Mussel‐Inspired Hydrogels for Self‐Adhesive Bioelectronics

Wearable and implantable bioelectronics are receiving a great deal of attention because they offer huge promise in personalized healthcare. Currently available bioelectronics generally rely on external aids to form an attachment to the human body, which leads to unstable performance in practical applications. Self‐adhesive bioelectronics are highly desirable for ameliorating these concerns by offering reliable and conformal contact with tissue, and stability and fidelity in the signal detection. However, achieving adequate and long‐term self‐adhesion to soft and wet biological tissues has been a daunting challenge. Recently, mussel‐inspired hydrogels have emerged as promising candidates for the design of self‐adhesive bioelectronics. In addition to self‐adhesiveness, the mussel‐inspired chemistry offers a unique pathway for integrating multiple functional properties to all‐in‐one bioelectronic devices, which have great implications for healthcare applications. In this report, the recent progress in the area of mussel‐inspired self‐adhesive bioelectronics is highlighted by specifically discussing: 1) adhesion mechanism of mussels, 2) mussel‐inspired hydrogels with long‐term and repeatable adhesion, 3) the recent advance in development of hydrogel bioelectronics by reconciling self‐adhesiveness and additional properties including conductivity, toughness, transparency, self‐healing, antibacterial properties, and tolerance to extreme environment, and 4) the challenges and prospects for the future design of the mussel‐inspired self‐adhesive bioelectronics.     

Recent Advances in Stimuli-Responsive Smart Membranes for Nanofiltration

Nanofiltration membrane plays an increasingly important role in many industrial applications, such as water treatment and resource recovery. The performance of the smart nanofiltration membrane is largely controlled by pore size, the Donnan effect, and surface wettability, which are determined by the function of stimuli-responsive components. Smart membranes, which contain stimuli-responsive components, are capable of changing their physical and chemical properties in response to changes in the environment so that the microstructure of the membrane will have more efficient performances and broader application prospects than the current traditional nanofiltration membranes. Herein, the preparation methods of stimuli-responsive membranes are described and they are systematically classified accordingly to their mechanisms. Moreover, the latest progress of stimuli-responsive membranes in nanofiltration and the main mechanism of each stimuli-response type through relevant examples are discussed and summarized. Finally, this review provides new insights into the remaining challenges and future directions of stimuli-responsive membranes. Fueled by advances in chemistry and materials science, it is expected to build a smart and efficient nanofiltration membrane platform for the benefit of mankind.     

Recent Advances of Cell Membrane‐Coated Nanomaterials for Biomedical Applications

Surface modification of nanomaterials is essential for their biomedical applications owing to their passive immune clearance and damage to reticuloendothelial systems. Recently, a cell membrane‐coating technology has been proposed as an ideal approach to modify nanomaterials owing to its facile functionalized process and good biocompatibility for improving performances of synthetic nanomaterials. Here, recent advances of cell membrane‐coated nanomaterials are reviewed based on the main biological functions of the cell membrane in living cells. An overview of the cell membrane is introduced to understand its functions and potential applications. Then, the applications of cell membrane‐coated nanomaterials based on the functions of the cell membrane are summarized, including physical barrier with selective permeability and cellular communication via information transmission and reception processes. Finally, perspectives of biomedical applications and challenges about cell membrane‐coated nanomaterials are discussed.     

2D Laminar Membranes for Selective Water and Ion Transport

Selective water and ion transport are essential in fields related to the environment, resources, energy, and more. Membranes, especially those constituted by 2D materials, are promising to control mass transport within nano‐ and sub‐nanoscales. When stacked together, the ultrathin nanosheets of these materials can build up laminar membranes with an ordered layer‐like structure. Numerous channels are thereby created among layers for fast and selective mass transport, which arouses huge research and application interests. This Review aims to present the latest theoretical and experimental advances of 2D laminar membranes for selective water and ion transport, covering three fundamental aspects. Starting with a concise introduction to the materials and assembly for laminar membranes, it then mainly focuses on systematically discussing the transport‐controlling effects caused by intrinsic membrane structure and extrinsic influences. The relation between these effects and current membrane selective performance as well as future membrane designs is then elucidated. The most urgent challenges and corresponding opportunities that emerge around 2D laminar membranes are highlighted thereafter.     

