antimicrobial surface antimicrobial surface an antimicrobial surface is coated by an antimicrobial agent that inhibits the ability of microorganisms to grow on the surface of a material. such surfaces are becoming more widely investigated for possible use in various settings including clinics, industry, and even the home. the most common and most important use of antimicrobial coatings has been in the healthcare setting for sterilization of medical devices to prevent hospital associated infections, which have accounted for almost 100,000 deaths in the united states. in addition to medical devices, linens and clothing can provide a suitable environment for many bacteria, fungi, and viruses to grow when in contact with the human body which allows for the transmission of infectious disease. antimicrobial surfaces are functionalized in a variety of different processes. a coating may be applied to a surface that has a chemical compound which is toxic to microorganisms. in alternative, it is possible to functionalize a surface by adsorbing a polymer or polypeptide and/or by changing its micro and nanostructure. an innovation in antimicrobial surfaces is the discovery that copper and its alloys (brasses, bronzes, cupronickel, copper-nickel-zinc, and others) are natural antimicrobial materials that have intrinsic properties to destroy a wide range of microorganisms. an abundance of peer-reviewed antimicrobial efficacy studies have been published regarding copper’s efficacy to destroy e. coli o157:h7, methicillin-resistant staphylococcus aureus (mrsa), staphylococcus, clostridium difficile, influenza a virus, adenovirus, and fungi. apart from the health industry, antimicrobial surfaces have been utilized for their ability to keep surfaces cleaned. either the physical nature of the surface, or the chemical make up can be manipulated to create an environment which cannot be inhabited by microorganisms for a variety of different reasons. photocatalytic materials have been used for their ability to kill many microorganisms and therefore can be used for self-cleaning surfaces as well as air cleaning, water purification, and antitumor activity. antimicrobial activity silver silver ions have been shown to react with the thiol group in enzymes and inactivate them, leading to cell death. these ions can inhibit oxidative enzymes such as yeast alcohol dehydrogenase. silver ions have also been shown to interact with dna to enhance pyrimidine dimerization by the photodynamic reaction and possibly prevent dna replication. the use of silver as an antimicrobial is well documented. copper the antimicrobial mechanisms of copper have been studied for decades and are still under investigation. a summary of potential mechanisms is available here: antimicrobial properties of copper#mechanisms of antibacterial action of copper. researchers today believe that the most important mechanisms include the following: elevated copper levels inside a cell causes oxidative stress and the generation of hydrogen peroxide. under these conditions, copper participates in the so-called fenton-type reaction — a chemical reaction causing oxidative damage to cells. excess copper causes a decline in the membrane integrity of microbes, leading to leakage of specific essential cell nutrients, such as potassium and glutamate. this leads to desiccation and subsequent cell death. while copper is needed for many protein functions, in an excess situation (as on a copper alloy surface), copper binds to proteins that do not require copper for their function. this "inappropriate" binding leads to loss-of-function of the protein, and/or breakdown of the protein into nonfunctional portions. organosilanes organosilanes create a network of electrically charged molecules on the surface, which rupture the cell wall on contact. this is due to their structure which consists of a hydrophobic element, and a cationic element. while the hydrophobic element may prevent adherence in the first place, it may also intercalate with the cell wall, whose rupture is aided by the cationic component. nutrient uptake the growth rate of e. coli and s. aureus was found to be independent of nutrient concentrations on non-antimicrobial surfaces. it was also noted that antimicrobial agents such as novaron ag 300 (silver sodium hydrogen zirconium phosphate) do not inhibit the growth rate of e. coli or s. aureus when nutrient concentrations are high, but do as they are decreased. this result leads to the possible antimicrobial mechanism of limiting the cell's uptake, or use efficiency, of nutrients. quaternary ammonium the quaternary ammonium compound dimethyloctadecyl (3-trimethoxysilyl propyl) ammonium