Biomicrofluidics

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Novel index for micromixing characterization and comparative analysis

Mranal Jain and K. Nandakumar

Biomicrofluidics 4, 031101 (2010); http://dx.doi.org/10.1063/1.3457121 (8 pages)

Online Publication Date: 2 July 2010

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The most basic micromixer is a T- or Y-mixer, where two confluent streams mix due to transverse diffusion. To enhance micromixing, various modifications of T-mixers are reported such as heterogeneously charged walls, grooves on the channel base, geometric variations by introducing physical constrictions, etc. The performance of these reported designs is evaluated against the T-mixer in terms of the deviation from perfectly mixed state and mixing length (device length required to achieve perfect mixing). Although many studies have noticed the reduced flow rates for improved mixer designs, the residence time is not taken into consideration for micromixing performance evaluation. In this work, we propose a novel index, based on residence time, for micromixing characterization and comparative analysis. For any given mixer, the proposed index identifies the nondiffusive mixing enhancement with respect to the T-mixer. Various micromixers are evaluated using the proposed index to demonstrate the usefulness of the index. It is also shown that physical constriction mixer types are equivalent to T-mixers. The proposed index is found to be insightful and could be used as a benchmark for comparing different mixing strategies.

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87.85.gf	Fluid mechanics and rheology
47.63.-b	Biological fluid dynamics
47.85.Np	Fluidics
87.80.Ek	Mechanical and micromechanical techniques

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Micro-optofluidic Lenses: A review

Nam-Trung Nguyen

Biomicrofluidics 4, 031501 (2010); http://dx.doi.org/10.1063/1.3460392 (15 pages) | Cited 10 times

Online Publication Date: 19 July 2010

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This review presents a systematic perspective on the development of micro-optofluidic lenses. The progress on the development of micro-optofluidic lenses are illustrated by example from recent literature. The advantage of micro-optofluidic lenses over solid lens systems is their tunability without the use of large actuators such as servo motors. Depending on the relative orientation of light path and the substrate surface, micro-optofluidic lenses can be categorized as in-plane or out-of-plane lenses. However, this review will focus on the tunability of the lenses and categorizes them according to the concept of tunability. Micro-optofluidic lenses can be either tuned by the liquid in use or by the shape of the lens. Micro-optofluidic lenses with tunable shape are categorized according to the actuation schemes. Typical parameters of micro-optofluidic lenses reported recently are compared and discussed. Finally, perspectives are given for future works in this field.

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42.79.Bh	Lenses, prisms and mirrors
47.85.Np	Fluidics
47.61.-k	Micro- and nano- scale flow phenomena
85.85.+j	Micro- and nano-electromechanical systems (MEMS/NEMS) and devices

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Preface to Special Topic: Surface Modification, Wetting, and Biological Interfaces (Guest Editors: John Ralston and Jingfang Zhou)

John Ralston and Jingfang Zhou

Biomicrofluidics 4, 032101 (2010); http://dx.doi.org/10.1063/1.3493644 (2 pages)

Online Publication Date: 30 September 2010

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This Special Topic section of Biomicrofluidics on “Surface Modification, Wetting, and Biological Interfaces,” is discussed. The topic is very timely and one that is tremendously relevant to the microfluidics and nanofluidics community.

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01.30.-y	Physics literature and publications
47.85.Np	Fluidics
87.80.-y	Biophysical techniques (research methods)

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Structural evolution of protein-biofilms: Simulations and experiments

Y. Schmitt, H. Hähl, C. Gilow, H. Mantz, K. Jacobs, O. Leidinger, M. Bellion, and L. Santen

Biomicrofluidics 4, 032201 (2010); http://dx.doi.org/10.1063/1.3488672 (18 pages)

Online Publication Date: 30 September 2010

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The control of biofilm formation is a challenging goal that has not been reached yet in many aspects. One unsolved question is the role of van der Waals forces and another is the importance of mutual interactions between the adsorbing and the adsorbed biomolecules (“critical crowding”). In this study, a combined experimental and theoretical approach is presented, which fundamentally probes both aspects. On three model proteins—lysozyme, α-amylase, and bovine serum albumin—the adsorption kinetics is studied experimentally. Composite substrates are used enabling a separation of the short- and the long-range forces. Although usually neglected, experimental evidence is given for the influence of van der Waals forces on the protein adsorption as revealed by in situ ellipsometry. The three proteins were chosen for their different conformational stabilities in order to investigate the influence of conformational changes on the adsorption kinetics. Monte Carlo simulations are used to develop a model for these experimental results by assuming an internal degree of freedom to represent conformational changes. The simulations also provide data on the distribution of adsorption sites. By in situ atomic force microscopy we can also test this distribution experimentally, which opens the possibility to, e.g., investigate the interactions between adsorbed proteins.

