Final Report: Sterilizing Medical Equipment With One Atmosphere Uniform Glow Discharge Plasma

EPA Contract Number: EPD04029
Title: Sterilizing Medical Equipment With One Atmosphere Uniform Glow Discharge Plasma
Investigators: Kelly-Wintenberg, Kimberly
Small Business: Atmospheric Glow Technologies
EPA Contact: Richards, April
Phase: I
Project Period: March 1, 2004 through August 31, 2004
Project Amount: $69,941
RFA: Small Business Innovation Research (SBIR) - Phase I (2004) RFA Text |  Recipients Lists
Research Category: Nanotechnology , SBIR - Nanotechnology , Small Business Innovation Research (SBIR)


The increase in the complexity of medical devices in the recent past has presented a myriad of challenges for those charged with providing sterile surgical instruments. Particular concerns include narrow bore lumens, multicomponent instruments that include electronics or fiber optics, and the reprocessing of single-use devices that contain heat- or chemical-sensitive polymers. Hospitals are increasingly looking to use alternative low-temperature technologies to meet their needs. Currently available low-temperature sterilization processes are associated with high capital costs and are out of reach for smaller healthcare facilities. The increase in the number of ambulatory surgical centers and dental clinics as well as the corresponding increase in the need for sterile instrumentation present a compelling opportunity to provide an economical means of rapidly sterilizing a wide range of devices. This, coupled with increasing environmental regulations, enhances the appeal of a process that can meet the outlined needs without creating hazardous waste or potentially dangerous toxins. Atmospheric Glow Technologies (AGT) proposes to provide an alternative sterilization technology based on the One Atmosphere Uniform Glow Discharge Plasma (OAUGDP™). The Atmospheric Plasma Sterilization (APS) test device will create reactive chemical species (RCS), which will be used to neutralize biocontaminants without inducing damage to medical device materials.

AGT envisions scaling the APS test device for sterilization applications in small clinics and dental offices, and eventually hospitals. Phase I served to support the early transition from laboratory feasibility tests using inoculated samples placed directly at the plasma exhaust to studies in which 3-dimensional objects were treated within a chamber. AGT has proof-of-principle for the neutralization of biocontaminants even on complex shapes in the presence of debris, the most conclusive proof being supplied by the successful completion of the Association of Official Analytical Chemists Sporicidal Activity Test (Official Method 966.04) in April of 2004. For this effort, AGT proposed to proceed with development towards sterilization within a larger chamber.

The goal of this research project was to investigate the feasibility and commercial potential of using OAUGDP™ for low-temperature sterilization of medical devices and instruments within a chamber of reasonable proportions. Progress toward this goal was supported by the following tasks:

  • Task 1: Fabricate an OAUGDP™ prototype that would function to sterilize contaminated medical equipment within a chamber.
  • Task 2: Inoculate stainless steel surgical instruments with microorganisms and assess the OAUGDP™ exposure time necessary to sterilize the device in the presence or absence of organic debris.
  • Task 3: Inoculate common sensitive medical equipment with microorganisms and assess the plasma exposure time necessary to sterilize the device in the presence or absence of organic debris.
  • Task 4: Evaluate any potential surface modifications occurring as a result of OAUGDP™ exposure through microscopic and analytical techniques.
  • Task 5: Prepare a final report summarizing the findings of this research project.

Summary/Accomplishments (Outputs/Outcomes):

AGT used its existing Atmospheric Plasma Decontamination System as a foundation for this research project. Briefly, the APS system consists of a plasma generator that creates RCS from air, a high voltage power supply that drives the plasma, an air pump that supplies air to the generator and ultimately expels the reactive chemistry, a fluid cooler and pump to remove waste heat from the plasma generator, and a heat exchanger to cool the exhaust stream of reactive chemicals. The antimicrobial chemicals in the exhaust stream are used to sterilize items.

Various parameters relevant to the operation of the APS test device were monitored during tests. These included high voltage, temperature at the sample location and of the coolant, chamber pressure, inlet gas pressure, exhaust pressure, airflow, relative humidity, and production of ozone.

Tests were performed to determine the values for airflow and humidity that would be pursued for this contract. Under the conditions of 9 mm Hg of water vapor at 2 cfm, with a voltage of 12 kV and a frequency of 6 kHz, Bacillus endospores on test strips were rapidly neutralized.

AGT’s prior sterilization efforts primarily focused on proof-of-principle, demonstrating that the reactive chemical species could be used to neutralize biocontaminants outside of the plasma volume with a directed exhaust flow. Because this research project provided for stepwise development of the plasma sterilization system toward practical application, it was necessary to demonstrate that sterilization could be achieved at various distances from the plasma. Ultimately, the aim is to provide a chambered sterilization system into which irregularly shaped items can be placed for low-temperature sterilization. Duplicate endospore strips were placed at distances of 5, 7, 9, and 12 inches from the plasma exhaust. Following standard plate counts, all samples were below the limit of detection for viability with the exception of one of the strips at the 9 inch location (650 endospores remained viable).

