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Bioinspired micro-/nanostructure fibers with a water collecting property Yuan Chen and Yongmei Zheng* We review the recent research on structure-induced water collecting properties of spider silk and bioinspired fibers. Since the capture silks of cribellate spiders have a unique wet-rebuilt structure with spindle-knots and joints for directional water collection, we were inspired to fabricate a series of artificial

Received 16th April 2014 Accepted 9th May 2014

gradient micro-/nanostructure fibers. These fibers display excellent functions, such as driving tiny water drops in certain directions, water capturing, multi-gradient cooperation effect, and wet-response to

DOI: 10.1039/c4nr02064b

environmental humidity. This review is helpful to the design of novel smart functional materials that can

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be extended to develop devices or systems for water collection, sensors, fluid-control, filters and others.

1

Introduction

Natural creatures have evolved their surfaces, over billions of years, to endow them with various unusual abilities to help them survive in tough conditions. Inspired by features of

Key Laboratory of Bio-Inspired Smart Interfacial Science and Technology of Ministry of Education, School of Chemistry and Environment, Beihang University, Beijing 100191, P., R. China. E-mail: [email protected]

Yuan Chen, born in 1987, is a PhD Student at the School of Chemistry and Environment, Beihang University, China. She received her BS degree from Tianjin University of Science and Technology in 2010, majoring in Chemistry Technology. She started to research the bioinspired materials in 2010. In 2013, she went to Lawrence Berkeley National Laboratory as a visiting scholar. She focuses on the controlled fabrication of bioinspired spider silks and water condensation. Now she has published four papers in different highlevel material journals and has obtained two national patents.

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biological surfaces, researchers have developed the design and fabrication of smart functional materials into a very promising eld of research. Because some creatures show a special wettability characteristic on their surfaces, for example, the lotus,1 water skipper,2 rose, buttery,3 cactus in the desert,4 spider silk,5 rice leaf and desert beetle,6 recently, much research has been focused on biological surfaces with water collecting properties. For example, the Namib Desert beetle can efficiently extract fog droplets from a moist breeze, which is a benet of

Yongmei Zheng, born in 1964, is currently a professor at the School of Chemistry and Environment and Key Laboratory of Bio-inspired Smart Interfacial Science and Technology of Ministry of Education, Beihang University. She received her BS degree (1987), MS degree (2000), and PhD degree (2003) from Jilin University, China. She rstly worked at the department of applied physics in Jilin University of technology (1987). She then worked as a postdoctoral fellow in Prof. Lei Jiang's group (2003–2005) and as a researcher at the Centre for National Nanoscience and Technology (2005–2007). She worked at the School of Chemistry and Environment in Beihang University (2008 to present). Her interests are focused on biological surfaces and bioinspired surfaces with gradient multistructures and wettability functions. Her research has been published in Nature, Advanced Materials, Angewandte Chemie, ACS Nano, Advanced Functional Materials, Small, Nanoscale, Chemical Communications, So Matter, Langmuir, Applied Physics Letters, etc., with 10 cover stories.

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the hydrophobic/hydrophilic patterned regions on its back.6 A tued plant from South Africa, Cotula fallax, has leaves with a hierarchical, three-dimensional layout, which also exhibit hydrophobicity. The smart leaf structure allows the plant to efficiently collect and channel water from fog to its stem.7 The capture silk of the cribellate spider, Uloborus walckenaerius, has a water-collecting ability resulting from a unique ber morphology aer wetting. The ‘wet-rebuilt’ bers are characterized by periodic spindle-knots made of random nanobrils and separated by joints made of aligned nanobrils. These structural features result in a surface energy gradient between the spindle-knots and the joints and a difference in Laplace pressure. Both factors act together to achieve continuous condensation and directional collection of water drops around the spindle-knots. Inspired by the spider silk, articial bres are designed to mimic the structural features of spider silk and exhibit its directional water collecting ability.5 Here, we review the recent research on structure-induced, wetness-controlling properties of spider silk or bioinspired bers. We highlight the capture silks of cribellate spiders, which have a unique wetrebuilt structure with spindle-knots and joints. Inspired by this structure's effect, we fabricated a series of articial gradient micro-/nanostructure bers to display functions such as driving tiny water drops in certain directions, water capturing, wetnessresponse on bioinspired bers, and multi-gradient cooperation effect. This review will be helpful in the design of novel smart functional materials that can be extended to develop devices or systems for water collection, sensors, uid control, lters and others.

1.1

Biophysical properties of spider silk

Aer millions of years of evolution, many creatures have become able to create a lamentous ber material. Filamentous ber materials have been used for thousands of years, in applications including: fabrics, medical wound dressings and military applications.8 You can nd spider silk from spider orb webs in many places around the world—under the trees, in the leaves. The silk ber radius ranges from a few micrometres to a few millimeters. It is difficult to distinguish the difference among spider silks using the naked eye. Silks have an amazing multiplicity of types and uses and have excellent mechanical properties and chemical compositions.9,10 Because of the diversity of spider silks, they are praised as unique natural biomaterials with characteristics at both the chemical and physical levels.11,12 Silk typically consists of two laments that are easily separated. Its coating consists of a varied number of small molecules of non-protein compounds, which are much thinner, e.g., the benchmark of spider silks, the dragline and frame silk of the orb weaving araneid spiders, such as the garden orb spider Araneus diadematus or the golden silk spider Nephila clavipes. Flagelliform silk can form the core laments of the orb web's capture thread. Another type of silk is produced from the aggregate gland to coat the capture thread laments. This silk is not hard, but an aqueous solution of small and highly hygroscopic peptides as well as sticky glycoproteins.13,14 The variety of spider silks open a window into the structure–

