Microfluidic Mold Research Articles

Improving the thickness uniformity of micro electroforming layer by megasonic agitation and the application

Throwing power can be used to quantitatively characterize the uniform distribution ability of electroforming layer thickness. From the perspective of throwing power, the effect of megasonic agitation on improving the thickness uniformity was studied under the experiment system of nickel containing electrolyte. Firstly, throwing power experiments were performed with different megasonic power densities. Experiment results show that throwing power increases with the increase in power density. When power density is 2.4 W/cm2, throwing power has a maximum value of 37.1% and increases 50.2% compared with the value of 24.7% obtained without megasonic. Then, the effect factors of throwing power, including electrolyte conductivity and cathodic polarizability, were investigated. The results reveal that megasonic has little effect on the cathodic polarizability, but it can increase the conductivity, which is conducive to improving thickness uniformity. Finally, to prove the feasibility of this method on micro metal devices, metal microfluidic chip molds were fabricated by megasonic micro electroforming. The result indicates that the thickness uniformity of the electroformed micro mold is improved by 43.4%.

3D Printing of Metallic Microstructured Mould Using Selective Laser Melting for Injection Moulding of Plastic Microfluidic Devices.

A new method, a 3D printing technique, in particular, selective laser melting (SLM), has been used to fabricate moulds for the injection moulding of thermoplastic microfluidic chips that are suitable for prototyping and early stage scale-up. The micro metallic patterns are printed on to a pre-finished substrate to form a microstructured mould. The dimensional accuracy, surface morphology, bonding strength between the printed patterns and substrate, as well as the microstructure of micro features were all characterized. A microfluidic mould was successfully printed and used directly for injection moulding of cyclic olefin copolymer (COC) microfluidic chips, which were used subsequently to successfully monitor nitrite concentrations in environmental water. The characterization indicated that this new process can be used for fast fabrication of mould tools for injection moulding/hot embossing microfluidic devices. It is faster, more flexible and less expensive than conventional micro-machining processes, although the accuracy and finish are still needed to improve though process optimization and hybrid SLM and machining processes.

Fabrication of metal microfluidic chip mold with coplanar auxiliary cathode in the electroforming process

Thickness nonuniformity is one of the prominent problems in the electroforming process of the metal microfluidic chip mold, which decreases the dimensional accuracy of the mold, and has a direct influence on the usability of the microfluidic chip. A novel method of auxiliary cathode for improving the thickness uniformity of electroformed mold is presented in this paper. Compared with the traditional method of the auxiliary cathode, a coplanar auxiliary cathode similar to the local microstructure of mold is used to fabricate the electroformed mold, and the thickness uniformity of electroformed mold is improved obviously. In addition, in order to determine the dimensions and placement of the auxiliary cathode, the computer-aided analysis is performed to simulate electroforming process. The simulated results show that with the help of a specially designed auxiliary cathode, the thickness uniformity is improved by about 53.2% compared with the result without the auxiliary cathode. Finally, based on the simulation, a microfluidic chip mold is fabricated by micro electroforming with the coplanar auxiliary cathode. The result shows that the thickness uniformity of the mold is improved by about 49.1%, which is in good agreement with the simulation result.

Physical thickness and group refractive index measurement of individual layers for double-stacked microstructures using spectral-domain interferometry

Abstract A non-destructive method for measuring the physical thickness and group refractive index of individual layers was proposed based on spectral-domain interferometry, which was realized to achieve real-time measurements using a mode-locked laser and an optical spectrum analyzer. As a double-stacked specimen, a microfluidic channel mold composed of a SU-8 photoresist and a silicon wafer was chosen. With areal scanning of the sample, a physical thickness map and a representative group refractive index value for each layer were obtained at the same time. The sample was measured 30 times consecutively at pre-determined points to estimate the repeatability of the physical thickness, for which the standard deviation was less than 10 nm. Moreover, a measurement comparison with two calibrated reference instruments was conducted. According to the comparative measurement results, physical thickness values obtained by the proposed method and with each comparative method were found to be in good agreement within expanded uncertainty levels.

