Technical Guidance on the Testing of Self-Disinfecting Surface Coatings against SARS-CoV-2

For conducting tests on the activity of the self-disinfecting surface coatings, it is important to consider and standardise the test materials of the surfaces intended for coating. As reported in The New England Journal of Medicine, SARS-CoV-2 was more stable on plastic and stainless steel than on copper and cardboard, and viable virus was detected up to 72 hours after deposition onto the former materials [3]. Stainless steel has been used as the reference material for testing the coating products on, due to the ease of using heat-treatment and various sterilisation methods for preparing the material. Several standards (including ISO22196, ISO21702 and JIS Z2801) also recommend the use of plastics as a test surface. Test material coated with a control product (vehicle) or untreated should be used as a control.

This guidance also considers the flexibility of using other test materials for testing the coatings on, such as, but not limited to, hardy board / calcium silicate panels, or even paper dry wall. These may be more representative materials used in an interior space. The selected test materials should be aligned with the intended use sites of the products (e.g. stainless steel lift buttons, metal grab poles, etc.).

NEA recommends that claims of effectiveness against COVID-19 virus are supported by robust scientific data directly against the causative agent itself (SARS-CoV-2) or against other coronaviruses (e.g. human coronavirus 229E, OC43, or a mouse coronavirus MHV (murine hepatitis virus)). This will ensure that the claims on reducing fomite-mediated transmission risk of COVID-19 are well-supported with direct evidence against coronaviruses.

Human coronavirus 229E has been most recommended by many international agencies. Therapeutic Goods Administration (TGA) of Australia recognises the use of human coronavirus 229E and MHV; US EPA indicates that the use of “another type of human coronavirus” similar to SARS-CoV-2 is requested, and addresses that the use of animal coronaviruses is not supported. Health Canada suggests that an indirect claim against SARS-CoV-2 can be allowed if the test was carried out against a specific coronavirus such as human coronavirus 229E, MERS-CoV, or SARS-CoV.

For measuring the virucidal activity, infectious assays (and not molecular methods such as PCR) should be used to measure the reduction in viral loads. These tests typically use tissue-culture based viral titration assays that measure the infectious titers of viruses.

In the efficacy test, the challenge virus is recovered from the treated surface to quantify the amount of virus reduction afforded by the product after the stipulated contact time. This is commonly achieved by flushing the challenged surface with a recovery media/buffer. Product leaching could occur if a coating product is removed alongside the challenge virus during recovery.

Product leaching may affect the interpretation of the test results in two ways. Firstly, the leached test product may continue to inactivate the virus recovered in the solution, resulting in an overestimated virus inactivation efficacy of the product after a stipulated contact time. Preliminary tests on selected products conducted at the Environmental Health Institute (EHI) of NEA showed that virus inactivation occurred in the recovery media (in solution), rather than on the coated surfaces. Secondly, the leached product may induce cellular toxicity on the cell monolayer used for virus quantification, making it difficult to distinguish the virus-induced cytopathic effect from the product-induced cytotoxic effect. If cytotoxicity of the test product is observed, higher dilution factor should be used to measure the actual viral load reduction (e.g. if cytotoxicity was observed at a dilution of 10^-3 in a TCID₅₀ assay, reduction of the virus infectious load must be demonstrated from dilutions of 10^-4 to 10^-7 to qualify a 3-log virus inactivation claim).

A neutralisation step should be performed immediately after the recovery of the challenge virus at the stipulated contact time. This can be achieved by (1) recovering the virus in a neutraliser solution (a buffer solution known to inhibit the effects of the test product), or (2) separating the recovered virus from the test product via gel filtration method.

A typical neutraliser solution referred to in ISO 21702 is Soya Casein Digest Lecithin Polysorbate (SCDLP) broth. CDC guidelines [4] also provide some examples of neutraliser solutions such as Letheen Media and Dey/Engley (D/E) Neutralising Media that can inactivate residual disinfectants. Alternatively, an appropriate chemical neutraliser may be used if the chemical nature of the test product could be neutralised by anionic-cationic interactions. Further examples of neutralisers can be found in ASTM E1054 and EN 1040. It is the responsibility of the suppliers to validate the effectiveness of their selected neutralisers on neutralising the residual antiviral activity of the test product in solution prior to the study, by following methods such as those stipulated in ASTM E1053 Standard Practice to Assess Virucidal Activity of Chemicals Intended for Disinfection of Inanimate, Nonporous Environmental Surfaces.

