Embedded antimicrobial agents protect cleanroom plastic surfaces
10/01/2007
Lower the risk of biocontamination without compromising material integrity
By Tracey Whitehouse, Aegis Environments
Repeating the word “microbe” in a cleanroom facility can be much like yelling “fire” in a crowded theater. Regardless of the industry the facility ultimately serves, the control of microorganisms is of critical importance in virtually any cleanroom due to the particulate nature of microorganisms. Manufacturers of cleanroom construction materials and facility managers themselves realize this importance as they are challenged and tasked daily to meet the high standards of asepsis set for their cleanroom facility. Deterioration, defacement, and cross-contamination are all real effects that occur from the microbial “corruption” of cleanroom surfaces (e.g., soft and hard walls, ceilings, flooring, and air duct systems). Most significantly for the clean environment, these surfaces can give refuge to microorganisms and ultimately act as a transfer site (vector), offering ideal environments for the proliferation and spread of microorganisms, which is detrimental to maintaining sterility.
The ability to make microbial-resistant surfaces in a cleanroom environment has advantages in many applications. Besides following the current protocols for controlling airborne particles, steps can be taken to protect the surfaces in a cleanroom. And, because every material introduced into a cleanroom is a potential source of contamination, cleanroom designers and specifiers must seek materials that can lower that risk without compromising material integrity. This discussion relates to how antimicrobials added to plastics used in cleanroom surfaces can help to mitigate this source of biocontamination.
The antimicrobial feature
Antimicrobials, by definition, kill, suppress, or inhibit the multiplication of microorganisms. On earth, there are hundreds and maybe thousands of chemistries that kill microorganisms. Many of these-such as arsenic, lead, tin, mercury, silver, plant extracts, and animal extracts-may be “natural,” but they can be highly toxic to people and the environment in most uses. An effective antimicrobial for the cleanroom industry can’t just kill or repel microorganisms; it must do so safely, over the life of the treated surface, and without negatively affecting the other important characteristics of the surface on which it is applied.
The vast majority of antimicrobials, like the ones found in nature, work by migrating or moving from the surface to which they are applied. This is the mechanism by which these migrating (leaching) antimicrobials poison a microbe. A leaching antimicrobial is strongest at the source and weakest as it travels away from that source. The outermost edge of that migration is called the “zone of inhibition.” This zone of inhibition is where resistant microbes are born because, like any living organism, microbes will take extreme measures to survive. As a result of the exposure to sub-lethal doses of antimicrobials, microbes can genetically and enzymatically reinvent themselves as “super-strains.” The very nature of these types of antimicrobials to migrate creates long-term antimicrobial durability challenges because eventually the microbes will no longer be affected by the antimicrobial that was tasked to control their proliferation.
An antimicrobial with a different mode of action incorporates a non-leaching bound antimicrobial organofunctional silane molecule (SiQuat). The mode of action relies on the agent remaining affixed to the substrate, killing microorganisms as they contact the surface to which it is applied. When applied, the agent actually polymerizes with the substrate, making the surface itself antimicrobial. When a microbe contacts the polymer matrix on the treated surface of the fabric or fiber, the cell is physically ruptured by the 18 carbon polymer chain of the primary molecule. Drawn downward toward the positively charged nitrogen atom in the molecule, the microbe cell is literally electrocuted and its cell wall physically disrupted. There is no defense, no non-lethal dose, and no survivors that could build immunity to this physical mode of action. No microbial adaptation occurs, and this mode of action achieves broad-spectrum effectiveness against bacteria, mold, and algae (see Fig. 1).
 Figure 1. Rendering of silane wall. Image courtesy of Aegis Environments. |
Once polymerized, the bound SiQuat does not migrate or create a zone of inhibition, so it does not set up conditions that allow for adapted organisms. Because it becomes part of the surface, it also does not cross the skin barrier, affect normal skin bacteria, or cause rashes or skin irritations.
Antimicrobial plastics used in cleanroom facilities
Achieving stability requires control of biocontamination in the design, specification, operation, and control of cleanrooms and associated critical environments. For many cleanroom facility managers in the microelectronics, pharmaceutical, and other industries, polyurethane (PU) based materials have proved to be a valuable asset to the cleanroom industry. Their durability, versatility, and ability to be easily cleaned have proved useful on hard surfaces such as furniture, insulation panels, sealants, and soft wall strips, panels, and framing. PU is also bonded to nylon knit for use in gloves and cleanroom suiting.
Biosafe Inc. (Pittsburgh, PA) has developed and patented a process that enables the inclusion of the SiQuat agent in a polymeric powder (HM 4100) compounded into plastics to impart durable antimicrobial properties throughout the component, inside and out. In testing with HM 4100 treated PU coated glass, a 6.82 log reduction against the test organism Escherichia coli with 1-hour contact time-at the 0.2 percent actives rate (ASTM E2149-01)-was demonstrated (see Table 1).
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Further testing was conducted to compare and determine the antibacterial efficacy of polyurethane plaques treated with 0.3 percent Biosafe HM 4100 against a 1 percent silver treatment. The samples were tested using the JIS Z 2801-200 test method with testing done at two separate 2-hour contact times against gram-negative Escherichia coli. Figure 2 demonstrates the efficacy of the bound HM 4100 on the treated PU as it relates to the silver sample.
Materials used in cleanrooms, particularly those used in panels and strips, rely on optically clear plastics to maintain a safe, blur-free work zone. New developments are underway to bring to market an antimicrobial plastic that not only imparts a biostatic surface but can also maintain the optical requirements needed for cleanroom window/panel specifications. Initial testing with an HM 4100 treated resin used in optically clear plastic manufacturing passed tests for yellowness, hazing, transmittance, and diffusion, among other properties, as compared to non-treated optically clear plastic. Virtually no optical differentiation was demonstrated (see Table 2).
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Specifying an antimicrobial in your cleanroom materials
To a cleanroom professional, nothing is more paramount than effective contamination control-not only for the safety of the staff, but also for the integrity of the product being manufactured. It is critical that these professionals review all materials that go into their cleanroom design. When researching antimicrobial plastics, the following points should be considered:
- Durability: Cleanroom materials require durable features. Plastics engineered for use in cleanroom and other aseptic facilities must have antimicrobial treatments that can survive the many abuses, namely abrasion from cleaning and product processing.
- Waste control/toxicity: Antimicrobials control a range of microbial pests, but their use must be managed so that they do not affect good and helpful microbes. Although heavy metals have long been rejected where they come into contact with the environment or human skin contact, silver-based products have unexpectedly made a resurgence.
- Spectrum of activity: Many materials are antimicrobial at the right concentration, but in cleanroom and medical applications, it is very important to strike a balance with broad range antimicrobial activity that is safe and economical. When integrating antimicrobial treatments into goods, this is even more important. A broad-spectrum antimicrobial will have activity at the deliverable concentration or contact concentration that kills or inhibits gram-positive bacteria, gram-negative bacteria, yeast, and mycelial fungi. Added spectra could include algae, virus, or other microbial pests.
- Adaptation: Any agent that affects a microorganism’s life has the potential to set up conditions where the microbial cells adapt or mutate into resistant types. This is undesirable in almost all settings but clearly should not be tolerated in a cleanroom facility. Consider the modes of actions of the antimicrobials you are considering before you make a final decision.
Tracey Whitehouse is a technical writer based in Midland, MI. Aegis Environments provides antimicrobial technologies and services to the building, construction, and textile industries (www.aegismicrobeshield.com).