Biomedical Engineering Reference
In-Depth Information
similar quality with the autograft across a 10-mm gap in the rabbit tibial nerve (Mo-
lander et al. 1983). Regenerates that formed across an 8-mm gap in the rat sciatic
nerve using tubes based on a copolymer of LA and ε-caprolactone were found to
behave equivalently to the autograft after 16 weeks (Robinson et al. 1991). Another
tube, based on a copolymer of LA and ε-caprolactone, which degraded faster than
the copolymer described immediately above (Robinson et al. 1991), was found to
be superior to the autograft after 11 weeks when used to bridge the 10-mm gap in
the rat sciatic nerve (den Dunnen et al. 1996). The two types of tube based on the
copolymer of LA and ε-caprolactone differed slightly in chemical composition but
significantly in porosity and in tube configuration; accordingly, it is not possible to
compare their performance directly and assign it unambiguously to their differences
in degradation rate. Equivalence to direct suturing was observed when a PGA tube
was used to bridge the approximately 0-mm gap in the median nerve of the monkey
(Tountas et al. 1993).
Using the data on critical axon elongation in Table
6.1
it is possible to reach
certain useful conclusions concerning some of the degradable tubes that have been
studied. Even though the investigators of degradable tubes did not employ a silicone
tube as an internal control, significant improvements over the silicone tube can be
deduced for several degradable tubes. This conclusion is reached on the basis of
estimates of the shift in critical length,
ΔL
, relative to the unfilled silicone tube stan-
dard (see Table
6.1
).
ΔL
values obtained in this manner were ≥ 3.7 for a plasticized
poly(lactic acid; Seckel et al. 1984), ≥ 3.7 for a copolymer of LA and ε-caprolactone
(Dunnen et al. 1993a, b), as well as ≤ 1.3 for an ethylene-vinyl acetate copolymer
(Aebischer et al. 1989). The inequality signs on the first two of the length shifts re-
ported above indicate a lower limit, suggesting that the 10-mm gap length employed
in these studies with the rat sciatic nerve (Seckel et al. 1984; den Dunnen et al.
1993a, b) may have been too short to adequately evaluate the regenerative activity
of these tubes.
6.4.6
Degradable Tubes Based on Natural Polymers
Components of the ECM have been used to fabricate implants used in PNS regen-
eration studies. In the organism, the ECM confers stiffness, strength, and, therefore,
stability of shape; it also provides strong regulatory activity to various cell types,
especially during development and healing of defects. The composition and struc-
ture of ECM varies among tissues. These matrices are typically highly hydrated
macromolecular networks composed of various amounts of glycoproteins such as
collagen, elastin, fibronectin, laminin, chondronectin, and other proteins; and pro-
teoglycans, macromolecules that comprise a protein core with glycosaminoglycan
(GAG) side chains, including chondroitin 6-sulfate, dermatan sulfate, and hepa-
ran sulfate. Macromolecular components of the ECM are synthesized in cells and
are secreted in the extracellular space where further physicochemical modification
takes place (e.g., crystallization and covalent crosslinking of collagen chains; Piez
and Reddi 1984).
