Antimicrobial Copper Cold Spray Coatings and SARS-CoV-2 Surface Inactivation

This article contextualizes how the antimicrobial properties and antipathogenic contact killing/inactivating performance of copper cold spray surfaces and coatings and can be extended to the COVID-19 pandemic as a preventative measure. Specifically, literature is reviewed in terms of how copper cold spray coatings can be applied to high-touch surfaces in biomedical as well as healthcare settings to prevent fomite transmission of SARS-CoV-2 through rapidly inactivating SARS-CoV-2 virions after contaminating a surface. The relevant literature on copper-based antipathogenic coatings and surfaces are then detailed. Particular attention is then given to the unique microstructurally-mediated pathway of copper ion diffusion associated with copper cold spray coatings that enable fomite inactivation.


INTRODUCTION
The objective of this review article is to situate the way in which the antipathogenic properties and antimicrobial contact killing/inactivating performance of copper cold spray coatings and surfaces can be extended to the COVID-19 pandemic as a preventative measure against transmission. Specifically, literature is reviewed from a materials performance and mechanistic perspective on how copper cold spray coatings are successfully able to be utilized as high-touch surfaces in biomedical as well as healthcare settings as an antimicrobial and viricidal material solution for enhanced prevention of fomite transmission of SARS-CoV-2 through rapidly inactivating SARS-CoV-2 virions after infecting a surface. For example, nursing homes, medical facilities, public 2873 transportation, and schools have become high traffic focal points for the spread and transmission of the SARS-CoV-2 virus during the current pandemic. Such settings house a significant volume of high touch surfaces on which the SARS-CoV-2 virus has been shown to be able to remain active and transmissible. By way of refitting the high touch surfaces of the most vulnerable locations and organizations with such antiviral copper cold spray coatings, our functionalized coatings would be able to contribute to the mitigation and prevention of SARS-CoV-2 infection.

COPPER AS AN OLIGODYNAMIC ANTIMICROBIAL MATERIAL
Numerable means of antimicrobial copper-containing materials and coatings production have been reported upon to date. Given the resurgence in reported research concerned with this topic, only a handful of current research articles are situated, as well as discussed below, within the array of antipathogenic copper surfaces, which have been reported upon to date. Accordingly, a hybrid poly-(lactide-co-glycolide), copper-oxide nanoparticle containing composite, and nanofiber-based scaffolding, was developed by Haider  subtilis. Each of the copper-based metallic glass composites were also found to completely eliminate the bacteria tested through a time-kill approach after no more than 250 minutes of direct contact and surface exposure.
Ciacotich et al. published an analysis of the antibacterial efficacy of an alloyed copper coating with silver as the alloying element in 18 . To perform a proper investigation of the antipathogenic performance, the copper-silver coating was subjected to testing conditions, which were defined by an EPA protocol of relevance, wherein a bacterial biofilm was imposed upon the surface of the alloyed copper-silver coating. Ciacotich et al. found that a Gram-positive bacterial biofilm was completely killed within 5 minutes of exposure time. Continued consideration of Ciacotich et al.'s hypothesis shows that the microorganism contact killing activity and behaviour associated with copper-silver coatings is a multifactorial/multivariate and complex process, which depends upon copper ion diffusion, bacterial cell oxidation, local variations in pH, and more. Turning towards commercially pure copper as the antimicrobial material of particular focus, Kocaman et al. produced biocidal wire arc sprayed copper coatings using a twin wire arc spray gun and a stainless-steel substrate surface 19  between Gram-positive and Gram-negative bacteria. Kocaman et al. also provided a reasonable hypothesis as to why the superbugs required greater exposure times for complete contact killing based upon the fact that superbugs have been found to have "thicker cell membranes, possibly causing a decrease in the rate of ion diffusion through the cell wall." It is the opinion of the authors of the present review article that the hypothesis provided by Kocaman et al. deserves continued investigation in future work. While discussion could continue, the final copper-containing approach, which will be noted herein before transitioning to SARS-CoV-2 and viral contact inactivation, concerns the work of Muralidharan et al. Specifically, a sulphonated poly-(ether-ether-ketone)copper film for antimicrobial applications was detailed by Muralidharan et al. 20 .
Another recent work of scholarship documented the use of Luminore CopperTouch TM coatings to inactivate SARS-CoV-2 on coated surfaces 21 . Unfortunately, the research by Mantlo et al. does not appear to delve too deeply into the realm of mechanisms associated with copper-mediated contact inactivation of SARS-CoV-2. Regardless, the Luminore CopperTouch TM surface was found to inactivate 99% of the SARS-CoV-2 titters within 2 hours while also inactivating 99.9% of the Ebola as well as Marburg viruses in that period of time as well. Consistent with our own claim that cuprous oxide (Cu2O) is likely to be just as effective as pure copper in diffusing the copper ions needed for viral contact inactivation 22 , recent work undertaken at Virginia Polytechnic Institute and State University has identified another copper-based coating that can also rapidly inactivate SARS-CoV-2 too 23 . Still, one of the most promising aspects of copper cold spray antipathogenic coatings relative to the coatings presented by Behzadinasab et al. and Mantlo et al. is the likelihood of even greater inactivation rates below 1-hour of exposure time given the dynamically recrystallized and severely plasticly deformed microstructure, which greatly enhances ion diffusivity.

