Thin Films Of Parylene Research Articles

Development of a Water Transmission Rate (WTR) Measurement System for Implantable Barrier Coatings.

While water vapor transmission rate (WVTR) measurement is standardly used to assess material permeability, a system able to quantify liquid water transmission rate (WTR) measurement is highly desirable for implantable thin film barrier coatings. Indeed, since implantable devices are in contact or immersed in body fluids, liquid WTR was carried out to obtain a more realistic measurement of the barrier performance. Parylene is a well-established polymer which is often the material of choice for biomedical encapsulation applications due to its flexibility, biocompatibility, and attractive barrier properties. Four grades of parylene coatings were tested with a newly developed permeation measurement system based on a quadrupole mass spectrometer (QMS) detection method. Successful measurements of gas and water vapor and the water transmission rates of thin parylene films were performed and validated, comparing the results with a standardized method. In addition, the WTR results allowed for the extraction of an acceleration transmission rate factor from the vapor-to-liquid water measurement mode, which varies from 4 to 4.8 between WVTR and WTR. With a WTR of 72.5 µm g m-2 day-1, parylene C displayed the most effective barrier performance.

A Study on Biocompatible Polymer-Based Packaging of Neural Interface for Chronic Implantation.

This paper proposed and verified the use of polymer-based packaging to implement the chronic implantation of neural interfaces using a combination of a commercial thermal epoxy and a thin parylene film. The packaging’s characteristics and the performance of the vulnerable interface between the thermal epoxy layer and polyimide layer, which is mainly used for neural electrodes and an FPCB, were evaluated through in vitro, in vivo, and acceleration experiments. The performance of neural interfaces—composed of the combination of the thermal epoxy and thin parylene film deposition as encapsulation packaging—was evaluated by using signal acquisition experiments based on artificial stimulation signal transmissions through in vitro and in vivo experiments. It has been found that, when commercial thermal epoxy normally cured at room temperature was cured at higher temperatures of 45 °C and 65 °C, not only is its lifetime increased with about twice the room-temperature-based curing conditions but also an interfacial adhesion is higher with more than twice the room-temperature-based curing conditions. In addition, through in vivo experiments using rats, it was confirmed that bodily fluids did not flow into the interface between the thermal epoxy and FPCB for up to 18 months, and it was verified that the rats maintained healthy conditions without occurring an immune response in the body to the thin parylene film deposition on the packaging’s surface.

An Inkjet Printed Flexible Electrocorticography (ECoG) Microelectrode Array on a Thin Parylene-C Film.

Electrocorticography (ECoG) is a conventional, invasive technique for recording brain signals from the cortical surface using an array of electrodes. In this study, we developed a highly flexible 22-channel ECoG microelectrode array on a thin Parylene film using novel fabrication techniques. Narrow (<40 µm) and thin (<500 nm) microelectrode patterns were first printed on PDMS, then the patterns were transferred onto Parylene films via vapor deposition and peeling. A custom-designed, 3D-printed connector was built and assembled with the Parylene-based flexible ECoG microelectrode array without soldering. The impedance of the assembled ECoG electrode array was measured in vitro by electrochemical impedance spectroscopy, and the result was consistent. In addition, we conducted in vivo studies by implanting the flexible ECoG sensor in a rat and successfully recording brain signals.

Novel Flexible Triboelectric Nanogenerator based on Metallized Porous PDMS and Parylene C

Triboelectric nanogenerators (TENGs) have recently become a powerful technology for energy harvesting and self-powered sensor networks. One of their main advantages is the possibility to employ a wide range of materials, especially for fabricating inexpensive and easy-to-use devices. This paper reports the fabrication and preliminary characterization of a novel flexible triboelectric nanogenerator which could be employed for driving future low power consumption wearable devices. The proposed TENG is a single-electrode device operating in contact-separation mode for applications in low-frequency energy harvesting from intermittent tapping loads involving the human body, such as finger or hand tapping. The novelty of the device lies in the choice of materials: it is based on a combination of a polysiloxane elastomer and a poly (para-xylylene). In particular, the TENG is composed, sequentially, of a poly (dimethylsiloxane) (PDMS) substrate which was made porous and rough with a steam-curing step; then, a metallization layer with titanium and gold, deposited on the PDMS surface with an optimal substrate–electrode adhesion. Finally, the metallized structure was coated with a thin film of parylene C serving as friction layer. This material provides excellent conformability and high charge-retaining capability, playing a crucial role in the triboelectric process; it also makes the device suitable for employment in harsh, wet environments owing to its inertness and barrier properties. Preliminary performance tests were conducted by measuring the open-circuit voltage and power density under finger tapping (~2 N) at ~5 Hz. The device exhibited a peak-to-peak voltage of 1.6 V and power density peak of 2.24 mW/m2 at ~0.4 MΩ. The proposed TENG demonstrated ease of process, simplicity, cost-effectiveness, and flexibility.

