Non-woven fabric-based microfluidic devices with hydrophobic wax barrier

This study proposed a novel method for the fabrication of non-woven based microfluidic devices with a wax hydrophobic barrier. Current microfluidic devices were fabricated with glass or polymer material, and paper is also widely used for the fabrication of low-cost microfluidic devices. The application of non-woven fabric based microfluidic devices provides a new option of bulk materials for microfluidics. Compared with the glass or polymer material used in microfluidics, non-woven fabric is low-cost, easy to process and disposable. Fluid can penetrate through the non-woven fabric material with capillary force without the requirement of external pumps. As fiber-based material, comparing with paper, non-woven fabric material is more durable with higher mechanical strength, and various types of non-woven fabric material also provide a board choice of surface chemical/physical properties for microfluidic applications. In this study, the hydrophilic non-woven fabric is chosen as the bulk material for microfluidic devices, a wax pattern transfer protocol is also proposed in this study for the deposition of hydrophobic barriers. For a demonstration of the proposed fabrication technique, a microfluidic mixer was also fabricated in this study.

     

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.     

Micro-structures fabrication on glasses for micro-fluidics by imprinting technique

Micro-channels for microfluidics were fabricated in soda-lime glass through imprinting technique, and then joined to another soda-lime glass slab by thermal assisted direct bonding (TADB). The joined samples were characterized before and after TADB by optical and scanning electron microscopy (SEM) and surface profilometer and shear strength test. The integrity of channels is maintained also after the bonding. The bonded interface between the two glass slabs was found to be without impurities, bubbles and cracks; good bonding strength of 32 MPa between two glasses was obtained as well. These techniques are simple and low cost, suitable for mass production of glass based micro-fluidic devices.     

Bonding of thermoplastic polymer microfluidics

Thermoplastics are highly attractive substrate materials for microfluidic systems, with important benefits in the development of low cost disposable devices for a host of bioanalytical applications. While significant research activity has been directed towards the formation of microfluidic components in a wide range of thermoplastics, sealing of these components is required for the formation of enclosed microchannels and other microfluidic elements, and thus bonding remains a critical step in any thermoplastic microfabrication process. Unlike silicon and glass, the diverse material properties of thermoplastics opens the door to an extensive array of substrate bonding options, together with a set of unique challenges which must be addressed to achieve optimal sealing results. In this paper we review the range of techniques developed for sealing thermoplastic microfluidics and discuss a number of practical issues surrounding these various bonding methods.     

Characterization and modeling of a microfluidic dielectrophoresis filter for biological species

Microfabricated interdigitated electrode array is a convenient form of electrode geometry for dielectrophoretic trapping of particles and biological entities such as cells and bacteria within microfluidic biochips. We present experimental results and finite element modeling of the holding forces for both positive and negative dielectrophoretic traps on microfabricated interdigitated electrodes within a microfluidic biochip fabricated in silicon with a 12-/spl mu/m-deep chamber. Anodic bonding was used to close the channels with a glass cover. An Experimental protocol was then used to measure the voltages necessary to capture different particles (polystyrene beads, yeast cells, spores and bacteria) against destabilizing fluid flows at a given frequency. The experimental results and those from modeling are found to be in close agreement, validating our ability to model the dielectrophoretic filter for bacteria, spores, yeast cells, and polystyrene beads. This knowledge can be very useful in designing and operating a dielectrophoretic barrier or filter to sort and select particles entering the microfluidic devices for further analysis.     

UV Laser Micromachining of Polymers for Microfluidic Applications

We have recently begun to explore the use of UV laser ablation micromachining to construct microfluidic devices in polymers. This technique can create microchannels rapidly and modify the resulting polymer surface in a single step. By ablating under different atmospheres, it is possible to alter both the surface chemistries and physical surface morphologies of the microchannels. We have employed electroosmotic flow measurements, chemical mapping, and optical microscopy to characterize the microfluidic devices. In addition, we have studied the parameters affecting the ablation, such as the laser wavelength, laser fluence, laser firing repetition rate, and the material being ablated.     

