Rapid large area fabrication of multiscale through-hole membranes

ODSEI Chip: An Open 3D Microfluidic Platform for Studying Tumor Spheroid-Endothelial Interactions

Current in vitro models of 3D tumor spheroids within the microenvironment have emerged as promising tools for understanding tumor progression and potential drug responses. However, creating spheroids with functional vasculature remains challenging in a controlled and high-throughput manner. Herein, a novel open 3D-microarray platform is presented for a spheroid-endothelium interaction (ODSEI) chip, capable of arraying more than 1000 spheroids on top of the vasculature, compartmentalized for single spheroid-level analysis of drug resistance, and allows for the extraction of specific spheroids for further analysis. As proof of concept, the crosstalk between breast cancer spheroids and vasculature is monitored, validating the roles of endothelial cells in acquired tamoxifen resistance. Cancer spheroids exhibited reduced sensitivity to tamoxifen in the presence of vasculature. Further analysis through single-cell RNA sequencing of extracted spheroids and protein arrays elucidated gene expression profiles and cytokines associated with acquired tamoxifen resistance, particularly involving the TNF-α pathway via NF-κB and mTOR signaling. By targeting the highly expressed cytokines (IL-8, TIMP1) identified, tamoxifen resistance in cancer spheroid can be effectively reversed. In summary, the ODSEI chip allows to study spheroid and endothelial interaction in various contexts, leading to improved insights into tumor biology and therapeutic strategies.

Artificial Intelligence-Assisted Automatic Raman-Activated Cell Sorting (AI-RACS) System for Mining Specific Functional Microorganisms in the Microbiome

The microbiome represents the natural presence of microorganisms, and exploring, understanding, and leveraging its functions will bring about significant breakthroughs in life sciences and applications. Raman-activated cell sorting (RACS) enables the correlation of phenotype and genotype at the single-cell level, offering a solution to the bottleneck in microbial community functional analysis caused by challenges in cultivating diverse microorganisms. However, current labor-intensive manual procedures fall short in catering to the demands of single-cell functional analysis in microbial communities. To address this issue, we developed an artificial intelligence-assisted Raman-activated cell sorting system (AI-RACS) that integrates precise single-cell positioning, automated data collection, optical tweezers capture, and single-cell printing to elevate microbial single-cell RACS from manual to automated, validating the efficacy of the system by isolating aluminum-tolerant microbes from acidic soil microbiota. Leveraging the AI-RACS framework, we sorted 13 strains from red soil samples under near-in situ conditions, with all demonstrating strong aluminum tolerance. AI-RACS efficiently segregates microbial cells from intricate environmental samples, investigating their functional attributes and presenting a novel tool for microbial research and applications.

Direct patterning of liquid materials on flat and curved substrates using flexible molds with through-hole and post arrays

Liquids undergo continuous deformation in the presence of external shear stresses; however, they are pinned between structures owing to their viscosity. Therefore, reshaping the liquids using their intrinsic material properties and structural interfaces is possible. In this study, we used the template-guided forming (TGF) method to reshape and produce oil patterns on flat and curved substrates. To produce oil patterns, we developed two oil patterning methods: direct heating-based oil patterning (DHOP) and solvent evaporation-based oil patterning (SEOP), which were characterized using various oils and solvents. To overcome the limitation of relying solely on liquid patterning that undergoes complete evaporation, we successfully fabricated liquid films using oil and nonpolar organic solvents that exhibit long-term stability. Therefore, achieving durability and control over the film thickness using nonpolar organic solvents has great potential for future applications in microfluidics. Furthermore, we demonstrated that the SEOP method in conjunction with TGF can produce various and unconventional patterns of an organic photoresist (SU-8), which cannot be produced through standard photolithography. Hence, we conclude that the proposed TFG-based oil pattering methods could be highly useful for producing unconventional and unprecedented patterns on flat and curved substrates for various applications, including microelectronics, optics, filtration and separation, biomedical engineering, and nanotechnology.

