Table of Contents
Related Titles
Title Page
Copyright
Preface
List of Contributors
Part I: Polymer Surfaces
Chapter 1: Part One: Polymer Surfaces
1.1 Introduction
1.2 Structuring and Modification of Interfaces by Self-Assembling Proteins
1.3 Structuring and Modification of Solid Surfaces via Printing of Biomolecules
1.4 Conclusion and Outlook
References
Chapter 2: Surface-Grafted Polymer Brushes
2.1 Introduction
2.2 Synthesis of Polymer Brushes
2.3 Stimuli-Responsive Polymer Brushes
2.4 Polyelectrolyte Brushes
2.5 Bio-Functionalized Polymer Brushes
Acknowledgment
References
Chapter 3: Inhibiting Nonspecific Protein Adsorption: Mechanisms, Methods, and Materials
3.1 Introduction
3.2 Underlying Forces Responsible for Nonspecific Protein Adsorption
3.3 Poly(Ethylene Glycol)
3.4 Surface Forces Apparatus (SFA)
3.5 Applications of Poly(Ethylene Glycol)
Summary
References
Chapter 4: Stimuli-Responsive Surfaces for Biomedical Applications
4.1 Introduction
4.2 Surface Modification Methodologies: How to Render Substrates with Stimuli Responsiveness
4.3 Exploitable Stimuli and Model Smart Biomaterials
4.4 Biomedical Applications of Smart Surfaces
4.5 Conclusions
Acknowledgments
References
Chapter 5: Surface Modification of Polymeric Biomaterials
5.1 Introduction
5.2 Effect of Material Surfaces on Interactions with Biological Entities
5.3 Surface Morphology of Polymeric Biomaterials
5.4 Surface Modifications to Improve Biocompatibility of Biomaterials
5.5 Surface Modifications to Improve Hemocompatibility of Biomaterials
5.6 Surface Modifications to Improve Antibacterial Properties of Biomaterials
5.7 Nanoparticles
References
Chapter 6: Polymer Vesicles on Surfaces
6.1 Introduction
6.2 Polymer Vesicles
6.3 Applications of Polymer Membranes and Vesicles as Smart and Active Surfaces
6.4 Current Limitations of Polymer Vesicles and Emerging Trends
6.5 Conclusions
Abbreviations and Symbols
References
Part II: Hydrogel Surfaces
Chapter 7: Protein-Engineered Hydrogels
7.1 Introduction to Protein Engineering for Materials Design
7.2 History and Development of Protein-Engineered Materials
7.3 Modular Design and Recombinant Synthesis Strategy
7.4 Processing Protein-Engineered Materials
7.5 Conclusion
References
Chapter 8: Bioactive and Smart Hydrogel Surfaces
8.1 Introduction
8.2 Mimicking the Extracellular Matrix
8.3 Hydrogels: Why Are They So Special?
8.4 Elastin-Like Recombinamers as Bioinspired Proteins
8.5 Perspectives
Acknowledgments
References
Chapter 9: Bioresponsive Surfaces and Stem Cell Niches
9.1 General Introduction
9.2 Stem Cell Niches
9.3 Surfaces as Stem Cell Niches
9.4 Conclusions
References
Part III: Hybrid & Inorganic Surfaces
Chapter 10: Micro- and Nanopatterning of Biomaterial Surfaces
10.1 Introduction
10.2 Photolithography
10.3 Electron Beam Lithography
10.4 Focused Ion Beam
10.5 Soft Lithography
10.6 Dip-Pen Nanolithography
10.7 Nanoimprint Lithography
10.8 Sandblasting and Acid Etching
10.9 Laser-Induced Surface Patterning
10.10 Colloidal Lithography
10.11 Conclusions and Perspectives
Acknowledgments
References
Chapter 11: Organic/Inorganic Hybrid Surfaces
11.1 Introduction
11.2 Calcium Carbonate Surfaces and Interfaces
11.3 Calcium Phosphate Surfaces and Interfaces
11.4 Silica Surfaces and Interfaces
11.5 Conclusion and Outlook
Acknowledgments
References
Chapter 12: Bioactive Ceramic and Metallic Surfaces for Bone Engineering
12.1 Introduction
12.2 Ceramics for Bone Replacement and Regeneration
12.3 Metallic Surfaces for Bone Replacement and Regeneration
12.4 Conclusions
References
Chapter 13: Plasma-Assisted Surface Treatments and Modifications for Biomedical Applications
13.1 Introduction
13.2 Surface Requisites for Biomedical Applications
13.3 Surface Functionalization of Inorganic Surfaces by Plasma Techniques
13.4 Applications of Plasma-Modified Surfaces in Biology and Biomedicine
13.5 Conclusions and Outlook
Acknowledgments
References
