Temperature-responsive Gels Research Articles

Relatively homogeneous network structures of temperature-responsive gels synthesized via atom transfer radical polymerization.

The network structures of poly(N-isopropylacrylamide) (PNIPAAm) gels prepared by atom transfer radical polymerization (ATRP) were compared with those prepared by free radical polymerization (FRP), as a conventional radical polymerization. Temperature-responsive shrinkage was observed in the PNIPAAm gels prepared by ATRP and FRP (ATRP and FRP gels), which depended on the cross-linker content. From the light-scattered intensities, 〈I〉T, measured at the different sample positions, we used the partial heterodyne method to determine the dynamic fluctuation, 〈I〉F, spatial component, 〈I〉C, and correlation length, ξ, of the ATRP and FRP gels, as a function of the cross-linker content and temperature. While there is little difference in 〈I〉F and ξ between the ATRP and FRP gels, 〈I〉C of the ATRP gel was smaller than that of the FRP gel. In addition, we calculated the standard deviation of 〈I〉T for the ATRP and FRP gels, as a function of temperature to quantify the inhomogeneity of the gel networks. The standard deviation revealed that increasing cross-linker content and temperature makes the gel networks more inhomogeneous. The dynamic light scattering (DLS) measurement used to characterize the gel network revealed that ATRP suppresses inhomogeneity more effectively than FRP. The standard deviation of the scattered intensity is used in this study to quantify the inhomogeneity of the network structures. Quantitative evaluations of the inhomogeneity of the network structures by the standard deviation of the scattered intensity are useful in the investigation of the structure-property relationships of gels.

Reduction and temperature-responsive hydrogel composed of hydroxyethyl disulfide-bis-glycidyl ether-crosslinked poly(hydroxyethyl acrylate-co-methyl methacrylate)

Reduction and temperature-responsive gels were prepared by crosslinking poly(hydroxyethyl acrylate-co-methyl methacrylate) (poly(HEA-co-MMA)) using hydroxyethyl disulfide-bis-glycidyl ether(HEDSGE) as a disulfide cross-linker. Poly(HEA-co-MMA) exhibited lower critical solution temperature (LCST) behavior when MMA content was 7.4 and 8.3 mole %. Energy dispersive X-ray and Raman spectroscopy revealed that the copolymer chains of the gels were cross-linked by disulfide bond (i.e., HEDSGE). The gels released their content in proportion to the concentration of a reducing agent (i.e., dithiothreitol). In addition, the gels released their payloads in a temperature dependent manner, possibly due to the LCST property of the copolymer. Highlights Reduction and thermo-responsive gels were prepared by crosslinking poly(HEA-co-MMA) using HEDSGE. Poly(HEA-co-MMA) exhibited LCST behavior when MMA content was 7.4 mole % and 8.3 mole %. Gels released their content in proportion to dithiothreitol (a reducing agent) concentration. Gels released their payloads in a temperature dependent manner.

A Review on pH and Temperature Responsive Gels in Drug Delivery

Anticancer drugs play important roles in cancer treatment. However, these drugs have many disadvantages such as poor solubility, high toxicity, and serious side effects like hair loss, nausea and vomiting, anemia etc. To overcome these drawbacks, many attempts have been made to develop novel controlled drug delivery systems. They can encapsulate the drug and release it to the cancer site without leaking into other sites. The employment of multi-responsive hydrogels as a drug delivery system have some advantages over other drug delivery systems due to their ease of preparation, high efficiency, high-water content, tunable physical, and biological properties. The most advantages of these hydrogels is the volume phase transitions in their cross-linked three-dimensional networks as exposure to external stimuli such as temperature, pH, pressure, electric field, magnetic field and light. There has been research on other drug delivery systems which can respond to changes in pH and temperature for targeted drug release. Among those,gels have been studied mostly for their dual responsiveness. This provides an update on progress of gel based dual pH and temperature responsive drug delivery systems. Various systems under these categories for targeted and controlled delivery of different classes of drugs such as ant diabetic and antibiotic drugs with special emphasis on anticancer drugs are discussed in this review.

Patterning non-equilibrium morphologies in stimuli-responsive gels through topographical confinement.

