Polymers in Regenerative Medicine E-Book

Manuel Monleon Pradas

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

Seitenzahl: 875

Veröffentlichungsjahr: 2015

Beschreibung

Biomedical applications of Polymers from Scaffolds to Nanostructures The ability of polymers to span wide ranges of mechanical properties and morph into desired shapes makes them useful for a variety of applications, including scaffolds, self-assembling materials, and nanomedicines. With an interdisciplinary list of subjects and contributors, this book overviews the biomedical applications of polymers and focuses on the aspect of regenerative medicine. Chapters also cover fundamentals, theories, and tools for scientists to apply polymers in the following ways: * Matrix protein interactions with synthetic surfaces * Methods and materials for cell scaffolds * Complex cell-materials microenvironments in bioreactors * Polymer therapeutics as nano-sized medicines for tissue repair * Functionalized mesoporous materials for controlled delivery * Nucleic acid delivery nanocarriers Concepts include macro and nano requirements for polymers as well as future perspectives, trends, and challenges in the field. From self-assembling peptides to self-curing systems, this book presents the full therapeutic potential of novel polymeric systems and topics that are in the leading edge of technology.

Leseprobe

POLYMERS IN REGENERATIVE MEDICINE

Biomedical Applications from Nano- to Macro-Structures

Edited By

Manuel Monleón Pradas

Centro de Biomateriales e Ingeniería TisularUniversitat Politècnica de ValènciaCIBER de Bioingeniería, Biomateriales y Nanomedicina (CIBER-BBN)Valencia, Spain

María J. Vicent

Polymer Therapeutics LaboratoryCentro de Investigación Príncipe FelipeValencia, Spain

Copyright © 2015 by John Wiley & Sons, Inc. All rights reserved

Published by John Wiley & Sons, Inc., Hoboken, New JerseyPublished simultaneously in Canada

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Library of Congress Cataloging-in-Publication Data:

Polymers in regenerative medicine : biomedical applications from nano- to macro-structures / edited by Manuel Monleón Pradas, Maria J. Vicent.  p. ; cm. Includes bibliographical references and index.

 ISBN 978-0-470-59638-8 (hardback)I. Monleón Pradas, Manuel, editor. II. Vicent, Maria J., editor.[DNLM: 1. Polymers. 2. Nanomedicine–trends. 3. Regenerative Medicine–trends. 4. Tissue Engineering–trends. QT 37.5.P7] R857.M3 610.28′4–dc23

    2014017656

Preface

Life expectancy has been continuously increasing and, consequently, human pathologies related to aging, such as musculoskeletal disorders, arthritis, nonhealing wounds, or neurodegenerative diseases, are becoming major health problems. Therefore, there is a need to identify novel strategies to improve the current therapeutic armory. This book presents a number of topics from polymer applications in the field of regenerative medicine, with a span from polymeric nanostructures to scaffolds. The full therapeutic potential of novel polymeric systems can only be developed through multidisciplinary collaborative research involving biologists, chemists, clinicians, and industries. This book tries to provide concepts and foundations to a general readership, as well as current applications and an overview of this exponentially growing field for experts.

Synthetic and natural polymers are compounds of great interest in many fields, especially in biomedical applications. In the past, they have been extensively used as excipients in traditional dosage forms, as materials for prostheses, valves, or contact lenses. More recently, their applications have been extended to sophisticated drug delivery systems and rationally designed scaffolds for cell therapy, so that interesting polymer structures for a variety of applications now cover the nanoscale in polymer therapeutics, the microscale in delivery systems, and the macroscale in hybrid cell-material constructs for tissue regeneration.

Polymeric materials are especially suited to interface with cells. Polymers are long-chain molecules that share basic features with biological macromolecules: both kinds of molecules deform with the inertial mechanism of conformational change and both are able to exhibit structure at a molecular level (the local sequence of different chemical monomers) and at a supramolecular and nano- to micrometer level (phase-separated domains, crystalline domains). More complex multimolecule arrangements leading to the macroscopic network structure of the extracellular matrix (ECM) represent a third level of structure, with typical dimensions ranging from tens to hundreds of microns.