ANTIMICROBIAL COPPER COLD SPRAY COATINGS
While antimicrobial copper coatings have been presented to the scientific community with greater regularity since the 20 th century, one of the first focused publications was published by Champagne and Helfritch 24 . Champagne and Helfritch's research illustrated how copper cold spray surfaces achieved greater antibacterial performance as a contact killing surface in comparison with two alternative copper surfaces, which were generated using plasma spray and wire arc spray. The demonstration by Champagne and Helfritch was achieved by way of inoculating MRSA on the three thermally sprayed surfaces for 2 hours, based on an EPA protocol of relevance, and the MRSA cells that survived were assayed. In comparison with the percentage of MRSA that survived 2 hours of direct contact and exposure to the plasma sprayed copper surface, which was greater than 10% when normalized against a stainless-steel control surface, the copper cold spray surface killed more than 99.999% of the inoculated antibiotic resistant MRSA.
Soon thereafter, Champagne and Helfritch reiterated their original findings 24 in subsequent papers explicitly focused upon highlighting the unique applications wellsuited for cold spray materials consolidation. That being said, Champagne and Helfritch did not stop exploring the antimicrobial efficacy associated with copper cold spray surfaces after publishing their original "proof of concept" study using MRSA as the test case. Rather, they subsequently partnered with academic researchers (Sundberg et al.) to demonstrate the way in which more rapid contact killing/inactivation rates can be achieved by utilizing a novel nanostructured spray-dried pure copper feedstock powder instead of the conventionally gas-atomized powder typically used in copper cold spray deposition 25  Thereafter, publications focusing on various material aspects such as surface roughness, surface species and surface chemistry, corrosion, and microstructure, were pursued by Sundberg et al. [26][27][28][29][30] Around the same time, Champagne, Sundberg, and Helfritch, coauthored a more focused document that reformulated their respective mechanistic framework for interpreting the postulated reasons as to why antimicrobial copper cold spray coatings antipathogenic efficacies outperform many alternative surface engineering solutions 31 . Not long after, a holistic framework for understanding the antipathogenic performance unique to copper cold spray coatings was presented by Sousa et al. 22 Beyond  36 . Additional work by da Silva et al. 37 resulted in self-sanitizing copper cold spray coatings that "completely inhibited after 10 [minutes] of direct contact between the bacteria and the coating surface."

MECHANISM OF ENHANCED ANTIVIRAL PERFORMANCE
Having established the non-negligible transmission pathway associated with SARS-CoV-2 and other viral pathogens via fomites wherein surface-to-hand and hand-to-mucous membrane inoculation occurs; copper as the most oligodynamic, antimicrobial, and naturally occurring elemental metal; and the application of copper cold spray materials consolidations and surfaces as an antipathogenic biomaterial, another critical question remains to adequately appreciate antimicrobial copper cold spray. Explicitly, one may wonder the following: "Why is antimicrobial copper cold spray able to achieve enhanced antiviral performance?" Irrespective of ribonucleic acid and/or nucleoprotein damage, membrane and/or membrane protein damage, or reactive oxygen species (ROS) formation and pathogen interaction is/are the primary mechanism underpinning copper's capacity to inactivate or kill pathogens in contact with a surface, the ability of a copper surface to readily diffuse copper ions to the microbe in contact with the surface dictates if any of the currently proposed damage pathways can be actualized.
That is to say, in most of the pathogen-copper killing/inactivation interactions reported upon to date, such as genomic damage, membrane disruption and damage, ROS, or atomic copper ion speciation 38 , microstructural and physical pathways for copper ion diffusion to the infectious agent from the copper material must be achieved. As briefly discussed earlier, Champagne et al. attributed the unique antipathogenic performance of copper cold spray coatings to the "extreme work hardening and correspondingly high dislocation density within the deposit… and ionic diffusion occurs principally through these dislocations…" associated with the supersonic particle consolidation process in 24 . dislocation driven copper ion diffusion framework for assessing the increased contact killing/inactivation rates for both viral and microbial pathogens relative to non-cold spray materials/solutions. As a result, the most current assessment and research provides readers with a detailed look into the role such material defects, i.e., dislocations, maintain relative to the role of grain-boundary mediated copper ion diffusion 22 .

CONCLUDING REMARKS AND FUTURE OUTLOOK
Considering the successful inactivation of Influenza A virions after exposure to copper cold spray coatings, in accordance with EPA evaluation protocols, and research demonstrating inactivation of SARS-CoV-2 after exposure to a conventional and less viricidal copper-based material (relative to copper cold spray surfaces), copper cold spray materials consolidations could quickly be optimized as a focused mitigation strategy for the COVID-19 pandemic. Deploying antipathogenic copper cold spray surfaces in the fight against SARS-CoV-2 and COVID-19 will simultaneously provide continued public health security by way of mitigating the magnitude of future pandemics through the prevention of fomite transmission from these coatings and microstructurally unique copper material.
Regardless of the fact that fomite transmission "isn't thought to be the main way the virus