Design and Analysis of a Novel Ultraviolet-C Device for Surgical Face Mask Disinfection.

This paper deals with the design of a compact sanitization device and the definition of a specific protocol for UV-C disinfection of a surgical face mask. The system was designed considering the material properties, face mask shape, and UV-C light distribution. DIALux software was used to evaluate the irradiance distribution provided by the lamps emitting in the UV-C range. The irradiance needed for UV-C-decontaminated bacteria and virus, and other contaminating pathogens, without compromising their integrity and guaranteeing inactivation of the bacteria, was evaluated. The face mask's material properties were analyzed with respect to UV-C exposure in terms of physicochemical properties, breathability, and bacterial filtration performance. Information on the effect of time-dependent passive decontamination at room temperature storage was provided. Single and multiple cycles of UV-C sanitization did not adversely affect respirator breathability and bacterial filtration efficiency. This multidisciplinal approach may provide important information on how it is possible to correctly sanitize a face mask and, in case of shortage, safely reuse the face mask.

Introduction

Face masks provide a physical barrier to reduce person-to-person virus or bacteria transmission in exhaled breath, which mostly originates from the respiratory microdroplets subsequent to sneezing and coughing,[1] and minimize the transmission of infectious diseases that are airborne or transmitted via respiratory droplets. Face masks are composed of a series of nonwoven polymeric layers designed to avoid the passage of bacteria and viruses in both directions thanks to their tailored pore geometry and large specific surface area.[2]

During the initial phases of the current pandemic COVID-19, a shortage in surgical mask supply was observed, requiring measures to conserve face masks after selecting an adequate disinfection method and procedure.[3] Environmental problems that could follow the actual pandemic period were also to be considered, due to the increase in production and consumption of face masks across the world.[1,4] Single-use polymeric materials have been identified as a significant source of plastics and plastic particle pollution in the environment.[5,6] The reuse of COVID-19 face masks after sanitization can be used as a novel solution to this emerging waste problem,[1,4] which can facilitate the shift towards an eco-efficient circular economy within the healthcare industry.

Ultraviolet-C (UV) surface disinfection has attracted tremendous attention, and many products have become available on the market, guaranteeing a complete disinfection in a short time.[7−9] Various public places with different levels of contaminated surface probabilities, from hospitals and healthcare facilities to restaurants and cafeterias, have started using UV-C surface disinfection systems. UV-C tools have demonstrated to be effective for microbial disinfection, and they are preferable to other methods (ozone, thermal treatment, plasma exposure, γ irradiation, etc), having several advantages, including rapid effectiveness and absence of chemical residual or risk exposing the users to toxic chemicals.

However, a limited understanding of the critical aspects of UV-C disinfection has led to inappropriate use of this promising technology. Dubious and nonscientific performance claims by some of the UV-C system designers and manufacturers are unfortunately widespread.[10]

Even though UV-C can be used to preserve our current supply of filtering face masks through cycles of decontamination and reuse,[11] UV-C radiation can potentially degrade TNT polymer layers and/or the polymer chemicophysical properties of the fibers; hence, it could affect the ability of a disposable facemak to protect the people.[1,7,8,10,12] In addition, soft, porous, irregular surfaces may present a challenge for UV-C decontamination due to the potential for areas of shadowing and absorption of organisms into sites with reduced delivery of UV-C.[13] However, any decontamination process may limit the functionality of the mask if it alters its fitting and filtering efficiency in terms of penetration and/or air flow resistance. Detailed studies are needed in order to analyze this aspect and define a specific protocol.[14]

The main aim of the present study is to provide the necessary information for developing UV-C devices for bacterial and virus disinfection on porous filtration materials such as surgical masks during the COVID-19 pandemic and beyond, preserving the polymer properties and face mask efficiency. The sanitization device was designed to obtain a uniform UV-C light distribution on the face mask surfaces for a safe sanitization process in terms of viral abatement and material characteristics. In order to estimate the irradiance distribution provided by a UV-C lamp on a three-dimensional (3D) area like the face mask surface, and in order to design the device in a right and safe way, the use of DIALux software was evaluated. Previous researchers have applied the following approaches: Baluja et al.[15] proposed a method to find the best configuration to decontaminate several masks at once, ensuring the appropriate dosage in shaded zones by DIALux Evo 8.2 simulations of the irradiance pattern inside a box. Ropathy et al.[16] reproduced the lighting in different room layouts in order to evaluate the effectiveness of a UV light-based surface disinfection system.

