This study presents a novel technology to manipulate micro-particles with the assistance from flexible polymer-based optically-induced dielectrophoretic (ODEP) devices. Bending the flexible ODEP devices downwards or upwards to create convex or concave curvatures, respectively, enables the more effective separation or collection of micro-particles with different diameters. The travel distances of the polystyrene beads of 40 μm diameter, as induced by the projected light in a given time period was increased by ~100%, which were 43.0 ± 5.0 and 84.6 ± 4.0 μm for flat and convex ODEP devices, respectively. A rapid separation or collection of micro-particles can be achieved with the assistance of gravity because the falling polystyrene beads followed the inclination of the downward and upward bent ODEP devices.
© 2011 OSA
In recent years, optically-induced dielectrophoresis (ODEP) has been recognized as a promising technique to manipulate micro- and nano-particles and cells in biomedical applications. The traditional complicated fabrication of micro-electrodes to generate the dielectrophoresis (DEP) force can be substituted by using “virtual” electrodes defined by a projected light beam [1,2]. Manipulation and separation of micro-particles, cells and beads are essential for a variety of applications such as sample pretreatment, cell manipulation and diagnosis [3–7]. With the ability to catch and control a single cell or particle by using appropriate optical patterns, ODEP has been developed as an enabling technique particularly in the manipulation, separation, collection, alignment, transportation and characterization of cells/particles [8–13]. Typically, ODEP chips utilize amorphous silicon (a-Si) as a photoconductive layer on a flat indium-tin-oxide (ITO) glass to generate the ODEP force [8–13]. However, the fabrication of a-Si usually utilizes a plasma-enhanced chemical vapor deposition (PECVD) process, which requires expensive and precision equipment and is a relatively high-temperature process. Accordingly, our previous works had successfully demonstrated the application of a thin-film bulk-heterojunction (BHJ) polymer (a mixture of regioregular poly(3-hexylthiophene) (P3HT)  and -phenyl C61-butyric acid methyl ester (PCBM)) as the photoactive layer on an ITO glass plate by using a solution-based, spin-coating process for the fabrication of polymer-based ODEP devices . When the polymer ODEP chips were illuminated by a projected light beam, the photo-induced charge carriers, which were created by the electron transfer of excitons at a donor/acceptor interface in the BHJ layer, disturb the uniformly-distributed electric field applied on the ODEP devices. A positive or negative DEP force was then generated by the “virtual” electrodes defined by the optical images from a computer-programmable projector and used to manipulate micro-particles or cells [16,17]. The BHJ polymer has a high absorption coefficient in the visible range and exhibits an efficient generation of charge carriers . The ODEP devices made of BHJ P3HT:PCBM as the photoactive layer have comparable performance when compared with their a-Si-based ODEP counterparts, enabling the selective manipulation of the micro-particles by varying the intensities and wavelengths (colors) of the projected light . With this advantage, promising and novel platforms using polymer-based ODEP devices could be realized. In this work, a versatile approach utilizing a flexible polymer-based ODEP chip was successfully demonstrated. Instead of using ITO glass as the substrate, the polymer ODEP device was fabricated on an ITO/polyethylene naphthalate (PEN) (YU-YI Inc., Taiwan) substrate which can be easily flexed or bent. The merits of this design (the polymer ODEP devices on the ITO/PEN substrate) take advantage of the pliability of the substrates by deforming the flexible polymer ODEP devices to enable a more effective manipulation and separation of micro-particles with assistance from gravity. Bending the flexible ODEP devices into a convex shape (downward direction) further facilitates the separation of micro-particles of various diameters, due to the polystyrene beads falling towards the downward incline of the flexible ODEP devices. The travel distances of the polystyrene beads of 40 μm diameter, as induced by the projected light in a set time period, was experimentally found to increase by ~100%, which were 43.0 ± 5.0 and 84.6 ± 4.0 μm for flat and convex ODEP devices, respectively. The flexible polymer ODEP device was superior to a device made from a-Si on a rigid substrate, making it an ideal platform for fast and selective manipulations in various biomedical applications. Our study highlights the new functionalities of a flexible polymer ODEP device with deformable curvature configurations to optimally enable the manipulation of micro-particles in the following featured applications.
2. Fabrication and experimental setup
Figure 1(a) schematically illustrates the configuration of the polymer-based ODEP device. The flexible polymer ODEP chip reported in this study used a donor/acceptor BHJ polymer as a light-active layer. The flexible polymer ODEP device was comprised of a top ITO/PEN thin film (with a sheet resistance < 15 Ω /cm2), a liquid solution containing the polystyrene beads and a bottom flexible ITO/PEN substrate coated with a poly(3,4- ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS, Baytron P, Bayer AG, Germany) film as the interface buffer layer [20–23], a BHJ P3HT (98.5% electronic grade, Rieke Metals, USA):PCBM(Nano-C, USA) polymer film as the photoactive layer. A 7.0 nm thick layer of lithium fluoride (LiF) was thermally evaporated on the P3HT:PCBM film in vacuum (~10−6 torr) as a water-oxygen resistant barrier.
