Simultaneous measurement of mechanical and surface properties in thermoresponsive, anchored hydrogel films

Hydrogel films have been used extensively in the preparation of biosensors and biomedical devices. The characteristics of the aqueous interface of the polymer layer are significant for the biosensor or device function; likewise, the changing mechanical properties of thermoresponsive polymers are an important feature that affects the polymer behavior. Atomic force microscopy was used here to characterize both the surface and the mechanical properties of polymeric hydrogel films prepared from a thermoresponsive terpolymer of N-isopropylacrylamide and acrylic acid with benzophenonemethacrylate as a photoreactive cross-linker comonomer. The force-distance curves thus obtained were analyzed to assess both the surface forces and the mechanical response that were associated with the hydrogel. These properties were investigated as a function of temperature, in water and in Tris buffer, for different degrees of polymer cross-linking. For samples in water, the distance over which the surface forces were effective was found to remain constant as the temperature was increased from 26 to 42 °C, even though the mechanical response indicated that the samples had been heated past the lower critical solution temperature, or LCST. The bulk of the polymer becomes less soluble above the LCST, although this does not seem to affect the surface properties. This may be due to the segregation of the acrylic acid-rich polymer segments near the gel surface, which is in agreement with reports for related systems.     

Electroactive hydrogels based on poly(acrylic acid) and polypyrrole

This study is concerned with the preparation of novel composite systems in which the hydrogel of crosslinked poly(acrylic acid) is a matrix and polypyrrole is an electroconducting component. The effects of synthesis conditions on the structure of the composite systems, their ability to swell in water, and mechanical characteristics are studied. The proposed synthesis procedure allows production of a bulk electroconducting phase of polypyrrole in the hydrogel matrix. The effect of crosslink density of hydrogel on the character of polypyrrole distribution in the matrix is studied. The above composites combine the electrical characteristics of polypyrrole with high elasticity and the ability to repeatedly swell the crosslinked poly(acrylic acid).     

Synthesis of millimeter‐sized hydrogel beads by inverse Pickering polymerization using starch‐based nanoparticles as emulsifier

The preparation of millimeter‐sized poly(acrylamide‐co‐acrylic acid) hydrogel beads via inverse Pickering emulsion polymerization using starch‐based nanoparticles (SNPs) as stabilizers is reported. Amphiphilic starch is fabricated by the introduction of butyl glycidyl ether groups and palmitate groups, and the hydrophobically modified SNPs are fabricated by a nanoprecipitation process. The obtained SNPs could adsorb at oil‐water interfaces to stabilize an inverse Pickering emulsion, and the effects of oil/water volume fraction ratio and SNP concentration on emulsions are comprehensively studied. Poly(acrylamide‐co‐acrylic acid) hydrogel beads with a size of approximately 1 mm are obtained by inverse Pickering emulsion polymerization stabilized by SNPs. The morphology and structure of hydrogel beads are extensively investigated, which confirms that SNPs locate on the surface of hydrogel beads and act as emulsifiers and network structures present inside the beads. Polymerization is also detected to investigate the potential formation mechanism of hydrogel beads. The pH‐responsive property of hydrogel beads and its potential application for drug delivery are also explored.     

Biodegradable and pH-sensitive hydrogels: Synthesis by crosslinking of N,N-dimethylacrylamide copolymer precursors

Novel pH-sensitive hydrogels containing azoaromatic crosslinks were synthesized by the crosslinking of polymeric precursors. First, a reactive polymeric precursor was synthesized by copolymerization of N,N-dimethylacrylamide, N-tert-butylacrylamide, acrylic acid, and N-methacryloylglycylglycine p-nitrophenyl ester. The hydrogel was prepared in the second step by the reaction of the polymeric precursor with N,N′-(ω-aminocaproyl)-4,4′-diaminoazobenzene. The hydrogels were characterized by the network structure, (that is, content of crosslinks, unreacted pendent groups, and cycles), the equilibrium swelling ratio as a function of pH, modulus of elasticity in compression, and the degradability in vitro. The results obtained indicated that the hydrogel network structure strongly depends on the reaction conditions such as polymer concentration, and the ratio of the reactive groups during the crosslinking reaction. The swelling and mechanical properties of hydrogels can be controlled by the modification of polymer backbone structure and/or the crosslinking density. The rates of hydrogel degradation depended on their degree of swelling. The higher the degree of swelling, the higher the degradability. The properties of the hydrogels suggest that they have a potential as carriers for colon-specific drug delivery. © 1994 John Wiley & Sons, Inc.     

