Tunable polymer microgel particles and their study using microscopy and real-time deformability cytometry

We report the preparation and mechanical properties of highly swellable, spherical polymer microgels synthesized by precipitation copolymerization of divinylbenzene-55 (DVB), 4-methylstyrene (4MS), and maleic anhydride (MA) at different cross-linker contents, in a range of methylethylketone (MEK) and heptane solvent mixtures. Microgels were characterized by optical and confocal microscopy, and their mechanical properties tested using real-time deformability cytometry (RT-DC), a technique developed to analyze cell properties by measuring deformation under shear stress. Hydrolysis of anhydride groups gave microgels with diameters ranging from 10 to 22 μm when swollen in saline, depending on vol% MEK and cross-linker loading. Young's moduli of the microgels could be tuned from 0.8 to 10 kPa by adjusting cross-linker content and MEK/heptane solvent composition, showing an inverse relationship between the effects of vol% MEK and %DVB on microgel properties. These microgels also show strain-stiffening in response to increasing shear stresses. Extension of the RT-DC method to the study of polymer colloids thus enables high-throughput analysis of microgels with tunable mechanical characteristics.     

Functionalised Microgels for Acrylic Coatings

Summary: Aqueous acrylic dispersions of hydroxy-functionalised copolymer microgel particles crosslinked with allyl methacrylate were synthesized by emulsion polymerization. The microgels were investigated as reactive polymer fillers in mixtures with a water-borne film-forming dispersion. Properties of coatings cast from mixtures of aqueous dispersion of hard microgel particles and film-forming water-borne dispersion were investigated. The swelling behaviour of microgels in selected solvents (aliphatic ketones) as a function of microgel composition is discussed as well. It was found that the swelling ability of microgels decreased with growing degree of crosslinking. Microgels comprising copolymerised butyl methacrylate swelled less in aliphatic ketones than microgels without this comonomer. This work was focused mainly on the influence of microgels incorporated in the commercial solvent-borne acrylic binders on the properties of coatings. It was shown that the application of microgels that were redispersed in acetone did not affect the surface appearance and transparency of coatings. Moreover, the presence of microgel network precursors accelerated film curing at ambient temperature and improved the final hardness of coatings.     

Microgel particles: The structure-property relationships and their biomedical applications

Microgels are crosslinked soft particles with a three-dimensional network structure that are swollen in a good solvent. They have frequently been termed “smart materials” since the size, softness, and interaction forces between particles are tunable by external stimuli such as temperature, pH, or magnetic and electric fields. It is this unique feature that has captured the interest of many scientists across a wide range of disciplines. This brief review covers the basic aspects of the relationships between the network structure and gel properties of the thermally sensitive poly(N-isopropylacrylamide) (pNIPAM) microgels including the phase transition process, the internal structure of microgels, and the phase behavior. Additionally, we highlight the impacts of microgels on the biomedical applications, especially in the gene delivery, cell matrix and differentiation of stem cells. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013, 51, 2995–3003     

Composite hydrogels with temperature sensitive heterogeneities: influence of gel matrix on the volume phase transition of embedded poly-(N-isopropylacrylamide) microgels

The thermo-responsive behaviour of poly-(N-isopropylacrylamide) (PNiPAM) microgels embedded in covalently cross-linked non-temperature-sensitive polyacrylamide (PAam) hydrogel matrixes with different compositions was investigated by using small angle neutron scattering (SANS). The composition of the composite hydrogel was varied by (a) increasing the cross-linker and acrylamide concentration leading to strong hydrogel matrixes and (b) by increasing the microgel concentration to obtain composite gels with an internal structure. Additionally we synthesized composite hydrogels by using γ-irradiation as initiation for the polymerisation. This leads to the formation of chemical bonds between the PNiPAM microgels and the surrounding polyacrylamide matrix. Thus it is possible to synthesize hydrogels without an additional cross-linker, as well as pure particle networks. Some samples were prepared at two different temperatures, below and above the volume phase transition temperature of PNiPAM, resulting in highly swollen or totally collapsed microgels during the incorporation step. The volume phase transition of microgels is not influenced by a hydrogel matrix with high acrylamide concentration independent of the preparation temperature. However, an increased cross-linker concentration leads to a corset like constraint on microgel swelling. Microgels, which are embedded in the collapsed state (at 50 °C), are not able to swell upon cooling, whereas microgels embedded in the swollen state can collapse upon heating. For samples with an increased microgel concentration, the close microgel packing was disturbed by the formation of the polyacrylamide matrix. The hydrogel matrix squeezes the microgels together and leads to partial aggregation. The experiments demonstrate how composite hydrogels with stimuli-sensitive heterogeneities can be prepared such that the full responsiveness of the embedded microgels is retained while the macroscopic dimensions of the gel are not affected by the volume phase transition of the microgels.     

