24 J. Bai et al.
contributing factor in the differences observed between the three sets of fracture
data is due to the variations in the dimensions of the microbridges. The volumetric
change of the microbridge depends on the time duration in which it is in contact
with water during the release step. A precise time control in the release step is
difficult to achieve.
2.3.6 Actuation of a Magnetic Spider Silk Microstructure
Spider drag-line silk is dissolved in a 1,1,1,3,3,3 hexafluoro-2 -propanol ( HFIP)
solution with a ratio of 1% w/w.
49,50
Ni particles with an average diameter of
2.6 μm were added to the spider-silk solution at a ratio of 15% w/w. A spin-on
process (30 seconds at 500 rpm) was used to form a Ni/spider-silk film on a silicon
substrate. Figure 2.12(a) shows the top view of the thin-film Ni/spider silk. While
porosity is evident throughout the film, this porosity can be reduced by decreasing
the weight ratios between the Ni particles and the spider-silk solution. The spider
silk matrix acts as a structural support, and the morphology of the Ni/spider-silk
film is dominated by the packing preferences of the Ni particles, which are likely
a result of Van der Waals forces between the particles. As shown in Fig. 2.12(b)
spider silk has a smoother surface and a lighter color than the Ni particles.
In order to demonstrate a Ni/spider-silk microstructure, a modified surface
micromachining process using a sacrificial etching technique was carried out to
fabricate a free-standing spider-silk microbridge. A plastic substrate was pre-
formed with a cavity using a conventional drilling process. A sugar solution
(50% wt) was poured into the cavity and air dried to form a sacrificial layer. The
Ni/spider-silk solution was then deposited using a 2 μl needle syringe, and air
a
b
Silk
Figure 2.12. SEM imaging of Ni-spider silk film (a)×90 top view (d)×10k top view.
SO13997_text.indd 34SO13997_text.indd 34 14/02/2011 11:19 AM14/02/2011 11:19 AM
Spider Silk as a MEMS Material 25
dried. Finally, the plastic substrate was immersed in water to ”etch” away the
sugar, leaving the free-standing Ni/spider-silk microbridge shown in Fig. 2.13
(3.5 mm × 1.3 mm × 0.2 mm).
A conventional M (magnetization) versus H (magnetizing field) hysteresis
loop was measured and plotted for the Ni/spider silk, as shown in Fig. 2.14.
A Quantum Design RSO SQUID magnetometer was used to characterize the
Ni/spider silk film at 300
K. The saturation magnetization (M
s
), coercivity (H
c
),
and remanence ratio (M
r
/M
s
) were found to be 0.028 emu/g,74O
e
and 13.4%,
respectively. These measurements were performed on a 14% w/w (Ni:spider-silk
solution) sample. Bulk Ni, on the other hand, has M
s
,H
c
and M
r
/M
s
values of 55
emu/g, 100 O
e
and 5%, respectively
51
. It can be seen that the M
s
of the Ni/spider
silk is significantly lower than that of the bulk Ni, indicating that the Ni/spider
silk is less magnetic. However, the values of H
c
and M
r
/M
s
fortheNi/spidersilk
are comparable to those of the bulk Ni, indicating that the Ni/spider silk material
retains ferromagnetic properties.
Figure 2.13. Fe/Ni 50%spider silk beam. For color reference, see page 254.
Figure 2.14. Hysteresis curve for 1:1 v/v iron spider silk sample. For color reference, see
page 254.
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