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87.15.B-	Structure of biomolecules
87.15.hp	Conformational changes
87.15.K-	Molecular interactions; membrane-protein interactions
87.15.R-	Reactions and kinetics
34.20.Gj	Intermolecular and atom-molecule potentials and forces
87.15.ak	Monte Carlo simulations

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Polymer brushes with nanoinclusions under shear: A molecular dynamics investigation

A. Milchev, D. I. Dimitrov, and K. Binder

Biomicrofluidics 4, 032202 (2010); http://dx.doi.org/10.1063/1.3396446 (8 pages)

Online Publication Date: 30 September 2010

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We use molecular dynamics simulations with a dissipative particle dynamics thermostat to study the behavior of nanosized inclusions (colloids) in a polymer brush under shear whereby the solvent is explicitly included in the simulation. The brush is described by a bead-spring model for flexible polymer chains, grafted on a solid substrate, while the polymer-soluble nanoparticles in the solution are taken as soft spheres whose diameter is about three times larger than that of the chain segments and the solvent. We find that the brush number density profile, as well as the density profiles of the nanoinclusions and the solvent, remains insensitive to strong shear although the grafted chains tilt in direction of the flow. The thickness of the penetration layer of nanoinclusions, as well as their average concentration in the brush, stays largely unaffected even at the strongest shear. Our result manifests the remarkable robustness of polymer brushes with embedded nanoparticles under high shear which could be of importance for technological applications.

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87.15.hg	Dynamics of intermolecular interactions
87.15.nr	Aggregation
87.15.ap	Molecular dynamics simulation
87.15.B-	Structure of biomolecules
61.25.hk	Polymer melts and blends
47.57.jb	Microemulsions
82.70.Dd	Colloids

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Investigations on the melting and bending modulus of polymer grafted bilayers using dissipative particle dynamics

Foram M. Thakkar and K. G. Ayappa

Biomicrofluidics 4, 032203 (2010); http://dx.doi.org/10.1063/1.3473720 (19 pages) | Cited 1 time

Online Publication Date: 30 September 2010

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Understanding the influence of polymer grafted bilayers on the physicomechanical properties of lipid membranes is important while developing liposomal based drug delivery systems. The melting characteristics and bending moduli of polymer grafted bilayers are investigated using dissipative particle dynamics simulations as a function of the amount of grafted polymer and lipid tail length. Simulations are carried out using a modified Andersen barostat, whereby the membrane is maintained in a tensionless state. For lipids made up of four to six tail beads, the transition from the low temperature L_β phase to the L_α phase is lowered only above a grafting fraction of G_f = 0.12 for polymers made up of 20 beads. Below G_f = 0.12 small changes are observed only for the HT₄ bilayer. The bending modulus of the bilayers is obtained as a function of G_f from a Fourier analysis of the height fluctuations. Using the theory developed by Marsh et al. [Biochim. Biophys. Acta 1615, 33 (2003)] for polymer grafted membranes, the contributions to the bending modulus due to changes arising from the grafted polymer and bilayer thinning are partitioned. The contributions to the changes in κ from bilayer thinning were found to lie within 11% for the lipids with four to six tail beads, increasing to 15% for the lipids containing nine tail beads. The changes in the area stretch modulus were also assessed and were found to have a small influence on the overall contribution from membrane thinning. The increase in the area per head group of the lipids was found to be consistent with the scalings predicted by self-consistent mean field results.

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87.16.dm	Mechanical properties and rheology
87.85.J-	Biomaterials

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Oxygen plasma treatment for reducing hydrophobicity of a sealed polydimethylsiloxane microchannel

Say Hwa Tan, Nam-Trung Nguyen, Yong Chin Chua, and Tae Goo Kang

Biomicrofluidics 4, 032204 (2010); http://dx.doi.org/10.1063/1.3466882 (8 pages) | Cited 6 times

Online Publication Date: 30 September 2010

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Rapid prototyping of polydimethylsiloxane (PDMS) is often used to build microfluidic devices. However, the inherent hydrophobic nature of the material limits the use of PDMS in many applications. While different methods have been developed to transform the hydrophobic PDMS surface to a hydrophilic surface, the actual implementation proved to be time consuming due to differences in equipment and the need for characterization. This paper reports a simple and easy protocol combining a second extended oxygen plasma treatments and proper storage to produce usable hydrophilic PDMS devices. The results show that at a plasma power of 70 W, an extended treatment of over 5 min would allow the PDMS surface to remain hydrophilic for more than 6 h. Storing the treated PDMS devices in de-ionized water would allow them to maintain their hydrophilicity for weeks. Atomic force microscopy analysis shows that a longer oxygen plasma time produces a smoother surface.