Chapter 14:Biological and Bioinspired Micro- and Nanostructured Adhesives
14.1 Introduction: Adhesion in Biological Systems
14.2 Fibrillar Contact Elements
14.3 Basic Physical Forces Contributing to Adhesion
14.4 Contact Mechanics
14.5 Larger Animals Rely on Finer Fibers
14.6 Peeling Theory
14.7 Artificial Adhesive Systems
14.8 Toward Smart Adhesives
Acknowledgment
References
Part IV: Cell–Surface Interactions
Chapter 15:Generic Methods of Surface Modification to Control Adhesion of Cells and Beyond
15.1 General Introduction
15.2 Survey on Generic Methods to Modify Material Surfaces
15.3 Results and Discussion
15.4 Summary and Conclusions
Acknowledgments
References
Chapter 16: Severe Deformations of Malignant Bone and Skin Cells, as well as Aged Cells, on Micropatterned Surfaces
16.1 Introduction
16.2 Experimental Methods
16.3 The Interaction of Bone Cells with Micropillars
16.4 The Deformation of Skin Cells as a Function of Their Malignancy
16.5 The Deformation of Fibroblasts of Different Cellular Ages
16.6 Discussion
16.7 Conclusions
Acknowledgments
References
Chapter 17: Thermoresponsive Cell Culture Surfaces Designed for Cell-Sheet-Based Tissue Engineering and Regenerative Medicine
17.1 Introduction
17.2 Characteristics of PIPAAm-Grafted Cell Culture Surfaces
17.3 Mechanisms of Cell Detachment from the Thermoresponsive Cell Culture Dish
17.4 Cell-Sheet-Based Tissue Engineering and Its Clinical Applications
17.5 Next-Generation Thermoresponsive Cell Culture Dishes
17.6 Conclusions
References
Chapter 18: Cell Mechanics on Surfaces
18.1 Introduction
18.2 What Is Elasticity and Stiffness?
18.3 Measuring and Quantifying Stiffness
18.4 Controlling Substrate Stiffness
18.5 Naturally Derived Scaffolds
18.6 Synthetic Scaffolds
18.7 Substrate Stiffness' Impact on Cell Behavior
18.8 When Stiffness In vivo Goes Awry: The Impact of Fibrosis on Function
18.9 Novel Surface Fabrication Techniques to Improve Biomimicry
18.10 Conclusion
Acknowledgment
Abbreviations
References
Chapter 19: Electrode–Neural Tissue Interactions: Immune Responses, Current Technologies, and Future Directions
19.1 Introduction
19.2 Immune Response to Neural Implants
19.3 Past and Current Neural Interfaces
19.4 Methods for Improvement of the Electrode–Tissue Interface
19.5 Conclusions and Future Directions
References
Index
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The Editors
Prof. Andreas Taubert
University of Potsdam
Institute of Chemistry
Karl-Liebknecht-Straße 24-25
14476 Potsdam-Golm
Germany
Prof. João F. Mano
University of Minho
3B's Research Group
Polymers Ave Park
S. Claudio do Barco
4806-909 Caldas das Taipas
Portugal
Prof. J. Carlos Rodr
guez-Cabello
Universidad de Valladolid
Ctro. Investigacion Cientifica
Paseo de Belén, s/n
47011 Valladolid
Spain
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Biomaterials are nowadays well-established. It is generally accepted that synthetic and semi-synthetic materials are an asset for, among others, medical doctors trying to improve the quality of life of their patients. This can for example be achieved by replacing damaged organs or tissues with artificial hips, knees, heart valves, blood vessels, and so forth. To successfully do this, however, the clinicians do need a solid understanding of how the artificial materials interact with the body of the patient and which biological feedback loops may be triggered or altered by, for example, implantation. As scientists and engineers around the world learn more about the finer details of how advanced materials developed in their laboratories behave under certain circumstances, they specifically have to learn about how their materials interact with a living organism. Although the merging of biology, medicine, chemistry, physics, and materials science is not a new topic anymore, the advent of biomimetic materials chemistry has fundamentally changed, or more accurately, extended, the field of materials for the biological and biomedical sciences.