In the past years, different decontamination devices have been developed to sanitize face masks, by applying UV-C light irradiation.[7] Many of these systems do not consider the 3D structure of the face mask and of the contamination; hence, the real and complete decontamination cannot be assured. Here, we propose a device designed to permit a real and safe decontamination of the whole face mask, with a decontamination cycle defined in terms of exposure time and irradiance, and we define the number of decontamination cycles guaranteeing face mask integrity, in order to avoid any physical–chemical degradation of the face mask properties. The novelty of this interdisciplinary work lies in its combining the knowledge of different fields (polymers, virus and bacteria, light distribution, and design), in order to develop an efficient system and to provide the scientific basis for further improvement.

This study contributes to enabling the reuse of surgical face masks, allowing a potential huge reduction of plastic wastes.

Materials and Methods

UV-C Device Design

UV-C Lamps and Measurements

UV-C lamps produced from a low-pressure mercury light source that emits short waves at 254 nm were used in this study: 15 Watt Philips, TUV T8, and 4 Watt Philips TUV 4W. The radiation intensity emitted by the UV-C lamps was measured and acquired with a radiometric probe LP471 TVC in the wavelength range between 220 and 280 nm, with a peak at 260 nm and quartz diffuser for cosine correction. The measuring range varies between 0.1 × 10–3 and 2000 W/m2.

Mechanical Design of the Sanitization Box

A customized sanitization box was designed and successively prototyped for the purpose of experimentally testing the sanitization equipment and protocol on the chosen face masks. The design of the equipment involved the following phases: (i) a conceptual design of the box based on the project requirements (e.g., need to guarantee the UV radiation confinement, need to guarantee the maximum exposure to UV radiation for both the external and internal surfaces of the face mask) to derive the functional and modular structures of the device and, finally, the product concept; (ii) an embodiment and detailed design of the box exploiting the CAD system SOLIDWORKS 2019 to define the most appropriate geometrical shapes and materials of all of the components, and to develop the virtual prototypes; and (iii) a critical design review phase to optimize the box model (e.g., miniaturization of the cavity and supports, most appropriate positioning of the internal component, material selection considering, for instance, the effects due to radiation reflections). A 3D view of the developed assembly is reported in Figure .

Figure 1

3D view of the sanitization device, with the lamp protection grid (red part) and the face mask support (green part).

The external box of the device, composed of two main parts, has gross dimensions of 244 × 124 × 98. The use of a plastic material (i.e., polyamide) has been preferred to assure lightness. As can be seen in Figure , the sanitization lamps are positioned in the central part at the top and bottom of the box, while in the left and right parts, specific spaces are dedicated to the housing of electronic devices and batteries.

To cover the two lamps and avoid accidental contacts, a protection grid has been designed (the red part in Figure ). The dimensioning of such a component has been done through a trade-off among two conflicting requirements: to guarantee a robust protection against physical contacts with lamps, and to minimally perturbate the UV radiation directed to the mask surfaces. To this purpose, different prototypes with different dimensions have been realized (through 3D printing), installed in the sanitization box, and then tested by measuring the percentage reduction of the UV-C light within the cavity by means of the radiometric probe LP471 TVC. The best compromise turned out to be a cylindrical protection grid that encloses the whole lamp, positioned at a minimum distance from the lamp’s external surface (i.e., 1 mm), with 16 holes and with rib dimensions of 2 × 2 mm2.