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07.10.Cm	Micromechanical devices and systems
47.85.Np	Fluidics
68.08.Bc	Wetting

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Herceptin functionalized microfluidic polydimethylsiloxane devices for the capture of human epidermal growth factor receptor 2 positive circulating breast cancer cells

Benjamin Thierry, Mahaveer Kurkuri, Jun Yan Shi, Lwin Ei Mon Phyo Lwin, and Dennis Palms

Biomicrofluidics 4, 032205 (2010); http://dx.doi.org/10.1063/1.3480573 (10 pages) | Cited 2 times

Online Publication Date: 30 September 2010

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Building on recent breakthroughs in the field of microfluidic-based capture of rare cancer cells circulating in the blood, the present article reports on the use of Herceptin functionalized PDMS devices designed to efficiently capture from blood cancer cells, overexpressing the tyrosine kinase human epidermal growth factor receptor (HER2). The identification of patients overexpressing HER2 is critical as it typically associates with an aggressive disease course in breast cancer and poor prognosis. Importantly, HER2 positive patients have been found to significantly benefit from Herceptin (Trastuzumab), a humanized monoclonal antibody (MAb) against HER2. Disposable PDMS devices prepared using standard soft lithography were functionalized by the plasma polymerization of an epoxy-containing monomer. The epoxy-rich thin film (AGEpp) thus created could be conjugated with Herceptin either directly or through a polyethylene glycol interlayer. The properties and reactivity toward the monoclonal antibody conjugation of these coatings were determined using x-ray photoelectron spectroscopy; direct conjugation provided a good compromise in reactivity and resistance to biologically nonspecific fouling and was selected. Using the breast cancer cell line SK-BR-3 as a model for cells overexpressing HER2, the immunocapture efficacy of the Herceptin functionalized PDMS was demonstrated in model studies. Validation studies confirmed the ability of the device to efficiently capture ( ∼ 80% capture yield) HER2 positive cells from full blood.

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87.85.Ox	Biomedical instrumentation and transducers, including micro-electro-mechanical systems (MEMS)
87.19.xj	Cancer
85.85.+j	Micro- and nano-electromechanical systems (MEMS/NEMS) and devices
87.16.-b	Subcellular structure and processes

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Surface patterning of bonded microfluidic channels

Craig Priest

Biomicrofluidics 4, 032206 (2010); http://dx.doi.org/10.1063/1.3493643 (13 pages)

Online Publication Date: 30 September 2010

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Microfluidic channels in which multiple chemical and biological processes can be integrated into a single chip have provided a suitable platform for high throughput screening, chemical synthesis, detection, and alike. These microchips generally exhibit a homogeneous surface chemistry, which limits their functionality. Localized surface modification of microchannels can be challenging due to the nonplanar geometries involved. However, chip bonding remains the main hurdle, with many methods involving thermal or plasma treatment that, in most cases, neutralizes the desired chemical functionality. Postbonding modification of microchannels is subject to many limitations, some of which have been recently overcome. Novel techniques include solution-based modification using laminar or capillary flow, while conventional techniques such as photolithography remain popular. Nonetheless, new methods, including localized microplasma treatment, are emerging as effective postbonding alternatives. This Review focuses on postbonding methods for surface patterning of microchannels.

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81.65.Cf	Surface cleaning, etching, patterning
52.77.Bn	Etching and cleaning
47.85.Np	Fluidics
47.60.Dx	Flows in ducts and channels
82.65.+r	Surface and interface chemistry; heterogeneous catalysis at surfaces
87.80.Ek	Mechanical and micromechanical techniques

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On-chip antibody immobilization for on-demand and rapid immunoassay on a microfluidic chip

Toshinori Ohashi, Kazuma Mawatari, and Takehiko Kitamori

Biomicrofluidics 4, 032207 (2010); http://dx.doi.org/10.1063/1.3437592 (7 pages) | Cited 4 times

Online Publication Date: 30 September 2010

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Immunoassay is one of the important applications of microfluidic chips and many methodologies were reported for decreasing sample/reagent volume, shortening assay time, and so on. Micro-enzyme-linked immunosorbent assay (micro-ELISA) is our method that utilizes packed microbeads in the microfluidic channel and the immunoreactions are induced on the beads surface. Due to the large surface-to-volume ratio and small analytical volume, excellent performances have been verified in assay time and sample/reagent volume. In order to realize the micro-ELISA, one of the important processes is the immobilization of antibody on the beads surface. Previously, the immobilization process was performed in a macroscale tube by physisorption of antibody, and long time (2 h) and large amount of antibody (or high concentration) were required for the immobilization. In addition, the processes including the reaction and washing were laborious, and changing the analyte was not easy. In this research, we integrated the immobilization process into a microfluidic chip by applying the avidin-biotin surface chemistry. The integration enabled very fast (1 min) immobilization with very small amount of precious antibody consumption (100 ng) for one assay. Because the laborious immobilization process can be automatically performed on the microfluidic chip, ELISA method became very easy. On-demand immunoassay was also possible just by changing the antibodies without using large amount of precious antibodies. Finally, the analytical performance was investigated by measuring C-reactive protein and good performance (limit of detection <20 ng/ml) was verified.