If a neutraliser is not available or ineffective, gel filtration method using Sephadex columns to separate the recovered virus from the test product may be considered (please refer to ASTM E1482 - 12(2017) Standard Practice for Use of Gel Filtration Columns for Cytotoxicity Reduction and Neutralization). 

Some standards have cited a contact time of up to 24 hours. Sufficient efficacy should be achieved within reasonably short time to effectively control the spread of COVID-19, as coating products are primarily targeted for use on high-touch surfaces.

Challenge virus could be applied using either a wet-film method (virus inoculum overlaid with a piece of film to prevent drying up) or a dry method (virus inoculum allowed to dry naturally after application) as described in several test standards/protocols. The wet-film method could potentially allow better contact of the product with the virus for a longer period of time, resulting in overestimated virus inactivation efficacy of the product.

The dry method mimics the real-world application where the virus-containing droplets are deposited onto surfaces and dry gradually. Efficacy data obtained from both the dry method and the wet-film method could be accepted if contact time is up to 15 minutes, as the droplets typically dry out after about 15 minutes. For contact times above 15 minutes, the dry method is preferred as it is more realistic.

Viruses on the surfaces decay with time naturally. The claimed antiviral activity of the product should be demonstrated beyond the natural decay of the virus on the tested surface (e.g. >3 log reduction above the natural decay of the virus on the specific test material). A control experiment on virus recovery from untreated surface should be conducted to assess if the coating imparts any additional benefit.

The suggested use sites in relation to contact times, together with a proposed antiviral efficacy value matrix in grouping/rating self-disinfecting surface coating products are shown below.

1.5.1 The value of short contact times (0 to 15 minutes):

A similarly high standard of contact times (within 10 minutes) expected of general surface disinfectants [5] should be applied to self-disinfecting surface coating products, as these products are typically targeted for use on high-touch surfaces.

A product within this category would allow the mitigation of virus spread via contaminated surfaces in the environment of high traffic volume, and a high virus prevalence in the population.

1.5.2 The value of longer contact times (>15 minutes to 2 hours), using dry method:

If a product requires >15 – 120 minutes to take effect, in places where there is high traffic volume (e.g. airport, public transport, work places and schools) and high prevalence of infected individuals, virus transmission by surface contamination can still occur regardless of the presence of the applied coating. However, the coating may reduce viral load and thus potentially reduce the virus transmission rate. This class of coating may be beneficial if logistical constraints prevent full wipe-down cleaning to be carried out at a frequency of 2 hours.

A product within this category may allow the mitigation of virus spread via contaminated surfaces in the environment of medium traffic volume, and a medium virus prevalence in the population.

This class of coating may be beneficial if logistical constraints prevent full wipe-down cleaning to be performed at a daily frequency.

1.5.3 The value of extended contact times (>2 to 24 hours), using dry method:
A product within this category may allow the mitigation of virus spread via contaminated surfaces in the environment of low traffic volume, and a low virus prevalence in the population.

1.5.4 A proposed antiviral efficacy value matrix:
We propose a matrix to rank the coating products into three classes based on efficacy level and contact time (Table 1). This matrix serves as a guide and is not intended to be prescriptive.

Table 1. Antiviral efficacy value matrix for self-disinfecting surface coating products.

Class A – high traffic volume and high virus prevalence.
Class B – medium traffic volume and medium virus prevalence.
Class C – low traffic volume and low virus prevalence.

Tests should be conducted to study the duration of residual virucidal effect (i.e. how long can the coating be left on the surfaces, and still retain sufficient virucidal activity). As self-disinfecting coating products are marketed on their long-lasting virus inactivation effect (weeks to months), the product’s durability and stability over time should be verified. Virus inactivation efficacy in the long term should be studied following multiple cleaning circles to simulate a defined duration of wearing, or by actual exposure to real-world conditions for an extended period. The duration tested should be aligned with the residual effect duration claimed by the products.

Figure 1 compares two products with identical short-term virus inactivation efficacy, but different durability. When surfaces coated with the two products, respectively, are repeatedly challenged with viruses, the product with long-lasting/durable effect continuously inactivates the deposited viruses with sustained efficacy, thus reducing the viral loads on the surface. Conversely, the product with diminishing durability is initially effective at reducing the viral loads, but gradually becomes ineffective, resulting in virus accumulation on the surface.

