Anisotropic Structure of Calcium-Induced Alginate Gels by Optical and Small-Angle X-ray Scattering Measurements

Tunable Viscoelasticity of Alginate Hydrogels via Serial Autoclaving

Alginate hydrogels are widely used as biomaterials for cell culture and tissue engineering due to their biocompatibility and tunable mechanical properties. Reducing alginate molecular weight is an effective strategy for modulating hydrogel viscoelasticity and stress relaxation behavior, which can significantly impact cell spreading and fate. However, current methods like gamma irradiation to produce low molecular weight alginates suffer from high cost and limited accessibility. Here, a facile and cost-effective approach to reduce alginate molecular weight in a highly controlled manner using serial autoclaving is presented. Increasing the number of autoclave cycles results in proportional reductions in intrinsic viscosity, hydrodynamic radius, and molecular weight of the polymer while maintaining its chemical composition. Hydrogels fabricated from mixtures of the autoclaved alginates exhibit tunable mechanical properties, with inclusion of lower molecular weight alginate leading to softer gels with faster stress relaxation behaviors. The method is demonstrated by establishing how viscoelastic relaxation affects the spreading of encapsulated fibroblasts and glioblastoma cells. Results establish repetitive autoclaving as an easily accessible technique to generate alginates with a range of molecular weights and to control the viscoelastic properties of alginate hydrogels, and demonstrate utility across applications in mechanobiology, tissue engineering, and regenerative medicine.

Human muscle stem cell responses to mechanical stress into tunable 3D alginate matrices

Preclinical data acquired for human muscle stem (hMuStem) cells indicate their great repair capacity in the context of muscle injury. However, their clinical potential is limited by their moderate ability to survive after transplantation. To overcome these limitations, their encapsulation within protective environment would be beneficial. In this study, tunable calcium-alginate hydrogels obtained through molding method using external or internal gelation were investigated as a new strategy for hMuStem cell encapsulation. The mechanical properties of these hydrogels were characterized in their fully hydrated state by compression experiments using Atomic Force Microscopy. Measured elastic moduli strongly depended on the gelation mode and calcium/alginate concentrations. Values ranged from 1 to 12.5 kPa and 3.9 to 25 kPa were obtained for hydrogels prepared following internal and external gelation, respectively. Also, differences in mechanical properties of hydrogels resulted from their internal organization, with an isotropic structure for internal gelation, while external mode led to anisotropic one. It was further shown that viability, morphological and myogenic differentiation characteristics of hMuStem cells incorporated within alginate hydrogels were preserved after their release. These results highlight that hMuStem cells encapsulated in calcium-alginate hydrogels maintain their functionality, thus allowing to develop muscle regeneration protocols to improve their therapeutic efficacy.

Alginate-metal cation interactions: Macromolecular approach

Alginates are a broad family of linear (unbranched) polysaccharides derived from brown seaweeds and some bacteria. Despite having only two monomers, i.e. β-d-mannuronate (M) and its C5 epimer α-l-guluronate (G), their blockwise arrangement in oligomannuronate (..MMM..), oligoguluronate (..GGG..), and polyalternating (..MGMG..) blocks endows it with a rather complex interaction pattern with specific counterions and salts. Classic polyelectrolyte theories well apply to alginate as polyanion in the interaction with monovalent and non-gelling divalent cations. The use of divalent gelling ions, such as Ca2+, Ba2+ or Sr2+, provides thermostable homogeneous or heterogeneous hydrogels where the block composition affects both macroscopic and microscopic properties. The mechanism of alginate gelation is still explained in terms of the original egg-box model, although over the years some novel insights have been proposed. In this review we summarize several decades of research related to structure-functionships in alginates in the presence of non-gelling and gelling cations and present some novel applications in the field of self-assembling nanoparticles and use of radionuclides.