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function relationships of many different properties. The major advantages of the excellent mechanical and biocompatibility of spider silk make its use widespread, such as in shing lines and wound covering material. Spider silks are undoubtedly the most excellent structural and functional materials in nature. 1.2

Wet-rebuilt structure of spider capture silks

Most of the work on spider silks is related to the experimental measurement of physical properties, including mechanical performance, especially elasticity, which has been reviewed previously. Recently, a novel wettability property of spider silks was revealed;5 it was found that the cribellate spiders' capture silks can change into a kind of microstructure that is unique from the original periodic puffs alternating with joints of a dry capture silk (Fig. 1a), which is composed of random nanobrils (Fig. 1b) along the periodic main-axis bres. The structures of nano-scale dimensions play an important role in water collection function. The wettability of spider silks is enhanced because of these highly hydrophilic nanobrils, which are favorable for condensing water drops. Spider silks have humidity-sensitive properties, and the structure of spider silks can be re-built at a higher humidity. It is interesting that the structure of spider silk changes when a dry spider silk is placed at higher humidity. Fig. 1c–g shows the scanning electron microscopy (SEM) images of wet-rebuilt spider silk. We can see that wetted spider silk has a more obvious spindle-knot and joint structure. Both spindle-knots and joints are composed of randomly distributed nanobrils, but there are some differences between spindle-knot and joint nanobrils. The nanobrils of spindle-knots show a rough and random conformation, and those of the joints form an anisotropic, aligned surface. Compared to the spindle-knots surface, the joint structure is smoother. These structural features lead to the formation of two driving forces, i.e., surface energy gradient and

Fig. 1 Structures of dry and wet-rebuilt cribellate spider silk.5 (a) Low magnification environmental SEM image of periodic puffs and joints surrounding two main-axis fibres. (b) Magnified image of puff composed of countless nanofibrils. (c) The apex angles and the periodicity length. The low-magnification and high-magnification images are added to show the structure of spindle-knots (d and e) and joints (f and g), respectively.

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a difference in Laplace pressure. This is why the water drops can move directionally. The nano-scale interactions, with their structural simplicity but functional complexity, give silks desirable and highly tunable properties.15–17 People always want to produce articial spider silks that have the same properties as natural spider silks through large-scale fabrication. A practical method is to use recombinant DNA technology. Aer a few years' study, researchers have synthesized some genes, hoping to realize the expression of spider silk protein. These genes have been implanted into yeast and bacteria, which were able to produce a replica of silk proteins. However, the protein folding and assembly process could not be well controlled. Thus, the various properties of the proteins were not comparable with spider silks. This eld of research is attracting the involvement of more and more scientists.8,14,16–22

1.3

Directional water collection ability

A wide range of applications have been found for micro-/ nanostructured surfaces, and they are attracting a great deal of attention. However, the micro-/nanostructured surfaces are affected by external conditions, for example, the adherence of snow or raindrops, oxidation, and friction drag; these phenomena are expected to be reduced or inhibited on such a surface.1,23–25 Many biological surfaces, including those in the plant and animal kingdoms, owe their unusual structural features to their micro- and nano-scale. These special structures control their interaction with water and leads to their wettability.2,26 It is the special structure that determines interaction with water, which achieves the wonderful wetting ability on surfaces, such as those of water striders,2 the lotus leaf,1 desert beetles,26 cicada wings,27 sh scales,28 buttery wings,3 etc. Now we have recognized that surface roughness gradient is important to creating these functional surfaces. A very interesting example is the Namib Desert beetle, whose back shows micrometer-sized patterns of hydrophobic and hydrophilic regions. The beetles make full use of these patterns to capture water from the air to meet its subsistence need. However, besides the two-dimensional surface, the onedimensional bers with unique wettability are rarely reported to direct the motion of droplets. Spider silks are found to have a strong ability to collect water from the air.29 Anyone who has seen a spider web in the early morning will notice the water droplets strung along its ne threads (Fig. 2a), which form beautiful dewdrops. Scientists are interested in this wonderful biological phenomenon. An experiment with human hair was conducted to mimic this phenomenon, but in fact, it failed. The water would not stick to it like spider silks. A type of Uloborus walckenaerius capture silk has received extensive attention. The reason is that this spider silk has a water collection ability attributed to a unique ber structure aer wetting, with the ‘wet-rebuilt’ bers characterized by spindle-knots made of random nanobrils and separated by joints made of aligned nanobrils with periodicity (Fig. 1c–g). Because of the different roughness of the spindle-knot and the

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Fig. 2 (a) Shiny water droplets on a spider's web. (b) In situ optical microscopic observation of directional water collection on spider silk in foggy conditions.5 The spider silk rebuilds a spindle-knot structure (A–C), and the water droplets condense on the rebuilt spider silks (D– F). Tiny water drops directionally move toward spindle-knots for water collection (G–I).