Rapid prototyping of shrinkable BOPS-based microfluidic devices

Biaxially oriented polystyrene (BOPS) is a commercialized packaging material, which has the advantages of biocompatibility, non-toxic, transparency, light-weight and cost-effective. Due to the stress accumulated from both directions in plane during the fabrication process, when BOPS was reheated above the glass transition temperature, an isotropic shrinkage will occur. This study proposed a low-cost and rapid prototyping method for the fabrication of BOPS-based microfluidics device. Both laser ablation and micro-milling were used for the fabrication of microchannels on the surface of the BOPS sheet, after thermal induced shrinkage, microchannels with finer microstructure could be achieved. For the sealing of fabricated microchannels on BOPS, two approaches were made using a layer of BOPS or a layer of polyester adhesive film. The thermal induced shrinkage and bonding strength were carefully studied in this study. Several microfluidic devices, including a droplet generator and a diffusion mixer were also fabricated for demonstration. The proposed fabrication method for BOPS-based microfluidics is simple, rapid, cost-effective and without the requirement of cleanroom facility, with help of thermal induced shrinkage, finer structure with high resolution could be achieved with conventional lab tools.     

PMMA microfluidic chip fabrication using laser ablation and low temperature bonding with OCA film and LOCA

A new PMMA microfluidic chip fabrication method by combining laser ablation technology with low-temperature bonding using optically clear adhesive (OCA) film and liquid optically clear adhesive (LOCA) was presented in this paper. The deformation and clogging issues of the microfluidic channel were well solved. The effective bonding area ratio of microfluidic chips could be greatly improved by adjusting bonding temperature and bonding time. The crevices around the microchannels were effectively eliminated by coating treatment of LOCA. The bonding strength and waterproof of PMMA microfluidic chips coating with/without LOCA were also evaluated in this paper.

     

Investigation and improvement of reversible microfluidic devices based on glass–PDMS–glass sandwich configuration

Reversibly assembled microfluidic devices are dismountable and reusable, which is useful for a number of applications such as micro- and nano-device fabrication, surface functionalization, complex cell patterning, and other biological analysis by means of spatial–temporal pattern. However, reversible microfluidic devices fabricated with current standard procedures can only be used for low-pressure applications. Assembling technology based on glass–PDMS–glass sandwich configuration provides an alternative sealing method for reversible microfluidic devices, which can drastically increase the sealing strength of reversibly adhered devices. The improvement mechanism of sealing properties of microfluidic devices based on the sandwich technique has not been fully characterized, hindering further improvement and broad use of this technique. Here, we characterize, for the first time, the effect of various parameters on the sealing strength of reversible PDMS/glass hybrid microfluidic devices, including contact area, PDMS thickness, assembling mode, and external force. To further improve the reversible sealing of glass–PDMS–glass microfluidic devices, we propose a new scheme which exploits mechanical clamping elements to reinforce the sealing strength of glass–PDMS–glass sandwich structures. Using our scheme, the glass–PDMS–glass microchips can survive a pressure up to 400 kPa, which is comparable to the irreversibly bonded PDMS microdevices. We believe that this bonding method may find use in lab-on-a-chip devices, particularly in active high-pressure-driven microfluidic devices.     

Desktop micromilled microfluidics

Micromilling is a proven method for prototyping microfluidic devices; however, high overhead costs, large machine footprints, an esoteric software stack, and nonstandard device bonding protocols may be hampering the widespread adoption of micromilling in the greater microfluidics community. This research exploits a free design-to-device software chain and uses it to explore the applicability of a new class of inexpensive, desktop micromills for fabricating microfluidic devices out of polycarbonate. We present an analysis framework for stratifying micromill’s spatial accuracy and surface quality. Utilizing this we concluded milling geometries directly on the substrate is advantageous to making molds out of the substrate, in terms of accuracy and minimum feature size. Moreover, we proposed a general procedure to calculate feedrate and spindle-speed for any sub-millimeter endmill based on a recommended load percentage. We also established stepover is the major parameter in determining the surface quality rather than spindle-speed and feedrate, showing low-cost mills are able to deliver high-quality surface finishes. Ultimately, we clarified the suitability of low-cost micromills and a cost-efficient assembly method in the field of microfluidics by demonstrating rate- and size-controlled microfluidic droplet generation.     