Self-Assembly of Structured Colloidal Gels for High-Resolution 3D Micropatterning of Proteins at Scale

Self-assembly, the spontaneous ordering of components into patterns, is widespread in nature and fundamental to generating function across length scales. Morphogen gradients in biological development are paradigmatic as both products and effectors of self-assembly and various attempts have been made to reproduce such gradients in biomaterial design. To date, approaches have typically utilized top-down fabrication techniques that, while allowing high-resolution control, are limited by scale and require chemical cross-linking steps to stabilize morphogen patterns in time. Here, a bottom-up approach to protein patterning is developed based on a novel binary reaction-diffusion process where proteins function as diffusive reactants to assemble a nanoclay-protein composite hydrogel. Using this approach, it is possible to generate scalable and highly stable 3D patterns of target proteins down to sub-cellular resolution through only physical interactions between clay nanoparticles and the proteins and ions present in blood. Patterned nanoclay gels are able to guide cell behavior to precisely template bone tissue formation in vivo. These results demonstrate the feasibility of stabilizing 3D gradients of biological signals through self-assembly processes and open up new possibilities for morphogen-based therapeutic strategies and models of biological development and repair.

Gelation and Orientation Dynamics Induced by Contact of Protein Solution with Transglutaminase Solution

Gel growth induced by contact of polymer solutions with crosslinker solutions yields an emerging class of anisotropic materials with many potential applications. Here, we report the case of a study on the dynamics in forming anisotropic gels using this approach with an enzyme as a trigger of gelation and gelatin as the polymer. Unlike the previously studied cases of gelation, the isotropic gelation was followed by gel polymer orientation after a lag time. The isotropic gelation dynamics did not depend on concentrations of the polymer turning into gel and of the enzyme inducing gelation, whereas, for the anisotropic gelation, the square of the gel thickness was a linear function of the elapsed time, and the slope increased with polymer concentration. The gelation dynamics of the present system was explained by a combination of diffusion-limited gelation followed by free-energy-limited orientation of polymer molecules.

Fabrication of bio-inspired anisotropic structures from biopolymers for biomedical applications: A review

The anisotropic features play indispensable roles in regulating various life activities in different organisms. Increasing efforts have been made to learn and mimic various tissues' intrinsic anisotropic structure or functionality for broad applications in different areas, especially in biomedicine and pharmacy. This paper discusses the strategies for fabricating biomaterials using biopolymers for biomedical applications with the case study analysis. Biopolymers, including different polysaccharides, proteins, and their derivates, that have been confirmed with sound biocompatibility for different biomedical applications are summarized, with a special focus on nanocellulose. Advanced analytical techniques for understanding and characterizing the biopolymer-based anisotropic structures for various biomedical applications are also summarized. Challenges still exist in precisely constructing biopolymers-based biomaterials with anisotropic structures from molecular to macroscopic levels and fitting the dynamic processes in native tissue. It is foreseeable that with the advancement of biopolymers' molecular functionalization, biopolymer building block orientation manipulation strategies, and structural characterization techniques, developing anisotropic biopolymer-based biomaterials for different biomedical applications would significantly contribute to a friendly disease-curing and healthcare experience.

One-Pot Hierarchical Structuring of Nanocellulose by Electrophoretic Deposition

The orientation control and the formation of hierarchical structures of nanoscale components, such as bionanofibers and nanosheets, have attracted considerable research interest with the aim of achieving sophisticated functional materials. Herein, we report a simple and flexible strategy for constructing sophisticated hierarchical structures through electrophoretic and electrochemical deposition. Cellulose nanofibers (CNFs), which are used as model materials, are deposited on an anode in an aqueous dispersion and seamlessly oriented from horizontal to vertical relatively to the electrode by adjusting the applied voltage between the electrodes. The oriented CNF hydrogels not only exhibit anisotropic mechanical properties but also form complex orientations and hierarchical structures, such as cartilage- and plant stem-like configurations in response to electrode shape and applied voltage. This simple and flexible technique is expected to be applicable to various materials and contribute to a wide range of fields that include biomimicry, functional nanomaterials, and sustainable and functional moldings.

