Macro- and Microscale Rheological Properties of Poly(vinyl alcohol) Aqueous Solutions

Gels That Serve as Mucus Simulants: A Review

Mucus is a critical part of the human body's immune system that traps and carries away various particulates such as anthropogenic pollutants, pollen, viruses, etc. Various synthetic hydrogels have been developed to mimic mucus, using different polymers as their backbones. Common to these simulants is a three-dimensional gel network that is physically crosslinked and is capable of loosely entrapping water within. Two of the challenges in mimicking mucus using synthetic hydrogels include the need to mimic the rheological properties of the mucus and its ability to capture particulates (its adhesion mechanism). In this paper, we review the existing mucus simulants and discuss their rheological, adhesive, and tribological properties. We show that most, but not all, simulants indeed mimic the rheological properties of the mucus; like mucus, most hydrogel mucus simulants reviewed here demonstrated a higher storage modulus than its loss modulus, and their values are in the range of that found in mucus. However, only one mimics the adhesive properties of the mucus (which are critical for the ability of mucus to capture particulates), Polyvinyl alcohol-Borax hydrogel.

Polypseudorotaxanes of Pluronic® F127 with Combinations of α- and β-Cyclodextrins for Topical Formulation of Acyclovir

Acyclovir (ACV) is one of the most used antiviral drugs for the treatment of herpes simplex virus infections and other relevant mucosal infections caused by viruses. Nevertheless, the low water solubility of ACV limits both its bioavailability and antiviral performance. The combination of block copolymer micelles and cyclodextrins (CDs) may result in polypseudorotaxanes with tunable drug solubilizing and gelling properties. However, the simultaneous addition of various CDs has barely been investigated yet. The aim of this work was to design and characterize ternary combinations of Pluronic® F127 (PF127), αCD and βCD in terms of polypseudorotaxane formation, rheological behavior, and ACV solubilization ability and controlled release. The formation of polypseudorotaxanes between PF127 and the CDs was confirmed by FT-IR spectroscopy, X-ray diffraction, and NMR spectroscopy. The effects of αCD/βCD concentration range (0–7% w/w) on copolymer (6.5% w/w) gel features were evaluated at 20 and 37 °C by rheological studies, resulting in changes of the copolymer gelling properties. PF127 with αCD/βCD improved the solubilization of ACV, maintaining the biocompatibility (hen’s egg test on the chorio-allantoic membrane). In addition, the gels were able to sustain acyclovir delivery. The formulation prepared with similar proportions of αCD and βCD provided a slower and more constant release. The results obtained suggest that the combination of Pluronic with αCD/βCD mixtures can be a valuable approach to tune the rheological features and drug release profiles from these supramolecular gels.

Water dynamics in physical hydrogels based on partially hydrophobized hyaluronic acid

The dynamics of hyaluronate-based hydrogels has been investigated by quasielastic neutron scattering (QENS). Hyaluronate (HYA) has been compared, in the same conditions of temperature and polymer concentration, to a chemically modified form, HYADD, in which the backbone has been grafted with a hexadecyl (C(16)) side-chain with a degree of substitution of about 2% (mol/mol). This modification increases the hydrophobicity of the polysaccharide and leads to a stable gel already at polymer concentration of 0.3% (w/v), yielding a viscosupplementation with less quantity of polysaccharide. The time-scale covered by our measurements probes both water and segmental biopolymer motions. In both systems, the local dynamics of the network in the ps time-scale is mostly due to local reorientational motions of side groups. Such motions are not significantly affected by the small amount of aliphatic chains forming the hydrophobic junctions in HYADD. The diffusivity of water in both HYA and HYADD coincides with that of pure water within the experimental uncertainty. This result confirms previous ones on the dynamics of water in HYA solutions and it is of relevance for biomedical applications of hyaluronate-based systems because it affects the diffusive processes of metabolites and their interaction with tissues.

Effects of microviscosity, dry electron scavenging, and protein mobility on the radiolysis of albumen hydrogel

Nanosecond pulse radiolysis experiments performed on the oxygen and nitrogen saturated thick fraction of egg white (which has an immeasurably high macroviscosity) produced a rate constant for hydrated electron reaction with oxygen equal to 1.7 × 10(10) M(-1) s(-1) at 18 °C, indicating that, due to the very low microviscosity, hydrated electrons are as mobile and reactive in the albumen hydrogel as in neat water. Also, the radiolytic yield for the hydrated electron (G-value) in the thick fraction of egg white (measured at the end of a 14 ns electron pulse) was found to be 86% of that determined in neat water, which can be attributed to the reaction of dry electrons with the protein constituents. Steady-state γ radiolysis studies on air-saturated thick and thin fraction (true solution) revealed that the immobility of egg white proteins disfavors reactions that result in molecular mass change.

Mobility of molecules and ions solubilized in protein gels: diffusion in the thick fraction of hen egg white

The thick fraction of hen egg white is a protein hydrogel with an immeasurably high viscosity composed of ∼90% water that can serve as a model system for mammalian mucous membrane. Measurements of the rate constants of diffusion-controlled reactions occurring within the gel (and corresponding activation energies) and electric conductivity revealed that the thick fraction of egg white can be envisioned as a 3D network comprising hydrated protein molecules (held by intermolecular S-S bridges) surrounded by water pools and channels (of nonuniform diameters) that have a microviscosity that is very similar to that of bulk water. This was corroborated by differential scanning calorimetry measurements that revealed that 16% of water is bound to proteins. The melting kinetics of ice crystallites (produced from the freezable water) indicates nonhomogeneous water pool size.