Microfluidic Mold Research Articles

A Single-Cell Interrogation System from Scratch: Microfluidics and Deep Learning.

Live-cell imaging using fluorescence microscopy enables researchers to study cellular processes in unprecedented detail. These techniques are becoming increasingly popular among microbiologists. The emergence of microfluidics and deep learning has significantly increased the amount of quantitative data that can be extracted from such experiments. However, these techniques require highly specialized expertise and equipment, making them inaccessible to many biologists. Here we present a guide for microbiologists, with a basic understanding of microfluidics, to construct a custom-made live-cell interrogation system that is capable of recording and analyzing thousands of bacterial cell-cycles per experiment. The requirements for different microbiological applications are varied, and experiments often demand a high level of versatility and custom-designed capabilities. This work is intended as a guide for the design and engineering of microfluidic master molds and how to build polydimethylsiloxane chips. Furthermore, we show how state-of-the-art deep-learning techniques can be used to design image processing algorithms that allow for the rapid extraction of highly quantitative information from large populations of individual bacterial cells.

Highly parallel bending tests for fungal hyphae enabled by two-photon polymerization of microfluidic mold

Highly parallel bending tests for fungal hyphae enabled by two-photon polymerization of microfluidic mold

Dimension compensation of printed master molds by a desktop LCD 3D printer for high-precision microfluidic applications.

Recent advances in low-cost liquid crystal display (LCD) 3D printing have popularized its use in creating microfluidic master molds and complete devices. However, the quality and precision of these fabrications often fall short of the rigorous standards required for advanced microfluidic applications. This study introduces a novel approach to enhance the dimensional accuracy of microchannels produced using a desktop LCD 3D printer. We propose a method for dimension compensation, optimize the printing parameters, and provide a straightforward post-treatment technique to ensure high-quality curing of polydimethylsiloxane (PDMS) in master molds made from photosensitive resin. Our investigation assesses the precision of 3D printing across three different scales of square cross-section microchannels by measuring their widths and heights, leading to the determination of optimal printing parameters that minimize dimensional errors. The dimensional errors are further reduced by introducing a series of dimension compensation factors, which correct the nominal dimensions of the microchannels by using the compensation factors in 3D printing. The dimensional accuracy is significantly improved after compensation even in fabricating complex microchannels of triangular cross-sections. Finally, a spiral channel of trapezoidal-like cross-section with tilted edges is fabricated for microfluidic application, and highly efficient particle separation is realized in the channel. The proposed method provides new insights for utilizing desktop LCD 3D printers to achieve high-accuracy microstructures necessary for advanced microfluidic applications.

Micro-milling tool mark removal mechanism by fluid jet polishing and its application for the precision manufacturing of micro-fluidic chip molds

The surface quality of micro-channels has a significant impact on their functional performance. Ultra-precision polishing of the micro-channel (mold) is essential to meet the requirements of the equipment. The tool marks resulting from the former manufacturing procedure such as micro-milling should be removed in the polishing process. Fluid jet polishing (FJP) is a promising method to achieve high surface quality as well as superior applicability to the small structure unit. However, the effect of the initial surface morphology and polishing parameters on the tool marks removal capacity in FJP is not clear. The manufacturing parameters are determined by trial and error approach, which is time-consuming and costly. In this paper, the tool mark removal capability of FJP was investigated theoretically and experimentally considering the effect of tool mark interval and abrasive size. Computational fluid dynamics (CFD) simulation of micro-channel polishing was conducted to investigate different abrasive impact angles with different abrasive sizes. The material removal characteristics at different stages were revealed to explain different polishing performance due to the variation of tool mark interval and abrasive size. This study can provide solid scientific basis for the ultra-precision finishing of micro-channel obtained by micro-milling and avoid invalid polishing due to the inappropriate initial surface.

A Non-Sacrificial 3D Printing Process for Fabricating Integrated Micro/Mesoscale Molds.

