Gelling kinetics and in situ mineralization of alginate hydrogels: A correlative spatiotemporal characterization toolbox

Microbeads as carriers for Bacillus pumilus: a biofertilizer focus on auxin production

The study aimed to develop a solid biofertilizer using Bacillus pumilus, focusing on auxin production to enhance plant drought tolerance. Methods involved immobilising B. pumilus in alginate-starch beads, focusing on microbial concentration, biopolymer types, and environmental conditions. The optimal formulation showed a diameter of 3.58 mm ± 0.18, a uniform size distribution after 15 h of drying at 30 °C, a stable bacterial concentration (1.99 × 109 CFU g-1 ± 1.03 × 109 over 180 days at room temperature), a high auxin production (748.8 µg g-1 ± 10.3 of IAA in 7 days), and a water retention capacity of 37% ± 4.07. In conclusion, this new formulation of alginate + starch + L-tryptophan + B. pumilus has the potential for use in crops due to its compelling water retention, high viability in storage at room temperature, and high auxin production, which provides commercial advantages.

Bioinspired mechanical mineralization of organogels

Mineralization is a long-lasting method commonly used by biological materials to selectively strengthen in response to site specific mechanical stress. Achieving a similar form of toughening in synthetic polymer composites remains challenging. In previous work, we developed methods to promote chemical reactions via the piezoelectrochemical effect with mechanical responses of inorganic, ZnO nanoparticles. Herein, we report a distinct example of a mechanically-mediated reaction in which the spherical ZnO nanoparticles react themselves leading to the formation of microrods composed of a Zn/S mineral inside an organogel. The microrods can be used to selectively create mineral deposits within the material resulting in the strengthening of the overall resulting composite.

Mineralized Microgels via Electrohydrodynamic Atomization: Optimization and In Vitro Model for Dentin-Pulp Complex

There is growing interest in the use of micro-sized hydrogels, including bioactive signals, as efficient platforms for tissue regeneration because they are able to mimic cell niche structure and selected functionalities. Herein, it is proposed to optimize bioactive composite microgels via electrohydrodynamic atomization (EHDA) to regenerate the dentin-pulp complex. The addition of disodium phosphate (Na2HPO4) salts as mineral precursors triggered an in situ reaction with divalent ions in solution, thus promoting the encapsulation of different amounts of apatite-like phases. Morphological analysis via image analysis of optical images confirmed a narrow distribution of perfectly rounded particles, with an average diameter ranging from 223 ± 18 μm to 502 ± 64 μm as a function of mineral content and process parameters used. FTIR, TEM, and EDAX analyses confirmed the formation of calcium phosphates with a characteristic Ca/P ratio close to 1.67 and a needle-like crystal shape. In vitro studies-using dental pulp stem cells (DPSCs) in crown sections of natural teeth slices-showed an increase in cell viability until 14 days, recording a decay of proliferation at 21 days, independent on the mineral amount, suggesting that differentiation is started, as confirmed by the increase of ALP activity at 14 days. In this view, mineralized microgels could be successfully used to support in vitro osteogenesis, working as an interesting model to study dental tissue regeneration.

Macroscopic Pattern Formation of Alginate Gels in a Two-Dimensional System

Macroscopic spatial patterns were formed in calcium alginate gels when a drop of a calcium nitrate solution was placed on the center of a sodium alginate solution on a petri dish. These patterns have been classified into two groups. One is multi-concentric rings consisting of alternating cloudy and transparent areas observed around the center of petri dishes. The other is streaks extending to the edge of the petri dish, which are formed to surround the concentric bands between the concentric bands and the petri dish edge. We have attempted to understand the origins of the pattern formations using the properties of phase separation and gelation. The distance between two adjacent concentric rings was roughly proportional to the distance from where the calcium nitrate solution was dropped. The proportional factor p increased exponentially for the inverse of the absolute temperature of the preparation. The p also depended on the concentration of alginate. The pattern characteristics in the concentric pattern agreed with those in the Liesegang pattern. The paths of radial streaks were disturbed at high temperatures. The length of these streaks shortened with increasing alginate concentration. The characteristics of the streaks were similar to those of crack patterns resulting from inhomogeneous shrinkage during drying.

