Effect of Shear Rate on the Orientation and Relaxation of a Vanillic Acid Based Liquid Crystalline Polymer.

In this study, we report on the visco-elastic response during start-up and cessation of shear of a novel bio-based liquid crystal polymer. The ensuing morphological changes are analyzed at different length scales by in-situ polarized optical microscopy and wide-angle X-ray diffraction. Upon inception of shear, the polydomain texture is initially stretched, at larger strain break up processes become increasingly important, and eventually a steady state texture is obtained. The shear stress response showed good coherence between optical and rheo-X-ray data. The evolution of the orientation parameter coincides with the evolution of the texture: the order parameter increases as the texture stretches, drops slightly in the break up regime, and reaches a constant value in the plateau regime. The relaxation of the shear stress and the polydomain texture showed two distinct processes with different timescales: The first is fast contraction of the stretched domain texture; the second is the slow coalescence of the polydomain texture. The timescale of the orientation parameter's relaxation matched with that of the slow coalescence process. All processes were found to scale with shear rate in the tested regime. These observations can have far reaching implications for the processing of liquid crystal polymers as they indicate that increased shear rates during processing can correspond to an increased relaxation rate of the orientation parameter and, therefore, a decrease in anisotropy and material properties after cooling.

Microfluidics-based single cell analysis reveals drug-dependent motility changes in trypanosomes.

We present a single cell viability assay, based on chemical gradient microfluidics in combination with optical micromanipulation. Here, we used this combination to in situ monitor the effects of drugs and chemicals on the motility of the flagellated unicellular parasite Trypanosoma brucei; specifically, the local cell velocity and the mean squared displacement (MSD) of the cell trajectories. With our method, we are able to record in situ cell fixation by glutaraldehyde, and to quantify the critical concentration of 2-deoxy-d-glucose required to completely paralyze trypanosomes. In addition, we detected and quantified the impact on cell propulsion and energy generation at much lower 2-deoxy-d-glucose concentrations. Our microfluidics-based approach advances fast cell-based drug testing in a way that allows us to distinguish cytocidal from cytostatic drug effects, screen effective dosages, and investigate the impact on cell motility of drugs and chemicals. Using suramin, we could reveal the impact of the widely used drug on trypanosomes: suramin lowers trypanosome motility and induces cell-lysis after endocytosis.

Optical trapping reveals propulsion forces, power generation and motility efficiency of the unicellular parasites Trypanosoma brucei brucei.

Unicellular parasites have developed sophisticated swimming mechanisms to survive in a wide range of environments. Cell motility of African trypanosomes, parasites responsible for fatal illness in humans and animals, is crucial both in the insect vector and the mammalian host. Using millisecond-scale imaging in a microfluidics platform along with a custom made optical trap, we are able to confine single cells to study trypanosome motility. From the trapping characteristics of the cells, we determine the propulsion force generated by cells with a single flagellum as well as of dividing trypanosomes with two fully developed flagella. Estimates of the dissipative energy and the power generation of single cells obtained from the motility patterns of the trypanosomes within the optical trap indicate that specific motility characteristics, in addition to locomotion, may be required for antibody clearance. Introducing a steerable second optical trap we could further measure the force, which is generated at the flagellar tip. Differences in the cellular structure of the trypanosomes are correlated with the trapping and motility characteristics and in consequence with their propulsion force, dissipative energy and power generation.

Flow loading induces oscillatory trajectories in a bloodstream parasite.

The dynamics of isolated microswimmers are studied in bounded flow using the African trypanosome, a unicellular parasite, as the model organism. With the help of a microfluidics platform, cells are subjected to flow and found to follow an oscillatory path that is well fit by a sine wave. The frequency and amplitudes of the oscillatory trajectories are dependent on the flow velocity and cell orientation. When traveling in such a manner, trypanosomes orient upstream while downstream-facing cells tumble within the same streamline. A comparison with immotile trypanosomes demonstrates that self-propulsion is essential to the trajectories of trypanosomes even at flow velocities up to ∼40 times higher than their own swimming speed. These studies reveal important swimming dynamics that may be generally pertinent to the transport of microswimmers in flow and may be relevant to microbial pathogenesis.

Impact of microscopic motility on the swimming behavior of parasites: straighter trypanosomes are more directional.

Microorganisms, particularly parasites, have developed sophisticated swimming mechanisms to cope with a varied range of environments. African Trypanosomes, causative agents of fatal illness in humans and animals, use an insect vector (the Tsetse fly) to infect mammals, involving many developmental changes in which cell motility is of prime importance. Our studies reveal that differences in cell body shape are correlated with a diverse range of cell behaviors contributing to the directional motion of the cell. Straighter cells swim more directionally while cells that exhibit little net displacement appear to be more bent. Initiation of cell division, beginning with the emergence of a second flagellum at the base, correlates to directional persistence. Cell trajectory and rapid body fluctuation correlation analysis uncovers two characteristic relaxation times: a short relaxation time due to strong body distortions in the range of 20 to 80 ms and a longer time associated with the persistence in average swimming direction in the order of 15 seconds. Different motility modes, possibly resulting from varying body stiffness, could be of consequence for host invasion during distinct infective stages.

Dominant genomic structures: detection and potential signal functions in the interferon-beta domain.

Eukaryotic genomes are divided into chromatin domains, which are thought to represent independent regulatory units. Typically, these domains are flanked by bordering elements that insulate the transcription unit from outside influences. Borders also demarcate the range of action for enhancer-like elements within the domain as they are formed around dominant genomic structures such as DNAse I hypersensitive sites (HS). Here we describe an efficient strategy to localize these elements. Our procedure is based on a computational method and predictions are verified by classical in vivo and in vitro procedures. Exemplified by the interferon-beta (IFN-beta) domain it proves its potential to provide novel insights into remote control principles of transcription. Sites with secondary-structure forming potential are localized by the analysis of stress-induced duplex destabilization (SIDD) properties and the associating factors are characterized by electrophoretic mobility shift assays (EMSA). These studies reveal far upstream factor binding sites within the IFN-beta domains of both humans and mice. A prominent example is YY1, a transcription factor that not only recognizes a core consensus motif, ATGG, but, in addition, the structural context, which is evident from characteristic imprints in the respective SIDD-profiles.