Shrinking Of Gels Research Articles

Lower critical concentration temperature as thermodynamic origin of syneresis: Case of kappa-carrageenan solution

Polysaccharide ubiquity is trimmed for applications of low syneresis impact. This syneresis may be crucial for specific applications that are very sensitive to gel dimension stability, namely, 3D scaffolds for cell culture for disease diagnosis and tissue engineering. We hypothesized that the syneresis origin results from the kappa-carrageenan (kC) polysaccharide thermodynamic instability, and we demonstrated this by measuring the critical (coil-to-coil contact) concentration as a function of temperature. The impact of 5 mM, 10 mM and 15 mM KCl salt on the critical concentration of the solution and the lower critical concentration temperature (LCCT) were particularly investigated. For the kC polysaccharide, the gelation temperature (Tg) falls at temperatures below the LCCT, which explains the shrinking or syneresis reaction of the polysaccharide gels. The gap between Tg and LCCT would be the thermomotive force of the syneresis of many colloidal gels.

A mechanical model of posterior vitreous detachment and generation of vitreoretinal tractions.

We propose a mechanical model of generation of vitreoretinal tractions in the presence of posterior vitreous detachment (PVD). PVD is a common occurrence with aging, and it consists in the separation of the vitreous body from the retina at the back pole of the eye, due to progressive shrinking of the vitreous gel. During this separation process, vitreoretinal tractions are generated at regions of high adhesion between the vitreous and the retina. Such tractions are mainly responsible for the creation of retinal tears, which can lead to retinal detachment. We describe the PVD evolution developing a continuum model of a shrinking soft body, representing the vitreous humor gel phase. In the model, the vitreous is surrounded by a membrane, stiffer than the bulk, the vitreous cortex, and it is contained within a rigid spherical domain, the vitreous chamber. The membrane is attached to the spherical wall and the adhesion strength is spatially non-uniform, increasing from the back to the front of the chamber, according to clinical observations. During the shrinking process, the vitreous undergoes elastic distortions, owing to the spatially variable adhesion on the wall, and this produces boundary tractions. We also consider the clinically relevant case of anomalous PVD, in which regions of focal adhesion between the vitreous and the retina exist, leading to the generation of strong, localized tractions. The model reproduces a PVD evolution in good qualitative agreement with clinical observations and makes it possible to correlate the shape of the detached vitreous with the intensity of vitreoretinal tractions.

Smart Hydrogel-Based Valves Inspired by the Stomata in Plants.

We report the design of hydrogels that can act as "smart" valves or membranes. Each hydrogel is engineered with a pore (about 1 cm long and <1 mm thick) that remains closed under ambient conditions but opens under specific conditions. Our design is inspired by the stomatal valves in plant leaves, which regulate the movement of water and gases in and out of the leaves. The design features two different gels, active and passive, which are attached concentrically to form a disc-shaped hybrid film. The pore is created in the central active gel, and the conditions for opening the pore can be tuned based on the chemistry of this gel. For example, if the active gel is made from N-isopropylacrylamide (NIPA), the actuation of the pore depends on the temperature of water relative to 32 °C, which is the lower-critical solution temperature (LCST) of NIPA. The concentric design of our hybrid provides directionality to the volumetric transition of the active gel, i.e., it ensures that the pore opens as the active gel shrinks. In turn, contact with hot water (T > 32 °C) opens the pore and allows the water to pass through the gel. Conversely, the pore remains closed when the water is cold (T < 32 °C). The gel thereby acts as a "smart" valve that is able to regulate the flow of solvent depending on its properties. We have extended the concept to other stimuli that can cause gel-swelling transitions including solvent composition, pH, and light. Additionally, when two different gel-based valves are arranged in series, the assembly acts as a logical "AND" gate, i.e., water flows through the valve-combination only if it simultaneously satisfies two distinct conditions (such as its pH being below a critical value and its temperature being above a critical value).

Polymer gels with associating side chains and their interaction with surfactants.

