Page 348 - Biomedical Engineering and Design Handbook Volume 1, Fundamentals

BIOPOLYMERS 325

TABLE 13.2 Medical Applications of Collagen
Specialty Application
Cardiology Heart valves
Dermatology Soft tissue augmentation
Dentistry Oral wounds
Biocoating for dental implants
Support for hydroxyapatite
Periodontal attachment
General surgery Hemostasis
Hernia repair
IV cuffs
Wound repair
Suture
Neurosurgery Nerve repair
Nerve conduits
Oncology Embolization
Orthopedic Bone repair
Cartilage reconstruction
Tendon and ligament repair
Ophthalmology Corneal graft
Tape or retinal attachment
Eye shield
Plastic surgery Skin replacement
Urology Dialysis membrane
Sphincter repair
Vascular Vessel replacement
Angioplasty
Other Biocoatings
Drug delivery
Cell culture
Organ replacement
Skin test
Source: Reproduced from F. H. Silver and A. K. Garg, “Collagen
characterization, processing, and medical applications,” in Handbook
of Biodegradable Polymers, A. J. Domb, J. Kost, and D. M. Wiseman,
(eds.). London: Harwood Academic Publishers, 1997, Chap. 17, p. 336.

preparation of materials and devices since it quickly resorbs in the moist environment of the body.
Water-insoluble collagen, however, is routinely used in the manufacture of medical devices. Water-
insoluble collagen is ground and purified to yield a powder that can be later processed into materials
and devices. Collagen cannot be melt processed and is, therefore, processed by evaporating water
from collagen suspensions. Insoluble collagen disperses well at pH between 2 and 4. Evaporating
1 percent suspensions forms collagen films. Freezing suspensions followed by lyophilizing (freeze
drying) forms sponges. Ice crystals form during freezing, which results in porosity after water is
removed during lyophilizing. Freezing temperature controls ice crystal size and 14-μm pores result
from freezing at −80°C and 100-μm pores at –30°C. Fibers and tubes are formed by extruding col-
lagen suspensions into aqueous solutions buffered at pH 7.5. 20
Collagen absorbs water readily in the moist environment of the body and degrades rapidly; there-
fore, devices are often cross-linked or chemically modified to make them less hydrophilic and to
reduce degradation. Viswanadham and Kramer showed that water content of untreated collagen hollow
fibers (15 to 20 μm thick, 400 μm outer diameter) is a function of humidity. The absorbed water plasticizes
collagen, lowering both the modulus and yield strength. Table 13.3 summarizes these results. Cross-linking
the fibers using UV radiation increased the modulus of the fibers. 21