Polydimethylsiloxane Structures Research Articles

Flow-through optical device based on silicone optical technology (SOT) for determination of iron in drinkable tap water

We propose a new concept “silicone optical technology (SOT)”, with low noise and simple integration of all optical components, and use a common matrix of polydimethylsiloxane (PDMS) as a prototype for rapid digital fabrication. This work involves the rapid design and fabrication of SOT optical modules designed to be coupled with a flow injection analysis system (SOT-FIA module), and their use in combination with a handheld photo absorbance meter “picoExplorer™” is also demonstrated. A digital fabrication method, by casting PDMS on the three-dimensional printed mold/frame, is developed. A simple structure of transparent PDMS mixed with the white pigment of titanium dioxide (TiO2) rutile particle (W-PDMS) optical core with a cladding of carbon black dispersed PDMS (K-PDMS) is fabricated. This structure performs as a tiny and straightforward optical filter and shows the ability to trap tilted incident light. The addition of the white pigment into PDMS results in a special enhancement of the optical sensitivity by increasing the amount of scattered light. This module is directly mounted on a white light emitting diode (LED) and the red-green-blue (RGB) sensor device from the picoExplorer at 45° to the flow channel to increase the detection length. The optical properties of the SOT-FIA module, which arise from the presence of white PDMS (W-PDMS) and the effect of the detection length, are investigated and show a dramatically enhanced sensitivity (30–45 times) of the optical detection compared with the calculation using Beer-Lambert's law. Moreover, lower than 1% of crosstalk is observed, which indicates the low noise level detection of the proposed SOT-FIA optical device. This proposed SOT-FIA module integrated with an LED detection system is developed to be combined with a flow injection analysis system for the colorimetric detection of iron in drinkable tap water samples. The results show excellent linearity, the ability for a wide range of chemical analysis, low detection limit, high percentage recovery, high precision and good agreement with the conventional spectrophotometric method. Furthermore, the tilted and tilt-shifted coupling alignments are newly developed and evaluated.

In Situ Inkjet Printing of the Perovskite Single-Crystal Array-Embedded Polydimethylsiloxane Film for Wearable Light-Emitting Devices.

Metal halide perovskites are promising light-emitting materials for applications such as wearable lighting devices and flat panel displays because of their high photoluminescence efficiency, high color purity, and facile solution processability. However, the intrinsic ambient instability and crystal friability issues have fundamentally hindered the practical applications of perovskites. Here, we solve this problem through a liquid to liquid self-encapsulation inkjet-printing technique. Perovskite inks are directly inkjet-printed into the liquid polydimethylsiloxane (PDMS) precursor to in situ form the self-encapsulation perovskite single-crystal-embedded PDMS structure. We show that the space-confined effect of the liquid PDMS precursor can significantly retard the perovskite crystallization process and promote the embedded growth of perovskite single crystals in PDMS. Benefiting from the sealing function of PDMS, the printed perovskite single crystals show excellent ambient stability and flexibility. Furthermore, we demonstrate that wafer-scale air-stable and flexible perovskite fluorescent patterns can be produced in PDMS by direct inkjet printing, which is cost-effective and free of complex microfabrication processes. This method provides a facile approach to scalable fabrication of air-stable and wearable perovskite fluorescent patterns, which will be of great significance for potential application in perovskite light-emitting diode display.

A Flexible Strain Sensor Based on the Porous Structure of a Carbon Black/Carbon Nanotube Conducting Network for Human Motion Detection.

High-performance flexible strain sensors are playing an increasingly important role in wearable electronics, such as human motion detection and health monitoring, with broad application prospects. This study developed a flexible resistance strain sensor with a porous structure composed of carbon black and multi-walled carbon nanotubes. A simple and low-cost spraying method for the surface of a porous polydimethylsiloxane substrate was used to form a layer of synergized conductive networks built by carbon black and multi-walled carbon nanotubes. By combining the advantages of the synergetic effects of mixed carbon black and carbon nanotubes and their porous polydimethylsiloxane structure, the performance of the sensor was improved. The results show that the sensor has a high sensitivity (GF) (up to 61.82), a wide strain range (0%–130%), a good linearity, and a high stability. Based on the excellent performance of the sensor, the flexible strain designed sensor was installed successfully on different joints of the human body, allowing for the monitoring of human movement and human respiratory changes. These results indicate that the sensor has promising potential for applications in human motion monitoring and physiological activity monitoring.

