Deformation Of Microgels Research Articles

Thin Films Constructed by Centrifugal Deposition of Highly Deformable, Charged Microgels.

Thin films composed entirely of microgel building blocks were fabricated using two kinds of self-cross-linked, oppositely charged microgels, via centrifugal deposition. Atomic force microscopy studies revealed that both microgels form very thin monolayer films due to a large degree of microgel deformation during deposition. Meanwhile, centrifugal deposition from a mixture of these two kinds of microgels resulted in the formation of microgel bilayers with a total thickness of around 20 nm. The film thickness increased linearly with the deposition time. Additionally, isotropic stretching/release by heating/cooling of the dried microgel films generated complicated buckling patterns, while anisotropic (uniaxial) stretching/release resulted in parallel buckling perpendicular to the stretching direction. The damage caused by anisotropic stretching and 100 °C treatment can be healed by addition of water, while damage caused via treatment at 150 °C cannot be healed due to the occurrence of polymer cross-linking, which inhibits the mobility of the microgel building blocks.

Transmission behavior of pNIPAM microgel particles through porous membranes

Microgels used for biomedical applications have to be purified to remove unreacted monomer, linear polymers and other impurities, some of which could be toxic. Purification is generally carried out by repeated dialysis against fresh water, which is extremely slow, or by ultracentrifugation, which is labor-intensive and expensive. Microfiltration and ultrafiltration are convection-driven, membrane based techniques which offer the potential for fast and scalable purification of microgel particles. Factors such as deformation of microgels and their responsiveness to environmental conditions such as pH and salt concentration would need to be considered while developing membrane-based purification processes. In this study, a model poly (N-isopropylacrylamide) or pNIPAM microgel was covalently tagged with Rhodamine B to enhance its detectability. The effects of salt concentration in the feed, membrane pore-size, and permeate flux on microgels transmission through different membranes were investigated. Both, the properties of the microgel particles, and operating conditions were found to significantly affect membrane filtration of pNIPAM microgel.

Ultrasoft, highly deformable microgels.

Microgels are colloidally stable, hydrogel microparticles that have previously been used in a range of (soft) material applications due to their tunable mechanical and chemical properties. Most commonly, thermo and pH-responsive poly(N-isopropylacrylamide) (pNIPAm) microgels can be fabricated by precipitation polymerization in the presence of the co-monomer acrylic acid (AAc). Traditionally pNIPAm microgels are synthesized in the presence of a crosslinking agent, such as N,N'-methylenebisacrylamide (BIS), however, microgels can also be synthesized under 'crosslinker free' conditions. The resulting particles have extremely low (<0.5%), core-localized crosslinking resulting from rare chain transfer reactions. AFM nanoindentation of these ultralow crosslinked (ULC) particles indicate that they are soft relative to crosslinked microgels, with a Young's modulus of ∼10 kPa. Furthermore, ULC microgels are highly deformable as indicated by a high degree of spreading on glass surfaces and the ability to translocate through nanopores significantly smaller than the hydrodynamic diameter of the particles. The size and charge of ULCs can be easily modulated by altering reaction conditions, such as temperature, monomer, surfactant and initiator concentrations, and through the addition of co-monomers. Microgels based on the widely utilized, biocompatible polymer polyethylene glycol (PEG) can also be synthesized under crosslinker free conditions. Due to their softness and deformability, ULC microgels are a unique base material for a wide variety of biomedical applications including biomaterials for drug delivery and regenerative medicine.

New insight into microgel-stabilized emulsions using transmission X-ray microscopy: nonuniform deformation and arrangement of microgels at liquid interfaces.

Microgel-covered interfaces, e.g., in emulsions, have attracted much interest lately. Different imaging techniques have been used to image these interfaces, either flat or curved, to investigate their properties and appearance. Techniques such as cryogenic scanning electron microscopy (cryo-SEM) and confocal microscopy have provided valuable insight into microgel-covered systems but still have some disadvantages such as part of the microgels being trapped in vitrified liquid or the need for fluorescent markers. Some of these disadvantages can be overcome by using transmission X-ray microscopy (TXM), which has the advantage of allowing the investigation of adsorbed and free microgels simultaneously. We used TXM to acquire tomographic image series of microgel-covered droplets and calculated 3D reconstructions from these image stacks. As a result, we could show that microgels deform anisotropically and penetrate the oil droplets in the hydrated state. Additionally, 3D reconstruction gives an idea of the arrangement of microgels adsorbed to oil droplets and reveals that droplet stabilization is possible without full coverage of the interface with polymer segments.

Impact of electrostatics on the adsorption of microgels at the interface of Pickering emulsions.