AGT is committed to the stepwise maturation of atmospheric plasma sterilization for practical application. A principal development step is the transition to a more open chamber within which larger samples that represent medical devices can be placed. Key issues to be addressed in this advancement include the increase in time that will be required for sterilization, the possibility that additional plasma generators will be required to increase the production of RCS, the uniformity of RCS flow within the chamber, and the penetration of RCS into crevices to inactivate microorganisms. Evaluation of all of these issues was beyond the scope of this 6-month contract; however, as detailed below, AGT was successful in implementing the existing APS test device with a chamber that provided a substantial increase in sterilization chamber volume.

AGT designed and fabricated a 0.33 ft 3 (8 " x 8 " x 9 " ) chamber of acrylic. The chamber has a port on top to which the plasma generator is attached. An additional port on the lower portion is provided for exhaust. Three wire-mesh tiers can be included for varying sample location. Following recovery experiments that verified that the loading and recovery protocol AGT was using provided a minimum of 1 x 106 endospores per hemostat, AGT initiated sterilization studies using the chambered APS test device. This test demonstrated that within 12 minutes, there are no viable endospores at the level of detection. Importantly, no reduction in viability could be attributed to airflow alone. AGT proposed testing the developing APS test device with contaminated polymers, as well as an indication of device compatibility with more sensitive medical materials. Upon validation of the inoculation/recovery protocol with high-density polyethylene (HDPE) coupons, AGT selected a 3-dimensional polymer model with which to continue studies. HDPE-barbed quick-disconnects were chosen. The connectors were washed, autoclaved, and then inoculated by pipetting the endospore suspension along the barbed area. Inoculated connectors were allowed to dry for 2 hours. Connectors showed an inoculation/recovery of 107 endospores per connector. Similar to the hemostats, neutralization of the bacterial endospores was achieved rather rapidly on the HDPE connectors. Following a 6- minute treatment, only 10 viable endospores were detected per mL of diluent. When the treatment time was extended to 12 minutes, no viable endospores could be detected. Again, no substantial reduction in viability could be attributed to the airflow conditions without the plasma energized.

Including an organic load was anticipated to be the most difficult portion of this research project. For sterilization to be achieved under these circumstances, the reactive chemical species must diffuse through the organic layer or through the barrier presented by a heavy inoculum. For a gaseous process that relies on short-lived reactive chemical species, this is particularly challenging because these metastables may recombine or be quenched prior to reaching the bacteria or endospores. In parallel research efforts, AGT has shown the potential of atmospheric plasma to penetrate extracellular matrices (complex multispecies biofilms) and residual proteins (media, serum proteins) to sterilize materials. These efforts, however, focused primarily on vegetative bacteria or were performed at minimal distances from the plasma.

AGT performed experiments in which Bacillus endospores were occluded by a 10 percent bovine serum albumin/0.65 percent NaCl challenge, a suspension of two gram-positive species of vegetative bacteria suspended in a lower organic load (residual tryptic soy broth), or a suspension of the gram-positive bacteria that had been extensively washed. In all cases, the organic load proved to be a significant barrier to neutralization of the endospores.

AGT took preliminary steps to begin addressing potential improvements in the test device that could enhance the performance in the presence of organic debris. These possibilities will be the focal point of Phase II.

The ultimate goal of this development work is to provide a safe alternative means of low-temperature sterilization that does not damage high-value, sensitive medical instrumentation. Although Phase I supported early development of the chambered APS test device, AGT also took the opportunity to gather important early evidence that the plasma sterilization process is compatible with different materials. Scanning electron microscopy, preliminary contact angle measurement analysis, and early energy dispersive spectroscopy analysis were performed and will serve as a basis for future work in Phase II.


There are several points to emphasize regarding the completion of Phase I and the extension of the work into Phase II.

  • AGT is encouraged by the success in transitioning from prototypes with more directed flow to a prototype with a sample chamber of 0.33 ft3. This push toward scaling the prototypes for practical application is an area that AGT will continue to pursue.
  • The ability to rapidly neutralize bacterial endospores on complex, 3-dimensional shapes within a chamber of reasonable proportions is significant.
  • Although materials compatibility tests are still in their initial stages, AGT continues to explore the inclusion of additional meaningful analytical techniques with which to evaluate the compatibility of OAUGDP™ with materials, especially those significant to the healthcare industry.
  • Although the inclusion of organic debris presented a barrier to endospore neutralization with this test device, AGT has formulated a systematic approach for Phase II to bridge this gap.

Supplemental Keywords:

Sterilization, medical equipment, One Atmosphere Uniform Glow Discharge Plasma, OAUGDP™, Atmospheric Plasma Sterilization test device, APS, reactive chemical species, RCS, Bacillus endospores, decontamination, high-density polyethylene, HDPE, SBIR,, Scientific Discipline, TREATMENT/CONTROL, Sustainable Industry/Business, Chemical Engineering, Physics, cleaner production/pollution prevention, Technology, New/Innovative technologies, Environmental Engineering, decontamination, medical equipment, antimicrobial surface preparation, clean technologies, environmentally friendly technology, medical equipment sterilization, nanotechnology, clean technology, sterilization, one atmosphere uniform glow discharge plasma