10. Microfluidics/Lab-on-a-Chip

An example of a wristwatch type biosensor based on microfluidics referred to as lab-on-achip system is shown in Fig. 8. The advantages of these systems are incorporating sample \$\* (%\*#\_z/!,.0%+\*\_z !0!0%+\*\_z\* z 0z\*(5/%/z+\*0+z+\*!z,(0"+.)^z\$!z\$%,z.!(%!/z+\*z)%¥ .+"(1% %/z\* z%\*2+(2!/z)\*%,1(0%+\*z+"z0%\*5z)+1\*0/z+"z"(1% /z%\*z)%.+\$\*\*!(/z1/%\*#z)%¥ crovalves. The test fluid is injected into the chip generally using an external pump or /5.%\*#!^z +)!z \$%,/z \$2!z !!\*z !/%#\*! z 3%0\$z \*z %\*0!#.0! z !(!0.+/00%((5w010! z %¥ phragm type micropump. The sample, which can have volume measured in nanoliters, "(+3/z0\$.+1#\$z)%.+"(1% %z\$\*\*!(/z2%z\*z!(!0.%z,+0!\*0%(z\* z,%((.5z0%+\*z1/%\*#z)%¥ crovalves (having various designs including membrane type) for various analyses. The fluid is preprocessed and then analyzed using a biosensor.

Figure 8. MEMS based biofluidic chip, commonly known as a lab-on-a chip that can be worn like a wristwatch [1].

\$!z%),(!)!\*00%+\*z+"z)%.+,1),/z\* z)%.+2(2!/z((+3/z"+.z"(1% z)\*%,1(0%+\*z\* z)1(¥ tiple sample processing steps in a single cassette. The three basic components of a mechanic valve are the actuator, the valve spring and the valve seat. The spring force keeps the valve shut in normally closed valves. In the case of normally open valves, the spring keeps the valve +,!\*z\* z3+.'/z#%\*/0z0\$!z010+.^zz/)((z/,.%\*#z+\*/0\*0z\*z!z.!(%6! z3%0\$zz/+"0z)0!¥

Silicon Carbide: A Biocompatible Semiconductor Used in Advanced Biosensors and BioMEMS/NEMS

http://dx.doi.org/10.5772/51811

371

Silicon Carbide: A Biocompatible Semiconductor Used in Advanced Biosensors and BioMEMS/NEMS http://dx.doi.org/10.5772/51811 371

chemical etching of p-type and n-type, 6H-SiCsubstrates [13]. Rosenbloom et al. reported the development of porous SiC membranes for use as protein filters [46]. The performance of the porous SiC membranes was evaluated using protein-containing solutions with proteins .\*#%\*#z%\*z)+(!1(.z3!%#\$0z".+)zDJ\_CCCz0+zKC\_CCCz(0+\*/zcd^z 0z3/z"+1\* z0\$0z0\$!z)!)¥ branes were able to pass proteins with molecular weights of up to 29 kDa and were able to exclude proteins in excess of 45 kDa. Moreover, the porous SiC membranes exhibited lower ,.+0!%\*z/+.,0%+\*z/z+),.! z 0+z+))!.%((5z2%((!z,+(5)!.w/! z)!).\*!/z/,!¥ %"%((5z !/%#\*! z"+.z,.+0!%\*z/+.,0%+\*\_z%\* %0%\*#z0\$!z,+0!\*0%(z"+.z%z)!).\*!/z%\*z%+¥

%.+)\$%\*! zFw%z)!).\*!/z(/+z,.+2% !z!4!((!\*0z/,!%)!\*/z0+z/01 5z0\$!z)!\$\*%¥ cal durability of 3C-SiC films since the adhesion of the film to the micromachined substrate is extremely high, due in part to the carbonization based growth process. Suspended 3C-SiC )!).\*!/z\$2!z,.+2!\*z 0+z!z \*z 00.0%2!z)!\$\*%(z /0.101.!z "+.z)%.+)\$%\*! z,.!/¥ sure sensors owing to their chemical inertness, mechanical durability and high temperature stability. In comparison with Si membranes, SiC membranes are much easier to fabricate [2].