The importance of electrostatics on microgel adsorption at a liquid interface is studied, as well as its consequence on emulsion stabilization. In this work, poly(N-isopropylacrylamide) (pNIPAM) microgels bearing different numbers of charges and various distribution profiles are studied, both in solution and at the oil-water interface of emulsion drops. Charged microgels are compared to neutral ones, and electrostatic interactions are screened by adding salt to the aqueous solution. In solution, electrostatics has a significant impact on microgel swelling, as induced by the osmotic pressure exerted by mobile counterions in the gel network. At the interface of drops, microgels pack in a hexagonal array, whose lattice parameter is independent of the number of charges and range of electrostatic interactions. Microgel morphology and packing are ruled only by the adsorption of the pNIPAM chain at the interface. Conversely, decreasing the charge density of microgels by the protonation of the carboxylic groups leads to unstable emulsions, possibly as a result of the impact of hydrogen bonding on microgel deformability.

Influence of microgel packing on raspberry-like heteroaggregate assembly

We describe the influence of microgel packing on colloidal-phase mediated heteroaggregation using poly(N-isopropylacrylamide) and poly(N-isopropylmethacrylamide) microgels with 1% mol or 5% mol N,N′-methylenebis(acrylamide) cross-linker. This system is uniquely designed to interrogate the influence of microgel structure and stiffness on microgel deformation at a curved interface by elminating the necessity of electrostatic charge pairing. Microgel monomer and cross-linker content is expected to influence deformation at a curved interface. Microgel deformation and swelling were characterized via atomic force microscopy (AFM) and viscometry. A systematic study of colloidal-phase mediated heteroaggregation was performed at varied effective volume fractions with all microgel compositions. Scanning electron microscopy (SEM) and qNano pore translocation experiments were used to asses the microgel coverage on the resultant raspberry-like particles (RLPs). Results reveal that microgel composition has a strong influence on the efficiency (as determined by microgel coverage) of RLP fabrication. The compositional effects appear to be related to the degree of microgel spreading/deformation at the interface, which is coupled to the influence of packing on assembly fidelity. These findings are widely applicable to systems where microgel deformation occurs at a curved interface. We also demonstrate that qNano pore translocation experiments can be used as a high-throughput method to analyze RLP microgel coverage.

Ultrasoft microgels displaying emergent platelet-like behaviours.

Efforts to create platelet-like structures for the augmentation of haemostasis have focused solely on recapitulating aspects of platelet adhesion; more complex platelet behaviours such as clot contraction are assumed to be inaccessible to synthetic systems. Here, we report the creation of fully synthetic platelet-like particles (PLPs) that augment clotting in vitro under physiological flow conditions and achieve wound-triggered haemostasis and decreased bleeding times in vivo in a traumatic injury model. PLPs were synthesized by combining highly deformable microgel particles with molecular-recognition motifs identified through directed evolution. In vitro and in silico analyses demonstrate that PLPs actively collapse fibrin networks, an emergent behaviour that mimics in vivo clot contraction. Mechanistically, clot collapse is intimately linked to the unique deformability and affinity of PLPs for fibrin fibres, as evidenced by dissipative particle dynamics simulations. Our findings should inform the future design of a broader class of dynamic, biosynthetic composite materials.

Influence of sulphate, chloride, and thiocyanate salts on formation of β-lactoglobulin–pectin microgels

Effects of sulphate, chloride, and thiocyanate salts on the heat-induced formation of protein-based microgels from β-lactoglobulin–pectin complexes were determined as a function of pH and protein-to-polysaccharide ratio. Aggregation temperatures were initially decreased at low ionic strength due to shielding of electrostatic interactions between β-lactoglobulin and pectin but increased with further increases in ionic strength. Turbidity of heated mixtures and associated sizes of formed microgels were increased with up to 75mmolkg−1 ionic strength. Aggregation and microgel formation were relatively increased in the presence of thiocyanate salts compared to chloride salts and relatively decreased in the presence of sulphate salts, indicating that the inverse Hofmeister series was relevant in this system. Topographical analysis of dried microgels by atomic force microscopy verified that microgels were smallest in the presence of sulphate salts and showed that added ions, particularly thiocyanate, increased the deformability of microgels during drying.

Pickering Emulsions Stabilized by Soft Microgels: Influence of the Emulsification Process on Particle Interfacial Organization and Emulsion Properties

This work reports a new evidence of the versatility of soft responsive microgels as stabilizers for Pickering emulsions. The organization of microgels at the oil-water interface is a function of the preparation pathway. The present results show that emulsification energy can be used as a trigger to modify microgel deformation at the oil-water interface and their packing density: high shear rates bring strong flattening of the microgels, whereas low shear rates lead to dense monolayers, where the microgels are laterally compressed. As a consequence, the resulting emulsions have opposite behavior in terms of flocculation, which arises from bridging between neighboring drops and is strongly dependent on their surface coverage. This strategy can be applied to any microgel which can sufficiently adsorb at low shear rates, i.e. small microgels or lightly cross-linked ones. The control of the organization of microgels at the interface does not only modify emulsion end-use properties but also constitutes a new tool for the development of Janus-type microgels, obtained by chemical modification of the adsorbed microgels.