joint, a surface energy gradient is produced on these two positions. At the same time, because of the different curvature of the two positions, there presents a difference of Laplace pressure. Under the cooperation of the two different driving forces, micrometer-sized water drops can achieve continuous condensation and directional collection of water drops around the spindle-knots. In addition, the stretch orientation of the nanobrous structure makes it easier to move the drops in the orientation direction. The movement of submillimeter-sized liquid drops has been reported to be facilitated by the surface energy gradients30,31 or the difference in Laplace pressure. But micrometer-sized drops have a larger hysteresis effect, which makes the movement more difficult. Studies have focused on overcoming the larger hysteresis effect to make the micrometersized drops move.32,33 Interestingly, the spider silks have successfully driven micrometer drops by tapping into both driving forces. These ndings could have implications for the design of materials for efficient chemical reactions and for water collection. When a single dry spider silk is placed in a high humidity environment, the silk structure will change, and at the same time, some water condenses on the spindle-knots and joints. The drops move further towards the spindle-knots. Fig. 2bA shows the water drops (indicated by white arrows), which rstly appear on the puffs. Then the puffs shrink and form spindle-knots and joints as the water drops grow (Fig. 2bB and C). The rebuilt-structure of wetting spider silk is the start of water collection (Fig. 2bD and E). Zheng and her colleagues5 divided the images into four regions (indicated by white dotted lines) to give a detailed explanation for the directional water collection on the wetting spider silk (Fig. 2bG–I). In this process, the spindle-knots play an important role as condensing site in the initial stage and as a collecting site for the smaller drops moving directionally from the joints. The joints serve as condensing sites. The water condenses on the joints and grows into droplets. Then they are transported from the joints to spindle-knots. When water drops complete condensation and collection on the spindle-knots and joints, the second process will start. This proceeds continuously, as the spider silks collect water. These images show us the

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cooperation between joints and spindle-knots acting as different sites, condensing sites and mainly as collecting sites, to realize the directional water collection. In fact, the silkworm silks and uniform Nylon bers exhibit very little of this water capturing property observed in the previously mentioned wetted spider silk. We have shown the roughness difference of the spindle-knots and the surfaces of the joints, which leads to surface energy gradients and a difference in Laplace pressure. Surface chemical composition and surface roughness are two factors that affect the surface energy gradient. The surface energy gradient drives the movement of water drops from low surface energy to the higher one on the spider silk. In Fig. 1, we have shown that the joint has a smaller axial-parallel roughness, which leads to a larger contact angle than that of the spindle-knots. Thus, the spindle-knots are more hydrophilic than the joints and have a higher surface energy. So a force is given by the surface energy gradient from a different ð Lk surface roughness, according to F ¼ gðcos qA  cos qR Þdl, Lj

where g is the surface tension of water, qA is the advancing angle, qR is the receding angle of the water drop on spider silk, and dl is the integrating variable from joint (Lj) to the spindleknot (Lk). The other driving force is the difference in Laplace pressure for directional water drop movement from joints to spindle-knots. The spindle-knot is a conical object, and a conical shape can give a difference in Laplace pressure (DP) because of its curvature gradient. According to the formula: ð r2 DP ¼  2g=ðr þ R0 Þ2 sin bdz, where g is the local radius, R0 r1

¼ (3V/4p)1/3 is the drop radius (V is the drop volume), b is the spindle-knot half apex-angle and z is the integrating variable along the diameter of the spindle-knot. The joints have a larger Laplace pressure. Spindle-knots have a low curvature site. The spindle-knots have a relatively smaller Laplace pressure. The result of different Laplace pressures is to propel the drop from joint sites to the spindle-knots. So the cooperation of surface energy gradient and difference in Laplace pressure results in directional water drop movement from joints to spindle-knots. Generally, capillary force is related to surface tension and characteristic length. This device allows us to control the driving force.34 Drops move along the parallel direction of an aligned surface topography rather than a randomly rough topography surface. This results from another inuencing factor related to a vapour–liquid–solid three-phase contact line (TCL). The TCL of the parallel direction of the surface is continuous with the aligned topography, and that of randomly rough topography is discontinuous. A continuous TCL will allow the drops to spread and move, but a discontinuous TCL will give rise to more signicant hysteresis effects. The TCL is continuous on the joint regions and discontinuous on the spindle-knot regions. The water drops move along joints with less hysteresis to spindle-knots with more hysteresis effects. This is the third force to drive the water movement from joints to spindle-knots. The different forces together contribute to the water capture, which constitutes the unique characteristic of spider silk.

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2 Fabrication of bioinspired fiber Spiders can be praised as the best ‘manufacturing engineers’ in biology. Spider silks possess micro- and nanoscale structures, such as nanobrils and nanocrystals. The nano- and microscale of spider silks plays an important role in achieving the excellent chemical and physical properties of spider silks. Unlike other biological materials, silk is not the result of cellular growth processes such as those that produce collagen, chitin, or keratin by continuous extrusion. Polymer extrusion is an industrial process and whereas, controlling structure is not yet a reality in industrial growth, bioinspired silks can be used as a template to design and produce a functional biomaterial. For the droplet movement, there are many methods to achieve this function. In drop movement design, there are two key factors to consider: one is the driving force; the other is the droplet-transporting capability. Some discoveries revealed that various mechanisms to drive drop movements are based on chemical gradient, thermal and electrical driving, or photoresponsive principles. However, there are some side effects when moving the drops in bioapplications.35 Spider silk gives us a smart revelation: to fabricate one-dimensional bioinspired bers to capture drops. An amazing phenomenon is that large water drops hang stably on spider silks on a foggy morning. Water collection ability has been observed as a fascinating property of spider capture silk. The cooperation of surface energy gradient and Laplace pressure drives the water drops to move from the joint of the silk towards the spindle-knots. Inspired by this study, some methods have been introduced to design and fabricate a kind of articial ber with the structural features of wet-rebuilt spider silks. Overall, the spider silks present multi-functionalities, and scientists have not revealed more about the secrets of spider silk. Inspired by the spider silks, we introduce typical methods to fabricate a series of articial spider silks with spindle-knots and joints. The surface nanostructures or chemical compositions are a subtle design. This research will help to nd a way for designing smart materials and devices that can drive water drops in a controlled manner. Fig. 3a shows the whole process of the dip-coating method. A Nylon ber is rstly immersed into a polymer solution and drawn out horizontally. Aer the Nylon ber is drawn out horizontally using a dip-coater machine, a cylindrical lm forms on the uniform ber surface. Because of the Rayleigh instability of the polymer solution, the lm spontaneously breaks up into polymer droplets. The polymer droplets become spindle-knots while being dried, and nally there are spindle-knots and joints on the ber.36,37 By optimizing conditions, e.g., solution concentration and drawn-out velocity, this method can control the size of spindle-knots, which affects the water collecting ability. This method can be developed to fabricate different smart materials, e.g., bioinspired silks with multi-gradient and multi-scale spindle-knots,38 using silkworm silk as the basic ber can increase the biodegradability and biocompatibility of the water capture materials.37 The dip-coating method is simple and economical for the fabrication of smart water collection

