Microgel Dispersions Research Articles

Fat-free fermented concentrated milk products as milk protein-based microgel dispersions: Particle characteristics as key drivers of textural properties.

The popularity of fat-free fermented concentrated milk products, such as fresh cheeses and high-protein yogurt, has increased over the recent years, attributed to greater availability and improvements in taste and texture. These improvements have been achieved through modifications and new developments in processing technologies, for example, higher heat treatment intensities and incorporating different membrane filtration technologies. Though numerous processing parameters are discussed in the literature, as well as reasons behind the developments, detailed examinations of how process modifications affect the final textural attributes of these products are lacking. To draw links between processing parameters and texture, we review the literature on fat-free fermented concentrated milk products from the perspective of fermented milk protein-based microgel particles as the basic structural unit. At each main processing step, relationships between process parameters, micro- and macrostructural and sensory (textural) properties are discussed.An overview of particle characteristics that drive structural changes at each processing step is developed in relation to textural characteristics. Using this approach of assessing relationships between structural characteristics of concentrated dispersions of fat-free fermented milk protein-based microgel particles and processing parameters provides a basic context for the selection of optimal parameters to achieve a desired texture.

Regenerated Cellulose Microgel: A Promising Reinforcing Agent and Gelator for Soft Matter

Unlike one-dimensional (1D) cellulose materials, such as cellulose nanofibers (CNFs), cellulose nanowhiskers, or microfibrillated cellulose, three-dimensional (3D) cellulose microgels have a multiscale structure composed of microscale particle size and nanoscale porous network. The hydrophilicity and microscale particle size enable homogeneous dispersion of microgels in water and render unique rheology property. While the nanoporous network of microgels assembled by the cellulose nanofiber allows diffusion and in situ polymerization of small molecules to form interpenetrating polymer network. Therefore, there is a huge potential for cellulose microgels as reinforcing agent and gelator for soft matter. In this work, as a proof of concept, cellulose microhydrogels (CMH) with narrow particle size distribution and tunable microstructure are prepared by applying high-speed shearing to chemically cross-linked regenerated cellulose macrogel. The as prepared microgels can be homogeneously dispersed in water, and form interpenetrating network with in situ polymerized polyacrylamide (PAM). This interpenetrating behavior between microgels and PAM accelerates the gelation process. Compared to CNFs reinforced PAM hydrogel, CMH/PAM hydrogel shows superior toughness in the compressive test with tunable mechanical properties by adjusting microstructure of microgel through tuning the cross-linking density. Cellulose microgel shows as a promising reinforcing agent and gelator for potential use in 3D printing of soft matter or as rheological modifier.

Crosslinked polymer encapsulated palladium nanoparticles for catalytic reduction and Suzuki reactions in aqueous medium

Acrylamide and N-isopropylacrylamide were copolymerized in the presence of a N,N-methylenebisacrylamide crosslinker to obtain poly(N-isopropylacrylamide-co-acrylamide) [P(NA)] polymer colloidal particles. Pd nano crystals with diameter of 4–8 nm were loaded into the [P(NA)] microgels by reduction of [PdCl4]-2 within dispersion of polymer microgels. The Pd NPs-loaded hybrid microgels were analysed by TEM, STEM, EDX and XRD. The catalytic ability of the Pd-[P(NA)] system was investigated towards reductive transformation of nitroarenes into corresponding aryl amines and Suzuki coupling transformation in a green solvent, H2O. The progress of catalytic reaction was examined by thin layer chromatography (TLC). Different reactants were effectively converted into their corresponding products with great to fabulous yields (extending from 75 to 97%) under gentle reaction conditions. The Pd-[P(NA)] catalyst is stable for long time and can be utilized numerous times without any notable loss in its catalytic action.

Microrheology of thermoresponsive poly(N-isopropylacrylamide) microgel dispersions near a substrate surface

HypothesisRelative to the bulk systems, the near-wall (<500 nm) rheological responses of soft poly(N-isopropylacrylamide) (PNIPAM) microgel dispersions may exhibit distinct dependence on the frequency (ω), temperature (T), and effective volume fraction (ϕeff) during the volume phase transitions. The microrheological behaviors are expected to be governed by the near-wall microstructure and its spatial heterogeneity. ExperimentsThe combination of active microrheometry (multipole magnetic tweezers) and total internal reflection microscopy (TIRM) was employed to probe the structure–rheology relationships of microgel dispersions near a substrate surface. The ω, T, and ϕeff-dependences of the storage modulus (G′), loss modulus (G"), and softness (J) were analyzed by power-law and Arrhenius-like scaling theories. The fluctuation of J was further analyzed to give a quantitative description of the inhomogeneity in the near-wall regions. Findings(1) Remarkable differences in the rheological behaviors between the bulk and near-wall cases are revealed, where the latter shows a segmented overlap behavior in ϕeff; (2) Five regimes of ϕeff that correspond to distinct physical states of the microgel dispersions are determined; (3) The near-wall local structures exhibit more heterogeneity in the glass and colloidal gel regimes as compared to the liquid regime.

