Biomedical Engineering Reference
In-Depth Information
adhesion and proliferation. This is because cell interactions with biomaterials
are heavily dependent on protein adsorption, and surface energy affects protein
adsorption onto biomaterials, which in turn affects cell adhesion. Different cell
types preferably bind to materials of differing wettability. For instance, most
likely due to altered protein adsorption, fibroblasts preferentially bind to
surfaces with a contact angle of 70ë (Tamada and Ikada, 1994) while endothelial
cells bind optimally to surfaces with a contact angle of 40ë (van Wachem et al.,
1987). Like mechanical strength, wettability is another design consideration that
can be optimized for specific tissue engineering applications.
Surface structures are another component that affects tissue growth on
engineered matrices. By printing biomimetic features that replicate cell features
on polymer conduits, researchers can influence cell growth and migration
(Bruder et al., 2007). Nanoscale surface modification can also positively influ-
ence cell growth on tissue engineering scaffolds (Miller et al., 2007). The role of
nanotechnology will be more thoroughly addressed in a later section in this text.
The interaction between cells and the ECM is bilateral. The ECM is built and
continually remolded by surrounding cells, and not only does the ECM provide
mechanical support to surrounding cells, but additionally, attributes such as the
elasticity of the ECM can affect cell differentiation. The relationship between
cells and the ECM has important implications for the design of matrices for
tissue engineering and regenerative medicine. Just by altering the mechanical
properties of the matrix, mesenchymal stem cells can be directed to differentiate
into neuronal, myogenic or osteogenic differentiation (Engler et al., 2006).
Unless cells can be incorporated into and remodel the matrix, the engineered
construct will not be able to effectively integrate into the surrounding living
tissue, and thus it will not be able to grow and develop as a native tissue.
Valvular implants in pediatric patients illustrate the importance of designing a
tissue engineering matrix that supports cellular ingrowth and remolding.
Traditional acelluar constructs are chemically stable, so in pediatric patients
they often fail due to their inability to adapt to a growing heart. However, by
using a decellularized allograft seeded with autologous cells, it is possible to
create a tissue engineered heart valve that develops with the patient (Cebotari et
al., 2006).
Except for certain tissues, angiogenesis must also be considered when
designing a matrix for tissue engineering and regenerative medicine. Without
proper matrix vascularization, the cells in tissue engineered constructs are con-
fined to a 100±200 m distance from an oxygen and nutrient source. At distances
greater than 100±200 m, the necessary nutrients for maintaining cell viability
can no longer reach the cells by simple diffusion. Incorporating vascularization
into a tissue engineered matrix will remove the limits of diffusion on tissue
development, but this problem is complicated by the fact that endothelial growth
is promoted by different surface wettability and porosity properties from other
tissue types. Attempts to produce a matrix that promotes vascularization include
