Lab-on-a-display: a new microparticle manipulation platform using a liquid crystal display (LCD)
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- Choi, W., Kim, S., Jang, J. et al. Microfluid Nanofluid (2007) 3: 217. doi:10.1007/s10404-006-0124-5
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This paper reports a new portable microfluidic platform, “lab-on-a-display,” that microparticles are manipulated by optoelectronic tweezers (OET) on a liquid crystal display (LCD). The OET has been constructed by assembling a ground layer, a liquid chamber, and a photoconductive layer. Without lens or optical alignments, the LCD image directly forms virtual electrodes on the photoconductive layer for dielectrophoretic manipulation. The lab-on-a-display was first realized by a conventional monochromatic LCD module and a light source brighter than 5,000 lux. It was successfully applied to the programmable manipulation of 45 μm polystyrene beads; more than 100 particles were transported with an optical image-driven control, following the moving edge of the image at every moment. The effects of bead size and bias voltage on the manipulation speed were also investigated. Due to the portability and compatibility for disposable applications, this new platform has potential for programmable particle manipulation or chip-based bioprocessing including cell separation and bead-based analysis.
KeywordsMicrofluidicsLab-on-a-displayLiquid crystal display (LCD)Optoelectronic tweezersDielectrophoresisParticle manipulation
A programmable DEP electrode array has become one of the key issues in DEP-based manipulation techniques. Manaresi et al. (2003) reported a complementary metal-oxide-semiconductor (CMOS) circuit-driven technique. It was demonstrated that the programmable DEP manipulator with individually addressable two-dimensional electrode array could be used for the parallel manipulations of biological cells and microscopic particles. Since the activated electrode patterns are movable and reconfigurable, it has many advantages such as single-particle addressing and selection, grap-and-drag motion, parallel manipulation, robustness from clogging and channel-less structure. However, it has a potential drawback for disposable applications due to high manufacturing cost. Integration of on-chip circuits increases the cost of the device, making it less attractive for disposable applications.
To deal with this drawback, optoelectronic tweezers (OET) were proposed by replacing the patterned electrodes with a pattern-less photoconductive layer (Chiou et al. 2005). In their approach, light-induced virtual electrodes on a photoconductive layer were used to make a DEP force by using the transmitted light signal generated from a digital micro-mirror device (DMD). This technique is potentially useful for biomedical applications, because the OET part can be used as a disposable cartridge-type for sample handling. The only electrical connections for the OET part would be two wires to supply an AC bias. However, the optical structure for DMD projection leads to difficulties in portable applications. The reflective structure resulted in complex structure. Additionally, the optical lens between DMD and OET requires optical alignment to focus the projected image on the photoconductive layer.
This paper aims to demonstrate a new lab-on-a-display platform for microscopic particle manipulation using a new optical structure. Unlike the projection display method based on DMD, we utilize direct image transfer on a liquid crystal display (LCD). A lab-on-a-display has no optical component between display (LCD) and OET, thus the OET part is just placed on the display device. Due to the elimination of lens and optical alignment, it would be more suitable for portable applications. In addition, this platform is relatively thin and tolerant to vibrations in real world application.
In this study, the lab-on-a-display was first realized by a conventional monochromatic LCD module. Prior to conducting the manipulation of microparticles, the feasibility of the lab-on-a-display was evaluated by electrical field simulation. We have also investigated various OET properties which depend on the light source, the LCD type (color or monochromatic), bias voltage, and microparticle size. Detailed experimental procedures and results of the lab-on-a-display prototype are reported herein.
2 Materials and methods
2.1 Design and microfabrication of a lab-on-a-display
Thickness comparisons of each component in the lab-on-a-display
2.2 Experimental setup
A film mask made from silver halide emulsion on a polyester film substrate (Han&All Tech Co., Ansan, Korea) was used as a static image pattern. Two types of LCD were investigated in this study. A color LCD (800 × 3 × 480 pixel array with 100 × 300-μm pixel size) was picked out from a 7-in LCD panel (TX18D11VM1CAA; Hitachi, Japan). Its dimensions were 102 mm in length and 163 mm in width. The thickness of the color LCD without backlight and with backlight was 2 and 11 mm, respectively. A 1.3-in monochromatic LCD (800 × 600 pixel array with 33-μm pixel pitch) module was taken out of a conventional projector (EMP-5300; Epson, Japan). Its dimensions were 42 mm in length, 40 mm in width, and 5 mm in thickness. The LCD module was operated by the LCD driver circuit of the projector. The image area of the LCD module was 26.4 mm × 19.6 mm, and the LCD images were drawn by using standard presentation software (Microsoft PowerPoint™) on a computer.
Plain polystyrene beads (PolySciences, PA, USA) were used for particle manipulation. The sample was prepared by dilution with deionized water to the final concentration of about 2.5 × 105 particles/mL. A sample droplet of 5–7 μL was sandwiched between the ground layer and the photoconductive layer using double-stick tape as a spacer, ensuring the liquid chamber of 120 μm thick. The electric bias voltage produced from a function generator (MXG-9802A; Seowon Family Co., Korea or AGF3022; Tektronix, USA) was applied across the ground layer and the photoconductive layer of the lab-on-a-display.
