Gel Shrinkage Research Articles

Cytocompatible gel formation of chitosan‐glycerol phosphate solutions supplemented with hydroxyl ethyl cellulose is due to the presence of glyoxal

To deliver and retain viable repair cells in a surgically prepared cartilage lesion, we previously developed an adhesive in situ-gelling cell carrier by suspending cells in a solution of hydroxyethyl cellulose (HEC), which was then mixed with chitosan-glycerol phosphate to form a chitosan-GP/HEC gel. The purpose of this study was to elucidate the mechanism of gelation to maximally control gel time and viability of encapsulated cells. We analyzed the role of osmolality, pH, gelation temperature, gel shrinkage, and HEC. A chitosan-GP solution at pH 6.8 with cytocompatible osmotic pressure (419 mOsm/kg) was achieved by lowering disodium GP concentration from 370 to 135 mM. This solution was still thermogelling but only at 73 degrees C. We next discovered that glyoxal, a common additive in ether cellulose manufacturing, was responsible for chitosan gelation. Monolayer cells survived and proliferated in up to 1 mM of glyoxal, however only a very narrow range of glyoxal concentration in chitosan-GP/HEC, 0.1-0.15 mM, permitted gel formation, cell survival, and cell proliferation. Chitosan gels containing HEC required slightly less glyoxal to solidify. Chitosan-GP/HEC loaded with viable chondrocytes formed an adhesive seal with ex vivo mosaic arthroplasty defects from sheep knee joints. In mosaic arthroplasty defects of live sheep, bleeding occurred beneath part of the hydrogel carrier, and the gel was cleared after 1 month in vivo. These data indicate that chitosan-GP/HEC is suitable as an adhesive and injectable delivery vehicle for clinical orthopedic applications involving single use treatments that guide acute cartilage repair processes.

Structure and Drug Release in a Crosslinked Poly(Ethylene Oxide) Hydrogel

Hydrogels are a continuously expanding class of pharmaceutical polymers designed for sustained or controlled drug release. The structure and intermolecular interactions in such systems define their macroscopic properties. The aim of this study was to investigate the mechanism of swelling, drug impregnation, and drug release from poly(ethylene oxide) (PEO) gel crosslinked by urethane bonds. A combination of SAXS/WAXS/SANS techniques enabled us to determine the phase transition between lamellar and extended gel network, and to apply different descriptions of crystallinity, based on lamellar and crystal lattice structures. It is shown that even low (1-7% w/w) loading of model drugs acetaminophen and caffeine, produced significant disorder in the polymer matrix. This effect was particularly pronounced for acetaminophen due to its specific ability to form complexes with PEO. The drug-release profiles were analyzed using a general cubic equation, proposed for this work, which allowed us to determine the gel hydration velocity. The results indicate that the release profiles correlate inversely with the polymer crystallinity.

Dynamic Hydrogels: Translating a Protein Conformational Change into Macroscopic Motion

Natural inspiration: A bioinspired functional material in the form of a hydrogel created by cross-linking an engineered version of calmodulin, a protein which undergoes a conformational change in response to ligand binding (see schematic representation), with a four-armed poly(ethylene glycol) molecule terminated with acrylate groups decreased in volume by 20 % when treated with the ligand trifluoperazine. Multiple cycles of gel swelling and shrinkage were possible.

Impact of Lipid Phase on Water Transfer in Food

Apparent water diffusivity has been studied on an emulsified food model made of gelatin and hydrogenized vegetable fat. Lipid content was varied from 0 to 50% w/w and three particle mean diameters were generated (40, 110, and 400 μm) in gelatin samples containing 75 g of water per 100 g of fat-free sample. In joined compartment experiments, fatty hydrated gels were dehydrated in contact with either a dry pure gelatin compartment or an industrial sponge cake. Water content profiles were used to determine the apparent water diffusivity value using Ruiz Cabrera et al.'s method,[1] which takes gel shrinkage into account.

