Polydimethylsiloxane Structures Research Articles

The molecular surface conformation of surface-tethered polyelectrolytes on PDMS surfaces

Surface properties of polymer materials are important for many applications such as biomedical materials, marine antifouling coatings, polymer membranes for biological and chemical molecule separations, and polymer adhesives. Surface properties are highly dependent on molecular surface structures. Presently, surface polymerization is one of the most effective methods to tailor and optimize molecular surface structures of many polymers. Herein, the surface structure of polydimethylsiloxane (PDMS) is modified by tethering polyelectrolytes (PEs) through surface-initiated ultraviolet (UV) polymerization. Using dimethylacrylamide, acrylic acid, and [2-(methacryloyloxy)ethyl]dimethyl(3-sulfopropyl)ammonium as monomers, cationic, anionic, and zwitterionic PEs are grafted onto PDMS surfaces using the “graft-from” method. Successful grafting of PEs onto PDMS surfaces has been verified by several analytical techniques, including Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, and contact angle measurement. In particular, sum-frequency generation (SFG) vibrational spectroscopy has been employed to probe the molecular structure of modified PDMS surfaces in both the dry and hydrated states. It was found that in air, the surface dominating –Si–CH3groups of unmodified PDMS segregate to the PDMS surface along with the grafted PEs. These methyl groups have similar orientations to those on the unmodified PDMS surface: they more or less stand up on the surface, along the surface normal. Upon exposure to water, only SFG signals from the surface-tethered PE chains on the modified PDMS surface are observed, showing a substantial surface restructuring behavior. More details regarding such surface restructuring behavior can be deduced through SFG data analysis. With numerous PDMS applications in aqueous environments, it is of particular importance to modify its surface structures and characterize such surface structures in aqueous environments in situ.

Gas permeation through a synthesized composite PDMS/PES membrane

In this paper, a defect-free polydimethylsiloxane (PDMS)-based composite membrane was synthesized and characterized. The membrane consisted of a thin PDMS film (4 μm) and polyethersulfone (PES) as a support material. At first, a macroporous PES support was prepared by phase inversion method and its thickness, porosity, pore size distribution and surface porosity were determined. PES characterization results confirmed suitability of the support for the composite membrane preparation. Then, PDMS solution, containing crosslinker and catalyst, was cast over the support. Sorption and permeation of C 3H 8, CO 2, CH 4 and H 2 through the synthesized composite membrane were measured at various upstream pressures and based on solution–diffusion mechanism, their diffusion coefficients were calculated. Flory–Huggins (FH) interaction parameters, χ, of the gases with the polymer matrix were calculated based on Flory–Rehner (FR) expression for the crosslinked PDMS membrane and discussed. The concentration-averaged FH interaction parameters of H 2, CH 4, CO 2 and C 3H 8 in the synthesized PDMS membrane were estimated to be 2.526, 0.102, 0.435 and 0.248, respectively. Higher χ values exhibited less favorable interactions between the gas and the membrane and vice versa. Chemical similarity of CH 4 with backbone structure of PDMS, led to the lowest concentration-averaged χ values for CH 4. Increasing feed pressure increased permeability, solubility and diffusion coefficients of bigger gases while decreased those of smaller ones. Hence, C 3H 8/gas ideal selectivity also increased with increasing feed pressure. Only, local effective diffusion coefficient of C 3H 8 increased with increasing penetrant concentration which indicated plasticization effect of C 3H 8 over the range of penetrant concentration studied.

Novel polydimethylsiloxane (PDMS) based microchannel fabrication method for lab-on-a-chip application

A novel method is presented for fabricating PDMS-based microchannel. The mold consists of a tunable PDMS structure rather than the commonly used SU-8 mold with fixed shape. This structure is prepared by transferring the desired microchannel pattern into one PDMS substrate via soft lithography and bonding it to a spin-coated PDMS membrane with the oxygen plasma activated technology. Through applying proper pressure to this structure and keeping it constant during the whole molding process, the resulted membrane deformation shape with rounded cross-sectional contour is totally replicated into the device layer, constituting the final main body of the microchannel. Besides the inherent feature of rounded cross-section provided by this method, the depth of the microchannel can be easily controlled by changing the pressure. At the same time, through designing different widths at different positions in one microchannel, it is straightforward to realize microchannel structure with multi-depth in one step using one photomask. Both of these characteristics have been successfully demonstrated in our experiments and it will definitely find wide applications in lab-on-a-chip area.