In addition to its outstanding mechanical properties, 3C-SiC thin films have unique electro- +,0%z,.+,!.0%!/z0\$0z\*z!z!4,(+%0! z"+.z%+)! %(z%)#%\*#^z \*z+\*2!\*0%+\*(z+,0%(z)%.+¥ scopy, diffraction effects limit the spatial resolution to one-half of the illuminating 32!(!\*#0\$^zz0!\$\*%-1!z'\*+3\*z/z\*!.w"%!( z/\*\*%\*#z+,0%(z)%.+/+,5zc
dz\$/z.!¥ cently emerged as a means to resolve images below the diffraction limit. NSOM utilizes a detector that is placed in very close proximity to a sample in order to detect evanescent "%!( /z//+%0! z3%0\$z0\$!z/1."!z+"z0\$!z/,!%)!\*^z1\$z\$.0!.%/0%/z)'!z
zz,+0!\*¥ tially powerful tool for biological imaging. However, a drawback of NSOM is that the AFMbased probe significantly limits the ability to image biological specimens in fluidic

An example of a wristwatch type biosensor based on microfluidics referred to as lab-on-achip system is shown in Fig. 8. The advantages of these systems are incorporating sample \$\* (%\*#\_z/!,.0%+\*\_z !0!0%+\*\_z\* z 0z\*(5/%/z+\*0+z+\*!z,(0"+.)^z\$!z\$%,z.!(%!/z+\*z)%¥ .+"(1% %/z\* z%\*2+(2!/z)\*%,1(0%+\*z+"z0%\*5z)+1\*0/z+"z"(1% /z%\*z)%.+\$\*\*!(/z1/%\*#z)%¥ crovalves. The test fluid is injected into the chip generally using an external pump or /5.%\*#!^z +)!z \$%,/z \$2!z !!\*z !/%#\*! z 3%0\$z \*z %\*0!#.0! z !(!0.+/00%((5w010! z %¥ phragm type micropump. The sample, which can have volume measured in nanoliters, "(+3/z0\$.+1#\$z)%.+"(1% %z\$\*\*!(/z2%z\*z!(!0.%z,+0!\*0%(z\* z,%((.5z0%+\*z1/%\*#z)%¥ crovalves (having various designs including membrane type) for various analyses. The fluid

filteration applications [13].

370 Physics and Technology of Silicon Carbide Devices

9. Biomedical imaging

environments [13, 47].

10. Microfluidics/Lab-on-a-Chip

is preprocessed and then analyzed using a biosensor.

Figure 8. MEMS based biofluidic chip, commonly known as a lab-on-a chip that can be worn like a wristwatch [1].

\$!z%),(!)!\*00%+\*z+"z)%.+,1),/z\* z)%.+2(2!/z((+3/z"+.z"(1% z)\*%,1(0%+\*z\* z)1(¥ tiple sample processing steps in a single cassette. The three basic components of a mechanic valve are the actuator, the valve spring and the valve seat. The spring force keeps the valve shut in normally closed valves. In the case of normally open valves, the spring keeps the valve +,!\*z\* z3+.'/z#%\*/0z0\$!z010+.^zz/)((z/,.%\*#z+\*/0\*0z\*z!z.!(%6! z3%0\$zz/+"0z)0!¥ rial such as rubber. The solution with soft materials offers a further advantage of excellent sealing characteristics. The leakage ratio can be improved from three to four orders of magnitude compared to those made of hard material such as silicon, glass, or silicon nitride. If the valve is designed for bistable operation, there is no need for a valve spring because the two valve states are controlled actively. Since the non-powered state is undefined, a valve spring can still be considered for the initial, nonpowered state to assure safe operation. A bistable valve spring allows the valve seat to snap into its working position. In this case, the actuator needs to be powered in a short period to have enough force to trigger the position change. The force generated by the spring is then high enough to seal the valve inlet [48].

