The data processing regarding AFM nanoindentation experiments on biological samples relies on the basic contact mechanics models like the Hertz model and the Oliver & Pharr analysis. Despite the fact that the two aforementioned techniques are assumed to provide equivalent results since they are based on the same underlying theory of contact mechanics, significant differences regarding the Young's modulus calculation even on the same tested sample have been presented in the literature. The differences can be even greater than 30% depending on the used model. In addition, when the Oliver & Pharr analysis is used, a systematic greater Young's modulus value is always calculated compared to the Hertzian analysis. In this paper, the two techniques are briefly described and two possible reasons that accurately explain the observed differences in the calculated value of the Young's modulus are presented.

The objective of this paper was to investigate the influence of UV irradiation on collagen D-band periodicity by using the AFM imaging and nanoindentation methods. It is well known than UV irradiation is one of the main factors inducing destabilization of collagen molecules. Due to the human's skin chronic exposure to sun light, the research concerning the influence of UV radiation on collagen is of great interest. The impact of UV irradiation on collagen can be studied in nanoscale using Atomic Force Microscopy (AFM). AFM is a powerful tool as far as surface characterization is concerned, due to its ability to relate high resolution imaging with mechanical properties. Hence, high resolution images of individual collagen fibrils and load-displacement curves on the overlapping and gap regions, under various time intervals of UV exposure, were obtained. The results demonstrated that the UV rays affect the height level differences between the overlapping and gap regions. Under various time intervals of UV exposure, the height difference between overlaps and gaps reduced from ~3.7 nm to ~0.8 nm and the fibril diameters showed an average of 8-10% reduction. In addition, the irradiation influenced the mechanical properties of collagen fibrils. The Young's modulus values were reduced per 66% (overlaps) and 61% (gaps) compared to their initial values. The observed alterations on the structural and the mechanical properties of collagen fibrils are probably a consequence of the polypeptide chain scission due to the impact of the UV irradiation.

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