A flexible strategy to fabricate trumpet-shaped porous PDMS membranes for organ-on-chip application

Porous polydimethylsiloxane (PDMS) membrane is a crucial element in organs-on-chips fabrication, supplying a unique substrate that can be used for the generation of tissue-tissue interfaces, separate co-culture, biomimetic stretch application, etc. However, the existing methods of through-hole PDMS membrane production are largely limited by labor-consuming processes and/or expensive equipment. Here, we propose an accessible and low-cost strategy to fabricate through-hole PDMS membranes with good controllability, which is performed via combining wet-etching and spin-coating processes. The porous membrane is obtained by spin-coating OS-20 diluted PDMS on an etched glass template with a columnar array structure. The pore size and thickness of the PDMS membrane can be adjusted flexibly via optimizing the template structure and spinning speed. In particular, compared to the traditional vertical through-hole structure of porous membranes, the membranes prepared by this method feature a trumpet-shaped structure, which allows for the generation of some unique bionic structures on organs-on-chips. When the trumpet-shape faces upward, the endothelium spreads at the bottom of the porous membrane, and intestinal cells form a villous structure, achieving the same effect as traditional methods. Conversely, when the trumpet-shape faces downward, intestinal cells spontaneously form a crypt-like structure, which is challenging to achieve with other methods. The proposed approach is simple, flexible with good reproducibility, and low-cost, which provides a new way to facilitate the building of multifunctional organ-on-chip systems and accelerate their translational applications.

Straightforward Cell Patterning with Ultra-Low Background Using Polydimethylsiloxane Through-Hole Membranes

Micropatterning is becoming an increasingly popular tool to realize microscale cell positioning and decipher cell activities and functions under specific microenvironments. However, a facile methodology for building a highly precise cell pattern still remains challenging. In this study, A simple and straightforward method for stable and efficient cell patterning with ultra-low background using polydimethylsiloxane through-hole membranes is developed. The patterning process is conveniently on the basis of membrane peeling and routine pipetting. Cell patterning in high quality involving over 97% patterning coincidence and zero residue on the background is achieved. The high repeatability and stability of the established method for multiple types of cell arrangements with different spatial profiles is demonstrated. The customizable cell patterning with ultra-low background and high diversity is confirmed to be quite feasible and reliable. Furthermore, the applicability of the patterning method for investigating the fundamental cell activities is also verified experimentally. The authors believe this microengineering advancement has valuable applications in many microscale cell manipulation-associated research fields including cell biology, cell engineering, cell imaging, and cell sensing.

Fabricating and Laminating Films with Through-Holes and Engraved/Protruding Structures for 3D Micro/Nanofluidic Platforms

Micro/nanofluidic devices have become popular for delicately processing biological, material, and chemical samples. However, their reliance on 2D fabrication schemes has hindered further innovation. Here, a 3D manufacturing method is proposed through the innovation of laminated object manufacturing (LOM), which involves the selection of building materials as well as the development of molding and lamination techniques. Fabrication of interlayer films is demonstrated with both multi-layered micro-/nanostructures and through-holes, using an injection molding approach and establishing strategic principles of film design. Utilization of the multi-layered through-hole films in LOM allows reducing the number of alignments and laminations by at least two times compared to conventional LOM. Using a dual-curing resin for film fabrication, a surface-treatment-free and collapse-free lamination technique is shown for constructing 3D multiscale micro/nanofluidic devices with ultralow aspect ratio nanochannels. The 3D manufacturing method enables the development of a nanochannel-based attoliter droplet generator capable of 3D parallelization for mass production, which implies the remarkable potential to extend numerous existing 2D micro/nanofluidic platforms into a 3D framework.