Methanol and Humidity Capacitive Sensors Based on Thin Films of MOF Nanoparticles.

The successful development of modern gas sensing technologies requires high sensitivity and selectivity coupled to cost effectiveness, which implies the necessity to miniaturize devices while reducing the amount of sensing material. The appealing alternative of integrating nanoparticles of a porous metal-organic framework (MOF) onto capacitive sensors based on interdigitated electrode (IDE) chips is presented. We report the deposition of MIL-96(Al) MOF thin films via the Langmuir-Blodgett (LB) method on the IDE chips, which allowed the study of their gas/vapor sensing properties. First, sorption studies of several organic vapors like methanol, toluene, chloroform, etc. were conducted on bulk MOF. The sorption data revealed that MIL-96(Al) presents high affinity toward water and methanol. Later on, ordered LB monolayer films of MIL-96(Al) particles of ∼200 nm were successfully deposited onto IDE chips with homogeneous coverage of the surface in comparison to conventional thin film fabrication techniques such as drop-casting. The sensing tests showed that MOF LB films were selective for water and methanol, and short response/recovery times were achieved. Finally, chemical vapor deposition (CVD) of a porous thin film of Parylene C (thickness ∼250-300 nm) was performed on top of the MOF LB films to fabricate a thin selective layer. The sensing results showed an increase in the water selectivity and sensitivity, while those of methanol showed a huge decrease. These results prove the feasibility of the LB technique for the fabrication of ordered MOF thin films onto IDE chips using very small MOF quantities.

Long‐wavelength infrared characteristics of multifunctional microfibrous thin films of Parylene C

AbstractCalculations show that structurally chiral, three‐dimensionally periodic, microfibrous thin films (μFTFs) of Parylene‐C can function as both polarization‐insensitive and circular‐polarization‐selective photonic crystals in the long‐wavelength infrared regime. Careful engineering is required for bandstop filters in order to avoid the numerous absorption peaks of bulk Parylene‐C. The photonic functionality of the Parylene‐C μFTFs is in addition to their phononic, hydrophobic, and biomedical functionalities.

Charge Carrier Distribution in Low-Voltage Dual-Gate Organic Thin-Film Transistors

Dual-gate organic thin-film transistors (DGOTFTs), which exhibit better electrical properties, in terms of on-current and subthreshold slope than those of single-gate organic thin-film transistors (OTFTs) are promising devices for high-performance and robust organic electronics. Electrical behaviors of high-voltage (&gt;10 V) DGOTFTs have been studied: however, the performance analysis in low-voltage DGOTFTs has not been reported because fabrication of low-voltage DGOTFTs is generally challenging. In this study, we successfully fabricated low-voltage (&lt;5 V) DGOTFTs by employing thin parylene film as gate dielectrics and visualized the charge carrier distributions in low-voltage DGOTFTs by a simulation that is based on finite element method (FEM). The simulation results indicated that the dual-gate system produces a dual-channel and has excellent control of charge carrier density in the organic semiconducting layer, which leads to the better switching characteristics than the single-gate devices.

