a Water Desalination and Reuse Center (WDRC), Division of Biological and Environmental Sciences and Engineering, King Abdullah University of Science and Technology (KAUST), Thuwal, Saudi Arabia. E-mail: Peng.Wang@kaust.edu.sa
With the development of modern industry and modern economies, environmental problems, especially water pollution and water scarcity, have become the most serious global challenges. In dealing with these challenges, various kinds of functionalized materials and devices are purposefully developed, fabricated, and utilized. It is clear that smart materials have not only provided effective strategies for solving environmental problems, but have also exhibited unprecedented advantages over traditional materials by integrating multifunctions and/or processes into one advanced device/material. In this book, we will present a broad collection of bioinspired smart materials and systems that are used in environmental problem solving. The topics of these chapters span from bioinspired fog collection, self-healing materials, responsive particle-stabilized emulsions, smart draw solutions in forward osmosis, slippery coating, insightful analysis of problems and opportunities for hydrophobic surfaces applied in real conditions, to superwetting materials for oil–water separation.
With the development of modern industry and modern economies, environmental problems, especially water pollution and water scarcity, have become the most serious global challenges.1–4 In dealing with these challenges, various kinds of functionalized materials and devices are purposefully developed, fabricated, and utilized.5–10 For instance, to clean up spilled oil from an oil leakage accident, absorbent materials have been widely used to selectively remove the oil from the aqueous system.9,10 Shown in Figure 1.1 are examples of absorptive pads used in the Kalamazoo River oil spill (Michigan, US, 2010) and the Huangdao oil pipeline explosion incident (Qingdao, China, 2013). These absorptive pads are generally made of recycled cellulose or polypropylene, and have highly porous structures.10 Upon the oil removal, the pads, which have the absorbed oil, are collected and disposed of by direct burning. In other areas, such as wastewater treatment and seawater desalination, membrane-based separation processes are essential, which necessitate specifically functionalized membranes, such as nanofiltration (NF), ultrafiltration (UF), and reverse osmosis (RO) membranes.11–16 Therefore, it is now a popular perception that the solutions to existing and future environmental problems highly rely on developments in material sciences.
The concept of smart materials, since its inception in 1990s,17,18 has emerged as a novel design concept, which promise a variety of new technologies, extending from shape-memory materials, artificial muscles, nanoscale motors, and biosensors, to new drug-delivery devices, etc. (Figure 1.2a).19–29 Smart materials are conventionally defined as materials that are designed to have one or more properties which can be significantly changed in a controlled fashion in response to external stimuli, such as stress, temperature, moisture, pH, electric fields, or magnetic fields.30–33 Piezoelectric materials34–38 and shape-memory materials17–20 (including shape-memory alloys and shape-memory polymers) are early types of smart materials. Piezoelectric materials produce electrical charges when subjected to dynamic strain (Figure 1.3a). The piezoelectric effect is understood as the linear electromechanical interaction between the mechanical and the electrical state in crystalline materials with no inversion symmetry.34 Piezoelectric materials have been used to capture and store vibrational energy that can later be used to power up small devices. Shown in Figure 1.3b is a hybrid-fiber nanogenerator based on the piezoelectric effect, which comprises zinc oxide nanowires (NWs) and a poly(vinylidene fluoride) (PVDF) polymer around a conducting fiber.38 The zinc oxide NWs serve as a piezoelectric-potential generator and also as an additive to enhance the surface contact area, which guides the formation of a uniform layer of piezoelectric polymer (i.e., PVDF) around the fiber during the dip-coating process. By elongating or bending the hybrid fiber, mechanical energy is converted into electricity owing to constructive piezoelectric-potential generation from the two components (i.e., PVDF and ZnO NWs). The unique structure of the hybrid fiber may inspire future research in wearable energy-harvesting technology.