Hollow Microneedles on a Paper Fabricated by Standard Photolithography for the Screening Test of Prediabetes

Microneedle (MN) is a novel technique of the biomedical engineering field because of its ability to evaluate bioinformation via minimal invasion. One of the urgent requirements for ground-breaking health care monitoring is persistent monitoring. Hollow microneedles are extremely attractive to extract skin interstitial fluid (ISF) for analysis, which makes them perfect for sensing biomarkers and facilitating diagnosis. Nevertheless, its intricate fabrication process has hampered its extensive application. The present research demonstrates an easy one-step preparation approach for hollow MNs on the foundation of the refraction index variations of polyethylene glycol diacrylate (PEGDA) in the process of photopolymerization. The fabricated hollow microneedle exhibited ideal mechanical characteristics to penetrate the skin. Hydrodynamic simulations showed that the liquid was risen in a hollow microneedle by capillary force. Furthermore, a paper-based glucose sensor was integrated with the hollow microneedle. We also observed that the MN array smoothly extracted ISF in vitro and in vivo by capillary action. The outcomes displayed the applicability of the MN patch to persistent blood glucose (GLU) monitoring, diagnosis-related tests for patients and pre-diabetic individuals.

Performance characterization of transparent and conductive grids one-step-printed on curved substrates using template-guided foaming

Next-generation electronic devices require electrically conductive, mechanically flexible, and optically transparent conducting electrodes (CEs) that can endure large deformations.

Large-scale investigation of single cell activities and response dynamics in a microarray chip with a microfluidics-fabricated microporous membrane

Microengineering technology involving microfabrication, micropatterning and microfluidics enables promising advances in single cell manipulation and analysis. Herein, we describe a parallel, large-scale, and temporal investigation of diverse single cell activities and response dynamics using a facile-assembled microwell array chip with a microfluidics-molded microporous membrane. We demonstrated that the versatility with respect to geometrical homogeneity and diversity of microporous membrane fabrication, as well as the stability, repeatability, and reproducibility rely on the well-improved molding. Serial and practical operations including controllable single cell trapping, array-like culture or chemical stimulation, and temporal monitoring can be smoothly completed in the chip. We confirmed that the microwell array chip allowed an efficient construction of a single cell array. Using the cell array, on-chip detection of single cell behaviours under various culture and drug therapy conditions to explore phenotypic heterogeneity was achieved in massive and dynamic manners. These achievements provide a facile and reliable methodology for fabricating microporous membranes with precise control and for developing universal microplatforms to perform robust manipulation and versatile analysis of single cells. This work also offers an insight into the development of easy to fabricate/use and market-oriented microsystems for single cell research, pharmaceutical development, and high-throughput screening.

Direct Single-Step Printing of Conductive Grids on Curved Surfaces Using Template-Guided Foaming

Advanced transparent conductors have been studied intensively in the aspects of materials, structures, and printing methods. The material and structural advancements have been successfully accomplished with various conductive nanomaterials and spring-like structures for better electrical conductivity and high mechanical flexibility of the transparent conductors. However, the capability to print submicrometer conductive patterns directly and conformally on curved surfaces with low processing cost and high throughput remains a technological challenge to achieve, primarily because of the original two-dimensional (2D) nature of conventional lithography processes. In our study, we exploit a liquid-mediated patterning approach in the development of flexible templates, enabling printing of curvilinear silver grids in a single-step and strain-free manner at a submicrometer resolution within several minutes with minimum loss of noble metals. The template can guide arrays of receding liquid-air interfaces on curved substrates during liquid evaporation, thereby generating ordered 2D foam structures that can confine and assemble silver nanoparticles in grid patterns. The printed silver grids exhibit suitable optical, electrical, and Joule-heating performances, enabling their application in transparent heaters. Our technique has the potential to extend the existing 2D micro/nanofluidic liquid-mediated patterning approach to three-dimensional (3D) control of liquid-air interfaces for low-cost all-liquid-processed functional 3D optoelectronics in the future.

