Shell Microgels Research Articles

Responsive core–shell microgels: Synthesis, characterization, and possible applications

ABSTRACTCore–shell microgels are of increasing interest as smart carriers of catalysts, as sensors, or as building blocks for colloidal superstructures. In the context of colloidal assemblies, photonic applications are probably the most promising ones. This progress report presents and discusses the most recent results in this area focusing on the last 2–3 years, and also gives some background information. In addition, potential perspectives of this area will be outlined. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2013, 51, 1073–1083

Multifunctionality in metal@microgel colloidal nanocomposites

The integration of metal nanoparticles within crosslinked polymeric microgels, in a well-defined core–shell structure, offers unique possibilities in various fields due to the potential to introduce multiple functionalities by tailoring the properties of the inorganic and the organic components. This Highlight describes recent developments related to hybrid nanocomposites comprising a metal core and a smart microgel shell. Different synthesis approaches for the preparation of such hybrid systems are described, including not only surface-initiated polymerization, but also in situ chemical reactions for core optimization and functionality addition. The main properties derived from the multifunctionality of these nanocomposites are also addressed and potential applications in the fields of catalysis and sensing are presented.

Preparation and characterization of OSA/CS core–shell microgel: in vitro drug release and degradation properties

Cationic polymers have been widely used as drug delivery systems. Herein, an oxidized sodium alginate/chitosan (OSA/CS) core–shell microgel was prepared via water-in-oil emulsion method. Morphological properties of the resulting microgel were determined by transmission electron microscopy, hydrodynamic diameter of the microgel was characterized by dynamic light scattering. The objective of this work was to achieve the colon-specific delivery of an antiulcerative colitis drug using a fully nontoxic carrier. 5-aminosalicylic acid (5-ASA) was chosen as a model drug, which is rapidly absorbed before entering the colon, thus it is necessary to develop a colon-specific delivery system for it. The in vitro drug release profile was established in buffer solutions with 0.1 M HCl/NaCl (pH 1.2) and 0.1 M phosphate buffered saline (pH 7.4) at 37 °C. The results indicated that this OSA/CS core–shell microgel inhibited the release of 5-ASA in stomach to a certain extent and is degradable in physiological conditions. Due to the excellent biocompatible nature of CS and OSA, this core–shell microgel has good biocompatibility and may have potential applications in oral controlled drug delivery systems.

Linearly thermoresponsive core–shell microgels: Towards a new class of nanoactuators

In this study we present novel core–shell microgels, with a shell made of poly(N-n-propylacrylamide) (pNNPAM) and a core consisting of poly(N-iso-propylmethacrylamide) (pNIPMAM), exhibiting a unique linear temperature response. The effect is produced by the large LCST gap of 23 °C between the shell- and the core-forming polymer. We demonstrate that the shell exhibits a temperature induced de-swelling process that is almost independent of the swelling properties of the core. Furthermore the active collapse of the shell forces a collapse of the core (which is known as the so-called “corset-effect”). In a region between 25 °C and 41 °C the response of the particles is directly proportional to the temperature. Moreover, the core properties were systematically varied, revealing the possibility to linearly change the magnitude of the linear swelling. Hence, these particles are very promising as piezo-like linear nano-actuators.

Protein Sorption to Charged Microgels: Characterizing Binding Isotherms and Driving Forces

We present a set of Langmuir binding models in which electrostatic cooperativity effects to protein sorption is incorporated in the spirit of Guoy-Chapman-Stern models, where the global substrate (microgel) charge state is modified by bound reactants (charged proteins). Application of this approach to lysozyme sorption to oppositely charged core-shell microgels allows us to extract the intrinsic, binding affinity of the protein to the gel, which is salt concentration independent and mostly hydrophobic in nature. The total binding affinity is found to be mainly electrostatic in nature, changes many orders of magnitude during the sorption process, and is significantly influenced by osmotic deswelling effects. The intrinsic binding affinity is determined to be about 7 k(B)T for our system. We additionally show that Langmuir binding models and those based on excluded-volume interactions are formally equivalent for low to moderate protein packing, if the nature of the bound state is consistently defined. Having appreciated this, a more quantitative interpretation of binding isotherms in terms of separate physical interactions is possible in the future for a wide variety of experimental approaches.