Before the fabrication process, the flexible thin films were cleaned by ultrasonic agitation with deionized water for 20 minutes. Then PEDOT:PSS was spin-coated on the cleaned ITO/PEN substrate at 4000 rpm and later annealed at 130°C for 30 minutes. The P3HT:PCBM active layer was prepared by spin-coating P3HT:PCBM solution (1:1, 5 wt % in 1,2-dichlorobenzene, stirring at 40°C for 24 hours [24,25]) at 400 rpm on the PEDOT:PSS/ITO/PEN substrate inside a nitrogen-filled glove box through a slow-growth process . The thickness of the P3HT:PCBM film was measured to be approximate 763 nm.
Another ITO/PEN substrate was bonded on top of the ITO/PEN coated with the polymer BHJ layer by using a double-sided adhesive tape (Adhesives Research, Inc., IS-8930-19, ~30 μm for devices to manipulate 10 and 20 μm or 3M Scotch®, 668, ~45 μm for 40 μm polystyrene beads) as the top electrode for the ODEP chips. The adhesive tape was also used as a spacer to maintain a gap between the substrates that can be filled with a deionized aqueous solution and polystyrene beads. Figure 1(b) presents the experimental setup and design in this study. An alternating-current (AC) voltage (30 Vpp, 100 KHz) was supplied from a function generator (Model 195, Wavetek, U.K.) connected with a power amplifier (790 SERIES, PCB Piezotronics, Inc, Taiwan). A polysilicon thin-film transistor (TFT) micro-lens projector (ViewSonic PJ1172, Japan) with a spatial resolution of 1024 pixels x 768 pixels was used as a light source, and the optical images were controlled by a commercial computer program.(FLASH, Adobe, USA). Moreover, a 50X objective lens (Nikon, Japan) was placed between the projector and the flexible polymer ODEP chip to concentrate and collimate a light beam on the surface of the flexible polymer ODEP chip as the virtual electrodes to manipulate the micro-particles. The manipulation, separation and collection of micro-particles were monitored by using a charge-coupled-device (CCD, SSC-DC80, Sony, Japan) camera. The flexible polymer ODEP chip was placed into a customized holder, which was composed of two polymethyl methacrylate (PMMA) boards and two set screws. The PMMA boards were designed with a parallel pattern of plow grooves so that the curvature of the flexible polymer ODEP chips can be set by the two PMMA boards, by turning the set screws to bring the two PMMA boards closer to each other. With this experimental setup, the flexible polymer ODEP chips could be bent to various convex (downward) or concave (upward) curvatures.
3. Results and discussion
3.1 Selective separation of micro-particles with different diameters and collection of micro-particles
The versatile functionality of polymer ODEP devices can be first demonstrated by the selective separation for micro-particles of different diameters. Figure 2(a) shows photographs and schematic drawings explaining the manipulation of polystyrene beads with different sizes (20 and 40 μm) at a frequency of 100 KHz and a voltage of 30 Vpp in the polymer ODEP chip without any induced curvature. The beads of various sizes were pre-collected near the center of the chip by projecting and moving the two white light beams (optical power: 0.551 μW/cm2), controlled by a computer program, from border to center. Initially (at 0 s), a light beam was projected at the center of the substrate on polymer beads of different sizes as illustrated in the photograph in Fig. 2(a) part (I). Figure 2(a) part (I’) is the corresponding schematic drawing for the manipulation of micro-particles in the flat ODEP chip. A negative DEP force was generated by the light beam to repel the micro-particles from the center. The magnitude of the ODEP force (FODEP) can be presented by the following equation.Eq. (1), the polystyrene beads with a diameter of 40 μm should exhibit a stronger FODEP (repulsive force) than the 20 μm beads because the FODEP is proportional to r3. As shown in Fig. 2(a) part (II), the photograph taken at 6.3 s after the illumination of the light beam, the travel distance for the polystyrene beads with 20 and 40 μm diameters were similar, approximately 43.0 ± 5.0 μm. Although the induced FODEP was able to repel the micro-particles away from the center of the substrate, the influence of the FODEP to affect the travel distance for polystyrene beads of 20 and 40 μm diameters was limited. The difference in the travel distance between the 20 and 40 μm beads was small and effective separation of these different diameter polystyrene beads could not be performed in the set time interval on the flat polymer ODEP chips, as depicted in Fig. 2(a) part (II’). However, when the flexible polymer ODEP chip was bent slightly into a convex shape, the resulting inclined planes facilitated the separation of micro-particles because the falling polystyrene beads followed the direction of gravity.