Conformational behaviour of hydrophobically modified (HM) polymer gels in solutions of nonionic surfactants is studied theoretically. A HM gel contains hydrophobic side chains (stickers) grafted to its subchains. Hydrophobic stickers are capable to aggregate into joint micelles with surfactant molecules. Micelles containing more than one sticker serve as additional physical cross-links of the network, and their formation causes gel shrinking. In the proposed theoretical model, the interior of the gel/surfactant complex is treated as an array of densely packed spherical polymer brushes consisting of gel subchains tethered to the surface of the spherical sticker/surfactant micelles. Effect of stickers length and grafting density, surfactant concentration and hydrophobicity on gel swelling as well as on hydrophobic association inside it is analyzed. It is shown that increasing surfactant concentration can result in a gel collapse, which is caused by surfactant-induced hydrophobic aggregation of stickers, and a successive gel reswelling. The latter should be attributed to a growing fraction of surfactants in joint aggregates and, hence, increasing number of micelles containing only one sticker and not participating in gel physical cross-linking. In polyelectrolyte (PE) gels hydrophobic aggregation is opposed by osmotic pressure of mobile counterions, so that at some critical ionization degree hydrophobic association is completely suppressed. Hydrophobic modification of polymers is shown to open new ways for controlling gel responsiveness. In particular, it is discussed that incorporation of photosensitive groups into gel subchains and/or surfactant tail could give a possibility to vary the gel volume by light. Since hydrophobic aggregation regularities in gels and solutions are common, we hope our findings will be useful for design of polymer based self-healing materials as well.

Ferrogels cross-linked by magnetic nanoparticles—Deformation mechanisms in two and three dimensions studied by means of computer simulations

Ferrogels, i.e., magnetic nanoparticles immersed into a swollen polymer gel, are materials which combine the magnetic properties of a ferrofluid with the elastic properties of a hydrogel. This makes it possible to control their shape and elasticity by means of external magnetic fields. Magnetic gels are therefore considered for medical and technical applications such as actuation and drug delivery systems. In this paper, we focus on a special class of magnetic gels in which the magnetic nanoparticles act as the cross-linkers of the polymer network. Experimentally this is achieved by functionalizing the nanoparticles' surface such that the polymers can covalently bind to it. In this way, a coupling between the orientation of the nanoparticles and the polymer matrix is created. When an external magnetic field is applied to such a system, the magnetic particles align to the field, and due to the coupling between the nanoparticles' orientation and the gel matrix, a strain is exerted on the polymers. This leads to a shrinking of the gel as a whole. In this paper, we explain how to extend a two-dimensional simulation model for such a system (Soft Matter 8 (2012) 9923 [18]) to three dimensions. We will show that, while an isotropic deformation was observed in the two-dimensional case, in three dimensions the gel shrinks anisotropically. This is due to the fact that orientational fluctuations of the nanoparticles are hindered in the direction perpendicular to an external magnetic field.

Modeling the Photoinduced Reconfiguration and Directed Motion of Polymer Gels

AbstractA remarkable feature of certain biological species is their ability to dramatically alter their shape in response to environmental cues. Here, a computational model for photoresponsive polymer gels that contain spirobenzopyran (SP) chromophores is developed, and it is shown that these materials can undergo 3D biomimetic shape changes under non‐uniform illumination. The SP moieties are hydrophilic in the dark, but become hydrophobic under illumination with blue light. Hence, with the incorporation of these chromophores into gels in aqueous solutions, light can be harnessed to control the gel's swelling or shrinking and, thereby, dynamically alter the gel's shape. The model is first validated by determining the effects of uniform illumination on the temperature‐induced volume phase transitions in these gels, and show good agreement between these results and available experimental data. These gels can also be patterned remotely and reversibly by illuminating the samples through photomasks and, thus, molded into a variety of shapes with feature sizes on the submillimeter length scale. Furthermore, by repeatedly rastering the light source over the sample, the system can be driven to exhibit another biomimetic behavior: sustained, directed motion. The introduction of a temperature gradient provides a means of further controlling this autonomous movement. The results point to a robust method for controllably reconfiguring the morphology of soft materials and driving the self‐organization of multiple reconfigurable pieces into complex architectures.

