Research on a large power thermal bubble micro-ejector with induction heating

The present study investigates a large power thermal bubble micro-ejector with induction heating device. The traditional thermal-bubble ejectors adopted resistors as the heating resources, it can only work with lower power and convey liquid with lower flow rate. Induction heating devices are adopted to replace the resistor for heating liquid in this paper. With this heating method, there is no physical contact between the heating core and the external power supply circuit. The liquid in the chamber of micro-ejector is heated by the induction heating device and changes from liquid phase to gas phase, generating vapor bubbles in the micro chamber of the micro ejector. The bubble expands rapidly and ejects droplets through the nozzle. The prototype of the micro-ejector is fabricated and experiments are carried out. Continuous droplets are ejected out from the nozzle as the applied AC current is 0.6–0.65 A with the power frequency of 100 kHz. The total volume of the continuous droplets is ranging from 18.84 to 49.87 nL, and the corresponding flow rate is about 0.52–1.36 μL/min. Furthermore, this new micro-ejector can be adopted in conveying of micro-scale liquid, the injection of trace drugs and the 3D printing.     

Microchannel emulsification for mass production of uniform fine droplets: integration of microchannel arrays on a chip

We present a novel microchannel emulsification (MCE) system for mass-producing uniform fine droplets. A 60 × 60-mm MCE chip made of single-crystal silicon has 14 microchannel (MC) arrays and 1.2 × 10⁴ MCs, and each MC array consists of many parallel MCs and a terrace. A holder with two inlet through-holes and one outlet through-hole was also developed for simply infusing each liquid and collecting emulsion products. The MCE chip was sealed well by physically attaching it to a flat glass plate in the holder during emulsification. Uniform fine droplets of soybean oil with an average diameter of 10 μm were reliably generated from all the MC arrays. The size of the resultant fine droplets was almost independent of the dispersed-phase flow rate below a critical value. The continuous-phase flow rate was unimportant for both the droplet generation and the droplet size. The MCE chip enabled mass-producing uniform fine droplets at 1.5 ml h⁻¹ and 1.9 × 10⁹ h⁻¹, which could be further increased using a dispersed phase of low viscosity.     

Design and fabrication of a flexible three-axial tactile sensor array based on polyimide micromachining

This paper describes the design and fabrication of a flexible three-axial tactile sensor array using advanced polyimide micromachining technologies. The tactile sensor array is comprised of sixteen micro force sensors and it measures 13 mm × 18 mm. Each micro force sensor has a square membrane and four strain gauges, and its force capacity is 0.6 N in the three-axial directions. The optimal positions of the strain gauges are determined by the strain distribution obtained form finite element analysis (FEA). The normal and shear forces are detected by combining responses from four thin-film metal strain gauges embedded in a polyimide membrane. In order to acquire force signals from individual micro force sensors, we fabricated a PCB based on a multiplexer, operational amplifier and microprocessor with CAN network function. The sensor array is tested from the evaluation system with a three-component load cell. The developed sensor array can be applied in robots’ fingertips, as well as to other electronic applications with three-axial force measurement and flexibility keyword requirements.     

Using a cross-flow microfluidic chip for monodisperse UV-photopolymerized microparticles

In this paper, we report a microfluidic chip containing a cross-junction channel for the manipulation of UV-photopolymerized microparticles. Hydrodynamic-focusing is used to form a series of using 365 nm UV light to solidify the hydrogel droplets. We were able to control the size of the hydrogel droplets from 75 to 300 μm in diameter by altering the sample and by changing the flow rate ratio of the mineral oil in the center inlet channel to that of the side inlet channels. We found that the size of the emulsions increases with an increase in average velocity of the dispersed phase flow (polymer solution flow). The size of the emulsions decreases with an average velocity increase of the continuous phase flow (mineral oil flow). Experimental data show that the emulsions are very uniform. The developed microfluidic chip has the advantages of ease of fabrication, low cost, and high throughput. The emulsions generated are very uniform and have good regularity.     