Scleroglucan: a versatile polysaccharide for modified drug delivery

Scleroglucan is a natural polysaccharide, produced by fungi of the genus Sclerotium, that has been extensively studied for various commercial applications (secondary oil recovery, ceramic glazes, food, paints, etc.) and also shows several interesting pharmacological properties. This review focuses its attention on the use of scleroglucan, and some derivatives, in the field of pharmaceutics and in particular for the formulation of modified-release dosage forms. The reported investigations refer mainly to the following topics: natural scleroglucan suitable for the preparation of sustained release tablets and ocular formulations; oxidized and crosslinked scleroglucan used as a matrix for dosage forms sensitive to environmental conditions; co-crosslinked scleroglucan/gellan whose delivery rate can be affected by calcium ions. Furthermore, a novel hydrogel obtained with this polysaccharide and borate ions is described, and the particular structure of this hydrogel network has been interpreted in terms of conformational analysis and molecular dynamics. Profound attention is devoted to the mechanisms involved in drug release from the tested dosage forms that depend, according to the specific preparation, on swelling and/or diffusion. Experimental data are also discussed on the basis of a mathematical approach that allows a better understanding of the behavior of the tested polymeric materials.     

Radiation Synthesis of Superabsorbent Hydrogels Based on Chitosan and Acrylic Acid for Controlled Drug Release

Superabsorbent hydrogels based on the natural polymer chitosan and acrylic acid (CS/AAc) was prepared using ⁶⁰Co gamma radiation as a source of initiation and crosslinking. The factors, which affect the preparation of CS/AAc hydrogels such as irradiation dose, CS/AAc ratios, and acrylic acid monomer concentrations, to get the best optimum conditions, were investigated. The kinetic studies of the swelling of CS/AAc hydrogel showed that it follows a Fickian type of water diffusion. The Fickian constant value ‘n’ was more than 0.5 with a high swelling capacity of 300 g/g as superabsorbent hydrogel. In addition, the suitability of CS/AAc hydrogel as carrier material for the drug Chlortetracycline-HCl has been investigated by adsorption isotherm studies. The performance of drug release from hydrogel systems, influenced by acrylic acid ratio and the effect of pH of the medium was studied.     

Comprehensive Examination of Mechanical and Diffusional Effects on Cell Behavior Using a Decoupled 3D Hydrogel System

Hydrogels possess several physical and chemical properties suitable for engineering cellular environments for biomedical applications. Despite recent advances in hydrogel systems for cell culture, it is still a significant challenge to independently control the mechanical and diffusional properties of hydrogels, both of which are well known to influence various cell behaviors when using hydrogels as 3D cell culture systems. Controlling the crosslinking density of a hydrogel system to tune the mechanical properties inevitably affects their diffusional properties, as the crosslinking density and diffusion are often inversely correlated. In this study, a polymeric crosslinker is demonstrated that allows for the adjustment of the degree of substitution of reactive functional groups. By using this polymeric crosslinker, the rigidity of the resulting hydrogel is controlled in a wide range without changing the polymer concentration. Furthermore, their diffusional properties, as characterized by their swelling ratios, pore diameters, and drug release rates, are not significantly affected by the changes in the degree of substitution. 3D cell studies using this hydrogel system successfully demonstrate the varying effects of mechanical properties on different cell types, whereas those in a conventional hydrogel system are more significantly influenced by changes in diffusional properties.     

Polymeric photoinitiators based on side-chain benzoin methyl ether and tertiary amine moieties for fast UV curable coatings

The synthesis is reported of a new series of polymeric photoinitiators obtained by copolymerization of a-methylolbenzoin methyl ether acrylate (MBA) with different N,N-dialkylamino alkyl acrylates. The copolymers have been fully characterized and employed in the photoinitiated polymerization and crosslinking reactions of a standard acrylic formulation for clear curable coatings. The photoinitiation activity of the above systems has been measured by using microwave dielectrometry and differential photocalorimetry, and compared with that found for the corresponding low-molecular-weight models. The results clearly indicate that the copolymeric systems display a remarkable decrease of the induction period and an improved overall activity in the UV curing of the acrylic coatings. An interpretation of the experimental findings is also reported.     

