PDMS chip

The casting method has mostly been employed to fabricate poly(dimethylsi-loxane) (PDMS) chips, but has also been used with partially polymerized PS liquid preparations [85],... 

FIGURE 2.12 Fabrication procedure of a PDMS chip (a) silicon master wafer with positive surface relief, (b) pre-mixed solution of Sylgard 184 and its curing agent poured over the master, (c) cured PDMS slab peeled from the master wafer, (d) PDMS slab punched with reservoir holes, and (e) ready-to-use device sealed with another slab of PDMS [159]. Reprinted with permission from the American Chemical Society. 

FIGURE 2.14 Electron micrograph of a turn in the channel fabricated in a PDMS chip, created by casting the polymer against a positive relief, which is made of photoresist patterned on a glass substrate. The roughness in the side wall arises from the limited resolution of the transparency used as a photomask in photolithography [1033]. Reprinted with permission from the American Chemical Society. 

PDMS chips have also been cast from a glass (not Si) master containing photoresist as the positive relief stmcture [556,960]. PDMS was also cast against... 

A two-mask process has been used to create a glass master containing two levels of positive photoresist relief structures. The master was used to cast a PDMS chip consisting of the channel/chamber (25-30 pm deep) and weir (7-12 pm clearance) [960]. Another two-mask process was used to create a Si molding master consisting of 3-pm-high Si relief stmctures and 25-pm-high photoresist relief structures [364]. 

A solid-object printer was used to produce a plastic molding mask for casting PDMS chips [184]. A melted thermoplastic build material (m.p. 80-90°C) was used as the ink for printing. This method provides an... 

The surface of native polymeric (e.g., PDMS) chips is hydrophobic. This surface property has caused problems in filling aqueous solutions in the PDMS channels, and in support solution transport based on electroosmostic flow (EOF). 

Graft polymerization on PDMS channel surface (not channel lumen) in enclosed channels was carried out by first adsorbing the photoinitiator (benzophe-none) on the PDMS surface, followed by UV-mediated photopolymerization. In this manner, the EOF stability of the PDMS channels lasts for 45 h [254]. Besides PDMS chips, PC chips have also been chemically modified by sulfonation (using S03 gas) to produce hydrophilic surfaces [255]. 

On the other hand, if a hydrophobic channel, the oxidized PDMS chip can be placed at 90°C in order to recover the hydrophobicity of the channels [260]. 

In PDMS chips, metal electrodes cannot easily be made on PDMS because of its pliable nature. So the Au/Cr microband electrodes have first been formed on a glass plate, which was then aligned and sealed with a PDMS channel plate [748],... 

Pt electrodes for CE high voltage were embedded into PDMS pre-polymer cast against a mold. Upon curing, the Pt electrodes were well positioned at the solution reservoirs of the PDMS chip [292]. Metal electrodes were pierced through the PDMS well when the chip was mounted [293]. 

Besides pumping, centripetal acceleration is created. A maximum fluid rotational velocity of up to 12 m/s, and a corresponding radial acceleration in excess of 106 g have been produced within a diamond-shaped microchamber (55 x 55 im). This notch chamber was constmcted along the side wall of an otherwise straight channel (30 pm wide, 30 pm deep) which was fabricated on a PDMS chip. This microstructure caused flow detachment at the opening of the notch, leading to recirculating flow of microvortex inside the notch [384]. 

Flow switching in a PDMS chip can be achieved by deflection from two side channels (see Figure 3.34) [359]. When there is zero hydrostatic pressure, the two streams of fluorescent beads flow equally to two channels. When the hydrostatic pressure from the bottom channel is greater (i.e., 2 mm liquid level differ-... 

Passive mixing by chaotic advection is demonstrated in a PDMS chip consisting of a winding channel (see Figure 3.43). An essential component is a water-... 

The hydrophobic passive valve was constructed at the junction of a narrow side channel and a wide main channel on a PDMS chip for liquid droplet... 

CGE separation of dsDNA was performed in a PDMS chip [183] and a PMMA chip [614], which used a sieving medium containing Pluronic (F127), a thermoreversible gel. 

