Biomedical Engineering Reference
In-Depth Information
and the optimal degradation half-life was 14 ± 7 days (Fig. 8.13) (Soller et al. 2012).
Insufficient data are available to identify optimal values for ligand densities for inte-
grins α1β1 and α2β1. The limited evidence suggests a guideline for ligand densities
that exceed 200 µΜ α1β1 or α2β1 ligands (Tzeranis et al. 2014). Modification of
any of the first two structural features, and possibly of the third as well, deactivates
DRT almost completely.
Although it is clear that these structural criteria are necessary, there is little evi-
dence that they are also sufficient. For example, there is little information on how to
decide the optimal level of pore volume fraction (currently set at 0.995; Chen 1982)
or the optimal fraction of collagen fibers that carry the native D-banding that has
been retained following treatment in acetic acid during preparation of DRT (Sylves-
ter et al. 1989). Although early reports of DRT described a chemical composition
based on the original graft copolymer of collagen and chondroitin 6-sulfate (gly-
cosaminoglycan, GAG), the GAG component was omitted in experimental scaffold
preparations in our MIT laboratory approximately after year 2000. Omission of
GAG was implemented in deference to reported inhibition of peripheral nerve re-
generation in the presence of certain glycosaminoglycans (Carbonetto and Cochard
1987). It has been reported that reaction of collagen with GAG increases the half-
life of collagen slightly (Yannas et al. 1975a) and has a modest delaying effect on
wound contraction (Shafritz et al. 1994) but it does not appear to affect the overall
regenerative outcome. In what follows, we will focus on the first three structural
features of DRT.
Processing steps during preparation of the scaffold have been shown to control
these critical structural features of the DRT structure. The pore structure is con-
trolled by the conditions during freezing (Dagalakis et al. 1980), especially by the
freezing temperature. Degradation rate in vivo depends on the conditions of cross-
linking that lead to specific levels of the crosslink density (Yannas et al. 1975b;
Huang and Yannas 1977), or, alternately, to specific levels of the average molecular
weight between crosslinks (Yannas 1981). Finally, the density of ligands for specif-
ic integrins depends on the presence of particular hexapeptides and is, therefore, a
property of the amino acid sequence of the basic collagen protein. Furthermore, ef-
ficient cell-scaffold binding also requires certain conformational features of native
collagen (gelatin is denatured collagen and lacks its triple helical conformation).
Quantitative understanding of the regenerative effect of structural features of
collagen scaffolds has been based on the use of “collagen libraries.” These special
collections of collagen scaffolds are especially useful as probes of regenerative ac-
tivity. Some of these scaffolds are regeneratively active but most are inactive. The
basic experimental strategy in using these libraries is to prepare a homologous se-
ries of scaffolds, with members that differ from each other only with respect to the
level of just one structural variable (reference variable). Examples of such reference
variables are the average pore size of the scaffold, which can be varied from 10 µm
to more than 500 µm, the scaffold half-life for in vivo degradation between 1 week
and more than 100 weeks, and the concentration level of ligands for specific cell
receptors, such as the collagen binding integrins α1β1 and α2β1 for contractile cells.
Each of these variables can be usually varied by adjusting just one of the processing