Control of colloidal interactions between microgels with stimulus-responsive properties

Microgels can switch their chemical/physical properties with external stimulus, and the colloidal behavior of microgels is strongly affected by interparticle interactions. In this article, we introduce smart microgels, focusing on Janus microgels and oscillating microgels developed by our group. Janus microgels show anisotropic shape and chemical/physical properties, and thus the structures of their flocs are also anisotropic. Oscillating microgels show autonomous swelling/deswelling and dispersing/flocculating oscillations through synchronization with chemical reactions. The interparticle interactions of these microgels are discussed. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013, 51, 3021–3026     

Aloe vera–eluting collagen I microgels: physicochemical characterization and in vitro biological performance

Microgels absorb and retain high amounts of solvents, especially water. Because of their size, and association, the release kinetics of active molecules from microgels is easier to control than in hydrogels. Collagen I is one of the most extensively investigated biomaterials, although the key process parameters to produce microgels must be understood well before they can be used in veterinary and human medicine. Emulsification-gelation is widely used to obtain microgels because of its ease of handling and high yields. The concentration of the biomaterial and the homogenization method are among the critical parameters in this method. In this work, we produced cytocompatible collagen I microgels by emulsification-gelation and evaluated the effect of three different concentrations and homogenization methods on their physicochemical, mechanical, and biological properties. As proof of concept, microgels were loaded with an Aloe vera extract and the loading efficiency and the polyphenol release kinetics, as well as their properties assessed. When the same homogenization method (e.g. magnetic stirring) was used, the size of the microgels decreased with an increase of collagen I concentration, and the size distribution increased. In addition, the size and size distribution of microgels prepared with the same collagen I concentration were smaller when produced by high-energy homogenization methods (shear stress and ultrasound) than with a low-energy one (magnetic stirring). Collagen I concentration and the homogenization method also influenced the zeta-potential, the enzymatic degradation, and the encapsulation efficiency of the microgels. Overall, we show that the size of these microgels can be fine-tuned by the collagen I concentration and the homogenization method. Moreover, the integration of microgels of different sizes into the same carrier platform will pave the way for the combination of active compounds with different release kinetics.     

Microgels formed by electrostatic and hydrophobic interaction of naphthaleneacetic acid with β-cyclodextrin-grafted polyethyleneimine

Microgels were prepared by physically cross-linking β-cyclodextrin-grafted polyethyleneimine (βCD-PEI) using a hydrophobic acidic compound, naphthaleneacetic acid (NAA). Under a strong acidic condition (e.g., pH 3.0), fibrous microgels were observed on a scanning electron microscope (SEM) possibly due to the intermolecular electrostatic interaction of NAA with PEI. In the range of pH 4.0 to pH 8.0, globular microgels were found possibly because an intramolecular electrostatic interaction prevails over an intermolecular interaction. At pH 9.0 and pH 10.0, neither fibrous nor globular microgels were observed due to lack of the electrostatic interaction and the hydrophobic interaction of NAA with βCD-PEI. The release of fluorescein isothiocyanate-dextran (FITC-dextran; M.W., 10,000) from the microgels increased with increasing pH. At pHs higher than pH 3.0, not only the diffusion of the solute, but also the dissolution of the microgels could contribute to the release.     

