Biomedical Engineering Reference
In-Depth Information
chapter is to provide an overview of the considerations for designing a tissue
engineering scaffold and some of the tools available to create improved matrices
for tissue engineering and regenerative medicine.
10.2 Design considerations for matrices
The ECM is an intricate meshwork of proteins that supports the growth and
stability of the local cells. In many cases it also provides structural support to the
entire organism. Consequently, when designing an artificial matrix one must
consider both mechanical strength and the construct's ability to provide a local
environment that encourages tissue growth and discourages infection and
excessive immune responses. To be an effective vehicle for tissue engineering
applications, matrices must be more than simply biocompatible. In addition to
not triggering an aggressive detrimental immune response leading to fibrous
encapsulation, the material used in matrix construction should encourage cell
growth.
The engineered matrix must provide a mechanical modulus in the range of
10±1500MPa for hard tissues such as bone or 0.4±350MPa for softer tissues in
neuronal applications to provide adequate mechanical support until native cells
can supplement or replace it with a natural ECM (Hollister, 2005). Not only does
this fact illustrate the mechanical aspect of designing matrices, but it also
suggests that there must be different materials and processing methods for
creating an ECM substitute for different tissues in the body. Mechanical strength
seems at the surface to be an easily achieved matrix characteristic when
designing a construct using materials with known mechanical properties. How-
ever, the mechanical strength of a construct is complicated by the introduction of
other design components that make a matrix that provides adequate structural
support and that also promotes cellular growth.
One such material property that can significantly alter mechanical properties is
porosity. Porosity increases the proliferation of cells into the construct and provides
the necessary avenues for cell motion and nutrient diffusion. Like the ideal
mechanical modulus for different tissues, optimal pore size is also tissue specific.
The optimal pore size for neovascularization is 5m, while for fibroblast ingrowth,
a pore size of 5±15m is optimal. Hepatocyte ingrowth is optimal at a 20m pore
size. Osteoid, skin regeneration and bone regeneration all have lager optimal
construct pore sizes at 40±100m, 20±125m and 100±350m, respectively
(Yang et al., 2001). Porosity and mechanical strength are two matrix characteristics
that are in conflict. Increased porosity decreases the overall mechanical strength of
the construct. This problem can be solved in various ways; one is using two
different polymers, a first to meet the mechanical demands of the application and a
second to encourage cell growth (Shao et al., 2009).
Another design consideration for optimal tissue engineering matrix design is
surface energy. Surface energy is an important component for controlling cell
