Page 32 - Optofluidics Fundamentals, Devices, and Applications

Basic Micr ofluidic and Soft Lithographic Techniques 13

2-3-3 Optical Properties of PDMS
PDMS is optically transparent from 240 to 1100 nm [19], and has a
refractive index around 1.41. It has negligible birefringence. It is
therefore possible to enclose optofluidic components in PDMS, and
couple light through PDMS, with minimal loss due to absorption.
Commercially available PDMS—Silgard 184—does, unfortunately,
contain nanoparticles of silica that introduce unwanted scattering
of light. In the devices we and others have fabricated, the thickness
of PDMS for enclosure of microfluidic components is limited
(usually < 1 cm), and thus scattering due to passage of light through
PDMS does not cause significant loss during the coupling of light
into and out of the devices.
We have not identified a polymer that lacks these scatterers,
and still possesses the other desirable qualities of PDMS. The Nor-
land optical adhesives (photocurable polyurethanes), for example,
contain no scattering particles, but they are not soft, and cannot be
processed the same way as PDMS. This need for an elastomeric
polymer with high optical transparency and easy sealability pres-
ents an opportunity for future research in material science. To sum-
marize, PDMS has attractive features that make it useful for a wide
range of applications in laboratory, and for prototyping in research,
though it may not be the ultimate material used in large-scale man-
ufacturing. Other polymers used for fabricating microfluidic sys-
tems include h-PDMS, photocurable perfluoropolyethers (PFPE),
cyclic olefin copolymer (a thermoplastic polymer), thermoset poly-
ester, polymethylmethacrylate, polycarbonate, and polyurethanes [31].
Each material has its own advantages and disadvantages; depend-
ing on the application, one material may be more suitable than the
other. For example, PFPEs, a class of fluoropolymers that are liq-
uids at room temperature, are chemically resistant (like Teflon).
They are compatible with organic solvents such as toluene and
dichloromethane (both of which swell PDMS). The fabrication pro-
cess for channels in PFPE involves procedures that are more com-
plicated than with PDMS, however. There is no simple procedure
for adhesive-free contact sealing, and these polymers are much
more expensive.

2-4 Fabrication of Microfluidic Systems in PDMS
Systems in PDMS are typically fabricated using techniques in soft
lithography [19]. Soft lithography involves the replication of a topo-
graphically defined (typically in photoresist) structure on a master in
a soft elastomer. The process can be carried out in ambient laboratory
conditions. Replication can also be repeated multiple times. Soft
lithography therefore enables rapid, simple, and inexpensive fabrica-
tion processes.