Lithium Extraction by Emerging Metal–Organic Framework-Based Membranes

Lithium is mainly extracted from brine and ores; however, current lithium mining methods require large amounts of chemicals, discharge many wastes, and can have serious environmental repercussions. Metal–organic framework (MOF)-based membranes have shown great potential in lithium extraction due to their uniform pore sizes, high porosities, and rich host–guest chemistry compared to other materials. In this review, the processes and disadvantages of current lithium extraction technologies are introduced. The structure features and corresponding design strategy of MOFs suitable for Li⁺ ion separations are presented. Following, recent advances of polycrystalline MOF membranes, mixed matrix membranes, and MOF channel membranes for lithium-ion separation are discussed in detail. Finally, opportunities for future developments and challenges in this emerging research field are presented.     

Strategic Design of Clay‐Based Multifunctional Materials: From Natural Minerals to Nanostructured Membranes

Nature not only carefully prepares ingenious raw materials but also continuously inspires and guides human beings to create a wide variety of intelligent materials. As the most abundant mineral resource on earth, clay minerals are no longer synonymous with ceramics and cements. Many natural clay minerals can be exfoliated into single‐ or few‐layered nanosheets with exquisite physicochemical properties, which can be reassembled into functional membranes with a macroscopic controllable size and microscopic ordered structure. They are thus used in many fields including chemistry, biology, energy, and environmental science. Strategic design represents one of the key processes to enhance the value of clay minerals and broaden their applications. In this work, the three frequently used approaches of exfoliation are highlighted and the six routes of assembly including casting, dip‐coating, spray coating, vacuum filtration, electrophoretic deposition, and 3D printing are compared. The corresponding principles and advantages are summarized. Representative applications of clay‐based multifunctional membranes in protection, separation, responsiveness, flexible electronics, and energy conversion are presented. The challenges and future perspectives of the clay‐based multifunctional membranes are discussed.     

Surface Charge Modification on 2D Nanofluidic Membrane for Regulating Ion Transport

2D nanofluidic membranes are capable of regulating ion transport toward various applications concerning energy and environment, which is primarily contributed by the excess charge on the interior surface of narrow nanoscale pores. However, there is still a lack of comprehensive summaries and discussions on the surface charge modification principles and strategies of 2D nanofluidic membranes, as well as the practical applications of charge-modified 2D nanofluidic membranes for regulating ion transport. In this review, the surface charge modification principles and charge modification methods of 2D nanofluidic membranes are first introduced in detail, which is of great significance for improving the ion regulation capability of membranes and realizing the design of nanochannel materials. Next, recent advances in the two typical applications of concentration cells and water treatment based on charge-modified 2D nanofluidic membranes are summarized. Finally, some challenges and prospects related to charge-modified 2D nanofluidic membranes are discussed to indicate directions for future research in this field. It is anticipated that this review will provide valuable strategies for the development of high-performance charge-modified 2D nanofluidic membranes toward energy and environment applications.     

High‐Efficiency Water‐Transport Channels using the Synergistic Effect of a Hydrophilic Polymer and Graphene Oxide Laminates

Graphene oxide (GO) laminates possess unprecedented fast water‐transport channels. However, how to fully utilize these unique channels in order to maximize the separation properties of GO laminates remains a challenge. Here, a bio‐inspired membrane that couples an ultrathin surface water‐capturing polymeric layer (<10 nm) and GO laminates is designed. The proposed synergistic effect of highly enhanced water sorption from the polymeric layer and molecular channels from the GO laminates realizes fast and selective water transport through the integrated membrane. The prepared membrane exhibits highly selective water permeation with an excellent water flux of over 10 000 g m^?2 h^?1, which exceeds the performance upper bound of state‐of‐the‐art membranes for butanol dehydration. This bio‐inspired strategy demonstrated here opens the door to explore fast and selective channels derived from 2D or 3D materials for highly efficient molecular separation.     

Wearable and Implantable Triboelectric Nanogenerators

Triboelectric nanogenerators (TENGs) are a promising technology to convert mechanical energy to electrical energy based on coupled triboelectrification and electrostatic induction. With the rapid development of functional materials and manufacturing techniques, wearable and implantable TENGs have evolved into playing important roles in clinic and daily life from in vitro to in vivo. These flexible and light membrane‐like devices have the potential to be a new power supply or sensor element, to meet the special requirements for portable electronics, promoting innovation in electronic devices. In this review, the recent advances in wearable and implantable TENGs as sustainable power sources or self‐powered sensors are reviewed. In addition, the remaining challenges and future possible improvements of wearable and implantable TENG‐based self‐powered systems are discussed.     