Study on the effect of structure of polydimethylsiloxane grafted with polyethyleneoxide on surface activities

Twelve kinds of polydimethylsiloxane grafted with polyethyleneoxide (PDMS-g-PEO) were synthesized by two-step reactions: the preparation of silicone backbone containing methyl-hydrogen siloxane units and the following hydrosilylation reactions with PEO containing allyl groups. When the effects of structure of PDMS-g-PEO on the surface properties were investigated by comparing the each effect caused by EO content, EO chain length or the size of hydrophobe, the size of hydrophobe was found to be a critical factor for deciding surface tension rather than the content of EO. Significant decrease in the contact angle of the aqueous solution was observed when EO content of PDMS-g-PEO or the surfactant concentration increased. On the other hand, the viscosity of the surfactant solution at constant concentration slightly increased as EO content increased, but the difference was not significant. Dynamic surface tension measurement showed a decrease in the surface tension of the aqueous surfactant solution with an increase in the surfactant concentration. In addition, much longer times were required to reach an equilibrium compared with conventional nonionic or ionic surfactants due to the lower mobility of PDMS-g-PEO with high molecular weight.

Tunable Pneumatic Microoptics

A new class of tunable and actuated microoptical devices is presented: pneumatic microoptics. Using microelectromechanical system fabrication technology extended by the use of polydimethylsiloxane (PDMS) membranes, tunable microlenses, and lens arrays, actuated micromirrors with large tilt angles and tip-tilt piston mirrors have been designed and fabricated. Actuation is by pressure: Gas- or liquid-filled microfluidic cavities are employed to distend the microfabricated PDMS structures which then act as a lens surface or as an actuator for a micromirror. Thermopneumatic actuation is also employed for completely integrated tunable optical systems in which all actuator and optical components are fabricated on-chip. The technology is particularly promising for microsystem applications in which significant movement is required but high voltages or external fields are impractical. [2007-0301].

Nanochannels fabricated in polydimethylsiloxane using sacrificial electrospun polyethylene oxide nanofibers

The authors have used electrospun polyethylene oxide nanofibers as sacrificial templates to form nanofluidic channels in polydimethylsiloxane (PDMS). By depositing fibers on silicon templates incorporating larger structures, the authors demonstrate that these nanochannels can be integrated easily with microfluidics. They use fluorescence microscopy to image channels filled with dye solution. The utility of the hybrid micro- and nanofluidic PDMS structures for single molecule observation and manipulation was demonstrated by introducing single molecules of λ-DNA into the channels. This nanofabrication technique allows the simple construction of integrated micro- and nanofluidic PDMS structures without lithographic nanofabrication techniques.

Variable Wave Plate via Tunable Form-Birefringent Structures

We propose a compact and low-cost design for a variable wave plate using tunable form-birefringent grating structures in polydimethylsiloxane (PDMS). The described device operates through PDMS deformation via electrostatically actuated transparent electrodes. The optical and mechanical properties of tunable form-birefringent structures are modeled using birefringence, rigorous coupled-wave analysis, and simple structural mechanics theory. The device is fabricated by a form of soft nanoimprint lithography. The optical properties are characterized for different structure geometries. Operating principles were verified through testing of the completed device. The experimental results and theoretical study show that an irregular grating structure performs better than a periodic subwavelength structure. [2007-0303].

Functionalization of poly(dimethylsiloxane) by chemisorption of copolymers: DNA microarrays for pathogen detection

Abstract A robust method for coating polydimethylsiloxane (PDMS) allowing covalent binding of biomolecules is reported. The coating is based on adsorption of a copolymer, copoly(DMA-NAS-MAPS), bearing functional groups. The polymer is synthesized by free radical polymerization; it self-adsorbs onto the PDMS very rapidly, typically in 30 min, even without any PDMS surface pre-treatment. PDMS slides coated by copoly(DMA-NAS-MAPS) covalently bind amino-modified DNA fragments and have been tested in bacterial genotyping experiments. The usefulness of this functional coating on PDMS structures in lab-on-chip devices powered by microfluidics has been demonstrated. The copoly(DMA-NAS-MAPS) coated PDMS slides were used as flexible components of a microcell in semi-automated DNA microarray experiments for rapid food pathogen identification.

Miniaturized Vision System for Microfluidic Devices

One of the biggest obstacles for lab-on-chip (LOC) devices is the miniaturization of large-scale devices and its methodologies. Miniaturization of the current microscopic technologies combined with image processing may bring significant advantages for LOC devices in the dynamic processes of sizing, positioning, flow control and cell manipulation at different time scales. Here, we propose a vision system boarded on a polydimethylsiloxane (PDMS) polymer-based chip, which can be utilized in a complex microfluidic network for continuous monitoring of mammalian egg and donor cells of sizes in the range of 10–100 μm. The developed prototype system has sufficient resolution and is accompanied with a robust detection method for cell-based microfluidic applications. To assess its performance, an image processing algorithm was applied, and the capability of the detection method was evaluated using 11- and 26-μm particles. The results show that the proposed optical system of monitoring and illumination can be effectively incorporated into PDMS structures aiming at LOC devices.