Biomimetic materials chemistry is essentially founded on the recognition that in many respects Nature is superior to human technology. It is much more “clever”, if you wish. Nature has developed strategies that go back millions of years to produce complex materials that still often outperform many materials made by man. As such, biomimetics is an important and inspiring field in its own right, but because the first contact between tissue and material is always at the surface, the surface of a material needs very special attention from scientists and engineers as well as from clinicians. Therefore, in the last decade a change in the paradigm of biomaterial design has taken place. While in the past, the predominant trend was the exploration of their value as biomaterial of many materials that were created for other applications, many times far away from medicine, nowadays a new generation of biomaterials, specifically designed for biomedical uses, is taking place. The one relevant fact of the success of such generation is the implication of biology in the foundations of the material design. Thas has allowed the creation of materials that not only provides of adequate mechanical compliance but that are able to directly interact with cells, creating by this way a totally new scenario in those materials mimic better than never the rich complexity and functionality of the natural extracellular matrix. As this new concepts are proven their efficiency in biomaterial design it is time to start summarizing all these efforts and compiling the ideas and work that are crafting that new trend.
Indeed, while there are numerous books on biomaterials as such, the editors of this book felt that there is a lack of a concise work summarizing the state of the art of biomimetic materials with a special focus on surface aspects. To a large extent, the book was inspired by the highly successful European Union-funded Research and Teaching Network “BioPolySurf,” which was initiated and coordinated by one of the editors (J.C.R.-C.) and brought together scientists from all over Europe and from chemistry, physics, polymer science, and biology and biomedicine. We therefore felt it timely to produce a concise yet complete book on how surfaces can be made and modified and how the different surfaces interact with biology in the broadest sense, from the basics of surface structuring to surfaces that can already nowadays be used for a specific application or even enable the communication with the macroscopic world, for example by integration into electronic circuitry. A number of the former BioPolySurf partners have agreed to contribute to the book, but we have also been lucky to find excellent contributors from almost every continent demonstrating that European projects do successfully act as nuclei for world-wide research networks.
The book starts out with a set of chapters on physical and chemical strategies for generating a specifically organized surface. Polymers for surface structuring (Alexander Böker, RWTH Aachen University), polymer brushes (Szczepan Zapotoczny, Jagiellonian University), PEGylated surfaces (Larry Unsworth, University of Alberta), stimuli-responsive surfaces (João Mano, University of Minho), biopolymer surfaces (Vasif Hasirci, METU Ankara), gradient surfaces (Muhammad N. Yousaf, York University), and polymer-based sensors (Wolfgang Meier, University of Basel) are the topics of what can be viewed as the first section of the book.
The overarching theme of what can be regarded as the second section is hydrogel surfaces. It contains contributions on synthetic protein-based hydrogels (Sarah Heilshorn, Stanford University), bioactive and smart hydrogel surfaces (Carlos Rodriguez-Cabello, University of Valladolid), and on bioresponsive surfaces and stem cell niches (Josep Planell, Catalonian Institute of Bioengineering). This section easily connects to the following part on organic/inorganic hybrid and inorganic surfaces with contributions on structure formation on small scales (Tobias Kraus, Leibniz Institute for New Materials), organic/inorganic hybrid surfaces (Andreas Taubert, University of Potsdam), inorganic surfaces (Maria Pau Ginebra, Barcelona Institute of Technology; Sanjay Mathur, University of Köln), and finally bionic surfaces and surface mechanics (Stanislav Gorb, University of Kiel).