Advances in Micro/Nanoporous Membranes for Biomedical Engineering

Porous membrane materials at the micro/nanoscale have exhibited practical and potential value for extensive biological and medical applications associated with filtration and isolation, cell separation and sorting, micro-arrangement, in-vitro tissue reconstruction, high-throughput manipulation and analysis, and real-time sensing. Herein, an overview of technological development of micro/nanoporous membranes (M/N-PMs) is provided. Various membrane types and the progress documented in membrane fabrication techniques, including the electrochemical-etching, laser-based technology, microcontact printing, electron beam lithography, imprinting, capillary force lithography, spin coating, and microfluidic molding are described. Their key features, achievements, and limitations associated with micro/nanoporous membrane (M/N-PM) preparation are discussed. The recently popularized applications of M/N-PMs in biomedical engineering involving the separation of cells and biomolecules, bioparticle operations, biomimicking, micropatterning, bioassay, and biosensing are explored too. Finally, the challenges that need to be overcome for M/N-PM fabrication and future applications are highlighted.

Three-Dimensional Human Liver-Chip Emulating Premetastatic Niche Formation by Breast Cancer-Derived Extracellular Vesicles

The liver is one of the most common sites of breast cancer metastasis and is associated with high lethality. Although the interaction between tumor cells and their microenvironment at metastatic sites has been recognized as a key regulator of tumor progression, the underlying mechanism is not fully elucidated. Here, we describe a three-dimensional (3D) microfluidic human liver-on-a-chip (liver-chip) that emulates the formation of a premetastatic niche to investigate the roles of breast cancer-derived extracellular vesicles (EVs) in liver metastasis. We demonstrate that breast cancer-derived EVs activate liver sinusoidal endothelial cells (LSECs) in the liver-chip, inducing endothelial to mesenchymal transition and destruction of vessel barriers. In addition, we show that transforming growth factor β1 (TGFβ1) in breast cancer-derived EVs upregulates fibronectin, an adhesive extracellular matrix protein, on LSECs, which facilitates the adhesion of breast cancer cells to the liver microenvironment. Furthermore, we observed that EVs isolated from triple-negative breast cancer (TNBC) patients with liver metastasis contain higher TGFβ1 levels and induce adhesion of more breast cancer cells to the 3D human liver-chip than do EVs isolated from healthy donors or nonmetastatic TNBC patients. These findings provide a better understanding of the mechanisms through which breast cancer-derived EVs guide secondary metastasis to the liver. Furthermore, the 3D human liver-chip described in this study provides a platform to investigate the mechanisms underlying secondary metastasis to the liver and possible therapeutic strategies.

Beyond Polydimethylsiloxane: Alternative Materials for Fabrication of Organ-on-a-Chip Devices and Microphysiological Systems

Polydimethylsiloxane (PDMS) is the predominant material used for organ-on-a-chip devices and microphysiological systems (MPSs) due to its ease-of-use, elasticity, optical transparency, and inexpensive microfabrication. However, the absorption of small hydrophobic molecules by PDMS and the limited capacity for high-throughput manufacturing of PDMS-laden devices severely limit the application of these systems in personalized medicine, drug discovery, in vitro pharmacokinetic/pharmacodynamic (PK/PD) modeling, and the investigation of cellular responses to drugs. Consequently, the relatively young field of organ-on-a-chip devices and MPSs is gradually beginning to make the transition to alternative, nonabsorptive materials for these crucial applications. This review examines some of the first steps that have been made in the development of organ-on-a-chip devices and MPSs composed of such alternative materials, including elastomers, hydrogels, thermoplastic polymers, and inorganic materials. It also provides an outlook on where PDMS-alternative devices are trending and the obstacles that must be overcome in the development of versatile devices based on alternative materials to PDMS.

