Biomedical Engineering Reference
In-Depth Information
A general method for pSi gradient etching is:
1. Highly doped silicon wafers need to be used to generate pSi gradients.
In our laboratory, we use p
11
-type silicon wafers (0.0005-0.001 O cm,
h
100
i
, boron doped) purchased from Virginia Semiconductors
(Fredericksburg, VA, USA).
2. HF electrolyte solutions are prepared using 49% aqueous HF and 100%
ethanol as a surfactant at a ratio of 1 : 1 HF : ethanol.
3. A custom-built circular Teflon etching cell was used for these experi-
ments using aluminium foil to contact the back of the Si wafer and a Pt
mesh as the counter-electrode.
4. The Pt electrode is placed perpendicular to the substrate at one end of
the etching cell. The substrate-electrode separation is critical for the
formation of pSi gradients. If the electrode is too far from the substrate,
a shallow pore-size gradient with a small maximum pore size will be
formed. However, positioning the substrate too close to the substrate
will result in electropolishing and complete pore collapse.
5. A current density of 115 mA cm
2
is applied for 60 s using a Keithley
2425 sourcemeter.
6. Following anodisation, samples should be rinsed with ethanol,
methanol, acetone and DCM and subsequently dried with a gentle
stream of nitrogen.
7. Following pSi formation, the hydride-terminated surface is highly
susceptible to oxidation. Subsequent reactions utilising Si-H chemistry
(e.g., hydrosilylations or
d
n
3
r
4
n
g
|
7
electrografting)
should be performed
immediately.
8. Thermal oxidation of the pSi gradients significantly increases their
stability in aqueous media. We found that a two-stage thermal oxi-
dation process resulted in the most stable pSi gradients, oxidising
them in a tube furnace at 400 1C for 30 min followed by an additional
oxidation step at 800 1C for 60 min.
.
Research by Khung et al.,
5
Low et al.,
41
Wang et al.
117,134
and Karlsson
et al.
123
has shown that cells are sensitive to the pore size in which they are
grown on. Khung et al.
5
produced a laterally graded pSi surface on p
11
-type
Si and found that the attachment and morphology of SK-N-SH neuro-
blastoma cells was critically dependent on pore size. They observed that on
100-300 nm pores, cells adhered poorly to the surface but could still interact
through cell-cell contacts, reducing the need for cell-substratum contact.
Length and thickness of neurite processes formed by the cells adherent on
pSi were also dependent on pore size. Khung et al.
5
observed that the cells
growing on the 1000-3000 nm pore size region had the largest cell size and
area, and were larger than what was observed on flat Si. The smallest
length and area of the cells was observed in the 50-100 nm pore range.
This study demonstrated that topography can have an effect on cell adhesion
and morphology. Clements et al.
108
observed that attachment of rat
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