Stomata‐Inspired Membrane Produced Through Photopolymerization Patterning

The programmed movements of responsive functional hydrogels have received much attention because of their abundant functions and wide range of engineering applications. In this study, an innovative stomata‐inspired membrane (SIM) is fabricated by using a temperature‐responsive hydrogel through a simple, cost‐effective, and high‐throughput patterned photopolymerization. Polymerization‐induced diffusion on the macroscale surface results in formation of a double‐parted polymer membrane with fine pores after single illumination. After heating the SIM, the less deformable thick frame supports the whole structure and the highly deformable thin base regulates pore shape. Among various SIM types, the slit pores of monocot SIM, which are lined up in parallel, exhibit the largest radius deformation. The morphological configuration of the SIM can be easily controlled by changing the photomask for a given application. As the developed SIM features the sensing‐to‐activation functions of stimuli‐responsive hydrogels and can be easily fabricated, this membrane can be potentially used for numerous practical applications, such as filter membranes with adjustable pores, membrane‐based sensors, membrane‐based actuators, and multifunctional membranes.     

Graphene‐Directed Supramolecular Assembly of Multifunctional Polymer Hydrogel Membranes

Polymer‐based nanoporous hydrogel membranes hold great potential for a range of applications including molecular filtration/separation, controlled drug release, and as sensors and actuators. However, to be of practical utility, polymer membranes generally need to be fabricated as ultrathin yet mechanically robust, have a large‐area yet be defect‐free and in some cases, their structure needs the capability to adapt to certain stimuli. These stringent and sometimes self‐conflicting requirements make it very challenging to manufacture such bulk nanostructures in a controllable, scalable and cost‐effective manner. Here, a versatile approach to the fabrication of multifunctional polymer‐based hydrogel membranes is demonstrated by a single step involving filtration of an aqueous dispersion containing chemically converted graphene (CCG) and a polymer. With CCG uniquely serving as a membrane‐ and pore‐forming directing agent and as a physical cross‐linker, a range of water soluble polymers can be readily processed into nanoporous hydrogel membranes through supramolecular interactions. With the interconnected CCG network as a robust and porous scaffold, the membrane nanostructure can easily be fine‐tuned to suit different applications simply by controlling the chemistry and concentration of the incorporated polymer. This work provides a simple and versatile platform for the design and fabrication of new adaptive supramolecular membranes for a variety of applications.     

Creation of Superhydrophobic Electrospun Nonwovens Fabricated from Naturally Occurring Poly(Amino Acid) Derivatives

Creation of superhydrophobic materials bio‐inspired by nature fascinates many scientists. One of the most intriguing challenges in this field is the fabrication of these materials using biopolymers from the viewpoint of green chemistry and environmental chemistry. Here, superhydrophobic and biodegradable nonwovens are constructed by electrospinning from a naturally occurring poly(amino acid), poly(γ‐glutamic acid) (γ‐PGA), modified with a hydrophobic α‐amino acid, l ‐phenylalanine. The contact angle of a water droplet on the materials is 154°, and the droplet remains stuck to the material surface even if it is inverted, clearly indicating a petal‐type superhydrophobic property. Biodegradability and post‐functionalization of the nonwovens as well as cell adhesion on the superhydrophobic materials are also evaluated. As far as we know, this is the first report on biodegradable materials exhibiting a petal‐type superhydrophobicity. The material design and processing shown here can be applied to various bioresources and such functional materials will become a new class of functional materials satisfying some of the requirements in green science.     

Autonomously Responsive Membranes for Chemical Warfare Protection

Stimuli‐responsive materials offer new opportunities to resolve long‐standing material challenges and are rapidly gaining pivotal roles in diverse applications. For example, smart protective garments that rapidly transport water vapor and autonomously block chemical threats are expected to enable an effective new paradigm of adaptive personal protection. However, the incorporation of these seemingly incompatible properties into a single responsive system remains elusive. Herein, a bistable membrane that can rapidly, selectively, and reversibly transition from a highly breathable state in a safe environment to a chemically protective state when exposed to organophosphate threats such as sarin is demonstrated. Dynamic response to chemical stimuli is achieved through the physical collapse of an ultrathin copolymer layer on the membrane surface, which efficiently gates transport through membrane pores composed of single‐walled carbon nanotubes (SWNTs). The adoption of nanometer‐wide SWNTs for ultrafast moisture conduction enables a simultaneous boost in size‐sieving selectivity and water‐vapor permeability by decreasing nanotube diameter, thereby overcoming the breathability/protection trade‐off that limits conventional membrane materials. Adaptive multifunctional membranes based on this platform greatly extend the active use of a protective garment and present exciting opportunities in many other areas including separation processes, sensing, and smart delivery.     

