Page 377 - Biomedical Engineering and Design Handbook Volume 1, Fundamentals

354 BIOMATERIALS

important for shock absorption. However, the part will need to be heavier because a simple analysis
reveals that the wall thickness of a tubular molding in 30 percent discontinuous CFR Nylon 6,6 needs
to be 7 times that in 60 percent continuous CFR epoxy at constant tube radius. This can be overcome
by using more advanced thermoplastics such as PEEK and longer fibers in injection molding. For
the prosthetic foot unit, various composites have been used with the objective of storing energy and
returning it during motion. The Flex-Foot unit has a long CFR epoxy composite beam that stores
flexural energy along the entire length of the prosthesis rather than just the foot unit and is tailored
to the individual patient characteristics and activity level. In the Carbon Copy 2 foot, the keel is made
of a posterior Kevlar-reinforced nylon block and anterior CFR plastic leaf springs, providing
two-stage resistance to flexion. Finally, a nylon-reinforced silicone elastomer has been used to make
a durable cover for the flexible foam that encases the prosthesis. In the area of orthotics designed to
support injured tissue, composites find use in new bandage-form splinting materials replacing the old
cotton fabric and plaster of paris method. These are commonly laminated fiberglass or polyester knitted
fabrics in partially cured polyurethane matrices. Besides strength, they have the advantages of
better x-ray transmission and lower water adsorption. However, these casts also produce more dust
during sawing and are harder to remove.

14.8.4 Soft-Tissue Engineering
Cross-linked hydrogel networks are suitable as a scaffold for skin regeneration due to their high
water content and barrier properties, but they have poor mechanical properties. The tensile strength
and break point of the hydrogel poly(2-hydroxyethyl methacrylate) (pHEMA) were dramatically
enhanced by reinforcing it with Spandex and gauze fibers, resulting in gels that withstood greater
forces before tearing. In cartilage repair, where high compressive and shear properties are desirable,
low-density linear polyethylene was melt coated onto a woven three-dimensional fabric of UHMWPE
fibers to produce a composite that had compressive behavior approximating that of natural carti-
lage. 28 Fibrous poly(glycolic acid) (PGA) felts and poly(lactide-co-glycolide) (PLGA) fibers have
been used as surfaces and scaffolds for cartilage cell growth, where the cells produce the bulk matrix
when implanted at the site of the cartilage defect. The fiber diameter, interfiber distance, and
biodegradation rate were important parameters that affected the quality of the neocartilage formed
from such constructs. In the area of vascular grafts, woven-fiber tubes made from polyester (Dacron)
have high permeability during implantation, resulting in severe blood leakage through the graft
walls. To avoid the lengthy preclotting time needed to overcome this leakage problem, various
impermeable materials are coated on such grafts to form a composite, where the matrix functions as
the sealant rather than the load bearer. For example, alginate and gelatin have been used to
thoroughly wet Dacron fibers and seal the vascular prosthesis, and over time they are biodegraded.
For hemodialysis vascular access, where the graft has to be resilient to frequent needle puncture, a
technique employed in the DIASTAT graft involves PTFE fibers sandwiched between layers of
porous expanded PTFE. The fibers are pushed aside where the needle enters, and after needle with-
drawal, the fibers create a baffle effect to reduce leaking blood velocity and improve resealing. 29

REFERENCES

1. M. M. Schwartz, Composite Materials Handbook, 2d ed. New York: McGraw-Hill, 1992.
2. S. L. Evans and P. J. Gregson, “Composite technology in load-bearing orthopaedic implants,” Biomaterials
19:1329–1342 (1998).
3. P. K. Mallick, Composites Engineering Handbook. New York: Marcel Dekker, 1997.
4. D. L. Wise, Human Biomaterials Applications. Totowa, N. J.: Humana Press, 1996.
5. E. E. Gdoutos, K. Pilakoutas, and C. A. Rodopoulos, Failure Analysis of Industrial Composite Materials.
New York: McGraw-Hill, 2000.