Jerky-Inspired Fabrication of Anisotropic Hydrogels with Widely Tunable Mechanical Properties

Jerky is a type of meat product traditionally produced using a hang-drying process to achieve desirable textural properties. Inspired by the jerky processing, we present a strategy for fabricating strong alginate hydrogels with highly anisotropic structures via stretching and drying under constant stress. The tunable stretching process endowed the alginate hydrogels with adjustable mechanical properties and structural features by promoting the orientation and aggregation of the constituent polymers. At a high water content of about 80%, the tensile strength of the obtained hydrogel was increased to 20 MPa, which was 10 times higher than that of the hydrogel without the stretching process. Moreover, these hydrogels can be favorably compared with other common structural materials. This paper introduces a facile strategy to tune the structural alignment and mechanical properties of hydrogels, which will expand the applicability of the natural hydrogels formed by non-covalent interactions.

Nanopillar Templating Augments the Stiffness and Strength in Biopolymer Films

Natural load-bearing mammalian tissues, such as cartilage and ligaments, contain ∼70% water yet can be mechanically stiff and strong due to the highly templated structures within. Here, we present a bioinspired approach to significantly stiffen and strengthen biopolymer hydrogels and films through the combination of nanoscale architecture and templated microstructure. Imprinted submicrometer pillar arrays absorb energy and deflect cracks. The produced chitosan hydrogels show nanofiber chains aligned by nanopillar topography, subsequently templating the microstructure throughout the film. These templated nanopillar chitosan hydrogels mechanically outperform unstructured flat hydrogels, with increases in the moduli of ∼160%, up to ∼20 MPa, and work at break of ∼450%, up to 8.5 MJ m^-3. Furthermore, the strength at break increases by ∼350%, up to ∼37 MPa, and it is one of the strongest hydrogels yet reported. The nanopillar templating strategy is generalizable to other biopolymers capable of forming oriented domains and strong interactions. Overall, this process yields hydrogel films that demonstrate mechanical performance comparable to that of other stiff, strong hydrogels and natural tissues.

DEM-Based Approach for the Modeling of Gelation and Its Application to Alginate

The gelation of biopolymers is of great interest in the material science community and has gained increasing relevance in the past few decades, especially in the context of aerogels─lightweight open nanoporous materials. Understanding the underlying gel structure and influence of process parameters is of great importance to predict material properties such as mechanical strength. In order to improve understanding of the gelation mechanism in aqueous solution, this work presents a novel approach based on the discrete element method for the mesoscale for modeling gelation of hydrogels, similarly to an extremely coarse-grained molecular dynamics (MD) approach. For this, polymer chains are abstracted as dimer units connected by flexible bonds and interactions between units and with the environment, that is, diffusion in implicit water, are described. The model is based on Langevin dynamics and includes an implicit probabilistic ion model to capture the effects of ion availability during ion-mediated gelation. The model components are fully derived and parameterized using literature data and theoretical considerations based on a simplified representation of atomistic processes. The presented model enables investigations of the higher-scale network formation during gelation on the micrometer and millisecond scale, which are beyond classical modeling approaches such as MD. As a model system, calcium-mediated alginate gelation is investigated including the influence of ion concentration, polymer composition, polymer concentration, and molecular weight. The model is verified against numerous literature data as well as own experimental results for the corresponding Ca-alginate hydrogels using nitrogen porosimetry, NMR cryoporometry, and small-angle neutron scattering. The model reproduces both bundle size and pore size distribution in a reasonable agreement with the experiments. Overall, the modeling approach paves the way to physically motivated design of alginate gels.

