Novel prototyping method for microfluidic devices based on thermoplastic polyurethane microcapillary film

Thermoplastic polyurethane microcapillary film (TPU-MCF), as a novel extruded product, inherently contains an array of circular micron-sized capillaries embedded inside the polymer matrix. With the aid of simple laser cutting and conventional sealing technologies, a rapid prototyping method for microfluidic devices is proposed based on the ready-made microstructure of MCFs. Two functionalized microfluidic devices: serpentine micromixer and multi-droplet generator, are rapidly fabricated to demonstrate the advantages and potential of employing this new method. The whole proof-of-concept fabrication process can be completed in 8–10 min in a simple way; each procedure is repeatable with stable performance control of microfluidic devices; and the material cost can be as low as $0.01 for each device. The TPU-MCF and this novel method are expected to provide a new perspective and alternative in microfluidic community with particular requirements.     

Rapid prototyping of single-layer microfluidic PDMS devices with abrupt depth variations under non-clean-room conditions by using laser ablation and UV-curable polymer

The growing demand for microfluidic analytical devices calls for fast, cost-effective and high-throughput fabrication methods. Here we report a low-cost rapid prototyping method for single-layer microfluidic PDMS devices with abrupt depth variations under non-clean-room conditions. Channel patterns with different user-designed depths ranging from micrometres to millimetres are engraved on a polymethylmethacrylate (PMMA) plate in one step based on a laser ablation approach. A UV-curable polymer, Norland Optical Adhesive (NOA) 81, is then used to replicate the channel patterns from the PMMA female mould and is finally used as the master for single-layer polydimethylsiloxane (PDMS) microfluidic devices. This rapid prototyping method can significantly facilitate the fast evaluation of proof of concept in microfluidic researches and small-scale mass production for commercialization applications.     

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.     

Rapid prototyping of flexible multilayer microfluidic devices using polyester sealing film

Microsystem Technologies - In this study, we proposed a novel fabrication method for rapid prototyping of multilayer flexible microfluidic devices using laser ablated polyester sealing films. The...     

Thin-film electrode based droplet detection for microfluidic systems

We report on a droplet-producing microfluidic system with electrical impedance-based detection. The microfluidic devices are made of polydimethylsiloxane (PDMS) and glass with thin film electrodes connected to an impedance-monitoring circuit. Immiscible fluids containing the hydrophobic and hydrophilic phases are injected with syringe pumps and spontaneously break into water-in-oil droplet trains. When a droplet passes between a pair of electrodes in a medium having different electrical conductivity, the resulting impedance change signals the presence of the particle for closed-loop feedback during processing. The circuit produces a digital pulse for input into a computer control system. The droplet detector allows estimation of a droplet's arrival time at the microfluidic chip outlet for dispensing applications. Droplet detection is required in applications that count, sort, and direct microfluidic droplets. Because of their low cost and simplicity, microelectrode-based droplet detection techniques should find applications in digital microfluidics and in three-dimensional printing technology for rapid prototyping and biotechnology.     

A Rapid and Low-Cost Procedure for Fabrication of Glass Microfluidic Devices

In this paper, we present a simple, rapid, and low-cost procedure for fabricating glass microfluidic chips. This procedure uses commercially available microscopic slides as substrates and a thin layer of AZ 4620 positive photoresist (PR) as an etch mask for fabricating glass microfluidic components, rather than using expensive quartz glasses or Pyrex glasses as substrates and depositing an expensive metal or polysilicon/amorphous silicon layer as etch masks in conventional method. A long hard-baking process is proposed to realize the durable PR mask capable of withstanding a long etching process. In order to remove precipitated particles generated during the etching process, a new recipe of buffered oxide etching with addition of 20% HCl is also reported. A smooth surface microchannel with a depth of more than 110 mum is achieved after 2 h of etching. Meanwhile, a simple, fast, but reliable bonding process based on UV-curable glue has been developed which takes only 10 min to accomplish the efficient sealing of glass chips. The result shows that a high bonding yield (~ 100%) can be easily achieved without the requirement of clean room facilities and programmed high-temperature furnaces. The presented simple fabrication process is suitable for fast prototyping and manufacturing disposable microfluidic devices.     

Interconnection blocks with minimal dead volumes permitting planar interconnection to thin microfluidic devices

We have previously described ‘Interconnection Blocks’ which are re-usable, non-integrated PDMS blocks which allowing multiple, aligned and planar microfluidic interconnections. Here, we describe Interconnection Block versions with zero dead volumes that allow fluidic interfacing to flat or thin side-walled microfluidic devices. These designs increase the number of materials, types of devices and applications for which Interconnection Blocks can be used. Average leak pressures of 4.7 bar were recorded and all individual leak pressures recorded were above the 2-bar threshold for microfluidic applications. Additionally, the new Interconnection Block designs demonstrate that micromilling, a practical microfabrication method, can produce useful geometries not readily made through clean room-based approaches.     