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87.15.R-	Reactions and kinetics
85.85.+j	Micro- and nano-electromechanical systems (MEMS/NEMS) and devices

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Single-cell attachment and culture method using a photochemical reaction in a closed microfluidic system

Kihoon Jang, Yan Xu, Yo Tanaka, Kae Sato, Kazuma Mawatari, Tomohiro Konno, Kazuhiko Ishihara, and Takehiko Kitamori

Biomicrofluidics 4, 032208 (2010); http://dx.doi.org/10.1063/1.3494287 (8 pages) | Cited 1 time

Online Publication Date: 30 September 2010

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Recently, interest in single cell analysis has increased because of its potential for improving our understanding of cellular processes. Single cell operation and attachment is indispensable to realize this task. In this paper, we employed a simple and direct method for single-cell attachment and culture in a closed microchannel. The microchannel surface was modified by applying a nonbiofouling polymer, 2-methacryloyloxyethyl phosphorylcholine (MPC) polymer, and a nitrobenzyl photocleavable linker. Using ultraviolet (UV) light irradiation, the MPC polymer was selectively removed by a photochemical reaction that adjusted the cell adherence inside the microchannel. To obtain the desired single endothelial cell patterning in the microchannel, cell-adhesive regions were controlled by use of round photomasks with diameters of 10, 20, 30, or 50 μm. Single-cell adherence patterns were formed after 12 h of incubation, only when 20 and 30 μm photomasks were used, and the proportions of adherent and nonadherent cells among the entire UV-illuminated areas were 21.3%±0.3% and 7.9%±0.3%, respectively. The frequency of single-cell adherence in the case of the 20 μm photomask was 2.7 times greater than that in the case of the 30 μm photomask. We found that the 20 μm photomask was optimal for the formation of single-cell adherence patterns in the microchannel. This technique can be a powerful tool for analyzing environmental factors like cell-surface and cell-extracellular matrix contact.

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87.17.Rt	Cell adhesion and cell mechanics
87.53.Ay	Biophysical mechanisms of interaction
87.80.Ek	Mechanical and micromechanical techniques
47.85.Np	Fluidics
82.50.-m	Photochemistry

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Gold nanoparticle-assisted single base-pair mismatch discrimination on a microfluidic microarray device

Lin Wang and Paul C. H. Li

Biomicrofluidics 4, 032209 (2010); http://dx.doi.org/10.1063/1.3463720 (9 pages)

Online Publication Date: 30 September 2010

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Two simple gold nanoparticle (GNP)-based DNA analysis methods using a microfluidic device are presented. In the first method, probe DNA molecules are immobilized on the surface of a self-assembled submonolayer of GNPs. The hybridization efficiency of the target oligonulceotides was improved due to nanoscale spacing between probe molecules. In the second method, target DNA molecules, oligonulceotides or polymerase chain reaction (PCR) amplicons, are first bound to GNPs and then hybridized to the immobilized probe DNA on a glass slide. With the aid of GNPs, we have successfully discriminated, at room temperature, between two PCR amplicons (derived from closely related fungal pathogens, Botrytis cinerea and Botrytis squamosa) with one base-pair difference. DNA analysis on the microfluidic chip avoids the use of large sample volumes, and only a small amount of oligonucelotides (8 fmol) or PCR products (3 ng), was needed in the experiment. The whole procedure was accomplished at room temperature in 1 h, and apparatus for high temperature stringency was not required.