A Practical Method for Rapid Microchannel Fabrication in Polydimethylsiloxane by Replica Molding without Using Silicon Photoresist

This paper describes a very simple and rapid method for fabrication of microfluidic devices on the material polydimethylsiloxane (PDMS). The fabrication process is greatly simplified by using a micromachined polymethylmethacrylate (PMMA) master instead of using a silicon photoresist. A PDMS pre-polymer mixture was cast over the PMMA master and cured to produce positive relief structures in PDMS which then served as a mold. This PDMS mold was conveniently used for multiple production of PDMS replicas containing the desired microstructures similar to those on the surface of the original PMMA master. Because our procedure does not require a photoresist, it minimizes the requirement for special equipment and decreases the total fabrication time. Moreover, the method offers the advantage of a wide range of feature size, specifically deeper channels unobtainable with silicon photoresist. Examination of the sidewalls of the channels revealed high fidelity in reproducing the PMMA master via the hardened-PDMS mold. Our protocol enables the production of multiple PDMS-based microfluidic devices in a low-cost and efficient manner.

Nanocomposite Conductive Elastomer: Microfabrication Processes and Applications in Soft-Matter MEMS Sensors

ABSTRACT3D micro-molded PDMS (polydimethylsiloxane) elastomer is widely used in MEMS. However, traditional PDMS is non-conductive and as a result is used in mostly structural applications. It is difficult to embed elastomeric, “stretchy” conductors and transduction elements in molded PDMS matrix. We report general methods for monolithic fabrication of multi-layer PDMS structures with embedded conductive and non-conductive elastomer elements. Conductive PDMS parts, made of carbon-nanotube-filled composite PDMS, can form internal elastomer wires, electrodes, heaters, and sensors. The process uses a series of PDMS patterning, micromolding, and bonding techniques. In this work we demonstrate elastomer strain gauges, capacitive pressure sensors, as well as microfluidic channels with integrated heaters and sensors.

Failure Investigation of Polymer Mechanical Micromponents

ABSTRACTThis paper reports typical failure modes of polymer microstructures. Polydimethylsiloxane (PDMS) microstructures were fabricated using replication-based methods. The mechanical collapses of these structures were observed using scanning electron microscopy. In this work, we classified the mechanical collapses based on their individual geometries and discussed the mechanism of each failure mode. We also worked on solutions to recover these failed PDMS structures. This work helps to better understand the collapsing mechanism and to facilitate the development of control strategies to enhance the mechanical reliability of polymer microdevices.

Polarization Independent Static Microlens Array in the Homeotropic Liquid Crystal Configuration

ABSTRACT We propose a novel microlens array structure in a homeotropic liquid crystal (LC) configuration. The symmetry of molecular ordering in the homeotropic alignment on a circular surface relief structure provides the polarization independent focusing characteristics of the LC microlens. The surface relief structure of poly-dimethylsiloxane (PDMS) supports an initial convex lens driving scheme of the LC microlens as well as easy modification with great aging property.

FT‐IR Study of the Hydrolysis and Polymerization of Tetraethyl Orthosilicate and Polydimethyl Siloxane in the Presence of Tetrabutyl Orthotitanate

In this work, we have used FT‐IR spectroscopy to study the hydrolysis and polymerization reactions of tetraethyl orthosilicate (TEOS) and polydimethyl‐siloxane (PDMS) in the presence of tetrabutyl orthotitanate (TBOT). These reactions are used for obtaining SiO2–PDMS–TiO2 organically modified silicates (Ormosils). In order to obtain semi‐quantitative information about such reactions, a deconvolution procedure of the FT‐IR spectra has been done by use of a computer program. Hydrolysis reactions have been characterized by Me–O–C (Me = Si, Ti) bonds, and polymerization reactions by Me–O–Me bonds. Instantaneous hydrolysis of TEOS has been observed, together with condensation reactions between Si–OH groups, which give crosslinked and linear Si–O–Si structures. The TBOT is also hydrolyzed, but the high acid concentration inhibits condensation reactions between Ti–OH groups. The PDMS also condenses mainly with Si–OH groups and probably with Ti–OH, finally forming Me–O–PDMS bonds. The formation of Si–O–Si crosslinked structures and also Me–O–PDMS structures continues until the end of reaction. The gelling time is dependent on TBOT concentration in the reaction medium and, therefore, polycondensation reactions are dependent on TBOT concentration.

