parylene-C Surfaces Research Articles

Modifications of Parylene by Microstructures and Selenium Nanoparticles: Evaluation of Bacterial and Mesenchymal Stem Cell Viability.

Parylene-based implants or coatings introduce surfaces suffering from bacteria colonization. Here, we synthesized polyvinylpyrrolidone-stabilized selenium nanoparticles (SeNPs) as the antibacterial agent, and various approaches are studied for their reproducible adsorption, and thus the modification of parylene-C–coated glass substrate. The nanoparticle deposition process is optimized in the nanoparticle concentration to obtain evenly distributed NPs on the flat parylene-C surface. Moreover, the array of parylene-C micropillars is fabricated by the plasma etching of parylene-C on a silicon wafer, and the surface is modified with SeNPs. All designed surfaces are tested against two bacterial pathogens, Escherichia coli (Gram-negative) and Staphylococcus aureus (Gram-positive). The results show no antibacterial effect toward S. aureus, while some bacteriostatic effect is observed for E. coli on the flat and microstructured parylene. However, SeNPs did not enhance the antibacterial effect against both bacteria. Additionally, all designed surfaces show cytotoxic effects toward mesenchymal stem cells at high SeNP deposition. These results provide valuable information about the potential antibacterial treatment of widely used parylene-C in biomedicine.

Enhenced cell adhesion on collagen I treated parylene-C microplates

On account of unique mechanical property and inertia, parylene-C has become a promising material for microdevices especially in three-dimensional microstructures loaded with cells. However, parylene-C is not favorable for cell adhesion, and a routine procedure is to modify it with a new adhesive layer. Herein, the parylene-C substrates with or without collagen Ӏ (Col-I) coating were adopted to estimate the influence of micro-environment change on cell attachment and spreading. After modification with Col-I, cauliflower-like particles presented on the substrate surface. Contact angle was significantly decreased after Col-I modification, which suggested the surface hydrophilicity was enhanced. Furthermore, cells cultured on parylene-C surface with Col-I treatment showed increased proliferation rate and spreading areas. In order to test the adhesion strength, a series of fixed size parylene-C microplates was fabricated, and cell suspension concentration was adjusted to culture a single cell on one microplate. The microplate was folded by the autogenous shrinkage force of cell. The folding angles of parylene-C microplates with Col-I treatment exhibited higher folding angle (112.6 ± 15.6°) than untreated samples (46.7 ± 5.9°). The work proved the existence of Col-I layer was particularly important, especially in analysis of cells mechanics using parylene-C microplate as a substrate.

Parylene-C coated microporous PDMS structure protecting from functional deconditioning of platelets exposed to cardiostimulants.

Most elderly patients after orthopedic and dental implant surgeries are exposed to cardiostimulants to reduce potential blood pressure-related risks of cardiovascular diseases. Such treatments lead to deconditioning of platelet function, which is an important factor in wound healing treatments. We introduced an innovative parylene-C coated microporous PDMS structure that can prevent the functional deconditioning of platelets caused by certain cardiostimulants. At different concentrations of cardiostimulants (IPR; isoprenaline and DA; dopamine), pre-activation, activation, and post-activation of platelets were intensively examined under mechanical and chemical stimulation mimicking the physiological environment on four different surfaces (glass, flat parylene-C coated glass (F-PPXC), microporous PDMS structure (P-PDMS), and parylene-C-coated microporous PDMS structure (S-PPXC)). The 3D microporous structure with parylene-C (S-PPXC) surface could attenuate the deconditioning of platelet function caused by IPR. Moreover, the S-PPXC surface further enhanced the DA-dependent stimulation of platelet function. The reason for this is that the 3D microporous structure with parylene-C S-PPXC induced stable and fast adhesion of platelets through increased surface roughness and softness, resulting in a significant enhancement of platelet activity. Therefore, we propose the use of functional S-PPXC surfaces as a novel strategy in the development of biomedical products.

