Automated tracking of colloidal clusters with sub-pixel accuracy and precision

Quantitative tracking of features from video images is a basic technique employed in many areas of science. Here, we present a method for the tracking of features that partially overlap, in order to be able to track so-called colloidal molecules. Our approach implements two improvements into existing particle tracking algorithms. Firstly, we use the history of previously identified feature locations to successfully find their positions in consecutive frames. Secondly, we present a framework for non-linear least-squares fitting to summed radial model functions and analyze the accuracy (bias) and precision (random error) of the method on artificial data. We find that our tracking algorithm correctly identifies overlapping features with an accuracy below 0.2% of the feature radius and a precision of 0.1 to 0.01 pixels for a typical image of a colloidal cluster. Finally, we use our method to extract the three-dimensional diffusion tensor from the Brownian motion of colloidal dimers.Comment: 20 pages, 8 figures. Non-revised preprint version, please refer to http://dx.doi.org/10.1088/1361-648X/29/4/04400

Microparticle assembly pathways on lipid membranes

Understanding interactions between microparticles and lipid membranes is of increasing importance, especially for unraveling the influence of microplastics on our health and environment. Here, we study how a short-ranged adhesive force between microparticles and model lipid membranes causes membrane-mediated particle assembly. Using confocal microscopy, we observe the initial particle attachment to the membrane, then particle wrapping, and in rare cases spontaneous membrane tubulation. In the attached state, we measure that the particle mobility decreases by 26%. If multiple particles adhere to the same vesicle, their initial single-particle state determines their interactions and subsequent assembly pathways: 1) attached particles only aggregate when small adhesive vesicles are present in solution, 2) wrapped particles reversibly attract one another by membrane deformation, and 3) a combination of wrapped and attached particles form membrane-mediated dimers, which further assemble into a variety of complex structures. The experimental observation of distinct assembly pathways induced only by a short ranged membrane-particle adhesion, shows that a cellular cytoskeleton or other active components are not required for microparticle aggregation. We suggest that this membrane-mediated microparticle aggregation is a reason behind reported long retention times of polymer microparticles in organisms.Comment: 20 pages, 11 figures (including supporting material

Chemotherapie des fortgeschrittenen Pankreaskarzinoms mit intraarteriell appliziertem Mitomycin C in Kombination mit intraarteriell, intravenös appliziertem Gemcitabine : eine Phase-II-Studie

