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.

Comparative Large Amplitude Oscillatory Shear (LAOS) Study of Ionically and Physically Crosslinked Hydrogels.

Hydrogels are highly versatile and widely applicable materials within various scientific, technological, and food sectors. Alginate and gelatin hydrogels, along with their crafted variations, are possibly the most common ones. However, the ionic crosslinking of alginate-Ca++ is a different gelation mechanism than the physical crosslinking of gelatin. In this work, we prepare alginate-Ca++ hydrogels using individual layer gelation and experimentally evaluate LAOS rheological behavior. We apply shear-stress decomposition using the MITlaos software and obtain the elastic and viscous contributions within the nonlinear response of the individual alginate-Ca++ layer. We compare these results with the nonlinear responses of the gelatin-alginate ex situ individual layer. The strain-sweep patterns are similar, with loss modulus overshoot. The applied shear can destroy the larger-scale structural units (agglomerate/aggregates), resulting in analogous patterns. However, the critical strain points are different. Based on the shear-thickening ratio T of the LAOS analysis, it can be assumed that the common feature of ex situ preparation, i.e., gelation as individual layers, provides a matching bulk microstructure, as the hydrogels differ significantly at a molecular-binding level.

Therapeutic Applications of Metal and Metal-Oxide Nanoparticles: Dermato-Cosmetic Perspectives.

Invention of novel nanomaterials guaranteeing enhanced biomedical performance in diagnostics and therapeutics, is a perpetual initiative. In this regard, the upsurge and widespread usage of nanoparticles is a ubiquitous phenomenon, focusing predominantly on the application of submicroscopic (< 100 nm) particles. While this is facilitated attributing to their wide range of benefits, a major challenge is to create and maintain a balance, by alleviating the associated toxicity levels. In this minireview, we collate and discuss particularly recent advancements in therapeutic applications of metal and metal oxide nanoparticles in skin and cosmetic applications. On the one hand, we outline the dermatological intrusions, including applications in wound healing. On the other hand, we keep track of the recent trends in the development of cosmeceuticals via nanoparticle engrossments. The dermato-cosmetic applications of metal and metal oxide nanoparticles encompass diverse aspects, including targeted, controlled drug release, and conferring ultraviolet and antimicrobial protections to the skin. Additionally, we deliberate on the critical aspects in comprehending the advantage of rheological assessments, while characterizing the nanoparticulate systems. As an illustration, we single out psoriasis, to capture and comment on the nanodermatology-based curative standpoints. Finally, we lay a broad outlook and examine the imminent prospects.

Custom-made rheological setup for in situ real-time fast alginate-Ca2+ gelation.

There is a growing interest in the in situ gelation of the alginate-Ca2+ system due to its remarkable applications. In this work, we record and evaluate the fast gelation kinetics of alginate-Ca2+ using a custom-made rheometric setup. This enables us to inject CaCl2 into the alginate while we perform the rheological measurements. We successfully measure the in situ gelation reaction from the early stages. As the alginate concentration is increased up to 3 wt.%, we observe a systematic increase of the elastic modulus, G'. Similarly, higher concentrations and injected volumes of CaCl2 increase the magnitude and initial growth rate of G'. At longer times, the growth rate of G' is lower. It decreases further very slowly, indicating that the chemical reaction requires quite a considerable amount of time to be completed. Finally, from the rheometric data, we estimate the average rates of the elastic modulus during the initial and quasi-steady-state stages.

Nonlinear rheological behavior of gelatin gels: In situ gels and individual gel layers filled with hard particles.

HYPOTHESIS: Previously, we examined the impact of two preparation procedures on the mechanical properties of native gelatin gels. In this work, we extend our research by considering hard spherical particles as fillers. We expect that the presence of these fillers significantly affects the viscoelastic response. EXPERIMENTS: We prepared fresh in situ gels and individual gel layers filled with glass micro-beads up to 1% w/w and aged for 24 and 48 h. The hydrogels were made of 3 and 5% w/w gelatin at 5 °C, and we performed large amplitude oscillatory shear (LAOS) tests. We analyzed the intracycle linear and nonlinear response using normalized Lissajous-Bowditch curves. Utilizing the MITlaos software, we decomposed the total intracycle stress into elastic and viscous contributions and calculated the Chebyshev harmonics coefficients. FINDINGS: The fresh in situ gels exhibit severe progressive stiffening during the strain sweeps and a subsequent sharp decrease of both moduli. The filled layers show smoother yielding than the in situ gels. The fillers increase the dynamic moduli, affect the terminal LAOS regime, and enhance the intracycle nonlinearities at higher concentrations. The Lissajous-Bowditch curves of the aged layers indicate elastoplastic behavior, which is more pronounced for the 48 h filled gel layer than the native counterpart.

Nonlinear rheological behavior of gelatin gels: In situ gels and individual layers.

The gelation procedure and the gelation time of gelatin gels may lead to apparently similar materials, however, with different rheological fingerprints under small and large oscillatory shear deformations. Here, in the first paper of this series, gelation of 3 and 5% w/w gelatin solutions at 5 °C was performed in situ on the rheometer plate and in custom-made casting modules to obtain individual gel layers. Large amplitude oscillatory shear (LAOS) tests were performed. The nonlinear deformation regime was qualitatively analyzed using normalized Lissajous-Bowditch curves. The MITlaos software was employed to decompose the total intracycle stress response and to calculate the Chebyshev coefficients ratios. The dynamic moduli of the fresh gels were measured directly after the in situ preparation and within a time frame until 1.5 h. In the strain sweeps, we observed intense stiffening followed by yielding above 200% strain. However, the individual gel layers aged for 24 and 48 h show different LAOS fingerprints. The extensive loops in the viscous Lissajous-Bowditch curves indicate an elastoplastic material response. Based on the overall nonlinear rheological response, we propose a structural formation that describes the behavior of the gels for the conditions studied here. In the second paper of this series, we give the impact of hard micro-fillers (glass beads) and we describe the nonlinear characteristics of the filled gels.

