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1. Core–shell droplets and microcapsules formed through liquid–liquid phase separation of a colloid–polymer mixture

   https://doi.org/10.1039/D1SM01091C  

   Dang, Steven ; Brady, John ; Rel, Ryle ; Surineni, Sreenidhi ; O’Shaughnessy, Conor ; McGorty, Ryan ( September 2021 , Soft Matter)

   Microcapsules allow for the controlled containment, transport, and release of cargoes ranging from pharmaceuticals to fragrances. Given the interest from a variety of industries in microcapsules and other core–shell structures, a multitude of fabrication strategies exist. Here, we report on a method relying on a mixture of temperature-responsive microgel particles, poly( N -isopropylacrylamide) (pNIPAM), and a polymer which undergo fluid–fluid phase separation. At room temperature this mixture separates into colloid-rich (liquid) and colloid-poor (gas) fluids. By heating the sample above a critical temperature where the microgel particles shrink dramatically and develop a more deeply attractive interparticle potential, the droplets ofmore »the colloid-rich phase become gel-like. As the temperature is lowered back to room temperature, these droplets of gelled colloidal particles reliquefy and phase separation within the droplet occurs. This phase separation leads to colloid-poor droplets within the colloid-rich droplets surrounded by a continuous colloid-poor phase. The gas/liquid/gas all-aqueous double emulsion lasts only a few minutes before a majority of the inner droplets escape. However, the colloid-rich shell of the core–shell droplets can solidify with the addition of salt. That this method creates core–shell structures with a shell composed of stimuli-sensitive microgel colloidal particles using only aqueous components makes it attractive for encapsulating biological materials and making capsules that respond to changes in, for example, temperature, salt concentration, or pH.« less
2. Two-Dimensional and Three-Dimensional Ultrathin Multilayer Hydrogels through Layer-by-Layer Assembly

   https://doi.org/10.1021/acs.langmuir.2c00630  

   Kozlovskaya, Veronika ; Dolmat, Maksim ; Kharlampieva, Eugenia ( January 2022 , Langmuir)

   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-bylayer polymer deposition with the capability for dramatic and reversible changes in volumes upon environmental triggers, a characteristic of chemically crosslinked 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 highlightsmore »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.« less
3. Static-state particle fabrication via rapid vitrification of a thixotropic medium

   https://doi.org/10.1038/s41467-021-23992-2  

   Kim, Sang Yup ; Liu, Shanliangzi ; Sohn, Sungwoo ; Jacobs, Jane ; Shattuck, Mark D. ; O’Hern, Corey S. ; Schroers, Jan ; Loewenberg, Michael ; Kramer-Bottiglio, Rebecca ( June 2021 , Nature Communications)

   Functional particles that respond to external stimuli are spurring technological evolution across various disciplines. While large-scale production of functional particles is needed for their use in real-life applications, precise control over particle shapes and directional properties has remained elusive for high-throughput processes. We developed a high-throughput emulsion-based process that exploits rapid vitrification of a thixotropic medium to manufacture diverse functional particles in large quantities. The vitrified medium renders stationary emulsion droplets that preserve their shape and size during solidification, and energetic fields can be applied to build programmed anisotropy into the particles. We showcase mass-production of several functional particles,more »including low-melting point metallic particles, self-propelling Janus particles, and unidirectionally-magnetized robotic particles, via this static-state particle fabrication process.

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4. PDMS polymerized high internal phase emulsions (polyHIPEs) with closed-cell, aqueous-filled microcavities

   https://doi.org/10.1039/c9sm01732a  

   Kataruka, Amrita ; Hutchens, Shelby B. ( December 2019 , Soft Matter)

   Emulsion templates can produce a wide range of unique microstructures via solidification of the continuous phase. Some of these structures result in unique, fluid-filled composites reminiscent of biological tissue when the templating droplets develop into closed-cell structures. However, the state-of-the-art falls short in replicating the mechanical and functional response of biological structures due to stiff, fragile, and bio-incompatible materials while lacking systematic processing parameters. This article describes the synthesis of high internal phase, closed-cell, polydimethylsiloxane (PDMS) elastomeric foams which simultaneously achieve biocompatibility, mechanical robustness, flexibility, and selective permeability. Water-in-oil high internal phase emulsions (HIPEs) stabilized by silica nano-particles (SNPs) providemore »the microstructural template, resulting in a >74% by volume aqueous phase (up to 82%). To overcome the prohibitive barrier to HIPE formation when using a mechanically-superior, but highly viscous commercial PDMS kit, we produce HIPE templates via centrifugation of low internal phase emulsions (LIPEs, <30% by volume dispersed phase). This oil phase crosslinks into an aqueous-filled (water + glycerol + NaCl) elastomeric composite. The composite's microstructural dependence on viscosity ratio, mixing speed, emulsifier concentration, and centrifugal force are systematically characterized. The resulting microstructured, fluid-filled elastomer composites exhibit mechanically robust and highly flexible behavior due to the excellent properties of the PDMS continuous phase.« less
5. Molecularly-ordered hydrogels with controllable, anisotropic stimulus response

   https://doi.org/10.1039/C9SM00763F  

   Boothby, Jennifer M. ; Samuel, Jeremy ; Ware, Taylor H. ( June 2019 , Soft Matter)

   Hydrogels which morph between programmed shapes in response to aqueous stimuli are of significant interest for biosensors and artificial muscles, among other applications. However, programming hydrogel shape change at small size scales is a significant challenge. Here we use the inherent ordering capabilities of liquid crystals to create a mechanically anisotropic hydrogel; when coupled with responsive comonomers, the mechanical anisotropy in the network guides shape change in response to the desired aqueous condition. Our synthetic strategy hinges on the use of a methacrylic chromonic liquid crystal monomer which can be combined with a non-polymerizable chromonic of similar structure to varymore »the magnitude of shape change while retaining liquid crystalline order. This shape change is directional due to the mechanical anisotropy of the gel, which is up to 50% stiffer along the chromonic stack direction than perpendicular. Additionally, we show that the type of stimulus to which these anisotropic gels respond can be switched by incorporating responsive, hydrophilic comonomers without destroying the nematic phase or alignment. The utility of these properties is demonstrated in polymerized microstructures which exhibit Gaussian curvature in response to high pH due to emergent ordering in a micron-sized capillary.« less