Tatiana Nikolaeva

Quantitative microstructural magnetic resonance imaging (MRI) can noninvasively characterize tissue configuration at micrometer scales, but clinical uptake is limited by validation and optimization in human-relevant scenarios. Organoids are powerful human-relevant tissue models, yet translation is hampered by lack of non-destructive, longitudinal microstructural assessment. Bridging these gaps, microstructural MRI of living organoids can accelerate MRI biomarker and organoid development and validation. Here, we address key obstacles to enable organoid microstructural MRI. First, we use a unique 28.2 T MRI system to achieve spatial resolution with adequate signal-to-noise ratio and feasible scan times. Second, we implement flexible acquisitions with fast readouts to expand multivariate experimental capacity. Third, we develop a workflow combining 3D MRI and 3D lightsheet microscopy for cross-modality anatomical comparison beyond 2D. Using this platform, we demonstrate microstructural MRI of cortical organoids with resolutions down to (20 µm ) ³ , revealing anisotropy, heterogeneity, maturation-dependent differences, and temporal changes in cortical organoids. Correlative lightsheet microscopy confirms correspondence to axonal and nuclear architecture. This platform enables live-organoid MRI as a complementary tool to human- and animal imaging for robust microstructural assessment.

Covalent organic frameworks (COFs) are porous materials with high surface areas, making them interesting for a large variety of applications including energy storage, gas separation, photocatalysis, and chemical sensing. Structural variation plays an important role in tuning COF properties. Next to the type of the building block core, bonding directionality, and linking chemistry, substitution of building blocks provides another level of synthetic control. Thorough characterization and comparison of various substitution patterns is relevant for the molecular engineering of COFs via rational design. To this end, we have systematically synthesized and characterized multiple combinations of several methylated and non-methylated building blocks to obtain a series of imine-based COFs. This includes the experimental assignment of the COF structure by solid-state NMR. By comparing the properties of all COFs, the following trends were found: (1) upon methylation of the aldehyde nodes, COFs show increased Brunauer-Emmett-Teller surface areas, reduced pore collapse, blue-shifted absorbance spectra, and ∼0.2 eV increases in their optical band gaps. (2) COFs with dimethylated amine linkers show a lower porosity. (3) In tetramethylated amine linkers, the COF porosity even further decreases, the absorbance spectra are clearly red-shifted, and smaller optical band gaps are obtained. Our study shows that methyl substitution patterns on COF building blocks are a handle to control the UV absorbance of the resulting frameworks.

The mechanical properties of many supramolecular materials are often determined by noncovalent interactions that arise from an interplay between chemical composition and molecular microstructural organization. The reversible nature of noncovalent interactions gives supramolecular materials responsive properties that are otherwise difficult to obtain, such as becoming rigid as a response to mechanical stress. How exactly noncovalent interactions emerge from microstructure, and how they might change in response to applied force or deformation, is not understood. Here we combine nuclear magnetic resonance (NMR) and rheology to directly probe the role of chain proximity in polymer complexes. We observe an increase in chain proximity in response to imposed flow, which we hypothesize to originate from enhanced hydrogen bonding. The chain proximity is directly correlated to rod climbing and shear banding. Flow persists only when applied stresses are low, suggesting a stress-induced thickening mechanism. We verify that hydrogen bond disruptors can turn off both the nontrivial flow behavior and the spectroscopic evidence of chain proximity. The combined rheo-NMR approach shows that it is possible to directly observe the molecular origins behind supramolecular mechanics, paving the way for further study into mechanochemical properties of supramolecular materials.

Performing rheo-microMRI velocimetry at high magnetic field with strong pulsed field gradients has clear advantages in terms of (chemical) sensitivity and resolution in velocities, time and space. To benefit from these advantages some artefacts need to be minimized. Significant sources of such artefacts are chemical shift dispersion due to the high magnetic field, eddy currents caused by the pulsed magnetic field gradients, and possible mechanical instabilities in concentric cylinder (CC) rheo-cells. These in particular hamper quantitative assessment of spatially resolved velocity profiles needed to construct local flow curves (LFCs) in CC geometries with millimeter gap sizes. A major improvement was achieved by chemical shift selective suppression of signals that are spectroscopically different from the signal of interest. By also accounting for imper-fections in pulsed field gradients, LFCs were obtained that were virtually free of artefacts. The approach to obtain quantita-tive LFCs in millimeter gap CC rheo-MRI cells was validated for a Newtonian and a simple yield stress fluids, which both showed quantitative agreement between local and global flow curves. The acquisition of LFCs during heterogeneous and transient flow of a fat crystal dispersion demonstrated that local constitutive laws can be assessed by rheo-microMRI at high magnetic field in a non-invasive, quantitative and real-time manner.

A detailed investigation was carried out on the modulation of the coupling between network formation and recrystallisation of oil-dispersed micronized fat crystal (MFC) nanoplatelets by varying oil composition, shear and temperature. Sunflower (SF) and bean (BO) oils were used as dispersing media for MFC nanoplatelets. Already during MFC dispersion production at high shear a significant increase of the average crystal thickness (ACT) could be observed, pointing to recrystallisation of the MFC nanoplatelets. More rapid recrystallisation of MFC occurred in the SF dispersion compared to the BO dispersion, which is attributed to higher solubility of MFC in the SF oil. When the dispersions were maintained under low shear in a narrow gap Couette geometry we witness two stages for recrystallisation (measured via rheo-SAXD) and development of local yield stress (measured via rheo-MRI). In the first stage, shear-enabled mass transfer induces rapid recrystallisation of randomly distributed MFC nanoplatelets, which is reflected in a rapid increase of ACT (rheo-SAXD). The formation of a space filling weak-link MFC network explains the increase in yield stress (assessed in real time by rheo-MRI). In this second stage, recrystallisation slows down and yield stress decreases due to formation of MFC aggregates in the weak link network, as observed by confocal Raman imaging. The high fractal dimension of the weak-link network indicates that aggregation takes place via a particle-cluster mechanism. The effects of oil type and shear on recrystallisation rate and network strength could be reproduced in a stirred bowl with a heterogeneous shear stress field, which opens perspectives for rational manipulation of MFC thickness and network strength under industrial processing conditions.

Dispersions of micronized fat crystals (MFCs) in oil form a weak-interaction network organized by crystal aggregates in a continuous net of crystalline nanoplatelets.