Flow Lithography Research Articles

Flexible liquid-diode microtubes from multimodal microfluidics

Directional transport of liquids is of great importance in energy saving, chemical/biomedical engineering, and microfluidics applications. Despite considerable progress in engineering different open surfaces to achieve liquid manipulation, the realization of diode-like liquid transport in enclosed spaces is still challenging. Here, a flexible diode microtube is presented for directional liquid transport within confined spaces using pulsed microfluidics. The microtubes exhibit sophisticated microstructures on the inner wall, replicated from a precisely controlled flow configuration in the microfluidic channel. Under the effect of asymmetric pinning and unbalanced Laplace pressure, such microtubes enable directional liquid transport in closed channels. More importantly, by integrating in situ flow lithography with the microfluidic system, segmented liquid diodes are fabricated as assembly units for the construction of fluidic-electronic circuits that perform logic operations. These results demonstrate the capacity of the present liquid-diode microtubes for flexible, directional, and programmable liquid transport. We believe that it can open an avenue for designing advanced fluidic circuit-based devices toward versatile practical applications.

Label-free multiplex detection of miRNA-assayed hydrogel barcode using a low-aspect-ratio micropore sensor

We report a rapid and label-free multiplex sensing of target miRNAs using hydrogel barcodes and a low-aspect-ratio micropore device. A 6-bit hydrogel barcode representing codes 0 and 1 was fabricated using flow lithography and functionalized with probe DNA. Subsequently, the probe DNA was assayed with the target miRNA at a specific concentration. The barcode particles in the solution were translocated through a low-aspect-ratio micropore, and the current signal between the cis and trans electrodes was measured. Therefore, the micropore sensor successfully detected the encoded data of the 6-bit hydrogel barcodes. We analyzed signals, such as overshoot, peak drop, and translocation time, to determine the appropriate parameter and subsequently the target miRNA concentration. We observed that the magnitude of the overshoot signal was significantly associated with the concentration of the target miRNA, matched with finite element method (FEM) analysis results. The micropore sensor individually detected the four types of miRNAs in conjunction with the encoded data and achieved a detection limit of 10 pM. In addition, this micropore sensor enabled multiplex sensing of four types of encoded barcodes assayed with different miRNAs in each sample at once.

Lithographic Microneedle-Motors from Multimodal Microfluidics for Cargo Delivery.

Micromotors have led to an unprecedented revolution in the field of cargo delivery. Attempts in this area trend toward enriching their structures and improving their functions to promote their further applications. Herein, novel microneedle-motors (MNMs) for active drug delivery through a flexible multimodal microfluidic lithographic approach are presented. The multimodal microfluidics is composed of a co-flow geometry-derived droplet fluid and an active cargo mixed laminar flow in a triangular microchannel. The MNMs with sharp tips and spherical fuel-loading cavities are obtained continuously from microfluidics with the assistance of flow lithography. The structural parameters of the MNMs could be precisely tailored by simply choosing the flow speed or the shape of the photomask. As the actives are mixed into the phase solution during the generation, the resultant MNMs are loaded with cargoes for direct applications without any extra complex operation. Based on these features, it is demonstrated that with sharp tips and autonomous movement, the MNMs can efficiently penetrate the tissue-like substrates, indicating the potential in overcoming physiological barriers for cargo release. These results indicate that the proposed multimodal microfluidic lithographic MNMs are valuable for practical active cargo delivery in biomedical and other relative areas.

Microfluidic Synthesis of Hydrogel Microparticles with Superparamagnetic Colloids Embedded at Prescribed Positions for Anticounterfeiting Applications (Adv. Mater. Interfaces 29/2022)

Hydrogel Microparticles In article number 2200899, Mengmeng Zhang, Hüseyin Burak Era, and co-workers built a microfluidic platform for continuous synthesis of hydrogel microparticles with superparamagnetic colloids trapped at prescribed positions through stop–flow lithography. The spatial positions of trapped colloids collectively function as a binary code matrix for product quality control, tracking, and anti-counterfeiting.

