Biomedical Engineering Reference
In-Depth Information
desirable environmental characteristics of biopolymers, they are rapidly finding use in
several nonmedical niche markets such as biodegradable monofilament fishing nets. The
more rapid and widespread introduction of biopolymers has been hindered by their high
price (up to ten times) compared to petroleum-based polymers.
Biopolymers also can be produced by chemically polymerizing naturally occurring mono-
mers. Although these polymers are not produced by biological systems, the fact that they are
derived from basic biological building blocks makes them biocompatible, nontoxic, and bio-
degradable. Lactic acid-based polymers (e.g. polyesters such as poly-L-lactic acid or poly-
L-glycolic acid) have been used widely for many years for medical devices ranging from
biodegradable sutures to tissue engineering scaffolds. Lactic acid is found in blood and mus-
cle tissue and is produced commercially by microbial fermentation of sugars such as glucose
or hexose. Poly-L-lactides are frequently used in combination with poly-L-glycolic acid.
EXAMPLE PROBLEM 5.5
What materials are preferred by surgeons for repairing large surgical or traumatic defects in
bone? What factors influence this decision? Search the Internet for companies that make bone
repair products. Go to the website of the American Academy of Orthopaedic Surgeons and the
American Society of Plastic Surgeons for answers to these questions.
Solution
Autograft bone is the first choice for surgeons for the repair of bony defects. The tissues are
vital and contain living cells and growth factors that are required for bone regeneration. Allograft
bone or demineralized allograft bone matrix is the second choice. Demineralized bone has advan-
tages over as-harvested allograft bone because it is flexible and can conform to the defect site,
resorbs more rapidly (within months as compared to years for nondemineralized allograft bone),
and releases the bone inductive proteins known as bone morphogenetic growth factors originally
discovered by Dr. M. R. Urist in the 1970s.
5.3 LESSONS FROM NATURE ON BIOMATERIAL
DESIGN AND SELECTION
Biomedical engineers are asked to design medical devices or systems that repair, moni-
tor, or assist the functions of the human body. Approaches that mimic or replicate nature's
techniques, known as biomimetics, are often at the heart of a successful medical device or
therapy. There is an incredible complexity to natural tissues and organs that is still far
beyond the capacity of scientists to replicate. Furthermore, the precise function of every
aspect of the tissues or organs is not known. For these reasons, it is very difficult to theoret-
ically design medical devices, and the field has progressed through a fair amount of trial
and error, using materials not synthesized specifically for a biomedical application. None-
theless, there are several general concepts that have emerged that provide design strategies
and guidance for a biomaterials scientist involved in tissue/organ repair and regeneration,
and this includes biomimetics.
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