Three-dimensional printing technology has been implemented in microfluidic mold fabrication due to its freedom of design, speed, and low-cost fabrication. To facilitate mold fabrication processes and avoid the complexities of the soft lithography technique, we offer a non-sacrificial approach to fabricate microscale features along with mesoscale features using Stereolithography (SLA) printers to assemble a modular microfluidic mold. This helps with addressing an existing limitation with fabricating complex and time-consuming micro/mesoscale devices. The process flow, optimization of print time and feature resolution, alignments of modular devices, and the advantages and limitations with the offered technique are discussed in this paper.

Fabrication of Microfluidic and Optofluidic molds with SPE- 60 Photoresist: Advantages and Challenges

Fabrication of Microfluidic and Optofluidic molds with SPE- 60 Photoresist: Advantages and Challenges

Labeling on a Chip of Cellular Fibronectin and Matrix Metallopeptidase-9 in Human Serum.

We present a microfluidic chip for protein labeling in the human serum-based matrix. Serum is a complex sample matrix that contains a variety of proteins, and a matrix is used in many clinical tests. In this study, the device performance was tested using commercial serum samples from healthy donors spiked with the following target proteins: cellular fibronectin (c-Fn) and matrix metallopeptidase 9 (MMP9). The microfluidic molds were fabricated using micro milling on acrylic and using stereolithography (SLA) three-dimensional (3D) printing for an alternative method and comparison. A simple quality control was performed for both fabrication mold methods to inspect the channel height of the chip that plays a critical role in the labeling process. The fabricated microfluidic chip shows a good reproducibility and repeatability of the performance for the optimized channel height of 150 µm. The spiked proteins of c-Fn and MMP9 in the human serum-based matrix, were successfully labeled by the functionalized magnetic nanoparticles (MNPs). The biomarker labeling occurring in the serum was compared using a simple matrix sample: phosphate buffer. The measured signals obtained by using a magnetoresistive (MR) biochip platform showed that the labeling using the proposed microfluidic chip is in good agreement for both matrixes, i.e., the analytical performance (sensitivity) obtained with the serum, near the relevant cutoff values, is within the uncertainty of the measurements obtained with a simple and more controlled matrix: phosphate buffer. This finding is promising for stroke patient stratification where these biomarkers are found at high concentrations in the serum.

Optimizing Immunofunctionalization and Cell Capture on Micromolded Hydrogels via Controlled Oxygen-Inhibited Photopolymerization.

With circulating tumor cells (CTCs) playing a critical role in cancer metastasis, the quantitation and characterization of CTCs promise to provide precise diagnostic and prognostic information in service of personalized therapies. However, as CTCs are extremely rare, high yield, high purity strategies are required to target and isolate CTCs from patient samples. Recently, we demonstrated the selective capture of CTCs upon antibody-functionalized polyethylene glycol diacrylate (PEGDA) hydrogels photopolymerized within polydimethylsiloxane (PDMS) microfluidic molds. Isolated CTC purity was subsequently enriched by selectively releasing desired cells from photodegradable hydrogel capture surfaces. However, the fabrication of these acrylate-based hydrogels by photopolymerization is subject to oxygen inhibition, which dramatically affects the physical and chemical properties of hydrogel interfaces formed in proximity to PDMS boundaries. To evaluate how antibody conjugation density and cell capture is impacted by fabrication parameters affected by oxygen inhibition, PEGDA hydrogel features were polymerized within PDMS micromolds under different UV exposure conditions and linker (acrylate-PEG-biotin) concentrations. Predictions of acrylate conversion throughout the hydrogel feature were performed using a 1D reaction-diffusion model that describes oxygen-inhibited photopolymerization. The functional consequences of photopolymerization parameters and solution stoichiometry on CTC capture were experimentally quantified and evaluated. Results show that hydrogel surfaces polymerized under shorter exposure times and with higher linker concentrations display superior functionalization and higher CTC capture efficiency. Conversely, highly cross-linked hydrogel surfaces polymerized under longer exposure times are insensitive to functionalization and display poor capture, regardless of linker concentration. By highlighting the importance of oxygen-inhibited photopolymerization, these findings provide guidelines to design micromolded hydrogels with controlled ligand expression. In addition to enhancing the selective cell capture capacity of immunofunctional hydrogels, the ability to quantifiably design hydrogel interfaces described here will improve the sensitivity of hydrogel biosensors, provide a platform to finely screen cell-matrix interactions, and generally enhance the fidelity of micromolded hydrogel features.