Modification of chemical performance of dopants in xerogel films with entrapped ionic liquid

The effect of ionic liquid (IL), 1-butyl-3-methylimidazolium chloride [C4mim][Cl], entrapment on the properties of the xerogel films is described. This has been investigated by evaluation of the behavior of sulfonephthalein pH indicators entrapped in a xerogel matrix formed by tetraethoxysilane (TEOS) and vinyltriethoxysilane (VTEOS) with co-entrapped ionic liquid. We report that co-doping of the ionic liquid with pH indicators within the xerogel matrix significantly modifies the performance of primary dopants such as acidic constants and pH indicating ranges but the spectral intensity depends on the structure of the indicator. Increase in pH indicating ranges in the presence of ionic liquid can be advantageously used for designing pH optodes with a wider determination range. IL–xerogel modified films showed increased wettability, hydrophilicity, reduced gel shrinkage and faster response times compared to the unmodified silica matrix. The reason for such modification is explained mainly based upon changes in the environment of the indicator caused by an increase in hydrophilicity in the presence of IL as well as ionic interactions between the indicator and IL. The long term stability of the xerogel films was investigated over at least fifteen months.

Drying and shrinkage of polymer gels

The polymer hydrogel was synthesized by photo-polymerization process (UV light, 60 oC) in presence of Photo-initiator (IrgacureR) and Cross-linker (NN'-methylene bisacrylamide; MBAM). In the present work, the drying of polymer hydrogel was carried out to study the effect of temperature, gel-sheet thickness, monomer ratio of acryl acid to acrylamide (AA/AM), concentration of MBAM and quantity of monomers. A correlation has been developed for modified sheet thickness as a function of contraction coefficient and degree of drying. Effective diffusivity was estimated from Fickian-diffusive model considering modified sheet thickness and was found to be in the range of 1.1 ´ 10-10-5.93 ´ 10-10 m2/s. The activation energy obtained using Arrhenius type equation was found to be in the range of 2979-10737 kJ/kmol H2O. The drying behavior shows an initial shoot-up in drying rate followed by constant rate and two falling rate periods.

Gentle cell trapping and release on a microfluidic chip by in situ alginate hydrogel formation

Microfluidic devices are increasingly used to perform biological experiments on a single-cell basis. However, long-term stability of cell positions is still an issue. A novel biocompatible method for cell entrapment and release on a microchip is presented. It is based on the controlled formation of an alginate hydrogel by bringing two laminar flows of alginate and calcium ions in the range of 2 mM to 40 mM into contact. The resulting growth of a gel bar is used to enclose and immobilize yeast cells. Adding ethylenediaminetetraacetic acid (EDTA) to the alginate solution allows for control of the hydrogel growth, and by varying the ratio of Ca(2+) to EDTA concentrations gel growth or gel shrinkage can be induced at will. Trapped cells are released during shrinkage of the gel. The trapping efficiency for different cell speeds is investigated and the properties of gel growth are discussed using a diffusion model. Precise positioning of a single cell is demonstrated. The technique presented allows not only the reversible immobilization of cells under gentle conditions but also offers the potential of long-term cell cultures as shown by on-chip incubation of yeast cells. The procedure may provide a simple and fully biocompatible technique for a multitude of innovative experiments on cells in microsystems.

Characterization of statically loaded tissue-engineered mitral valve chordae tendineae.

Chordae tendineae are essential to the proper function of the mitral valve. Native chordae contain a dense collagenous core and an outer elastin sheath. We have been using the principle of directed collagen gel shrinkage to fabricate tissue-engineered mitral valve chordae. Because the microstructure of biologic tissues determines their mechanical behavior, the morphology of collagen and elastin in tissue-engineered chordae should mimic that of native chordae. The objective of this study, therefore, was to examine the morphology of our tissue-engineered constructs in comparison to native chordae. A collagen-cell suspension was cast into silicon rubber wells with microporous anchors at the ends and cultured in an incubator. The anchors allowed shrinkage to occur only transverse to the long axis of the wells, thus creating highly aligned collagen fibril constructs. The collagen constructs were cultured for 8 weeks and characterized mechanically, histologically, and biochemically at different culture time points. Histologic sections showed that in all mature constructs collagen fibers were oriented parallel to the long axis of the constructs. At the edge of the tissue collagen fibers were in general straight, whereas in the middle of the tissue they were wavy. Transmission electron microscopy showed a progressive increase in the density and longitudinal orientation of collagen fibrils with culture time. Light and scanning electron microscopy showed the presence of an elastin sheath around the collagen core. Immunostaining demonstrated that smooth muscle cells differentiate during tissue development and TUNEL assay showed that cells in the interior of the constructs undergo apoptosis. This study has demonstrated that collagen-cell constructs, with material properties and microstructure similar to native mitral valve chordae, can be developed using static culture.

