Enhancement of the surface free energy of PDMS for reversible and leakage-free bonding of PDMS–PS microfluidic cell-culture systems

Polydimethylsiloxane (PDMS) has become one of the most widely used materials in the fabrication of microfluidic systems bonded onto glass substrates, especially for cell biology applications. However, PDMS is often unsuitable for building microfluidic systems onto polystyrene (PS) which is the preferred substrate in most cell-culture protocols. In particular, PS is required for culturing many stem cell and primary cell types. Here, we propose a novel approach to building PDMS–PS microfluidic cell-culture systems, specifically realizing a strong and reversible bonding of PDMS on PS without using chemical agents which can have negative effects on cell viability. Our strategy to strengthen the bonding of PDMS to PS surfaces is to increase the surface free energy (SFE) by adjusting the mixing ratio of PDMS base to curing agent and by treating the surfaces of PDMS and PS with O₂ plasma and annealing. Our results show that using this method for PDMS–PS bonding, we are able to produce reliable reversible and leakage-free PDMS–PS microfluidic cell-culture systems.     

Computing non-self-intersecting offsets of NURBS surfaces

A new approach for the computation of non-self-intersecting offset surface of a single G₁ continuous NURBS surface has been presented. The approach recognizes special surfaces, i.e. planes, spheres, cones and cylinders, and offsets them precisely. An approximate offset surface within the specified tolerance is computed for a general free form surface. The method for a general free form surface consists of (1) sample offset surface based on second derivatives; (2) eliminate sample points which can give self-intersections; (3) surface fitting through the remaining sample points; and (4) removal of all the removable knots of the surface. The approach checks for self-intersections in the offset surface and removes the same automatically, if any. The non-self-intersecting offsets for surface of extrusion and surface of revolution are obtained by removing the self-intersections in the offset generator and profile curves respectively using point sampling, cleaning of sampled points, curve fitting and knot removal. The approach has better control on error. It generates offset surface with less number of control points and degree. The methodology works only for a class of problems where in the offset of a single G₁ surface is still a single connected surface without having any holes. The offset methodology has been demonstrated through three types of surfaces namely surface of revolution, surface of extrusion and a general free form surface. This approach has been extensively used in creation of offset surfaces of composite laminate components. The presented approach can also be used to check for self-intersections in any general surface and to remove the same, if any, with little modifications, as long as the cleaned surface is a single connected surface.     

Effects of temperature on particle trajectories inside hard disk drives

The presence of particles, which can intrude into the gas bearing, is one of the most common factors in the failure of hard disk drives (HDDs). Previous works investigated particle trajectories inside air-filled drives without considering temperature effects on the distribution of particles. Actually, especially for the submicron particle, particle trajectories and trapping status are affected by the temperature gradient since the thermophoretic force cannot be ignored. In this paper, considering major heat generation components such as the spindle motor and voice coil motor (VCM), trajectories and trapping status for Al₂O₃ particles inside a 2.5 inch helium-filled drive are simulated by the commercial computational fluid dynamics solver FLUENT with user-defined functions (UDFs). The trapping criterion for Al₂O₃ particles is used as boundary conditions for different colliding surfaces. The results reveal that particles in the air-filled drive will more likely degrade the head–disk interface (HDI) reliability. In addition, after considering the temperature, the particle trapping rate by the disk decreases both inside the air-filled drive and the helium-filled drive. And its reduction inside the air-filled drive is larger. Moreover, small particles will more likely degrade the HDI reliability since they can follow the rotatory flow well and have more chance to collide with the disk surface, and then easily attach onto the disk surface.

     

