Bio-Inspired, Smart, Multiscale Interfacial Materials

Bio-Inspired, Smart, Multiscale Interfacial Materials

(Parte 1 de 5)

DOI: 10.1002/adma.200800836

Bio-Inspired, Smart, Multiscale Interfacial Materials**

By Fan Xia and Lei Jiang*

1. Introduction

Learning from Nature means taking ideas from Nature and developing novel functional materials based on these concepts, as has been the case for, e.g., bio-inorganic materials (biomineralization),[1] bioinspired, multiscale structured materials (chiral morphologies),[2] bio-nanomaterials (bio-nanoparticles),[3] hybrid organic/inorganic implant materials (bonelike composites),[4] and smart biomaterials.[5,6] These bioinspired, smart materials are attracting more and more interest because of their unique properties, which have paved the way to many real-world applications, e.g., biomimetic fins,[7] actively moving polymers,[8] neural memory devices,[9] smart micro-/nanocontainers for drug delivery,[10] various biosensors,[1–13] dual/multi-responsive materials.[14,15] Also, many of these smart materials have surfaces that dynamically alter theirphysicochemical properties in response to changes in their environmental conditions and a triggered control of interfacial properties at the solid/water interface can be found in ion channels,[13] directional surface motions,[16] and bioinspired, smart surfaces with controllable wettability and adhesion.[17,18] The self-cleaning effect of lotus leaves,[19] the anisotropic de-wetting behavior of rice leaves,[20] the superhydrophobic forces exerted by a water strider’s leg,[21] the attachment mechanism of geckos,[2] and many other natural phenomena are all related to unique micro- and nanostructures on surfaces.[23–28] The creation of such complex functionalities in bioinspired materials depends on well-ordered multiscale structures. Here, we present a strategy for the design of bioinspired, smart, multiscale, interfacial (BSMI) materials based on this concept.

This review is organized into five sections. The first section gives a brief introduction to BSMI materials, while the second section summarizes the basic guidelines for their preparation. The third section provides a comprehensive overview of synthetic routes to four different types of BSMI materials: bioinspired functional, multifunctional, simple responsive, and smart interfacial materials. Lastly, an overview of potential applications of BSMI materials is followed by an outlook for future methods for the fabrication of BSMI materials.

2. Design of BSMI Materials

Smart materials reversibly response to both internal and external, environmental stimuli.[29,30] BSMI materials focus on

In this review a strategy for the design of bioinspired, smart, multiscale interfacial (BSMI) materials is presented and put into context with recent progress in the field of BSMI materials spanning natural to artificial to reversibly stimuli-sensitive interfaces. BSMI materials that respond to single/dual/multiple external stimuli, e.g., light, pH, electrical fields, and so on, can switch reversibly between two entirely opposite properties. This article utilizes hydrophobicity and hydrophilicity as an example to demonstrate the feasibility of the design strategy, which may also be extended to other properties, for example, conductor/insulator, p-type/n-type semiconductor, or ferromagnetism/anti-ferromagnetism, for the design of other BSMI materials in the future.

Center for Molecular Sciences, Institute of Chemistry Chinese Academy of Sciences Beijing, 100190 (P.R. China) E-mail:

F. Xia Graduate School of the Chinese Academy of Sciences Beijing, 100864 (P.R. China)

[**] The authors thank the National Research Fund for Fundamental Key

Projects (2007CB936403) and the National Natural Science Foundation of China (20571077) for continuing financial support. Support through the Chinese Academy of Sciences is gratefully acknowledged.

interfacial response systems. As shown in Figure 1 we suggest five principles for the design of these materials: i) Selection of a unique property found in biology, which can serve as ‘‘bio’’–inspiration; i) Understanding of the correlation between multiscale structure and macroscopic properties, because the correlation between the physico-chemical properties of a biological system and its multiscale structure is crucial for the design process of BSMI materials later on; i) Design and synthesis of suitable target molecules, since selecting appropriate molecules (key building) is important for realizing the desired response properties; iv) Design of a two-way response using bistable states, given that cooperative response processes inbiologicalsystemschangebetweenbistable states based on multiple weak interactions, which are reversible; v) Construction of a binary cooperative complementary interface: in order to control the physical and chemical properties, these properties in bistablestatesshouldbedifferentormayas well be totally opposite. While there are many mutually compensating properties, e.g.,hydrophilic/hydrophobic;conductive/ insulating, convex/concave, p-type/n-type,