Accurate Double-Height Micromolding Method for Three-Dimensional PolyDiMethylSiloxane Structures

A fabrication method for making accurate double-height micromolds is presented. Fine features of the micromold are transferred to a three-dimensional poly-dimethylsiloxane (PDMS) microfluidic membrane. The accuracy of features is within 1.55% and the maximum aspect ratio is 7. In this work, the first layer of the micromolds is made directly on silicon wafers by inductively coupled plasma reactive ion etching (ICP-RIE). The second layer is added by photolithography of SU-8 negative photoresist on top of the first layer. This method allows the fabrication of micromolds having wall dimensions as small as 3 µm that was not previously achievable. Such dimensions are required in biological microfluidic systems to reduce the amount of chemicals or to confine cells to a desired position.

On-column polymer-imbedded graphite inlet electrode for capillary electrophoresis coupled on-line with flow injection analysis in a poly(dimethylsiloxane) interface.

A method for coupling an electrophoretic driven separation to a liquid flow, using conventional fused-silica capillaries and a soft polymeric interface is presented. A novel design of the electrode providing high voltage to the electrophoretic separation was also developed. The electrode consisted of a conductive polyimide/graphite imbedded coating immobilized onto the capillary electrophoresis (CE) column inlet. This integrated electrode gave the same separation performance as a commonly used platinum electrode. The on-column electrode also showed good electrochemical stability in chronoamperometric experiments. In addition, with this electrode design, the electrode position relative to the inlet end of the CE column will always be constant and well defined. The on-line flow injection analysis (FIA)-CE system was used with electrospray ionization (ESI)-time of flight (TOF)-mass spectrometry detection. The preparation of the PDMS (poly(dimethylsiloxane)) interface for FIA-CE is described in detail and used for initial tests of the on-column polymer-imbedded graphite inlet electrode. In this interface, a pressure-driven liquid flow, a make up CE electrolyte and a CE column inlet meet in a two-level cross (95 microm ID) in the PDMS structure, enabling independent flow characterization.

Using polydimethylsiloxane as a thermocurable resist for a soft imprint lithography process

Nanoimprint lithography has been investigated using polydimethylsiloxane as a thermocurable resist. This novel process allowed us to reduce both pressure (<10 bars) and temperature (80 °C) when compared to a conventional imprinting process with a thermoplastic polymer resist such as polymethylmethacrylate. Using a new formulation of the elastomeric material, we have demonstrated high quality imprinting of both micronic and nanometric structures with no evidence of any viscous flow problems. The excellent etching resistance of the polydimethylsiloxane structures to a reactive ion etching silicon process and the compatibility with a lift-off procedure for pattern transfer are also presented.

Cell Placement and Neural Guidance Using a Three-Dimensional Microfluidic Array

Several fabrication techniques for making three-dimensional arrays of micro-wells for biological cell patterning and single-neuron guidance are presented. Methods for making complex 3d high-aspect-ratio structures in poly-dimethylsiloxane (PDMS) membrane are explored. In this work, three-dimensional micro-molds are made directly on silicon wafers though inductively coupled plasma reactive ion etching (ICP-RIE), and also using SU-8 negative photoresist. Cell placement is achieved through an array of 50-µm square holes in a 150–100 µm thick PDMS membrane, which is placed on a glass substrate. Vertical holes in the membrane are linked by horizontal tunnels on the glass side of the membrane, for use in neural guidance or delivery of drugs or nutrients. The effectiveness of the membrane for cell placement, growth and guidance was tested using fluorescent yeast cells and PC12 neuronal cells.

Structural investigation of polydimethylsiloxane–vanadate hybrid materials

The structure of polydimethylsiloxane–vanadate (PDMSV) hybrid materials has been fully investigated. In particular, the use and interpretation of thermogravimetric analysis, IR, Raman and multinuclear NMR (1H, 13C, 17O, 29Si, 51V) spectroscopies allowed an accurate description of their structure on a nanometric scale. The networks are composed of dimethylsiloxane segments with an average size of five to six units linked to monomeric vanadate units VO(OSiMe2..)3. Both units have been identified in cross-linked areas and in end groups. The dissolution and the degradation behaviour of the PDMSV materials are also described.

Molding of deep polydimethylsiloxane microstructures for microfluidics and biological applications.

Here we demonstrate the microfabrication of deep (> 25 microns) polymeric microstructures created by replica-molding polydimethylsiloxane (PDMS) from microfabricated Si substrates. The use of PDMS structures in microfluidics and biological applications is discussed. We investigated the feasibility of two methods for the microfabrication of the Si molds: deep plasma etch of silicon-on-insulator (SOI) wafers and photolithographic patterning of a spin-coated photoplastic layer. Although the SOI wafers can be patterned at higher resolution, we found that the inexpensive photoplastic yields similar replication fidelity. The latter is mostly limited by the mechanical stability of the replicated PDMS structures. As an example, we demonstrate the selective delivery of different cell suspensions to specific locations of a tissue culture substrate resulting in micropatterns of attached cells.