Transparent Polydimethylsiloxane Research Articles

Research on the selective adhesion characteristics of polydimethylsiloxane layer

Polydimethylsiloxane (PDMS) is a widely used inexpensive, non-toxic material which has many advantages. But it is generally considered that PDMS does not adhere to the other substrates without the special treatments due to its low surface energy. However, in this paper, it is the first time that we found that the PDMS adhered on the silicon dioxide substrate to form an adhesive layer only by the routine cast molding process. The mechanism of the PDMS adhesion on the silicon dioxide substrate during the process was illustrated in detail. The smooth, thin, transparent, hydrophobic and selective PDMS adhesion layer can be used as a functional coating to improve the special performances of the micro/nano devices. Finally, as an example, a facile approach is proposed to realize the superhydrophobic surfaces by combining the SiO2 microstructure with the PDMS adhesion layer.

Design, Fabrication, and Testing of a Microfluidic Device for Thermotaxis and Chemotaxis Assays of Sperm.

Infertile couples needing assisted reproduction are increasing, so a fundamental understanding of motile sperm migration is required. This paper presents an advanced microfluidic device for sperm motion analysis utilizing chemotaxis and thermotaxis simultaneously for the first time. The proposed device is a transparent polydimethylsiloxane- and glass-based microfluidic chip system providing a low-cost, useful, and disposable platform for sperm analysis. The concentration gradient of the chemoattractant (acetylcholine) and the temperature difference are formed along the microchannel. The temperature gradient is generated and controlled by a microheater and microsensor. Thermotactic and chemotactic responses of mouse sperm were examined using the proposed device. Experimental results show that motile mouse sperm are attracted more sensitively under integrated conditions of chemotaxis and thermotaxis rather than individual conditions of chemotaxis and thermotaxis. This sperm analysis device is expected to be a useful tool for the study of mammalian sperm migration and the improvement of artificial insemination techniques.

Conductive and transparent submicron polymer lens array fabrication for electrowetting applications

In this study, we have demonstrated a simple and inexpensive process to fabricate electrowetting lens arrays with various curvatures (micron to submicron) on conductive and transparent polydimethylsiloxane (PDMS) molds without additional metal layers. The microlens arrays were fabricated using one-step dual diffuser lithography process, which utilizes a pair of diffusers to diffract the incident rays of UV light at wide angles before approaching the photoresist. Dimensional control and high fill factor was achieved by just varying the exposure energy and gap between the patterns in the photomask, respectively. The patterns were replicated in conductive and transparent Ag(n)-PDMS (5–20% Ag) with 15 μm thickness. High conductivity of 4.6 × 10−1 S/m and high transmission efficiency of 90% was demonstrated by Ag(n)-PDMS molds. Micro-nanolens arrays fabricated by the optimized corelation were utilized to demonstrate switchable wettability behavior of water droplet at different applied voltages. The electrowetting microlens array fabrication method introduced in this work has high potential to be incorporated in optoelectronics and biomedical devices.

A skin-like stretchable colorimetric temperature sensor

Wearable and stretchable physical sensors that can conformally contact on the surface of organs or skin provide a new opportunity for human-activity monitoring and personal healthcare. Particularly, various attempts have been made in exploiting wearable and conformal sensors for thermal characterization of human skin. In this respect, skin-mounted thermochromic films show great capabilities in body temperature sensing. Thermochromic temperature sensors are attractive because of their easy signal analysis and optical recording, such as color transition and fluorescence emission change upon thermal stimuli. Here, desirable mechanical properties that match epidermis are obtained by physical crosslinking of polydiacetylene (PDA) and transparent elastomeric polydimethylsiloxane (PDMS) networks. The resulting PDA film displayed thermochromic and thermofluorescent transition temperature in the range of 25–85°C, with stretchability up to 300% and a skin-like Young’s modulus of ~230 kPa. This easy signal-handling provides excellent references for further design of convenient noninvasive sensing systems.