Microfluidic Synthesis of Hydrogel Microparticles with Superparamagnetic Colloids Embedded at Prescribed Positions for Anticounterfeiting Applications

AbstractA microfluidic platform for continuous synthesis of hydrogel microparticles with superparamagnetic colloids (SPCs) embedded at prescribed positions is described. The shape of the cross‐linked microparticle is independently controlled by stop–flow lithography, whereas the position of trapped SPCs are dictated by virtual magnetic moulds made of 2D nickel patches facilitating magnetic trapping. The spatial positions of trapped SPCs collectively function as a binary code matrix for product authentication. Analytical and finite element methods are combined to optimize the trapping efficiency of SPCs by systematically investigating magnetic field microgradients produced by nickel patches. It is envisioned that the proposed magnetic microparticles will contribute to the development of soft matter inspired product quality control, tracking and anti‐counterfeiting technologies.

Fabrication, Flow Assembly, and Permeation of Microscopic Any-Shape Particles.

Today, millimeter-sized nonspherical any-shape particles serve as flexible, functional scaffold material in chemical and biochemical reactors tailoring their hydrodynamic properties and active surface-to-volume ratio based on the particle's shape. Decreasing the particle size to smaller than 100μm would be desired as it increases the surface-to-volume ratio and promotes a particle assembly based on surface interactions, allowing the creation of tailored self-assembling 3D scaffolds. This study demonstrates a continuous high-throughput fabrication of microscopic 3D particles with complex shape and sub-micron resolution using continuous two-photon vertical flow lithography. Evolving from there, in-channel particle fabrication into a confined microfluidic chamber with a resting fluid enables the precise fabrication of a defined number of particles. 3D assemblies with various particle shapes are fabricated and analyzed regarding their permeability and morphology, representing convective accessibility of the assembly's porosity. Differently shaped particles highlight the importance of contact area regarding particle-particle interactions and the respective hydraulic resistance of an assembly. Finally, cell culture experiments show manifold cell-particle interactions promising applicability as bio-hybrid tissue. This study pushes the research boundaries of adaptive, responsive, and permeable 3D scaffolds and granular media by demonstrating a high throughput fabrication solution and a precise hydrodynamic analysis method for micro-particleassemblies.

Microfluidic flow assembly system with magnetic clamp for unlimited geometry in millimetric hydrogel film patterning

Polymeric sheets with geometrically selective patterns and functions can be used for many labeling and sensing applications. Conventional fabrication techniques such as templated-based printing require cumbersome, multiple and intensive steps, pose difficulty towards automation, and thus are mostly applied for simple array patterned sheets. Alternative microfluidics-based methods, such as flow lithography is limited to creating parallel patterns. We develop a microfluidic assembly fabrication based on flow lithography to make millimetric polymeric sheets with geometrically complex and selective patterns. This new fabrication is empowered by integrating a magnetically actuated clamp in the microfluidic channel. The integration of this magnetic clamp in combination of flow lithography allows rapid flow-assisted exchanges of multiple compositions during sequence patterning. Resolution is limited only by the quality of the UV photolithography source and the microscope optics. Using our method, we demonstrate the synthesis of enclosed geometrically complex hydrogels film with the inclusion of choice chemical functionalizations for pH and temperature sensing, fluorescent markers and for biomolecule sensing, fluorescence and enzymatic activity.

Flow lithography for structured microparticles: fundamentals, methods and applications.

Structured microparticles, with unique shapes, customizable sizes, multiple materials, and spatially-defined chemistries, are leading the way for emerging 'lab on a particle' technologies. These microparticles with engineered designs find applications in multiplexed diagnostics, drug delivery, single-cell secretion assays, single-molecule detection assays, high throughput cytometry, micro-robotics, self-assembly, and tissue engineering. In this article we review state-of-the-art particle manufacturing technologies based on flow-assisted photolithography performed inside microfluidic channels. Important physicochemical concepts are discussed to provide a basis for understanding the fabrication technologies. These photolithography technologies are compared based on the structural as well as compositional complexity of the fabricated particles. Particles are categorized, from 1D to 3D particles, based on the number of dimensions that can be independently controlled during the fabrication process. After discussing the advantages of the individual techniques, important applications of the fabricated particles are reviewed. Lastly, a future perspective is provided with potential directions to improve the throughput of particle fabrication, realize new particle shapes, measure particles in an automated manner, and adopt the 'lab on a particle' technologies to other areas of research.

