Serological assays can detect anti-SARS-CoV-2 (SARS2) antibodies, but their sensitivity often comes at the expense of specificity. Here we developed a Tripartite Automated Blood Immunoassay (TRABI) to assess the IgG response against SARS2. Calibration was per-formed with 90 prepandemic and 55 virologically and clinically confirmed COVID-19 sam-ples. Posterior probabilities of seropositivities were calculated from 3x8 measurements of logarithmically diluted samples against the ectodomain and the receptor-binding domain of the spike protein and the nucleoprotein. We then performed 760'320 assays on 5'503 prepandemic and 26'177 copandemic samples from hospital patients and healthy blood donors. We found 176 seropositive samples between December 2019 and May 2020. The seroprevalence increased conspicuously in March 2020 but plateaued in late April at 0.8-1.6% in both cohorts, indicating an equilibrium between new infections and the waning of immunity. This points to a high effectiveness of containment measures and/or to unex-pectedly rapid loss of humoral responses.

The clinical outcome of SARS-CoV-2 infections, which can range from asymptomatic to lethal, is crucially shaped by the concentration of antiviral antibodies and by their affinity to their targets. However, the affinity of polyclonal antibody responses in plasma is difficult to measure. Here we used microfluidic antibody affinity profiling (MAAP) to determine the aggregate affinities and concentrations of anti–SARS-CoV-2 antibodies in plasma samples of 42 seropositive individuals, 19 of which were healthy donors, 20 displayed mild symptoms, and 3 were critically ill. We found that dissociation constants, Kd, of anti–receptor-binding domain antibodies spanned 2.5 orders of magnitude from sub-nanomolar to 43 nM. Using MAAP we found that antibodies of seropositive individuals induced the dissociation of pre-formed spike-ACE2 receptor complexes, which indicates that MAAP can be adapted as a complementary receptor competition assay. By comparison with cytopathic effect–based neutralisation assays, we show that MAAP can reliably predict the cellular neutralisation ability of sera, which may be an important consideration when selecting the most effective samples for therapeutic plasmapheresis and tracking the success of vaccinations.

Abstract3D scaffolds in the form of hydrogels and microgels have allowed for more native cell‐culture systems to be developed relative to flat substrates. Native biological tissues are, however, usually spatially inhomogeneous and anisotropic, but regulating the spatial density of hydrogels at the microscale to mimic this inhomogeneity has been challenging to achieve. Moreover, the development of biocompatible synthesis approaches for protein‐based microgels remains challenging, and typical gelation conditions include UV light, extreme pH, extreme temperature, or organic solvents, factors which can compromise the viability of cells. This study addresses these challenges by demonstrating an approach to fabricate protein microgels with controllable radial density through microfluidic mixing and physical and enzymatic crosslinking of gelatin precursor molecules. Microgels with a higher density in their cores and microgels with a higher density in their shells are demonstrated. The microgels have robust stability at 37 °C and different dissolution rates through enzymolysis, which can be further used for gradient scaffolds for 3D cell culture, enabling controlled degradability, and the release of biomolecules. The design principles of the microgels could also be exploited to generate other soft materials for applications ranging from novel protein‐only micro reactors to soft robots.

The clinical outcome of SARS-CoV-2 infections can range from asymptomatic to lethal, and is thought to be crucially shaped by the quality of the immune response which includes antibody titres and affinity for their targets. Using Microfluidic Antibody Affinity Profiling (MAAP), we determined the aggregate affinities and concentrations of anti-SARS-CoV-2 antibodies in plasma samples of 42 seropositive individuals, 23 of whom were confirmed to be SARS-CoV-2-positive by PCR testing. We found that dissociation constants (Kd) of anti-RBD antibodies spanned more than two orders of magnitude from 80 pM to 25 nM, despite having similar antibody concentrations. Individual patients showed progressively higher antibody concentrations but constant Kd values, suggesting that affinities did not mature over time. 33 sera showed affinities higher than that of the CoV2 spike for its ACE2 receptor. Accordingly, addition of seropositive plasma to pre-formed spike-ACE2 receptor complexes led to their dissociation. Finally, we observed that the RBD of HKU1, OC43, and SARS-CoV coronaviruses, but not unrelated control proteins, were able to compete substantially with the RBD of SARS-CoV-2 in solution. Therefore, the affinity of total plasma immunoglobulins to SARS-CoV-2 is an indicator of the quality of the immune response to SARS-CoV-2, and may help select the most efficacious samples for therapeutic plasmapheresis.

Many biological molecules are by their nature amphiphilic and have the ability to act as surfactants, stabilizing interfaces between aqueous and immiscible oil phases. In this paper, we explore the adsorption kinetics of surfactin, a naturally occurring cyclic lipopeptide, at hexadecane/water interfaces and compare and contrast its adsorption behaviour with that of synthetic alkyl benzene sulfonate isomers, through direct measurements of changes in interfacial tension upon surfactant adsorption. We access millisecond time resolution in kinetic measurements by making use of droplet microfluidics to probe the interfacial tension of hexadecane droplets dispersed in a continuous water phase through monitoring their deformation when the droplets are exposed to shear flows in a microfluidic channel with regular corrugations. Our results reveal that surfactin rapidly adsorbs to the interface, thus the interfacial tension equilibrates within 300 ms, while the synthetic surfactants used undergo adsorption processes at an approximately one order of magnitude longer timescale. The approach presented may provide opportunities for understanding and modulating the adsorption mechanism of amphiphiles on a variety of interfaces in the context of life sciences and industrial applications.