The contributions in the first part of the book, “Methods for synthetic extracellular matrices and scaffolds,” comprise those topics that are more directly related to the tissue engineering and regenerative applications of polymer structures, where their micro- and macrostructures have more importance. Key questions permitting a rational design (Chapter 1) and selection of materials (Chapters 2 and 6) for scaffolds with adequate interactions with the biological interphase (Chapters 3–5) are addressed, as well as specific techniques and applications where scaffolds drive the therapeutic output, and organ replacement is discussed in Chapter 11. A closer look is then given in Part B, “Nanostructures for tissue engineering,” to the effect of modifications at the nanoscale, a hot topic in the design of nanomedicines for tissue repair, a field of exponential growth. Here the selection of polymers as active components of nanostructures together with the understanding of the solution conformation of natural and synthetic materials (Chapter 8) with self-assembled properties at the nanoscale (Chapter 7) is of crucial importance to better design therapies in regenerative medicine. These materials should be able to efficiently deliver to the targeted site the bioactive agents of different nature including small drugs, peptides, proteins (Chapters 8 and 10), or even oligonucleotide sequences (Chapter 9).

Chapter 1 addresses the performance of polymers as materials for tissue engineering scaffolds. These synthetic tridimensional structures provide grafted cells with a niche and with adequate mechanical and chemical stimuli and thus can promote the process of tissue regeneration. Various mechanical, physicochemical, biological, and structural requirements posed on these structures are discussed, and how to match them through bulk and surface chemistry and by means of different porogenic techniques are elaborated. Questions arising from the interplay between composition, function, and structure are discussed, and the most important parameters for a physical and biological characterization of scaffold performance are presented. The possibilities afforded by polymerization chemistry and/or subsequent processing or treatment make polymers such unique materials for tissue engineering scaffolds.

Many polymers from natural sources have found application in tissue engineering and regenerative medicine. Chapter 2 presents a comprehensive overview of them, as well as examples of their application and clinical use. Their origin varies from marine crustacean and algae, as well as mammalian, plants, and microorganism-processed products. These polymers have good biodegradability, usually low-inflammatory response, and reduced cytotoxicity, which make them so interesting. The properties and main uses of naturally derived polyesters, polysaccharides (chitosan, agarose, alginates, starch, hyaluronate, and others), protein-based polymers (silk, collagen, fibrin, and others) are discussed, and emphasis is given to the responsive nature of these polymers and to their modification in order to obtain sensitive biomaterial systems for tissue engineering. Stimuli–responsive or “smart” polymeric systems are polymers that undergo strong physical or chemical property changes responding to small changes in environmental conditions of a physical (e.g., temperature, light, mechanical stress, or electric field) or chemical (e.g., pH or ionic strength) nature.

Various aspects of the interaction between polymer surfaces and cells are covered in Chapters 3 and 4. This is a central problem in the understanding of the regeneration process assisted by synthetic materials. The recognition of an alien surface by a cell is mediated through its membrane receptors that interact with the adsorbed protein layer on the surface. A thorough discussion of the processes of protein adsorption, cell adhesion, and matrix remodeling phenomena at the cell–material interface is presented in Chapter 3. Cell adhesion is the first step of the regeneration process and plays a fundamental role in subsequent cell differentiation, growth, viability, and phenotype expression. The nature of the adsorbed layer of proteins on a polymer surface dictates the initial cellular response and, eventually, the fate of a synthetic material when it is placed in a biological environment. The chapters review the role of surface chemistry and patterning on the phenomenon of fibrillogenesis of adsorbed ECM proteins such as laminin and fibronectin, and the different experimental techniques to follow protein adsorption. The fundamentals of cell adhesion on synthetic polymers are also presented in these chapters. The role of the different adhesion structures is examined, especially of focal adhesions, fibrillar adhesions, and focal complexes. These are multidomain molecules that can interact with several distinct partner molecules, and they are decisive for the proliferative or migratory response of cells and for the generation of the forces governing the mechanosensory processes in cells. The influence of mechanical, topographical, and chemical properties of the synthetic surface on focal adhesion kinase, a signaling protein contributing to integrin control of cell motility, survival, and proliferation, is specifically addressed in Chapter 4.