Highly stable flexible organic field-effect transistors with Parylene-C gate dielectrics on a flexible substrate

Abstract Poly(chloro-p-xylene), or Parylene-C, is a polymeric insulating material that has good physical and chemical properties, such as a high dielectric strength, a pin-free surface, and good mechanical/chemical stability, but is difficult to apply to top-contact-structured OFETs since its hydrophobic and very rough surfaces hinder the growth of organic semiconductor crystals and promote the formation of interface traps. Herein, we applied a blend of PS and TIPS-PEN dissolved in 1,2,3,4 tetrahydronaphthalene to overcome these limitations of Parylene-C. To confirm the influence of this system, we analyzed the morphologies of crystals grown on Parylene-C surfaces modified by various organic and polymer materials, including methacryloxypropyltrimethoxysilane, hexamethyldisilazane, and dimethylchlorosilane-terminated polystyrene. Our investigation showed the ability of the PS:TIPS-PEN blend system to be used to overcome the above-described limitations of Parylene-C, and to manufacture top-contact OFETs displaying stable operation under gate-bias stress. Notably, we applied Parylene-C and this blend system in practical flexible OFETs that displayed highly stable properties.

Selective PEGylation of Parylene-C/SiO2 Substrates for Improved Astrocyte Cell Patterning

Controlling the spatial distribution of glia and neurons in in vitro culture offers the opportunity to study how cellular interactions contribute to large scale network behaviour. A recently developed approach to cell-patterning uses differential adsorption of animal-serum protein on parylene-C and SiO2 surfaces to enable patterning of neurons and glia. Serum, however, is typically poorly defined and generates reproducibility challenges. Alternative activation methods are highly desirable to enable patterning without relying on animal serum. We take advantage of the innate contrasting surface chemistries of parylene-C and SiO2 to enable selective bonding of polyethylene glycol SiO2 surfaces, i.e. PEGylation, rendering them almost completely repulsive to cell adhesion. As the reagents used in the PEGylation protocol are chemically defined, the reproducibility and batch-to-batch variability complications associated with the used of animal serum are avoided. We report that PEGylated parylene-C/SiO2 substrates achieve a contrast in astrocyte density of 65:1 whereas the standard serum-immersion protocol results in a contrast of 5.6:1. Furthermore, single-cell isolation was significantly improved on PEGylated substrates when astrocytes were grown on close-proximity parylene-C nodes, whereas isolation was limited on serum-activated substrates due tolerance for cell adhesion on serum-adsorbed SiO2 surfaces.

In-situ observation of dewetting processes of evaporating microdroplets on parylene-C surfaces

In-situ observation of dewetting processes of evaporating microdroplets on parylene-C surfaces

Fabrication of flexible microlens array through vapor-induced room temperature dewetting on plasma treated Parylene-C.

Microlens arrays have been widely used as an essential component in many optical systems. The conventional fabrication techniques, such as ink-jet printing, photoresist thermal reflow, grayscale photolithography, and LIGA process have some drawbacks in aspects of process complexity and high fabrication cost. This paper presents a low-temperature, rapid and cost-effective method to successfully generate SU-8 microlens arrays on biocompatible Parylene-C surfaces. Vapor-induced SU-8 dewetting has been achieved on chemically modified Parylene-C surfaces that were selectively pre-treated with O(2) or SF(6) plasma. By this technique, the SU-8 droplets are self-organized onto desired positions to form a microlens array. This method is also suitable for production of microlens array on curved surfaces. The progress of dewetting has been studied by both experiment and simulation.

Super Hydrophobic Parylene-C Produced by Consecutive <inline-formula> <tex-math notation="TeX">${\rm O}_{2}$ </tex-math></inline-formula> and <inline-formula> <tex-math notation="TeX">${\rm SF}_{6}$ </tex-math></inline-formula> Plasma Treatment

