Soft Lithography Research Articles

Single-cell impedance cytometry of anticancer drug-treated tumor cells exhibiting mitotic arrest state to apoptosis using low-cost silver-PDMS microelectrodes.

Chemotherapeutic drugs such as paclitaxel and vinblastine interact with microtubules and thus induce complex cell states of mitosis arrest at the G2/M phase followed by apoptosis dependent on drug exposure time and concentration. Microfluidic impedance cytometry (MIC), as a label-free and high-throughput technology for single-cell analysis, has been applied for viability assay of cancer cells post drug exposure at fixed time and dosage, yet verification of this technique for varied tumor cell states after anticancer drug treatment remains a challenge. Here we present a novel MIC device and for the first time perform impedance cytometry on carcinoma cells exhibiting progressive states of G2/M arrest followed by apoptosis related to drug concentration and exposure time, after treatments with paclitaxel and vinblastine, respectively. Our results from impedance cytometry reveal increased amplitude and negative phase shift at low frequency as well as higher opacity for HeLa cells under G2/M mitotic arrest compared to untreated cells. The cells under apoptosis, on the other hand, exhibit opposite changes in these electrical parameters. Therefore, the impedance features differentiate the HeLa cells under progressive states post anticancer drug treatment. We also demonstrate that vinblastine poses a more potent drug effect than paclitaxel especially at low concentrations. Our device is fabricated using a unique sacrificial layer-free soft lithography process as compared to the existing MIC device, which gives rise to readily aligned parallel microelectrodes made of silver-PDMS embedded in PDMS channel sidewalls with one molding step. Our results uncover the potential of the MIC device, with a fairly simple and low-cost fabrication process, for cellular state screening in anticancer drug therapy.

Ordered non-close packed colloidal array with morphology control.

Non-close packed (NCP) colloidal arrays find wide applications in the fields of photonics, optical chip fabrication, nano sphere lithography and so on. However, unlike their close packed counterparts, such arrays cannot be obtained by direct self-organization of colloidal particles and require specialized techniques involving plasma/reactive ion etching, electric field driven assembly, substrate stretching or precise positioning of the particles. In this article, we present a facile template guided approach for fabricating ordered NCP arrays of colloidal particles. First, we employ soft lithography to replicate self-assembled hexagonal close packed (HCP) arrays of 'larger colloidal particles' (LPs) to obtain a topographically patterned positive and/or negative replica of the initial array. These replicas are then used as templates to spin coat 'smaller colloidal particles' (SPs), which may even have some degree of poly-dispersity, to obtain ordered NCP arrays. We further show that pattern morphology can be modulated based on whether a single or a double replicated template is used to confine the SPs, the concentration (Cn) of the SPs in the casting solution as well as the relative commensuration of the diameter of the SPs (ds) with that of the LPs (dL). Finally, we show that such NCP arrays can be transferred onto any flat surface by UVO mediated colloidal transfer printing.

Development of cell counting module using soft lithography technology

Development of cell counting module using soft lithography technology

Fabrication of Centimeter-Scale Plasmonic Nanoparticle Arrays with Ultranarrow Surface Lattice Resonances.

Plasmonic surface lattice resonances (SLRs) supported by metallic nanoparticle (NP) arrays show diverse applications including nanolasers, sensors, photocatalysis, and nonlinear optics. However, to rationally fabricate high-quality plasmonic NP arrays with ultranarrow SLR line widths over large areas remains challenging. This article describes a general approach for the efficient fabrication of centimeter-scale inorganic NP arrays with precisely controlled NP size, composition, position, and lattice geometry. This method combines the processes of solvent-assisted soft lithography and in situ site-specific NP growth to reproducibly create many replicates of NP arrays without utilizing cleanroom and specialized equipment. For demonstration, we show that Au NP arrays exhibit ultranarrow SLRs with a line width of 4 nm and a quality factor of 218 toward the theoretical limit.

Accurate Transfer of Individual Nanoparticles onto Single Photonic Nanostructures.

