Biomedical Engineering Reference
In-Depth Information
antiviral and terminal sterilization processing steps are necessary to provide
safety assurance and, in most cases, to achieve regulatory approvals (Hodde and
Hiles, 2002).
6.4
The extreme importance of processing
ECM materials are full of proteins and other biologically important molecules
that can be rinsed away, altered, destroyed, or simply rendered mute by the
many steps of processing in an attempt to make them safe for human use. Many
of these steps can affect the mechanical and physical properties of the resultant
grafts, but perhaps more subtle are their effects on the biological properties both
for short-term complications and for long-term outcomes in the recipients. These
effects can also extend to cell-culture applications where a cell-friendly matrix is
of utmost importance for cellular tissue engineering.
6.4.1 Process similarities and differences
Virtually all commercially viable tissue harvesting processes involve one or
more steps of mechanical isolation, chemical washes, thorough rinses, dehydra-
tion, freezing, and gas or radiation sterilization. Not every graft material
includes every step, but all grafts undergo processes that alter them from their
original, in situ composition and structure. The extent of this alteration is one of
the main determinants that affect its fate in the patient.
Mechanical steps involve cutting, delaminating, or grinding the source organ
or tissue to form a sheet or powdered material that can be further processed. This
is often done at a location away from the manufacturing facility, such as a
packing plant or morgue, in order to facilitate transport and allow for quick
storage methods to be used.
Chemical steps often used are focused on the goals of decellularization,
disinfection, and sometimes crosslinking for mechanical `stabilization.'
Decellularization is an art that ultimately means that no living cells survive
the process. Lipid content measurements, histologic methods, or DNA extrac-
tions can all be used to help quantify the extent of decellularization, but there is
little consensus on exact definitions of what constitutes complete and necessary
decellularization. For example, DNA is often left behind in decellularized grafts,
but in the absence of strands long enough to code for complete genes or known
viruses, little or no harm can be attributed to DNA remnants.
Chemical disinfection is of utmost importance in the processing of tissues to
make them safe. Most regulatory bodies reviewing processes for animal tissues
require the manufacturers to provide extensive data on the overkill of various
resistant viruses that might be found in their source animals. This involves spike
and recovery studies with several logs of kill built into the process as a safety
factor (Hodde and Hiles, 2002). Achieving this extensive overkill of bacteria and
