5: Capacitive Thin-Film Hygrometers
Capacitive hygrometers measure the dielectric activity of non-conductive polymers, which changes in direct proportion to relative humidity. The dielectric activity shifts of the polymers are measured as capacitance changes. These accurate hygrometers give precise capacitive measurements within +/-3%.
#6: Resistive Thin-Film Hygrometers
Resistive hygrometers function similarly to capacitive hygrometers. Instead of measuring changes in capacitance, resistive hygrometers measure changes in resistance. Resistive hygrometers do this using an organic polymer whose electrical resistance changes in logarithmic proportion to the air’s relative humidity. These accurate hygrometers yield precise measurements within +/- 5%.
How do you calibrate humidity monitors?
Calibration confirms that hygrometers are suitable for their intended use. Regular calibration (via a calibration program) ensures that hygrometer devices are running accurately during use. Monitors used during sterilization, manufacturing, storage, and transport of medical devices should be consistently calibrated. For hygrometer use, the manufacturer’s certificate of calibration may be used until the expiration date. For humidity-monitoring devices, measurement accuracy and responsiveness are part of the calibration process, along with a dew point standard to confirm humidity measurements. The relative humidity is expressed as a percentage. Thus, humidity accuracy is typically reported as a +/- error from the true relative humidity value. Overall, hygrometers must have the appropriate responsiveness and accuracy for their humidity monitoring application before use.
Summary
Overall, sterility can be assured only by using a validated sterilization process under current good manufacturing practices (cGMP). Humidity can negatively affect medical devices during sterilization, product shipping, and product storage. For this reason, humidity monitoring is critical to creating and maintaining product sterility over time. Unlike with temperature monitoring, humidity effects are delayed by product packaging and require more time for their impact to be observed. Humidity monitoring devices (hygrometers) measure relative humidity. The relative humidity is dependent on temperature and is the ratio of water vapor pressure in air to the air’s saturated water vapor pressure (at the given temperature). Six various technologies can measure relative humidity: 1) sling psychrometers, 2) hair hygrometers, 3) infrared hygrometers, 4) dew point hygrometers, 5) capacitive thin-film hygrometer, and 6) resistive thin-film hygrometers. The most sensitive hygrometers are infrared hygrometers. However, dewpoint hydrometers are often used to calibrate other hygrometers. All in all, ensure you choose a contract testing organization that can support you with appropriate sterilization process development and validations for your unique medical device or product needs.
Ethide Labs is a contract testing organization specializing in Sterilization Validations and Bioburden Testing. Ethide Labs also offers Microbiology Testing, Sterility Testing, Bacterial Endotoxin Testing, EO Residual Testing, Cytotoxicity Testing, Environmental Monitoring & Package Integrity Testing services for medical device companies and allied industries. Ethide is an ISO 13485 certified facility.
References
United States Pharmacopeial Convention. <1229> Sterilization of Compendial Articles. Rockville, MD, USA. 2021. (USPC <1229>).
United States Pharmacopeial Convention. <1118> Monitoring Devices Time Temperature And Humidity. Rockville, MD, USA. 2021. (USPC <1118>).
