Biomedical Engineering Reference
In-Depth Information
the use of electrical signals to guide nerve growth, groove patterns and bio-
mimetic features resembling Schwann cells have been used to encourage growth
through matrices.
The more one understands the mechanisms of cell±matrix interactions, the
more interesting the problem of designing an effective vehicle for tissue growth
becomes. The rest of this chapter will examine some of the tools available to
researchers and clinicians when designing a proper scaffold for
tissue
engineering.
10.3 Materials for natural matrices
The use of natural matrices has the intrinsic benefit of already possessing the
surface topography of natural tissues. Two common natural matrix sources are
porcine urinary bladder and porcine small intestinal submucosa (Mendelson and
Schoen, 2006). Before implantation, natural matrices are sterilized and decellu-
larized, leaving only a protein-based, commonly collagen, scaffold. Natural
matrices are especially advantageous when the function of the tissue is mech-
anical, because maintaining the ECM maintains the function of the transplant.
An example of a tissue that this is especially true for this is a heart valve.
Completely decellularized heart valves perform the same mechanical function as
a native cellularized heart valve. However, while natural matrices must be
remolded out of their native forms, they are notoriously harder to work with than
synthetic thermoplastics such as polylactic acid and polyurethane that can be
easily heated and molded. Two other problems facing the use of natural matrices
for tissue engineering and regenerative medicine are the poor and static
mechanical properties of natural matrices and batch-to-batch variation (Yang et
al., 2001).
For the use of natural matrices, the matrices must be processed and stored
effectively. Any remaining cell or protein debris can trigger an immune response
and lead to failure of the implant, so decellularization is a central component of
processing natural matrices. Decellularization can be accomplished using
enzymatic treatment, detergent-based methods, freeze-drying or osmotic
gradients (Schmidt et al., 2007). Once free of debris, the challenging task
becomes finding an effective method to store the matrices before implantation.
They can be stored hydrated or dried using vacuum or freeze-drying. If dried, the
matrices can simply be rehydrated a few minutes before use. Although both
storage methods are currently used, that does not make them equally effective.
Procine urinary bladder scaffolds stored in a freeze-dried state can promote more
cell growth than the same material stored hydrated (Freytes et al., 2008).
All sources for natural matrices do not create equally effective matrices for
tissue engineering. Porcine decellularized scaffolds have a lower propensity to
attract cellular ingrowth than those from humans and the porcine scaffolds
generate a more severe immune response (Rieder et al., 2005). In addition to the
