Semi-batch control over functional group distributions in thermoresponsive microgels

A Novel Approach for the Synthesis of Responsive Core-Shell Nanogels with a Poly(N-Isopropylacrylamide) Core and a Controlled Polyamine Shell

Microgel particles can play a key role, e.g., in drug delivery systems, tissue engineering, advanced (bio)sensors or (bio)catalysis. Amine-functionalized microgels are particularly interesting in many applications since they can provide pH responsiveness, chemical functionalities for, e.g., bioconjugation, unique binding characteristics for pollutants and interactions with cell surfaces. Since the incorporation of amine functionalities in controlled amounts with predefined architectures is still a challenge, here, we present a novel method for the synthesis of responsive core-shell nanogels (dh < 100 nm) with a poly(N-isopropylacrylamide) (pNIPAm) core and a polyamine shell. To achieve this goal, a surface-functionalized pNIPAm nanogel was first prepared in a semi-batch precipitation polymerization reaction. Surface functionalization was achieved by adding acrylic acid to the reaction mixture in the final stage of the precipitation polymerization. Under these conditions, the carboxyl functionalities were confined to the outer shell of the nanogel particles, preserving the core's temperature-responsive behavior and providing reactive functionalities on the nanogel surface. The polyamine shell was prepared by the chemical coupling of polyethyleneimine to the nanogel's carboxyl functionalities using a water-soluble carbodiimide (EDC) to facilitate the coupling reaction. The efficiency of the coupling was assessed by varying the EDC concentration and reaction temperature. The molecular weight of PEI was also varied in a wide range (Mw = 0.6 to 750 kDa), and we found that it had a profound effect on how many polyamine repeat units could be immobilized in the nanogel shell. The swelling and the electrophoretic mobility of the prepared core-shell nanogels were also studied as a function of pH and temperature, demonstrating the successful formation of the polyamine shell on the nanogel core and its effect on the nanogel characteristics. This study provides a general framework for the controlled synthesis of core-shell nanogels with tunable surface properties, which can be applied in many potential applications.

Precise dapagliflozin delivery by cardiac homing peptide functionalized mesoporous silica nanocarries for heart failure repair after myocardial infarction

Myocardial infarction (MI) may cause irreversible damage or destroy to part of the heart muscle, affecting the heart’s ability and power to pump blood as efficiently as before, often resulting in heart failure (HF). Cardiomyocyte death and scar formation after MI may then trigger chronic neurohormonal activation and ventricular remodeling. We developed a biocompatible and mono-dispersed mesoporous silica nanoparticles (MSN) divergent porous channel for dapagliflozin (DAPA) loading. After surface modification of the cardiac-targeting peptides, the novel drug delivery system was successfully homed, and precisely released drugs for the hypoxic and weak acid damaged cardiomyocytes. Our biocompatible MSN- based nanocarriers for dapagliflozin delivery system could effective cardiac repair and regeneration in vivo, opening new opportunities for healing patients with ischemic heart disease in clinical.

Methacrylic acid based microgels and hybrid microgels

Methacrylic acid based microgels have got much consideration in the last two decades because of their potential uses in different fields owing to their responsive behaviour towards external stimuli. Synthesis, properties and uses of methacrylic acid based microgels and their hybrids have been critically reviewed in this article. With minute change in external stimuli such as pH and ionic strength of medium, these microgels show quick swelling/deswelling reversibly. The methacrylic acid based microgels have been widely reported for applications in the area of nanotechnology, drug delivery, sensing and catalysis due to their responsive behaviour. A critical review of current research development in this field along with upcoming perception is presented here. This discussion is concluded with proposed probable future studies for additional growth in this field of research.

One-pot surfactant-free modulation of size and functional group distribution in thermoresponsive microgels

Control over the size and functional group distribution of soft responsive hydrogel particles is essential for applications such as drug delivery, catalysis and chemical sensing. Traditionally, targeted functional group distributions are achieved with semi-batch techniques which require specialized equipment, while the preparation of size-tailored particles typically involves the use of surfactants. Herein, we present a simple and robust surfactant-free method for the modulation of size and carboxylic acid functional group distribution in poly(N-isopropylacrylamide) thermoresponsive microgels, employing reaction pH as the single experimental parameter. The varying distributions of carboxylic acid residues arise due to differences in kinetic reactivity, which are a function of the degree of dissociation of methacrylic acid, and thus of reaction pH. Incorporated charged residues induce a surfactant-like action during the particle nucleation stage, and impact the final particle size. Characterization with dynamic light scattering, and electron microscopy consistently supports the pH-tailored morphology of the microgels. A mathematical model which accounts for particle deformation on the imaging substrate also shows excellent agreement with the experimental results.