Transparent-flexible-multimodal triboelectric nanogenerators for mechanical energy harvesting and self-powered sensor applications

Triboelectric nanogenerators (TENGs) harvest and convert mechanical energy to electrical energy. TENGs that are transparent and flexible can be applied to various (opto-)electronic devices supporting finger- or pen-based touchscreen inputs. This paper presents a transparent, flexible TENG that harvests mechanical tapping energy (typically discarded) by simple placement on touchscreen devices. The developed TENG consists of flexible and transparent conducting electrodes (FTCE) with high transmittance (> 93%) and low sheet resistance (18.5 Ω/sq), and transparent 3D-hierarchical polydimethylsiloxane (PDMS) with porous pyramid-patterns. In this study, the developed TENG directly powered eight light-emitting diodes (LEDs) by harvesting the mechanical energy produced by tapping with a touch pen while playing a smartphone game. We also used the transparent TENG as a transparent single-electrode-based, self-powered raindrop detection sensor on a window for a smart home. Our results indicate that the proposed TENG can be used not only as an effective mechanical energy harvester for transparent, flexible, and next-generation optoelectronics devices but also as a self-powered sensor for future Internet-of-Things applications.

Dictionary learning-based reverberation removal enables depth-resolved photoacoustic microscopy of cortical microvasculature in the mouse brain

Photoacoustic microscopy (PAM) capitalizes on the optical absorption of blood hemoglobin to enable label-free high-contrast imaging of the cerebral microvasculature in vivo. Although time-resolved ultrasonic detection equips PAM with depth-sectioning capability, most of the data at depths are often obscured by acoustic reverberant artifacts from superficial cortical layers and thus unusable. In this paper, we present a first-of-a-kind dictionary learning algorithm to remove the reverberant signal while preserving underlying microvascular anatomy. This algorithm was validated in vitro, using dyed beads embedded in an optically transparent polydimethylsiloxane phantom. Subsequently, we demonstrated in the live mouse brain that the algorithm can suppress reverberant artifacts by 21.0 ± 5.4 dB, enabling depth-resolved PAM up to 500 µm from the brain surface.

Research of detection depth for graphene-based optical sensor

Graphene-based optical sensors have been developed for research into the biological intercellular refractive index (RI) because they offer greater detection depths than those provided by the surface plasmon resonance technique. In this Letter, we propose an experimental approach for measurement of the detection depth in a graphene-based optical sensor system that uses transparent polydimethylsiloxane layers with different thicknesses. The experimental results show that detection depths of 2.5 μm and 3 μm can be achieved at wavelengths of 532 nm and 633 nm, respectively. These results prove that graphene-based optical sensors can realize long-range RI detection and are thus promising for use as tools in the biological cell detection field. Additionally, we analyze the factors that influence the detection depth and provide a feasible approach for detection depth control based on adjustment of the wavelength and the angle of incidence. We believe that this approach will be useful in RI tomography applications.

Deformation Measurements of Randomly Rough Surfaces

Measurements of surface deformations as part of the “Contact Mechanics Challenge” were collected using digital image correlation (DIC). For these experiments, a scaled version (1000×) of the periodic and random roughness surface provided for the “Contact Mechanics Challenge” was used. A 100 mm × 100 mm scale replica of the surface, approximately 10 mm thick, was 3D-printed using an opaque polymethylmethacrylate and pressed into contact against flat, transparent polydimethylsiloxane (PDMS) sheets with dead weight loads. Four different formulations of PDMS were used, and the resulting elastic moduli ranged from 64 kPa to 2.1 MPa. The DIC technique was used in situ to measure the deformation of the PDMS surface at each load increment from 22.5 to 450 N. Surface deformations in and out of contact were measured across the entire apparent area of contact and overlaid with the measurements of contact area to provide a complete description of the surface profile during loading. A direct comparison between these experiments and the simulations regarding the gap within the contact at a reduced pressure of 0.164 agrees to within ±10% when normalized to the maximum gap.

Design and analysis of a microfluidic colour-changing glasses controlled by shape memory alloy (SMA) actuators

In this work, a microfluidic colour-changing glasses controlled by shape memory alloy (SMA) actuators is designed and presented by circulating colour liquids in the microfluidic channels on the lens. For liquids controlling, a unidirectional NiTi SMA spring is employed to design and fabricate a simple microfluidic actuator, which can provide the maximum restoring force of 5.27 N and the fastest recovering rate of 2.71 mm/s. After the responding characteristics of the SMA controller are tested and calculated under different conditions, the pressure changes at the channel inlet are investigated in liquids circulation process for different channel designs. This liquid colour-changing glasses show fast response, rapid prototyping and high controllability compared with the traditional solid colour-changing way. Meantime, the impacts of different channel designs on response performances are validated, which can provide foundations for further optimization of the channel designs. Soft lithography technology is applied for the fabrication of the colour-changing layer made of transparent silicone PDMS (polydimethylsiloxane) instead of conventional mechanical machining. Organosilicon coating based on methyltrimethoxysilane and phenyltrimethoxysilane (PTMS) is firstly fabricated for modification of PDMS surface to increase the wear resistance of the colour-changing lens.