Correlation between Dielectric/Electric Properties and Cross-Linking/Charge Density Distributions of Thermally Sensitive Spherical PNIPAM Microgels

Thermally sensitive poly(N-isopropylacrylamide) (PNIPAM) spherical microgels with different charge distribution were prepared by different monomer and comonomer feeding methods. The frequency (f, 10–2–107 Hz) and temperature (T, 5–50 °C) dependent complex dielectric/electric properties of these microgels were analyzed by dielectric relaxation spectroscopy. Each microgel can be treated as a large multicharged macroion surrounded by many small counterions, and the relative displacement and the diffusion of counterions contribute to the permittivity and conductivity of the microgel dispersion. Both the dielectric permittivity at f > 103 Hz and the conductivity at f > 106 Hz decrease when the microgels shrink at temperatures higher than the lower critical solution temperature (LCST) of PNIPAM due to the shrink-induced counterion dissociation. The variation of either permittivity or conductivity with temperature reveals a sharper transition for the microgels or the microgel shell with a more uniform charge dis...

In Vitro Degradation and Drug-Release Properties of Water-Soluble Chitosan Cross-Linked Oxidized Sodium Alginate Core–Shell Microgels

Hydrogels based on sodium alginate (SA) have already been widely used in biomedical applications using Ca2+ as a cross-linker; however, these hydrogels tend to disintegrate in electrolyte solutions. To solve this problem, we present a kind of oxidized sodium alginate (OSA) microgel using water-soluble chitosan (WSC) as a cross-linker. This microgel was successfully prepared via an emulsion cross-linking technique at room temperature. The microgel was cross-linked by the formation of both Schiff base bonds and inter-polyelectrolyte complexes, which can efficiently eliminate the disintegration of the microgel in electrolyte solutions. Morphological properties of the resulting microgels were determined by transmission electron microscopy (TEM), hydrodynamic diameters of the microgels were characterized by dynamic light scattering (DLS). The objective of this work was to achieve the colon-specific delivery of an anti-ulcerative colitis drug. 5-Aminosalicylic acid (5-ASA) was chosen as a model drug and the in vitro drug-release profile was established in buffer solutions with 0.1 M HCl/NaCl (pH 1.2) and 0.1 M phosphate-buffered saline (PBS, pH 7.4) at 37°C. The microgel was incubated in 0.1 M PBS (pH 7.4) at 37°C to determine its degradation behavior. Cell cytotoxicity (tested by MTT assay) showed that this microgel had no significant cytotoxicity. These results indicated that this microgel prepared by introducing WSC into OSA may have potential applications in oral controlled drug-delivery systems. Therefore, the OSA/WSC microgel may be a useful carrier for the colon-specific delivery of anti-inflammatory drugs including 5-ASA and the enhanced therapeutic effect of ulcerative colitis.

ChemInform Abstract: Hybrids from Polymer Colloids and Metallic Nanoparticles: A Novel Type of “Green” Catalyst

AbstractReview: 82 refs.

One-Step Preparation of Uniform Cane-Ball Shaped Water-Swellable Microgels Containing Poly(N-vinyl formamide)

In this study we report the preparation of a new family of core-shell microgels that are water-swellable and have a morphology that is controllable by particle composition. Here, nearly monodisperse core-shell PNVF-xGMA [poly(N-vinylformamide-co-glycidyl methacrylate)] particles (where x is the weight fraction of GMA used) were prepared via nonaqueous dispersion (NAD) polymerization in one step. The shells were PGMA-rich and were cross-linked by reaction of epoxide groups (from GMA) with amide groups (from NVF). The core of the particles was PNVF-rich. A bifunctional cross-linking monomer was not required to prepare these new microgels. The particles had a remarkable "cane-ball"-like morphology with interconnected ridges, and this could be controlled by the value for x. The particle size was tunable over the range 0.8-1.8 μm. Alkaline hydrolysis was used to hydrolyze the PNVF segments to poly(vinylamine), PVAM. The high swelling pressure of the cationic cores caused shell fragmentation and release of some of the core polymer when the hydrolyzed particles were dispersed in pure water. The extent to which this occurred was controllable by x. Remarkably, the PGMA-rich shells could be detached from the hydrolyzed particles by dispersion in water followed by drying. The hydrolyzed PNVF-0.4GMA particles contained both positively and negatively charged regions and the dispersions appeared to exhibit charge-patch aggregation at low ionic strengths. The new cross-linking strategy used here to prepare the PNVF-xGMA particles should be generally applicable for amide-containing monomers and may enable the preparation of a range of new water-swellable microgels.