Stimuli-responsive "smart" polymers have generated significant interest for introducing dynamic control into the properties of antifouling coatings, smart membranes, switchable adhesives and cell manipulation substrates. Switchable surface morphologies formed by confining stimuli-responsive gels to topographically structured substrates have shown potential for a variety of interfacial applications. Beyond patterning the equilibrium swelling behavior of gels, subjecting stimuli-responsive gels to topographical confinement could also introduce spatial gradients in the various timescales associated with gel deformation, giving rise to novel non-equilibrium morphologies. Here we show how by curing poly(N-isopropylacrylamide) (pNIPAAm)-based gel under confinement to a rigid, bumpy substrate, we can not only induce the surface curvature to invert with temperature, but also program the transient, non-equilibrium morphologies that emerge during the inversion process through changing the heating path. Finite element simulations show that the emergence of these transient morphologies is correlated with confinement-induced gradients in polymer concentration and position-dependent hydrostatic pressure within the gel. To illustrate the relevance of such morphologies in interfacial applications, we show how they enable us to control the gravity-induced assembly of colloidal particles and microalgae. Finally, we show how more complex arrangements in particle assembly can be created through controlling the thickness of the temperature-responsive gel over the bumps. Patterning stimuli-responsive gels on topographically-structured surfaces not only enables switching between two invertible topographies, but could also create opportunities for stimuli ramp-dependent control over the local curvature of the surface and emergence of unique transient morphologies. Harnessing these features could have potential in the design of multifunctional, actuatable materials for switchable adhesion, antifouling, cell manipulation, and liquid and particle transport surfaces.

Mechanical properties of temperature-responsive gels containing ethylene glycol in their side chains.

The mechanical properties of temperature-responsive and biocompatible poly(oligo-ethylene glycol methyl ether methacrylate)-based gels were investigated using dynamic viscoelasticity measurements so as to find applications in tissue and biomedical engineering. The gels were copolymerized using two ethylene glycol methacrylate monomers with diethylene glycol side chains: diethylene glycol methacrylate (MeO2MA), which contains two ethylene oxide units, and oligo-ethylene glycol methyl ether methacrylate (OEGMA) with either four or five ethylene oxide units. The storage (G') and loss (G'') moduli of these gels exhibit unique temperature-responsive behavior and depend on the copolymerization ratio. In MeO2MA-rich gels, phase separation occurred with increasing temperature, resulting in a significant increase in G' and the disappearance of the frequency dependence of G''. Although phase separation of OEGMA-rich gels was also observed with increasing temperature, it resulted in only a slight increase in the storage modulus due to the steric hindrance of the side chain. The mechanical properties of these gels are thus found to be strongly affected by a slight difference in the number of ethylene oxide groups in their side chains.

Structure–property relations for temperature-responsive gels

A model is developed for equilibrium unconstrained swelling of thermo-responsive gels that takes into account formation of hydrophobic aggregates above the volume phase transition temperature (VPTT). The solid phase of a gel is treated as a polymer network with two types of junctions: covalent bonds that are formed under cross-linking polymerization of a pre-gel solution, and physical junctions that are developed due to aggregation of hydrophobic segments above VPTT. Adjustable parameters are found by fitting experimental data on poly(N-isopropylacrylamide) gels with various molar fractions of monomers and cross-linker and various routes of cross-linking (chemical reaction versus irradiation), furnished with sacrificial chains in pre-gel solutions, and synthesized under phase separation condition in mixed solvents. The ability of the model to describe equilibrium swelling diagrams is confirmed by numerical simulation.

Mechanical response and equilibrium swelling of temperature-responsive gels

A model is developed for the mechanical behavior of thermo-responsive gels that accounts for an increase in concentration of physical cross-links between chains driven by phase separation. Governing equations are derived for equilibrium water uptake under unconstrained and constrained swelling, as well as for the elastic response of swollen gels. These relations are applied to the analysis of swelling diagrams on NIPA, VCL, PEG, and NIPA-DMIAA gels. The ability of the model to predict the growth of elastic moduli above the volume phase transition temperature is confirmed by numerical simulation.