Molecular mobility in micellar complexes of a nonionic surfactant and the poly(acrylic acid)-based hydrogel

The interaction between the polyelectrolyte gel of crosslinked poly(acrylic acid) (PAA) and nonionic surfactant Brij 58 based on poly(ethylene glycol) (C₁₆H₃₃(CH₂CH₂O)₂₀OH) is studied. It is established that poly(acrylic acid)-surfactant complexes are formed. Nondissociated carboxyl groups of poly(acrylic acid) and oxygen atoms of the surfactant are involved in the complexation. Surfactant micelles are a kind of bridge that connects polymer chains. The presence of the surfactant decreases the equilibrium swelling of the hydrogel. The spin probe method is employed to determine the local mobility of the hydrocarbon core of a micelle in the complex. It was shown that the local mobility is independent of the hydrogel crosslink density and is much lower in acidic than in alkaline media. In acidic media, much more surfactant molecules of micelles are involved in the complexation than in alkaline media. However, even in alkaline media, surfactant micelles cannot leave the hydrogel, while the spin probes located in micelles are at equilibrium with the spin probes present in the external aqueous medium. The prospects for applying the considered systems as carriers for controlled release drugs are discussed.     

Polymeric flocculants processing by accelerated electron beams and microwave heating

Results obtained by accelerated electron beam, microwave and simultaneous microwave and electron beam application in the chemistry of acrylamide and acrylic acid copolymers (polymeric flocculants used for wastewater treatment) are presented. Comparative results concerning the molecular weight and Huggins’ constant for the acrylamide and acrylic acid copolymers obtained by classical heating, microwave heating, electron beam irradiation and simultaneous microwave and electron beam treatment are reported. Microwave heating produces high water solubility of the polymeric flocculants but median molecular weight values. Electron beam irradiation gives high molecular weight values but associated with a cross-linked structure (poor water solubility) while microwave energy addition to electron beam energy gives simultaneously high molecular weight values and high water solubility.     

Penetration of3H-glycerol trinitrate through silicone-hydroxyethyl methacrylate matrices

The possibility of use of a powder-like polymeric hydrogel of 2-hydroxyethyl methacrylate with 33 wt.% methacrylic acid as potential pharmaceutical adjuvant in the form of a composite powdered material, with silicone rubber in the form of matrices for transdermal therapeutic system has been studied. The effect of solvents and the content of composite hydrogel in matrices on the rate of permeation of an antianginic drug, radioactive labeled glycerol trinitrate (GTN) was evaluated. The results show that hydrogel copolymers are prospective adjuvants for transdermal therapeutic systems.     

A comparative study of the potential of acrylic and sol-gel polymers for molecular imprinting

The successful molecular imprinting of 2-aminopyridine (2-apy) in bulk polymerisations of acrylic and sol-gel based polymers has been achieved. Both polymeric systems reveal varying degrees of affinity in rebinding the original template as well as a number of structural analogues. Rebinding was conducted in chloroform, acetonitrile and methanol in order to assess the role of hydrogen bonding in imprinting. The acrylic imprinted polymer retained approximately 50% of the template in rebinding studies in chloroform compared to 100% for the sol-gel. However, this higher affinity for the sol-gel was accompanied by a higher degree of non-specific binding. While the acrylic polymer performed poorly in acetonitrile, the sol-gel maintained a high degree of discrimination.The acrylic polymer exhibited little discrimination between imprinted and reference polymers for 3-aminopyridine (3-apy) indicating the high selectivity of the MIP polymer for 2-apy relative to 3-apy. This selectivity was reduced in acetonitrile. Selectivity of the sol-gel for 2-apy in chloroform was poor as 3-apy was retained to a similar degree. Comparable results were obtained in acetonitrile. 4-Aminopyridine (4-apy) bound strongly to all polymers in all solvents and proved very difficult to remove due to the high degree of non-specific binding for both polymeric matrices.     

