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 4 substantiated the microstructurally de- pendent conjecture that had been put forth [17] . Remarkably, Champagne et al. found that the Cu cold spray surfaces killed greater than 99.999% of inoculat- ed MRSA. Two years later, in 2015, a nov- el nanostructured Cu cold spray surface was found to inactivate 99.3% of inocu- lated Influenza A virions after two hours of exposure [18] . Such an inactivation rate was greater than that achieved when the conventional copper cold spray con- solidation was utilized, which was orig- inally reported upon in 2013, and had reached 97.7% inactivation. As will be discussed shortly hereafter, the nano- structured Cu cold spray surfaces were consolidated using a novel spray-dried feedstock material in comparison with the conventional use of gas-atomized feedstock powder. In addition to study- ing the virial inactivation rates achieved via Cu cold spray as a function of the mi- crostructure of the resultant coatings, the conventional Cu cold spray coating process was redeployed two addition- al times using two additionally gas-at- omized and pure copper powders from different powder suppliers. This was re- portedly pursued by Champagne et al. in 2015 to ensure that the MRSA-based contact killing capacity of the consoli- dated surfaces could be reproduced for verification and validation of the ob- servations first reported upon in 2013. Each additional version of the Cu cold spray coatings produced using conven- tional feedstock powder was found to be consistent with the original findings, wherein each of the surfaces reached a bacterial reduction rate greater than 99.9% of the inoculated MRSA following two hours of direct contact [18] . Nearly six years after Champagne et al.’s proof-of-concept study was pub- lished, da Silva et al. reported upon the antimicrobial properties of anoth- er conventionally deposited copper cold spray coating [19] . They found that their Cu cold spray coating was able to effectively achieve nearly complete inhibition of Staphylococcus aureus ’s growth after ten minutes of continu- ous contact with the Cu surface. One month after da Silva et al.’s work was published, Sousa et al. reinvigorat- ed the interest in the antipathogenic nanostructured Cu cold spray coatings by way of exploring the nanomechani- cal behavior of the consolidated surfac- es through spherical nanoindentation stress-strain curve analysis [14] . Sousa et al. explored the mechanical properties of the novel nanostructured as well as conventional Cu cold spray coatings to assess their mechanical suitability and durability for common touch surfac- es in hospital rooms that require resis- tance to plastic deformation and to also probe a hypothesis presented by Cham- pagne et al., which concerned the use of hardness and/or strength to assess the amount of defect-mediated Cu ion dif- fusion pathways present in the coatings (Fig. 2). With the aforementioned in mind, Champagne et al. continued to criti- cally examine their own research by way of synthesizing work reported be- tween 2013 and 2019 through the lens of a hypothesis that identified the dis- location densities brought about by the cold spray deposition process as the microstructural feature most respon- sible for the antibacterial and antiviral functionality of the coatings [20] . As has been discussed elsewhere, the claim by Champagne et al. that dislocation den- sity generation serves as cold spray’s “application-dependent mechanism for antimicrobial effectiveness” has been refined and has refocused upon the de- gree of dynamic recrystallization grain refinement experienced by copper during cold spray processing [16] . In oth- er words, Champagne et al., in 2019, reported dislocation density as the mi- crostructural constituent responsible for enhanced antiviral contact inacti- vation or antibacterial contact killing, whereas Sousa et al. combined disloca- tion density with grain-boundary med- itated atomic Cu diffusion. Regardless, Fig. 3 attests to the viral inactivation Fig. 2 — (a) Chemically etched, cross-sectional scanning electron micrograph of the nanostructured Cu cold spray coating; (b) chemically etched, cross-sectional scanning electron micrograph of the conventional Cu cold spray coating; and the refined indentation stress vs. strain curves for the nanostructured and conventional Cu cold spray coatings in the cross- sectional orientations shown in (a) and (b). Adapted from Journal of Thermal Spray Technology [15] .