Hydroxyapatite-reinforced alginate fibers with bioinspired dually aligned architectures

Biological materials have excellent mechanical properties due to their organized structures from nano- to macro-scale. Artificial manufacture of materials with anisotropic microstructures still remains challenging. We described a stress-induced method to fabricate anisotropic alginate fibers. Organic-inorganic composite fibers were obtained by incorporating aligned hydroxyapatite (HAP) nanowires into the alginate fiber. Detailed structural characterization revealed the bone-like structure of the HAP-reinforced alginate fibers. Tensile test results showed that the maximum Young's modulus and tensile strength were 4.3 GPa and 153.8 MPa, respectively. A multiscale reinforcing mechanism is proposed after the discussion of the structure-property relationship: highly ordered and compacted nanofibrils aligned along the longitudinal direction at the microscale, and two kinds of alginate gels with different mechanical behaviors at the nanoscale coexisted (acidic alginate gel and calcium-alginate gel). This work validates the effectiveness of the bioinspired fabrication strategy, which inspires further manufacturing and optimization of materials for diverse applications.

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.

Mechanism of Hydration and Hydration Induced Structural Changes of Calcium Alginate Aerogel

The most relevant properties of polysaccharide aerogels in practical applications are determined by their microstructures. Hydration has a dominant role in altering the microstructures of these hydrophilic porous materials. To understand the hydration induced structural changes of monolithic Ca-alginate aerogel, produced by drying fully cross-linked gels with supercritical CO₂, the aerogel was gradually hydrated and characterized at different states of hydration by small-angle neutron scattering (SANS), liquid-state nuclear magnetic resonance (NMR) spectroscopy, and magic angle spinning (MAS) NMR spectroscopy. First, the incorporation of structural water and the formation of an extensive hydration sphere mobilize the Ca-alginate macromolecules and induce the rearrangement of the dry-state tertiary and quaternary structures. The primary fibrils of the original aerogel backbone form hydrated fibers and fascicles, resulting in the significant increase of pore size, the smoothing of the nanostructured surface, and the increase of the fractal dimension of the matrix. Because of the formation of these new superstructures in the hydrated backbone, the stiffness and the compressive strength of the aerogel significantly increase compared to its dry-state properties. Further elevation of the water content of the aerogel results in a critical hydration state. The Ca-alginate fibers of the backbone disintegrate into well-hydrated chains, which eventually form a quasi-homogeneous hydrogel-like network. Consequently, the porous structure collapses and the well-defined solid backbone ceases to exist. Even in this hydrogel-like state, the macroscopic integrity of the Ca-alginate monolith is intact. The postulated mechanism accounts for the modification of the macroscopic properties of Ca-alginate aerogel in relation to both humid and aqueous environments.

Effect of the C-terminal amino acid of the peptide on the structure and mechanical properties of alginate-peptide hydrogels across length-scales

Alginate is a natural anionic polysaccharide that exhibits excellent biocompatibility and biodegradability. Alginate hydrogels have many different applications in the field of regenerative medicine especially when peptides are conjugated to the alginate backbone. Here, we systematically investigate the effect of six arginine-glycine-aspartic acid (RGD)-containing peptides, G6KRGDY/S, A6KRGDY/S and V6KRGDY/S, on the macroscopic and microscopic physical properties and spatial organization of alginate-peptides hydrogels. Using rheology, small angle X-ray scattering and nanoindentation measurements we show a strong correlation between the macroscopic-bulk properties and the microscopic-local properties of the alginate-peptide hydrogels. Furthermore, our results indicate that the identity of the amino acid at the C-terminal of the peptide plays a major role in determining the structure and mechanical properties of the hydrogel across length-scales, where the presence of tyrosine (Y) terminated peptides introduce more junction-zones and consequently larger stiffness than those terminated with serine (S).

Controlled Loading and Release of Beta-Lactoglobulin in Calcium-Polygalacturonate Hydrogels

