© 2003 ASM International. All Rights Reserved. www.asminternational.org Handbook of Materials for Medical Devices (#06974G) CHAPTER 1 Overview of Biomaterials and Their Use in Medical Devices A BIOMATERIAL, as defined in this hand- shapes, have relatively low cost, and be readily book, is any synthetic material that is used to available. replace or restore function to a body tissue and Figure 1 lists the various material require- is continuously or intermittently in contact with ments that must be met for successful total joint body fluids (Ref 1). This definition is somewhat replacement. The ideal material or material restrictive, because it excludes materials used combination should exhibit the following prop- for devices such as surgical or dental instru- erties: ments. Although these instruments are exposed A biocompatible chemical composition to to body fluids, they do not replace or augment • avoid adverse tissue reactions the function of human tissue. It should be noted, Excellent resistance to degradation (e.g., cor- however, that materials for surgical instru- • rosion resistance for metals or resistance to ments, particularly stainless steels, are reviewed biological degradation in polymers) briefly in Chapter 3, “Metallic Materials,” in Acceptable strength to sustain cyclic loading this handbook. Similarly, stainless steels and • endured by the joint shape memory alloys used for dental/endodon- A low modulus to minimize bone resorption tic instruments are discussed in Chapter 10, • High wear resistance to minimize wear- “Biomaterials for Dental Applications.” • debris generation Also excluded from the aforementioned defi- nition are materials that are used for external prostheses, such as artificial limbs or devices Uses for Biomaterials (Ref 3) such as hearing aids. These materials are not exposed to body fluids. One of the primary reasons that biomaterials Exposure to body fluids usually implies that are used is to physically replace hard or soft tis- the biomaterial is placed within the interior of sues that have become damaged or destroyed the body, and this places several strict restric- through some pathological process (Ref 3). tions on materials that can be used as a bioma- Although the tissues and structures of the body terial (Ref 1). First and foremost, a biomaterial perform for an extended period of time in most must be biocompatible—it should not elicit an people, they do suffer from a variety of destruc- adverse response from the body, and vice versa. tive processes, including fracture, infection, and Additionally, it should be nontoxic and noncar- cancer that cause pain, disfigurement, or loss of cinogenic. These requirements eliminate many function. Under these circumstances, it may be engineering materials that are available. Next, possible to remove the diseased tissue and the biomaterial should possess adequate physi- replace it with some suitable synthetic material. cal and mechanical properties to serve as aug- Orthopedics. One of the most prominent mentation or replacement of body tissues. For application areas for biomaterials is for orthope- practical use, a biomaterial should be amenable dic implant devices. Both osteoarthritis and to being formed or machined into different rheumatoid arthritis affect the structure of freely © 2003 ASM International. All Rights Reserved. www.asminternational.org Handbook of Materials for Medical Devices (#06974G) 2 / Handbook of Materials for Medical Devices movable (synovial) joints, such as the hip, knee, with artificial arteries. As shown in Table 1, shoulder, ankle, and elbow (Fig. 2). The pain in polymers are the material of choice for vascular such joints, particularly weight-bearing joints prostheses (see Chapter 7, “Polymeric Materi- such as the hip and knee, can be considerable, als,” in this handbook for further details). and the effects on ambulatory function quite Ophthalmics. The tissues of the eye can devastating. It has been possible to replace these suffer from several diseases, leading to reduced joints with prostheses since the advent of anes- vision and eventually, blindness. Cataracts, for thesia, antisepsis, and antibiotics, and the relief example, cause cloudiness of the lens. This may of pain and restoration of mobility is well be replaced with a synthetic (polymer) intraoc- known to hundreds of thousands of patients. ular lens (Table 1). Materials for contact lenses, The use of biomaterials for orthopedic because they are in intimate contact with the tis- implant devices is one of the major focal points sues of the eye, are also considered biomateri- of this handbook. In fact, Chapters 2 through 7 als. As with intraocular lenses, they too are used and Chapter 9 (refer to Table of Contents) all to preserve and restore vision (see