An optimised embossing process can use the same master over again without any
wearing of its features or compromise of the replica quality. Moreover, the method
has short processing times (40 min per device), which makes it suitable for rapid
prototyping of devices on small to medium scale [ 1 ].
2.4 Micromilling
Micromilling is a subtractive fabrication process based on the mechanical removal
of material by a cutting tool from a stock piece, also known as substrate or
workpiece, resulting in micrometre-sized features. It is a direct method enabling
transfer of 3D designs into bulk material, which makes it suitable for machining
masters used in hot embossing, injection moulding and resin casting or as an
immediate microfluidic device. The micromilling is applied for rapid prototyping
in polymer and metal substrates. Resulting architectures can be utilized multiple
times as masters for casting resins such as polydimethylsiloxane (e.g., PDMS)
and/or can be integrated in the final version of a device. Typically, for microfab-
rication purposes, dimensions of the features fall in the range from 25μm; also can
decrease down to 3μm depending on the class and price range [ 1 ]. The lower price
range machines provide comparable quality and accuracy as their mid-range coun-
terparts (Fig.3.6a). However, the midrange milling machines come with extra
functionalities such as automatic holders and substrate aligners. Variations of
milling machine designs exist where additional degrees of freedom are available,
i.e.rotational axle. The development of milling has shifted the lower limit bound-
aries even further down (15μm) lateral obtained with basic functionality CNC, that
is available for 15 k USD, thus bringing mechanical methods closer to the level of
precision obtained with soft lithography. When automation is required together
with high accuracy the price can exceed 200 k but guarantees better quality and
high throughput.
2.5 Choice of Instrument and Relation to Material
The choice of cutting tools, substrates and milling parameters is driven by the
intended functionality of the final product, also by the design complexity and stage
of design development. At the beginning of the design process, when a concept is
sought and single devices are investigated direct manufacturing in polymer might
be sufficient. Metal substrates (e.g., brass, copper) are used for milling moulds in
advanced design stages that intend multiple replications and produce large number
of devices. The choice of substrate relates to the final functionality of a microfluidic
device and is typically difficult because of the large variety of substrates. Choosing
milling-tools, however, is straightforward and depends on the features that need to
be fabricated, milled material maximum spindle speed.
94 N. Dimov