chloride (si-qac) has been found to have antimicrobial activity when covalently bonded to a surface. many other quaternary ammonium compounds are known to have antimicrobial properties (e.g. alkyldimethylbenzylammonium chloride and didecyldimethylammonium chloride). these last two are membrane active compounds; against s. aureus the first forms a single monolayer coverage of the s. aureus cells on the outer membrane, while the second forms a double monolayer. this leads to cell leakage and total release of the intracellular potassium and 260 nm-absorbing pools in this order. selectivity by definition, "antimicrobial" refers to something that is detrimental to a microbe. because the definition of a microbe (or microorganism) is very general, something that is "antimicrobial" could have a detrimental effect against a range of organisms ranging from beneficial to harmful ones, and could include mammalian cells and cell types typically associated with disease such as bacteria, viruses, protozoans, and fungi. selectivity refers to the ability to combat a certain type or class of organism. depending on the application, the ability to selectively combat certain microorganisms while having little detrimental effect against others dictates the usefulness of a particular antimicrobial surface in a given context. bactericides a main way to combat the growth of bacterial cells on a surface is to prevent the initial adhesion of the cells to that surface. some coatings which accomplish this include chlorhexidine incorporated hydroxyapatite coatings, chlorhexidine-containing polylactide coatings on an anodized surface, and polymer and calcium phosphate coatings with chlorhexidine. antibiotic coatings provide another way of preventing the growth of bacteria. gentamicin is an antibiotic which has a relatively broad antibacterial spectrum. also, gentamicin is one of the rare kinds of thermo stable antibiotics and so it is one of the most widely used antibiotics for coating titanium implants. other antibiotics with broad antibacterial spectra are cephalothin, carbenicillin, amoxicillin, cefamandole, tobramycin, and vancomycin. copper and copper alloy surfaces are an effective means for preventing the growth of bacteria. extensive u.s. epa-supervised antimicrobial efficacy tests on staphylococcus aureus, enterobacter aerogenes, methicillin-resistant staphylococcus aureus (mrsa), escherichia coli 0157:h7, and pseudomonas aeruginosa have determined that when cleaned regularly, some 355 different epa-registrered antimicrobial copper alloy surfaces: continuously reduce bacterial contamination, achieving 99.9% reduction within two hours of exposure; kill greater than 99.9% of gram-negative and gram-positive bacteria within two hours of exposure; deliver continuous and ongoing antibacterial action, remaining effective in killing greater than 99.9% of bacteria within two hours; kill greater than 99.9% of bacteria within two hours, and continue to kill 99% of bacteria even after repeated contamination; help inhibit the buildup and growth of bacteria within two hours of exposure between routing cleaning and sanitizing steps. see: antimicrobial copper touch surfaces for main article. viral inhibitors influenza viruses are mainly spread from person to person through airborne droplets produced while coughing or sneezing. however, the viruses can also be transmitted when a person touches respiratory droplets settled on an object or surface. it is during this stage that an antiviral surface could play the biggest role in cutting down on the spread of a virus. glass slides painted with the hydrophobic long-chained polycation n,n dodecyl,methyl-polyethylenimine (n,n-dodecyl,methyl-pei) are highly lethal to waterborne influenza a viruses, including not only wild-type human and avian strains but also their neuraminidase mutants resistant to anti-influenza drugs. copper alloy surfaces have been investigated for their antiviral efficacies. after incubation for one hour on copper, active influenza a virus particles were reduced by 75%. after six hours, the particles were reduced on copper by 99.999%. also, 75% of adenovirus particles were inactivated on copper (c11000) within 1 hour. within six hours, 99.999% of the adenovirus particles were inactivated. fungal inhibitors a chromogranin a-derived antifungal peptide (cga 47–66, chromofungin) when embedded on a surface has been shown to have antifungal activity by interacting with the fungal membrane and thereby penetrating into the cell. additionally, in vitro studies have demonstrated that such an antifungal coating is able to inhibit the growth of yeast candida albicans by 65% and completely stop the