The last section builds a bridge to biology and engineering as it comprises as set of chapters on cell-surface interactions and on the use of surfaces for cell engineering. Individual chapters discuss self-assembled monolayers and layer-by-layer surfaces as model substrates for cell adhesion (Thomas Groth, University of Halle), topography effects (Günter Reiter, University of Freiburg), surfaces for cell sheet production (Teruo Okano & Jun Kobayashi, Tokyo Women's Medical University), cell mechanics on surfaces (Adam Engler, University of California, San Diego), and electrode-tissue interactions (Mohammad Reza Abidian, Pennsylvania State University).
In summary, the editors believe that the selection of topics covers the key aspects of biomaterials surface science, from the chemistry and physics of fairly simple surfaces to highly structured, complex, and often multifunctional and multiresponsive surfaces that are well-adapted to a certain biological materials problem and beyond.
At this point it is important to acknowledge that this book would not have been possible without the help of all the authors mentioned above and their respective teams. These colleagues all agreed on a fairly tight schedule and delivered high quality overviews over their respective field of expertise within the general topic of biomaterials surface science. It is these people who advance the field not only by performing outstanding research, but also by letting others participate in their knowledge that make being a scientist a real pleasure.
Last but not least, the editors would also like to acknowledge the very efficient and friendly staff at Wiley-VCH, Dr. Martin Preuss, Dr. Bente Flier, and Ms. Bernadette Gmeiner, who provided much needed support over the course of the entire production process.
Andreas Taubert
João F. Mano
J. Carlos Rodríguez-Cabello
List of Contributors
Part I
Polymer Surfaces
The use of proteins as an alternative for synthetic structures for the formation of new materials is a highly active topic in the research field [1, 2]. Properties and structures of proteins are generally well understood and this enables their use for other systems and in different settings/environments than the ones they are originally designed for [3]. Proteins themselves already display interesting properties with respect to catalytic activity, storage capabilities, and in being available in a wide variety of shapes and sizes. When introduced into systems not comprising a natural setting for these structures, the properties can be used, for example, to influence interfacial properties, for serving as a template for the deposition of inorganic materials, in modifications with synthetic moieties, or in combination with other biological structures.
Here we show different approaches and highlights of proteins at interfaces and the utilization in producing novel hybrid structures using their catalytic or coordinating properties for mineralization processes at liquid–liquid as well as liquid–solid interfaces. Additionally, at liquid–solid interfaces, a more localized degree of organization can be achieved via various deposition processes into a wide variety of patterns. The creation of patterns of biological species, including proteins, peptide fragments, antibodies, nucleotides, and so on, on solid surfaces allows for the development of biosensors and affinity essays.
Nature offers a great diversity of proteins building complex superstructures that serve as a matrix for the growth of different materials. The process of biomineralization differs from organism to organism. In many cases, organisms build their biominerals by preorganizing a proteinous matrix that is subsequently mineralized. The mineralization can be guided by the insoluble matrix (by binding to crystals as well as by constraining the available space) and by soluble proteins and low-molecular-weight agents binding to the growing crystallites. We only discuss some very special systems in this chapter; a more general overview about biomineralization is given in the literature [4, 5]. Classical examples of proteins involved in biomineralization are collagen on the one hand, a protein assembling into fibrils and fibers [6, 7], and chitin on the other hand, assembling into different phases and also forming nematic phases [8, 9]. But there are also proteins that do not self-assemble in solution but at interfaces.
Self-organized protein structures on solid and liquid interfaces can be used for the tailored production of materials. The protein can serve as a starting point for nucleation, but it can also be a part of the forming material, yielding a composite material.
We discuss the assembly of long-chain polyamines occurring together with silaffins, the assembly of silicateins and hydrophobins, as well as some examples of possible modifications of the adsorbed proteins.