Microgel Particles at Interfaces: Phenomena, Principles, and Opportunities in Food Sciences

The use of soft microgel particles for stabilizing emulsions has captured increasing attention across a wide range of disciplines in the past decades. Being soft, the nanoparticles, which are spherical in solution, undergo a structure change when adsorbed at the oil-water interface. This morphology change leads to the special dynamic properties of interface layers and packing structures, which then alter the interfacial tension and rheological properties of the interface. In addition, emulsions stabilized by these particles, known as Pickering emulsions, can be triggered by changing a variety of environmental conditions, which is especially desirable in industrial applications such as oil transportation processes and biphasic catalysis, where the emulsions can be stabilized and destabilized on demand. Although many studies of the behavior of soft microgel nanoparticles at interfaces have been reported, there are still many challenges in gaining a full understanding of the structure, dynamics, and effective interactions between microgels at the interface. In this Feature Article, we address some of the most important findings and problems in the field. They include the adsorption kinetics of soft microgel particles, particle conformation at the interface, pH and thermal responsiveness, and the interfacial rheological properties of soft-particle-occupied interfaces. We also discuss some potential benefits of using emulsions stabilized by soft particles for food applications as an alternative to conventional surfactant-based systems. We hope to encourage further investigation of these problems, which would be very beneficial to extending this knowledge to all other related soft matter systems.

Direct Monitoring of Microgel Formation during Precipitation Polymerization of N-Isopropylacrylamide Using in Situ SANS

Poly(N-isopropylacrylamide) microgels have found various uses in fundamental polymer and colloid science as well as in different applications. They are conveniently prepared by precipitation polymerization. In this reaction, radical polymerization and colloidal stabilization interact with each other to produce well-defined thermosensitive particles of narrow size distribution. However, the underlying mechanism of precipitation polymerization has not been fully understood. In particular, the crucial early stages of microgel formation have been poorly investigated so far. In this contribution, we have used small-angle neutron scattering in conjunction with a stopped-flow device to monitor the particle growth during precipitation polymerization in situ. The average particle volume growth is found to follow pseudo-first order kinetics, indicating that the polymerization rate is determined by the availability of the unreacted monomer, as the initiator concentration does not change considerably during the reaction. This is confirmed by calorimetric investigation of the polymerization process. Peroxide initiator-induced self-crosslinking of N-isopropylacrylamide and the use of the bifunctional crosslinker N,N'-methylenebisacrylamide are shown to decrease the particle number density in the batch. The results of the in situ small-angle neutron scattering measurements indicate that the particles form at an early stage in the reaction and their number density remains approximately the same thereafter. The overall reaction rate is found to be sensitive to monomer and initiator concentration in accordance with a radical solution polymerization mechanism, supporting the results from our earlier studies.

Dynamically Cross-Linked Self-Assembled Thermoresponsive Microgels with Homogeneous Internal Structures

The internal morphology of temperature-responsive degradable poly(N-isopropylacrylamide) (PNIPAM) microgels formed via an aqueous self-assembly process based on hydrazide and aldehyde-functionalized PNIPAM oligomers is investigated. A combination of surface force measurements, small angle neutron scattering (SANS), and ultrasmall angle neutron scattering (USANS) was used to demonstrate that the self-assembled microgels have a homogeneously cross-linked internal structure. This result is surprising given the sequential addition process used to fabricate the microgels, which was expected to result in a densely cross-linked shell-diffuse core structure. The homogeneous internal structure identified is also significantly different than conventional microgels prepared via precipitation polymerization, which typically exhibit a diffuse shell-dense core structure. The homogeneous structure is hypothesized to result from the dynamic nature of the hydrazone cross-linking chemistry used to couple with the assembly conditions chosen that promote polymer interdiffusion. The lack of an internal cross-linking gradient within these degradable and monodisperse microgels is expected to facilitate more consistent drug release over time, improved optical properties, and other potential application benefits.

Dual-responsive biocompatible microgels as high loaded cargo: understanding of encapsulation/release driving forces by NMR NOESY

The suitability of biocompatible microgels as a new cosmetic carrier has been demonstrated through their ability of encapsulation/release of cosmetic active molecules.

Responsive Particle-Stabilized Emulsions: Formation and Applications

Responsive Pickering emulsions have attracted increasing attention over the last decade. These ‘surfactant-free’ emulsions are stabilized by particulate stabilizers and their properties and stability can be controlled by applying stimuli to the system. The excellent stability of Pickering emulsions makes them even more beneficial when they are compared to conventional emulsions which are stabilized by low molecular weight surfactants or amphiphilic polymers. Different responsive Pickering emulsions systems have been developed and reported by researchers. For example, they include pH responsiveness, magnetic responsiveness, thermo-responsiveness, ion-specific systems and photo-responsiveness. In this chapter, the formation and stabilization of such emulsions are discussed, with examples of different categories of particulate stabilizers, including inorganic, biological and polymeric particles. The discussion then moves on to the applications of such responsive emulsions in the pharmaceutical industry, petroleum processing, extraction and Pickering emulsion polymerization.

