Gels, Volume 3, Issue 4 (December 2017) – 12 articles

This research article will describe the design and use of polyelectrolyte hydrogel particles as internal curing agents in concrete and present new results on relevant hydrogel-ion interactions. When incorporated into concrete, hydrogel particles release their stored water to fuel the curing reaction, resulting in reduced volumetric shrinkage and cracking and thus increasing concrete service life. The hydrogel’s swelling performance and mechanical properties are strongly sensitive to multivalent cations that are naturally present in concrete mixtures, including calcium and aluminum. Model poly(acrylic acid(AA)-acrylamide(AM))-based hydrogel particles with different chemical compositions (AA:AM monomer ratio) were synthesized and immersed in sodium, calcium, and aluminum salt solutions. The presence of multivalent cations resulted in decreased swelling capacity and altered swelling kinetics to the point where some hydrogel compositions displayed rapid deswelling behavior and the formation of a mechanically stiff shell. Interestingly, when incorporated into mortar, hydrogel particles reduced mixture shrinkage while encouraging the formation of specific inorganic phases (calcium hydroxide and calcium silicate hydrate) within the void space previously occupied by the swollen particle. Full article

This paper presents a rheological study describing the effects of shear on the flow curves of colloidal gels prepared with different concentrations of fumed silica (4%, 5%, 6%, and 7%) and a hydrophobic solvent (Hydrocarbon fuel, JP-8). Viscosity measurements as a function of time were carried out at different shear rates (10, 50, 100, 500, and 1000 s⁻¹), and based on this data, a new structural kinetics model was used to describe the system. Previous work has based the analysis of time dependent fluids on the viscosity of the intact material, i.e., before it is sheared, which is a condition very difficult to achieve when weak gels are tested. The simple action of loading the gel in the rheometer affects its structure and rheology, and the reproducibility of the measurements is thus seriously compromised. Changes in viscosity and viscoelastic properties of the sheared material are indicative of microstructural changes in the gel that need to be accounted for. Therefore, a more realistic method is presented in this work. In addition, microscopical images (Cryo-SEM) were obtained to show how the structure of the gel is affected upon application of shear. Full article

The swelling of polyelectrolyte hydrogels usually depends on the pH, and if the pH is high enough degradation can occur. A microindentation device was developed to dynamically test these processes in whey protein isolate hydrogels at alkaline pH 7–14. At low alkaline pH the shear modulus decreases during swelling, consistent with rubber elasticity theory, yet when chemical degradation occurs at pH ≥ 11.5 the modulus decreases quickly and extensively. The apparent modulus was constant with the indentation depth when swelling predominates, but gradients were observed when fast chemical degradation occurs at 0.05–0.1 M NaOH. In addition, these profiles were constant with time when dissolution rates are also constant, the first evidence that a swollen layer with steady state mechanical properties is achieved despite extensive dissolution. At >0.5 M NaOH, we provide mechanical evidence showing that most interactions inside the gels are destroyed, gels were very weak and hardly swell, yet they still dissolve very slowly. Microindentation can provide complementary valuable information to study the degradation of hydrogels. Full article

Hydrogels contain ample amounts of water, with the water-to-solid ratio sometimes reaching tens of thousands of times. How can so much water remain securely lodged within the gel? New findings imply a simple mechanism. Next to hydrophilic surfaces, water transitions into an extensive gel-like phase in which molecules become ordered. This “fourth phase” of water sticks securely to the solid gel matrix, ensuring that the water does not leak out. Full article

Here, we report the synthesis and functionalization of five different shapes of Au nanoparticles (NPs), namely nanorods, tetrahexahedral, bipyramids, nanomakura, and spheres with PEG and poly (N-isopropylacrylamide)-acrylic acid (pNIPAm-AAc) hydrogels. The anisotropic NPs are synthesized using seed-mediated growth in the presence of silver. The NPs have been characterized using Dynamic Light Scattering (DLS), zeta potential measurements, UV-Visible spectrophotometry (UV-Vis), and Scanning Transmission Electron Microscopy (S(T)EM). Cyt C was loaded into the PEG-hydrogel-coated AuNPs using a modified breathing-in method. Loading efficiencies (up to 80%), dependent on particle geometry, concentration, and hydrogel content, were obtained. Release experiments conducted at high temperature (40 °C) and acidic pH (3) showed higher release for larger sizes of PEG-hydrogel-coated AuNPs, with temporal transition from spherical to thin film release geometry. AuNP shape, size, number density, and hydrogel content are found to influence the loading as well as release kinetics of Cyt C from these systems. Full article

Polyampholytes are a class of polymers made up of positively and negatively charged monomer subunits. Polyampholytes offer a unique tunable set of properties driven by the interactions between the charged monomer subunits. Some tunable properties of polyampholytes include mechanical properties, nonfouling characteristics, swelling due to changes in pH or salt concentration, and drug delivery capability. These characteristics lend themselves to multiple biomedical applications, and this review paper will summarize applications of polyampholyte polymers demonstrated over the last five years in tissue engineering, cryopreservation and drug delivery. Full article

