Human-derived acellular matrices for dermal replacement - Biomaterials for Treating Skin Loss - page 157

Furthermore, the components identified through biochemical analysis are shown not
only to be retained in the RTM, but also to possess their native structural architecture.
Taken together, the data suggest that the RTM is likely to retain the cell recognition
sequences and structural porosity necessary to allow the migration, adhesion,
proliferation and differentiation of host cell populations within the graft.
Molecular detail may also be obtained through the use of transmission electron
microscopy (TEM). Such technology allows visualization of individual collagen
fibers. As shown in Fig. 10.8(a), the acellular dermal matrix comprises almost
exclusively collagen fibers arranged in numerous bundles. These bundles are
constructed from the complex fibrillar network reminiscent of that seen following
H&E staining ( Fig. 10.6(a)) . At higher magnification they exhibit their usual
repetitive banding pattern characteristic of the arrangement of the individual
collagen molecules within each fiber (Fig. 10.8(b)).
As previously noted, one detrimental consequence of improper processing is
cross-linking between individual collagen fibers which can lead to poor functional
outcomes. Differential scanning calorimetry (DSC) can be used to measure the
thermodynamic properties of the RTM and demonstrate the effect that different
preservation techniques can have on the matrix. Collagen melt temperature and the
enthalpic change associated with the melt transition are two properties that are
greatly affected by various matrix modifications, including cross-linking. DSC
analysis of AlloDerm RTM demonstrates peak melting at roughly 63 ºC ( Fig.
10.9) . This peak temperature is consistent with melting of purified collagen type I
(Miles and Ghelashvili, 1999) and is comparable with unprocessed dermis.

10.3.2 Mechanical analyses

The dermis provides the majority of mechanical properties of skin. Just as the
biochemical and structural properties of a matrix affect its functionality in vivo , so
too do its mechanical properties (Badylak, 2007; Clark et al. , 2007). Proper
processing of native tissue in the production of acellular RTMs is therefore
required to preserve the mechanical properties of the native tissues. Because
LifeCell's RTMs have been explored for use in loading environments, the me-
chanical properties of the RTM have been investigated and quantified by a variety
of specific metrics. Uniaxial tensile extension on more than 100 donor lots of RTM
demonstrates a significant positive correlation between tissue thickness and ulti-
mate load per cm width as tested over a range of 25 to near 500 N cm -1 ( Fig.
10.10a) . Clinically, overall strength is not dependent solely upon the intrinsic
strength of the tissue, but also on how strongly the RTM can be fixed to the host.
Suture pull analysis has been performed to determine how well the processed RTM
retains sutures under tension. Polyblend (FiberWire) sutures in a mattress stitch
configuration were pulled through test samples. The tissue-suture composite was
fixed in the testing frame by securing the tissue in the lower grip and looping the
suture around a metal hook in the upper grip. The composite was extended