Responsive Membranes and Materials E-Book

• Herausgeber: John Wiley & Sons
• Kategorie: Fachliteratur
• Sprache: Englisch

Seitenzahl: 712

Veröffentlichungsjahr: 2012

Beschreibung

The development of new multifunctional membranes and materials which respond to external stimuli, such as pH, temperature, light, biochemicals or magnetic or electrical signals, represents new approaches to separations, reactions, or recognitions. With multiple cooperative functions, responsive membranes and materials have applications which range from biopharmaceutical, to drug delivery systems to water treatment.

This book covers recent advances in the generation and application of responsive materials and includes:

• Development and design of responsive membranes and materials
• Carbon nanotube membranes
• Tunable separations, reactions and nanoparticle synthesis
• Responsive membranes for water treatment
• Pore-filled membranes for drug release
• Biologically-inspired responsive materials and hydrogels
• Biomimetic polymer gels

Responsive Membranes and Materials provides a cutting-edge resource for researchers and scientists in membrane science and technology, as well as specialists in separations, biomaterials, bionanotechnology, drug delivery, polymers, and functional materials.

Leseprobe

Contents

Cover

Title Page

Copyright

Dedication

Preface: Overview of the Book Highlighting Responsive Behaviour

List of Contributors

Chapter 1: Oligonucleic Acids (“Aptamers”) for Designing Stimuli-Responsive Membranes

1.1 Introduction

1.2 Aptamers – Structure, Function, Incorporation, and Selection

1.3 Characterization Techniques for Aptamer-Target Interactions

1.3.1 Measuring Overall Structural Changes of Aptamers Using QCM-D

1.3.2 Measuring Overall Structural Changes of Aptamers Using DPI

1.4 Aptamers – Applications

1.4.1 Electromechanical Gates

1.4.2 Stimuli-Responsive Nucleic Acid Gates in Nanoparticles

1.4.3 Stimuli-Responsive Aptamer Gates in Nanoparticles

1.4.4 Stimuli-Responsive Aptamer-Based Gating Membranes

1.5 Outlook

Acknowledgements

References

Chapter 2: Emerging Membrane Nanomaterials – Towards Natural Selection of Functions

2.1 Introduction

2.2 Ion-Pair Conduction Pathways in Liquid and Hybrid Membranes

2.3 Dynamic Insidepore Resolution Towards Emergent Membrane Functions

2.4 Dynameric Membranes and Materials

2.4.1 Constitutional Hybrid Materials

2.4.2 Dynameric Membranes Displaying Tunable Properties on Constitutional Exchange

2.5 Conclusion

Acknowledgements

References

Chapter 3: Carbon Nanotube Membranes as an Idealized Platform for Protein Channel Mimetic Pumps

3.1 Introduction

3.2 Experimental Understanding of Mass Transport Through CNTs

3.2.1 Ionic Diffusion and Gatekeeper Activity

3.2.2 Gas and Fluid Flow

3.3 Electrostatic Gatekeeping and Electro-osmotic Pumping

3.3.1 Biological Gating

3.4 CNT Membrane Applications

3.5 Conclusion and Future Prospects

Acknowledgements

References

Chapter 4: Synthesis Aspects in the Design of Responsive Membranes

4.1 Introduction

4.2 Responsive Mechanisms

4.3 Responsive Polymers

4.3.1 Temperature-Responsive Polymers

4.3.2 Polymers that Respond to pH, Ionic Strength, Light

4.4 Preparation of Responsive Membranes

4.5 Polymer Processing into Membranes

4.5.1 Solvent Casting

4.5.2 Phase Inversion

4.6 In Situ Polymerization

4.6.1 Radiation-Based Methods

4.6.2 Interpenetrating Polymer Networks (IPNs)

4.7 Surface Modification Using Stimuli-Responsive Polymers

4.8 “Grafting to” Methods

4.8.1 Physical Adsorption – Non-covalent

4.8.2 Chemical Grafting – Covalent

4.8.3 Surface Entrapment – Non-covalent, Physically Entangled

4.9 “Grafting from” – a.k.a. Surface-Initiated Polymerization

4.9.1 Photo-Initiated Polymerization

4.9.2 Atom Transfer Radical Polymerization

4.9.3 Reversible Addition-Fragmentation Chain Transfer Polymerization

4.9.4 Other Grafting Methods

4.9.5 Summary of “Grafting from” Methods

4.10 Future Directions

References

Chapter 5: Tunable Separations, Reactions, and Nanoparticle Synthesis in Functionalized Membranes

