Bożena Gładyszewska
Method for testing selected mechanical properties of thin-film biomaterials 
The dissertation presents a new method for testing the mechanical properties of thin-film biomaterials, i.e. the method of randomly distributed markers.
According to literature data, the key strength properties of materials are Young’s modulus, Poisson’s ratio and critical stress. Thus, the main purpose of this work was to find out whether the basic mechanical properties of thin-film biomaterials can be determined experimentally, avoiding the negative impact of edge effects, commonly encountered in stress tests. The experiment was preceded by a review of the existing research methods, to determine whether they can be adapted for the purposes of biomaterial analysis or whether a new method should be developed, based on experimental results.
One of the most advanced methods employed to assess mechanical properties, namely the Fourier analysis of deformation of a periodic gold array deposited onto the tested material, was verified in the study. However, it was found that the method is subject to certain important limitations since the deposition of a gold array requires the use of specialist equipment (a vacuum chamber enabling cathode sputtering). Moreover, gold arrays cannot be deposited onto wet materials, and the obtained results may be unreliable if the near-surface layer of the analyzed material is stiffened by the deposited gold array. Therefore, it was necessary to propose a new method, free from the above disadvantages.
The method described in the dissertation is based on an analysis of the image and positions of m arkers randomly distributed across the tested material. A test stand was designed and constructed in order to perform measurements and verify the effectiveness of the proposed method. The seed coats of raw and blanched beans as well as fresh, dried and frozen broad beans were used as experimental material. Stress tests of the seed costs of beans and broad beans were also conducted during the germination process, i.e. a process based on natural watering of seeds and seed coats, significantly affecting the mechanical properties of the analyzed material. The above thin-film biomaterials were perfectly suited for the purpose of this study since they differ considerably with respect to their mechanical properties.
It was confirmed that the method of randomly distributed markers allows to avoid numerous limitations and errors characteristic of other methods applied to test the mechanical properties of biological materials. Its main advantage is the fact that the results are independent of the effects occurring at specimen edges, i.e. close to the clamps of a testing machine. This method permits measurements within a well-defined, selected point of the analyzed material. Even in case of partial specimen damage the experiment can be continued and changes in the positions of markers distributed across the undamaged area can be observed. This is possible due to the application of constant force increment instead of constant strain increment, applied in the majority of classical testing machines.
The applications of the proposed method are not limited to seed coats. Such tests can be performed on all types of materials from which thin-film specimens can be extracted. This method may be successfully employed in the case of such biomaterials as the leaves, stems and culms of plants, the peels and skins of fruits and vegetables, animal skin or root vegetables. It follows that the method of randomly distributed markers may be widely used for the purposes of both agricultural and materials science. First of all, however, it constitutes a new, efficient tool for testing the mechanical properties of thin-film biomaterials.