Controlled integration of metallic nanoparticles (NPs) onto photonic nanostructures enables the realization of complex devices for extreme light confinement and enhanced light-matter interaction. For instance, such NPs could be massively integrated on metal plates to build nanoparticle-on-mirror (NPoM) nanocavities or photonic integrated waveguides (WGs) to build WG-driven nanoantennas. However, metallic NPs are usually deposited via drop-casting, which prevents their accurate positioning. Here, we present a methodology for precise transfer and positioning of individual NPs onto different photonic nanostructures. Our method is based on soft lithography printing that employs elastomeric stamp-assisted transfer of individual NPs onto a single nanostructure. It can also parallel imprint many individual NPs with high throughput and accuracy in a single step. Raman spectroscopy confirms enhanced light-matter interactions in the resulting NPoM-based nanophotonic devices. Our method mixes top-down and bottom-up nanofabrication techniques and shows the potential of building complex photonic nanodevices for multiple applications ranging from enhanced sensing and spectroscopy to signal processing.

Improving droplet microfluidic systems for studying single bacteria growth.

Antimicrobial resistance remains a global threat with ~ 5 million deaths in 2019 alone and 10 million deaths projected every year by 2050. Current tools employed in the analysis of bacteria can be time inefficient, leading to delayed diagnosis and treatment. In this work, we develop a microfluidic setup capable of bacteria incubation and detection of growth in ~ 2h. We fabricated polydimethylsiloxane (PDMS) microchips via soft lithography, enclosed microchannels by plasma bonding to glass, and utilized PDMS blocks for simplified connection of devices to a flow system. We generated uniform droplets enclosing zero, one or two bacteria within our devices, and incubated droplet-encapsulated bacteria with 100 × lower concentrations of a fluorescence probe of bacterial growth compared to prior work. We assessed bacterial growth via laser induced fluorescence after room temperature incubation for 2h and obtained a range of signals corresponding to droplets with or without bacteria. Our devices allow for online droplet incubation, monitoring, detection, and tracking. Developing microfluidic chips for single bacteria studies will improve the analysis and treatment of antimicrobial resistance.

EMSCs‐Seeded Micro‐Stripe Patterned Polycaprolactone Promoting Sciatic Nerve Regeneration

AbstractTransplantation of a synthetic polymer nerve conduit carrying stem cells is promising for treating peripheral nerve injury. In this study, a micropatterned inner surface of polycaprolactone (PCL) nerve conduit cellularized with nasal respiratory mucosa‐derived ectoderm mesenchymal stem cells (EMSCs) is adopted for peripheral nerve regeneration. The PCL microstructure grating is fabricated by soft nanoimprint lithography (NIL), whose templates can be easily transferred from the Ebeam photoresist pattern. The most suitable microstructure is carefully chosen to support EMSCs alignment and nerve regeneration. In vitro, the micropatterned PCL nerve conduit greatly enhanced the directional alignment of EMSCs and the MBP and S100 expression of myelin proteins. Additionally, the micropatterned conduit preseeded with EMSCs significantly stimulated the axon‐directed extension of neural stem cells. In vivo, micropatterned PCL conduits cotransplanted with EMSCs enhanced motor function and promoted sciatic nerve regeneration and extension in a rat model of sciatic nerve defect. Taken together, this “all‐in‐one” nerve guidance conduit makes sciatic nerve regeneration and morphological repairing accelerated, which has a potential clinical application in neural tissue engineering.

Mechano-Node-Pore Sensing: A Rapid, Label-Free Platform for Multi-Parameter Single-Cell Viscoelastic Measurements.

Cellular mechanical properties are involved in a wide variety of biological processes and diseases, ranging from stem cell differentiation to cancer metastasis. Conventional methods for measuring these properties, such as atomic force microscopy (AFM) and micropipette aspiration (MA), capture rich information, reflecting a cell's full viscoelastic response; however, these methods are limited by very low throughput. High-throughput approaches, such as real-time deformability cytometry (RT-DC), can only measure limited mechanical information, as they are often restricted to single-parameter readouts that only reflect a cell's elastic properties. In contrast to these methods, mechano-node-pore sensing (mechano-NPS) is a flexible, label-free microfluidic platform that bridges the gap in achieving multi-parameter viscoelastic measurements of a cell with moderate throughput. A direct current (DC) measurement is used to monitor cells as they transit a microfluidic channel, tracking their size and velocity before, during, and after they are forced through a narrow constriction. This information (i.e., size and velocity) is used to quantify each cell's transverse deformation, resistance to deformation, and recovery from deformation. In general, this electronics-based microfluidic platform provides multiple viscoelastic cell properties, and thus a more complete picture of a cell's mechanical state. Because it requires minimal sample preparation, utilizes a straightforward electronic measurement (in contrast to a high-speed camera), and takes advantage of standard soft lithography fabrication, the implementation of this platform is simple, accessible, and adaptable to downstream analysis. This platform's flexibility, utility, and sensitivity have provided unique mechanical information on a diverse range of cells, with the potential for many more applications in basic science and clinical diagnostics.