The processes of cell–material interaction in vivo, though, are much more complex than any of the experimental situations that can be reproduced in vitro. Many cell types coexist in any tissue, and the cross-talk processes between them through different kinds of signals are to a large extent unknown. An attempt to come closer to more realistic scenarios involves the use of bioreactors, where cells and materials can be combined with different signaling molecules under culture conditions that can be controlled in ways that try to resemble aspects of the natural cell microenvironment: nutrient flow, mechanical stresses, concentration gradients, different gas diffusion, etc., including coculture systems. This problem is addressed in Chapter 5, the last of the first, “macro” part of our book, with emphasis given to the dynamic character of the processes that lead to the consideration of the bioreactor, the cells, and the soluble and synthetic materials as a hybrid system.

The second part of the book, “Nanostructures for tissue engineering,” includes contributions addressing topics where the molecular and nanoscale dimensions of the materials play a dominant role, as is the case of therapeutics. Bioactive nanostructures molecularly crafted to signal cells or carry therapeutic agents to specific cells have great potential to regenerate tissues and cure disease. The chemistry of such nanostructures should allow them to interact specifically with cell receptors or intracellular structures.

The first example of the importance of nanostructures shows the application of self-curing formulations for hard as well as soft tissue regeneration (Chapter 6), which react chemically in the human body and allow targeting and controlled release of bioactive components. Self-curing systems based on macromolecular architectures can be applied locally and can act as antibacterial, antimicrobicide, or anti-inflammatory agents. Although very promising steps have been already achieved, to obtain real biomimetic systems that could be integrated in the natural ECM with adequate biofunctionality and biodegradability is still a challenge. The ECM is therefore a main target for consideration in future development of bioactive and biodegradable formulations that should be able to act as controlled reservoirs of bioactive agents, controlled release matrices and, in addition, as the adequate scaffolds for the development of natural regenerated tissues and organs. The application of physical interactions between macromolecular systems and the selective chemical reactions will be key factors for the evolution of these active materials at the nanoscale.

The organization of these nanostructures at larger length scales comparable to cells and large colonies of cells will also be critical to their function. Chapter 7 describes an extensive family of amphiphilic molecules that self-assemble into supramolecular nanofibers with capacity to display a large diversity of signals to cells. “Self-assembly” is the spontaneous arrangement of molecules into stable patterns by the driving force of noncovalent interactions such as hydrogen bonds, ionic bonds, electrostatic bonds, and van der Waals interactions. Regular alternating hydrophobic and hydrophilic residues in short peptide molecules create two distinct surfaces, one hydrophobic and the other hydrophilic, resulting in β-sheet structures in water. They are water soluble and form soft hydrogels when a change in ionic strength and/or the pH of the solution occurs due to salts or buffers. As a result, a network of interweaving nanofibers of around 10 nm diameter is formed, with many features in common with the ECM. Furthermore, the versatility of the modification of these materials permits their functionalization with signaling sequences to instruct cells in different ways. This chapter illustrates the use of these systems to regenerate axons in the central nervous system for spinal cord injuries, bone, and blood vessels in cardiovascular therapies. With the appropriate supramolecular design, these nanostructures could also be used in stem cell, cancer, and gene therapies.