Preparation and characterization of structured hydrogel microparticles based on cross-linked hyperbranched polyglycerol

The aim of this work was to obtain well-defined HyPG-MA (methacrylated hyperbranched polyglycerol) microparticles with uniform sizes. Therefore, three different preparation methods were evaluated. First, we assessed a micromolding technique using rigid SU-8 (a photoresist based on epoxies) grids. Independent of the surface treatment of the SU-8 grid or the type of polymer used, approximately 50% of the microgels remained attached to the SU-8 grid or broke into smaller particles during the release process in which drying of the gels was followed by a sonication process. Although 90% methacrylate conversion could be obtained, this method has some additional drawbacks as the obtained dried microgels did not rehydrate completely after the drying step. Second, a soft micromolding technique was evaluated using elastomeric PDMS (poly(dimethyl siloxane)) grids. The use of these flexible grids resulted in a high yield (80-90% yield; >90% methacrylate conversion) of microgels with a well-defined size and shape (squares 100 microm x 100 microm x 50 microm or hexagons with ? 30 microm and a thickness of 20 microm) without the occurrence of water evaporation. However, a number of particles showed a less-defined shape as not all grids could be filled well. The microgels showed restricted swelling, implying that these gels are dimensionally stable. Third, an alternative method referred to as photolithography was evaluated. This method was suitable to tailor accurately the size and shape of HyPG-MA microgels and additionally gained 100% yield. Well-defined HyPG-MA microgels in the size range of 200-1400 microm (thickness of 6, 20, or 50 microm), with a methacrylate conversion of >90%, could easily be prepared by adding an inhibitor (e.g., 1% (w/v) of vitamin C) to the polymer solution to inhibit dark polymerization. Microgels in the size range of 30-100 microm (>90% conversion) could only be obtained when applying the photomask in direct contact with the polymer solution and using a higher (i.e., 2% (w/v)) concentration of vitamin C. Additionally, the microgels showed limited swelling, indicating that rather dimensionally stable particles were obtained. In conclusion, this paper shows that photolithography and soft micromolding, as compared to rigid micromolding, are the most appropriate techniques to fabricate structured HyPG-MA microgels with a tailorable and well-defined size and shape. These microgels have great potential in tissue engineering and drug delivery applications.     

A novel UV-sensitive photopolymerization system with microgel matrix

A series of crosslinked microgels with quaternary ammonium ions on the surface was prepared by quaternization with N,N-dimethylbenzylamine in the presence of microgel particles prepared by emulsion copolymerization of styrene (St), chloromethylstyrene (CMS) and divinylbenzene (DVB). Microgels with diameters in the range of 15–100 nm were successfully dispersed in organic solvents such as 2-methoxyethanol and 2-ethoxyethanol without an emulsifier. A photosensitive layer was formed by coating a photosensitive solution on a grained aluminum plate. The solution was comprised of the microgels, the multifunctional monomer and standard ultraviolet (UV) photoinitiators, such as 2,4-diethyl thioxanthone (DETX)/ethyl p-di-ethylaminobenzoate (EPA). This gave a heterogeneous photosensitive layer which produced good polymer patterns after exposure to UV light followed by development in tap water. A typical polymer layer, consisting of the microgels (poly(styrene-co-N,N-dimethylbenzylvinyl-benzylammonium chloride-co-divinylbenzene)), DETX/EPA, and the multifunctional acrylate monomers, exhibited photosensitivity of 0.06 mJ/cm² for UV light. This sensitivity is much higher than the homogeneous photopolymerization system with an analogous composition.     

