Mirko Nitschke

Surface characterisation of NH3 plasma treated polyamide 6 foils

Abstract Nitrogen-containing plasmas are widely used to improve wettability, printability, bondability, and biocompatibility of polymer surfaces. Plasma-treatments fed with NH3 give rise to N-functionalities, such as amino (NH2), imino (CHNH), cyano (CN) and others on polymers, plus oxygen-containing groups due to post-plasma atmospheric oxidation. This work deals with NH3 plasma treatment of PA 6 foils and the evaluation of surface modification as a function of treatment time. The introduced functionalities were observed by streaming potential measurements (surface charge), X-ray photoelectron spectroscopy analysis (nature of introduced functionalities), atomic force microscopy (surface topography), and contact angle measurement (assessment of treatment effect). The results show that the introduction of N-containing groups is increasing with longer treatment time only to a certain extent where the negative effect of surface destruction prevails over the positive effect of introduction of functional groups. The treatment causes a shift of the isoelectric point (IEP) toward pH of 6.2 as compared to 4.2 found for the untreated foil. If the treatment time is longer than 1 min the IEP is shifted to lower pH, the number of amino groups on the surface is reduced and the contact angle is increased.

Durable surface modification of poly(tetrafluoroethylene) by low pressure H2O plasma treatment followed by acrylic acid graft polymerization

In order to obtain a durable and homogeneous surface functionalization of poly(tetrafluoroethylene) (PTFE), a two step procedure based on low pressure plasma treatment followed by a grafting reaction was used. PTFE foils and plasma deposited fluorocarbon polymer films with a chemical structure close to PTFE were investigated. The fluoropolymer surfaces were treated with microwave water vapor plasma to introduce functional groups and radicals. The generated radicals were utilized as reactive species for the gas phase graft polymerization of acrylic acid. A homogeneous and stable poly(acrylic acid) (pAAc) layer with a thickness of about 70 nm was formed. The influence of the grafting period and the acrylic acid vapor pressure on the amount of grafted pAAc was studied. The properties of the modified surfaces were characterized by X-ray photoelectron spectroscopy, atomic force microscopy, Fourier transform infrared spectroscopy, spectroscopic ellipsometry, electrokinetic measurements and dynamic contact angle measurements. The grafted surfaces can be used for a covalent protein immobilization.

Stability and ageing of plasma treated poly(tetrafluoroethylene) surfaces

Abstract In this study CO 2 , H 2 /H 2 O and H 2 O low pressure plasma treatment of poly(tetrafluoroethylene) (PTFE) foils and of thin plasma deposited fluorocarbon polymer (PDFP) films with a structure close to PTFE was investigated. The properties of the plasma were analyzed by mass spectroscopy (MS) and optical emission spectroscopy (OES). The modified fluorocarbon surfaces were characterized by X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), fourier transform infrared (FTIR) spectroscopy, spectroscopic ellipsometry, electrokinetic measurements and dynamic contact angle measurements in order to find optimized treatment conditions. The results of the surface modification were compared with respect to the efficiency of the plasma treatment and the stability of the modification effect at different ambient conditions. It was shown that the H 2 O plasma treatment is the most effective process for the intended modification. The hydrophobic PTFE surface was converted into a more hydrophilic one. The introduced radicals after the H 2 O plasma treatment can be utilized subsequently for post plasma reactions such as grafting processes.

Interrelations between charging, structure and electrokinetics of nanometric polyelectrolyte films.

