Miha Škarabot

Assembly and control of 3D nematic dipolar colloidal crystals.

Topology has long been considered as an abstract mathematical discipline with little connection to material science. Here we demonstrate that control over spatial and temporal positioning of topological defects allows for the design and assembly of three-dimensional nematic colloidal crystals, giving some unexpected material properties, such as giant electrostriction and collective electro-rotation. Using laser tweezers, we have assembled three-dimensional colloidal crystals made up of 4 μm microspheres in a bulk nematic liquid crystal, implementing a step-by-step protocol, dictated by the orientation of point defects. The three-dimensional colloidal crystals have tetragonal symmetry with antiparallel topological dipoles and exhibit giant electrostriction, shrinking by 25-30% at 0.37 V μm(-1). An external electric field induces a reversible and controllable electro-rotation of the crystal as a whole, with the angle of rotation being ~30° at 0.14 V μm(-1), when using liquid crystal with negative dielectric anisotropy. This demonstrates a new class of electrically highly responsive soft materials.

Human stefin B readily forms amyloid fibrils in vitro

Human stefin B (cystatin B) is an intracellular cysteine proteinase inhibitor broadly distributed in different tissues. Here, we show that recombinant human stefin B readily forms amyloid fibrils in vitro. It dimerises and further oligomerises, starting from the native-like acid intermediate, I(N), populated at pH 5. On standing at room temperature it produces regular (over 4 microm long) fibrils over a period of several months. These have been visualised by transmission electron microscopy and atomic force microscopy. Their cross-sectional diameter is about 14 nm and blocks of 27 nm repeat longitudinally. The fibrils are smooth, of unbranched surface, consistent with findings of other amyloid fibrils. Thioflavin T fluorescence spectra as a function of time were recorded and Congo red dye binding to the fibrils was demonstrated. Adding 10% (v/v) trifluoroethanol resulted in an increased rate of fibrillation with a typical lag phase. The finding that human stefin B, in contrast to the homologue stefin A, forms amyloid fibrils rather easily should promote further studies of the proteins behaviour in vivo, and/or as a model system for fibrillogenesis.

Interactions of micro-rods in a thin layer of a nematic liquid crystal

We present an experimental analysis of the topological properties of a nematic liquid crystal in the vicinity of small cylindrical objects (micro-rods), dispersed in the nematic liquid crystal 5CB (4-n-pentyl-4-cyanobiphenyl). Depending on the type of liquid crystal anchoring on the surfaces of micro-rods, we have observed two types of the symmetry in the surrounding liquid crystal molecules: dipolar and quadrupolar. Using the manipulation of micro-rods with laser tweezers, the strength and separation dependencies of various pair interaction potentials have been determined. Our results are in qualitative agreement with previous theoretical predictions for 2D interactions of micro-rods in the nematic liquid crystals.

Dielectric, ferroelectric, piezoelectric, and electrostrictive properties of K0.5Na0.5NbO3 single crystals

The dielectric, ferroelectric, piezoelectric, and electrostrictive properties of K0.5Na0.5NbO3 single crystals (KNN s.c.) prepared by solid-state crystal growth are reported. The dielectric constant (e), dielectric losses (tan δ), remanent polarization (Pr), and coercive field (Ec) for KNN s.c. in the [13¯1] direction at room temperature are 1015, 1%, 17 μC/cm2, and 24 kV/cm, respectively. The influence of 180° domains to the linear piezoelectric response and quadratic electrostrictive response of KNN s.c. is discussed. The piezoelectric coefficient d33 and the electrostrictive coefficient M33 of KNN s.c. measured using atomic force microscopy at 2 Hz was 80 pm/V and 2.59×10−14 m2/V2, respectively. The extremely high M33 value can be explained by the extrinsic strain from the domain-wall motion. The properties of the surrounding polycrystalline KNN ceramic are added for comparison.

Design of 2D Binary Colloidal Crystals in a Nematic Liquid Crystal

In this paper, we examine directed self-assembly in a 2D binary system of dipolar and quadrupolar colloidal particles with normal surface boundary conditions, dispersed in the nematic liquid crystal. Using the laser tweezers, we assembled a large variety of stable 2D colloidal crystal structures. In all analyzed structures, the particles, their surface treatment and the cell conditions were the same, which gives us the ability to systematically follow the evolution of colloidal assembly when many particles are present. We present an analogy between molecular self-assembly and organization of colloidal microspheres in liquid crystalline medium to extend the strategy for designing colloidal crystalline structures of different level of complexity.

