Soft Lithography Techniques Research Articles

Beetle wing inspired fabrication of nanojunction based biomimetic SERS substrates for sensitive detection of analytes

ABSTRACT Surface architecture belonging to flora and fauna is preferred because of their unique three dimensional micro/nanostructures, which can be utilised as novel support in SERS based chemical detection. In this work, we have fabricated biomimetic SERS substrates by replicating the wings of two different beetles with interesting surface morphology by employing soft lithography technique. Micro-bowls and nanogaps between globular microstructures on the replicated artificial wings surface have been utilised for aggregation of nanoparticles to create electromagnetic hot spots and enhanced interaction of nanoparticles with analyte molecules. The surface of these substrates has been engineered to form a homo-nanojunction between gold thin film and gold nanoparticles, which makes it more SERS active. The remarkable performance of these nano-junction based substrates was examined for SERS based sensitive detection of an organic dye, methylene blue, and a fungicide, carbendazim. This work paves way for the design and development of substrates using biomimetic approaches.

Opto-Electrical Investigation of Zn Metal-Doped CdS and Their Application in Soft Lithographic Technique

Un-doped CdS and Zn metal-doped CdS (0.1-0.5 M) were synthesized by chemical precipitation method. Properties like optical band gap and conductivity were determined by different characterization techniques. UV-Visible spectroscopy was applied to estimate the band gap where it showed a significant blue shift due to quantum confinement effect. Size determined by scanning electron microscopy (SEM) was found to be in nanorange from 41 to 60 nm for both un-doped and metal doped CdS nanocrystals. EDAX confirmed the doping by showing peak for Zn. XRD (X-ray diffraction) showed the lattice structure to be cubic for the synthesized nanoparticles and conductivity for un-doped CdS was 7.69E-6 Ω-1m-1 where it changes to 9.25E-6 Ω-1 m-1 and 7.98E-6 Ω-1m-1 for 0.2 M and 0.5 M Zn-doped CdS, respectively. In order to obtain good results, nanocrystals having highest conductivity are used in soft lithographic technique. Here µTM (micro transfer-molding) was followed for patterning with PDMS (poly-(dimethylsiloxane)) mold and silicon substrate for better results.

A low-cost, composite collagen-PDMS material for extended fluid retention in the skin-interfaced microfluidic devices

The advancement of soft, wearable microfluidic devices relies on the microfabrication of polydimethylsiloxane (PDMS) using soft lithography techniques. However, thin 3D microstructures made of PDMS limit long-term storage of aqueous samples and reduce the accuracy of onboard sensing modalities within the platform because of the material's high permeation of water vapor. We studied a composite material of collagen microparticles and PDMS that greatly reduces water evaporation while maintaining the properties of a soft elastomer required for skin-interfaced microfluidics. The collagen-PDMS material is biocompatible, affordable, and non-toxic. We reduced permeability by 80.2% by building a film containing 30 wt% collagen microparticles. Mechanical properties, such as elastic modulus and bonding efficacy, can vary as a function of particle concentration in the films. The skin-interfaced collagen-PDMS microfluidic devices increase sweat retention by 45% through 9 h compared with pure PDMS. This material can greatly improve the long-term sample storage of epidermal devices.

Design of a Tele-operable Soft Actuator

Communication delay between robotic control systems and the robotic actuator is a major issue since the soft robotic motion requires controls to be precise and have quick responses. This is also essential for real time control of robots where the user should be able to send and receive signals with minimum delay to emulate a real time control. Specifically, in soft robotics , the communication delay could become a bigger issue due to the fact that soft robots require mode of pressure exerted into the actuator to make any kind of motion. A delay in such instances could cause the whole system to malfunction and even could cause the actuator to explode in the case of rapid and subtle changes in internally applied pressure. In this research, available IoT (Internet of Things) related systems and models have been studied and suitable IoT system was designed. The main focus of this research was to study the communication delay in IoT embedded hand gesture controlled soft robot finger (actuator). Two separate data processing units have been introduced in the master side and in the slave side to rapid capturing and sending data. By using this approach, processing delay is expected to be minimized. Transmission delay, propagation delay and queuing delay are found to be handled through external factors. These factors could be changed by using various kinds of servers and service providers. Soft elastomer actuator was designed and developed using standard rapid prototyping and soft lithography techniques generally used in soft robotics. Testing, data collecting and analyzing the system are carried out at various locations in Sri lanka and finally the Round Trip Delay Time (RTD) for each case was compared.