Fabrication of microfluidic chip mold based on through-mask electrochemical etching and micro electroforming process

During the fabrication of nickel microfluidic chip mold insert, the problem of poor interfacial adhesion strength between electroformed layer and substrate often appeared which seriously reduced the service lifetime of the micro mold. In this paper, a novel pretreatment method of pickling and through-mask electrochemical etching was proposed to improve the interfacial adhesion strength of nickel microfluidic chip molds. Micro beams were used for exploring the effect of different pretreatment parameters on improving the adhesion strength. The adhesion strength was measured through a micro shearing test. The test results show that the mean interfacial adhesion strength without pretreatment is 52.1 MPa, and the interfacial adhesion strengths of other three groups with different pretreatment parameters are 103.4 MPa, 158.2 MPa and 229.7 MPa, respectively. The result indicated that pickling and electrochemical etching can effectively improve the adhesion strength between electroformed layer and substrate. Besides, in order to find the most suitable fabrication parameters for nickel mold insert, lateral corrosion, etching depth and etching surface area were also analyzed. The measured results show that the adhesion strength almost increases linearly with the increase of etching surface area in a scope of 1.50 mm2 to 2.07 mm2. Overall consideration, when a double cross-shaped microfluidic chip mold insert was fabricated by pretreatment of pickling for 20 s and electrochemical etching for 5 min, the micro mold insert has higher interfacial adhesion strength and the substrate has a moderate lateral corrosion, which contributes to extending the service life of microfluidic chip mold.

3D-glass molds for facile production of complex droplet microfluidic chips.

In order to leverage the immense potential of droplet microfluidics, it is necessary to simplify the process of chip design and fabrication. While polydimethylsiloxane (PDMS) replica molding has greatly revolutionized the chip-production process, its dependence on 2D-limited photolithography has restricted the design possibilities, as well as further dissemination of microfluidics to non-specialized labs. To break free from these restrictions while keeping fabrication straighforward, we introduce an approach to produce complex multi-height (3D) droplet microfluidic glass molds and subsequent chip production by PDMS replica molding. The glass molds are fabricated with sub-micrometric resolution using femtosecond laser machining technology, which allows directly realizing designs with multiple levels or even continuously changing heights. The presented technique significantly expands the experimental capabilities of the droplet microfluidic chip. It allows direct fabrication of multilevel structures such as droplet traps for prolonged observation and optical fiber integration for fluorescence detection. Furthermore, the fabrication of novel structures based on sloped channels (ramps) enables improved droplet reinjection and picoinjection or even a multi-parallelized drop generator based on gradients of confinement. The fabrication of these and other 3D-features is currently only available at such resolution by the presented strategy. Together with the simplicity of PDMS replica molding, this provides an accessible solution for both specialized and non-specialized labs to customize microfluidic experimentation and expand their possibilities.

An Electrically Tunable Terahertz Plasmonic Device Based on Shape Memory Alloys and Liquid Metals

AbstractAn electrically tunable terahertz (THz) plasmonic device is designed and fabricated using liquid metals (eutectic gallium indium) and shape memory alloy wires (Flexinol). The liquid metal is injected into the voids of a poly(dimethyl) siloxane microfluidic mold forming a periodic array of subwavelength apertures, while the wires are inserted into the elastomer below the metal plane. When a DC voltage is applied to the wires, they contract via Joule heating, reducing the aperture periodicity and blueshifting the transmission resonances of the device. When the voltage is removed, the wires cool and elongate back to their original length, allowing the transmission spectrum to return to its original state. The magnitude of this change depends upon the applied voltage. The device is shown to thermally cycle between the relaxed state and the fully contracted state reproducibly over at least 500 thermal cycles. The asymmetric geometry of the device and the contraction process yield transmission properties that are unexpected: two closely spaced resonances, where both resonances correspond to the same scattering indices, and an increase in the transmission amplitude of the lowest order resonance upon contraction. Numerical simulations are used to understand these features.

