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 5 rate of cold sprayed Cu vs. structural Cu, irrespective of the scientifically reduc- ible microstructural feature most able to enhance atomic Cu ion diffusivity from the consolidated coatings for the purpose of contact killing/inactivation applications. Moreover, the virus-length-scale and bacterial-length-scale surface rough- ness of the conventional Cu and nano- structured Cu cold spray surfaces have also been reported [13] . Performed us- ing atomic force microscopy (AFM) and 3D-confocal microscopy, the research by Sundberg et al. aimed to procure a more holistic understanding of the mechanisms underpinning the nano- structured Cu cold spray surface’s en- hanced antiviral performance when compared with the conventional Cu cold spray coatings’ efficacy. Coupled AFM-confocal microscopy-based char- acterization of the nanostructured Cu and conventional Cu cold spray con- solidations enabled prospective sur- face-phenomena and surface-mediated mechanisms underlying the antiviral ef- ficacies to be investigated. Soon there- after, Sundberg et al. continued their alternative analysis via the incorpora- tion of corrosion studies and surface effects to thoroughly probe the ionic copper chemical reactivities associat- ed with the conventional Cu and nano- structured Cu cold spray surfaces [21] . Turning to the research published to date and concerned with function- alized antimicrobial copper cold spray coatings since the global COVID-19 pan- demic took effect, four additional re- search articles of immediate relevance are considered next. In large part, two studies [16,22] aimed to initialize the for- mulation of a properties-processing- structure-performance framework for antimicrobial copper cold spray. Also, additional microscopy-based, mechan- ically based, and chemically/physic- ally based characterization of the nano- structured and conventional copper coatings, single-particle deposits, and feedstock materials was performed and reported upon [15] . In terms of mi- crostructural inspection, Fig. 4 pres- ents the cross-sectional renderings of the polycrystallinity associated with the conventional copper cold sprayed coatings. As for Cu cold spray research that was published and specifically con- cerned with SARS-CoV-2 as the viral pathogen being explicitly studied, Huta- soit et al.’s testing resulted in 96% inac- tivation of the novel coronavirus within two hours of exposure to a convention- al Cu cold spray surface via direct con- tact of the pathogen with their coatings. Hutasoit et al. also demonstrated that 99.2% inactivation was achieved at a five-hour exposure period. At the same time, Hutasoit et al. found that an- nealing the as-deposited convention- al Cu cold spray coating resulted in a decrease in the anti- pathogenic efficacy, which is consistent with the framework presented by Sousa et al. [16] . In any case, the speed at which the coatings were applied, as well as the 45-de- gree angle relative to the stainless-steel push plate substrate, likely prevented the feedstock powder from being maximally de- formed and therefore incapable of reaching optimal atomic Cu ion diffusivity. Thus, inactivation rates with more idealized processing parameters would likely achieve great- er than 96% reduction of SARS-CoV-2 at the two-hour mark [4] . Lastly, Hutasoit et al.’s recorded reduction percentage at two hours is also consistent with the as-deposited conventional copper cold spray coating procured for Influenza A inactivation, wherein a 97.3% reduction was achieved for another viral agent [18] . Around the same time, Lucas et al. reported a greater than seven log decrease in microbes exposed to a conventional copper cold spray coat- ing; wherein noteworthy decreases in Fig. 3 — Percent of Influenza A that remained active as a function of exposure time to Cu cold spray surfaces in comparison with a traditionally structural copper component. Adapted from Champagne et al. [20] . Fig. 4 — Cross-sectional high-resolution scanning electron micrographs of conventional copper cold spray material consolidations. The samples were prepared using a focused ion beam milling system. Two of the micrographs were adapted from Sundberg et al. [13] .

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