Polyacrylamide-based Hydrogels Research Articles

Ni ferrite highly organized as humidity sensors

Synthesis and characterization of nickel ferrite using polyacrylamide-based hydrogels as template agents for obtaining NiFe2O4 by the sol–gel autocombustion method was studied, the investigation representing a novelty in the field.The polyacrylamide-based hydrogel used as a template agent, was synthesized by an original simultaneous polymerization-crosslinking method, using a monofunctional crosslinking agent. The ferrite was synthesized by the sol–gel autocombustion method, using citric acid as a chelating/fuel agent. The as-obtained hydrogel was characterized by IR, SEM and thermogravimetric techniques, as well as swelling behavior. As-synthesized ferrites were characterized by IR, SEM, XRD techniques, as well as by electrical and magnetic properties. The electrical investigations were carried out in different humidity conditions in order to highlight the influence of humidity on electrical properties, in this respect the prepared NiFe2O4 with highly organized structure was tested for applications as humidity sensors.

Rheological and mechanical behavior of polyacrylamide hydrogels chemically crosslinked with allyl agarose for two-dimensional gel electrophoresis

Two-dimensional (2-D) gel electrophoresis currently represents one of the most standard techniques for protein separation. In addition to the most commonly employed polyacrylamide crosslinked hydrogels, acrylamide–agarose copolymers have been proposed as promising systems for separation matrices in 2-D electrophoresis, because of the good resolution of both high and low molecular mass proteins made possible by careful control and optimization of the hydrogel pore structure. As a matter of fact, a thorough understanding of the nature of the hydrogel pore structure as well as of the parameters by which it is influenced is crucial for the design of hydrogel systems with optimal sieving properties. In this work, a series of acrylamide-based hydrogels covalently crosslinked with different concentrations of allyl agarose (0.2–1%) is prepared and characterized by creep-recovery measurements, dynamic rheology and tensile tests, in the attempt to gain a clearer understanding of structure–property relationships in crosslinked polyacrylamide-based hydrogels. The rheological and mechanical properties of crosslinked acrylamide–agarose hydrogels are found to be greatly affected by crosslinker concentration. Dynamic rheological tests show that hydrogels with a percentage of allyl agarose between 0.2% and 0.6% have a low density of elastically effective crosslinks, explaining the good separation of high molecular mass proteins in 2-D gel electrophoresis. Over the same range of crosslinker concentration, creep-recovery measurements reveal the presence of non-permanent crosslinks in the hydrogel network that justifies the good resolution of low molecular mass proteins as well. In tensile tests, the hydrogel crosslinked with 0.4% of allyl agarose exhibits the best results in terms of mechanical strength and toughness. Our results show how the control of the viscoelastic and the mechanical properties of these materials allow the design of mechanically stable hydrogels with improved sieving ability in protein electrophoresis over a wide range of molecular masses.

Organic-inorganic interpenetrating polymer networks and hybrid polymer materials prepared by frontal polymerization

Novel polyacrylamide-based hydrogels containing 3-(trimethoxysilyl)propyl methacrylate and/or tetraethoxy silane were synthesized by means of frontal polymerization, using ammonium persulfate as initiator, N,N′-methylene bisacrylamide as crosslinking agent and dimethyl sulfoxide as solvent. The obtained samples were treated at pH of 2 or 5 to induce the sol–gel reaction and evaluate their swelling behavior in the conditions. The occurrence of this reaction was assessed by solid-state NMR. Moreover, the thermal properties of the dry materials were studied by differential scanning calorimetry and thermal gravimetric analysis, and their water-contact angles were measured. It was found that the amount of Si affects the extent of swelling and the hydrophilicity of the resulting materials. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013, 51, 4618–4625

Tailoring the adhesion properties of polyacrylamide-based hydrogels. Application for skin contact

In the present study, we investigated the adhesive performances of polyacrylamide-based hydrogels intended to be used as dermatological patches. Accordingly, we have prepared conventional copolymer poly(acrylamide-co-hydroxyethyl methacrylate) hydrogels and nanocomposite copolymer poly(acrylamide-co-hydroxyethyl methacrylate) hydrogels filled with poly(butyl acrylate) nanoparticles. We evaluated their adhesive properties when they were applied to different substrates (equivalent human skin, stainless steel) using a probe tack test. The adhesion energy was found to be related to the chemical composition and the rheological properties of the hydrogels which were also measured.