A reusable, isoporous through-hole membrane filter for airborne particulate matter removal

This work introduces a new cleanable (and thus reusable) membrane air filter. The membrane filters were fabricated using a soft-lithographic technique, and they are isoporous having through-hole pores. The soft-lithographic technique can be applied to the fabrication of the membranes on various supporting structures such as meshes, or even on freestanding isoporous membranes in an additive fashion leading to bi- or tri-layered membranes. When freestanding single layer membrane filters are used for the removal of PM2.5 contained in the air stream, isoporous through-hole membranes show 60–80% (depending on pore size) filtration efficiency with low pressure drop (less than 40 Pa), which is quite contrary to the common belief that the filtration mechanism would be size sieving. Using a bi-layer configurations of 800 nm isopore membrane layers on top of 5, 10, or 25 μm membranes, these filters’ removal efficiency is >96%, but with a bit high (200–300 Pa) pressure drop. To mitigate the trade-off between efficiency and pressure drop in single and bi-layer isoporous membrane filters, multiple 5 μm-pore membranes with a varying separation gap of 3–15 mm have shown an efficiency of up to 98% with pressure a drop of less than 100 Pa. The main filtration mechanisms, inertial impaction and interception, were identified based on experimental data and finite element analysis. Importantly, the membrane filters can easily be cleaned by wiping with wet tissue or by spraying them with water/ethanol (9:1) mixture, which enables many subsequent re-uses. In addition, the thermoset nature of the membrane material shows high temperature stability, exhibiting ~5% shrinkage at 350 °C and ~15% shrinkage at 400 °C. The membrane can also endure very low temperatures (−80 °C via dry ice), which makes the operating temperature range of the membranes very wide. Due to their simple soft-lithographic fabrication process and their good efficiency and reasonable pressure drop, the isoporous through-hole membrane filters described here present compelling alternative to more conventional filters used for the removal of airborne particulate contaminants.

Tunneling diodes based on polymer infiltrated vertically aligned carbon nanotube forests

We report the fabrication and characterization of metal–insulator–metal diodes incorporating vertically aligned carbon nanotube (VACNT) arrays encased in polymer for applications in high frequency optoelectronics. Polydimethylsiloxane (PDMS) and epoxy infiltrating media are used in this study. VACNT forests are embedded with polymer to form a planarized surface over which an array of tunneling diodes is fabricated. Diodes comprising Al2O3 and HfO2 dielectric multilayers achieve highly nonlinear and asymmetric current-voltage characteristics. Results show that asymmetry in excess of 92 can be achieved with multi-insulator barrier tuning, though there is a strong correlation between asymmetry, resistance, and device longevity. With our best performing and most stable device structure (PDMS–VACNT/Al2O3-HfO2-Al2O3-HfO2/PEDOT:PSS), we provide a demonstration of optical-to-d.c. rectification at 638 nm, realizing a current responsivity of 0.65 µA W−1. Our approach to fabricating these VACNT diode arrays is facile and highly scalable. It is capable of being integrated with solution-processed materials and soft lithography techniques to create flexible devices for optical and infrared detection.

Design and Fabrication of Low-cost Microfluidic Channel for Biomedical Application

This paper presents the design, simulation and low-cost fabrication of microfluidic channel for biomedical application. Channel is fabricated using soft lithography technique. Printed Circuit Board (PCB) is used to make the master for the channel. Channel pattern is transferred on PCB plate using toner transfer technique followed by ferric chloride etching. Paper also discusses, the issues involved in PCB based master fabrication and their viable solutions. Glass is used as substrate material and the channel is made of Sylgard 184 Polydimethylsiloxane (PDMS). Channel is interfaced with a syringe pump to observe the fluid flow. To predict the behavior of the channel, FEM simulation is performed using COMSOL Multiphysics 5.2a. There is a good match between the theoretical, simulation and test results. Finally, to test the biocompatibility of the channel, genomic DNA is passed through the channel and gel electrophoresis analysis is performed.

Prototype of a Smart Microfluidic Platform for the Evaluation of SARS-Cov-2 Pathogenesis, Along with Estimation of the Effectiveness of Potential Drug Candidates and Antigen-Antibody Interactions in Convalescent Plasma Therapy.