The COMOSS has also been fabricated on a PDMS chip for CEC separation of FITC-labeled peptides (Figure 6.25). However, in the CEC separation of a mixture of rhodamine and fluorescein, a broad rhodamine peak was obtained, but fluorescein did not have this problem. This was possibly because the neutral rhodamine had diffused into the PDMS substrate, as illustrated in the fluorescent image in Figure 6.26 [360]. In another report, CEC separation of a peptide mixture was performed on a PDMS chip after cerium(IV)-catalyzed polymerization of the stationary phase within the channels [646]. 

FIGURE 6.25 Scheme of a COMOSS column incorporated in a PDMS chip for CEC separation. A reverse-phase coating (poly[styrenesulfonic acid]) was bonded to the COMOSS column after silanization treatment of the PDMS surface. Since the usual solvent (toluene) for silanization cannot be used in PDMS, the surfactant, SDS, was used to help dissolve the silanes [360]. Reprinted with permission from Wiley-VCH Verlag. 

Pre-column OPA derivatization was also employed to analyze biogenic amines prior to MEKC separation on a PDMS chip [654]. Pre-column OPA derivatization and MEKC were also performed on a glass chip to analyze amino acids. Usually, OPA was used for fluorescent detection. However, in this report, amperometric detection was used as the OPA derivatives were also electroactive. Voltage (needed for separation) programming was used to decrease the migration time of late migrating species [655]. 

Separation of two proteins (BSA, IgG) by anion-exchange chromatography was demonstrated on a PDMS chip packed with beads [662]. 

Fraction collection of fragments from a 100-bp DNA ladder was also achieved in a PDMS chip [664] or a glass chip. In the latter example, a small reversed field was maintained in the separation column to halt or slow down later migrating DNA in order to assist collection of a DNA fraction [665]. In one report, a single peak from the IEF separation channel has been isolated and transferred to a subsequent channel by means of microfluidic valve control [449],... 

An IEF/CGE separation for proteins has been achieved on a PDMS chip. Microfluidic valves were used to prevent intermixing between the two separation buffers used in IEF and CGE separations. The IEF ampholyte was very sensitive to high buffer concentration, but a small amount of ampholyte in the CGE did not affect its separation resolution [449]. 

In one report, the detection optical fiber (coupled to a blue LED light source) and the microavalanche photodiode were both embedded in a PDMS chip to detect proteins [692]. 

A planar optical waveguide was integrated within a PDMS chip for fluorescent detection. The waveguide consists of a 150-nm-thick silicon nitride layer deposited on a 2.1-pm-thick Si02 buffer layer on a Si substrate. The rabbit IgG was... 

To increase the absorbance detection sensitivity, both a collimating lens and a detection slit have been used in a PDMS chip to reduce collection of scattered light [707], An improved S/N ratio was also obtained by using tapered channel waveguides as the collimators as well as the elliptical lens for absorbance detection [708],... 

Optical absorbance detection was also achieved using a CCD-based spectrophotometer. Two food dyes (FD C Blue 1 dye and FD C Red 3 dye) were first separated by open-tubular liquid chromatography and then detected on a PDMS chip. The LODs for the blue and red dyes were determined to be 80 pM and 200 pM, respectively [569]. 

A liquid prism was created on a PDMS chip for detection based on absorption and refractive index shift. The liquid prism was formed by filling a hollow triangular-shaped chamber with a liquid sample. Excitation and emission were arranged at the minimum deviation configuration. At a low concentration of fluorescein (< 100 pM), excitation light from an optical fiber was absorbed by the molecule, but there was no shift in the excitation maximum. In this absorption-only mode, the LOD of fluorescein was 6 pM. At higher concentration (i.e., 100-1000 pM), there is an additional shift in the excitation maximum This leads to a much sharper decrease in the measured intensity, which is more than can be accounted for simply by the absorption effect [714]. 

The change in the refractive index gradient (RIG) between adjacent laminar flow streams is employed as a MW sensor, as shown in Figure 7.19. A sample was mixed with a mobile phase in a PDMS chip. The two laminar flow streams... 

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