Formation of a Crack-Free, Hybrid Skin Layer with Tunable Surface Topography and Improved Gas Permeation Selectivity on Elastomers Using Gel-Liquid Infiltration Polymerization

Highly Robust Interfacially Polymerized PA Layer on Thermally Responsive Semi-IPN Hydrogel: Toward On-Demand Tuning of Porosity and Surface Charge

Hydrogel composites with skin layer that allows fast and selective rejection of molecules possess high potential for numerous applications, including sample preconcentration for point-of-use detection and analysis. The stimuli-responsive hydrogels are particularly promising due to facile regenerability. However, poor adhesion of the skin layer due to swelling-degree difference during continuous swelling/deswelling of the hydrogel hinders its further development. In this work, a polyamide skin layer with strong adhesion was fabricated via gel-liquid interfacial polymerization (GLIP) of branched polyethyleneimine (PEI) with trimesoyl chloride (TMC) on a cross-linked N-isopropyl acrylamide hydrogel network containing dispersed poly sodium acrylate (PSA), while the traditional m-phenylenediamine (MPD)-TMC polyamide layer readily delaminates. We investigated the mechanistic design principle, which not only resulted in strong anchoring of the polyamide layer to the hydrogel surface but also enabled manipulation of the surface morphology, porosity, and surface charge by tailoring interfacial reaction conditions. The polyamide/hydrogel composite was able to withstand 100 cycles of swelling/deswelling without any delamination or a significant decrease in its rejection performance of the model dye, i.e., methylene blue. Regeneration can be done by deswelling the swollen beads at 60 °C, which also releases any loosely bound molecules together with absorbed water. This work provides insights into the development of a physically and chemically robust skin layer on various types of hydrogels for applications such as preconcentration, antifouling-coating, selective compound extraction, etc.

Harnessing Wrinkling Patterns Using Shape Memory Polymer Microparticles

Shape memory polymers (SMPs) are the simplest and most attractive alternatives for soft substrates of typical bilayer wrinkle systems because of shape fixity and recovery capabilities. Herein, we have successfully programmed large compressive strains in chemical cross-linking shape memory polystyrene (PS) microparticles via nanoimprint lithography, which acted as the substrate of a wrinkle system using a gold nanoparticle (Au NP) film as the top layer. When triggered by two different stimuli (direct heating and toluene vapors), the thin Au NP film could transform into various wrinkle structures atop the recovered PS particles. In addition, we also investigated the evolution mechanisms of wrinkling by heating and toluene vapors and tuned the wrinkled surfaces through altering the Au NP thickness and stimulation methods (direct heating and toluene vapors), which utilized the structural adjustability of Au NPs to program the amplitude, wavelength, and morphology of the wrinkles. The concept presented here provides a cost-effective approach to realize the surface wrinkling and can be extended to other available SMPs.

Stretchable Electronics Based on PDMS Substrates

Stretchable electronics, which can retain their functions under stretching, have attracted great interest in recent decades. Elastic substrates, which bear the applied strain and regulate the strain distribution in circuits, are indispensable components in stretchable electronics. Moreover, the self-healing property of the substrate is a premise to endow stretchable electronics with the same characteristics, so the device may recover from failure resulting from large and frequent deformations. Therefore, the properties of the elastic substrate are crucial to the overall performance of stretchable devices. Poly(dimethylsiloxane) (PDMS) is widely used as the substrate material for stretchable electronics, not only because of its advantages, which include stable chemical properties, good thermal stability, transparency, and biological compatibility, but also because of its capability of attaining designer functionalities via surface modification and bulk property tailoring. Herein, the strategies for fabricating stretchable electronics on PDMS substrates are summarized, and the influence of the physical and chemical properties of PDMS, including surface chemical status, physical modulus, geometric structures, and self-healing properties, on the performance of stretchable electronics is discussed. Finally, the challenges and future opportunities of stretchable electronics based on PDMS substrates are considered.