In general, proteins are constructed from amino acids, some of them possessing apolar side chains, while others have polar or charged side chains. The best way to keep the apolar amino acids away from the surrounding aqueous phase is in most case the creation of a hydrophobic core. This core enables most of the apolar side chains to interact with each other via van der Waals forces, while the other amino acids can interact with surrounding water molecules. This construction is stable in solution – but it is not necessarily stable when the protein approaches an interface. The apolar phase might be a much more favorable surrounding to many apolar groups compared to the hydrophobic core. The resulting surface activity is different compared to classical surfactants as the protein does not necessarily present apolar groups to the surrounding phases in its native state. Thus, the protein often has to rearrange itself at the interface, so that the hydrophobic core turns inside out into the apolar phase, with the other groups remaining in contact with the aqueous phase. This leads to an energetically favored state of the protein that also reduces the interfacial tension. Obviously, this process often leads to dramatic changes in the secondary structure, making the adsorption irreversible or leading at least to a high activation energy for desorption. Adsorption can be analyzed with different models, which often distinguish between the diffusion to the interface and the process of rearrangement,sometimes including different conformations at the interface [10, 11].
We discuss silaffins as the first protein taking part in biomineralization processes. Silaffins consist of a phosphorylated backbone and polyamine side chains. These molecules occur in diatoms, in which they help to build various structures of silica being as beautiful as highly organized. These silaffins occur together as a mixture with other substances in nature, and the accompanying long-chain polyamines are especially important. These long-chain polyamines cannot be classified as proteins, but we discuss them in this chapter as they have functions similar to proteins that take part in the biomineralization of diatoms. While proteins such as collagen assemble into solid structures, the long-chain polyamine (most likely the crucial factor for typical structure formation of silica in diatoms [12]) phase separates into small droplets in aqueous solution. Silica precipitates on the surface of these droplets, embedding a fraction of the polyamines. When a critical amount of polyamine is co-precipitated within the silica, the droplet breaks down into smaller droplets because of the changes in phosphate concentration and pH value, and the precipitation proceeds afterwards at the freshly built surfaces. The silaffins do not give structure to the material, but accelerate the precipitation of silica. By this process, a hierarchical hexagonal material is built [12–14].
Hydrophobins are proteins capable of forming organized structures at interfaces via self-assembly. Filamentous fungi excrete these small, globular proteins that assemble into various structures. Many different hydrophobins exist, showing only a weak similarity in sequence, but exhibiting a typical pattern of eight cysteine residues building four disulfide bridges. These bridges also stabilize the secondary structures as some of the cysteines lie within helices or sheets [17]. Hydrophobins are commercially available in large amounts [18]. Despite their different sequences, fungi use different hydrophobins to lower the surface tension against water as well as to hydrophobize their spores and fruitbodies. This enables them to grow their fruitbodies out of the substrate into air or to infect new substrates coming from air [19, 20].
Hydrophobins are divided into two classes differing in terms of aggregate stability, as shown in Figure 1.1. Hydrophobins of class I build typical rod-shaped aggregates, termed rodlets, having a width of around 10 nm and a length of 100–250 nm [21]. These rodlets assemble into films at interfaces being extremely robust to detergents and fluctuations in pH; solubilization of the aggregates and their films is only possible via treatment with trifluoroacetic acid. In contrast, class II hydrophobins do not build rodlets, and their films are less stable and can easily be dissolved. These films also form characteristic patterns, although on a smaller lengthscale compared to the rodlets formed by class I hydrophobins [17, 20, 21].
Figure 1.1 (a) Atomic force microscopic (AFM) image of rodlets formed by the HGFI hydrophobin from Grifola frondosa. Rodlet formation is characteristic of class I hydrophobins. The rodlets were formed at the air–water interface in a Langmuir trough by multiple compression and lifted on a solid support for imaging, as described in [15]. (b) A surface membrane of HFBI imaged by AFM showing an organized structure. The film was formed at the air–water interface and lifted onto a mica support, as described in [16].
Source: Figure and description are taken from [17], reprinted with permission of Elsevier, Copyright 2009.
Furthermore, all hydrophobins possess a hydrophobic patch that is important for their surface activity. While most proteins have a hydrophobic core and a hydrophilic surface, the hydrophobic patch of hydrophobins is located on the surface. This leads to enhanced surface activity, as the protein is an amphiphile in its native state [17, 21]. Additionally, hydrophobins can rearrange at the interface like any other protein and they therefore adsorb irreversibly at the interface or need at least a much higher amount of energy to desorb in comparison to common surfactants [10, 11].
