Spider Silk as a MEMS Material 13
2.2 THIN-FILM SPIDER SILK PREPARATION
Spider dragline silk harvested from the golden orb weaving spider N. clavipes was
obtained based on a traditional forced silking technique.
30,10,29
The silk was dis-
solved in a 1,1,1,3,3,3 hexafluoro-2-propanol (HFIP) solution with a ratio of either
1% w/w or 0.5% w/w.
30
The solution was allowed to dissolve for approximately
24 hours at ambient temperature before spin-coating onto a silicon wafer. A 10 ml
droplet was spun at approximately 500 rpm for 30 seconds and then air dried for
further thin-film mechanical measurements.
Iron spider silk thin film: Iron pentacarbonyl was purchased from Sigma
Aldrich. Liquid iron pentacarbonyl has a low boiling point and is sensitive to UV
light; therefore it has to be refrigerated and shielded from UV light. The mixture
was diluted in 1:1 v/v (spider silk solution vs. Fe(CO)
5
) ratio. The mixture was
then deposited onto the silicon substrate using a pipette. The sample was then
exposed under an Entela model UVG-54 handheld short wave UV lamp. This was
performed for 2 to 3 hours. A thin film was formed from the droplet.
2.3 CHARACTERIZATION
2.3.1 SEM and EDS
Scanning electron microscopy was used to examine the quality of thin-films.
Figure 2.1 shows a cross-sectional view of a thin spider silk film of two dilutions:
1% and 0.5%. It can be seen from the SEM image that porosity exists throughout
the film, and there is no significant porosity difference between the two diluted
solutions (1% and 0.5% w/w). These voids are presumably left behind by evapo-
ration of the HFIP solvent (1,1,1,3,3,3 hexafluoro-2-propanol) during spinning and
at the drying stage.
SEM images were also taken to examine magnetic spider silk film deposited
on a silicon substrate (Fig. 2(a)). Figure 2.2 shows a SEM image of 1% v/v silk
mixed with Fe(CO)
5
. The sample appeared to exist in two phases - as a film and
as a porous/fibrous-like area. It is not entirely clear why the formation of the
Thin Film Spider Silk
Si
Pores
Asperities
a b
Figure 2.1. Thin film cross section of (a) 1% w/w (b) 0.5% w/w spider silk.
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Spider Silk as a MEMS Material 15
A
B
C
Figure 2.3. Image of the Area Used by X-ray EDS.
Table 2. 1 Fe wt% found using X-ray EDS at Three
Locations Indicated in Fig. 2.3.
ABC
FE(wt%) 2.07 1.52 3.55
In order to determine the nature of the spheres, EDS was used to examine
the elemental composition and concentrations in films made of iron pentacarbonyl
doped spider silk and iron pentacarbonyl alone. For comparative purposes in this
study, the UV exposure method outlined forironspidersilkfilmwasperformed
on both films. The sample areas are shown in Fig. 2.3 and the EDS results are
tabulated in Table 2.1. EDS was focused on the centres of both types of particles
under SEM. It is important to note that EDS examines a small volume that is at
least 1 mm in diameter. In the case of spheres from iron spider silk samples,
it is difficult to separate the data from the spheres versus the data from the
fibrous material underneath. This will be addressed in later sections. The initial
concentration of iron contained in the iron spider silk mixture was calculated to
be 13.9 wt%, however, EDS shows 1.52 wt% iron in the solid film. A much higher
concentration of iron (36.2 wt%) was found using the pure Fe(Co)
5
. Significant
amounts of nitrogen and carbon are also found in the film since nitrogen is one
of the basic building blocks of amino acids, thus indicating the presence of spider
silk. The small concentration of fluorine is a result of HFIP. Similar concentrations
of fluorine for regenerated spider silk samples have previously been reported.
29
Two types of iron oxide occur naturally, Fe
2
O
3
and Fe
3
O
4
,bothhaving
ferromagnetic properties.
39
Nano iron oxide particles of 10-30 nm diameter can
be produced thermally from Fe(CO)
5
.
14
The orange/copper colour of spheres
observed from photolysis of iron pentacarbonyl have an iron concentration of
36 wt%, which is similar to that if Fe
2
O
3,
also orange/copper in color.
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