Periodic Photobleaching with Structured Illumination for Diffusion Imaging

The use of periodically structured illumination coupled with spatial Fourier-transform fluorescence recovery after photobleaching (FT-FRAP) was shown to support diffusivity mapping within segmented domains of arbitrary shape. Periodic comb-bleach patterning of the excitation beam during photobleaching encoded spatial maps of diffusion onto harmonic peaks in the spatial Fourier transform. Diffusion manifests as a simple exponential decay of a given harmonic, improving the signal to noise ratio and simplifying mathematical analysis. Image segmentation prior to Fourier transformation was shown to support pooling for signal to noise enhancement for regions of arbitrary shape expected to exhibit similar diffusivity within a domain. Following proof-of concept analyses based on simulations with known ground-truth maps, diffusion imaging by FT-FRAP was used to map spatially resolved diffusion differences within phase-separated domains of model amorphous solid dispersion spin-cast thin films. Notably, multi-harmonic analysis by FT-FRAP was able to definitively discriminate and quantify the roles of internal diffusion and exchange to higher mobility interfacial layers in modeling the recovery kinetics within thin amorphous/amorphous phase-separated domains, with interfacial diffusion playing a critical role in recovery. These results have direct implications for the design of amorphous systems for stable storage and efficacious delivery of therapeutic molecules.

Automated summary

The use of periodically structured illumination coupled with spatial Fourier-transform fluorescence recovery after photobleaching (FT-FRAP) allows for diffusivity mapping within segmented domains of arbitrary shape. By encoding spatial maps of diffusion onto harmonic peaks in the spatial Fourier transform, diffusion imaging by FT-FRAP improves signal to noise ratio and simplifies mathematical analysis. This technique has been applied to map spatially resolved diffusion differences within phase-separated domains of model amorphous solid dispersion spin-cast thin films, with implications for the design of stable storage and efficient delivery of therapeutic molecules.