Preparation of anisotropic multiscale micro-hydrogels via two-photon continuous flow lithography

HypothesisPolymeric anisotropic soft microparticles show interesting behavior in biological environments and hold promise for drug delivery and biomedical applications. However, self-assembly and substrate-based lithographic techniques are limited by low resolution, batch operation or specific particle geometry and deformability. Two-photon polymerization in microfluidic channels may offer the required resolution to continuously fabricate anisotropic micro-hydrogels in sub-10 µm size-range. ExperimentsHere, a pulsed laser source is used to perform two-photon polymerization under microfluidic flow of a poly(ethylene glycol) diacrylate (PEGDA) solution with the objective of realizing anisotropic micro-hydrogels carrying payloads of various nature, including small molecules and nanoparticles. The fabrication process is described via a reactive-convective-diffusion system of equations, whose solution under proper auxiliary conditions is used to corroborate the experimental observations and sample the configuration space. FindingsBy tuning the flow velocity, exposure time and pre-polymer composition, anisotropic PEGDA micro-hydrogels are obtained in the 1–10 μm size-range and exhibit an aspect ratio varying from 1 to 5. Furthermore, 200 nm curcumin-loaded poly(lactic-co-glycolic acid) (PLGA) nanoparticles and 100 nm ssRNA-encapsulating lipid nanoparticles were entrapped within square PEGDA micro-hydrogels. The proposed approach could support the fabrication of micro-hydrogels of well-defined morphology, stiffness, and surface properties for the sustained release of therapeutic agents.

Fabrication of ceramic particles from preceramic polymers using stop flow lithography

Stop flow lithography (SFL) combines aspects of microfluidic and photolithography to continuously fabricate particles with uniform planar shapes as dictated by a mask. In this work we aim to expand the palette of materials suitable for SFL processing by investigating the use of UV-crosslinkable preceramic polymers to make ceramic particles. A commercially available methacrylated-polysiloxane was used as the preceramic polymer and was mixed with 2.5 wt% Irgacure 651 photoinitiator. A simple SFL system was assembled to continuously fabricate UV-crosslinked preceramic polymer particles in the shape of hexagons, triangles, and gears with diameters ranging from 100 to 200 μm and thicknesses of 74 μm +/- 4 μm. Particles were harvested from the excess preceramic solution, cleaned and then pyrolyzed at 1000 °C to transform them into silicon oxycarbide ceramic particles. Particle shape was maintained during pyrolysis despite a ∼80 % linear shrinkage due to the removal of acryl and methyl side groups, as confirmed via FTIR. After pyrolysis the outer diameters of the SiOC particles ranged from 20 to 40 μm with thicknesses of 10 μm–12 μm. Pyrolyzed particles were successfully recovered and dispersed in water. This work demonstrates a robust path for the fabrication of ceramic particles with specific shapes from preceramic polymers via SFL.

Scanning two-photon continuous flow lithography for the fabrication of multi-functional microparticles.

In this work, we demonstrate the high-throughput fabrication of 3D microparticles using a scanning two-photon continuous flow lithography (STP-CFL) technique in which microparticles are shaped by scanning the laser beam at the interface of laminar co-flows. The results demonstrate the ability of STP-CFL to manufacture high-resolution complex geometries of cell carriers that possess distinct regions with different functionalities. A new approach is presented for printing out-of-plane features on the microparticles. The approach eliminates the use of axial scanning stages, which are not favorable since they induce fluctuations in the flowing polymer media and their scanning speed is slower than the speed of galvanometer mirror scanners.

Rapid fabrication of sieved microwells and cross-flow microparticle trapping

The use of microwells is popular for a wide range of applications due to its’ simplicity. However, the seeding of conventional microwells, which are closed at the bottom, is restricted to gravitational sedimentation for cell or particle deposition and therefore require lengthy settling times to maximize well occupancy. The addition of microfluidics to the capture process has accelerated cell or particle dispersion and improved capture ability but is mostly limited to gravitationally-driven settling for capture into the wells. An alternative approach to conventional closed-microwells, sieved microwells supersedes reliance on gravity by using hydrodynamic forces through the open pores at the bottom of the microwells to draw targets into the wells. We have developed a rapid fabrication method, based on flow lithography techniques, which allows us to easily customize the mesh pore sizes in a simple two-step process. Finally, by combining this microwell design with cross-flow trapping in a microfluidic two-layered channel, we achieve an 88 ± 6% well occupancy in under 10 s.