Shape-memory materials are featured by their ability to recover their original shape from a significant and seemingly plastic deformation when a particular stimulus is applied, which is known as the shape-memory effect.17–20,39–41 Shape-memory materials have been utilized in many fields, from aerospace engineering (e.g., in deployable structures and morphing wings) to medical devices (e.g., in stents and filters). Figure 1.3c and d show an example of a reversible bidirectional shape-memory free-standing copolymer network with two types of crystallizable domains that is capable of a fully reversible bidirectional shape-memory effect.41 This copolymer network is made of a multiphase copolyester urethane and consists of one set of crystallizable domain that determines the shape-shifting geometry, while the other domain of the network provides the thermally controlled actuation capability. This reversible bidirectional shape-memory technology can be inspirational to the design of many smart devices such as self-sufficient grippers and reversible fastening devices.
In essence, the response mechanism of smart materials lies in the change in molecular movement in response to external stimuli, which brings about the macroscopic property change of the materials. Following this line of thought, the origin of all smart materials is biomimicking, as nature is the ultimate builder of machines.30 Therefore, in the comprehensive view, ‘smart materials’ describe materials which are astute or ‘operating as if by intelligence’, and they can also widely refer to materials that are fabricated from bioinspirations, mimicking nature’s procedures, structure, or strategy (Figure 1.2b). The simplest of organisms in nature can utilize atomic-scale engineering to build systems from fundamental chemistry into tissue with functional gradation to alter properties, integrated with electrical, chemical, and mechanical capabilities.42 Many fascinating biological processes are systems that exhibit motor functions that perform mechanical transformations triggered by a fuel input. Different triggering stimuli, such as adenosine triphosphate (ATP) fuel, a pH gradient, and light signals, are common activators of biological motors.22 For instance, the myosin–actin couple represents an ATP-driven motor that is translated to macroscopic mechanical functions of muscles.43,44
Inspired by biomotors, researchers have artificially fabricated smart molecular motor or nanoscale motor (nanomotor) systems, and further developed artificial muscles and robots.21–23,43–50 Ingenious supramolecular architectures that enable the signal-triggered mechanical translocation of molecular units have been developed. Figure 1.4a shows a linear vectorial translocation of a molecular unit on a molecular wire.22 The system consists of a crown ether threaded onto a wire that includes an ammonium group and a bipyridinium group as binding sites and two bulky groups that act as stoppers at the ends of the wire. The crown ether is stabilized on the ammonium site in the form of structure (I). Deprotonation of the ammonium unit (chemical stimulus) results in the shuttling of the crown ether to the less-favored bipyridinium unit to form (II), whereas acidification of the system, and protonation of the amine site, reshuttles the crown ether ring to the protonated site to re-form (I). Beside this molecular-level motor, nanomotors with macroscopic movement have been developed after Whitesides and coworkers first reported self-propelled movement of centimeter-sized plates,51 capable of converting chemical energy into autonomous movement.21,23 Several propelling mechanisms have been employed for the realization of the movement of the nanomotors, including bubble recoil, bubble implosion, Brownian ratchet, thermal gradient, interfacial tension, bipolar electrophoresis, and self-diffusiophoresis.52–57Figure 1.4b shows an example of the autonomous movement of a nanomotor. The nanomotor is prepared by selectively entrapping catalytically active platinum nanoparticles within polymer stomatocytes nanocavities and subsequently using catalysis as a driving force for movement.58 Hydrogen peroxide can access the inner stomatocyte cavity, where it is decomposed by the active catalyst (the entrapped platinum nanoparticles) into oxygen and water. This generates a rapid discharge, which induces thrust and directional movement.
