Mussels use a variety of 3, 4-dihydroxyphenyl-l-alanine (DOPA) rich proteins specifically tailored to adhering to wet surfaces. Synthetic polypeptide analogues of adhesive mussel foot proteins (specifically mfp-3) are used to study the role of DOPA in adhesion. The mussel-inspired peptide is a random copolymer of DOPA and N5 -(2-hydroxyethyl)-l-glutamine synthesized with DOPA concentrations of 0–27 mol% and molecular weights of 5.9–7.1 kDa. Thin films (3–5 nm thick) of the mussel-inspired peptide are used in the surface forces apparatus (SFA) to measure the force–distance profiles and adhesion and cohesion energies of the films in an acetate buffer. The adhesion energies of the mussel-inspired peptide films to mica and TiO2 surfaces increase with DOPA concentration. The adhesion energy to mica is 0.09 μJ m−2 molDOPA−1 and does not depend on contact time or load. The adhesion energy to TiO2 is 0.29 μJ m−2 molDOPA−1 for short contact times and increases to 0.51 μJ m−2 molDOPA−1 for contact times >60 min in a way suggestive of a phase transition within the film. Oxidation of DOPA to the quinone form, either by addition of periodate or by increasing the pH, increases the thickness and reduces the cohesion of the films. Adding thiol containing polymers between the oxidized films recovers some of the cohesion strength. Comparison of the mussel-inspired peptide films to previous studies on mfp-3 thin films show that the strong adhesion and cohesion in mfp-3 films can be attributed to DOPA groups favorably oriented within or at the interface of these films.

Mussels have a remarkable ability to attach their holdfast, or byssus, opportunistically to a variety of substrata that are wet, saline, corroded, and/or fouled by biofilms. Mytilus edulis foot protein-5 (Mefp-5) is one of several proteins in the byssal adhesive plaque of the mussel M. edulis. The high content of 3,4 dihydroxyphenylalanine (Dopa) (~30 mol%) and its localization near the plaque-substrate interface have often prompted speculation that Mefp-5 plays a key role in adhesion. Using the surface forces apparatus, we show that on mica surfaces Mefp-5 achieves an adhesion energy approaching Ead = ~− 14 mJ/m2. This exceeds the adhesion energy of another interfacial protein, Mefp-3, by a factor of 4–5 and is greater than the adhesion between highly oriented monolayers of biotin and streptavidin. The adhesion to mica is notable for its dependence on Dopa, which is most stable under reducing conditions and acidic pH. Mefp-5 also exhibits strong protein-protein interactions with itself as well as with Mefp-3 from M. edulis.

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The pathway of the biologically active molecule hydrogen peroxide (H 2 O 2 ) from the plasma generation in the gas phase by an atmospheric pressure argon plasma jet, to its transition into the liquid phase and finally to its inhibiting effect on human skin cells is investigated for different feed gas humidity settings. Gas phase diagnostics like Fourier transformed infrared spectroscopy and laser induced fluorescence spectroscopy on hydroxyl radicals ( • OH) are combined with liquid analytics such as chemical assays and electron paramagnetic resonance spectroscopy. Furthermore, the viability of human skin cells is measured by Alamar Blue ® assay. By comparing the gas phase results with chemical simulations in the far field, H 2 O 2 generation and destruction processes are clearly identified. The net production rate of H 2 O 2 in the gas phase is almost identical to the H 2 O 2 net production rate in the liquid phase. Moreover, by mimicking the H 2 O 2 generation of the plasma jet with the help of an H 2 O 2 bubbler it is concluded that the solubility of gas phase H 2 O 2 plays a major role in generating hydrogen peroxide in the liquid. Furthermore, it is shown that H 2 O 2 concentration correlates remarkably well with the cell viability. Other species in the liquid like • OH or superoxide anion radical (O •− 2 ) do not vary significantly with feed gas humidity.