Microstructured Macromaterials Based on IPN Microgels.

This study investigates the formation of microstructured macromaterials from thermo- and pH-sensitive microgels based on interpenetrating networks of poly-N-isopropylacrylamide (PNIPAM) and polyacrylic acid (PAA). Macromaterials are produced as a result of the deposition of microgel particles and subsequent crosslinking of polyacrylic acid subnetworks to each other due to the formation of the anhydride bonds during annealing. Since both PNIPAM and PAA are environment-sensitive polymers, one can expect that their conformational state during material development will affect its resulting properties. Thus, the influence of conditions of preparation for annealing (pH of the solution, the temperature of preliminary drying) on the swelling behavior, pH- and thermosensitivity, and macromaterial inner structure was investigated. In parallel, the study of the effect of the relative conformations of the IPN microgel subnetworks on the formation of macromaterials was carried out by the computer simulations method. It was shown that the properties of the prepared macromaterials strongly depend both on the temperature and pH of the PNIPAM-PAA IPN microgel dispersions. This opens up new opportunities to obtain materials with pre-chosen characteristics and environmental sensitivity.

Volume Fraction Measurement of Soft (Dairy) Microgels by Standard Addition and Static Light Scattering

The volume fraction of the dispersed phase in concentrated soft (dairy) microgels, such as fresh cheese, is directly related to structure and rheology. Measurement or modeling of volume fraction for soft and mechanically sensitive microgel dispersions is problematic, since responsiveness and rheological changes upon mechanical input for these systems limits application of typical functional relationships, i.e., using apparent viscosity. In this paper, we propose a method to measure volume fraction for soft (dairy) microgel dispersions by standard addition and volume-weighted particle size distributions obtained by static light scattering. Relative particle volumes are converted to soft particle volume fraction, based on spiked standard particle volumes. Volume fractions for two example microgel dispersions, namely, differently produced fresh cheeses, were evaluated before and after post-treatments of tempering and mechanical processing. By selecting the size of standard particles based on size ratios and the levels of the mixing ratios/relative fractions, the method could be applied robustly within a wide range of particle sizes (1 to 500 μm) and multimodal size distributions (up to quadmodal). Tempering increased the volume fraction for both example microgel dispersions (P < 0.05). Subsequent mechanical treatment reduced the volume fraction back to the starting value before tempering (P < 0.05). Furthermore, it was shown that the increase and successive decrease in apparent viscosity with tempering and mechanical post-treatments is not exclusively due to particle aggregation and breakdown, but to volume changes of each particle. For environmentally responsive soft matter, the proposed method is promising for measurement of volume fraction.

Synthesis and In Situ Gelation Behavior of Thermoresponsive Poly(N-isopropylacrylamide)/Chitosan Microgels

Thermoresponsive microgels composed of poly(N-isopropylacrylamide) (PNIPAM) and chitosan (CS) were synthesized by soap-free emulsion polymerization of N-isopropylacrylamide monomer in the presence of CS, using an anionic radical initiator and a chemical crosslinker. The resulting PNIPAM/CS microgels contracted in an aqueous phase at elevated temperature, exhibiting a negatively thermoresponsive nature with a volume phase transition temperature (VPTT) of about 34.6 °C. The microgel dispersions with a relatively high polymer content gelated when being heated across their VPTT due to their attractive hydrophobic interactions. We systematically investigated this thermo-induced in situ gelation behavior by small-deformation oscillatory rheological measurements within the linear viscoelastic region. The dynamic temperature ramp sweeps revealed that the gelation temperature was greatly influenced by the polymer content of the microgel dispersion, decreasing with increasing polymer content. The isothermal time sweeps showed that the gelation occurred more quickly at either higher polymer content or higher isothermal heating temperature. The relaxation dynamics for the as-formed gel were also studied by isothermal frequency sweeps. It was found that the gels exhibited a frequency-independent storage modulus in the lower frequency range, and the greater the polymer content of the microgel dispersions, the more elastic the gels that formed.