Flat and microstructured polymeric membranes in organs-on-chips

In recent years, organs-on-chips (OOCs) have been developed to meet the desire for more realistic in vitro cell culture models. These systems introduce microfluidics, mechanical stretch and other physiological stimuli to in vitro models, thereby significantly enhancing their descriptive power. In most OOCs, porous polymeric membranes are used as substrates for cell culture. The polymeric material, morphology and shape of these membranes are often suboptimal, despite their importance for achieving ideal cell functionality such as cell-cell interaction and differentiation. The currently used membranes are flat and thus do not account for the shape and surface morphology of a tissue. Moreover, the polymers used for fabrication of these membranes often lack relevant characteristics, such as mechanical properties matching the tissue to be developed and/or cytocompatibility. Recently, innovative techniques have been reported for fabrication of porous membranes with suitable porosity, shape and surface morphology matching the requirements of OOCs. In this paper, we review the state of the art for developing these membranes and discuss their application in OOCs.

A Review on Porous Polymeric Membrane Preparation. Part II: Production Techniques with Polyethylene, Polydimethylsiloxane, Polypropylene, Polyimide, and Polytetrafluoroethylene

The development of porous polymeric membranes is an important area of application in separation technology. This article summarizes the development of porous polymers from the perspectives of materials and methods for membrane production. Polymers such as polyethylene, polydimethylsiloxane, polypropylene, polyimide, and polytetrafluoroethylene are reviewed due to their outstanding thermal stability, chemical resistance, mechanical strength, and low cost. Six different methods for membrane fabrication are critically reviewed, including thermally induced phase separation, melt-spinning and cold-stretching, phase separation micromolding, imprinting/soft molding, manual punching, and three-dimensional printing. Each method is described in details related to the strategy used to produce the porous polymeric membranes with a specific morphology and separation performances. The key factors associated with each method are presented, including solvent/non-solvent system type and composition, polymer solution composition and concentration, processing parameters, and ambient conditions. Current challenges are also described, leading to future development and innovation to improve these membranes in terms of materials, fabrication equipment, and possible modifications.

Controlled open-cell two-dimensional liquid foam generation for micro- and nanoscale patterning of materials

Liquid foam consists of liquid film networks. The films can be thinned to the nanoscale via evaporation and have potential in bottom-up material structuring applications. However, their use has been limited due to their dynamic fluidity, complex topological changes, and physical characteristics of the closed system. Here, we present a simple and versatile microfluidic approach for controlling two-dimensional liquid foam, designing not only evaporative microholes for directed drainage to generate desired film networks without topological changes for the first time, but also microposts to pin the generated films at set positions. Patterning materials in liquid is achievable using the thin films as nanoscale molds, which has additional potential through repeatable patterning on a substrate and combination with a lithographic technique. By enabling direct-writable multi-integrated patterning of various heterogeneous materials in two-dimensional or three-dimensional networked nanostructures, this technique provides novel means of nanofabrication superior to both lithographic and bottom-up state-of-the-art techniques.

Design rules for a tunable merged-tip microneedle

This publication proposes the use of an elasto-capillarity-driven self-assembly for fabricating a microscale merged-tip structure out of a variety of biocompatible UV-curable polymers for use as a microneedle platform. In addition, the novel merged-tip microstructure constitutes a new class of microneedles, which incorporates the convergence of biocompatible polymer micropillars, leading to the formation of a sharp tip and an open cavity capable of both liquid trapping and volume control. When combined with biocompatible photopolymer micropillar arrays fabricated with photolithography, elasto-capillarity-driven self-assembly provides a means for producing a complex microneedle-like structure without the use of micromolding or micromachining. This publication also explores and defines the design rules by which several fabrication aspects, such as micropillar dimensions, shapes, pattern array configurations, and materials, can be manipulated to produce a customizable microneedle array with controllable cavity volumes, fracture points, and merge profiles. In addition, the incorporation of a modular through-hole micropore membrane base was also investigated as a method for constitutive payload delivery and fluid-sampling functionalities. The flexibility and fabrication simplicity of the merged-tip microneedle platform holds promise in transdermal drug delivery applications.

