Method for preparation of planar alginate hydrogels by external gelling using an aerosol of gelling solution

Experimental Advances in the Real-Time Recording of Cross-Linking Alginate In Situ Gelation: A Review

Alginate-based hydrogels are promising smart materials widely employed in the food, bioengineering, and energy sectors. The development and optimization of their production require a thorough knowledge of gelation. In recent years, advanced experimental procedures have been developed for real-time cross-linking alginate reaction monitoring. Novel methods, such as customized rheometric setups, enable the recording of mechanical properties and morphological changes during hydrogel formation. These innovative techniques provide important insights into the gelation stages, the reaction rate, the diffusion of cross-linker to polymer chains, and the homogeneity of the gelling structures. Based on real-time experimental data, kinetic models are developed to enhance comprehension of the reaction mechanism and, eventually, to predict the gelation progress. The aim is to enable better control of the characterization of both the complex gelation and the propagated structures. This review aspires to present a comprehensive overview and evaluation of the breakthrough innovations of the real-time in situ recording of cross-linking alginate hydrogels and bead formation. A detailed analysis of the pioneering experimental developments provides a deep comprehension of the alginate gelation, including the parameters controlling the reaction.

Pulsatile Drug Delivery System Triggered by Acoustic Radiation Force

Since biological systems exhibit a circadian rhythm (24-hour cycle), they are susceptible to the timing of drug administration. Indeed, several disorders require a therapy that synchronizes with the onset of symptoms. A targeted therapy with spatially and temporally precise controlled drug release can guarantee a considerable gain in terms of efficacy and safety of the treatment compared to traditional pharmacological methods, especially for chronotherapeutic disorders. This paper presents a proof of concept of an innovative pulsatile drug delivery system remotely triggered by the acoustic radiation force of ultrasound. The device consists of a case, in which a drug-loaded gel can be embedded, and a sliding top that can be moved on demand by the application of an acoustic stimulus, thus enabling drug release. Results demonstrate for the first time that ultrasound acoustic radiation force (up to 0.1 N) can be used for an efficient pulsatile drug delivery (up to 20 μg of drug released for each shot).

Identification of Molecular Fluorophore as a Component of Carbon Dots able to Induce Gelation in a Fluorescent Multivalent-Metal-Ion-Free Alginate Hydrogel

We introduce a simple approach to fabricate fluorescent multivalent metal ion-free alginate hydrogels, which can be produced using carbon dots accessible from natural sources (citric acid and L-cysteine). Molecular fluorophore 5-oxo-2,3-dihydro-5H-[1,3]-thiazolo[3,2-a] pyridine-3,7-dicarboxylic acid (TPDCA), which is formed during the synthesis of carbon dots, is identified as a key segment for the crosslinking of hydrogels. The crosslinking happens through dynamic complexation of carboxylic acid groups of TPDCA and alginate cages along with sodium ions. The TPDCA derived hydrogels are investigated regarding to their thermal, rheological and optical properties, and found to exhibit characteristic fluorescence of this aggregated molecular fluorophore. Moreover, gradient hydrogels with tunable mechanical and optical properties and controlled release are obtained upon immersion of the hydrogel reactors in solutions of divalent metal ions (Ca2+, Cu2+, and Ni2+) with a higher affinity to alginate.

Development of a microchannel emulsification process for pancreatic beta cell encapsulation

In this study, we developed a high‐throughput microchannel emulsification process to encapsulate pancreatic beta cells in monodisperse alginate beads. The process builds on a stirred emulsification and internal gelation method previously adapted to pancreatic cell encapsulation. Alginate bead production was achieved by flowing a 0.5–2.5% alginate solution with cells and CaCO₃ across a 1‐mm thick polytetrafluoroethylene plate with 700 × 200 μm rectangular straight‐through channels. Alginate beads ranging from 1.5–3 mm in diameter were obtained at production rates exceeding 140 mL/hr per microchannel. Compared to the stirred emulsification process, the microchannel emulsification beads had a narrower size distribution and demonstrated enhanced compressive burst strength. Both microchannel and stirred emulsification beads exhibited homogeneous profiles of 0.7% alginate concentration using an initial alginate solution concentration of 1.5%. Encapsulated beta cell viability of 89 ± 2% based on live/dead staining was achieved by minimizing the bead residence time in the acidified organic phase fluid. Microchannel emulsification is a promising method for clinical‐scale pancreatic beta cell encapsulation as well as other applications in the pharmaceutical, food, and cosmetic industries.