Contact CMOS imaging of gaseous oxygen sensor array

We describe a compact luminescent gaseous oxygen (O₂) sensor microsystem based on the direct integration of sensor elements with a polymeric optical filter and placed on a low power complementary metal-oxide semiconductor (CMOS) imager integrated circuit (IC). The sensor operates on the measurement of excited-state emission intensity of O₂-sensitive luminophore molecules tris(4,7-diphenyl-1,10-phenanthroline) ruthenium(II) ([Ru(dpp)₃]²⁺) encapsulated within sol-gel derived xerogel thin films. The polymeric optical filter is made with polydimethylsiloxane (PDMS) that is mixed with a dye (Sudan-II). The PDMS membrane surface is molded to incorporate arrays of trapezoidal microstructures that serve to focus the optical sensor signals on to the imager pixels. The molded PDMS membrane is then attached with the PDMS color filter. The xerogel sensor arrays are contact printed on top of the PDMS trapezoidal lens-like microstructures. The CMOS imager uses a 32 × 32 (1024 elements) array of active pixel sensors and each pixel includes a high-gain phototransistor to convert the detected optical signals into electrical currents. Correlated double sampling circuit, pixel address, digital control and signal integration circuits are also implemented on-chip. The CMOS imager data is read out as a serial coded signal. The CMOS imager consumes a static power of 320 μW and an average dynamic power of 625 μW when operating at 100 Hz sampling frequency and 1.8 V DC. This CMOS sensor system provides a useful platform for the development of miniaturized optical chemical gas sensors.     

Electronic and structural properties of low-index L12–Al3Zr surfaces by first-principle calculations

The atomic relaxations, electronic properties and surface energies of low index L1₂-Al₃Zr surfaces were studied by using the first-principles method based on the density functional theory. Five low index surfaces with different terminations are studied systematically. The study shows that atomic relaxations occur mainly within the outermost two layers. The stoichiometric (111), non-stoichiometric (110) and (001) surfaces with different terminations are investigated. The (111) surface which has the lowest surface energy and is independent on the chemical potential of Al atom is found to be the most thermodynamically stable surface. For the non-stoichiometric surfaces, the AlZr terminated (001) surface and Al terminated (110) surface are metastable under an Zr-rich and Al-rich condition respectively. These results are in consistant with the results of density of states. The lattice misfit between α-Al and L1₂-Al₃Zr is not more than 1.65% by our calculation, indicating L1₂-Al₃Zr is a potent effective heterogeneous nucleating agent for α-Al.     

Low-cost polymer microfluidic device for on-chip extraction of bacterial DNA

A polymer microfluidic device for on-chip extraction of bacterial DNA has been developed for molecular diagnostics. In order to manufacture a low-cost, disposable microchip, micropillar arrays of high surface-to-volume ratio (0.152 μm⁻¹) were constructed on polymethyl methacrylate (PMMA) by hot embossing with an electroformed Ni mold, and their surface was modified with SiO₂ and an organosilane compound in subsequent steps. To seal open microchannels, the organosilane layer on top plane of the micropillars was selectively removed through photocatalytic oxidation via TiO₂/UV treatment at room temperature. As a result, the underlying SiO₂ surface was exposed without deteriorating the organosilane layer coated on lateral surface of the micropillars that could serve as bacterial cell adhesion moiety. Afterwards, a plasma-treated PDMS substrate was bonded to the exposed SiO₂ surface, completing the device fabrication. To optimize manufacturing throughput and process integration, the whole fabrication process was performed at 6 inch wafer-level including polymer imprinting, organosilane coating, and bonding. Preparation of bacterial DNA was carried out with the fabricated PDMS/PMMA chip according to the following procedure: bacterial cell capture, washing, in situ lysis, and DNA elution. The polymer-based microchip presented here demonstrated similar performance to Glass/Si chip in terms of bacterial cell capture efficiency and polymerase chain reaction (PCR) compatibility.     

On-chip manipulation of nonmagnetic particles in paramagnetic solutions using embedded permanent magnets

This study develops a method for embedding permanent magnets into poly(dimethylsiloxane) (PDMS)-based microfluidic chips. Magnets can be brought very close to the planar microchannels for enhanced magnetic field and field gradients, which enables on-chip continuous-flow manipulation of nonmagnetic particles in typical paramagnetic solutions. We performed a systematic study of the transport of polystyrene particles suspended in manganese (II) chloride (MnCl₂) solutions through a rectangular microchannel. Owing to their smaller magnetization than the suspending fluid, particles experience negative magnetophoresis and are deflected away from the magnet. The effects of particle position (relative to the magnet), particle size, MnCl₂ salt concentration, and fluid flow velocity on the horizontal magnetophoretic deflection are examined using a combined experimental and theoretical approach. The experimental results agree quantitatively with the predictions of an analytical model. The demonstrated nonmagnetic particle deflection may be used with the potential to focus and sort cells in lab-on-a-chip for bio-applications.     