Peristaltic patterns for swelling and shrinking of soft cylindrical gels

We propose a variational method for determining the surface patterns of cylindrical gels for both swelling and shrinking. Exact solutions are calculated for the initial stages of such peristaltic instabilities. The morphology and the formation mechanisms depend on a competition between bulk elastic energy and surface tension.

Light-induced spatial control of pH-jump reaction at smart gel interface

We proposed here a ‘smart’ control of an interface movement of proton diffusion in temperature- and pH-responsive hydrogels using a light-induced spatial pH-jump reaction. A photoinitiated proton-releasing reaction of o-nitrobenzaldehyde (NBA) was integrated into poly(N-isopropylacrylamide-o-2-carboxyisopropylacrylamide) (P(NIPAAm-co-CIPAAm)) hydrogels. NBA-integrated hydrogels demonstrated quick release of proton upon UV irradiation, allowing the pH inside the gel to decrease below the pKa of P(NIPAAm-co-CIPAAm) within a minute. The NBA-integrated gel was shown to shrink rapidly upon UV irradiation without polymer “skin layer” formation due to a uniform decrease of pH inside the gel. Spatial control of gel shrinking was also created by irradiating UV light to a limited region of the gel through a photomask. The interface of proton diffusion (“active interface”) gradually moved toward non-illuminated area. The apparent position of “active interface”, however, did not change remarkably above the LCST, while protons continuously diffused outward direction. This is because the “active interface” also moved inward direction as gel shrank above the LCST. As a result, slow movement of the apparent interface was observed. The NBA-integrated gel was also successfully employed for the controlled release of an entrapped dextran in a light controlled manner. This system is highly promising as smart platforms for triggered and programmed transportation of drugs.

Diffusion of poly(ethylene glycol) and ectoine in NIPAAm hydrogels with confocal Raman spectroscopy

The diffusion behavior of poly(ethylene glycol) (PEG) in N-isopropylacrylamide (NIPAAm) hydrogels was investigated using confocal Raman spectroscopy with regard to temperature (25°C, 30°C and 35°C), PEG concentration (10 and 40 wt.%), PEG molecular weight (2,000 and 12,000 g/mol) and addition of the compatible solute ectoine (0.1 and 2 wt.%). Swelling and shrinking of the gels was observed by means of confocal Raman spectroscopy. The swelling behavior of NIPAAm gels in aqueous solutions of PEG and ectoine was found to resemble the swelling behavior in pure water with regard to temperature, i.e., the gel shrinks with increasing temperature. However, the presence and concentration of PEG and ectoine influence the swelling behavior by lowering the volume phase-transition temperature of the gel and facilitating shrinking. In some cases, a re-swelling of the gel was observed after the initial shrinking at the onset of PEG diffusion, which can be explained by PEG changing the chemical potential in the gel phase as it diffuses into the sample allowing the water to re-enter. The expulsion of water from the gel during shrinking and the so-caused increase of PNIPAAm and PEG concentrations in some cases led to the PEG diffusion seemingly being faster in more shrunken gels despite of their higher diffusion resistance.

Evaluation of the stimulus response of hydroxyapatite/ calcium alginate composite gels

