The rheological properties of weak gels of poly(vinyl alcohol) and sodium borate

ROS-responsive hydrogels: from design and additive manufacturing to biomedical applications

Hydrogels with intricate 3D networks and high hydrophilicity have qualities resembling those of biological tissues, making them ideal candidates for use as smart biomedical materials. Reactive oxygen species (ROS) responsive hydrogels are an innovative class of smart hydrogels, and are cross-linked by ROS-responsive modules through covalent interactions, coordination interactions, or supramolecular interactions. Due to the introduction of ROS response modules, this class of hydrogels exhibits a sensitive response to the oxidative stress microenvironment existing in organisms. Simultaneously, due to the modularity of the ROS-responsive structure, ROS-responsive hydrogels can be manufactured on a large scale through additive manufacturing. This review will delve into the design, fabrication, and applications of ROS-responsive hydrogels. The main goal is to clarify the chemical principles that govern the response mechanism of these hydrogels, further providing new perspectives and methods for designing responsive hydrogel materials.

Self-Healable, Injectable Hydrogel with Enhanced Clotrimazole Solubilization as a Potential Therapeutic Platform for Gynecology

Injectable, self-healing hydrogels with enhanced solubilization of hydrophobic drugs are urgently needed for antimicrobial intravaginal therapies. Here, we report the first hydrogel systems constructed of dynamic boronic esters cross-linking unimolecular micelles, which are a reservoir of antifungal hydrophobic drug molecules. The selective hydrophobization of hyperbranched polyglycidol with phenyl units in the core via ester or urethane bonds enabled the solubilization of clotrimazole, a water-insoluble drug of broad antifungal properties. The encapsulation efficiency of clotrimazole increases with the degree of the HbPGL core modification; however, the encapsulation is more favorable in the case of urethane derivatives. In addition, the rate of clotrimazole release was lower from HbPGL hydrophobized via urethane bonds than with ester linkages. In this work, we also revealed that the hydrophobization degree of HbPGL significantly influences the rheological properties of its hydrogels with poly(acrylamide-ran-2-acrylamidephenylboronic acid). The elastic strength of networks (G_N) and the thermal stability of hydrogels increased along with the degree of HbPGL core hydrophobization. The degradation of the hydrogel constructed of the neat HbPGL was observed at approx. 40 °C, whereas the hydrogels constructed on HbPGL, where the monohydroxyl units were modified above 30 mol %, were stable above 50 °C. Moreover, the flow and self-healing ability of hydrogels were gradually decreased due to the reduced dynamics of macromolecules in the network as an effect of increased hydrophobicity. The changes in the rheological properties of hydrogels resulted from the engagement of phenyl units into the intermolecular hydrophobic interactions, which besides boronic esters constituted additional cross-links. This study demonstrates that the HbPGL core hydrophobized with phenyl units at 30 mol % degrees via urethane linkages is optimal in respect of the drug encapsulation efficiency and rheological properties including both self-healable and injectable behavior. This work is important because of a proper selection of a building component for the construction of a therapeutic hydrogel platform dedicated to the intravaginal delivery of hydrophobic drugs.

Composite Dynamic Hydrogels Constructed on Boronic Ester Cross-Links with NIR-Enhanced Diffusivity

Dynamic hydrogels with thermosensitive cross-links are highly promising platforms for "on-demand" drug delivery systems. However, there is a problem with triggering a response in their whole volume, which reduces their efficiency. To achieve better thermoresponsiveness, a graphene oxide-filled composite hydrogel based on boronic ester cross-links, composed of hyperbranched polyglycidol, HbPGL, and poly(acrylamide-ran-2-acrylamidephenylboronic acid), poly(AM-ran-2-AAPBA), has been constructed. The homogeneous embedment of graphene oxide (GO) in the network assured near-infrared (NIR)-photothermal response in its bulk due to the rapid light-to-heat conversion. The rate and amplitude of materials response increase with graphene oxide concentration. The temperature of the hydrogel containing graphene oxide at a concentration of 13.2 mg/mL increased from 36.6 to 41 °C in 29 s upon NIR irradiation. The network diffusivity and the extent of its change with temperature can be regulated by the length of the applied boronic acid-based cross-linking agent. The hydrogel constructed on the shorter copolymer (M_n = 23 000 g/mol) displayed a significant increase in diffusivity with temperature. A diffusion ordered NMR study revealed that the diffusion coefficient determined for niacin, a model drug encapsulated in the hydrogel, increased from 6.09 × 10^-10 at 25 °C to 1.28 × 10^-9 m²/s at 41 °C. In the case of the hydrogel constructed on the longer acrylamide copolymer (M_n = 43 000 g/mol), in which physical entanglements stabilize the network, the change of encapsulated niacin diffusion coefficient was significantly smaller, i.e., from 3.83 × 10^-10 at 25 °C to 6.63 × 10^-10 m²/s at 41 °C. The possibility of on-demand NIR-regulated diffusivity of the reported boronic ester-based hydrogels makes them promising candidates for controlled drug delivery platforms.

