A la recherche de la Kryptonite du COVID : La meilleure lampe UV pour la désinfection du virus COVID-19

Une collaboration de recherche entre le NIST et le DHS révèle les meilleures longueurs d’onde de la lumière UV pour COVID-19 virus disinfection.

To disinfect a surface, you can illuminate it with a blast of ultraviolet (UV) light, which is at a bluer wavelength than the human eye can see. But to specifically inactivate SARS-CoV-2, the virus that causes COVID-19, which wavelengths are best? And how much radiation is sufficient?

Two main obstacles must be overcome for scientists to answer those questions. First, they need to completely separate the virus from extraneous substances in the environment. Second, they must illuminate the virus with a single wavelength of UV light at a time, with minimal changes to the experimental setup between tests.

A recent study overcame both these obstacles and completed what may be the most thorough test ever conducted of how several different UV and visible wavelengths affect SARS-CoV-2. The research was a collaboration between the National Institute of Standards and Technology (NIST) and the National Biodefense Analysis and Countermeasures Center (NBACC), a U.S. Department of Homeland Security (DHS) Science and Technology Directorate laboratory, 

In a new paper recently published in the journal Applied Optics, the collaborators describe their novel system for projecting a single wavelength of light at a time onto a sample of COVID-19 virus in a secure laboratory. Classified as Biosafety Level 3 (BSL-3), the lab is designed for studying microbes that are potentially lethal when inhaled. Their experiment tested more wavelengths of UV and visible light than any other study with the virus that causes COVID-19 to date.

So, what is COVID’s kryptonite? As it turns out, nothing special: The SARS-CoV-2 virus is susceptible to the same wavelengths of UV light as other viruses such as those that cause the flu. The most effective wavelengths were ones in the “UVC” range between 222 and 280 nanometers (nm). UVC light (full range from 200 to 280 nm) is shorter than the UVB wavelengths (280 to 315 nm) that cause sunburn.

Scientists also showed that the virus’s surroundings can have a protective effect on the virus. In the experiments, it took a smaller UV dose to inactivate viruses when they were placed in pure water than when they were placed in simulated saliva, which contains salts, proteins, and other substances found in actual human saliva. Suspending the virus in simulated saliva creates a situation similar to real-world scenarios involving sneezes and coughs. This detail may make the findings more directly informative than those of previous studies.

“I think one of the big contributions of this study is that we were able to show that the kind of idealized results we see in most studies don’t always predict what happens when there’s a more realistic scenario at play,” said Michael Schuit of NBACC. “When you have material like the simulated saliva around the virus, that can reduce the efficacy of UV decontamination approaches.”

Manufacturers of UV disinfection devices and regulators can use these results to help inform how long surfaces in medical settings, airplanes, or even liquids should be irradiated to achieve the inactivation of the SARS-CoV-2 virus.

“Right now, there’s a big push to get UVC disinfection into the commercial atmosphere,” said NIST researcher Cameron Miller. “Long-term, hopefully this study will lead to standards and other methodologies for measuring UV dose required to inactivate SARS-CoV-2 and other harmful viruses.”

This project built upon earlier work the NIST team did with another collaborator on inactivating microorganisms in water.

Shed a Little Light

Depending on the wavelength, UV light damages pathogens in different ways. Some wavelengths can damage microbes’ RNA or DNA, causing them to lose the ability to replicate. Other wavelengths can break down proteins, destroying the virus itself.

Even though people have known about UV light’s disinfection abilities for more than a hundred years, there’s been an explosion in UV disinfection research in the past decade. One reason is that traditional sources of UV light sometimes contain toxic materials such as mercury. Recently, use of nontoxic LED lamps as a UV light source has mitigated some of these concerns.

For this study, the NIST collaborators worked with biologists at NBACC, whose research informs biodefense planning on biological threats such as anthrax and Ebola virus.

“What NBACC was able to do was grow the virus, concentrate it, and remove everything else,” Miller said. “We were trying to get a clear message of how much light we need to inactivate just the SARS-CoV-2 virus.”

In the study, the team tested the virus in different suspensions. In addition to using the saliva mimic, scientists also put the virus in water to see what happened in a “pure” environment, without components that could shield it. They tested their virus suspensions both as liquids and as dried droplets on steel surfaces, which represented something that an infected person might sneeze or cough out.

NIST’s job was to direct the UV light from a laser onto the samples. They were looking for the dose required to kill 90% of the virus.

