Preface

This introductory text deals predominately with calcium phosphate-based bioceramic materials that are now ubiquitously used in clinical applications to coat the surfaces of metallic endoprosthetic and dental implants that aim at replacing lost body parts or restoring functions to diseased or damaged tissues of the human body. The authors have written the text from a materials scientist's point of view. Hence, its main subject matter concerns the technology of coating deposition as well as the description of properties of bioceramic coatings including their in vitro alteration and testing in contact with simulated body fluids. We will also provide some salient information on in vivo coating-tissue interactions within the natural environment of the living body. Relevant information gained from experimental animal models will be described, without diving too deeply into the biomedical, physiological and endocrinological background.

Calcium phosphates are harbingers of life. They play a paramount role on Earth as one of the essential basic building blocks of living matter. Hydroxyapatite-collagen composite scaffolds provide the mechanical supporting strength and resilience of the gravity-defying bony skeletons of all vertebrates. The dentine and enamel of teeth are likewise based on these materials. However, natural biological apatite-collagen composites provide not only strength but also flexibility, their porous structure allowing exchange of essential nutrients, and a biologically compatible resorption and precipitation behaviour under appropriate physical and chemical conditions that control the build-up by osteoblasts and resorption by osteoclasts within bony matter. Hence, the calcium-deficient defect hydroxyapatite in bone is a reservoir of phosphorus that can be delivered to the body on demand (Pasteris, Wopenka and Valsami-Jones, 2008).

Nevertheless, if one considers the low abundance of phosphorus in the Earth's crust of slightly less than 0.1 mass%, it is a remarkably odd and puzzling choice of Nature to construct many critical pathways of both plant photosynthesis and animal metabolism around this exceedingly rare element (Westheimer, 1987; Filippelli, 2008). Apart from building up the skeleton of vertebrates, biological phosphate compounds are engaged in fuelling the energetic requirements of the photosynthetic pathway of plants called the Calvin-Benson cycle as well as the intercellular energy transfer within the mitochondria of animals that both rely on adenosine triphosphate (ATP). ATP releases the energy needed to sustain the metabolic processes when reduced to adenosine diphosphate (ADP). Hence, this unique energetic contribution of the phosphate groups is central to the functioning of ATP, arguably the most abundant biological molecule in Nature. Furthermore, deoxyribonucleic acid (DNA) as the carrier of the genetic information code owes its double helical structure to phosphate ester bridges that link the two strands of the helix, and are composed of the four nucleobases, the purine-based adenine and guanine, and the pyrimidine-based thymine and cytosine. Lastly, phospholipid bilayers are the main structural components of all cellular membranes that isolate the cell interior from its surrounding, potentially hostile environment. Most phospholipids contain a glycerol-derived diglyceride, a phosphate group, and a simple organic molecule such as choline, a quaternary 2-hydroxy-N,N,N-trimethylethanammonium salt.

The inorganic calcium phosphate minerals most ubiquitously occurring in Nature belong to the apatite group in its many crystal chemical expressions such as hydroxyapatite, fluorapatite and chlorapatite as well as other calcium orthophosphates such as monetite, brushite and whitlockite. While in the past there has been general agreement that these calcium phosphate-based minerals are the most important reservoirs supplying life on Earth with essential phosphorus, more recently feldspars came into focus as a hidden source of phosphorus. It happens that in feldspars P5+ is able to replace tetrahedrally coordinated Si4+ by coupled substitution with Al3+ to maintain charge balance, that is (London et al., 1990; Manning, 2008). Considering the abundance of feldspars in the Earth's crust, and the easy accessibility for plants and soil biota of their P-containing weathering products, predominately clays, feldspars may indeed be a much more significant source of phosphorus than apatites (Parsons, Lee and Smith, 1998).

