Interpenetrating network formation in gellan--agarose gel composites

Hydrogels: An overview of its classifications, properties, and applications

The review paper starts with the introduction to hydrogels along with broad literature survey covering different modes of synthesis including high energy radiation methods. After that, paper covered broad classification of the hydrogels depending upon the basis of their source of origin, method of synthesis, type of cross-linking present and ionic charges on bound groups. Another advanced category response triggered hydrogels, which includes pH, temperature, electro, and light and substrate responsive hydrogels was also studied. Presented paper summarises chemical structure, properties, and synthesis of different kinds of hydrogels. Main focus was given to the preparation super absorbents such as: Semi-interpenetrating networks (semi-IPNs), Interpenetrating networks (IPNs) and cross-linked binary graft copolymers (BGCPs). The weak mechanical properties and easy degradation limit the uses of bio-based -hydrogels in biomedical field. Their properties can be improved through different chemical and physical methods. These methods were also discussed in the current research paper. Also, it includes development of hydrogels as controlled drug delivery devices, as implants and biomaterials to replace malfunctioned body parts along with their use in several other applications listed in the literature. Literature survey on the application of hydrogels in different fields like biomedical, nano-biotechnology, tissue engineering, drug delivery and agriculture was also carried out.

New insights into food hydrogels with reinforced mechanical properties: A review on innovative strategies

Enhancement on the mechanical properties of hydrogels leads to a wider range of their applications in various fields. Therefore, there has been a great interest recently for developing new strategies to reinforce hydrogels. Moreover, food gels must be edible in terms of both raw materials and production. This paper reviews innovative techniques such as particle/fiber-reinforced hydrogel, double network, dual crosslinking, freeze-thaw cycles, physical conditioning and soaking methods to improve the mechanical properties of hydrogels. Additionally, their fundamental mechanisms, advantages and disadvantages have been discussed. Important biopolymers that have been employed for these strategies and also their potentials in food applications have been summarized. The general mechanism of these strategies is based on increasing the degree of crosslinking between interacting polymers in hydrogels. These links can be formed by adding fillers (oil droplets or fibers in filled gels) or cross-linkers (regarding double network and soaking method) and also by condensation or alignment of the biopolymers (freeze-thaw cycle and physical conditioning) in the gel network. The properties of particle/fiber-reinforced hydrogels extremely depend on the filler, gel matrix and the interaction between them. In freeze-thaw cycles and physical conditioning methods, it is possible to form new links in the gel network without adding any cross-linkers or fillers. It is expected that the utilization of gels will get broader and more varied in food industries by using these strategies.

Interaction of Mannitol and Sucrose with Gellan Gum in Freeze-Dried Gel Systems

The effect of sucrose and mannitol addition to low-acyl (LA) gellan gum gels at both the molecular and macroscopic levels prior to, and after freeze-drying has been investigated. It has been shown that the gel network order as well as the mechanical properties are changed with the solute content, especially in the case of sucrose. The freeze-dried gel structure, containing either mannitol or sucrose, was studied, reporting for the first time the interaction of mannitol with the gellan gum gel. The generated freeze-dried gel network was evaluated in terms of porosity, pore size and wall thickness distributions. The solute physical state was correlated the water activity trend as a function of the solute content. Since mannitol is crystalline, the water activity decreases, in contrast with the amorphous sucrose. The rehydration mechanism was investigated and associated with the solute release from the structure. Specifically, the material properties (surface and bulk) as well as the role of the dissolution medium over time were assessed. It was found that the rehydration for both the gellan/sucrose and gellan/mannitol systems was highly influenced by the additive content, as an increase in water uptake was measured up to 10 wt%. A further increase in solute led to a considerable drop in the rehydration rate and extent due to the change in the freeze-dried structure, with smaller pores and with higher wall thickness values.

Polyacrylamide Ferrogels with Magnetite or Strontium Hexaferrite: Next Step in the Development of Soft Biomimetic Matter for Biosensor Applications

Magnetic biosensors are an important part of biomedical applications of magnetic materials. As the living tissue is basically a "soft matter." this study addresses the development of ferrogels (FG) with micron sized magnetic particles of magnetite and strontium hexaferrite mimicking the living tissue. The basic composition of the FG comprised the polymeric network of polyacrylamide, synthesized by free radical polymerization of monomeric acrylamide (AAm) in water solution at three levels of concentration (1.1 M, 0.85 M and 0.58 M) to provide the FG with varying elasticity. To improve FG biocompatibility and to prevent the precipitation of the particles, polysaccharide thickeners-guar gum or xanthan gum were used. The content of magnetic particles in FG varied up to 5.2 wt % depending on the FG composition. The mechanical properties of FG and their deformation in a uniform magnetic field were comparatively analyzed. FG filled with strontium hexaferrite particles have larger Young's modulus value than FG filled with magnetite particles, most likely due to the specific features of the adhesion of the network's polymeric subchains on the surface of the particles. FG networks with xanthan are stronger and have higher modulus than the FG with guar. FG based on magnetite, contract in a magnetic field 0.42 T, whereas some FG based on strontium hexaferrite swell. Weak FG with the lowest concentration of AAm shows a much stronger response to a field, as the concentration of AAm governs the Young's modulus of ferrogel. A small magnetic field magnetoimpedance sensor prototype with Co_68.6Fe_3.9Mo_3.0Si_12.0B_12.5 rapidly quenched amorphous ribbon based element was designed aiming to develop a sensor working with a disposable stripe sensitive element. The proposed protocol allowed measurements of the concentration dependence of magnetic particles in gels using magnetoimpedance responses in the presence of magnetite and strontium hexaferrite ferrogels with xanthan. We have discussed the importance of magnetic history for the detection process and demonstrated the importance of remnant magnetization in the case of the gels with large magnetic particles.