The quick formation of stable layers for different hydrophobins is followed by the decrease in interfacial tension and the increase in the dilatational modulus [18, 22]. The underlying processes can be understood by molecular dynamics (MD) simulations [23], and also by visualizing the structure via AFM and SEM, as can be seen in Figure 1.1 [17, 19]. The structures of HFBI show regular and nearly hexagonal features at liquid interfaces. The lattice parameters can be varied by the preparation technique of the interface or by protein engineering [24].
Films of the artificial hydrophobin H*Protein B on silica can serve as a template for the growth of layers of TiO2, consisting of polycrystalline anatase. The protein films are prepared by immersing a piranha-cleaned silicon wafer into a buffered hydrophobin solution at different temperatures for various periods. Afterwards, the coated wafer is transferred to an aqueous solution of titania at a controlled temperature to grow the titanium layer. The protein film does not only serve as a nucleation point, but as IR spectra show, it also gets incorporated into the layer of titanium dioxide. The roughness of the film can be controlled by the deposition time, and the mechanical strength in terms of hardness and Young's modulus was found to be much greater compared to layers prepared by chemical bath deposition [25]. This shows clearly that the use of proteins is not just another route to prepare materials, but rather a route to produce composite materials with superior properties.
Films of hydrophobin on liquid interfaces can serve as a matrix for subsequent mineralization (an example structure is shown in Figure 1.2). For example, an oil-in-water emulsion stabilized with the artificial hydrophobin H*Protein B can serve as a template for the creation of mineral microcapsules. In the first step, the protein adsorbs to the oil–water interface. Several oils are applicable for this process, and many of them work in the subsequent mineralization. The interfacial tension between oil and water is the important parameter that determines whether mineralization will take place or not. This knowledge enables to choose an oil that will match the desired properties of each process without having to test different oils in a screening. The protein is mineralized by a saturated solution of calcium phosphate with a suitable pH of 7.4 for the precipitation of hydroxyapatite, yielding oil-filled mineral capsules with a shell of hydroxyapatite. This process has several advantages. In most cases, the oil can be removed easily after the synthesis of the capsules, but it can also be used to solubilize compounds and keep them inside the capsules. Moreover, the process works under mild reaction conditions, and the resulting mineral phase is the same as in bones (nanocrystalline hydroxyapatite); consequently, the probability of getting a biocompatible material is high. The capsules can also withstand high temperatures up to 900 °C, and in addition to that, their morphology is tunable by thermal treatment. The morphology changes in two ways. First, the small crystallites begin to sinter together – this affects the mechanical properties as well as the porosity. Second, the mineral phase seems to change at high temperatures, accompanied by a drastic change in morphology (shown in Figure 1.2b). This enhances the scope of these capsules, as they could also be used as microreactors in processes that take place at elevated temperatures [22].
Figure 1.2 Capsules synthesized from a hydrophobin-stabilized emulsion: (a) intact mineral capsule after 68 days of mineralization from a perfluorooctane/water emulsion; (b) capsules prepared from a perfluorooctane/water emulsion, sintered for 1 h at 900 °C. Structure of the mineral shell of capsules in dependence of oil: (c) perfluorooctane/water (50 days of mineralization); (d) silicone oil/water after partial washing with heptane, remains of silicone oil cover the surface at the right-hand side of the image (17 days of mineralization).
Source: Figures and descriptions are taken from [22], reprinted with permission of The Royal Society of Chemistry, Copyright 2011.
The biomimetic character of this approach of synthesis is not just the use of the protein as a simple matrix to start the mineralization. Proteins refold at interfaces to optimize the contact of hydrophobic groups with the apolar phase and the contact of the hydrophilic groups with the polar phase. It is feasible that the reorganization depends on the character of the apolar phase. The experiments show that the morphology of the mineral changes for different oils. This indicates that the protein does not just start the mineralization by heterogeneous nucleation, but also influences the mineral growth – a concept that is frequently used by matrix proteins in nature [5, 22].