Syneresis of self-crowded calcium-alginate hydrogels as a self-driven athermal aging process

The assembly of biopolymers into a hydrated elastic network often goes along with syneresis, a spontaneous process during which the hydrogel slowly shrinks and releases solvent. The tendency to syneresis of calcium-alginate hydrogels, widely used biocompatible materials, is a hindrance to applications for which dimensional integrity is crucial. Although calcium-induced aggregation of specific block-sequences has been long known as the microscopic process at work in both primary cross-linking and syneresis, the nature of the coupling between these structural events and the global deswelling flow has remained so far elusive. We have tackled this issue within the regime of entangled pregels that yield highly cross-linked, self-crowded hydrogels with stiff networks. Using an original, stopped-flow extrusion experiment, we have unveiled a robust, stretched-exponential kinetics of shrinking, spanning more than six decades of time and quasi-independent of the alginate concentration. A careful analysis of the puzzling dynamical features of syneresis in these gels has led us to propose that due to the network rigidity, the calcium-fueled, random collapse events that drive solvent locally, are not thermally activated but rather controlled by the average poroelastic flow itself, according to a self-sustained mechanism described here for the first time.

Self-Organized Spatiotemporal Mineralization of Hydrogel: A Simulant of Osteon

Nature creates fascinating self-organized spatiotemporal patterns through the delicate control of reaction-diffusion dynamics. As the primary unit of cortical bone, osteon has concentric lamellar architecture, which plays a crucial role in the mechanical and physiological functions of bone. However, it remains a great challenge to fabricate the osteon-like structure in a natural self-organization way. Taking advantage of the nonequilibrium reaction in hydrogels, a simple mineralization strategy to closely mimic the formation of osteon in a mild physiological condition is developed. By constructing two reverse concentration gradients of ions from periphery to interior of cylindrical hydrogel, spatiotemporal self-organization of calcium phosphate in concentric rings is generated. It is noteworthy that minerals in different layers possess diverse contents and crystalline phases, which further guide the adhesion and spread of osteoblasts on these patterns, resembling the architecture and cytological behavior of osteon. Besides, theoretical data indicates the predominate role of ion concentrations and pH values of solution, in good accordance with experimental results. Independent of precise instruments, this lifelike method is easily obtained, cost-efficient, and effectively imitates the mineral deposition in osteon from a physiochemical view. The strategy may be expanded to develop other functional material patterns via spatiotemporal self-organization.

Gelation process of carboxymethyl chitosan-zinc supramolecular hydrogel studied with fluorescence imaging and mathematical modelling

Herein we report on a detailed study about the gelation kinetics of carboxymethyl chitosan-zinc (CMCh-Zn) supramolecular hydrogel by taking advantage of its intrinsic fluorescence property. A specific gelation device is designed and the gel front can be directly visualized under 365 nm UV light. The results show that when increasing Zn²⁺ concentration from 0.1 M to 1.0 M, the apparent diffusion coefficient increases gradually from 2.72 × 10^-6 cm²/s to 4.50 × 10^-6 cm²/s. The gelation kinetics then is described with a "zero order" mathematical model, proving that the gel thickness is related to the square root of the gelation time and the diffusion step is the controlling step of the gelation process. Later a more advanced model, developed in 1D geometry and solved numerically, is used to describe and predict experimental results, proving its reliability and the correct description of all the phenomena involved in the gelation process of CMCh-Zn hydrogel.

Tailoring the assembly of collagen fibers in alginate microspheres

The application of microspheres instead of bulk hydrogels in cell-laden biomaterials offers multiple advantages such as a high surface-to-volume-ratio and, consequently, a better nutrition and oxygen transfer to and from cells. The preparation of inert alginate microspheres is facile, quick, and well-established and the fabrication of alginate-collagen microspheres has been previously reported. However, no detailed characterization of the collagen fibrillogenesis in the alginate matrix is available. We use second-harmonic imaging microscopy reflection confocal microscopy and turbidity assay to study the assembly of collagen in alginate microspheres. We show that the assembly of collagen fibers in a gelled alginate matrix is a complex process that can be aided by addition of small polar molecules, such as glycine and by a careful selection of the gelling buffer used to prepare alginate hydrogels.

