The Simons Observatory: metamaterial microwave absorber and its cryogenic applications.

Controlling stray light at millimeter wavelengths requires special optical design and selection of absorptive materials that should be compatible with cryogenic operating environments. While a wide selection of absorptive materials exists, these typically exhibit high indices of refraction and reflect/scatter a significant fraction of light before absorption. For many lower index materials such as commercial microwave absorbers, their applications in cryogenic environments are challenging. In this paper, we present a new tool to control stray light: metamaterial microwave absorber tiles. These tiles comprise an outer metamaterial layer that approximates a lossy gradient index anti-reflection coating. They are fabricated via injection molding commercially available carbon-loaded polyurethane (25% by mass). The injection molding technology enables mass production at low cost. The design of these tiles is presented, along with thermal tests to 1 K. Room temperature optical measurements verify their control of reflectance to less than 1% up to 65∘ angles of incidence, and control of wide angle scattering below 0.01%. The dielectric properties of the bulk carbon-loaded material used in the tiles is also measured at different temperatures, confirming that the material maintains similar dielectric properties down to 3 K.

Exhaled aerosol increases with COVID-19 infection, age, and obesity.

COVID-19 transmits by droplets generated from surfaces of airway mucus during processes of respiration within hosts infected by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) virus. We studied respiratory droplet generation and exhalation in human and nonhuman primate subjects with and without COVID-19 infection to explore whether SARS-CoV-2 infection, and other changes in physiological state, translate into observable evolution of numbers and sizes of exhaled respiratory droplets in healthy and diseased subjects. In our observational cohort study of the exhaled breath particles of 194 healthy human subjects, and in our experimental infection study of eight nonhuman primates infected, by aerosol, with SARS-CoV-2, we found that exhaled aerosol particles vary between subjects by three orders of magnitude, with exhaled respiratory droplet number increasing with degree of COVID-19 infection and elevated BMI-years. We observed that 18% of human subjects (35) accounted for 80% of the exhaled bioaerosol of the group (194), reflecting a superspreader distribution of bioaerosol analogous to a classical 20:80 superspreader of infection distribution. These findings suggest that quantitative assessment and control of exhaled aerosol may be critical to slowing the airborne spread of COVID-19 in the absence of an effective and widely disseminated vaccine.

A New Natural Defense Against Airborne Pathogens.

We propose the nasal administration of calcium-enriched physiological salts as a new hygienic intervention with possible therapeutic application as a response to the rapid and tenacious spread of COVID-19. We test the effectiveness of these salts against viral and bacterial pathogens in animals and humans. We find that aerosol administration of these salts to the airways diminishes the exhalation of the small particles that face masks fail to filter and, in the case of an influenza swine model, completely block airborne transmission of disease. In a study of 10 human volunteers (5 less than 65 years and 5 older than 65 years), we show that delivery of a nasal saline comprising calcium and sodium salts quickly (within 15 min) and durably (up to at least 6 h) diminishes exhaled particles from the human airways. Being predominantly smaller than 1 µm, these particles are below the size effectively filtered by conventional masks. The suppression of exhaled droplets by the nasal delivery of calcium-rich saline with aerosol droplet size of around 10 µm suggests the upper airways as a primary source of bioaerosol generation. The suppression effect is especially pronounced (99%) among those who exhale large numbers of particles. In our study, we found this high-particle exhalation group to correlate with advanced age. We argue for a new hygienic practice of nasal cleansing by a calcium-rich saline aerosol, to complement the washing of hands with ordinary soap, use of a face mask, and social distancing.