More than a decade ago, Dr. William Leiva, senior program manager of sterile reprocessing at Medtronic, observed something troubling. He noticed that some sterile barrier systems (SBS), like peel patches, looked “brittle” or “toasted” after sterilization.
This observation led Leiva to investigate the nuances of the steam sterilization cycle and the impact of superheated steam on sterile barrier systems. With support from the AAMI Foundation in the form of a KILMER grant, he focused on the effects of superheated steam on sterile barrier systems (SBS), such as peel pouches.
Leiva’s research is undergirded by basic differences in steam quality and performance. Wet saturated steam has a “tremendous amount of energy” in the form of latent heat. The moisture allows the steam to transfer energy to medical devices during sterilization. But dry saturated steam has lower enthalpy, or ability to transfer heat. Further, superheated steam has no enthalpy.
While steam sterilization cycles tend to follow the same pattern, and the sterilization plateau is constant despite some regional differences, real-world circumstances can lead to variance. Leiva also pointed out that larger steam sterilizers typically have an inner chamber that, if over-pressurized, can create superheated steam.
The impact of steam of inadequate quality, such as superheated steam, can be substantial. Leiva also noted that there is major variance between hospital and industrial settings. Hospitals usually have “different loads every single day,” which can lead to different steam diffusion characteristics. Latent heat is “significantly reduced” in the event of reduced saturation.
Wet loads remain the primary cause for load reprocessing, and increasing jacket pressure enables dryer loads. The rise of new kinds of polymers and load configurations also creates complexities. Leiva also stated that steam sterilizers calculate temperature based on pressure, meaning that if superheated steam is present, the actual temperature within the chamber is likely higher than indicated.
Study Aims and Structure
Faced with these issues, Levia adopted three research goals:
- First, validate a reproducible method to generate superheated steam in a test laboratory vessel.
- Second, expose samples to control and experimental superheated steam conditions.
- Third, conduct performance testing of the sterile barrier after exposure to conditions of both control and superheated steam.
The goal? Determine “if the sterile barrier is compromised” or if it maintains sterility.
After subjecting the SBS systems to a test sterilization cycle of 15 minutes exposure, 10 minutes of drying, and a two-minute vacuum time at both 121 degrees Celsius and 132 degrees Celsius, Leiva conducted three tests on the SBS systems exposed to superheated steam.
- A visual inspection, finding that some SBS systems were damaged at both 121 degrees Celsius.
- A peel test, finding that superheated steam made the packaging weaker, and that the packages used in the experiment had a lower peel force.
- A bubble test in accordance with ASTM F2096, finding significant correlation of experimental conditions and failure of the bubble test.
The SBS systems showed signs of damage at both 121 and 132 degrees Celsius.
Study Conclusions
Leiva concluded that the damage suggests that superheated steam does pose risks to sterile barrier systems. The proposed method was validated and delivered statistically significant results on cycle performance for both control and experimental sample groups. Levia also found a strong positive relationship between experimental conditions and decreased performance of sterile barriers. The visually damaged sterile barrier systems are comparable to both paper and plastic SBS observed in hospital production vessels.
Leiva noted that relying solely on built-in physical parameters may lead to approval of cycles with inadequate exposure conditions, saying, “We may be missing some information.”
What’s Next?
Leiva concluded by suggesting several concrete steps for future research that could provide a better understanding of issues surrounding SBS and superheated steam. First, he proposed including an assessment of cycle parameters, including cycle temperature, cycle drying time and drying pressure. Second, assessing different sterile barrier systems to determine reliability for short storage could be useful.