5.1 Introduction

5.2 Membrane Functionalization

5.2.1 Chemical Modification

5.2.2 Surface Initiated Membrane Modification

5.2.3 Cross-Linked Hydrogel (Pore Filled) Membranes

5.2.4 Layer by Layer Assemblies

5.3 Applications

5.3.1 Water Flux Tunability

5.3.2 Tunable Separation of Salts

5.3.3 Charged-Polymer Multilayer Assemblies for Environmental Applications

5.4 Responsive Membranes and Materials for Catalysis and Reactions

5.4.1 Iron-Functionalized Responsive Membranes

5.4.2 Responsive Membranes for Enzymatic Catalysis

Acknowledgements

References

Chapter 6: Responsive Membranes for Water Treatment

6.1 Introduction

6.2 Fabrication of Responsive Membranes

6.2.1 Functionalization by Incubation in Liquids

6.2.2 Functionalization by Incorporation of Responsive Groups in the Base Membrane

6.2.3 Surface Modification of Existing Membranes

6.3 Outlook

References

Chapter 7: Functionalization of Polymeric Membranes and Feed Spacers for Fouling Control in Drinking Water Treatment Applications

7.1 Membrane Filtration

7.2 Fouling

7.3 Improving Membrane Performance

7.3.1 Plasma Treatment

7.3.2 Ultraviolet (UV) Irradiation

7.3.3 Membrane Modification by Graft Polymerization

7.3.4 Ion Beam Irradiation

7.4 Design and Surface Modifications of Feed Spacers for Biofouling Control

7.5 Conclusion

Acknowledgements

References

Chapter 8: Pore-Filled Membranes as Responsive Release Devices

8.1 Introduction

8.2 Responsive Pore-Filled Membranes

8.3 Development and Characterization of PVDF-PAA Pore-Filled pH-Sensitive Membranes

8.3.1 Membrane Gel Incorporation (Mass Gain)

8.3.2 Membrane pH Reversibility

8.3.3 Membrane Water Flux as pH Varied from 2 to 7.5

8.3.4 Effects of Gel Incorporation on Membrane Pure Water Permeabilities at pH Neutral and Acidic

8.3.5 Estimation and Calculation of Pore Size

8.4 pH-Sensitive Poly(Vinylidene Fluoride)-Poly(Acrylic Acid) Pore-Filled Membranes for Controlled Drug Release in Ruminant Animals

8.4.1 Determination of Membrane Diffusion Permeability (PS) for Salicylic Acid

8.4.2 Applicability of the Fabricated Pore-Filled Membranes on the Salicylic Acid Release and Retention

References

Chapter 9: Magnetic Nanocomposites for Remote Controlled Responsive Therapy and in Vivo Tracking

9.1 Introduction

9.1.1 Nanocomposite Polymers

9.1.2 Magnetic Nanoparticles

9.2 Applications of Magnetic Nanocomposite Polymers

9.2.1 Thermal Actuation

9.2.2 Thermal Therapy

9.2.3 Mechanical Actuation

9.2.4 In Vivo Tracking and Applications

9.3 Concluding Remarks

References

Chapter 10: The Interactions between Salt Ions and Thermo-Responsive Poly (N-Isopropylacrylamide) from Molecular Dynamics Simulations

10.1 Introduction

10.2 Computational Details

10.3 Results and Discussion

10.4 Conclusion

Acknowledgements

References

Chapter 11: Biologically-Inspired Responsive Materials: Integrating Biological Function into Synthetic Materials