Incorporation of fluorescent microgels inside calcite single crystals

Single crystals have been shown to incorporate guest materials (e.g., gel networks and functional particles) and the incorporated materials endow the crystal hosts with non-intrinsic properties. The acquisition of the non-intrinsic crystal properties impels us to diversify the incorporated guest materials and thus to spread the potential applications of single-crystalline materials. Here, fluorescent NIPAM-based microgels have been selected as the guest materials to be incorporated into the crystal hosts. In the presence of microgels, calcite crystallize from either solution or gel media have been examined. Solution-grown crystals exclude the microgels, while two gel-grown methods have proved effective for incorporating these microgels inside calcite single crystals. On one hand, an additional macroscopic agarose gel-network is introduced to trap the microgels. Crystallization in the agarose gel leads to incorporation of agarose gel-network together with the trapped microgels. The microgel concentration in the crystal reaches 33 mg/mL. On the other hand, dispersions of microgels are concentrated to about 352 mg/mL and the microgels aggregate into non-flowing macroscopic gels (termed as macrogels). Crystallization in a macrogel results in its incorporation. By showing the incorporation of microgels through physical entrapment, this work provides a new platform to functionalize single crystals by the microgels.

Oxidation-Responsive Emulsions Stabilized by Cleavable Metallo-Supramolecular Cross-Linked Microgels.

An original route to develop an advanced class of microgel emulsifiers containing stimulable metallo-supramolecular instead of frozen covalent cross-links is reported. The poly(N-isopropylmethacrylamide) (PNiPMAM) chains of the microgel are connected by iron(II)-bis(terpyridine) coordination supramolecular complexes that can be cleaved on demand, leading to unique properties both at interfaces and in volume. The microgel synthesis is not demanding, and the characterization of its supramolecular structure can be precisely achieved by standard methods. Singularly, interfaces of an oil-in-water emulsion stabilized by the supramolecular particles can be triggered at the molecular scale by oxidation of Fe(II) to Fe(III), leading to emulsion breaking. In bulk, we show that a microgel dispersion can indeed be transformed into a polymer solution upon oxidation. Our study paves the way to the discovery of unusual microgel properties as our proof-of-concept can be extended to different supramolecular chemistry and architecture.

Observation of Unusual Thermoresponsive Volume Phase Transition Behavior of Cubic Poly(N-isopropylacrylamide) Microgels.

Here, we report the observation of an unusual thermoresponsive volume phase transition behavior of cubic poly(N-isopropylacrylamide) (PNIPAM) microgels. Cubic PNIPAM microgels with a mean edge size of 125 ± 41 nm were synthesized via electrochemical-initiated radical polymerization with a photovoltaic cell as power supply. In turbidity and laser light scattering studies on dilute aqueous dispersions of these cubic microgels, both the light attenuation and hydrodynamic radius variations with temperature reveal an additional transition at about 25.0 °C, besides the widely reported volume phase transition at the PNIPAM LCST that is typically found for (quasi-)spherical microgels. This unusual thermoresponsive volume phase transition behavior of the cubic microgels can be elucidated by using a core-corona model, with the contribution from each part varying at different temperatures. The finding is also checked by thermal analysis.

Large scale aggregation in magnetic colloids induced by high frequency magnetic fields

The paper presents evidence of large scale aggregation in an aqueous dispersion of magnetic microgels, induced by 100 kHz AC magnetic fields with amplitude ranging from 40 Oe to 120 Oe. The microgels with 250 nm z-average hydrodynamic diameter and 44 emu/g saturation magnetization are composed by 8 nm magnetite nanoparticles imbedded in (3-acrylamidopropyl)-trimethylammonium chloride (APTAC) matrix. The magnetically induced aggregation was investigated by means of light extinction (LE) and small angle light scattering (SALS) at room temperature. The SALS experiments show the formation and growth of field direction elongated aggregates with thickness in the range of a few microns and length in the range of tens to hundreds microns. The extinction experiments show the increasing of aggregate total volume with increasing magnetic field amplitude. It was estimated that, at the saturation of the aggregation process in 120 Oe amplitude field, the aggregates consist of about 80% of the total amount of magnetic microgels within the sample. These findings have potential interest for magnetic composite applications in magnetic hyperthermia and heat transfer applications.

A Way to Predict Gold Nanoparticles/Polymer Hybrid Microgel Agglomeration Based on Rheological Studies.