Electrohydrodynamic inkjet printing of Polydimethylsiloxane (PDMS)

The current fabrication methods for micro and nano scale Polydimethylsiloxane (PDMS) devices either require high environmental conditions or sophisticated procedures at high cost. Electrohydrodynamic inkjet (EHD) printing is a potential method to effectively fabricate micro scale PDMS devices because EHD printing is environment-friendly, low-cost, compatible to various inks, and most importantly, high resolution. However, little research has been conducted to study EHD printing of PDMS-based ink. In this paper, EHD printing of PDMS has been investigated. The effects of several critical parameters on the printing process were studied. The dimension of the printed patterns and the printing frequency were preciously controlled by the voltage parameters. The patterns with a network structure was demonstrated. The results indicated that the voltage amplitude and the pulse frequency could both control the dimensions of droplets and the printing frequency. As future work, we plan to develop a simulation tool to predict printing quality of PDMS and optimize printing process to fabricated 3D PDMS structures in micro scale.

A flexible precise volume sensor based on metal-on-polyimide electrodes sandwiched by PDMS channel for microfluidic systems

This paper reports a flexible precise volume sensor with metal-on-polyimide (PI) electrodes to substitute for the peripheral ration pump of a microfluidic system, thus beneficial for integration and miniaturization. This in-channel volume sensor consists of multi-electrode pairs, and it can perform volume measurement of the fluid flowing through microchannels by testing the resistance variation of the electrode pairs, which makes the device possible to help automatically control the sample volume in the mixing and reacting processes inside a microfluidic chip without peripheral ration pumps. The electrode pairs of the sensor are fabricated on flexible PI surface directly by inkjet printing. Then, the electrodes with the PI substrate are transferred and sandwiched by a polydimethylsiloxane (PDMS) substrate layer and a PDMS channel layer to form the flexible precise volume sensor. This method overcomes the challenge of patterning metals on PDMS and the sandwiched PDMS–PI–PDMS structure is beneficial for integration with other PDMS-based microfluidic chips. The effects of electrode-tip shapes and numbers of the electrode pairs are also investigated. A novel calculating method is proposed to obtain more precise results when different numbers of electrode pairs are used in different situations. According to the experimental results, the more electrode pairs are used in the same spacing, the better measurement precision can be obtained. The volume sensor with optimized electrode-tip and multi-electrode pairs can detect the fluid volume in nanolitre scales with the relative error of < 0.8%. This work exhibits the potential to form a total lab-on-a-chip without peripheral ration pumps.

Biomimetic-inspired micro-nano hierarchical structures for capacitive pressure sensor applications

Capacitive pressure sensors are an important part of flexible/stretchable electronic skin applications. The fabrication of most of the reported polydimethylsiloxane (PDMS) based capacitive pressure sensors involves complex techniques and expensive tools. To improve the capacitive sensing properties of PDMS, a bio-mimicking hierarchical micro- and nanostructure that are facile, cost-effective, and scalable is presented in this work. Red rose petals are known to consist of hierarchical micropapillae and nanofolds. So, the artificial fabrication of these micro- and nanostructures provides a necessary roughness for pressure sensors. The sensors comprise both single and double layers of well-defined PDMS structures with indium tin oxide (ITO) electrodes. This capacitive pressure sensor allows the sensitive detection of both static and dynamic external stimuli. By successfully mimicking rose petals whose hemisphere contains micropapillae and nanofolds, ultra-sensitive capacitive pressure sensors show a sensitivity of 0.055 kPa−1 over a wide pressure range (0.5–10 kPa) with a fast response time (<200 ms) and high stability. The sensing performance also checked for capability to accurately pulse monitoring at the human wrist which demonstrated the bilayer pressure sensor has promising application in wearable electronics.

Intermediate layer-based bonding techniques for polydimethylsiloxane/digital light processing 3D-printed microfluidic devices

There is profound potential in developing novel microfluidic devices by integrating 3D-printed structures with polydimethylsiloxane (PDMS) to reap the advantages. One important aspect of the successful development of such microfluidic devices is to achieve a uniform and strong bond between PDMS and the 3D-printed structure. In this paper, we present the measurement results of the bonding strength between PDMS and digital light processing 3D-printed structures using intermediate layer-based bonding techniques. Five different intermediate layers were investigated: double-sided tape, a PDMS/tape composite, UV glue, (3-Aminopropyl) triethoxysilane (APTES), and sputter-coated SiO2. A simple burst test structure was designed and fabricated. Out of five available resins from the manufacturer, we chose the yellow resin to compare the bonding strengths with those intermediate layers. The burst test system consisted of a computer-controlled solenoid valve, pressure regulators, and a microscope for inspection. The sputter-coated SiO2 intermediate layer with a thickness of 77 nm demonstrated the highest bonding strength compared to other intermediate layers. The burst pressure with this SiO2 intermediate layer was greater than 436.65 kPa. Furthermore, we measured the burst pressure using the same SiO2 intermediate layer process for two other resins (emerald and black). The burst pressures for these were less than that of the yellow resin. The results indicate that the intermediate layer thickness and oxygen treatment processes have to be optimized for each resin.