Enantiomeric separation by open-tubular capillary electrochromatography using bovine-serum-albumin-conjugated graphene oxide–magnetic nanocomposites as stationary phase

In this work, a novel chip-based enantioselective open-tubular capillary electrochromatography (OT-CEC) was developed employing bovine serum albumin (BSA)-conjugated graphene oxide–magnetic nanocomposites (GO/Fe₃O₄) as stationary phase. GO/Fe₃O₄ nanocomposites with high magnetic responsivity, excellent solubility, and high dispersibility in water were prepared through a facile and controllable in situ chemical deposition strategy. BSA was then adsorbed onto the GO/Fe₃O₄ surface to form GO/Fe₃O₄/BSA conjugates, which were then locally packed into PDMS microchannels with the help of magnets. The resultant GO/Fe₃O₄/BSA conjugates not only have the magnetism of Fe₃O₄ NPs that make them easily manipulated by an external magnetic field, but also have the larger surface and excellent biocompatibility of graphene, which can incorporate much more biomolecules and well maintain their biological activity. In addition, the successful BSA decorations endowed GO/Fe₃O₄/BSA conjugates with pH-tunable water solubility related to the isoelectric point of BSA (pI 4.7) and led to enhanced stability against high ionic strength. Compared with the native PDMS microchannel, the modified surfaces exhibited more stable and suppressed electroosmotic mobility and less nonspecific adsorption toward analytes. Successful separation of tryptophan, threonine, and propranolol enantiomers were achieved in less than 80 s with resolution factors of 1.22, 1.9, and 2.1, respectively, utilizing a separation length of 37 mm coupled with in column amperometric detection. The presented on-chip enantioselective OT-CEC protocol simplifies the protein immobilization methodology and has the potential to provide a platform for high-throughput screening of enantiomer candidates.     

Zone electrophoresis of proteins in poly(dimethylsiloxane) (PDMS) microchip coated with physically adsorbed amphiphilic phospholipid polymer

The zone electrophoresis of protein in poly(dimethylsiloxane) (PDMS) microchip coated with the physically adsorbed amphiphilic phospholipid polymer (PMMSi) was investigated. PMMSi was composed of 2-methacryloyloxyethyl phosphorylcholine (MPC) and 3-(methacryloyloxy) propyltris (trimethylsiloxy) silane (MPTSSi) units in a random fashion. The membrane of PMMSi can be formed on the PDMS surface by a simple and quick dip-coating method. The membrane showed high hydrophilicity and good stability in water, as determined by contact angle measurement, fourier-transformed infrared absorption by attenuated total reflection (ATR-FTIR), and X-ray photoelectron spectroscopy (XPS) analysis. High suppression of protein adsorption to the PDMS surface and reduction in electroosmotic flow (EOF) were achieved by PMMSi coating due to an increase of hydrophilicity, and a decrease of the ζ-potential on the surface of PDMS. For zone electrophoresis, the PMMSi30 containing 30 % hydrophilic MPC was the most suitable molecular design in terms of the stability of the coated membrane on PDMS surface. The average value of EOF mobility of PDMS microchip coated with PMMSi30 was 1.4 × 10^?4 cm² V^?1 s^?1, and the RSD was 4.1 %. Zone electrophoresis of uranine was further demonstrated with high repeatability and reproducibility. Separation of two FITC-labeled proteins (BSA and insulin) was performed with high efficiency and resolution compared with non-treated PDMS microchip.     

Fabrication of long-term hydrophilic surfaces of poly(dimethyl siloxane) using 2-hydroxy ethyl methacrylate

In the present work, 2-hydroxy ethyl methacrylate (HEMA) was used to modify surface of poly(dimethyl siloxane) (PDMS) elastomer. Fourier transform infrared spectroscopy (FTIR) and wetting angle measurements were used for the analysis of modified surface and hydrophilic stability of PDMS. Results of the surface reconstruction reveal that long-term hydrophilic surfaces of PDMS can be achieved by use of HEMA.     