Double dynamic hydrogels formed by wormlike surfactant micelles and cross-linked polymer

HYPOTHESIS

Interpenetrating networks consisting of a polymer network with dynamic cross-links and a supramolecular network allow obtaining hydrogels with significantly enhanced mechanical properties.

EXPERIMENTS

Binary hydrogels composed of a dynamically cross-linked poly(vinyl alcohol) (PVA) network and a transient network of entangled highly charged mixed wormlike micelles (WLMs) of surfactants (potassium oleate and n-octyltrimethylammonium bromide) were prepared and studied by rheometry, SANS, USANS, cryo-TEM, and NMR spectroscopy.

FINDINGS

Binary hydrogels show significantly enhanced rheological properties (a 3400-fold higher viscosity and 27-fold higher plateau modulus) as compared to their components taken separately. This is due to the microphase separation leading to local concentrating of PVA and WLMs providing larger number of polymer-polymer contacts for cross-linking and longer WLMs with more entanglements. Such materials are very promising for the application in many areas, ranging from enhanced oil recovery to biomedical uses.

Enhanced surface decontamination of radioactive Cs by self-generated, strippable hydrogels based on reversible cross-linking

A self-generated, strippable hydrogel containing adsorbents was developed to remove the radioactive cesium from surfaces by adsorption for wide-area surface decontamination. Two aqueous polymeric solutions of polyvinyl alcohol (PVA) and phenylboronic-acid-grafted alginate (PBA-Alg) were easily applied to surfaces and subsequently self-generated a hydrogel based on the PBA-diol ester bond. Compared to the strippable coating and chemical gels, the PBA-diol ester bond-based hydrogel was easily peeled off the surfaces without a drying step due to its high elasticity, which is more practical and time saving. The resulting hydrogel displayed high ¹³⁷Cs removal efficiencies of 91.61% for painted cement, 97.505% for aluminum, 94.05% for stainless steel, and 53.5% for cement, which was 2.3 times higher than that of Decongel due to the presence of the adsorbent in the hydrogel having an excellent Cs distribution coefficient (3.34 × 10⁴ mL/g). Moreover, the volume of radioactive waste generated after the surface decontamination could be reduced by a simple magnetic separation of the adsorbent from the used hydrogel, which can reduce the waste disposal cost. Therefore, our hydrogel system has great potential as a new, cost-effective surface decontaminant in various nuclear industry fields including wide-area environmental remediation after a nuclear accident or terrorist attack.

Closantel nano-encapsulated polyvinyl alcohol (PVA) solutions

The influence of closantel on the rheological and physicochemical properties (particle size and by UV-Vis absorption spectroscopy) of PVA aqueous solutions is studied here. About 1% PVA aqueous solutions were prepared by varying the closantel content. The increase of closantel content led to a reduction in the particle size of final solutions. All the solutions were buffered at pH 7.4 and exhibited shear-thinning behavior. Furthermore, in oscillatory flow, a "solid-like" type behavior was observed for the sample containing 30 μg/mL closantel. Indicating a strong interaction between the dispersed and continuous phases and evidencing an interconnected network between the nanoparticle and PVA, this sample also showed the highest shear viscosity and higher shear thinning slope, indicating a more intrincate structure disrupted by shear. In conclusion, PVA interacts with closantel in aqueous solution and the critical concentration for closantel encapsulation by PVA was about 30 μg/mL; above this concentration, the average particle size decreased notoriously which was associated to closantel interacting with the surface of the PVA aggregates and thus avoiding to some extent direct polymer-polymer interaction.

Selective formation of organo, organo-aqueous, and hydro gel-like materials from partially hydrolysed poly(vinyl acetate)s based on different boron-containing crosslinkers

Viscoelastic, gel-like, polymeric dispersions (HVPDs) can be prepared by crosslinking polyols with borax or boric acid in water under alkaline conditions. Rheologically similar HVPDs have been prepared in organic liquids containing no water or hydroxylic groups through crosslinking partially or fully hydrolysed poly(vinyl acetate)s with trimethyl borate, boric acid, or borax. The organo-HVPDs are water-sensitive and rheoreversible on exposure to water. They were characterised rheologically and by solution and solid-state (11)B NMR spectroscopy. Spectroscopic analyses show the presence of mono- and di-diol crosslinks, as well as non-crosslinked boron species in HVPDs prepared with trimethyl borate or boric acid. The number of crosslinks in organo-HVPDs prepared with borax increased over the course of several days. Results from solution and solid-state (11)B NMR spectroscopy are comparable; no solid-like component was detectable. We demonstrate that hydro, organo, or organo-aqueous HVPDs can be obtained from partially hydrolysed poly(vinyl acetate)s by 'tuning' the structure of the boron-based crosslinker.