With this setup, the collaboration was able to measure how the virus responded to 16 different wavelengths spanning from the very low end of the UVC, 222 nm, all the way up into the middle part of the visible wavelength range, at 488 nm. Researchers included the longer wavelengths because some blue light has been shown to have disinfecting properties.

No Piece of Cake

Getting the laser light onto the samples in a secure lab was not trivial. Researchers in a BSL-3 lab wear scrubs and hoods with respirators. Leaving the lab requires a long shower before changing back into civilian clothes.

Equipment such as the team’s expensive laser would have had to undergo a considerably more severe sterilization procedure.

“It’s sort of a one-way door,” Miller said. “Anything coming out of that lab has to be either incinerated, autoclaved [heat-sterilized], ou désinfectés chimiquement avec de la vapeur de peroxyde d’hydrogène. Donc, emmener notre laser de 120 000 $ n’était pas l’option que nous voulions utiliser.”

Au lieu de cela, les chercheurs du NIST ont conçu un système où le laser et une partie de l’optique se trouvaient dans un couloir à l’extérieur du laboratoire. Ils ont fait passer la lumière par un câble à fibre optique de 4 mètres de long qui traversait un joint sous une porte du laboratoire. Une pression négative maintenait le flux d’air du couloir vers le laboratoire et empêchait toute fuite vers l’extérieur.

Le laser ne produisait qu’une seule longueur d’onde à la fois et était entièrement accordable, de sorte que les chercheurs pouvaient produire la longueur d’onde de leur choix. Mais comme la lumière se courbe à des angles différents en fonction de sa longueur d’onde, ils ont dû créer un système de prisme qui modifiait l’angle auquel la lumière entrait dans la fibre pour qu’elle s’aligne correctement. Pour modifier l’angle de sortie, il fallait tourner manuellement un bouton qu’ils avaient créé pour ajuster la position d’un prisme. Ils ont essayé de rendre le tout aussi simple que possible, avec un nombre minimal de pièces mobiles.

“Le dispositif que l’équipe du NIST a mis au point nous a permis de tester rapidement une très large gamme de longueurs d’onde différentes, toutes à des longueurs d’onde très contrôlées et précises”, a déclaré Schuit. “Si nous avions essayé de faire le même nombre de longueurs d’onde sans ce système, nous aurions dû jongler avec un tas de types de dispositifs différents, chacun d’entre eux ayant produit des bandes d’onde de différentes largeurs. Ils auraient nécessité des configurations différentes, et il y aurait eu beaucoup de variables supplémentaires dans le mélange.”

La manipulation de la lumière a nécessité des miroirs et des lentilles, mais les chercheurs l’ont conçue pour en utiliser le moins possible, car chacun d’entre eux entraîne une perte d’intensité pour la lumière UV.

Pour les matériaux qui devaient entrer dans le laboratoire pour projeter la lumière de la fibre sur les échantillons de virus COVID, l’équipe a essayé d’utiliser des pièces peu coûteuses. “Nous avons imprimé en 3D beaucoup de choses”, a déclaré le physicien du NIST Steve Grantham, un membre clé de l’équipe avec Thomas Larason du NIST. “Ainsi, rien n’était vraiment cher et si nous ne l’utilisons plus jamais, ce n’est pas un gros problème”.

Même la communication entre la zone du laser et l’intérieur du laboratoire était difficile car les gens ne pouvaient pas entrer et sortir à leur guise, ils ont donc employé un système d’interphone câblé.

Malgré ces difficultés, le système a étonnamment bien fonctionné, a déclaré M. Miller, d’autant plus qu’ils n’ont eu que quelques mois pour le mettre en place. “Il y a quelques domaines que nous pourrions probablement améliorer, mais je pense que nos gains seraient minimes”, a déclaré Miller.

L’équipe du NIST prévoit d’utiliser ce système pour des études futures d’autres virus et micro-organismes que les biologistes des laboratoires de haute sécurité pourraient vouloir mener.

“Quand le prochain virus arrivera, ou n’importe quel pathogène qui les intéressera, tout ce que nous aurons à faire sera d’enrouler le système laser là-haut, de pousser une fibre en dessous, et ils le connecteront à leur système de projecteur”, a déclaré Miller. “Nous sommes donc prêts pour la prochaine fois.”

Ce travail a été financé en partie par le Department of Homeland Security Science and Technology Directorate.