The effect of the aspect ratio on the hydrophobicity of microstructured polydimethylsiloxane (PDMS) robust surfaces

In this study, we have designed and fabricated robust hydrophobic surfaces that are composed of various micropillar arrays and investigated the effect of the aspect ratio (feature height/feature size) of the micropillar on the wettability of the fabricated surfaces. The robust, micropillar-arrayed surfaces were designed to yield the same Wenzel and Cassie water contact angles (CAs). According to our design rule, one can achieve an enhanced hydrophobic surface by increasing the height of the micropillars. The designed hydrophobic surfaces were fabricated by polydimethylsiloxane (PDMS) replica molding with photolithographically micropatterned SU-8 masters. The hydrophobicity properties of the fabricated PDMS surfaces were fully characterized theoretically and experimentally. From the theoretical and experimental results, it was found that the micropillars of an intrinsically hydrophobic material with a high aspect ratio enhance the hydrophobicity of the surface by increasing the surface roughness (in view of the Wenzel state) and the opportunities for the entrapment of air beneath a water droplet (the Cassie state).     

Hyper-miniaturization of monodisperse alginate–TiO2 composite particles with densely packed TiO2 nanoparticles

We report a novel technique to fabricate alginate–TiO₂ composite particles with densely packed TiO₂ nanoparticles. Using a microfluidic device, monodisperse sodium alginate droplets containing low-density TiO₂ nanoparticles (1 or 5 w/v%) were formed in the oil phase. The sodium alginate droplets formed in the oil phase were subsequently placed on a Ca²⁺-loaded agarose-gel plate to induce shrinkage by water removal (from the droplets to the Ca²⁺-loaded agarose-gel plate) and gelation by Ca²⁺ transport (from the Ca²⁺-loaded agarose-gel plate to the droplets). Thus, the produced alginate–TiO₂ composite particles containing densely packed TiO₂ nanoparticles were significantly smaller than the microchannel. We also investigated the optimal conditions to successfully produce spherical composite particles by varying the oil phases, surfactants, calcium concentrations and gel strength of the agarose-gel plate. Moreover, our method could decrease the probability of channel clogging that often occurs when a colloidal suspension (e.g., nanoparticles) is used as the dispersed phase. This method facilitates the stable production of monodisperse alginate–inorganic composite particles for a wide range of applications.     

Multi-scale,multi-depth lithography using optical fibers for microfluidic applications

This paper proposes and demonstrates a method for multi-scale, multi-depth three-dimensional (3D) lithography. In this method, 3D molds for replicating microchannels are fabricated by passing a non-focused laser beam through an optical fiber, whose tip is immersed in a droplet of photopolymer. Line width is adjustable from 1 to 980 µm using eight kinds of optical fibers with different core diameters. The height of line drawing can be controlled by adjusting the distance between the tip of the optical fiber and a substrate. The surface roughness (R_a, R_z) of a single line and plane was evaluated. The method was employed to fabricate a 3D mold of a microchannel containing tandem chambers, which was then successfully replicated in PDMS. Multi-scale, multi-depth 3D lithography can provide a simple, flexible tool for producing PDMS microfluidic devices.     

Fabrication of optically smooth, through-wafer silicon molds for PDMS total internal reflection-based devices

This paper presents a systematic approach to fabricate optically smooth, through-wafer silicon (Si) molds for polymer optical devices, in particular poly(dimethylsiloxane) (PDMS) total internal reflection (TIR)-based devices. First, the Si molds were fabricated by an optimized, through-wafer deep reactive ion etching (DRIE) process to achieve small roughness. To further reduce the roughness, the Si molds were then oxidized and etched in BHF for three times to achieve surface roughness average (R _a) and root mean square (RMS) roughness below 25 nm while peak-to-valley (P–V) roughness is below 150 nm. We monitored the surface roughness and morphology of the sidewalls of Si mold through three cycles of oxidation and BHF etching using field emission scanning electron microscopy (FESEM) and atomic force microscopy (AFM). We found that further oxidation and BHF etching might not have much effect in further reducing the roughness while the device feature definitions might be compromised. Finally, the PDMS TIR-based devices replicated from the Si molds were evaluated by means of FESEM, AFM and by imaging of the fluorescent evanescent spots.     