The wetting behavior of polymeric biomaterials is of great importance for biomedical research and pharmaceutical applications. Tailoring of polymer surface wettability is particularly effective to address biomedical issues such as biofouling control and biocompatibility improvement. In this paper, we conducted comprehensive experiments and analytical modeling to understand the effects of a consecutive-O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> -SF <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sub> plasma treatment on the super hydrophobicity of parylene-C. Experimentally, super hydrophobic parylene-C surfaces with a maximum water contact angle of ~169° have been successfully achieved. Atomic force microscopy and X-ray photoelectron spectroscopy results strongly suggest that the modification of surface wettability can be attributed to the variation in surface roughness and the plasma-induced surface chemistry. Analytically, a transition of droplet status from the Wenzel state to the Cassie state on hydrophobic parylene-C surfaces has been demonstrated after sufficient roughening by O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> plasma pre-treatment. The surface morphology of plasma-treated parylene-C films has also been analyzed and the hexagonal-close-packed model of downward crowns shows the best agreement with experimental results. Our simple and time-efficient treatment eliminates the need for creating well-defined patterns, is completely compatible with conventional microfabrication techniques, and can also be applied to curved parylene surfaces.

Plasma Enhanced Hydrophobicity of Parylene-C Surfaces for a Blood Contacting Pressure Sensor

A bio- and haemocompatible packaging strategy is proposed for an implantable pressure sensor for continuous direct blood pressure monitoring. A commercial sub-millimeter piezoresistive sensor was attached to an alumina substrate and a double coating of PDMS and parylene-C was applied. The packaged device has dimensions of 2.6x3.6x1.8mm. A surface modification technique using O2 and SF6 plasma has been employed to tune the haemocompatibility of the packaged sensor. By enhancing the intrinsic hydrophobicity of parylene it is possible to increase plasma proteins adsorption and minimize platelets adhesion on the surface. Contact angle measurements were performed on parylene-coated PDMS samples after plasma treatment, using water and whole pig blood. A maximum contact angle of 134° has been achieved. The influence of different plasma exposure times on the surface wettability has been investigated. To study the stability of the plasma treated surfaces, the contact angle was measured immediately and at regular intervals.

Effect of O2, Ar/H2 and CF4 plasma treatments on the structural and dielectric properties of parylene-C thin films

Plasma treatment of parylene-C surfaces not only causes structural modification of the surface during the plasma exposure, but also leaves active sites on the surfaces, which decreases the dielectric properties. In this work, the effects of oxygen, argon/hydrogen and fluorine plasma treatment on the surface and dielectric properties of parylene-C thin films were investigated using Fourier transform-IR spectroscopy, energy dispersive x-ray analysis and dielectric spectroscopy measurement. The results showed that the plasma treatment successfully introduced fluorine functional groups and decreased the oxygen content on the parylene-C surfaces. It appears that the replacement of oxygen and hydrogen by fluorine atoms led to a decrease in the local orientational polarizability of parylene-C. Consequently, it was found that the atmospheric fluorine plasma-treated parylene-C possessed lower dielectric characteristics, 16% lower than the untreated parylene-C at industrial frequencies (10–104 Hz). The Ar/H2 plasma treatment is also an experimental means to reduce the dielectric properties and to decrease the oxygen content in parylene-C. In contrast, the oxygen plasma increases the dielectric constant and can cause deterioration of the leakage current associated with carbon depletion showing C–O and C=O formation. CF4 and Ar/H2 plasma treatment does not significantly affect the long molecular motion (α-relaxation). Additional extrinsic oxygen content due to O2 plasma treatment in the parylene-C structure reproduces the increase in the time constant of both the short (β-relaxation) and long molecular motion.

Size-Selective Sliding of Sessile Drops on a Slightly Inclined Plane Using Low-Frequency AC Electrowetting

When placed on an inclined solid plane, drops often stick to the solid surface due to pinning forces caused by contact angle hysteresis. When the drop size or the plane's incline angle is small, the drop is difficult to slide due to a decrease in gravitational force. Here we demonstrate that small drops (0.4-9 μL) on a slightly inclined plane (~12°, Teflon and parylene-C surface) can be mobilized through patterned electrodes by applying low-frequency ac electrowetting under 400 Hz (110-180 V(rms)), which has a mechanism different from that of the high-frequency ac method that induces sliding by reducing contact angle hysteresis. We attribute the sliding motion of our method to a combination of contact angle hysteresis and interfacial oscillation driven by ac electrowetting instead of the minimization of contact angle hysteresis at a high frequency. We investigated the effects of ac frequency on the sliding motion and terminal sliding of drops; the terminal sliding velocity is greatest at resonance frequency. Varying the electrowetting number (0.21-0.56) at a fixed frequency (40 Hz) for 5 μL drops, we found an empirical relationship between the electrowetting number and the terminal sliding velocity. Using the relationship between the drop size and ac frequency, we can selectively slide drops of a specific size or merge two drops along an inclined plane. This simple method will help with constructing microfluidic platforms with sorting, merging, transporting, and mixing of drops without a programmable control of electrical signals. Also, this method has a potential in heat transfer applications because heat removal capacity can be enhanced significantly through drop oscillation.

