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 micro sloping structures of SU-8 by substrate penetration lithography

In this paper, three-dimensional (3D) micro sloping structures were fabricated by ordinary mask pattern and diffraction phenomenon. Especially, we fabricated the structures with SU-8 negative photoresist and substrate penetration lithography. In this method, exposure is performed arranging in order of a mask, a substrate and the SU-8 resist. There is a gap that is equal to the thickness of the substrate between resist and mask. In narrow slit of mask, resist is less exposed than usual because of Fraunhofer diffraction. The amount of exposure depends on slit width so that the height of SU-8 resist can be controlled. A 173 μm height of structure was obtained in the case of 27 μm width slit and 24.2 μm height of structure was obtained in the case of 7.4 μm width slit. By using this method, high aspect ratio 3D SU-8 structures with smooth sloping were fabricated in the length of 100–300 μm and in the height of 50–200 μm with rectangular triangle mask pattern. In the same way, there is influence of Fresnel diffraction on edge of aperture so that micro taper structures were fabricated. A lot of taper structures were fabricated by the method to make the surface repellency. The contact angle was achieved more than 160° in this study.     

Low temperature carbon nanotube and hexagonal diamond deposition with photo-enhanced chemical vapor deposition

Deposition of carbon nanotube and hexagonal diamond thin films at low substrate temperature with photo-enhanced chemical vapor deposition is described here. Extensive experimentation is conducted to optimize the catalyst layer utilized for deposition by varying Al/Ni/Al metal layer thicknesses on SiO₂ coated Si substrates. The coated substrates are annealed to transform the thin metal layers into nanoparticles. Suitable catalyst layer thicknesses obtained are 3/2/3, 5/1/5 and 5/3/5 nm for Al/Ni/Al sandwich metal layers. Suitable annealing conditions are in the range of 350–450 °C for substrate temperature and from 0.22 to 10 Torr for chamber pressure in ammonia ambient for 25 min. Carbon tetrachloride (CCl₄) is used as a carbon precursor in this work. Argon to CCl₄ flow ratio is varied in 1.5–19 range, chamber pressure is varied in 3–10 Torr range, and the substrate temperature is varied in 350–450 °C range. Carbon nanotubes (CNT) growth is observed at lower chamber pressure, lower partial pressure of CCl₄, lower substrate temperature and for thin Ni catalyst layer. The optimal CNT deposition condition observed is 5 Torr total chamber pressure, 9:1 partial pressure ratio of Ar to CCl₄, 400 °C substrate temperature and 5/1/5 nm thick Al/Ni/Al catalyst layers. The hexagonal diamond deposition is observed at a higher chamber pressure, higher partial pressure of CCl₄, higher substrate temperature and for a thicker Ni catalyst layer. The optimal condition for hexagonal diamond deposition observed is 10 Torr total chamber pressure, 7:3 partial pressure ratio of Ar to CCl₄, 450 °C substrate temperature and 5/3/5 nm thick Al/Ni/Al catalyst sandwich layers.     

Development of a nanofluidic preconcentrator with precise sample positioning and multi-channel preconcentration

A nanofluidic preconcentrator with the capability of rapidly preconcentrating and precisely positioning protein bands in multiple microchannels has been developed for highly sensitive detection of biomolecules. A novel electrical resistive network model is developed to guide the design of the nanofluidic preconcentrator which consists of a PDMS slab bonded with a glass slide. In the prototype design, two microchannels (23 mm long, 25–50 μm wide, and 5–15 μm deep), one preconcentration microchannel and one ground microchannel are connected in the middle via 16 nanochannels (25–50 μm long, 25 μm wide, and 50–80 nm deep). With two sets of optimal voltage settings applied on the opposite ends of the nanofluidic chip, the ion depletion region and electrokinetic trapping were generated to carry out the preconcentration. With the optimal voltage settings (30–30 V) predicted by the model, the ionic current of the nanochannel in our optimized preconcentrator was adjusted to be greater than the threshold value (3.9 nA) needed for the occurrence of the preconcentration, and a preconcentration factor >10⁵ was achieved in 5 min. The sample positioning capability of the preconcentrator was demonstrated by adjusting the applied voltages and moving the preconcentrated protein bands to multiple sites by a distance from several micrometers to several millimeters in the preconcentration channel. The multi-channel preconcentration capability was also demonstrated by preconcentrating two protein bands in two separate microchannels. In this work, the resistive network model was developed and validated to optimize nanofluidic preconcentrators for rapid, high throughput and highly sensitive sensing of low abundance analytes.     