F. Xia, L. Jiang / Bio-Inspired, Smart, Multiscale Interfacial Materials

Figure 1. Schematic of the design process (learning from nature). Four design routes lead to four types of materials: bio-inspired functional materials; bio-inspired multifunctional materials; bioinspired, simple-responsive materials; and bio-inspired, smart, interfacial materials according. The design process contains five principles. The first and second route each contain three principles, the third contains four, and the forth contains all five principles.

Lei Jiang is currently a professor at the Institute of Chemistry, Chinese Academy of Sciences (ICCAS). He received his BS degree (1987), MS degree (1990) and Ph.D. degree (1994) from Jilin University of China (Jintie Li’s group). He then worked as a postdoctoral fellow in Prof. Akira Fujishima’s group in Tokyo University. In 1996, he worked as a senior researcher in Kanagawa Academy of Sciences and Technology under Prof. Kazuhito Hashimoto. He joined ICCAS as part of the Hundred Talents Program in 1999. His scientific interest is focused on bioinspired surface and interfacial materials.

Fan Xia is currently a Ph.D. student at ICCAS. He received his B.Sc. degree in chemistry from Huazhong University of Science and technology, China in 2003. In 2003, he joined Prof. Lei Jiang’s group and received his M.S. degree in 2005. His current scientific interests are focused on fabricating intelligent materials and understanding their structure-related special surface physical and chemical properties.

Adv. Mater. 2008, 20, 2842–2858 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 2843

REVIEW oxidizing/reducing, ferromagnetic/anti- ferromagnetic, and so on, this paper only demonstrates the feasibility of our strategy using the concept of hydrophobicity/hydrophilicity.

The selection of different principles when designing a material, that is following different routes during the design process, will create different materials. Hence, the four routes depicted in Figure 1 lead to different types of bio-inspired materials. The following sections will discuss each principle (Section 2) and review selected examples and routes of design processes (Section 3).

2.1. Unique Properties in Biological Systems

Selecting unique properties in biological systems and using them as bioinspiration is the first step in learning from nature. Many scientific questions can be deduced from nature’s smart responses to external factors. In general, biological systems are organic-inorganic hybrid composite materials which respond to external stimuli, e.g., smell, vision, hearing are responses to scent,[31] light,[32] and sound,[3] respectively, but these biological response systems are too complicated to be imitated directly. However, recently some less complex, but nonetheless special features in biology received intense attention (Fig. 2), such as, the selfcleaning effect of lotus leaves and duck feathers,[34,35] the non-fogging, superhydrophobic compound eyes of mosquitoes,[36] the locomotion of geckos and octopuses via highly adhesive feet and suckers,[37,38] the non-wetting phenomenon of water striders walking on water,[39] the color of peacock feathers, butterfly wings, and beetle shells which is caused by a periodic microstructure,[40–42] the special nanostructures causing anti-reflectivity in cicada’s wings and moth’s compound eyes,[43,4] and lastly the special photonic reflectivity of sponge spurs due to their unique microstructure.[45] All these features are suitable for bio-inspiration.

2.2. Correlation between Multiscale Structure and Property

Interestingly, many unique properties are related to special micro- and nanostructures (second and third row in Fig. 2) and understanding the correlation between the unique properties of a biological system and its micro- and nanometer-scale structure is critical for the fabrication of novel materials.