Originating in China during December 2019, the novel corona-virus, SARS-CoV-2, has created mayhem worldwide in a very short time. The outbreak has been so rapid and widespread that the only option to treat the patients was administering drugs already available in the market like chloroquine/hydroxychloroquine (an antimalarial drug) and remedesivir. A large number of patients have been cured but the attribution to survival by these drugs has been controversial. Till date, we do not have any specific drug or vaccine available for COVID-19 and the pandemic seems to be far from over. To handle the current challenges posed by the outbreak effectively, we need to employ innovative interdisciplinary approaches. Organ-on-chip (OOC), particularly lung-on-chip, is one such approach which combines the potential of microfluidics, cell culture and molecular biology into a single miniaturised platform. The device is realized to be capable of simulating in-vivo physiological responses of an organ. In the current study, an OOC, which is a multichannel 3D cell culture microfluidic device, is made via soft lithography technique, using polydimethylsiloxane-polymer and diverse polymeric porous/semipermeable membranes. Several polymer membranes i.e. PDMS, polyvinylidene fluoride (PVDF), nitrocellulose, polyester etc., integrated into the microdevices, were efficiently explored to realize their better cell-adhesion and viability property. We also propose for the application of a simple, smart and cost-effective lung-on-chip platform to study the SARS-CoV-2 pathogenesis in humans, drug toxicity testing and provide insights into antigen–antibody interactions. This platform will enable us to study multiple phenomena at a micro-level generating more reliable data and a better understanding of the underlying mechanisms of SARS-CoV-2 infection and pathogenesis.

A device for investigation of natural cell mobility and deformability.

A microfluidic device made of polydimethylsiloxane was developed for continuous evaluation of natural migration mobility of many eukaryotic cells in relaxed and deformed state. The device was fabricated by standard photolithography and soft lithography techniques using the SU-8 3010 negative photoresist on a glass wafer as the master mold. The simple flow-free device exploits the chemotactic movement of cells through a set of mechanical barriers in the direction of concentration gradients of attractants. The barriers are formed by arrays of circular cross-section pillars with decreasing spacing 7, 5, and 3μm. To pass through the obstacles, the cells are deformed and change their cytoskeletal architecture. The instantaneous migration velocities of cells are monitored in a time-lapse setup of the scanning confocal microscope. Thus, the cellular deformability and migratory activity can easily be evaluated. The functionality of the device was tested with model HeLa cells stably transfected with fluorescent Premo FUCCI Cell Cycle Sensor. The designed device has the potential to be implemented for testing the tendency of patients' tumors to metastasis.

Enhanced Protein Crystallization on Nafion Membranes Modified by Low-Cost Surface Patterning Techniques

In this work, the influence of surface topography on protein crystallization over Nafion® is investigated. Two types of Nafion® based membranes were modified by soft lithographic techniques in orde...

Novel Silicon Titanium Diboride Micropatterned Substrates for Cellular Patterning

Both hard material photolithography and soft lithography are widely used for patterned cell culture. Soft lithography techniques enable bioactive molecule incorporation, however complex surface modifications are required to introduce specific ligands or proteins in conventional photolithography. In this study, we demonstrate human umbilical vein cell (HUVEC) and adult bone marrow derived mesenchymal stem cell (MSC) patterning on titanium diboride (TiB2) layers deposited on silicon (Si) substrates by electron-beam evaporation and micropatterned using photolithography. Micropatterned cell growth specificity on geometric shapes of circle and/or lines is achieved via differential growth factors adsorption in the presence of heparin. Specifically, the deposited films of TiB2 showed increased stiffness, hardness, hydrophilicity and surface charge when compared to background Si. These substrates were found to be compatible with HUVEC and MSC viability, based on biomarker expression and RNA-sequence transcriptome analysis. Cell-type dependent, micropattern selective cell growth, such as contact guidance, alignment, and durotaxis, were observed. In addition, MSC clustering was achieved, enabling a three-dimensional (3D) aggregate based microenvironment during culture. This study clearly demonstrates the potential of microfabricated Si and TiB2 biomaterials for patterned cell culture in vitro, independent of any additional surface modification.

The Fabrication of Micro-/Nano-Structural Antirefraction Coatings Based on the PDMS Film for Solar Cells Application

A simple and low-cost methodology to fabricate micro/nanostructure anti-refraction coatings (ARCs) of solar cells is introduced. The nanostructure ARCs are generated by O2 Plasma surface modification technique for a flexible polydimethylsiloxane (PDMS) polymeric film, the microstructure ARCs are obtained by soft imprint lithography, and the micro-nano complex structure PDMS ARCs are fabricated by soft imprint lithography technique combined with surface modification technique. The measured results show that these PDMS ARCs can decrease reflection loss of light from the solar cells and increase the solar cells photovoltaic conversion efficiency.