Stimuli-responsive gels have been actively studied and applied to a variety of devices such as drug carriers, artificial muscles and actuators. In this research study, calcium alginate gels containing hydroxyapatite were prepared in one step by adding a diammonium hydrogenphosphate–1% alginate solution to a calcium chloride solution and evaluated for pH dependence and electrochemical properties as stimuli-responsive gels. When the composite gel was immersed in a solution of pH 2–10 or subjected to an applied voltage, the composite gel with a high content of hydroxyapatite tended to maintain its volume. The composite gels immersed in a pH indicator solution changed color with an applied voltage. As a result, it was revealed that alteration of the pH of composite gels occurred by electro-osmosis and electrophoresis. Swelling and shrinking of the composite gels were controlled by the external stimuli of pH, voltage and the content of hydroxyapatite; therefore, it is proposed that composite gels would be good bone-filling material having the function of drug carriers. Calcium alginate gels containing hydroxyapatite were prepared in one step and evaluated with regard to pH dependence and electrochemical property for applying to drug carriers. The composite gels immersed in a pH indicator solution changed its color by applying voltage. By these results, it was revealed that pH alteration of composite gels occurred by electro-osmosis and electrophoresis. Swelling and shrinking of the composite gels were controlled by external stimuli of pH and voltage and the content of hydroxyapatite.

Properties of a poly(acrylamide‐co‐diallyl dimethyl ammonium chloride) hydrogel synthesized in a water–ionic liquid binary system

AbstractA novel copolymer hydrogel, poly(acrylamide‐co‐diallyl dimethyl ammonium chloride), was prepared by the radical copolymerization of acrylamide and diallyl dimethyl ammonium chloride in an ionic liquid (IL)–water binary system in the presence of the crosslinker N,N′‐methylene bisacrylamide. The equilibrium swelling ratios of the hydrogels synthesized in the IL–water binary system increased with the content of IL and were remarkably higher than that of the gel synthesized in water. Differential scanning calorimetry measurements showed that the glass‐transition temperatures of the dry hydrogels that were synthesized in the IL–water binary system were remarkably lower than that of the gel synthesized in pure water. The mechanical properties of the gels synthesized in both water and the IL–water binary system were characterized with a universal material‐testing machine. The results show that fracture toughness of the hydrogels was improved when they were synthesized in the IL–water binary system. The gel shrank under a direct‐current electric field. The response rates of the gels that were synthesized with the IL–water binary system were faster than that of the gel synthesized in water. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2010

Shrinking Behavior of Surfactant‐Grafted Thermosensitive Gels and the Mechanism of Rapid Shrinking

AbstractSurfactant‐grafted hydrogels with a fast response to temperature were prepared. In order to clarify the mechanism of rapid shrinking, the effects of the grafted surfactant and the homogeneity of the main chain were investigated. Poly(NIPAAm‐co‐S180A) gels prepared using a chemical cross‐linker (bis‐PNS gels) exhibited rapid shrinking, as did PNS gels prepared by γ‐ray irradiation (γ‐PNS gels). This suggested that the rapid shrinking of the PNS gel did not depend on the homogeneity of the main‐chain structure. The shrinking kinetics of the bis‐PNS gels depended on the amount of the introduced surfactant, which means that shrinking is enhanced by micelle formation as a dynamic driving force. From the analysis by dynamic light scattering (DLS) and scanning microscopic light scattering (SMILS), it was suggested that the micelle structure, which induced rapid shrinking, existed in the bis‐PNS gel.magnified image

Preparation of fast response superabsorbent hydrogels by radiation polymerization and crosslinking of N-isopropylacrylamide in solution

Macroporous temperature-responsive poly( N-isopropylacrylamide) (PNIPAAm) hydrogels with high equilibrium swelling and fast response rates were obtained by a 60Co γ- and electron beam (EB) irradiation of aqueous N-isopropylacrylamide (NIPAAm) monomer solutions. The effect of irradiation temperatures, the dose, the addition of a pore-forming agent on the swelling ratio, and the kinetics of swelling and shrinking of the PNIPAAm gels was studied. The gels synthesized above the LCST exhibited the highest equilibrium swelling (300–400) and fastest response rate measured by minutes. Scanning electron microscope (SEM) pictures revealed that the gels synthesized above the LCST have larger pores than those prepared at temperatures below the LCST. The gels showed a reversible response to cyclical changes in temperature and might be used in a pulsed drug delivery device. The gels synthesized above the LCST exhibited the highest testosterone propionate release.

