In this paper, we report an AFM study on the supramolecular structures adopted by the synthetic polypentapeptide poly(ValGlyGlyValGly), whose monomeric sequence is an abundant, simple building block of elastin. The polypeptide was analyzed by deposition from both methanolic and aqueous suspensions, showing different behaviors. In methanol, the polypeptide is able to evolve, in a time-dependent way, from layers to ribbons to beaded filaments. When the equilibrium is reached, the formation of well-defined dendritic structures is also observed. This restructuring of the polypentapeptide seems to be reminiscent of a sort of Rayleigh instability. When deposited from aqueous suspensions, the polypeptide self-assembles either in fibrillar networks or in amyloid-like patterns, both of them being found in elastin or elastin-related polypeptides. As a general finding, poly(ValGlyGlyValGly) seems to constitute an excellent mimetic of the supramolecular properties of native elastin.

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Calibration of atomic force microscope (AFM) cantilevers is necessary for the measurement of nanonewton and piconewton forces, which are critical to analytical applications of AFM in the analysis of polymer surfaces, biological structures and organic molecules. We have developed a compact and easy-to-use reference artefact for this calibration. This consists of an array of dual spiral-cantilever springs, each supporting a polycrystalline silicon disc of 170 µm in diameter. These were fabricated by a two-layer polysilicon surface micromachining method. Doppler interferometry is used to measure the fundamental resonant frequency of each device accurately. We call such an array a microfabricated array of reference springs (MARS).These devices have a number of advantages. Firstly, modelling the fundamental resonant frequencies of the devices is much more straightforward than for AFM cantilevers, because the mass and spring functions are isolated in different parts of the structure. Secondly, the spring constant of each spring is in linear proportion to the mass of the device, given that the resonant frequency is measured accurately. The thickness and hence the mass can be measured accurately by AFM or interferometry.The array spans the range of spring constant important in AFM, allowing almost any AFM cantilever to be calibrated easily and rapidly. The design of the MARS makes it much less sensitive to uncertainties in its dimensions, which is expected to lead to an improvement, in principle, of approximately a factor of three compared to the most accurate previous methods of spring constant calibration, because the spring constant is proportional to the a critical thickness (after resonant frequency has been measured) rather than the cube of a critical thickness, as for a reference cantilever.

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