Biomedical Engineering Reference
In-Depth Information
PDLLA in the syringe gradient gradually increased as the syringe was filled.
Whilst the 24 h attachment of osteoblast-like MC3T3-E1 cells was similar
across the entire gradient, cell proliferation was found to be faster on the
PDLLA-rich end. This could be attributed to the fact that the PDLLA-rich end
of the gradient was smoother than that of the PLLA-rich end. Similarly, van
Sliedregt et al.
157
found that epithelial cells, osteosarcoma cells and fibro-
blasts proliferated faster on PDLLA surfaces as opposed to PLLA surfaces.
Conversely, Ishaug-Riley et al. found that human articular cartilage chon-
drocyte attachment was faster on PLLA compared to PDLLA.
162
As high-
lighted previously, the elasticity of a substrate has the ability to direct the
lineage of stem cells.
2,54,67
Through altering the cross-linking density to
obtain a soft or rigid surface, cells could be directed towards neuronal cells
or to bone cells.
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10.2.1.5 Gradients of Coating Thickness
The thickness of a coating has also been shown to affect the attachment of
cells.
7,99,100
Whilst a thickness gradient is not strictly a topography gradient,
it is discussed in this section for simplicity.
Smith et al.
99,100
and Crosby et al.
163
have produced thickness gradients
via the use of a flow coater. A Si wafer was mounted onto a motorised stage.
A knife-edge was then placed just above the surface at a 51 angle to the
substrate, and a drop of polymer solution was placed under the knife. As the
stage was moved, the knife spread the polymer solution over the substrate.
Through increasing the stage velocity and upon solvent evaporation (which
occurs in seconds) a thickness gradient was formed. The velocity of the stage
determined the thickness of the polymer layer, with film thicknesses ranging
from 25-500 nm. Poly(styrene-b-methyl methacrylate) co-polymer thickness
gradients were formed using this technique on silicon wafers. It has previ-
ously been observed that the methacrylate preferentially segregates to the
substrate polymer interface whereas the styrene preferentially migrates to
the air interface.
164,165
In this example, whilst the researchers were inten-
tionally altering the thickness of the polymer film, a secondary effect, namely
phase separation between the two polymers was also observed. As the
thickness changed across the gradient surface, the surface changed from
smooth, to discreet islands arising from phase separation, and then back to
smooth again. Film thickness was measured with a UV-visible interfer-
ometer, whereby a contour map was formed through recording measure-
ments at discreet intervals across the surface.
.
10.2.2 Chemical Gradients
Chemical gradients allow for a diverse range of surface properties to be cre-
ated through the use of different functional groups. Polymers are increasingly
being used for biomaterial and biomedical applications due to their cheap
synthesis and ease of structural manipulation, which allows for the creation of
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