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.

An active one-particle microrheometer: Incorporating magnetic tweezers to total internal reflection microscopy

We present a novel microrheometer by incorporating magnetic tweezers in the total internal reflection microscopy (TIRM) that enables measuring of viscoelastic properties of materials near solid surface. An evanescent wave generated by a solid∕liquid interface in the TIRM is used as the incident light source in the microrheometer. When a probe particle (of a few micrometers diameter) moves near the interface, it can interact with the evanescent field and reflect its position with respect to the interface by the scattered light intensity. The exponential distance dependence of the evanescent field, on the one hand, makes this technique extremely sensitive to small changes from z-fluctuations of the probe (with a resolution of several nanometers), and on the other, it does not require imaging of the probe with high lateral resolution. Another distinct advantage is the high sensitivity in determining the z position of the probe in the absence of any labeling. The incorporated magnetic tweezers enable us to effectively manipulate the distance of the embedded particle from the interface either by a constant or an oscillatory force. The force ramp is easy to implement through a coil current ramp. In this way, the local viscous and elastic properties of a given system under different confinements can therefore be measured by resolving the near-surface particle motion. To test the feasibility of applying this microrheology to soft materials, we measured the viscoelastic properties of sucrose and poly(ethylene glycol) solutions and compared the results to bulk rheometry. In addition, we applied this technique in monitoring the structure and properties of deformable microgel particles near the flat surface.

Poly(N-isopropylacrylamide) microgels at the oil–water interface: adsorption kinetics

Understanding the adsorption behaviors of soft poly(N-isopropylacrylamide) (PNIPAM) microgels to the oil–water interface has become increasingly important both in terms of fundamental science and applications of microgels as multi-stimuli responsive emulsion stabilizers. In the present work, we used pendant drop tensiometry to trace the evolution of oil–water interfacial tensions. We investigated two PNIPAM microgels with different cross-link density as well as poly(styrene-co-NIPAM) particles. We found that the adsorption of microgels from the aqueous phase to the oil–water interface is dominated by two steps. Microgels first diffuse to the oil–water interface and this diffusion process depends on microgel concentration in the bulk phase. The second process involves the deformation and spreading of microgels at the interface. The second process depends strongly on microgel deformability. The behavior of the different microgel systems is compared with conventional Pickering stabilizers and proteins. Our results demonstrate that the softness of the microgels dominates their properties at the oil–water interface. The change of microgel shape at the interface resembles the unfolding transitions observed with proteins. On the other hand, microgels are distinctly different from conventional, rigid Pickering stabilizers.

Unraveling the 3D Localization and Deformation of Responsive Microgels at Oil/Water Interfaces: A Step Forward in Understanding Soft Emulsion Stabilizers

Responsive microgels are deformable submicrometer cross-linked polymeric hydrogel particles that are used as a novel class of emulsion stabilizers. Their flexibility and the triggering of conformational changes by external stimuli lead to several advantages compared to rigid particles used in conventional Pickering emulsions. Despite their rapidly increasing use, several key aspects relating to microgel microstructure and localization at liquid interfaces are still unexplored. We present here a novel characterization that employs freeze-fracture shadow-casting cryo-SEM to disclose quantitative 3D information on the deformation and protrusion of microgels at water/oil interfaces. Despite the bulk pH response (swelling), we report here the unexpected absence of size and vertical position changes as a function of pH at liquid interfaces and interpret the results using simple arguments that link the particle interfacial activity, solvation, and internal deformation. These results pave the way to a deeper understanding of a novel class of soft materials.

Structure Optimization and Mechanical Model for Microgel-Reinforced Hydrogels with High Strength and Toughness

In this work, the mechanical behavior of sparsely cross-linked, neutral polyacrylamide (PAAm) hydrogels containing densely cross-linked polyelectrolyte microgels of poly(2-acrylamido-2-methylpropanesulfonic sodium) (PNaAMPS) were studied systematically by varying the formulations. The microgel-reinforced (MR) hydrogels have a two-phase composite structure, where the disperse phase is the rigid double-network (DN) microgels, and the continuous phase is the soft PAAm matrix. At the optimal formulation, the MR gels showed high mechanical strength and toughness, comparable to conventional DN hydrogels. The two critical parameters for the substantial enhancement of mechanical strength and toughness of MR gels are the concentration of PNaAMPS microgel and the molar ratio of the PAAm to the PNaAMPS in the microgel phase. Selective dyeing of the embedded microgels in MR gels allowed for visualization of the deformation of microgels, and we found that the local strain of microgels was much smaller than the global strain applied on MR gels; this indicates that isostress model (Reuss’s model) is more suitable than isostrain model (Voigt’s model) for this composite system.