We show here how the structure of polygalacturonate (polyGalA) hydrogels cross-linked by Ca²⁺ cations via external gelation controls the loading and release rate of beta-lactoglobulin (BLG), a globular protein. Hydrogels prepared from a polyGalA/BLG solution are found to be similar to those obtained from a polyGalA solution in our previous study (Maire du Poset et al. Biomacromolecules 2019, 20 (7), 2864-2872): they exhibit similar transparencies and gradients of mechanical properties and polyGalA concentrations. The nominal BLG/polyGalA ratio of the mixtures is almost recovered within the whole mixed hydrogel despite such strong concentration gradients, except in the part of the hydrogels with the largest mesh size, where more BLG proteins are present. This gradient enables one to tune the amount of protein loaded within the hydrogel. At a local scale, the proteins are distributed evenly within the hydrogel network, as shown by small-angle neutron scattering (SANS). The release of proteins from hydrogels is driven by Fickian diffusion, and the release rate increases with the mesh size of the network, with a characteristic time of a few hours. The specific structure of these polysaccharide-based hydrogels allows for control of both the dosage and the release rate of the loaded protein and makes them good candidates for use as oral controlled-delivery systems.

Tuning the Structure of Galacturonate Hydrogels: External Gelation by Ca, Zn, or Fe Cationic Cross-Linkers

We show here how the nature of various divalent cations M²⁺ (Ca²⁺, Zn²⁺, or Fe²⁺) influences the structure and mechanical properties of ionotropic polygalacturonate (polyGal) hydrogels designed by the diffusion of cations along one direction (external gelation). All hydrogels exhibit strong gradients of polyGal and cation concentrations, which are similar for all studied cations with a constant ratio R = [M²⁺]/[Gal] equal to 0.25, showing that every M²⁺ cation interacts with four galacturonate (Gal) units all along the gels. The regions of the hydrogels formed in the early stages of the gelation process are also similar for all cations and are homogeneous, with the same characteristic mesh size (75 ± 5 Å, as measured by small angle neutron scattering (SANS)) and the same storage modulus G' (∼5 × 10⁴ Pa). Conversely, in the regions of the gels formed in later stages of the process there exist differences in mechanical properties, turbidity, and local structure from one cation to another. Zn(II)-polyGal and Fe(II)-polyGal hydrogels display mesoscopic heterogeneities, more marked in case of Fe than for Zn, that are not present in Ca(II)-polyGal hydrogels. This comes from the mode and the strength of association between the cation and the Gal unit (bidentate for Ca²⁺ and monodentate "egg-box" for Zn²⁺ and Fe²⁺). Cross-links formed by Zn²⁺ and Fe²⁺ have a higher stability (lower ability to untie and reform) that induces the formation of local heterogeneities in the early stages of the gelation process whose size progressively increases during the gel growth, a mechanism that does not occur for cross-links made by Ca²⁺ that are less stable and enable possible reorganizations between polyGal chains.

Strong Deformation of Ferrofluid-Filled Elastic Alginate Capsules in Inhomogenous Magnetic Fields

We present a new system based on alginate gels for the encapsulation of a ferrofluid drop, which allows us to create millimeter-sized elastic capsules that are highly deformable by inhomogeneous magnetic fields. We use a combination of experimental and theoretical work in order to characterize and quantify the deformation behavior of these ferrofluid-filled capsules. We introduce a novel method for the direct encapsulation of unpolar liquids by sodium alginate. By adding 1-hexanol to the unpolar liquid, we can dissolve sufficient amounts of CaCl₂ in the resulting mixture for ionotropic gelation of sodium alginate. The addition of polar alcohol molecules allows us to encapsulate a ferrofluid as a single phase rather than an emulsion without impairing ferrofluid stability. This encapsulation method increases the amount of encapsulated magnetic nanoparticles resulting in high deformations of approximately 30% (in height-to-width ratio) in inhomogeneous magnetic field with magnetic field variations of 50 mT over the size of the capsule. This offers possible applications of capsules as actuators, switches, or valves in confined spaces like microfluidic devices. We determine both elastic moduli of the capsule shell, Young's modulus and Poisson's ratio, by employing two independent mechanical methods, spinning capsule measurements and capsule compression between parallel plates. We then show that the observed magnetic deformation can be fully understood from magnetic forces exerted by the ferrofluid on the capsule shell if the magnetic field distribution and magnetization properties of the ferrofluid are known. We perform a detailed analysis of the magnetic deformation by employing a theoretical model based on nonlinear elasticity theory. Using an iterative solution scheme that couples a finite element/boundary element method for the magnetic field calculation to the solution of the elastic shape equations, we achieve quantitative agreement between theory and experiment for deformed capsule shapes using the Young modulus from mechanical characterization and the surface Poisson ratio as a fit parameter. This detailed analysis confirms the results from mechanical characterization that the surface Poisson ratio of the alginate shell is close to unity, that is, deformations of the alginate shell are almost area conserving.