Optically transparent superhydrophobic polydimethylsiloxane by periodic surface microtexture

To achieve surface microtexture on poly(dimethysiloxane) (PDMS), laser processing of an aluminum sample and replication through hot embossing to a polycarbonate sheet, followed by PDMS thermal curing on both the etched aluminum and embossed polycarbonate is used. Surface topology measurements of the microtextured aluminum reveal microtexture height near 12μm, with slightly smaller microtexture height on the replicated surfaces. These PDMS samples exhibit optical transparency between 43% and 72% while retaining static contact angles above 150°, as the percent area without microtexture is increased from 53.8% to 81%. While samples replicated from the aluminum master exhibit roll-off angles of below 2°, samples produced using a PC master exhibit an almost complete loss of roll-off angle performance. This is a consequence of the microtexture profile being inverted at each embossing step. Optical analysis of evaporating water droplets revealed a strong pinning force at the smooth to microtexture interface, which also affects the observed roll-off angle. Samples with an untextured surface area above 81% exhibit a transition in wetting dynamics, resulting in a decrease in the static contact angle to near 130° and a change in the roll-off angle to values near 65° as exhibited on a smooth PDMS surface. The fabrication process is found to be repeatable over many months, without degradation in performance of the microtexture master or PDMS samples.

UV‐Blocking Photoluminescent Silicon Nanocrystal/Polydimethylsiloxane Composites

It is a challenge to synthesize transparent polydimethylsiloxane (PDMS) materials that can completely absorb the light energy of ultraviolet (UV) A, B, and C regions. Herein, near‐infrared (NIR) photoluminescent PDMS composites with hydrogen‐terminated silicon nanocrystals (ncSi:H) or decyl‐terminated silicon nanocrystals (ncSi‐decyl) fabricated by combination of hydrosilylation and polymer encapsulation are reported. Their morphologies, optical and mechanical properties, and thermal stabilities are investigated. It is interesting to find that these PDMS composites filled with even a very small amount of ncSi:H or ncSi‐decyl have unprecedented UV‐blocking properties and superior thermal stabilities as compared to the PDMS reference. In particular, the ncSi‐decyl/PDMS composite possesses impressive photoluminescence. Furthermore, ncSi‐decyl are favorable for reinforcing the mechanical properties of the PDMS composites as compared to the PDMS reference and the ncSi:H/PDMS composite. The enhancement in the mechanical properties and thermal stabilities of these novel PL PDMS composites depends upon the creation of crosslinkable sites and entanglement interactions between the PDMS chains and the ncSi. These results suggest that potential applications for NIR photoluminescent ncSi/PDMS composites will likely be found in the fields of advanced UV‐blocking textiles, cosmetics, photofluids, stretchable electronic devices, anticounterfeiting materials, biological imaging, and diagnostics.

In Vitro Studies on a Microfluidic Sensor with Embedded Obstacles Using New Antibacterial Synthetic Compounds (1-TDPPO) Mixed Prop-2-en-1-one with Difluoro Phenyl.

This paper describes the use of an analytical microfluidic sensor for accelerating chemo-repellent response and strong anti-bacterial 1-(Thien-2-yl)-3-(2, 6-difluoro phenyl) prop-2-en-1-one (1-TDPPO). The chemically-synthesized antimicrobial agent, which included prop-2-en-1-one and difluoro phenyl groups, was moving through an optically transparent polydimethylsiloxane (PDMS) microfluidic sensor with circular obstacles arranged evenly. The response, growth and distribution of fluorescent labeling Pseudomonas aeruginosa PAO1 against the antimicrobial agent were monitored by confocal laser scanning microscope (CLSM). The microfluidic sensor along with 1-TDPPOin this study exhibits the following advantages: (i) Real-time chemo-repellent responses of cell dynamics; (ii) Rapid eradication of biofilm by embedded obstacles and powerful antibacterial agents, which significantly reduce the response time compared to classical methods; (iii) Minimal consumption of cells and antimicrobial agents; and (iv) Simplifying the process of the normalization of the fluorescence intensity and monitoring of biofilm by captured images and datasets.

