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This text for advanced undergraduate and graduate students covers the fundamental relationships between the structure and properties of materials and biological tissues. The successful integration of material and biological properties, shape, and architecture to engineer a wide range of optimized designs for specific functions is the ultimate aim of a biomaterials scientist. Relevant examples illustrate the intrinsic and tailored properties of metal, ceramic, polymeric, carbon-derived, composite, and naturally derived biomaterials.

Fundamentals of Biomaterials is written in a single voice, ensuring clarity and continuity of the text and content. As a result, the reader will be gradually familiarized with the field, starting with materials and their properties and eventually leading to critical interactions with the host environment. Classical and novel examples illuminate topics from basic material properties to tissue engineering, nanobiomaterials, and guided tissue regeneration.

This comprehensive and engaging text:

integrates materials and biological properties to understand biomaterials function and design

provides the basics of biological tissue components and hierarchy

includes recent topics from tissue engineering and guided tissue regeneration to nanoarchitecture of biomaterials and their surfaces

contains perspectives/case studies from widely-recognized experts in the field

features chapter-ending summaries to help readers to identify the key, take-home messages.



1. Introduction

Marcus Vitruvius Pollio was a Roman architect and engineer of the first century BC. Vitruvius is one of the first people who looked at the human body as an object that has dimensions and proportions and regarded it as a symbol of harmony that he observed between the various human body parts. The Vitruvian Man diagram which Leonardo da Vinci has drawn (Fig. 1.1) was inspired by Marcus Vitruvius and has now come to symbolize the biomedical field maybe because the biomaterials scientists and engineers are trying to create a harmony between the natural organs and tissues and their artificial counterparts that we prepare in the lab.
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2. Properties of Solids

Properties of the solids are very important for the biomaterials field because all biomaterials including metals, ceramics, and the very soft ones like hydrogels used in contact lenses, cell printing, or tissue engineering applications are solids. Soft biomaterials also need to be studied like the less hydrated and much harder biomaterials such as pyrolytic carbon used in the heart valves or hydroxyapatite of bone-like implants.
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3. Metals as Biomaterials

Metals are generally hard, opaque, shiny, malleable, ductile, and conductive materials. Organization of the atoms in solid metals is generally close-packed surrounded by others, having crystal structures as body-centered cubic (bcc), face-centered cubic (fcc), or hexagonal close-packed (hcp). Outer shell electrons are delocalized and free to move and form a kind of cloud around atoms. Meanwhile atoms stay together due to the electrostatic interactions created among each other. This kind of bond is named as metallic bond. Since the outer shell electrons are not strongly bonded to the total structure, metals can easily loose them in chemical reactions and form cations. Electrostatic interactions among cations and anions form salts which are soluble in aqueous media. Metals can form alloys by mixing them with other metallic elements at the molecular level. The main purpose of forming alloys is to enhance some properties of the metal such as make it less brittle, harder, and more resistant to corrosion or have a more desirable color and luster. Metals have several properties that are specific to them, including malleability, which allows the shaping of metal into implants, and ductility, which refers to the ability to draw out metal in the shape of wire and is an important property in allowing the manufacture of intramedullary rods, screws, and long stems.
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4. Ceramics

Ceramics are inorganic materials that are composed of metallic and nonmetallic elements which are bonded to each other with ionic or covalent bonds. Ionic bonds are strong and directional, and therefore ceramics have melting temperatures higher than those of metals and polymers which have metallic or covalent bonds, respectively. Ceramics are produced from materials in powder form by application of heat. They are hard, strong, and brittle. Since they do not have any free electrons, they are poor conductors of heat and electricity. There are numerous combinations of the metallic and nonmetallic groups, and the most commonly known nonmetallic groups are oxides, hydrides, carbides, phosphates, sulfides, and silicates. Aluminum oxides, calcium phosphates, and titanium nitrides are in this class. Carbon-based materials such as carbon, graphite, diamond, and graphene are sometimes classified as members of the ceramics group, but in this book, they will be presented in another chapter (Chap. 6).
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5. Polymers as Biomaterials

The chemical reaction in which high molecular weight molecules are formed from monomers is known as polymerization reaction. Polymerization can proceed according to two different mechanisms, chain growth (or addition) and step growth (or condensation) polymerization. A distinction can be made between condensation and addition mechanisms of polymerization; in condensation polymerization, two functional groups bond with each other, generally by releasing a small molecule such as H2O, while in addition polymerization, double bonds of monomers react without releasing any molecule. However, there are some condensation reactions where no molecule release occurs such as polyurethane polymerization. Meanwhile, these two methods are not exclusive; both approaches can be used in the polymerization of the same monomer, or the same polymer can be prepared by two or more different monomers through both of these approaches, as long as suitable groups are available for individual polymerizations. Nylon 6 is a polymer with repeating units ~NH(CH2)5CO- and can be synthesized either from 6-aminocaproic acid by condensation or from caprolactam by addition polymerization [1]. There are also some new polymerization techniques as click polymerization, atom transfer radical polymerization (ATRP), and reversible addition-fragmentation chain transfer polymerization (RAFT) which became popular in the last decades. In these techniques, some special catalysts and agents are used to obtain specially designed polymers with controlled molecular weights (Fig. 5.1).
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6. Carbon as a Biomaterial

