Prototyping of microfluidic systems with integrated ... - DTU Nanotech

4 Introduction
molecule in the detector might easily be lost in the noise of the detector
electronics.
The time plotted on the graph is the time it takes for a molecules
to diffuse the given distance. This time is important, since the analysis
result is not reliable, if the concentration unintentionally changes e.g.
from the one side of a channel to the other, due to lack of time for the
molecules to diffuse, and reach an equilibrium concentration throughout
the entire channel width. It is seen that a molecule can diffuse 1 µm in
1 ms, and that diffusion on the largest scale in the graph, 1 mm, takes
in the order of 500 s or 8 minutes.
The optimal size of a microfluidic total analysis system is therefore
determined by a volume big enough to yield statistically correct results,
and with a number of molecules large enough to be detected by the
detector system available, but small enough to make diffusion time,
and thereby the time it takes to perform the analysis, short. Also, the
shrinking of the system allows parallelisation of the analysis, since more
units can be packaged into a given area. By comparing and evaluating
the parameters listed above, the optimal system size can be found for a
given application.
Furthermore, the optimal size for microfluidic systems is also determined
by the manufacturing processes. Limits in terms of e.g. size,
accuracy, and cost also influence heavily on the structure size in commercially
available systems.
Altogether, system dimensions between 10 and 100 µm seem to be
the best compromise between the factors of Fig. 1.1.
1.2 Prototyping of microfluidic systems
The scope of the work presented in the present thesis has been to
investigate, adapt and improve existing direct prototyping methods for
use with the polymer Topas.
Progress in metallurgy and material science in the last decade has
made miniaturisation of traditional end mills down to 5 µm possible.
Using high precision step motors for positioning of high speed air turbines,
micro milling has become a viable method for manufacturing small
quantities of structures with feature sizes down to the micrometer range.
For prototyping of e.g. microfluidic systems, this method has proven
to be an alternative to standard clean room processes, due to its low