Another important component of the sanitization box is the support (the green part in Figure ) needed to maintain the face mask in the right position during the sanitization cycle, assuring to expose to radiation most of the mask’s internal and external surfaces without creases. The developed framework has dimensions of 100 × 75 × 18 mm3 (excluding the fixing pins needed to assemble the support with the box) and it is composed of ribs of dimensions 3 × 3 mm2. As can be seen from the Supporting Information Figure S1, the support is composed of rounded ribs and has a draft angle of 18.5° to favor the face mask positioning and removal (less than 1 min for each operation), so that the upper part has external dimensions of 100 × 65 mm2.

Irradiance Simulation

To ensure proper decontamination, it is crucial to study, simulate, and estimate the irradiance distribution on the face mask surface provided by the lamps and evaluate the right lamp/mask distance to assure a safe sanitization procedure in a short time. The software DIALux Evo 9.2[17] was used to this aim; this tool is commonly used for architectural lighting and for the design of indoor, outdoor, and street lighting systems. Here it was applied to the UV-C light distribution. In particular, the followed methodology derives from previous research studies,[15,16,18] where a correlation between Lux (lx) (output of the simulations) and the UV-C intensity in μW/cm2 was established. In the present paper, the correlations found from Arines[15,18] were taken into account: in order to get the isolines in μW/cm2, it is necessary to multiply by 100 the radiant flux of the lamps in W, before introducing the value-correspondent item of the tool.

Furthermore, in order to validate the used method and to define the right photometric solid distribution of the lamp, preliminary photometric measurements, by radiometric probe, were carried out in a parallelepiped box internally covered with aluminum (internal dimensions 55 × 24 × 15 cm3) and with a UV-C lamp inside (Philips TUV T8, total power of 15 W and UV-C radiation power of 4.9 W); the measured values were compared with the simulation results. Different test surfaces were identified with the UV-C lamp disposed along the longer dimension (55 cm) in all of the configurations. In DIALux models, two parameters control the way in which light reflects on the walls: a coefficient of 73% was chosen for the metallic surface of the walls (reflectance factor), and a value of 50% was chosen for the reflective coating.[15,16,18] The lighting spatial distribution of the lamp’s irradiance, defined thanks to these measurements, was used for the definitive model of the UV-C disinfection device. Figures S2 and S3 in SI show the models of the tested boxes developed in DIALux: in Figure S2 the preliminary box, and in Figure S3 the final UV-C-based disinfection device. Figure represents the characteristics of the photometric solid distribution of the UV-C selected lamp: it was chosen based on the preliminary tests.

Figure 2

UV-C germicidal lamp: photometric spectrum (maximum emission at 250 nm), and the selected photometric spatial distribution (emitted candle for kilolumen of the lighting flux).

The final project of the disinfection box (Figure ) was modeled in AutoCAD and imported in DIALux. The same boundary conditions were assumed for the input power of the lamps and the features of the devices’ casing. In this final configuration, the mask was schematically represented by means of three rectangular surfaces that are placed between the two lamps (Figure ).

Figure 3

(a) 3D view of the calculation surfaces inside the box; (b) the two-dimensional (2D) and 3D maps of the simulated irradiance values inside the device.

Surgical Face Mask

Commercial-grade surgical face mask were selected for this study. Face mask are based on three layers of nonwoven fabric microstructures of thermoplastic polymers: the external and internal layers are based on spunbond poly(ethylene terephthalate) (PET), while the internal layer is based on melt-blown poly(butylene terephthalate) (PBT).

Sanitization Evaluation

Artificial Contamination of Type II Surgical Masks with ϕ6 as the Biological Indicator

The artificial contamination was performed to achieve massive contamination of the mask. A suspension of ϕ6 with a concentration of ≈109 PFU/mL was aerosolized for 2.5 min on each side of the mask. The mask was placed in the Andersen impactor connected to a vacuum pump with a flow rate at 28.3 L/min, permitting the aerosolized virus to reach directly the mask.