11.1 Introduction

11.2 Biomimetics in Biotechnology

11.3 Hinge-Motion Binding Proteins

11.4 Calmodulin

11.5 Biologically-Inspired Responsive Membranes

11.6 Stimuli-Responsive Hydrogels

11.7 Micro/Nanofabrication of Hydrogels

11.8 Mechanical Characterization of Hydrogels

11.9 Creep Properties of Hydrogels

11.10 Conclusion and Future Perspectives

Acknowledgements

References

Chapter 12: Responsive Colloids with Controlled Topology

12.1 Introduction

12.2 Inert Core/Responsive Shell Particles

12.3 Responsive Core/Responsive Shell Particles

12.4 Hollow Particles

12.5 Janus Particles

12.6 Summary

References

Chapter 13: Novel Biomimetic Polymer Gels Exhibiting Self-Oscillation

13.1 Introduction

13.2 The Design Concept of Self-Oscillating Gel

13.3 Aspects of the Autonomous Swelling–Deswelling Oscillation

13.4 Design of Biomimetic Actuator Using Self-Oscillating Polymer and Gel

13.4.1 Ciliary Motion Actuator (Artificial Cilia)

13.4.2 Self-Walking Gel

13.4.3 Theoretical Simulation of the Self-Oscillating Gel

13.5 Mass Transport Surface Utilizing Peristaltic Motion of Gel

13.6 Self-Oscillating Polymer Chains and Microgels as “Nanooscillators”

13.6.1 Solubility Oscillation of Polymer Chains

13.6.2 Self-Flocculating/Dispersing Oscillation of Microgels

13.6.3 Viscosity Oscillation of Polymer Solution and Microgel Dispersion

13.6.4 Attempts of Self-Oscillation under Acid- and Oxidant-Free Physiological Conditions

13.7 Conclusion

References

Chapter 14: Electroactive Polymer Soft Material Based on Dielectric Elastomer

14.1 Introduction to Electroactive Polymers

14.1.1 Development History

14.1.2 Classification

14.1.3 Electronic Electroactive Polymers

14.1.4 Ionic Electroactive Polymers

14.1.5 Electroactive Polymer Applications

14.1.6 Application of Dielectric Elastomers

14.1.7 Manufacturing the Main Structure of Actuators Using EAP Materials

14.1.8 The Current Problem for EAP Materials and their Prospects

14.2 Materials of Dielectric Elastomers

14.2.1 The Working Principle of Dielectric Elastomers

14.2.2 Material Modification of Dielectric Elastomer

14.2.3 Dielectric Elastomer Composite

14.3 The Theory of Dielectric Elastomers

14.3.1 Free Energy of Dielectric Elastomer Electromechanical Coupling System

14.3.2 Special Elastic Energy

14.3.3 Special Electric Field Energy

14.3.4 Incompressible Dielectric Elastomer

14.3.5 Model of Several Dielectric Elastomers

14.4 Failure Model of a Dielectric Elastomer

14.4.1 Electrical Breakdown

14.4.2 Electromechanical Instability and Snap-Through Instability

14.4.3 Loss of Tension

14.4.4 Rupture by Stretching

14.4.5 Zero Electric Field Condition

14.4.6 Super-Electrostriction Deformation of a Dielectric Elastomer

14.5 Converter Theory of Dielectric Elastomer

14.5.1 Principle for Conversion Cycle

14.5.2 Plane Actuator

14.5.3 Spring-Roll Dielectric Elastomer Actuator

14.5.4 Tube-Type Actuator

14.5.5 Film-Spring System

14.5.6 Energy Harvester

14.5.7 The Non-Linear Vibration of a Dielectric Elastomer Ball

14.5.8 Folded Actuator

References

Chapter 15: Responsive Membranes/Material-Based Separations: Research and Development Needs

15.1 Introduction

15.2 Water Treatment

15.3 Biological Applications

15.4 Gas Separation and Additional Applications

References

Index

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Library of Congress Cataloging-in-Publication DataResponsive membranes and materials/D. Bhattacharyya … [et al.]. p. cm. Includes bibliographical references and index. ISBN 978-0-470-97430-8 (cloth) 1. Membranes (Technology) 2. Separation (Technology) I. Bhattacharyya, D. (Dibakar), 1941– TP159.M4R47 2012 660′.2842–dc23 2012022662

A catalogue record for this book is available from the British Library.

ISBN: 9780470974308

D. Bhattacharyya: To Gale, my wife, for her encouragement and understanding; To my graduate and undergraduate students, for making academic life very stimulating; To my grandchildren, Nathan, Madeline, Lila, and Zoe for bringing additional joy in my life.

Thomas Schäfer: With gratitude to Belén, Maria, Monika, and Rainer for constant company. In memory of Conny.

S. R. Wickramasinghe: To my parents, for their encouragement; Xianghong, for her support and optimism; and Aroshe, for reminding me of the excitement each day holds.