导电水凝胶的制备

作为一种新型的功能材料,导电水凝胶已经引起广泛的关注。本文根据目前的研究现状,将导电水凝胶大致分为聚电解质导电水凝胶,酸掺杂导电水凝胶,无机物添加导电水凝胶以及导电高分子基导电水凝胶等几大类,并综述了它们的制备方法。另外,由于大分子体系的导电高分子和水凝胶都有着独特和重要的性能,这使得它们具有广阔的应用价值。所以,本文在综述导电水凝胶制备进展的同时着重综述了导电高分子基导电水凝胶的制备进展。     

Application of sensitive hydrogels in chemical and pH sensors

In the present work, pH-sensitive poly(vinyl alcohol)/poly(acrylic acid) (PVA/PAA) blends as well as hydrogels based on poly(N-isopropylacrylamide) (PNIPAAm), which are sensitive to organic solvent concentration in aqueous solutions, were used in silicon micromachined sensors. A sensitivity of approximately 15 mV/pH was obtained for a pH sensor with a 50 μm thick PVA/PAA hydrogel layer in a pH range above the acid exponent of acrylic acid (pK_a=4.7). The output voltage versus pH-value characteristics and the long-term signal stability of hydrogel-based sensors were investigated and the measurement conditions necessary for high signal reproducibility were determined. The influence of the preparation conditions of the hydrogel films on the sensitivity and response time of the chemical and pH sensors is discussed.     

Preparation and characterization of a novel stimuli-responsive nanocomposite hydrogel with improved mechanical properties

A novel stimuli-responsive organic/inorganic nanocomposite hydrogel (NC hydrogel) with excellent mechanical properties was synthesized by in situ polymerization of 2-(2-methoxyethoxy) ethyl methacrylate (MEO(2)MA), oligo (ethylene glycol) methacrylate (OEGMA) and acrylic acid (AAc), as the polymeric matrix (PMOA), and fibrillar attpulgite (AT), as the reinforcer and cross-linker. The effect of the AT content on the mechanical properties for the swollen and dried NC hydrogels was determined by tensile testing and dynamic mechanical analysis (DMA), respectively. The tensile testing results showed that the incorporation of AT nanoparticles significantly enhanced the mechanical properties of NC hydrogels. As the content of AT increased, the tensile strength, tensile modulus and effective cross-linked chain density increased. The DMA results showed that the storage modulus of AT/PMOA NC hydrogels was increased and the glass transition temperatures shifted to higher temperature compared to the pure PMOA hydrogel, which further indicated that the enhancement of mechanical property depended upon the presence and content of AT. In addition, the faster swelling rates of the NC hydrogels were observed in comparison with the corresponding physically cross-linked PMOA hydrogel, except for 1% AT/PMOA sample. However, the deswelling kinetics of NC hydrogels was obviously retarded.     

Synthesis and characterization of magnetic poly(acrylic acid) hydrogel fabricated with cobalt nanoparticles for adsorption and catalytic applications

The present study aimed to synthesize poly(acrylic acid) hydrogel embedded with magnetic cobalt (Co) nanoparticles and to investigate their potential in adsorption and catalysis. The hydrogel was prepared by facile free radical polymerization reaction and Co nanoparticles were fabricated within hydrogel by reducing Co (II) ions using NaBH₄ as reducing agent. Co nanoparticles within hydrogel system imparted magnetic properties to the resulting composite gel and also increased the adsorption capacity. The swelling study of hydrogel was carried out by gravimetric analysis. Different functional groups were identified by Fourier Transform Infrared Spectroscopy and Transmission Electron Microscopy analysis was done to investigate dispersion of Co nanoparticles in hydrogel. The bare hydrogel along with Co nanoparticles loaded gel were tested as adsorbent systems for the removal of a cationic dye (methylene blue) from aqueous solution. 95% removal of methylene blue was achieved with a highest adsorption capacity of 836.5 mg/g of adsorbent. The famous adsorption isotherms were used to evaluate adsorption data. Results showed that Freundlich isotherm model was followed with R² value of 0.95. The hydrogel was also used for catalytic reduction in a toxic pollutant, i.e., 4-nitrophenol. Experimental data for 4-nitrophenol reduction followed pseudo first order kinetics model. Activation energy and apparent rate constant were calculated as 9.24 kJ/mol and 0.24 min⁻¹, respectively. Recycling of the magnetic poly(acrylic acid) hydrogel fabricated with Cobalt nanoparticles was carried out for four consecutive cycles and no significant loss in catalytic activity was observed.