ϕ6

Bacteriophage ϕ6 DSM21518 derives from the microbial collection of the Leibniz Institute DSMZ-German Collection of Microorganisms and Cell Cultures GmbH (Braunschweig, Germany), and was propagated using the double-layer agar method. The lower layer consists of Trypticasein Soy Agar (TSA, Condalab) and the upper layer consists of 5 mL of Trypticasein Soy Broth (TSB, Condalab) supplemented with 0.9% agar (Difco) mixed with 100 μL of Pseudomonas syringae (DSM21482) incubated overnight at 28 °C in TSB. Subsequently, after pouring the soft agar, 100 μL of bacteriophage solution ϕ6 was inoculated by spot. The plates were then incubated overnight at 28 °C. The plaque formed, ϕ6, was recovered with 4 mL of viral buffer (Na2HPO4 7 g/L, KH2PO4 3 g/L, NaCl 5 g/L, 1 mM MgSO4, 1 mM CaCl2). After the viral buffer was added, the plate was stirred at 50 rpm for 4 h, at the end of which the soft agar was scraped and purified by centrifugation at 6000 rpm for 5 min to separate the host cell particulate and bacteriophage. The supernatant was filtered through a 0.22 μm Millex-GP syringe filter unit and used as stock. Stocks in glycerol and DMSO (10% vol/vol) were prepared of the ϕ6 solution, stored at −80 °C.

Real Contamination

The masks were worn for periods between 2 and 4 h by different operators to contaminate the mask with a microbiological population as much representative as possible in terms of quantity and quality.

Bioburden Test and Disinfection Efficiency

The bioburden test (according to standard EN 14683:2019[19]) was used to estimate the totally viable microorganisms present on the sample before (for the control) and after the sanitization treatment. The mask was inserted into the bottle (Duran Schott, Mainz-Germany) containing the extraction liquid (viral buffer for ϕ6 contamination: Na2HPO4 7 g/L, KH2PO4 3 g/L, NaCl 5 g/L, 1 mM MgSO4, 1 mM CaCl2; peptone water for natural contamination: peptone 1 g/L, NaCl 5 g/L, surfactant polysorbate 20 2 g/L). It was then stirred for 5 min at 250 rpm. At the end of incubation, the quantification of the virus was evaluated by counting the lysis plaques; each lysis plate is defined as a plaque forming unit (PFU) and corresponds to a virion unit. Several aliquots of extraction liquid together with 100 μL of P. syringae and 6 mL of 0.90% soft agar in sterile 20 mL tubes were after a little mixing poured onto Petri dishes containing TSA. The plates were incubated for 24 h at 28° C. In the case of natural contamination, after the incubation, several aliquots (1–100 mL) of the liquid were filtered through a sterile 0.45 μm microbiological filter (Whatman) by using a filtration system (CombiSart Multi-Branch System, Sartorius, Germany). The filter was then placed on Petri dishes (Ø60 mm) of TSA for quantification of the bacteria, and Saboraud agar (SDA) was for quantification of the fungi; the plates were incubated for 24 h at 37 °C and for 48/72 h at 28 °C. After incubation, the quantification of colony-forming units (CFUs) and PFUs and the disinfection efficiency were evaluated by means of eqs and 2wherewhere

CFU or PFU = colony-forming units (plaque forming units) found on the filter

V = filtered volume (or spread volume)

VTOT = total volume of extraction liquid (viral buffer or peptone water)

Ab. Log = logarithmic abatement

C = UFC found on a mask exposed to UV radiation

C0 = UFC found on untreated mask (control)

UV-C Sanitization Effect on Face Mask Materials

To study the effect of UV-C irradiation on polymeric TNT material properties, the face masks were irradiated with 20, 40, 60, 80, and 100 J/cm2 of UV-C radiation, and the material characterization results were compared with the non-irradiated masks in terms of infrared spectroscopy, wettability, and morphology.