Mimicking flower petals to fabricate self-cleaning and antireflective polymer surfaces

This work explored a facile way to fabricate superhydrophobic and antireflective polymer surfaces using Canna indica flower petals. A simple, inexpensive two-step soft-lithography technique was employed to replicate the surface structures of C. indica flower petals onto two different polymer surfaces namely, polydimethylsiloxane (PDMS) — a hydrophobic polymer and resorcinol–formaldehyde (RF) xerogel — a hydrophilic polymer. First, a negative replica was prepared in PDMS using the flower petal as a template. Second, the negative PDMS replica was later used as a stamp to prepare a positive replica in both PDMS and RF xerogel. These replicated structured surfaces greatly influenced the wettability properties. The contact angle of the replicated PDMS surfaces increased to the nearly superhydrophobic values of 145° (negative PDMS replica) and 144° (positive PDMS replica) compared with that of the plain surface, which is 101°. Interestingly, the contact angle of the replicated RF surface significantly increased to 151° (superhydrophobic) compared with that of the planar RF surface, which is hydrophilic (68°). Furthermore, the replicated polymer surfaces exhibited not only excellent superhydrophobic properties but also antireflective properties. These multifunctional surfaces with superhydrophobicity as well as an omnidirectional antireflective property may find a wide range of applications, such as solar energy harvesting, optical displays and marine applications.

Substantial improvement on electrical energy harvesting by chemically modified/sandpaper-based surface modification in micro-scale for hybrid nanogenerators

Herein, a unique energy harvester made of triboelectric and piezoelectric effects made of a dual perovskite solid system by substituting a trace amount of perovskite BaTiO3 into the parent material K0.5Na0.5NbO3 material. The combined system (1 − x) K0.5Na0.5NbO3- x BaTiO3 at x = 0, 0.02, 0.04, 0.06 and 0.08 is lead-free and eco-friendly in nature. The doping concentration KNN- 0.02 BTO shows high remnant polarization and electrical response. The active layer of the hybrid device is made of mixing KNN- 0.02 BTO into the PDMS matrix. The electrical response of the composite film is studied systematically with the addition of various weight percentages of nanoparticles into the PDMS matrix, where 10 wt% shows the high electrical output. The electrical responses with respect to surface roughness have been studied, and the roughness has been created using cost-effective soft lithographic techniques. The individual components, such as TENG and PENG, in comparison with the hybrid device, had been systematically studied, and the hybrid performance is high compared to individual components. The PCF-NG device generates a maximum electrical output of ≈610 V/13.7 μA with a power density of 0.55 W/m2 at load resistance of 100 MΩ. Further, the device demonstrates capability in powering low power electronic devices such as capacitor charging, LED lit up, and powering wristwatches. This work would provide an opportunity for developing a stable and highly reliable built-in power source for various applications and would lead a path towards battery-free smart electronics.

Fast fabrication of a 3D prototyping microfluidic device for liquid cross-flow and droplet high-throughput generation

Microfluidic applications require the optimization of chip structure and dimensions. The quick and easy fabrication of chips plays an important role in the progress of research. However, the standard soft lithography technique requires several hours to produce a replication mold. Shortening the mold fabrication time still faces some challenges. We present a fast and easy method to fabricate a positive mold for multi-layer or 3D microdevice construction. Single-layer positive molds with triangular, rectangular, and circular patterns were produced in less than 50 min. A depth-to-width ratio is achieved in the range of 0.6–2.4 and the minimum pattern size reached 50 µm. One additional layer only increases the total time by 5 min. Based on the fabricated positive molds, we created microchips with different depth channels and 3D structures, realizing the cross-flow of multiple liquids and the large-scale production of mono-disperse droplets with controllable size. By using the formed droplets as templates, hydrogel particles with controllable structures were synthesized uniformly. The established method is fast, simple, and compatible with standard soft lithography equipment, displaying the potential to accelerating research progress.

Replication of the Surface Wettability of Plant Leaves with Different Surface Morphologies Using Soft Lithography

Naturally occurring surfaces which display interesting surface wetting properties such as superhydrophobicity, superoleophobicity and directional wettability are found in most aquatic plants. These phenomena arise from the micro/nano structures present on these surfaces complimented by their surface chemistry. The replication of such surface structures is highly pursued due to their potential in applications such as self-cleaning (Lotus effect), microfluidic devices and antifouling surfaces. Soft lithography is a technique that has been used to replicate micro/nano structures with varying degrees of success. Nevertheless, in the context of natural surfaces, the technique has been mostly limited to the replication of the lotus leaf structure. Therefore, a systematic investigation could be fruitful since it has the potential to be scaled up to replicate different surface structures as well as large-area patterning. In this study, the feasibility of soft lithography technique on natural leaf surfaces was investigated using five plant species with different surface morphologies. The negatives of these primary molds were replicated using polydimethylsiloxane (PDMS) and the final positive replica was successfully replicated from PDMS while using hydroxypropyl methylcellulose (HPMC) as an anti-stick layer. The structures were characterized based on SEM images and contact angle measurements. Additionally, the effect of HPMC was also investigated. The technique can easily be extended to broader applications in other areas that require micro/nanostructured surfaces such as anti-reflection coatings, chemical sensors and anti-microbial surfaces.

