Elastic and Flow Properties of Densely Packed Binary Microgel Mixtures with Size and Stiffness Disparities

Self-Healing of Charged Microgels in Neutral and Charged Environments

The softness of microgels depends on many aspects, such as particle characteristic lengths, sample concentration, chemical composition of the sample, and elastic moduli of the particle. Here, the response to crowding of ionic microgels is studied. Charged and uncharged ionic microgels are studied in concentrated suspensions of both neutral and ionic microgels with the same swollen size. The combination of small-angle X-ray and neutron scattering with contrast variation allows us to probe both the particle-to-particle arrangement and the response of individual ionic microgels to crowding. When the ionic microgels are uncharged, initial isotropic deswelling followed by faceting is observed. Therefore, the ionizable groups in the polymeric network do not affect the response of the ionic microgel to crowding, which is similar to what has been reported for neutral microgels. In contrast, the kind of microgels composing the matrix plays a key role once the ionic microgels are charged. If the matrix is composed of neutral microgels, a pronounced faceting and negligible deswelling is observed. When only charged ionic microgels are present in the suspension, isotropic deswelling without faceting is dominant.

How Softness Matters in Soft Nanogels and Nanogel Assemblies

Softness plays a key role in determining the macroscopic properties of colloidal systems, from synthetic nanogels to biological macromolecules, from viruses to star polymers. However, we are missing a way to quantify what the term "softness" means in nanoscience. Having quantitative parameters is fundamental to compare different systems and understand what the consequences of softness on the macroscopic properties are. Here, we propose different quantities that can be measured using scattering methods and microscopy experiments. On the basis of these quantities, we review the recent literature on micro- and nanogels, i.e. cross-linked polymer networks swollen in water, a widely used model system for soft colloids. Applying our criteria, we address the question what makes a nanomaterial soft? We discuss and introduce general criteria to quantify the different definitions of softness for an individual compressible colloid. This is done in terms of the energetic cost associated with the deformation and the capability of the colloid to isotropically deswell. Then, concentrated solutions of soft colloids are considered. New definitions of softness and new parameters, which depend on the particle-to-particle interactions, are introduced in terms of faceting and interpenetration. The influence of the different synthetic routes on the softness of nanogels is discussed. Concentrated solutions of nanogels are considered and we review the recent results in the literature concerning the phase behavior and flow properties of nanogels both in three and two dimensions, in the light of the different parameters we defined. The aim of this review is to look at the results on micro- and nanogels in a more quantitative way that allow us to explain the reported properties in terms of differences in colloidal softness. Furthermore, this review can give researchers dealing with soft colloids quantitative methods to define unambiguously which softness matters in their compound.

Aspiration-assisted freeform bioprinting of mesenchymal stem cell spheroids within alginate microgels

Aspiration-assisted freeform bioprinting (AAfB) has emerged as a promising technique for precise placement of tissue spheroids in three-dimensional (3D) space enabling tissue fabrication. To achieve success in embedded bioprinting using AAfB, an ideal support bath should possess shear-thinning behavior and yield-stress to facilitate tight fusion and assembly of bioprinted spheroids forming tissues. Several studies have demonstrated support baths for embedded bioprinting in the past few years, yet a majority of these materials poses challenges due to their low biocompatibility, opaqueness, complex and prolonged preparation procedures, and limited spheroid fusion efficacy. In this study, to circumvent the aforementioned limitations, we present the feasibility of AAfB of human mesenchymal stem cell (hMSC) spheroids in alginate microgels as a support bath. Alginate microgels were first prepared with different particle sizes modulated by blending time and concentration, followed by determination of the optimal bioprinting conditions by the assessment of rheological properties, bioprintability, and spheroid fusion efficiency. The bioprinted and consequently self-assembled tissue structures made of hMSC spheroids were osteogenically induced for bone tissue formation. Alongside, we investigated the effects of peripheral blood monocyte-derived osteoclast incorporation into the hMSC spheroids in heterotypic bone tissue formation. We demonstrated that alginate microgels enabled unprecedented positional accuracy (∼5%), transparency for visualization, and improved fusion efficiency (∼97%) of bioprinted hMSC spheroids for bone fabrication. This study demonstrates the potential of using alginate microgels as a support bath for many different applications including but not limited to freeform bioprinting of spheroids, cell-laden hydrogels, and fugitive inks to form viable tissue constructs.

A facile evanescent-field imaging approach for monitoring colloidal gel evolution near a surface

A facile evanescent-field imaging approach is developed to probe the aggregation behavior of near-wall colloids/clusters during colloidal gel evolution. Total internal reflection microscope (TIRM) images are directly utilized to access the structural relaxation time via density-fluctuation theory. The behaviors of cluster-cluster aggregation and physical aging of the colloidal gel networks are resolved in both time and space under fractal scaling criteria.

Glass and Jamming Rheology in Soft Particles Made of PNIPAM and Polyacrylic Acid

The phase behaviour of soft colloids has attracted great attention due to the large variety of new phenomenologies emerging from their ability to pack at very high volume fractions. Here we report rheological measurements on interpenetrated polymer network microgels composed of poly(N-isopropylacrylamide) (PNIPAM) and polyacrylic acid (PAAc) at fixed PAAc content as a function of weight concentration. We found three different rheological regimes characteristic of three different states: a Newtonian shear-thinning fluid, an attractive glass characterized by a yield stress, and a jamming state. We discuss the possible molecular mechanisms driving the formation of these states.