Sylvia Daunert: I would like to dedicate this book to my students, colleagues, and mentors who constantly inspire and challenge me to be a creative scientist. Also, many thanks to all whom supported me and contributed throughout the writing and editing processes; it has been a pleasure working with all of you!

Preface: Overview of the Book Highlighting Responsive Behaviour

D. Bhattacharyya, T. Schäfer, S. Daunert, and S. R. Wickramasinghe

The integration of knowledge from the life sciences field with synthetic membranes and materials to create stimuli-responsive behaviour is an important area of science and engineering. Martien, et al. (Nature Materials, 2010) wrote: “Responsive polymer materials can adapt to surrounding environments, regulate transport of ions and molecules, change wettability and adhesion of different species on external stimuli, or convert chemical and biochemical signals into optical, electrical, thermal and mechanical signals, and vice versa”. Touchscreens, light-emitting diodes, etc. are everyday devices that rely on stimuli-responsive materials. However, the response to stimuli can also be explored at the molecular scale for controlling mass transport or creating motion. The fabrication of membranes and materials which can respond to pH, temperature, light, biochemicals, and so on is an important aspect of this book. While reading this text, a multitude of stimuli and responses are taking place in the reader's body, be it for exchanging information through the dendrites and axons of neurons, or for the homeostatic control of the trillions of cells in the hosts. Our health and function entirely relies on the fine-tuned interplay of varied kinds of stimuli-triggered responses at the molecular level, which create a cascade of concerted actions that result in our body working as a perfectly fine-tuned machine. We are far from being able to reproduce the complexity of natural stimuli responsive systems, but in recent years a growing scientific community has been concerned with creating responsive systems that allow us, at the molecular level, to control global release, separation, or actuation as a response to an external stimulus. Similar to the architecture of high-rise buildings or the evolution of living systems, the underlying idea is to use a bottom-up approach for assembling individually characterized elements, molecules, or molecular constructs, which together execute a controllable function. The beauty of such systems emerges when molecular building blocks of very diverse responses are rationally designed and subsequently assembled in order to yield a fine-tuned global system response – that emulates the beauty which Nature in its mastery accomplishes incessantly.

This book is about such constructs, with a particular focus on responsive membranes and materials. It comprises contributions which range from the synthesis of stimuli-responsive membranes and colloids to their applications at very different scales; from self-assembled systems with molecular recognition capabilities within nanopores, to the combination of bulk materials that alter either the effective pore diameter or restrict entrance into pores. Some examples are summarized in Figure 1. Whatever their concept or final use, stimuli-responsive membranes and materials cannot be understood without bearing in mind that their response upon interaction with a stimulus results in a more favourable energetic state, translated as a decrease of the Gibbs energy. This is the basis for understanding under what conditions a system may undergo alterations, or elicit a “response”, in order to release energy. The reader's attention is drawn to the role of the total chemical potential. It is the sum of the internal chemical potential – comprising parameters such as density or activity – and the external chemical potential – referring to an external force field such as an electrostatic, magnetic, luminescent, or gravitational field.

Figure 1 Examples of some stimuli-responsive systems for tuning the permeability to selectivity of nanopores to self-organization and drug release.

In this context, responses to stimuli are the result of driving forces that can have very different origins. As a consequence, on the one hand this provides a large degree of freedom for fine-tuning responses through the creation of subtle interplay between different kinds of forces. On the other hand, this also means that responses must not be designed or interpreted without accounting for all possible variable parameters, as otherwise a responsive system might naturally fail. For example, DNA-aptamers can selectively bind to target molecules, the stimulus, and thereby undergo conformational changes as a specific response. This can be explored for gating mechanisms in nanopores (Chapter 1). However, a significant increase in temperature can result in similar conformational changes as an unspecific response in which case, the DNA-aptamer loses its function. Proteins embedded in hydrogels can bind to specific ions resulting in an overall swelling of the hydrogel; however, external pressure through shear forces might strongly counteract this response (Chapter 11) and partially frustrate the responsive function. Conversely, opposing forces or interactions might also be systematically exploited for designing stimulus-responsive materials such as Janus particles (Chapter 12) or self-oscillating polymer gels (Chapter 13). These examples demonstrate that a thorough understanding of the underlying phenomena is indispensable as it provides a vast playground for creatively designing responsive membranes and materials.