Micro liquid rotor operated by surface-acoustic-wave

A micro liquid rotor operated by surface acoustic waves (SAWs) was proposed and fabricated. The liquid rotor has two interdigital transducers (IDTs) that generate SAWs, in order to increase efficiency of rotating the liquid. The IDTs were fabricated by patterning Al/Cr on a LiNbO₃ substrate. Moreover, the liquid rotor has a cylindrical liquid pool of 4 mm diameter and 200 μm depth. A sidewall of the liquid pool was made of epoxy-based negative photoresist on the substrate. As characteristics of the liquid rotor, we investigated relationships between electric power applied to the IDTs and angular velocity of the liquid or its temperature. Through experimentation, it was shown that the angular velocity and the temperature increase with increasing the electric power. When the fabricated liquid rotor was applied electric power of 2 W, the liquid of 5 μL rotated at 330 rad/s (3,100 rpm) and its temperature reached 65°C.     

A cell electrofusion microfluidic chip with micro-cavity microelectrode array

A new cell electrofusion microfluidic chip with 19,000 pairs of micro-cavity structures patterned on vertical sidewalls of a serpentine-shaped microchannel has been designed and fabricated. In each micro-cavity structure, the two sidewalls perpendicular to the microchannel are made of SiO₂ insulator, and that parallel to the microchannel is made of silicon as the microelectrode. One purpose of the design with micro-cavity microelectrode array is to obtain high membrane voltage occurring at the contact point of two paired cells, where cell fusion takes place. The device was tested to electrofuse NIH3T3 and myoblast cells under a relatively low voltage (~9 V). Under an AC electric field applied between the pair of microelectrodes positioned in the opposite micro-cavities, about 85–90 % micro-cavities captured cells, and about 60 % micro-cavities are effectively capable of trapping the desired two-cell pairs. DC electric pulses of low voltage (~9 V) were subsequently applied between the micro-cavity microelectrode arrays to induce electrofusion. Due to the concentration of the local electric field near the micro-cavity structure, fusion efficiency reaches about 50 % of total cells loaded into the device. Multi-cell electrofusion and membrane rupture at the end of cell chains are eliminated through the present novel design.     

PMMA to Polystyrene bonding for polymer based microfluidic systems

A thermal bonding technique for Poly (methylmethacrylate) (PMMA) to Polystyrene (PS) is presented in this paper. The PMMA to PS bonding was achieved using a thermocompression method, and the bonding strength was carefully characterized. The bonding temperature ranged from 110 to 125 °C with a varying compression force, from 700 to 1,000 N (0.36–0.51 MPa). After the bonding process, two kinds of adhesion quantification methods were used to measure the bonding strength: the double cantilever beam method and the tensile stress method. The results show that the bonding strength increases with a rising bonding temperature and bonding force. The results also indicate that the bonding strength is independent of bonding time. A deep-UV surface treatment method was also provided in this paper to lower the bonding temperature and compression force. Finally, a PMMA to PS bonded microfluidic device was fabricated successfully.     

Rapid polymer microchannel fabrication by hot roller embossing process

Using thermoplastic polymers as substrate material is an attractive approach to develop low-cost, disposable microfluidic devices. This study investigates a simple and rapid polymer replication method of fabricating microchannels by a hot roller embossing process. The hot roller embosser used in this study was modified from a commercially available film laminator, and the roller micromold was fabricated by spin coating an SU-8 layer on a flexible copper sheet. A straight microchannel measuring 5?cm long, 200?μm wide, and 41.4?μm deep was used to evaluate the imprinting performance on cyclic olefin copolymer and polyvinylchloride film. This study also investigates the effects of hot roller embossing temperature, rolling speed, and embossing pressure on the microchannel depth and geometry transfer efficiency.     