Another type of important smart materials is the self-healing material.59–63 The inspiration for self-healing materials also comes from biological systems, which have the ability to heal after being wounded. Self-healing materials can be polymers, metals, ceramics, and their composites. When damaged through thermal, mechanical, ballistic, or other means they have the ability to heal and restore the material to its original set of properties. This is a very valuable characteristic to design into a material since it effectively expands the lifetime use of the product and has desirable economic and human safety attributes. White and coworkers pioneered a structural polymeric material with the ability to autonomously heal cracks, as illustrated in Figure 1.5a.64 This material incorporates a microencapsulated healing agent that is released upon crack intrusion. Polymerization of the healing agent is then triggered by contact with an embedded catalyst, bonding the crack faces. Their fracture experiments have revealed as much as 75% recovery in toughness.
Human skin has a unique structure that enables minor damage to be healed effectively. The outer epidermal layer of skin is composed of multiple sublayers that work in concert to continually rebuild the surface of the skin, whereas the underlying dermal layer supplies the epidermis with nutrient-laden blood and regulates temperature.65 A cut in the skin triggers blood flow from the capillary network in the dermal layer to the wound site, rapidly forming a clot that serves as a matrix through which cells and growth factors migrate as healing ensues. Owing to the vascular nature of this supply system, minor damage to the same area can be healed repeatedly. By mimicking the human skin, White et al. reported the use of microvascular networks for autonomic healing of structural damage by exploring a microvascular coating–substrate architecture (Figure 1.5b).66 To this end, an epoxy coating is deposited on a ductile substrate that contains a pervasive three-dimensional (3D) microvascular network. Solid catalyst particles are incorporated within the coating and the network is filled with a liquid healing agent. After damage occurs in the coating, healing agent wicks from the microchannels into the crack(s) through capillary action, and interacts with the catalyst particles in the coating to initiate polymerization, rebonding the crack faces autonomously. After a sufficient time period, the cracks are healed and the structural integrity of the coating is restored. Clearly, with this smart self-healing ability, materials can have longer lifetimes and lower production and maintenance costs.
Bioinspired interfacial material is another important type of smart materials.67–73 A famous example of the bioinspired interfacial materials is lotus-leaf inspired superhydrophobic self-cleaning material.74,75 Since Jiang et al. first revealed that the combination of spatial micro- and nanometer-scale hierarchical structures and proper chemical composition results in the superhydrophobic self-cleaning effect of lotus leaf,75 numerous efforts have been devoted to this area and many artificial superhydrophobic surfaces have been developed, which allowed for a series of applications, such as self-cleaning surfaces, marine coatings, anti-adhesive coatings, microfluidic channels with reduced flow resistance, and so forth.67–76 Furthermore, in combination with the responsive polymer materials, smart responsive surfaces were developed.77–80 Jiang et al. first demonstrated reversible switching between superhydrophilicity and superhydrophobicity.77 By grafting temperature-responsive polymer of poly(N-isopropylacrylamide) (PNIPAM) onto a rough substrate, thermo-responsive switching between superhydrophilicity and superhydrophobicity was realized. As shown in Figure 1.6a, with increasing temperature the water contact angle on such surface increases. This effect can be explained by the competition between intermolecular and intramolecular hydrogen bonding below and above the lower critical solution temperature (LCST). At temperatures below the LCST the predominantly intermolecular hydrogen bonding contributes to the hydrophilicity of PNIPAM films whereas at temperatures above the LCST increased intramolecular hydrogen bonding results in hydrophobic films (Figure 1.6b). Later on, Jiang et al. further demonstrated that the PNIPAM surface could effectively control the platelet adhesion in vitro.78 They grafted the polymer onto silicon nanowire arrays via surface-initiated atom transfer radical polymerization and found that the as-prepared surface showed largely reduced platelet adhesion in vitro both below and above the LCST of PNIPAM (∼32 °C), while a smooth PNIPAM surface exhibited anti-adhesion to platelets only below the LCST (Figure 1.6c). Contact angle and adhesive force measurements on oil droplets in water demonstrated that the nanoscale topography kept a relatively high ratio of water content on the as-prepared surface and played a key role in largely reducing the adhesion of platelets (Figure 1.6d). The results have potential in the applications of PNIPAM in the fields of biomaterials and biomedicine under human physiological temperature and provide a new strategy for fabricating new blood-compatible materials.
