PDMS Structures Research Articles

Hydrophobicity Recovery of Polydimethylsiloxane Treated with Oxygen Plasma and Ion Implantation

One of a few disadvantage properties of polymethylsiloxane (PDMS) is its hydrophobicity which can hinder its fluidic applications. Oxygen plasma treatment on PDMS surfaces is a conventional method in rendering them hydrophilic, however hydrophobicity returns within days after treatment. This study proposes oxygen ion implantation at the micrometer scale inside PDMS structures to ensure lasting hydrophilicity. The PDMS slabs were treated with 20 keV oxygen ion with 1 × 1015 ions/cm2 fluence and inductively coupled oxygen plasma generated at a power of 28 W and frequency of 13.56 MHz. The chemical composition of the treated PDMS surface was monitored by X-ray photoelectron spectroscopy (XPS) and it was found that oxygen concentration remain high after three days of oxygen plasma treatment on the PDMS. Although the surface oxygen concentration in the oxygen ion implanted PDMS was not as high as that oxygen plasma treated PDMS, the oxygen concentration level was remained higher than that of the untreated PDMS surface for four weeks.

An integrated gas-liquid droplet microfluidic platform for digital sampling and detection of airborne targets

The use of microfluidic droplets has become ubiquitous in many Lab-on-a-Chip (LOC) applications ranging from material synthesis to novel biochemical sensing. In this paper, we introduce a new droplet-based approach that incorporates a gas phase for generating liquid droplet microreactors in a microfluidic flow-focusing format. We demonstrate the subsequent on-chip transition, collection and handling of the droplets in a secondary liquid carrier inside a multilayer PDMS structure. The presented technique has potential applications in capturing and probing airborne particles and gaseous vapors using high surface-to-volume picoliter droplets. The discrete microfluidic gas-liquid interfaces created in this approach, greatly facilitate absorption and up-concentration of a gaseous target analyte into the droplet volume. The chip-based format of the units also allows for different microfluidic modules and analytical techniques to be integrated in this platform for droplet probing, providing highly-sensitive LOC detection systems. Here, we demonstrate the basic principles of sample partitioning with gas-liquid droplets by capturing and detection of vaporized ammonia at different gaseous concentrations using Nessler’s reaction inside the droplets. The results of this work provide a simple and robust quantification approach for determining gaseous ammonia which can be further expanded to other gas-phase analytes in next generation of airborne target detectors for human breath analysis and environmental monitoring.

A practical fabrication method of the gecko-inspired easy-removal skin adhesives

Abstract Easy-removal skin adhesives require a robust reversible adhesion. This requirement is addressed in this study with the fabrication of PDMS micro-patterned surfaces that inspired by gecko feet. The design of these gecko-inspired structures were aimed to maximize the ratio between pull-off strength and peel strength. They were fabricated using the laser cutting technology which is typically used for industrial manufacturing applications. Several kinds of PDMS specimens in triangular, square and hexagonal patterns, as well as triangular, square, diamond and circular cross-sections were made. The wetting properties of the gecko-inspired surfaces were evaluated by contact angle measurements. Pull-off strength and peel strength measurements were performed against a silicon skin substitute. Multiple attachments were achieved on a range of preloads. The averaged pull-off strength under a preload 10 N for 10 s can reach up to approximately five times of peel strength when the peel angle is 30 degree. Also compared with conventional Band-Aids, a slight enhancement in attachment ability and a significant decrease in peel strength between the skin and the adhesives were consistently observed. Therefore, the fabrication of the gecko-inspired structures on the micro-molding of PDMS appeared to offer a near-practical way for manufacturing an easy-removal skin adhesives, albeit in its present form with a comparable adhesion strength and a decreased peel strength. The originality of this work is the reverse de-molding approach based on the combination of the cost- and time-efficient laser cutting methods and the Teflon film as the mold material, which avoid the limitation caused by taking PDMS structure out of the molds, so that provide more variations of the tip geometry. As such, a further development of this fabrication method might be of significant interest in a number of practical applications in skin tissue industrial design.