Figure 2 gives an overview of the chapters of this book and their main emphasis and is intended as a quick reference. The book starts with three chapters (1 to 3) dealing with the formation of responsive hybrid materials through modification of suitable support structures. The aim is to mimic molecular transport across cell membranes, rather than relying on bulk responses. Chapter 1 explores the capacity of existing building blocks, DNA-aptamers, to undergo conformational changes upon specific binding to a target molecule. If embedded within a fine-tuned nanoporous support structure, it is shown how these receptors can trigger the release or permeation of compounds depending on the presence of a target molecule. The chapter also gives a glimpse of the analytical tools employed for verifying the dimensional changes that DNA-aptamers undergo during this process. Chapter 2 describes a methodology to create self-organized supramolecular structures in which simple building blocks are allowed to self-assemble under the influence of an external stimulus in order to achieve hybrid materials of desired selectivity, for example for selective ion-transport. Here, the concept of evolution is employed in order to upregulate the function of a membrane through adaptive adjustment in the presence of a target solute. Chapter 3 focuses on the modification of the front tip of carbon nanotubes with functional molecules to serve as “gatekeepers” and function as ideal transport channels. The proposed system benefits from the fast fluid flow through the cores of the nanotubes combined with a high density of selective receptors at their tips, mimicking molecular transport across biological membrane transporter proteins.

Figure 2 Overview of the chapters of this book and a rough division by their main emphasis. Naturally, all chapters overlap in one aspect or another given that the common theme is the formation or investigation of responsive materials.

Modification of materials requires knowledge of the tools and methods needed to achieve the desired properties in the materials, and Chapter 4 discusses routes to surface modifications for producing responsive membranes. Different grafting methods are presented such as photo-initiated polymerization, atom transfer radical polymerization (ATRP, and reversible addition-fragmentation chain transfer polymerization RAFT), which will also be explored in subsequent chapters (5–7). Chapter 5 spans the gap from the synthesis of responsive polymers to their final application, introducing in this way the engineering component of such systems. After an overview of membrane modification techniques, it describes applications (including layer by layer assembly) concerning tunability of water flux and separation of salts (compare to Chapter 2). The chapter then goes well beyond these more obvious applications by introducing responsive (temperature and pH) membranes and hydrogels for nanoparticle synthesis and degradation of contaminants in aqueous solutions. Chapters 6 and 7 further focus on responsive membranes in water treatment, given its global importance and the fact that fouling phenomena strongly affect otherwise economic membrane separations. Linking with Chapter 4, common synthesis strategies for membrane modification as well as magnetically driven micromixers are presented in Chapter 6 and their effect on water filtration is discussed. Chapter 7 further elaborates on fouling control of the membrane surface and feed spacers, while introducing other membrane surface treatments such as ultraviolet, plasma, or surface irradiation by ion beams and chemically induced free radical polymerization.

In addition to the aforementioned emerging applications, controlled release has traditionally been the predominant field for responsive membranes and materials. While the related physico-chemical phenomena are the same, important differences can exist with regard to particular requirements for the final applications. For example, responsive membranes for water treatment must be easily available on a large scale, thus requiring membrane fabrication to be as straightforward as possible. Furthermore, the membranes should be relatively robust and maintain their level of performance despite possible variations in the composition of the (aqueous) feed solutions. In contrast, materials for controlled release mainly find their application in relatively stable physiological conditions and on an individually small scale where biocompatibility is vital. Comparing the various chapters of this book will allow the reader to become aware of one crucial aspect of responsive membranes and materials, which is often ignored at the early stage of technology developments, and that is the engineering requirements of the final application. Chapter 8 deals with pore-filled hybrid membranes capable of responding to changes in pH, and elaborates on methods to estimate the resulting pore sizes. The membranes are intended for drug release applications. The use of a magnetic field as an external stimulus is described in Chapter 9 with a focus on magnetic nanoparticles that are incorporated into a polymeric host matrix. It is shown how a magnetic field can generate responses in various ways depending on whether it is used in a static or alternating mode.