Microbial-induced calcium carbonate precipitation: an experimental toolbox for in situ and real time investigation of micro-scale pH evolution

Concrete is the second most consumed product by humans, after water. However, the production of conventional concrete causes more than 5% of anthropogenic CO₂ emissions and therefore there is a need for emission-reduced construction materials. One method to produce a solid, concrete-like construction material is microbial-induced calcium carbonate precipitation (MICP). To get a better understanding of MICP it is important to be able to follow local pH changes in dissolution and precipitation processes of CaCO₃. In this work we present a new method to study processes of MICP at the micro-scale in situ and in real time. We present two different methods to monitor the pH changes during the precipitation process of CaCO₃. In the first method, the average pH of small sample volumes is measured in real time, and pH changes are subsequently correlated with processes in the sample by comparing to optical microscope results. The second method is introduced to follow local pH changes at a grain scale in situ and in real time. Furthermore, local pH changes during the dissolution of CaCO₃ crystals are monitored. We demonstrate that these two methods are powerful tools to investigate the pH changes for both MICP precipitation and CaCO₃ dissolution for knowledge-based improvement of MICP-based material properties.

We present two novel experimental methods to follow global and local pH changes on a microscale in bio-cementation processes.

Stabilisation of amorphous calcium phosphate in polyethylene glycol hydrogels

Acellular polymer-calcium phosphate composites are promising bone graft materials. Hydrogels are suitable for providing a temporary matrix, while calcium phosphate minerals serve as ion depots for calcium and phosphate required for de novo bone formation. Crystalline calcium phosphates are stable under biological conditions and are commonly used in such scaffolds. However, the low solubility of these phases reduces the availability of free ions and potentially obstructs the remodelling necessary for the formation of mineralised tissue. Here, we investigate two different strategies to stabilise amorphous calcium phosphates in a synthetic polyethylene glycol-based hydrogel matrix. In vitro experiments mimicking an injectable application showed that amorphous calcium phosphate (ACP) of variable stability was formed in the hydrogel matrices. In additive-free composites, ACP transformed into brushite within minutes. Citrate or zinc additives were found to stabilise the formed ACP phase to different degrees. In the presence of citrate, ACP was stable for at least 2 h before it transformed into hydroxyapatite within 3-20 days. Partial calcium substitution with zinc (Zn/Ca = 10%) produced zinc-doped ACP of high stability that did not show signs of crystallisation for at least 20 days. The presented methods and findings open new possibilities for the design of novel injectable synthetic bone graft materials. The possibility to produce ACP with tailorable stability promises great potential for creating temporary scaffolds with good osteogenic properties. STATEMENT OF SIGNIFICANCE: Synthetic hydrogel-calcium phosphate (CaP) composites are promising biomaterials to replace human- and animal-derived bone scaffolds. Most reported hydrogel-CaP composite materials employ crystalline CaP phases that lack the osteoinductive properties of autograft. Stabilising amorphous calcium phosphates (ACP) could overcome this limitation, readily delivering calcium and phosphate ions and facilitating remodelling into new bone tissue. The design of synthetic hydrogel-ACP scaffolds, however, requires more understanding of the mineralisation processes in such matrices. This study presents a model system to characterise the complex mineral formation and transformation processes within a hydrogel matrix. We demonstrate a facile route to produce self-mineralising injectable synthetic hydrogels and prove two different strategies to stabilise ACP for different periods within the formed composites.

