Prolonged morphometric study of barnacles grown on soft substrata of hydrogels and elastomers.

A long-term investigation of the shell shape and the basal morphology of barnacles grown on tough, double-network (DN) hydrogels and polydimethylsiloxane (PDMS) elastomer was conducted in a laboratory environment. The elastic modulus of these soft substrata varied between 0.01 and 0.47 MPa. Polystyrene (PS) (elastic modulus, 3 GPa) was used as a hard substratum control. It was found that the shell shape and the basal plate morphology of barnacles were different on the rigid PS substratum compared to the soft substrata of PDMS and DN hydrogels. Barnacles on the PS substratum had a truncated cone shape with a flat basal plate while on soft PDMS and DN gels, barnacles had a pseudo-cylindrical shape and their basal plates showed curvature. In addition, a large adhesive layer was observed under barnacles on PDMS, but not on DN gels. The effect of substratum stiffness is discussed in terms of barnacle muscle contraction, whereby the relative stiffness of the substratum compared to that of the muscle is considered as the key parameter.

Motility and structural polymorphism of polymer-actin complex gel.

We report a soft gel machine reconstructed from muscle proteins. We have found that chemically cross-linked polymer-actin complex gel can move on myosin coated surface with a velocity as high as that of native F-actin, by coupling to ATP hydrolysis. Additionally, it is shown that the velocity and motional pattern of polymer-actin complex gel depends on the morphology of polymer-complex gels. Since the designing of functional actuator into well-defined size and morphology is important, the structural behavior of polymer-actin complexes has been investigated. This result shows that the morphology and growth size of polymer-actin complex can be controlled by change of electrostatic interaction between F-actins and polycations. Our results indicate that bio actuator with desired shape can be created by using polymer-actin complex.

Sensing surface mechanical deformation using active probes driven by motor proteins.

Studying mechanical deformation at the surface of soft materials has been challenging due to the difficulty in separating surface deformation from the bulk elasticity of the materials. Here, we introduce a new approach for studying the surface mechanical deformation of a soft material by utilizing a large number of self-propelled microprobes driven by motor proteins on the surface of the material. Information about the surface mechanical deformation of the soft material is obtained through changes in mobility of the microprobes wandering across the surface of the soft material. The active microprobes respond to mechanical deformation of the surface and readily change their velocity and direction depending on the extent and mode of surface deformation. This highly parallel and reliable method of sensing mechanical deformation at the surface of soft materials is expected to find applications that explore surface mechanics of soft materials and consequently would greatly benefit the surface science.

Cultivation of endothelial cells on adhesive protein-free synthetic polymer gels.

Various hydrogels without modification by any cell adhesive proteins have been investigated as cell scaffolds. The present study shows that bovine fetal aorta endothelial cells can adhere, spread, proliferate, and reach confluence on poly(acrylic acid), poly(sodium p-styrene sulfonate), and poly(2-acrylamido-2- methyl-1-propanesulfonic sodium) gels, whereas cells reach subconfluence on poly(vinyl alcohol) and poly(methacrylic acid) gels. The proliferation behavior was sensitive to both hydrogel charge density and crosslinker concentration. The relationship between cell proliferation and zeta potential of gels was discussed. It was found that hydrogels with a negative zeta potential higher than about 20 mV facilitates cell proliferation.

Buckling of Microtubules on a 2D Elastic Medium.

We have demonstrated compression stress induced mechanical deformation of microtubules (MTs) on a two-dimensional elastic medium and investigated the role of compression strain, strain rate, and a MT-associated protein in the deformation of MTs. We show that MTs, supported on a two-dimensional substrate by a MT-associated protein kinesin, undergo buckling when they are subjected to compression stress. Compression strain strongly affects the extent of buckling, although compression rate has no substantial effect on the buckling of MTs. Most importantly, the density of kinesin is found to play the key role in determining the buckling mode of MTs. We have made a comparison between our experimental results and the 'elastic foundation model' that theoretically predicts the buckling behavior of MTs and its connection to MT-associated proteins. Taking into consideration the role of kinesin in altering the mechanical property of MTs, we are able to explain the buckling behavior of MTs by the elastic foundation model. This work will help understand the buckling mechanism of MTs and its connection to MT-associated proteins or surrounding medium, and consequently will aid in obtaining a meticulous scenario of the compression stress induced deformation of MTs in cells.