From the above examples, we can see that the concept of smartness in material design rationally integrates multiple synergistic and advanced functions into one single material or device, which helps broaden the application and elongate the lifetime of the materials. Therefore, the development of smart materials represents the future of material design and fabrication. Therefore, it is not a surprise that smart materials have also set foot in the environmental community, and have shown great promises in coming up with novel and next generation solutions to the grand environmental challenges. In turn, the interaction between environmental science with materials science has also promoted the development of smart materials, and a series of novel smart materials and their applications in environmental area have been explored. For instance, forward osmosis (FO) is a promising membrane technology for seawater desalination and wastewater treatment at low energy cost.15,81–83 FO is a naturally occurring osmosis-driven process, and consists of two key components: (i) a semipermeable membrane that allows the permeation of water while rejecting salts and other unwanted elements; and (ii) a draw solution that is able to generate sufficiently high osmotic pressure to draw water from the feed solution across the membrane. In an applicable FO desalination process, the draw solute should be recycled and regenerated in economical and efficient ways. The use of smart draw agents with facile recovery methods can effectively lower the energy consumption of the process at a great potential.84–90 To this end, Lu et al. reported a new design of thermoresponsive magnetic nanoparticles (TMNPs) to significantly improve their osmolality for seawater desalination (Figure 1.7).90 In this work, magnetic nanoparticles (MTPs, i.e., Fe3O4) are functionalized with a copolymer, poly(sodium styrene-4-sulfonate)-co-poly(N-isopropylacrylamide) (PSSS-PNIPAM). This copolymer integrates two functions: poly(sodium styrene-4-sulfonate) (PSSS) as a polyelectrolyte can dissociate to give a large number of ions in solution and to provide high osmolality, whereas thermoresponsive PNIPAM facilitates draw-solute regeneration via particle aggregation at enhanced temperatures. The MNPs functionalized with PSSS-PNIPAM successfully draw water from seawater. In addition, regeneration of the MNP-based draw solution can be achieved with magnetic separation assisted by mild heating.
In the field of oil-spill cleanup, smart materials also show tremendous advantages over conventional methods. A bioinspired smart membrane with superwetting behavior can easily realize gravity-driven oil–water separation, which is of great importance to facilitate the oil-spill cleanup, contributing to much reduced response time and operation cost.91–94 For instance, inspired by the superhydrophobic lotus leaf, Jiang and coworkers in 2004 first prepared superhydrophobic and superoleophilic mesh for gravity-driven oil–water separation, by depositing low-surface-energy material of polytetrafluoroethylene (PTFE) on stainless steel mesh.95 The coated mesh showed a water contact angle greater than 150°, and diesel contact angle of ∼0°, and thus can be used effectively for the separation of oil and water, allowing oil to pass though while retaining water. Later, researchers gradually realized that although this superhydrophobic and superoleophilic material is effective for the separation of an oil–water mixture, this ‘oil-removing’ type of material can be easily fouled or even blocked by oils because of their intrinsic oleophilic property. The adhered oils, especially high-viscosity oils, seriously affect the separation efficiency after usage for a limited period of time. In addition, oils adhered or absorbed are hard to remove, which results in secondary pollution during the post-treatment process as well as a waste of both oil and oleophilic materials. To solve this problem, inspired by the oil-repellent capability of fish scale (Figure 1.8a),96 Jiang and coworkers recently fabricated a novel superhydrophilic and underwater superoleophobic hydrogel-coated mesh (Figure 1.8b).97 This novel ‘water-removing’ type of material has completely opposite wettability to traditional hydrophobic and oleophilic materials, and thus overcomes the easy-fouling and recycling problem because, in essence: (a) it allows water to pass, which effectively avoids or reduces the possibility of the membrane being clogged by the viscous oil; (2) it prevents the formation of the water barrier between the membranes and the oil phase, which would otherwise occur with the conventional hydrophobic and oleophilic separation materials due to the fact that water is generally heavier than oil phase and it thus prevents the contact between oil and separation membranes.97–100 They have shown that this novel material can selectively separate water from oil/water mixtures such as vegetable oil, gasoline, diesel, and even crude oil/water mixtures effectively (more than 99%) and without any extra power.
