Biomedical Engineering Reference
In-Depth Information
potential mechanism indicated that differences in degradation behaviors
between polymers are not significant, but that the elastic modulus is a
critical parameter, being relevant to biology at the microscopic (cellular)
level and possibly also having an impact at macroscopic (tissue/organ)
scales. They concluded that the elastic modulus is a property that should be
considered in the development and optimization of synthetic biopolymers
for tissue engineering.
Recently, MSCs have provided striking evidence that ECM elasticity
influences differentiation. Indeed, multipotent cells are able to start a
transdifferentiation process towards very soft tissues, such as nervous tissue,
when the elastic modulus (E) of the substrate is about 0.5 kPa. Intermediate
stiffness (
10 kPa) addresses cells toward a muscle phenotype and harder E
~
(
30 kPa) to cartilage/bone (Engler et al., 2006). This should address
intelligent design of new biopolymers intended for specific applications
(Mitragotri and Lahann, 2009). Biopolymers presently used in tissue
engineering are extremely stiff. PLA has a bulk elasticity of E
≥
1GPa, ten
thousand times stiffer than most soft tissues. Thus, the engineering of soft
tissue replacements
~
requires biopolymers
softer
than those presently
available.
Poly(butylene/thiodiethylene succinate) block copolymers (PBSPTDGS)
were prepared by reactive blending of the parent homopolymers (PBS and
PTDGS) in the presence of Ti(OBu)
4
(Soccioa et al., 2008). The random
copolymer, characterized by the lowest crystallinity degree, exhibits the
lowest elastic modulus and the highest deformation at break. When
evaluated for indirect cytotoxicity, films of block PBSPTDGS30 and
random PBSPTDGS240 copolymers appeared entirely biocompatible. In
addition, the cellular adhesion and proliferation of H9c2 cells (Tantini et al.,
2006) (derived from embryonic rat heart) seeded and grown up to 14 days in
culture over the same films demonstrated that these new materials might be
of interest for tissue engineering applications.
The biocompatibility of neat PLLA, PLLA/nHA and PLLA/mHA
composite scaffolds were evaluated in-vitro by observing the behavior of
stained MSCs cultured in close contact with scaffolds (Nejati et al., 2008).
Cell growth in material-free organ cultures can be separated into four stages.
Cells adhered on the surface of the composite in a round shape during the
first two days. The round cell then attached, spread and proliferated into the
inner pores of the scaffold, exhibiting morphologies ranging from spindle
shaped to polygonal. After one week, the cells reached confluence on the
material while the material-free group did not reach this status (Fig. 17.8).
The representative cell culture micrographs of cell attachment into the
scaffolds after seven days were observed. It was seen that the round-shaped
cells attached and proliferated to the scaffold's surface, became spindle like
and then migrated through the pores. The number of round-shaped cells is