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87.80.Ek	Mechanical and micromechanical techniques
87.85.Rs	Nanotechnologies-applications
87.14.gk	DNA
82.80.-d	Chemical analysis and related physical methods of analysis

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DNA separation by cholesterol-bearing pullulan nanogels

Keisuke Kondo, Noritada Kaji, Sayaka Toita, Yukihiro Okamoto, Manabu Tokeshi, Kazunari Akiyoshi, and Yoshinobu Baba

Biomicrofluidics 4, 032210 (2010); http://dx.doi.org/10.1063/1.3479997 (8 pages)

Online Publication Date: 30 September 2010

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We present an application of a novel DNA separation matrix, cholesterol-bearing pullulan (CHP) nanogels, for microchip electrophoresis. The solution of the CHP showed a unique phase transition around 30 mg/ml and formed gel phase over this critical concentration. This gel phase consists of the weak hydrophobic interactions between the cholesterols could be easily deformed by external forces, and thus, loading process of the CHP nanogels into microchannels became easier. The high concentration of the CHP nanogels provided excellent resolutions especially for small DNA fragments from 100 to 1500 bp. The separation mechanism was discussed based on Ogston and Reptation models which had developed in gels or polymer solutions. The result of a single molecule imaging gave us an insight of the separation mechanism and the nanogel structures as well.

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87.80.Qk	Biochemical separation processes
87.85.J-	Biomaterials
87.85.Rs	Nanotechnologies-applications
87.14.gk	DNA
87.15.Tt	Electrophoresis
87.15.N-	Properties of solutions of macromolecules
82.70.Gg	Gels and sols

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Electrical properties with relaxation through human blood

S. Abdalla, S. S. Al-ameer, and S. H. Al-Magaishi

Biomicrofluidics 4, 034101 (2010); http://dx.doi.org/10.1063/1.3458908 (16 pages) | Cited 2 times

Online Publication Date: 8 July 2010

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The present work aims to study the effects of the blood-microstructure on the electrical conduction from two different but correlated properties: Electrical and mechanical (viscosity), and to derive useful parameters for the evaluation of electrical conduction as a function of the blood viscosity. ac-conductivity and dielectric constant of normal and diabetic blood are measured in the frequency range 10 kHz–1 MHz at the room temperature. An empirical relation relating the resistivity and viscosity of the blood has been presented. The results show that a microfluidic device is a viable and simple solution for determination of electrical and rheological behaviors of blood samples.

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87.85.jc	Electrical, thermal, and mechanical properties of biological matter
87.85.gf	Fluid mechanics and rheology
87.80.Ek	Mechanical and micromechanical techniques

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Optoelectrofluidic field separation based on light-intensity gradients

Sanghyun Lee, Hyun Jin Park, Jin Sung Yoon, and Kwan Hyoung Kang

Biomicrofluidics 4, 034102 (2010); http://dx.doi.org/10.1063/1.3463716 (12 pages) | Cited 2 times

Online Publication Date: 14 July 2010

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Optoelectrofluidic field separation (OEFS) of particles under light -intensity gradient (LIG) is reported, where the LIG illumination on the photoconductive layer converts the short-ranged dielectrophoresis (DEP) force to the long-ranged one. The long-ranged DEP force can compete with the hydrodynamic force by alternating current electro-osmosis (ACEO) over the entire illumination area for realizing effective field separation of particles. In the OEFS system, the codirectional illumination and observation induce the levitation effect, compensating the attenuation of the DEP force under LIG illumination by slightly floating particles from the surface. Results of the field separation and concentration of diverse particle pairs (0.82–16 μm) are well demonstrated, and conditions determining the critical radius and effective particle manipulation are discussed. The OEFS with codirectional LIG strategy could be a promising particle manipulation method in many applications where a rapid manipulation of biological cells and particles over the entire working area are of interest.

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87.80.Cc	Optical trapping
87.50.ch	Electrophoresis/dielectrophoresis and other mechanical effects
87.17.-d	Cell processes

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Patterning nanowire and micro-/nanoparticle array on micropillar-structured surface: Experiment and modeling

Chung Hsun Lin, Jingjiao Guan, Shiu Wu Chau, Shia Chung Chen, and L. James Lee

Biomicrofluidics 4, 034103 (2010); http://dx.doi.org/10.1063/1.3474639 (15 pages) | Cited 2 times

Online Publication Date: 4 August 2010

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DNA molecules in a solution can be immobilized and stretched into a highly ordered array on a solid surface containing micropillars by molecular combing technique. However, the mechanism of this process is not well understood. In this study, we demonstrated the generation of DNA nanostrand array with linear, zigzag, and fork-zigzag patterns and the microfluidic processes are modeled based on a deforming body-fitted grid approach. The simulation results provide insights for explaining the stretching, immobilizing, and patterning of DNA molecules observed in the experiments.

