Fortunately, advances in antimicrobial technology are providing the health care sector and other areas concerned with transmission of infections with new opportunities to control the growth and spread of germs. Standard infection prevention practices, such as hand washing and disinfection will continue to be critical, but some hospitals are now expanding the role of antimicrobial agents from therapeutic to preventative. In short, they are providing built-in antimicrobial protection for the entire patient environment from the scrubs the care givers wear to every element in the patient room including the architectural paint, floor and furniture coatings, curtains, bathroom fixtures, patient bed, and any other patient high-touch points. The success of this preventative approach depends primarily on using an antimicrobial agent that is effective, safe, durable, cost efficient and adaptable to a variety of materials. Silver, revered for thousands of years as a natural healing metal and preservative, is an excellent fit for this set of requirements.

Silver: Trusted Protection Not Tarnished by Time

Silver’s inherent antimicrobial properties have a long historical track record. As early as 1200 B.C., the ancient Phoenicians stored water in silver bottles to prevent spoilage from microbes. By 500 B.C., Greeks and Romans routinely used silver vessels for water purification. In the fourth century B.C., Hippocrates, the father of modern medicine, noted the healing benefits and anti-disease properties of silver. As they trekked westward, 19th century American pioneers used silver to keep their water safe and prevent common ailments such as dysentery, colds and flu. They also used silver dollars in their milk containers to slow bacterial growth. The 1800s also saw silver used directly for medicinal purposes: as silver sutures for surgical wounds, as a silver nitrate solution to prevent blindness in newborns, and to treat typhoid and anthrax. Before the onset of antibiotics, silver compounds were used to prevent infection during World War I. During the 1920s, over 3 million prescriptions were written annually for medicinal silver.

Silver’s impressive history continues today. Because it is recognized as one of the most non-toxic and safest of nature’s metals, silver is routinely used in neonatal eye drops to prevent eye infection. It is the preferred antimicrobial in wound care because silver’s oligodynamic biocidal action enables it to reduce infections without promoting antibiotic resistance. Silver sulfadiazine is the most popular treatment for burns in U.S. burn centers. Outside the health care sector, silver is used in surfaces such as cutting boards and table tops to help protect against food contamination, in sport and military clothing to reduce bacteria-causing odors, in outdoor furniture and rugs to prevent material degradation, and in water purification filters used by international airlines and NASA.

Figure 1 Click to enlarge

Nanotechnology: Less is More
 

Various antimicrobial technologies have been commercialized for some time. These older technologies, however, have inherent limitations in terms of meeting today’s high-performance demands of health care facilities and other locations where infection is a concern. The latest breakthrough in silver antimicrobial products – nanoscale silver additives – effectively resolves these issues. Nanotechnology combines engineering and science to make materials that are smaller than 100 nanometers. To put that size into perspective, the width of a human hair is about 100,000 nanometers while the average size of bacteria and viruses is generally 2,000 and 100 nanometers, respectively. When materials are engineered at nanoscale, they exhibit unique properties not present in their bulk form.

The key to nanoscale silver’s antimicrobial advantage is found in how it works. Ionic silver is the active form of silver that is most effective at eliminating bacteria, mold and fungus. Moisture activates silver nanoparticles, which release silver ions that attack microbes. These silver ions inhibit bacteria by stopping bacterial energy metabolism and electrolyte transport, by slowing or stopping DNA replication or by binding to the bacterial cell wall causing it to collapse or burst. As such, silver ions are active against a broad range of gram-positive and gram-negative bacteria, have a low risk for bacteria resistance, and are effective in low concentrations.

Figure 2 Click to enlarge

Reducing silver to nanoscale sizes provides several important benefits not found in silver’s bulk form. The first is related to antimicrobial performance. The key to optimizing the use of silver as an antimicrobial is to maximize the production of the silver ions that target and eliminate microbes. This is achieved by reducing the average size of the silver particle, which exponentially increases the collective surface area available for silver ion production (Figure 1). Hence, unlike bulk silver-based antimicrobial solutions that depend on adding increasingly more silver to be effective, nanoscale silver can achieve the same or better levels of efficacy using considerably less silver, thereby reducing cost and the potential for environmental impact.

Nanoscale size brings other advantages to silver. Silver nanoparticles can be covalently bonded to molecules in the products in which they are integrated. In this way, the silver particles are less susceptible to being washed out or worn off over time. Under the proper application techniques, their antimicrobial capabilities are effective for the expected life of the host material. In addition, silver nanoparticles are so small that they don’t impact mechanical properties (Figure 2). And because silver nanoparticles are metallic, they retain a number of key attributes of bulk silver, such as thermal stability during processing and UV-color stability. Finally, nanoparticle silver additives can be produced in integration-ready formulas, compatible with virtually any manufacturing process across a wide variety of host materials, including coatings, foams and engineered plastics as well as natural and synthetic fibers and fabrics. Specific to coatings, nanoparticle silver additives for liquid systems enable formulators to “just add and mix” at numerous steps during the manufacturing process. Powder additives are also available for integration with other performance additives typically compounded into powder coatings, such as flame retardants and colorants.