Self-assembly of nematic colloids

Dispersions of colloidal particles in nematic liquid crystals show new classes of interparticle forces, which are anisotropic, long range, and several thousand times stronger than van der Waals forces in water-based colloids. These forces are responsible for a variety of new self-assembled colloidal microstructures, which cannot be observed in isotropic solvents, such as chains of microspheres and cellular soft solid materials. Basic principles of particle self-assembly in 2D nematic colloids are discussed, showing this is a novel paradigm in colloid science, which could lead to new approaches of colloidal self-assembly for photonic devices.

Amyloid fibril formation by human stefin B: influence of pH and TFE on fibril growth and morphology

As shown before, human stefin B (cystatin B) populates two partly unfolded species, a native-like state at pH 4.8 and a structured molten globule state at pH 3.3 (high ionic strength), from each of which amyloid fibrils grow. Here, we show that the fibrils obtained at pH 3.3 differ from those at pH 4.8 and that those obtained at pH 3.3 (protofibrils) do not transform readily to mature fibrils. In addition we show that amorphous aggregates are also a source of fibrils. The kinetics of amyloid fibril formation at different trifluoroethanol (TFE) concentrations were measured. TFE accelerates fibril growth at predenaturational concentrations of the alcohol. At concentrations higher than 10%, the fibrillar yield decreases proportionately as the population of an all α-helical, denatured form of the protein increases. At an optimum TFE concentration, the lag and the growth phases are observed, similarly to some other amyloidogenic proteins. Morphology of the protein species at the beginning and the end of the reactions was observed using atomic force microscopy and transmission electron microscopy. Final fibril morphologies differ depending on solvent conditions.

Different propensity to form amyloid fibrils by two homologous proteins-Human stefins A and B: searching for an explanation.

By using ThT fluorescence, X‐ray diffraction, and atomic force microscopy (AFM), it has been shown that human stefins A and B (subfamily A of cystatins) form amyloid fibrils. Both protein fibrils show the 4.7 Å and 10 Å reflections characteristic for cross β‐structure. Similar height of ∼3 nm and longitudinal repeat of 25–27 nm were observed by AFM for both protein fibrils. Fibrils with a double height of 5.6 nm were only observed with stefin A. The fibrils width for stefin A fibrils, as observed by transmission electron microsopy (TEM), was in the same range as previously reported for stefin B (Žerovnik et al., Biochem Biophys Acta 2002;1594:1–5). The conditions needed to undergo fibrillation differ, though. The amyloid fibrils start to form at pH 5 for stefin B, whereas in stefin A, preheated sample has to be acidified to pH < 2.5. In both cases, adding TFE, seeding, and alignment in a strong magnetic field accelerate the fibril growth. Visual analysis of the three‐dimensional structures of monomers and domain‐swapped dimers suggests that major differences in stability of both homologues stem from arrangement of specific salt bridges, which fix α‐helix (and the α‐loop) to β‐sheet in stefin A monomeric and dimeric forms. Proteins 2004;55:000–000.

Direct observation of interaction of nanoparticles in a nematic liquid crystal

The topological properties and interactions of nano colloids in the nematic liquid crystal pentylcyanobiphenyl (5CB) have been studied. We find that silica particles, surface-functionalized with N,N-dimethyl-N-octadecyl-3-aminopropyl trimethoxysilyl chloride induce a dipolar structure of 5CB around them even for 125 nm diameter colloids. The 125 nm dipolar colloidal particle is accompanied by a clearly resolvable hyperbolic point defect and behaves hydrodynamically as a much bigger particle because of the elastic distortion around it. This has a surprising consequence that the pair-binding energy of the dipolar nanocolloids is as high as 700 kBT for 125 nm colloids and is practically size-independent in the range from 125 nm to 500 nm.

The design trend in tissue-engineering scaffolds based on nanomechanical properties of individual electrospun nanofibers

This paper especially highlights the finding that the mechanical properties of polymeric nanofibers can be tuned by changing the fiber size as well as the composition. For this purpose, the bending Youngs modulus was determined using atomic force microscope by involving single-material (polyvinyl alcohol (PVA), polyethylene oxide (PEO 400K)) and composite nanofibers (polyvinyl alcohol/hyaluronic acid (PVA/HA), polyethylene oxide/chitosan (PEO 400K/CS)). The mechanical property, namely the bending Youngs modulus, increases as the diameter of the fibers decreases from the bulk down to the nanometer regime (less than 200 nm). The ranking of increasing stiffness according to the AFM measurements of the three-point beam bending test are in agreement, and can be ranked: PEO 400K<PVA/HA≈PVA<PEO<400K/CS. According to our results, CS-based nanofibers are the stiffest (15 GPa) and the most resilient to erosion in an aqueous medium. Consequently, they possess the most appropriate attributes for bone, tendon, and cartilage tissue scaffold engineering. Nanofibers based on PVA (6 GPa) and PEO (3 GPa) are more elastic (a smaller bending Youngs modulus) and therefore are the most suitable for skin and wound tissue scaffolds.