A highly important research effort is currently being made in simulating the physico-chemical behaviour of bulk materials such as polymers by molecular dynamics (MD) simulation. Although still far from substituting experimental evidence, increasingly refined MD models accompanied by enhanced computational capacities will enable experiments made in silico, paving the way to systematic and automated screening algorithms for the optimization of material properties and, as a consequence, diminishing considerably the need for material and time-intensive experimental trial and error approaches. Chapter 10 provides such an example for the state-of-the-art of MD simulations by describing the interaction of salt ions with a thermo-responsive polymer.

Chapter 11 extends the biomimicking concepts outlined in previous chapters to the use of hybrid bulk materials which have biological recognition moieties incorporated in their polymeric structure. The chapter describes protein-hydrogel systems which can act as sensors or valves upon interaction with targets. It draws attention to the mechanical characterization of such hydrogels which is of utmost importance for practical applications. From the previous chapters it can be seen that responsive systems can be fabricated in various forms, be it as surfaces on membranes or particles, or inside pores. The final application determines which is the most efficient or appropriate overall strategy. Chapter 12 gives an extensive overview on the fabrication of responsive polymer colloids and how their topology can be controlled depending on the final application. It also presents particular opportunities in colloidal responsive systems such as Janus and patchy particles. The use of polymer gels as self-oscillating systems is described in Chapter 13. Using the Belousov–Zhabotinsky reaction as a stimulus, its oscillation is converted into a continuous swelling–deswelling of a polymer gel and it is shown how under defined experimental conditions this can be explored by allowing the gel to “walk” autonomously. Finally, Chapter 14 introduces electroactive polymers which deform in an electrical field. The chapter gives extensive examples of such dielectric elastomers together with their characterization and a theoretical description of the underlying thermodynamic phenomena.

The book closes with Chapter 15, summarizing the developments and research needs in the predominant fields of application of responsive materials, and providing an outlook onto the vast opportunities which lie ahead for this fascinating multi-disciplinary field of materials research.

List of Contributors

Barboiu, Mihail Adaptive Supramolecular Nanosystems Group, Institut Européen des Membranes – UMR CNRS 5635, France

Bhattacharyya, D. Department of Chemical and Materials Engineering, University of Kentucky, USA

Daunert, Sylvia Miller School of Medicine, Department of Biochemistry and Molecular Biology, University of Miami, USA

Dickson, James Department of Chemical Engineering, McMaster University, Canada

Dow, Elizabeth S. Directorate for Engineering, National Science Foundation, USA

Du, Hongbo Department of Chemical Engineering, University of Arkansas, USA

Escobar, Isabel C. Department of Chemical and Environmental Engineering, The University of Toledo, USA

Gaulding, Jeffrey C. School of Chemistry and Biochemistry and Petit Institute for Bioengineering and Bioscience, Georgia Institute of Technology, USA

Gorey, Colleen Department of Chemical and Environmental Engineering, The University of Toledo, USA

Hausman, Richard Department of Chemical and Environmental Engineering, The University of Toledo, USA

Hawkins, Ashley M. Department of Chemical and Materials Engineering, University of Kentucky, USA

Herman, Emily S. School of Chemistry and Biochemistry and Petit Institute for Bioengineering and Bioscience, Georgia Institute of Technology, USA

Hilt, J. Zach Department of Chemical and Materials Engineering, University of Kentucky, USA

Hinds, Bruce Department of Chemical and Materials Engineering, University of Kentucky, USA

Hu, Kang Research and Development, Land O'Lakes, Inc., USA

Husson, Scott M. Department of Chemical and Biomolecular Engineering and Center for Advanced Engineering Fibers and Films, Clemson University, USA

Khatwani, Santosh Miller School of Medicine, Department of Biochemistry and Molecular Biology, University of Miami, USA

Leng, Jinsong Centre for Composite Materials, Science Park of Harbin Institute of Technology (HIT), People's Republic of China

Lewis, Scott R. Department of Chemical and Materials Engineering, University of Kentucky, USA

Liu, Liwu Department of Astronautical Science and Mechanics, Harbin Institute of Technology (HIT), People's Republic of China

Liu, Yanju Department of Astronautical Science and Mechanics, Harbin Institute of Technology (HIT), People's Republic of China

Lyon, L. Andrew School of Chemistry and Biochemistry and Petit Institute for Bioengineering and Bioscience, Georgia Institute of Technology, USA

