Mohamed N. Rahaman - Materials for Biomedical Engineering

Polymers are generally composed of long‐chain molecules formed by repeated bonding of a large number of small molecules. The simplest example is polyethylene (abbreviated PE) whose molecular chains consist a large number (several hundred to several thousand) of ethylene (H 2C=CH 2) molecules bonded together. Polymers show low strength and low elastic modulus. In contrast to metals and ceramics, they show a time‐dependent mechanical response. This means that the measured mechanical properties of polymers, such as strength and elastic modulus, are dependent on the duration or rate of the mechanical testing procedure, or on the temperature at which the test is performed ( Chapter 4). A given polymer, such as PMMA, for example, can show a range of mechanical behavior, from ductile to brittle, depending on the rate or the temperature of the mechanical test. Unless they are brittle, polymers typically show good fatigue resistance. Due to their low hardness, the resistance of polymers to abrasive wear is low.

Polymers generally have low density (~1 g/cm 3), low electrical conductivity, and low thermal conductivity. A clear advantage of polymers over metals and ceramics is their ease of fabrication. Polymers can be easily formed into the requisite shape and microstructure using conventional processing or additive manufacturing (3D printing) methods. Another advantage of polymers is their compositional flexibility. Polymers can not only be synthesized with the requisite composition but their composition can also be easily modified to achieve more desirable properties, such as degradation rate.

Overall, the human body, except for bone and teeth, is composed of soft tissues (and organs). When compared to metals and ceramics, polymers can be more easily designed to approximate the structure and properties of these soft tissues. Consequently, polymers find considerable use as biomaterials. Because of the ease in synthesizing compositions with a controllable degradation rate, polymers also find considerable use as biomaterials for drug delivery.

Composites are composed of two or more physically distinct materials or phases ( Chapter 12). Synthetic composites used as biomaterials are composed of one or more of the primary classes of materials (metals, ceramics, and polymers), and consist of a continuous phase (the matrix) and a dispersed phase (the reinforcing phase). While composites are abundant in nature, synthetic composites find only limited use as biomaterials. A well‐known example of a natural composite is bone, produced by embedded cells and composed of an inorganic (ceramic) phase of fine lath‐like particles of composition approximating that of hydroxyapatite and an organic phase composed of collagen.

Synthetic composites are often of interest when a single material cannot provide the desired combination of properties. For example, polymers have an advantage of ease of fabrication but, because of their weak mechanical properties, they are not suitable as implants to heal defects in structural bone that have to support a significant physiological stress. To better approximate the mechanical properties of bone, polymers can be reinforced with a strong material, such as a ceramic in the form of particles or fibers. Use of particles composed of hydroxyapatite or bioactive glass can also enhance the functionality of the polymer matrix, such as its bioactivity.

As ceramics are brittle, they are sometimes reinforced with another phase, typically strong ceramic particles or fibers, to improve their resistance to fracture, a property quantified by a measurable parameter called the fracture toughness. For example, zirconium oxide (ZrO 2) is incorporated into alumina (Al 2O 3) to form a composite, referred to as zirconia‐toughened alumina (ZTA), an improved form of which is called alumina matrix composite (AMC). Because of its better fracture resistance (higher fracture toughness) and better wear resistance, AMC has now replaced Al 2O 3as a ceramic femoral head material in hip implants.

Charts provide a succinct way to show a direct comparison of material properties. As a mechanical function is often a major consideration in many biomedical applications, charts that compare mechanical properties can be useful at the outset of materials selection and design. Several types of mechanical properties are available and, depending on the application, some properties can be more important than others. Figure 1.5shows one type of material property chart, which depicts the measured strength versus elastic modulus for the primary classes of synthetic materials and for selected natural materials. The properties of the synthetic materials shown in Figure 1.5(and in most mechanical property charts) are those for the dense materials, that is, the materials have zero or near zero porosity. As the presence of porosity lowers the mechanical properties of a material, one approach to modifying the properties of strong synthetic materials in order to achieve more optimal properties for use as biomaterials is to incorporate a controlled amount of porosity into them. In comparison, incorporation of a strong phase into polymers to form a composite provides a way to improve their mechanical properties.

Figure 1.5 Strength versus elastic modulus for the three major classes of synthetic materials used as biomaterials (ceramics, metals, and polymers). The range shown for each class of material is approximate and for the dense (or almost dense) material (zero or close to zero porosity). The presence of porosity in these materials will lead to a reduction in these and other mechanical properties. The properties of some human tissues are shown for comparison.

Modern implants for total hip replacement ( Figure 1.1b) provide a useful example of the design and selection of biomaterials. While these implants do not reflect the aforementioned trend in which biological sciences are playing a more significant role in biomaterials design, they take into account some advances, such as the response of cells and tissues to implanted biomaterials. Although the hip implant has not changed considerably in its overall geometry since Sir John Charnley first implanted it into patients around 1960, the incorporation of advances in materials science, engineering, and biological principles into materials selection has resulted in the creation of longer‐lasting implants having better performance. Currently, approximately half a million total hip joint replacement surgeries are performed annually in the United States and elsewhere in the world, which provide an improved quality of life for this significant number of people.

The human hip joint consists essentially of a femoral head with an approximately spherical surface, which articulates within a socket in the acetabular bone ( Figure 1.6a). A prosthetic hip implant attempts to reproduce these geometrical features ( Figure 1.6b). It consists of three major parts, a femoral stem, a femoral head (also called a ball), and an acetabular cup (socket) composed of a liner and a shell ( Figure 1.7). The stem is inserted into the femur to provide stability for the femoral head, which articulates in the liner of the acetabular cup that is secured to the pelvic bone. Important factors in materials design and selection for these three components of the hip implant are discussed in the following sections.