Biomedical Engineering Reference
In-Depth Information
beginning of the 1980s, research had moved on and the technique of micro-manufacturing
was used to create actuators made up of polycrystalline silicon used in disk read heads.
At the end of the 1980s, the potential of MEMS was widely recognized and their appli-
cations increasingly entered the fields of micro-electronics and biomedicine.
MEMS-based sensors are a class of devices that build very small electrical and mechan-
ical components on a single chip. MEMS-based sensors are a crucial component in
automotive electronics, medical equipment, hard disk drives, computer peripherals, wire-
less devices, and smart portable electronics such as cell phones and PDAs.
For automotive safety, acceleration sensors provide crash detection for efficient deploy-
ment of forward and side airbags, as well as other automotive safety devices. Accelerom-
eters are also used in electronic stability control (ESC) to measure the lateral acceleration
of the vehicle and to help drivers maintain control during potentially unstable driving con-
ditions. Acceleration sensors are used to detect whether or not the car is moving, in order
to save power, and to measure wheel speed and/or direction of rotation. In cell phones,
MEMS products activate different features by using the more natural hand movements of
tilting, rather than pushing several buttons.
In specialized healthcare monitoring applications, pressure sensors provide key patient
diagnostics. MEMS technology utilizes different sensing technologies, including capac-
itive, piezoresistive, and optical. For example, piezoresistive three-dimensional tactile
sensor arrays, suitable for robotic applications, have been developed by Mei
et al
.[3].
A polymer-based MEMS tactile sensor has been used for texture classification by Kim
et al
. [10]. In a novel design by R. Ahmadi
et al
. [11], a magnetic resonance imaging
(MRI)-compatible optical-fiber MEMS tactile sensor was developed, without any need to
use the array of this sensor to measure the distributed tactile information.
2.7 Piezoresistive Sensors
Change in resistivity of a semiconductor due to applied mechanical stress is called piezore-
sistivity and is a widely used sensor principle. Nowadays, this principle is used in the
MEMS field for a wide variety of sensing applications, including accelerometers, pressure
sensors [12], gyro rotation rate sensors [13], tactile sensors [14], flow sensors, sensors for
monitoring the structural integrity of mechanical elements [15], and chemical/biological
sensors.
In metal, resistance change is caused by an alteration in its geometry (length and cross
section) resulting from applied mechanical stress. In semiconductors, in addition to the
geometric effect, the resistance of the material itself also changes due to the change in
the mechanical state of stress. Consequently, there are two important ways by which
the resistance value can change with applied strain. The magnitude of resistance change
originating from the piezoresistive effect is much greater than what is achievable from
the change in geometry. In particular, doped
1
silicon exhibits remarkable piezoresistive
response characteristics among all known piezoresistive materials [16, 17].
The fact that the resistivity of semiconductor silicon may change under applied strain
is fascinating. In semiconductors, changes in inter-atomic spacing resulting from strain
1
In semiconductor production, doping is intentionally introducing impurities into an extremely pure (also referred
to as intrinsic) semiconductor for the purpose of modulating its electrical properties.
