Mohamed N. Rahaman - Materials for Biomedical Engineering

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.

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.

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.

Except for carbon, ceramics are compounds of metallic and nonmetallic elements, such as Al 2O 3or silicon nitride (Si 3N 4), 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 refers to the catastrophic shattering of an object into two or more pieces under sufficiently high mechanical stresses. Despite their brittleness, ceramics can be engineered to function safely and reliably over long durations under high stresses if these stresses are compressive in nature. Ceramics are not recommended for use in high‐stress applications when the applied stress has an appreciable tensile or bending component. In general, ceramics show high hardness and wear resistance.

The density of ceramics is lower than that of most metals, often by a factor of two or more, which means that for the same geometry, a ceramic implant will not be as heavy as a metal implant. Ceramics are generally poor conductors of electricity and heat, and those ceramics used in technological applications normally have a high degree of chemical inertness. On the other hand, certain compositions, such as hydroxyapatite and β‐tricalcium phosphate, composed of the same ions as the mineral constituent of bone, show some reactivity in the physiological environment.

Ceramics are generally more difficult to fabricate than metals and far more difficult to fabricate than polymers. Forming ceramics into useful objects having the requisite external shape and microstructure often requires high fabrication temperatures, approximately several hundred degrees Celsius. Due to their high hardness, ceramics are expensive and difficult to machine into the desired shape and surface smoothness after their fabrication.

Overall, major limitations of ceramics are their brittleness, and the difficulty and cost of fabricating them into useful objects. On the other hand, ceramics have high compressive strength, stiffness, hardness, and wear resistance. Consequently, ceramics generally find use in applications where the applied stress is much lower than their strength or mainly compressive in nature, and where high wear resistance and chemical inertness are required.