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.
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
Figure 2.12. SEM imaging of Ni-spider silk ﬁlm (a)×90 top view (d)×10k top view.
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