Pathways and challenges towards a complete characterization of microgels

Due to their controlled size, sensitivity to external stimuli, and ease-of-use, microgel colloids are unique building blocks for soft materials made by crosslinking polymers on the micrometer scale. Despite the plethora of work published, many questions about their internal structure, interactions, and phase behavior are still open. The reasons for this lack of understanding are the challenges arising from the small size of the microgel particles, complex pairwise interactions, and their solvent permeability. Here we describe pathways toward a complete understanding of microgel colloids based on recent experimental advances in nanoscale characterization, such as super-resolution microscopy, scattering methods, and modeling.

Two-step yielding behavior of densely packed microgel mixtures with chemically dissimilar surfaces and largely different sizes

Steady-state flow and elastic behavior is investigated for the moderately concentrated binary suspensions of soft microgels (pastes) with chemically dissimilar surfaces, and various degrees of size- and stiffness disparities. The pastes of poly(N-isopropyl acrylamide) (N) and poly(N-isopropyl methacrylamide) (NM) microgels with different values of yield strain γc (γNc > γNMc) are employed as the components. For the single microgel pastes (φ ≈ 1 where φ is apparent volume fraction), the values of γc are governed by the chemical species of constituent polymer in microgel surface whereas γc is insensitive to cross-link density and particle size. We demonstrate that the binary N/NM pastes with large size disparity (RN/NM = DN/DNM < 0.26 where D is the microgel diameter) at low φN (φN: weight fraction of small N microgels) exhibit the peculiarities in several rheological aspects, i.e., the two-step yielding in steady-state flow, and their values of γc and equilibrium shear modulus (G0) being equivalent to those of the single large NM microgel paste. These peculiarities are attributed to the characteristic packing resulting from large size disparity in which all or almost of the small N microgels tend to be accommodated in the gap between the large NM microgels even in moderately concentrated state. This characteristic packing substantially masks the contribution of the small N microgels at low φN, explaining the φN-independent G0 and γc as well as the first yielding governed solely by the large NM microgels. The second yielding results from the emerged contribution of the small N microgels expelled out from the gap by the positional rearrangements after the first yielding. The binary homo-N/N pastes with the similarly large size disparity at low φsmall also exhibit the φsmall-independent values of G0, but they show one-step yielding, indicating that the two-step yielding requires not only sufficiently large size disparity but also chemical dissimilarity (different values of γc) between the two components.

Concentration dependence of the dynamics of microgel suspensions investigated by dynamic light scattering

The dynamics of colloidal gel particle suspensions, i.e., microgel suspensions, has been investigated by dynamic light scattering (DLS) over a wide concentration range from the (I) dilute (φ < φcp) to the (II) intermediate (φ ≈ φcp) and (III) high concentration regions (φ ≫ φcp), where φ and φcp are the volume fraction of the gel particles in the suspension and the random close packing fraction, φcp ≈ 0.64, respectively. The time-intensity correlation function exhibited a distinct change with increasing φ, i.e., from ergodic behaviour (region I and II) to nonergodic behaviour (region III). A mode transition from translational (region I) to cooperative diffusion (the so-called gel mode) (region II) was also observed due to the soft and deformable nature of the microgels. Different from the dynamics of hard colloidal glass suspensions, the gel mode remained even at φ ≫ φcp. By using the ensemble-averaged time-correlation function, IE, we quantify the relationship between φ and their dynamics, and show that the soft microgels are deswollen in the densely packed state.

Viscosity of mixtures of protein aggregates with different sizes and morphologies

Protein aggregates were generated by thermal denaturation of whey protein isolates. Depending on the heating conditions, fractal aggregates of various sizes or microgels were obtained. The osmotic compressibility and correlation length of mixtures of fractal aggregates of different sizes were found to be close to the weighted averages of the individual components at the same concentration. The viscosity of these mixtures can be described by a logarithmic mixing law using the weight fraction and the viscosity of the individual components. The same mixing law describes the behavior of mixtures of fractal aggregates and microgels. The effect of the type of protein was investigated by mixing fractal aggregates formed by whey and soy protein isolates. It is suggested that the viscosity of the mixtures is determined by the cooperative movement over length scales much larger than the size of the aggregates.

Hydrogel Microellipsoids that Form Robust String-Like Assemblies at the Air/Water Interface

Soft colloidal particles such as hydrogel microspheres assemble at air/water or oil/water interfaces, where the soft colloids are highly deformed and their surface polymer chains are highly entangled with each other. Herein, we report the formation of robust one-dimensional, string-like colloidal assemblies through self-organization of hydrogel microspheres with shape anisotropy at the air/water interface of sessile droplets. Shape-anisotropic hydrogel microspheres were synthesized via two-step polymerization, whereby a hydrogel shell was formed onto preformed rigid microellipsoids. The shape anisotropy of the hydrogel microspheres was confirmed by transmission electron microscopy and high-speed atomic force microscopy as well as by light-scattering measurements. The present findings are crucial for the understanding of natural self-organization phenomena, where "softness" influences microscopic assembled structures such as those of Nostoc bacteria.

Non-Thermoresponsive Decanano-sized Domains in Thermoresponsive Hydrogel Microspheres Revealed by Temperature-Controlled High-Speed Atomic Force Microscopy

Despite the tremendous efforts devoted to the structural analysis of hydrogel microspheres (microgels), many details of their structures remain unclear. Reported in this study is that thermoresponsive poly(N-isopropyl acrylamide) (pNIPAm)-based microgels exhibit not only the widely accepted core-shell structures, but also inhomogeneous decanano-sized non-thermoresponsive spherical domains within their dense cores, which was revealed by temperature-controlled high-speed atomic force microscopy (TC-HS-AFM). Based on a series of experiments, it is concluded that the non-thermoresponsive domains are characteristic for pNIPAm microgels synthesized by precipitation polymerization, and plausible structures for microgels prepared by other polymerization techniques are proposed.