Direct comparison of the rheology of model hard and soft particle glasses

The effects of particle softness and the role of the outer shell mechanics on the linear viscoelasticity and yielding behaviour of colloidal glasses are critically assessed using three different model colloidal particles: (i) sterically stabilized PMMA particles with model hard sphere interactions, (ii) core–shell microgels with a deformable PNIPAM outer shell and (iii) ultra-soft star-like micelles with inter-penetrable multi-arms. The volume fraction dependence of the elastic modulus and the yield stress reflects the softness of the effective inter-particle potential. The yield strain exhibits distinct non-monotonic volume fraction dependence for hard spheres below close packing whereas for both soft particles it increases above close packing due to particle softness. Stress overshoots in start-up shear show a common increase with shear rate in all systems. However, the stress overshoots are significantly stronger in star-like micelles due to transient arm entanglements. In relation with that similar stress peaks are detected within the period of the large amplitude oscillatory shear only in star-like micelles. Finally, we discuss the scaling exponents for the G′ and G′′ decrease at large oscillatory strain amplitudes and their relation with steady shear stress.

Core–shell microgels as “smart” carriers for enzymes

We present a thermodynamic study of the adsorption of lysozyme on a negatively charged core–shell microgel at pH 7.2. The carrier particles consist of a polystyrene core onto which a charged poly(N-isopropylacrylamide-co-acrylic acid) network is attached. Isothermal titration calorimetry (ITC) is used to investigate the temperature and salt dependence of lysozyme binding. Our ITC analysis unequivocally shows that the adsorption of lysozyme onto the charged gel is driven by entropy. The addition of salt strongly decreases the binding affinity, indicating significant electrostatic contributions to the adsorption process. However, at high salt concentrations, substantial protein binding with unaltered entropies is still observed pointing to large contributions from hydrophobic interactions. Furthermore, the calorimetric analysis suggests that protonation of lysozyme takes place upon binding. This is directly shown by analysis of the enzymatic activity of adsorbed lysozyme. It was found that the activity is enhanced about ∼3.5 times, indicating that lysozyme has taken up approximately one proton when entering the gel. The entire set of data demonstrates that core–shell microgels present “smart” colloidal carriers for lysozyme that enhance its activity.

Glucose- and temperature-responsive core–shell microgels for controlled insulin release

In this paper, a novel core–shell microgel was prepared based on poly(N-isopropylacrylamide-co-3-acrylamidophenylboronic acid-co-dextran-maleic acid) (p(NIPAM-co-AAPBA-co-Dex-Ma)) coated silica nanoparticles via radical polymerization. The core–shell microgels presented glucose- and temperature-sensitivity, and the shell thickness could be easily regulated by the molar ratio of the comonomers. The polymer shell was active in drug loading as it underwent a distinct transition from a swollen state to a collapsed state as the temperature increased. Insulin was applied as a model drug and the behaviors of insulin loading/release were investigated. The insulin loading was temperature-dependent and up to 20%. The cumulative release of insulin in vitro showed that insulin release was dependent on glucose concentration. The glucose sensitivity of the microgels could be adjusted simply by adjusting the pH. The cytotoxicity assay indicated that the core–shell microgels have good biocompatibility. These results showed that the core–shell microgels have potential for a controlled insulin release system.

Core–shell microgels as model colloids for rheological studies

We review recent work done on the rheology of thermosensitive suspensions. These systems consist of aqueous suspensions of core–shell particles having a solid polystyrene core and a shell of thermosensitive crosslinked poly(N-isopropylacrylamide) (PNIPA). In cold water the thermosensitive PNIPA-network is swollen leading to a high effective volume fraction of the particles in suspension. Approaching the volume transition at 32 °C the network shrinks by expelling water. Hence, the effective volume fraction can be adjusted by the temperature. We demonstrate that these suspensions are a well-characterized model system for the study of the flow behavior of concentrated suspensions. In particular, experimental work done on this system can be compared to the predictions of the mode-coupling theory (MCT) of the fluid-to-glass transition. Excellent agreement is found demonstrating that MCT captures the essential features of the dynamics of flowing suspensions. In particular, MCT predicts a melting of the glass by shear which is fully corroborated by the experimental data.

Preparation and characterization of ellipsoidal-shaped thermosensitive microgel colloids with tailored aspect ratios

Prolate model colloids with defined properties can be obtained by the stretching of spherical polymeric particles, which is well-known for polystyrene and poly(methyl methacrylate) latices. The present study aims to extend this approach to functional core–shell particles in order to achieve a new class of anisotropic colloidal materials where both the aspect ratio and the effective volume fraction can be controlled by temperature. We describe the synthesis and characterization of these functional anisotropic core–shell particles consisting of a polystyrene (PS) core onto which a crosslinked thermoresponsive microgel shell of poly(N-isopropylmethacrylamide) (PNIPMAm) was grafted. Embedded into a film of polyvinylalcohol (PVA), the spherical composite microgels are heated above the glass transition temperature (Tg) of the polystyrene core and then stretched with different deformations γ. During the stretching, the particles adopt an elongated shape, that they retain after cooling to room temperature as confirmed by small-angle X-ray scattering performed on the films. Anisotropic composite microgels with aspect ratios ranging from 2.2 to 6.5 are recovered after dissolution of the PVA and purification, and are fully characterized by diverse methods such as transmission electron microscopy, confocal microscopy and light scattering. The temperature sensitivity of these anisotropic composite microgels is maintained as confirmed by dynamic light scattering and cryogenic electron microscopy performed below and above the volume phase transition of the shell.