Harvesting of Living Cell Sheets by the Dynamic Generation of Diffractive Photothermal Pattern on PEDOT

Near infrared (NIR) photothermal pattern on conductive polymer film enables unique approaches to harvest large‐area cell sheets with various patterns without the use of patterned culture dish. The NIR photothermal pattern is generated from a patterned optical lens (POL), which creates a dynamic near IR light pattern and the corresponding photothermal pattern (PTP) on the polymer film. The POL is prepared from transparent polydimethylsiloxane designed to generate various light patterns. The PTPs allow a noninvasive harvest of cultured cells as an intact living cell sheet with a high harvesting efficiency (ηcell > 100). Various PTPs are generated by the diffraction of NIR light through POLs having different micropatterns, which afford cell sheets with a desired pattern without changing the original cell morphology at cultured state. Furthermore, a large‐area living cell sheet is obtained with a detached area larger than 19 cm2, which is the largest living cell sheet up to date. Further optical engineering of the harvesting system allows multiple productions of cell sheets with one dose of light. It is possible to harvest cell sheets not only from human fibroblast cells but also from human adipose‐derived stem cells, indicating that the method can be applied to engineer various cells.

A fast processing route of aspheric polydimethylsiloxane lenses array (APLA) and optical characterization for smartphone microscopy

A fast processing route of aspheric polydimethylsiloxane (PDMS) lenses array (APLA) is proposed via the combined effect of inverted gravitational and heat-assisted forces. The fabrication time can be dramatically reduced to ~30s, compared favorably to the traditional duration of ~2 hours of repeated cycles of addition-curing processes. In this paper, a low-cost flexible lens can be fabricated by repeatedly depositing, inverting, curing a hanging transparent PDMS elastomer droplet on a previously deposited curved structure. Complex structures with aspheric curve features and various focal lengths can be successfully produced and the fabricated 4 types of APLA have various focal lengths in the range of 7.03mm, 6.00mm, 5.33mm, and 4.43mm, respectively. Empirically, a direct relationship between the PDMS volume and focal lengths of the lenses can be experimentally deducted. Using these fabricated APLA, an ordinary commercial smartphone camera can be easily transformed to a low-cost, portable digital microscopy (50×magnification) such that point of care diagnostic can be implemented pervasively.

Robust and Stable Transparent Superhydrophobic Polydimethylsiloxane Films by Duplicating via a Femtosecond Laser-Ablated Template.

Realizing superhydrophobicity, high transparency on polydimethylsiloxane (PDMS) surface enlarges its application fields. We applied a femtosecond laser to fabricate well-designed structures combining microgrooves with microholes array on mirror finished stainless steel to form a template. Then liquid PDMS was charged for the duplicating process to introduce a particular structure composed of a microwalls array with a certain distance between each other and a microprotrusion positioned at the center of a plate surrounded by microwalls. The parameters such as the side length of microwalls and the height of a microcone were optimized to achieve required superhydrophobicity at the same time as high-transparency properties. The PDMS surfaces show superhydrophobicity with a static contact angle of up to 154.5 ± 1.7° and sliding angle lower to 6 ± 0.5°, also with a transparency over 91%, a loss less than 1% compared with plat PDMS by the measured light wavelength in the visible light scale. The friction robust over 100 cycles by sandpaper, strong light stability by 8 times density treatment, and thermal stability up to 325 °C of superhydrophobic PDMS surface was investigated. We report here a convenient and efficient duplicating method, being capable to form a transparent PDMS surface with superhydrophobicity in mass production, which shows extensive application potentials.

Enhanced triboelectrification of the polydimethylsiloxane surface by ultraviolet irradiation

Study of the triboelectric charging effect has recently gained much attraction by proposing a new potential technical application in the field of energy harvesting. Transparent polydimethylsiloxane (PDMS) has some advantages in employing the triboelectric effect due to good conformity at nanometer scale and the simple fabrication process. In this study, we demonstrate that UV irradiation can enhance the performance of a PDMS-based nanotribogenerator. Contact atomic force microscopy combined with Kelvin probe force microscopy enables an in-depth investigation of the effect of UV illumination on local triboelectric charge generation and its decay in PDMS. We found that UV exposure not only facilitates triboelectric charge generation but also enhances charge redistribution, which is related to the wettability of the PDMS surface. This study provides insights into the fundamental understanding and design of triboelectric generator devices.

