Description of Instrumentation and Procedures - Advances in Biochemical Engineering Biotechnology - page 10

milled into the plastic base to a depth of 0.5 mm. Electrically-insulated thermi-
stors in direct contact with flow stream were placed outside the enzyme cell after
the porous polyethylene filters. The enzyme cell was charged with the enzyme
preparation prior to compaction. Ready access to the enzyme compartment
makes replacement of enzymes possible.

2.2.2
Microcolumn Sensor

The microcolumn sensor (inner diameter x length: 0.6 15 mm) was construct-

ed of stainless-steel tubing and is free of auxiliary components (see Fig. 4).
Microbead thermistors (thermistors shaped like a bead) were directly moun-
ted in the reference and measurement probes on the outer surface of the inlet
and outlet gold tubings, using heat-conducting epoxy. Both the length (about
200 mm) and inner diameter (0.15 mm) of the inlet tubing between the sample
valve and the column were minimized, in order to reduce sample dispersion
during transport in the flow system. Similarly, in order to reduce heat loss,
simple and short connections between the column and the inlet or outlet
tubings were required.

¥

2.3
Microsystems

A planar substrate, such as silicon wafer, could be micromachined by a sequen-
ce of deposition and etching processes. This results in three-dimensional micro-
structures which can be implemented in cavities, grooves, holes, diaphragms,
cantilever beams etc. The process referred to as silicon micromachining often
employs anisotropic etchants such as potassium hydroxide and ethylene diamine
pyrocatechol. The crystallographic orientation is important as the above-men-
tioned etchants show an etch-rate anisotropy. The ratio for the (100)-, (110)- and
(111)- planes is typically 100 : 16 : 1. The technique of electrochemical etch stop
could be applied for control of the microstructural dimensions. An alternative

Fig. 4. Illustration of a miniaturized ET system (refer to text for details)