On Porosity of Carbon Aerogels from Sol-Gel Polymerization of Phenolic Novolak and Furfural

Carbon aerogels were prepared by sol-gel polymerization of phenolic novolak and furfural followed by supercritical drying and pyrolysis. The porosity and morphologies of carbon aerogels were characterized by nitrogen adsorption, apparent density, He- pycnometer method, and transmission electronic microscopy (TEM). Effect of ratios of phenolic novolak to furfural (Ra) and total concentration of reactants (C) in sol-gel step on porosity and morphologies of carbon aerogels was investigated. The carbon aerogels synthesized are rich in meso- and macropores. The Ra determines the cross-linking density of polymers, thereby the compatibility of the polymers, and ultimately the shrinkage of gels in the drying and pyrolysis. The network sizes and the porosity of organic and carbon aerogels are mainly determined by Ra. The C has no effect on volume shrinkage of gels in drying and pyrolysis and has only dilute effect in determining bulk density of organic and carbon aerogels, and ultimately the porosity of carbon aerogels. Conversion of mesopores to micro- and macropores is observed, which is related to combination of C and Ra, and determines the partition of micro-, meso- and macropores.

Stress During Drying of Two Stone Consolidants Applied in Monumental Conservation

The object of this paper is to evaluate behaviour during drying of two stone consolidants: Wacker OH and Tegovakon V, containing tetraethoxysilane. During drying, the gel network contracts due to capillary pressure generated by solvent evaporation. When the consolidant dries inside the stone porous structure, the shrinkage is constrained in all three dimensions. In these conditions, the dried gel suffers a high stress that could cause it to crack. When there is a free surface, as for a consolidant layer on the surface of a pore, the stress can relax in the direction normal to the surface. In this case, the stress is controlled by network rigidity. The rigidity of the gel network has been evaluated by mercury porosimetry, while pore size, which controls capillary pressure, has been determined by nitrogen adsorption. The shrinkage of gels under mercury pressure is characterised by high moduli. This fact suggests a high rigidity of the networks. The small pore radii found in the network (<3 nm) indicate that high capillary pressures are generated within the gel network.

A constitutive equation for viscoelastic porous gels during syneresis and drying

In this investigation, a constitutive equation was developed for viscoelastic porous gels during drying process based on the mechanism of shrinkage. One important application for the analysis is to predict the residual stress developed during the drying of gels in the sol–gel fabrication process. The gels can be modeled as porous continuum material, where the solid phases are viscoelastic material, and the pores are filled with some kinds of liquid. During drying, the large capillary forces generated in the tiny (∼3–10 nm) pores serve as the main driving force for the shrinkage of gels, whereas the applied forces and hydrostatic pressure from the liquid and also contribute to the deformation process. Finally, the tensile residual stress in the thin film that developed in the drying was predicted based on the constitutive equation. It is hopeful that the developed viscoelastic constitutive equation can be adapted into a FEM code to predict the residual stress and optimum design the fabrication parameters without cracking of the specimen.

Application of mercury porosimetry to the study of xerogels used as stone consolidants

Alkoxysilanes, low-viscosity monomers that polymerize into the porous network of stone by a sol–gel process, are widely used in the restoration of stone buildings. We have used the mercury porosimetry technique to characterize changes in microstructure of three granites following their consolidation with two popular commercial products (Wacker OH and Tegovakon V). The suitability of this technique is questioned because a surprising increase of stone porosity is observed. In order to investigate the feasibility of porosimetry, we analyze the behavior of xerogels prepared from the two commercial products, under mercury pressure. Gels are basically compacted and not intruded by mercury. Thus, the increase of stone porosity after consolidation can actually be associated with gel shrinkage. Mercury porosimetry, therefore, has been found unsuitable for characterizing the microstructure of consolidated rocks. However, it can be employed usefully to evaluate shrinkage of gels under mercury pressure, which permits the behavior of a consolidant during the process of drying in stone to be predicted. It is a key factor because many problems of consolidants are related to their drying process within the stone. Gels under study exhibit a high rigidity and an elastic behavior, as consequence of their microporous structure. Finally, the reduction in the porous volume of gels after the porosimetry test demonstrates that the shrinkage mechanism is based on pore collapse.