Compliant Ultrasound Proximity Sensor for the Safe Operation of Human Friendly Robots Integrated with Tactile Sensing Capability

The robot proximity and tactile sensors can be categorized into two groups: grip sensors and safety sensors. They have different performance requirements. The safety sensor should have long proximity range and fast response in order to secure enough response time before colliding with ambient objects. As for the tactile sensing function, the safety sensor need to be fast and compliant to mitigate the impact from a collision. In order to meet these requirements, we proposed and demonstrated a compliant integrated safety sensor suitable to human-friendly robots. An ultrasonic proximity sensor and a piezoelectric tactile sensor made of PVDF films have been integrated in a compliant PDMS structure. The implemented sensor demonstrated the maximum proximity range of 35 cm. The directional tolerance for 30 cm detection range was about ±15° from the normal axis. The integrated PVDF tactile sensor was able to detect various impacts of up to 20 N in a controlled experimental setup.

Fabrication of a microchannel using chain-shaped alginate hydrogel fibers as sacrificial structure

In this research, we proposed a method to fabricate a PDMS chain-shaped microchannel by using a chain-shaped alginate hydrogel fiber as sacrificial structure. Embedding and dissolving a chain-shaped alginate hydrogel fiber in a PDMS structure, we succeeded in formation of a chain-shaped microchannel. When we infused 2 different colors of ink into this microchannel from 2 inlets, we achieved the control of flow regimes between laminar and vibrated flows according to their flow rates. Therefore, we believe that the chain-shaped alginate hydrogel fiber will be a useful tool for a simple and low-cost preparation of PDMS microchannels with complex shapes.

Porous PDMS structures for the storage and release of aqueous solutions into fluidic environments

Typical microfluidic systems take advantage of multiple storage reservoirs, pumps and valves for the storage, driving and release of buffers and other reagents. However, the fabrication, integration, and operation of such components can be difficult. In particular, the reliance of such components on external off-chip equipment limits their utility for creating self-sufficient, stand-alone microfluidic systems. Here, we demonstrate a porous sponge made of polydimethylsiloxane (PDMS), which is fabricated by templating microscale water droplets using a T-junction microfluidic structure. High-resolution microscopy reveals that this sponge contains a network of pores, interconnected by small holes. This unique structure enables the sponge to passively release stored solutions very slowly. Proof-of-concept experiments demonstrate that the sponge can be used for the passive release of stored solutions into narrow channels and circular well plates, with the latter used for inducing intracellular calcium signalling of immobilised endothelial cells. The release rate of stored solutions can be controlled by varying the size of interconnecting holes, which can be easily achieved by changing the flow rate of the water injected into the T-junction. We also demonstrate the active release of stored liquids into a fluidic channel upon the manual compression of the sponge. The developed PDMS sponge can be easily integrated into complex micro/macro fluidic systems and prepared with a wide array of reagents, representing a new building block for self-sufficient microfluidic systems.

Copy Number Variation Analysis of Circulating Tumor Cells at a Single Cell Level Based on Hydrogel Encapsulation

