Biomaterials for MEMS

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: [e-mail]

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

SO13997_text.indd 19SO13997_text.indd 19 26/01/2011 3:49 PM26/01/2011 3:49 PM

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.

SO13997_text.indd 20SO13997_text.indd 20 26/01/2011 3:49 PM26/01/2011 3:49 PM