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87.80.Ek	Mechanical and micromechanical techniques
87.85.Rs	Nanotechnologies-applications
87.85.Ox	Biomedical instrumentation and transducers, including micro-electro-mechanical systems (MEMS)
87.14.gk	DNA
81.16.Rf	Micro- and nanoscale pattern formation

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A dielectrophoretic chip with a roughened metal surface for on-chip surface-enhanced Raman scattering analysis of bacteria

I-Fang Cheng, Chi-Chang Lin, Dong-Yi Lin, and Hsien-Chang Chang

Biomicrofluidics 4, 034104 (2010); http://dx.doi.org/10.1063/1.3474638 (11 pages) | Cited 14 times

Online Publication Date: 5 August 2010

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We present an analysis of the results of in situ surface-enhanced Raman scattering (SERS) of bacteria using a microfluidic chip capable of continuously sorting and concentrating bacteria via three-dimensional dielectrophoresis (DEP). Microchannels were made by sandwiching DEP microelectrodes between two glass slides. Avoiding the use of a metal nanoparticle suspension, a roughened metal surface is integrated into the DEP-based microfluidic chip for on-chip SERS detection of bacteria. On the upper surface of the slide, a roughened metal shelter was settled in front of the DEP concentrator to enhance Raman scattering. Similarly, an electrode-patterned bottom layer fabricated on a thin cover-slip was used to reduce fluorescence noise from the glass substrate. Gram positive (Staphylococcus aureus) and Gram negative (Pseudomonas aeruginosa) bacteria were effectively distinguished in the SERS spectral data. Staphylococcus aureus (concentration of 10⁶ CFU/ml) was continuously separated and concentrated via DEP out of a sample of blood cells. At a flow rate of 1 μl/min, the bacteria were highly concentrated at the roughened surface and ready for on-chip SERS analysis within 3 min. The SERS data were successfully amplified by one order of magnitude and analyzed within a few minutes, resulting in the detection of signature peaks of the respective bacteria.

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87.80.Kc	Electrochemical techniques
87.80.Dj	Spectroscopies
87.80.Ek	Mechanical and micromechanical techniques
87.85.fk	Biosensors
47.85.Np	Fluidics

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Microliter-bioreactor array with buoyancy-driven stirring for human hematopoietic stem cell culture

Camilla Luni, Hope C. Feldman, Michela Pozzobon, Paolo De Coppi, Carl D. Meinhart, and Nicola Elvassore

Biomicrofluidics 4, 034105 (2010); http://dx.doi.org/10.1063/1.3380627 (13 pages) | Cited 2 times

Online Publication Date: 11 August 2010

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This work presents the development of an array of bioreactors where finely controlled stirring is provided at the microliter scale (100–300 μl). The microliter-bioreactor array is useful for performing protocol optimization in up to 96 parallel experiments of hematopoietic stem cell (HSC) cultures. Exploring a wide range of experimental conditions at the microliter scale minimizes cost and labor. Once the cell culture protocol is optimized, it can be applied to large-scale bioreactors for stem cell production at the clinical level. The controlled stirring inside the wells of a standard 96-well plate is provided by buoyancy-driven thermoconvection. The temperature and velocity fields within the culture volume are determined with numerical simulations. The numerical results are verified with experimental velocity measurements using microparticle image velocimetry (μPIV) and are used to define feasible experimental conditions for stem cell cultures. To test the bioreactor array’s functionality, human umbilical cord blood-derived CD34⁺ cells were cultured for 7 days at five different stirring conditions (0.24–0.58 μm/s) in six repeated experiments. Cells were characterized in terms of proliferation, and flow cytometry measurements of viability and CD34 expression. The microliter-bioreactor array demonstrates its ability to support HSC cultures under stirred conditions without adversely affecting the cell behavior. Because of the highly controlled operative conditions, it can be used to explore culture conditions where the mass transport of endogenous and exogenous growth factors is selectively enhanced, and cell suspension provided. While the bioreactor array was developed for culturing HSCs, its application can be extended to other cell types.

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87.80.Ek	Mechanical and micromechanical techniques
87.17.Uv	Biotechnology of cell processes
85.85.+j	Micro- and nano-electromechanical systems (MEMS/NEMS) and devices
47.85.Np	Fluidics

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Numerical study of in situ preconcentration for rapid and sensitive nanoparticle detection

Kai Yang and Jie Wu

Biomicrofluidics 4, 034106 (2010); http://dx.doi.org/10.1063/1.3467446 (15 pages) | Cited 1 time

Online Publication Date: 12 August 2010

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This paper presents a numerical study of a preconcentrator design that can effectively increase the binding rate at the sensor in a real time manner. The particle enrichment is realized by the ac electrothermal (ACET) effect, which induces fluid movement to carry nanoparticles toward the sensor. The ACET is the only electrical method to manipulate a biological sample of medium to high ionic strength (>0.1 S/m, e.g., 0.06× phosphate buffered saline). The preconcentrator consists of a pair of electrodes striding over the sensor, simple to implement as it is electrically controlled. This preconcentrator design is compatible and can be readily integrated with many types of micro- to nanosensors. By applying an ac signal over the electrodes, local vortices will generate a large velocity perpendicular to the reaction surface, which enhances transport of analytes toward the sensor. Our simulation shows that the binding rate at the sensor surface is greatly enhanced. Our study also shows that the collection of analytes will be affected by various parameters such as channel height, inlet velocity, and sensor size, and our results will provide guidance in optimization of the preconcentrator design.