Reversible adsorption of calcium ions by imprinted temperature sensitive gels

With the aim of developing polymeric gels sensitive to external stimuli and able to reversibly adsorb and release divalent ions, copolymer gels of N-isopropylacrylamide (NIPA) and methacrylic (MAA) monomers were prepared. We chose calcium as a target divalent ion. Two MAAs form a complex with a calcium ion, and the NIPA component allows the polymers to swell and shrink reversibly in response to temperature. The adsorbing site develops an affinity to target ions when the adsorbing molecules come into proximity, but when they are separated, the affinity diminishes. To enhance the affinity to calcium, an imprinting technique was applied using Ca2+ and Pb2+ ions as templates in methylsulfoxide and dioxane media, respectively. The adsorption capacity of the imprinted gels was compared with that of the nonimprinted gels, and the effects of the templates, the solvents, and the amount of methacrylic monomers used in the synthesis and the medium temperature over the Ca2+ adsorption capacity of the gels from aqueous solutions were evaluated. The analysis of the adsorption revealed that (a) the adsorption can be described by the Langmuir isotherms; (b) there is an approximately linear relationship between saturation and methacrylic monomer concentration; (c) the affinity depends on the degree of gel swelling or shrinkage that can be switched on and off by temperature; (d) in the shrunken state, the affinity depends approximately linearly on the MAA concentration in the imprinted gels, whereas in the nonimprinted gels it is proportional to the square of MAA concentration; (e) the imprinted gels adsorb more than the nonimprinted gels when MAA concentration is less than that of permanent cross linkers. The success of imprinting of CaMAA2 and PbMAA2 complex is evidence for memory of such complex onto the weakly cross-linked gel.

Release Behavior of Ketoprofen from Poly(Acryloyl-L-proline Methyl Ester) Gels Having Different Crosslinked Networks

In order to clarify the relationship between the crosslinked structure of thermo-responsive polymer and drug release profile, polymer gels based on acryloyl-L-proline methyl ester (A-ProOMe) were synthesized in a mixture of water and acetone by the following two methods: a simultaneously occurring process of radiation-induced polymerization and crosslinking without a crosslinker (self-bridging method), and radiation-induced polymerization in the presence of the crosslinker tetradecaethylene glycol dimethacrylate (cosslinker method). The pronounced gap in thermo-response between two A-ProOMe gels, with an apparent degree of crosslinking of 11 for 1-propanol, shows a different shrinking pattern in the initial stage of time. The gels, which were obtained with the self-bridging method and the crosslinker method, were kept constant at a swelling ratio of 17 in water at 0°C for all systems. However, those values fell to 0.5 and 4, respectively, at 10 min after the temperature was increased to 37°C. The release mechanism of ketoprofen from two gel devices showed an anomalous (non-Fickian) transport, in which the release of ketoprofen with a low water-solubility could be directly related to the rapid release of water accompanying a gel shrinkage.

Reversible phase transitions in polymer gels induced by radiation forces.

Many polymer gels undergo reversible, discontinuous volume changes in response to changes in the balance between repulsive intermolecular forces that act to expand the polymer network and attractive forces that act to shrink it. Repulsive forces are usually electrostatic or hydrophobic in nature, whereas attraction is mediated by hydrogen bonding or van der Waals interactions. The competition between these counteracting forces, and hence the gel volume, can thus be controlled by subtle changes in parameters such as pH (ref. 4), temperature, solvent composition or gel composition. Here we describe a more direct influence on this balance of forces, by showing that the radiation force generated by a focused laser beam induces reversible shrinkage in polymer gels. Control experiments confirm that the laser-induced volume phase transitions are due to radiation forces, rather than local heating, modifying the weak interactions in the gels, in agreement with previous observations of light-induced chain association in polymer solutions. We find that, owing to shear-relaxation processes, gel shrinkage occurs up to several tens of micrometres away from the irradiation spot, raising the prospect that the combination of stimuli-responsive polymer gels and laser light might lead to new gel-based systems for applications such as actuating or sensing.

Net shape ceramic microcomponents by modified sol-gel casting

Development of ceramic microcomponents can open up the current silicon-based MEMS (micro-electro-mechanical systems) technology to new applications, especially for use in high temperature, chemically reactive environments. We report a study of sol–gel technologies applied to the fabrication of microcomponents. Organic/inorganic hybrid materials, particularly silica-based hybrids, were developed by sol–gel processing. A weak silica network was made possible by using acid catalyst, low functionality organic precursor in the sol preparation. Since the weak network has a high tendency to collapse, the result is a denser material. Incorporation of organic ligands modifies the surface chemistry of gel network so that a greater drying stress was developed and a denser hybrid structure was achieved without high temperature treatment. In addition, the incorporation of organic ligands prevented formation of cracks. Nanoscale oxide particles were dispersed and incorporated into the gel network by surface condensation. The incorporation of titania nanoparticles into sols greatly reduced the volume shrinkage of gels and enhanced the mechanical strength of the components. In this work, silica sol was also applied to coat metallic microcomponents and a thin uniform coating was formed.