Single-step replicable microfluidic check valve for rectifying and sensing low Reynolds number flow

This paper presents a new microfluidic check valve well suited for low Reynolds number flow rate sensing, micropump flow rectification, and flow control in lab-on-a-chip devices. The valve uses coupling between fluid movement in a channel and an elastomeric column (flap) suspended in the fluid path to generate a strong anisotropic flow resistance. Soft lithography-based molding techniques were used to fabricate the valve, allowing for a low-cost, single-step fabrication process. Three valves—having heights of 25, 50, and 75 μm, respectively—were fabricated and experimentally evaluated; the best of them demonstrated a maximum fluidic diodicity of 4.6 at a Reynolds number of 12.6 and a significant diodicity of 1.6 at the low Reynolds number of 0.7. The valve’s notable low Reynolds number response was realized by adopting a design methodology that balances the stiffness of the elastomer flap and adhesion forces between the flap and its seat. A pair of elastomer check valves integrated with a miniature membrane actuator demonstrated a flow rectification efficiency of 29.8%. The valve’s other notable features include a wide bandwidth response, the ability to admit particles without becoming jammed, and flow rate sensing capability based on optical flap displacement measurements.     

Micro-droplet formation in non-Newtonian fluid in a microchannel

Micro-droplet formation from an aperture with a diameter of micrometers is numerically investigated under the cross-flow conditions of an experimental microchannel emulsification process. The process involves dispersing an oil phase into continuous phase fluid through a microchannel wall made of apertured substrate. Cross-flow in the microchannel is of non-Newtonian nature, which is included in the simulations. Micro-droplets of diameter 0.76–30 μm are obtained from the simulations for the apertures of diameter 0.1–10.0 μm. The simulation results show that rheology of the bulk liquid flow greatly affects the formation and size of droplets and that dispersed micro-droplets are formed by two different breakup mechanisms: in dripping regime and in jetting regime characterized by capillary number Ca. Relations between droplet size, aperture opening size, interfacial tension, bulk flow rheology, and disperse phase flow rate are discussed based on the simulation and the experimental results. Data and models from literature on membrane emulsification and T-junction droplet formation processes are discussed and compared with the present results. Detailed force balance models are discussed. Scaling factor for predicting droplet size is suggested.     

AC-pulse modulated electrohydrodynamic jet printing and electroless copper deposition for conductive microscale patterning on flexible insulating substrates

This paper presents a novel micro-manufacturing method for fabrication of electrical features and patterns on highly insulating substrates and flexible substrates based on high-resolution AC-pulse modulated electrohydrodynamic jet (e-jet) printing of silver nanoink as seed layer followed by electroless copper deposition. Traditional ink jet printing method is limited in printing resolution which is determined by dimension of printing nozzle and dimension of droplets. Traditional e-jet printing has the disadvantage of residual charge problem especially for highly insulating substrates which cannot dredge remained charge of printed droplets, resulting in distorted electrostatic field and low printing controllability. Meanwhile, for printing of liquid phase ink, feature resolution contradicts with the required thickness, which is a key factor of conductivity of printed patterns. In this paper, a novel AC-modulated e-jet printing technique is applied to neutralize charges on substrates by switching polarity of consequent droplets for direct printing of high-resolution conductive silver patterns on insulating substrates. Electroless copper deposition is introduced in the fabrication process to solve the thickness problem of the resulting features. Variables of fabrication process, including amplitude and frequency of AC-pulsed voltage, plotting speed, curing temperature, number of layers, concentration of solution for copper growth, were identified to achieve reliable and conductive printed patterns. Sub-20 µm silver tracks with resistivity about 3.16 times of bulk silver were successfully fabricated. We demonstrated that ac-pulse modulated e-jet printing followed by electroless copper deposition can produce high resolution conductive patterns with improved thickness on insulating substrates and flexible substrates, which can be applied to direct printing and micro scale patterning for flexible electronics and wearable devices applications.     

