Periodic Reporting for period 2 - Cell-Lasers (Intracellular lasers: Coupling of optical resonances with biological processes) Reporting period: 2021-11-01 to 2023-04-30 Summary of the context and overall objectives of the project The goal of the Cell-Lasers project is to develop and embed several types of lasers into cells to study the cell biology to an unprecedented accuracy and multiplexing ability. The specific objectives are to study:• forces acting within cells,• properties of natural cavities in lipid droplets and• intracellular chemical environment.In the long term Cell-Lasers aims to transform the bio-integrated lasers from being a pure scientific curiosity into powerful tool for the study of biophysical and biochemical processes taking place on a single cell level. Cell-Lasers will significantly improve measurement of several biochemical and biophysical parameters inside cells. By elucidating processes in cells which also lead to diseases, Cell-Lasers will have a strong and long-term impact on medical and social level, by improving quality of life. Lasers inside cells are also very interesting to broad audience and can have large media impact. Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far Interfacial tension is important in a number of biological processes. Therefore, a precise in situ measurement of interfacial tension is very important. A simple, fast and very precise technique to measure interfacial tension between two immiscible liquids based on optical resonances was demonstrated (Fig. 1). A microdroplet is generated at the end of a glass microcapillary, submerged in a continuous liquid phase, and its size changes are monitored with nanometer precision via the optical resonances, while simultaneously applying finely tunable pressure through the microcapillary. Interfacial tension was determined from the size of the droplet and the pressure in the microcapillary at equilibrium. Droplets as small as 8 microns were used, thus requiring extremely small sample volume.The results of this study were published in a peer reviewed article: Gregor Pirnat, and Matjaž Humar, Whispering Gallery-Mode Microdroplet Tensiometry, Advanced Photonics Research 2, 2100129 (2021), DOI: 10.1002/adpr.202100129Coupling of light with the complex soft matter structures inserted into a laser cavity was studied into detail. This study provides experimental and simulation insights into this coupling. Complex tunable microlasers emitting structured light were made from self-assembled topological liquid crystal superstructures containing topological defects inserted into a thin Fabry-Pérot microcavity (Fig. 2). The topology and geometry of the liquid crystal superstructure determine the structuring of the emitted light by providing complex three dimensionally varying optical axis and order parameter singularities, also affecting the topology of the light polarization.The results of this study were published in a peer reviewed article: Miha Papič, Urban Mur, Kottoli Poyil Zuhail, Miha Ravnik, Igor Muševič, and Matjaž Humar, Topological liquid crystal superstructures as structured light lasers, Proceedings of the National Academy of Sciences 118, e2110839118 (2021), DOI: 10.1073/pnas.2110839118Despite numerous novel methods for optical imaging in strongly scattering biological tissues, imaging at single-cell resolution beyond the ballistic light transport regime remains very challenging. We demonstrated that optical microcavity probes embedded inside cells enable three-dimensional localization and tracking of individual cells over extended time periods, as well as sensing of their environment, at depths well beyond the light transport length (Fig. 3). This is achieved by utilizing unique spectral features of the whispering-gallery modes, which are unaffected by tissue scattering, absorption and autofluorescence. In addition, microcavities can be functionalized for simultaneous sensing of various parameters, such as temperature or pH value, which extends their versatility beyond the capabilities of standard fluorescent labels. First author of the paper Aljaž Kavčič presented the results of this work in his master’s thesis, for which he was awarded »Prešernova nagrada« of University of Ljubljana.The results of this study were published in a peer reviewed article: Aljaž Kavčič, Maja Garvas, Matevž Marinčič, Katrin Unger, Anna Maria Coclite, Boris Majaron, Matjaž Humar, Deep tissue localization and sensing using optical microcavity probes, Nature Communications 13, 1269 (2022), DOI: 10.1038/s41467-022-28904-6 Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far) The work will continue on all three main objectives:• forces acting within cells,• properties of natural cavities in lipid droplets and• intracellular chemical environment.On all of these three topics we already have very promising results. Some of the results are already summarized in manuscripts and will be submitted soon. (left) The principle of the experiment. (right) Localized microcavities and their sizes. Figure shows the experimental configuration of a complex soft matter laser. Figure shows the principle of interfacial tension measurement.