Construction of ordered structure in polysaccharide hydrogel: A review

Hydrogels are three-dimensional, hydrophilic, polymeric networks, held together by a variety of physical or chemical crosslinks. Among the numerous polymers that can be employed to fabricate hydrogel, polysaccharides have attracted enormous attention due to their peculiar properties that make them suitable for various applications. Compared with homogeneous hydrogels, hydrogels with ordered structures on various length scales are endowed with excellent properties and promising applications in materials science. In the present review, a wide range of techniques were introduced and discussed, which had been utilized to construct ordered hierarchical structures in polysaccharide hydrogels. These techniques focused on the construction of multi-layered and orientated structure, which are two typical and very important forms of hierarchical structure.

Anisotropic Multi-channel Collagen Gel (MCCG) Guides the Growth Direction of the Neurite-like Processes of PC12 Cells

Hydrogels made of various materials using a variety of methods have been extensively studied for use in tissue engineering, and collagen is one of the most common material used for its biocompatibility due to it being a major component of the extracellular matrix (ECM). Furthermore, the alignment of collagen fibres has been shown to direct the growth of neurites, an important criterion for engineering nervous tissues. The Multi-channel Collagen Gel (MCCG) has collagen fibres aligned circumferentially around the channel structures of the gel, and we predicted that the MCCG could guide the growth direction of neurites. In this study, we showed that the growth pathway of the neurite-like processes of PC12 cells were guided in MCCG but not in normal collagen gel (COL). The gelation of collagen gels are known to be affected by ionic concentrations, and hence we also investigated the effects of different concentrations of NaCl on the properties of MCCG. We found that, despite differences in channel density, spacing between channels, and degree of collagen fibre alignment, all MCCGs had similar guiding properties on the growth of neurites. Therefore, we believe that anisotropic MCCG could be a useful biomaterial for neural tissue engineering in the future.

Analysis of Heterogeneous Gelation Dynamics and Their Application to Blood Coagulation

We present a scaling model based on a moving boundary picture to describe heterogeneous gelation dynamics. The dynamics of gelation induced by different gelation mechanisms is expressed by the scaled equation for the time taken for development of the gel layer with a few kinetic coefficients characterizing the system. The physical meaning obtained by the analysis for a simple boundary condition from the standpoint of the phase transition shows that the time development of the gelation layer depends on whether the dynamics of the order parameter expressing the gelation of the polymer solution is fast or slow compared with the diffusion of the gelators in the heterogeneous gelation. The analytical method is used to understand the coagulation of blood from various animals. An experiment using systems with plasma coagulation occurring at interfaces with calcium chloride solution and with packed erythrocytes is performed to provide the data for model fitting and it is clarified that a few key kinetic coefficients in plasma coagulation can be estimated from the analysis of gelation dynamics.

Tuning the mechanical properties of alginate-peptide hydrogels

Alginate, a polysaccharide that gels in the presence of divalent ions, has been used in the field of regenerative medicine to facilitate cell growth in impaired tissues by providing an artificial bio-surrounding similar to the natural extra cellular matrix (ECM). Here, we present a systematic investigation of the effect of three arginine-glycine-aspartic acid (RGD)-containing peptides, G6KRGDY, A6KRGDY and V6KRGDY, on the physical properties of alginate-peptide hydrogels. Rheology measurements showed that the storage modulus of the alginate-A6KRGDY and alginate-V6KRGDY gels is an order of magnitude higher than that of the alginate-G6KRGDY gel. Small angle X-ray scattering (SAXS) measurements suggest that the difference in the mechanical properties of the gels is due to the formation of larger peptide junction zones in addition to the ones formed by calcium ions. These findings indicate that the peptides' ability to self-assemble in aqueous solution is a significant factor in tuning the stiffness of the alginate/peptide hybrid hydrogels.