A model describing the internal structure of core/shell hydrogels

We apply a theory to the constrained swelling of gel particles, explicitly accounting for the propagation of elastic forces through the particle. This approach, together with conventional thermodynamics of gel swelling, allows modelling of the equilibrium state of gels with properties that are spatially inhomogeneous. In our case we consider both a discrete inhomogeneity in the form of assigning different water solubilities to the core and shell domains of the particle, and a continuous inhomogeneity in allowing the density of chemical cross-links to vary gradually through the network. The model is used to understand the behaviour of temperature-sensitive poly(N-isopropyl acrylamide) core/poly(N-isopropyl methacrylamide) shell microgels investigated in an earlier experimental study. How the swelling of the core and shell is affected by the presence of each other at different temperatures is investigated and explained from a mechanical and thermodynamic perspective.

Cononsolvency of Poly(N,N-diethylacrylamide) (PDEAAM) and Poly(N-isopropylacrylamide) (PNIPAM) Based Microgels in Water/Methanol Mixtures: Copolymer vs Core−Shell Microgel

The cononsolvency effect on the swelling of different stimuli-sensitive microgels was investigated in water/methanol mixtures. Poly(N,N-diethylacrylamide) (PDEAAM) and poly(N-isopropylacrylamide) (PNIPAM) homopolymer microgels, a PDEAAM-core−PNIPAM-shell microgel, and a poly(N,N-diethylacrylamide-co-N-isopropylacrylamide) (P(DEAAM−NIPAM)) microgel have been synthesized. The core−shell and the copolymer microgel have similar compositions; the PDEAAM core of the PDEAAM-core−PNIPAM-shell particle contributes 80% to the total mass, and the copolymer consists of 75% DEAAM and 25% PNIPAM. They were characterized with dynamic light scattering (DLS) and small-angle neutron scattering (SANS). Microgels containing PNIPAM show the cononsolvency effect, and the particle size reaches a minimum at a methanol fraction of xmethanol = 0.2. Moreover, in core−shell microgels, the cononsolvency effect can be limited to one compartment of the particle. A core−shell microgel with a PDEAAM core and PNIPAM shell reveals a sharp ...

Synthesis and characterization of thermoresponsive and biocompatible core–shell microgels based on N-isopropylacrylamide and carboxymethyl chitosan

The objective of the study is to report thermally sensitive and biocompatible capability core–shell microgels. These core–shell microgels composed of a chemically crosslinked poly( N-isopropylmethacrylamide) (PNIPAM) core and a carboxymethyl chitosan (CMCS) shell were designed and synthesized by seed emulsion polymerization in this work. Due to the excellent biocompatible nature of CMCS, these PNIPAM/CMCS hybrid microgels have good biocompatibility. The chemical component of the PNIPAM/CMCS particles was confirmed by comparing the Fourier-transform infrared spectra (FTIR) of PNIPAM and PNIPAM/CMCS particles. Transmission electron microscopy (TEM) picture shows the core–shell structure of the resulted PNIPAM/CMCS nanocomposite particles. Thermally induced deswelling of PNIPAM/CMCS water dispersions which occurred in the increase of the temperature in the region of the lower critical solution (LCST) was investigated by means of dynamic light scattering (DLS). The thermal study by using differential scanning calorimetry (DSC) revealed that volume-phase transition temperatures of the physical microgel increased first with an increase in CMCS and slightly increased thereafter with a further increase in CMCS.