A simple and cost-effective method for fabrication of integrated electronic-microfluidic devices using a laser-patterned PDMS layer

We report a simple and cost-effective method for fabricating integrated electronic-microfluidic devices with multilayer configurations. A CO₂ laser plotter was employed to directly write patterns on a transferred polydimethylsiloxane (PDMS) layer, which served as both a bonding and a working layer. The integration of electronics in microfluidic devices was achieved by an alignment bonding of top and bottom electrode-patterned substrates fabricated with conventional lithography, sputtering and lift-off techniques. Processes of the developed fabrication method were illustrated. Major issues associated with this method as PDMS surface treatment and characterization, thickness-control of the transferred PDMS layer, and laser parameters optimization were discussed, along with the examination and testing of bonding with two representative materials (glass and silicon). The capability of this method was further demonstrated by fabricating a microfluidic chip with sputter-coated electrodes on the top and bottom substrates. The device functioning as a microparticle focusing and trapping chip was experimentally verified. It is confirmed that the proposed method has many advantages, including simple and fast fabrication process, low cost, easy integration of electronics, strong bonding strength, chemical and biological compatibility, etc.     

A universal approach for irreversible bonding of rigid substrate-based microfluidic devices at room temperature

We present a simple and universal method for irreversibly bonding rigid substrate-based microfluidic devices at room temperature. In this method, a pre-patterned self-adhesive film covers and seals the area of the rigid substrate containing microchannels to create a closed microfluidic system, and then an adhesive-assisted sandwich bonding is used to reinforce the strength of bonding. The bonding can be achieved in 10 min at room temperature without requiring cleanroom facilities, complex surface modification, or employing rigorous cleaning. Despite its simplicity, this bonding method can create high-performance microfluidic devices with burst pressures over 2 MPa, but without channel clogging or microstructure deformation. The universality of this bonding method is demonstrated by applying it to the production of microfluidic devices with various rigid substrates. The simplicity, low cost, and universality of our method should allow it to be adopted by researchers lacking access to cleanroom facilities.     

SU-8 microchannels for live cell dielectrophoresis improvements

In this work a novel highly precise SU-8 fabrication technology is employed to construct microfluidic devices for sensitive dielectrophoretic (DEP) manipulation of budding yeast cells. A benchmark microfluidic live cell sorting system is presented, and the effect of microchannel misalignment above electrode topologies on live cell DEP is discussed in detail. Simplified model of budding Saccharomyces cerevisiae yeast cell is presented and validated experimentally in fabricated microfluidic devices. A novel fabrication process enabling rapid prototyping of microfluidic devices with well-aligned integrated electrodes is presented and the process flow is described. Identical devices were produced with standard soft-lithography processes. In comparison to standard PDMS based soft-lithography, an SU-8 layer was used to construct the microchannel walls sealed by a flat sheet of PDMS to obtain the microfluidic channels. Direct bonding of PDMS to SU-8 surface was achieved by efficient wet chemical silanization combined with oxygen plasma treatment of the contact surface. The presented fabrication process significantly improved the alignment of the microstructures. While, according to the benchmark study, the standard PDMS procedure fell well outside the range required for reasonable cell sorting efficiency. In addition, PDMS delamination above electrode topologies was significantly decreased over standard soft-lithography devices. The fabrication time and costs of the proposed methodology were found to be roughly the same.

     

Stamp-and-stick room-temperature bonding technique for microdevices

Multilayer MEMS and microfluidic designs using diverse materials demand separate fabrication of device components followed by assembly to make the final device. Structural and moving components, labile bio-molecules, fluids and temperature-sensitive materials place special restrictions on the bonding processes that can be used for assembly of MEMS devices. We describe a room temperature "stamp and stick (SAS)" transfer bonding technique for silicon, glass and nitride surfaces using a UV curable adhesive. Alternatively, poly(dimethylsiloxane) (PDMS) can also be used as the adhesive; this is particularly useful for bonding PDMS devices. A thin layer of adhesive is first spun on a flat wafer. This adhesive layer is then selectively transferred to the device chip from the wafer using a stamping process. The device chip can then be aligned and bonded to other chips/wafers. This bonding process is conformal and works even on surfaces with uneven topography. This aspect is especially relevant to microfluidics, where good sealing can be difficult to obtain with channels on uneven surfaces. Burst pressure tests suggest that wafer bonds using the UV curable adhesive could withstand pressures of 700 kPa (7 atmospheres); those with PDMS could withstand 200 to 700 kPa (2-7 atmospheres) depending on the geometry and configuration of the device.     

Fabrication of microfluidic structures in quartz via micro machining technologies

Microfluidic channels have been created for quartz material using micromechanical manufacturing technologies such as micro laser machining, micro ultrasonic machining, and ultra-precision machining. Ultra-precision machining has been used to manufacture cross-junction channels 14 µm wide and 28 µm deep with a three-dimensional triangle cross-section. Micro laser machining has been used to manufacture U-shaped and -shaped microfluidic channels. Deep holes and microfluidic channels with a high slenderness ratio (width/depth) can be obtained by using micro ultrasonic machining technology. These three machining techniques are compared with respect to surface profiles and machining quality.