Nanomedicine has been defined as “the use of nanosized tools for the diagnosis, prevention, and treatment of disease and to gain increased understanding of the complex underlying pathophysiology of disease. The ultimate goal is improved quality-of-life.” Currently, about 40 nanoproducts for health care are in routine use. Among the nanotechnologies explained here, “polymer therapeutics” is blooming as the most successful first-generation nanomedicine (Chapter 8). Polymer conjugates differ from other nanopharmaceuticals that simply entrap, solubilize, or control drug release without resorting to chemical conjugation; it sums the advantage of small size, typically <25 nm, which enables better access to the biological targets that so many other nanocarriers cannot attain. Clinical proof-of-concept for polymer–protein conjugates is already a fact, but recent advances in polymer chemistry and the techniques available for physicochemical and biological characterization are enabling for the first time in-depth analysis of the critical polymer therapeutic characteristics governing their structure–activity relationships. Initial studies to date cover a broad spectrum of pathologies, trying to seek treatments for chronic and debilitating diseases of increasing population of older age (i.e., diabetes, hypertension, infections, digestive track diseases, or rheumatoid arthritis). At present, Cimzia® (rheumatoid arthritis), Macugen® (age-related macular degeneration), and Krystexxa® (chronic gout) are in routine clinical use. Within this context, a very promising research approach looks at polymer therapeutics as a tool to promote tissue repair. Applications in wound healing, bone resorption, or ischemia/reperfusion injuries are described in Chapter 8. More ambitious targets, such as cardiac-tissue regeneration or neurodegenerative disorders, are the focus of ongoing research projects, and first studies already in process should come to fruition in the near future. Although still in its infancy, gene therapy can have a major role in this area. Indeed, stem cells can be genetically engineered by means of adequate nanovectors (viral or nonviral) to direct their ex vivo and in vivo behavior. Gene therapy as regenerative medicine is still facing considerable delivery challenges (e.g., safety and inefficiency); however, the latest advances in gene nanocarrier design have led to a few nanoconstructs with acceptable toxicities and efficacies and is described Chapter 9. This considerable progress has been fostered by new emerging materials designed in response to our deeper understanding of the biological barriers in gene delivery. Moreover, nanomaterials can now be used in combination with physical methods to increase further their efficacy in vitro and in vivo. With current techniques, and expecting further advances in the following years, successful application of synthetic gene nanocarriers to regenerative medicine seems to be both a desirable and a reasonable goal.

Organic polymeric materials can be combined with inorganic, giving rise to new “intelligent” hybrid materials possessing unique advantages. Chapter 10 discusses the use of mesoporous silica nanoparticles functionalized to become gated receptacles for controlled drug delivery. These mesoporous supports can be capped and synthesized as nanometric particles, resulting in suitable materials for the design of “nanodevices” for on-command delivery applications. The molecular “gates” are sensitive to a variety of stimuli, and the system is thus able to deliver active molecules or pharmaceuticals with high control.

As a concluding overview, Chapter 11 ponders the challenges and opportunities in the field. The chapter summarizes the requirements at the macro- and nanoscale for the polymers to be used in clinical applications and the technologies facilitating the process. Future advances in tissue engineering and regenerative medicine will depend on the development of “smart biomaterials” that actively participate in functional tissue regeneration. Engineering the mechanical, physical, and biological properties of these materials requires unique experimental, theoretical, and computational approaches necessarily based on a profound understanding of their structure at the nanoscale.

The volume of knowledge in these fields grows steadily, and proposals, alternatives, and solutions for many problems accumulate in the pages of journals. Although some tissue engineering approaches have already demonstrated practical application, clinical translation in regenerative medicine progresses only slowly, for various reasons. A much more dynamic translatory effort will surely result in real breakthroughs and, one must hope, actual advances in new health treatments.

The editors wish finally to express their deep gratitude to all authors and collaborators who have made this book possible.

María J. VicentPolymer Therapeutics Lab., Centro de Investigación Príncipe Felipe, Valencia, Spain

Manuel Monleón PradasUniversitat Politècnica de València, SpainandCIBER de Bioingeniería, Biomateriales y Nanomedicina (CIBER-BBN), Spain

Contributors

María Rosa Aguilar, Biomaterials Group, Polymeric Nanomaterials, Biomaterials Department, Institute of Polymer Science and Technology (CSIC), Madrid; and Biomedical Research Networking Centre in Bioengineering, Biomaterials and Nanomedicine (CIBER-BBN), Madrid, Spain

Maria J. Alonso, Center for Research in Molecular Medicine and Chronic Diseases (CIMUS), Department of Pharmacy and Pharmaceutical Technology, and Institute for Health Research (IDIS), University of Santiago de Compostela, Campus Vida, Santiago de Compostela, Spain

George Altankov, Institute for Bioengineering of Catalonia, Barcelona; and Institució Catalana de Recerca i Estudis Avançats (ICREA), Barcelona, Spain

Ana Armiñán, Polymer Therapeutics Lab, Centro de Investigación Príncipe Felipe, Valencia, Spain

Anthony Atala, Wake Forest Institute for Regenerative Medicine, Wake Forest School of Medicine, Winston-Salem, NC, USA