Resistive Pulse Analysis of Microgel Deformation During Nanopore Translocation.

Deformation of 570-nm radius poly(N-isopropylacrylamide-co-acrylic acid) microgels passing through individual 375- to 915-nm radius nanopores in glass has been investigated by the resistive-pulse method. Particle translocation through nanopores of dimensions smaller than the microgel yields electrical signatures reflecting the dynamics of microgel deformation. Translocation rates, and event duration and peak shape, are functions of the conductivities of microgel and electrolyte. Our results demonstrate that nanopore resistive-pulse methods provide new fundamental insights into microgel permeation through porous membranes.

Electrical signature of the deformation and dehydration of microgels during translocation through nanopores

The resistive-pulse sensing technique was used to investigate the deformation and dehydration of individual 570 nm radius poly(N-isopropylacrylamide-co-acrylic acid) microgels during their translocation through a glass membrane containing a single conical nanopore with orfice radii ranging from 200 to 700 nm. Microgel translocation rates were found to be dependent on both the applied pressure and the pore radius, and a translocation threshold pressure was found to be a function of the pore size. Importantly, current–time traces resulting from translocation events demonstrate changes in the conductivity of the microgel, due to compression and partial dehydration during translocation. A minimum nanopore-to-microgel radius ratio of ∼0.4 is observed for translocation, suggesting a theoretical limit imposed by the compressibility of the microgel and Columbic repulsion with the pore walls. Our results provide fundamental insight into microgels including their compressibility and conductivity, as well as the ability of soft particles to permeate porous membranes and pass through voids of dimensions smaller than the particles themselves.

Soft microgels as Pickering emulsion stabilisers: role of particle deformability

We synthesised soft uncharged microgels made of poly(N-isopropylacrylamide) of variable cross-linking degrees and we probed their efficiency as stabilisers to fabricate oil-in-water emulsions. Interestingly, such particles undergo a swollen-to-collapsed transition above a critical temperature. By combining several microscopy methods and by exploiting the limited coalescence process, we were able to determine both the particle concentration and structure at the interface, as a function of the cross-linking density. Being deformable, the initially spherical microgels adopt a “fried egg-like” structure when adsorbed at the oil/water interface. As expected, the interfacial deformation is increasingly pronounced as the cross-linking degree decreases. The most deformable microgels tend to form 2D connected networks characterised by significant overlapping of the peripheral parts. When the deformability is lost, by increasing the cross-linking density or the temperature, the stabilisation efficiency is considerably reduced. Our results strongly suggest that emulsion stability is mainly determined by the microgels' deformability and we discuss the origin of that empirical link in terms of lateral overlapping and interfacial elasticity.

Controlling Porosity within Colloidal Heteroaggregates

Heteroaggregates of cationic poly(2-vinylpyridine) microgels and anionic polystyrene latex particles have been made by mixing dilute, aqueous suspensions. The growth of the heteroaggregates was arrested by the addition of anionic silica particles that adsorbed to the free surface of the cationic microgel particles. The resulting heteroaggregates were then concentrated by vacuum filtration, freeze-dried, and characterized by mercury porosimetry and electron microscopy. The inclusion of soft, deformable microgels resulted in heteroaggregates with higher porosity than obtained with heteroaggregates of anionic and cationic latex particles. Control of the pore volumes within the freeze-dried filter cakes was demonstrated by two approaches. In the first approach, heteroaggregation at a constant KCl concentration of 0.01 mM was arrested at different times after mixing the latex and microgel particles, thereby limiting the size of the aggregates. The porosity of the resulting filter cake increased from 61 to 65 vol % as the aggregation time increased from 15 to 120 s. In the second technique, the aggregation time prior to arrest was maintained at 120 s while the KCl concentration was varied between 0.01 and 10 mM. The pore volume of the aggregates decreased from 65 to 57 vol % as the electrolyte concentration increased, a trend explained in terms of the effect of the Debye length on the aggregation process.

Rheological aging and rejuvenation in microgel pastes

We have studied experimentally the rheological behavior of concentrated suspensions of soft deformable microgels below the yield point. We have found history-dependent effects which are interpreted in terms of aging and rejuvenation phenomena, analogous to those existing in glassy systems. The stress amplitude controls the long-time memory and determines the slow evolution of the suspensions.