A series of novel temperature responsive hydrogels were synthesized by free radical polymerization with varying content of chrysin multiacrylate (ChryMA). The goal was to study the impact of this novel polyphenolic-based multiacrylate on the properties of N-isopropylacrylamide (NIPAAm) hydrogels. The temperature responsive behavior of the copolymerized gels was characterized by swelling studies, and their lower critical solution temperature (LCST) was characterized through differential scanning calorimetry (DSC). It was shown that the incorporation of ChryMA decreased the swelling ratios of the hydrogels and shifted their LCSTs to a lower temperature. Gels with different ChryMA content showed different levels of response to temperature change. Higher content gels had a broader phase transition and smaller temperature response, which could be attributed to the increased hydrophobicity being introduced by the ChryMA. Full article

Thixotropy is one of the oldest documented rheological phenomenon in colloid science and may be defined as an increase of viscosity in a state of rest and a decrease of viscosity when submitted to a constant shearing stress. This behavior has been exploited in recent years to prepare injectable hydrogels for application in drug delivery systems. Thixotropic hydrogels may be profitably used in the field of regenerative medicine, which promotes tissue healing after injuries and diseases, as the molten hydrogel may be injected by syringe and then self-adapts in the space inside the injection site and recovers the solid form. We will focus our attention on the preparation, properties, and some applications of biocompatible thixotropic hydrogels. Full article

In this review, the importance of water in hydrogel (HG) properties and structure is analyzed. A variety of methods such as ¹H NMR (nuclear magnetic resonance), DSC (differential scanning calorimetry), XRD (X-ray powder diffraction), dielectric relaxation spectroscopy, thermally stimulated depolarization current, quasi-elastic neutron scattering, rheometry, diffusion, adsorption, infrared spectroscopy are used to study water in HG. The state of HG water is rather non-uniform. According to thermodynamic features of water in HG, some of it is non-freezing and strongly bound, another fraction is freezing and weakly bound, and the third fraction is non-bound, free water freezing at 0 °C. According to structural features of water in HG, it can be divided into two fractions with strongly associated and weakly associated waters. The properties of the water in HG depend also on the amounts and types of solutes, pH, salinity, structural features of HG functionalities. Full article

We investigated the release behavior of glucagon-like peptide-1 (GLP-1) from a biodegradable injectable polymer (IP) hydrogel. This hydrogel shows temperature-responsive irreversible gelation due to the covalent bond formation through a thiol-ene reaction. In vitro sustained release of GLP-1 from an irreversible IP formulation (F(P1/D+PA₄₀)) was observed compared with a reversible (physical gelation) IP formulation (F(P1)). Moreover, pharmaceutically active levels of GLP-1 were maintained in blood after subcutaneous injection of the irreversible IP formulation into rats. This system should be useful for the minimally invasive sustained drug release of peptide drugs and other water-soluble bioactive reagents. Full article

Bio-based release systems for pro-angiogenic growth factors are of interest, to overcome insufficient vascularization and bio-integration of implants. In this study, we investigated heparin-functionalized hydrogels based on gelatin type A or albumin as storage and release systems for vascular endothelial growth factor (VEGF). The hydrogels were crosslinked using carbodiimide chemistry in presence of heparin. Heparin-functionalization of the hydrogels was monitored by critical electrolyte concentration (CEC) staining. The hydrogels were characterized in terms of swelling in buffer solution and VEGF-containing solutions, and their loading with and release of VEGF was monitored. The equilibrium degree of swelling (EDS) was lower for albumin-based gels compared to gelatin-based gels. EDS was adjustable with the used carbodiimide concentration for both biopolymers. Furthermore, VEGF-loading and release were dependent on the carbodiimide concentration and loading conditions for both biopolymers. Loading of albumin-based gels was higher compared to gelatin-based gels, and its burst release was lower. Finally, elevated cumulative VEGF release after 21 days was determined for albumin-based hydrogels compared to gelatin A-based hydrogels. We consider the characteristic net charges of the proteins and degradation of albumin during release time as reasons for the observed effects. Both heparin-functionalized biomaterial systems, chemically crosslinked gelatin type A or albumin, had tunable physicochemical properties, and can be considered for controlled delivery of the pro-angiogenic growth factor VEGF. Full article

The innate ability of poly(N-isopropylacrylamide) (PNIPAAm) thermo-responsive hydrogel to copolymerize and to graft synthetic polymers and biomolecules, in conjunction with the highly controlled methods of radical polymerization which are now available, have expedited the widespread number of papers published in the last decade—especially in the biomedical field. Therefore, PNIPAAm-based hydrogels are extensively investigated for applications on the controlled delivery of active molecules, in self-healing materials, tissue engineering, regenerative medicine, or in the smart encapsulation of cells. The most promising polymers for biodegradability enhancement of PNIPAAm hydrogels are probably poly(ethylene glycol) (PEG) and/or poly(ε-caprolactone) (PCL), whereas the biocompatibility is mostly achieved with biopolymers. Ultimately, advances in three-dimensional bioprinting technology would contribute to the design of new devices and medical tools with thermal stimuli response needs, fabricated with PNIPAAm hydrogels. Full article