Figure 2(b) parts (I) and (I’) present photographs and corresponding explanation schematic illustrations taken at 0 s when the light beam was projected on the polystyrene beads (20 and 40 μm) which were pre-collected near the center of convexly bent flexible polymer ODEP chip. At 6.3 s after applying illumination, the travel distances, as shown in Fig. 2(b) parts (II) and (II’), were measured to be approximate 40.3 ± 5.0 and 84.6 ± 4.0 μm for the 20 and 40 μm beads, respectively. The difference in the travel distance for the different diameter beads was around 40.3 ± 6.0 μm, significantly larger than the result shown in Fig. 2(a) part (II). Accordingly, at 12.0 s, two light beams were projected onto the flexible substrate to distinctly isolate the different diameter beads and to confine the 20 μm polystyrene beads to the center region of the chip. The 40 μm beads can be continuously rolling down the inclined plane to the edge of the image as illustrated in Fig. 2(b) parts (III) and (III’). Alternatively, the rolling length of the polystyrene beads can be controlled by varying the curvatures of the flexible substrates and the particles stop to move at the edges. The success of this technique to selectively separate the micro-particles of various diameters, as presented in Fig. 2(b) (Media 1), was mainly due to the flexible chip configuration. This unique feature cannot be accomplished by rigid a-Si-based ODEP chips.
When the flexible polymer ODEP chip was bent upward into a concave shape, it can be used as an accumulator to collect the micro-particles as depicted in Fig. 2(c), which are photographs and corresponding schematic explanations for the collection of 40 μm polystyrene beads. From 0 to 12.0 s, Fig. 2(c) parts (I) and (I’) to Fig. 2(c) parts (II) and (II’), the randomly scattered 40 μm polystyrene beads were collected by two opposing light beams moving towards the center of the substrate, which was at the bottom of the concavely flexed chip. After turning off the lights, the polystyrene beads remained at the bottom of the chip as shown in Fig. 2(c) parts (III) and (III’), because the motion of the particles was constrained by the concave curvature. Our results demonstrated that the identical flexible polymer ODEP chip could be configured into convex or concave curvatures at appropriate times for the multiple sorting functionalities of micro-particles.
Figure 3 presents the results for the average travel distance and total force, after 1.5 s, of 10 μm polystyrene beads in concave, flat, and convex-curved flexible polymer ODEP chips. A projected light beam (white light) with a line width of 20 μm was used to induce the movement of the micro-particles. The average travel distance was measured to be 43.6 ± 3.0 μm, corresponding to a FODEP of 5.43 pN on the flat chip. Note that the FODEP was calculated from Stokes’ law, i.e., FODEP where r is the radius of the spherical particle, η is the dynamic viscosity of the fluid, and ν is the maximum drag velocity [27,28]. The travel distances and total force were 24.2 ± 4.5 μm and 0.81 pN, 35.6 ± 5.0 μm and 2.56 pN on the concave-shaped substrate with curvature of 1.09 1/cm and 0.27 1/cm, respectively. The travel distances and total force were 46.5 ± 5.0 μm and 5.92 pN, 60.3 ± 6.0 μm and 9.99 pN on the convex-shaped substrates with curvature of 0.17 1/cm and 0.25 1/cm, respectively. The differences mainly result from the external deformation applied on the flexible chips. The travel distance of the micro-particles was enhanced or hindered by the inclined planes in the convex or concave-curved devices, respectively, due to gravity.
When the flexible polymer ODEP chip was not bent, the micro-particle was only repelled by the FODEP. The FODEP, as extracted from the result in Fig. 2(a), for the 40 μm polystyrene beads on the flat substrate were estimated to be 5.29 pN and the schematic plot was illustrated in the plot in Fig. 4(a) . When the flexible polymer ODEP chip had a convex curvature, an additional force was generated in the direction parallel to the substrate which pushed the beads falling down the inclined plane, as depicted in Fig. 4(b). The net force (Fnet) was attributed to the weight of the bead (344.82 pN) minus the buoyancy force (328.40 pN) and the parallel force was equal to the Fnet times the sine of the bending angle of the flexible chip (6.88° in this study). The parallel force was calculated to be 1.97 pN for 40μm polystyrene beads, which is approximate ~40% of FODEP. Accordingly, the 40 μm beads encountered a parallel force in the travel direction due to gravity on the convex substrate, which resulted in more effective separation for micro-particles on the flexible ODEP chips. The travel distance was increased from 43.0 ± 5.0 μm to 84.6 ± 4.0 μm for 40 μm beads on the flat- and convex-shaped ODEP chip as illustrated in Figs. 2(a) and 2(b). The gravitational force can enhance or reduce the travel distance on the micro-particles, depending on whether the surface was convex or concave-shaped substrates. The former indicates that the particle was repelled by the FODEP at the beginning, then assisted by the parallel force in the inclined plane since the surface was convex. Conversely, in the concave configuration, the parallel force in the inclined plane diminished the influence of the FODEP, resulting in a relatively smaller travel distance. As shown in Fig. 3, when the chip had a convex curvature with a curvature of 0.17 and 0.25 1/cm, respectively. The average travel distance of a 10 μm bead was increased by approximately ~25% (43.6 ± 3.0 to 46.5 ± 5.0) and ~50% (43.6 ± 3.0 to 60.3 ± 6.0 μm), enabling a faster and more effective manipulation. Conversely, the average travel distance was decreased by approximate ~55% and ~35% when the chip had a concave curvature with a curvature of 1.09 and 0.27 1/cm, respectively. Additionally, the total force as depicted in Fig. 3 was changes with the curvature due to the additional parallel force in the inclined plane.