Safe and cost-effective rapid-prototyping of multilayer PMMA microfluidic devices

The rapid translation of research from bench to bedside, as well as the generation of commercial impact, has never been more important for both academic and industrial researchers. It is therefore unsurprising that more and more microfluidic groups are investigating research using a wide range of thermoplastics which can be readily translated to large-scale manufacturing, if the technology is taken to commercialisation. While structuring, via additive or subtractive manufacturing, is becoming relatively easy through the use of commercial-grade devices, reliable and fast assembly remains a challenge. In this article, we propose an innocuous and cost-effective, under 2-min technique which enables the bonding of multiple poly(methyl methacrylate) layers. This bonding technique relies on the application of small amounts (10 µl/cm²) of ethanol, low temperatures (70 °C) and relatively low pressures (~1.6 MPa). Our characterisation analysis shows that using a bonding time of 2 min is enough to produce a strong bond able to withstand pressures always above 6.2 MPa (with mean of 8 MPa, highest reported in the literature), with minimal channel deformation (<5%). This technique, which we demonstrate on assembly comprising up to 19 layers, presents an exciting improvement in the field of rapid prototyping of microfluidic devices, enabling extremely fast design cycles in microfluidic research with a material compatible with mass manufacturing, thus allowing a smoother transition from the laboratory to the market. Beyond the research community, this robust prototyping technique has the potential to impact on the deliverability of other scientific endeavours including educational projects and will directly fuel the fluidic maker movement.     

Miniaturized DNA amplification platform with soft-lithographically fabricated continuous-flow PCR microfluidic device on a portable temperature controller

Fabrication of 3D microfluidic devices is normally quite expensive and tedious. A strategy was established to rapidly and effectively produce multilayer 3D microfluidic chips which are made of two layers of poly(methyl methacrylate) (PMMA) sheets and three layers of double-sided pressure sensitive adhesive (PSA) tapes. The channel structures were cut in each layer by cutting plotter before assembly. The structured channels were covered by a PMMA sheet on top and a PMMA carrier which contained threads to connect with tubing. A large variety of PMMA slides and PSA tapes can easily be designed and cut with the help of a cutting plotter. The microfluidic chip was manually assembled by a simple lamination process.The complete fabrication process from device design concept to working device can be completed in minutes without the need of expensive equipment such as laser, thermal lamination, and cleanroom. This rapid frabrication method was applied for design of a 3D hydrodynamic focusing device for synthesis of gold nanoparticles (AuNPs) as proof-of-concept. The fouling of AuNPs was prevented by means of a sheath flow. Different parameters such as flow rate and concentration of reagents were controlled to achieve AuNPs of various sizes. The sheet-based fabrication method offers a possibility to create complex microfluidic devices in a rapid, cheap and easy way.

     

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.     

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.     

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.

     

Fast fabrication of microfluidic devices using a low-cost prototyping method

Conventional ways to produce microfluidic devices cost a lot due to the requirements for cleanroom environments and expensive equipment, which prevents the wider applications of microfluidics in academia and in industry. In this paper, a dry film photoresist was utilized in a simple way to reduce the fabrication cost of microfluidic masters. Thus, a fast prototyping and fabrication of microstructures in polydimethylsiloxane microchips through a replica molding technology was achieved in a low-cost setting within 2.5 h. Subsequently, major manufacturing conditions were optimized to acquire well-resolved microfluidic molds, and the replicated microchips were validated to be of good performance. A T-junction channel microchip was fabricated by using a dry film master to generate water droplets of uniform target size. Meanwhile, a gated injection of fluorescein sodium and a contactless conductivity detection of Na⁺ were both performed in a crosslink channel microchip via capillary electrophoresis, in other words, this fast prototyping and fabrication method would be an efficient, economical way to embody structural design into microfluidic chips for various applications.     

A novel filtration method integrated on centrifugal microfluidic devices

A method has been developed that integrates filters directly into centrifugal microfluidic devices. This technique is suitable for both rapid prototyping and commercial applications. Commercially available filter paper was sealed into the centrifugal microfluidic device with a simple manual fabrication procedure. The method was validated using soil slurry in water and a variety of filter papers with pore sizes ranging from 0.7 to 11 μm. Filtration times of 4 s to several minutes were obtained for 100 μL samples depending on the type of filter paper and rotation rate utilized. The validity of the method was demonstrated by assessing the amount of light lost due to the scatter or absorption caused by particles in the filtered sample while the device was in motion. Filtration and sedimentation were compared and after 30 min of centrifugation, sedimentation had not removed particles as well as filtration. This technique opens up centrifugal microfluidic devices to a wide range of samples.     