Improvement of neuronal cell adhesiveness on parylene with oxygen plasma treatment

We improved adhesiveness of a neuron-like cell, PC12, on a Parylene-C surface by O(2) plasma treatment which changes the surface from hydrophobic to hydrophilic. Neural cell adhesiveness on the plasma-treated Parylene-C was more than twenty times better compared to non-treated Parylene-C and it was close to that on a conventional polystyrene tissue-culture dish.

Surface chemical immobilization of parylene C with thermosensitive block copolymer brushes based onN‐isopropylacrylamide andN‐tert‐butylacrylamide: Synthesis, characterization, and cell adhesion/detachment

Poly(N-isopropylacrylamide) (pNIPAM), poly(N-tert-butylacrylamide) (pNTBAM), and their copolymer brushes were covalently immobilized onto parylene C (PC) surfaces via surface initiated atom transfer radical polymerization (ATRP). Contact angle measurement between 13 and 40°C showed that the hydrophobicity of the modified PC surfaces was thermally sensitive. Among these samples, PC grafted with pNIPAM (PC-NI), PC grafted with pNTBAM (PC-NT) and PC grafted with copolymer brushes containing pNTBAM and pNIPAM (PC-NT-NI) exhibited the lower critical solution temperature (LCST) at 29, 22, and 24°C, respectively. Cytocompatibility study for the modified surfaces was performed by 5 days human skin fibroblast culture at 37°C. Data showed that only a very small amount of cells adhered on the PC and PC-NI surfaces, while a significantly higher amount of cell adhesion and growth was observed on PC-NT and PC-NT-NI surfaces. Furthermore, cell detachment at the temperatures of 24 and 6°C were studied after the substrates were cultured with cells at 37°C for 24 h. The results showed that the cells on PC-NI formed the aggregations and loosely attached on the substrate after 30-min culture at 24°C, while no significant cell detachment was observed for PC-NT and PC-NT-NI samples at this temperature. By continuing the cell culture for additional 100 min at 6°C for PC-NT and PC-NT-NI, about 10 and 35% of the cells were found detached respectively, and the unattached cells aggregated on the substrate. In comparison, cells cultured on the tissue culture petri dish (TCP) exhibited no quantity and morphology changes at the culture temperatures of 37, 24, and 6°C. This study showed that: (1) immobilization of PC with nonthermal sensitive pNTBAM could provide PC surface thermal sensitive hydrophilicity; (2) the chlorines on the polymer brushes of PC-NT could be used to further initiate the ATRP pNIPAM and form block copolymer brushes; (3) the incorporation of pNTBAM into pNIPAM on PC-NT-NI could change the surface thermal hydrophilicity property, and be further applied to decrease the LCST of the modified PC surface; (4) grafted pNIPAM brushes on PC-NI by ATRP showed very low cell adhesion and proliferation in 5 days fibroblast culture at 37°C, and cell detached at 24°C; (5) the incorporation of pNTBAM into pNIPAM on PC-NT-NI decreased the thermal sensitivity of cell adhesion/detachment, cell detached at 6°C, but the cell adhesion and proliferation were significantly improved at a wide temperature range.

