Biomedical Engineering Reference
In-Depth Information
2.4.5
Temperature Sensor
Temperature sensors are an important component for integrated circuits to become
viable in medical diagnostics. Cells must often be viable for extended periods
of time, requiring on-chip incubation and temperature monitoring. Analytes and
physiological fluids must frequently be maintained at specific temperatures for
proper chemical activation. Biochemical reactions are also often initiated and
terminated by temperature changes-a well-known example being polymerase chain
reaction (PCR). To this end, recent IC/microfluidic chips have had multiple tem-
perature sensors integrated into their architecture, allowing the temperature to be
monitored independently over different regions of the chip [
12
]. They consist of
micron-scale thermistors built under the IC surface. They work by monitoring
temperature-dependent resistance changes and calibrating them to a temperature
scale. A feedback loop with an external cooling apparatus then maintains the chip
at the desired temperature.
2.4.6
Microwave Dielectric Heating
Microwave dielectric heating has been developed for warming small, micron-sized
droplets in oil [
32
]. Electric fields in the 1-3-GHz bandwidth - in the microwave
spectrum - are transmitted across droplets, causing rapid and well controlled heating
of the droplets without heating the surrounding environment directly. The advantage
of heating such small-length scales is the high surface-to-volume ratio, allowing
rapid thermal cycling in the 15-ms range. The technology is scalable onto an
integrated circuit platform, with each individual pixel acting as a microwave heater.
This enables the highly localized heating and incubation of droplets and cells.
2.4.7
Capacitance Sensor
Sensing the capacitative coupling between a pixel and an object on top can be used to
inform the device where droplets and cells are located. This is seen in Fig.
2.16
.This
is critical for giving the chip feedback capabilities and is a method for error checking
that a cell or droplet indeed moved to the position it was instructed. Capacitative
sensing can eliminate the need for the hybrid integrated circuit/microfluidic chip
to be placed under an optical microscope. In Fig.
2.16
, capacitative sensors were
embedded under every pixel, giving high-resolution images of cells and beads
resting on the chip's surface [
17
]. The sensors discerned differences in electrical
properties in the medium directly above the chip, and mapped them as grayscale
images. Fig.
2.16
shows the image obtained from the chip's embedded sensors next
to an image of the same region taken with an optical microscope.