Linear viscoelastic and microstructural properties of native male human skin and in vitro 3D reconstructed skin models.

The study reports first ever account of measurements of linear viscoelastic moduli under small amplitude oscillatory shear deformations, for commercially available juvenile and aged in vitro 3D reconstructed skin models. The results were compared with those of native male whole human and dermis-only foreskin samples, catering to a wide age group from 0.5 to 68 years, including samples from a 23-year-old male abdomen. In the strain sweep tests, the dermis of the juvenile/young age group assumed a higher intrinsic elastic modulus than the whole skin. A reverse qualitative trend was noted for the adult/aged age group. Confirmed by the histological examination of the stained cross-sections, this is attributed to the nascent epidermal differentiation and the high fiber density of dermal collagen. The oscillation frequency sweeps exposed a greater dependence of the elasticity on the frequency for the native male dermis foreskin samples as compared to the whole skins, irrespective of age. This is anticipated since the extremely structured epidermis confers higher resistance to the whole skins towards intracycle deformations compared to the dermis, thereby storing smaller elastic energy. The 3D skin models examined in this work exhibited a broader linear viscoelastic region, a larger viscoelasticity, and much higher dynamic moduli, compared to the native skin. The rheological trends are a significant addition to the literature and may be used as a reference for the design of next generation of scaffolds.

Surface tension-induced gel fracture. Part 1. Fracture of agar gels.

This work involves an experimental investigation of the spreading of liquids on gel layers in the presence of surfactants. Of primary interest is the instability that accompanies the cracking of gels through the deposition and subsequent spreading of a drop of surfactant solution on their surfaces. This instability manifests itself via the shaping of crack-like spreading "arms", in formations that resemble starbursts. The main aim of this study is to elucidate the complex interactions between spreading surfactants and underlying gels and to achieve a fundamental understanding of the mechanism behind the observed phenomenon of the cracking pattern formation. By spreading SDS and Silwet L-77 surfactant solutions on the surfaces of agar gels, the different ways that system parameters such as the surfactant chemistry and concentration and the gel strength can affect the morphology and dynamics of the starburst patterns are explored. The crack propagation dynamics is fitted to a power law by measuring the temporal evolution of the length of the spreading arms that form each one of the observed patterns. The values of the exponent of the power law are within the predicted limits for Marangoni-driven spreading on thick layers. Therefore, Marangoni stresses, induced by surface tension gradients between the spreading surfactant and the underlying gel layer, are identified to be the main driving force behind these phenomena, whereas gravitational forces were also found to play an important role. A mechanism that involves the "unzipping" of the gel in a manner perpendicular to the direction of the largest surface tension gradient is proposed. This mechanism highlights the important role of the width of the arms in the process; it is demonstrated that a cracking pattern is formed only within the experimental conditions that allow S/Δw to be greater than G', where S is the spreading coefficient, Δw is the change in the width of the crack, and G' is the storage modulus of the substrate.

Surface tension-induced gel fracture. Part 2. Fracture of gelatin gels.

The spreading of surfactants on gel layers has been found to be accompanied by an intriguing instability which involves the formation of crack-like patterns on the surface of the gel. In an attempt to extend the findings on the spreading on agar gels presented in part 1 of this series, this paper examines the case of surfactant spreading on gelatin, which is a characteristic example of a protein-based gel. Aqueous solutions of Silwet L-77 of varying concentrations were spread on thick gelatin layers of varying concentrations. The resulting pattern formation was found to have many similarities to the corresponding phenomenon on agar. In terms of spreading dynamics, the values of the spreading exponent, n, of the power law L(t) ~ kt(n), which describes the temporal evolution of the cracks, are similar to those of the agar case, within the predicted limits for surface tension gradient-induced spreading on thick films, highlighting the dominant presence of Marangoni stresses. However, the values of the spreading coefficient, k, are much smaller compared to those measured during the spreading on agar. Further observations are linked with the rheological properties of gelatin, which are also measured in detail.

Rheological study of synovial fluid obtained from dogs: healthy, pathological, and post-surgery, after spontaneous rupture of cranial cruciate ligament.

In the present study synovial fluid (SF) obtained from the stifle joint of healthy adult dogs and of dogs after cranial cruciate ligament rupture was analyzed regarding its rheological characteristics according to the condition of the joint. The viscoelastic and shear flow properties were measured at 25 and 38 degrees C. The results showed that the healthy SF exhibits practically temperature independent viscosity curve and satisfactory viscoelastic characteristics, i.e. G' > G'', over frequencies of 0.05-5 Hz, and characteristic relaxation time lambda of the order of magnitude of 100 s. Creep measurements demonstrate that the zero shear viscosity was in the range of 10-100 Pa s. In shear flow viscosity measurements, by increasing gamma from 10(-4) s(-1) up to 10(3) s(-1), non-Newtonian shear thinning behavior was observed and the viscosity values were decreased from 10(3) to 0.1 Pa s. On the contrary, in pathological conditions of cranial cruciate ligament rupture (CCLR), the measured viscosity was found drastically reduced, i.e. between 100 and 10 mPa s. The CCLR synovial fluid, similar to healthy SF, exhibits insignificant temperature dependence. The present study showed also that about one week after a surgery for CCLR repair the SF exhibits non-Newtonian behavior of dilute polymers. After two weeks from the operation, however, the rheological behavior converges to the one of healthy SF.