Materials for Biomedical Engineering. Mohamed N. Rahaman
Figure 1.9 Illustrative summary of the evolution of biomaterials classified in terms of historical periods.
Prior to World War II
A prosthetic toe formed from wood and leather (both naturally available materials), has been found in an Egyptian mummy dating back to around 1000 BC, while one of the earliest prosthetic arms, composed of iron, is reported to have been used by a Roman general around 200 BC. Archeologists have reported the use of linen sutures by the early Egyptians and catgut sutures in the Middle Ages in Europe. Artificial eyeballs made from gold several centuries ago to replace eyeballs lost as a result of disease or injury evolved into the use of glass by the Venetians in the sixteenth century and, later, in other parts of Europe. Prosthetic teeth, made from gold or carved from dog or calf teeth, were anchored with gold wire or gold plate to form dental restorations which have been found in tombs from the Etruscan civilization dating back to 300–800 BC. Dental implants composed of iron are reported to have been found in a human skeleton dating back to approximately 200 BC in France.
A Few Decades After World War II
With the introduction of plastics (synthetic polymers) in the early twentieth century and, as materials designed for use in World War II became more widely available, the first few decades following the World War II saw a rapid growth in the use of synthetic materials as biomaterials. Based on observations showing that fragments of PMMA from shattered canopies of fighter planes in World War II were well tolerated in the eyes of pilots, intraocular lenses were created and implanted in humans by Sir Harold Ridley around 1949. Synthetic arteries (referred to as vascular grafts) made from parachute fabric (silk or nylon), were implanted in a patient by Arthur Voorhees in 1952 to replace a ruptured aneurysm. Sir John Charnley pioneered the hip implant, composed of synthetic metals and polymers, in the late 1950s. An improved version of the implant, composed of stainless steel and high molecular weight PE, showed favorable success rates when implanted in patients the early 1960s. Improved versions of intraocular lenses, vascular grafts, and hip implants, along with several devices created from these chemically inert biomaterials are now implanted in millions of patients.
Contemporary Period
The last 30–40 years have seen a rapid advance in the development and application of biomaterials. Prior to this period, biomaterials were selected from mechanically durable and chemically stable materials that were available off the shelf. These biomaterials were designed to serve a mainly mechanical (or physical) function. The last few decades have seen a significant shift in emphasis in the design of biomaterials (Section 1.3). Whereas materials science played a dominant role in biomaterials design and selection previously, this contemporary period is marked by biological sciences playing a role of significance comparable to that of materials science. Advances in biological sciences are utilized in designing biomaterials that serve not just a mechanical function but also a biological function.
Contemporary biomaterials are degradable or bioactive. Instead of remaining in their original chemically inert form in the body, implants are now being designed to degrade at a controllable rate, be replaced by newly forming tissue, and eventually disappear completely. Additionally, degradable implants can deliver ions and biomolecules to target specific cells and tissues in vivo, to control the behavior of cells, or to stimulate cells to create new tissues and organs in the body. In this way, these biomaterials can stimulate the body to heal itself.
Examples of contemporary biomaterials include composites composed of polylactic acid and β‐tricalcium phosphate for fracture fixation (Figure 1.1j) and calcium phosphate cements that set in situ for healing bone defects. The incorporation of a drug‐eluting function in a stent is now becoming common to reduce the tendency for endothelial proliferation and the adherence and clumping of blood cells, thereby improving the long‐term efficacy of the stent in treating vascular disease (Section 1.3.1).
Biomaterials are an important component in some tissue engineering (or regenerative medicine) approaches to create functional tissues and organs. At the same time, advances in science and technology are being used to design these biomaterials with greater complexity in structural and chemical characteristics to better mimic the structure of biological tissues. In addition to degradable or bioactive solids, water‐filled polymers known as hydrogels, synthetic, or natural, find considerable use in tissue engineering approaches, particularly for creating soft tissues. The stiffness of hydrogels and their structural characteristics can be modified to better mimic those of soft tissues. Functional skin substitutes for healing burns are already available commercially, while there are considerable research and development efforts to create bone, cartilage, liver, and pancreas for implantation in the body.
In addition to medical applications, contemporary biomaterials are playing a significant role in the pharmaceutical industry. In the form of 3D objects or particles of size ~1 μm to a few hundred microns (referred to as microparticles), degradable biomaterials have been used for several decades as devices to deliver drugs in a controlled manner to a particular site in the body. Hydrogels that respond to environmental conditions such as pH or temperature are another type of biomaterials that find use as carriers in drug delivery. Emphasis has been shifting in the last couple of decades toward more precise drug targeting in which the drug molecules are targeted specifically to the cells or tissues where they will exert their desired effect. This is achieved by combining the drug molecules with a suitable biomaterial to which is added a cell‐specific antibody or other targeting molecule that has an affinity for the targeted cells or tissues. Increasingly, the biomaterial in these applications is in the form of fine particles, of size smaller than 50–100 nm, referred to as nanoparticles. The combination of chemotherapy drugs and nanoparticles is under development as an alternative technique to radiotherapy or chemotherapy to treat cancer cells. Techniques that rely on the magnetic or optical properties of specially designed nanoparticles are also under development for treating tumors.
Contemporary biomaterials are now being designed and developed for use in diagnostic imaging of cells and tissues. One area of particular interest is the detection and real‐time imaging of cancer cells. Here again, biomaterials in the form of nanoparticles, particularly nanoparticles that become highly fluorescent when irradiated with light, are playing a significant role. Nanoparticles that simultaneously combine a drug delivery function and a diagnostic imaging function are under development to provide a technique for real‐time diagnosis and treatment of cancer.
1.7 Biomaterials – An Interdisciplinary Field
The significant advances made in the development of contemporary biomaterials, described in the previous section, would not have been possible without the collaboration among researchers from a variety of fields and this collaboration will grow in future. Scientists, engineers, and clinicians working within their respective fields do not possess the range of skills and expertise that are often required to design, create, and evaluate new or improved biomaterials with better performance and functionality for use in medicine and dentistry. The field of biomaterials is a truly interdisciplinary (or multidisciplinary) field that encompasses collaborations from more than one of the following fields: materials science, biological sciences, chemistry, biomedical engineering (or bioengineering), traditional engineering fields such as chemical engineering, mechanical engineering, and electrical engineering, biochemistry, and the medical and dental fields.
1.8 Concluding Remarks
In this chapter, we presented an introductory overview of biomaterials and the biomaterials field. Materials have been used to repair the human body since antiquity, but their use and degree of sophistication have increased significantly over time, particularly