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Scheme of methods to fabricate bioinspired fibers, (a) dipcoating, (b) electro-dynamic, (c) fluid-coating and (d) micro-fluidic. (e) Illustration of bioinspired fibers in functions such as directional driving, selective driving, stronger collection, and reversibility of environmental wetting-response. Fig. 3

materials. Dhinojwala et al.39 reported functional microthreads fabricated by the dip-coating method using poly(dimethylsiloxane) solution. Not only was the morphology of this bioinspired material similar to the spider capture silk, but so were the adhesive properties. The method controls well the morphology and structure of the functional spindle-knots by varying the capillary number. The dip-coating method of producing functional bers provides a great guarantee for its wide range of applications. A coaxial electrospinning (co-ESP) method can be used to fabricate various microscopic core–shell or tubular bers with various materials.43,52 A sprayable outer uid imprints a series of heterogeneous beads, and the spinnable inner uid forms the centric ber. The components of the inner and outer uids should be controlled to form the bead-on-string, hetero-structured bers with tunable compositions. Fig. 3b illustrates the coaxial electrospining approach. The spinnable inner uid with high viscosity, and the sprayable outer uid with low viscosity, combine and work through the inner and outer needle, respectively. Then, bead-on-string, hetero-structured bers are collected on aluminum foil.43,53 For the preparation of these bead-on-string, hetero-structured bers, the two uids (inner/ outer) cannot be miscible. The electrohydrodynamic process can prevent them from mixing for a short time. It is hard for the spinnable outer uid with low viscosity to envelope the inner uid using an immiscible solvent system. Thus, it is not possible to use two uids that easily separate in the jetting process to fabricate the bead-on-string, hetero-structured bers. The other factor affecting the formation of bead-on-string, hetero-structured bers is the ow rate of the inner uid. If it is too high, the spindle-knot structure decreases or even disappears. However, if it is too low, the uid is difficult to shape and drips from the nozzle. Increasing the inner uid decreases the relative content of the outer uid, which will result in the decrease of the spindle-knot structure. In this design, the surface energy difference is considered. As described previously, these two uids should be miscible. However, the interface action can be manipulated by choosing appropriate solute

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components. The viscous drag force exerted by the inner uid can be mitigated by the outer polymer with a higher surface energy and the inner polymer with a lower surface energy. During the electrohydrodynamic jetting process, the outer polymer is conducive to spontaneous Rayleigh break-up. In addition, the higher surface energy component can prevent the lower energy uid wetting and adhesion during the solvent evaporation. This is a facile and powerful method to prepare novel bioinspired articial spider silk bers. It opens the eld of electrohydrodynamic jetting to a much wider range of surface chemistries and applications. The method is simple, and it offers the possibility to quickly fabricate one-dimensional, hetero-structured ber materials on a large scale. Bioinspired materials are fabricated to realize the watercollection ability by mimicking the micro-/nanostructure features of spider silk. In our previous studies, bioinspired articial spider silk bers were fabricated into periodic spindleknots by the dip-coating and coaxial electrospinning method. However, the dip-coating method is limited because of its discontinuity. We cannot obtain a very long length of ber using this method with a single ‘immersing-in/drawing-out’ operation. Furthermore, it is hard to get a single bioinspired ber by the electrospinning method. These limitations require us to nd a new method to fabricate durable and inexpensive bioinspired articial spider silk bers and make the bioinspired material suitable for large-scale practical applications. Here, we introduce another method, called the uid-coating method, to continuously fabricate periodic spindle-knots on Nylon bers on a large scale. Fig. 3c shows the schematic illustration of the uid coating method for the large-scale fabrication of bioinspired articial spider silk bers. In order to avoid gravityinduced liquid ow, a Nylon ber is fed horizontally through a polymer solution reservoir. The ber is xed, with one end connected to a rolling motor to drag the ber through the polymer solution reservoir. Two capillary tubes are used to guide the ber. When the ber is steadily drawn out of the reservoir at a given velocity, the polymer solution coats onto the ber surface at the end of the capillary tube. A charge coupled device (CCD) camera is used to record the whole fabrication process. These investigations demonstrate that the uid coating method can be used to fabricate bioinspired articial spider silk bers with a structure similar to the spider silks.44 The uid coating on the ber has been studied for a long time as a very common method.45–50 The applications of uid coating range from the improvement of the mechanical properties of bers, e.g., using copper bers, carbon bers or shing line ber98 as the basic bers, to the corrosion protection of metal wire and even includes simple candle-making. Using this coating method, it is necessary to consider how to control the lm thickness under these fabrication conditions. The viscous force makes the liquid move with the ber. The surface tension tends to drive the liquid back to the solution reservoir. The ratio of these two opposing forces largely affects the lm thickness. The capillary number (Ca) can be described as follows: Ca ¼ hV/g, where h, V and g are the solution viscosity, the ber drawing velocity and the surface tension of the solution, respectively. As for a given polymer solution, the solution viscosity and surface