High-water-content mouldable polyvinyl alcohol-borax hydrogels reinforced by well-dispersed cellulose nanoparticles: dynamic rheological properties and hydrogel formation mechanism

Cellulose nanoparticle (CNP) reinforced polyvinyl alcohol-borax (PB) hydrogels were produced via a facile approach in an aqueous system. The effects of particle size, aspect ratio, crystal structure, and surface charge of CNPs on the rheological properties of the composite hydrogels were investigated. The rheological measurements confirmed the incorporation of well-dispersed CNPs to PB system significantly enhanced the viscoelasticity and stiffness of hydrogels. The obtained free-standing, high elasticity and mouldable hydrogels exhibited self-recovery under continuous step strain and thermo-reversibility under temperature sweep. With the addition of cellulose I nanofibers, a 19-fold increase in the high-frequency plateau of storage modulus was obtained compared with that of the pure PB. CNPs acted as multifunctional crosslinking agents and nanofillers to physically and chemically bridge the 3D network hydrogel. The plausible mechanism for the multi-complexation between CNPs, polyvinyl alcohol and borax was proposed to understand the relationship between the 3D network and hydrogel properties.

Structural and mechanical properties of "peelable" organoaqueous dispersions with partially hydrolyzed poly(vinyl acetate)-borate networks: applications to cleaning painted surfaces

The preparation and structural characterization of a family of viscoelastic dispersions of borate cross-linked, 80% hydrolyzed poly(vinyl acetate) (80PVAc) in aqueous-organic liquids are presented. Correlations between mechanical properties (from rheological measurements) and the degree and nature of cross-linking (from (11)B NMR spectroscopy) are reported, and the results are used to assess their potential as low-impact cleaning agents for the surfaces of paintings. Because the dispersions can be prepared at room temperature by simple procedures from readily available materials and can contain up to 50% (w/w) of an organic liquid, they offer important advantages over previously described cleaning agents that are based on fully hydrolyzed PVAc (i.e., poly(vinyl alcohol). The mechanical properties of the various aqueous-organic dispersions, as determined quantitatively by rheological investigations and qualitatively by their ease of removal from a solid surface (i.e., the so-called "peel-off" ability) have been tuned systematically by varying the amount of organic liquid, its structure, and the concentrations of borax and 80PVAc. The (11)B NMR studies demonstrate that the concentration of borate ions actively participating in cross-linking increases significantly with the amount of organic liquid in the mixture. The degree of cross-linking remains constant when the 80PVAc and borax concentrations are varied, as long as their ratios are kept constant. Some of the 80PVAc-borax dispersions have been tested successfully as cleaning agents on the surface of a 16th-17th century oil-on-wood painting by Lodovico Cardi, "Il Cigoli", that was covered by a brown patina and on the surface of a Renaissance wall painting by Vecchietta in Santa Maria della Scala, Siena, Italy, that had a degraded polyacrylate coating from a previous conservation treatment.

Cosolvent gel-like materials from partially hydrolyzed poly(vinyl acetate)s and borax

A gel-like, high-viscosity polymeric dispersion (HVPD) based on cross-linked borate, partially hydrolyzed poly(vinyl acetate) (xPVAc, where x is the percent hydrolysis) is described. Unlike hydro-HVPDs prepared from poly(vinyl alcohol) (PVA) and borate, the liquid portion of these materials can be composed of up to 75% of an organic cosolvent because of the influence of residual acetate groups on the polymer backbone. The effects of the degree of hydrolysis, molecular weight, polymer and cross-linker concentrations, and type and amount of organic cosolvent on the rheological and structural properties of the materials are investigated. The stability of the systems is explored through rheological and melting-range studies. (11)B NMR and small-angle neutron scattering (SANS) are used to probe the structure of the dispersions. The addition of an organic liquid to the xPVAc-borate HVPDs results in a drastic increase in the number of cross-linked borate species as well as the agglomeration of the polymer into bundles. These effects result in an increase in the relaxation time and thermal stability of the networks. The ability to make xPVAc-borate HVPDs with very large amounts of and rather different organic liquids, with very different rheological properties that can be controlled easily, opens new possibilities for applications of PVAc-based dispersions.

