Microfluidics - Introduction to BioMEMS - page 123

3.5 Fabrication of Microluidic Channels
Although nature may have had billions of years to optimize its microluidic systems, engineers
have had only a few decades. Lithographic methods were irst used for microluidics in the 1980s,
but have been largely supplanted by methods that make less expensive inal devices. Although
some microluidic devices can be very simple, there has been increasing emphasis on creating
luidic systems that allow multistep processes or highly parallel processing of multiple samples.
As a consequence, there is a premium on fabrication methods that allow complexity of function
through multiplexing, crossing of luid paths, valving, and other sophisticated architectures
typically found in macroscopic luidic systems like petroleum reining plants.
3.5.1 The Building Materials
Dozens of diferent materials have been used for microluidics. Here, we will just mention a few
of the most common ones along with their advantages and disadvantages.
3.5.1.1 The “Historical” Materials: Silicon and Glass
Most of the initial microluidic devices were made using silicon because the original microluidics
researchers were refugees from microelectronics research. Silicon channels have generally been
made using conventional photolithography, so pits and channels can be formed in either one or
both sides of a Si wafer. he process usually proceeds through the design of one or more photo-
masks, and photoresist spinning, patterning, and chemical etching (either wet or dry). To make
a complete luidic system, these etched parts must be at least partially sealed. Although gluing is
possible with very large crude devices, the most successful devices have used either Si-Si bonding
or anodic bonding to a glass with a very similar coeicient of thermal expansion such as Pyrex
(because the bonding is done at very high temperatures, a thermal coeicient mismatch would
crack the wafer on cooling). he oxide of Si is silica, which has a well-deined pH-dependent surface
charge, making it very useful for a variety of microluidic processes like electro-osmotic pumping.
Etched Si-based devices can be made with very ine features, but are expensive, fragile, and
do not hold up to strongly alkaline solutions, which can etch the Si and its oxide coating. Silicon
is nearly opaque in the visible and UV regions of the spectrum, so optical methods can only be
used by imaging through glass layers bonded to it, generally eliminating transmission monitor-
ing, but allowing luorescence imaging. he fact that Si is also a semiconductor, and therefore
not a good electrical insulator, is also a handicap in one very common microluidic applica-
tion—electrophoresis, in which large voltage drops are oten required. For capillary electropho-
resis on a chip, all-glass devices are the norm. he isotropically etched glass parts can be sealed
by apposing two or more parts and increasing the temperature of the entire device to nearly
the melting temperature of the glass. he etching processes for both Si and glasses requires
very reactive and dangerous chemicals, such as hydroluoric acid, best done in a professionally
stafed microfabrication laboratory; selective glass etching poses additional challenges because
very few materials resist a deep glass etch. Consequently, the inal devices tend to be expensive.
his is ine for devices to be used for days to years (for example, an implanted microluidic device
or an environmental monitor), but not so good for single-use applications (like a point-of-care
diagnostics device for use with human blood). Also, the high-temperature sealing processes
(upwards of 300°C) destroy any organic molecules in or on the devices, so any biomolecular or
antifouling coatings for the devices must be applied ater assembly.
3.5.1.2 The Advent of Plastics
For low-cost luidic devices (micro or macro), the selection of choice in the industry has long
been injection molding. Many thermoplastics (including inexpensive ones like polystyrene and
polypropylene) can be molded into devices with extremely small features, as long as the topol-
ogy allows removal by opening a mold. Although the end product parts can have a cost of pen-
nies each, the cost of making the irst mold is in the tens of thousands of dollars, so the irst