3 Results and discussion
3.1 Simulation of electric field distribution
As a proof-of-concept, we simulated an electric field distribution in the liquid layer of the lab-on-a-display. The electric field was calculated by using a commercial CFD solver (CFD − ACE+; ESI US R&D Inc., Huntsville, AL, USA) in the condition of 22 V AC bias at 100 kHz. Because the photoconductivity of hydrogenated amorphous silicon photoconductor was larger than the dark conductivity for more than three orders of magnitude (Ryu et al. 2001), we assumed the illuminated region and non-illuminated region of photoconductive layer as a conductor and an insulator, respectively.
In order to transport the particle across a long distance, the LCD image should be changed dynamically according to a proper program sequence. If the image is static, a particle would move only for a limited distance. For a long movement the image pattern needs to be continuously changed because a trapped particle would move according to the edge of moving image. For example, a particle can move from the left end to the right end of the liquid chamber if the image edge moves from the left to the right of the LCD. Figure 3b shows the liquid layers at three sequential moments while the bright image extends from the left side to the right side.
3.2 Light-induced virtual electrode generation
Instead of the color LCD, we thus tested a monochromatic LCD module which has no color filter (Fig. 5c). The LCD image was visible through both the glass side and the photoconductive side, and the beads were transported by the LCD light-induced DEP. When white light was illuminated, the color of bright regions through the photoconductive side was also red. Here, the monochromatic LCD gave two additional changes such as the reduction of the LCD pixel size and unwanted image pattern. Since the pixel size was shrinked from 100 μm × 300 μm to 33 μm × 33 μm, smaller virtual electrodes would enable more precise control of position. As shown in Fig. 5c, the image pattern (“I”) on the LCD module was not the same with the intended image pattern. Two pixels in every six pixels were always bright and dark, respectively, while the rest four pixels in every six pixels were controllable according to the programmed image by a computer. In addition, the dark pixels out of the image pattern were visible as a black vertical line through the photoconductive layer; however, the bright pixels in the image pattern were invisible through the photoconductive layer. The difference of this microscopic image pattern was inferred to have its origin in the LCD driver circuits, because the image without microscope looked like the original on computer screen: the black vertical lines by the dark pixels were invisible.
3.3 Microparticle manipulation using dynamic LCD image pattern
Overall thickness of the lab-on-a-display was about 6.5 mm (Table 1). This dimension may be applicable to the portable applications. The thickness of the prototype is determined mostly by the thicknesses of LCD and glass substrate; the LCD was 5 mm thick and the two glass substrates are 0.7 mm each. In addition, the weight of lab-on-a-display was mainly contributed to the LCD. The LCD was 14.9 g in weight, while the other layers were about 3.4 g in weight. Therefore, a thinner LCD would lead to reduce the size and weight of lab-on-display device.
In this study, the manipulation area of the lab-on-a-display was 26.4 mm × 19.6 mm, which larger than other programmable DEP manipulators. The reported manipulation areas of CMOS technique and DMD technique were 8 mm × 8 mm and 1.3 mm × 1.0 mm, respectively. The manipulation area can be more extended than current prototype without technical difficulties, since a lot of larger LCD panels which can be implemented after removing color filters are commercially available already. Larger manipulation areas or a larger number of virtual electrodes are affordable to many potential applications, e.g., high throughput application in cell separation or drug discovery.
However, current prototype was thought to be hard for single particle manipulation because of image blurring. The image from the 33-μm-size pixel displays up to 200 μm wide, when a microscopic focus is set at the beads in the liquid chamber. It seems that the light from the LCD is scattered to all direction while it pass through glass substrate of the LCD or the OET. To conduct single particle (or cell) manipulation, this image blurring should be reduced to less than or similar with the particle (or cell) size. One of the possible methods to decrease image blurring is using a thinner LCD module. Another approach is the use of a thinner substrate for the photoconductive layer in the OET (e.g., 150-μm-thick plastic film substrate instead of 700-μm-thick glass substrate).
Specifications of the demonstrated lab-on-a-display
High temperature poly silicon
42 mm × 40 mm × 5 mm
26.4 mm × 19.6 mm (1.3 in diagonal)
33 μm × 33 μm
Pixel array size
800 × 600
37.5 mm × 25.0 mm
Key layer of the photoconductive layer
1-μm-thick hydrogenated amorphous silicon
22 V @ 100 kHz
26.4 mm × 19.6 mm
Maximum number of virtual electrodes
The lab-on-a-display platform based on a new optical structure was designed and demonstrated. Numerical simulation and experimental results indicated that the direct transfer of a LCD image could enable the OET manipulation. From the photoconductivity measurement experiment, the fabricated photoconductive layer showed good property for virtual electrode formation. A number of microparticles were transported by the monochromatic LCD and light-patterned virtual electrodes. As the bead velocity increased proportionally to the voltage squared, there was a microparticle size dependency of its speed. This new platform is anticipated to be useful for lab-on-a-chip and diagnostic instruments since it enables programmable 2-D manipulation as well as provides portability and compatibility for disposable applications.
This research was supported by the Nano/Bio Science & Technology Program (M10536090002-05N3609-00210) of the Ministry of Science and Technology (MOST), Korea. The authors also thank CHUNG Moon Soul Center for BioInformation and BioElectronics, KAIST. The microfabrication works were performed at the Digital Nanolocomotion Center (KAIST, Daejeon, Korea) and the TFT-LCD Research Center (Kyung Hee University, Seoul, Korea).