Oxidized dextrins as alternative crosslinking agents for polysaccharides: application to hydrogels of agarose-chitosan

Hydrogel networks that combine suitable physical and biomechanical characteristics for tissue engineering scaffolds are in demand. The aim of this work was the development of hydrogel networks based on agarose and chitosan using oxidized dextrins as low cytotoxicity crosslinking agents, paying special attention to the study of the influence of the polysaccharide composition and oxidation degree of the dextrins in the final characteristics of the network. The results show that the formation of an interpenetrating or a semi-interpenetrating polymer network was mainly dependent on a minimum agarose content and degree of oxidation of dextrin. Spectroscopic, thermal and swelling analysis revealed good compatibility with an absence of phase separation of polysaccharides at agarose:chitosan proportions of 50:50 and 25:75. The analysis of atomic force microscopy images showed the formation of a fibrillar microstructure whose distribution within the crosslinked chitosan depended mainly on the crosslinker. All materials exhibited the viscoelastic behaviour typical of gels, with a constant storage modulus independent of frequency for all compositions. The stiffness was strongly influenced by the degree of oxidation of the crosslinker. Cellular response to the hydrogels was studied with cells of different strains, and cell adhesion and proliferation was correlated with the homogeneity of the samples and their elastic properties. Some hydrogel formulations seemed to be candidates for tissue engineering applications such as wound healing or soft tissue regeneration.

Redox- and pH-responsive polymer gels with reversible sol–gel transitions and self-healing properties

Redox- and pH-responsive polymer gel with self-healing property was prepared by crosslinking of benzhydrazide-containing polytriazole with a disulfide-containing dialdehyde.

Tissue-engineered articular cartilage exhibits tension-compression nonlinearity reminiscent of the native cartilage

The tensile modulus of articular cartilage is much larger than its compressive modulus. This tension-compression nonlinearity enhances interstitial fluid pressurization and decreases the frictional coefficient. The current set of studies examines the tensile and compressive properties of cylindrical chondrocyte-seeded agarose constructs over different developmental stages through a novel method that combines osmotic loading, video microscopy, and uniaxial unconfined compression testing. This method was previously used to examine tension-compression nonlinearity in native cartilage. Engineered cartilage, cultured under free-swelling (FS) or dynamically loaded (DL) conditions, was tested in unconfined compression in hypertonic and hypotonic salt solutions. The apparent equilibrium modulus decreased with increasing salt concentration, indicating that increasing the bath solution osmolarity shielded the fixed charges within the tissue, shifting the measured moduli along the tension-compression curve and revealing the intrinsic properties of the tissue. With this method, we were able to measure the tensile (401±83kPa for FS and 678±473kPa for DL) and compressive (161±33kPa for FS and 348±203kPa for DL) moduli of the same engineered cartilage specimens. These moduli are comparable to values obtained from traditional methods, validating this technique for measuring the tensile and compressive properties of hydrogel-based constructs. This study shows that engineered cartilage exhibits tension-compression nonlinearity reminiscent of the native tissue, and that dynamic deformational loading can yield significantly higher tensile properties.

Fluctuation-induced forces in critical crosslinked polymer blends

In this paper, we report on the computation of the induced forces in crosslinked polymer blends, with immersed small colloidal particles (nanoparticles) or confined to two parallel plates (film). We assume that the particles or the walls prefer to be attracted by one polymer, close to the spinodal temperature where a microphase separation takes place. This is the so-called “critical adsorption”. As an assumption, the particle diameter or the film thickness is considered to be small enough in comparison with the size of microdomains (“mesh size”). The critical fluctuations of the crosslinked mixture induce a pair potential between particles located in the non-preferred phase or between the confining walls. The purpose is to recall how this Casimir pair potential can be determined, as a function of the interparticle distance or the walls separation. To achieve calculations, use is made of an extended de Gennes model that takes into account the colloid-polymer and polymer-wall interactions. Finally, the obtained results are compared to those relatively to uncrosslinked polymer blends in the same geometries, and the main conclusion is that the induced force is reduced by the presence of permanent crosslinks.