Simple action potential measurement of cardiac cell sheet utilizing electronic sheet

In this study, a device for measuring the action potential of cardiac cell sheets was developed. The action potential was measured using a device comprising a 2-µm-thick parylene film with a silver electrode printed on it, which was referred to as the “electronic sheet.” The thin parylene film exhibits high biocompatibility and flexibility. Therefore, it demonstrates promise for biomedical microelectromechanical system applications. In this study, a cell sheet was used because the interest it had garnered in regenerative medicine for creating cardiac tissue in vitro similar to that in vivo. A high-efficiency drug development system can be realized by combining cell sheet technology and fabricating a flexible electronic sheet. The action potential of a cardiac cell sheet from a rat was measured using the as-developed flexible electronic sheet.

A flexible electrostatic kinetic energy harvester based on electret films of electrospun nanofibers

This paper reports a paper-based electrostatic kinetic energy harvester (e-KEH) implementing multilayered electret films based on electrospun nanofibrous material. It is the first time that a fully flexible electret-based e-KEH is reported. The proposed electret, PVDF-PTFD nanofibrous covered by Parylene C, has a faster stabilization of surface potential than a planar thin film of Parylene C, and a higher stability of charge storage. With a maximum force of 0.5 N and a 3-layer electret, the device capacitance increases from 25 to 100 pF during a pressing operation. Working with the optimal resistive load of 16 MΩ, the device pressed manually delivers a peak instantaneous power up to 45.6 μW and an average energy of 54 nJ/stroke, corresponding to a peak instantaneous power density of 7.3 μW cm−2 and an average energy density of 8.6 nJ cm−2/stroke. Within 450 manual strokes, a 10 nF capacitor is charged up to 8.5 V by the prototype through a full-wave diode bridge. On a 1 μF capacitor, the energy delivery of 9.9 nJ/stroke has been obtained with a 10 Hz pressing movement excited by a vibrator with a maximum force of 0.5 N.

Selective Insulation of Carbon Nanotubes

We demonstrate a method for the selective encapsulation of carbon nanotubes in thin parylene films using iron as a sacrificial lift‐off layer. The iron serves as an inhibitor of parylene deposition and prevents the parylene molecules from linking, thus facilitating selective area coating after lift‐off.

Smooth-surfaced flexible wall shear stress sensor fabricated by film transfer technology

We propose a smooth-surfaced flexible wall shear stress sensor fabricated by a newly developed thin film transfer technology to reveal the state of wall turbulence. This sensor consists of thin film types of thermal shear stress sensors and a PDMS (polydimethylsiloxane) mold structure. The sensor has a high spatial and temporal resolution comparable to that of the turbulence change because the micro-heaters of the sensing element were fabricated on thin parylene film less than 3μm thick to reduce heat capacity, and a cavity for heat insulation was formed under the heaters. The sensor can precisely measure the wall shear stress thanks to the smooth sensor surface. In this study, we established the sensor fabrication process by using film transfer technology, and we fabricated three types of sensors with film thicknesses of 0.5, 1.0, and 3.0μm in order to confirm the change in response characteristics due to the differences in film thickness. We evaluated the response characteristics and the shear stress detection function of the fabricated sensors. As a result, the cut-off frequency became faster as the film sensor thickness became thinner—it was 1221Hz with a 0.5-μm thick sensor. In addition, the square of the sensor output E2 was proportional to the shear stress to the 0.85-th power τ0.85.

Dielectric Properties of and Charge Transport in Columnar Microfibrous Thin Films of Parylene C

Parylene-C microfibrous thin films ( $\mu $ FTFs), grown using oblique-angle physicochemical vapor deposition, were examined for potential use as low-relative-permittivity (i.e., low- $\kappa $ ) interlayer dielectrics in integrated circuits and, more importantly, flexible electronics. These films were characterized using capacitance–voltage–temperature (CVT) and current–voltage–temperature experiments at different temperatures (ranging from 298 to 420 K) and frequencies (ranging from 1 kHz to 1 MHz). Field emission scanning electron microscopy revealed the Parylene C $\mu $ FTFs to be highly porous. Consequently, their $\kappa $ values are at least 20% lower than those of bulk Parylene C. The dependences of $\kappa $ on frequency and temperature suggest that molecular dipole oscillations are responsible for charge polarization in the $\mu $ FTFs. The dc leakage current in the $\mu $ FTFs at temperatures not exceeding 100 °C (~373 K) was found to arise from Poole–Frenkel (PF) emissionmechanism with a barrier energy of about 0.77 eV. Moreover, when fitted to the PF model, the experimental data yielded high-frequency values $\kappa _{\infty }$ of $\kappa $ in agreement with those obtained from CVT experiments, thus confirming our identification of PF as themajor responsiblemechanism and confirming the low- $\kappa $ characteristic of microfibrous Parylene C. The ac current transport in the $\mu $ FTFs was found attributable to small-polaron-tunneling hopping conduction and characterized by the power law $ \omega ^{s}$ , with $s\in [{0.082,0.85}]$ increasing with temperature. AC conduction in the $\mu $ FTFs is temperature-activated with an activation energy that decreases from 0.020 to 0.012 eV as frequency increases from 1 kHz to 1 MHz.