Highly transparent flexible polydimethylsiloxane films – a promising candidate for optoelectronic devices

AbstractIn the present work, a siloxane based flexible substrate for optoelectronic applications with improved flexibility and high temperature withstanding capacity has been fabricated and the details are reported. The structural and optical properties of the substrate were characterized and the optical transparency is found to be about 90% − 95% in the visible region. The substrate withstands temperatures up to 300 °C without significant degradation and has a low thermal coefficient of expansion. Fourier transform infrared spectral data show the materials in the prepared substrate and SEM studies show the surface and cross‐sectional characteristics of the substrate. It has good mechanical strength, can withstand a high breaking load of 7 N, has a average tensile strength of 48.86 MPa and high average tensile modulus of 127.32 and also possesses excellent surface smoothness and resistance to both chemical and moisture with better rigidity; it will be a long lasting alternative for flexible plastic substrates. The optical characteristics such as refractive index (n) and extinction coefficient (k) are also reported. © 2016 Society of Chemical Industry

Morphological Design of Gold Nanopillar Arrays and Their Optical Properties

The fabrication of large areas of gold nanopillar arrays by combining nanosphere lithography and electrodeposition of metals is demonstrated here for the first time. Both the morphology and the surface density of the nanopillars can be finely tuned by controlling the size of the colloidal beads used in the template, the sintering time of the template and the electrodeposition time of metals. The colloidal templates are removed after electrodeposition, and the nanopillar arrays can be transferred onto a transparent polydimethylsiloxane substrate and optically characterized. Our focus here is on the ease and versatility of this technique to prepare large two-dimensional arrays of plasmonic gold nanopillars and their resulting optical properties. By making use of finite difference time domain numerical simulations, the excitation of localized surface plasmon resonances of the individual nanopillar with a strong dipolar character is observed, which is characteristic of such uncoupled metallic nanostructures. ...

Optofluidic bioimaging platform for quantitative phase imaging of lab on a chip devices using digital holographic microscopy

We propose a versatile 3D phase-imaging microscope platform for real-time imaging of optomicrofluidic devices based on the principle of digital holographic microscopy (DHM). Lab-on-chip microfluidic devices fabricated on transparent polydimethylsiloxane (PDMS) and glass substrates have attained wide popularity in biological sensing applications. However, monitoring, visualization, and characterization of microfluidic devices, microfluidic flows, and the biochemical kinetics happening in these devices is difficult due to the lack of proper techniques for real-time imaging and analysis. The traditional bright-field microscopic techniques fail in imaging applications, as the microfluidic channels and the fluids carrying biological samples are transparent and not visible in bright light. Phase-based microscopy techniques that can image the phase of the microfluidic channel and changes in refractive indices due to the fluids and biological samples present in the channel are ideal for imaging the fluid flow dynamics in a microfluidic channel at high resolutions. This paper demonstrates three-dimensional imaging of a microfluidic device with nanometric depth precisions and high SNR. We demonstrate imaging of microelectrodes of nanometric thickness patterned on glass substrate and the microfluidic channel. Three-dimensional imaging of a transparent PDMS optomicrofluidic channel, fluid flow, and live yeast cell flow in this channel has been demonstrated using DHM. We also quantify the average velocity of fluid flow through the channel. In comparison to any conventional bright-field microscope, the 3D depth information in the images illustrated in this work carry much information about the biological system under observation. The results demonstrated in this paper prove the high potential of DHM in imaging optofluidic devices; detection of pathogens, cells, and bioanalytes on lab-on-chip devices; and in studying microfluidic dynamics in real time based on phase changes.

INVITED] Total-internal-reflection-based photomask for large-area photolithography

Photolithography has been widely implemented with a photomask in contact or in close proximity to the photoresist layer. The flatness of the substrates is a crucial factor to guarantee the quality of the entire patterned photoresist (PR) layer especially for large-area photolithography. However, some substrates, such as sapphire wafers, do not possess highly uniform thickness as silicon wafer does. In this work, we demonstrate that a flexible polydimethylsiloxane (PDMS) photomask with optical total-internal-reflection structure can effectively circumvent this problem for mass production. Different from conventional photomask that the light is blocked by the patterned reflective/absorbing materials, the distributions of light intensity on the PR is engineered by the geometric structure of the transparent PDMS photomask. We demonstrate that 4in. patterned sapphire wafers can be successfully fabricated by using this PDMS photomask, which can be easily integrated into the present techniques in industry for mass production of substrates for GaN-based optoelectronic devices.