Designing responsive microgels for drug delivery applications

Microgels based on thermally responsive polymers have been widely investigated in the context of controlled release applications, with increasing recent interest on developing a clearer understanding of what physical, chemical, and biological parameters must be considered to rationally design a microgel to deliver a specific drug at a specific rate in a specific physiological context. In this contribution, we outline these key design parameters associated with engineering responsive microgels for drug delivery and discuss several recent examples of how these principles have been applied to the synthesis of microgels or microgel-based composites. Overall, we suggest that in vivo assessment of these materials is essential to bridge the existing gap between the fascinating properties observed in the lab and the practical use of microgels in the clinic. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013, 51, 3027–3043     

From microfluidics to hierarchical hydrogel materials

Over the past two decades, microfluidics has made significant contributions to material and life sciences, particularly via the design of nano-, micro- and mesoscale materials such as nanoparticles, micelles, vesicles, emulsion droplets, and microgels. Unmatched in control over a multitude of material parameters, microfluidics has also shed light on fundamental aspects of material design such as the early stages of nucleation and growth processes as well as structure evolution. Exemplarily, polymer hydrogel particles can be formed via microfluidics with exact control over size, shape, functionalization, compartmentalization, and mechanics that is hardly found in any other processing method. Interestingly, the utilization of microfluidics for material design largely focuses on the fabrication of single entities that act as reaction volume for organic and cell-free biosynthesis, cell mimics, or local environment for cell culturing. In recent years, however, hydrogel design has shifted towards structures that integrate a large variety of functions, e.g., to address the demands for sensing tasks in a complex environment or more closely mimicking architecture and organization of tissue by multiparametric cultures. Hence, this review provides an overview of recent literature that explores microfluidics for fabricating hydrogel materials that go well beyond common length scales as well as the structural and functional complexity of microgels necessary to produce hierarchical hydrogel structures. We focus on examples that utilize microfluidics to design microgel-based assemblies, on microfluidically made polymer microgels for 3D bioprinting, on hydrogels fabricated by microfluidics in a continuous fashion, like fibers, and on hydrogel structures that are shaped by microchannels.     

Isothermal Titration Calorimetry of Molecularly Imprinted Polymer Nanospheres

Ultrasensitive isothermal titration calorimetry was used to generate thermodynamic data to assess the binding properties of molecularly imprinted polymer microgels. Microgels were imprinted using L ‐boc‐phenylalanine anilide (L ‐BFA) and then utilized in binding experiments with a variety of probe molecules, structurally closely related to the template molecule. Significant differences were observed between the binding enthalpy of the original template L ‐BFA and those of D ‐BFA, L ‐boc‐phenylalanine, L ‐boc‐tryptophane, and L ‐boc‐tyrosine.     

Synthesis of monodisperse biodegradable microgels in microfluidic devices

Microgels are promising materials in drug delivery and biomedicine. Although monodisperse microgels would offer considerable advantages, most microgels investigated and used today are polydisperse in size. We report on the fabrication of 10 mum sized monodisperse microgels by emulsifying an aqueous dextran-hydroxyethyl methacrylate (dex-HEMA) phase within an oil phase at the junction of microfluidic channels. Dex-HEMA microgels are biodegradable and are ideally suited for the controlled delivery of proteins.     

pH/温度双重敏感性微凝胶的合成与应用

智能微凝胶是一类具有独特物理、化学性质的聚合物网络和溶剂组成的体系, 在众多的领域具有潜在的应用.本文介绍了近年来pH和温度双重敏感性微凝胶的研究概况、制备方法及其在药物控制释放、分离纯化和光子晶体方面的应用.     

On the crucial importance of the pH for the formation and self-stabilization of protein microgels and strands

Stable suspensions of protein microgels are formed by heating salt-free β-lactoglobulin solutions at concentrations up to about C = 50 g·L(-1) if the pH is set within a narrow range between 5.75 and 6.1. The internal protein concentration of these spherical particles is about 150 g·L(-1) and the average hydrodynamic radius decreases with increasing pH from 200 to 75 nm. The formation of the microgels leads to an increase of the pH, which is a necessary condition to obtain stable suspensions. The spontaneous increase of the pH during microgel formation leads to an increase of their surface charge density and inhibits secondary aggregation. This self-stabilization mechanism is not sufficient if the initial pH is below 5.75 in which case secondary aggregation leads to precipitation. Microgels are no longer formed above a critical initial pH, but instead short, curved protein strands are obtained with a hydrodynamic radius of about 15-20 nm.     