3D Printing of Micromolds and Microfluidic Devices

Over the past 30 years there has been a steady increase in interest in polymeric microfluidics and lab-on-a-chip technologies. The global microfluidics market is projected to reach USD 8.78 Billion by 2021 from USD 3.65 Billion in 2015, at a CAGR of 19.2% during the forecast period (2016 to 2021) [1]. While many polymers have been employed to realize microfluidic devices, polydimethylsiloxane (PDMS), a silicone based elastomer, has been widely used because of its biocompatibility, low cost, low toxicity, high oxidative and thermal stability, optical transparent, low permeability to water, low electrical conductivity, and ease of micropatterning [2,3, 4,5,6,7]. However, microfabrication of PDMS based microfluidic devices involves fabrication of micromolds which need expenisve infrastruce, such as clean room, photolithography equipment, masks etc. [8, 9, 10, 11, 12, 13]. Previously we had presented 3-D printing of complex MEMS structures [14] and other devices [15,16]. In this paper, we present fabrication of microfluidic molds and devices by employing 3D printing technology that are otherwise time consuming and difficult to manufacture with state of the art 2-D MEMS fabrication technology. Figure 1 shows optical micrograph of 3D printed micromold channels.

Rapid fabrication of rounded microchannels via extrusion printing of molds using a thixotropic ink

This paper reports a direct-write strategy that rapidly produces rounded cross-sectional molds for casting of microfluidic channels in polydimethylsiloxane (PDMS). Robotically controlled microextrusion of a thixotropic ink through a micronozzle onto a substrate surface generates user-defined positive relief structures that serve as molds. Printed lateral resolutions of less than 10μm can be achieved by using a micronozzle with dimensions of a few micrometers and microscopic visual feedback. The cross-sectional geometries of the microchannels can be easily adjusted by regulating parameters such as the printing speed, applied pressure, micronozzle-substrate distance, and number of stacks. As a result of the “liquid rope coiling” effect, this technique provides a straightforward way of producing a serpentine microchannel. To demonstrate the feasibility of extrusion printing, several complex microfluidic structures were cast in PDMS from their printed molds. This strategy provides a simple but flexible approach to on-demand prototyping of microfluidic molds with controllable, rounded cross-sections for microfluidic structures.

Burr Reduction of Micro-milled Microfluidic Channels Mould Using a Tapered Tool

Moulds with micro-sizes features needed for many applications, such as for hot embossing, can be manufactured using micro-milling process. However, the burrs formed in the micro-milling process are a challenge that needs to be addressed. The burr sizes are comparable to the micro-milled feature sizes and the common types of burr seen being the top/side and exit burrs. The use of a tapered geometry micro-milling tool is investigated in this paper that enables reduction in both the top and exit burrs. The straight and tapered micro-milling tools of various angles are used and the burrs formed are observed. Micro-milling experiments are conducted on an aluminium alloy by producing common positive features seen in the mould for the production of polymer microfluidic devices. The results show that the burr reduction can be attributed due to the increase of the taper angle. It is seen that the tapered tool not only substantially reduces the top burrs, but also leaves behind inclined walls which further help in reducing exit burrs formed during the subsequent finish face milling. Furthermore, embossing trials performed with the micro-milled tapered geometry moulds show improved performance not only because burrs are reduced and also because the taper eases mould release.

Microfluidically Reconfigured Wideband Frequency-Tunable Liquid-Metal Monopole Antenna

A microfluidically reconfigured wideband frequency-tunable liquid-metal monopole antenna is presented. The antenna operation relies on continuous moving of the liquid-metal volume over the capacitively coupled microstrip line feed network with a micropump unit. To maximize the capacitive coupling at the feed point, the antenna is realized by bonding microfluidic channel molds prepared in polydimethylsiloxane (PDMS) polymer with 1-mil-thick liquid crystal polymer (LCP) substrate. The concept is demonstrated through the design and experimentation of a 4:1 frequency-tunable monopole antenna that operates from 1.29 to 5.17 GHz. The presented monopole antenna can also be utilized as an element to form wideband frequency-tunable high-gain antenna apertures without necessitating any additional micropump units. This is accomplished by resorting to strategically meandered or interconnected microfluidic channels. This concept is demonstrated through the design and experimentation of a $4 \times 1$ microfluidically controlled monopole array capable of providing 2:1 frequency tuning range from 2.5 to 5 GHz.

