January_2021_AMP_Digital

A D V A N C E D M A T E R I A L S & P R O C E S S E S | J A N U A R Y 2 0 2 1 3 3 Cu 1+ and Cu 2+ . An extreme example of a class of aerobic microorganisms (un- der ideal conditions) follows from the aerobic methane-oxidizing bacterium known as methanotrophs. More specif- ically, methanotrophs are a gram-neg- ative and methane oxidative form of bacteria with more than thirty Cu-con- taining proteins [8] . As such, researchers have attempted to invoke methano- trophs as being a potential surrogate for understanding how microbes gener- ally uptake or remove Cu in a regulated fashion [9] . However, in doing so, research- ers appeared to be unaware of the fact that the critical Cu-methanobactin re- sponsible for the uptake and remov- al of Cu ions in methanotrophs have been shown to be unable to be inter- nalized by non-methanotrophs, such as E. coli , in contrast with the internal- ization of Cu-methanobactin from one methanotroph to another [8] . As such, one cannot consider methanotrophs as suitable surrogate microbes for garner- ing wide-ranging Cu regulation mech- anisms across various pathogens, as has previously been suggested else- where [9] . Accordingly, attention will be refocused back toward the vast span of Cu-sensitive pathogens, which are also non-methanotrophs, to potential- ly identify prospective commonalities underpinning Cu-regulation in micro- bial agents of interest. Therefore, one may consider the fact that gram-nega- tive bacterial cells have been shown to commit metabolic self-destruction [10] , wherein “the outer membrane is the initial target and Fenton reactions be- tween respiration generated oxygen radicals and copper ions results in the generation of reactive oxygen spe- cies,” thus leading to “metabolic sui- cide” as expressed by Warnes et al. [11] . In contrast with gram-negative bacte- ria, Warnes et al. found that viral cop- per contact inactivation efficacy (via viral RNA destruction) was proportion- al to the Cu-content associated with the dry Cu-surfaces that were inoculat- ed with noroviruses. Warnes et al. also observed a reduction in the total copy number of viral-protein-genome-linked proteins, which are required for infec- tivity, with increasing copper content relative to a stainless-steel control [10] . The work by Warnes et al. that was just discussed is concerned with the vi- ral inactivation mechanism associated with a non-enveloped viral pathogen that was directly exposed to and inoc- ulated upon copper surfaces. However, SARS-CoV-1, MERS, and SARS-CoV-2 are examples of enveloped viruses, rather than non-enveloped viruses, and there- fore react with Cu ions in a manner that is uniquely distinguishable from non-enveloped pathogens – precisely because of the presence of said enve- lope. Therefore, one currently hypoth- esized mechanism for copper contact inactivation of enveloped coronavi- ruses was achieved via the use of hu- man coronavirus 229E [12] . In this case, Warnes et al. found that “Exposure to copper destroyed the viral genomes and irreversibly affected virus morphol- ogy, including disintegration of enve- lope and dispersal of surface spikes… the inactivation… was enhanced by re- active oxygen species generation” on the copper surface. While it is appreciable to note that the microbiologists who penned the references cited above also observed the fact that the contact killing or in- activation rates vary as a function of Cu content, less intuitive material-lev- el characteristics are also known to af- fect the antipathogenic performance of Cu surfaces too. Such characteristics include surface roughness [13] , micro- structure [14–16] , and the like. For exam- ple, Champagne et al. hypothesized that the distinctive microstructures af- filiated with pure copper arc sprayed, plasma sprayed, and cold sprayed, sur- faces would yield dissimilar antipatho- genic activity as a direct consequence of the microstructures associated with each material processing condition. Examination of microstructures asso- ciated with each of the three thermal- ly sprayed coatings led to the following observation made by Champagne et al. [17] : “[differences] in microstructure are evident, suggesting that differenc- es in biological activity may also oc- cur.” Such an observation is consistent with Champagne et al.’s hypothesis that antipathogenic performance depends upon not only copper content but also the microstructure associated with the deposited copper. Figure 1 presents the cross-sectional renderings of the resul- tant microstructures just discussed via optical microscopy. ANTIPATHOGENIC COPPER COLD SPRAY COATINGS Concerning the plasma spray- based, cold spray-based, and arc spray- based antipathogenic surfaces devel- oped by Champagne et al. and detailed above, the resultant percent of MRSA that survived direct contact through two hours of inoculation on the surfaces Fig. 1 — (a) Cross-sectional optical micrograph of the Cu wire arc sprayed coating; (b) cross-sectional optical micrograph of the Cu plasma sprayed coating; and (c) cross-sectional optical micrograph of the conventional Cu cold spray consolidation. Adapted fromChampagne et al. [17] .