Background While the overall stiffness of the lens has been measured in a number of studies, the knowledge about the stiffness distribution within the lens is still limited. The purpose of this study was to determine the stiffness gradient in the human crystalline lens. A secondary purpose was to determine whether the stiffness gradient depends on age. Methods The local dynamic stiffness was measured in 10 human crystalline lenses (age range: 19 to 78 years). The lenses were stored at −70°C before being measured. The influence of freezing on the mechanical properties has been determined in a previous study. A small oscillating probe was used to measure the local dynamic shear modulus as a measure of lens stiffness. The measurements were taken in the cross-sectional plane through the lens equator. Results The local dynamic shear modulus varied with location for all tested lenses. The central stiffness of the oldest lens (78 years) was 10 4 times higher than the youngest (19 years) lens. The equatorial stiffness of the oldest lens was 10 2 times higher than the youngest lens. For the older lenses, the centre was 5.8-210 times stiffer than the periphery, as opposed to earlier results described by Fisher (1971), who found that the periphery was up to 3 times softer than the centre for lenses younger than 70-years-old. For the three youngest lenses (19 to 49 years), the periphery was 2.2-16.6 times stiffer than the centre. Conclusions The dynamic stiffness of the crystalline lens varies with location within the lens. The stiffness gradient depends on the age of the lens. The results of the 10 lenses indicate that the stiffness of both centre and periphery increase with age, but at a different rate.

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Adsorption and aggregation of carbosiloxane dendrimers on mica and pyrolytic graphite were investigated by scanning force microscopy (SFM). The aggregation process started from (i) single molecules which coagulated to (ii) clusters and (iii) fluid droplets followed by formation of (iv) a complete layer on the solid substrate. The molecules were displayed as a globular particle with a diameter of about 2.5 nm. Tapping SFM of the liquid was possible due to the fact that the dendrimer undergoes a transition to a viscoelastic state below the tapping frequency of about 360 kHz. Dynamic shear compliance experiments have shown a plateau of 5 -lo-' Pa-' around this frequency. Dendrimer droplets slowly spread into polygonal lamellae with a thickness of two molecular layers. The structures indicate a rather regular dense packing of the globular molecules.

A carbosilane dendrimer containing hydroxyl terminal groups, which showed two types of wetting in dependence of the substrate used, was studied by tapping force microscopy. Due to the preferential adsorption of the hydroxyl groups, the dendrimer displayed autophobic spreading on mica, whereas a substrate which was first coated with a semifluorinated polymer was only partially wetted. In both cases, submicrometer-sized droplets were deposited on the surface. Microscopic contact angles were measured and compared with macroscopic values obtained by a standard sessile drop technique. The comparison showed lower values for the microscopic angle which were explained by deformation of the droplets caused by the tapping tip. The oscillatory motions of the tip intermittently touching a viscoelastic sample were calculated using a simple model of the tapping mode. The indentation of the tip into the sample and the induced phase shift relative to the oscillations in air were determined from the model, showing a good agreement with experimental results. Compared to traditional methods, this approach offers advantages such as (i) three-dimensional visualization of the whole droplet, (ii) submicrometer resolution of the structure near the three-phase boundary, and (iii) accurate determination of small contact angles.

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Soluble propene/ethene/CO terpolymers (EPEC) with ultrahigh molecular weight (up to 1.2 x lo6 g/mol) were prepared by the use of dicationic palladium(I1) phosphine catalysts and an optimized amount of water as activator. When the molar ratio of ethene/CO to propene/CO is below 50 mol-9, the terpolymers are thermoplastic elastomers with excellent properties. Above this ratio the terpolymers are crystalline thermoplastics. The ultrahigh molecular weight elastomers are highly soluble in organic solvents such as CH2CI, and CHCl, .

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