High Throughput Techniques for the Investigation of Cell–Material Interactions - Biointerfaces: Where Material Meets Biology - page 273

mesenchymal stem cells on pSi gradients spanning pore sizes of 900 nm to
10 nm was optimal at a pore diameter of 19 11 nm. In a complementary
experiment, Wang et al. 135 investigated the response of three different MSC
lines on pSi gradient. Rat bone marrow-derived MSCs were found to be more
sensitive to pore size than human bone marrow-derived MSCs. Additionally,
human adipose-derived MSCs were shown to attach more strongly to a pore
size of

d n 3 r 4 n g | 7

B

300 nm than on flat Si substrates. These studies clearly demon-
strate that cells derived from different sources display remarkedly different
cellular response to the same topography cues and highlight the complexity
of defining specific topographical cues to drive a specific cellular response.
Karlsson et al. 123 investigated the effect of pore size of pSi on the attachment
of human serum albumin. A non-linear curve was observed for the ad-
sorption of HSA on pSi with preferential attachment found in the largest
pore size region of the gradient. Wang et al. 134 also found that a decrease in
the ridge roughness of n-type pSi gradients lead to an increase in the at-
tachment and spreading of rat MSCs. These results demonstrate the utility
of pSi to quickly and simply study the effect of nanotopography on cell be-
haviour. Whilst the use of pSi gradients has many advantages in terms of its
ease of fabrication and further functionalisation, the stability of the films is
clearly an issue for long-term cell culture studies. For this reason, a stable
substrate with tuneable porous gradients is highly desirable.

10.2.1.2 Porous Alumina Gradients
Porous alumina (pAl) is formed through anodisation of aluminium (Al) foil
in aqueous acids. A current is applied between the anode (Al substrate) and
cathode (usually lead or platinum) and traditionally the electrodes are pos-
itioned parallel to each other to create an evenly dispersed electric field and
therefore a uniform pore size across the surface. Phosphoric acid, oxalic acid
and sulfuric acid are the most commonly used electrolytes for pAl formation.
Based on previous research, smaller pore diameters are achieved when using
oxalic acid compared to phosphoric acid. 47 The pore diameter, interpore
distance and pore wall width can be optimised through adjusting the ano-
disation conditions. Ordered hexagonal patterns formed under certain pAl
etching conditions are thought to be attributed to the stress-driven interface
caused by repulsive forces between neighbouring pores. 136 In recent years,
two-step anodisations have become increasingly popular as a means to
create intricate patterned architectures including triangular or square
shaped pores,

.

checkerboard patterns and complex internal pore

structures. 136
The first pAl gradient was described by Kant et al. 47 whereby the electrodes
were positioned at an angle of 451 to each other to achieve nonlinear ano-
disation. The formation of pAl gradients can be explained by the gradual
change in current density, similarly observed in pSi gradient formation,
arising from the non-parallel electrodes as well as a gradual temperature
change across the surface during the anodisation process. Two different