Nanofiltration Membranes with Modified Active Layer Using Aromatic Polyamide Dendrimers

The modification of a commercial nanofiltration (NF) membrane (TFC‐S) with shape‐persistent dendritic molecules is reported. Amphiphilic aromatic polyamide dendrimers (G1–G3) are synthesized via a divergent approach and used for membrane active layer modification by direct percolation. The permeate samples collected from the percolation experiments are analyzed by UV‐visible spectroscopy to monitor the influence of dendrimer generations on percolation behavior and active layer modification. Further characterization of modified membranes by Rutherford backscattering spectrometry and atomic force microscopy techniques reveals a relatively low‐level accumulation of dendrimers inside the original TFC‐S NF membrane active layer and subsequent formation of a coating of pure aramide dendrimers on top of the active layer. A PES‐PVA ultrafiltration membrane is used as a control membrane support (without an NF active layer) showing that structural compatibility between the dendrimers and support plays an important role in the membrane modification process. The performance of the modified TFC‐S membrane is evaluated on the basis of the rejection abilities for a variety of water contaminants having a range of molecular size and chemistry. As the water flux is inversely proportional to the thickness of the active layer, the amount of dendrimers deposited for specific contaminants are optimized to improve the solute rejection while maintaining high water flux.     

Mussel‐Inspired Polydopamine‐Treated Reinforced Composite Membranes with Self‐Supported CeOx Radical Scavengers for Highly Stable PEM Fuel Cells

The physical and chemical degradations of a state‐of‐the‐art proton exchange membrane (PEM) composed of a perfluorinated sulfonic acid (PFSA) ionomer and polytetrafluoroethylene (PTFE) reinforcement are induced through the repeated expansion/shrinkage of the ionomer and free radical attacks. Such degradations essentially originate from the loose structure of the materials and the low interactive binding force among the PEM constituents. In this study, the need for simplified design principles of adhesives led to the use of mussel‐inspired polydopamine (PD) as an interfacial modifier for the fabrication of highly durable PEM. Indeed, a self‐polymerized dopamine layer acts as an interfacial glue, and enables efficient impregnation of a hydrophilic PFSA ionomer into porous hydrophobic PTFE with high packing density, resulting in strong adhesion between the PTFE and the PFSA polymers in the membrane. In addition, the redox property of the PD end groups spontaneously reduces the partial Ce salts in the ionomer solution and anchors them to the PD@PTFE substrate as defective cerium oxide (CeO_x) nanoparticles, reducing the dissolution and subsequent migration under cell operations. Finally, a CePD@PTFE membrane shows outstanding durability in fuel cells under an accelerated humidity cycling test with a reduction in the degree of physical and chemical failures.     

Double Stimuli‐Responsive Isoporous Membranes via Post‐Modification of pH‐Sensitive Self‐Assembled Diblock Copolymer Membranes

Double stimuli‐responsive membranes are prepared by modification of pH‐sensitive integral asymmetric polystyrene‐b‐poly(4‐vinylpyridine) (PS‐b‐P4VP) diblock copolymer membranes with temperature‐responsive poly(N‐isopropylacrylamide) (pNIPAM) by a surface linking reaction. PS‐b‐P4VP membranes are first functionalized with a mild mussel‐inspired polydopamine coating and then reacted via Michael addition with an amine‐terminated pNIPAM‐NH₂ under slightly basic conditions. The membranes are thoroughly characterized by nuclear magnetic resonance (¹H‐NMR), Fourier transform infrared spectroscopy and X‐ray‐induced photoelectron spectroscopy. Additionally dynamic contact angle measurements are performed comparing the sinking rate of water droplets at different temperatures. The pH‐ and thermo‐double sensitivities of the modified membranes are proven by determining the water flux under different temperature and pH conditions.     