Design, fabrication and testing of a high performance silicon piezoresistive Z-axis accelerometer with proof mass-edge-aligned-flexures

This paper presents design, fabrication and testing of a quad beam silicon piezoresistive Z-axis accelerometer with very low cross-axis sensitivity. The accelerometer device proposed in the present work consists of a thick proof mass supported by four thin beams (also called as flexures) that are connected to an outer supporting rim. Cross-axis sensitivity in piezoresistive accelerometers is an important issue particularly for high performance applications. In the present study, low cross-axis sensitivity is achieved by improving the device stability by placing the four flexures in line with the proof mass edges. Various modules of a finite element method based software called CoventorWare^™ was used for design optimization. Based on the simulation results, a flexure thickness of 30 μm and a diffused resistor doping concentration of 5 × 10¹⁸ atoms/cm³ were fixed to achieve a high prime-axis sensitivity of 122 μV/Vg, low cross-axis sensitivity of 27 ppm and a relatively higher bandwidth of 2.89 kHz. The designed accelerometer was realized by a complementary metal oxide semiconductor compatible bulk micromachining process using a dual doped tetra methyl ammonium hydroxide etching solution. The fabricated accelerometer devices were tested up to 13 g static acceleration using a rate table. Test results of fabricated devices with 30 μm flexure thickness show an average prime axis sensitivity of 111 μV/Vg with very low cross-axis sensitivities of 0.652 and 0.688 μV/Vg along X-axis and Y-axis, respectively.     

Development and characterisation of integrated microfluidics on waveguide-based photonic platforms fabricated from hybrid materials

This article reports on a detailed investigation of sol–gel processed hybrid organic–inorganic materials for use in lab-on-a-chip (LoC) applications. A particular focus on this research was the implementation of integrated microfluidic circuitry in waveguide-based photonic sensing platforms. This objective is not possible using other fabrication technologies that are typically used for microfluidic platforms. Significant results on the surface characterisation of hybrid sol–gel processed materials have been obtained which highlight the ability to tune the hydrophilicity of the materials by careful adjustment of material constituents and processing conditions. A proof-of-principle microfluidic platform was designed and a fabrication process was established which addressed requirements for refractive index tuning (essential for waveguiding), bonding of a transparent cover layer to the device, optimized sol–gel deposition process, and a photolithography process to form the microchannels. Characterisation of fluid flow in the resulting microchannels revealed volumetric flow rates between 0.012 and 0.018 μl/min which is characteristic of capillary-driven fluid flow. As proof of the integration of optical and microfluidic functionality, a microchannel was fabricated crossing an optical waveguide which demonstrated that the presence of optical waveguides does not significantly disrupt capillary-driven fluid flow. These results represent the first comprehensive evaluation of photocurable hybrid sol–gel materials for use in waveguide-based photonic platforms for lab-on-a-chip applications.     

Direct pattern formation of bacterial cells using micro-droplets generated by electrohydrodynamic forces

Bacterial cells were patterned efficiently on a nutrient-coated membrane in a direct, rapid, and cost-effective manner using electro-hydrodynamically generated micro-droplets. By varying the viscosity and electrical conductivity of the bacterial printing solution, uniform droplets were obtained. Escherichia coli cells were directly printed onto a membrane filter, followed by overnight incubation on an agar plate which created line patterns of bacterial colonies with a width of approximately 160 μm. Optimization of the concentration of the chemical components [i.e. the ethylene glycol (EG), phosphate buffer (PB), and sodium chloride (NaCl)] in the processing bacterial solution allowed successful growth and patterning of the cells. The optimal conditions to achieve the most effective cell growth and patterning on these printed surfaces were an EG concentration of 40 vol% and a concentration as low as 10 mM of PB or NaCl in the printing solution. Cells passing through the electric nozzle during the printing process remained viable.     

Development of a MEMS microvalve array for fluid flow control

A microelectromechanical system (MEMS) microvalve array for fluid flow control is described. The device consists of a parallel array of surface-micromachined binary microvalves working cooperatively to achieve precision how control on a macroscopic level. Flow rate across the microvalve array is proportional to the number of microvalves open, yielding a scalable high-precision fluidic control system. Device design and fabrication, using a one-level polycrystalline silicon surface-micromachining process combined with a single anisotropic bulk etching process are detailed. Performance measurements on fabricated devices confirm feasibility of the fluidic control concept and robustness of the electromechanical design. Air-flow rates of 150 ml/min for a pressure differential of 10 kPa were demonstrated. Linear flow control was achieved over a wide range of operating flow rates. A continuum fluidic model based on incompressible low Reynolds number flow theory was implemented using a finite-difference approximation. The model accurately predicted the effect of microvalve diaphragm compliance on flow rate. Excellent agreement between theoretical predictions and experimental data was obtained over the entire range of flow conditions tested experimentally     