Curvature facilitates podocyte culture in a biomimetic platform

Most kidney diseases begin with abnormalities in glomerular podocytes, motivating the need for podocyte models to study pathophysiological mechanisms and new treatment options. However, podocytes cultured in vitro face a limited ability to maintain appreciable extents of differentiation hallmarks, raising concerns over the relevance of study results. Many key properties such as nephrin expression and morphology reach plateaus that are far from the in vivo levels. Here, we demonstrate that a biomimetic topography, consisting of microhemispheres arrayed over the cell culture substrate, promotes podocyte differentiation in vitro. We define new methods for fabricating microscale curvature on various substrates, including a thin porous membrane. By growing podocytes on our topographic substrates, we found that these biophysical cues augmented nephrin gene expression, supported full-size nephrin protein expression, encouraged structural arrangement of F-actin and nephrin within the cell, and promoted process formation and even interdigitation compared to the flat substrates. Furthermore, the topography facilitated nephrin localization on curved structures while nuclei lay in the valleys between them. The improved differentiation was also evidenced by tracking barrier function to albumin over time using our custom topomembranes. Overall, our work presents accessible methods for incorporating microcurvature on various common substrates, and demonstrates the importance of biophysical stimulation in supporting higher-fidelity podocyte cultivation in vitro.

Large-scale patterning of living colloids for dynamic studies of neutrophil-microbe interactions

Neutrophils are the first white blood cells to respond to microbes and to limit their invasion of the body. However, the growth of the microbes into colonies often challenges the neutrophils ability to contain them. To study the interactions between neutrophils and microbial colonies, we designed an assay for arranging microbes in clusters of controlled size (i.e. living colloids). The patterned microbes in the living colloid are mechanically trapped inside the wells and fully accessible to neutrophils. Using the assay, we studied the interactions between human neutrophils and Candida albicans and Staphylococcus aureus, two common human pathogens. We also probed the susceptibility of C. albicans colloids to caspofungin, a common antifungal drug.

Large-scale fabrication of free-standing and sub-μm PDMS through-hole membranes

Free-standing polydimethylsiloxane (PDMS) through-hole membranes have been studied extensively in recent years for chemical and biomedical applications. However, robust fabrication of such membranes with sub-μm through-holes, and at a sub-μm thickness over large areas is challenging. In this paper, we report a robust and simple method for large-scale fabrication of free-standing and sub-μm PDMS through-hole membranes, combining soft-lithography with reactive plasma etching techniques. First, arrays of sub-μm photoresist (PR) columns were patterned on another spin-coated sacrificial PR layer, using conventional photolithography processes. Subsequently, a solution of PDMS : hexane at a 1 : 10 ratio was spin-coated over these fabricated arrays. The cured PDMS membrane was etched in a plasma mixture of sulfur hexafluoride (SF6) and oxygen (O2) to open the through-holes. This PDMS membrane can be smoothly released with a supporting ring by completely dissolving the sacrificial PR structures in acetone. Using this fabrication method, we demonstrated the fabrication of free-standing PDMS membranes at various sub-μm thicknesses down to 600 ± 20 nm, and nanometer-sized through-hole (810 ± 20 nm diameter) densities, over areas as large as 3 cm in diameter. Furthermore, we demonstrated the potential of the as-prepared membranes as cell-culture substrates for biomedical applications by culturing endothelial cells on these membranes in a Transwell-like set-up.

"Open-top" microfluidic device for in vitro three-dimensional capillary beds

We introduce a novel microfluidic device to co-culture a blood vessel network and cell tissues in an in vivo-like niche. Our "open-top" microfluidic device is composed of microchannels with micropores in the ceiling, which provides direct fluid access from reservoir to microchannel. Fluid connections through micropores afford novel advantages, including: i) the long-term culture of large-scale microvessel network, ii) access of different fluids to inner and exterior sides of the microvessel, and iii) co-culturing of the microvessel network and small cell tissue. In this study, we have successfully assembled microvessels with 5 mm channel widths. We were also able to mimic capillary bed conditions by co-culturing microvessels with cancer spheroids. Intimate contact between the cancer spheroid and microvessel caused vessel recruitment and an increase in vessel formation, and affected vessel morphology. We expect this device to be used as a novel platform for vascularized tissue models.