Considering the importance of the structure of bone as a biocomposite of Ca-deficient defect hydroxyapatite and triple helical strands of collagen I, it is not surprising that as early as about 40 years ago synthetic hydroxyapatite was suggested as a biocompatible artificial material for incorporation in the human body. Hydroxyapatite was used in the form of densified implants for dental root replacement (Denissen and de Groot, 1979) and as a suitable material for filling bone cavities, for fashioning skeletal prostheses (Hulbert et al., 1970) and for coatings hip endoprosthetic devices (Ducheyne et al., 1980; León and Jansen, 2009). Since then research into the biomedical application of calcium phosphate as osseoconductive coatings has virtually exploded. Many deposition methods were experimentally and some, eventually, clinically evaluated that range from biomimetical processing routes intended to mimic Nature's low temperature, template-mediated biomineralisation pathways (Bryksin et al., 2014) to surface-induced mineralisation (SIM), to electrochemical and electrophoretic deposition, to plasma-assisted metal-organic chemical vapour deposition (PA-MOCVD), to atmospheric plasma spraying (APS) or suspension plasma spraying (SPS) (Campbell, 2003). This treatise will review many of these deposition techniques and will thus provide up-to-date information on the resulting bioceramic coatings, their structure, composition and biomedical functions (see Heness and Ben-Nissan, 2004; Sarkar and Banerjee, 2010; Ducheyne et al., 2011; Heimann, 2012; Dorozhkin, 2012; Zhang, 2013; Surmenev, Surmeneva and Ivanova, 2014). In short, the present book intends to act as a primer to introduce non-specialists to the wide-reaching field of bioceramic coatings that are being designed, developed and tested with the aim to alleviate medical deficiencies and the associated suffering of millions of people afflicted with joint and dental maladies.

During the last several decades, research into bulk bioceramics and bioceramic coatings has emerged as a hot topic among materials scientists. Virtually thousands of papers can now be found in relevant journals (see Appendix) and on the Internet. Attempting to treat this vast field in an encyclopaedic fashion is clearly impossible as each day new contributions are being published with ever-increasing speed and regularity. Hence, trying to keep abreast with these developments is akin to shooting at a very fast moving target. The best that one can do is to provide snapshots of currently available information and attempting to separate the wheat from the chaff whenever possible. To paraphrase the resigning comment by the great German poet Johann Wolfgang von Goethe, uttered in his autobiography 'Out of my Life: Poetry and Truth': 'Such (.) work will never be finished; one has to declare it finished when one has done the utmost in terms of time and circumstances'.

As a parting glance, it should be mentioned that during the preparation of the text, three imaginary readers have intently looked over our shoulder: an interested layperson, a professional working in the area of the subject matter of this treatise, and a diligent student whose interest and knowledge are located somewhere in-between. The layperson may not be conversant with many of the subtleties expounded throughout our text but may be eager to penetrate deeper into the subject of bioceramic coatings. Hence, to somewhat relieve this potential reader from the burden of looking up non-familiar analytical techniques and special scientific terms in other textbooks or encyclopaedias, we have provided in the Chapters 5 and 7 short explanations that precede the more detailed descriptions of coating deposition techniques, and characterisation and testing procedures.

Our second imaginary reader is the professional who may look into specific chapters to extract expert knowledge. He or she will act as a thorough if not harsh critic of our endeavour, and will undoubtedly castigate us for having left out crucial aspects of the subject matter treated in this book. This expert may also criticise us for having used inappropriate terms and faulty connections among materials science and biomedical facts. Alas, we used such possibly scientifically shaky explanations to satisfy the limited level of understanding of imaginary reader #1. The expert may also accuse us of having skimmed over the deep subtleties of the subject, and, in particular, not having given due consideration to those aspects in which he or she has earned scientific standing and international acclaim. However, during the vast progress made in developing increasingly sophisticated techniques to design and engineer bioceramic materials including coatings, many unexplored vestiges and nooks and crannies have been left behind the speedily advancing battle lines that require additional and more detailed studies. Some of the content of this book has been devoted to 'mopping up' such neglected research topics. These topics notwithstanding, we are much aware of deficiencies in our approach and hence ask imaginary...