Inhibition of the transport of HIV in vitro using a pH-responsive synthetic mucin-like polymer system

In conjunction with the routine role of delivering the active ingredient, carefully designed drug delivery vehicles can also provide ancillary functions that augment the overall efficacy of the system. Inspired by the ability of the cervicovaginal mucus to impede the movement of HIV virions at acidic pH, we have engineered a pH-responsive synthetic polymer that shows improved barrier properties over the naturally occurring cervicovaginal mucus by inhibiting viral transport at both acidic and neutral pH. The pH-responsive synthetic mucin-like polymer is constructed with phenylboronic acid (PBA) and salicylhydroxamic acid (SHA), each individually copolymerized with a 2-hydroxypropyl methacrylamide (pHPMA) polymer backbone. At pH 4.8, the crosslinked polymers form a transient network with a characteristic relaxation time of 0.9 s and elastic modulus of 11 Pa. On addition of semen, the polymers form a densely crosslinked elastic network with a characteristic relaxation time greater than 60 s and elastic modulus of 1800 Pa. Interactions between the PBA-SHA crosslinked polymers and mucin at acidic pH showed a significant increase in elastic modulus and crosslink lifetime (p < 0.05). A transport assay revealed that migration of HIV and cells was significantly impeded by the polymer network at pH ≥ 4.8 with a diffusion coefficient of 1.60 x 10(-4) μm(2)/s for HIV. Additionally, these crosslinked polymers did not induce symptoms of toxicity or irritation in either human vaginal explants or a mouse model. In summary, the pH-responsive crosslinked polymer system reported here holds promise as a class of microbicide delivery vehicle that could inhibit the transport of virions from semen to the target tissue and, thereby, contribute to the overall activity of the microbicide formulation.

New frontiers in materials science for art conservation: responsive gels and beyond

The works of art and artifacts that constitute our cultural heritage are subject to deterioration, both from internal and from external factors. Surfaces that interact with the environment are the most prone to aging and decay; accordingly, soiling is a prime factor in the degradation of surfaces and the attendant disfigurement of a piece. Coatings that were originally intended to protect or contribute aesthetically to an artwork should be removed if they begin to have a destructive impact on its appearance or surface chemistry. Since the mid-19th century, organic solvents have been the method of choice for cleaning painted surfaces and removing degraded coatings. Care must be taken to choose a solvent mixture that minimizes swelling of or leaching from the original paint films, which would damage and compromise the physical integrity of all the layers of paint. The use of gels and poultices, first advocated in the 1980s, helps by localizing the solvent and, in some cases, by reducing solvent permeation into underlying paint layers. Unfortunately, it is not always easy to remove gels and their residues from a paint surface. In this Account, we address the removal problem by examining the properties of three classes of innovative gels for use on artwork--rheoreversible gels, magnetic gels, and "peelable" gels. Their rheological properties and efficacies for treating the surfaces of works have been studied, demonstrating uniquely useful characteristics in each class: (1) Rheoreversible gels become free-flowing on application of a chemical or thermal "switch". For art conservation, a chemical trigger is preferred. Stable gels formed by bubbling CO(2) through solutions of polyallylamine or polyethylenimines (thereby producing ammonium carbamates, which act as chain cross-links) can be prepared with a wide range of solvent mixtures. After solubilization of varnish and dirt, addition of a weak acid (mineral or organic) displaces the CO(2), and the resulting free-flowing liquid can be removed gently. (2) Incorporation of magnetic, coated-ferrite nanoparticles into polyacrylamide gels adds functionality to a versatile system comprising oil-in-water microemulsions, aqueous micellar solutions, or xerogels that act as sponges. The ferrite particles allow the use of magnets both to place the gels precisely on a surface and to lift them from it after cleaning. (3) Novel formulations of poly(vinyl alcohol)-borate gels, which accept a range of organic cosolvents, show promise for swelling and dissolving organic coatings. This family of gels can be quite stiff but can be spread. They are non-sticky and have sufficient strength to be removed by peeling or lifting them from a sensitive surface. These three classes of gels are potentially very important soft materials to augment and improve the range of options available for conserving cultural heritage, and their interesting chemical-physical properties open a rich area for future scientific investigation.

