Complete pH-Dependent Shape Recovery in Cubical Hydrogel Capsules after Large Osmotic Deformations

Real and In Silico Microgels Show Comparable Bulk Moduli Below and Above the Volume Phase Transition

The compressibility of soft colloids influences their phase behavior and flow properties, especially in concentrated suspensions. Particle compressibility, which is proportional to the reciprocal of the bulk modulus K, is a key parameter for soft polymer-based particles that can be compressed in crowded environments. Here, microgels with different degrees of cross-linking, i.e., softness, are investigated below and above their volume phase transition temperature (VPTT). By combining molecular dynamics simulations with small-angle neutron scattering with contrast variation, a change in the particle bulk moduli of two orders of magnitude is observed. The degree of cross-linking has a significant impact on the bulk modulus of the swollen microgel, while above the VPTT the values of K are almost independent of the cross-linking density. The excellent agreement between experimental results and simulations also highlight that the model microgels from computer simulations possess both the internal architecture and the elastic properties of real polymeric networks. This paves the way to a systematic use of simulations to investigate the behavior of dense microgel suspensions below and above their VPTT.

Polymer Complex Multilayers for Drug Delivery and Medical Devices

Polymer complex multilayers (PCMs) can be engineered into various structures with tunable properties via layer-by-layer (LBL) assembly driven by noncovalent forces. Due to their ease of preparation, capability of integrating multiple functional components, and excellent substrate compliance, biocompatible PCMs as coating materials or individual entities have attracted extensive attention in biomedical applications. This Spotlight on Applications presents recent progress on PCMs applied for drug delivery and medical devices. We provide several examples to address the importance of using PCM platforms to achieve controlled drug delivery including stimuli-triggered release, sustained release, and spatiotemporal sequential release. The effects of PCM coatings on the bioresponse regulation and performance enhancement of implantable devices are also highlighted. Moreover, the design and fabrication of flexible electrical and optical elements modified with LBL PCMs have been discussed, which demonstrates the great potential to advance emerging wearable devices for disease monitoring and health management.

Two-Dimensional and Three-Dimensional Ultrathin Multilayer Hydrogels through Layer-by-Layer Assembly

Stimuli-responsive multilayer hydrogels have opened new opportunities to design hierarchically organized networks with properties controlled at the nanoscale. These multilayer materials integrate structural, morphological, and compositional versatility provided by alternating layer-by-layer polymer deposition with the capability for dramatic and reversible changes in volumes upon environmental triggers, a characteristic of chemically cross-linked responsive networks. Despite their intriguing potential, there has been limited knowledge about the structure-property relationships of multilayer hydrogels, partly because of the challenges in regulating network structural organization and the limited set of the instrumental pool to resolve structure and properties at nanometer spatial resolution. This Feature Article highlights our recent studies on advancing assembly technologies, fundamentals, and applications of multilayer hydrogels. The fundamental relationships among synthetic strategies, chemical compositions, and hydrogel architectures are discussed, and their impacts on stimuli-induced volume changes, morphology, and mechanical responses are presented. We present an overview of our studies on thin multilayer hydrogel coatings, focusing on controlling and quantifying the degree of layer intermixing, which are crucial issues in the design of hydrogels with predictable properties. We also uncover the behavior of stratified "multicompartment" hydrogels in response to changes in pH and temperature. We summarize the mechanical responses of free-standing multilayer hydrogels, including planar thin coatings and films with closed geometries such as hollow microcapsules and nonhollow hydrogel microparticles with spherical and nonspherical shapes. Finally, we will showcase potential applications of pH- and temperature-sensitive multilayer hydrogels in sensing and drug delivery. The knowledge about multilayer hydrogels can advance the rational design of polymer networks with predictable and well-tunable properties, contributing to modern polymer science and broadening hydrogel applications.

Resolving the different bulk moduli within individual soft nanogels using small-angle neutron scattering

The bulk modulus, K, quantifies the elastic response of an object to an isotropic compression. For soft compressible colloids, knowing K is essential to accurately predict the suspension response to crowding. Most colloids have complex architectures characterized by different softness, which additionally depends on compression. Here, we determine the different values of K for the various morphological parts of individual nanogels and probe the changes of K with compression. Our method uses a partially deuterated polymer, which exerts the required isotropic stress, and small-angle neutron scattering with contrast matching to determine the form factor of the particles without any scattering contribution from the polymer. We show a clear difference in softness, compressibility, and evolution of K between the shell of the nanogel and the rest of the particle, depending on the amount of cross-linker used in their synthesis.

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.