Circulating tumor cells (CTCs) are defined as tumor cells circulating in peripheral blood of patients with metastatic cancer. CTCs have potential as a biomarker for cancer diagnosis or drug discovery. Recently, much attention has been paid to genomic analyses such as copy number variation (CNV) analysis of CTCs, because it will provide important information related to treatment for personalized therapy. These analyses should be performed at a single cell level because of the heterogeneity of cancer cells. However, CTCs are extremely rare cell in blood (1 in 109blood cells), so highly efficient technique for CTC recovery and single-cell isolation is required. Our group has developed “microcavity array system” on which rare CTCs can be efficiently captured from whole blood on the basis of differences in the size and deformability between tumor and blood cells. Furthermore, a novel technique for a single cell manipulation using a photo-polymerized hydrogel, “hydrogel-based cell encapsulation” has been demonstrated. In this study, to evaluate the performance of “microcavity array system” and “hydrogel-based cell encapsulation”, whole genome amplification (WGA) was performed using single-CTCs isolated by our proposed method. Furthermore, the utility of the proposed method in CNV analysis were evaluated. The microcavity array (φ= 8 μm, 4×103 cavities) was created by electroforming technique as microfabricated filter. The microcavity array was fabricated with PDMS structure equipped with a vacuum microchannel to apply negative pressure for cell entrapment. A549 cells (lung cancer cell line), SKBR-3 cells (breast cancer cell line) or MCF-7 cells (breast cancer cell line) were used as model CTCs. These cells were stained with fluorescent dyes, and suspended in PBS or healthy whole blood. These cell suspensions were introduced into the microcavity array at a flow rate of 150 µl/min. After cell recovery process, the number of cancer cells captured on the microcavity array was counted under a fluorescence microscope. As a results, more than 95% of cancer cells was successfully recovered on the microcavity array from PBS and whole blood. Then, poly (ethylene glycol) diacrylate (PEGDA) hydrogel was introduced onto the MCA. An excitation light (λ=365 nm) was irradiated to the targeted cancer cells for 30 seconds, leading to cross-linking reaction of PEGDA hydrogel and encapsulation of a single-cell. Solidified PEGDA hydrogel can be easily recovered from microcavity array by tweezers. Fluorescent microscopic analysis revealed that targeted single-cells were successfully encapsulated with PEGDA hydrogel. The hydrogel-based cell encapsulation process for 20 single-cells was completed within 10 min. To evaluate the effect of hydrogel on genome amplification, hydrogels with single-cells were subjected to the WGA, and the yields of WGA products were measured by PicoGreen assay. Genome amplification was also successfully carried out in all single-cells encapsulated on the hydrogel, and the yields of WGA products were the same with those obtained by using the micromanipulation as a conventional method. The amplification bias of WGA was evaluated based on the success rate of PCR amplification using 9 different primer sets (multiple loci). The PCR amplification of 9 fragments revealed that no significant bias was observed between WGA samples from single-cells isolated by hydrogel encapsulation and micromanipulation. Based on these results, we concluded that our single cell isolation method could be utilized for WGA at single cell level. Furthermore, quantitative PCR was performed to evaluate CNV analysis using SKBR-3 and MCF-7 cells, which have different HER2 status. As a result, the level of HER2 copy number in WGA products obtained from single-cells was similar with that from bulk analysis (103cells).Determination and monitoring of HER2 status in genome is well known to serve as a useful biomarker for diagnosis of breast cancer. Therefore, our proposed method for CNV analysis of CTCs is expected to be widely utilized towards real-time cancer diagnosis without repeating surgical biopsy.

Structural Properties and Antifungal Activity against Candida albicans Biofilm of Different Composite Layers Based on Ag/Zn Doped Hydroxyapatite-Polydimethylsiloxanes.

Modern medicine is still struggling to find new and more effective methods for fighting off viruses, bacteria and fungi. Among the most dangerous and at times life-threatening fungi is Candida albicans. Our work is focused on surface and structural characterization of hydroxyapatite, silver doped hydroxyapatite and zinc doped hydroxyapatite deposited on a titanium substrate previously coated with polydimethylsiloxane (HAp-PDMS, Ag:HAp-PDMS, Zn:HAp-PDMS) by different techniques: Scanning Electron Microscopy (SEM), Glow Discharge Optical Emission Spectroscopy (GDOES) and Fourier Transform Infrared Spectroscopy (FTIR). The morphological studies revealed that the use of the PDMS polymer as an interlayer improves the quality of the coatings. The structural characterizations of the thin films revealed the basic constituents of both apatitic and PDMS structure. In addition, the GD depth profiles indicated the formation of a composite material as well as the successful embedding of the HAp, Zn:HAp and Ag:HAp into the polymer. On the other hand, in vitro evaluation of the antifungal properties of Ag:HAp-PDMS and Zn:HAp-PDMS demonstrated the fungicidal effects of Ag:HAp-PDMS and the potential antifungal effect of Zn:HAp-PDMS composite layers against C. albicans biofilm. The results acquired in this research complete previous research on the potential use of new complex materials produced by nanotechnology in biomedicine.