Hydrophobins form stable films on hydrophobic solid as well as on liquid surfaces. The adsorption onto solid surfaces is often characterized by contact angle measurements. The contact angle of the hydrophobin EAS dissolved in water on Teflon (pure system: 108 ± 2°) changes to 48 ± 10° by adsorption of hydrophobin. The binding to the surface is strong, as the contact angle is still 62 ± 8° after washing with a hot solution of sodium dodecyl sulfate [19]. A commercially available class I hydrophobin can build remarkable stable layers on oxidized silica, changing the contact angle of water from 0° to 67°. This behavior emphasizes the amphiphilic properties of the molecule, as it is able to turn an apolar surface into a much more polar one and vice versa. The film withstands temperatures up to 90 °C without dissolving and shows a regular structure [25]. Films made of hydrophobin are also feasible to protect silicon against etching by alkaline solutions; hydrophobins can therefore serve as an alternative to classical lithography masks [26]. The microscopic structure of hydrophobin films was already explored by MD simulations. These simulations identified the important parts for the binding to hydrophobic surfaces for SC3 [23] and HFBII (a snapshot of the stable conformation at a silicone surface is shown in Figure 1.3) [27]. This knowledge enables molecular engineering to tailor the adsorption properties of these proteins to specific requirements.
Figure 1.3 (a,b) Representative tightly bound HFBII/Si(1 1 1) interface. The full hydrophobic patch is colored blue with the most adhesive residues colored red. Figure (b) gives a zoomed-in perspective view with the near-silicon methyl carbons shown as transparent van der Waals's spheres.
Source: Figure and description are taken with modifications from [27], reprinted with permission of Springer, Copyright 2011.
Silicateins are enzymes extracted from marine sponges, in which they hydrolyze different silica precursors under ambient conditions and physiological pH, without being very substrate-specific [28]. Special care has to be taken when immobilizing silicateins, as they become inactive if their secondary structure changes because of adsorption or constrictions in mobility of the active center. These constraints can be fulfilled using a spacing layer between matrix and silicatein layer. A quite general approach is to use a polymer layer together with a spacer. In the original system, nitrilotriacetic acid (NTA) binds to a gold surface via its thiol groups. The acidic groups, localized at the other end of the molecule, complex a nickel ion, which can be subsequently complexed by a His-tag attached to a silicatein (see Figure 1.4 for the adapted modification of WS2 rods) [29]. The silicatein keeps its catalytic activity, which is discussed in detail later. This system has also been adapted to use polymer layers [30], Fe2O3 [31], WS2 [32], or TiO2 [33] as a matrix. The variety of matrix materials shows that this system is well studied for many cases. There are also more straightforward ways that also preserve the catalytic activity of silicatein: gold surfaces can be modified with cystamine or cysteamine via their thiol groups. Afterwards, glutardialdehyde is added to link the silicatein covalently to the amine layer. Subsequently, the surface is mineralized by the addition of silica precursors [34]. All these processes share the need for a spacer in contrast to the systems at liquid/liquid interfaces described previously.
Figure 1.4 (a) Schematic representation of the fabrication of the biotitania/NT-WS2 nanocomposite. In the first step, the WS2 nanotube is functionalized with the multifunctional polymer ligand (gray) by complexation through Ni
groups. The NTA tripod ligand is bound to the side groups of the polymer. In the next step, the silicatein-containing His-tag is attached to the NTA ligand by complexation of Ni
ions through the His-tag. Finally, the water-stable precursor of titanium is hydrolyzed by the immobilized silicatein. (b) Scanning force microscopy (SFM) height image of surface-functionalized WS2 nanotubes. (c) Corresponding phase-contrast image shows the material contrast on the nanotubes. (d) Overview SEM image demonstrating the deposition of titania onto WS2 nanotubes. (e) Enlarged (HRSEM, high-resolution scanning electron microscopy) view of titania-coated WS2 nanotubes.
Source: Figures and description are taken with modifications from [32], reprinted with permission of WILEY-VCH, Copyright 2009.
We described several ways to bind silicatein to various surfaces without diminishing its activity. This remaining catalytic activity can be used to precipitate various materials from precursors, giving rise to several hybrid materials under mild reaction conditions: WS22Figure 1.52Figure 1.5