Biomaterials for MEMS

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.

SO13997_text.indd 21SO13997_text.indd 21 26/01/2011 3:49 PM26/01/2011 3:49 PM

SO13997_text.indd 24SO13997_text.indd 24 14/02/2011 11:28 AM14/02/2011 11:28 AM

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.

SO13997_text.indd 23SO13997_text.indd 23 26/01/2011 3:49 PM26/01/2011 3:49 PM