A new laser device for ultra-rapid and sustainable aerosol sterilization

Abstract

The current COVID-19 pandemic has highlighted the importance of aerosol-based transmission of human pathogens; this therefore calls for novel medical devices which are able to sterilize contaminated aerosols. Here we describe a new laser device able to sterilize droplets containing either viruses or bacteria. Using engineered viral particles, we determined the 10,600 nm wavelength as the most efficient and exploitable laser source to be manufactured in a commercial device. Given the lack of existing working models to reproduce a human aerosol containing living microbial particles, we developed a new system mimicking human droplet formation and preserving bacterial and viral viability. This evidenced the efficacy of 10,600 nm laser light to kill two aerosol transmitted human pathogens, Legionella pneumophila and SARS-CoV-2. The minimal exposure time of <15 ms was required for the inactivation of over 99% pathogens in the aerosol; this is a key element in the design of a device that is safe and can be used in preventing inter-individual transmission. This represents a major advantage over existing devices, which mainly aim at either purifying incoming air by filters or sterilizing solid surfaces, which are not the major transmission routes for airborne communicable diseases.

Keywords: Aerosol; Air sterilization; Laser light; Legionella pneumophila; SARS-CoV-2.

Graphical abstract

Fig. 1

Identification of the most effective laser wavelength in inactivating AAV and LV vectors, a. Schematic representation of an Adeno-Associated Virus (upper image) and a Lentivirus (lower image). b. Schematic representation of the experimental procedure used to compare the effect of multiple laser wavelengths on both AAV and LV vector transduction efficiency. c, d. Quantification of viral vector activity (defined as the ratio between EGFP⁺ green area and Hoechst⁺ nuclear area) upon transduction of HEK293 cells with a drop of either AAV-EGFP (1 × 10⁶ vg/cell) or LV-EGFP (1 × 10³ iu/cell) vectors in control conditions (CTRL) or exposed to the indicated laser wavelengths for 2 s. * P < 0.05; *** P < 0.001; **** P < 0.0001. Cells not exposed to any vector represent the negative control (NEG CTRL). The lower detection threshold in this assay is 10⁴ vg/cell and 10 iu/cell for AAV and LV, respectively (Supplementary Fig. 1). e, f. Representative images of HEK293 cells corresponding to the graphs in panels c and d.

Fig. 2

Generation of a new nebulizer and laser device capable of sterilizing human-like aerosol contaminated by LV particles, a. Schematic representation of the experimental setup to evaluate LV vector activity upon nebulization with multiple commercial and custom systems. b. Quantification of LV-EGFP activity after nebulization. Multiple aliquots of the same LV-EGFP preparation were aerosolized using multiple systems, which exploit ultrasounds, Venturi effect and vacuum, as well as with our new nebulizer. The same volume of the generated aerosol was condensed by cooling and transferred to HEK293 cells. The lower detection threshold in this assay is 10 LV-EGFP iu/cell. c. Schematic representation of our nebulizer, in which a function generator operates at 3 Hz with 41% duty cycle to activate a piston air pump. A low-pass filter tube keeps a constant airflow, attenuated in pression. d. Representative images of an aerosol generated by our nebulizer (upper panel) and by a speaking individual. e. Distribution of droplet size generated by either our nebulizer or a speaking individual. f. Schematic representation of our laser prototype assembled with the new nebulizer. The aerosol was either exposed to laser for 12.5 ms or directly threw into the cooling tank for condensation. 1. Laser source; 2. Beam expander 5X; 3. Safety cover; 4. Laser chamber; 4a Aerosol inlet; 4b. Aerosol outlet; 5. Cooling tank for treated aerosol condensation; 6. Head power meter (which blocks the laser beam); 7. Control chamber (no laser); 7a. Control aerosol inlet; 7b Control aerosol outlet; 8. Cooling tank for control aerosol condensation. The real prototype is shown in Supplementary Fig. 3.

Fig. 3

Aerosolized viral vectors and human pathogens are efficiently inactivated by 10,600 nm laser light, a. Representative images of HEK293 cells infected with LV-EGFP at 48 h after infection. LV-EGFP was nebulized and irradiated using our prototype and condensed. Samples from tanks n. 8 (CTRL, control) and n.5 (laser-treated) were eventually added to cells. b. Quantification of LV activity, measured as EGFP/Hoechst area. The lower detection threshold in this assay is 10 LV-EGFP iu/cell. ****P < 0.0001. c. Representative images of cytopathic effect (white plaques) of SARS-CoV-2 on Vero cells in 48-well plates. Both control (CTRL, green dashed line) and laser-treated (red dashed line) samples were plated on Vero cells at multiple dilutions for quantification of plaque formation. d. Representative images of L. pneumophila colonies on Agar plates. Both control (CTRL) and laser-treated samples were plated on Agar at multiple dilutions for quantification of bacterial viability.

Fig. 4

Implementation of the new laser technology in real settings, a. Schematic drawing showing the core of the proposed laser technology, with indication of its major components. b. Schematic representation of air inflow and outflow from the core system. As indicated by the red arrows, the air is aspirated by the fan at the end of the system to enter the sterilization chamber, where it is exposed to laser light. Purified air exists from both sides of the fan. c. Schematic representation of a real device inserted into a false ceiling. Air flow is indicated by the red arrows. d. Schematic representation of a real device inserted into the ceiling of an elevator. e. Schematic representation of a real device designed to be placed in a populated room. Air flow is indicated by the red arrows. f. Schematic representation of a real device designed to be placed in a populated room, hanging from a lateral wall. Air flow is indicated by the red arrows.