A facile strategy to integrate robust porous aluminum foil into microfluidic chip for sorting particles

High efficiency integration of functional microdevices into microchips is crucial for broad microfluidic applications. Here, a device-insertion and pressure sealing method was proposed to integrate robust porous aluminum foil into a microchannel for microchip functionalization which demonstrate the advantage of high efficient foil microfabrication and facile integration into the microfluidic chip. The porous aluminum foil with large area (10 × 10 mm²) was realized by one-step femtosecond laser perforating technique within few minutes and its pores size could be precisely controlled from 3 μm to millimeter scale by adjusting the laser pulse energy and pulse number. To verify the versatility and flexibility of this method, two kinds of different microchips were designed and fabricated. The vertical-sieve 3D microfluidic chip can separate silicon dioxide (SiO₂) microspheres of two different sizes (20 and 5 μm), whereas the complex stacking multilayered structures (sandwich-like) microfluidic chip can be used to sort three different kinds of SiO₂ particles (20, 10 and 5 μm) with ultrahigh separation efficiency of more than 92%. Furthermore, these robust filters can be reused via cleaning by backflow (mild clogging) or disassembling (heavy clogging).     

Dielectrophoretically assembled particles: feasibility for optofluidic systems

This work presents the dielectrophoretic manipulation of sub-micron particles suspended in water and the investigation of their optical responses using a microfluidic system. The particles are made of silica and have different diameters of 600, 450, and 250 nm. Experiments show a very interesting feature of the curved microelectrodes, in which the particles are pushed toward or away from the microchannel centerline depending on their levitation heights, which is further analyzed by numerical simulations. In doing so, applying an AC signal of 12 V_p–p and 5 MHz across the microelectrodes along with a flow rate of 1 μl/min within the microchannel leads to the formation of a tunable band of particles along the centerline. Experiments show that the 250 nm particles guide the longitudinal light along the microchannel due to their small scattering. This arrangement is employed to study the feasibility of developing an optofluidic system, which can be potentially used for the formation of particles-core/liquid-cladding optical waveguides.     

A novel technique for fabrication of micro- and nanofluidic device with embedded single carbon nanotube

This paper describes a novel technique for fabrication of micro- and nanofluidic device that consists of a carbon nanotube (CNT) and a polydimethylsiloxane (PDMS) microchannel. Single CNT was placed at desired locations using dielectrophoresis (DEP) and PDMS microchannel was constructed on the aligned CNT via photolithography and soft lithography techniques. This technique enables a CNT to be seamlessly embedded in a PDMS microchannel. Moreover, controlling the PDMS curing condition enables the construction of the device with or without a CNT (the device without CNT has a trace nanochannel in PDMS). Preliminary flow tests such as capillary effect and pressure-driven flow were performed with the fabricated devices. In the capillary effect tests, the flow stopped at the nanochannel in both devices. In the pressure-driven flow lower flow resistance was observed in the device with a CNT.     

Nano-microscale moulding of some metal plates with high strength Ni–W alloy moulds

High strength nanocrystalline and amorphous Ni-14–24 at. % W alloys with their tensile strengths of about 3 GPa have been prepared by electrodeposition. Nano-microscale Ni–W alloy mould inserts, consisting of line and space structures with the line-widths of 700 and 300 nm and the height of 200 nm, were fabricated. High compression stress moulding of some metal plates of pure-Al, SUS-316L stainless steel and Zr₆₉Cu₁₆Ni₅Al₁₀ bulk metallic glass with the Ni–W alloy inserts was carried out at room temperature. In the case of the pure-Al under moulding stress of about 350 MPa, full moulding was achieved with the depths of about 200 nm approximately equal to the height of the inserts. Repeat moulding of 200 cycles did not cause any noticeable change or degradation of the Ni–W alloy inserts. In the case of the SUS-316L stainless steel under the moulding stress of about 1 GPa, however, the nano-microscale moulding was not achieved. This may be due to the high strain hardening ability of the SUS-316L stainless steel during plastic deformation. In the case of the Zr₆₉Cu₁₆Ni₅Al₁₀ bulk metallic glass with a high yielding stress of about 1.5 GPa and no strain hardening ability, full moulding was almost achieved successfully under the high moulding stress of about 2 GPa.     

