properties between an implant and the host tissue can lead to adverse biological effects. The use of a strong metal implant to repair a bone defect can, for example, lead to resorption and weakening of the surrounding bone and, eventually, to bone fracture. The mechanical properties of a material can also influence the response of cells and, thus, they can determine the ability to regenerate a specific tissue. For example, a material that is optimal for regenerating bone typically would not be suitable for regenerating a soft tissue such as cartilage.
Stability in the biological environment: A biomaterial should be nondegradable or should degrade at a desirable rate for the intended application. For a degradable biomaterial, a guideline often found in the literature is that the implant should degrade at a rate comparable to the rate at which new tissue is being formed. However, this can often be difficult to achieve in practice.
Ease of fabrication: Typically, a biomaterial will have an external shape (or geometry), complex or simple, that is similar to the tissue or organ to be replaced or regenerated. Biomaterials can be 3D objects, fibers, coatings, films, or particles, depending on the application. The biomaterial might be required to have internal structural features that are also important. These internal structural features relate to the way in which the components or phases, such as the solid phase and porosity, are arranged within the biomaterial. The structure at a microscale and nanoscale, referred to as the “microstructure” and “nanostructure,” respectively, are often the major structural features of interest. The microstructure (or nanostructure) can be simple or complex, depending on the application. Materials used to create biomaterials should be capable of being formed economically into the desired external shape and internal microstructure (and/or nanostructure). In common with other technologies, the use of additive manufacturing to create biomaterials, particularly with complex shape and microstructure, has been increasing rapidly.
Ability to be sterilized: A biomaterial should be capable of being sterilized by one of the sterilization methods that employ heat (steam autoclaving or dry heat), gas, radiation, or electron beam treatment prior to their use in studies in vitro or implantation in vivo. Lack of sterility will invariably lead to infection and destruction of cells, followed by potentially catastrophic failure in vivo.
Other physicochemical properties: The applications of biomaterials are many and, consequently, one or more additional properties can be particularly important in certain applications. The capacity to absorb water can be crucial to the use of biomaterials in applications such as drug delivery and tissue engineering. It depends on the composition and the structure of the biomaterial. As water is composed of polar molecules, the capacity to interact with, and absorb, water molecules is favored by the presence of ionic charges, polar groups, or a combination of the two, in the molecular structure of the biomaterial. The biomaterial should also have an interconnected but rather expandable molecular structure to allow the migration of water molecules into the structure.
Whether a material is electrically conducting or insulting is important to the function of some medical devices. Electrically conducting metals are, for example, important for use as electrodes in pacemakers and neural stimulators. In comparison, electrically insulating materials are typically used as coatings to isolate or insulate sensitive electronic devices. The ability to conduct electrical signals is quantified by the electrical conductivity of the material or, less commonly, by the electrical resistivity, which is the inverse of the electrical conductivity. The capacity of a material to respond to a magnetic field is important for its ability to function in some treatments such as hyperthermia treatment of tumors and in diagnostic imaging.
The use of materials in devices such as contact lenses and intraoptical lenses is crucially dependent on their ability to transmit light (i.e. their transparency). Whether a material can conduct heat or not is quantified by its thermal conductivity. The thermal expansion coefficient quantifies the expansion or contraction of a material upon heating or cooling. These thermal properties are normally important for biomaterials that are subjected to sizable temperature changes during manufacture or use. As some biomaterials, particularly natural materials, can deteriorate when heated, their maximum processing or use temperature can also be important.
1.4 Properties of Materials
Biomaterials, as noted earlier, are designed to have some desirable combination of properties, depending on the application. In the selection of a biomaterial for a specific application, it is worth recognizing at the outset that:
The primary classes of synthetic materials, metals, ceramics and polymers, and natural materials have vastly different intrinsic (or inherent) properties;
Within each class, the measured properties can cover a wide range.
A useful starting point is a qualitative understanding of the intrinsic properties of these different classes of materials. Then, we will present the intrinsic properties of these classes of synthetic materials and compare them with those of some natural materials. In Chapter 2, the basic principles for understanding the intrinsic properties of these classes of materials in terms of their atomic structure are presented.
1.4.1 Intrinsic Properties of Metals
Metals are composed of single elements (such as Ti) or a combination of elements, forming alloys such as brass, which is an alloy of copper and zinc. Most metals show excellent mechanical properties, such as high strength, high stiffness, high ductility, and good fatigue resistance. Strength refers to the ability of a material to support an applied load (or mechanical stress) without breaking. While stiffness refers to the ability to resist deformation when subjected to an applied load, the elastic modulus is a more effective and more widely used measure of a material’s stiffness. Ductility refers to the ability to deform rather than shatter catastrophically, particularly when the applied stress becomes sufficiently high. Fatigue resistance refers to the ability to withstand repeated cyclic loading without fracturing. Most metals generally show moderate hardness and moderate resistance to abrasion or wear, somewhere between ceramics and polymers. The majority of metals have a high density, higher than ceramics and much higher than polymers. The excellent electrical and thermal conductivity of metals is well known.
Except for the noble metals such as gold, silver, and platinum, most pure metals corrode in an aqueous environment, such as the physiological environment. Consequently, most metals cannot be used as implantable biomaterials. On the other hand, a protective oxide surface layer forms rapidly on some metals upon exposure to an oxidizing environment, which passivates them from corrosion. These passivated metals, such as Ti, certain Ti alloys, and stainless steel, have a high resistance to corrosion in the normal physiological environment. Because of their excellent mechanical properties and corrosion resistance, they find considerable use in a variety of orthopedic and dental applications, such as fracture fixation plates, total joint replacement, and dental implants.
Ease of fabrication, as noted earlier, is also an important factor in the selection of a material for use as a biomaterial. Metals can be formed with reasonable ease into 3D objects, coatings and films using conventional fabrication methods that are widely used in the metallurgical industry. Additive manufacturing, also referred to as 3D printing, now provides another method to produce metals with the requisite external shape and microstructure for use as biomaterials.
Overall, metals are normally selected for use as biomaterials when excellent mechanical properties, high electrical conductivity, or a combination of both must be guaranteed. Suitable metals have a high resistance to corrosion in the physiological environment, such as certain noble metals or metals passivated by a protective oxide surface layer.
1.4.2 Intrinsic Properties of Ceramics
Except for carbon, ceramics are compounds of metallic and nonmetallic elements, such as Al2O3 or silicon nitride (Si3N4 ), for example. Although ceramics used in technological or engineering applications typically show better strength and elastic modulus than most metals, a distinctive drawback is their inherent brittleness. Brittleness