Infrared Spectroscopy

The face masks’ TNT nonwoven fabrics were chemically characterized by Fourier infrared spectroscopy (FT-IR) in the 400–4000-cm–1 range by a Jasco FT-IR 615 spectrometer. Pristine and sanitized face masks were analyzed in ATR mode with a germanium crystal.

Water Contact Angles (WCA)

The water contact angles (WCA) were assessed using the sessile drop method in air by an FTA1000 Analyzer (Dinwiddie Street, Portsmouth, VA). Deionized water drops of 20 μL (high-performance liquid chromatography grade water) were placed on the face mask, and measurements were recorded 10 s after the liquid made contact with the surface. Five independent determinations at different sites were averaged. Neat and exposed face masks were analyzed.

Morphological Analysis

The morphological analysis of the surfaces of the spunbond and melt-blown TNT and of the fracture sections before and after sanitization was carried out by means of scanning electron microscopy (SEM) measurements, using the SEM Jeol instrument JSM 5200. Before analysis, all of the samples were metallized with gold, using the Balzers MED010 instrument. Furthermore, the sections were made after freezing the samples.

Breathability and Bacterial Filtration Efficiency (BFE)

The effect of multiple disinfection cycles on the face mask integrity was analyzed in terms of breathability and BFE. The breathability measurements were performed according to the UNI EN 14683:2019 standard[19] using the GBN701 differential pressure tester. To carry out the measurements, the masks were deprived of the rubber bands and cut laterally so that the fabric is stretched out without creases. The measurement was repeated on five distinct points representative of the entire mask and the breathability was measured on each point of the masks.

The BFE was measured as described in UNI EN 14683:2019 standard,[19] modified by using the Escherichia coli ATCC 9637 as the model organism. A suspension of E. coli was aerosolized using a nebulizer (Xearpro Bulldog Aero) through a pyrex glass aerosolization chamber (680 mm 80 mm) attached to a 6-stage Andersen impactor (TE-10–800 Tisch Environmental) connected to a vacuum pump with a flow rate at 28.3 L/min. The bacterial suspension concentration was adjusted to provide a value of 1700–2700 colony-forming units (CFUs) in the positive control test (without mask). A specimen of the mask material was clamped between a six-stage cascade impactor and an aerosol chamber. The BFE of the mask is given by the number of colony-forming units passing through the medical face mask material and expressed as a percentage of the number of colony-forming units present in the challenge aerosol according to eq

Results and Discussion

Irradiance Measurements and Simulation Results

Figure S4 shows the results of the simulations performed on the rectangular-shaped test box. The irradiance isolines obtained from the simulations are represented: the red points correspond to the measurement positions and in black the measured values are shown. For the first lighting distribution (Figure S4a), at a distance of about 15 cm, the measured irradiance values are similar to the simulated values only under the lamp, whereas the distribution of the irradiance has an opposite behavior in comparison with the measurements, with a difference of up to 30%. The photometric solid is not appropriate and a second one was therefore considered: a better correspondence between the measurements and the program outputs is observed in this case (Figure S4b) with maximum differences of about 3–4%. Based on these results, the second type of UV-C-wave germicidal tube with a larger and flattened spatial distribution showed a suitable irradiance behavior, and it was selected to study the UV-C light arrangement for the disinfection device (Figure ).

The final configuration with a lower total volume of the device, an aluminum internal surface, and two lamps symmetrically disposed allows a more uniform distribution of the irradiance on the sanitized surfaces. Without supporting grids, both measurements and simulations were carried out. A good correspondence above all in the central surface of the mask was obtained (2200–3000 μW/cm2 measured vs 2600 μW/cm2 simulated), with maximum differences of about 13–15%. With the supporting grids of the lamps, a uniform value of 2100 μW/cm2 is obtained in the central area of the mask (surface n.2); values of 1100–1000 μW/cm2 are obtained in the border-bent zones (surfaces n.1 and n.3), as shown in Figure .