Studies of Parylene C Microfibrous Thin Films Electrical Properties

We have fabricated and measured the room-temperature capacitance in metal-insulator-metal (MIM) structures utilizing the Parylene C films as the insulators in the frequency range 1 kHz to 1 MHz. The Parylene-C films were of variable thicknesses and were either spin-on dense films or columnar microfibrous thin films (μFTFs) grown using oblique angle physicochemical vapor deposition. The values of the vapor tilt angle, χv, used were 30°, 60°, or 90°. Expectedly, columnar microfibrous thin film of Parylene C is observed to have lower values of k than the dense one. However, it is observed that the dielectric constant of the μFTFs is increased with increasing deposition tilt angle χv. This is explained in terms of film porosity as a function of χv, which is enhanced in films deposited at lower tilt angles. The dependence of k on measurement frequency suggests that molecular dipole effects dominate charge polarization in the films.

Spontaneously Formed Wrinkled Substrates for Stretchable Electronics Using Intrinsically Rigid Materials

The preparation of a stretchable substrate was carried out by blending poly (ethylene glycol)-block-poly (propylene glycol)-block-poly(ethylene glycol) and polydimethylsiloxane (PDMS) to enhance the stretchability and the surface energy of the elastic film. The deposition of a thin parylene film on this modified PDMS led to the formation of a spontaneously wrinkled surface, resulting in a highly stretchable substrate. Rigid Au conductors were deposited on the resultant wrinkles substrates, and could be stretched up to 20% strain. In addition, stretchable pentacene thin-film transistors were successfully fabricated on the wrinkled substrate without the formation of stiff islands.

Crystallinity and mechanical effects from annealing Parylene thin films

Parylene is commonly used as thin film polymer for MEMS devices and smart materials. This paper investigates the impact on bulk properties due to annealing various types of Parylene films. A thin film of Parylene N, C and a hybrid material consisting of Parylene N and C were deposited using a standard Gorham process. The thin film samples were annealed at varying temperatures from room temperature up to 300°C. The films were analyzed to determine the mechanical and crystallinity effects due to different annealing temperatures. The results demonstrate that the percentage of crystallinity and the full-width-half-maximum value on the 2θ X-ray diffraction scan increases as the annealing temperature increases until the melting temperature of the Parylene films was achieved. Highly crystalline films of 85% and 92% crystallinity were achieved for Parylene C and N respectively. Investigation of the hybrid film showed that the individual Parylene films behave independently to each other, and the crystallinity of one film had no significant impact to the other film. Mechanical testing showed that the elastic modulus and yield strength increase as a function of annealing, whereas the elongation-to-break parameter decreases. The change in elastic modulus was more significant for Parylene C than Parylene N and this is attributed to the larger change in crystallinity that was observed. Parylene C had a 112% increase in crystallinity compared to a 61% increase for Parylene N, because the original Parylene N material was more crystalline than Parylene C so the change of crystallinity was greater for Parylene C.

Co-Design Method and Wafer-Level Packaging Technique of Thin-Film Flexible Antenna and Silicon CMOS Rectifier Chips for Wireless-Powered Neural Interface Systems.