Elena Aznar, Centro de Reconocimiento Molecular y Desarrollo Tecnológico (IDM), Unidad Mixta Universidad Politécnica de Valencia-Universidad de Valencia, Valencia; and CIBER de Bioingeniería, Biomateriales y Nanomedicina (CIBER-BBN), Valencia, Spain

Erea Borrajo, Center for Research in Molecular Medicine and Chronic Diseases (CIMUS), Department of Pharmacy and Pharmaceutical Technology, Department of Physiology, School of Medicine, and Institute for Heath Research (IDIS), University of Santiago de Compostela, Campus Vida, Santiago de Compostela, Spain

Marco Cantini, Biomedical Engineering Research Division, School of Engineering, University of Glasgow, Glasgow, UK

Estela Climent, Centro de Reconocimiento Molecular y Desarrollo Tecnológico (IDM), Unidad Mixta Universidad Politécnica de Valencia-Universidad de Valencia, Valencia; and CIBER de Bioingeniería, Biomateriales y Nanomedicina (CIBER-BBN), Valencia, Spain

Dunia Mercedes García Cruz, Center for Biomaterials and Tissue Engineering, Universitat Politècnica de Valencia, Valencia, Spain

Stergios C. Dermenoudis, Laboratory of Biomechanics & Biomedical Engineering, Mechanical Engineering & Aeronautics Department, University of Patras, Rion, Greece

M.T. Fernandez Muiños, Department of Bioengineering, IQS-School of Engineering, Ramon Llull University, Barcelona, Spain

Andrés J. García, Woodruff School of Mechanical Engineering, Petit Institute for Bioengineering and Bioscience, Georgia Institute of Technology Atlanta, GA, USA

Marcos Garcia-Fuentes, Center for Research in Molecular Medicine and Chronic Diseases (CIMUS), Department of Pharmacy and Pharmaceutical Technology, and Institute for Health Research (IDIS), University of Santiago de Compostela, Campus Vida, Santiago de Compostela, Spain

Cristina González-García, Woodruff School of Mechanical Engineering, Petit Institute for Bioengineering and Bioscience, Georgia Institute of Technology Atlanta, GA, USA

Jorge Luis Escobar Ivirico, Center for Biomaterials and Tissue Engineering, Universitat Politècnica de Valencia, Valencia, Spain

Sang Jin Lee, Wake Forest Institute for Regenerative Medicine, Wake Forest School of Medicine, Winston-Salem, NC, USA

Ana Vallés Lluch, Center for Biomaterials and Tissue Engineering, Universitat Politècnica de Valencia, Valencia, Spain

João F. Mano, 3Bs Research Group-Biomaterials, Biodegradables and Biomimetics, University of Minho, Guimarães; and ICVS/3Bs, PT Government Associated Laboratory, Braga/Guimarães, Portugal

Ramón Martínez-Máñez, Centro de Reconocimiento Molecular y Desarrollo Tecnológico (IDM), Unidad Mixta Universidad Politécnica de Valencia-Universidad de Valencia, Valencia; Departamento de Química, Universidad Politécnica de Valencia, Valencia; and CIBER de Bioingeniería, Biomateriales y Nanomedicina (CIBER-BBN), Valencia, Spain

Yannis F. Missirlis, Laboratory of Biomechanics & Biomedical Engineering, Mechanical Engineering & Aeronautics Department, University of Patras, Rion, Greece

Laura Mondragon, Centro de Reconocimiento Molecular y Desarrollo Tecnológico (IDM), Unidad Mixta Universidad Politécnica de Valencia-Universidad de Valencia, Valencia; and CIBER de Bioingeniería, Biomateriales y Nanomedicina (CIBER-BBN), Valencia, Spain

Mariana B. Oliveira, 3Bs Research Group-Biomaterials, Biodegradables and Biomimetics, University of Minho, Guimarães; and ICVS/3Bs, PT Government Associated Laboratory, Braga/Guimarães, Portugal

Manuel Monleón Pradas, Center for Biomaterials and Tissue Engineering, Universitat Politècnica de Valencia, Valencia; and Biomedical Research Networking Center in Bioengineering, Biomaterials and Nanomedicine (CIBER-BBN), Valencia, Spain

