Introduction 5
substrate. These commonly-used manufacturing methods have been implemented
for conventional industries and matured with values in cost effectiveness and mass
production. They are now being considered, with some adjustments and fine-
tuning in procedures, processes, apparatus, and formula, for advanced bioMEMS
and biomaterial fabrication and manufacturing.
1.3 BIOMEDICAL MICRODEVICES
MEMS community has been focusing on many biological applications, and the
field of BioMEMS has grown rapidly and significantly. Some of the demon-
strated devices and fabrication materials are summarized in Table 1.2. This
summary only serves as an example of devices and materials that are related
to the BioMEMS fields and not purposely excluding other materials that have
been demonstrated in MEMS devices. Most of the MEMS applications focus
on non-implantable applications, such as diagnostic tools or tissue engineering
devices. These devices or applications have fewer concerns toward material uses
and device architecutures. Implantable MEMS drug delivery devices have also
been demonstrated.
25
Commercial efforts have been pursuit (MicroCHIPS, Inc.,
Bedford, MA, USA). Micro-scale silicon reservoirs filled with drugs are sealed by
thin metallic films. The metallic film is connected to a battery. When a voltage
is applied, electrochemical reaction causes corrosion on the film and drug can
be released by diffusion. Another implantable MEMS device is a micro strain
gauge for applications in orthopedics. Thin-film strain gauges are fabricated and
connected to an RF module.
26
The powering and sensor data transfer are done
using electromagnetic induction wirelessly. Microelectrodes have also been used
to record or stimulate neurological activities in brain.
27
Bulk micromachined
silicon needle arrays integrated with CMOS circuitry were used to record neuronal
signal from the motor cortex and the signals were amplified and transmitted to
a computer. The brain prosthetic system serves as a brain-computer interface to
control artificial limbs with specific neuronal activities. Similarly, smaller arrays of
microneedles can also be used with integrated circuits for electrical stimulations
on neurons for tremor control or pain management.
28,29
Recently, researchers
from ETH have demonstrated elegantly a MEMS device for ophthalmological
applications.
30
A contact lens was attached with a thin-film metal inductive
coil and metallic strain gauges. An ASIC chip was mechanically thinned down
to 50 μm thick and attached onto the contact lens to regulate power and data
transmission. The pressure in eyes then could be monitored continuously in vivo
for a long period of time.
These implantable devices often face tremendous technical challenges. First
of all, the size requirements for implantation as the body cavity may be small
present a design and fabrication challenge for the functionalities required. MEMS
technology offers great advantages to address this issue. Secondly, in order for the
implant to stay inside the body in a long period of time, biocompatibility needs to
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6 M. Chiao and J.-C. Chiao
Table 1. 2 Biomaterials used in MEMS and microfluidic devices.
Diagnostic Tools Actuators Tissue Engineering
semiconductor material
silicon PCR
32
microneedles;
33
neuron growth
34
surgical tools
35
polymers
PDMS PCR;
36
flow cytometry
37
drug delivery;
38
cell and neuron growth
39, 40
pumps
4143
hydrogel cell-interacting matrix
44
polyimide microneedles
45
other materials
iron-based magnetic particles magnetic tagging
46
pumps
43
piezoelectric material (PZT; ZnO) acoustic wave sensor
47
pumps
48
titanium microneedles
49, 50
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