Biochips – fundamentals and applications - Electrochemical Sensors, Biosensors and Their Biomedical Applications - page 349

FIGURE 11.28

Silicon wafer-based electronic biochip (10 10 arrays) (see Plate 11 for color version).

leaving a negative charge region in the glass interface towards the silicon substrate, and
then the two wafers are electrostatically attracted to each other. The presence of oxy-
gen ions at the wafer interface will oxidize the bottom substrate and hence establish a
bond in between, so the wafers won't fall apart even if the electric fi eld disappears. The
bond is so strong that any attempt to break the bond will lead to fracture of the wafers.
In addition to the two methods mentioned above, several other alternatives like adhe-
sive, eutectic, and glass frit are also investigated.
Figure 11.28 (see Plate 11 for color version) shows a silicon wafer-based x-y
addressable bioarray that was made in the author's lab.

11.4.2.3 Fabrication technology for ceramic or plastic substrate
Large-scale biochip fabrication can be accomplished not only by lithographic tech-
niques but also using screen-printing (thick fi lm) processes [111] or popular PCB tech-
nology. The screen-printing technology relies on printing patterns of conductors and
insulators onto the surface of planar (plastic or ceramic) substrate. The screen-printing
process involves several steps including placement of the ink onto a pattern screen or
stencil, followed by forcing it through the screen with the aid of a squeegee, and dying/
curing the printed patterns [112]. Such a process yields mass-producible (uniform and
disposable) electrodes of different shapes or sizes. The electrochemical reactivity and
overall performance of screen-printed electrodes are dependent upon the composition
of the ink employed and on the printing and curing conditions (e.g. pressure, tem-
perature). Disposable potentiometric sensors can be fabricated by combination of ion-
selective electrode slides require microliter (10-15 L) sample volumes, and are ideally

suited for various decentralized applications. Mass-produced potentiometric sensor
arrays are also being developed in connection with future high speed clinical analyz-
ers. These are being combined with advanced materials (e.g. hydrogels) that obviate
the need for internal fi lling solutions. The screen-printing technology requires lower
capital and production costs than the thin fi lm lithographic approach, but is limited to

µ