Structure and properties of gels

A brief review is presented of the structure and properties of inorganic gels. The mechanism of formation of gels and precipitates involves the aggregation of polymers or particles. If bonds form irreversibly, fractal aggregates grow and eventually fill space, resulting in a continuous elastic gel network; if the bonding is reversible, then the aggregates reorganize and become more compact, leading to precipitates. The existence of capillary pores in cement paste indicates that C-S-H is a precipitate, rather than a gel. Chemical reactions continue after gelation, leading to increasing rigidity and, in some cases, to spontaneous shrinkage (called syneresis). During drying, capillary pressure on the order of 10 to 100 MPa develops and produces huge shrinkage of compliant gels. As the gel shrinks, its modulus increases by several orders of magnitude; shrinkage stops when the network is stiff enough to resist the capillary forces. For gels, the modulus increases in proportion to the volume fraction of solids to a power between ∼3 and 4; consequently, the evolution of the modulus during drying and the final shrinkage can be estimated. Warping and cracking do not depend on the magnitude of the capillary pressure, but only on the gradient in pressure within the body. Slow drying reduces the gradients and prevents damage. The stresses and strains during drying can be calculated in terms of the permeability, physical dimensions, and viscoelastic properties of the gel. A variety of techniques have been developed to measure the latter properties, the simplest of which is a beam-bending measurement. When this method is applied to a rod of cement paste, the permeability and modulus are obtained in a matter of minutes.

Evolution in Structural and Optical Properties of Stannic Oxide Xerogel upon Heat Treatment

Upon consecutive heat treatments at increasing temperatures, the microstructure of solution–sol–gel‐derived stannic oxide (SnO2) xerogel evolves in three stages: (I) below 300°C, characterized by extensive dehydroxylation and gel shrinkage with little grain growth and surface loss; (II) between 300° and 500°C, by extensive crystallization, leading to dramatic surface loss (by 70%); and (III) above 500°C, by grain growth. Concurrently, the UV‐absorption edge shows red shifts during stages I and II and blue shifts during stage III, resulting in distinct color variations. The edge displacement bears a close correlation with a Raman “defect band” at ∼305–328 cm−1.

P-Nitrophenol permeability and temperature characteristics of an acryloyl-L-proline methyl ester-based porous gel membrane

Thermoresponsive porous gel membranes were synthesized by a simultaneously occurring process consisting of radiation-induced polymerization and crosslinking in aqueous solutions at various concentrations of acryloyl-L-proline methyl ester(A-ProOMe) without a crosslinker. Permeation of p-nitrophenol (PNP) through a thermoresponsive porous gel membrane obtained at a monomer concentration of 80% (w/w) drastically reduced around 14°C, the lower critical solution temperature (LCST) of linear poly(A-ProOMe) in water, from 0.60 × 10−3 cm/min at 10°C to no permeation at 18°C, accompanied by changes in both size and shape of pores associated with gel shrinkage. Moreover, it was found that porous gel membranes with a porosity of approximately 60% had a greater PNP permeability constant through porous gel membranes with mutually connected pores obtained at a monomer concentration of 50% (w/w) than individually supported pores obtained at a monomer concentration of 70% (w/w). © 1998 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 36: 1495–1500, 1998

Tin oxide gel shrinkage during CO 2 supercritical drying

Shrinking behavior of tin oxide gels during solvent exchange in CO 2 supercritical drying was investigated. The drying process consisted of two solvent-exchanging steps, including a first replacement of water by acetone and a second of acetone by liquid CO 2. Gels prepared from a basic sol with a ζ-potential of −75 mV exhibited extensive shrinkage during both steps, while those from an acidic hydrous precipitate of lower potential (| ζ-potential|<35 mV) shrank only during the second step. The shrinkage during the first step correlates with the ξ-potential reduction with increasing acetone concentration, while that during the second step for both gels can be attributed to osmotic compressive pressure that arises from an increase in the liquid–solid (gel skeleton) interfacial energy with increasing CO 2 concentration. Dramatically different aerogel microstructures were obtained by controlling the gel surface potential in conjunction with different combinations of solvent-evaporating and supercritical drying.