     

Fabrication of polystyrene microfluidic devices using a pulsed CO<Subscript>2</Subscript> laser system

In this article, we described a simple and rapid method for fabrication of droplet microfluidic devices on polystyrene substrate using a CO₂ laser system. The effects of the laser power and the cutting speed on the depth, width and aspect ratio of the microchannels fabricated on polystyrene were investigated. The polystyrene microfluidic channels were encapsulated using a hot press bonding technique. The experimental results showed that both discrete droplets and laminar flows could be obtained in the device.     

Moving shot,an affordable and high-throughput setup for direct imaging of fast-moving microdroplets

Droplet microfluidics have a great potential in chemical and biomedical applications, due to their high throughput, versatility, and massive parallelism. To enhance their throughput, many devices based on the droplet microfluidics are using a flow-through configuration, in which the droplets are generated, transported, and analyzed in a continuous stream with a high velocity. Direct imaging of moving droplets is often necessary to characterize the spatiotemporal dynamics of the chemical reaction and physical process in the droplets. However, due to the motion blur caused by the movement of the droplets during exposure, an expensive high-speed camera is required for clear imaging, which is cost prohibitive in many applications. In this paper, we are presenting ‘Moving shot’ to demonstrate direct imaging of fast-moving droplets in microfluidic channels at an affordable cost. A microfluidic device is translated at the same velocity but in the opposite direction of moving droplets in it, so that the droplets are stationary with respect to the objective lens. With this approach, we can image fluorescent droplets moving at 0.34 cm s⁻¹ with an exposure time up to 2 s without motion blur. We strongly believe that the proposed technique can enable cost-effective and high-throughput imaging of fast-moving droplets in a microfluidic channel.

     

Fabrication of microstructures on different materials by diode-pumped solid state laser writing for microfluidics applications

Laser writing attached many attentions for fabrication micro-channels in microfluidics devices and lab-on-chip devices for biomedical applications. In this study, micro-channels were fabricated on different materials as masters using nanosecond diode-pumped solid state (DPSS) laser writing for imprinting on glass and polymer microfluidics devices. Good quality microstructures were fabricated on silicon, nickel alloy, cooper/brass and alumina, respectively by laser writing which proved that the nanosecond DPSS laser is suitable for rapid prototyping and rapid manufacturing of surface microstructures on different substrates as mask-less exposure system of imprinting.     

A microvalve for hybrid microfluidic systems

A hybrid valve for lab on chip applications is presented. The valve is assembled by bonding poly (methyl methacrylate), PMMA, and silicon-based elastomers. The process used to promote the hybrid bonding includes the deposition of an organosilane (TMSPM) on the thermoplastic polymer, PMMA to interface PMMA and elastomers. For this study, a membrane in ELASTOSIL^? is bonded in correspondence of the end of two microfluidic channels of a fabricated PMMA microfluidic chip. Prior the bonding, a plasma etching process has been used to remove the TMSPM in a confined circular area. This process made possible to bond selectively the edge of a membrane leaving free to move its central part. Actuating the membrane with an external positive pressure or vacuum is possible, respectively, to obstruct or to connect the microfluidic channels. The microvalve may be simply integrated in microfluidic devices and permits the control of microvolumes of fluid in processes such as transport, separation, and mixing. The deposition of the TMSPM, the bonding of the valve and its actuation has been characterized and tested. The flow rate control of liquids through the valve has been characterized. The results have been discussed and commented. The valve can stand up to 14 psi without showing leakages.     

Low-cost rapid prototyping of glass microfluidic devices using a micromilling technique

A method is proposed for rapid prototyping of glass microfluidic devices utilizing a commercial micromilling machine. In the proposed approach, micromilling is performed with the glass substrates immersed in cool water, which could efficiently remove debris and increase the life of milling tools. We also investigate the effects of spindle speed, feed rate, cutting depth, cooling mode, and tool type on finished channel geometries, bottom surface roughness, and burring along the channel sides. It was found that low cutting depths, high spindle speeds and low feed rate produce smoother channels. Several functional microfluidic devices were demonstrated with this rapid prototyping method. The results confirm that the proposed micromilling technique represents a viable solution for the rapid and economic fabrication of glass-based microfluidic chips. We believe that this method will greatly improve the accessibility of glass microfluidic devices to researchers.