Biomedical Engineering Reference
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Figure 10.8
SEM images of pAl gradient formed through anodic etching of Al
substrates in the presence of (A) oxalic acid and (B) phosphoric as the
electrolyte at increasing distances from the electrode. Numbers 1 to 5 in
(A) represent locations on the optical image (inset f). Scale bar
¼
1 mm
and 500 nm (insets).
Figure adapted from Kant et al.
47
.
electrolytes, oxalic acid and phosphoric acid, were used in this study to
achieve different pore geometries and features. Brush-like structures were
observed in the region with the shortest separation from the cathode, which
can be explained as remnants of pore wall structures that have collapsed
(Figure 10.8). As the distance between the anode and cathode was increased,
the pore size decreased in a non-linear manner. Whilst the technique
demonstrated the successful formation of a pAl gradient, there are obvious
limitations with this method including reproducibility, extending the gra-
dient beyond 10 mm in length and varying the pore size distribution.
Dronov et al.
137
explored the possibility of changing the pore size of uni-
form pAl using a chemical etching technique. Here, a uniform pAl surface
was generated with a small pore size and large interpore distance. Substrates
were then immersed in 5% phosphoric acid solutions for different time
periods to achieve varying degrees of pore enlargement. An extension of this
study was described by Wang et al.
138
who prepared gradient pore size pAl
substrates via a two-step process. A uniform pAl substrate of a small pore
size was first prepared. pAl gradients were subsequently prepared by dipping
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