Photoswitchable gel assembly based on molecular recognition

The formation of effective and precise linkages in bottom-up or top-down processes is important for the development of self-assembled materials. Self-assembly through molecular recognition events is a powerful tool for producing functionalized materials. Photoresponsive molecular recognition systems can permit the creation of photoregulated self-assembled macroscopic objects. Here we demonstrate that macroscopic gel assembly can be highly regulated through photoisomerization of an azobenzene moiety that interacts differently with two host molecules. A photoregulated gel assembly system is developed using polyacrylamide-based hydrogels functionalized with azobenzene (guest) or cyclodextrin (host) moieties. Reversible adhesion and dissociation of the host gel from the guest gel may be controlled by photoirradiation. The differential affinities of α-cyclodextrin or β-cyclodextrin for the trans-azobenzene and cis-azobenzene are employed in the construction of a photoswitchable gel assembly system.

Molecular Imprinting of Maltose Binding Protein: Tuning Protein Recognition at the Molecular Level

Protein imprinting in hydrogels is one approach for developing artificial receptors capable of specific recognition and binding of a target molecule. Through selection of monomers with side groups that can interact with the target protein and control over the degree of cross-linking, the architecture and spatial distribution of interaction points can be optimized for a target protein. Here we report on the imprinting of polyacrylamide-based hydrogels with maltose binding protein (MBP). To design the optimum architecture, we analyze the distribution of surface amino acid residues on the protein surface. We show that the selectivity of MBP recognition is increased by incorporating monomers that can introduce sites for hydrogen bonding, hydrophilic interactions, and electrostatic interactions. MBP-imprinted films showed high specificity and could discriminate between reference proteins with similar molecular weight, dimensions, and isoelectric point.

Covalent grafting of polyacrylamide-based hydrogels to a polypropylene surface activated with functional polyperoxide

Polyacrylamide (PAAm)-based hydrogels covalently bonded to a polypropylene (PP) substrate have been synthesized using modification of the PP with a new functional polyperoxide (FPP). The FPP initiates radical polymerization from the PP’s surface by decomposing the peroxide groups. Two different approaches for forming the hydrogel network have been used. In the first, a grafted hydrogel network is formed by radical polymerization of the acrylamide on the peroxidized PP substrate (“grafting from”) in the presence of N, N′-methylene-bis-acrylamide (a cross-linking agent) and potassium persulfate (an additional initiator in bulk). In the second approach, a grafted hydrogel network is synthesized by: (i) grafting the PAAm macromolecules from the peroxidized PP substrate and (ii) following intermolecular condensation of the surface-grafted PAAm and adding to the reactive bulk PAAm and poly- N-(hydroxymethyl)acrylamide (used as a cross-linking macromolecule), resulting in the formation of a hydrogel network grafted to the PP substrate. The covalent attachment of hydrogel networks has been verified for both approaches by contact angle measurements, dye adsorption by the hydrogel residues after network removal and extraction, and mechanical failure tests. The intermolecular condensation method has been found to be more advantageous for the formation of covalently bonded hydrogels on the PP substrate, while grafted network formation by radical polymerization was detected only for a limited range of monomer and cross-linking agent concentrations.

Tailoring the mechanical properties of polyacrylamide-based hydrogels

Here, we studied the mechanical properties of polyacrylic acid-co-polyacrylamide (pAAc/pAAm) and polyacrylate-co-polyacrylamide (pNaAc/pAAm)-based hydrogels as a function of composition and total polymer content. While both polyacrylate and polyacrylic acid are weakly-charged polyelectrolytes with carboxylate side groups, they exhibit different behavior as copolymers with polyacrylamide. In swelling studies the highest degree of volumetric expansion occurs in the polyacrylate-rich hydrogels. Rheological measurements indicate that the shear storage modulus of the hydrogels typically increases with the percentage of polyacrylamide for a given polymer volume fraction; however, the simultaneous strengthening and embrittling effects of polyacrylamide as a copolymer are more dramatic for the pNaAc/pAAm hydrogels.