A study on a hybrid structure flexible electro-rheological microvalve for soft microactuators

In order to realize a fluidic soft microactuator with a built-in control valve, this paper presents a cantilever type flexible electro-rheological microvalve (FERV) with a hybrid flow channel structure made from polydimethylsiloxane (PDMS) and SU-8. The hybrid structure provides high flexibility with the PDMS structure while only slight expansion occurred under high pressure with the SU-8 structure. In addition, its flexible electrodes are realized by UV-curable PEDOT:PSS (poly(3,4-ethylenedioxythiophene) polystyrene sulfonate) that is a flexible conductive polymer and can be fabricated by simple and fast fabrication process without high-cost equipment. The proposed FERV can control the flow rate of the electro-rheological fluid (ERF) through the flow channel by changing its apparent viscosity with an applied electric field. FEM simulations were conducted to demonstrate the flexural rigidity of the designed FERV and compare it with the previous FERVs. Developing micro-electro-mechanical systems (MEMS) processes using the photolithography technique, the FERV was successfully fabricated and its characteristics were experimentally clarified. The results showed the feasibility of the proposed FERV in the soft microactuator application.

Structural Formation of Oil-in-Water (O/W) and Water-in-Oil-in-Water (W/O/W) Droplets in PDMS Device Using Protrusion Channel without Hydrophilic Surface Treatment.

This paper presents a simple method of droplet formation using liquids that easily wet polydimethylsiloxane (PDMS) surfaces without any surface treatment. Using only structural features and uniform flow focusing, Oil-in-Water (O/W) and Water-in-Oil-in-Water (W/O/W) droplets were formed in the full PDMS structure. Extrusion channel and three-dimensional flow focusing resulted in effective fluidic conditions for droplet formation and the droplet size could be precisely controlled by controlling the flow rate of each phase. The proposed structure can be utilized as an important element for droplet based research, as well as a droplet generator.

Effects of Pulse Interval and Dosing Flux on Cells Varying the Relative Velocity of Micro Droplets and Culture Solution

Microdroplet dosing to cell on a chip could meet the demand of narrow diffusion distance, controllable pulse dosing and less impact to cells. In this work, we studied the diffusion process of microdroplet cell pulse dosing in the three-layer sandwich structure of PDMS (polydimethylsiloxane)/PCTE (polycarbonate) microporous membrane/PDMS chip. The mathematical model is established to solve the diffusion process and the process of rhodamine transfer to micro-traps is simulated. The rhodamine mass fraction distribution, pressure field and velocity field around the microdroplet and cell surfaces are analyzed for further study of interdiffusion and convective diffusion effect. The cell pulse dosing time and drug delivery efficiency could be controlled by adjusting microdroplet and culture solution velocity without impairing cells at micro-traps. Furthermore, the accuracy and controllability of the cell dosing pulse time and maximum drug mass fraction on cell surfaces are achieved and the drug effect on cells could be analyzed more precisely especially for neuron cell dosing.

Single layered swastika-shaped flexible linear-to-circular polarizer using textiles for S-band application

A new swastika-shaped single layered flexible linear-to-circular polarizer is proposed for Cube Satellite application in S-band. The proposed linear-to-circular polarizer is designed fully using ShieldIt Super conductive textile on a lightweight felt substrate. Principle of operation is discussed comprehensively with an equivalent circuit model and its interaction with different orthogonal polarized wave components along with a succinct study of surface currents in different polarizations. Another linear-to-circular polarizer implemented on Polydimethylsiloxane (PDMS) substrate is designed and compared with the textile-based design. A preliminary deployment scheme is also proposed and studied by analyzing its performance under different bending conditions. Both textile and PDMS structures are confirmed to be operational within the range of 2.39 to 2.45 GHz. The textile polarizer is then prototyped and validated to be exhibiting similar conversion efficiency (CE) of 98% (simulated) and 93% (measured) at 2.45 GHz. Moreover, the fractional bandwidth defined by their 90% CE limits are 7.35% (from 2.36 to 2.55 GHz) in simulations and 6.6% (from 2.36 to 2.52 GHz) in measurements. The resulting axial ratio produced is 1.4 dB (in simulations) and 2 dB (in measurements), indicated its suitability for S-band application.

"Do-it-in-classroom" fabrication of microfluidic systems by replica moulding of pasta structures.