Carbon is an element found abundantly in the Earth’s crust and in the human body. The various bonding capabilities enable it to form so many different varieties of compounds including the many gases, liquids, and solids. The carbon compounds constitute the nutrients, the organic energy sources, the building materials for plants, and many other molecules in the body. Since all living species are hydrocarbon based, carbon basically is the element of life if water is the molecule of life. Carbon-derived compounds like diamond, graphite, and graphene are made of only one element, and the method of their production is different than the commercially available ceramics since the melting temperature of carbon is very high.
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7. Building Blocks of the Human Body

The human body can be considered to be a combination of very complex groups of systems which function smoothly. When this organization is examined from the constituent molecules upward toward the systems, the lowest layer is amino acids, nucleotides, saccharides, and lipids. Upon their combination, proteins and enzymes, polynucleotides, polysaccharides, and lipoid structures are formed. These, in return, form the cells, tissues, organs, organ systems, and finally, the living organism, the human body.
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8. Composites as Biomaterials

Composites are materials which contain more than one component with different physical, chemical, and structural characteristics, and each component contributes to the final product to reach a desirable composition and property. Research on engineered composites was started in the 1960s, and the first composites were produced for automobile and aerospace industries to produce tough, mechanically strong, and stable materials and to have the performance exceed the requirements. The definition of a composite is “combination of two materials in which one of them serves as the reinforcing phase (in the form of fibers, sheets, or particles) embedded in the second material, the matrix.”
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9. Fundamentals of Human Biology and Anatomy

The human body has different structural levels of organization, starting with molecules and macromolecules at the lowest level and increasing in both size and complexity to cells, tissues, organs, and eventually to the systems that make up the whole organism.
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10. Tissue-Biomaterial Interactions

When a foreign material is implanted into the body, tissue responds to the material by showing allergic, toxic, or carcinogenic responses. Meanwhile the tissue creates various effects on the material such as corrosion, degradation, or deterioration. The tissue-material interface is critical for a proper implant performance because this is where the two systems meet and the success of the implant is decided. In order to understand the response of the tissues and the implant to each other, the properties of a typical biomaterial surface has to be known.
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11. Biocompatibility

One of the first definitions of a biomaterial was that of the Clemson University Advisory Board for Biomaterials [1] which was “a systemically and pharmacologically inert substance designed for implantation within or incorporation with living systems.” Biocompatibility was defined by Williams (2008) as “the ability of a material to perform with an appropriate host response in a specific application” [2]. According to ASTM on the other hand, it is defined as a “comparison of the tissue response produced through the close association of the implanted candidate material to its implant site within the host animal to that tissue response recognized and established as suitable with control materials.” Another definition of biocompatibility provided was “The condition of being compatible with living tissue by virtue of a lack of toxicity or ability to cause immunological rejection” [3].
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12. Hemocompatibility

Hemocompatibility is a specific and advanced state of biocompatibility which is especially important for blood interfacing biomaterials. It is important due to its systemic consequences, mainly a blood clot traveling to distant sites and causing unforeseen problems. Any biomaterial which is shown to be biocompatible may not necessarily be hemocompatible, but a hemocompatible material has to be biocompatible. This is because the components in the blood and the processes that take place in it are so different than the rest of those in other tissues and that this issue deserves a separate treatment. In order to understand hemocompatibility, we should first look at the circulatory system and the elements of the circulatory system.
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13. Sterilization of Biomaterials

Sterilization is a necessary and an important part of the manufacture of any biomaterial because infection is the last thing a patient who has undergone an implant placement surgery needs. This would hamper the healing process, probably make the implant unsuccessful and make the patient incapable of dealing with this additional health problem, while the actual surgery might have been a very serious one such as a cardiovascular implantation. Sterilization is needed because the implants could be carrying harmful microorganisms due the implant production environment being not sufficiently sterile, the operating theater may not be microorganism-free, or the surgical instruments could be contaminated. The solution for the last problem is proper sterilization of the implant. Of course in the operating room, all the instruments used during the operation should also be sterilized. The selection of an appropriate sterilization method is an important issue. The goal of sterilization is to reduce the amount of microorganisms found in the surgical environment and on the devices to an internationally acceptable level. However, the methods used to achieve this could have a significant effect on the physical, chemical, and toxicological properties of a biomaterial and as a result on its performance; therefore care should be exercised in the selection of the correct method of sterilization.
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14. Biomaterials and Devices in Soft Tissue Augmentation