In the case of the ‘‘lotus effect’’, the phenomenon of superhydrophobicity observed in lotus leaves and duck feathers,[34,35] the original hypothesis was that both high contact angle (superhydrophobicity) and low sliding angle (low adhesion) were a result of surface roughness caused by micrometer-scale papillae and epicuticular wax. More recently however, the discovery of novel micro- and nanometer-scale hierarchical structures on the surfaces of lotus leaves, i.e., branch-like nanostructures on top of the micropapillae, let to the proposal that these hierarchical structures might be the reason for the uniqueproperties observed. The combination of special structure and corresponding chemical composition

F. Xia, L. Jiang / Bio-Inspired, Smart, Multiscale Interfacial Materials

Figure 2. Multiscale structure in biology. Four types of interesting biological properties can be found in nature: a) self-cleaning properties: lotus leaf, duck feather, and mosquitos eye (from left to right), b) mechanical properties: gecko feet, octopus suckers, and water strider, c) color through structure: peacock feather, butterfly wings, and beetle shells, and d) optical properties: cicada wings, moth compound eyes, and sponge spur. In each case the first row shows a photograph of the biological feature, while the second and third row show scanning electron microscopy (SEM) images of corresponding micro- and nanometer-scale structures.

leads to the unique properties observed in the lotus leaves, which is also the case for many of the following examples.

Researchers proposed that micro- and nanometer-scale hierarchical structures on biological surfaces, as exemplified by the scanning electron microscopy (SEM) images in Figure 2a (second and third row), can cause special phenomena such as the self-cleaning effect of lotus leaves and the anti-fogging properties of the compound eyes of mosquitoes.[36]

Geckos are known for their excellent ability to climb walls and run on ceilings (highly adhesive ‘‘gecko’’ feet),[37] and octopuses for their ability to anchor their body to the substratum and hold their prey (octopus suckers).[38] Meanwhile, water striders stand effortlessly and walk quickly on water due to their non-wetting legs. Figure 2b shows oriented, micrometer-scale, needle- shaped setae on the legs of water striders,[39] which are considered to be the origin of their superhydrophobicity.

The color of the feathers of a peacock, the wings of a butterfly, or the shell of a beetle,[40–42] is caused by microstructures that exhibit periodic variations in dielectric constant in one, two, or three dimensions (SEM images in Fig. 2c), with the period being in the order of the wavelength of the corresponding color. Color through structure is can be found in a significant number of animals, particularly those living in weakly lit environments, and in cases where the synthesis of a particular pigment is biochemically too expensive, e.g., green in beetles.[46]

Nanostructures found in the wings of cicadas and the compound eyes of moths (SEM images in Fig. 2d) can minimize reflectivity over a broad range of angles and frequencies,[43,4] throughgraduallymatchingtheopticalimpedanceofonemedium withitsneighboracrosstheinterface.Thisisachievedthroughthe integrationofarraysoftaperedelements,socallednipplearrays, into the boundary. Such nipple arrays are commonly found on arthropodal ommatidialsurfacesandarecurrently the subjectof considerable research activities. Likewise, the special photonic reflectivityofspongespursisascribedtouniquemicrostructureas well.[45]

In summary, many unique properties found in nature can be attributed to hierarchical structures. The practical realization of complex functionalities in bio-inspired materials depends on well-ordered, multiscale structures (micro- and macrostructures) producedby variousphysical andchemical methods,and is a crucial point in the design of novel BSMI materials. Therefore, learning from nature, grasping underlying algorithms, and applying them to artificial processes is the path pursued by many scientists.[47] Based on the correlation between property and multiscale structure of a biological system, artificial materials with similar or novel properties can be designed.

2.3. Design and Synthesis of Target Molecules

Apart from the multiscale structure, the design and synthesis of suitable target molecules is another important principle in the design of novel BSMI materials. Target molecules are required to be functional or be responsive to external stimuli with the capability to change their conformational structure and/or properties upon being exposed to an external stimulus. External stimuli can be classified into two types: physical and chemical stimuli. While the former includes light, electricity, magnetic fields, heat, and stress, amongst many others, the latter includes acids/bases, oxidizing/reducing agents, electrochemical forces, etc. The interest in target molecules has increased exponentially recently due to their promising potential. Figure 3 lists some of the many smart molecules described in the literature, i.e., thermo-, glucose-, pH-, electricity- or photo-responsive molecules.[48–53] If polarity, conformation, functional group of an interfacial molecule change, the physical properties of the stimuli responsive molecules change accordingly.

In summary, there are many functional and responsive molecules that are excellent building blocks for the fabrication of smart materials. Depending on the design, the synthesis, and the combination of the building blocks, different functional, single responsive and even dual/multiresponsive materials are possible. Some examples are listed in Section 3.

