Page 332 - Biomedical Engineering and Design Handbook Volume 1, Fundamentals

CHAPTER 13

BIOPOLYMERS

Christopher Batich
University of Florida, Gainesville, Florida
Patrick Leamy
LifeCell Corporation, Branchburg, New Jersey

13.1 INTRODUCTION 309
13.2 POLYMER SCIENCE 310
13.3 SPECIFIC POLYMERS 319
13.4 A NOTE ON TISSUE ENGINEERING
APPLICATIONS 336
REFERENCES 337

13.1 INTRODUCTION

Polymers are large molecules synthesized from smaller molecules, called monomers. Most polymers
are organic compounds with carbon as the base element. Plastics are polymers that are rigid solids
at room temperature and generally contain additional additives. Some common plastics used in
biomedical applications are polymethyl methacrylate for intraocular lenses, braided polyethylene
terephthalate for vascular grafts, and ultrahigh-molecular-weight polyethylene for the articulating
surfaces of orthopedic implants. Polymers, and biopolymers in particular, encompass a much broader
spectrum than plastics alone. Biopolymers include synthetic polymers and natural polymers such as
proteins, polysaccharides, and polynucleotides. This chapter covers only the most commonly used
examples in each class, but will provide references to more specific sources.
Many useful polymers are water soluble and used as solutions. Hyaluronic acid is a naturally
occurring high-molecular-weight polymer found in connective tissues and is used to protect the iris
and cornea during ophthalmic surgery. Polyvinyl pyrrolidinone is a synthetic polymer used as a
1
binder or additive in 25 percent of all pharmaceuticals. Hydrogels are another class of polymer that
has many biomedical applications. Hydrogels are polymers that swell in water but retain their overall
shape. They are therefore soft and moist, and mimic many natural tissues. The most well-known
hydrogel series is poly(hydroxyethyl methacrylate) (PHEMA) and PHEMA copolymers which are
used in soft contact lenses.
Gelling polymers are hydrogels that can be formed in situ using chemical or physical bonding of
polymers in solution. Alginates, for instance, are acidic polysaccharides that can be cross-linked
using divalent cations such as calcium. Other examples of gelling polymers are the poloxamers that
can gel with an increase in temperature. Alginates are widely used in cell immobilization, and polox-
amers as coatings to prevent postsurgical adhesions.
Elastomers are low-modulus polymers that can reversibly deform up to many times (some over
500 percent) their original size. Silicones and polyurethanes are common elastomeric biopolymers.

309