Das Adenokarzinom des Pankreas ist das fünfthäufigste Malignom der westlichen Länder mit einer sehr schlechten Prognose. Zum Zeitpunkt der Diagnosestellung ist das Pankreaskarzinom wegen der frühen lokalen Infiltration und Metastasierung meist nicht mehr kurativ behandelbar. 80 % aller Pankreaskarzinome werden in einem fortgeschrittenen Krankheitsstadium diagnostiziert. Die Therapie des fortgeschrittenen Pankreaskarzinoms gestaltet sich problematisch. Bislang führten systemische chemotherapeutische Ansätze nicht zur erhofften Verlängerung der Lebenszeit. In der vorliegenden Phase II Studie wurden in der Abteilung für Allgemein- und Gefäßchirurgie der Universitätsklinik Frankfurt am Main 17 Patienten mit lokal fortgeschrittenem oder metastasiertem Pankreaskarzinom mit einer regional/systemischen Kombinationschemotherapie behandelt. Ein Therapiezyklus bestand aus einer 3ominütigen intraarteriellen Infusion von 8,5 mg/m2 Mitomycin C und aus einer 60minütigen intraarteriellen Infusion von 500 mg/m² Gemcitabine über einen transfemoralen Truncuskatheter an den Tagen 1 und 22, gefolgt von einer systemischen lnfusion von 500 mg/rn² Gemcitabine über 30 Minuten an den Tagen 8, 15, 29 und 36. Die Komhinationschemotherapie war nebenwirkungs- und komplikationsarm. Schwere Nebenwirkungen im NCI-Stadium III/IV wurden im Verlauf von 37 Therapiezyklen mit 74 regionalen Applikationen und 148 systemischen Applikationen in 9 % der Applikationen als Beeinträchtigung der Knochenmarksfunktion (Leukopenie, Thrombopenie und Hämoglobinabfall) und in 15 % der Applikationen als Leberfunktionsstörungen (Erhöhung von Transaminasen, alkalischer Phosphatase und Bilirubin) beobachtet. Die Nebenwirkungen nach regionalen Therapien waren mit den Nebenwirkungen nach venösen Therapien vergleichbar. Kein Patient verstarb an den Folgen einer Nebenwirkung. Ein Patient erlitt nach einer regionalen Chemotherapie einen kompletten Verschluß der Arteria iliaca externa sinistra. Der Therapieerfolg wurde anhand von Computertomographien (CT) und Tumormarker CA 19-9 Bestimmungen nach jedem Chemotherapiezyklus beobachtet und gemäß den WHO-Kriterien beurteilt. Nach CT-Kriterien zeigten vier Patienten eine Regression. Eine komplette Remission wurde bei einer Patientin, eine partielle Remission bei drei Patienten beobachtet. Eine radiologische Remissionsrate von 24 % konnte errechnet werden. Bei fünf Patienten ließ sich unter Therapie keine Größenzunahme des Tumors erkennen (Stable disease). Das Tumorwachstum war bei neun Patienten progredient. Bei sieben Patienten konnte unter Therapie ein Tumormarkerrückgang von mehr als 50 % evaluiert werden (Remissionsrate 41%). Insgesamt zeigten sich zwei komplette Remissionen mit Sinken des Tumormarkers CA 19-9 in den Referenzbereich, d.h. unter 37 µg/ml, und fünf partielle Remissionen. Bei fünf Patienten war der Verlauf des Tumormarkers CA 19-9 unter Therapie stabil (Stable disease). Der Tumormarker CA 19-9 stieg progredient bei fünf Patienten. Das mediane Überleben nach der Kombinationschemotherapie betrug 9,1 Monate (95 % CI: 6-12 Monate). Das mediane Überleben war für Patienten ohne Fernmetastasen (n = 7) mit 15 Monaten (95 % CI: 3-23 Monate) signifikant (p = 0,037) länger als für Patienten mit Fernmetastasen (n = 10) mit 6,3 Monaten (95 % CI: 4,6-12 Monate). Die mediane progressionsfreie Zeit während der Kombinationschemotherapie betrug 4,6 Monate (95 % CI: 2,1-8,7 Monate). Das radiologische Ansprechen (24 %) und die mediane Überlebenszeit (9,1 Monate) dieser regional/systemischen Kombinationschemotherapie waren im Vergleich zu systemischen Standardchemotherapien mit Gemcitabine, die radiologische Remissionsraten von 6,3 % bis 12 % und mediane Überlebenszeiten von 4,8 bis 6,6 Monaten zeigten, erhöht. Die Studie wird aufgrund ihres hohen klinischen Nutzens weiter fortgesetzt. Eine randomisierte Phase III Studie, die die vorliegende regional/systemische Kombinationschemotherapie (Mitomycin C 8,5 mg/m2 Gemcitabine 500 mg/m2) mit dem systemischen Standardverfahren (Gemcitabine 1000 mg/m2) vergleicht, wird angestrebt

Diffusion-based height analysis reveals robust microswimmer-wall separation

Microswimmers typically move near walls, which can strongly influence their motion. However, direct experimental measurements of swimmer-wall separation remain elusive to date. Here, we determine this separation for model catalytic microswimmers from the height dependence of the passive component of their mean-squared displacement. We find that swimmers exhibit "ypsotaxis", a tendency to assume a fixed height above the wall for a range of salt concentrations, swimmer surface charges, and swimmer sizes. Our findings indicate that ypsotaxis is activity-induced, posing restrictions on future modeling of their still debated propulsion mechanism