Design and Fabrication of Multichannel PDMS Microfluidic

Microfluidic delivers miniaturized fluidic networks for processing liquids in the microliter range. In the recent years, lab-on-chip (LOC) is become a main tool for point-of-care (POC) diagnostic especially in the medical field. In this paper, we presented a design and fabrication on multi disease analysis using single chip via delivery of fluid with the multiple transducers is the pathway of multi-channel microfluidic based LOC’s. 3 in 1 nano biosensor kit was attached with the microfluidic to produce nano-biolab-on-chip (NBLOC). The multi channels microfluidic chip was designed including the micro channels, one inlet, three outlet and sensor contact area. The microfluidic chip was designed to include multiplex detection for pathogen that consists of multiple channels of simultaneous results. The LOC system was designed using Design Spark Mechanical software and PDMS was used as a medium of the microfluidic. The microfluidic mold and PDMS microfluidic morphological properties have been characterized by using low power microscope (LPM), high power microscope (HPM) and surface profiler. The LOC system physical was experimental by dropping food coloring through the inlet and collecting at the sensor contact area outlet.

Cheap, versatile, and turnkey fabrication of microfluidic master molds using consumer-grade LCD stereolithography 3D printing

The recent development of 3D printers allowed a lot of limitations in the field of microfabrication to be circumvented. The ever-growing chase for smaller dimensions has come to an end in domains such as microfluidics, and the focus now shifted to a cost-efficiency challenge. In this paper, the use of a high-resolution stereolithography LCD 3D printer is investigated for fast and cheap production of microfluidic master molds. More precisely, the UV LED array and the LCD matrix of the printer act as an illuminator and a programmable photomask for soft lithography. The achieved resolution of around 100 μm is mainly limited by the pixel geometry of the LCD matrix. A tree-shape gradient mixer was fabricated using the presented method. It shows very good performances despite the presence of sidewall ripples due to the uneven pixel geometry of the LCD matrix. Any design can be brought from concept to realization in under 2 h. Given its sub-€1000 cost, this method is a very good entry point for labs wishing to explore the potential of microfluidic devices in their experiments, as well as a teaching tool for introducing students to microfluidics.

A profile shaping and surface finishing process of micro electrochemical machining for microstructures on microfluidic chip molds

In the lithography-based fabrication process of microstructures on microfluidic chip molds, the shortcoming of limited substrate materials seriously shortens the service lifetime of the mold. This research focuses on a profile shaping and surface finishing process of micro electrochemical machining (ECM) for microstructures with targeted dimensional and geometrical characteristics on the mold steel S136H. The effects of parameters on the material removal region and single-layer depth are firstly investigated. The sidewall taper of the machined groove increases with the machining voltage and decreases with the scanning velocity. Besides, the bottom surface quality is improved with increasing path spans. Then, the mathematical models of the material removal region and single-layer depth are analyzed, which are regarded as guidance for path planning and parameter optimization. In ECM experiments, a relatively higher machining voltage (20 V) is utilized to rapidly machine basic profiles of the designed flow resistor in the profiles shaping procedure. Sloping sidewalls and rough surfaces are finished by using a lower machining voltage (14 V) and a higher scanning velocity (500 μm s−1) in the surface finishing procedure. As a result, the specially designed flow resistor with a dimensional deviation < 10 μm, and surface roughness Ra < 500 nm is obtained. The proposed ECM process is proved to machine microstructures with high precision and curve profiles on the mold steel.

