Fabrication technologies - Micromixers Fundamentals Design and Fabrication - page 145

channels and fills them automatically due to capillary forces. After curing and peeling off the PDMS
master, polymer structures remain on the substrate surface. This technique can be used to pattern
silicon and glass with different materials [86] . Solvent-assisted micromolding uses a solvent to wet the
PDMS stamp and soften the structural polymer. The solvent only dissolves the structural polymer and
not PDMS. The stamp is pressed on a polymer film, which dissolves in the solvent and fills the gaps
between relief structures of the stamp. After dissipation and evaporation of the solvent, solid polymer
remains on the substrate [86] .

4.2.2.4 Fast prototyping of micromixers with soft lithography
Soft lithography is a popular method for rapid prototyping of microfluidic devices. The majority of
reported lab-scale prototypes were fabricated with this method. PDMS is a good device material
because it has a number of useful properties: low cost, low toxicity, transparency from the visible
wavelengths into the near ultraviolet wavelengths, and chemical inertness. The fabrication cost is
further reduced by using high-resolution laser-printed transparency mask. This type of low cost allows
feature size on the order of 20 m.

First, PDMS is mixed from the two commercially available prepolymers. The weight ratio of
the base and the curing agent is usually 10:1. A low-cost simple way is to fabricate the solid
master structuring an SU-8 layer using lithography ( Fig. 4.16 (a)). Glass posts are placed on the
SU-8 master to define the access holes and the reservoirs. These reservoirs and access holes can
also be punched later in the PDMS layer. The PDMS mixture is poured into the master and stands
for a few minutes to self-level. The whole set is then cured at relatively low temperature (from
60 Cto80 C) for several hours. After peeling off and having surface treatment with low-
temperature oxygen plasma ( Fig. 4.16 (b)), the structured PDMS membrane is brought into
contact with clean glass, silica, or another piece of surface-activated PDMS ( Fig. 4.16 (c)). The
sealed channel can withstand pressures up to five bars. Without surface treatment, PDMS also
forms a watertight seal when pressed against itself, glass, or most other smooth surfaces. These
reversible seals are useful for detachable fluidic devices, which are often required in research and
prototyping.

m

FIGURE 4.16

Fabrication of microchannels with soft lithography: (a) spin coating a silicon wafer with SU-8, UV exposure with
a clear field mask, development of SU-8 master; (b) pouring PDMS on the mold, curing, and peeling off the
PDMS part; and (c) surface treatment of PDMS in oxygen plasma and bonding to glass.