Quantitative characterization of the focusing process and dynamic behavior of differently sized microparticles in a spiral microchannel

In this paper, a spiral microchannel was fabricated to systematically investigate particle dynamics. The focusing process or migration behavior of different-sized particles in the outlet region was presented. Specifically, for focused microparticles, quantitative characterization and analysis of how particles migrate towards the equilibrium positions with the increase in flow rate (De = 0.31–3.36) were performed. For unfocused microparticles, the particle migration behavior and the particle-free region’s formation process were characterized over a wide range of flow rates (De = 0.31–4.58), and the emergence of double particle-free regions was observed at De ≥ 3.36. These results provide insights into the design and operation of high-throughput particle/cell filtration and separation. Furthermore, using the location markers pre-fabricated along with the microchannel structures, the focusing or migration dynamics of different-sized particles along the spiral microchannel was systematically explored. The particle migration length effects on focusing degree and particle-free region width were analyzed. These analyses may be valuable for the optimization of microchannel structures. In addition, this device was successfully used to efficiently filter rare particles from a large-volume sample and separate particles of two different sizes according to their focusing states.     

Numerical simulation of the capillary flow in the meander microchannel

Pumping in microfluidic devices is an important issue in actuating fluid flow in microchannel, especially that capillary force has received more and more attractions due to the self-driven motion without external power input. However, less 2D simulation was done on the capillary flow in microchannel especially the meander microchannel which can be used for mixing and lab-on-a-chip (LOC) application. In this paper, the numerical simulation of the capillary flow in the meander microchannel has been studied using computer fluid dynamic simulation software CFD-ACE⁺. Different combinations of channel width in the X-direction denoted as W_x and Y-direction denoted as W_y were designed for simulating capillary flow behavior and pressure drop. The designed four types of meander microchannels (W_x × W_y) were 100 × 100 μm, 100 × 200 μm, 50 × 200 μm, and 50 × 400 μm. In this simulation results, it is found that the capillary pumping speed is highly depending on the channel width. The large speed change occurs at the turning angle of channel width change from W_x to W_y. The fastest pumping effect is found in the meander channel of 100 × 100 μm, which has an average pumping speed of 0.439 mm/s. The slowest average flow speed of 0.205 mm/s occurs in the meander channel of 50 × 400 μm. Changing the meander channel width may vary the capillary flow behavior including the pumping speed and the flow resistance as well as pressure drop which will be a good reference in designing the meander microchannels for microfluidic and LOC application.     

Simple and cost-effective fabrication of two-dimensional plastic nanochannels from silica nanowire templates

Nanofluidic systems are attracting a great deal of interest due to their fundamental significance and potential applications in chemistry, biology and physics. However, high fabrication cost, expensive equipments and complicated fabrication process of most current fabrication techniques prevent lots of researchers from entering the nanofluidic field. Here we present a quick, simple and cost-effective method for fabricating two-dimensional (2D) nanochannel in polycarbonate (PC) substrates. Silica nanowires, taper-drawn from commercially available single-mode fiber were used as templates and embedded in the PC substrate by hot embossing. The nanochannels were created after removing the nanowires by hydrofluoric acid (HF) etching. Poly(dimethylsiloxane) (PDMS) was used to seal the nanochannel reversibly. Nanochannels with widths range from 100 to 900 nm and lengths up to several millimeters were obtained. Various nanostructures including integrated micro and nanochannels, nanochannel array, bent nanochannel and cross-shaped nanochannel were fabricated and characterized by fluorescent microscope, scanning electron microscope (SEM) and atomic force microscope (AFM).     

Low power highly sensitive platform for gas sensing application

This paper reports a low power miniaturized MEMS based integrated gas sensor with 36.84 % sensitivity (ΔR/R₀) for as low as 4 ppm (NH₃) gas concentration. Micro-heater based gas sensor device presented here consumes very low power (360 °C at 98 mW/mm²) with platinum (Pt) micro-heater. Low powered micro-heater is an essential component of the metal oxide based gas sensors which are portable and battery operated. These micro-heaters usually cover less than 5 % of the gas sensor chip area but they need to be thermally isolated from substrate, to reduce thermal losses. This paper elaborates on design aspects of micro fabricated low power gas sensor which includes ‘membrane design’ below the microheater; the ‘cavity-to-active area ratio’; effect of silicon thickness below the silicon dioxide membrane; etc. using FEM simulations and experimentation. The key issues pertaining to process modules like fragile wafer handling after bulk micro-machining; lift-off of platinum and sensing films for the realization of heater, inter-digitated-electrodes (IDE) and sensing film are dealt with in detail. Low power platinum microheater achieving 700 °C at 267 mW/mm² are fabricated. Temperature calculations are based on experimentally calculated thermal coefficient of resistance (TCR) and IR imaging. Temperature uniformity and localized heating is verified with infrared imaging. Reliability tests of the heater device show their ruggedness and repeatability. Stable heater temperature with standard deviation (σ) of 0.015 obtained during continuous powering for an hour. Cyclic ON–OFF test on the device indicate the ruggedness of the micro-heater. High sensitivity of the device for was observed for ammonia (NH₃), resulting in 40 % response for ~4 ppm gas concentration at 230 °C operating temperature.     

