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Measuring the Elastic Properties of Thin Polymer Films with the Atomic Force Microscope Jan Domke and Manfred Radmacher* Lehrstuhl fu ¨ r Angewandte Physik, Ludwig-Maximilians Universita ¨ tMu ¨ nchen, Amalienstrasse 54, 80799 Mu ¨ nchen, Germany Received December 1, 1997. In Final Form: March 16, 1998 The elastic properties of thin gelatin films were investigated with the atomic force microscope (AFM). The degree of swelling and thus the softness of the gelatin can be tuned by immersing it in mixtures of propanol and water. Therefore, we have chosen gelatin films as a model system to characterize the measurement of elasticity of thin and soft samples. The major aim of this study was to investigate the influence of the film thickness on the apparent elastic (Young’s) modulus. Thus, we prepared wedge- shaped samples with a well-defined thickness of up to 1 μ m. The Young’s modulus of our samples was between 1 MPa and 20 kPa depending on the degree of swelling. The elasticity was calculated by analyzing the recorded force curves with the help of the Hertz model. We show that the calculated Young’s modulus is dependent on the local film thickness and the applied loading force of the AFM tip. Thus, the influence of the hard substrate on the calculated softness of the film can be characterized as a function of indentation. It was possible to determine the elastic properties of gelatin films with a thickness down to 50 nm and a Young’s modulus of 20 kPa. Introduction The atomic force microscope (AFM) 1,2 is a rather new andpowerfulmethodforstudyingbiologicalsamples.With the ability of imaging proteins, DNA, or even whole cells in their physiological environment, 3,4 the AFM is an importanttoolfortheinvestigationofbiologicalprocesses on the nanometer scale. The AFM can not only be used for imaging the topography of surfaces but also for measuring forces on the molecular level. The AFM has been used for measuring binding forces between single molecules 5 andforobservingconformationalchangeswhen stretching single molecules. 6 Because it is also possible to apply well-defined small forces on a sample, the AFM was soon used as a nanoindenter measuring elastic properties, 7 thismethodhasalsobeenappliedtobiological samples. 8 - 10 Forinvestigatingtheelasticity,forcecurves are taken: the sample is compressed by the indenting AFM tip and the elastic response of the sample under this loading force is analyzed. The so-called force-mapping mode is a very sensitive tool for measuring interaction forces such as adhesion 11 or electrostatics 12 with a lateral resolution of only a few nanometers. When applying this technique to living cells, 13,14 the combination of an ex- tremely soft and very thin sample of only a few hundreds of nanometers in the peripheral parts creates a serious problem; that is, the AFM tip can completely compress the sample. Thus, the tip will “feel” the underlying stiff substrate. Consequently, the calculated or apparent Young’s modulus is too high and the sample appears too
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