Hydrodynamic interactions between squirmers near walls: far-field dynamics and near-field cluster stability.

Confinement increases contacts between microswimmers in dilute suspensions and affects their interactions. In particular, boundaries have been shown experimentally to lead to the formation of clusters that would not occur in bulk fluids. To what extent does hydrodynamics govern these boundary-driven encounters between microswimmers? We consider theoretically the symmetric boundary-mediated encounters of model microswimmers under gravity through far-field interaction of a pair of weak squirmers, as well as the lubrication interactions occurring after contact between two or more squirmers. In the far field, the orientation of microswimmers is controlled by the wall and the squirming parameter. The presence of a second swimmer influences the orientation of the original squirmer, but for weak squirmers, most of the interaction occurs after contact. We thus analyse next the near-field reorientation of circular groups of squirmers. We show that a large number of swimmers and the presence of gravity can stabilize clusters of pullers, while the opposite is true for pushers; to be stable, clusters of pushers thus need to be governed by other interactions (e.g. phoretic). This simplified approach to the phenomenon of active clustering enables us to highlight the hydrodynamic contribution, which can be hard to isolate in experimental realizations.

Vascular imaging with contrast agent in hard and soft tissues using microcomputed-tomography.

Vascularization is essential for many tissues and is a main requisite for various tissue-engineering strategies. Different techniques are used for highlighting vasculature, in vivo and ex vivo, in 2-D or 3-D including histological staining, immunohistochemistry, radiography, angiography, microscopy, computed tomography (CT) or micro-CT, both stand-alone and synchrotron system. Vascularization can be studied with or without a contrast agent. This paper presents the results obtained with the latest Skyscan micro-CT (Skyscan 1272, Bruker, Belgium) following barium sulphate injection replacing the bloodstream in comparison with results obtained with a Skyscan In Vivo 1076. Different hard and soft tissues were perfused with contrast agent and were harvested. Samples were analysed using both forms of micro-CT, and improved results were shown using this new micro-CT. This study highlights the vasculature using micro-CT methods. The results obtained with the Skyscan 1272 are clearly defined compared to results obtained with Skyscan 1076. In particular, this instrument highlights the high number of small vessels, which were not seen before at lower resolution. This new micro-CT opens broader possibilities in detection and characterization of the 3-D vascular tree to assess vascular tissue engineering strategies.

Clinical features and outcome of sphenoid sinus aspergillosis: A retrospective series of 15 cases.

INTRODUCTION: Sphenoid sinusitis is uncommon, and aspergillus infections represent a minority of these cases. This study was designed to describe the characteristics of this disease and present a review of the literature. PATIENTS AND METHOD: Retrospective study from 2004 to 2010 based on 15 patients managed at Nantes University Hospital. Risk factors and history, symptoms, intranasal examination, imaging, histological and mycological results as well as analysis of the response to treatment and outcome were analysed. RESULTS AND DISCUSSIONS: Patients were aged between 14 to 78 years, almost 75% of patients were older than 50 and 73% of patients were women. The most common symptoms were headache [80%], nasal blockage or discharge [33%], and recurrent mild epistaxis [20%]. Predisposing factors were immunodepression in three patients, with no cases of diabetes. Two patients had a history of intranasal surgery and one had a history of facial trauma. Nasal endoscopy was normal in 40% of cases. CT was suggestive of sphenoid sinus aspergillosis in more than one half of cases [8/15] and demonstrated osteolysis in four patients. An incorrect preoperative diagnosis of mucocele was proposed in three patients. Histological examination demonstrated spore-forming structures in every case, but culture was positive in only four cases. Only two patients required antifungal therapy, including one patient with invasive aspergillosis. CONCLUSION: Chronic noninvasive sphenoid sinus aspergillosis appears to be a benign disease, essentially affecting women and patients over the age of 50 years. Symptoms are fairly non-specific. Imaging and histological examination are essential for management. The invasive form is a serious disease requiring rapid, multidisciplinary management.