Local Structure of Ca2+ Alginate Hydrogels Gelled via Competitive Ligand Exchange and Measured by Small Angle X-Ray Scattering

Alginates, being linear anionic co-polymers of 1,4-linked residues β-d-ManA (M) and α-l-GulA (G), are widely applied as hydrogel biomaterials due to their favourable in vivo biocompatibility and convenient ionic crosslinking. The "egg-box" model is the prevailing description of the local structure of junction zones that form between the alginate chains and divalent cations, such as Ca2+, when ionic gelation occurs. In the present study we address to what extent signatures of lateral dimerization and further lateral association of junction zones also represent a valid model for the gelation of alginate using the recently reported method of competitive ligand exchange of chelated Ca2+ ions as a method for introducing gelling ions at constant pH. Small angle X-ray scattering with a q range from 0.1 to 3.3 nm-1 was employed to determine local structure in the hydrogel, using a custom-made fluid sample cell inserted in the X-ray beam. The scattering volume was intended to be localized to the contact zone between the two injected aqueous alginate solutions, and data was captured to resolve the kinetics of the structure formation at three different conditions of pH. The data show evolution of the local structure for the Ca2+ induced formation of junction zones in an alginate with 68% G residues, characterized by cross-sectional radii that could be accounted for by a two-component, broken rod like model. The evolution of the two component weight fractions apparently underpinned the connectivity, as reflected in the rheological data.

Novel In Situ Gelling Hydrogels Loaded with Recombinant Collagen Peptide Microspheres as a Slow-Release System Induce Ectopic Bone Formation

New solutions for large bone defect repair are needed. Here, in situ gelling slow release systems for bone induction are assessed. Collagen-I based Recombinant Peptide (RCP) microspheres (MSs) are produced and used as a carrier for bone morphogenetic protein 2 (BMP-2). The RCP-MSs are dispersed in three hydrogels: high mannuronate (SLM) alginate, high guluronate (SLG) alginate, and thermoresponsive hyaluronan derivative (HApN). HApN+RCP-MS forms a gel structure at 32 ºC or above, while SLM+RCP-MS and SLG+RCP-MS respond to shear stress displaying thixotropic behavior. Alginate formulations show sustained release of BMP-2, while there is minimal release from HApN. These formulations are injected subcutaneously in rats. SLM+RCP-MS and SLG+RCP-MS loaded with BMP-2 induce ectopic bone formation as revealed by X-ray tomography and histology, whereas HApN+RCP-MS do not. Vascularization occurs within all the formulations studied and is significantly higher in SLG+MS and HApN+RCP-MS than in SLM+RCP-MS. Inflammation (based on macrophage subset staining) decreases over time in both alginate groups, but increases in the HApN+RCP-MS condition. It is shown that a balance between inflammatory cell infiltration, BMP-2 release, and vascularization, achieved in the SLG+RCP-MS alginate condition, is optimal for the induction of de novo bone formation.

Insights into gelation kinetics and gel front migration in cation-induced polysaccharide hydrogels by viscoelastic and turbidity measurements: Effect of the nature of divalent cations

Polysaccharide-based hydrogels were prepared by the diffusion of various divalent cations (X²⁺) into the polygalacturonate (polyGal) solution through a dialysis membrane. The diffusion of various divalent cations (Mg²⁺, Ca²⁺, Zn²⁺ and Ba²⁺) was investigated. The polyGal gel growth was studied as a function of the initial cation concentration by both viscoelastic and turbidity measurements. We have demonstrated for the first time that the determination of the spatiotemporal variation of turbidity during the gelation process allowed to study the gel front migration. For Ca-polyGal, Zn-polyGal and Ba-polyGal, the gel front migration was characterized by the presence of a peak at the sol/gel interface. This peak was not observed in the case of Mg-polyGal where the gel was not formed. The apparent diffusion coefficient of the gel front (D_app) which was calculated from the evolution of this peak increased when the initial cation concentration was increased. Moreover, we have suggested a gelation mechanism based on the presence of a threshold molar ratio R* (=[X²⁺]/[Galacturonic unit]) in which some point-like crosslinks are precursors of the formation of dimers and multimers inducing the contraction of the gel and thus the formation of the gel front.

Evaluation of disintegrants functionality for orodispersible mini tablets

OBJECTIVE

This work evaluates the functionalities of different superdisintegrants (SD) for manufacturing orodispersible mini tablets (ODMT) by direct compression.

