Mechanical properties of Bombyx mori silkworm silk: viscoelasticity, structural and molecular origin

This thesis is a comparative experimental and theoretical work focused on the mechanical properties of Bombyx mori silk on macroscopic and molecular levels. We did study the response of silk to a mechanical perturbation by monitoring changes of the microscopic structure in the crystalline regions as well as the molecular dynamics of the amorphous phase. In order to achieve this goal \insitu scattering experiments were performed: synchrotron X-ray radiation and cold neutrons were used as probes of the structure and dynamics, respectively. Much attention was devoted to the less studied temporal evolution of the surveyed phenomena. Accordingly our stretching experiments covered several time intervals, ranging from fractions of a second up to several days. Because such environmental parameters as temperature and humidity strongly influence the thermo-mechanical properties of silk the measurements were done at controlled environmental conditions and were repeated at several humidities. The theoretical analysis of our experimental data resulted in the development of a set of interrelated viscoelastic, structural and dynamical models. These models allow to describe the behaviour of stretched silk in the examined range of forces and elongations over almost the whole range of humidity. Particularly, they help to clarify the basic principles of the viscoelastic linear and non-linear behaviour of silk as well as the geometrical and structural aspects of the elongation in the crystalline regions. Generally, a link between the macroscopic viscoelastic behaviour and the mechanism at the molecular length scales has been established. With our experimental techniques and analyses we could separate the mechanical properties of the crystalline region of silk from those of the amorphous part, as well as show their interplay. In our opinion, interesting novel results were also obtained concerning the long-time mechanical relaxation of silk and molecular diffusion processes. Both phenomena exhibit definitive memory effects and can be well described by the models based on fractional calculus. Despite the fact that these phenomena belong to very different temporal and spacial levels the fractional characteristics of these phenomena are very similar.Die vorgelegte Arbeit ist eine komparative theoretische und experimentelle Untersuchung der mechanischen Eigenschaften von Bombyx mori Seide auf makroskopischer und auf molekularer Ebene. Die Reaktion der Seide auf mechanische Einwirkungen wurde erforscht mittels der Beobachtung der Variationen der mikroskopischen Struktur der kristallinen Regionen und der Molekulardynamik der amorphen Phase. Zu diesem Zweck wurden insitu Streuungsexperimente durchgeführt, bei denen die Struktur mittels Synchrotronstrahlung erkundet wurde. Hingegen wurden die molekulardynamischen Eigenschaften der amorphen Phase mittels Neutronenstreuung untersucht. Besonderes viel Aufmerksamkeit widmeten wir den noch wenig erforschten Aspekten der zeitlichen Entwicklung der vorliegenden Phänomene. Dementsprechend wurden unsere Dehnungsexperimente auf mehreren Zeitskalen durchgeführt, mit Dauern von einigen Sekunden bis zu einigen Tagen. Auch wurden unsere Experimente in einer streng kontrollierten Umgebung durchgeführt, da die Temperatur und die Feuchtigkeit der Umgebung die mechanischen Eigenschaften der Seide stark beeinflussen. Um den Einfluß des Wassergehaltes auf die mechanischen Eigenschaften der Seide genauer zu untersuchen wurden die Messungen bei verschiedenen Feuchtigkeiten wiederholt. Aus der theoretischen Analyse unserer experimentalen Ergebnisse resultierte eine Reihe von zusammenhängenden, viskoelastischen, strukturellen und dynamischen Modellen. Dies erlaubte uns das Verhalten von gedehnten Seidenfasern in den untersuchten Kraft-Dehnungs-Bereichen effektiv zu beschreiben. Insbesonders ermöglichte es uns sowohl die Grundprinzipien des linearen und nicht linearen viskoelastischen Verhaltens von Seide als auch die geometrischen und strukturellen Aspekte der Dehnung der kristallinen Bereiche zu klären. Generell konnte der Zusammenhang zwischen dem makroskopisch-viskoelastischen Verhalten und den Wirkungsmechanismen auf der molekularen Ebene etabliert werden. Mittels unserer experimentellen Techniken und unserer theoretischen Analyse waren wir in der Lage die mechanischen Eigenschaften der kristallinen Bereiche von den Eigenschaften der amorphen Regionen zu trennen und ihre Wechselwirkung zu zeigen. Unserer Meinung nach verdienen einige unserer neuen Ergebnisse aus dem Bereich der langzeit-viskoelastischen-Relaxation und der molekularen Diffusionsprozesse besondere Aufmerksamkeit. Beide Phänomene zeigen einen Memory-Effekt und sind beschreibbar mit Modellen der differential-fraktionellen Analyse. Trotz der Tatsache, dass beide Phänomene zu verschiedenen zeitlichen und räumlichen Ebenen gehören, zeigen ihre fraktionellen Charakteristika große Ähnlichkeiten

Multiple Mechanical Gradients are Responsible for the Strong Adhesion of Spider Attachment Hair

Wandering spiders climb vertically and walk upside-down on rough and smooth surfaces using a nanostructured attachment system on their feet. The spiders are assumed to adhere by intermolecular van der Waals forces between the adhesive structures and the substrate. The adhesive elements are arranged highly ordered on the hierarchically structured attachment hair (setae). While walking, it has been suggested that the spiders apply a shear force on their legs to increase friction. However, the detailed mechanical behavior of the hair's structures during attachment and detachment remains unknown. Here, gradients of the mechanical properties of the attachment hair on different length scales that have evolved to support attachment, stabilize adhesion in contact, and withstand high stress at detachment, examined by in situ experiments, are shown. Shearing helps to self-align the adhesive elements with the substrate. The study is anticipated to contribute to the development of optimized artificial dry adhesives

Cosmos 2044

The effects of microgravity and hind limb suspension on the enzyme patterns are assessed within a slow twitch muscle (soleus) and a fast twitch muscle (tibialis anterior). Studies were made on 95 soleus fibers and about 300 tibialis anterior (TA) fibers. Over 2200 individual enzyme measurements were made. Six key metabolic enzymes (hexokinase, pyruvate kinease, citrate kinase, beta-hydroxyacyl CoA dehydrogenase, glucose-6-P dehydrogenase, and aspartate aminotransferase) plus glutaminase and glutamate decarboxylase, as well as glutamate, aspartate, and GABA, were measured in 11 regions of the hippocampal formation of synchronous, flight, and tail suspension rats. Major differences were observed in the normal distribution of each enzyme and amine acid, but no substantive effects of either microgravity or tail suspension on these patterns were clearly demonstrated