Biomedical Engineering Reference
In-Depth Information
features to an underlying substrate pattern because the pattern of the swelled stamp is
diicult to predict and evolves with time as the solvent is allowed to evaporate before
stamping. When illing microchannels (not bonded to a surface) with a swelling solu-
tion, the microchannels might detach, spilling their contents onto undesired areas;
hence, swelling can be a limiting factor for patterning certain polymers from solvent-
based precursors using microluidic channels. Examples of “strong” solvents that
swell PDMS are ethanol, acetone, hexane, and tetrahydrofuran, whereas examples
of solvents that hold acceptably in microchannels without breaking the conformal
seal are N-methylpyrrolidinone, dimethylformamide, or dimethylsulfoxide. Swelling
of PDMS can be advantageous as a strategy to open the pores of PDMS and intro-
duce chemicals; benzophenone (a photoactivatable cross-linker) mixed with 10% ace-
tone has been introduced into PDMS (as deep as ~50-μm-thick layers) to chemically
immobilize other polymers such as PEG, a protein adhesion deterrent.
High permeability to gases and luids
. Values that can be found in the literature for gas
permeability for PDMS are on the order of several hundred Barrers (1 Barrer = 10
−10
cm
3
[STP] cm/[cm
2
s cmHg]) for air's gases (N
2
, CO
2
, and O
2
), with the permeability to N
2
and
CO
2
being similar and the permeability to O
2
being approximately three times higher
than N
2
and CO
2
. In comparison, the values for a plastic such as PMMA are two to four
orders of magnitude smaller (a fraction of a Barrer). hese values vary wildly with the
degree of cross-linking (which determines the porosity of the elastomer). Anecdotally,
common short-chain oils such as citronella can be readily smelled through a PDMS mem-
brane. Water also difuses readily through PDMS (difusion coeicient,
D
~3-6 × 10
9
/
m
2
s
−1
), compared with
D
~2 × 10
6
/m
2
s
−1
for PMMA, three orders of magnitude smaller.
High thermal expansion coeicient
. PDMS expands considerably when heated com-
pared with most solids, in particular, compared with the silicon mold. In the com-
mon range of curing temperatures from
T
1
= 20°C to
T
2
= 70°C, the thermal expansion
coeicients of PDMS and silicon are approximately
C
= 3.1 × 10
−4
K
−1
and 2.6 × 10
−6
K
−1
, so during the cooling process, PDMS shrinks about 100 times more than silicon,
by a factor of
C
(
T
2
−
T
1
) = 1.55% and 0.013%, respectively.
1.6.3 Microstamping
his method, originally named
microcontact printing
and popularly referred to as
microstamp-
ing
, was irst devised to “print” organic molecules. Microstamping is conceptually equivalent to
the old rubber stamp-based printing technique called “lexography
.
” It essentially consists of the
contact-transfer of the material of interest from a microfabricated rubber stamp onto a surface
only on the areas contacted by the stamp (
Figure 1.18
). In the original paper, a PDMS stamp
soaked with alkanethiols was used to selectively form an alkanethiol self-assembled monolayer
that resisted a gold etchant only on the areas contacted by the stamp. hus, in contrast to photo-
lithography, microstamping is an
additive
patterning method. Because of its additive nature,
microstamping can be applied to nonplanar surfaces. In-registry multilayer patterning can be
achieved by backing a thin stamp with a rigid glass support.
he most attractive aspects of microstamping are:
1. Replica-molding the stamp from the master is straightforward, nonhazardous, and
inexpensive (in terms of both materials cost and human labor).
2. he molding procedure leaves the master intact; thus, the most expensive part of
the process, photolithography—or any other method used to pattern the master—is
needed only once.
3. he patterning procedure usually does not damage the sample or the stamp (thus it
can be reused).
