Plasma polymer film designs through the eyes of ToF-SIMS.

Time-of-flight secondary ion mass spectrometry (ToF-SIMS) is increasingly used for the detailed chemical characterization of complex organic materials. Of particular interest in biointerface materials, it provides the accurate molecular information on their surface, a prerequisite for the understanding of subsequent interaction with biomaterials. Plasma polymer films are promising biointerface materials, as tuning the deposition parameters allows the control over film stability and density of surface functional groups. However, the optimization of these film properties not only requires a detailed characterization of the film chemistry, but also that of the deposition mechanisms. Here, ToF-SIMS is used within its different operation modes to investigate those on several plasma polymer film designs. The detailed information on surface molecular chemistry, interface conformation, vertical and lateral chemical and cross-linking gradients is gathered and linked to the underlying deposition mechanisms. In combination with other techniques, the interpretation and understanding of the final functional property of the films in terms of protein adsorption and site-specific binding is achieved.

Composition and Stability of Plasma Polymer Films Exhibiting Vertical Chemical Gradients.

Controlling the balance between stability and functional group density in grown plasma polymer films is the key to diverse applications such as drug release, tissue-engineered implants, filtration, contact lenses, microfluidics, electrodes, sensors, etc. Highly functional plasma polymer films typically show a limited stability in air or aqueous environments due to mechanisms like molecular reorganization, oxidation, and hydrolysis. Stabilization is achieved by enhancing cross-linking at the cost of the terminal functional groups such as -OH and -COOH, but also -NH2, etc. To overcome such limitations, a structural and chemical gradient was introduced perpendicular to the surface plane; this vertical gradient structure is composed of a highly cross-linked base layer, gradually changing into a more functional nanoscaled surface termination layer. This was achieved using CO2/C2H4 discharges with decreasing power input and increasing gas ratio during plasma polymer deposition. The aging behavior and stability of such oxygen-functional vertical gradient nanostructures were studied in air and in different aqueous environments (acidic pH 4, neutral pH ≈ 6.2, and basic pH 10). Complementary characterization methods were used, including angle-resolved X-ray photoelectron spectroscopy (ARXPS) and time-of-flight secondary ion mass spectrometry (ToF-SIMS) as well as water contact angle (WCA) measurements. It was found that in air, the vertical gradient films are stabilized over a period of months. The same gradients also appear to be stable in neutral water over a period of at least 1 week. Changes in the oxygen depth profiles have been observed at pH 4 and pH 10 showing structural and chemical aging effects on different time scales. The use of vertical gradient plasma polymer nanofilms thus represents a novel approach providing enhanced stability, thus opening the possibility for new applications.

Enhancing Surface Plasmon Resonance Detection Using Nanostructured Au Chips.

The increase of the sensitivity of surface plasmon resonance (SPR) refractometers was studied experimentally by forming a periodic relief in the form of a grating with submicron period on the surface of the Au-coated chip. Periodic reliefs of different depths and spatial frequency were formed on the Au film surface using interference lithography and vacuum chalcogenide photoresists. Spatial frequencies of the grating were selected close to the conditions of Bragg reflection of plasmons for the working wavelength of the SPR refractometer and the used environment (solution of glycerol in water). It was found that the degree of refractometer sensitivity enhancement and the value of the interval of environment refractive index variation, Δn, in which this enhancement is observed, depend on the depth of the grating relief. By increasing the depth of relief from 13.5 ± 2 nm to 21.0 ± 2 nm, Δn decreased from 0.009 to 0.0031, whereas sensitivity increased from 110 deg./RIU (refractive index unit) for a standard chip up to 264 and 484 deg./RIU for the nanostructured chips, respectively. Finally, it was shown that the working range of the sensor can be adjusted to the refractive index of the studied environment by changing the spatial frequency of the grating, by modification of the chip surface or by rotation of the chip.

Suppression of Hydrophobic Recovery by Plasma Polymer Films with Vertical Chemical Gradients.

Vertical chemical gradients extending over a few nanometers were explored. The gradients are based on plasma-polymerized oxygen-containing ethylene (ppOEt) films. Using plasma conditions with low CO2/C2H4 ratio and high energy input, cross-linked films were deposited as base layer, while increasing CO2 and lowering energy input resulted in less cross-linked yet highly functional films as applied as top layer. Aging studies indicate that, in particular, for very thin gradient structures, the cross-linked subsurface zone effectively hinders reorientation of the surface functional groups, thus restricting hydrophobic recovery and oxidation effects.

Simultaneous immobilization of heparin and gentamicin on polypropylene textiles: a dual therapeutic activity.

The aim of this work was to prepare a nonwoven polypropylene (PP) textile functionalized with bioactive molecules in order to improve simultaneously anticoagulation and antibacterial properties. The immobilization of either heparin (anticoagulation agent) or gentamicin (aminoglycoside class antibiotic) alone has already been proven to be effective on PP nonwoven textiles. In this work, we managed to go further, by immobilizing both heparin and gentamicin at the same time on one unique textile. A successive immersion in different heparin and gentamicin bathes successfully led to a dual drug coated textile, as confirmed by several characterization techniques (Fourier transform infrared-attenuated total reflectance, X-ray photoelectron spectroscopy, and scanning electron microscopy). The immobilization times were varied in order to determine the best compromise between cytocompatibility, anticoagulant effect, and antimicrobial activity. Short immersion times in gentamicin solutions confer very good antimicrobial activity to the textile and avoid cytotoxicity, whereas long immersion times in heparin solution were necessary to observe a significant anticoagulant effect.