VAT photopolymerization 3D printing optimization of high aspect ratio structures for additive manufacturing of chips towards biomedical applications

Organ-on-chip and Lab-on-chip are microfluidic devices widely applied in the biomedical field. They are traditionally produced by soft lithography: starting from a mold fabricated by optical photolithography, a Polydimethylsiloxane (PDMS) device is obtained by casting and baking. While this technique offers the possibility to produce features with high resolution, it is not flexible enough to respond to the necessity of customization and prototyping. In this study, we propose as alternative the production of devices by digital light processing (DLP), a vat photopolymerization technology, in combination with a commercially available, biocompatible resin. Studying the process factors by a statistical methodology called Design of Experiment (DoE), we were able to achieve small features with high aspect ratio (60). DoE method allowed us to have a deep understanding of the process without the need of any physical inspection of the involved phenomena, and to generate empirical models, correlating the process factors to the dimensions of the final printed object. We proved that this optimization was beneficial also in terms of transparency (evaluated by UV-Vis spectrophotometry), and mechanical strength (evaluated by a compression test) of the printed resin. Finally, a proof-of-concept microfluidic device was fabricated, sealed to a PDMS membrane through an oxygen plasma treatment, and tested against leakage on a microfluidic circuit for one week. As result, we proved that DLP printing is not only a suitable method to develop microfluidic devices, but if correctly optimized it can also reproduce small features in the order of tens of micrometers rapidly.

Scalable large-area mesh-structured microfluidic gradient generator for drug testing applications.

Microfluidic concentration gradient generators are useful in drug testing, drug screening, and other cellular applications to avoid manual errors, save time, and labor. However, expensive fabrication techniques make such devices prohibitively costly. Here, in the present work, we developed a microfluidic concentration gradient generator (μCGG) using a recently proposed non-conventional photolithography-less method. In this method, ceramic suspension fluid was shaped into a square mesh by controlling Saffman Taylor instability in a multiport lifted Hele-Shaw cell (MLHSC). Using the shaped ceramic structure as the template, μCGG was prepared by soft lithography. The concentration gradient was characterized and effect of the flow rates was studied using COMSOL simulations. The simulation result was further validated by creating a fluorescein dye (fluorescein isothiocanate) gradient in the fabricated μCGG. To demonstrate the use of this device for drug testing, we created various concentrations of an anticancer drug-curcumin-using the device and determined its inhibitory concentration on cervical cancer cell-line HeLa. We found that the IC50 of curcumin for HeLa matched well with the conventional multi-well drug testing method. This method of μCGG fabrication has multiple advantages over conventional photolithography such as: (i) the channel layout and inlet-outlet arrangements can be changed by simply wiping the ceramic fluid before it solidifies, (ii) it is cost effective, (iii) large area patterning is easily achievable, and (iv) the method is scalable. This technique can be utilized to achieve a broad range of concentration gradient to be used for various biological and non-biological applications.

Influence of spiral topographies on human limbal-derived immortalized corneal epithelial cells.

Cells migrate from the limbus to the corneal epithelium following a centripetal pathway. Corneal epithelial cells tend to orientate in spiral or vortex patterns. However, when cultured in-vitro, limbal derived corneal epithelia do not tend to align or migrate in a spiral pattern. Here, we used soft lithography to create silk fibroin substrates with spiral topographies that direct the human limbal-derived immortalized corneal epithelial cells (hTCEpi) to form a spiral orientation. The impact of this topography on the cells was then characterized. The spiral patterns affected cytoskeletal organization, cell spreading, and nuclei shapes. Spiral width and numbers had a significant impact on proliferation of cells, their focal adhesion, their chromatin condensation, and number of actin filaments. Immunocytochemical staining showed that the spiral pattern enhanced the expression of markers associated with limbal stem cells. The current work illustrates micro spiral patterns can serve to control the nature of limbal derived epithelial cells by providing relevant biophysical cues.

