High Throughput Techniques for the Investigation of Cell–Material Interactions - Biointerfaces: Where Material Meets Biology - page 279

have to be investigated individually. Lampin et al. 52 have prepared PMMA
roughness substrates using alumina sandblasting whereby the sand grain
size dictated the roughness of the surface. Topography characterisation was
carried out using profilometry. Although these surfaces were not gradients, it
was found that attachment of vascular and corneal cells increased with in-
creasing surface roughness. Whilst this result contradicts the findings of
Kunzler et al., it should be noted that the surfaces prepared by Lampin et al.
showed an increase in hydrophobicity with roughness which may have in-
fluenced cell attachment. Additionally, the surface chemistry in the two
studies was completely different, which, given the sensitivity of cells to
surface chemistry, would undoubtedly influence the cellular response.

d n 3 r 4 n g | 7

10.2.1.4 Surface Elasticity Gradients
The elasticity of a substrate surface has also been shown to influence cellular
responses. 2,54,87,93,157-160 There have been several methods employed to
generate elasticity gradients including gradient co-polymerisation of two
monomers, 87 a gradient in the concentration of curing agent 159,161 and ap-
plying a temperature gradient during PDMS curing. 158
Tse et al. 93 created a stiffness gradient of polyacrylamide by UV curing
through a photomask. A radial gradient photomask was positioned in be-
tween the UV source and monomer-substrate assembly, with the degree of
cross-linking decreasing moving from the centre to the outside of the sub-
strate. This decrease in cross-linking leads to an increase in the elasticity of
the hydrogel. A uniform layer of collagen was covalently bound to the gra-
dient hydrogels to help facilitate cell attachment. MSCs were found towards
the stiffer region of the gradient. Using PDMS stiffness gradients, it was
found that that osteogenesis of rat mesenchymal stem cells (rMSCs) was
strongly dependent on the PDMS stiffness as indicated by the calcium
mineralisation observed in the stiffer regions of the gradient. 158 Several
modifications to this method described by Vincent et al. 159 involved using a
linear gradient photomask to vary the degree of curing, or a custom-built
microfluidic gradient maker to vary the cross-linker concentration. Fol-
lowing deposition of a uniform Fn adsorption layer, an in depth analysis of
MSC durotaxis (directed migration of cells up a stiffness gradient) was per-
formed. Whilst uniform attachment was observed within hours of cell
seeding, after 3 days cells were found to migrate towards stiffer regions of
the gradient.
Poly( L -lactic acid) (PLLA) and poly( DL -lactic acid) (PDLLA) gradients were
prepared by Simon et al. 87 These polymers differ only in their tacticity (i.e.,
their stereochemistry) leading to PLLA being crystalline and PDLLA being
amorphous. Hence, PLLA has a higher modulus (amount of force required to
deform a material). PLLA-PDLLA gradients were prepared using a syringe
system. PDLLA monomer was slowly introduced into a beaker containing
PLLA, whilst a syringe was simultaneously withdrawing a solution of the
mixed monomer composition in the beaker. In this way, the concentration of

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