     

Continuous fabrication of biocatalyst immobilized microparticles using photopolymerization and immiscible liquids in microfluidic systems

We report a novel technique for manufacturing polymeric microparticles containing biocatalysts by the behavior of immiscible liquids in microfluidic systems and in situ photopolymerization. The approach utilizes a UV-polymerizable hydrogel/enzyme solution and an immiscible oil solution. The oil and hydrogel solutions form emulsions in pressure-driven flow in microchannels at high values of the dimensionless capillary number (Ca). The resultant hydrogel droplets are then polymerized in situ via exposure to 365 nm UV light. This technique allows for the generation of monodisperse particles whose size can be controlled by the regulation of flow rates. In addition, both manufacturing microparticles and immobilizing biocatalysts can be performed simultaneously and continuously.     

Synthesis and Swelling Behavior of Acrylate‐Based Hydrogels

The purpose of this investigation was to report the synthesis of a novel pH‐sensitive acrylate‐based hydrogel by polymerizing the comonomers 2‐hydroxyethyl methacrylate, HEMA, acrylic acid, AA, and sodium acrylate, NaAc. The NaAc component was obtained by neutralization of AA with sodium hydroxide. Hydrogels were obtained by free radical copolymerization in aqueous solution in the presence of redox initiators, Na₂S₂O₈/Na₂S₂O₅, and ethylene glycol dimethacrylate, EGDMA, crosslinker. The copolymers were synthesized by varying neutralization percent of AA in the range of 10–100. The swelling behavior of the copolymeric gels were investigated as a function of pH, temperature, ionic strength, and AA neutralization percent. The polymer mesh size, ξ, molecular weight between crosslinks, M_c , and crosslinking density, q, were determined by using the Flory‐Rehner equation in the pH range of 2–8 as 8.78–48.8 Å, 209–2667 g/mol, and 0.046–0.59, respectively. The diffusional exponent value, n, of the synthesized hydrogel was found to be 0.59, indicating a non‐Fickian diffusion mechanism. It can be concluded that the hydrogel demonstrated a sharp change in its water absorbency, mesh size and molecular weight between crosslinks of the network with a change in pH of the swelling media. The latter properties suggest strong consideration of these hydrogels for use as oral drug delivery systems and ion‐exchangers for removal of metal ions from aqueous media, owing to the carboxylate groups within the polymeric network.     

Stabilisation Effect of Calcium Ions on Polymer Network in Hydrogels Derived from a Lyotropic Phase of Hydroxypropylcellulose

The process of crosslinking of the hydrogel, derived from lyotropic liquid crystalline (LLC) phases of hydroxypropylcellulose/acrylic acid-water, by calcium ions was studied by means of Raman and dielectric spectroscopy. Formation of salt by poly(acrylic acid) and calcium ions, resulting in hydrogel crosslinking, induces differences in Raman spectra of the hydrogel before and after the crosslinking. The crosslinking results in significant increase in the activation energy of β-relaxation of poly(acrylic acid). This is a direct consequence of restriction in motions of carboxylic groups of poly(acrylic acid) due to calcium salt formation. Thus, the crosslinking improves polymer network stability in the hydrogel in the swollen state.     

A biospecific polymeric carrier for polypeptide drugs

Polymeric systems for targeting polypeptide medicinal drugs onto the wall of the small intestine after oral administration were synthesized. These systems are composed of particles of ovomucoid-modified polyacrylamide hydrogel with insulin physically immobilized in its bulk. In vitro experiments have demonstrated the enhanced stability of the immobilized polypeptide against proteolysis and the affinity of polymeric particles to lectin-containing surfaces. The efficiency of the systems obtained was demonstrated in experiments with animals and diabetes patients. The effrct of these systems upon oral administration was qualitatevely similar to that of subcutaneously administered native insulin.__________Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 11, pp. 2504–2509, November, 2004.