Flow-driven Surface Instabilities of Tubular Chitosan Hydrogel

Chitosan-based hydrogel wound dressing: From mechanism to applications, a review

As promising biomaterials, hydrogels are widely used in the medical engineering field, especially in wound repairing. Compared with traditional wound dressings, such as gauze and bandage, hydrogel could absorb and retain more water without dissolving or losing its three-dimensional structure, thus avoiding secondary injury and promoting wound healing. Chitosan and its derivatives have become hot research topics for hydrogel wound dressing production due to their unique molecular structure and diverse biological activities. In this review, the mechanism of wound healing was introduced systematically. The mechanism of action of chitosan in the first three stages of wound repair (hemostasis, antimicrobial properties and progranulation), the effect of chitosan deacetylation and the molecular weight on its performance are analyzed. Additionally, the recent progress in intelligent and drug-loaded chitosan-based hydrogels and the features and advantages of chitosan were discussed. Finally, the challenges and prospects for the future development of chitosan-based hydrogels were discussed.

Shape Evolution of Precipitate Membranes in Flow Systems

Chemical gardens are macroscopic structures that form when a salt seed is submerged in an alkaline solution. Their thin precipitate membranes separate the reactant partners and slow down the approach toward equilibrium. During this stage, a gradual thickening occurs, which is driven by steep cross-membrane gradients and governed by selective ion transport. We study these growth dynamics in microfluidic channels for the case of Ni(OH)₂ membranes. Fast flowing reactant solutions create thickening membranes of a nearly constant width along the channel, whereas slow flows produce wedge-shaped structures that fail to grow along their downstream end. The overall dynamics and shapes are caused by the competition of reactant consumption and transport replenishment. They are reproduced quantitatively by a two-variable reaction-diffusion-advection model which provides kinetic insights into the growth of precipitate membranes.

Flow-driven synthesis of calcium phosphate-calcium alginate hybrid chemical gardens

Systems far-from-equilibrium self-assemble into spatiotemporal structures. Here, we report on the formation of calcium alginate gardens along with their inorganic hybrids when a sodium alginate solution containing sodium phosphate in various compositions is injected into a calcium chloride reservoir. The viscoelastic properties of the membranes developed are controlled by the injection rate, while their thickness by the amount of sodium phosphate besides diffusion. Inorganic hybrid membranes with constant thickness are synthesized in the presence of a sufficient amount of sodium phosphate. The electrochemical characterization of the membranes suggests that the driving force is the pH-gradient developing along the two sides; hence, the cell potential can be controlled by the addition of alkaline sodium phosphate into the sodium alginate solution.

Pattern selection of directionally oriented chitosan tubes

The growth of viscoelastic curved materials, inspired by biological systems, may give rise to various complex structures. One of the simplest ways to control the pattern formation is to vary the orientation of the reaction vessel while keeping all other experimental conditions constant. Here, we report the self-organization of soft chitosan tubes by injecting acidic chitosan sol into a pool of sodium hydroxide solution, where the adhesive force between the gel and container keeps the tubules on the bottom of the reactor. The horizontal growth of the tubular structure undergoes spontaneous symmetry breaking, where instabilities develop on the surface of the chitosan tubules. Transformation of folds into wrinkles and finally to a smooth tube takes place by varying the orientation of the container. In addition to characterizing the evolving structures, we have also shown that the linear growth rate of the tube scales with the tilt angle of the container from the horizontal.

Formation and growth of lithium phosphate chemical gardens

We show that a chemical garden can be developed from an alkaline metal precipitate using a flow-driven setup. By injecting sodium phosphate solution into lithium chloride solution from below, a liquid jet appears, on which a precipitate grows forming a structure resembling a hydrothermal vent. The precipitate column continuously builds upward until a maximum height is reached. The vertical growth then significantly slows down while the tube diameter still increases. The analysis of the growth profiles has revealed a linear dependence of volume growth rate on the injection rate, hence yielding a universal growth profile. The expansion in diameter, localized at the tip of the structure, scales with a power law suggesting that the phenomenon is controlled by both diffusion and convection.

Co-encapsulation of vitamin D and rutin in chitosan-zein microparticles

There is a growing interest in co-encapsulating multiple species to harness potential synergy between them, enhance their stability and efficacy in various products. The aim of this work was to co-encapsulate vitamin D3 and rutin inside chitosan-zein microparticles using a simple and easily scalable process for food fortification. This was achieved via anti-solvent precipitation coupled with spray-drying. Free-flowing powders of spherical microparticles with wrinkled surface and particle size < 10 μm were obtained. The encapsulation efficiency was 75% for vitamin D3 and 44% for rutin and this could be attributed to their different molecular size and affinity to the aqueous phase. The physicochemical properties were characterized by X-Ray powder diffraction and Fourier transform infrared spectroscopy. The two crystalline bioactive compounds were present in the microparticles in amorphous form, which would allow for better bioavailability when compared to non-encapsulated crystalline solid. Therefore, the obtained microparticles would be suitable for use as food ingredient for vitamin D3 fortification, with the co-encapsulated rutin acting as stability and activity enhancer.

Perovskite chemical gardens: highly fluorescent microtubes from self-assembly and ion exchange

We report the shape-preserving conversion of self-assembled CaCO3 microtubes to PbCO3 and MAPbBr3 perovskite.

Hierarchical Self-Assembly of Metal-Ion-Modulated Chitosan Tubules

Soft materials such as gels or biological tissues can develop via self-assembly under chemo-mechanical forces. Here, we report the instantaneous formation of soft tubular structures with a two-level hierarchy by injecting a mixture of inorganic salt and chitosan (CS) solution from below into a reactor filled with alkaline solution. Folding and wrinkling instabilities occur on the originally smooth surface controlled by the salt composition and concentration. Liesegang-like precipitation patterns develop on the outer surface on a μm length scale in the presence of calcium chloride, while the precipitate particles are distributed evenly in the bulk as corroborated by X-ray μ-CT. On the other hand, barium hydroxide precipitates out only in the thin outer layer of the CS tubule when barium chloride is introduced into the CS solution. Independent of the concentration of the weakly interacting salt, an electric potential gradient across the CS membrane develops, which vanishes when the pH difference between the two sides of the membrane diminishes.

Flow-driven Surface Instabilities of Tubular Chitosan Hydrogel

Spatial structures break their symmetry under the influence of shear stress arising from fluid flow. Here, we present surface instabilities appearing on chitosan tubes when an acidic solution of chitosan with various molecular weight is injected into a pool of sodium hydroxide solution. At slow flow rates wrinkle-to-fold transition takes place along the direction of the flow yielding a banded structure. For greater injection rates we observe coexisting modes of wrinkles and folds which are stabilized to periodic wrinkles when the alkaline concentration is increased. The instabilities are characterized by the scaling laws of the pattern wavelength and amplitude with the tube characteristics. Our experimental adaptation of mechanical instabilities provides a new in situ method to create soft biomaterials with the desired surface morphology without the use of any prefabricated templates.