Dimensional effects of polymer pillar arrays on hydrophobicity

This paper reports experimental studies on the dimensional effects of pillar arrays on surface hydrophobicity. Completely different behaviours are observed on intrinsically hydrophobic and hydrophilic materials. In order to experimentally observe the Cassie-to-Wenzel transition, circular pillar arrays with varying diameters and spacing are fabricated on PDMS and SU-8 surfaces. Wetting experiments show that the PDMS arrays demonstrate clear Cassie-to-Wenzel transition as the spacing increases. The critical transition spacing for different diameters shows a close match to theoretical predictions based on a force balance model. The transition model also predicts the maximum contact angles. By comparison, the dimensional effects on an intrinsically hydrophilic surface (SU-8) show smaller contact angles without a transition phase. These differences come from the inherent wettability of the two polymers. Without further coatings or treatment, the PDMS structures can achieve contact angles close to 170° while the SU-8 arrays can only achieve contact angles up to 140°.

Protein patterning by microcontact printing using pyramidal PDMS stamps.

Micro-contact printing, μCP, is a well-established soft-lithography technique for printing biomolecules. μCP uses stamps made of Poly(dimethylsiloxane), PDMS, made by replicating a microstructured silicon master fabricated by semiconductor manufacturing processes. One of the problems of the μCP is the difficult control of the printing process, which, because of the high compressibility of PDMS, is very sensitive to minute changes in the applied pressure. This over-sensitive response leads to frequent and/or uncontrollable collapse of the stamps with high aspect ratios, thus decreasing the printing accuracy and reproducibility. Here we present a straightforward methodology of designing and fabricating PDMS structures with an architecture which uses the collapse of the stamp to reduce, rather than enlarge the variability of the printing. The PDMS stamp, organized as an array of pyramidal micro-posts, whose ceiling collapses when pressed on a flat surface, replicates the structure of the silicon master fabricated by anisotropic wet etching. Upon application of pressure, depending on the size of, and the pitch between, the PDMS pyramids, an air gap is formed surrounding either the entire array, or individual posts. The printing technology, which also exhibits a remarkably low background noise for fluorescence detection, may find applications when the clear demarcation of the shapes of protein patterns and the distance between them are critical, such as microarrays and studies of cell patterning.Electronic supplementary materialThe online version of this article (doi:10.1007/s10544-016-0036-4) contains supplementary material, which is available to authorized users.

Optimization of a PDMS structure for energy harvesting under compressive forces

This paper reports the optimisation of a PDMS structure for energy harvesting applications. The PDMS structure was optimised using ANSYS simulation. The fabrication processes were also optimised for maximum PDMS ferroelectret energy harvesting performance. The optimised PDMS structure was fabricated using a 3D-printed plastic mould. The ANSYS simulation and experiment results demonstrated the variation in energy harvesting performance of the PDMS ferroelectret depending upon the different void geometry. The optimised operating frequency is determined by the geometry of the voids inside ferroelectrets and the Young's modulus of the PDMS. The peak voltage and energy generated per strike under a compressive load was obtained experimentally as a function of force applied and frequency. The experimental maximum peak output power of an optimised PDMS structure with an area of 2×2 cm2 was 0.46 μW across an optimum load resistance of 21 MΩ under a square wave force of 800 N and 17 Hz.