2.4. Two-way Responses via Bistable States

Although the conformation or orientation of a responsive molecule can change under external stimuli, this response on a molecular level is only the primary response in a biological system. The response process in biological systems is based on multi-weak-interactions, which are entirely reversible. In fact, biological systems at all levels of organization respond to external stimuli via multi-weak-interactions that act as reversible switches between cooperative bistable (multistable) states.

A very typical examples is the multi-weak-interaction among hemoglobin, oxygen, and carbon dioxide (Fig. 4).[54] There are two major conformations (bistable states) of hemoglobin, R state and T state, which induce two types of multi-weak-interaction between hemoglobin and oxygen. In the T state hemoglobin and oxygen are loosely bound, which results in the release of O2 to organs and tissues, while hemoglobin in the R state and oxygen bind together strongly, which facilitates O2 uptake in the lungs. These enthalpy-driven processes of O2 uptake and release depend on the switch in multi-weak-interaction (hydrogen bonding) between the two cooperative bistable states of hemoglobin (T state and R state) and oxygen, which is a fully reversible process.

Designing entirely reversible response systems with cooperative multi-weak-interactions is another crucial point in the design of novel BSMI materials. Switch processes between cooperative multi-weak-interactions can either be entropy- or enthalpy-driven, whereby most switch processes in nature are driven by enthalpy, like the reversible, pH-responsive i-motif

F. Xia, L. Jiang / Bio-Inspired, Smart, Multiscale Interfacial Materials

Adv. Mater. 2008, 20, 2842–2858 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 2845

REVIEW F. Xia, L. Jiang / Bio-Inspired, Smart, Multiscale Interfacial Materials

Figure 3. Typical stimuli-responsive molecules. a) thermo-, b) glucose-, c) pH-, d) electro-, and e) photo-responsive molecules. 2846 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Adv. Mater. 2008, 20, 2842–2858

DNA conformation change for example.[5] Section 3 details some of these processes.

2.5. Binary Cooperative Complementary Interfaces

The purpose of constructing a cooperative response with bistable states is to control the physical and chemical properties. Therefore, the physical or chemical properties of the bistable states should be different or even complementary to each other. Figure 5 shows some physical and chemical properties that complement each other, e.g., hydrophilic and hydrophobic, conducting and insulating, convex and concave, p-type and n-type semiconductors, oxidizing and reducing, and ferromagnetic and antiferromagnetic behaviour. Recently, the novel concept of binary cooperative complementary nanometer-scale interfacial materials was proposed, i.e., materials that exhibit two complementary properties on the nanometer-scale and can switch reversibly between both properties on the ‘‘macroscale’’ under certain conditions.[56] Some recent results,[57–61] indicate that this concept isusefulforthedesign ofnovelinterfacialmaterials.Undercertain conditions, these interfaces exhibit unexpected properties, providing a huge potential to be explored theoretically and many potential novel applications. Hydrophilicity and hydrophobicity are the two opposite extremes in wettability of a surface and artificial switches between hydrophilic and hydrophobic or even superhydrophilic and superhydrophobic that can be triggered by external stimuli, such as temperature, pH, light, etc.,[62–6] are excellent examples of BSMI materials.

3. Examples of BSMI Materials Design

Taking the lotus leaf as an example, its superhydrophobic and self-cleaning properties can serve as bio-inspiration. A great number of research focuses on the correlation between microstructure and macroscale property, and novel findings of micro- and nanometer-scale hierarchical structures on the surface of the lotus leaf, i.e., branch-like nanostructures on top of the micropapillae, enable us to explain such unique properties.[34] Since then, a number of researches on superhydrophobic surfaces were motivated by the aim to mimic nature and a lot of effort was put into the development of various artificial superhydrophobic surfaces.[67–71] Generally, surfaces with a static contact angle (CA) higher than 1508 are defined as superhydrophobic surfaces. As shown in Figure 6, six different CA states and hystereses are possible for superhydrophobic surfaces: a) Wenzel’s state (water droplets pin the surface in a wet-contact mode), b) Cassie’s super-

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