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1. 4D Printing of Multi‐Stimuli Responsive Protein‐Based Hydrogels for Autonomous Shape Transformations

   https://doi.org/10.1002/adfm.202011012  

   Narupai, Benjaporn; Smith, Patrick T.; Nelson, Alshakim (March 2021, Advanced Functional Materials)

   Abstract Stimuli responsive hydrogels that can change shape in response to applied external stimuli are appealing for soft robotics, biomedical devices, drug delivery, and actuators. However, existing 3D printed shape morphing materials are non‐biodegradable, which limits their use in biomedical applications. Here, 3D printed protein‐based hydrogels are developed and applied for programmable structural changes under the action of temperature, pH, or an enzyme. Key to the success of this strategy is the use of methacrylated bovine serum albumin (MA–BSA) as a biodegradable building block to Pickering emulsion gels in the presence ofN‐isopropylacrylamide or 2‐dimethylaminoethyl methacrylate. These shear‐thinning gels are ideal for direct ink write (DIW) 3D printing of multi‐layered stimuli‐responsive hydrogels. While poly(N‐isopropylacrylamide) and poly(dimethylaminoethyl methacrylate) introduce temperature and pH‐responsive properties into the printed objects, a unique feature of this strategy is an enzyme‐triggered shape transformation based on the degradation of the bovine serum albumin network. To highlight this technique, protein‐based hydrogels that reversibly change shape based on environmental temperature and pH are fabricated, and irreversibly altered by enzymatic degradation, which demonstrates the complexity that can be introduced into 4D printed systems. 

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2. Amphiphilic Heterografted Molecular Bottlebrushes with Tertiary Amine‐Containing Side Chains as Efficient and Robust pH‐Responsive Emulsifiers

   https://doi.org/10.1002/anie.202315424  

   Kelly, Michael T.; Chen, Zhan; Russell, Thomas P.; Zhao, Bin (November 2023, Angewandte Chemie International Edition)

   Abstract By combining the unique characteristics of molecular bottlebrushes (MBBs) and the properties of stimuli‐responsive polymers, we show that MBBs with randomly grafted poly(n‐butyl acrylate) and pH‐responsive poly(2‐(N,N‐diethylamino)ethyl methacrylate) (PDEAEMA) side chains are efficient and robust pH‐responsive emulsifiers. Water‐in‐toluene emulsions were formed at pH 4.0 and disrupted by increasing the pH to 10.0. The emulsion generation and disruption was reversible over the ten cycles investigated, and the bottlebrushes remained intact. The exceptional emulsion stability stemmed from the high interfacial binding energy of MBBs, imparted by their large molecular size and Janus architecture at the interface, as evidenced by the interfacial jamming and wrinkling of the assemblies upon reducing the interfacial area. At pH 10.0, PDEAEMA became water‐insoluble, and the MBBs desorbed from the interface, causing de‐emulsification. Consequently, we have shown that the judicious design of MBBs can generate properties of particle emulsifiers from their large size, while the responsiveness of the MBBs enables more potential applications. 

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3. 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 of 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. 

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4. Building Supracolloidal Fibers from Zwitterion‐Stabilized Adhesive Emulsions

   https://doi.org/10.1002/adfm.201804325  

   Santa Chalarca, Cristiam F.; Letteri, Rachel A.; Perazzo, Antonio; Stone, Howard A.; Emrick, Todd (September 2018, Advanced Functional Materials)

   Abstract Oil‐in‐water droplets stabilized with polymer zwitterions (PZWs) exhibit salt‐responsive aggregation–disaggregation behavior. Here, a method to shape these droplets is described, starting from their aggregated state, into supracolloidal fibers by simply extruding them into aqueous media. The effect of salt concentration, in both the initial emulsion and the aqueous medium, on the ability of the emulsions to form fibers is examined. After fiber formation, a transition from well‐defined macroscopic structures to noninteracting droplet dispersions can be triggered, simply by increasing the salt concentration of the aqueous environment. The interdroplet energy of adhesion and emulsion rheology correlate qualitatively with salt concentration and thus impact the ability of the emulsions to be shaped by extrusion. The interdroplet adhesion is dependent on both salt concentration and polymer composition, which allows tailoring of conditions to trigger fiber disaggregation. Finally, fibers with variable compositions along their length are prepared by sequential loading and extrusion of emulsions containing oil phases of differing densities. 

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5. 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 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. 

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