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87.80.-y	Biophysical techniques (research methods)
87.85.gf	Fluid mechanics and rheology
87.80.Kc	Electrochemical techniques
82.80.Fk	Electrochemical methods

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Negative dielectrophoretic capture of bacterial spores in food matrices

Mehti Koklu, Seungkyung Park, Suresh D. Pillai, and Ali Beskok

Biomicrofluidics 4, 034107 (2010); http://dx.doi.org/10.1063/1.3479998 (15 pages) | Cited 8 times

Online Publication Date: 17 August 2010

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A microfluidic device with planar square electrodes is developed for capturing particles from high conductivity media using negative dielectrophoresis (n-DEP). Specifically, Bacillus subtilis and Clostridium sporogenes spores, and polystyrene particles are tested in NaCl solution (0.05 and 0.225 S/m), apple juice (0.225 S/m), and milk (0.525 S/m). Depending on the conductivity of the medium, the Joule heating produces electrothermal flow (ETF), which continuously circulates and transports the particles to the DEP capture sites. Combination of the ETF and n-DEP results in different particle capture efficiencies as a function of the conductivity. Utilizing 20 μm height DEP chambers, “almost complete” and rapid particle capture from lower conductivity (0.05 S/m) medium is observed. Using DEP chambers above 150 μm in height, the onset of a global fluid motion for high conductivity media is observed. This motion enhances particle capture on the electrodes at the center of the DEP chamber. The n-DEP electrodes are designed to have well defined electric field minima, enabling sample concentration at 1000 distinct locations within the chip. The electrode design also facilitates integration of immunoassay and other surface sensors onto the particle capture sites for rapid detection of target micro-organisms in the future.

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87.15.Tt	Electrophoresis
87.50.ch	Electrophoresis/dielectrophoresis and other mechanical effects
82.45.-h	Electrochemistry and electrophoresis
47.85.Np	Fluidics
87.80.Ek	Mechanical and micromechanical techniques
47.63.-b	Biological fluid dynamics

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Focusing and trapping of DNA molecules by head-on ac electrokinetic streaming through join asymmetric polarization

Jung-Rong Du and Hsien-Hung Wei

Biomicrofluidics 4, 034108 (2010); http://dx.doi.org/10.1063/1.3481468 (13 pages)

Online Publication Date: 19 August 2010

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In this work, invoking join asymmetric ac polarization using double half-quadrupole electrodes in a symmetric arrangement, we demonstrate a head-on ac electro-osmotic streaming capable of focusing and trapping DNA molecules efficiently. This is manifested by the observation that picomolar DNA molecules can be trapped into a large crosslike spot with at least an order of magnitude concentration enhancement within just half a minute. We identify that the phenomenon is a combined result of the formation of two prefocused DNA jets flowing toward each other, dipole-induced attraction between focused DNA molecules, and dielectrophoretic trap on the spot. With an additional horizontal pumping, we observe that the trap can transform into a peculiar pitchfork streaming capable of continuous collection and long-distance transport of concentrated DNA molecules. We also show that the same electrode design can be used to direct assembly of submicrometer particles. This newly designed microfluidic platform not only has potentials in enhancing detection sensitivity and facilitating functional assembly for on-chip analysis but also provides an added advantage of transporting target molecules in a focused and continuous manner.

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87.80.Ek	Mechanical and micromechanical techniques
47.85.Np	Fluidics
87.14.gk	DNA
87.15.Tt	Electrophoresis

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Contraction and extension of Vorticella and its mechanical characterization under flow loading

Moeto Nagai, Hiroshi Asai, and Hiroyuki Fujita

Biomicrofluidics 4, 034109 (2010); http://dx.doi.org/10.1063/1.3481777 (11 pages) | Cited 1 time

Online Publication Date: 26 August 2010

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We have studied the contraction and extension of Vorticella convallaria and its mechanical properties with a microfluidic loading system. Cells of V. convallaria were injected to a microfluidic channel (500 μm in width and 100 μm in height) and loaded by flow up to ∼ 350 mm s⁻¹. The flow produced a drag force on the order of nanonewton on a typical vorticellid cell body. We gradually increased the loading force on the same V. convallaria specimen and examined its mechanical property and stalk motion of V. convallaria. With greater drag forces, the contraction distance linearly decreased; the contracted length was close to around 90% of the stretched length. We estimated the drag force on Vorticella in the channel by calculating the force on a sphere in a linear shear flow.