Streaming current, surface conductivity and swelling data of poly(acrylic acid) (PAA) and poly(ethylene imine) (PEI) thin films are analyzed on the basis of the theory for diffuse soft interfaces (J.F.L. Duval, R. Zimmermann, A. L. Cordeiro, N. Rein, C. Werner, Langmuir 25 (2009) 10691). Focus is put on ways to unravel the electroosmotic and migration contributions of the measured surface conductivity, which is crucial for appropriate electrokinetic analysis of films carrying high densities of dissociable groups. Results demonstrate that the osmotically-driven swelling of the PAA films with increasing pH is accompanied by an increase in diffuseness for the interphasial polymer segment density distribution. This heterogeneity is particularly marked at low ionic strength with a non-monotonous dependence of the streaming current on pH and the presence of a maximum at pH∼6.5. The analysis of the PEI films evidences heterogeneous swelling with lowering pH, i.e. upon protonation of the amine groups. The characteristic decay length in the interphasial PEI segment density distribution is found to be nearly independent of the pH, which is in line with the moderate swelling determined by ellipsometry. A critical discussion is given on the strengths and limitations of electrokinetics/surface conductivity for quantifying the coupled electrohydrodynamic and structural properties of moderately to highly swollen polyelectrolyte thin films.

Temperature dependent physicochemical properties of poly(N-isopropylacrylamide-co-N-(1-phenylethyl) acrylamide) thin films

The physicochemical properties of thermo-responsive polymer films are dynamically altered upon changes in environmental conditions. We report on the design and detailed characterization of a novel thermo-responsive polymer film with a temperature transition tuned to fit applications related to the control of marine biofouling. A copolymer consisting of poly(N-isopropylacrylamide) (PNIPAAm) and N-(1-phenylethyl) acrylamide (PEAAm) was synthesized and immobilized as a thin film onto Teflon AF surfaces using a low pressure argon plasma treatment. The temperature dependent physicochemical properties of the thermo-responsive film were thoroughly characterized and the impact of sea water on the film properties was investigated. The immobilized thermo-responsive film exhibits a reversible swelling/deswelling with temperature. Atomic force microscopy showed no morphological changes with varying temperature. Streaming current measurements performed above and below the transition temperature of the thermo-responsive hydrogel indicated that the charging of the polymer/aqueous solution interface is mainly determined by the preferential water ion adsorption at the Teflon AF surface. Inverse contact angles measured using captive air bubbles and analysed by axisymmetric drop shape analysis (ADSA) supported the intrinsic properties of the thermo-responsive film, as surface hydrophilicity decreased with increasing temperature. The advancing water contact angle decreased with increasing temperature, which may be explained by the different molecular mobility at different temperatures, allowing or hampering the re-orientation of hydrophobic segments at the solid–liquid and solid–fluid interfaces. These new films will allow investigations on the interaction of microorganisms with environmentally sensitive surfaces.

Polypropylene surface functionalization with chitosan

Chitosan coatings on oxygen-plasma pre-treated polypropylene (PP) surfaces were formed to improve their wettability, dyeing behavior and reactivity without altering material bulk properties. XPS, electrokinetic potential and contact angle measurements as well as dye uptake tests were carried out for surface characterization of modified PP, evaluation of chitosan coatings stability, and the effects of temperature and pH on coatings formation. About 20–30% of the total amount of chitosan immobilized on PP was found to be covalently bonded to the plasma pre-treated surface through the heat induced reactions with oxygen-containing functional groups at T > 80°C that corresponded to 47% of surface coverage. Subsequent cross-linking reaction with epichlorohydrin proved to be an efficient way to reduce the susceptibility of chitosan coatings to acidic hydrolysis.

Plasma modification of polytetrafluoroethylene for immobilization of the fibrinolytic protein urokinase

Abstract The modification of surface properties is necessary to improve the blood compatibility of polytetrafluoroethylene (PTFE). This paper describes the concept of a modification of PTFE surfaces. It consists of three steps: first, water vapour microwave plasma modification; second, acrylic acid vapour-phase post-grafting; and third, immobilization of the fibrinolytic protein urokinase. During or after each step, the modification results were characterized by zeta potential, ellipsometry, wetting measurements and X-ray photoelectron spectroscopy.