Scalable attoliter monodisperse droplet formation using multiphase nano-microfluidics

We demonstrate a robust method to produce monodisperse femtoliter to attoliter droplets by using a nano-microfluidic device. Two immiscible liquids are forced through a nanochannel where a steady nanoscopic liquid filament forms, thinning close to the nanochannel exit to a microchannel due to the capillary focusing. When the nanoscopic filament enters the microchannel, monodisperse droplets are formed by capillary instability. In a certain range of physical parameters and geometrical configurations, the droplet size is only determined by the nanochannel height and independent of liquid flow rates and ratios, surfactants, and continuous phase viscosity. By using nanochannels with a height of 100–900 nm, 0.4–3.5 μm diameter droplets (volume down to 30 aL) have been produced. The generated droplets are stable for at least weeks.     

Laser polishing and 2PP structuring of inside microfluidic channels in fused silica

This study presents the development of post-processing steps for microfluidics fabricated with selective laser etching (SLE) in fused silica. In a first step, the SLE surface—even inner walls of microfluidic channels—can be smoothed by laser polishing. In addition, two-photon polymerization (2PP) can be used to manufacture polymer microstructures and microcomponents inside the microfluidic channels. The reduction in the surface roughness by laser polishing is a remelting process. While heating the glass surface above softening temperature, laser radiation relocates material thanks to the surface tension. With laser polishing, the RMS roughness of SLE surfaces can be reduced from 12 µm down to 3 nm for spatial wavelength λ < 400 µm. Thanks to the laser polishing, fluidic processes as well as particles in microchannels can be observed with microscopy. A manufactured microfluidic demonstrates that SLE and laser polishing can be combined successfully. By developing two-photon polymerization (2PP) processing in microchannels we aim to enable new applications with sophisticated 3D structures inside the microchannel. With 2PP, lenses with a diameter of 50 µm are processed with a form accuracy rms of 70 nm. In addition, this study demonstrates that 3D structures can be fabricated inside the microchannels manufactured with SLE. Thanks to the combination of SLE, laser polishing and 2PP, research is pioneering new applications for microfluidics made of fused silica.     

Disposable microfluidic blood cuvette for measuring hemoglobin concentration

This paper presents a new air-bubble free microfluidic blood cuvette for the measurement of hemoglobin concentration. The microfluidic blood cuvette was filled with blood samples by capillary force, and hemoglobin levels in the blood were determined by measuring absorbance at the wavelength of 530 nm. Two different microfluidic blood cuvettes with dual and single sidewall microchannels were investigated. The microfluidic blood cuvette was fabricated using a polymethyl methacrylate substrate and a dry film photoresist. During the blood-filling process, air was trapped in the dual-sided wall-type cuvettes, while no air trapping occurred in the single sidewall-type cuvettes. The sensitivity of the hemoglobin measurements was more linear in a 105 μm deep microchannel than in a 35 μm deep microchannel.     

A microfluidic device for thermal particle detection

We demonstrate the use of heat to count microscopic particles. A thermal particle detector (TPD) was fabricated by combining a 500-nm-thick silicon nitride membrane containing a thin-film resistive temperature detector with a silicone elastomer microchannel. Particles with diameters of 90 and 200 μm created relative temperature changes of 0.11 and ?0.44 K, respectively, as they flowed by the sensor. A first-order lumped thermal model was developed to predict the temperature changes. Multiple particles were counted in series to demonstrate the utility of the TPD as a particle counter.