Circular Microfluidic Channels Research Articles

Compact single fiber optical tweezer–micropipette system for completely noninvasive cell sorting

Bridging optical tweezers and microfluidics can form a multifunctional platform, which can overcome the difficulties of precise manipulation in hydrodynamic flow in a noninvasive method. However, when integrated into a microfluidic chip, the fiber optic tweezer loses its flexibility. Here, we propose a compact single fiber optical tweezer–micropipette system. It can sort particles by differences in shape and refractive index in a completely noninvasive way while retaining the flexibility, high selectivity, and precision of a fiber optical tweezer. Compact microfluidic channels are formed by combining two different-diameter micropipettes. The internal diameter of the circular microfluidic channel is less than 30 μm. Furthermore, we calculated the trapping and pushing regions of Yeast and Chlorella and achieved the separation of Yeast from Chlorella in the experiments. We did 90 sets of tests on the sorting accuracy of single fiber optical tweezer (SFOT). SFOT was able to distinguish between two types of particles in each test. With the advantages of high selectivity, high accuracy, low optical power consumption, and compact structure, such methods can be used in the fields of optical separation, cell transportation, cell sorting, and single-cell analysis.

Capturing individual charged microparticles and cells with microelectrodes in a circular microfluidic channel

Capture and, subsequently, in situ characterization of microparticles generally require complex fabrication processes and sophisticated techniques in microfluidic devices. In this paper, we present a simple and inexpensive technique for capturing and applying chemical agents to individual charged microparticles with the same glass microelectrode in a circular microfluidic channel. The microfluidic device features a thin poly-dimethyl-siloxane (PDMS) layer (100 µm, thickness) above the circular channel (diameter, 50 µm) so that the tip of the glass microelectrode can enter into the channel by penetrating through the thin PDMS layer. When an electric field is applied across the tip of the microelectrodes, the tip can capture charged microparticles or living cells that are flowing through the channel. Meanwhile, the same glass microelectrode can be used to apply chemical agents to the captured particles or living cells by filling the solution of the chemical agent into its tip. This simple design method offered simple fabrication without the need for any specialized equipment for the manipulation of charged particles or living cells in controlled microenvironments.

JMEMS Letters Fabrication of Self-Sealed Circular Microfluidic Channels in Glass by Thermal Blowing Method

We report fabrication of circular glass microchannels with radius less than 100 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu \text{m}$ </tex-math></inline-formula> using the glass blowing technique. Silicon trenches filled with nitrogen are used for microchannel fabrication. We report an interesting reflow regime where the flowing glass fills the silicon trenches to form a completely circular microchannel. Circular microchannels have symmetric velocity profile and do not suffer from flow stagnation problems observed in rectangular channels. Besides circular shape, fabrication conditions of the semi-circular and elliptically shaped channels are also discussed. A mathematical model explaining the observation is also presented and is compared with the experimental results. [2021-0183]

Toward Vasculature in Skeletal Muscle-on-a-Chip through Thermo-Responsive Sacrificial Templates

Developing new approaches for vascularizing synthetic tissue systems will have a tremendous impact in diverse areas. One area where this is particularly important is developing new skeletal muscle tissue systems, which could be utilized in physiological model studies and tissue regeneration. To develop vascularized approaches a microfluidic on-chip design for creating channels in polymer systems can be pursued. Current microfluidic tissue engineering methods include soft lithography, rapid prototyping, and cell printing; however, these have limitations such as having their scaffolding being inorganic, less desirable planar vasculature geometry, low fabrication efficiency, and limited resolution. Here we successfully developed a circular microfluidic channel embedded in a 3D extracellular matrix scaffolding with 3D myogenesis. We used a thermo-responsive polymer approach with micromilling-molding and designed a mixture of polyester wax and paraffin wax to fabricate the sacrificial template for microfluidic channel generation in the scaffolding. These findings will impact a number of fields including biomaterials, biomimetic structures, and personalized medicine in the future.

Orthogonal co-cultivation of smooth muscle cell and endothelial cell layers to construct in vivo-like vasculature.

Development of a three-dimensional (3D) vascular co-cultivation system is one of the major challenges to provide an advanced analytical platform for studying blood vessel related diseases. To date, however, the in vivo-like vessel system has not been fully realized due to the difficulty of co-cultivation of the cells with orthogonal alignment. In this study, we report the utilization of microfabrication technology to construct biomimetic 3D co-cultured vasculature. First, microwrinkle patterns whose direction was perpendicular to the axis of a circular microfluidic channel were fabricated, and vascular smooth muscle cells (VSMCs) were cultured inside the microchannel, leading to an in vivo-like circumferential VSMC layer. Then, human umbilical vein endothelial cells (HUVECs) were co-cultured on the circumferentially aligned VSMC, and the success of double layer formation of HUVEC-VSMC in the circular microchannel could be monitored. After HUVEC cultivation, we applied shear flow in order to induce the orientation of HUVEC parallel to the axis, and the analysis of orientation angle and spreading area of HUVECs indicated that they were changed by shear stress to be aligned to the direction of flow. Thus, the HUVEC and VSMC layer could be aligned with a distinct direction. The expression level of VE-Cadherin located at the boundary of HUVECs implies in vivo-like vascular behavior. The proposed in vitro microfluidic vascular assay platform would be valuable for studying vascular diseases with high reliability due to in vivo-likeness.

