Biomaterials for MEMS

Characterization of Biomaterials 227

at this critical length scale. It has been used to study the hierarchical structure of a

biological material such as a bone. In particular it plays a crucial role in studying

the lowest hierarchical level, within the nanometer scale, where the distribution

of organic fibers (collagen) and crystalline mineral nano-particles has a major

influence for mechanical optimization of bone.

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10.3 SURFACE ANALYSIS METHODS

The surface structure characterization of a biomaterial presents different analyt-

ical challenges since the composition and spacial arrangement of the molecular

segments on the surface may not be representative of the bulk structure. In

addition, the surface is the interface where the biomaterial meets and interacts

with the molecular constituents of the biological host setting (e.g., bone, soft tissue,

blood, cells, body fluids). These interactions can modify the material and result in

macroscopic events in the biological media which ultimately determine whether

or not an MEMS device will have successful performance in the host biological

environment. Therefore, surface characterization will be discussed in more detail

in this chapter.

The properties that are of interest in the characterization of biomaterial sur-

faces include the chemical structure, the hydrophilicity or hydrophobicity, the

presence of ionic groups, the morphology and topography.

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Varying degrees of

information about these properties can be obtained using different analysis meth-

ods including microscopic, spectroscopic and thermodynamic and other methods.

We aim to provide an overview on some of the specific techniques frequently used

in the characterization of biomaterial surfaces. In addition, discussions and refer-

ences are made to emerging methods such as rapid Raman spectroscopy, optical

coherence tomography (OCT), and multi-photon excitation microscopy that have

recently come to the forefront of biomedical research and possess great potentials

for studying the interactions of biomaterials with real biological environment

in vivo.

10.3.1 Microscopic Methods

Microscopy techniques often used in biomaterial analysis include scanning elec-

tron microscopy (SEM), transmission electron spectroscopy (TEM), scanning tun-

neling microscopy (STM) and atomic force microscopy (AFM).

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Recent develop-

ments in light microscopy include confocal laser scanning microscopy (CLSM) and

optical near-field microscopy.

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10.3.1.1 Electron Microscopy

An electron microscope is a type of microscope that uses electrons to illuminate

a specimen in a vacuum environment and create an enlarged image. Electron

microscopes have much greater resolving power than light microscopes. Electron

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228 H. Zeng

microscopy can be performed in either transmission or secondary electron modes.

In transmission electron microscopy (TEM),

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images are formed by diffraction

or phase contrast mechanisms. The detectors are phosphor screens coupled

with digital CCD cameras. Unlike the TEM, where electrons of the high energy

beam form the image of the specimen, the scanning electron microscope (SEM)

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produces images by detecting low energy secondary electrons which are emitted

from the surface of the specimen due to excitation by the primary electron beam. In

the SEM, the electron beam is rastered across the sample, with detectors building

up an image by mapping the detected signals with beam positions. SEM and TEM

are commonly used for studying both the surface morphology of and the cellular

response to biomaterials.

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SEM sample preparation involves fixation (when proteins, cells, or tissues

are present), followed by drying, attachment to a metallic stub, and then coating

with a metal prior to data collection. The thin metallic coating is typically 20 to

30 nm in thickness. Common conductive metals used include gold, platinum,

or gold/palladium alloy. It should be noted that the drying and metal coating

processes used in the preparation of some polymeric materials might alter surface

morphology, particularly those surfaces that may undergo changes in a hydrated

environment. In addition to imaging the surface morphology of biomaterials, the

SEM can be combined with other analysis methods such as energy dispersive X-

ray analysis to determine elemental distribution

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and IR and Raman spectroscopy

to monitor surface modification procedures.

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Figure 10.2 shows an example

application of using SEM to assess the effect of sterilization on biomaterial surface

morphology.

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TEM sample preparation involves fixation (if biological materials are present),

processing, embedding and sectioning. Samples must be very, very thin (50 nm–

90 nm). This is achieved by embedding samples in hard plastic resins and

using very sharp diamond knife ultramicrotomes. TEM provides extraordinary

resolution of organelle and membrane structures in single cells. There is probably

Figure 10.2. SEM micrograph of as-deposited surface finish of sputter deposition elemen-

tal titanium (left side) compared with SEM micrograph of surface finish of autoclaved

sputter deposition elemental titanium (right side). Pictures were taken from a candidate

biomaterial for developing implantable MEMS devices. With permission from.

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