On the structural stability of guanosine-based supramolecular hydrogels

Supramolecular hydrogels formed from the self-assembly of low molecular weight derivatives are very attractive systems, because of their potential applications in nano- and bio-technology. In this paper, the stable and transparent hydrogels observed in binary mixtures of guanosine derivatives (G), namely guanosine 5'-monophosphate (GMP) and guanosine (Gua), dissolved in water (at volume fractions larger than 0.95), were investigated by microscopy techniques and Small Angle X-ray Scattering (SAXS). The results confirm the presence of G-quadruplexes, chiral cylindrical rods obtained by the regular stacking of self-assembled planar cyclic guanosine quartets. However, the addition of Gua determines the formation of very stable hydrogels able to trap large amounts of water (up to a volume fraction of 0.99) and characterised by an unusual anisotropic order. A modified lateral helix-to-helix interaction pattern, tuned by Gua, is suggested to be responsible for the supramolecular gelation and the stability of the hydrogels during swelling.

Bioinspired microstructures of chitosan hydrogel provide enhanced wear protection

We describe the fabrication of physical chitosan hydrogels exhibiting a layered structure. This bilayered structure, as shown by SEM and confocal microscopy, is composed of a thin dense superficial zone (SZ), covering a deeper zone (DZ) containing microchannels orientated perpendicularly to the SZ. We show that such structure favors diffusion of macromolecules within the hydrogel matrix up to a critical pressure, σ_c, above which channels were constricted. Moreover, we found that the SZ provided a higher wear resistance than the DZ which was severely damaged at a pressure equal to the elastic modulus of the gel. The coefficient of friction (CoF) of the SZ remained independent of the applied load with μ_SZ = 0.38 ± 0.02, while CoF measured at DZ exhibited two regimes: an initial CoF close to the value found on the SZ, and a CoF that decreased to μ_DZ = 0.18 ± 0.01 at pressures higher than the critical pressure σ_c. Overall, our results show that internal structuring is a promising avenue in controlling and improving the wear resistance of soft materials such as hydrogels.

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.

Swim bladder collagen forms hydrogel with macroscopic superstructure by diffusion induced fast gelation

Marine collagen has been attracting attention as a medical material in recent times due to the low risk of pathogen infection compared to animal collagen. Type I collagen extracted from the swim bladder of Bester sturgeon fish has excellent characteristics such as high denaturation temperature, high solubility, low viscosity and an extremely fast rate to form large bundle of fibers under certain conditions. These specific characteristics of swim bladder collagen (SBC) permit us to create stable, disk shaped hydrogels with concentric orientation of collagen fibers by the controlled diffusion of neutral buffer through collagen solution at room temperature. However, traditionally used animal collagens, e.g. calf skin collagen (CSC) and porcine skin collagen (PSC), could not form any stable and oriented structure by this method. The mechanism of the superstructure formation of SBC by a diffusion induced gelation process has been explored. The fast fibrillogenesis rate of SBC causes a quick squeezing out of the solvent from the gel phase to the sol phase during gelation, which builds an internal stress at the gel-sol interface. The tensile stress induces the collagen molecules of the gel phase to align along the gel-sol interface direction to give this concentric ring-shaped orientation pattern. On the other hand, the slow fibrillogenesis rate of animal collagens due to the high viscosity of the solution does not favor the ordered structure formation. The denaturation temperature of SBC increases significantly from 31 °C to 43 °C after gelation, whereas that of CSC and PSC were found to increase a little. Rheology experiment shows that the SBC gel has storage modulus larger than 15 kPa. The SBC hydrogels with thermal and mechanical stability have potential as bio-materials for tissue engineering applications.