Miniature and Reconfigurable CPW Folded Slot Antennas Employing Liquid-Metal Capacitive Loading

Microfluidic channels filled with liquid metal are used to realize miniature and reconfigurable CPW folded slot antennas. The method is based on employing the reactive loading effect of fluid metal bridges on top of the CPW slot antenna. As a result of this reactive loading, the frequency of the antenna reduces and the antenna is miniaturized by a factor of 85%. Also, by changing the configuration of filled and empty channels, each channel can be used as a switch. By using two pairs of microfluidic channels, three different frequency bands of 2.4, 3.5, and 5.8 GHz can be achieved. This translates to a switching ratio ( $\bm{f}_{\bm{TR}}=\bm{f}_{\mathbf{2}}/\bm{f}_{\mathbf{1}}$ ) of more than 2.5. The antenna is realized using common PCB techniques for the antenna circuit board and three-dimensional (3-D) printing technology for PDMS-based microfluidics structure. The antenna circuit board and the PDMS structure are bonded to each other using a very thin spin-coated PDMS layer. Design methodology, simulation, and measurement results of both antenna prototypes are presented. Both the miniature and reconfigurable antennas have similar radiation patterns to a normal CPW folded slot antenna and show low cross-polarization levels at all operating frequencies.

Self-assembly of polydimethylsiloxane structures from 2D to 3D for bio-hybrid actuation

This work aims to demonstrate the feasibility of a novel approach for the development of 3D self-assembled polydimethylsiloxane structures, to be used as engineered flexible matrices for bio-hybrid actuation. We described the fabrication of engineered bilayers, organized in a 3D architecture by means of a stress-induced rolling membrane technique. Such structures were provided with ad hoc surface topographies, for both cell alignment and cell survival after membrane rolling. We reported the results of advanced finite element model simulations, predicting the system behavior in terms of overall contraction, induced by the contractile activity of muscle cells seeded on the membrane. Then, we tested in vitro the structure with primary cardiomyocytes to evaluate the real bio-actuator contraction, thus validating the simulation results. At a later stage, we provided the samples with a stable fibronectin coating, by covalently binding the protein on the polymer surface, thus enabling long-term cultures with C2C12 skeletal muscle cells, a more controllable cell type. These tests revealed cell viability and alignment on the rolled structures, but also the ability of cells to differentiate and to form multinucleated and oriented myotubes on the polymer surface, also supported by a fibroblast feeder layer. Our results highlighted the possibility of developing 3D rolled PDMS structures, characterized by different mechanical properties, as novel bio-hybrid actuators.

A flexible multimodal tactile display array for virtual shape and texture

Multimodal tactile display systems that are capable of delivering versatile tactile information effectively are of great interest for virtual reality and tele-operation applications. In this paper, we present a new flexible multimodal tactile display system that delivers shape and texture information simultaneously using a single actuator. The proposed tactile display is actuated by a combination of electromagnetic and pneumatic forces and thus exhibits high tactile force to deliver shape information and large vibration amplitude to deliver texture information. In addition, the proposed tactile display device was implemented on a flexible PDMS structure to allow for tight attachment on any curved body to transfer reliable tactile information. By applying a current pulse of 300 mA for 5 ms, we readily controlled the state of the tactile actuator and measured the actuation displacement of 900 μm and the generated force of 20.1 gf. The average vibration amplitude of 30 μm was measured over a wide dynamic range (25---650 Hz) with an applied current of 74 mA. Lastly, by delivering various tactile information to users, we verified the functionalities of the implemented 4 ? 1 multimodal tactile display array. User study results showed that the fabricated multimodal tactile display system successfully delivered both shape information with an accuracy of 73 % and texture information with an accuracy of 90 %. This work demonstrates the potential of our multimodal tactile display system for uses in various applications such as tele-operation, tactile communication, and visual presentation for visually impaired.

μ-biomimetic flow-sensors—introducing light-guiding PDMS structures into MEMS

In the area of biomimetics, engineers use inspiration from natural systems to develop technical devices, such as sensors. One example is the lateral line system of fish. It is a mechanoreceptive system consisting of up to several thousand individual sensors called neuromasts, which enable fish to sense prey, predators, or conspecifics. So far, the small size and high sensitivity of the lateral line is unmatched by man-made sensor devices. Here, we describe an artificial lateral line system based on an optical detection principle. We developed artificial canal neuromasts using MEMS technology including thick film techniques. In this work, we describe the MEMS fabrication and characterize a sensor prototype. Our sensor consists of a silicon chip, a housing, and an electronic circuit. We demonstrate the functionality of our -biomimetic flow sensor by analyzing its response to constant water flow and flow fluctuations. Furthermore, we discuss the sensor robustness and sensitivity of our sensor and its suitability for industrial and medical applications. In sum, our sensor can be used for many tasks, e.g. for monitoring fluid flow in medical applications, for detecting leakages in tap water systems or for air and gas flow measurements. Finally, our flow sensor can even be used to improve current knowledge about the functional significance of the fish lateral line.