Guiding of emulsion droplets in microfluidic chips along shallow tracks defined by laser ablation

We demonstrate controlled guiding of nanoliter emulsion droplets of polar liquids suspended in oil along shallow hydrophilic tracks fabricated at the base of microchannels located within microfluidic chips. The tracks for droplet guiding are generated by exposing the glass surface of polydimethylsiloxane (PDMS)-coated microscope slides via femtosecond laser ablation. The difference in wettability of glass and PDMS surfaces together with the shallow step-like transverse topographical profile of the ablated tracks allows polar droplets wetting preferentially the glass surface to follow the track. In this study, we investigate guiding of droplets of two different polar liquids (water/ethylene glycol) with and without surfactant suspended in an oil medium along surface tracks of different depths of 1, 1.5, and 2 \(\upmu\)m. The results of experiments are also verified with computational fluid dynamics simulations. Guiding of droplets along the tracks as a function of the droplet composition and size and the surface profile depth is evaluated by analyzing the trajectories of moving droplets with respect to the track central axis, and conditions for stable guiding are identified. The experiments and numerical simulations indicate that while the track topography plays a role in droplet guiding using 1.5- and 2-\(\upmu\)m deep tracks, for the case of the smallest track depth of 1 \(\upmu\)m, droplet guiding is mainly caused by surface energy modification along the track rather than the presence of a topographical step on the surface. Our results can be exploited to sort passively different microdroplets mixed in the same microfluidic chip, based on their inherent wetting properties, and they can also pave the way for guiding of droplets along reconfigurable tracks defined by surface energy modifications obtained using other external control mechanisms such as electric field or light.     

Flexible piezoelectric micromachined ultrasonic transducer (pMUT) for application in brain stimulation

We propose a new flexible piezoelectric micromachined ultrasonic transducer (pMUT) array integrated on flexible polydimethylsiloxane (PDMS) that can be used in studying brain stimulation by ultrasound. To achieve the technical demands of a high sound pressure level and flexibility, a diaphragm-type piezoelectric ultrasound transducer array was manufactured with 55 μm-thick bulk lead zirconate titanate (PZT) that was thinned after bonding with a silicon wafer. The ultrasound transducer array was then strongly bonded onto a PDMS substrate using an oxygen-plasma treatment followed by precise dicing with a fixed pitch to achieve flexibility. The radius of curvature was smaller than 5 mm, which is sufficient for attachment to the surface of a mouse brain. After a thinning process for the PZT layer, we observed that the PZT layer still maintained a high ferroelectric property. The measured remnant polarization (P_r) and coercive field (E_c) were 28.26 μC/cm² and 79 kV/cm, respectively. The resonant frequencies of fabricated pMUT elements with different membrane sizes of 700, 800, 900, 1200 μm in diameter were measured to be 694.4, 565.4, 430.8, and 289.3 kHz, respectively. By measuring the ultrasound output pressure, a pMUT showed a sound intensity (I_sppa) of 44 mW/cm² at 80 V, which is high enough for low-intensity ultrasound brain stimulation.

     

Effect of surface charges of nanoparticles on response current of enzyme electrode for single use

The response current of glucose oxidase (GOD) electrode has been enhanced by the nanoparticles (NPs), but the mechanism of enhancement remains unclear. The effect of surface charges of NPs on the response current of GOD-electrode enhanced by NPs was studied by using the electrophoresis and the determination of zeta potential. The results indicate that, besides the inherent surface effect of NPs, the surface charges are essential for the enhancement of response current of enzyme electrode. More GOD molecules can be adsorbed on the surface of SiO₂ NPs, because GOD molecules hold surface charge whose property was opposite to that of SiO₂ NPs, but the same as that of Au NPs. When Au NPs and SiO₂ NPs are mixed with the ratio of 1:1 in mol, the combined particles can carry out both functions of the two kinds of NPs, and enhance response performance of GOD-electrode greatly.     

Modeling surface complexation at the colloid/electrolyte interface

A computer program was developed to model the electrical double and triple layer on amphoteric oxide surfaces. It was tested for the adsorption of Co²⁺ ions on colloidal spherical hematite (α-Fe₂O₃) particles in a NaNO₃ suspension. The procedure is general and with a few modifications the program can be applied to other chemical equilibrium problems.     

Computational simulation of gas flow and heat transfer near an immersed object in fluidized beds

To improve the understanding of the heat transfer mechanism and to find a reliable and simple heat-transfer model, the gas flow and heat transfer between fluidized beds and the surfaces of an immersed object is numerically simulated based on a double particle-layer and porous medium model. The velocity field and temperature distribution of the gas and particles are analysed during the heat transfer process. The simulation shows that the change of gas velocity with the distance from immersed surface is consistent with the variation of bed voidage, and is used to validate approximately dimensional analysing result that the gas velocity between immersed surface and particles is 4.6U_mf/ε_mf. The effects of particle size and particle residence time on the thermal penetration depth and the heat-transfer coefficients are also discussed.