Dual stimuli-responsive oligo(ethylene glycol)-based microgels: insight into the role of internal structure in volume phase transitions and loading of magnetic nanoparticles to design stable thermoresponsive hybrid microgels

Design of multi-responsive biocompatible P(MEO₂MA-co-OEGMA-co-MAA) microgels and their hybrid magnetic couterparts.

Electrophoretic behavior of microgel-immobilized polyions

Electrophoretic behavior was studied for N-isopropylacrylamide (NIPA) microgels, into which different amounts of poly(acrylic acid) (PAAc) were physically entrapped. Copolymer microgels of NIPA with acrylic acid (AAc) were also studied as a control. Electrophoretic mobility was measured in 0.1 M NaCl solution at 25 °C as a function of pH, using an electrophoretic light scattering technique. The mobility of the copolymer microgel whose COOH groups are fully ionized agreed with that of PAAc when its ionization degree (α(n)) is close to the mole fraction (f(AAc)) of the AAc unit in the copolymer gel. There was good agreement between the mobility values of the copolymer microgel and the linear NIPA/AAc copolymer when their AAc contents are very close to each other. However, the mobility of the microgel with immobilized PAAc was higher than that of the copolymer microgel, even when there was no difference in the AAc content for both microgels. Moreover, the immobilized PAAc showed a higher mobility than the free PAAc when its α(n) is equal to f(AAc) in the immobilized system. No correlation was observed between the mobility and the hydrodynamic radius. These results were discussed in terms of the free draining model (FDM) for the electrophoresis of polyelectrolytes. It became apparent that the mobility difference depending upon whether (i) the PAAc ions are in the cage of the NIPA network or (ii) the AAc units are copolymerized with the network chain is due to the structural difference of the segments considered in the FDM.

Controlling the synthesis and characterization of micrometer-sized PNIPAM microgels with tailored morphologies

This Article presents the controlling synthesis and characterization of micrometer-sized, multiresponsive poly(N-isopropylacrylamide-co-methacrylic acid) (PNIPAM-MAA) microgel particles. By combining semibatch and temperature-programmed surfactant-free precipitation polymerization, we have successfully developed a novel approach to the preparation of temperature- and pH-responsive PNIPAM microgels with a dense-shell (DS), dense-core (DC), or homogeneous (HOMO) structure. We then investigated the interaction between the synthesized microgels and some fluorescent dye molecules using confocal laser scanning microscopy (CLSM). Our results have qualitatively revealed that the cross-linkers and the functional carboxylic groups (-COOH) could be homogeneously distributed, predominately localized inside the core, or concentrated near the surface of the synthesized microgels. Moreover, pH-responsive swelling behaviors of the microgels were investigated and discussed with titration and CLSM data. We found that the swelling capability is strongly dependent on the morphology of the PNIPAM microgel. Besides the absorption of fluorescent molecules, the synthesized microgels also showed a strong affinity for fluorescently labeled polypeptide, even at a relatively high salt concentration.

Responsive emulsions stabilized by stimuli-sensitive microgels: emulsions with special non-Pickering properties

Recent studies revealing the unique properties of microgel-stabilized responsive emulsions are discussed, and microgels are compared to classical rigid-particle Pickering stabilizers. Microgels are strongly swollen, lyophilic particles that become deformed at the oil-water interface and protrude only a little into the oil phase. Temperature- and pH-sensitive microgels allow us to prepare temperature- and pH-sensitive emulsions and thus enable us to prepare and break emulsions on demand. Although such emulsions are sensitive to pH, the stabilization of droplets is not due to electrostatic repulsion, instead the viscoelastic properties of the interface seem to dominate droplet stability. Being soft and porous, microgels behave distinctly differently from rigid particles at the interface: they are deformed and strongly flattened especially in the case of oil-in-water emulsions. The microgels are located mainly on the water side of the interface for both oil-in-water and water-in-oil emulsions. In contrast to rigid, solid particles, the behavior of microgels at oil-water interfaces does not depend only on the interfacial tension but also on the balance among the interfacial tension, swelling, elasticity, and deformability of the microgel, which needs to be considered. It is obvious that microgels as soft, porous particles are significantly different from classical rigid colloidal stabilizers in Pickering emulsions and we suggest avoiding the term Pickering emulsion when swollen microgels are employed. Microgel-stabilized emulsions require the development of new theoretical models to understand their properties. They open the door to new sophisticated applications.