A new HPF specimen carrier adapter for the use of high-pressure freezing with cryoscanning electron microscope: two applications: stearic acid organization in a hydroxypropyl methylcellulose matrix and mice myocardium

Comparative Analysis of Electron Microscopy Techniques for Hydrogel Microarchitecture Characterization: SEM, Cryo-SEM, ESEM, and TEM

Hydrogels have emerged as a versatile class of materials with broad applications in biomedical engineering, drug delivery, and tissue engineering. Understanding their intricate structures and morphologies is crucial for tailoring their properties to meet specific biomedical needs. It has been clearly established that the composition and microarchitecture of the materials play a critical role in essential cellular mechanisms such as mechanosensing, adhesion, and remodeling. This question is essential in tissue engineering, where precisely characterizing the microarchitecture of the materials used to model the cell microenvironment is a critical step to ensure the reproducibility and relevance of reconstructed tissues. In this study, we present a comprehensive comparison of four advanced electron microscopy techniques, namely, scanning electron microscopy, cryo-scanning electron microscopy, environmental scanning electron microscopy, and transmission electron microscopy, to observe the hydrogel microarchitecture, including a comparison of the sample preparation methods for each technique. Each technique's specific advantages and limitations are discussed in detail, highlighting their unique capabilities in characterizing the hydrogel structures. We illustrate this study with two semisynthetic hydrogels, such as gelatin methacrylate and hyaluronic acid methacrylate. Moreover, we delve into the critical sample preparation steps necessary for each method, emphasizing the need to preserve the hydrogel's native state while obtaining high-resolution images. This comparative analysis aims to select the most suitable electron microscopy technique for their hydrogel studies, fostering deeper insights into the design and development of advanced biomaterials for tissue engineering applications.

Mucus from human bronchial epithelial cultures: rheology and adhesion across length scales

Mucus is a viscoelastic aqueous fluid that participates in the protective barrier of many mammals' epithelia. In the airways, together with cilia beating, mucus rheological properties are crucial for lung mucociliary function, and, when impaired, potentially participate in the onset and progression of chronic obstructive pulmonary disease (COPD). Samples of human mucus collected in vivo are inherently contaminated and are thus poorly characterized. Human bronchial epithelium (HBE) cultures, differentiated from primary cells at an air-liquid interface, are highly reliable models to assess non-contaminated mucus. In this paper, the viscoelastic properties of HBE mucus derived from healthy subjects, patients with COPD and from smokers are measured. Hallmarks of shear-thinning and elasticity are obtained at the macroscale, whereas at the microscale mucus appears as a heterogeneous medium showing an almost Newtonian behaviour in some extended regions and an elastic behaviour close to boundaries. In addition, we developed an original method to probe mucus adhesion at the microscopic scale using optical tweezers. The measured adhesion forces and the comparison with mucus-simulants rheology as well as mucus imaging collectively support a structure composed of a network of elastic adhesive filaments with a large mesh size, embedded in a very soft gel.

3D printing of biocompatible low molecular weight gels: Imbricated structures with sacrificial and persistent N-alkyl-d-galactonamides

HYPOTHESIS

We have shown earlier that low molecular weight gels based on N-heptyl-d-galactonamide hydrogels can be 3D printed by solvent exchange, but they tend to dissolve in the printing bath. We wanted to explore the printing of less soluble N-alkyl-d-galactonamides with longer alkyl chains. Less soluble hydrogels could be good candidates as cell culture scaffolds.

EXPERIMENTS

N-hexyl, N-octyl and N-nonyl-d-galactonamide solutions in dimethylsulfoxide are injected in a bath of water following patterns driven by a 2D drawing robot coupled to a z-platform. Solubilization of the gels with time has been determined and solubility of the gelators has been measured by NMR. Imbricated structures have been built with N-nonyl-d-galactonamide as a persistent ink and N-hexyl or N-heptyl-d-galactonamide as sacrificial inks. Human mesenchymal stem cells have been cultured on N-nonyl-d-galactonamide hydrogels prepared by cooling or by 3D printing.

FINDINGS

The conditions for printing well-resolved 3D patterns have been determined for the three gelators. In imbricated structures, the solubilization of N-hexyl or N-heptyl-d-galactonamide occurred after a few hours or days and gave channels. Human mesenchymal stem cells grown on N-nonyl-d-galactonamide hydrogels prepared by heating-cooling, which are stable and have a fibrillar microstructure, developed properly. 3D printed hydrogels, which microstructure is made of micrometric flakes, appeared too fragile to withstand cell growth.

Microstructural and rheological investigation of upcycled metal-organic frameworks stabilized Pickering emulsions

HYPOTHESIS

Stabilizing Pickering emulsions with metal-organic frameworks (MOFs) is a known way to incorporate them into hierarchically porous materials. Studies generally focus on their final properties and emulsion microstructures are rarely precisely described. Our hypothesis was that characterizing the microstructural and rheological properties of Pickering emulsions stabilized solely by Al-based MOFs (MIL-96) particles would provide insights into how to control their stability and workability for potential industrial applications.

EXPERIMENTS

MIL-96(Al) particles, obtained from Li-ion battery waste were used to stabilize paraffin-in-water Pickering emulsions. The influence of the formulation parameters (paraffin/water volume ratio and MIL-96(Al) content) were investigated and the emulsions were analysed using optical microscopy, cryo-scanning electron microscopy and rheological measurements.

FINDINGS

MIL-96(Al) efficiently stabilized paraffin-in-water emulsions with up to 80% of internal phase. The emulsions with a low paraffin volume fraction had large droplets and a fluid gel-like texture. The emulsions with higher paraffin volume fractions were more compact and had two-step flow curves. In this system, excess MIL-96(Al) particles aggregated in the continuous phase as flocs interact with particles adsorbed at the paraffin-water interface, creating a secondary network that has to be broken for flow to resume. This behaviour may be interesting to investigate in other MOF-stabilized emulsions.

How an organogelator can gelate water: gelation transfer from oil to water induced by a nanoemulsion

A hydrogel can be formed by an organogelator in the presence of a nanoemulsion. It is expected that this is due to a gelation transfer from oil to water. The system started with an oil-in-water nanoemulsion prepared according to a phase inversion temperature (PIT) process. Into this nanoemulsion consisting of Kolliphor® RH40 and Brij® L4 as surfactants, and Miglyol® 812 as oil and water, we introduced the organogelator 12-hydroxyoctadecanoic acid (12-HOA) in the oil phase. After cooling at room temperature, a slow reversible gelation of the water phase occurred with persistence of the nanoemulsion. This thermally reversible system was investigated using various techniques (rheology, turbidimetry, optical and electron microscopies, scattering techniques). Successive stages appeared during the cooling process after the nanoemulsion formation, corresponding to the migration and self-assembly of the organogelator from the oil nanodroplets to the water phase. According to our measurements and the known self-assembly of 12-HOA, a mechanism explaining the formation of the gelled nanoemulsion is proposed.