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Endothelial monolayers physiologically adapt to flow and flow-induced wall shear stress, attaining ordered configurations in which elongation, orientation, and polarization are coherently organized over many cells. Here, with the flow direction unchanged, a peculiar bi-stable (along the flow direction or perpendicular to it) cell alignment is observed, emerging as a function of the flow intensity alone, while cell polarization is purely instructed by flow directionality. Driven by the experimental findings, the parallelism between endothelia is delineated under a flow field and the transition of dual-frequency nematic liquid crystals under an external oscillatory electric field. The resulting physical model reproduces the two stable configurations and the energy landscape of the corresponding system transitions. In addition, it reveals the existence of a disordered, metastable state emerging upon system perturbation. This intermediate state, experimentally demonstrated in endothelial monolayers, is shown to expose the cellular system to a weakening of cell-to-cell junctions to the detriment of the monolayer integrity. The flow-adaptation of monolayers composed of healthy and senescent endothelia is successfully predicted by the model with adjustable nematic parameters. These results may help to understand the maladaptive response of in vivo endothelial tissues to disturbed hemodynamics and the progressive functional decay of senescent endothelia., Advanced Science, 9 (16), ISSN:2198-3844

The generation of traction forces and their transmission to the extracellular environment supports the disseminative migration of cells from a primary tumor. In cancer cells, the periodic variation of nuclear stiffness during the cell cycle provides a functional link between efficient translocation and proliferation. However, the mechanical framework completing this picture remains unexplored. Here, the Fucci2 reporter was expressed in various human epithelial cancer cells to resolve their cell cycle phase transition. The corresponding tractions were captured by a recently developed reference-free confocal traction-force microscopy platform. The combined approach was conducive to the analysis of phase-dependent force variation at the level of individual integrin contacts. Detected forces were invariably higher in the G1 and early S phases than in the ensuing late S/G2, and locally colocalized with high levels of paxillin phosphorylation. Perturbation of paxillin phosphorylation at focal adhesions, obtained through the biochemical inhibition of focal adhesion kinase (FAK) or the transfection of nonphosphorylatable or phosphomimetic paxillin mutants, significantly diminished the force transmitted to the substrate. These data demonstrate a reproducible modulation of force transmission during the cell cycle progression of cancer cells, instrumental to their invasion of dense environments. In addition, they delineate a model in which paxillin phosphorylation supports the mechanical maturation of adhesions relaying forces to the substrate., Molecular Biology of the Cell, 29 (21), ISSN:1939-4586, ISSN:1059-1524