In view of the diversity of oil–water mixtures and the complexity of oil-spill incidents, an on-demand or highly controllable oil–water separation is highly desirable, i.e., the separation of the oil–water mixture can be controlled so to let either oil or water pass through in one system. Such a controllable separation entails a smart surface with controlled oil wettability, or more desirably a smart surface that switches its oil wettability in response to external stimuli in aqueous media.101–105 Wang and coworkers for the first time demonstrated a smart surface with switchable superoleophilicity and superoleophobicity in aqueous media for controllable oil–water separation, as illustrated in Figure 1.9.101 To obtain a smart surface with switchable oil wettability in aqueous media, especially between superoleophobicity and superoleophilicity, the chemistry on the surface should be delicately designed such that it comprises both hydrophilic and oleophilic/hydrophobic characteristics, with either characteristic becoming dominantly exposed over the other in response to environmental conditions. By employing a block copolymer-grafting strategy that uses a block copolymer comprising pH-responsive poly(2-vinylpyridine) and oleophilic/hydrophobic polydimethylsiloxane blocks (i.e., P2VP-b-PDMS) to functionalize inexpensive and easily available materials, including non-woven textiles and polyurethane sponges, surfaces with switchable superoleophilicity and superoleophobicity in aqueous media were prepared. The P2VP block on the grafted block copolymer can alter its wettability and its conformation via protonation and deprotonation in response to the pH of the aqueous media, which in turn provides controllable and switchable access of oil by the oleophilic PDMS block on the surface, thus realizing a smart surface with switchable oil wettability in aqueous media. With such a functionalized surface, highly controllable oil–water separation was realized. It is anticipated that such a surface with controlled oil wettability would offer great promise in the design and fabrication of intelligent materials for advanced applications.
Besides the superwetting membranes, bioinspired nanomotors are used for the removal of oil from water. Wang et al. demonstrated the use of artificial nanomotors for effective interaction, capture, transport, and removal of oil droplets from an aqueous medium. The catalytic microtubular Au/Ni/PEDOT/Pt motors were fabricated by electrochemical deposition and further functionalized with alkanethiols to form a hydrophobic monolayer (SAM) on the outer gold surface of the microtube (Figure 1.10(A)).106 The strong interactions between the alkanethiol chains and oil from the solution enable micromotors to capture oil droplets and transport them. The authors showed that the resultant SAM-coated Au/Ni/PEDOT/Pt microsubmarine displayed continuous interaction with large oil droplets and was capable of loading and transporting multiple small oil droplets (Figure 1.10(B) and (C)). Such a bioinspired nanomotor has great potential in the autonomous removal of emulsified oil droplets, which is a very demanding task for conventional oil spill cleanup methods.
From these examples, it is clear that smart materials have not only provided effective strategies for solving environmental problems, but have also exhibited unprecedented advantages over traditional materials by integration of multifunction and/or processes into one advanced device/material. In the rest of this book, we will present a broad collection of bioinspired smart materials and systems that are used in environmental problem solving. The topics of these chapters span from bioinspired fog collection, self-healing materials, responsive particle-stabilized emulsions, smart draw solution in forward osmosis, slippery coatings, insightful analysis of problems and opportunities of hydrophobic surface application in real condition, to superwetting materials for oil–water separation. We hope this book will provide an inspiration for readers to further explore smart materials to solve environmental problems.
© The Royal Society of Chemistry 2016 (2016)