Frederick G. Yamagishi

Investigations of plasma-polymerized films as primers for Parylene-C coatings on neural prosthesis materials

Abstract A need exists to protect neural prosthesis devices from corrosion by biofluids in order to enhance their usable lifetime. Parylene-C is useful and biocompatible polymer coating, but its adhesion to metals used in these devices is not sufficient to achieve the necessary lifetimes. We developed plasma-polymerized hydrocarbon films to act as primer layers for enhancing the adhesion of Parylene-C to metallic surfaces. It was found that the metal surface should be clean and that the wet and dry adhesion of the overcoating is a function of the chemical nature of the surface. Thus, excellent wet and dry adhesion was obtained on clean tantalum and silicon surfaces which had been overcoated with a very thin layer of silicon dioxide. In each case this thin layer was overcoated with plasma-polymerized methane and then Parylene-C. Since the latter two processes are free radical processes, covalent bonds are created to enhance the adhesion. Cleaning procedures and careful reaction conditions are necessary. These are described.

Plasma-polymerized films as moisture barriers for alkali halide optics☆

Abstract The nature of the alkali halide surface is of paramount importance in enabling plasma-polymerized films to act as moisture barriers and protective coatings. We developed an effective in situ surface passivation method that uses the active species formed in a plasma derived from Freon gases to remove surface hydroxide groups with chlorine or fluorine atoms. A plasma-polymerized ethane protective coating deposited onto a passivated NaCl window provided protection from damage by water vapor at 98% relative humidity (RH) for at least 100 h. Furthermore, thin films of a variety of surfactants derived from fatty acids on NaCl and KBr were also useful passivating agents. Lifetimes of up to 10 days were obtained on NaCl at 88% RH. Plasma-polymerized films contained long-lived trapped free radicals which reacted with oxygen when the film was exposed to air. This generated oxygen functionality in the polymer which decreased the hydrophobicity and increased the IR absorption. Methods to eliminate these radicals are discussed.

Photochemical and Thermal Stability Studies on a Liquid Crystal Mixture of Cyanobiphenyls

Liquid crystals (LCs) have unique properties that allow them to be used in information display devices.1 In particular, LCs are useful for displays used in wrist watches, calculators, message boards, flat-panel television, and large-screen projection systems.2 The choice of materials is often dictated by the desired application. The use of LCs in electro-optical devices exposed to high-intensity light must meet not only the usual criteria for these devices such as birefringence, dielectric and conductivity anisotropy, and alignment qualities, but must be photochemically stable to long exposures of visible light and thermally stable to heat generated by exposure to visible and near infrared light. Large-screen projection displays that use liquid crystal light valves3 have more severe photochemical stability requirements to achieve long lifetime displays than many other devices. Even very small absorption “tails” of the LCs that extend into the visible region of the spectrum can cause lifetime problems (should photo-decomposition occur) because of the high intensity of light used. Figure 1 shows a light valve projection system.3

Conductive Polymer-Based Transducers as Vapor-Phase Detectors

Publisher Summary This chapter discusses the development of conductive polymer-based transducers for the detection of volatile organic compounds (VOCs) and other gaseous pollutants for application in environmental monitoring. In the experiment described in the chapter, highly sensitive sensor elements derived from polyaniline (PANI) are demonstrated for NO2 and other VOCs. A figure of merit is defined to allow the comparison of the response of different sensors to particular challenge vapors at various concentrations. The use of excess silane coupling agents to modify the surface of the sensor substrates in combination with excess of a sulfonic acid used to convert PANI to its conductive state results in highly sensitive, stable, and reversible VOC sensors, along with some polythiophene derivatives, for the detection of ketones, esters, and aromatic hydrocarbons. Individual sensors show different sensitivities to each class of VOC used in the study so that an array of these sensors generated a characteristic signature for each class demonstrating selectivity and classification.