Critical microphase properties of crosslinked polymer blends with quenched random impurities

We extend published works dealing with microphase separation in crosslinked polymer blends to the case where these are surrounded by random impurities. To study their influence on critical microphase properties, from a static and kinetics point of view, we first assume that the (real) disorder caused by impurities is quenched. Second, the replica theory is used to study such critical properties, upon the impurities concentration and their interaction strength. More precisely, we compute the spinodal temperature and structure factor. We find that the spinodal temperature is shifted towards its lower and higher values, for attractive and repulsive impurities, respectively. The obtained expression for the static structure factor suggests that, contrarily to repulsive impurities, the crosslinked mixture scatters better in the presence of attractive ones. Thereafter, the study is extended to kinetics of microphase separation, when the mixture is impregnated by small random impurities. Kinetics is investigated through the growth rate, and in particular, we demonstrate that the latter is increased by the presence of repulsive impurities. This is natural, since these play a stabilizer role. Finally, the discussion is extended to crosslinked polymer blends immersed in a good solvent, which induces drastic changes of the critical microphase properties.

Mixed biopolymer aqueous solutions--phase behaviour and rheology

Mixed biopolymer solutions are found in many food systems and household products, and are also employed in industrial processes such as bio-separation and purification. They display a rich phase behaviour, ranging from association and precipitation to the more common segregative phase separation into two liquid phases. Understanding the underlying physics of their phase behaviour and of the rheology-morphology relationships of the resulting phases is a topic of interest and importance in terms of being able to reliably design and produce products containing mixed biopolymer solutions and predicting their behaviour. The science of mixed biopolymer solutions is complicated by the fact that they are ternary systems, typically comprising mostly water, and that the biopolymers themselves are liable to structural transitions such as gelation. Both of these factors can play an important role in the phase behaviour of the mixtures, and the morphology of the resulting phases. In the following, an introduction is given to the physics of mixed biopolymer solutions and the behaviour of their phases, with a view to highlighting the unique aspects of such materials in comparison to other liquid-liquid mixtures, such as emulsions and polymer blends, and also the more interesting topics for future research in these fascinating materials.

Theory of microphase separation in crosslinked polymer blends immersed in a θ-solvent

The aim of this work is a theoretical study of the effects of the solvent quality on the microphase separation in crosslinked polymer blends, from a static and kinetics point of view. More precisely, we assume that the crosslinked mixture is trapped in a θ-solvent. The static microphase properties are studied through the static structure factor. The latter is computed using an extended blob model, where the crosslinked unlike chains can be viewed as sequences of blobs. We demonstrate that the presence of the θ-solvent simply leads to a multiplicative renormalization of these properties, and the renormalization factors are powers of the overall monomer volume fraction. Second, we investigate the early kinetics of the microphase separation, via the relaxation rate, τ(q), which is a function of the wave number q (at fixed temperature and monomer volume fraction). We first show that the kinetics is entirely controlled by local motions of Rouse type, since the slow motions are frozen out by the presence of crosslinks. Using the blob model, we find an explicit form for the growth rate Ω(q) = τ(q)⁻¹, which depends, in addition to the wave number q , on the overall monomer volume fraction, Φ. Also, we discuss the effect of initial entanglements that are trapped when the system is crosslinked. In fact, these play the role of true reticulation points, and then, they quantitatively contribute to the microseparation phenomenon. Finally, the results are compared to their homologous relatively to the molten state and to the good solvent case. The main conclusion is that the quality of the solvent induces drastic changes of the microphase properties.

Release of theophylline from polymer blend hydrogels

Polymer blending is a simple yet attractive method to obtain combined physical and mechanical properties of polymers. In this paper, three types of blend hydrogels were prepared, each by physically blending two different natural polymers, and a model drug, theophylline (TPH), was immobilized into these hydrogels for the studies of drug release. The release profiles of TPH from various types of hydrogels were determined by UV-vis absorption measurement at 272 nm. The experimental results show that the releases of TPH from these hydrogels are dependent upon the composition of the hydrogel, the type of component, the possible interactions between two component polymers, as well as external temperature. All the release profiles clearly demonstrate a temperature effect. Among the three blend hydrogels, the slowest release was observed from the blend hydrogel of gelatin and agar with a weight ratio of 1:1. The drug release patterns and release mechanisms have been discussed by considering the possible molecular interactions and gel network structures.

Interpenetrating network formation in agarose--kappa-carrageenan gel composites

Thermal, mechanical, turbidity, and microscope evidence is provided which strongly suggests molecular interpenetrating network (IPN) formation by mixtures of the seaweed polysaccharides agarose and kappa-carrageenan. Over a range of ionic strength, and potassium content, there is no evidence for synergistic coupling of the networks, and simple phase separation (demixing) can definitely be ruled out. At low ionic strength, where the agarose gels first, differential scanning calorimetry evidence shows some influence of the carrageenan on the agarose ordering enthalpy, particularly at higher polymer concentrations. As the potassium level is increased, however, and the order of gelling is reversed, this effect disappears. Cure behavior for the systems at high ionic strength can be described as a simple summation of the pure component contributions. At low ionic strength, on the other hand, the modulus behavior is more complex, suggesting either a modification, in the mixture, of the kappa-carrageenan gelling parameters or a more complex modulus additivity rule.