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tension cannot be changed. The coating behaviors change based on the different drawing velocities. At low velocities, the dynamic meniscus is stable and the lm thickness generally obeys the Landau–Levich–Derjaguin theory,51 called the ‘viscocapillary region’. When increasing the drawing velocity, the thickness of coated lm also increases accordingly. At a very high velocity, it is important to consider the effect of inertia. We call this regime the ‘viscoinertial regime’. This condition is unstable for the dynamic meniscus. The uid-coating method can be used to fabricate bioinspired bers with spindle-knots and joints on a large scale. These durable, bioinspired articial spider silk bers can be applied in water collection and the directional transport of liquid droplets. Lee et al.54 used a microuidic process comprising a number of individually controllable inlets to generate a spinning process that mimics the spinning process of spiders. This micro-uidic technology includes a microuidic chip and a digital controller to fabricate bers with complex morphologies and tunable composition,54–59 as shown in Fig. 3d. A valve is used to control the uid volume in each inlet channel, which can code the chemical composition of the ber. It is easy to tune the number and dimension of the coded region. In addition, the microuidic technology can fabricate bers with controllable structure and topography. The bioinspired bers with periodic spindle-knots can be generated using microuidic technology with different diameters and lengths of the spindle-knots, which can be realized by operating the valve. By changing the material during coding, the porosity structure on the spindleknots can be fabricated. The coding has many potential applications including: biosensors, high-throughput screening and the spatio-temporal cell positioning of heterogeneous cells for tissue engineering. The as-prepared bers have been shown to have many applications, such as in water collection, purication, tissue engineering, drug delivery, and new textiles. The fabrication methods of bioinspired spider silks can be developed to design multifunctional materials. For instance, Hou et al.60 used the Rayleigh instability technique and breath gure method to fabricate porous hump bers. The features of the pores can be controlled by changing the resin reaction conditions. A variety of hump bers with smooth, less porous, homogenously porous, gradient porous and dented microstructures are fabricated successfully, as shown in Fig. 4a. A carbon ber was immersed in the resin solution (epoxy E-44 and diethylenetriamine mixed in the ratio of 10 : 1) and fabricated by a dip-coating method. Because of the increase of solution viscosity with the reaction time, the process results in different morphologies of the bioinspired bers. At 5 min, the surface is smooth because of evaporation of the solvent aer a short reaction time. At 10 min, the viscosity increases, and the condensed water droplet is immobilized on the center of a bead. Large micropores appear on the bead. Aer 15 min, the epoxy solution changes into a gel and makes the condensed water droplets hard to move. This results in a homogenous micropore distribution. Aer more reaction time, about 20 min, the sinking of condensed water droplets is limited because of the high viscosity, which leads to a gradated roughness structure. Beyond 20 min, some dented areas are formed, as the

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Fig. 4 (a) SEM images of the polymer bioinspired fibers fabricated after different reaction times.60 (b) Intact and helically cracked spindle knots after calcination.63

condensed water droplets coalesce. Besides reaction time, the relative humidity is also a factor that inuences the porous structure. These porous materials have a variety of applications, e.g., biosensors, membranes, microreactors and devices.61,62 Jiang et al.63 fabricated smooth, screw-shaped cracks on dipcoating core–shell spindle-knots, as shown in Fig. 4b. Aer calcination, the smooth surfaces of the spindle-knots change into screw-shaped cracks. The results are of great signicance for material fracture techniques and fracture mechanisms. This is also a novel development for the dip-coating method. The spindle-knot structure opens a broader perspective to the design of bioinspired materials. Compared with other methods, such as template-based synthesis,40 vapor-phase synthesis,41 solution-phase deposition42 and CoESP,41 the simplicity and scalability of this method allows rapid and easy large-scale fabrication. The spider's web hangs shiny with water drops. This beautiful sight attracts us to try to reveal how to design functional bers which are similar to spider silks. We have developed some fabricating methods such as dip-coating, electrodynamic methods, uid-coating and microuidic methods. Fig. 3e shows a summary of the design strategies and applications of bioinspired functional bers with unique wettability. Designing structural features of the ber surface can produce novel functional bers, e.g., bers with characteristics of directional driving for droplets, or intelligent catching and releasing of droplets. Heterogeneous chemical components or temperature responses cause selective driving. Some light, pH, or thermal stimuli molecules modied on the ber surfaces can also control the direction of movement of the water droplets. Slope and curvature effects achieve stronger collection and higherefficiency liquid transportation along the ber. In addition, the smart bioinspired functional bers with unique wettability can

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have segmented swelling and shrinking with environmental reversibility. The various fabrication methods of functional bers ensure that they can be used for a wide range of applications. In the next sections details about water collection properties of bioinspired functional bers and some applications for this novel material are given.