Mechanical and responsive properties of temperature-responsive gels prepared via atom transfer radical polymerization

Temperature-responsive poly(N-isopropylacrylamide) (PNIPAAm) gels were prepared via atom transfer radical polymerization (ATRP), and their mechanical and responsive properties were investigated from the viewpoint of their network homogeneity.

Temperature Responsive Gels Based on Pluronic F127 and Poly(vinyl alcohol)

Aqueous mixtures of Pluronic F127 and poly(vinyl alcohol) (20% polymer concentration in the system) were prepared and investigated by rheology and laser particle size distribution, in order to obtain temperature responsive gels for biomedical applications. The influence of the gelation conditions and system composition on the viscoelastic parameters and particle size distribution (which are very sensitive to the sol−gel transition process) was followed. The investigations realized at physiological temperature (37 °C) showed the thixotropic profile of the gels, the yield stress induced by the Pluronic presence, and the high elasticity evidenced in the creep-recovery tests.

Investigation of temperature-swing adsorption of aromatic compounds in water using polyampholyte gels with hydrophilic nonionic groups

The adsorption and desorption behaviors of bisphenol-A in water have been studied using the temperature-responsive polyampholyte gels composed of sodium styrene sulfate (SSS), vinylbenzyl trimethylammonium chloride (VBTA), and N,N′-dimethylacrylamide (DMAA). The resultant behaviors of the SSS-VBTA-DMAA gels of three different monomer compositions are compared with those of the SSS-VBTA gel that can adsorb bisphenol-A at room temperature and desorb the pre-adsorbed bisphenol-A at higher temperature as in our previous report (Morisada et al., Adsorption 2008, 14, 621), to elucidate the effect of DMAA on the adsorption property of the polyampholyte gel. The adsorption isotherms, which were described by Henry's law, indicated that the adsorption amounts of bisphenol-A onto the SSS-VBTA-DMAA gels at 298 K are much larger than those at 353 K. At 298 K, the Henry's adsorption constant for the SSS-VBTA-DMAA gel with the lowest mole fraction of DMAA was larger than that for the SSS-VBTA gel, whereas a further increase in the DMAA mole fraction resulted in a decrease in the adsorption constant. Moreover, the temperature-swing adsorption experiments demonstrated that the SSS-VBTA-DMAA gel can repeatedly adsorb bisphenol-A at 298 K and desorb some of the pre-adsorbed bisphenol-A at 353 K, where the desorption amount of bisphenol-A is larger than that from the SSS-VBTA gel. The results obtained here show that the addition of DMAA to the SSS-VBTA gel slightly improves the performance of the gel as a temperature-responsive adsorbent. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010

Temperature responsive hydroxypropyl cellulose for encapsulation

This work focuses on the use of temperature responsive gels (TRGs) (polymeric hydrogels with a large temperature-dependent change in volume) for flavor retention at cooking temperatures. Specifically, we have studied a gel with a lower critical solution temperature (LCST) that swells at low temperatures and collapses at high temperatures. In the collapsed state, the polymer acts as a transport barrier, keeping the volatile flavors inside. We have successfully synthesized a cellulose gel that exhibits this volume change and have encapsulated an oil phase inside the gel. The flavor-loaded encapsulated oil exhibited an increased release time when compared to similar gelatin capsules.

Body temperature-responsive gels derived from hydroxypropylcellulose bearing lignin II: adsorption and release behavior

The lower critical solution temperature (LCST) of hydroxypropylcellulose bearing lignin (HPC-L) prepared from unbleached pulp depends on the amount of residual lignin. An HPC-L gel having thermal properties reflective of original HPC-L was prepared using ethyleneglycol diglycidylether as a crosslinker, as previously reported [Uraki et al. (2004) Carbohydr. Polym. 58:123–130], and the volume transition temperature was detected as an endothermic peak by differential scanning calorimetry (DSC). The adsorption and release behavior of the guest molecules to/from this gel was then examined. When the adsorption of cationic and anionic guests was compared, cationic methylene blue (MB) was adsorbed in larger amounts than anionic methyl orange (MO). In addition, MB adsorption into the HPC-L gel was greater than MB adsorption into the HPC gel prepared from commercially available HPC. This suggests that residual lignin affects the adsorption of organic dyes. Significant differences were not observed with respect to the release of MB from HPC-L at 38 °C and lower temperatures. In the adsorption of surfactants, marked adsorption at around the critical micelle concentration of the ionic surfactants and gel swelling were observed. Such swelling did not occur in the aqueous nonionic surfactant solution, although the nonionic surfactant was adsorbed into the gel. Gel swelling may have been caused by the electrostatic repulsion of the ionic surfactants adsorbed onto the polymer chains within the gel structure.

