Fabrication technologies - Micromixers Fundamentals Design and Fabrication - page 142

4.2.2.2 Hot embossing
Hot embossing was widely used for the fabrication of simple microchannels. The technique uses
a master mold and a flat polymer substrate. The polymer substrate is heated above the glass transition
temperatures of the substrate material. The glass temperature is typically in the range from 50 Cto
150 C. Embossing force (0.5 to 2 kN/cm 2 ) is then applied on the substrate under vacuum conditions.
Before release, the master and the substrate are cooled under the applied embossing force. The entire
hot-embossing process takes about few minutes [83 85] . The vacuum is needed due to the formation

of gas bubbles in the small structures. The vacuum also prevents corrosion of the master. The drawback
of this technique compared to injection molding is the relatively long cycle time on the order of several
minutes.
The most important parameters of hot embossing are embossing temperature, deembossing
temperature, embossing pressures, and hold time. Table 4.8 lists the typical values of these parameters
for hot embossing of PMMA and PC. Hot embossing can only make open channel structures.
Fabricating covered channels and fluidic interconnects needs additional packaging techniques, such as
thermal bonding to a sheet of the same material at temperatures above the glass temperature.
Injection compression molding combines the advantages of both injection and hot embossing. The
polymer melt is first injected into the mold. The mold melt is then compressed to shape the final part.
The low viscosity of the melt results in good filling in the molded part.

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4.2.2.3 Soft lithography
Soft lithography is a direct pattern transfer technique. The term “soft” refers to an elastomeric stamp
with patterned relief structures on its surface. Polydimethylsiloxane (PDMS) has been used
successfully as the elastomeric material. PDMS exhibits unique properties suitable for this purpose.
PDMS has an inorganic siloxane backbone with organic methyl groups attached to silicon (see
Fig. 4.13 ). Both prepolymers and curing agents are commercially available. PDMS has high optical
transparency above a wavelength of 230 nm and low self-fluorescence. PDMS has a low interfacial
free energy, which avoids molecules of most polymers sticking on or reacting with its surface. The
interfacial free energy of PDMS can be manipulated with plasma treatment. The modified surface
properties of PDMS are needed for certain applications. PDMS is stable against humidity and
temperature. This material is optically transparent and can be cured by UV light. PDMS is an elas-
tomer and can therefore attach on nonplanar surfaces. PDMS is mechanically durable. These char-
acteristics make PDMS an ideal material for soft lithography [86] .
PDMS also has a number of drawbacks, such as swelling, shrinking, and elastic deformation. The
design of a PDMS part should consider the shrinking effect upon curing. A number of organic solvents
can swell PDMS as well. Furthermore, elastic deformation can limit the aspect ratio of the designed

FIGURE 4.13

Chemical structures of PDMS.