Biomedical Engineering Reference
In-Depth Information
by virtue of their weakly cross-linked polymer chains. Accordingly, the three most widely used
methods for micromolding polymers are
injection molding
(for thermoset and thermoplastic
polymers),
hot embossing
(for thermoplastic polymers), and
sot lithography
(for elastomeric
polymers).
1.5.1 Injection Molding
If the polymer is applied onto the mold from the molten phase, it is usually a viscous liquid that
does not low well, so the mold cannot have narrow, deep features that might otherwise trap
bubbles. he molten polymer, in addition, degrades quickly in air, so it is best to quickly apply
the polymer at high pressure using a pneumatic piston—a technique dubbed
injection molding
.
A vast majority of the polymeric objects that surround us—toys, furniture, bottles, etc., as well
as the widely used polystyrene petri dishes and lasks for cell culture—are molded by injection
molding. Unfortunately, the required pressures are incompatible with the brittle silicon sub-
strates used in photolithography, so the master must be fabricated on a more solid substrate,
usually a metal surface, onto which the photoresist pattern is transferred by etching (the photo-
resist does not survive the injection molding temperatures). he large diferences in thermal
expansion between the mold and the polymer can make release diicult as the temperature is
lowered. In addition, melting of the polymers generates hazardous vapors that must be dealt
with, typically by using a fume exhaust system that is exorbitantly expensive to install.
1.5.2 Hot Embossing
When only surface features on a lat polymer substrate are sought, heating the mold above the
glass transition temperature (
T
g
) of a thermoplastic polymer and pressing it onto the polymer
substrate results in embossed features—a method termed
hot embossing
. (he technique is also
known as
nanoimprinting
or
nanoimprint lithography
in the nanotechnology community,
but “hot embossing” is more descriptive.) Compared with injection molding, hot embossing
does not require pneumatic injection and does not produce appreciable vapor amounts, so it can
be easily implemented on a laboratory benchtop. he success of the replication is determined by
the
T
g
(the lower the better) and by the diference in thermal expansion coeicient with respect
to the mold (the smaller the better). Examples of polymers that replicate well with hot embossing
are poly(methyl methacrylates) (PMMA;
T
g
= 106°C), polycarbonates (
T
g
= 150°C), polystyrenes
(
T
g
= 80°C-100°C), and cyclic olein copolymers (
T
g
= 138°C). All these materials have a similar
linear thermal expansion coeicient in the range of 60 to 90 × 10
−6
K
−1
. Hot embossing is the
“poor man's version” of injection molding—its main disadvantage is that the molding of deep
features produces large displacements of material that have to be redeposited somewhere, usu-
ally at the top of the trenches, forming nonplanar accumulations that are feature-dependent.
1.5.3 Curable Polymers
If the polymer is applied onto the mold as a solvent-based solution, shrinking of the polymer as
the solvent is evaporated can be problematic—for example, residual stress, resulting in warping
ater release, and loss of resolution. hus, a favored strategy is to
create
(or “cure”) the polymer
irreversibly
on
the mold by using the monomer solution and a cross-linker or initiator. Epoxies,
polyurethanes, and silicones are examples of
thermosetting polymers
(also called
thermosets
)
that can be formed this way. (Despite the name, the curing reaction can be done by heat, a
chemical reaction, or irradiation.)
A particularly successful micromolding polymer is the elastomer PDMS, a type of silicone
rubber which is thermally cured as a two-component mixture. PDMS micromolding is the basis
of a family of sister techniques that George Whitesides and colleagues at Harvard ingeniously
baptized as “
sot lithography
”
(from the sotness of PDMS). hese techniques have had such an
enormous impact on BioMEMS that they deserve a section of their own.
