Biomedical Engineering Reference
In-Depth Information
the optimal role of biomaterials in tissue regeneration that continue to motivate biomate-
rials research and new product development.
Over most of history, minimal understanding of the biological mechanisms of tissues
meant that the biomedical engineering approach was to completely replace the damaged
body part with a prosthetic—a simple, nonbiologically active piece of hardware. As our
understanding of developmental biology, disease, and healthy tissue structure and function
improved, the concept of attempting to repair damaged tissues emerged. More recently, with
the advent of stem cell research, the medical field believes it will be possible to regenerate
damaged or diseased tissues by cell-based tissue engineering approaches (see Chapter 6).
The notion of a biomaterial has evolved over time in step with changing medical concepts.
Williams in 1987 defined a biomaterial as “a nonviable material used in a medical device,
intended to interact with biological systems.” This definition still holds true today and encom-
passes the earliest use of biomaterials for replacing form (e.g., wooden leg, glass eye), as well
as the current use of biomaterials in regenerative medical devices such as a biodegradable
scaffold used to deliver cells for tissue engineering. While the definition has remained the
same, there have been dramatic changes in understanding of the level of interaction of bio-
materials with the biological system (in this case, the human body). The expectations for
biomaterial function have advanced from remaining relatively inert in the body, to being “bio-
active” and not blocking regeneration, to providing biological cues that initiate and guide
regeneration. Now there are biomaterials that can initiate a biological response after implanta-
tion such as cell adhesion, proliferation, or more excitingly, the differentiation of a stem cell
that may one day lead to regeneration of a whole organ.
Due to the complexity of cell and tissue reactions to biomaterials, it has proven advanta-
geous to look to nature for guidance on biomaterials design, selection, synthesis, and fabri-
cation. This approach is known as biomimetics. Within the discipline of biomaterials,
biomimetics involves imitating aspects of natural materials or living tissues such as their
chemistry, microstructure, or fabrication method. This does not always lead to the desired
outcome, since many of the functionalities of natural tissues are as yet unknown. Further-
more, the desirable or optimal properties of a biomaterial vary enormously, depending
on where they will be used in the body. Therefore, in addition to presenting general strate-
gies for guiding tissue repair by varying the chemistry, structure, and properties of bioma-
terials, this chapter includes application-specific biomaterials solutions for several of the
major organ systems in the body and for drug delivery applications. This chapter also
includes a section on the regulatory approval process and the testing required that play
an essential role in establishing and ensuring the safety and efficacy of medical products.
5.2 BIOMATERIALS: TYPES, PROPERTIES,
AND THEIR APPLICATIONS
There is a wide choice of possible biomaterials to use for any given biomedical applica-
tion. The engineer must begin by selecting which general class of material to use. The four
basic classes or types of materials are metals, ceramics/glasses, polymers, and composites,
which are mixtures of any of the first three types of materials. Natural materials such as
animal heart valves are made of proteins that have a repeating polymeric-type structure
