Preface
The rational design of biomaterials plays a crucial role on the success of devices for different medical applications. The investigation of ex novo chemical synthesis as well as the use of unconventional technological strategies allows manipulating biomaterials into appropriate two-dimensional (2D) and three-dimensional (3D) forms, in order to tailor the main physical, chemical, structural, and biological properties requested to achieve desired clinical efficacy. As a function of their peculiar properties—that is, degradation, mechanical response, biocompatibility, fluid transport, and absorption—biomaterials from natural or synthetic source can be properly manipulated to (a) design innovative devices to support lost/dysfunctional tissues or organs, (b) mimic extracellular matrix (ECM) by temporary 3D substrates able to guide neo tissue formation and organization, or (c) fabricate nano-sized systems with controlled interfaces and/or molecular release for innovative therapeutic uses.
This book offers a large but punctual overview of innovative platforms based on nanostructured biomaterials, focussing the attention on the peculiar chemistry of used biomaterials—by a well-consolidated classification including polymers, ceramics, and metals. For this purpose, the book includes 15 chapters, divided into four different subsections. An introductory section is mainly aimed at introducing the current state of art for the use of biomaterials in tissue repair and regeneration (Chapter 1), also taking into account basic regulatory aspects (Chapter 2) and future targets in clinical use (Chapter 3). The other three sections singularly addressed the recent discoveries on the use of polymers and their composites (Chapters 4–7), ceramics (Chapters 8–11), and metals (Chapters 12–15) for the design of devices for relevant medical needs.
In each section, the peculiar role and the impact of nanotechnology and nanomaterials in helping to tackle today’s urgent challenges and to achieve more effective treatments as well as more successful diagnoses is emphasized.
Introducing biomaterials for tissue 1 repair and regeneration
Vincenzo Guarino*, Michele Iafisco†, Silvia Spriano‡
1.1 Introduction
Over the last decade, there has been a significant progress toward the development of biomaterials to be used for the fabrication of innovative devices for a wide variety of biomedical applications. The manipulation of material chemistry and processing technologies allows for the design of tailor-made systems with peculiar mechanical/ morphological/surface/functional properties which can be tailored on the specific application of interest. In particular, current state of the art in biomaterials design is continuously evolving to offer a portfolio of innovative devices to support the functionalities of natural tissues. In recent years, there has been increasing emphasis on materials that could be used in tissue repair and engineering areas. After an early empirical phase of biomaterials selection based on availability, some attempts were primarily focused on either achieving structural/mechanical performance or on rendering biomaterials inert and thus unrecognizable as foreign bodies by the immune system. Hence, biomaterials were used as implants in the form of sutures, bone plates, joint replacements, ligaments, vascular grafts, heart valves, intraocular lenses, dental implants, and medical devices like pacemakers and biosensors [1, 2]. Secondly, biomaterials have increased their relevance as elementary unit to design synthetic frameworks, namely, scaffolds, matrices, or foams—with features at micro-, submicro- and nanoscale—able to guide the in vitro and/or in vivo mechanisms during the regeneration process of natural tissues. More recently, particular interest has been addressed to biomaterials at the nanoscale for the enormous opportunities to exploit peculiar properties (i.e., extended surface area, high surface to volume ratios, high reactivity) to design a large variety of conventional or unconventional devices with improved interfaces with cells and macromolecules, toward the definition of innovative therapeutic/ diagnostic/theragnostic therapies.
To date, several criteria have been used to select biomaterials, mainly based on their chemistry, molecular weight, solubility, shape and structure, hydrophilicity/hydrophobicity, lubricity, surface energy, water absorption degradation, and erosion mechanism.
Commonly, materials for biomedical use can be divided into three principal classes: metals, ceramics, and polymers—natural or synthetic ones, respectively [3]. Herein, we would discuss the main features of biomaterials, basically remarking how the peculiar properties of materials influence specific functions, thus directing their use toward different fields of applications in tissue repair and regeneration.
Biomaterials: Basic concepts
1.2.1 Metals
The applications of metals as biomaterials are varied and range from surgical equipment components to bones substitutions, artificial joints or valves, fracture fixation devices, stents, and dentures. Most are used in orthopedic and dental surgery, but cardiovascular applications are also widely diffused.
The main causes of failure of metal implants were, and partially still are, wear, low fracture toughness or strength, stress shielding, fibrous encapsulation, inflammation, and infections. The evolution of metallic biomaterials was targeted as first to biocompatibility and mechanical issues (to face wear, low fracture toughness or strength, stress shielding), then to fast osseointegration (to face fibrous encapsulation), and recently to multifunctional properties (with a focus on modulation of inflammation and limitation of the risk of infection), as summarized in Fig. 1.1.
Metallic materials have important mechanical characteristics, with respect to other materials, such as high elastic modulus (100–200 GPa) and high yield strength (300– 1000 MPa), thus make it possible to build structures capable of supporting high loads without large elastic deformations or permanent plastic deformations. As last, because of good ductility, when the applied stress exceeds the yield strength, the metal structure shows plastic deformation rather than brittle breaking, allowing the replacement of the deformed implant before it breaks. Adequate mechanical strength, under static or cyclic loading (fatigue), currently is an almost met demand by metal biomaterials (0.5% of people experiencing hip prosthesis breakage). On the other side, a low elastic modulus is of interest in bone contact applications where stress shielding effect has to be avoided and this is still an open issue.
Concerning the biological requirements, biocompatibility (i.e., strictly related to high corrosion resistance), stable anchorage with tissues, and bioactive behavior (to avoid fibrous encapsulation) are the main needs. Even though currently used bulk metal materials usually have good biocompatibility and corrosion resistance, a still unmet issue is related to minimization of the friction forces and wear in the artificial joints in order to avoid toxic wear debris and to guarantee a long implant life. Stable anchorage and bioactive behavior were widely investigated and several surface treatments, coatings, and functionalization processes were developed and are in use, even if some un-met issues are still remaining. Any innovation concerning surfaces must avoid introducing further complications due to, for instance, excessive surface roughness, stress concentration factors, or material changing due to added thermal treatments.