Plasma generation by household microwave oven for surface modification and other emerging applications

A simple and inexpensive method to generate plasma using a kitchen microwave oven is described in this paper. The microwave-generated plasma is characterized by spectroscopic analysis and compared with the absorption spectra of a gas discharge tube. A Paschen-like curve is observed as the microwave plasma initiation time is plotted as a function of the pressure of the plasma chamber. We have also demonstrated that this microwave-generated air plasma can be used in a multitude of applications such as: (a) surface modification of a substrate to change its wettability; (b) surface modification to change electrical/optical properties of a substrate; and (c) enhancement of adhesive forces for improved bonding of polymeric microfluidic molds, such as bonding polydimethylsiloxane (PDMS) chips to glass covers. These simple techniques of plasma generation and subsequent surface treatment and modification applications may bring new opportunities leading to new innovations not only in advanced labs, but also in undergraduate and even high school research labs.

Aerosol-jet-printed, conformable microfluidic force sensors

SummaryForce sensors that are thin, low-cost, flexible, and compatible with commercial microelectronic chips are of great interest for use in biomedical sensing, precision surgery, and robotics. By leveraging a combination of microfluidics and capacitive sensing, we develop a thin, flexible force sensor that is conformable and robust. The sensor consists of a partially filled microfluidic channel made from a deformable material, with the channel overlaying a series of interdigitated electrodes coated with a thin, insulating polymer layer. When a force is applied to the microfluidic channel reservoir, the fluid is displaced along the channel over the electrodes, thus inducing a capacitance change proportional to the applied force. The microfluidic molds themselves are made of low-cost sacrificial materials deposited via aerosol-jet printing, which is also used to print the electrode layer. We envisage a large range of industrial and biomedical applications for this force sensor.

Evaluation of Lateral and Vertical Dimensions of Micromolds Fabricated by a PolyJet™ Printer.

PolyJet™ 3D printers have been widely used for the fabrication of microfluidic molds to replicate castable resins due to the ease to create microstructures with smooth surfaces. However, the microstructures fabricated by PolyJet printers do not accurately match with those defined by the computer-aided design (CAD) drawing. While the reflow and spreading of the resin before photopolymerization are known to increase the lateral dimension (width) of the printed structures, the influence of resin spreading on the vertical dimension (height) has not been fully investigated. In this work, we characterized the deviations in both lateral and vertical dimensions of the microstructures printed by PolyJet printers. The width of the printed structures was always larger than the designed width due to the spreading of resin. Importantly, the microstructures designed with narrow widths failed to reproduce the intended heights of the structures. Our study revealed that there existed a threshold width (wd′) required to achieve the designed height, and the layer thickness (a parameter set by the printer) influenced the threshold width. The thresholds width to achieve the designed height was found to be 300, 300, and 500 μm for the print layer thicknesses of 16, 28, and 36 μm, respectively. We further developed two general mathematical models for the regions above and below this threshold width. Our models represented the experimental data with an accuracy of more than 96% for the two different regions. We validated our models against the experimental data and the maximum deviation was found to be <4.5%. Our experimental findings and model framework should be useful for the design and fabrication of microstructures using PolyJet printers, which can be replicated to form microfluidic devices.