Rapid fabrication of microfluidic PDMS devices from reusable PDMS molds using laser ablation

The conventional fabrication methods for microfluidic devices require cleanroom processes that are costly and time-consuming. We present a novel, facile, and low-cost method for rapid fabrication of polydimethylsiloxane (PDMS) molds and devices. The method consists of three main fabrication steps: female mold (FM), male mold (MM), and chip fabrication. We use a CO2 laser cutter to pattern a thin, spin-coated PDMS layer for FM fabrication. We then obtain reusable PDMS MM from the FM using PDMS/PDMS casting. Finally, a second casting step is used to replicate PDMS devices from the MM. Demolding of one PDMS layer from another is carried out without any potentially hazardous chemical surface treatment. We have successfully demonstrated that this novel method allows fabrication of microfluidic molds and devices with precise dimensions (thickness, width, length) using a single material, PDMS, which is very common across microfluidic laboratories. The whole process, from idea to device testing, can be completed in 1.5 h in a standard laboratory.

Study on Improving Thickness Uniformity of Microfluidic Chip Mold in the Electroforming Process.

Electroformed microfluidic chip mold faces the problem of uneven thickness, which decreases the dimensional accuracy of the mold, and increases the production cost. To fabricate a mold with uniform thickness, two methods are investigated. Firstly, experiments are carried out to study how the ultrasonic agitation affects the thickness uniformity of the mold. It is found that the thickness uniformity is maximally improved by about 30% after 2 h electroforming under 200 kHz and 500 W ultrasonic agitation. Secondly, adding a second cathode, a method suitable for long-time electroforming is studied by numerical simulation. The simulation results show that with a 4 mm width second cathode used, the thickness uniformity is improved by about 30% after 2 h of electroforming, and that with electroforming time extended, the thickness uniformity is improved more obviously. After 22 h electroforming, the thickness uniformity is increased by about 45%. Finally, by comparing two methods, the method of adding a second cathode is chosen, and a microfluidic chip mold is made with the help of a specially designed second cathode. The result shows that the thickness uniformity of the mold is increased by about 50%, which is in good agreement with the simulation results.

Multi-depth valved microfluidics for biofilm segmentation

Bacterial biofilms present a societal challenge, as they occur in the majority of infections but are highly resistant to both immune mechanisms and traditional antibiotics. In the pursuit of better understanding biofilm biology for developing new treatments, there is a need for streamlined, controlled platforms for biofilm growth and evaluation. We leverage advantages of microfluidics to develop a system in which biofilms are formed and sectioned, allowing parallel assays on multiple sections of one biofilm. A microfluidic testbed with multiple depth profiles was developed to accommodate biofilm growth and sectioning by hydraulically actuated valves. In realization of the platform, a novel fabrication technique was developed for creating multi-depth microfluidic molds using sequentially patterned photoresist separated and passivated by conformal coatings using atomic layer deposition. Biofilm thickness variation within three separately tested devices was less than 13% of the average thickness in each device, while variation between devices was 23% of the average thickness. In a demonstration of parallel experiments performed on one biofilm within one device, integrated valves were used to trisect the uniform biofilms with one section maintained as a control, and two sections exposed to different concentrations of sodium dodecyl sulfate. The technology presented here for multi-depth microchannel fabrication can be used to create a host of microfluidic devices with diverse architectures. While this work focuses on one application of such a device in biofilm sectioning for parallel experimentation, the tailored architectures enabled by the fabrication technology can be used to create devices that provide new biological information.

Replication of microchannel structures in WC–Co feedstock using elastomeric replica moulds by hot embossing process