Textural, mechanical, and microstructural properties of restructured pimiento alginate-guar gels

Textural, mechanical, microstructural, and thermal properties of reconstituted pimiento alginate-guar gels subjected to thermal and mechanical stresses during pasteurization process were investigated. Alginate-guar gelling system at ratio 2:1 at different calcium chloride concentrations (2-8%) and varying acid conditions including citric and lactic acid 1% were evaluated. Textural profile analysis parameters viz. hardness, springiness, gumminess, cohesiveness, adhesiveness as well as mechanical properties, structural, thermal, and morphological characteristics of pimiento strips were examined. Gel strength and elasticity of pimiento strips were increased at higher calcium levels. Fracturability was decreased clearly revealed the gel system, regain its strength in spite of high pulp content (25%) and can maintain its own structure. Increase in hardness and reduction in springiness showed loss of elasticity, which may be attributed to the gel shrinkage during thermal processing, making it more compact and dense. Therefore, the restructured pimiento strips were completely thermo stable at pasteurization temperature. By increasing alginate and calcium level in the pimiento strip, glass transition temperature was reduced from 112 to 98°C. Fourier transform infrared spectroscopy studies confirmed that the gel network structure at acidic conditions was stable and acid type did not has significant effect on the chemical interactions. The microstructural results showed ellipsoidal and compact structure in which can be an evidence of high elasticity and rigidity of pimiento gels. These results completely approved the high elasticity and rigidity of the pimiento strips and their ability to sustain successfully thermal and mechanical stresses with appreciable textural integrity during processing such as pasteurization. PRACTICAL APPLICATIONS: Restructured pimiento strip for stuffing into cocktail olives is a challenging problem in binary composite hydrogels of alginate and guar gum. The strips should be designed to be strong enough to handle mechanically and cut easily without tearing. Furthermore, it had to withstand to pasteurization process and be flexible to bend like a hairpin into the pit hole of olive. In addition, it also had low syneresis and shrinkage that the strips not fall out of the olive hole during storage. In the view point of marketing and nutritional aspects, it is more advantageous to develop a restructured fruit gel systems several times higher than the limiting 10% pulp because of its weak structure. Therefore, alginate-gel systems containing 25% pimiento pulp were evaluated at different levels of calcium chloride and thermal processing.

Alginate-Based Edible Films and Coatings for Food Packaging Applications

Alginate is a naturally occurring polysaccharide used in the bio industry. It is mainly derived from brown algae species. Alginate-based edible coatings and films attract interest for improving/maintaining quality and extending the shelf-life of fruit, vegetable, meat, poultry, seafood, and cheese by reducing dehydration (as sacrificial moisture agent), controlling respiration, enhancing product appearance, improving mechanical properties, etc. This paper reviews the most recent essential information about alginate-based edible coatings. The categorization of alginate-based coatings/film in food packaging concept is formed gradually with the explanation of the most important titles. Emphasis will be placed on active ingredients incorporated into alginate-based formulations, edible coating/film application methods, research and development studies of coated food products and mass transfer and barrier characteristics of the alginate-based coatings/films. Future trends are also reviewed to identify research gaps and recommend new research areas. The summarized information presented in this article will enable researchers to thoroughly understand the fundamentals of the coating process and to develop alginate-based edible films and coatings more readily.

Structural changes in alginate-based microspheres exposed to in vivo environment as revealed by confocal Raman microscopy

A next-generation cure for type 1 diabetes relies on immunoprotection of insulin-producing cells, which can be achieved by their encapsulation in microspheres made of non-covalently crosslinked hydrogels. Treatment success is directly related to the microsphere structure that is characterized by the localization of the polymers constituting the hydrogel material. However, due to the lack of a suitable analytical method, it is presently unknown how the microsphere structure changes in vivo, which complicates evaluation of different encapsulation approaches. Here, confocal Raman microscopy (CRM) imaging was tailored to serve as a powerful new tool for tracking structural changes in two major encapsulation designs, alginate-based microbeads and multi-component microcapsules. CRM analyses before implantation and after explantation from a mouse model revealed complete loss of the original heterogeneous structure in the alginate microbeads, making the intentionally high initial heterogeneity a questionable design choice. On the other hand, the structural heterogeneity was conserved in the microcapsules, which indicates that this design will better retain its immunoprotective properties in vivo. In another application, CRM was used for quantitative mapping of the alginate concentration throughout the microbead volume. Such data provide invaluable information about the microenvironment cells would encounter upon their encapsulation in alginate microbeads.

