Biopolymers - Polymer Science and Engineering - page 515

material variations, biomedical polymers will not be discussed. Reference [2]
gives more detailed coverage of biomedical polymers.
According to the definition of biopolymers given above, complex carbon-
based macromolecular arrangements such as DNA, RNA, and proteins do not fall
under this definition. Although many refer to these molecules as natural polymers
or biopolymers since they are produced by living organisms, this is not entirely
accurate due to the fact that they contain many different types of repeating units
connected in somewhat complex sequences. Precisely speaking, they are biomac-
romolecules. In biomacromolecules, covalent bonds exist between different types
of amino acids, parallel to the covalent bonds that connect monomer groups
within a polymer. DNA, RNA, and proteins have unique structures. One example
is DNA's double helix which is caused by secondary bonds and interactions such
as hydrogen bonds, ionic bonds, van der Waals forces, and hydrophobic interac-
tions. Due to the strong influence of secondary bonds and interactions on the
structure of these complex macromolecules, the environment plays a significant
role in their conformation, which in turn determines functionality. A slight change
in environmental conditions such as pH or temperature may cause a protein to
change conformation and lose functionality, known as protein denaturing. Some
diseases, like Alzheimer's, can be traced back to changes in protein conformation
which resulted in a loss of function. For further reading on protein conformation
diseases or the fundamentals of biochemistry, consult [2,3] .
There are two major groups of protein structures, fibrous and globular, present
in the body. Fibrous proteins have more regularity in their monomer sequences
than the other type of protein. Some examples of fibrous proteins include keratin
(hair, nails, feathers, horns), collagen (connective tissue), fibroin (silk), and myo-
sin (muscle) [4,5] . The combination of these fibrous proteins into plastics is a
topic of current interest. Other proteins such as soy protein and wheat gluten are
used in bioplastics and biomedical applications [4,6] .
13.2 Natural Polymers
Natural polymers are polymers produced in living organisms and as such are
essential to all life. Some of these natural polymers can be extracted and modified
into plastics for commercial use, known as bio-based polymers or bioplastics.
Extraction and modification of these polymers from living organisms will be dis-
cussed in the next section.
13.2.1 Natural Rubbers
Natural rubber is probably the most widely used natural polymer in modern life.
It is a polyterpene synthesized by enzymatic polymerization of isopentenylpyro-
phosphate [5,7] and the repeat unit structure is isoprene (1-17). Natural rubber
consists of 97% cis-1,4-polyisoprene (1-18) and exists in the natural latex form in