Page 29 - Optofluidics Fundamentals, Devices, and Applications

10 Cha pte r T w o

PDMS is elastomeric, it is possible to form optical components whose
dimensions can be tuned mechanically. Stretching or compressing a
surface-relief grating or Fresnel lens made of PDMS, for example,
changes the periodicity of the grooves on the grating or the lens, and
the respective diffraction pattern generated or the focal properties of
the lens [22,23].
2-3-2 Surface Chemistry of PDMS
The surface of PDMS is hydrophobic as it contains repeating units of
–O-Si(CH ) −groups. By exposing it to oxygen or air plasma, this sur-
3 2
face can be made hydrophilic. Exposure to plasma introduces silanol
(Si–OH) groups, and destroys methyl groups (Si–CH ). Plasma-
3
oxidized PDMS can be wetted by aqueous, polar solvents, and eutec-
tic gallium-indium, a liquid metal alloy. On standing, a hydrophilic,
oxidized PDMS surface becomes hydrophobic, as the surface recon-
structs and as non-crosslinked components of the prepolymer bloom
to the surface. It is possible to keep PDMS that has been plasma-
treated hydrophilic indefinitely by keeping the surfaces in contact
with water or polar organic solvents.
The silanol groups on the surface of PDMS allow it to react with a
wide range of silanes (Si–R) that are terminated with important func-
tional groups (i.e., R = NH , COOH, SH). By using different functional
2
groups, it is possible to adjust the surface of PDMS to be hydrophilic
or hydrophobic, or to introduce other reactive groups. Grafting a
poly(ethylene glycol)di-(triethoxy)silane onto an oxidized PDMS sur-
face makes the surface hydrophilic permanently, and reduces non-
specific adsorption of proteins. Silanizing oxidized PDMS with an
amino-terminated silane (aminopropyltriethoxysilate) provides a reac-
tive surface for a bifunctional cross-linker for protein attachment [24].
These modified polar surfaces can, however, become hydrophobic
again through blooming of mobile, nonpolar siloxanes. Application
of a sol-gel coating may be more protective, but has not been exten-
sively developed [25].
Irreversible Sealing
It is simpler to seal channels made in PDMS than channels that are
made in glass, silicon, or thermoplastics, as high temperatures, pres-
sures, and voltages are not required. For example, sealing glass to
glass or silicon to silicon requires high temperatures (~600 C for glass;
o
>800°C for silicon) and/or voltages (500–1500 V for anodic bonding
of glass). Sealing of channels in PDMS can be performed in ambient
laboratory conditions. By exposing the surface of PDMS and the sur-
face of the substrate to an air- or oxygen-based plasma, PDMS chan-
nels can be sealed irreversibly to PDMS, glass, silicon, polystyrene,
polyethylene, or silicon nitride [24]. Plasma oxidization produces
silanol groups on PDMS, and –OH-containing functional groups on
the other materials. When the surfaces are brought into contact, the