Page 353 - Biomedical Engineering and Design Handbook Volume 1, Fundamentals

330 BIOMATERIALS

Cellulose is a partially crystalline polysaccharide and is the chief constituent of plant fiber. Cotton
is the purest natural form of cellulose containing 90 percent cellulose. Cellulose decomposes before
melting and therefore cannot be melt processed. It is insoluble in organics and water and can only
be dissolved in strong basic solutions. Regenerated cellulose, also known as rayon, is cellulose that
has been precipitated from a basic solution. Cellulose is used in bandages and sutures. Cuprophan is
cellulose precipitated from copper hydroxide solutions to form hemodialysis membranes.

Cellulose Acetate. Degradation: bioinert.

CH Ac CH Ac
2
2
O O
H H O
H H
Ac H O Ac H O Ac = O C CH 3
H H
H Ac H Ac n

Cellulose acetate

Cellulose acetate is a modified cellulose that can be melt processed. Cellulose acetate membranes
are used for hemodialysis.

Nylon 6,6. Degradation: slow bioerosion.

O O H H
C (CH ) C N (CH ) N
2 4
2 6
n

Poly(hexamethylene adipimide) is also known as nylon 6,6 since its repeat unit has two 6 carbon
sequences per repeat unit. Nylon is tough, abrasion resistant, and has a low coefficient of friction,
making it a popular suture material. 27 Nylon 6,6 is hydrophilic and absorbs water when placed in
28
tissues or in humid environments (9 to 11 percent water when fully saturated ). Absorbed water acts
as a plasticizer, increasing the ductility and reducing the modulus of nylon 6,6. Nylon bioerodes at
a very slow rate. Nylon 6,6 implanted in dogs lost 25 percent of its tensile strength after 89 days and
83 percent after 725 days. 29

Nylon 6: Poly(caprolactam). Degradation: slow bioerosion.

O H
C (CH ) N
2 5

Nylon 6 has similar properties to nylon 6,6, the primary difference being that nylon 6 has a lower
melting temperature and is more moisture sensitive.