Fabrication of circular microfluidic channels through grayscale dual-projection lithography

Circular microfluidic channels are in great demand since they are more realistic in mimicking physiological flow systems, generating axis-symmetrical flow, and achieving uniform shear stress. A typical microchannel with rectangular cross section can induce non-physiological gradients of shear rate, pressure, and velocity. This paper presents a novel method of fabricating microfluidic channels with circular and elliptical cross sections through grayscale dual-projection lithography. Our method utilizes two projecting systems to expose grayscale image face-to-face and simultaneously polymerize the photocurable material. The cross-sectional profiles of the fabricated microchannels are consistent with mathematical predictions and, therefore, demonstrate the capability of controlling the channel shapes precisely. Customized circular microchannels can be generated with complex features such as junctions, bifurcations, hierarchies, and gradually changed diameters. This method is capable of fabricating circular channels with a wide range of diameters (39 μm–2 mm) as well as elliptical channels with a major-to-minor axis ratio up to 600%. Microfluidic devices with circular cross sections suitable for particle analysis were made as a demonstrative application in nanoparticle binding and distribution within a mimetic blood vessel. A ready-to-use microfluidic device with customized circular channels can be fabricated within 1 h without the need of clean room or expensive photolithography devices.

Circumferential alignment of vascular smooth muscle cells in a circular microfluidic channel

The circumferential alignment of human aortic smooth muscle cells (HASMCs) in an orthogonally micropatterned circular microfluidic channel is reported to form an in vivo-like smooth muscle cell layer. To construct a biomimetic smooth muscle cell layer which is aligned perpendicular to the axis of blood vessel, a half-circular polydimethylsiloxane (PDMS) microchannel is first fabricated by soft lithography using a convex PDMS mold. Then, the orthogonally microwrinkle patterns are generated inside the half-circular microchannel by a strain responsive wrinkling method. During the UV treatment on a PDMS substrate with uniaxial 40% stretch and a subsequent strain releasing step, the microwrinkle patterns perpendicular to the axial direction of the circular microchannel are generated, which can guide the circumferential alignment of HASMCs during cultivation. The analysis of orientation angle, shape index, and contractile protein marker expression indicates that the cultured HASMCs reveal the in vivo-like cell phenotype. Finally, a fully circular microchannel is produced by bonding two half-circular microchannels, and the HASMCs are cultured circumferentially inside the channels with high alignment and viability for 5 days. These results demonstrated the creation of an in vivo-like 3D smooth muscle cell layer in the circular microfluidic channel which can provide a bioassay platforms for in-depth study of HASMC biology and vascular function.

Circular Beam-Steering Reconfigurable Antenna With Liquid Metal Parasitics

A novel antenna reconfiguration mechanism based on the displacement of liquid metal sections is presented. The liquid nature of the moving parts of the antenna helps avoid the main disadvantage of mechanically-actuated reconfigurable antennas which is the mechanical failure of their solid parts due to material fatigue, creep or wear. Furthermore, the displacement of liquid elements can be more effectively performed than in the case of solid materials by applying precise microfluidic techniques such as continuous-flow pumping or electrowetting. The reconfiguration mechanism is demonstrated through the design, fabrication and measurement of a radiation pattern reconfigurable antenna. This antenna operates at 1800 MHz with 4.0% bandwidth and is capable of performing beam-steering over a 360° range with fine tuning. The antenna is a novel circular Yagi-Uda array, where the movable parasitic director and reflector elements are implemented by liquid metal mercury (Hg). The parasitics are placed and rotated in a circular microfluidic channel around the driven element by means of a flow generated and controlled by a piezoelectric micropump. The measured results demonstrate good performance and the applicability of the microfluidic system.

Fabrication of circular microfluidic channels by combining mechanical micromilling and soft lithography

The fabrication of microfluidic channels with complex three-dimensional (3D) geometries presents a major challenge to the field of microfluidics, because conventional lithography methods are mainly limited to rectangular cross-sections. In this paper, we demonstrate the use of mechanical micromachining to fabricate microfluidic channels with complex cross-sectional geometries. Micro-scale milling tools are first used to fabricate semi-circular patterns on planar metallic surfaces to create a master mold. The micromilled pattern is then transferred to polydimethylsiloxane (PDMS) through a two-step reverse molding process. Using these semi-circular PDMS channels, circular cross-sectioned microchannels are created by aligning and adhering two channels face-to-face. Straight and serpentine-shaped microchannels were fabricated, and the channel geometry and precision of the metallic master and PDMS molds were assessed through scanning electron microscopy and non-contact profilometry. Channel functionality was tested by perfusion of liquid through the channels. This work demonstrates that micromachining enabled soft lithography is capable of fabricating non-rectangular cross-section channels for microfluidic applications. We believe that this approach will be important for many fields from biomimetics and vascular engineering to microfabrication and microreactor technologies.

A rapid and simple fabrication method for 3-dimensional circular microfluidic channel using metal wire removal process

In this article, we introduce a rapid and simple fabrication method to realize a 3-dimensional (3-D) microfluidic channel with a near-perfect circular cross section. This new concept of fabrication method is defined by metal wire removal process, where the metal wire such as a thin soldering wire for the 3-D circular shape is commercially available. For the microfluidic channel mold, PDMS (polydimethylsiloxane) was poured on several shapes such as 3-D circular, helix, and double helix shapes, of soldering wire and solidified. The soldering wire was then melted out by heating. With the two-step process, rapidly and simply fabricated 3-D circular microfluidic channels can be obtained. CPAE (endothelial cell line) cells were cultured inside the channel to evaluate the biocompatibility of the fabricated microfluidic channel. Our method will be very useful in making various circular shapes of 3-D microfluidic devices that need multi-depth and round corners inside the channel.