Self‐sealing thermoplastic fluoroelastomer enables rapid fabrication of modular microreactors

AbstractA novel fluorinated soft thermoplastic elastomer (sTPE) for microfluidics is presented. It allows the rapid fabrication of microfluidic devices through a 30‐second hot embossing cycle at 220°C followed by self‐sealing through simple conformal contact at room temperature, or with baking. The material shows high chemical resistance, particularly in comparison to polydimethylsiloxane (PDMS), to many common organic solvents and can be rapidly micropatterned with high fidelity using a variety of microfluidic master molds thanks to its low mechanical stiffness. Self‐sealing of the material is reversible and withstands pressures of up to 2.8 bar with room temperature sealing and four bar with baking at 185°C for 2 hours. The elastomeric, transparent sTPE exhibits material characteristics that make it suited for use as a microreactor, such as low absorption, surface roughness and oxygen permeability, while also allowing a facile and scalable fabrication process. Modular microfluidic devices, leveraging the fast and reversible room temperature self‐sealing, are demonstrated for the generation of water droplets in a toluene continuous phase using T‐junctions of variable size. The sTPE offers an alternative to common microfluidic materials, overcoming some of their key drawbacks, and giving scope for low‐cost and high‐throughput devices for flow chemistry applications.

A combined 3D printing/CNC micro-milling method to fabricate a large-scale microfluidic device with the small size 3D architectures: an application for tumor spheroid production

The fabrication of a large-scale microfluidic mold with 3D microstructures for manufacturing of the conical microwell chip using a combined projection micro-stereolithography (PµSL) 3D printing/CNC micro-milling method for tumor spheroid formation is presented. The PµSL technique is known as the most promising method of manufacturing microfluidic chips due to the possibility of creating complex three-dimensional microstructures with high resolution in the range of several micrometers. The purpose of applying the proposed method is to investigate the influence of microwell depths on the formation of tumor spheroids. In the conventional methods, the construction of three-dimensional microstructures and multi-height chips is difficult, time-consuming, and is performed using a multi-step lithography process. Microwell depth is an essential parameter for microwell design since it directly affects the shear stress of the fluid flow and the diffusion of nutrients, respiratory gases, and growth factors. In this study, a chip was made with microwells of different depth varying from 100 to 500 µm. The mold of the microwell section is printed by the lab-made PµSL printer with 6 and 1 µm lateral and vertical resolutions. Other parts of the mold, such as the main chamber and micro-channels, were manufactured using the CNC micro-milling method. Finally, different parts of the master mold were assembled and used for PDMS casting. The proposed technique drastically simplifies the fabrication and rapid prototyping of large-scale microfluidic devices with high-resolution microstructures by combining 3D printing with the CNC micro-milling method.

Aerosol-Jet Printed Conformable Microfluidic Force Sensors

Force sensors that are thin, low cost, flexible and compatible with commercial microelectronic chips are of great interest for use in biomedical sensing, precision surgery, robotics and in various “smart control” technologies. By leveraging a novel combination of microfluidics and capacitive sensing we have developed a thin, flexible force sensor that is conformable and robust. The sensor consists of a partially filled microfluidic channel made from a deformable material, with the channel overlaying a series of interdigitated electrodes that are coated with a thin polymer layer for insulation. When a force is applied to a fluid reservoir at the base of the channel, the fluid is displaced along the channel, passing over the electrodes and thus inducing a capacitance change, which is proportional to the amount of force applied. The microfluidic molds themselves are made of low-cost sacrificial material inks deposited via aerosol-jet printing, which is also used to print the electrode layer. The sensors exhibit a nearly linear change in capacitance in response to external load, with a sensitivity of up to 3.8 pF/N and a force range of up to 9 N achieved with the present design. We quantify the effects of changing the electrode morphology, liquid permittivity, sensor thickness and geometry, as well as the thickness of the insulation layer over the electrodes, on sensor range and sensitivity, using a combination of experiments and finite element analysis simulations. We further demonstrate how the sensor can be used to remotely control a robotic clamp. The conformable force sensors developed here are thin, flexible and facilitate easy signal processing. The fabrication processes are low-cost, amenable to fast prototyping and mass manufacturing. We envisage a large range of industrial and biomedical applications for this novel force sensor.