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85.85.+j	Micro- and nano-electromechanical systems (MEMS/NEMS) and devices
47.85.Np	Fluidics
87.16.-b	Subcellular structure and processes
47.63.-b	Biological fluid dynamics
87.85.gf	Fluid mechanics and rheology

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Microfluidic parallel circuit for measurement of hydraulic resistance

Sungyoung Choi, Myung Gwon Lee, and Je-Kyun Park

Biomicrofluidics 4, 034110 (2010); http://dx.doi.org/10.1063/1.3486609 (9 pages) | Cited 1 time

Online Publication Date: 31 August 2010

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We present a microfluidic parallel circuit that directly compares the test channel of an unknown hydraulic resistance with the reference channel with a known resistance, thereby measuring the unknown resistance without any measurement setup, such as standard pressure gauges. Many of microfluidic applications require the precise transport of fluid along a channel network with complex patterns. Therefore, it is important to accurately characterize and measure the hydraulic resistance of each channel segment, and determines whether the device principle works well. However, there is no fluidic device that includes features, such as the ability to diagnose microfluidic problems by measuring the hydraulic resistance of a microfluidic component in microscales. To address the above need, we demonstrate a simple strategy to measure an unknown hydraulic resistance, by characterizing the hydraulic resistance of microchannels with different widths and defining an equivalent linear channel of a microchannel with repeated patterns of a sudden contraction and expansion.

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47.80.Fg	Pressure and temperature measurements
47.60.Dx	Flows in ducts and channels
47.85.Np	Fluidics
85.85.+j	Micro- and nano-electromechanical systems (MEMS/NEMS) and devices

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A new perspective on in vitro assessment method for evaluating quantum dot toxicity by using microfluidics technology

Sanjeev Kumar Mahto, Tae Hyun Yoon, and Seog Woo Rhee

Biomicrofluidics 4, 034111 (2010); http://dx.doi.org/10.1063/1.3486610 (8 pages) | Cited 1 time

Online Publication Date: 24 September 2010

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In this study, we demonstrate a new perspective on in vitro assessment method for evaluating quantum dot (QD) toxicity by using microfluidics technology. A new biomimetic approach, based on the flow exposure condition, was applied in order to characterize the cytotoxic potential of QD. In addition, the outcomes obtained from the flow exposure condition were compared to those of the static exposure condition. An in vitro cell array system was established that used an integrated multicompartmented microfluidic device to develop a sensitive flow exposure condition. QDs modified with cetyltrimethyl ammonium bromide/trioctylphosphine oxide were used for the cytotoxicity assessment. The results suggested noticeable differences in the number of detached and deformed cells and the viability percentages between two different exposure conditions. The intracellular production of reactive oxygen species and release of cadmium were found to be the possible causes of QD-induced cytotoxicity, irrespective of the types of exposure condition. In contrast to the static exposure, the flow exposure apparently avoided the gravitational settling of particles and probably assisted in the homogeneous distribution of nanoparticles in the culture medium during exposure time. Moreover, the flow exposure condition resembled in vivo physiological conditions very closely, and thus, the flow exposure condition can offer potential advantages for nanotoxicity research.

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87.85.Rs	Nanotechnologies-applications
87.17.-d	Cell processes
87.80.Ek	Mechanical and micromechanical techniques

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The effect of flap parameters on fluid rectification in a microfluidic diode

Kunwar Pal Singh and Manoj Kumar

Biomicrofluidics 4, 034112 (2010); http://dx.doi.org/10.1063/1.3492403 (17 pages) | Cited 1 time

Online Publication Date: 27 September 2010

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We have studied the effect of flap parameters on fluid rectification in a microfluidic diode. We use Navier–Stokes equations and arbitrary Lagrangian–Eulerian formulation to obtain dynamics of fluid flow and motion of the flap. The flap opens during forward flow and seals against a stopper during reverse flow. This allows flow in the forward direction and prevents it in the reverse direction. The rectifier is fluidic analog to a semiconductor diode in function because it rectifies fluid flow. Velocity-pressure (V-P) curves analog to the current-voltage (I-V) curves of the electronic diode has been obtained. The effect of the flap parameters, such as length, thickness, and Young’s modulus has been found out. The transient response of the flap and fluid flow under oscillating pressure driven flow has also been obtained.

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87.80.Ek	Mechanical and micromechanical techniques
47.85.Np	Fluidics
87.85.Ox	Biomedical instrumentation and transducers, including micro-electro-mechanical systems (MEMS)
85.85.+j	Micro- and nano-electromechanical systems (MEMS/NEMS) and devices

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