Cristina Martínez Ramos, Center for Biomaterials and Tissue Engineering, Universitat Politècnica de Valencia, Valencia, Spain

Patricia Rico, Center for Biomaterials and Tissue Engineering, Universitat Politècnica de València, Valencia; and CIBER de Bioingeniería, Biomateriales y Nanomedicina, Valencia, Spain

Julio San Román, Biomaterials Group, Polymeric Nanomaterials, Biomaterials Department, Institute of Polymer Science and Technology (CSIC), Madrid; and Biomedical Research Networking Centre in Bioengineering, Biomaterials and Nanomedicine (CIBER-BBN), Madrid, Spain

Manuel Salmerón-Sánchez, Biomedical Engineering Research Division, School of Engineering, University of Glasgow, Glasgow, UK

Félix Sancenón, Centro de Reconocimiento Molecular y Desarrollo Tecnológico (IDM), Unidad Mixta Universidad Politécnica de Valencia-Universidad de Valencia, Valencia; Departamento de Química, Universidad Politécnica de Valencia, Valencia; and CIBER de Bioingeniería, Biomateriales y Nanomedicina (CIBER-BBN), Valencia, Spain

C.E. Semino, Department of Bioengineering, IQS-School of Engineering, Ramon Llull University, Barcelona, Spain

Pilar Sepúlveda, Fundación Hospital La Fe, Valencia, Spain

Blanca Vázquez, Biomaterials Group, Polymeric Nanomaterials, Biomaterials Department, Institute of Polymer Science and Technology (CSIC), Madrid; and Biomedical Research Networking Centre in Bioengineering, Biomaterials and Nanomedicine (CIBER-BBN), Madrid, Spain

María J. Vicent, Polymer Therapeutics Lab, Centro de Investigación Príncipe Felipe, Valencia, Spain

Anxo Vidal, Center for Research in Molecular Medicine and Chronic Diseases (CIMUS), Department of Physiology, School of Medicine, and Institute for Heath Research (IDIS), University of Santiago de Compostela, Campus Vida, Santiago de Compostela, Spain

Part AMethods for Synthetic Extracellular Matrices and Scaffolds

1Polymers as Materials for Tissue Engineering Scaffolds

Ana Vallés Lluch1, Dunia Mercedes García Cruz1, Jorge Luis Escobar Ivirico1, Cristina Martínez Ramos1 and Manuel Monleón Pradas1,2

1Center for Biomaterials and Tissue Engineering, Universitat Politècnica de Valencia, Valencia, Spain

2Biomedical Research Networking Center in Bioengineering, Biomaterials and Nanomedicine (CIBER-BBN), Valencia, Spain

1.1 The Requirements Imposed by Application on Material Structures Intended as Tissue Engineering Scaffolds

The discovery of the multipotent nature of different kinds of stem cells opened new horizons for therapeutics for surgery and for medicine in general. Current treatments for diseased or injured tissues or organs range from transplantation from allogenic or xenogenic donors or reconstruction by transfer of autologous tissues to the use of nonbiological either implanted or extracorporeal devices. None of these strategies is free of inconveniencies (shortage and effectiveness of donors, clinical complications, need of immunosuppressive drugs, tumors formation, etc.). The hope to understand stem cells in their behavior up to the point of directing their differentiation toward desired lineages is at the base of the different new therapeutical strategies encompassed by regenerative medicine.

This process of cell differentiation is triggered and governed by a multiplicity of factors and stimuli, which cells receive from an immediate environment made from other interacting cells and from the extracellular matrix (ECM). In cases of severe loss or degeneration of tissue, the sites of intended regeneration have lost their basic structures, and thus new grafted cells, even if having the right properties in vitro, fail to regenerate functional tissue in vivo. At this point, synthetic tridimensional structures, so-called scaffolds, may be of help, by providing grafted cells with a niche and adequate mechanical and chemical stimuli. As an example, cardiac tissue regeneration in cases of myocardial infarction and subsequent ventricular remodeling has been recently addressed by cell transplantation and cell sheet engineering, with a variety of cell populations and supply methods [1, 2]. Common difficulties found include lack of functional integration and a low survival of the grafted cells. These shortcomings could be overcome by means of biomaterials into which the cells would be encapsulated [3, 4].