proliferation of filamentous fungus neurospora crassa. copper and copper alloy surfaces have demonstrated a die-off of aspergillus spp., fusarium spp., penicillium chrysogenum, aspergillus niger and candida albicans fungal spores. hence, the potential to help prevent the spread of fungi that cause human infections by using copper alloys (instead of non-antifungal metals) in air conditioning systems is worthy of further investigation. surface modification surface roughness the physical topology of a surface will determine the viable environment for bacteria. it may affect the ability for a microbe to adhere to its surface. textile surfaces, tend to be very easy for microbes to adhere due to the abundance of interstitial spacing between fibers. figure 1: wenzel model wenzel model was developed to calculate the dependence that surface roughness has on the observed contact angle. surfaces that are not atomically smooth will exhibit an observed contact angle that varies from the actual contact angle of the surface. the equation is expressed as: c o s θ o b s = r ∗ c o s θ =r*cos\theta } where r is the ratio of the actual area of the surface to the observed area of a surface and θ is the young's contact angle as defined for an ideal surface. see wetting. based on physical modification of the surface, antiviral surface can be designed by decorating micropillars on the surface. chemical modification grafting polymers onto and/or from surfaces antimicrobial activity can be imparted onto a surface through the grafting of functionalized polymers, for example those terminated with quaternary amine functional groups, through one of two principle methods. with these methods—“grafting to” and “grafting from”—polymers can be chemically bound to a solid surface and thus the properties of the surface (i.e. antimicrobial activity) can be controlled. quaternary ammonium ion-containing polymers (pqa) have been proven to effectively kill cells and spores through their interactions with cell membranes. a wealth of nitrogenous monomers can be quaternized to be biologically active. these monomers, for example 2-dimethylaminoethyl methacrylate (dmaema) or 4-vinyl pyridine (4-vp) can be subsequently polymerized with atrp. thus antimicrobial surfaces can be prepared via “grafting to” or “grafting from” mechanisms. grafting onto grafting to involves the strong adsorption or chemical bonding of a polymer molecule to a surface from solution. this process is typically achieved through a coupling agent that links a handle on the surface to a reactive group on either of the chain termini. although simple, this approach suffers from the disadvantage of a relatively low grafting density as a result of steric hindrance from the already-attached polymer coils. after coupling, as in all cases, polymers attempt to maximize their entropy typically by assuming a brush or mushroom conformation. thus, potential binding sites become inaccessible beneath this “mushroom domain”. figure 2: schematic of grafting density. pre-synthesized polymers, like the pdmeama/ptmspma block copolymer, can be immobilized on a surface (i.e. glass) by simply immersing the surface in an aqueous solution containing the polymer. for a process like this, grafting density depends on the concentration and molecular weight of the polymer as well as the amount time the surface was immersed in solution. as expected, an inverse relationship exists between grafting density and molecular weight. as the antimicrobial activity depends on the concentration of quaternary ammonium tethered to the surface, grafting density and molecular weight represent opposing factors that can be manipulated to achieve high efficacy. grafting from this limitation can be overcome by polymerizing directly on the surface. this process is referred to as grafting from, or surface-initiated polymerization (sip). as the name suggests, the initiator molecules must be immobilized on the solid surface. like other polymerization methods, sip can be tailored to follow radical, anionic, or cationic mechanisms and can be controlled utilizing reversible addition transfer polymerization (raft), atom transfer radical polymerization (atrp), or nitroxide-mediated techniques. a controlled polymerization allows for the formation of stretched conformation polymer structures that maximize grafting density and thus biocidal efficiency. this process also allows for high density grafting of high molecular weight polymer which further improves efficacy. superhydrophobic surfaces a superhydrophobic surface is a low energy, generally rough surface on which water has a contact angle of >150°. nonpolar materials such as hydrocarbons traditionally