Macroporous Zeolitic Membrane Bioreactors

A simple and effective method is described for the fabrication of robust zeolitic membranes with three‐dimensional (3D) interconnected macroporous structures. The membranes were prepared by electrostatically seeding mesoporous silica sphere (MSS) self‐assembled films with silicalite‐1 nanoparticles, followed by hydrothermal treatment. The membrane thickness, which is determined by the MSS film thickness, can be easily adjusted from tens to hundreds of micrometers by varying the concentration of the MSS dispersion and the solution volume. Biomacromolecule‐functionalized macroporous zeolitic membrane bioreactors were subsequently prepared via the layer‐by‐layer (LbL) electrostatic assembly of polyelectrolytes and enzyme (catalase) on the 3D macroporous membranes. The enzyme‐modified membranes with interconnected macroporous structures display enzyme loading amounts and activities that are one order of magnitude higher than corresponding 3D zeolite films with closed macropores, and approximately three orders of magnitude higher than their non‐porous planar film counterparts assembled on silica substrates. The enzyme loadings and activities were found to be approximately linearly dependent on the thicknesses of the membranes. Furthermore, the immobilized enzyme exhibits enhanced reaction stability in comparison with enzyme in bulk solution. These membranes are potentially useful for separations as they could be used to simultaneously perform reaction and separation steps.     

Amino‐Functionalized Silica Membranes for Enhanced Carbon Dioxide Permeation

Functionalization of silica membranes is important for enhancing surface interactions with specific chemicals in order to enhance separations. It is important to develop synthesis strategies that allow control over the density and the surface chemistry of the functional group in order to tailor the membrane separation properties. In this paper we investigate the ability of amino functionalization to enhance CO₂ transport in silica membranes. Specifically, we examine three synthesis techniques for functionalizing silica membranes with amino groups that result in different surface chemistries of the silica membranes. Silica membranes are amino‐functionalized by atomic layer deposition (ALD) with aminopropyldimethylethoxysilane (APDMES), ethylenediamine (EDA)‐assisted APDMES ALD, and direct attachment of aminopropyltriethoxysilane (APTES) from the liquid phase. Three different reaction schemes are presented and verified by using Fourier‐transform infrared (FTIR) spectroscopy. The FTIR measurements were performed on silica powders that were processed using the same reaction conditions as the membranes used in this study. The differences in reaction schemes are correlated with changes in the CO₂ facilitation characteristics. It is found that high loadings of amino groups, in which interaction with the silica surface is minimized, promote the highest CO₂ transport.     

Membranes in Solar-Driven Evaporation: Design Principles and Applications

Solar-driven evaporation process brings exciting opportunities to recover clean water and resources in a sustainable way from diverse sources like seawater and wastewater. Separation membranes, as a vital material in many environmental and energy applications, can contribute significantly to this process owing to their structural features. However, the unique roles of membranes in solar evaporator construction and process design are seldom recognized and not summarized yet from scientific principles and application demands, which forms the motivation of this review. Herein, the roles of membranes in different processes based on solar-driven evaporation are focused and the design principles of membrane materials and devices to meet the requirements of these applications are discussed. Fabrication strategies for photothermal membranes are introduced primarily, followed by a discussion on how to design membrane materials, devices, and processes to pursue optimal performance and realize advanced functions accompanied by evaporation. Furthermore, the future of this field is forecast with both challenges and opportunities.     

Visible‐Light‐Activated Photocatalytic Films toward Self‐Cleaning Membranes

Membranes are among the most promising means of delivering increased supplies of fit‐for‐purpose water, but membrane fouling remains a critical issue restricting their widespread application. Coupling photocatalysis with membrane separation has been proposed as a potentially effective approach to reduce membrane fouling. However, commonly used materials in photocatalysis limit use of low‐cost sources such as sunlight due to their large bandgaps. There are few examples of in situ photocatalytic self‐cleaning of membranes, with removal from the filtration system and ex situ illumination being more common. In this work, a visible‐light‐activated photocatalytic film prepared by nitrogen doping into the lattice of TiO₂ is deposited on commercial ceramic membranes via atomic layer deposition. The synergy between membrane separation and redox reactions between organic pollutants and reactive oxygen species produced by the visible‐light‐activated layer offers a possibility for stable and sustainable membrane operation under in situ solar irradiation.