In this work, a detailed rheological study of hybrid poly(acrylamide-co-acrylic acid) P(AAm-co-AAc) aqueous microgel dispersions is performed. Our intention is to understand how the presence of gold nanoparticles, AuNP, embedded within the microgel matrix, affects the viscoelastic properties, the colloidal gel structure formation, and the structure recovery after cessation of the deformation of the aqueous microgel dispersions. Frequency sweep experiments confirmed that hybrid microgel dispersions present a gel-like behavior and that the presence of AuNP content within microgel matrix contributes to the elasticity of the microgel dispersions. Strain sweep test confirmed that hybrid microgels aqueous dispersion also form colloidal gel structures that break upon deformation but that can be recovered when the deformation decreases. The fractal analysis performed to hybrid microgels, by applying Shih et al. and Wu and Morbidelli’s scaling theories, evidenced that AuNP significantly affects the colloidal gel structure configuration ending up with the formation of agglomerates or microgel clusters with closer structures in comparison to the reference P(AAm-co-AAc) aqueous microgel dispersions.

Controlled Release of Glucose from Orally Delivered Temperature- and pH-Responsive Polysaccharide Microparticle Dispersions

Dual-responsive polysaccharide microgel dispersions were synthesized by the self-assembly of a temperature-responsive water-soluble cellulose ether, hydroxypropyl cellulose (HPC), and a pH-responsi...

Approach to the preparation of temperature‐sensitive poly(N‐isopropylacrylamide)/polyethylenimine microgel

A novel route was developed to the preparation of temperature-sensitive poly(N-isopropylacrylamide)/polyethylenimine (PNIPAM/PEI) microgels by the radical copolymerisation of N-isopropylacrylamide with acryloylated PEI in aqueous media. Here, the acryloylated polyethylenimine (PEI), used as a macromonomer, was synthesised by a dehydration condensation reaction between PEI and acrylic acid. The as-prepared microgels exhibited well-defined core/shell structure with PNIPAM-rich core surrounded by PEI-rich shell. The microgels showed the volume phase transition temperature near physiological temperature, which can be easily regulated by the pH of microgel dispersion.

PH-Sensitive Polyampholyte Microgels of Poly(Acrylic Acid-co-Vinylamine) as Injectable Hydrogel for Controlled Drug Release.

pH-sensitive polyampholyte microgels of poly(acrylic acid-co-vinylamine) (P(AA-co-VAm)) were developed as an injectable hydrogel for controlled drug release. The microgels of P(AA-co-VAm) were prepared via inverse suspension polymerization of acrylic acid and N-vinylformamide followed by hydrolysis of poly(N-vinylformamide) (PNVF) chains of the resultant microgels under basic condition. The pH-sensitivity of the P(AA-co-VAm) microgels in zeta potential and swelling ratio were investigated using a zeta potential analyzer and optical microscope. The results showed that both the zeta potential and the swelling ratio of the microgels were highly affected by the solution pH. By changing the pH of P(AA-co-VAm) microgel dispersion, the interparticle interaction and the swelling ratio of the microgels could be well adjusted and a colloidal hydrogel could be fabricated at moderate pH, showing a pH-triggered reversible fluid-gel transition. Using the polyampholyte P(AA-co-VAm) microgels as an injectable hydrogel drug release system, a sustained drug release could be achieved, indicating the great potentials of the pH-sensitive P(AA-co-VAm) microgels for controlled drug delivery.

What do stirred yogurt microgels look like? Comparison of laser diffraction, 2D dynamic image analysis and 3D reconstruction

Abstract Stirred yogurts can be considered as concentrated dispersions of microgels. The size, shape and fractal dimension of these microgels are known to have a direct impact on textural and sensory properties of stirred yogurts, consequently their thorough characterization is of interest. Different techniques can be used including laser diffraction (LD), 2D dynamic image analysis or 3D reconstruction from z-stack confocal images. The aim of this study was to compare the ability of the three techniques to describe the size, shape and fractal dimension of the stirred yogurt microgels. Two stirred yogurts with different compositions, one fat free (0.1%) and one high fat (9.3%), were used. The microgel size distributions obtained were similar using LD, 2D image analysis and 3D reconstruction. Additionally, 2D image analysis and 3D reconstruction enabled visualization of the microgels and access to their shape through morphological factors such as roughness index. The microgels observed were non-spherical, rough and heterogeneous in shape. All three techniques also made it possible to determine the fractal dimension of the microgels, but 2D image analysis displayed lower values than LD and 3D reconstruction.