In this paper, a co-design method and a wafer-level packaging technique of a flexible antenna and a CMOS rectifier chip for use in a small-sized implantable system on the brain surface are proposed. The proposed co-design method optimizes the system architecture, and can help avoid the use of external matching components, resulting in the realization of a small-size system. In addition, the technique employed to assemble a silicon large-scale integration (LSI) chip on the very thin parylene film (5 μm) enables the integration of the rectifier circuits and the flexible antenna (rectenna). In the demonstration of wireless power transmission (WPT), the fabricated flexible rectenna achieved a maximum efficiency of 0.497% with a distance of 3 cm between antennas. In addition, WPT with radio waves allows a misalignment of 185% against antenna size, implying that the misalignment has a less effect on the WPT characteristics compared with electromagnetic induction.

Focus-tunable low-power electrowetting lenses with thin parylene films.

Electrowetting lenses with record low power consumption (microwatts) have been demonstrated using high-quality parylene AF-4 dielectric layers and large dodecyl sulfate ions. Water and propylene glycol are interchanged as the polar liquid to enable diverging and converging lens operation achievable with the application of 15V. The optical quality of the lenses is comparable to conventional microlenses and the tuning exhibits very little (<0.5°) contact angle hysteresis.

Effects of Parylene Coating on Electron Transport in Pristine Suspended Carbon Nanotube Field-Effect-Transistors

Carbon nanotube (CNT) field effect transistors (FETs) are anticipated to provide a viable alternative to silicon as CMOS technology begins to reach ultimate scaling lengths. With high carrier mobilities, current density, and tunable bandgaps, CNTs can allow for the growth of the transistor industry beyond traditional materials. However, these nanotubes are sensitive to their surroundings and require a protective passivation layer to isolate them from contamination by their external environment. Thin parylene films have been shown to be a practical passivation layer for CNT devices as a flexible, chemically inert, high dielectric strength, pinhole-free material. In this paper, we perform electrical characterization of single suspended CNT FETs before and after passivation with parylene-C thin-film deposition. Analysis shows moderate changes in the threshold voltage, subthreshold slope, and ON/OFF ratio with parylene passivation, indicating minimal effect on the CNT FET overall performance.

Frequency- and temperature-dependent storage and loss moduli of microfibrous thin films of Parylene C

Elastic cyclic loading of ~110µm-thick Parylene C thin films comprising parallel slanted microfibers was performed in the frequency range 5–200Hz, to determine the dependences of their storage and loss moduli on the temperature and the frequency. The glass-transition temperature was identified as about 65°C from the temperature-maximum of the loss modulus. A low-frequency peak (at 115–135Hz) in the variation of the loss modulus is due to the resonance of an individual microfiber loaded by the remaining microfibers in the film.

Electrochemical impedance spectroscopy based MEMS sensors for phthalates detection in water and juices

Phthalate esters are ubiquitous environmental and food pollutants well known as endocrine disrupting compounds (EDCs). These developmental and reproductive toxicants pose a grave risk to the human health due to their unlimited use in consumer plastic industry. Detection of phthalates is strictly laboratory based time consuming and expensive process and requires expertise of highly qualified and skilled professionals. We present a real time, non-invasive, label free rapid detection technique to quantify phthalates' presence in deionized water and fruit juices. Electrochemical impedance spectroscopy (EIS) technique applied to a novel planar inter-digital (ID) capacitive sensor plays a vital role to explore the presence of phthalate esters in bulk fluid media. The ID sensor with multiple sensing gold electrodes was fabricated on silicon substrate using micro-electromechanical system (MEMS) device fabrication technology. A thin film of parylene C polymer was coated as a passivation layer to enhance the capacitive sensing capabilities of the sensor and to reduce the magnitude of Faradic current flowing through the sensor. Various concentrations, 0.002ppm through to 2ppm of di (2-ethylhexyl) phthalate (DEHP) in deionized water, were exposed to the sensing system by dip testing method. Impedance spectra obtained was analysed to determine sample conductance which led to consequent evaluation of its dielectric properties. Electro-chemical impedance spectrum analyser algorithm was employed to model the experimentally obtained impedance spectra. Curve fitting technique was applied to deduce constant phase element (CPE) equivalent circuit based on Randle's equivalent circuit model. The sensing system was tested to detect different concentrations of DEHP in orange juice as a real world application. The result analysis indicated that our rapid testing technique is able to detect the presence of DEHP in all test samples distinctively.