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3 Wetting-controlling properties 3.1. Size effect and water capturing In our previous studies, we successfully reproduced these watercontrolling structural features on bioinspired articial spider silks. This material offers new insights in to designing water collection materials. Next, we describe how the fabrication conditions affect the geometric parameters of the spindle-knots and, furthermore, the size, thereby affecting the water collection ability. The spindle-knots/joints structures can be fabricated under optimized conditions to control the size effect of the spindle-knots on water collection ability. Fig. 5a shows the optical images used to clarify the effect of spindle-knot size on water collection ability. Three bioinspired articial bers with four spindle-knots and a uniform ber as shown in Fig. 5aA–D are tested under the same fog ow and the tests recorded by CCD at the same distance from the bers. Fig. 5aE illustrates the total volume of all the water droplets on a section of a bioinspired articial ber at different collection times. At 12 s, the bioinspired articial ber with the largest spindle-knots has collected 35 nL, while it has only collected about 3 nL of water on the uniform ber. This result indicates that the spindle-knot size affects the water collection ability,

Fig. 5 (a) Optical images and results of water collection ability vs. time of different-sized bioinspired fibers.64 (b) Stable three-phase contact line on an artificial humped fiber during the growth of a hanging drop.36 (c) Unstable three-phase contact line on a uniform fiber during the growth of a hanging drop.36

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and the bioinspired articial bers collect more water than smooth bers under the same conditions. This interesting observation shows that the water collection ability keeps in step with the size of the spindle-knots. These investigations show that we can evaluate the water collection ability of bioinspired articial spider silks with different spindle-knot sizes. Using the dip-coating method, we can fabricate the bioinspired articial bers with different sizes by controlling the solution viscosity, ber drawing velocity and solution surface tension.64 In addition, the bioinspired silks with multi-gradient and multi-scale spindle-knots are fabricated with a dip-coating Rayleigh instability technique and a water droplet template method.38 These investigations show that we can control the fabrication of periodic spindle-knots on bio-inspired articial spider silks. Based on these fabrication methods, the bioinspired articial bers with different spindle-knot sizes can be applied in collecting fog by quickly transporting the condensed water droplets away and freeing the original place on the bre for a new cycle of condensation. If we consider the hanging drops formed during the water condensing system, a large hanging drop is nally formed aer the continuous coalescence of small, condensed drops on an articial humped ber. The force of a hanging drop is balanced between the capillary adhesion and the gravity of the droplet.31 Solid–liquid adhesion is an important aspect of surface wettability. Bioinspired bers with periodic spindle-knots have been shown to have much stronger adhesion to liquid droplets than smooth bers. Fig. 5b shows the stable TCL on an articial humped ber during the growth of a hanging drop. Because of the spindle-knot structure, there is an unusual TCL axially along the bioinspired ber surface. The spindle-knot structures can improve the stability of TCL by combining the ‘slope’ and ‘curvature’ effects, which provide sufficient capillary adhesion to pin the hanging drops. We have shown that the size of spindle-knots can be controlled by changing the conditions of the fabrication methods. Thus, the TCL length can be modied. However, the TCL on a uniform ber is unstable during the growth of a hanging drop. The contact length contracts as the hanging-drop volume increase (Fig. 5c). This investigation offers a deep insight into solid–liquid adhesion of curve structured bers, and it is helpful to the design of smart materials for promising applications in water collection engineering or microdevices.36 These studies demonstrate that the geometrical structure of bioinspired bers induces much stronger water hanging ability than that of uniform bers, which is attributed to the special geometry that offers an effectively increasing TCL length or limits the contact length to be shortened. Hou et al.65 further discussed water collection related to the periodicity of geometry on the articial humped ber. They revealed the length of the TCL in conjunction with the threshold volume (Vthr) of a hanging drop at two different modes, e.g., when a single spindle-knot collects and when two spindle-knots collect together. It has been found that articial humped bers can hang a larger-volume water drop than Nylon bers because of ‘slope’ and ‘curvature’ effects. Fig. 6 further illustrates that the presence of two spindle-knots can generate higher collection efficiency and hang a larger water drop than

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Illustrations of water droplet formation on bioinspired fibers with spindle-knots.65 (a) Fiber with one spindle-knot, (b) fiber with two spindle-knots.

Fig. 6

one spindle-knot. This results from the larger TCL length and increased capturing sites on two spindle-knots (Fig. 6b) compared to the one spindle-knot (Fig. 6a). Two spindle-knots work together, which increases not only the length of TCL, but also the contact length. The study on TCL of the articial humped bers helps to design a smart one-dimensional ber material and enhances its water collection efficiency.

3.2. Water droplet movement reversibility Liquid transport on surfaces is considerably signicant in certain applications,66 e.g., fog harvesting, the design and operation of microuidics, ltration,67 integrated DNA analysis devices68,69 and condensers. Liquid transport can be achieved by chemical, thermal, and electrochemical methods.70–76 Inspired by the spider silks, a series of articial gradient bers were fabricated by designing chemical compositions and surface nanostructures. The water drop can move towards or away from the spindle-knots by optimizing the cooperation of multigradients (e.g., curvature, chemical, and roughness in difference) on the ber surfaces. Fig. 7a shows the tiny water drop movement on the prepared bers with different kinds of spindle-knots in an environment with high relative humidity of more than 95%. A ber is covered with a rough poly(methylmethacrylate) (PMMA) spindle-knot and a smooth PMMA joint. The tiny water drops rstly condensed randomly and were then driven toward the spindle-knot. There is a roughness gradient along the ber. There are rough porous nanostructures on the spindle-knot, and smooth structures on the joints. Based on the principle of surface energy gradients, a water drop tends to be driven towards the higher surface energy or the more wettable region. As a result, if the articial hump is divided into two parts, one covering poly(styrene) (PS) with a lower surface energy than PMMA which is the other part, the water droplets should be driven away from the spindle-knot. The nanostructures can enhance the hydrophobicity of the PS spindle-knots. The chemical gradient is considered to have a strong action on water droplets and drives them away from the PS spindle-knot. However, in the second experiment, tiny water droplets randomly condense around the spindle-knot and then