Structural color and rheology of self-assembled poly(N-isopropylacrylamide-methacrylic acid) microgels in water

Microgels are excellent building blocks for photonic band gap materials (PBG) in water. Aqueous dispersions of temperature and pH responsive microgels composed by isopropylacrylamide (NIPA, 92 mol%) and methacrylic acid (MA, 8 mol%) show structural color in the concentration range of 44–88 mg/mL. At temperatures below the LCST of pNIPAM in water (∼32 °C), the dispersions self-organize as an elastic solid, with concentration-dependent refractive index. At 22 °C the color changes from red to blue when increasing the concentration. This is due to an increase in the elastic modulus. A transition to a liquid phase (characterized by a loss of the elastic modulus and of the structural color) occurs at a temperature just below the LCST. The color is a result of the diffraction of the visible light that do not propagate within the PBG as shown by the confocal reflectivity images of the microgel dispersions.

Microgels as viscosity modifiers influence lubrication performance of continuum.

Biocompatible microgels have been demonstrated to act as excellent lubricants, however, the influence of the continuum on their overall mechanical performance has been neglected so far. In this work, the mechanical performance of colloidal whey protein microgels (hydrodynamic diameter ∼100 nm measured using dynamic light scattering and atomic force microscopy) of different rigidity dispersed in Newtonian (buffer and corn syrup) or complex non-Newtonian fluids (xanthan gum) is investigated for the first time via rheology and soft tribology. Dispersions of both soft microgels (G' ∼ 100.0 Pa) and hard microgels (G' ∼ 10.0 kPa) were observed to act as thickeners in buffer as well as in low viscosity corn syrup and correspondingly reduced the friction, latter decreased as a function of the increased rigidity of the microgels. Differently, in high viscosity continuum, the microgels acted as thinning agents and increased the friction. In the lubrication limit, microgels in buffer or corn syrup behaved as Newtonian fluids with effective viscosity corresponding to their second Newtonian plateau value (η∞). However, the lubrication performance of the microgels dispersed in the complex fluid (xanthan gum) could not be described quantitatively by η∞. For the low viscosity xanthan gum, the microgels had no influence on friction. Nevertheless, for the high viscosity counterparts, the soft microgels acted as thinning agents whilst the hard microgels accelerated the onset of elastohydrodynamic regime. This study demonstrates that microgels act as viscosity modifiers directly influencing the tribological performance, depending upon a subtle interplay of rheological properties of the particles and continuum.

Versatile oligo(ethylene glycol)-based biocompatible microgels for loading/release of active bio(macro)molecules.

The present study aims in the understanding of the effect of oligo(ethylene glycol)-based biocompatible microgels inner structure on the encapsulation/release mechanisms of different types of cosmetic active molecules. For that, multi-responsive microgels were synthesized using three types of cross-linkers: ethylene glycol dimethacrylate (EGDMA), oligo(ethylene glycol) diacrylate (OEGDA) and N,N-methylenebisacrylamide (MBA). The inner morphology of the microgels synthesized was studied by 1H-nuclear magnetic resonance (1H NMR) and small-angle neutron scattering (SANS) techniques and no effect of cross-linker type on microgel microstructure was observed in the case of analysing purified microgel dispersions. Moreover, all the microgels synthesized presented conventional swelling/de-swelling behavior as a function of temperature and pH. Two hydrophobic, one hydrophilic, and one macromolecule as cosmetic active molecules were effectively loaded into different microgel particles via hydrophobic interactions and hydrogen-bonding interactions between -OH groups of active molecules and ether oxygens of different microgel particles. Their release profiles as a function of cross-linker type used and encapsulated amounts were studied by Peppas-Sahlin model. No effect of the cross-linker type was observed due to the similar inner structure of all the microgels synthesized.

(Invited) Self-Organization of Functional Hydrogel Microspheres at the Fluid Interface

The unique drying behavior of aqueous droplets that contain soft hydrogel microspheres (microgels) upon evaporation will be presented. The author’s group has previously reported that uniformly ordered thin films are obtained from drying poly(N-isopropyl acrylamide) microgel dispersions, in comparison with the ring-shaped deposits that are obtained from drying solid microsphere dispersions [1]. In this presentation, effects of the size, chemical structure, and crosslinking densities, i.e. “softness” of the microgels, substrates and so forth on the phenomenon were systematically investigated. For the macro- and microscopic observation of the drying behavior of various microgel dispersions, an optical microscope and a digital camera were used. The results suggested that the convection in the aqueous droplets plays an dominant role for the transportation of the microgels to the air/water interface. On the basis of these finding, a design guideline for the rapid formation of uniform thin films of microgels will be demonstrated [2]. In addition, recently further experimental results will be discussed. Partially reproduced with permission from [2] Copyright (2018) American Chemical Society. Reference [1] K. Horigome et al. Langmuir 2012, 28, 12962−12970. [2] M. Takizawa et al. Langmuir 2018, 34, 4515−4525.