Influence of ionic liquids as additives on sol–gel immobilized lipase

The immobilization of lipase from Candida rugosa, using ionic liquids as additives to protect the inactivation of lipase by released alcohol and shrinking of gel during sol–gel process, was investigated. The influence of various factors, such as structure of ionic liquids, content of ionic liquids and types of precursor in the sol–gel process on the activity and stability of immobilized lipase was also studied. The highest hydrolytic activity of immobilized lipase was obtained when the hydrophilic ionic liquid, [C 2mim][BF 4], was used as an additive, while the highest stability of immobilized lipase was obtained by using hydrophobic ionic liquid, [C 16mim][Tf 2N]. Therefore, the binary mixtures of these ionic liquids as additives were used to obtain the optimal immobilized lipase, which shows both high activity and stability. The hydrolysis and esterification activities of lipase co-immobilized with the mixture of 1:1 at molar ratio of [C 2mim][BF 4] and [C 16mim][Tf 2N] were 10-fold and 14-fold greater than in silica gel without ionic liquids (ILs), respectively. After 5 days incubation of this immobilized lipase in n-hexane at 50 °C, 84% of initial activity was remained, while the residual activity of the lipase immobilized without ILs was 28%.

Stable Incorporation of Gold Nanorods into N-Isopropylacrylamide Hydrogels and Their Rapid Shrinkage Induced by Near-Infrared Laser Irradiation

In this study, we prepared gold nanorod (NR)-embedded N-isopropylacrylamide (NIPAM) hydrogels and studied their volume phase transition behavior induced by near-infrared (near-IR) laser irradiation utilizing the photothermal conversion characteristics of the NRs. When poly(ethylene glycol)-modified NRs were used for the preparation of composite gels, the NRs showed marked dispersion stability in the gel. Near-IR laser irradiation of the gel (cylindrical shape, diameter = 140 microm) under the following conditions, NR concentrations in the gel > or =100 microM and laser irradiation power > or =490 mW, resulted in shrinkage of the gel in the following manner: (1) waist formation around the irradiation spot and (2) growth of the waist along the axial directions of the gel. The gel shrinking induced by near-IR irradiation occurred much more rapidly than that afforded by a temperature jump, because the former was not accompanied by the skin layer formation, which disturbs the rapid shrinking of the gels. When a composite gel containing the model drug (rhodamine-labeled dextran) was irradiated with a near-IR laser, the rapid release of the drug was observed. Taking advantage of the high spatial resolution of the irradiation point, we further achieved the irradiation-point-specific release of the drug from one such gel.

Using ionic liquids to stabilize lipase within sol–gel derived silica

Ionic liquids (ILs) were used as additives to protect the inactivation of enzymes by released alcohol and shrinking of gel during the sol–gel process. The Candida rugosa lipases immobilized by using ILs in sol–gel process showed higher activity and stability than lipase immobilized without ILs. The hydrolytic and esterification activities of lipase coimmobilized with ILs were 5-fold and 16-fold greater than in silica gel without ILs. After 5 days incubation of lipase coimmobilized with 1-octyl-3-methylimidazolium bis[(trifluoromethyl)sulfonyl]amide at 50 °C, residual activity of lipase was about 80% of initial activity, while the lipase immobilized without ILs was completely inactivated. ILs may act as a template during gelation and reduce shrinkage of gel by pore filling. They can also behave as a stabilizer to protect the enzyme from the inactivation.