Figure 1. Illustration of the effect of product durability on viral load reduction

Stability of the coatings may be affected by the ambient environment (e.g. temperature, humidity) and by the mechanical and chemical interactions during cleaning. The stability of the surface coats under use conditions should be studied to inform the frequency (i.e. time intervals required) for re-application of the products.

Durability tests should include, but are not limited to, 1) simulated cleaning in accordance with the cleaning regimes, 2) abrasion test, 3) scratch test, 4) repeated swabbing test to mimic use conditions in high traffic areas. To account for the outdoor environmental conditions in Singapore, durability test in a high temperature, high humidity environment chamber (30-33 °C, 70-100% humidity) should be performed in parallel. The durability tests conducted should be aligned with the product claims set out by the companies.

ISO 21702:2019 “Measurement of antiviral activity on plastics and other non-porous surfaces” describes the inoculation of the test surface with 400 µL of test virus which is subsequently covered with a film to prevent drying over the contact time of up to 24 hours. This protocol may not reflect the actual conditions as the test surface is kept moist (with the application of a film) throughout the test duration. Modification of this method could be considered to allow the use of a small volume of virus inoculum, followed by natural drying over the contact time.

US EPA’s Interim Method for Evaluating the Efficacy of Antimicrobial Surface Coatings [6] is a method currently under development. This method can be modified to include tests on viruses. The test protocol encompasses neutralisation test, challenge inoculum allowed to dry, and durability tests (abrasion and chemical tests).

We encourage testing laboratories and companies to consider all the highlighted points stated above when evaluating the products.

NEA continues to receive and review data provided by suppliers of self-disinfecting surface coating products. Our assessment process follows the flowchart presented in Figure 2. Briefly, the study methodology is first assessed to determine if it is appropriate for testing a self-disinfecting surface coating product. For example, methods that test the product as an in-solution disinfectant, instead of a coating, are deemed inappropriate. A neutralisation test is also required to be considered an appropriate protocol. To ensure that claims on reducing fomite-mediated transmission risk of COVID-19 are well-supported with direct evidence against coronaviruses, NEA accepts efficacy data against the causative agent itself (SARS-CoV-2) or other coronaviruses (e.g. human coronavirus 229E, OC43, and MHV). If contact times are up to 15 minutes, wet-film method is acceptable. However, dry method will be requested for contact times >15 minutes – 24 hours. Finally, durability test needs to be supported by efficacy data showing sustained virus inactivation efficacy after a certain duration of weathering (e.g. simulated cleaning, abrasion test, and chemical test), which should be aligned with the durability claims of the product.

Figure 2. Evaluation process for self-disinfecting surface coating products

Self-disinfecting surface coating products are designed to be used on surfaces exposed to high human traffic and in high-touch zones. The safety aspects of these coating products, and their effects on human health and environment should therefore be considered.

Toxicity of the test product or active ingredients and their potential risks to human health and the environment should be noted. For example, toxicity studies can include (1) acute toxicity studies (e.g. acute oral toxicity, acute dermal toxicity, acute inhalation toxicity, primary dermal irritation, primary eye irritation, and skin sensitization) to assess the potential immediate hazard to human health; (2) chronic toxicity studies (e.g. carcinogenicity, mutagenicity, and reproductive toxicity) to assess the potential long-term hazard to human health; (3) ecotoxicity studies (e.g. acute avian oral toxicity, acute toxicity to freshwater fish, acute toxicity to freshwater invertebrates (daphnia magna), and toxicity to fresh water algae) to assess potential adverse effects on non-target organisms in the environment.

At present, most of self-disinfecting surface coating products exhibit inactivation activities by releasing the active ingredients from the surfaces (e.g. heavy-metal impregnated surfaces) [1]. Repeated exposure to the eluted active ingredients may cause adverse effects to humans and environments. To study the potential leaching of the active ingredients from the surfaces, zone of inhibition test [7] [8] could be conducted as follows. A bacterial strain of interest is grown in pure culture, followed by spreading a suspension of the pure culture evenly over the face of a sterile agar plate. The surface coat is applied to the centre of the agar plate and incubated for 18-24 hr (or longer if necessary), at a temperature suitable for the test bacteria. If active ingredient leaches from the surface coat into the agar and then exerts a growth-inhibiting effect, a clear zone (the zone of inhibition) appears around the surface coat. The method is not classically quantitative, but the diameter of the zone of inhibition could be measured to reflect the degree of leaching.