Controlled Adhesion and Growth of Long Term Glial and Neuronal Cultures on Parylene-C

This paper explores the long term development of networks of glia and neurons on patterns of Parylene-C on a SiO2 substrate. We harvested glia and neurons from the Sprague-Dawley (P1–P7) rat hippocampus and utilized an established cell patterning technique in order to investigate cellular migration, over the course of 3 weeks. This work demonstrates that uncontrolled glial mitosis gradually disrupts cellular patterns that are established early during culture. This effect is not attributed to a loss of protein from the Parylene-C surface, as nitrogen levels on the substrate remain stable over 3 weeks. The inclusion of the anti-mitotic cytarabine (Ara-C) in the culture medium moderates glial division and thus, adequately preserves initial glial and neuronal conformity to underlying patterns. Neuronal apoptosis, often associated with the use of Ara-C, is mitigated by the addition of brain derived neurotrophic factor (BDNF). We believe that with the right combination of glial inhibitors and neuronal promoters, the Parylene-C based cell patterning method can generate structured, active neural networks that can be sustained and investigated over extended periods of time. To our knowledge this is the first report on the concurrent application of Ara-C and BDNF on patterned cell cultures.

Chemical surface modification of parylene C for enhanced protein immobilization and cell proliferation

To introduce the adhesion site of proteins and/or cells on parylene C (PC)-coated medical devices that can be used as implantable biosensors or drug delivery capsules, the PC surfaces were initially modified by the Friedel–Crafts acylation reaction to generate active chlorines. These chlorines were then employed to initiate the atom transfer radical polymerization of tert-butyl acrylate (TBA) and form a polymer brush layer of polyTBA on PC; the acrylate groups in the polymer brushes were hydrolyzed to carboxylic acid groups and further activated into succinimidyl ester groups via the 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide/N-hydroxysuccinimide coupling reaction. The PC surface grafted with polymer brushes and activated by succinimide showed efficient attachment of proteins, including gelatin, contortrostatin (CN) and bovine serum albumin (BSA), all at high density on the PC surface. The CN density on the surface was evaluated for both monolayer and polymer brush-based coatings. Based on fluorescence measurements, the polymer brush gives a 60-fold higher surface protein density than the monolayer-based system. Gelatin was used as a model protein and covalently coated onto the modified PC surface for cell culture study. Substrates with gelatin coating showed a significantly higher cell attachment and proliferation in 7 days cultures as compared to the uncoated substrates. In addition, a conventional photolithography technique was coupled with the surface chemistry to successfully pattern the BSA labeled with fluorescein isothiocyanate on the modified PC surfaces.

Human fibroblast attachment on fibrous parylene-C thin-film substrates

Although the polymeric form of parylene-C is used in many medical devices, the mechanistic nature of cellular attachment to polymeric parylene-C is not clear. We examined the effects of (i) substrate morphology, (ii) surface wettability and (iii) presence of serum proteins on fibroblast attachment. A physicochemical vapor deposition technique was implemented to deposit flat parylene-C substrates as well as fibrous substrates of three different morphologies: slanted columnar, chevronic and chiral. Flat parylene-C surfaces were moderately hydrophobic while fibrous substrates were superhydrophobic. Pretreatment with oxygen plasma changed the substrate surfaces from hydrophobic to superhydrophilic. The attachment efficiency of human fibroblast cells to the flat and three fibrous thin-film parylene-C substrates was investigated. Fibroblast attachment was better on fibrous substrates than on flat substrates, and oxygen plasma pretreatment facilitated fibroblast attachment on all four morphologies. Serum proteins also facilitated cell attachment on all substrates. The combination of oxygen plasma pre-treatment and serum proteins increased fibroblast adhesion in an additive manner on flat, but not on fibrous parylene-C substrates. The morphology of cell–substrate interactions differed between fibrous and flat parylene-C substrates.

Directed assembly of high density single-walled carbon nanotube patterns on flexiblepolymer substrates

We report an effective technique for the controlled assembly of single-walledcarbon nanotubes (SWNTs) and demonstrate organized high density networkarchitectures on soft polymeric substrates. We utilize the surface energy differentialbetween a plasma treated (hydrophilic) parylene-C surface and a photoresist(hydrophobic) surface to create microscale patterns of SWNT networks on a 10 µm thick parylene-C substrate. The large scale fabrication of patterned SWNTstructures presented is achieved by performing site-selective fluidic assembly ofSWNTs. Electrically continuous nanotube network micro-arrays as small as 4 µm wide that areup to 1500 µm long with controlled separation have been fabricated by dissolving the photoresist afterassembly. Electrical and mechanical characterization of nanotube networks on the flexiblesubstrate in both static and dynamic modes indicates that the structure can handle bothcompressive and tensile deformations with no hysteresis. The technology presented hasimmediate applications in making thin film transistors, interconnects and sensors onflexible substrates.

