2. Materials and methods
To compare influence of EB irradiation on different respirators two different polypropylene masks: 3M 1863+ conforming to the high FFP3 standard and 3M 9101E conforming to the FFP1 standard were chosen for the investigation. Masks were sealed in plastic bags and irradiated with doses 12 kGy and 25 kGy in air atmosphere, at ambient temperature. Electron beam irradiation of samples was carried out using 10 MeV, 10 kW linear electron accelerator “Elektronika”. Delivered doses were confirmed using B3 radiochromic foil dosimeter measured with a flat bed scanner and RisoScan software, with uncertainties evaluated at 8%. The masks were irradiated in a single layer to minimise dose gradient and ensure uniformity of delivered doses. For both tested samples the dose increase inside the samples was below 1 %.
Doses were selected taking into account assumption the possible variability in viral load in used masks and its random distribution among products. On the base of microbial contamination found in surgical masks one can realise that the standard deviation of the bioburden is higher than the mean N: 47 ± 56 cfu/ml/piece for inside mask area and 166 ± 199 cfu/ml/piece from mask outside area [14]. This results from the variability of environments where masks are used and differences in the level of the bioburden. On the base of the maximum of 1000 microorganisms should be present in the product, decontamination dose (a dose required for 5 or 6 order of magnitude reduction of bioburden) was calculated as 12 kGy [14]. Moreover, masks were also irradiated with standard sterilization dose 25 kGy which is defined as sterilization dose according to VDmax method given in ISO 11137-2 standard “Sterilization of health care products — Radiation — Part 2: Establishing the sterilization dose”.. Masks that were not irradiated were used as control samples.
SEM images of the masks layers were obtained, using a Hitachi TM-100 scanning electron microscope with an accelerating voltage of 15.0kV. Samples for the SEM examination were prepared according to a standard procedure, fixed with conductive glue, and coated with a thin layer of gold. The samples were examined at a magnification of 500×.
Thermogravimetric analysis (TGA) of masks samples was used to determine the thermal stability and possible degradation of respirators materials was conducted with a Q500 TGA (TA Instruments) thermogravimetric analyser in the temperature range 37–700oC at a heat ingrate of 10oC per minute, under a constant flow (60 mL/min) of nitrogen gas.
To determine the initial separation efficiency of the tested respirators samples before and after irradiation the high quality test bench MFP NanoPlus (PALAS GmbH) was used. The main elements of this test-rig are: UGF2000 generator which is able to generated nanoobject from liquid solution, cascade of impactors which were used to cut-off the largest particles (which were not object of this research), the bipolar neutralizer Kr-85, used to ensure equilibrium charge distribution on particles, DEMC classification column, pneumatic filtration chamber and universal fluid condensation particle counter UF-CPC. The UF-CPC together with the DEMC classification column form a system called U-SMPS. It allows to classify and count particles in size range from 20 to 200 nm. Scheme of MFP nanoPlus is presented in Fig.1.
Fractional and overall filtration effectiveness was determined for solid particles (KCl nanocrystals) as well as oil nanodroplets of Di-Ethyl-Hexyl-Sebacat (DEHS). For tests circular samples with diameter of 60 mm were punched from tested respirators. All filtration test were performed at the air flow rate 95 L/min and the air face velocity 0.559 m/s. For this experiments It was required to performed experiments in a sequence of measurements without (upstream) and with (downstream) a filtrating material in the tested chamber. One series consisted of two upstream measurements and two downstream measurements. There were carried out two series of measurements for each masks material. Next, the average value of the filtration effectiveness was calculated and presented on diagrams below. The time of a single measurement of upstream and downstream was always 380 sec. For this time, the interval between measurements was included and it was 60 sec. Such a long time of a single measurement was necessary to correctly classify and count nanoparticles. Moreover, during the tests there were also determined the pressure drops across the tested materials and their initial overall filtration efficiency. Size distribution of the generated aerosols used in the experiments is presented in Fig. 2.