Heart failure is a condition in which the heart is unable to provide adequate blood flow to the vital organs of the human body. Up to date, the most effective treatment is heart transplantation. Availability of donor hearts, however, is limited. Consequently, the number of patients in the heart transplant waiting list rises every year. Recently, an artificial solution has been developed, involving the implantation of ventricular assist devices, in order to provide temporary support to the patient’s heart and bridge the time to transplantation. A ventricular assist device (VAD) is an implantable electromechanical apparatus, responsible for assisting the function of a failing heart. Device implantation, however, is tied to poor survival rates of patients. Causal role have the severe thromboembolic events which, in turn, lead to device malfunction and patient death. These adverse phenomena, triggered by the direct contact between blood and synthetic material, necessitate the administration of intense and lifelong anticoagulation therapies. Coating the luminal surface of the device with a fully hemocompatible blood-foreign interface would halt coagulation and simultaneously improve patient survivability and life quality. Additionally, a hemocompatible surface would provide opening for VADs to be used as a destination therapy. To this extent, optimal protection could be represented by an autologous endothelial cell layer, the natural interfacial layer between blood and tissue, through the process of surface endothelialization. The Zurich Heart project, aims at improving contemporary VAD designs (System modification) as well as developing new concepts for VADs (Alternative systems). The work presented in this thesis is part of the System modification track. The long term goal is to develop a fully hemocompatible ventricular assist device. Hence, we envision to generate and maintain a functional endothelium on the luminal surface of the device. Realization of the abovementioned objective requires, however, to successfully tackle bottlenecks associated with endothelialization of devices. In the first part of this thesis, such challenges are identified and discussed. In the next two chapters, we introduce strategies, utilizing rationally designed surface topographies, to enhance endothelial cell retention under realistic hemodynamic conditions. In the last chapter, we focus on understanding the responses of human endothelia under supraphysiological magnitudes of wall shear stress. Regarding the clinical integration of cardiovascular devices, the paucity of source cells is a potentially calamitous scenario. Therefore, the development of surface engineering strategies to achieve full endothelialization, while minimizing the amount of endothelial cells required to seed the surface, is necessary. Stable endothelialization is the outcome of the interaction between endothelial cells, flow-generated wall shear stress and the substrate topography. In the 2nd part of the thesis, a novel strategy is presented and validated, based on the use of optimized surface topographies, combined with confined islands of seeded endothelial cells. With this approach, when approximately half of the substrate is covered with endothelial cells, the time to full endothelialization, compared to an unstructured surface, is almost halved. These results demonstrate a novel approach on the endothelialization of cardiovascular devices featuring partial endothelial cell seeding prior to implantation and exploiting the wound healing potential of endothelia to yield prompt endothelialization in situ. The second step towards full endothelialization of devices is the investigation of endothelia responses under pro-inflammatory signaling, expected in cardiovascular patients. Pro-inflammatory milieu, in conjunction with high magnitudes of wall shear stress, could potentially compromise endothelial integrity and survival. In the 3rd part of this thesis, we deal with this underlying danger by challenging human endothelial monolayers with the pro-inflammatory factor TNF-α under realistic hemodynamic conditions. Moreover, we demonstrate that the simple contact between endothelial cells and an optimized surface geometry can inhibit NF-kB activation downstream of TNF-α, yielding eventually increased stability of cell-to-cell junctions and focal adhesions. Importantly, the suggested topographic modifications can be implemented on a range of artificial substrates, enabling their endothelialization under the expected device operational conditions. Endothelial cell function under physiological flow conditions has been the scope of extensive research the past years. Information is incomplete, however, regarding the responses of confluent endothelia under supraphysiological wall shear stresses. The last part of this thesis investigates a differential response of the endothelium under these conditions. Supraphysiological magnitudes of wall shear stress, drive human endothelia monolayers to a stable perpendicular to the flow orientation. Importantly, this observation finds a common physiological reference to the valvular endothelial cells, which also exhibit a vertical to the flow phenotype. Pre-aligning endothelia with physiological levels of wall shear stress, and then exposing them to supraphysiological WSS magnitude, leads the endothelium to an unstable state, obtaining a random phenotype. This isotropic orientation decreases the resistance of endothelial cells to supraphysiological wall shear stress and results in loss of endothelium connectivity. Last, we report on the temporal evolution of the traction force fingerprint during endothelial phenotypic alteration. In particular, we utilize a traction force microscopy platform, previously developed from researchers in our laboratory.

Στόχος της εργασίας είναι η παραμετρική μελέτη μεταλλικών τρισδιάστατων πλαισίων με συνδέσμους δυσκαμψίας, ανθεκτικούς σε λυγισμό (BRB: Buckling Restrained Braces). Τα πρώτα κεφάλαια της εργασίας αποτελούν το θεωρητικό υπόβαθρο της διατριβής και αναφέρονται στα ζητήματα που αφορούν τον αντισεισμικό σχεδιασμό των μεταλλικών κατασκευών. Στο υπόβαθρο αυτό βασίστηκε τόσο ο σχεδιασμός του δείγματος των κατασκευών όσο και η επιλογή των υπό διερεύνηση παραμέτρων. Το αντικείμενο της εργασίας καθαυτό παρουσιάζεται στα τρία τελευταία κεφάλαια όπου γίνεται η περιγραφή του σχεδιασμού των τελικών κτιρίων ώστε να επιτευχθεί φυσική μονοαξονική εκκεντρότητα και στη συνέχεια η ανάλυση των κατασκευών αυτών με μη γραμμικές αναλύσεις. Από αυτές δημιουργείται μια βάση δεδομένων από την οποία εξάγονται συμπεράσματα που αφορούν τη συσχέτιση της εκκεντρότητας με τα μεγέθη απόκρισης των κατασκευών. The subject of this thesis is the non linear analysis of 3D steel frames with buckling restrained braces (BRB).