Soft tissues constitute a significant portion of the human body, and the ECM of the soft tissues are made up of mainly collagen, as the strong material, hyaluronic acid and chondroitin sulfate as the water and form retaining materials, and elastin as the elasticity component. Biomaterials are designed and used to support their performance and to substitute them when there is a need. The following are the systems and materials to do just that.
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15. Biomaterials and Devices in Hard Tissue Augmentation

Bone loss or damage can result from illnesses such as cancer and osteoporosis or by trauma sustained in an accident. The lost bone has to be substituted using alternative materials or methods such as tissue engineering or additive manufacturing, while damages like cracks or breaks are repaired by use of implants such as fracture fixation devices in the form of screws, wires, pins, staples, or more complex load-bearing devices until healing is achieved. There are five types of bone in the human body. These are the long bones, femur (upper leg), tibia (lower leg), and humerus (upper arm); short bones, carpals (wrist) and tarsals (ankle); flat bones, scapula (shoulder bone), sternum (chest bone), cranium (skull), pelvis, and ribs; irregular bones, vertebrae, sacrum (lower end of the vertebra), and mandible (lower jaw); and sesamoid bones, bones in tendons (patella and pisiform).
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16. Blood Interfacing Applications

Blood is a type of connective tissue and is the most difficult one to bring an implant in contact with because in addition to being biocompatible, a material has to be hemocompatible to serve as a successful implant. Hemocompatibility is defined as the property of a material not to elicit thrombosis and blood coagulation, loss or damage to platelets and other blood elements, or in short, a biomaterial’s property should not to initiate any adverse effects on blood constituents or functions. A broad series of tests are conducted in situ, in vitro, and in vivo before a material is tested on humans for its hemocompatibility. The main types of adverse effects are thrombosis (blood clotting), damage to blood cells (e.g., hemolysis of erythrocytes), adherence and decrease of blood elements such as platelets, and immune responses initiated through the various complement activation pathways.
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17. Controlled Release Systems

Drugs are bioactive agents used to treat or prevent diseases and illnesses through chemical action in the body. In order for a drug to be effective, it has to be at the site of cause of the illness, whether it is an infection, a blockage of an artery, pain, or some other malfunction of the bodily organs or tissues due to genetic causes or a trauma or aging. In order for the drug to reach this target site, it has to be introduced to the body (administration), cross barriers (distribution), get modified by the enzymes within the body (metabolism), and be removed from the body (elimination or excretion). All these processes affect the rate, dose at the target, efficacy, and the fate of the drug. In order for the drug to have prolonged or predetermined period of presence and sufficiently high concentration and to be localized specifically at the target tissue, “controlled release systems” are designed. All the abovementioned topics have to be discussed before a controlled release system can be designed.
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18. Tissue Engineering and Regenerative Medicine

Biomaterials and biomedical devices can be constructed of a variety of materials, and depending on the end use, incorporation of bioactive species such as drugs, enzymes, growth factors, and other molecules is possible. Until the last 15 years, a complete biological entity such as a cell was not incorporated into the biomedical devices. Most of these devices were generally expected and designed to be stable, to have service lives long enough to serve as long as the host lived, except for a few cases such as resorbable sutures and short-duration implants. However, the thought of biodegradable cell-seeded devices that would completely integrate with the biological system during the wound healing process was very appealing because these implants were to be designed to blend with the tissues in the body, and this would be a cure and would not leave behind any traces after a certain implantation period. As a result of these important advantages, this approach became a very appealing solution for many problems arising from the long-term implantation of durable materials. This new field, now called “tissue engineering,” is supported by a number of interdisciplinary fields (Fig. 18.1). The main components of tissue engineering are a scaffold or a cell carrier, mature or stem cells, and bioactive molecules such as growth factors (Fig. 18.2). Meanwhile cell therapies were introduced into the field of novel therapeutic tools where the main difference from tissue engineering was the absence of the scaffold. Over time these two fields together started to be called regenerative medicine.
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19. Nano- and Microarchitecture of Biomaterial Surfaces

Richard Feynman (winner of 1965 Nobel Prize in Physics) gave a talk at the American Physical Society meeting on December 29, 1959, titled “There’s Plenty of Room at the Bottom” at the California Institute of Technology (CalTech) upon which the whole discussion on the topic of nanotechnology started.
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