Monitoring nanoprobe diffusion in osmotically-stressed hydrogels

ABSTRACTWe have developed an optical chamber for studying the effect of swelling or shrinking of a gel on the translational diffusion of fluorescent nanoprobes embedded in the gel. On one side of the chamber, the gel is in contact with a hydrating or dehydrating polymeric solution through a porous membrane, allowing control of the rate of hydration or dehydration of the gel. On the other side, a laser beam is focused into the gel to excite the fluorescence of the nanoprobes, which is continuously monitored to reveal possible structural changes of the stressed gel. Using fluorescence correlation spectroscopy we measure correlation functions of the nanoprobes at various times following the contact of the gel with the hydrating or dehydrating solution, and determine changes of both the average concentration and the apparent diffusion time of the nanoprobes as the gel is shrinking or swelling. We have tested the chamber using the fluorophore, TAMRA (MW = 430 Da), embedded in a poly(vinyl-alcohol) gel that is being dehydrated by a solution of poly(vinyl-pyrrolidone) (28% w/w). As expected TAMRA moves slower as the gel shrinks. However, the changes in the diffusion time of TAMRA as a function of the PVA concentration of the shrinking gel appear to be different than those measured on TAMRA diffusing in PVA gels prepared at different PVA concentrations but with the same cross-link density.

In situ Electrochemical Fluorescence Studies of PPV

Conjugated phenylene-vinylene polymers are widely used in organic light-emitting and photovoltaic devices. The comprehension of the optical properties upon charge injection is of crucial importance for the improvement of such organoelectronic devices. The processes of electrochemical doping, electrolyte diffusion, and degradation have been studied by cyclic voltammetry and chronoamperometric methods. Kinetic studies by in situ fluorescence spectroscopy have been used for the determination of the mobility of charge carriers in the polymer making used of electrochemical Stern-Volmer analysis. The mobility of holes for MDMO-PPV measured by this method was 2.5 x 10(-7) cm2 V s(-1). Non-Faradic variations of the fluorescence after doping-dedoping cycles have been related to morphological changes in the polymeric layer. The evolution of the fluorescence obeys a first-order kinetics law, similarly to the trend of the variation of volume during gel shrinking.

Molecular thermodynamics for swelling of a mesoscopic ionomer gel in 1 : 1 salt solutions

For a microphase-separated diblock copolymer ionic gel swollen in salt solution, a molecular-thermodynamic model is based on the self-consistent field theory in the limit of strongly segregated copolymer subchains. The geometry of microdomains is described using the Milner generic wedge construction neglecting the packing frustration. A geometry-dependent generalized analytical solution for the linearized Poisson-Boltzmann equation is obtained. This generalized solution not only reduces to those known previously for planar, cylindrical and spherical geometries, but is also applicable to saddle-like structures. Thermodynamic functions are expressed analytically for gels of lamellar, bicontinuous, cylindrical and spherical morphologies. Molecules are characterized by chain composition, length, rigidity, degree of ionization, and by effective polymer-polymer and polymer-solvent interaction parameters. The model predicts equilibrium solvent uptakes and the equilibrium microdomain spacing for gels swollen in salt solutions. Results are given for details of the gel structure: distribution of mobile ions and polymer segments, and the electric potential across microdomains. Apart from effects obtained by coupling the classical Flory-Rehner theory with Donnan equilibria, viz. increased swelling with polyelectrolyte charge and shrinking of gel upon addition of salt, the model predicts the effects of microphase morphology on swelling.

A concept for miniaturized 3-D cell culture using an extracellular matrix gel

This paper presents a novel method to embed, anchor, and cultivate cells in a controlled 3-D flow-through microenvironment. This is realized using an etched silicon pillar flow chamber filled with extracellular matrix (ECM) gel mixed with cells. At 4 degrees C, while in liquid form, ECM gel is mixed with cells and injected into the chamber. Raising the temperature to 37 degrees C results in a gel, with cells embedded. The silicon pillars both stabilize and increase the surface to volume ratio of the gel. During polymerization the gel shrinks, thus creating channels, which enables perfusion through the chip. The pillars increase the mechanical stability of the gel permitting high surface flow rates without surface modifications. Within the structure cells were still viable and proliferating after 6 days of cultivation. Our method thus makes it possible to perform medium- to long-term cultivation of cells in a controlled 3-D environment. This concept opens possibilities to perform studies of cells in a more physiological environment compared to traditional 2-D cultures on flat substrates.