Figure 4

Image of the final sanitization device.

The simulation results of the box with the face mask support yielded a radiation intensity of about 2000 mW/cm2 at the distance of about 1.5 cm. The resulting radiation dose, the main parameter to be considered in the surface sanitization procedure, can be determined by eq Hence, by modulating the exposure time a radiation dose ranging from 1.2 J/cm2 (10 min) to 3.6 J/cm2 (30 min) can be obtained in the sanitization device. Studies in the literature[13] have shown that a dose of more than 1 J/cm2 is needed to guarantee a good level of decontamination.

Sanitization Box Prototype

According to the illuminotechnical simulation results, the sanitization box prototype was designed with the following geometrical and material characteristics:

External dimensions: 244 × 124 × 90 mm3

Dimensions of the internal cavity: 157 × 115 × 65 mm3

Lamp + supports dimensions: 155 × 20 × 25 mm3

Lamp position: centered in the internal cavity

Distance between the lamp surface and mask surface: 15 mm

The main functional groups composing the prototype are the following:

a plastic box composed of the main body and a cover, joint through two metallic hinges that allow the opening and closing of the sanitization box. The internal surface of the box has been entirely covered with a 0.5 mm thick aluminum layer in order to maximize the reflections of the UV-C radiation, and thus guarantee an as-uniform-as-possible distribution within the sanitization cavity, as used in the DIALux simulation tool;

an interlock that avoids the lamp power up when the cover is not completely closed, satisfying the safety requirements during the device use;

two lamp protection grids realized in polycarbonate through a 3D printer;

a face mask support realized in polycarbonate through a 3D printer;

a lighted button integrating a light-emitting diode (LED) able to light up in four different colors (blue, green, yellow, and red) to indicate four different device states (stand-by, ready to be used, sanitization in progress, and alarm).

Figure shows the image of the prototyped sanitization device.

Sanitization Effects

Bacteriophage ϕ6 has been chosen as a biological indicator to test the optimal exposure dose for one sanitization cycle to be able to obtain a 3-log-reduction of the viral load. Use of enveloped viruses like bacteriophage ϕ6 is desirable because it may capture the potential effects of a viral envelope on the virus’s interaction with its environment, such as on a surface or in aerosol, which may impact the UV disinfection performance.[20] Moreover, the behavior of ϕ6 under UV-C irradiation has been described in detail and compared with the Sars COV virus,[21] appearing more resistant with respect to other enveloped viruses, thus suggesting ϕ6 as a practical and realistic conservative virus surrogate where use of coronaviruses is not feasible. According to Nicolau et al.,[22] no study on UV irradiation on surgical masks have reported the use of ϕ6 as a biological indicator; conversely, it was recently used on N95 respirator UV-C disinfection protocols.[7,23,24]

Masks artificially polluted by ϕ6 aerosolization resulted in a viral dose of ca 106 PFU/mask, while masks really polluted by wearing by several operators in different indoor and outdoor environments resulted in a bacterial load of ca 105 CFU/mask.

Figure shows the effect of different UV-C doses on log abatement of real and ϕ6 contamination on surgical masks. During this experiment, a limited reduction of microbial load was observed (up to −1.4 LOG) even with a high UV dose in both typologies of contamination. Besides the small reduction, increasing doses of UV irradiation did not retrieve an increasing reduction of residual microbial load after the UV treatment, thus indicating some limitation in the UV irradiation effect.

Figure 5

Biological agent abatement under different UV-C doses on surgical masks subjected to real (□ symbol) and ϕ6 (x symbol) contamination using a prototype without face mask support.

The data underlined the shadowing as one of the main setbacks for UV-C’s sanitization method; this problem occurs when parts of the masks are poorly irradiated, and the object possesses inner layers where microorganisms can remain. This aspect is claimed as one of the main drawbacks able to impact UV-C’s germicidal capability.[22] An upgraded version of the prototype has been developed to address this limitation, including a face mask support, accurately designed in order to overcome these problems and able to keep open the whole mask’s superficial area (the green part in Figure ).