Semi-batch control over functional group distributions in thermoresponsive microgels

Thermosensitive poly(N-isopropylacrylamide-co-methacrylic acid) (poly(NIPAM-co-MAA)) microgels were prepared via semi-batch free radical copolymerization in which the functional monomer (methacrylic acid) was continuously fed into the reaction vessel at various speeds. Microgels with the same bulk MAA contents (and thus the same overall compositions) but different radial functional group distributions were produced, with batch copolymerizations resulting in core-localized functional groups, fast-feed semi-batch copolymerizations resulting in near-uniform functional group distributions, and slow-feed semi-batch copolymerizations resulting in shell-localized functional groups. Functional group distributions in the microgels were probed using titration analysis, electrophoresis, and transmission electron microscopy. The induced functional group distributions have particularly significant impacts on the pH-induced swelling and cationic drug binding behavior of the microgels; slower monomer feeds result in increased pH-induced swelling but lower drug binding. This work suggests that continuous semi-batch feed regimes can be used to synthesize thermoresponsive microgels with well-defined internal morphologies if an understanding of the relative copolymerization kinetics of each comonomer relative to NIPAM is achieved.     

Cononsolvency of poly-N-isopropyl acrylamide (PNIPAM): Microgels versus linear chains and macrogels

Poly-N-isopropyl acryl amide (PNIPAM) is swollen in both pure water and pure methanol but collapses in mixtures of these solvents. In this review, this cononsolvency of PNIPAM in water/methanol mixtures is discussed. Experimental studies of linear PNIPAM chains and macrogels are compared to microgels. Theoretical studies are presented based on molecular dynamics simulation and quantum mechanical calculations as well as semi-empirical models. The different explanations for the cononsolvency available in the literature are introduced. Experiments show that all PNIPAM species collapse and re-swell at comparable methanol fraction in the mixture. Cross-linker density of macrogels and microgels has only slight influence on cononsolvency, whereas chain length of linear chains has a significant influence. Microgels provide advantages to study cononsolvency by en'abling a broader experimental approach. Furthermore, multi-sensitive microgels can be prepared, which contain compartments sensitive to different stimuli.     

Particulate and continuum mechanics of microgel pastes: effect and non-effect of compositional heterogeneity

Microgels are deformable colloids that can be packed by external compression; such packing transforms a suspension of loose microgels into a viscoelastic paste with mechanical properties controlled by the elasticity of the constituent particles. We aim to understand how the presence of microgel particles with different individual elastic moduli affects this interplay in heterogeneous microgel packings. We do this by preparing microgel pastes that contain both soft, loosely cross-linked and stiff, densely cross-linked microgel particles and probe their shear elasticity. We consider particle packing fractions that cover the range from particles at the onset of contact to particles that are strongly packed, deformed, and deswollen to investigate the transition from a particulate suspension to a macrogel-type system. These studies reveal that the elasticity of heterogeneous microgel suspensions at low packing is due to the response of the soft, easily deformable microgel particles alone, whereas at high packing both soft and stiff microgels linearly add to the paste elasticity. This fundamental difference is due to the fundamentally different origin of elasticity at different microgel packing; whereas the soft particle interaction potential dominates the suspension mechanics at low microgel packing, rubber-like elasticity that equally reflects both soft and stiff contributions governs the mechanics of the same samples at high microgel packing.     

变温反相悬浮聚合制备温度敏感性聚合物微凝胶

报道了一种利用变温的途径制备具有温度敏感性聚合物凝胶微粒的悬浮聚合方法.以正庚烷为连续相,过硫酸铵和四甲基乙二胺为引发剂,采用将具有反向温敏性的可降解大分子单体水溶液在低温下分散好以后再升高到聚合温度的变温反相悬浮聚合的方法制备了反向温敏性的可降解微凝胶.该方法避免了由于分散相在聚合温度下发生物理凝胶化所导致的分散困难等问题.考察了微凝胶的温敏性、粒径分布和降解行为等,并研究了油水比对微凝胶形貌的影响.