Biomaterials for MEMS

20 J. Bai et al.

slows down. This is shown by overlapping of the five minute spectrum with the

40 minute spectrum.

Finally, a small shift in wave number at the 1823 peak is shown between the

one minute and five minute spectra. Since the wave number shifts when the

sample is in different states (such as liquid, solid or gas), it is likely that the shift

here is a result of the sample not being completely dried when the spectrum was

taken after one minute of UV exposure.

For the magnetic spider silk material, the formation of the observed spheres

is a direct result of mixing iron pentacarbonyl with the regenerated spider silk

solution. The spheres contain a concentration of Fe that is slightly higher than the

rest of the material. Fibrous and porous material containing a low concentration

of iron also formed as a result of this mixed solution. In order to further explore

the reason behind this phenomenon, an FTIR was performed to study any binding

that could have occurred between the iron particles and the spider silk proteins.

2.3.4 Squid

A SQUID was used to characterize the magnetic hysteresis of the material.

Figure 2.8 illustrates the M vs. H hysteresis plot for a solid iron spider silk

sample at 300 K. The saturation magnetization is found at 1.4 × 10

−5

emu/g,

which is 105 times smaller than the previously reported level for polymer coated

iron particles.

2

This is not surprising, as the iron wt% in the reported particles is

around 21%; whereas the amount of Fe in this sample is only around 1.5–3 %. The

remanence ratio of 16.6% and coercivity of 30 Oe was also found. These two values

are in the same range as the ones reported for polymer coated iron particles.

2.3.5 Micromachining and Mechanical Testing of a Spider Silk

Microbridge

The mechanical testing results of the film performed by a nano indentation

machine [mts] are summarized in Table 2.2. The elastic modulus and hardness

decrease as the % w/w ratio of silk to solvent decreases. This is likely due to

the difference in fibroin concentration. However, an advantage of the 0.5% w/w

solution is its film consistency, reduction in surface roughness, and its increased

sensitivity to the effects of spin-coating speed. As shown in Fig. 2.1(a), the film

porosity appears to be less prominent in the upper region compared to the lower

region of the cross section. Mechanical testing data were mostly collected in

the upper region of the film thickness (the first 500 nm), and this could be a

contributing factor to the high Young’s modulus value, which is not common in

porous materials.

Data on the viscoelastic properties (loss tangent) of the film were obtained

for the first time in this experiment. The loss tangent showed no dependency on

% w/w dilution in the range used and was found to be similar to the loss tangent

of human bone specimens (0.05).

38

The data collected using the nano indentation

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Spider Silk as a MEMS Material 21

Figure 2.8. Sacrificial etching process steps to fabricate spider silk microbridge. For color

reference, see page 253.

Table 2. 2 Mechanical testing of thin film spider silk

Sample Elastic Modulus Hardness Viscoelasticity

(GPa) [std] (GPa) [std] (loss tangent) [std]

1% w/w 7.251[0.494] 0.276 [0.005] 0.0436 [0.0065]

0.5% w/w 4.41 [0.342] 0.182 [0.012] 0.182 [0.012]

machine takes into consideration the substrate effect on thin film tests. This has

been previously documented in many studies.

4,26

Using a widely accepted rule of

thumb, the intrinsic property of the film can be reasonably detected with minimal

effect from the substrate when the indentation depth is kept to 10% of the film

thickness.

37

Only consistent mechanical property data within this region is used in

this work. In addition, the thicknesses of the films used are in the range of 5–10 μm,

which should improve the accuracy of the nano indentation data.

In order to demonstrate a micromachined spider silk microstructure, a mod-

ified surface micromachining process with a sacrificial etching technique

36

was

carried out to fabricate a free-standing spider silk microbridge. Figure 2.8(a),

shows a silicon substrate that is anisotropically etched to form a cavity. Sugar

solution (50 wt%) was poured into the cavity and air dried to form a sacrificial

layer (Fig. 2.8(b)). The spider silk solution was then deposited using a 2 ml needle

syringe and air dried (Fig. 2.8(c)). Finally, the silicon substrate was immersed in

water to “etch” away the sugar (Fig. 8(d)), leaving the free-standing spider silk

microbridge shown in Fig. 2.8(d). Figure 2.9(a) shows a SEM microphoto of the

spider silk microbridge (800 × 800 × 40 μm

3

).

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