Generally speaking, scaffolds must assist the regeneration process, performing as an artificial cellular environment during some stages of the tissue regeneration [5, 6]. Either in vitro or in vivo, they must replace as best as possible some of the functions of the ECM: they must (i) contribute to the structural and mechanical integrity of the diseased tissue, (ii) serve as a means of transport of nutrients and wastes and facilitate vascularization, (iii) act as a spatial guide for cell spreading and organization, and (iv) transduce mechanical or biochemical stimuli, and eventually transport, store, and deliver active molecules that effect the expression of the phenotype. Besides these functions, in defining the requirements on materials intended as scaffolds, two other sets of factors must be taken into account: those deriving from the specificity of the application (in vitro or in vivo, temporal or permanent, etc.) and those related to processability and manufacture (sizes and shapes of the implants, sterilization procedures).

Function, specific application and processability considerations thus define a number of requisite properties of mechanical, physicochemical, biological, and structural nature. From the mechanical side, strength (resistance to failure) and stiffness (characterized by shear, tensile, or compressive moduli) are the most important properties to be addressed. Modulus values as different as those of brain and bone determine a wide interval of magnitude, and mechanotransduction of signals to the cells depends significantly on this property, especially on the surface moduli. The most important physicochemical properties of scaffold materials are their degradable or stable nature, their permeability and diffusivity to fluids and gases, and their hydrophilic or hydrophobic nature. Material surfaces possess also specific biologically relevant properties: their chemical functionalities may be directly involved in the surface nucleation of compounds such as bone hydroxyapatite, or they may adsorb ECM proteins in different conformations, thus affecting cell adhesion, spreading, and proliferation. Lastly, microstructural properties of the materials, such as their pore volume fraction, pore connectivity and geometry (shape, dimensions of the pores, regularity) are critical for the scaffold’s final performance. The scaffold’s ability to host cells in required numbers, to allow vascularization throughout it, or to guide and organize spatially cell growth in specific ways, depends crucially on these properties.

Our ways to meet these mechanical, physicochemical, biological, and structural requirements is through bulk and surface chemistry for the first three, and through different porogenic techniques for the fourth. A material with a given overall chemical composition may be, furthermore, in very different physical states: it may be a random or a block copolymer, it may be an interpenetrated network, it may be semicrystalline or amorphous, vitreous or rubbery under physiological conditions. These possibilities are afforded by polymerization chemistry and/or subsequent processing or treatment, and make polymers such unique materials for tissue engineering applications.

The intended end uses of the scaffolds are widely different. Scaffolds may be implanted empty (acellular) when they are expected to be invaded and colonized by the cells in a short period of time; otherwise, it is necessary to seed the scaffolds with the appropriate cells before implantation or even to preculture them in vitro within the scaffold [7]. A bioreactor can be helpful in this stage to recreate in vitro some dynamic conditions and the mechanical and/or chemical stimuli that the cells would receive in vivo. Scaffolds may be chemically modified in order to direct cell anchorage or differentiation, through addition of proteins, peptides, growth factors (GFs), hormones, enzymes, or other regulators of the cell behavior [8–11]. Several methods for the controlled release of factors from scaffolds have been developed [12–14].

Polymer materials are especially suited to interface with cells. Being formed by long chain molecules, they share some basic properties with biological macromolecules. At the most fundamental level, both kinds of molecules deform with the inertial mechanism of conformational change, which gives rise to molecular dynamics with characteristic relaxation phenomena at long time scales. Moreover, both biological and synthetic macromolecules are able to exhibit structure at a subnanometer, molecular level (the local arrangement of different chemical monomers) and at a supramolecular, nano- to micrometer level: phase-separated domains, crystalline domains. And the more complex multimolecular associations leading to the macroscopic network structure of the ECM can be mimicked to some extent by the porous architecture of the polymer scaffolds. This represents a third level of structure, with typical dimensions ranging from tens to hundreds of microns.

