High Throughput Techniques for the Investigation of Cell–Material Interactions - Biointerfaces: Where Material Meets Biology - page 270

d n 3 r 4 n g | 7

Figure 10.6 Schematic of pore formation.

Pores are readily formed in the Si surface of highly doped substrates.
Boron is electron deficient and hence positive holes are present in the bulk
silicon. These holes act as charge carriers and facilitate the interaction with
F ions in the electrolyte solution (Figure 10.6). These positive holes move
towards the negative electrode and in doing so, weaken the bonds in the
surface silicon layer. This allows for the F ions to attack and displace Si on
the surface. Once the first Si has been dislodged, a pit is formed in the
surface and the electric field is concentrated at the tip of these pits. As a
result, the positive holes are attracted to this tip and hence the pores are
deepened, rather than widened to make larger pores. The depth of the pores
scales with the etching time.
A gradient porous surface can be achieved through placing the platinum
(Pt) electrode at one end of the exposed surface as demonstrated in
Figure 10.7A. The current density decreases at increasing distances from the
electrode and hence pores are the largest in the region closest to the elec-
trode and smallest in the region furthest from the electrode. The pore range
that can be achieved with this technique varies with the set etching con-
ditions. However, it has been shown that gradients with pores ranging from
1.5 mm to 2 nm 46 or 3 mm to 70 nm 5 can be achieved over a distance of
1.5 cm. A detailed description of pSi gradient preparation can be found
below. Khung et al. 5 has produced laterally graded pSi as shown in
Figure 10.7. In such experiments, the pore range could be customised
through the adjustment of the current density. Collins et al. 46 have also
prepared lateral pSi gradients. Using profilometry, they demonstrated that
film thickness decreased at increasing distances from the electrode. In the
examples presented above, the produced gradients were found to be non-
linear, 5,46,123,124 whereby the pore size rapidly decreased near the electrode
followed by a steady plateau in the pore size. The non-linear nature of the
gradient could be attributed to the non-linear decrease in current density
across the substrate.
Freshly etched pSi is terminated by silicon-hydride (Si-H) bonds which
are unstable and prone to oxidation and corrosion in biological fluids. 41,124
For some applications, such as drug delivery, this may be a desired effect.
However, for other biomaterials applications, such as transplantable scaf-
folds, retaining the stability of the underlying porous layer is critical.

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