usability. Accessibility to equipment and techniques are also indispensable part of
the selection process. Scale of production is another factor, which plays role in
method selection. Last but not least comes available time for manufacturing and
testing complex variations in the device architecture. It is important to think about
both experimental and fabrication time as factors that need to be considered when
choosing a fabrication strategy. Minimizing the fabrication guarantees more project
time on experiments.
Fabrication methods can be grouped in direct: those translating the designed
features in one rapid step; and indirect—translation occurs in multiple stages
between the creation of the technical drawing and the production of the features
(Fig.3.2). An example for direct method is laser ablation, where a computer-aided
design (CAD) file is sent to the laser, and the fabrication can start after a definition
of the power, speed and firing rate of the laser head.
The choice of microfabrication methods depends (1) on the functionality that
you want to achieve with your device and (2) on the overall operational conditions.
A system designer takes in consideration the extreme working range of conditions
where the device should remain operational. Typically, for laboratory based bio-
logical investigations there are no extremums in temperature as most applications
run below 40C. But if your microfluidic chip is meant as reusable, autoclavable
and resistant to polar aliphatic solvents, such as acetone for instance, then a proper
material should be selected (Table3.1). This means that a material must meet the
following criteria: glass transition temperature above 121 C, so it can be
autoclaved; plus, it must have high resistance towards solvents. The suitable
candidates, polycarbonate (PC) and cyclic olefin copolymer (COC), are then
considered with regards to all available fabrication techniques. Some additional
requirement is necessary to narrow down the list of possible fabrication candidates
and to select one. Such selection criterion is the maximum possible resolution of the
fabrication method. If we return to the hypothetical case study, and for argument
sake a minimum feature size of 50μm is considered for fully-fledged device, then
the viable options are milling, hot-embossing casting and moulding. However,
economical and time constrains play significant role in the decision making process
Fig. 3.1 Design iterations.
The design cycle begins
with list of target
functionalities, followed by
calculation of the critical
dimensions, technical
computer-aided drawing,
determination of suitable
materials and fabrication
methods that can obtain
those dimensions
86 N. Dimov