Human corneal endothelial cell sheets for transplantation: Thermo-responsive cell culture carriers to meet cell-specific requirements

Corneal endothelial diseases lead to severe vision impairment, motivating the transplantation of donor corneae or corneal endothelial lamellae, which is, however, impeded by endothelial cell loss during processing. Therefore, one prioritized aim in corneal tissue engineering is the generation of transplantable human corneal endothelial cell (HCEC) layers. Thermo-responsive cell culture carriers are widely used for non-enzymatic harvest of cell sheets. The current study presents a novel thermo-responsive carrier based on simultaneous electron beam immobilization and cross-linking of poly(vinyl methyl ether) (PVME) on polymeric surfaces, which allows one to adjust layer thickness, stiffness, switching amplitude and functionalization with bioactive molecules to meet cell type specific requirements. The efficacy of this approach for HCEC, which require elaborate cell culture conditions and are strongly adherent to the substratum, is demonstrated. The developed method may pave the way to tissue engineering of corneal endothelium and significantly improve therapeutic options.

Sulfated glyco-block copolymers with specific receptor and growth factor binding to support cell adhesion and proliferation

We report on the successful preparation of thin glyco-block copolymer films with a combined thermoresponsive and heparin-like functionality. The copolymers were synthesized from poly(N-isopropylacrylamide) and glucose units and were covalently fixed onto glass supports by means of low pressure plasma cross-linking. The thin films retain the thermoresponsive characteristics of poly(N-isopropylacrylamide) with a transition temperature around 33 degrees C. Additionally, it could be shown that sulfation of the glucose moieties introduces a heparin-like functionality to the films. An increase in binding of basic fibroblast growth factor (bFGF) as well as specific adhesion of endothelial cells and hematopoietic progenitor cells could be demonstrated. The functional coupling of bFGF to the glyco-block copolymer surfaces was further proven by the dose-dependent response of endothelial cell proliferation. The results show that the newly synthesized glyco-block copolymers allow for the preparation of biomimetic surfaces with dual functionalities of thermoresponsive and heparin-like characteristics for the application in cell culture experiments with specific binding and release of heparin-binding growth factors and cell adhesion receptors.

Cultivation of an immortalized human corneal endothelial cell population and two distinct clonal subpopulations on thermo-responsive carriers.

BackgroundRecently, it was possible to show that human corneal endothelial cells (HCEC) can be cultured on thermo-responsive polymer substrates, and can be harvested as entire cell sheets without losing viability. We sought to study HCEC sheet cultivation on such cell culture carriers under serum-free conditions as the next consequential step in developing methods for generation of corneal endothelial cell transplants.MethodsAn immortalized heterogenous HCEC population and two immortalized, clonally grown HCEC lines (HCEC-B4G12 and HCEC-H9C1) were cultured on thermo-responsive substrates under serum-supplemented and serum-free culture conditions. Cell sheets were characterized by phase contrast microscopy and by immunofluorescent staining for ZO-1, Na+,K+-ATPase, and vinculin.ResultsAll tested HCEC populations were able to adhere, spread and proliferate on thermo-responsive substrates under serum-supplemented conditions. Under serum-free conditions, pre-coating of the polymer substrates with ECM proteins was necessary to facilitate attachment and spreading of the cells, except in the case of HCEC-B4G12 cells. The heterogenous HCEC population formed closed monolayers, properly localized ZO-1 to lateral cell borders, and had moderate vinculin levels under serum-free, and higher vinculin levels under serum-supplemented culture conditions. HCEC-B4G12 cells formed closed monolayers, showed proper localization of ZO-1 and Na+,K+-ATPase to lateral cell borders, and had high vinculin levels irrespective of culture conditions. In contrast, HCEC-H9C1 cells had lowest vinculin levels under serum-supplemented, and higher vinculin levels under serum-free culture conditions. ZO-1 was detected throughout the cytoplasm under both culture conditions. These loosely adherent cells were only able to form a closed monolayer under serum-supplemented conditions.ConclusionsSerum-free production of HCEC sheets is possible. The extremely adherent clonal HCEC line B4G12 produced higher vinculin levels than the other two tested HCEC populations, and showed strong adherence to the thermo-responsive, polymeric culture substratum irrespective of culture conditions. This cell line closely resembles terminally differentiated HCEC in vivo, and was found to be particularly suitable for further studies on HCEC cell sheet engineering.