SERS-active linear barcodes by microfluidic-assisted patterning

Simple, low-cost, robust, and scalable fabrication of microscopic linear barcodes with high levels of complexity and multiple authentication layers is critical for emerging applications in information security and anti-counterfeiting. This manuscript presents a novel approach for fabrication of microscopic linear barcodes that can be visualized under Raman microscopy. Microfluidic channels are used as molds to generate linear patterns of end-grafted polymers on a substrate. These patterns serve as templates for area-selective binding of colloidal gold nanoparticles resulting in plasmonic arrays. The deposition of multiple taggant molecules on the plasmonic arrays via a second microfluidic mold results in a linear barcode with unique Raman fingerprints that are enhanced by the underlying plasmonic nanoparticles. The width of the bars is as small as 10 μm, with a total barcode length on the order of 100 μm. The simultaneous use of geometric and chemical security layers provides a high level of complexity challenging the counterfeiting of the barcodes. The additive, scalable, and inexpensive nature of the presented approach can be easily adapted to different colloidal nanomaterials and applications.

A novel abrasive water jet machining technique for rapid fabrication of three-dimensional microfluidic components.

Microfluidic lab-on-a-chip devices are usually fabricated using replica molding, with poly(dimethylsiloxane) (PDMS) casting on a mold. Most common techniques used to fabricate microfluidic molds, such as photolithography and soft lithography, require costly facilities such as a cleanroom, and complicated steps, especially for the fabrication of three-dimensional (3D) features. For example, an often-desired 3D microchannel feature consists of intersecting channels with depth variations. This type of 3D flow focusing geometry has applications in flow cytometry and droplet generation. Various manufacturing techniques have recently been developed for the rapid fabrication of such 3D microfluidic features. In this paper, we describe a new method of mold fabrication that utilizes water jet cutting technology to fabricate free-standing structures on mild steel sheets to make a mold for PDMS casting. As a proof-of-concept, we use this fabrication technique to make a PDMS chip that has a 3D flow focusing junction, an inlet for the sample fluid, two inlets for the sheath fluid, and an outlet. The flow focusing junction is patterned into the PDMS slab with an abrupt, nearly stepwise change to the depth of the microchannel junction. We use confocal microscopy to visualize the 3D flow focusing of a sample flow using this geometry, and we also use the same geometry to generate water-in-oil droplets. This alternative approach to create microfluidic molds is versatile and may find utility in reducing the cost and complexity involved in fabricating 3D features in microfluidic devices.

Abrasive water and slurry jet micro-machining techniques for fabrication of molds containing raised free-standing micro-features

The demand for metallic micro-molds that can be used for inexpensive mass production of polymeric microfluidic chips is increasing. Existing manufacturing techniques such as soft-lithography and photolithography can require multiple time-consuming steps, especially when the aim is to create three-dimensional features. In this study, the feasibility of using abrasive water jet machining (AWJM) and abrasive slurry jet machining (ASJM) to fabricate such micro-molds in Al6061-T6 and SS316 was studied. Jet raster scans under various combinations of process parameters were used in order to machine micro-pockets containing free-standing structures, representing molds for casting microfluidic chips with channel networks. As expected, for both materials and using both ASJM and AWJM, the pocket roughness decreased as the distance between adjacent raster scans (step size) decreased, but the lowest waviness occurred at an intermediate step size. The best quality pockets were achieved on SS316 using ASJM with the intermediate step size and the highest possible slurry mass flow rate. Unmasked machining could not be used to fabricate molds with sharp-edged intersecting features, and a novel hybrid AWJM/ASJM masked machining technique was thus introduced. An undercut and an undesirable erosion near the edges of the mask formed if the position of the last raster scan closest to the mask was not carefully controlled. Possible reasons for these phenomena were discussed in terms of the likelihood of jet deflection off the machining kerf and mask, and the resulting erosion due to secondary slurry flow. By careful selection of the process parameters, it was demonstrated that high quality molds with both single and intersecting free-standing structures at multiple heights could be fabricated, thus making three-dimensional microfluidic chip mold fabrication feasible.