Chapter Two
Spider Silk as a MEMS Material
J. Bai
,W.C.Chu
, J.-C. Chiao
and M. Chiao
Department.of Mechanical Engineering, The University of British Columbia, Canada
School of Environmental Health, The University of British Columbia, Canada
Department of Electrical Engineering, University of Texas at Arlington, USA
§
E-mail: muchiao@mech.ubc.ca
A thin-film made of dragline spider silk is produced and fabricated at room temper-
ature using a spin-cast technique; the film’s micromachining, optical and mechanical
characterization is obtained. The mechanical strength of spider silk film is 5 to
10 times greater than the reported strength of hydrogel. Magnetic spider silk thin-film
is also made by photolysis of iron pentacarbonyl mixed with spider silk in solution. A
free-standing microcantilever beam is fabricated and actuated under a magnetic field.
2.1 INTRODUCTION
Due to the limitations of polysilicon, in particular, its lack of flexibility in biomed-
ical applications and its expensive processing cost, numerous studies have been
undertaken to investigate alternate materials for microelectromechanical process-
ing and applications. Therefore, polymeric materials like polymethylsiloxane
(PDMS) and hydrogel have been used to fabricate micromechanical components
such as microfluidic valves and micropump diaphragms. These materials do offer
more economical fabrication and elasticity than polysilicon does; however, their
mechanical strength is not high enough for many biomedical applications.
1
On the other hand, spider silk, a natural biopolymer, has been shown to
possess good biomaterial qualities and can potentially be engineered to change its
mechanical and chemical properties to fit certain application requirements. Spider
silk is one of the strongest natural materials with a high tensile strength and tough-
ness. The amount of energy required to break spider silk is three times higher than
Kevlar
18
and more than 25 times higher than steel.
8
These preliminary results have
led to much research on fabricating spider silk for artificial ligaments, tendons,
Biomaterials for MEMS, Edited b y M. Chiao and J.-C. Chiao
Copyright © 2011 by Pan Stanford Publishing Pte. Ltd.
www.panstanford.com
978-981-4241-46-5
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12 J. Bai et al.
surgical sutures or biodegradable membranes, bullet proof vests and structural
applications. Moreover, silk based scaffolds for bone formation offer options to
address the limitation of existing materials, such as collagen (which has poor me-
chanical properties), poly(lactiv-glucolic) acid (which induces inflammation), and
hydroxypatite (which is not completely biodegradable).
32
Biomembranes made of
spider silk film can also be used for pumping or as valves in microfluidic channels.
Spider silk’s robust structure and modifiable surface chemistry make it an ideal
candidate as a chemical sensor or actuator. Recombinant spider silk proteins
in plants and goat milk have been successfully demonstrated
16,27
to increase the
commercialization potential of spider silk dramatically.
Dragline silk is a type of spider silk made up of two different but similar
proteins (fibroins). In the secondary structure of these proteins, alanine-rich
regions organize into beta-sheets which form the core of crystalline structures
held together by hydrogen bonds. The glycine rich regions are less ordered.
It is this combination of crystalline and amorphous regions that is responsible
for the strength and extensibility of spider silk.
1,15,33
Regenerated spider silk is
first harvested from spiders, dissolved into solvents and then re-spun through an
orifice. Mechanical properties, such as the toughness of the regenerated silk, rely
largely on the assembly process of the proteins during re-spinning and drying.
However, the tensile strength of native silk is found to be three times higher than
that of regenerated spider silk.
30
Many microelectromechanical systems (MEMS) require a magnetic aspect to
be incorporated in the function; therefore, studies to incorporate magnetic prop-
erty components into MEMS structures have been undertaken.
7
For example, thin-
film NiFe attached to a polysilicon micro cantilever beam was shown to actuate
under a magnetic field
13
More recently, polydimethylsiloxane (PDMS) was mixed
with NdFeB particles to form a magnetic membrane used in a micro pump.
41
Several magnetic oxide nanoparticles, including Fe
2
O
3
and magnetite, have been
synthesized with particle sizes between 4–16 nm by using microemulsion, elec-
trochemical deposition, and other methods.
11,35
Composite films consisting of
iron-iron oxide have also been achieved through chemical vapour deposition of
iron pentacarbonyl.
19
More importantly, polymer coated magnetic nanoparticles
have been fabricated using thermal deposition in the presence of ammonia and
polymeric dispersants.
2,3
However, most of these magnetic incorporation methods
require high temperature, high pressure or very expensive instrumentation to aid
in the fabrication.
This chapter will describe a new magnetic spider silk composite fabricated
using a regenerated spider silk matrix and iron pentacarbonyl (Fe(CO)
5
) at room
temperature and atmospheric pressure. Nanoparticles of iron oxides were made
via photolysis of iron pentacarbonyl using UV light instead of thermal or chemical
decomposition. Fourier Transformed Infrared Spectroscopy (FTIR) was used to
characterize the interaction of iron pentacarbonyl with spider silk proteins. A
micro cantilever beam made of iron oxide nanoparticles in thin-film spider silk
was fabricated. The micro beam was actuated using magnetic field.
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