METHODS

Twenty-three formulations varying in SD type, concentration, and lubricant were used to manufacture ODMT. The ODMT were then characterized for the following properties: friability, porosity, tensile strength, in vivo and in vitro disintegration time (DT).

RESULTS

The results show that the presence, type, and concentration of SD did not influence friability, porosity, or tablet tensile strength. With regards to in vivo DT, only cross-linked poly (vinyl pyrrolidone) improved DT in all the tested formulations. Results also showed that when using microcrystalline cellulose (MCC) above 20% in the formulation, DT is longer. Cross-linked carboxymethyl cellulose accelerates DT when the MCC content is less than 20%. As for cross-linked carboxymethyl starch and calcium alginate showed no improvement on DT. Results for in vitro DT were all shorter than in vivo results and there was no correlation with the in vivo evaluation.

CONCLUSIONS

This study shows that there is a need to develop better in vitro testing that precisely simulates in vivo conditions and that are adapted to ODMT. This standardization of the test methods for ODMTs must be accompanied by an improvement in the comprehension of SD mechanisms.

A correlative spatiotemporal microscale study of calcium phosphate formation and transformation within an alginate hydrogel matrix

The modification of soft hydrogels with hard inorganic components is a method used to form composite materials with application in non-load-bearing bone tissue engineering. The inclusion of an inorganic component may provide mechanical enhancement, introduce osteoconductive or osteoinductive properties, or change other aspects of interactions between native or implanted cells and the material. A thorough understanding of the interactions between such components is needed to improve the rational design of such biomaterials. To achieve this goal, model systems which could allow study of the formation and transformation of mineral phases within a hydrogel network with a range of experimental methods and high spatial and time resolution are needed. Here, we report a detailed investigation of the formation and transformation process of calcium phosphate mineral within an alginate hydrogel matrix. A combination of optical microscopy, confocal Raman microspectroscopy and electron microscopy was used to investigate the spatial distribution, morphology and crystal phase of the calcium phosphate mineral, as well as to study transformation of the mineral phases during the hydrogel mineralization process and upon incubation in a simulated body fluid. It was found, that under the conditions used in this work, mineral initially formed as a metastable amorphous calcium phosphate phase (ACP). The ACP particles had a distinctive spherical morphology and transformed within minutes into brushite in the presence of brushite seed crystals or into octacalcium phosphate, when no seeds were present in the hydrogel matrix. Incubation of brushite-alginate composites in simulated body fluid resulted in formation of hydroxyapatite. The characterization strategy presented here allows for non-destructive, in situ observation of mineralization processes in optically transparent hydrogels with little to no sample preparation.

STATEMENT OF SIGNIFICANCE

The precipitation and transformations of calcium phosphates (CaP) is a complex process, where both formation kinetics and the stability of different mineral phases control the outcome. This situation is even more complex if CaP is precipitated in a hydrogel matrix, where one can expect the organic matrix to modulate crystallization by introducing supersaturation gradients or changing the nucleation and growth kinetics of crystals. In this study we apply a range of characterization techniques to study the mineral formation and transformations of CaP within an alginate matrix with spatiotemporal resolution. It demonstrates how a detailed investigation of the mineral precipitation and transformations can aid in the future rational design of hydrogel-based materials for bone tissue engineering and studies of biomineralization processes.

Versatile, cell and chip friendly method to gel alginate in microfluidic devices

Alginate is used extensively in microfluidic devices to produce discrete beads or fibres at the microscale. Such structures may be used to encapsulate sensitive cargoes such as cells and biomolecules. On chip gelation of alginate represents a significant challenge since gelling kinetics or physicochemical conditions are not biocompatible. Here we present a new method that offers a hitherto unprecedented level of control over the gelling kinetics and pH applied to the encapsulation of a variety of cells in both bead and fibre geometries. This versatile approach proved straightforward to adjust to achieve appropriate solution conditions required for implementation in microfluidic devices and resulted in highly reliable device operation and very high viability of several different encapsulated cell types for prolonged periods. We believe this method offers a paradigm shift in alginate gelling technology for application in microfluidics.