Chapter 7, deal with the materials and performance associ- “Polymeric Materials,” in this handbook for ated with orthopedic implants. As shown in details). Table 1, a variety of metals, polymers, and Dental Applications. Within the mouth, ceramics are used for such applications. both the tooth and supporting gum tissues can Cardiovascular Applications. In the car- be readily destroyed by bacterially controlled diovascular, or circulatory, system (the heart diseases. Dental caries (cavities), the demineral- and blood vessels involved in circulating blood ization and dissolution of teeth associated with throughout the body), problems can arise with the metabolic activity in plaque (a film of mucus heart valves and arteries, both of which can be that traps bacteria on the surface of the teeth), successfully treated with implants. The heart can cause extensive tooth loss. Teeth in their valves suffer from structural changes that pre- entirety and segments of teeth both can be vent the valve from either fully opening or fully replaced or restored by a variety of materials closing, and the diseased valve can be replaced (Table 1). A thorough review of these materials with a variety of substitutes. As with orthopedic can be found in Chapter 10, “Biomaterials for implants, ceramics (carbons, as described in Dental Applications,” in this handbook. Chapter 6, “Ceramic Materials,” in this hand- Wound Healing. One of the oldest uses of book), metals, and polymers are used as materi- implantable biomaterials can be traced back to als of construction (Table 1). the introduction of sutures for wound closure. Arteries, particularly the coronary arteries The ancient Egyptians used linen as a suture as and the vessels of the lower limbs, become far back as 2000 b.c. Synthetic suture materials blocked by fatty deposits (atherosclerosis), and include both polymers (the most widely syn- it is possible in some cases to replace segments thetic suture material) and some metals (e.g., a. b. c. Fig. 1 Implant material requirements in orthopedic applications. Source: Ref 2 © 2003 ASM International. All Rights Reserved. www.asminternational.org Handbook of Materials for Medical Devices (#06974G) Chapter 1: Overview of Biomaterials and Their Use in Medical Devices / 3 stainless steels and tantalum). Chapter 7, “Poly- Another important wound-healing category meric Materials,” in this handbook discusses the is that of fracture fixation devices. These inc- characteristics and properties of synthetic suture lude bone plates, screws, nails, rods, wires, and materials. other devices used for fracture treatment. Although some nonmetallic materials (e.g., car- bon-carbon composite bone plates) have been investigated, almost all fracture fixation devices used for orthopedic applications are made from Table 1 Examples of medical and dental metals, most notably stainless steels (see Chap- materials and their applications ter 3, “Metallic Materials,” in this handbook for Material Principal applications details). Drug-Delivery Systems. One of the fastest Metals and alloys growing areas for implant applications is for 316L stainless steel Fracture fixation, stents, surgical devices for controlled and targeted delivery of instruments CP-Ti, Ti-Al-V, Ti-Al-Nb, Ti- Bone and joint replacement, drugs. Many attempts have been made to incor- 13Nb-13Zr, Ti-Mo-Zr-Fe fracture fixation, dental porate drug reservoirs into implantable devices implants, pacemaker for a sustained and preferably controlled re- encapsulation Co-Cr-Mo, Cr-Ni-Cr-Mo Bone and joint replacement, lease. Some of these technologies use new poly- dental implants, dental meric materials as vehicles for drug delivery. restorations, heart valves Chapters 7, “Polymeric Materials,” and 9, Ni-Ti Bone plates, stents, orthodontic wires “Coatings,” in this handbook describe these Gold alloys Dental restorations materials. Silver products Antibacterial agents Platinum and Pt-Ir Electrodes Hg-Ag-Sn amalgam Dental restorations Types of Biomaterials (Ref 1) Ceramics and glasses Alumina Joint replacement, dental Most synthetic biomaterials used for implants implants Zirconia Joint replacement are common materials familiar to the average Calcium phosphates Bone repair and augmentation, materials engineer or scientist (Table 1). In gen- surface coatings on metals eral, these materials can be divided into the fol- Bioactive glasses Bone replacement Porcelain Dental restorations lowing categories: metals, polymers, ceramics, Carbons Heart valves, percutaneous and composites. devices, dental implants Polymers Polyethylene Joint replacement Polypropylene Sutures PET Sutures, vascular prosthesis Polyamides Sutures PTFE Soft-tissue augmentation, vascular prostheses Polyesters Vascular prostheses, drug- delivery systems Polyurethanes