Snakeskin-Inspired Elastomers with Extremely Low Coefficient of Friction under Dry Conditions

Soft elastomers are critical to a broad range of existing and emerging technologies. One major limitation of soft elastomers is the large friction of coefficient (COF) due to inherently large adhesion and internal loss. In applications where lubrication is not applicable, such as soft robotics, wearable electronics, and biomedical devices, elastomers with inherently low dry COF are required. Inspired by the low COF of snakeskins atop soft bodies, this study reports the development of elastomers with low dry COF by growing a hybrid skin layer with a strong interface with a large stiffness gradient. Using a solid-liquid interfacial polymerization (SLIP) process, hybrid skin layers are imparted onto elastomers, which reduces the COF of the elastomers from 1.6 to 0.1, without sacrificing the bulk compliance and ductility of elastomer. Compared with existing surface modification methods, the SLIP process offers spatial control and ability to modify flat, prepatterned, curved, and inner surfaces, which is essential to engineer multifunctional skin layers for emerging applications.

Zwitterionic Hydrogel-Impregnated Membranes with Polyamide Skin Achieving Superior Water/Salt Separation Properties

Support-free nonporous membranes have emerged as a new material platform for osmotic pressure-driven processes due to its insusceptibility to internal concentration polarization (ICP). Herein, we demonstrate high-performance membranes of zwitterionic hydrogels impregnated in porous membranes with a skin layer of highly cross-linked polyamides on both sides prepared by gel-liquid interfacial polymerization (GLIP). Such a configuration eliminates the pores and thus ICP, while the thin polyamide layer provides high salt rejection but negligible resistance to the water transport compared with the hydrogels. The polyamide skin layers are characterized using scanning electron microscopy and atomic force microscopy. The effect of the hydrogel compositions and polyamide formation conditions on the water/salt separation properties is thoroughly investigated. Example membranes show water permeance and salt rejection comparable to state-of-the-art commercial forward osmosis membranes and essentially no ICP.

Advanced Soft Materials, Sensor Integrations, and Applications of Wearable Flexible Hybrid Electronics in Healthcare, Energy, and Environment

Recent advances in soft materials and system integration technologies have provided a unique opportunity to design various types of wearable flexible hybrid electronics (WFHE) for advanced human healthcare and human-machine interfaces. The hybrid integration of soft and biocompatible materials with miniaturized wireless wearable systems is undoubtedly an attractive prospect in the sense that the successful device performance requires high degrees of mechanical flexibility, sensing capability, and user-friendly simplicity. Here, the most up-to-date materials, sensors, and system-packaging technologies to develop advanced WFHE are provided. Details of mechanical, electrical, physicochemical, and biocompatible properties are discussed with integrated sensor applications in healthcare, energy, and environment. In addition, limitations of the current materials are discussed, as well as key challenges and the future direction of WFHE. Collectively, an all-inclusive review of the newly developed WFHE along with a summary of imperative requirements of material properties, sensor capabilities, electronics performance, and skin integrations is provided.

Controlled Growth of Polyamide Films atop Homogenous and Heterogeneous Hydrogels using Gel-Liquid Interfacial Polymerization

Controlled growth of crosslinked polyamide (PA) thin films is demonstrated at the interface of a monomer-soaked hydrogel and an organic solution of the complementary monomer. Termed gel-liquid interfacial polymerization (GLIP), the resulting PA films are measured to be chemically and mechanically analogous to the active layer in thin film composite membranes. PA thin films are prepared using the GLIP process on both a morphologically homogeneous hydrogel prepared from poly(2-hydroxyethylmethacrylate) (PHEMA) and a phase-separated, heterogeneous hydrogel prepared from poly(acrylamide) (PAAm). Two monomer systems are examined: trimesoyl chloride (TMC) reacting with m-phenylene diamine (MPD) and TMC reacting with piperazine (PIP). Unlike the self-limiting growth behavior in TFC membrane fabrication, diffusion-limited, continuous growth of the PA films is observed, where both the thickness and roughness of the PA layers increase with reaction time. A key morphological difference is found between the two monomer systems using the GLIP process: TMC/MPD produces a ridge-and-valley surface morphology whereas TMC/PIP produces nodule/granular structures. The GLIP process represents a unique opportunity to not only explore the pore characteristics (size, spacing, and continuity) on the resulting structure and morphology of interfacially polymerized thin films, but also a method to modify the surface of (or encapsulate) hydrogels.