Cut-Off UV Light Filter to Prevent the Negative Slope of the Soft Lithography Hard Mold Walls

Cut-Off UV Light Filter to Prevent the Negative Slope of the Soft Lithography Hard Mold Walls

Overview of Advanced Micro-Nano Manufacturing Technologies for Triboelectric Nanogenerators

In the era of the Internet of Things, various electronics play an important role in information interaction, in which the power supply is an urgent problem to be solved. Triboelectric nanogenerator (TENG) is an emerging mechanical energy harvesting technology that can serve as a power source for electronics, which is developing towards high performance, miniaturization and integration. Herein, the advanced micro-nano manufacturing technologies are systematically reviewed for TENGs. First, film preparation such as physical vapor deposition, chemical vapor deposition, electrochemical deposition, electrospinning and screen printing for triboelectric layers are introduced and discussed. Then, surface processing, such as soft lithography, laser ablation, inductively coupled plasma and nanoimprint for micro-nano structures on the surface of triboelectric layers are also introduced and discussed. In addition, micro-electromechanical system fabrication for TENG devices such as acoustic and vibration sensors, is introduced, and their current challenges are analyzed. Finally, the challenges of the advanced micro-nano manufacturing technologies for the TENGs are systematically summarized, and further development is prospected.

Flexible Superhydrophobic Microlens Arrays for Humid Outdoor Environment Applications.

A microlens array (MLA) is an essential optical imaging device in the applications of augmented and virtual realities. The imaging of MLA would become blurry in a humid outdoor atmosphere. While the incorporation of superhydrophobicity to MLA would prevent the adhesion of droplets, the complex structure and the multiple fabrication process reduce the capability of optical imaging of MLA. Herein, a flexible superhydrophobic MLA with good optical imaging capability is successfully fabricated by the combination of 3D direct laser writing (DLW) and soft lithography. 3D DLW allows the fabrication of MLA with a hierarchical pillar array (h-MLA) in one step, which ensures good optical properties of the resulting polydimethylsiloxane (PDMS) h-MLA. The resulting h-MLAs with pitches ranging between 50 and 100 μm are superhydrophobic from which water droplets slide away at a sliding angle smaller than 15.6° and bounce off from the surface. Meanwhile, the hierarchical pillar array has a limited impact on the imaging capability and the field of view of h-MLA. With an optimized pitch of 60 μm, h-MLA has a transparency as good as MLA. Moreover, PDMS h-MLA retains excellent optical and superhydrophobic properties when bent and in an extremely humid environment. We believe that the proposed h-MLA could find applications in outdoor environments.

A Thin Format Vision-Based Tactile Sensor With a Microlens Array (MLA)

Vision-based tactile sensors have been widely studied in the robotics field for high spatial resolution and compatibility with machine learning algorithms. However, the currently employed sensor’s imaging system is bulky, limiting its further application. Here, we present a microlens array (MLA)-based vision system to achieve a low thickness format of the sensor package with high tactile sensing performance. Multiple micromachined microlens units cover the whole elastic touching layer and provide a stitched clear tactile image, enabling high spatial resolution with a thin thickness of 5 mm. The thermal reflow and the soft lithography method ensure the uniform spherical profile and smooth surface of microlenses. Both optical and mechanical characterizations demonstrated the sensor’s stable imaging and excellent tactile sensing, enabling precise 3-D tactile information, such as displacement mapping and force distribution with an ultracompact thin structure.

Optical Manipulation of Incident Light for Enhanced Photon Absorption in Ultrathin Organic Photovoltaics.

We attempted to improve the photon absorption of the photoactive layer in organic photovoltaic (OPV) devices by device engineering without changing their thickness. Soft nanoimprinting lithography was used to introduce a 1D grating pattern into the photoactive layer. The increase in photocurrent caused by the propagating surface plasmon-polariton mode was quantitatively analyzed by measuring the external quantum efficiency in transverse magnetic and transverse electric modes. In addition, the introduction of an ultrathin substrate with a refractive index of 1.34 improved photon absorption by overcoming the mismatched optical impedance at the air/substrate interface. As a result, the power conversion efficiency (PCE) of an ultrathin OPV with a 400 nm grating period was 8.34%, which was 11.6% higher than that of an unpatterned ultrathin OPV, and the PCE was 3.2 times higher at a low incident light angle of 80°, indicating very low incident light angle dependence.

