# An Efficient UV-C Disinfection Approach and Biological Assessment Strategy for Microphones
## 3 Results
### UV-C Disinfection Assessment  
UV-C inactivation of M13 and influenza virus in solution: To get first insight into the efficacy of the device and to allow extrapolating our findings to pathogenic viruses, we placed quartz cuvettes containing bacteriophage M13 or PR8 H1N1 influenza virus in the device (Supplementary Information S3) at the position in front (irradiance: 7.185 \(\mu\)W/cm\({}^{2}\)) and simultaneously irradiated the liquid samples for 10 min resulting in a UV dose of 4.311 mWs/cm\({}^{2}\). The initial concentration before irradiation was 5.0 \(\times\) 10\({}^{7}\) PFU/mL for both viruses. In all the samples, no active viruses were detectable upon irradiation, indicating an efficacy of over 99.99%, which concurs with the theoretical calculation of irradiance and viral susceptibility towards UV-C. The results indicate that M13 is a suitable model virus for these tests. Regarding foam disinfection, to determine the UV-C disinfection efficacy, samples of foam of 1 cm\({}^{2}\) were used. The samples were contaminated via a spraying device to mimic aerosol formation and deposition. The spraying device was placed at a distance and angle that allowed us to cover two foam samples with aerosols simultaneously during a short burst. One sample served as control and the other sample was treated with UV-C (Figure 3). For the spraying deposition, it was found that more liquid was deposited than in the initial recovery experiments, where 10 \(\mu\)L aliquots of M13 suspension were applied to the foam by pipetting. Therefore, the M13 concentration used for the contamination of foams by spraying was reduced to 10\({}^{11}\) PFU/mL. The exact volume could not be determined, but, the extractability and comparison between the control and sample remain correct. For the inoculated foam samples, the viruses were extracted and the viral loads of the disinfected sample and the respective control were compared to assess the reduction of PFU in percentage. Figure 3 shows reductions in the M13 titers of 99.3%, 99.2%, and 99.0% in the technical replicates. On average it was found that a 99.2% reduction in the viral titer was achieved by 10 min of UV-C irradiation with an irradiance of 7.185 \(\mu\)W/cm\({}^{2}\), resulting in a UV dose of 4.311 mWs/cm\({}^{2}\). We regard this as an important finding, because the foam is considered to be the most inaccessible part of the microphone that may shield microorganisms against UV-C due to its porosity. The efficacy of UV-C disinfection for 20 min was only marginally increased compared to disinfection for 10 min and, therefore, all further experiments involved 10 min UV-C disinfection. Regarding the UV-C disinfection assessment of infected microphones, because UV-C disinfection of microphone foams was very effective, we next evaluated the UV-C disinfection of microphones as a whole. To this end, the method of spraying and virus extraction was adapted to contaminate the whole microphone (Figure 4). Also in this case, two microphones were sprayed at the same time, receiving an equal dose of M13 bacteriophage. One microphone served as the untreated control, and the other one was subjected to 10 min of UV-C irradiation. Subsequently, the microphones were extracted in a similar fashion as the foam, but by using a larger container holding 400 mL of water. These experiments were performed three times independently with technical replicates. The PFU reduction on whole microphones upon UV-C irradiation was determined by comparing the control to the 10-min irradiated microphone, showing an average reduction in infectious virus particles of 99.8%, 99.1%, and 98.9% for the different technical replicates and an overall average reduction of 99.3%. Because the microphones themselves will affect the UV dose that reaches the surfaces and because there are different materials used at different positions within the microphone, a direct location dependent dose-response identification does not illustrate the true operating conditions.