Universal motion of mirror-symmetric microparticles in confined Stokes flow

Comprehensive understanding of particle motion in microfluidic devices is essential to unlock additional technologies for shape-based separation and sorting of microparticles like microplastics, cells, and crystal polymorphs. Such particles interact hydrodynamically with confining surfaces, thus altering their trajectories. These hydrodynamic interactions are shape dependent and can be tuned to guide a particle along a specific path. We produce strongly confined particles with various shapes in a shallow microfluidic channel via stop flow lithography. Regardless of their exact shape, particles with a single mirror plane have identical modes of motion: in-plane rotation and cross-stream translation along a bell-shaped path. Each mode has a characteristic time, determined by particle geometry. Furthermore, each particle trajectory can be scaled by its respective characteristic times onto two master curves. We propose minimalistic relations linking these timescales to particle shape. Together these master curves yield a trajectory universal to particles with a single mirror plane.

Multi-Color Luminescence Transition of Upconversion Nanocrystals via Crystal Phase Control with SiO2 for High Temperature Thermal Labels.

Upconversion nanocrystals (UCNs)‐embedded microarchitectures with luminescence color transition capability and enhanced luminescence intensity under extreme conditions are suitable for developing a robust labeling system in a high‐temperature thermal industrial process. However, most UCNs based labeling systems are limited by the loss of luminescence owing to the destruction of the crystalline phase or by a predetermined luminescence color without color transition capability. Herein, an unusual crystal phase transition of UCNs to a hexagonal apatite phase in the presence of SiO2 nanoparticles is reported with the enhancements of 130‐fold green luminescence and 52‐fold luminance as compared to that of the SiO2‐free counterpart. By rationally combining this strategy with an additive color mixing method using a mask‐less flow lithography technique, single to multiple luminescence color transition, scalable labeling systems with hidden letters‐, and multi‐luminescence colored microparticles are demonstrated for a UCNs luminescence color change‐based high temperature labeling system.

Complex microparticle architectures from stimuli-responsive intrinsically disordered proteins

The controllable production of microparticles with complex geometries is useful for a variety of applications in materials science and bioengineering. The formation of intricate microarchitectures typically requires sophisticated fabrication techniques such as flow lithography or multiple-emulsion microfluidics. By harnessing the molecular interactions of a set of artificial intrinsically disordered proteins (IDPs), we have created complex microparticle geometries, including porous particles, core-shell and hollow shell structures, and a unique ‘fruits-on-a-vine’ arrangement, by exploiting the metastable region of the phase diagram of thermally responsive IDPs within microdroplets. Through multi-site unnatural amino acid (UAA) incorporation, these protein microparticles can also be photo-crosslinked and stably extracted to an all-aqueous environment. This work expands the functional utility of artificial IDPs as well as the available microarchitectures of this class of biocompatible IDPs, with potential applications in drug delivery and tissue engineering.

Direct Conjugation of Streptavidin to Encoded Hydrogel Microparticles for Multiplex Biomolecule Detection with Rapid Probe-Set Modification.

Encoded hydrogel microparticles synthesized via flow lithography have drawn attention for multiplex biomarker detection due to their high multiplex capability and solution-like hybridization kinetics. However, the current methods for preparing particles cannot achieve a flexible, rapid probe-set modification, which is necessary for the production of various combinations of target panels in clinical diagnosis. In order to accomplish the unmet needs, streptavidin was incorporated into the encoded hydrogel microparticles to take advantage of the rapid streptavidin–biotin interactions that can be used in probe-set modification. However, the existing methods suffer from low efficiency of streptavidin conjugation, cause undesirable deformation of particles, and impair the assay capability. Here, we present a simple and powerful method to conjugate streptavidin to the encoded hydrogel microparticles for better assay performance and rapid probe-set modification. Streptavidin was directly conjugated to the encoded hydrogel microparticles using the aza-Michael addition click reaction, which can proceed in mild, aqueous condition without catalysts. A highly flexible and sensitive assay was developed to quantify DNA and proteins using streptavidin-conjugated encoded hydrogel microparticles. We also validated the potential applications of our particles conducting multiplex detection of cancer-related miRNAs.