Rapid soda-lime glass etching process for producing microfluidic channels with higher aspect ratio

This paper presents a simple method to produce microfluidic channels in soda-lime glasses with the aspect ratio >0.5 utilizing a modified wet etching protocol. A low-cost positive photoresist (PR) layer is used as the etching mask for the wet etching process. Prior to the PR and primer coating procedure, a UV activation process is adopted for enhancing the binding strength of the hexamethyldisilazane primer layer and the glass substrate, resulting in an better adhesion for the PR layer. A fast etching recipe is also developed by increasing the acidity and the temperature of the buffered oxide (BOE) etchant. Since the photoresist etching mask does not peel during the etching process shortly, the structure of the etching mask forms a barrier and results in a different diffusion rate for the etchant inside the etched trench structure. A slower etching rate for the glass is observed at the undercut region such that the proposed anisotropic etching pattern can be achieved. Results show that the etching rate of the modified glass etching process is as high as 7.7 μm/min which is much faster than that of pure BOE etchant (0.96 μm/min). Sealed microfluidic channel with the aspect ratio of around 0.62 is produced with the developed method. The method developed in the present study provides a rapid and efficient way to produce microfluidic channels with higher aspect ratio.     

Optimization and application of dry film photoresist for rapid fabrication of high-aspect-ratio microfluidic devices

Fabrication of high-aspect-ratio PDMS microfluidic devices with conventional SU-8 based soft photolithography is challenging, and often, the thickness of the master from which PDMS replicas are molded is non-uniform. Here, we present an optimized, low cost, fast prototyping microfabrication technique to make deep (up to 500 μm) and high-aspect-ratio (up to 10) microfluidic channels by producing masters by laminating a single or multiple layers of a thin dry film photoresist onto metal wafers. In particular, we explore the required exposure energy for different film thicknesses as well as the highest achievable channel depths and aspect ratios. The homogeneity of the depth of PDMS channels formed using these masters is quantified and found to be remarkably uniform over distances of 20 mm or more. The importance of the processing parameters, such as the exposure energy and development time on final feature size, wall angle, and channel aspect ratio, is investigated. In addition, we report some failure cases, the potential reasons, and strategies for making optimized devices. Potentially, deep microfluidic channels with a wide range of aspect ratios can be used to make long, homogenous separation devices that can be used in cell sorting, filtration, and flow cytometry. We believe the protocols we outline here will be of great utility to the microfluidics community.     

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.     

Advanced adaptive feed control for CNC machining

In computer numerical control (CNC) machining, the tool feed rate is crucial for determining the machining time. It also affects the degree of tool wear and the final product quality. In a mass production line, the feed rate guides the production cycle. On the other hand, in single-time machining, such as for molds and dies, the tool wear and product quality are influenced by the length of machining time. Accordingly, optimizing the CNC program in terms of the feed rate is critical and should account for various factors, such as the cutting depth, width, spindle speed, and cutting oil. Determining the optimal tool feed rate, however, can be challenging given the various machine tools, machining paths, and cutting conditions involved. It is important to balance the machining load by equalizing the tool's load, reducing the machining time during no-load segments, and controlling the feed rate during high load segments. In this study, an advanced adaptive control method was designed that adjusts the tool feed rate in real time during rough machining. By predicting both the current and future machining load based on the tool position and time stamp, the proposed method combines reference load control curves and cutting characteristics, unlike existing passive adaptive control methods. Four different feed control methods were tested including conventional and proposed adaptive feed control. The results of the comparative analysis was presented with respect to the average machining load and tool wear, the machining time, and the average tool feed speed. When the proposed adaptive control method was used, the production time was reduced up to 12.8% in the test machining while the tool life was increased.     

Xurography: rapid prototyping of microstructures using a cutting plotter

This paper introduces xurography, or "razor writing," as a novel rapid prototyping technique for creating microstructures in various films. This technique uses a cutting plotter traditionally used in the sign industry for cutting graphics in adhesive vinyl films. A cutting plotter with an addressable resolution of 10 /spl mu/m was used to cut microstructures in various films with thicknesses ranging from 25 to 1000 /spl mu/m. Positive features down to 35 /spl mu/m and negative features down to 18 /spl mu/m were cut in a 25 /spl mu/m thick material. Higher aspect ratios of 5.2 for positive features and 8 for negative features were possible in a 360 /spl mu/m thick material. A simple model correlating material properties to minimum feature size is introduced. Multilayered microstructures cut from pressure sensitive and thermal activated adhesive films were laminated in less than 30 min without photolithographic processes or chemicals. Potential applications of these microstructures are explored including: shadow masking, electroplating, micromolds for PDMS, and multilayered three-dimensional (3-D) channels. This inexpensive method can rapidly prototype microfluidic devices or tertiary fluid connections for higher resolution devices. [1488].     

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.