Patterning Catalyst Layers with Microscale Features by Soft Lithography Techniques for Proton Exchange Membrane Fuel Cells

Microtransfer molding (μTM) and microcontact printing (μCP) techniques were demonstrated for the preparation of platinum-based catalysts in hexagonally arranged patterns to achieve cathode catalyst...

Fabrication of wavy micromixer using soft lithography technique

Microfluidic devices are enormously used in different engineering applications such as biological, chemical, electronics, mechanical, etc. Manufacturing of microfluidic devices becomes one of the emerging areas of research in the current times. Manufacturing of microfluidic devices is a challenging issue via general fabrication technologies. Several types of materials and methods are in use for fabricating microfluidic devices for different kinds of uses. Micromixer is an essential component to integrate any microfluidic device. In this paper, a fast process for the fabricating of micromixer with poly-di-methyl-siloxane (PDMS) is presented and elaborated. Two types of wavy micromixers, namely raccoon, and serpentine are fabricated. The complete fabrication process of the micromixer has been conducted with major two steps, namely, mold preparation, and fabrication. Initially, the mold of the micromixer of raccoon, and serpentine type with proper dimensions is produced using CO2 assisted laser machining on poly-methyl-meth-acrylate (PMMA), and then using these molds, micromixer of PDMS is prepared by soft lithography technique. Micromixer with accurate and precious dimensions as per the required design can be fabricated using the proposed process for different purposes. This article may be helpful for the researchers to get the proper knowledge related to the fabrication of wavy micromixer in a fast and cost-effective way.

Chemical Lift-Off Lithography of Metal and Semiconductor Surfaces.

Chemical lift-off lithography (CLL) is a subtractive soft-lithographic technique that uses polydimethylsiloxane (PDMS) stamps to pattern self-assembled monolayers of functional molecules for applications ranging from biomolecule patterning to transistor fabrication. A hallmark of CLL is preferential cleavage of Au-Au bonds, as opposed to bonds connecting the molecular layer to the substrate, i.e., Au-S bonds. Herein, we show that CLL can be used more broadly as a technique to pattern a variety of substrates composed of coinage metals (Pt, Pd, Ag, Cu), transition and reactive metals (Ni, Ti, Al), and a semiconductor (Ge) using straightforward alkanethiolate self-assembly chemistry. We demonstrate high-fidelity patterning in terms of precise features over large areas on all surfaces investigated. We use patterned monolayers as chemical resists for wet etching to generate metal microstructures. Substrate atoms, along with alkanethiolates, were removed as a result of lift-off, as previously observed for Au. We demonstrate the formation of PDMS-stamp-supported bimetallic monolayers by performing CLL on two different metal surfaces using the same PDMS stamp. By expanding the scope of the surfaces compatible with CLL, we advance and generalize CLL as a method to pattern a wide range of substrates, as well as to produce supported metal monolayers, both with broad applications in surface and materials science.

Microfluidic diffraction phase microscopy and its application in parasites measurement

This paper proposes a method of using diffraction phase microscopy combined with microfluidic chip to quantitatively measure waterborne parasites. A diffraction phase microscopy system is built up by combining interferometry with optical microscope to achieve high sensitivity real-time measurement of parasites. Based on soft lithographic techniques, a double-layered microfluidic chip with U-shaped trapping structures is designed and fabricated for high throughput single parasites trapping. Ficoll solution with the same refractive index as polydimethylsiloxane (PDMS) is introduced into the microfluidic chamber to eliminate significant artifacts in phase imaging originating from diffraction at the edges of trapping structures. The accuracy of the system is verified using standard polystyrene microspheres of different diameters, and the error of maximum phase shift does not exceed 3%. 100 Giardia Lamblia (G. Lamblia) cysts and 100 Cryptosporidium Parvum (C. Parvum) oocysts are measured using this system. The phase maps of the parasites are obtained from the interferograms. The morphological parameters and quantitative optical volume difference distribution of the two kind of waterborne parasites are obtained by analyzing the quantitative phase maps. Quantitative data provides the basis for understanding their physiological characteristics. The microfluidic diffraction phase microscopy system has simple structure, good stability and high measurement accuracy, and has great potential for real-time monitoring and label-free quantitative studies of single microorganism.