Figure shows the effect of different UV-C doses on the log-reduction of real and ϕ6 contamination on surgical masks by applying the upgraded prototype, integrated with a specific face mask support (Figure S1). The values on the x-axis show the irradiance dose obtained by applying the simulated UV-C value to the exposure time.

Figure 6

Biological agent abatement under different UV-C doses on surgical masks subjected to real (□ symbol) and ϕ6 (x symbol) contamination using an upgraded version of prototype with face mask support.

According to the mechanical design of the sanitization box and the biological results that permitted to study the effect of different UV-C doses on the log-reduction of real and ϕ6 contamination, the sanitization protocol was set to guarantee a log abatement value of 3 corresponding to a 30 min long cycle of irradiation, which according to radiometric probe measurements is equal to a 2.8 J/cm2 irradiance dose. These data are in accordance with the recently published results.[13]

Face Mask Properties Analysis

Face mask polymeric properties were tested in terms of infrared spectroscopy, wettability, and microstructure, after exposure to increased irradiance UV-C values, from 20 to 100 J/cm2, which are comparable from 7 to 35 sanitization cycles, respectively.

Infrared spectra (Figure ) show the typical characteristics of the absorption spectra of the neat PBT polymer: i.e., 3000–2800 cm–1; for the C–H aliphatic stretching vibration of saturated and unsaturated compounds, the C=O vibrational band at around 1716 cm–1, the symmetric and asymmetric C-O-C stretching modes in the 1200–1100 cm–1, spectral range, and modes assigned to the aromatic ring at 870 and 725 cm–1 are found. After UV-C exposure, no pronounced changes in the molecular structure of PBT under the conditions studied are observed. In the region 1600–1700 cm–1, some new bands near the C=O peak could be observed, which are attributed to the formation of degradation byproducts, including conjugated aldehydes and conjugated ketones, due to the degradation and oxidative effects of UV-C radiation. This band is evident only after 80 J/cm2 exposure; it increases with irradiation time[25,26] and is due to the photoproducts of PBT.[27] The band at 1690 cm–1 is not evident for the lower UV-C dose irradiance. Different studies have proposed that the photo-oxidation mechanism of PBT involves the formation of ester derivatives, perester derivatives, aliphatic acids, benzaldehyde, and benzoic acid in the presence of oxygen.[26]

Figure 7

Infrared spectra in the ATR spectra mode of the unexposed and exposed external layers of surgical face masks after increased irradiance.

Wettability

Figure shows the water contact angle images and values of pristine and UV-C-treated TNT surface with 80 and 100 J/cm2. TNT nonwoven fabrics, used to develop surgical face masks, have hydrophobic properties. Wettability and droplet adherence are important properties that were expected to provide an indication about the likelihood of droplet transmission through face masks. The selected TNT shows a WCA of 109°; the exposure to UV-C radiation for a short time does not modify this value; in fact, TNT exposed to 80 J/cm2 has the same WCA value. By increasing the irradiance dose, a decrease of the WCA can be observed. A reduction of 30% was measured at 100 J/cm2; this reduction is confirmed by the infrared analysis, performed in ATR mode. The surface wettability of PBT measured by the contact angle is modified by the irradiation.[28] The surface photo-degradation increases the water wettability of the initially hydrophobic polymers. A shorter irradiation dose could not modify the surface. UV-C irradiation of PBT under air atmosphere could generate new functional groups, such as lactones, aldehydes, and anhydrides, that better interact with the water molecules, showing a decreased WCA.

Figure 8

WCA angle images and values of unexposed and exposed external layers of surgical masks after increased irradiance dose.