1.2 Composition and Function

1.2.1 General Considerations

1.2.1.1 The Influence of Surface Chemistry

The fate of an implant is determined by the host tissue reaction to it, and this is mainly a matter of surface interactions, chemical and topological [15, 16]. Cells react to events in their environment as a consequence of signaling processes transduced by cell membrane receptors. These are large molecules that bind to or react with chemical functionalities of the environmental molecules in specific ways, triggering a number of subsequent cellular processes [17]. The usual foreign body reaction to implanted synthetic material consists in a number of processes ending in the isolation of the implant, through its encapsulation in high-density fibrotic tissue. This circumstance may in some applications imply the failure of the implant as it makes impossible a functional continuous integration of the grafted cells in the site of regeneration. The first stages in the encapsulation process involve the adsorption of ECM proteins onto the foreign surface, and the interaction of cell–membrane receptors with them [18]. The conformation of the adsorbed proteins thus may play an important role in the fate of an implanted scaffold. Since the cell–material interaction is always mediated by the ECM proteins adsorbed on the material’s surface, the chemical and topological properties of the surface responsible for the adsorbed conformation of the proteins will always be determinant for the biological performance of a scaffold [19]. Cell adhesion and cell spreading, especially at early stages of the process, will depend on those properties.

The features of surface chemistry having the greatest influence in this respect are the hydrophilic–hydrophobic balance of functionalities, the surface charges, their spatial distribution on the surface, and the surface stiffness.

The more or less hydrophilic character of a surface is determined by the presence in its composition of hydrophilic functionalities (such as –OH, –COOH, –NH2, –SH, polar groups, or bound ions) and by their mobility (large in the rubber state, very impeded in the glassy state). ECM proteins adsorb poorly onto highly hydrophilic surfaces, and consequently the cells adhere with difficulty, especially at the earliest times of contact. Adsorption sites of this kind of surfaces are preferentially occupied by water molecules, in a labile dynamic equilibrium that is difficult competing protein adsorption. In this situation, proteins adsorb, if at all, in small amounts and with a typically globular conformation, which minimizes the area of their interface with the material and thus the free energy. Correspondingly, cells attach, if at all, in small amounts and with a rounded shape, with a poorly developed cytoskeleton, frequently preferring cell-to-cell associations over cell–surface contacts. By contrast, on more hydrophobic surfaces ECM proteins tend to adsorb in larger amounts and with more extended conformations, now energetically preferred since the protein–material interaction destroys the water–surface bond, decreasing the free energy. This causes cell-binding sequences in the adsorbed proteins to be more accessible and, as a consequence, cells attach to hydrophobic surfaces more, and with more extended shapes, numerous processes, and larger focal adhesions and a developed cytoskeleton.

The state of affairs just described for hydrophilic and for hydrophobic surfaces applies, as a general rule, to the early stages of the cell––material interaction process. With time, different situations may arise as a consequence of new processes competing with those just described, such as the progressive build-up of protein–protein interactions (fibrillogenesis) [20] and the cell remodeling of the ECM. The early interactions, however, may be critical for the success of the implant since it is they who determine cell invasion, neovascularization, and the foreign-body response.

The spatial distribution of the surface functionalities is also important for the processes just mentioned. Since proteins have both hydrophilic and hydrophobic domains, the shapes they acquire upon adsorption may depend on the presence on the material surface of alternatingly distributed hydrophilic and hydrophobic domains at a scale that matches the separation of those domains in the proteins. Block copolymers, interpenetrating polymer networks, blends, or nanocomposite organic–organic or organic–inorganic materials are systems whose phase distribution can be tailored at the nanometer scale relevant for the protein–surface, and thus for cell–surface, interaction. The issue of surface topography is also controllable to some degree by factors such as the sizes of the phase-separated nanodomains in heterogeneous material surfaces. These may include crystallites of different sizes, alternating with amorphous domains in semicrystalline polymers, or nanophases of different chemical and mechanical properties in block copolymers, polymer blends, or interpenetrated polymer networks. Protein adsorption, and hence cell early adhesion on the material, is very sensitive to these purely physical features of a surface; nano- and microroughness of the surface topography favors protein adsorption, acting as nucleation points for the adsorption process by diminishing the interfacial surface tension.