The Synthesis of Polyacrylamide-Based Multi-component Hydrogels. A Neural Network Modeling

The present paper deals with the synthesis and swelling behavior of the semi- and interpenetrated multi-component networks based on polyacrylamide. The networks were prepared by a “single step” process of polymerization/crosslinking and are constituted by a polyacrylamide matrix and a biodegradable interpenetrated polymer. The synthesis was carried out in aqueous medium, for pre-established conditions of temperature, duration and concentrations of the polymerization system components (monomer, initiator, crosslinking agent, inclusion polymer). The yield in crosslinked polymer, the swelling degree and the swelling rate constant were the parameters of interest in this experimental study. In order to evaluate the quantitative influence of the reaction conditions upon yield and swelling degree, different classes of neural networks are tested. Due to their performance, Jordan Elman networks–a special type of networks–were chosen. A trial and error method leads to a neural network topology with two hidden layers and different activation functions in context, intermediate and output layers. The predictions provided by this model are in good agreement with the experimental data. A general modeling strategy, useful for different polymerization processes, is also developed.

Mechanically Reinforcing Polyacrylate/Polyacrylamide Hydrogels through the Addition of Colloidal Particles

AbstractPolyacrylamide is a popular material for many bio-related applications ranging from electrophoretic separation to cellular supports. A limitation of polyacrylamide-based hydrogels, however, is their mechanical compliance. The current study examines the effect of colloidal particles as a reinforcing filler phase to enhance the mechanical stiffness of polyacrylamide-polyacrylate hydrogels. Measurements with oscillatory rheology show that for a fixed polymer volume fraction, the presence of colloidal particles with various surface modifications generally results in an increase of the shear storage modulus of the hydrogel-particle composite. Interestingly, this study indicated that no discernable trends can be linked between the values of the shear storage modulus and the particle surface characteristics.

Production of arrays of cardiac and skeletal muscle myofibers by micropatterning techniques on a soft substrate

Micropatterning and microfabrication techniques have been widely used to pattern cells on surfaces and to have a deeper insight into many processes in cell biology such as cell adhesion and interactions with the surrounding environment. The aim of this study was the development of an easy and versatile technique for the in vitro production of arrays of functional cardiac and skeletal muscle myofibers using micropatterning techniques on soft substrates. Cardiomyocytes were used for the production of oriented cardiac myofibers whereas mouse muscle satellite cells for that of differentiated parallel myotubes. We performed micro-contact printing of extracellular matrix proteins on soft polyacrylamide-based hydrogels photopolymerized onto functionalized glass slides. Our methods proved to be simple, repeatable and effective in obtaining an extremely selective adhesion of both cardiomyocytes and satellite cells onto patterned soft hydrogel surfaces. Cardiomyocytes resulted in aligned cardiac myofibers able to exhibit a synchronous contractile activity after 2 days of culture. We demonstrated for the first time that murine satellite cells, cultured on a soft hydrogel substrate, fuse and form aligned myotubes after 7 days of culture. Immunofluorescence analyses confirmed correct expression of cell phenotype, differentiation markers and sarcomeric organization. These results were obtained in myotubes derived from satellite cells from both wild type and MDX mice which are research models for the study of muscle dystrophy. These arrays of both cardiac and skeletal muscle myofibers could be used as in vitro models for pharmacological screening tests or biological studies at the single fiber level.