After nearly 30 years of research on the hydrophobic interaction, the search for the hydrophobic force law is still continuing. Indeed, there are more questions than answers, and the experimental data are often quite different for nominally similar conditions, as well as, apparently, for nano-, micro-, and macroscopic surfaces. This has led to the conclusion that the experimentally observed force–distance relationships are either a combination of different ‘fundamental’ interactions, or that the hydrophobic force-law, if there is one, is complex – depending on numerous parameters. The only unexpectedly strong attractive force measured in all experiments so far has a range of D ≈ 100–200 Å, increasing roughly exponentially down to ~ 10–20 Å and then more steeply down to adhesive contact at D = 0 or, for power-law potentials, effectively at D ≈ 2 Å. The measured forces in this regime (100–200 Å) and especially the adhesive forces are much stronger, and have a different distance-dependence from the continuum VDW force (Lifshitz theory) for non-conducting dielectric media. We suggest a three-regime force-law for the forces observed between hydrophobic surfaces: In the first, from 100–200 Å to thousands of ångstroms, the dominating force is created by complementary electrostatic domains or patches on the apposing surfaces and/or bridging vapour cavities; a ‘pure’ but still not well-understood ‘long-range hydrophobic force’ dominates the second regime from ~ 150 to ~ 15 Å, possibly due to an enhanced Hamaker constant associated with the ‘proton-hopping’ polarizability of water; while below ~ 10–15 Å to contact there is another ‘pure short-range hydrophobic force’ related to water structuring effects associated with surface-induced changes in the orientation and/or density of water molecules and H-bonds at the water–hydrophobic interface. We present recent SFA and other experimental results, as well as a simplified model for water based on a spherically-symmetric potential that is able to capture some basic features of hydrophobic association. Such a model may be useful for theoretical studies of the HI over the broad range of scales observed in SFA experiments.

This work focused on qualitative and quantitative detection of oxygen free radicals in liquids after plasma treatment with an atmospheric pressure argon plasma jet by electron paramagnetic resonance spectroscopy (EPR). For the treatment, a shielded plasma jet, where the active effluent zone is surrounded by a protective gas curtain was used.

Finding a solution for air species contamination of atmospheric pressure plasmas in plasma medical treatment is a major task for the new field of plasma medicine. Several approaches use complex climate chambers to control the surrounding atmosphere. In this paper, ambient species are excluded in plasma-human-skin-cell treatment by ensheathing the plasma jet effluent with a shielding gas. Not only does this gas curtain protect the plasma jet effluent from inflow of air species but it also, more importantly, allows controlling the effluent reactive species composition by adjusting the mixture of the shielding gas. In the present investigations, the mixture of nitrogen to oxygen within the gas curtain around an argon atmospheric pressure plasma jet (kinpen) is varied. The resulting reactive plasma components produced in the jet effluent are thus either oxygen or nitrogen dominated. With this gas curtain, the effect of reactive oxygen species (ROS) and reactive nitrogen species (RNS) on the cell viability of indirectly plasma-treated HaCaT skin cells is studied. This human keratinocyte cell line is an established standard for a skin model system. The cell viability is determined by a fluorometric assay, where metabolically active cells transform nonfluorescent resazurin to the highly fluorescent resorufin. Plasma jet and gas curtain are characterized by numerical flow simulation as well as by optical emission spectroscopy. The generation of nitrite within the used standard cell culture medium serves as a measure for generated RNS. Measurements with the leukodye dichlorodihydrofluorescein diacetate show that, despite a variation of the shielding gas mixture, the total amount of generated reactive oxygen plus nitrogen species is constant. It is shown that a plasma dominated by RNS disrupts cellular growth less than a ROSdominated plasma.Index Terms-Atmospheric pressure plasma jet, gas curtain, plasma liquid interaction, plasma medicine, reactive nitrogen species (RNS), reactive oxygen plus nitrogen species (RONS), reactive oxygen species (ROS), skin cells.

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