Complex hydrogel systems composed of polymers, liposomes, and cyclodextrins: implications of composition on rheological properties and aging

Rheological properties of complex hydrogels containing different amounts of liposomes and/or cyclodextrin (CD) were evaluated. Sonicated unilamellar vesicles (SUV) were loaded in a hydrogel composed of Carbopol 974 NF and hydroxyethylcellulose (Natrosol 250 HX). Phosphatidylcholine (PC) and hydrogenated-PC (HPC) liposomes, both mixed with cholesterol in a 2:1 lipid/chol mol ratio, were used. In some cases, hydroxypropyl-beta-cyclodextrin was also added (100 or 400 mg/mL). Gels were incubated at 40 degrees C/75% humidity for 7 days or 1 month to evaluate the effect of aging on their rheological properties. FTIR and DSC studies were performed to investigate possible interactions between the polymers and CD molecules at different CD concentrations. Static and dynamic rheological measurements were carried out. All gels had shear-thinning behavior (fitted well by the Cross model) with the exception of gels containing high concentrations of CD that were transformed into nonflowing elastic sticky solids, especially after aging. The more pronounced elastic behavior of gels containing 400 mg/mL CD is reflected by the higher values of relaxation strengths over all relaxation times. Complete interaction between polymers and CD, in the high-CD-content gels, as proven by FTIR and DSC studies, explains the dominating contribution of CD on gel characteristics. The addition of liposomes to such CD-containing gels has a substantial effect on their rheological properties, which are dependent on the liposome type (HPC/chol liposomes > PC/chol) and the lipid/CD ratio. This is explained by the "neutralization" of some CD molecules that prefer to interact with chol molecules that they extract from the lipid membranes. Gels with a high CD concentration (400 mg/mL) are almost insensitive to aging, whereas all other gels become slightly more elastic and less viscous as aging proceeds.

Poly(vinyl alcohol)-borate hydro/cosolvent gels: viscoelastic properties, solubilizing power, and application to art conservation

We report the development of a new type of hydrogel in which a cosolvent has been added to the water component. The gel networks are based on the well-known poly(vinyl alcohol)-borate systems (PVA-borate). However, it is shown that the rheological and solubilizing properties of the hydrogels can be modified drastically by the addition of a cosolvent. The studies have focused on 1-propanol as the added liquid, although it is shown that others (propylene carbonate, 1-pentanol, cyclohexanone, and 2-butanol) are amenable to making modified hydrogels as well. In addition to the rheological measurements, the gels have been investigated by differential scanning calorimetry (free water index) and determination of their solubilizing power. Finally, the gels have been applied to clean and oxidized varnish (patina) from the surface of a XVI-XVII century oil-on-wood painting by Ludovico Cardi detto il Cigoli. The mode of cleaning by and removal of the PVA-borate water/1-propanol gel from the painted surface demonstrate several advantages over other gels used in art conservation.

Chemorheology of phenylboronate-salicylhydroxamate crosslinked hydrogel networks with a sulfonated polymer backbone

Hydrogel networks crosslinked with polymer-bound phenylboronic acid (PBA) and salicylhydroxamic acid (SHA) demonstrate pH-reversible gel behavior due to the pH-dependent equilibrium of the crosslinking moieties that form the gel network. Furthermore, the pH at which gels behave dynamically can be controlled by use of a polyelectrolyte backbone. Here we report on the frequency-dependent chemorheological characterization of PBA-SHA crosslinked hydrogel networks with a sulfonated polymer backbone. Our results suggest that the anionic nature of the polymers allows reversible crosslinking at neutral pH that an otherwise neutral-backboned PBA-SHA crosslinked network cannot, and that these charge-induced dynamics can be effectively screened by ions in solution. Moreover, moduli-frequency data can effectively be reduced into a single master curve with a neutral-backboned PBA-SHA gel data set as the reference condition.

Rheological properties of protein-surfactant based gels

Water-based protein-surfactant gels, formed by mixing bovine serum albumin (BSA) and sodium dodecyl sulfate in water, were investigated by rheological methods. The measurements were performed for many different protein-to-surfactant ratios as a function of the applied frequency, stress, or strain, as well as by changing the temperature, in the range between 15 and 65 degrees C. The rheological behavior of the gels as a function of applied frequency is interpreted in terms of the overlapping of at least two viscoelastic relaxation processes. The rheological results indicate the presence of thermal transitions from essentially viscous to mainly elastic regimes, in analogy with the thermal gelation processes observed in polymer solutions. The thermal gelation threshold in the present system is modulated by the protein/surfactant ratio. Differential scanning calorimetry measurements were also performed to determine whether thermal gelation is somehow concomitant to protein denaturation. The results indicate that the thermal denaturation of BSA in protein-surfactant based gels occurs at slightly higher temperatures than in the bulk. Scanning electron microscopy indicates the occurrence in the gel structure of globules formed by the arrangement of fibrils.