A pore-scale investigation of microplastics migration and deposition during unsaturated flow in porous media

Microplastics are ubiquitous in the natural environment and have the potential to endanger the natural environment, ecology and even human health. A series of microfluidic experiments by using soft lithography technology were carried out to investigate the effect of flow rate, particle volume fraction, particle size and pore/throat ratio on microplastics migration and deposition at the pore scale. We discovered a range of deposition patterns of the spherical microplastics from no particle deposition, to discontinuous particle layer, and to continuous particle layers in the retained liquid in the pores, depending on the particle size and volume fraction. Several metrics, including air saturation, probability of particle detainment, expansion ratio and thickness of residual liquid, were quantified to examine the role of various parameters on particle migration and retention of microplastics. At low flow rate (Q = 0.05 μL/min), microplastics migration and deposition were sensitive to changes in particle volume fraction, particle size and pore/throat ratio. In contrast, at high flow rates (Q > 5 μL/min), the migration and retention of particles were mainly controlled by strongly channelized air invasion pattern, while the particle volume fraction, particle size and pore/throat size ratio have only secondary influence. At intermediate range of flow rates, microplastics migration and deposition were dramatically impacted by flow rate, particle volume fraction, particle size and pore/throat ratio. This work improves the understanding of the mechanisms of particle migration and retention in porous media and can provide a reference for more accurate assessment of the exposure levels and times of microplastics in soil and groundwater systems.

Preparation Methods and Functional Characteristics of Regenerated Keratin-Based Biofilms.

The recycling, development, and application of keratin-containing waste (e.g., hair, wool, feather, and so on) provide an important means to address related environmental pollution and energy shortage issues. The extraction of keratin and the development of keratin-based functional materials are key to solving keratin-containing waste pollution. Keratin-based biofilms are gaining substantial interest due to their excellent characteristics, such as good biocompatibility, high biodegradability, appropriate adsorption, and rich renewable sources, among others. At present, keratin-based biofilms are a good option for various applications, and the development of keratin-based biofilms from keratin-containing waste is considered crucial for sustainable development. In this paper, in order to achieve clean production while maintaining the functional characteristics of natural keratin as much as possible, four important keratin extraction methods-thermal hydrolysis, ultrasonic technology, eco-friendly solvent system, and microbial decomposition-are described, and the characteristics of these four extraction methods are analysed. Next, methods for the preparation of keratin-based biofilms are introduced, including solvent casting, electrospinning, template self-assembly, freeze-drying, and soft lithography methods. Then, the functional properties and application prospects of keratin-based biofilms are discussed. Finally, future research directions related to keratin-based biofilms are proposed. Overall, it can be concluded that the high-value conversion of keratin-containing waste into regenerated keratin-based biofilms has great importance for sustainable development and is highly suggested due to their great potential for use in biomedical materials, optoelectronic devices, and metal ion detection applications. It is hoped that this paper can provide some basic information for the development and application of keratin-based biofilms.

A simple and accessible approach for processing photopolymer master molds for the fabrication of microfluidic polydimethylsiloxane devices

The use of three-dimensional (3D) printing for fabrication of master molds for microfluidic devices is very attractive due to its availability and simplicity and replaces the standard methods of soft lithography. However, the commercially available photopolymer resins inhibit the curing of polydimethylsiloxane (PDMS), preventing reliable replication of 3D printed master mold structures. Here, we present a simple and safe method to post-process 3D printed photopolymer master molds for PDMS microfluidic devices. This approach expands the possibilities of prototyping microfluidic PDMS devices for a wider research community without complex post-processing tools currently required for fabrication of 3D photopolymer master molds.

Parallelization of Microfluidic Droplet Junctions for Ultraviscous Fluids.

The parallelization of multiple microfluidic droplet junctions has been successfully achieved so that the production throughput of the uniform microemulsions/particles has witnessed considerable progress. However, these advancements have been observed only in the case of a low viscous fluid (viscosity of 10-2 -10-3 Pa s). This study designs and fabricates a microfluidic device, enabling a uniform micro-emulsification of an ultraviscous fluid (viscosity of 3.5 Pa s) with a throughput of ≈330 000 droplets per hour. Multiple T-junctions of a dispersed oil phase, split from a single inlet, are connected into the single post-crossflow channel of a continuous water phase. In the proposed device, the continuous water phase undergoes a series circuit, wherein the resistances are continuously accumulated. The independent corrugations of the dispersed oil phase channel, under the theoretical guidance, compromise such increased resistances; the ratio of water to oil flow rates at each junction becomes consistent across T-junctions. Owing to the design being based on a fully 2D interconnection, single-step soft lithography is sufficient for developing the full device. This easy-to-craft architecture contrasts with the previous approach, wherein complicated 3D interconnections of the multiple junctions are involved, thereby facilitating the rapid uptake of high throughput droplet microfluidics for experts and newcomers alike.