Table 1 Click to enlarge

Nanoscale Silver: Proven Performance with New Breakthroughs

While recent advances in nanotechnology have improved its performance and flexibility, nanoscale silver itself has an established track record of use and a proven safety record with respect to both human health and the environment. The FDA has approved the use of silver nanoparticles in medical products such as wound care dressings and catheters to prevent bacterial colonization in the devices. Also, the International Oeko-Tex® Association, a respected independent testing institution, has tested and classified nanoscale silver additives as not harmful to human health. In non-public health applications, nanoscale silver colloids have been FIFRA registered and used safely to control algae over six decades. And carbon filters impregnated with nanoscale silver are commonly used to protect our drinking water supply.

In fact, it is estimated that over 50 percent of EPA-registered silver products are based on nanoscale silver. These longstanding uses provide a clear understanding of the life-cycle and “downstream” speciation of nanoscale silver; silver in waste or natural environments is immediately converted to inert silver minerals such as silver sulfide, thus mitigating the risk of unintended environmental impacts. That said, manufacturers of antimicrobial products must ensure that the claims and use patterns of their products are in compliance with appropriate regulatory policies if the benefits of antimicrobial protection are to be fully realized.

Breakthroughs in dispersion technology now enable nanoscale silver to be readily integrated into many common coating systems, resulting in durable antimicrobial performance in dry film. Laboratory tests have shown that levels as low as 250 ppm (0.025% w/w) of nanoparticle silver are effective in flexible and breathable systems. Harder, less breathable and more hydrophobic systems require higher loadings (Table 1).

Various blooming agents can improve the surface activity of silver in systems. Due to the effectiveness of silver nanoparticles at very low concentrations, there is no impact on dry times, film formation, gloss, or abrasion resistance. In addition, since silver nanoparticles do not thermally or UV degrade, the applicator does not need to make any changes to post curing processes.

Table 2 Click to enlarge

Antimicrobial Landscape in Coatings

Given their broad facility-based applications, coatings with antimicrobial performance can play an increasingly important role in protecting high-touch surfaces against the spread of infection-causing bacteria. Antimicrobial products, such as silver zeolites, silver halides salts and organic biocides, have been incorporated into coatings for a number of years. However, current antimicrobial performance standards, particularly in the health care arena, are substantially higher than in the past in terms of durability, efficacy and safety. These traditional antimicrobials have been challenged in meeting these tougher standards.

Zeolite-based antimicrobial technology has been available since the 1980s. Zeolites are ceramic cages, typically 2-200 µm in size and filled with 1-10% pre-ionized silver. Because silver zeolites quickly release their silver ion reservoir via ion exchange in the presence of sodium, this technology provides relatively short-term antimicrobial protection when cleaned regularly with traditional sodium-laden detergents. In addition, the larger particle size of silver zeolites can cause them to settle out of solution and negatively impact dry film properties such as UV stability and abrasion resistance.

Silver halide salts are also used as an antimicrobial but tend to have limited efficacy due to their low solubility. As a consequence, silver halides tend to require high add rates to achieve significant performance. At these levels, the salts also have a significant negative impact on dry film properties, especially UV color stability. This is not unexpected as silver halides are the basis of traditional photographic chemistry.

Table 3 Click to enlarge

Organic biocide additives are used in the majority of water-based coatings for the protection of the emulsion itself. Organic biocides are effective against specific algae and fungus that can contaminate and degrade the emulsion during processing and in-can storage. However, these biocides are not always equally effective against a broader range of bacteria. They typically degrade or deactivate during application or are wiped away from the dry film once the coating is applied, providing only several months of antimicrobial protection. Some of the organic biocides have the potential to be harmful to humans and Congress is currently considering a consumer ban for one such organic biocide, triclosan.

Organic biocides along with residual solvents, leveling/flow agents, plasticizers, and catalysts can yield a coating that has antimicrobial properties right out of the can. It is very important, therefore, that formulators develop and perform accelerated weather/aging on the dry films in order to determine the effectiveness of an antimicrobial additive. Tables 2 and 3 provide data on aged and unaged polyester coatings. The results demonstrate the importance of aging tests in identifying the durability of antimicrobial effects among competing antimicrobial solutions.

New Role, New Opportunities

The growing financial and human impact of microbe-causing infections in the hospital and beyond is creating new opportunities for the coatings market sector. Their presence in a variety of high-touch point products that can harbor germs opens up a new role in providing preventative antimicrobial protection wherever the public gathers but most particularly in the health care arena. Performance gaps in older antimicrobial technologies, especially in terms of durability and flexibility, have, up to now, prevented coatings formulators from becoming a part of a comprehensive solution to prevent the spread of infection-causing germs. Nanoscale silver additives that use the latest advances in nanotechnology can bridge these gaps. Silver nanoparticles are integrated at the molecular level and designed for controlled ion release, thereby permitting long-term local silver concentrations above the antimicrobial threshold.

They continue to work effectively against microbes – bacteria, mold and fungus – for the expected life of the product. In addition, nanoscale silver is stable, will not thermally or UV degrade, has no impact on other physical properties of the product, and can be easily integrated into the manufacturing process. Most importantly, they have been proven safe and eco-friendly. The combination of coatings with silver nanoparticle-based antimicrobial protection can present a strong line of defense in locations where infection control is of concern.

*Figures and tables courtesy of NanoHorizons Inc. For more information, visit www.SmartSilver.com or e-mail industrial@NanoHorizons.com.