Biomedical Engineering Reference
In-Depth Information
collagen scaffold (Soller et al. 2012). It has been explained (see Chap. 8) on the
basis of the observation that skin defects and peripheral nerve defects heal by the
same mechanism. In both cases, contractile cells apply mechanical forces that close
the defects; a collagen scaffold with optimized structure applies contraction block-
ade in the nerve stumps, a process that appears to suffice for inducing regeneration
of high quality. The optimal degradation half-life corresponds to the critical time
range when contractile cells in both skin and peripheral nerve defects have just
populated the defect and are therefore now vulnerable to being bound on ligands
on the scaffold surface, with resulting cancellation of the macroscopic mechanical
force (Soller et al. 2012).
Use of tubes fabricated from fibronectin mats gave regenerative activity equiva-
lent to that of the nerve autograft after about nine weeks when it was used to bridge
the 10-mm gap in the rat sciatic nerve (Whitworth et al. 1995). In another study, a
transected rat sciatic nerve was treated by wrapping a laminin sheet around the zero-
mm defect; in a control group, the stumps were connected by suturing. After about
17 weeks, it was observed that the laminin sheet was as effective as direct sutur-
ing in supporting functional recovery (Kauppila et al. 1993). The limited evidence
available on tubes fabricated from fibronectin and laminin does not readily lead to
generalizations.
6.4.7
Semipermeable Tubes
Acrylic copolymer tubes that allowed transfer of molecules with a molecular weight
(MW) cutoff of 50 kDa did not show a significant difference in regenerative activ-
ity compared with impermeable silicone tubes (Hurtado et al. 1987; Knoops et al.
1990); however, a small increase in relative level of innervation of the same acrylic
copolymer tube relative to the silicone tube was observed in another study (Aebi-
scher et al. 1988). The comparisons were made between tubes with different chemi-
cal composition and, for this reason, they do not lead to unambiguous conclusions
on the effect of protein-permeability of the tubes on regenerative activity.
Studies of silicone tubes that were either impermeable or were permeable to
particles with diameter less than 1.2 μm led to the conclusion that the frequency
of innervation was the same in both groups (Jenq and Coggeshall 1985a, c; Jenq
et al. 1987). Use of silicone tubes that had relatively small holes measuring 5 μm,
and were confirmed to be cell-permeable, showed the same level of reinnervation
as did silicone tubes that had macroscopic holes (Jenq and Coggeshall 1985c; Jenq
et al. 1987). Comparison of silicone tubes incorporating macroscopic holes with
impermeable silicone tubes showed that the numbers of both myelinated and non-
myelinated axons in the distal stump were significantly higher when the tubes had
holes (Jenq and Coggeshall 1985c). In addition, tubes with macroscopic holes al-
lowed formation of bridges of connective tissue that passed through the holes and
reached all the way to the connective tissue on the surface of the regenerated trunk,
suggesting the possibility that connective tissue cells, probably fibroblasts that had