Influence of Architecture on the Interaction of Negatively Charged Multisensitive Poly(N-isopropylacrylamide)-co-Methacrylic Acid Microgels with Oppositely Charged Polyelectrolyte: Absorption vs Adsorption

Two sets of core-shell microgels composed of temperature-sensitive poly(N-isopropylacrylamide) (PNiPAM) with different spatial distribution of pH-sensitive methacrylic acid (MAA) groups were prepared. The cores consist of either PNiPAM (neutral core; nc) or PNiPAM-co-MAA (charged core; cc). A charged shell existing of PNiPAM-co-MAA was added to the neutral core (yielding neutral core-charged shell; nccs), on the charged core, on the other hand, a neutral shell of PNiPAM was added (charged core-neutral shell; ccns). Complexes of these microgels with positively charged poly(diallyldimethylammonium chloride) (PDADMAC) of different molar masses were prepared. The amount of bound polyelectrolyte was quantified, and the microgel-polyelectrolyte complexes were characterized with respect to electrophoretic mobility and hydrodynamic radius. The penetration of polyelectrolyte into the microgel was also monitored by means of lifetime analysis of a fluorescent dye covalently bound to poly(L-lysine) providing information on the probe's local environment. The architecture of the microgel has a significant influence on the interaction with oppositely charged polyelectrolyte. Complexes with microgel with the charged shell tend to flocculate at charge ratios of 1 and are thus similar to polyelectrolyte complexes with rigid colloidal particles. Complexes with microgels that consist of a charged core and a neutral shell show very different properties: They are still temperature sensitive and reveal an influence of the polyelectrolyte's chain length. Low molecular weight PDADMAC can penetrate through the neutral shell into the charged core, and thus nearly no charge reversal occurs. The high-MW polyelectrolyte does not penetrate fully and leads to charge reversal. The results demonstrate that microgels are able to absorb or adsorb polyelectrolytes depending on the polyelectrolyte's chain length and the microgels architecture. Complexes with different surface properties and different colloidal stability can be prepared, and polyelectrolytes can be encapsulated in the microgel core. Thus, multisensitive core-shell microgels combine permeability and compartmentalization on a nanometer length scale and provide unique opportunities for applications in controlled uptake and release.

Thermally Induced Phase Transition of Glucose-Sensitive Core−Shell Microgels

Four series of poly(N-isopropylacrylamide) (PNIPAM) (core)/poly(N-isopropylacrylamide-co-3-acrylamidophenylboronic acid) (P(NIPAM-AAPBA)) (shell) microgels were synthesized by the modification of PNIPAM (core)/poly(N-isopropylacrylamide-co-acrylic acid) (P(NIPAM-AA)) (shell) microgels with 3-aminophenylboronic acid (APBA). Their thermosensitive behaviors were studied by dynamic light scattering. Two or three phase transitions were detected depending on the shell thickness. These transitions were confirmed by the first derivative plot of the turbidity data. The first transition occurring at about 17 degrees C was assigned to that of the P(NIPAM-AAPBA) shell, whereas the second and third ones, which occur at about 22 and 28 degrees C, respectively, were assigned to that of the PNIPAM core. These results indicate that the influences of a shrunk P(NIPAM-AAPBA) shell on the different parts of the PNIPAM core are different. As the outer part, or the "shell" part of the PNIPAM core, directly connects with the P(NIPAM-AAPBA) shell, its phase transition temperature is reduced to a larger degree as compared with that of the inner part, or the "core" part. Glucose-induced swelling was observed for all the microgels, indicating their glucose-sensitivity. However, the degree of glucose-induced swelling is much smaller than that of the pure P(NIPAM-AAPBA) microgels.

A novel multi-responsive polyampholyte composite hydrogel with excellent mechanical strength and rapid shrinking rate

Series of hydrophilic core–shell microgels with cross-linked poly(N-isopropylacrylamide) (PNIPAAm) as core and poly(vinyl amine) (PVAm) as shell are synthesized via surfactant-free emulsion polymerization. Then, the microgels are treated with a small amount of potassium persulfate (KPS) to generate free radicals on the amine nitrogens of PVAm, which subsequently initiate the graft copolymerization of acrylic acid (AA), acryloyloxyethyl trimethyl ammonium chloride (DAC), and acrylamide (AAm) onto microgels to prepare multi-responsive composite hydrogels. The composite hydrogels consist of cross-linked ungrafted polyampholyte chains as the first network and microgels with grafted polyampholyte chains as graft point and second network and show surprising mechanical strength and rapid response rate. The investigation shows the compress strength of composite hydrogels is up to 17–30 MPa, which is 60–100 times higher than that of the hydrogel matrix. The composite hydrogel shows reversible switch of transmittance when traveling the lowest critical temperature (LCST) of microgels. When the composite hydrogel swollen in pH 2.86 solution at ambient condition is immersed into the pH 7.00 solution at 45 °C, a rapid dynamic shrinking can be observed. And the character time ( τ) of shrinking dynamic of composite hydrogel is 251.9 min, which is less than that of hydrogel matrix ( τ = 2273.7 min).