Here, we describe a novel method for fabrication of microfluidic structures in classroom environments. This method is based on replica moulding of pasta structures in polydimethylsiloxane. Placing pasta structures on a petroleum jelly base layer enables templating round-shaped structures with controllable cross-sectional profiles. The pasta structures can be easily deformed and combined to create more complex 3D microfluidic structures. Proof-of-concept experiments indicate the capability of this method for studying the mixing of neighbouring flows, generation of droplets, lateral migration of particles, as well as culturing, shear stress stimulation, and imaging of cells. Our "do-it-in-classroom" method bridges the gap between the classroom and the laboratory.

Fabrication and thermoresistive behavior characterization of three-dimensional silver-polydimethylsiloxane (Ag-PDMS) microbridges in a mini-channel

Polydimethylsiloxane (PDMS) microstructures, such as micro-pillars, have versatile applications as actuators in microfluidic devices. With the addition of functional materials, e.g., silver (Ag) particles, PDMS structures can also attain sensing properties such as electrical conductance. In this paper, we introduce a fabrication technique using sacrificial agar to develop novel double-side hinged PDMS and Ag-PDMS microstructures, called three-dimensional microbridges, suspended in the middle of a PDMS channel. The technique offers simple and consistent fabrication of high aspect ratio (∼44) microbridges with diameters of 90–200 μm. The design has advantages of yielding electrically-conductive Ag-PDMS microbridges in the bulk of the channel with accessible terminals at channel sidewalls for connection to external electrical instruments. This allowed us to investigate the electrical performance of Ag-PDMS microbridges and their thermoresistive behavior for future applications as temperature sensors. It was found that 70% Ag-PDMS microbridges with various diameters have electrical resistances in the range of 10–110 Ω, while demonstrating an exponential increase in resistance in response to the temperature of the fluid rising from 18 °C to 51 °C. These functionalized and three dimensional microbridges can be embedded in microfluidic devices and utilized as integrated sensors for measuring the bulk properties of fluids or the existence of target biological substances in the fluid.

Highly Stretchable Supercapacitors Enabled by Interwoven CNTs Partially Embedded in PDMS

We present flexible and stretchable supercapacitors composed of interwoven carbon nanotubes (CNTs) embedded in polydimethylsiloxane (PDMS) substrates. CNTs are grown using atmospheric-pressure chemical vapor deposition (APCVD) on a Si/SiO2 substrate and then partially embedded into PDMS. This unique process permits a rapid and facile integration of the interwoven CNT–PDMS structure as a flexible and stretchable supercapacitor electrode with a high level of integrity under various strains. The electrochemical properties of the supercapacitors are measured in 30% KOH solution and with a poly(vinyl alcohol) (PVA)–KOH gel electrolyte (i.e., all-solid-state flexible supercapacitor). The measured capacitance of the supercapacitor is 0.6 mF/cm2 in 30% KOH solution and is 0.3 mF/cm2 with a PVA–KOH gel electrolyte at a scan rate of 100 mV/s, showing a consistent performance under stretching from 0% to 200% and bending/twisting angles from 0° to 180°. The stretching test is performed for 200 cycles from 0% to 100%,...

Flexible Optical Fiber Fabry–Perot Interferometer Based Acoustic and Mechanical Vibration Sensor

We have proposed and demonstrated a novel flexible optical fiber Fabry–Perot interferometer (FPI) based vibration sensor with broadband frequency response. The sensor consists of a flexible FP cavity formed by soft splicing using polydimethylsiloxane (PDMS) material with low Young's Modulus and a compact fiber cantilever of low moment inertia as the vibration receiver. Acoustic or mechanical waves experienced by the fiber cantilever can result in obvious change of the FPI cavity length and finally be transduced to the intensity variation of the sensor signal, which makes the sensor highly sensitive to external acoustic or mechanical vibrations. Taking advantage of the superior elasticity of PDMS and compact structure of cantilever, a broadband frequency response for vibration detection has been achieved together with high sensitivity without extra external vibration receiver. The vibration detection is theoretically analyzed, and acoustic vibrations up to 20 kHz and damped mechanical vibrations from tens of hertz to tens of kilohertz, covering both the sound and part of ultrasound range, have been experimentally detected. Moreover, each event of the ball hitting and rebounding has been accurately identified by our sensor, showing excellent vibration sensing only by a thin optical fiber. We believe such a broadband, compact and cost-effective sensor would be a promising candidate for monitoring both acoustic and mechanical vibrations.