Flexible pentacene ion sensitive field effect transistor with a hydrogenated silicon nitride surface treated Parylene top gate insulator

We report on the realization of flexible ion sensitive organic field effect transistors based on pentacene on which Parylene-C was deposited as top gate dielectric. In order to create proton sensitive sites at the insulator/electrolyte interface, Parylene-C surface has been covered with a thin layer of hydrogenated silicon nitride (SiN:H) deposited by photochemical vapor deposition at moderate temperature. The combination of Parylene and SiN:H enables the realization of highly reproducible and good performance transistors as well as ion sensitive sensors with an excellent pH response both in the acidic and alkaline pH range in a nearly all plastic technology.

Cell and Protein Compatibility of Parylene-C Surfaces

Parylene-C, which is traditionally used to coat implantable devices, has emerged as a promising material to generate miniaturized devices due to its unique mechanical properties and inertness. In this paper we compared the surface properties and cell and protein compatibility of parylene-C relative to other commonly used BioMEMS materials. We evaluated the surface hydrophobicity and roughness of parylene-C and compared these results to those of tissue culture-treated polystyrene, poly(dimethylsiloxane) (PDMS), and glass. We also treated parylene-C and PDMS with air plasma, and coated the surfaces with fibronectin to demonstrate that biochemical treatments modify the surface properties of parylene-C. Although plasma treatment caused both parylene-C and PDMS to become hydrophilic, only parylene-C substrates retained their hydrophilic properties over time. Furthermore, parylene-C substrates display a higher degree of nanoscale surface roughness (>20 nm) than the other substrates. We also examined the level of BSA and IgG protein adsorption on various surfaces and found that surface plasma treatment decreased the degree of protein adsorption on both PDMS and parylene-C substrates. After testing the degree of cell adhesion and spreading of two mammalian cell types, NIH-3T3 fibroblasts and AML-12 hepatocytes, we found that the adhesion of both cell types to surface-treated parylene-C variants were comparable to standard tissue culture substrates, such as polystyrene. Overall, these results indicate that parylene-C, along with its surface-treated variants, could potentially be a useful material for fabricating cell-based microdevices.

Generation of static and dynamic patterned co-cultures using microfabricated parylene-C stencils

Many biological processes, such as stem cell differentiation, wound healing and development, involve dynamic interactions between cells and their microenvironment. The ability to control these dynamic processes in vitro would be potentially useful to fabricate tissue engineering constructs, study biological processes, and direct stem cell differentiation. In this paper, we used a parylene-C microstencil to develop two methods of creating patterned co-cultures using either static or dynamic conditions. In the static case, embryonic stem (ES) cells were co-cultured with fibroblasts or hepatocytes by using the reversible sealing of the stencil on the substrate. In the dynamic case, ES cells were co-cultured with NIH-3T3 fibroblasts and AML12 hepatocytes sequentially by engineering the surface properties of the stencil. In this approach, the top surface of the parylene-C stencil was initially treated with hyaluronic acid (HA) to reduce non-specific cell adhesion. The stencil was then sealed on a substrate and seeded with ES cells which adhered to the underlying substrate through the holes in the membrane. To switch the surface properties of the parylene-C stencils to cell adhesive, collagen was deposited on the parylene-C surfaces. Subsequently, a second cell type was seeded on the parylene-C stencils to form a patterned co-culture. This group of cells was removed by peeling off the parylene-C stencils, which enabled the patterning of a third cell type. Although the static patterned co-culture approach has been demonstrated previously with a variety of methods, layer-by-layer modification of microfabricated parylene-C stencils enables dynamic patterning of multiple cell types in sequence. Thus, this method is a promising approach to engineering the complexity of cell-cell interactions in tissue culture in a spatially and temporally regulated manner.