Özalp, Veli Cengiz POLYMAT, University of the Basque Country (UPV/EHU), Spain

Puleo, David A. Center for Biomedical Engineering, University of Kentucky, USA

Qian, Xianghong Department of Chemical Engineering, University of Arkansas, USA

Schäfer, Thomas POLYMAT, University of the Basque Country (UPV/EHU), Spain and IKERBASQUE, Basque Foundation for Science, Spain

Serrano-Santos, María Belén POLYMAT, University of the Basque Country (UPV/EHU), Spain and DEEEA, Universitat Rovira i Virgili, Spain

Smuleac, Vasile Department of Chemical and Materials Engineering, University of Kentucky, USA

Turner, Kendrick Miller School of Medicine, Department of Biochemistry and Molecular Biology, University of Miami, USA

Wesson, Rosemarie D. Directorate for Engineering, National Science Foundation, USA

Wickramasinghe, S. R. Ralph E. Martin Department of Chemical Engineering, University of Arkansas, USA and Lehrstuhl für Technische Chemie II, Universität Duisburg-Essen, Germany

Williams, Sonya R. Directorate for Engineering, National Science Foundation, USA

Xiao, Li Department of Chemical and Materials Engineering, University of Kentucky, USA

Yang, Qian Lehrstuhl für Technische Chemie II, Universität Duisburg-Essen, Germany

Yoshida, Ryo Department of Materials Engineering, School of Engineering, The University of Tokyo, Japan

Zhang, Zhen Centre for Composite Materials, Science Park of Harbin Institute of Technology (HIT), People's Republic of China

1

Oligonucleic Acids (“Aptamers”) for Designing Stimuli-Responsive Membranes

Veli Cengiz Özalp1, María Belén Serrano-Santos1,2 and Thomas Schäfer1,3

1POLYMAT, University of the Basque Country (UPV/EHU), Spain

2DEEEA, Universitat Rovira i Virgili, Spain

3IKERBASQUE, Basque Foundation for Science, Spain

1.1 Introduction

Stimulus-responsive materials can be understood as materials that change their properties upon exposure to a stimulus in various ways: they may undergo a physical bulk change, for example, as occurs in shape memory polymers upon a temperature change (Figure 1.1a); they may modify their overall (bulk) physico-chemical properties, as do, for example, ionic liquids of switchable polarity upon exposure to a gas (Figure 1.1b); or they may consist only partially of responsive segments that are incorporated into an otherwise non-responsive support structure which may change both their physical and their physico-chemical properties, such as can be observed in shrinkable polymer brushes when exposed to light (Figure 1.2a). The significant difference between the latter and the former two, which are bulk responses, is the fact that in principal only a local, selective, and specific action of a stimulus is required in order to trigger a change in the responsive part of the material. While you are reading these lines Mother Nature is continuously doing this in your body using responsive transporter proteins which are embedded in the otherwise non-responsive lipid bilayer of the cell membrane, without the need for your whole body to be exposed to light or undergo a dramatic change in temperature. In a similar manner to what Nature achieves so ingeniously in your cell membranes, so this chapter deals with porous artificial membranes and particles which use the molecular recognition capacity of oligonuclic acids as a kind of a “gatekeeper” for triggering a local change in permeability or release of solutes.

Figure 1.1 (a) Response of shape-memory polymers to a temperature change and (b) an ionic liquid–water mixture to the exposure to carbon dioxide.

Figure 1.2 (a) Light-responsive polymer brushes and (b) hydrogels which undergo a volume change upon a change of pH.

Common strategies for designing stimuli-responsive membranes and particles that change their permeability or release rate are based on bulk stimuli such as light, pH, ionic strength, and temperature, as well as the action of an electric or magnetic field [1]. While such bulk stimuli may act locally, for example in the case of the irradiation of azo-groups of polymer brushes leading to a reversible shrinking (Figure 1.2a), they may also affect bulk solutions and materials, for example in the case of using temperature or pH as a stimulus for the reversible shrinking of hydrogels (Figure 1.2b). Acting on a bulk when indeed a local action is required means wasting energy and resources as well as limiting the degrees of freedom during the design of such systems: for example, pH and temperature as stimuli must remain within the physiological conditions when the respective responsive materials are to be used in the human body; stimulation by light may in this case not even be an option. Furthermore, if the desired response in the material is only required upon the appearance of a defined molecule or cell, as is the case for targeting of drugs, such bulk stimuli can in fact be highly inefficient.