Page 28 - Optofluidics Fundamentals, Devices, and Applications

Basic Micr ofluidic and Soft Lithographic Techniques 9

Property Characteristics Consequence
Optical Transparent; UV cutoff, Optical detection from
240 nm 240–1100 nm
Electrical Insulating; breakdown Allows embedded circuits;
7
voltage, 2 × 10 V/m intentional breakdown to open
connections
Mechanical Elastomeric; tunable Conforms to surfaces; allows
Young’s modulus, typical actuation by reversible
value of ~750 kPa deformation; facilitates release
from molds
Thermal Insulating; thermal Can be used to insulate heated
conductivity, 0.2 W/(m · K); solutions; does not allow
coefficient of thermal dissipation of resistive heating
expansion, 310 μm/ from electrophoretic separation
(m · °C); stable up to
~300°C
Interfacial Low surface free energy Replicas release easily from
~ 20 erg/cm 2 molds; can be reversibly sealed
to materials; not wetted by
water unless oxidized to Si-OH
presenting surface
Permeability Low permeability to liquid Contains aqueous solutions in
water; permeable to gases channels; allows gas transport
and nonpolar organic through the bulk material;
solvents incompatible with many organic
solvents
Reactivity Inert; can be oxidized by Unreactive toward most
exposure to a plasma reagents; surface can be
etched; can be modified to be
hydrophilic and also reactive
toward silanes
Toxicity Nontoxic Can be implanted in vivo;
supports mammalian cell growth

(Adapted from J. C. McDonald and G. M. Whitesides, “Poly(dimethylsiloxane) as a mate-
rial for fabricating microfluidic devices,” Acc. Chem. Res., 35, (2002), 491–499.)

TABLE 2-1 Physical and Chemical Properties of PDMS
releases from features of a mold—including undercut features—without
damaging them or itself. Microfluidic channels and other features on
the micron scale can therefore be reproduced with high fidelity in
PDMS by replica molding. Using composite stamps composed of two
layers—a stiff layer supported by a flexible layer of PDMS, replica-
tion of features below 1 nm has been demonstrated [20,21]. Because