Rheological characterisation of alginate-like exopolymer gels crosslinked with calcium.

Bacterial alginate-like exopolymers (ALE) gels have been used in this work as a model for the extracellular polymeric matrix of biofilms. Aim was to relate the mechanical properties and strength of this matrix that make biofilms as persistent to cleaning as they are, to the complex cohesive molecular interactions involved. Mechanical properties of the gels as a function of CaCO3 concentration were investigated using dynamic and static rheology. Gels with relatively low CaCO3 concentrations, between 100 µmol and 300 µmol per g ALE, were found to exhibit similar viscoelastic behaviour as real biofilms, with elastic moduli between 50 Pa and 100 Pa and dissipation factors between 0.2 and 0.3. Increasing CaCO3 concentrations resulted in an increase of the elastic modulus up to 250 Pa, accompanied by an increase in brittleness. At a CaCO3 concentration of 1250 µmol per g ALE this trend stopped, probably due to disturbance of the continuous ALE network by precipitation of salts. Therefore, overdosing of Ca salts can be an adequate approach for the removal of biofouling. All gels exhibited permanent strain hardening under medium strain, and their mechanical properties showed dependency on their strain history. Even after application of an oscillatory strain with 200% amplitude that caused the gel structure to collapse, the gels recovered 65 to 90% of their original shear modulus, for the major part within the first 20 s. Recovery was slightly less for gels with high CaCO3 concentration. In creep tests fitted with a Burgers model with multiple Kelvin elements at least three different interactions in the ALE gels could be distinguished with characteristic retardation times in the range of 10, 100 and 1000 s. Further identification of the mechanisms underlying the gel mechanics will allow the development of targeted strategies to undermine the mechanical strength of biofouling and aid the cleaning process.

Improved piercing of microneedle arrays in dermatomed human skin by an impact insertion method.

An electrical applicator was designed, which can pierce short microneedles into the skin with a predefined velocity. Three different shapes of microneedles were used, namely 300 mum assembled hollow metal microneedle arrays, 300 mum solid metal microneedle arrays and 245 mum hollow silicon microneedle arrays. The latter are available as 4x4, 6x6 and 9x9 arrays. When using a velocity of 1 or 3 m/s reproducible piercing of dermatomed and full thickness human skin was evident from the appearance of blue spots on the dermal side of the skin after Trypan Blue treatment and the presence of fluorescently labeled particles in dermatomed skin. Manual piercing did not result in the appearance of blue spots. Transport studies revealed that i) piercing of microneedles with a predefined velocity into human skin resulted in a drastic enhancement of the Cascade Blue (CB, Mw 538) transport, ii) A higher piercing velocity resulted in a higher CB transport rate, iii) The CB transport rate was also dependent on the shape of the microneedles and iv) no difference in transport rate was observed between 4x4, 6x6 and 9x9 hollow silicon microneedle arrays.

Stick-slip motion in spite of a slippery contact: do we get what we see in atomic friction?

Atomic force microscopy provides direct atomic-scale access to friction. In this paper, unexpected and potentially dramatic consequences of the tip elasticity are discussed. Under certain natural conditions an essentially new, nontrivial regime can be entered. Although the tip appears to perform typical stick-slip motion, the tip-surface contact is fully "lubricated" by fast thermal motion of the tip apex. The interpretation of the observations needs to be changed completely in this case.

Synthesis of 18F-labelled VP 16-213 and podophyllotoxin using acetyl hypofluorite.

The fluorination of the antitumour agents VP 16-213 and podophyllotoxin using [18F]acetyl hypofluorite is described. Overall labeling yields of 10% and 20% respectively were obtained. The F-atom is found to be introduced into the E-moiety of the compounds, apparently without unwanted conformational changes.