A polymeric high-throughput pressure-driven micromixer using a nanoporous membrane

This article presents a polymeric high-throughput, pressure-driven nanofluidic mixer utilizing a nanoporous charge-selective Nafion membrane. The device has no movable parts and is fabricated in PMMA by means of laser machining and thermal bonding. Mixing is achieved by strong vortices occurring above the nanoporous membrane when applying an electric field. These vortices are caused by electroconvection in the concentration polarization zone. The mixer is operating at Peclet number as high as 63.5 × 10³ allowing rapid mixing at a high throughput. The design and fabrication of the mixer is simple, reproducible, and of low cost. The fabrication approach presented in this article can be easily transferred to roll-to-roll technology for mass fabrication.     

Design and fabrication of a micro Alvarez lens array with a variable focal length

A 5 × 5 micro Alvarez lens array mold was fabricated using a 5-axis ultraprecision diamond machine and an Alvarez lens array was manufactured by injection molding process. Unlike conventional processes for asymmetrical element fabrication such as small tool grinding, this research demonstrates slow tool servo broaching process that allows the entire Alvarez lens array to be accurately machined on a metal mold in a single operation. To further reduce manufacturing cost, injection molding was used to fabricate the Alvarez lens arrays. The mold and molded lenses were both measured using an optical profiler. All measured profiles showed a good agreement with design and surface roughness also indicated an optical surface finish. The functionality of the molded polymeric lens arrays was achieved when the focal lengths were varied by laterally translating the molded Alvarez lens array pair. This research is a demonstration of the capability of fabricating complex optics using the same approach.     

Chitosan microfiber fabrication using a microfluidic chip and its application to cell cultures

In this study, a poly-methyl-methacrylate (PMMA) microfluidic chip with a 45° cross-junction microchannel is fabricated using a CO₂ laser machine to generate chitosan microfibers. Chitosan solution and sodium tripolyphosphate (STPP) solution were injected into the cross-junction microchannel of the microfluidic chip. The laminar flow of the chitosan solution was generated by hydrodynamic focusing. The diameter of laminar flow, which ranged from 30 to 50 μm, was controlled by changing the ratio between chitosan solution and STPP solution flow rates in the PMMA microfluidic chip. The laminar flow of the chitosan solution was converted into chitosan microfibers with STPP solution via the cross-linking reaction; the diameter of chitosan microfibers was in the range of 50–200 μm. The chitosan microfibers were then coated with collagen for cell cultivation. The results show that the chitosan microfibers provide good growth conditions for cells. They could be used as a scaffold for cell cultures in tissue engineering applications. This novel method has advantages of ease of fabrication, simple and low-cost process.     

A roller embossing process for rapid fabrication of microlens arrays on glass substrates

This paper reports an innovative technique for rapid fabrication of polymeric microlens arrays based on UV roller embossing process. In this method, a thin flat mold is fabricated by electroforming of nickel against a microlens master. The thin Ni mold with microlens cavities is then wrapped onto cylinder to form the roller. During rolling operation, the roller pressing and dragging the UV-curable photopolymer layer on the glass substrate through the rolling zone, the microlens array is formed. At the same time, the microlens array is cured by the UV light radiation while traveling through the rolling zone. The technique can be developed to an effective roll-to-roll process at room temperature and with low pressure. In this study, a roller embossing facility with UV exposure capacity has been designed, constructed and tested. Under the proper processing conditions, the 100×100 arrays of polymeric microlens, with a diameter of 100 μm, a pitch of 200 μm and a sag height of 21 μm can be successfully fabricated.     