Probing mechanical properties of fully hydrated gels and biological tissues

A longstanding challenge in accurate mechanical characterization of engineered and biological tissues is maintenance of both stable sample hydration and high instrument signal resolution. Here, we describe the modification of an instrumented indenter to accommodate nanomechanical characterization of biological and synthetic tissues in liquid media, and demonstrate accurate acquisition of force–displacement data that can be used to extract viscoelastoplastic properties of hydrated gels and tissues. We demonstrate the validity of this approach via elastoplastic analysis of relatively stiff, water-insensitive materials of elastic moduli E>1000 kPa (borosilicate glass and polypropylene), and then consider the viscoelastic response and representative mechanical properties of compliant, synthetic polymer hydrogels (polyacrylamide-based hydrogels of varying mol%-bis crosslinker) and biological tissues (porcine skin and liver) of E<500 kPa. Indentation responses obtained via loading/unloading hystereses and contact creep loading were highly repeatable, and the inferred E were in good agreement with available macroscopic data for all samples. As expected, increased chemical crosslinking of polyacrylamide increased stiffness ( E⩽40 kPa) and decreased creep compliance. E of porcine liver (760 kPa) and skin (222 kPa) were also within the range of macroscopic measurements reported for a limited subset of species and disease states. These data show that instrumented indentation of fully immersed samples can be reliably applied for materials spanning several orders of magnitude in stiffness ( E=kPa–GPa). These capabilities are particularly important to materials design and characterization of macromolecules, cells, explanted tissues, and synthetic extracellular matrices as a function of spatial position, degree of hydration, or hydrolytic/enzymatic/corrosion reaction times.

Neural Network Modeling Applied to Polyacrylamide Based Hydrogels Synthetized By Single Step Process

This paper presents a series of experimental data obtained from the synthesis of polyacrylamide-based hydrogels and a general neural network methodology that accomplishes the modeling and optimization of the polymerization process. Using direct neural network modeling, the variation of the main parameters in the synthesis of polyacrylamide-based hydrogels (polymerization yield and maximum swelling degree) was modeled in correlation with reactant concentrations, temperature, and reaction time. The predictions of the network, verified against initial training data and other testing data in the domain of the reaction conditions, were quite precise. Inverse neural modeling determines, in a facile manner and with good results, the initial reaction conditions, which lead to a preestablished reaction yield and maximum swelling degree. This optimization method is more advantageous compared to a difficult classical procedure that requires a good mathematical model and an optimization solving technique.

Ultrafine Hydrogel Nanoparticles: Synthetic Approach and Therapeutic Application in Living Cells