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move toward the rough PS hump from two sides of the joint. This is an unexpected result, because the droplets run from the hydrophilic PMMA joints toward the hydrophobic PS spindleknot. What is interesting is that roughness cannot enhance the chemical gradient to drive the drops towards the more wettable region (smooth PMMA joint). In this case, another factor needs be taken into account to understand this unusual phenomenon. In order to move, a droplet must overcome the resistant force due to contact angle hysteresis, which is always opposite to the moving direction.77 To get rid of the inuence of roughness, the articial humped bers are treated in a solvent atmosphere to be smoother on the ber surfaces, like the behavior of tiny water drops on these bers in the third experiment. In the fourth experiment, smooth hydrophobic PS spindle-knots and smooth hydrophilic PMMA joints were investigated. Water drops were driven from the spindle-knots to the joints. In summary, for a tiny water drop on an articial spindle-knot ber, the total force determines the directional driving of the drops, comprising the chemical force from the chemical gradient, the hysteresis resistance, and the curvature gradient, together with the Laplace force. A kind of smart ber was designed by using N-isopropylacrylamide (NIPAAm) polymer with temperature-responsive hydrogels.78 The spindle-knot surfaces on these bers have characteristics of curvature, roughness and changeable wettability as well. The tiny water droplets can be manipulated reversibly in a directional movement towards or away from the spindle-knots in certain humidity conditions by changing the temperature (above or below lower critical solution temperature (LCST) of the polymer). The block copolymers of PMMA-bpoly(N-isopropylacrylamide) (PNIPAAm) were synthetized by using an atom transfer radical polymerization method to obtain a temperature-responsive wettability effect. The water contact angles on the PMMA-b-PNIPAAm surface can be changed with temperature. This indicates that the wettability can be switched via a conformational change of the NIPAAm molecular chain at low/high temperatures. Using a dip-coating method, the polymer solution is replaced with PMMA-b-PNIPAAm solution. The wettability of the ber surface is controlled by the LCST because of the PMMA-b-PNIPAAm along the ber. The forces from surface gradients determine the direction of the water droplet movement. There are three forces to be considered: wettability gradient force81 (FW), the Laplace force79,80 (FL) and the hysteresis force82–84 (FH), which inuence the motion of droplets on the bioinspired articial ber. The wettable gradient acts on the droplet when condensed on the spindle-knot surface, as shown in Fig. 7b. The M and L are the ‘more wettable’ and ‘less wettable’ bers, respectively, which are controlled by regulating temperature and humidity, which can be manipulated reversibly. At a lower LCST (Fig. 7bA), FW is consistent with FL at the directional forces acting on the droplets. The droplet is driven towards the more wettable part (Fig. 7bA and C). At lower LCST (Fig. 7bB), because of the PNIPAAm molecular chain changing, the FW points to the joint, which is opposite to the FL. FH plays an important role in the transport of the water droplet. The whole droplet is dragged away from the spindle-knot or partly towards the more wettable side (Fig. 7bD). Fig. 7bC and D shows

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Fig. 7 (a) In situ optical observation of driving tiny water drops with controllable direction on artificial spider silks. Tiny water drops were driven

toward the knots with rough PMMA and PS. After the fibers were smoothed, drops were driven towards the knots with smooth PMMA spindleknots or away from the knots on the fibers with smooth PS spindle-knots.77 (b) Schematics of motion behavior of a water droplet at different temperature conditions. (c) Illustration of tiny droplet directional gathering after visible (Vis) or ultraviolet (UV) irradiation.77

the motion behavior of the water droplet in low relative humidity. This study is signicant for developing smart materials to control uid transport or for realizing liquid collection at high efficiency. Hou et al.85 designed a bioinspired ber by using an azobenzene polymer material to control the water driving direction. By the cooperation of roughness, curvature and photo-responsive wettability aer Vis or UV irradiation, the water droplets move away from or towards the spindle-knot (Fig. 7c). This research offers a new insight into the function of materials, for example, directional water driving, heat transfer and water collecting devices.

difference in Laplace pressure on the gradual spindle-knots and drop coalescence release energy. In the rst state, with the cooperation of surface energy in the gradient and the difference in Laplace pressure, the tiny water droplets are driven towards the spindle-knots. With the coalescence of tiny water droplets, a bigger water drop is transported towards the bigger spindleknot for a more stable condition because of the forces of differences in Laplace pressure along the gradient spindleknots and drop coalescence release energy at the second state.