Elimination of Unreacted Acrylate Double Bonds in the Polymer Networks of Microparticles Synthesized via Flow Lithography.

Flow lithography (FL), a versatile technique used to synthesize anisotropic multifunctional microparticles, has attracted substantial interest, given that the resulting particles with complex geometries and multilayered biochemical functionalities can be used in a wide variety of applications. However, after this process, there are double bonds remaining from the cross-linkable groups of monomers. The unreacted cross-linkable groups can affect the particles' biochemical properties. Here, we verify that the microparticles produced by FL contain a significant number of unreacted acrylate double bonds (UADBs), which could cause irreversible biochemical changes in the particle and pernicious effects to biological systems. We also confirm that the particles contain a considerable number of UADBs, regardless of the various synthetic (lithographic) conditions that can be used in a typical FL process. We present an effective way to eliminate a substantial amount of UADBs after synthesis by linking biochemically inert poly(ethylene glycol) based on click chemistry. We verify that eliminating UADBs by using this click chemistry approach can efficiently resolve problems, such as the occurrence of random reactions and the cytotoxicity of UADBs.

Affinity-Enhanced CTC-Capturing Hydrogel Microparticles Fabricated by Degassed Mold Lithography.

Technologies for the detection and isolation of circulating tumor cells (CTCs) are essential in liquid biopsy, a minimally invasive technique for early diagnosis and medical intervention in cancer patients. A promising method for CTC capture, using an affinity-based approach, is the use of functionalized hydrogel microparticles (MP), which have the advantages of water-like reactivity, biologically compatible materials, and synergy with various analysis platforms. In this paper, we demonstrate the feasibility of CTC capture by hydrogel particles synthesized using a novel method called degassed mold lithography (DML). This technique increases the porosity and functionality of the MPs for effective conjugation with antibodies. Qualitative fluorescence analysis demonstrates that DML produces superior uniformity, integrity, and functionality of the MPs, as compared to conventional stop flow lithography (SFL). Analysis of the fluorescence intensity from porosity-controlled MPs by each reaction step of antibody conjugation elucidates that more antibodies are loaded when the particles are more porous. The feasibility of selective cell capture is demonstrated using breast cancer cell lines. In conclusion, using DML for the synthesis of porous MPs offers a powerful method for improving the cell affinity of the antibody-conjugated MPs.

Generation of 3D Microparticles in Microchannels with Non-rectangular Cross Sections

Flow lithography in a conventional rectangular microchannel is limited to fabrication of particles for which only the shapes of top perimeters are controlled. We present a flow lithography technique for fabrication of microparticles of diverse 3D shapes and multiple layers using non-rectangular microchannels with designed cross sections that allow the creation of complex shapes and diverse cross-sectional shapes. Variations in cross-sectional shape allow high-throughput, on-demand production of microparticles in unconventional shapes such as tetrahedrons and pyramids. Multilayered 3D particles were generated in an enlarging triangular channel combined with on-chip PDMS valves, which allow particle alignment and fluid exchange. These 3D microparticles are expected to further expand the wide variety of applications of microparticles, especially in drug delivery and tissue engineering fields.

Intelligent Detection Platform for Simultaneous Detection of Multiple MiRNAs Based on Smartphone.

As a marker of malignant tumors, miRNA is closely related to the occurrence and metastasis of tumors. How to achieve rapid and sensitive real-time detection is important for clinical prevention and treatment of cancer. In this study, an intelligent detection platform based on smartphone image processing technology made point-of-care testing a reality. This new smart approach could detect multiple targets simultaneously and sensitively. Hydrogel microparticles of different coding modes (shapes, numbers) were prepared by flow lithography to detect different miRNAs. After sandwich immunoassays, different shapes of hydrogels showed different fluorescence intensities depending on their targets. Images were captured by a smartphone and then analyzed by image recognition processing software installed on the smartphone. The concentration of miRNA was obtained within 10 s. The entire reaction process did not exceed 2 h. This intelligent and portable detection platform for miRNAs was reliable and the limit of detection reached the femtomole level. This work provided a demonstration of intelligent, portable, real-time detection of tumor markers.