Multidirectional flexible force sensors based on confined, self-adjusting carbon nanotube arrays

We demonstrate a highly sensitive force sensor based on self-adjusting carbon nanotube (CNT) arrays. Aligned CNT arrays are directly synthesized on silicon microstructures by a space-confined growth technique which enables a facile self-adjusting contact. To afford flexibility and softness, the patterned microstructures with the integrated CNTs are embedded in polydimethylsiloxane structures. The sensing mechanism is based on variations in the contact resistance between the facing CNT arrays under the applied force. By finite element analysis, proper dimensions and positions for each component are determined. Further, high sensitivities up to 15.05%/mN of the proposed sensors were confirmed experimentally. Multidirectional sensing capability could also be achieved by designing multiple sets of sensing elements in a single sensor. The sensors show long-term operational stability, owing to the unique properties of the constituent CNTs, such as outstanding mechanical durability and elasticity.

Three-axis scanning force sensor with liquid metal electrodes

Endoscopic palpation is a developing technology that shows promise in identifying early stage small tumors located beneath the surface of an organ. This technology requires sensors to detect the stiffness difference between stiff tumors and soft healthy tissue. This paper proposes a three-axis ballpoint-pen-type force sensor that can scan the surface to measure both shear and reactive forces, which can be used to calculate the distribution of stiffness. The sensor design includes five capacitors, four of which are formed at the lateral side of the cylindrical silicone rubber body. Fabrication of the electrodes is achieved by filling the pockets of a molded polydimethylsiloxane structure with liquid metal. The manufactured sensor was capable of identifying the magnitude and orientation of an applied shear force at 1 N. Detection of a stiff region was also successfully demonstrated.

3D Printing by Multiphase Silicone/Water Capillary Inks.

3D printing of polymers is accomplished easily with thermoplastics as the extruded hot melt solidifies rapidly during the printing process. Printing with liquid polymer precursors is more challenging due to their longer curing times. One curable liquid polymer of specific interest is polydimethylsiloxane (PDMS). This study demonstrates a new efficient technique for 3D printing with PDMS by using a capillary suspension ink containing PDMS in the form of both precured microbeads and uncured liquid precursor, dispersed in water as continuous medium. The PDMS microbeads are held together in thixotropic granular paste by capillary attraction induced by the liquid precursor. These capillary suspensions possess high storage moduli and yield stresses that are needed for direct ink writing. They could be 3D printed and cured both in air and under water. The resulting PDMS structures are remarkably elastic, flexible, and extensible. As the ink is made of porous, biocompatible silicone that can be printed directly inside aqueous medium, it can be used in 3D printed biomedical products, or in applications such as direct printing of bioscaffolds on live tissue. This study demonstrates a number of examples using the high softness, elasticity, and resilience of these 3D printed structures.

Bonding Nafion® with polydimethysiloxane: A versatile approach towards ion-exchange membrane microfluidic devices

Polydimethylsiloxane (PDMS) is one of the most widely used materials for the fabrication of microfluidic devices. The uncomplicated integration of ion-exchange membranes, such as Nafion®, would facilitate manifold electrochemical processes in microfluidics. However, the Nafion ionomer does not directly bond with cross-linked PDMS and the assembly of the membrane and PDMS substrate still remains a technological challenge. This work reports a straightforward procedure using a simple bifunctional silane as crosslinking agent to directly integrate Nafion membranes into PDMS structures. The procedure applies a plasma treatment for both Nafion membrane and PDMS substrate to generate reactive surface functional groups that covalently bond with the functional groups of the silane, resulting in the formation of a strong chemical bonding across the interface. Peel tests prove the robust and uniform bonding that is also stable in acidic solutions. The ion conductivity of the bonded Nafion is comparable to that of the commercial Nafion.

Organic Halides and Nanocone Plastic Structures Enhance the Energy Conversion Efficiency and Self-Cleaning Ability of Colloidal Quantum Dot Photovoltaic Devices

This paper presents solid-state ligand exchange of spin-coated colloidal lead sulfide quantum dot (PbS QD) films by methylammonium iodide (MAI) and integration of them in depleted heterojunction solar (DHS) devices having an antireflecting (AR) nanocone plastic structure. Time-resolved photoluminescence measurements determine a shorter lifetime of the charge carries on a semiconductor (TiO2) electron transfer layer for the MAI-passivated QD films as compared with those with long-chain aliphatic or short thiol ligands. Consequently, the DHS device yields improved power conversion efficiency (>125%) relative to oleic-acid-passivated PbS QD films. Using anodized aluminum oxide templates, an inverted nanocone polydimethylsiloxane structure was also prepared and utilized as an AR layer in the DHS device. The solar cells exhibit an energy conversion efficiency of 7.5% with enhanced water-repellant ability.