Valve seats represent a large challenge to microvalve design and fabrication. The valve seat should satisfy two requirements: low leakage and high resistance against particulate contamination. For a minimum leakage rate, the valve should be designed with a large sealing area, which must be extremely flat. Softer materials such as rubber or other elastomers are recommended for the valve seat. Resistance against particles can be realized in many ways. First, a hard valve seat can simply crush the particles. For this purpose, the valve requires actuators with large force such as piezostacks and hard coating layer for the valve seat. Sec ond, the particles can be surrounded and sealed by a soft coating on the valve seat. Third, small particle traps such as holes or trenches can be fabricated on the valve seat or on the opposite valve base. A combination of the third measure with the first and second measures is recommended, so that tiny particles can be trapped and burred after being crushed by the large actuation force. For the first and third measures, the valve seat needs to be coated with hard, wear-resistant material such as silicon nitride, silicon carbide, or diamond [9].

Blood or other aqueous solutions can be pumped into the system where various processes are performed. If the adhesion between the microchannel surface and the biofluid is high, the biomolecules will stick to the microchannel surface and restrict flow. In order to facilitate flow, the microchannel surfaces with low bioadhesion are required. Fluid flow in polymer channels can produce triboelectric surface potential which may affect the flow. Polymers are known to generate surface potential and the magnitude of the potential varies from one polymer to another [1]. Conductive surface layers such as SiC can be deposited on the polymer channels to reduce triboelectric effects. Compared with its crystalline counterparts, amorphous-SiC is particularly attractive for microfluidics and related lab-on-a-chip applications because it can be deposited on a much wider range of substrate materials while retaining a high level of chemical inertness.

These features are the way for on-line monitoring of several processes in many application fields. A large new area is that of micrototal analysis systems (ul AS) or lab-on-a chip, where attempts are made to completely integrate biochemical systems on one silicon or glass chip. A currently emerging development is that of miniaturized integrated physical chemosensors and biosensors. Rather than applying a chemical interface to determine the biochemical properties of fluids, physical properties or phenomena in the fluid are used. Persisting problems with such a chemically sensitive interface layer, such as poor reproducibility, drift, ageing and contamination, are circumvented in this approach. All these devices require new technological approaches for the fabrication of small channels, novel integrated microdetectors, and other components. Most compounds used for biochemical analysis do not possess a fluorescent functionality, and thus labeling with a fluorescent marker is required. Recently, the on-chip integration of electrochemical and conductivity detection has been reported [49]. A schematical drawing of the chip is shown in Fig. 9.

Figure 9. Drawing of a microchemical detector containing electrophoresis separation and conductivity detection [49]

Conductivity detection can be used for on-chip measurements. To avoid electrolysis and electrode fouling when the solution was in contact with the measurement electrodes, contactless conductivity detection was proposed. A four-electrode capacitively coupled (contactless) detector has been integrated on a Pyrex glass chip for detection of peptides (1 mM) and cations (5 mM K+, Na+, Li+). The Al electrode (500 nm Al/100 nm Ti) was deposited in a 600- nm-deep trench and was covered with a thin dielectric layer (30-nm SiC). The other parts of the channel were covered and insulated with SigNy (160 nm). This four-electrode configuration allows for sensitive detection at different background conductivities without the need for adjustment of measurement frequency. In contactless mode, the dielectric thickness should be small [50]. Iliescu et al. [51] recently explored the use of a-SiC membranes as structures for chip-based cell culturing. The optical properties of the SiC film supported the use of classical fluorescence microscopy and thus were ideal for cell culturing studies. NHzf was used to reduce the native oxide on the SiC surfaces. The NHzF surface treatment resulted in greater cell density on the a-SiC samples as compared with untreated surfaces. Collectively, these achievements show the potential for SiC in highly functional lab-on-a-chip devices.
