Biomaterials for MEMS

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

41−43

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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