An investigation of PDMS structures for optimized ferroelectret performance

This paper reports the ANSYS simulation and fabrication processes for optimising PDMS ferroelectret performance. The proposed model extends the previously published analytical models and compares this with simulation of individual void geometry. The ferroelectret material is fabricated from PDMS using 3D-printed plastic moulds. The analytical model and Ansys simulation results predict the variation in performance of the PDMS ferroelectret with the different void geometry and surface charge density. The theoretical maximum piezoelectric coefficient d33 achieved was about 220 pC/N. The experimental maximum d33 obtained was 172 pC/N.

Effects of the structures of end groups of pendant polydimethylsiloxane attached to a polyurethane copolymer on the low temperature toughness

End groups with different structures were grafted to polyurethane (PU) using poly(dimethylsiloxane) (PDMS) as a spacer. The low-temperature toughness of the PUs was tested at −30°C, and the structure selectivities of the end groups for low-temperature toughness were compared. The PDMS functioned as a flexible linker that connected the end groups to the PUs. The tensile strength of the PU generally improved despite the grafting of PDMS and end groups. The conventional shape recovery ratio at 45°C remained greater than 90%, regardless of the content and structure of PDMS and the end group. The PU that contained an adamantyl group (cubic) or a naphthyl group (planar rectangle) showed instant recovery, even at −30°C, but the PU that contained a phenyl (planar square) or phenoxyphenyl (bent squares) group required warming to 0°C for a similar degree of recovery but showed improvement over a linear PU without any end group. The characteristic structure of the end group was responsible for the selective low-temperature toughness. The low-temperature toughness results and the thermal and mechanical properties of the PUs are discussed. POLYM. ENG. SCI., 55:1931–1940, 2015. © 2014 Society of Plastics Engineers

Synthesis and Thermal Stability of Crosslinked Carbazole-Substituted Poly(Dimethylsiloxane) for Led Encapsulation

A series of carbazole substituted poly(dimethylsiloxane) (PDMS) was synthesized by hydrosilylation reaction of 9-vinyl carbazole into PDMS structure. It was then crosslinked with vinyl trimethoxysilane by condensation reaction. The fabricated product increased in refractive index with increasing carbazole content. The higher carbazole content also produced high thermal stability with an increase in Tg and degradation temperature. This behaviour owe much to the carbazole moiety as well as the formation of crosslink network These features contribute to potential use as electronic devices encapsulants and coatings.

Real-time In-situ Lactate Monitoring in 3D Multi-cellular Spheroid Cultures by Using Enzyme-based Biosensors in Hanging Drop Networks

We present the integration of enzyme-based lactate biosensors into a microfluidic network of hanging drops, in which spherical microtissues can be formed and cultured over time. The addition of biosensors enables continuous in-situ monitoring of effects upon applying different culturing conditions or dosing compounds to the 3D tissue structures. The integration has been achieved by using a hybrid fabrication method, in which sensor modules on glass are embedded into a microfluidic PDMS structure. The lactate biosensors enabled real-time measurements of lactate produced by human colon carcinoma spheroids.

Trapping self-propelled micromotors with microfabricated chevron and heart-shaped chips

We demonstrate that catalytic micromotors can be trapped in microfluidic chips containing chevron and heart-shaped structures. Despite the challenge presented by the reduced size of the traps, microfluidic chips with different trapping geometries can be fabricated via replica moulding. We prove that these microfluidic chips can capture micromotors without the need for any external mechanism to control their motion.