Laponite as a rheology modifier of alginate solutions: Physical gelation and aging evolution

The rheological behavior of alginate and Laponite/alginate solutions was studied. It was observed that the Cross viscosity model successfully describes the steady-state shear behavior of this polysaccharide. The scaling behavior analyzed for the entangled regime is in good agreement with polyelectrolyte solutions (G_e∼c_p^3/2), with interactions generated between the alginate and the charged surfaces of the Laponite platelets. Therefore, the effect of Laponite as a rheology modifier is influenced by the alginate concentration. Higher alginate concentrations hindered the formation of the house of cards microstructure. Frequency sweep tests were performed to analyze the transition from solid-like to liquid-like behavior in a solid-like dominated domain. Soft physical gels were obtained at low alginate concentrations. The gel point was determined (1.65wt.% of alginate and 2wt.% of Laponite) through the Kramers-Krönig damping factor, and time sweep tests revealed the evolution of the storage (G') and loss modulus (G″) as functions of the waiting time (t_w). The growing elasticity revealed that Laponite/alginate solutions undergo aging.

Novel control of gel fraction and enhancement of bonding strength for constructing 3D architecture of tissue engineering scaffold with alginate tubular fiber

Alginate tubular fiber has been successfully prepared via coaxial fluid crosslink mode, which is potentially used for the construction of vascularized tissue engineering scaffolds (VTES). However, its elastic and smooth surface is negative for the adhesion of fibers. In this study, the gel fractions were controlled in a novel way of two-step crosslink process in order to meet the needs of each processing link. Based on such consideration, an appropriate formulation was selected to direct write single fiber, which ensured the tubular structure with enough gel portion as well as adhesion between fibers with the reserved sol. Finally, the integrity of the scaffolds had a further development within the 2nd crosslink bath process, which would help to solve the question of poor shear resistance for hydrogel scaffolds.

Interpenetration of Natural Polymer Aerogels by Supercritical Drying

Natural polymers, such as alginate and gelatin, can be used to produce scaffolds for tissue engineering applications; but, their mechanical and biochemical performance should be improved. A possible solution to obtain this result, is the generation of multi-component scaffolds, by blending two or more polymers. One way to realize it, is the formation of an interpenetrating polymer network (IPN). In this work, the interpenetration of alginate and gelatin hydrogels has been successfully obtained and preserved by supercritical CO₂ (SC-CO₂) drying performed at 200 bar and 35 °C, using different blend compositions: from alginate/gelatin = 20:80 v/v to alginate/gelatin = 80:20 v/v. The process allowed modulation of morphology and mechanical properties of these blends. The overall result was made possible by the supercritical drying process that, working at zero surface tension, allows preserving the hydrogels nanostructure in the corresponding aerogels.

A reactor-like spinneret used in 3D printing alginate hollow fiber: a numerical study of morphological evolution

In this paper, a reactor-like spinneret is proposed to generate a continuous hollow hydrogel fiber. In order to reliably control the deforming dynamics, the components of the spinneret are standardized in order to ease the online observation of morphological evolution. We found that not only did a co-flow occur in the tubular space, but a relatively large shrinkage of the shell layer at the outlet also occurred. Whereupon a weak coupling of the velocity field and diffusion-reacting co-flow was developed to describe the monitored co-flow morphology and to simulate the intermediate state of the concentration field, as well as to calculate the shrinkage profile with an integral formula. And a critical isogram [G]cri was determined to correspond to the morphological segmental feature, to trigger gelation and shrinkage as a threshold of solubility and the integral upper limit of the shrinkage region. Experimental evidence indicates that: the simulation is able to effectively predict the inner diameter of the hollow fiber; the transient inner diameter of the fiber at the outlet is expanded by approximately 70 μm (co-flow distance = 15 mm) as compared to the initial fluid dynamics value, and that the relative mean error of the simulated inner diameter was less than 8%. The proposed study provides deeper insight into the printing of hollow fibers and other gelling processes which utilize a reactor-like spinneret.

Size- and shape-dependent foreign body immune response to materials implanted in rodents and non-human primates

The efficacy of implanted biomedical devices is often compromised by host recognition and subsequent foreign body responses. Here, we demonstrate the role of the geometry of implanted materials on their biocompatibility in vivo. In rodent and non-human primate animal models, implanted spheres 1.5 mm and above in diameter across a broad spectrum of materials, including hydrogels, ceramics, metals and plastics, significantly abrogated foreign body reactions and fibrosis when compared with smaller spheres. We also show that for encapsulated rat pancreatic islet cells transplanted into streptozotocin-treated diabetic C57BL/6 mice, islets prepared in 1.5-mm alginate capsules were able to restore blood-glucose control for up to 180 days, a period more than five times longer than for transplanted grafts encapsulated within conventionally sized 0.5-mm alginate capsules. Our findings suggest that the in vivo biocompatibility of biomedical devices can be significantly improved simply by tuning their spherical dimensions.

Precipitation and surface adsorption of metal complexes during electropolishing. Theory and characterization with X-ray nanotomography and surface tension isotherms

Using X-ray nano-tomography, a gel-like phase formed at the tungsten surface during electropolishing was discovered and attributed to a complex process of precipitation and sintering of tungsten-based nuclei.