BUDDY D. RATNER, ALLAN S. HOFFMAN, FREDERICK J. SCHOEN, JACK E. LEMONS

BIOMATERIALS AND BIOMATERIALS SCIENCE

Biomaterials Science: An Introduction to Materials in Medicine addresses the properties and applications of materials (synthetic and natural) that are used in contact with biological systems. These materials are commonly called biomaterials. Biomaterials, an exciting field with steady, strong growth over its approximately half century of existence, encompasses aspects of medicine, biology, chemistry, and materials science. It sits on a foundation of engineering principles. There is also a compelling human side to the therapeutic and diagnostic application of biomaterials. This textbook aims to (1) introduce these diverse elements, particularly focusing on their interrelationships rather than differences and (2) systematize the subject into a cohesive curriculum.

We title this textbook Biomaterials Science: An Introduction to Materials in Medicine to reflect, first, that the book highlights the scientific and engineering fundamentals behind biomaterials and their applications, and second, that this volume contains sufficient background material to guide the reader to a fair appreciation of the field of biomaterials. Furthermore, every chapter in this textbook can serve as a portal to an extensive contemporary literature. The magnitude of the biomaterials endeavor, its interdisciplinary scope, and examples of biomaterials applications will be revealed in this introductory chapter and throughout the book.

Although biomaterials are primarily used for medical applications (the focus of this text), they are also used to grow cells in culture, to assay for blood proteins in the clinical laboratory, in equipment for processing biomolecules for biotechnological applications, for implants to regulate fertility in cattle, in diagnostic gene arrays, in the aquaculture of oysters, and for investigational cell-silicon “biochips.” How do we reconcile these diverse uses of materials into one field? The common thread is the interaction between biological systems and synthetic or modified natural materials.

In medical applications, biomaterials are rarely used as isolated materials but are more commonly integrated into devices or implants. Although this is a text on materials, it will quickly become apparent that the subject cannot be explored without also considering biomedical devices and the biological response to them. Indeed, both the effect of the materials/device on the recipient and that of the host tissues on the device can lead to device failure. Furthermore, a biomaterial must always be considered in the context of its final fabricated, sterilized form. For example, when a polyurethane elastomer is cast from a solvent onto a mold to form the pump bladder of a heart assist device, it can elicit different blood reactions than when injection molding is used to form the same device. A hemodialysis system serving as an artificial kidney requires materials that must function in contact with a patient’s blood and also exhibit appropriate membrane permeability and mass transport characteristics. It also must employ mechanical and electronic systems to pump blood and control flow rates.

Because of space limitations and the materials focus of this work, many aspects of device design are not addressed in this book. Consider the example of the hemodialysis system. The focus here is on membrane materials and their biocompatibility; there is little coverage of mass transport through membranes, the burst strength of membranes, flow systems, and monitoring electronics. Other books and articles cover these topics in detail.

The words “biomaterial” and “biocompatibility” have already been used in this introduction without formal definition. A few definitions and descriptions are in order and will be expanded upon in this and subsequent chapters.

A definition of “biomaterial” endorsed by a consensus of experts in the field, is:

If the word “medical” is removed, this definition becomes broader and can encompass the wide range of applications suggested above.

If the word “nonviable” is removed, the definition becomes even more general and can address many new tissue-engineering and hybrid artificial organ applications where living cells are used.

“Biomaterials science” is the physical and biological study of materials and their interaction with the biological environment. Traditionally, the most intense development and investigation have been directed toward biomaterials synthesis, optimization, characterization, testing, and the biology of host–material interactions. Most biomaterials introduce a nonspecific, stereotyped biological reaction. Considerable current effort is directed toward the development of engineered surfaces that could elicit rapid and highly precise reactions with cells and proteins, tailored to a specific application.

Indeed, a complementary definition essential for understanding the goal (i.e., specific end applications) of biomaterials science is that of “biocompatibility.”

Examples of “appropriate host responses” include the resistance to blood clotting, resistance to bacterial colonization, and normal, uncomplicated healing. Examples of specific applications include a hemodialysis membrane, a urinary catheter, or a hip-joint replacement prosthesis. Note that the hemodialysis membrane might be in contact with the patient’s blood for 3 hours, the catheter may be inserted for a week, and the hip joint may be in place for the life of the patient.

This general concept of biocompatilility has been extended recently in the broad approach called “tissue engineering” in which in-vitro and in-vivo pathophysiological processes are harnessed by careful selection of cells, materials, and metabolic and biomechanical conditions to regenerate functional tissues.

Thus, in these definitions and discussion, we are introduced to considerations that set biomaterials apart from most materials explored in materials science. Table 1 lists a few applications for synthetic materials in the body. It includes many materials that are often classified as “biomaterials.” Note that metals, ceramics, polymers, glasses, carbons, and composite materials are listed. Such materials are used as molded or machined parts, coatings, fibers, films, foams and fabrics. Table 2 presents estimates of the numbers of medical devices containing biomaterials that are implanted in humans each year and the size of the commercial market for biomaterials and medical devices.

TABLE 1 Some Applications of Synthetic Materials and Modified Natural Materials in Medicine

TABLE 2 The Biomaterials and Healthcare Market—Facts and Figures (per year) (U.S....