Biomedical Engineering Reference
In-Depth Information
Hydrogels are water-swollen, cross-linked polymeric structures that have received sig-
nificant attention because of their many applications in biomedical applications. Hydrogels
are prepared by cross-linking the polymer chains while in an aqueous solution by irradia-
tion or chemical methods to create a water-filled polymer network. The most widely used
hydrogel is cross-linked polyhydroxyethylmethacrylate (PHEMA). The PHEMA structure
has a water content similar to living tissue, has resistance to degradation, is not absorbed by
the body, withstands heat sterilization without damage, and can be prepared in a variety of
shapes and forms. Applications of hydrogels include contact lenses, drug delivery vehicles,
wound healing adhesives, sexual organ reconstruction materials, artificial kidney membranes,
and vocal cord replacement materials. The amount of cross-linking within hydrogels is a criti-
cal parameter that affects swelling ratios and mechanical strength.
Quite a variety of techniques are employed in forming polymer medical devices. The
technique depends on several factors such as whether the material is thermosetting or ther-
moplastic, and, if thermoplastic, the temperature at which it softens. Thermosetting poly-
mers must be prepared as a liquid linear polymer and then cured in a mold. They cannot
be molded after this step. Thermoplastic polymers can be molded repeatedly (by compres-
sion, injection, extrusion, etc.), cast, and formed into fibers or films by extrusion followed
by drawing or rolling to improve properties such as strength. Later in this chapter many
methods will be described for forming three-dimensional porous structures from resorbable
polymers that are suitable for tissue engineering applications. The ability to form complex
shapes and control the structure at many length scales, as well as the fact they are degrad-
able, make polymers the number one biomaterial type of choice for tissue engineering.
EXAMPLE PROBLEM 5.3
What material is preferred to produce a blood bag? A dialysis bag? What design parameters
are involved?
Solution
PVC has been used for blood bags since the 1950s. Since PVC is naturally brittle, phthalate
plasticizers are used to make it flexible. These leach out over time from the plastic bag and into
the liquid that they contain. When fed in large quantities to rats, the plasticizers can cause cancer;
therefore, other plastics are being investigated. Dialysis bags are made of low-density polyethyl-
ene (LDPE). In these examples, materials selection has been governed by the fact that the material
must be flexible, chemically stable, and relatively inert.
5.2.4 Composites
Composite materials consist of two or more distinct parts. Although a pure material may
have distinct structural subunits such as grains or molecules, the term
is reserved
for materials consisting of two of more chemically distinct constituents that are separated
by a distinct interface. Examples of composites used in biomedical applications include
carbon fiber-reinforced polyethylene and hydroxyapatite particle-reinforced polylactic acid
composite
