Page 346 - Biomedical Engineering and Design Handbook Volume 1, Fundamentals

BIOPOLYMERS 323

Since cross-linking is chemically mild and easily accomplished, calcium cross-linked alginate is
commonly used for cell immobilization. Cells are immobilized to prevent immune response in vivo
and to prevent cells from traveling from the desired location in vivo. Immobilization is most often
accomplished by adding cells to a sodium alginate solution, followed by dripping the solution into a
calcium chloride solution to cross-link the alginate and entrap cells.

Gelatin. Degradation: biodegradable. Gelatin is a protein prepared by hydrolyzing type I collagen
using aqueous acids or bases. Collagen is discussed further in the section on hydrogels. Hydrolysis
involves disruption of the collagen tertiary triple helix structure and reduction of molecular weight
to yield gelatin that is soluble in warm water. Following hydrolysis, gelatin is purified and dried to
yield a powder. Contrary to the poloxamers, gelatin solutions (>0.5 weight percent) gel with a reduc-
tion in temperature. Gelatin gels melt between 23 and 30°C and gelatin solutions set around 2 to 5°C
lower than the melting point. Gelatin is used as a tablet coating or capsule materials as an enteric
coating to control the release rate of drugs. Gelatin sponges are similar to collagen sponges and are
used as hemostatic agents.

Fibrin. Degradation: biodegradable. Fibrin is the monomer formed from fibrinogen in the blood
when a clot is formed. It is a protein that first polymerizes and then cross-links during clot formation,
and has been isolated and used as a biological adhesive and matrix for tissue engineering. The
gel formation involves mixing fibrinogen with the gelling enzyme (thrombin) and a second calcium-
containing solution. Speed of gellation is controlled by concentrations. Biodegradation occurs fairly
rapidly due to natural enzymatic activity (fibrinolysis) resulting from plasmin in tissue. Fibrin is used
as a soft tissue adhesive and is used in tissue scaffolds.

13.3.3 Hydrogels
Hydrogels are materials that swell when placed in aqueous environments, but maintain their overall
shape. Hydrogels can be formed by cross-linking nearly any water-soluble polymer. Many natural
materials such as collagen and chitosan (derived from chitin) absorb significant amounts of water
and can be considered to be hydrogels. Hydrogels are compliant since the polymer chains have high
mobilities due to the presence of water. Hydrogel mechanical properties are dependent on water
content. Modulus and yield strength decrease with water content, while elongation tends to increase.
Hydrogels are lubricious due to their hydrophilic nature. Hydrogels resist protein absorption and
microbial attack due to their hydrophilicity and dynamic structure.

Poly(hydroxyethyl methacrylate). Degradation: bioinert.

CH 3
CH 2 C
n
C O
O CH CH OH
2
2
Poly(hydroxyethyl methacrylate) (PHEMA) is a hydrogel generally cross-linked with ethylene glycol
dimethacrylate (which is normally present as a contaminant in the monomer). PHEMA’s hydrogel
properties such as resistance to protein adsorption and lubricity make it an ideal material for contact
lenses. Hydrated PHEMA gels have good oxygen permeability, which is necessary for the health of
the cornea. PHEMA is copolymerized with polyacrylic acid (PAA) or poly(N-vinyl pyrrolidinone)
to increase its water absorbing capability.