Although the zone of inhibition test can only be conducted against bacteria, the test result is representative to demonstrate the intrinsic leaching property of the surface coat. Prior to zone of inhibition test, antibacterial efficacy studies of the surface coat should be conducted to demonstrate sufficient efficacy against selected bacteria. At least three different bacteria should be tested in the antibacterial efficacy studies and the subsequent zone of inhibition tests to demonstrate the non-specificity of the test results.

If leaching is detected from the zone of inhibition test, quantitative chemical analysis of the eluate should be performed to determine the amount of the eluted active ingredient. Subsequently, risk assessment should be conducted to assess the exposure levels of the released active ingredient and the resulted potential risks to public health, in order to justify the safe use of the surface coat.

Self-disinfecting surface coatings have been marketed as products that can help reduce the transmission of SARS-CoV-2 by fomite in this pandemic. Many products do not come with robust scientific evidence to support their claims, due to the lack of a standard in the market for these “new breed” of products. The aim of this document is to provide guidance on the robust scientific assessment of efficacy, safety and quality of self-disinfecting surface coatings, and to help encourage scientific and policy discussions with the aim of establishing a Singapore or International Standard for assessing these products.

[1] M. Ahonen, A. Kahru, A. Ivask, K. Kasemets, S. Koljalg, P. Mantecca, I. V. Vrcek, M. M. Keinanen-Toivola and F. Crijns, “roactive approach for safe use of antimicrobial coatings in healthcare settings: Opinion of the COST action network AMiCI.,” International Journal of Environmental Research and Public Health, vol. 14, p. 366, 2017. 

[2] D. J. Weber and W. A. Rutala, “Self-disinfecting surfaces: Review of current methodologies and future prospects.,” American Journal of Infection Control, vol. 41, pp. S31-S35, 2013. 

[3] N. van Doremalen, T. Bushmaker, D. H. Morris, M. G. Holbrook, A. Gamble, B. N. Williamson, A. Tamin, J. L. Harcourt, N. J. Thornburg, S. I. Gerber, J. O. Lloyd-Smith, E. de Wit and V. J. Munster, “Aerosol and surface stablility of SARS-CoV-2 as compared with SARS-CoV-1,” NEJM, vol. 382, no. 16, pp. 1564-1567, 2020. 

[4] CDC, “Guideline for Disinfection and Sterilization in Healthcare Facilities,” 2008. [Online]. Available: https://www.cdc.gov/infectioncontrol/guidelines/disinfection/disinfection-methods/regulatory-framework.html.

[5] “List of Household Products and Active Ingredients for Surface Disinfection of the COVID-19 Virus,” [Online]. Available: https://www.nea.gov.sg/our-services/public-cleanliness/environmental-cleaning-guidelines/guidelines/interim-list-of-household-products-and-active-ingredients-for-disinfection-of-covid-19.

[6] “US-EPA Interim Method for Evaluating the Efficacy of Antimicrobial Surface Coatings,” [Online]. Available: https://www.epa.gov/pesticide-analytical-methods/antimicrobial-testing-methods-procedures-interim-method-evaluating.

[7] M. Laboratory, “Zone of inhibition test for antimicrobial activity,” 17 July 2020. [Online]. Available: https://microchemlab.com/test/zone-inhibition-test-antimicrobial-activity.

[8] C. Wang, J. Wu, L. Li, C. Mu and W. Lin, “A facile preparation of a novel nonleaching antimicrobial waterborne polyurethane leather coating functionalized by quaternary phosphonium salt.,” Journal of Leather Science and Engineering, vol. 2, p. 2, 2020. 

[9] “Interim Guidance - Review for Products Adding Residual Efficacy Claims,” [Online]. Available: https://www.epa.gov/pesticide-registration/interim-guidance-review-products-adding-residual-efficacy-claims.

[10] T. G. Administration, “TGA instructions for disinfectant testing, Australian government, Department of Health, Therapeutic Goods Administration,” 2020.