Microbial trench-based optofluidic system for reagentless determination of phenolic compounds

Phenolic compounds are one of the main contaminants of soil and water due to their toxicity and persistence in the natural environment. Their presence is commonly determined with bulky and expensive instrumentation (e.g. chromatography systems), requiring sample collection and transport to the laboratory. Sample transport delays data acquisition, postponing potential actions to prevent environmental catastrophes. This article presents a portable, miniaturized, robust and low-cost microbial trench-based optofluidic system for reagentless determination of phenols in water. The optofluidic system is composed of a poly(methyl methacrylate) structure, incorporating polymeric optical elements and miniaturized discrete auxiliary components for optical transduction. An electronic circuit, adapted from a lock-in amplifier, is used for system control and interfering ambient light subtraction. In the trench, genetically modified bacteria are stably entrapped in an alginate hydrogel for quantitative determination of model phenol catechol. Alginate is also acting as a diffusion barrier for compounds present in the sample. Additionally, the superior refractive index of the gel (compared to water) confines the light in the lower level of the chip. Hence, the optical readout of the device is only altered by changes in the trench. Catechol molecules (colorless) in the sample diffuse through the alginate matrix and reach bacteria, which degrade them to a colored compound. The absorbance increase at 450 nm reports the presence of catechol simply, quickly (~10 min) and quantitatively without addition of chemical reagents. This miniaturized, portable and robust optofluidic system opens the possibility for quick and reliable determination of environmental contamination in situ, thus mitigating the effects of accidental spills.

Anisotropic swelling in hydrogels formed by cooperatively aligned megamolecules

A cyanobacterial polysaccharide, sacran, which has a high molecular length over 30 μm, forms in-plane oriented film by casting. The film creates uniaxially-swelling hydrogels with a micrometer thickness.

π-Conjugated polymer anisotropic organogel nanofibrous assemblies for thermoresponsive photonic switches

The present work demonstrates one of the first examples of π-conjugated photonic switches (or photonic wave plates) based on the tailor-made π-conjugated polymer anisotropic organogel. New semicrystalline segmented π-conjugated polymers are designed with rigid aromatic oligophenylenevinylene π-core and flexible alkyl chain along the polymer backbone. These polymers are found to be self-assembled as semicrystalline or amorphous with respect to the number of carbon atoms in the alkyl units. These semicrystalline polymers produce organogels having nanofibrous morphology of 20 nm thickness with length up to 5 μm. The polymer organogel is aligned in a narrow glass capillary, and this anisotropic gel device is further demonstrated as photonic switches. The glass capillary device behaves as typical λ/4 photonic wave plates upon the illumination of the plane polarized light. The λ/4 photonic switching ability is found to be maximum at θ = 45° angle under the cross polarizers. The orthogonal arrangements of the gel capillaries produce dark and bright spots as on-and-off optical switches. Thermoreversibility of the polymer organogel (also its xerogel) was exploited to construct thermoresponsive photonic switches for the temperature window starting from 25 to 160 °C. The organic photonic switch concept can be adapted to large number of other π-conjugated materials for optical communication and storage.

Structural regime identification in ionotropic alginate gels: influence of the cation nature and alginate structure

The morphologies of several ionotropic alginate hydrogels and aerogels were investigated by SAXS according to the nature of the divalent metal cation (Mn(2+), Co(2+), Zn(2+), Cu(2+)) and the guluronic fraction of the alginate. All alginate hydrogel and aerogel samples show isotropic small-angle X-ray scattering. Gelation results from cooperative associations of cations and chain segments and yields different nanostructures, that is, nanofibrillar morphology or multiple junction morphology, according to cation type and eventually mannuronic/guluronic ratio. Therefore, Mn and Cu gels present the same morphology whatever the guluronic ratio, whereas Co and Zn gels yield different nanostructures. In the size range investigated by SAXS (~10-200 Å), the structure of aerogels obtained by CO(2) supercritical drying is found to be inherited from the morphology of the parent hydrogel whatever the initial structural regime.