New Nanophase Separated Intelligent Amphiphilic Conetworks and Gels

AbstractAmphiphilic conetworks (APCN) are new materials composed of covalently bonded otherwise immiscible hydrophilic and hydrophobic polymer chains. The amphiphilic nature of these new crosslinked polymers is indicated by their swelling ability in both hydrophilic and hydrophobic solvents. Special synthetic techniques have been developed for the preparation of these new unique materials, such as poly(2‐hydroxyethyl methacrylate)‐l‐polyisobutylene (PHEMA‐l‐PIB), poly(methacrylic acid)‐l‐polyisobutylene (PMAA‐l‐PIB) and poly(N,N‐dimethylaminoethyl methacrylate)‐l‐polyisobutylene (PDMAEMA‐l‐PIB) (‐l‐ stands for linked by). Due to their unique architecture, macrophase separation of the immiscible components is prevented by the chemical bonding in the conetworks. As a results, phase separation leads to nanodomains with usually 2‐20 nm domain sizes as shown by AFM measurements. The nanophase separated morphology may also lead to smart temperature responsive gels with high mechanical stability, such as in the case of poly(N,N‐dimethylaminoethyl methacrylate)‐l‐polyisobutylene APCNs as discovered during these studies. In another approach, poly(2‐hydroxyethyl methacrylate)‐l‐polyisobutylene and poly(methacrylic acid)‐l‐polyisobutylene APCNs were prepared by a special two‐step process. The new PMAA‐l‐PIB polyelectrolyte APCNs possess smart (intelligent) reversible pH‐responsive properties in aqueous media. These unique conetwork structures and properties of these new emerging materials may lead to numerous new potential applications, such as smart materialk products, sustained drug release matrices, biomaterials, nanohybrids, nanotemplates etc.

Body temperature-responsive gels derived from hydroxypropylcellulose bearing lignin

Hydroxypropylcellulose bearing lignin (HPC-L) prepared from softwood by the atmospheric acetic acid pulping followed by derivatization showed the lower critical solution temperature (LCST) of approximate 38 °C in 2% aqueous solution, which were 5° lower than that of pure HPC with a similar molecular mass to HPC-L. Two types, urethane and epoxy, of chemical gels were prepared from HPC-L and HPC using hexamethylene diisocyanate and polyethylene glycol diglycidyl ether as crosslinkers, respectively. The urethane-type gels from both HPC's have undergone shrinking at less than 20 °C. This thermoresponse of the gels did not reflect LCST of the sources. Such low temperature shrinking was brought about by the hydrophobicity of crosslinker. On the other hand, the epoxy-type gels showed a very similar volume transition temperature (VTT) upon shrinking to LCST; the epoxy-type HPC-L gel showed VTT at 38 °C, while the corresponding HPC gel had VTT at 46 °C. This difference in VTT as well as LCST was attributable to hydrophobicity of lignin dangled to HPC. Thus, the body temperature-responsive gel can be prepared from HPC-L with the water-soluble crosslinker.

Dewatering Fine Coal Slurries by Gel Extraction

ABSTRACT Gel extraction is evaluated as a novel technique for dewatering fine coal slurries. This technique uses temperature-responsive gels to absorb water from slurries at low temperatures; after separation of the swollen gel from the dewatered slurry, the gel is heated slightly above ambient temperature, which causes it to release the water it absorbed. The gel can then be recycled. The equilibrium and kinetic properties of poly(N-isopropylacrylamide) gel were evaluated for utility in this process. The gels effectively dewatered slurries to around 70 wt% solids; performance was not a strong function of particle size, though coarser slurries (−16 mesh) could be dewatered to greater extents than the finer slurries (325 = 400 mesh). The gels showed no sign of deterioration over a period of 2 months and 20 cycles.