Soft and stiff normal modes in floppy colloidal square lattices

Floppy microscale spring networks are widely studied in theory and simulations, but no well-controlled experimental system currently exists. Here, we show that square lattices consisting of colloid-supported lipid bilayers functionalized with DNA linkers act as microscale floppy spring networks. We extract their normal modes by inverting the particle displacement correlation matrix, showing the emergence of a spectrum of soft modes with low effective stiffness in addition to stiff modes that derive from linker interactions. Evaluation of the softest mode, a uniform shear mode, reveals that shear stiffness decreases with lattice size. Experiments match well with Brownian particle simulations and we develop a theoretical description based on mapping interactions onto linear response to describe the modes. Our results reveal the importance of entropic steric effects, and can be used for developing reconfigurable materials at the colloidal length scale

Interface geometry of binary mixtures on curved substrates

Motivated by recent experimental work on multicomponent lipid membranes supported by colloidal scaffolds, we report an exhaustive theoretical investigation of the equilibrium configurations of binary mixtures on curved substrates. Starting from the J\"ulicher-Lipowsky generalization of the Canham-Helfrich free energy to multicomponent membranes, we derive a number of exact relations governing the structure of an interface separating two lipid phases on arbitrarily shaped substrates and its stability. We then restrict our analysis to four classes of surfaces of both applied and conceptual interest: the sphere, axisymmetric surfaces, minimal surfaces and developable surfaces. For each class we investigate how the structure of the geometry and topology of the interface is affected by the shape of the substrate and we make various testable predictions. Our work sheds light on the subtle interaction mechanism between membrane shape and its chemical composition and provides a solid framework for interpreting results from experiments on supported lipid bilayers.Comment: 26 pages, 10 figure

Brownian motion of flexibly-linked colloidal rings

Ring, or cyclic, polymers have unique properties compared to linear polymers, due to their topologically closed structure that has no beginning or end. Experimental measurements on molecular ring polymers are challenging due to their polydispersity in molecular weight and the presence of undesired side products such as chains. Here, we study an experimental model system for cyclic polymers, that consists of rings of flexibly-linked micron-sized colloids with $n$=4..8 segments. We characterize the conformations of these flexible colloidal rings and find that they are freely-jointed up to steric restrictions. We measure their diffusive behavior and compare it to hydrodynamic simulations. Interestingly, flexible colloidal rings have a larger translational and rotational diffusion coefficient compared to colloidal chains. In contrast to chains, their internal deformation mode shows slower fluctuations for $n\lesssim 8$ and saturates for higher values of $n$. We show that constraints stemming from the ring structure cause this decrease in flexibility for small $n$ and infer the expected scaling of the flexibility as function of ring size. Our findings could have implications for the behavior of both synthetic and biological ring polymers, as well as for the dynamic modes of floppy colloidal materials

Thermodynamic equilibrium of binary mixtures on curved surfaces

We study the global influence of curvature on the free energy landscape of two-dimensional binary mixtures confined on closed surfaces. Starting from a generic effective free energy, constructed on the basis of symmetry considerations and conservation laws, we identify several model-independent phenomena, such as a curvature-dependent line tension and local shifts in the binodal concentrations. To shed light on the origin of the phenomenological parameters appearing in the effective free energy, we further construct a lattice-gas model of binary mixtures on non-trivial substrates, based on the curved-space generalization of the two-dimensional Ising model. This allows us to decompose the interaction between the local concentration of the mixture and the substrate curvature into four distinct contributions, as a result of which the phase diagram splits into critical sub-diagrams. The resulting free energy landscape can admit, as stable equilibria, strongly inhomogeneous mixed phases, which we refer to as antimixed states below the critical temperature. We corroborate our semi-analytical findings with phase-field numerical simulations on realistic curved lattices. Despite this work being primarily motivated by recent experimental observations of multi-component lipid vesicles supported by colloidal scaffolds, our results are applicable to any binary mixture confined on closed surfaces of arbitrary geometry.Comment: 20 Pages, 7 Figures; comments and references added, typos correcte