A study of PZT valveless micropump with asymmetric obstacles

The results of a study on a new type of PZT valveless micropump with asymmetric obstacles are presented in this paper. The valveless micropump was made through a MEMS fabrication process by using simply only one photo mask. Asymmetric obstacles are used for the flow directing device instead of the diffuser/nozzle elements used in previous studies. In this study, numerical simulations were also carried out to evaluate the design and the performance of the new micropump. The main feature of the present micropump is that it has a uniform cross-section area across the micro-channel, which gives many advantages. The differential pressure head and the pumping flow rate can be adjusted easily by using obstacles of different shapes and changing the PZT operating frequency without changing the dimensions of the micro-channel. In this experiment, the performance of the micropump was evaluated by measuring the pressure head difference and the flow rate as the input voltage ranged from 20 to 40 V, a range much lower than those in previous studies. The pumping pressure can reach a maximum of 1.2 kPa, and the maximum net volume flow rate is 156 μl/min. These test data indicate that this micropump fulfills the demands for most micro-fluidic systems. Moreover, the present device can be easily applied to complex systems with combinations of several pumps and microchannels in the future.     

Low-voltage electroosmotic pumping using porous anodic alumina membranes

This study demonstrated electroosmotic pumping with high flow rate per unit area at a rather low applied voltage by using alumina nano-porous membrane. The platinum mesh electrode is perpendicular to, and has direct contact with the nano-channel inlet for proving uniform electric field and for reducing the electric voltage drop in the reservoir. The measured flow rate versus electrolyte (KCl) concentration reveals two distinct characteristics. First, the flow rate is usually high at low concentrations (10⁻⁵ to 10⁻⁷ M) in which a maximum value occurs. Second, a remarkable drop of flow rate is seen when the concentration surpasses 10⁻⁴ M. The maximum flow rate achieved from this study is 0.09 mL/min V cm² and the energy transfer efficiency is 0.43% at an operation voltage of 20 V. The mesh electrodes with 33 wire spacing are capable of providing an uniform electric field, the nano-porous membrane with a low electrolyte concentration provides the environment for strong overlapping of electric double layer, in association with the thin alumina membrane, leading to a high flow rate at a rather low applied voltage (20–80 V). The flow rate is comparable to the existing results whereas the corresponding operation voltage of this study is about one to two orders lower than most of the existing results.     

Development of a nanostructural microwave probe based on GaAs

In order to develop a new structural microwave probe, we studied the fabrication of an AFM probe on a GaAs wafer. A waveguide was introduced by evaporating Au film on the top and bottom surfaces of the GaAs AFM probe where a tip 7 μm high with a 2.0 aspect ratio was formed and the dimensions of the cantilever were 250 × 30 × 15 μm. The open structure of the waveguide at the tip of the probe was obtained by FIB fabrication. An AFM image and profile analysis for a standard sample, obtained by the fabricated GaAs microwave probe and a commercial Si AFM probe, indicate that the fabricated probe has a similar capability for measurement of material topography as compared to the commercial probe.     

Biodegradable submicrometric sieves in PLLA fabricated by soft lithography

Sieves are membranes with a regular array of uniform pores that present low flow resistance. Because of such characteristics they are promising devices for filtration, separation of particles by size and drug delivery control systems. In this paper, we propose and demonstrated the use of a soft lithography process for fabrication of biodegradable sieves in PLLA (poly-l-lactide) with pores in the scale of hundred of nanometers. The fabrication process is suitable for mass production and submicrometric pore diameters can be fabricated with homogeneity of about 15%. The PLLA self sustained sieve can be integrated to PLLA capsules, compounding a drug delivery systems or implants.     

Development of ‘Multi-arm Bioprinter’ for hybrid biofabrication of tissue engineering constructs

This paper highlights the development of ‘Multi-arm Bioprinter (MABP)’ capable of concurrent multi-material deposition with independent motion path and dispensing parameters including deposition speed, material dispensing rate, and nozzle travel velocity for use in tissue engineering. In this research, the system is designed to concurrently print a filament structure and deposit cell spheroids between the filaments to create a hybrid structure to support the cell spheroids in three dimensions (3Ds). This process can be used with multiple cell types and is capable of reducing the fabrication time while using optimized dispensing parameters for each material. A novel method of dispensing the crosslinking solution using a co-axial nozzle was also developed and demonstrated in this paper. Cell-laden structures were fabricated through concurrent deposition of cell-encapsulated filaments and with cell spheroids to validate this concept. Rheology studies were then conducted to determine the effects of crosslink flow on filament width, hydrogel dispensing pressure on filament width, and dispensing time interval on spheroid diameter.