3.2 The effect of light color on travel distance
Figure 5(a) illustrates the maximum drag velocity (vd) and the induced ODEP force (FODEP) for 20 μm polystyrene beads, in the flat polymer ODEP chip, induced by a light beam with a width of 20 μm at three different wavelengths (colors corresponding to red, green and blue) and the white light illumination. Note that all light beams have the same optical power (0.115 μW/cm2). The inset shows the ultraviolet-visible (UV-Vis) absorption spectrum of the P3HT:PCBM film and spectra for the three primary blue, green, and red colors of the projected light beams. It was noted that the bead motion, as induced by the green light beam, usually had the larger average travel distance than by those induced by the blue, red, and white color beams. This was because the spectrum of the green light beam had the largest overlap with the absorption spectrum of the P3HT:PCBM film, as compared to the blue and the red wavelengths, as shown in the inset of Fig. 5(a). The higher absorption of the photoactive layer by the green light beam in the ODEP chip induces a FODEP with a larger magnitude, therefore, contributes to the longer travel distance of the micro-particles. Figure 5(b) presents the the average travel distance for 10 μm polystyrene beads, after 1.5 s, in the concave-shaped polymer ODEP chip, induced by a light beam with widths of 15, 20, and 25 μm, respectively, at the white light illumination. It was also noted that the light beams with a width of 25 μm generated the longest average travel distance for 10 μm polystyrene beads, as compared to 15 and 20 μm wide beams. This indicated that the line width had to cover the entire bead to generate a sufficient FODEP .
3.3 The Effect of Light Intensity on Travel Distance
On the other hand, the manipulation of the 10 μm polystyrene beads in a convex-shaped polymer ODEP chip by the white projected light beam (20 μm width) of different optical power was conducted (Fig. 6 ). The travel distance of polystyrene beads was found to be changed with the optical power, which was 26.5 ± 2.5, 41.3 ± 3.0, and 42.7 ± 3.0 μm for the light of optical power 0.291, 0.477, and 0.551 μW/cm2, respectively, due to the variation in the magnitude of the induced FODEP. The manipulation of micro-particles in flexible polymer ODEP chips, therefore, could be fine-tuned by adjusting the wavelength (color), width, and the optical power of the projected light beams.
This study, for the first time, reported a new platform on a flexible ITO/PEN substrate for the manipulation, separation and collection of micro-particles using the FODEP, with the integration of gravity assistance, which improved the effectiveness of manipulation. Instead of using a-Si and other solid substrates, the proposed new device used a P3HT:PCBM blended polymer thin film on a flexible ITO/PEN substrate. Experimental results showed that the travel distance for 40 μm polystyrene beads on a convex-shaped ODEP chip was 84.6 ± 4.0 μm, which was much higher than the flat ODEP chip. This indicated that the gravity force can be used effectively when the ODEP chip is flexible. Moreover, the flexible polymer ODEP chip can be also bent with a concave curvature, allowing micro-particles/cells to be collected at the bottom of chip. Accordingly, the curvature of the substrate enables a more effective separation (comparing the results of Figs. 2(a) and (b)) or collection (comparing the results of Figs. 2(b) and 2(c)) of particles within a time interval. The curvatures can also be fine-tuned to control the rolling length of the polystyrene particles in the inclined plane. Our results indicate the development of flexible polymer ODEP devices has superior advantages and multiple functions than those devices made of a-Si on a rigid substrate and may provide a new platform for future biological applications.
The author G. B. Lee would like to thank the National Science Council (NSC) in Taiwan (NSC 99-2120-M-007-015) and Toward World-class University Project for their financial support. The author T. F. Guo would like to thank the NSC in Taiwan (NSC99-2113-M-006-008-MY3) and the Asian Office of Aerospace Research and Development (AOARD-10-4054) for financially supporting this research. The authors contributed equally to this work.
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