Hot embossing is a net shaping process that is able to produce the micro-components of polymers with intrinsic and complex shapes at lower cost compared with machining and injection moulding. However, the emboss of hard metals, such as WC-Co, is more challenging due to their high thermal conductivity and ease of agglomeration. Thus, a WC-Co alloy mixed with a wax-based binder feedstock was selected. The formed feedstock exhibited pseudo-plastic flow and was successfully embossed (green part). Here, we developed a novel process that is used to replicate polymer microfluidic chips while simultaneously reducing the channel surface roughness of the mould insert, yielding optical-grade (less than 100 nm surface roughness) channels and reservoirs. This paper concerns the replication of metallic microfluidic mould inserts in WC-Co and the parameters associated with feedstock formation via a hot embossing process. A suitable formulation for micro-powder hot embossing has been established and characterised by thermogravimetric analyses and measurements of mixing torques to verify and quantify the homogeneity of the proposed feedstocks. The relative density of the samples increased with processing temperature, and almost fully dense materials were obtained. In this work, the effects of the sintering temperature on the physical properties were systematically analysed. The evolution of the metal surface morphology during the hot embossing process was also investigated. The results indicate that the feedstock can be used to manufacture micro-fluidic die mould cavities with a low roughness, proper dimensions and good shape retention. The shrinkage of the sintered part was approximately 19-24% compared with that of the brown part.

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.

The manufacturing and replication of microfluidic mould inserts by the hot embossing process

In this paper, the manufacturing of metallic microfluidic mould inserts is studied based on the hot embossing process. The feedstocks are prepared and analysed. The quality of the feedstock resulting from the mixing process is found to depend on numerous parameters, such as the mixing time, the mixing temperature, the shear rate and the powder loading. The present mixing study is conducted with copper powders. The thermal and rheological properties of the selected feedstock under various mixing conditions are determined and used to establish the necessary conditions for mixing, hot embossing and debinding to obtain the copper microfluidic mould inserts. Sintering at different temperatures is conducted under vacuum. The effects of the sintering temperatures are evaluated based on the sintered microstructures. In this work, the dimensional variations in the microfluidic samples, in particular the dimensional shrinkage, global warpage and surface roughness at each stage of the process, are quantified and compared in detail.

Characterization of rapid PDMS casting technique utilizing molding forms fabricated by 3D rapid prototyping technology (RPT)

In this work we characterize a novel possibility for PDMS (PolyDiMethylSiloxane) casting/ micromolding methods with the utilization of molding forms fabricated by a commercially available novel acrylic photopolymer based 3D printing method. The quality and absolute spatial accuracy of 1) different 3D printing modes (‘matt’ vs. ‘glossy’); 2) the molded PDMS structures and 3) the subsequently produced complementary structures made of epoxy resin were investigated. The outcome of these two form transfer technologies were evaluated by the cross sectional analysis of open microfluidic channels (trenches) with various design. Our results reveal the spatial accuracy in terms of real vs. CAD (Computer Aided Design) values for the 3D printed acrylic structures and the limits of their form transfer to PDMS, then to epoxy structures. Additionally the significant differences between the various spatial directions (X, Y, Z) have been characterized, and the conclusion was drawn that the ‘glossy’ printing mode is not appropriate for 3D printing of microfluidic molds.

Adhesive bonding by SU-8 transfer for assembling microfluidic devices

SU-8 is largely used to make microfluidic molds or components, but mainly for producing high-precision and thermally stable structures. We present a versatile method that employs SU-8 as glue to perform an adhesive bonding between micro-patterned structures. More in general, this technique enables an easy assembly of microfluidic devices, which can also be made by different materials, where selective bonding is required. The adhesive bonding is achieved by transferring a thin layer of SU-8 5 (thickness ≤15 μm) on a substrate by means of a polyimide foil. The method is described in detail and an example of its application is given. Finally, a shear test is carried out to prove sufficient adhesion strength for microfluidic applications.

Laser Cladding of Micro-Fluidic Channel Mold

Laser cladding of micro-fluidic channels mold was performed using Nd:YAG laser and synchronous powder feeder. Influences of laser power and powder feed rate on clad layer geometrical dimensions and qualities were investigated. Results show that powder feed rate 1.5-2.5 g/min used obtains micro clad layers which meet geometrical dimensions’ requirement of micro-fluidic channels mold and combination of parameters laser power 400 W and powder feed rate 2.0 g/min achieves micro clad layer with better clad quality. Scanning paths of micro-fluidic channels mold was planned through CAD-CAM software. Using optimum parameters combination, multilayer laser cladding experiment was carried out and a sample of micro-fluidic channels mold was fabricated with expectative structure and hardness. A little milling and polishing makes the sample meet technical requirements, 0.2mm in height and 0.3mm in width. Fabricating a mold takes 15-20 min totally.