The entry of drug payloads into the circulatory system is met by rapid removal by resident macrophages. The almost immediate drop in circulating levels of the therapeutic agent has been identified as a major obstacle for efficient drug delivery. Various physical properties determine the fate of NP-based (NP= nanoparticle) drug-delivery platforms in vivo. Size, surface charge, and surface hydrophobicity of the particles all affect the ability of macrophages to remove them from circulation and, hence, their bioavailability. Large particles (> 200 nm diameter) are more easily captured by macrophages. Ultrafine nanoparticles can avoid resident macrophage capture and are thus more likely to be eventually removed from the body by renal clearance, thereby decreasing the potential for bioaccumulation. The use of ultrafine materials of appropriate size and composition for the delivery of potent and effective therapeutics may mitigate some of the problems associated with drug delivery to the central nervous system (CNS) as well as management of other diseases. Ultrafine polyacrylamide-based hydrogels are an example of the many interesting materials used for drug delivery in vivo. The neutral surface properties of the hydrophilic polyacrylamide-based nanoplatforms reduce uptake by macrophage and make them a candidate for development as a useful therapeutic entity. Lower protein adsorption and high water content further decrease the opsonization by plasma proteins in the bloodstream and aid in the evasion of circulating and tissue-based macrophages. The size of polyacrylamide-based nanoparticles can be easily modulated by synthetic approaches. Furthermore, polyacrylamide can easily copolymerize with other monomers to introduce functional groups; thus, targeting tags can be incorporated to guide drug delivery. All this suggests that polyacrylamide-based hydrogels may be used as a novel class of safe nanosized drug carriers for the in vivo delivery of a variety of therapeutic moieties. Photodynamic therapy (PDT), a promising new treatment for certain types of cancer, has received considerable attention in recent years. The principle of PDT is that optically excited photosensitizer molecules, localized in cancer tissues, transfer their energy to molecular oxygen to form highly reactive singlet oxygen (O2), which kills living cells. The subcellular localization of photosensitizers depends on the photosensitizer3s nature and the cell lines studied. Among the “second-generation” photosensitizers, meta-tetra(hydroxyphenyl)chlorin (mTHPC) has recently received extensive attention because of its high phototoxicity at very low concentrations, or low light levels, and it has recently been approved in the European Union for head and neck cancer therapy. However, a specific formulation is required to deliver the hydrophobic drug. This is because the injectable mTHPC, for PDT clinical trials and for most in vivo studies reported in the literature, still uses the standard solution (ethanol/PEG400/water) formulation. In this way, after administration, the hydrophobic molecules are diluted in the biological environment. Eventually the mTHPC molecules precipitate out and stick onto cell or tissue surfaces, causing side effects. Although a few reports show the encapsulation of mTHPC into sub-200-nm silica or poly(lactic-co-glycolic acid) (PLGA) nanoparticles, size and surface issues, as well as application in a biological environment, remain open. Herein, a novel synthetic approach is described for obtaining ultrafine hydrophilic polyacrylamide-based nanoparticles. Used as an example, mTHPC-encapsulating nanoparticles are synthesized. The behaviors of the drug molecule, within nanoparticles and in solution, are chemically compared for their phototoxicity. Finally, their effectiveness in killing living cells was evaluated. It is well-known that for inverse microemulsions the droplet size is determined by the molar ratio of water to surfactant, [W0]/[surfactant]. [7] Also, droplet stability during polymerization depends on the volume of the water droplets. The volume reduction of the water pool and the increase in surfactant, in the inverse microemulsion, makes the droplet more rigid and thus decreases the possibility of droplet collisions during polymerization. Thus, small-size nanoparticles can be obtained. Experiments have shown that by changing the molar ratio of water to surfactants one could control both size and shape of the nanoparticles. We chose to emulsify both the monomers and cross-linkers directly into hexane, without water, to get the smallest polyacrylamide nanoparticles. First, the ultrafine blank polyacrylamide particles were synthesized. Acrylamide (AAm) monomer and cross-linker N,N-methylenebis(acrylamide) (MBA) were suspended in a hexane solution containing surfactant (AOT= [*] Dr. D. Gao, Prof. R. Kopelman Department of Chemistry University of Michigan Ann Arbor, MI 48109-1055 (USA) Fax: (+1)734-936-2778 E-mail: kopelman@umich.edu

Rheological properties of polyacrylamide-based hydrogels

Rheological properties of hydrogels of polyacrylamide-based polyelectrolytes were studied. The dependence of the rheological characteristics of the gels on the content of an organic modifier and parameters of a mechanical action was disclosed. The range of parameters where the hydrogels of modified polyacrylamide demonstrated the highest mechanical strength was determined.

Preparative-scale isoelectric trapping separations in methanol–water mixtures

The typically low aqueous solubilities of small, hydrophobic organic ampholytic molecules limit the production rates that can be achieved in their isoelectric trapping (IET) separations and call for the use of hydro-organic mixtures as solvents. The compatibility of methanol–water mixtures and poly(ethylene terephthalate) substrate-supported isoelectric polyacrylamide hydrogels, developed for binary IET separations in a Gradiflow BF200IET unit, was investigated. The isoelectric polyacrylamide-based hydrogels retained their functional and mechanical integrities when the methanol concentration in the hydro-organic solvent mixture was kept at or below 25% (v/v). The utility of the hydro-organic media was demonstrated in the purification of a hydrophobic ampholytic compound, technical grade 4-hydroxy-3-(morpholinomethyl) benzoic acid. Production rates as high as 7 mg/h were achieved using small, 15 cm 2 active surface area isoelectric membranes.