It is known that aromatic polyesters exposed to UV light, in vacuum or inert atmosphere, undergo photo-degradation and/or photo-cross-linking reactions. Irradiation of PBT at 254 nm results in the formation of benzoic acid, benzaldehyde, formate, and alcohol end groups via Norrish I mechanisms, and benzoic acid, vinyl, and γ -butyrolactone end groups via Norrish II mechanisms. All of these functions have been previously identified by IR spectroscopy.[26]

Figure shows the SEM images of the internal meltblow layer of a surgical face mask, by comparing the pristine materials with those exposed to 100 J/cm2 UV-C radiation. The internal polymeric layer comprises small voids compared to the other two layers and it acts as a filter, stopping harmful and unwanted particles from entering the body.[1] The microstructure of the layer is an important parameter to consider since the porosity and surface area of the fibers in the face mask dictate their breathability and filterability. The SEM images did not demonstrate any observable change of the general structure of the surgical masks. UV-C did not damage the filtration layer even when the exposure time was up to 100 J/cm2. Although UV-C is proven to facilitate the degradation of thermoplastic polymers, a short exposure targeted at disinfection of the mask would not cause significant structural damage to the filtration layer.[29]

Figure 9

SEM images of the internal meltblow layer of a surgical face mask at different resolutions of unexposed (A) and exposed surface at 100 J/cm2 (B).

Breathability and BFE

To investigate whether UV-C exposure in air affects the medical face mask’s efficiency, breathability and bacterial filtration efficiency were analyzed, after single and multiple sanitization cycles. Table shows the breathability and BFE values on the original face mask and after one and five sanitization cycles. Five cycles were selected in order to obtain a maximum irradiance dose of less than 20 J/cm2, which was proven to not modify the infrared spectra, morphology, and wettability of the TNT polymeric tissue, and to safely decontaminate the face masks. Table underlines that the face mask efficiency in terms of breathability performances (resistance to air flow) and bacterial efficiency penetration were not significantly affected regardless of the UV-C decontamination process until 5 cycles of decontamination. Compared with untreated masks, both values remained within the variability range of measurements made on the untreated masks. Variation in these measurements is mainly due to the intrinsic local heterogeneity of the fibrous structure composing the filter tissue of the masks. Thus, we can conclude that the defined sanitization multiple cycle does not alter the structure of the nonwoven media.

Table 1

Breathability and BFE Values of the Face Masks, after Single and Multiple Sanitization Cycles, Compared with the Original Ones

face mask	breathability (Pa/cm2)	BFE (%)
original	46.2 ± 0.9	99.95 ± 0.05
1 cycle	46.0 ± 1.2	99.92 ± 0.03
5 cycles	46.7 ± 1.1	99.85 ± 0.09

Conclusions

Decontamination of surgical masks has become an alternative strategy to regenerate basic protective equipment so as to control the spread of diseases under the current difficult situation. In the present research, ultraviolet germicidal irradiation has been demonstrated to have potential for use in disinfecting medical face masks, which could allow the mask to be used safely multiple times during a public health emergency. However, we demonstrated the importance of designing specific support in order to assure a real and uniform UV-C distribution on all face mask surfaces. DIALux software was successfully applied to simulate and study the spatial distribution of the UV-C light on the different surfaces of the face masks.

Furthermore, it is important to understand how UV-C affects the polymeric layer of the mask, and especially whether UV-C degrades the protection offered by the respirator. Our results suggest that UV-C could be used to effectively disinfect disposable masks for reuse, but the maximum number of disinfection cycles will be limited and connected to the UV-C dose required to inactivate the pathogen. Our results demonstrate that we can safely decontaminate face masks for almost 5 times, without affecting the face mask performance in terms of breathability and bacterial filtration efficiency and surface polymeric properties in terms of microstructure, wettability, and chemical properties. Our device, methodology, and sanitization protocol, if put into practice, can provide several innovative ideas that we believe represent meaningful contributions to the field of UV-C decontamination devices. We have proven that the lifetime of medical face masks can be extended. This multidisciplinal approach provides important information on how often a given face mask may be safely reused.