Morphogenesis of liposomes caused by polycation-induced actin assembly formation.

We investigated the effects of polycation-mediated actin assembly on the morphological transformation of the lipid vesicle membrane by spatiotemporally controlling actin assembly. By triggering the radical polymerization of the cationic monomer using UV irradiation, we achieved a varied photoinduced assembly of actin in bulk solution. Furthermore, we designed liposomes containing actin and cationic monomers. In these actin-encapsulated liposomes, various actin assemblies were formed by UV irradiation similar to that observed in bulk solution. Moreover, morphogenesis of actin-encapsulated liposomes was observed in liposomes encapsulated with G-actin but not with F-actin. This result indicates that a dynamic polymerization of G-actin is important for vesicle protrusion.

Actin network formation by unidirectional polycation diffusion.

We show that F-actins form three-dimensional giant network under uni-directional diffusion of polycations, at a dilute actin concentration (0.01 mg/mL) that only bundles are formed by homogeneous mixing with polycations. The mesh size of the actin network depends on polycation concentration and ionic strength, while bundle thickness of network depends only on ionic strength, which indicates that actin network is formed through nucleation-growth mechanism. The mesh size and the bundle thickness are determined by nucleus concentration and nucleus size, respectively. The atomic force microscopy measurement correlates the elasticity of the actin network, E, with the mesh size, xi, as E approximately xi-1, while the bundle thickness, D dependence of E cannot be described by a simple scaling relation. E approximately D6.5 when D is small and E approximately D0.1 when D is large. Our study on the self-assembly of actin network under asymmetric polycation condition would provide the crucial insight into the organization of biopolymers in polarized condition of cell.

Polarity and motility of large polymer-actin complexes.

The polarity of polymer-actin complexes obtained by mixing F-actin with synthetic polymers carrying positive charges such as poly(L-lysine), x,y-ionene bromide polymers, and poly(N-[3-(dimethylamino)propyl]acrylamide) (PDMAPAA-Q) have been investigated. Actin complexes formed with poly(L-lysine) and PDMAPAA-Q, which carry charges on their side chains, show a higher polarity than those formed with x,y-ionene bromide polymers, which have charges on their chain backbones. All these polymer-actin complex gels show motility on the surfaces coated with myosin by coupling to adenosine 5'-triphosphate hydrolysis. A linear correlation between the polarity of polymer-actin complex gels and the motility is observed.

Morphology of actin assemblies in response to polycation and salts.

F-actins are semi-flexible polyelectrolytes and can be assembled into a large polymer-actin complex with polymorphism through electrostatic interaction with polycations. This study investigates the structural phase behavior and the growth of polymer-actin complexes in terms of its longitudinal and lateral sizes in various polycation and KCl concentrations for a constant actin concentration. Our results show that the longitudinal growth and lateral growth of polymer-actin complexes, initiated by a common nucleation process, are dominated by different factors in subsequent growth process. This induces the structural polymorphism of polymer-actin complexes. Major factors to influence the polymorphism of polymer-actin complexes in polyelectrolyte systems have been discussed. Our results indicate that the semiflexible polyelectrolyte nature of F-actins is important for controlling the morphology and growth of actin architectures in cells.

Growth of large polymer-actin complexes.

Polymer-actin complexes as large as 10-50 microm with filamentous, branched, stranded, and ring shapes are obtained when fluorescent phalloidin-labeled F-actin is mixed with some synthetic polymers carrying positive charges such as poly-L-lysine, x,y-ionene bromide polymers. All growth of these complexes occurs cooperatively at some certain critical polymer concentrations, regardless of the chemical structure of the polymer, while the morphology of the complexes is substantially influenced by the chemical structure of the polymer. Poly-Lys-actin complex grows preferentially along the filament axis even above the critical concentration. 3,3-ionene-actin complexes show completely homogeneous filaments below the critical concentration but forms bundles at a higher concentration. Occasionally, ring shape complexes can be observed in the 6,6-ionene-actin complex.