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IntroductionPhotomodifiable azopolymer nanotopographies represent a powerful means of assessing how cells respond to rapid changes in the local microenvironment. However, previous studies have suggested that azopolymers are readily photomodified under typical fluorescence imaging conditions over much of the visible spectrum. Here we assess the stability of azopolymer nanoridges under 1-photon and 2-photon imaging over a broad range of wavelengths. MethodsAzopolymer nanoridges were created via microtransfer molding of master structures that were created using interference lithography. The effects of exposure to a broad range of wavelengths of light polarized parallel to the ridges were assessed on both a spinning-disk confocal microscope and a 2-photon fluorescence microscope. Experiments with liveDictyostelium discoideumcells were also performed using alternating cycles of 514-nm light for photomodification and 561-nm light for fluorescence imaging. Results and DiscussionWe find that for both 1-photon and 2-photon imaging, only a limited range of wavelengths of light leads to photomodification of the azopolymer nanotopography. These results indicate that nondestructive 1-photon and 2-photon fluorescence imaging can be performed over a considerably broader range of wavelengths than would be suggested by previous research. 

Triplet–triplet annihilation upconversion (TTA-UC) is a process that shows promise for applications such as energy-harvesting and light-generation technologies. The irradiance dependent performance of TTA-UC systems is typically gauged using a graphical analysis, rather than a detailed model. Additionally, kinetic models for TTA-UC rarely incorporate mass conservation, which is a phenomenon that can have important consequences under experimentally relevant conditions. We present an analytical, mass-conserving kinetic model for TTA-UC, and demonstrate that the mass-conservation constraint cannot generally be ignored. This model accounts for saturation in TTA-UC data. Saturation complicates the interpretation of the threshold irradiance I th , a popular performance metric. We propose two alternative figures of merit for overall performance. Finally, we show that our model can robustly fit experimental data from a wide variety of sensitized TTA-UC systems, enabling the direct and accurate determination of I th and of our proposed performance metrics. We employ this fitting procedure to benchmark and compare these metrics, using data from the literature. 

Abstract Asymmetric nanotopography with sub-cellular dimensions has recently demonstrated the ability to provide a unidirectional bias in cell migration. The details of this guidance depend on the type of cell studied and the design of the nanotopography. This behavior is not yet well understood, so there is a need for a predictive description of cell migration on such nanotopography that captures both the initiation of migration, and the way cell migration evolves. Here, we employ a three-dimensional, physics-based model to study cell guidance on asymmetric nanosawteeth. In agreement with experimental data, our model predicts that asymmetric sawteeth lead to spontaneous motion. Our model demonstrates that the nanosawteeth induce a unidirectional bias in guidance direction that is dependent upon actin polymerization rate and sawtooth dimensions. Motivated by this model, an analysis of previously reported experimental data indicates that the degree of guidance by asymmetric nanosawteeth increases with the cell velocity.