3.3. Micro-drop fast transport Excellent micro-structures have been designed to realize water collection, inspired by natural biological surfaces such as the beetle's back,6 the cactus4 and spider silks.5 Based on water collection research, Zheng et al.86 proposed a novel design to fabricate a spindle-knot structure via a special dip-coating method, which breaks the limit of water transport distance. The dip-coating method is different from the common fabrication method. A uniform Nylon ber xed on a support is soaked into a polymer solution with an angle to the horizontal line and then drawn out at a given velocity. It results in the gradual decrease of sizes of the spindle-knots. Because of the angle to the horizontal liquid level, the solution lm on the ber is obviously non-uniform, and forms different-sized spindle-knots. This gradual structure produces a unique water directional movement channel, which breaks the limit of distance between the two spindle-knots. As shown in Fig. 8, the water droplet moves directionally across four gradual spindle-knots for a distance of 5.0 mm in 130 s. There are three factors which drive the directional movement of water droplets along the gradient ber, based on this research. These include capillary adhesion, This journal is © The Royal Society of Chemistry 2014

Fig. 8 Water drops' directional movement for a long distance on a

bioinspired fiber with gradient structures.86 (a) Water transport in a directional movement. (b) The details of the water drops in directional movement.

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At the third state, the water droplet hanging between two spindle-knots tends to slide from the smaller spindle-knot to the bigger one because of the difference in capillary adhesion force. Multi-level cooperation of the three forces drives the transport of micro-drops quickly for a long distance. This creative bioinspired material realizes not only water collection, but also water transport. This novel material has many potential applications in uid control87–89 and aerosol systems.90 3.4. Wetting response The articial humped bers can respond to environmental humidity to collect water. For example, the bioinspired spider silks produced by the coaxial electrospinning method show an interesting response to environmental change because of their alternating hydrophilic and hydrophobic surface characteristics. This swelling behavior of the spindle-knot is similar to that of the capture silk of cribellate spiders in a humid environment.43 The bead-on-string heterostructured bers are fabricated using a spinnable inner uid with a high viscosity (PS, low surface energy) and sprayable outer uid with a low viscosity (poly(ethylene glycol), high surface energy). Because of their alternating hydrophilic and hydrophobic surface characteristics, the bioinspired bers have a reversible environmental response property, which swells up with increasing relative humidity (RH) and shrink with decreasing RH (Fig. 9a). This smart ber could be used as a microreactor in biological medicine, for phase separation and nanoscale ordered structures. Environmental reversibility has attracted some scientists to research smart bers to collect/release water from fog. Yang et al.92 reported a versatile and simple sponge-like cotton fabric

Review

with PNIPAAm modication. The PNIPAAm-cotton fabric is triggered by temperature variations (LCST) to swell/ shrink. This temperature environmental reversibility behavior results in the autonomous collection/release of water from a humid atmosphere. The water collection material with water reversibility could be used for water ow-conduction and purication. It could also be applied for collecting water from fog and converting it into recyclable water. The bioinspired silks respond to the environmental humidity by collecting water by means of the cooperative driving forces resulting from individual spindle-knots. We reveal that multilevel spindle-knots play a role in increasing the efficiency of water collection from humid environments. Fig. 9b shows the water collection processes of articial spindle-knotted ber (Fig. 9bA) and uniform ber (Fig. 9bB) under the same humidity of ~70% at the same temperature of 5  C with time and the relationship curves of net-volume versus time of the articial spindle-knotted ber (Fig. 9bC) and the uniform ber (Fig. 9bD). Comparing the water collection ability of the bioinspired, articial spindleknotted ber and the uniform ber, the former (>80 nL) collects 33 times more water than the latter (2.4 nL). The bioinspired spindle-knotted ber displays a stronger water collection ability than the uniform ber in air. This fully illustrates that the bioinspired articial ber with multi-level structure can improve water collection efficiency. By the cooperation of surface energy gradient and difference in Laplace pressure, the drops complete a circling process of condensation / coalescence / transport / transfer of pinning to achieve high efficiency in movement. The bioinspired silk shows a high-efficiency water collection ability in the humid environment.91

4 Outlook

(a) In situ optical observation of shrinkage of the “beads” with decreasing RH, (A) 90%, (B) 70%, (C) 35%.43 (b) Water collection of (A) bioinspired spindle-knotted fiber and (B) uniform fiber and (C) the relationship of the net volumes of drops with time on the two different fibers.91 Fig. 9

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We have reviewed the research in recent years on bioinspired wetted spider silks and their water collection functions at the micro-level. Bioinspired gradient bers can be fabricated successfully from micro-level to nano-level by dip-coating, electrodynamics, uid-coating and microuidic methods. Thus, many functions such as water collection and tiny drop driving are displayed excellently. Facing increasing demand from society, novel functional materials need to be developed continuously. This review is just the beginning of further research in the eld of bioinspired materials. We are looking forward to the development of many challenges and opportunities. Firstly, nature gives us numerous creative clues for designing biomaterials. More and more new functions are waiting to be discovered. Secondly, the wettability of surfaces are controlled effectively by the stimulus-responsive molecules. It is expected that future investigations will not only use materials responsive to stimuli including light, pH, or thermal stimuli to improve the ber's control of the moving direction of water droplets,93 but also design asymmetric geometric structures and different style gradient features onto one-dimensional bers. Finally, bioinspired micro and nanostructure bers achieve completely unidirectional transport of liquid, which realizes fast and highly-efficient tasks. The bioinspired materials are expected to be applied in high efficiency water

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collection devices,91,92 humidity sensors,93,94 uid-control,95 micro-reactor devices96 and other devices or systems.97

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Acknowledgements This work is supported by the National Natural Science Foundation of China (21234001), the National Research Fund for Fundamental Key Project (2013CB9330001), the Doctoral Fund of Ministry of Education of China (20121102110035) and the Innovation Foundation of BUAA for phD Graduates.

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Review

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nanostructure fibers with a water collecting property.

We review the recent research on structure-induced water collecting properties of spider silk and bioinspired fibers. Since the capture silks of cribe...
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