Micropipette aspiration method for characterizing biological materials with surface energy.

Many soft biological tissues possess a considerable surface stress, which plays a significant role in their biophysical functions, but most previous methods for characterizing their mechanical properties have neglected the effects of surface stress. In this work, we investigate the micropipette aspiration method to measure the mechanical properties of soft tissues and cells with surface effects. The neo-Hookean constitutive model is adopted to describe the hyperelasticity of the measured biological material, and the surface effect is taken into account by the finite element method. It is found that when the pipette radius or aspiration length is comparable to the elastocapillary length, surface energy may distinctly alter the aspiration response. Generally, both the aspiration length and the bulk normal stress decrease with increasing surface energy, and thus neglecting the surface energy would lead to an overestimation of elastic modulus. Through dimensional analysis and numerical simulations, we provide an explicit relation between the imposed pressure and the aspiration length. This method can be applied to determine the mechanical properties of soft biological tissues and organs, e.g., livers, tumors and embryos.

Hierarchical chirality transfer in the growth of Towel Gourd tendrils.

Chirality plays a significant role in the physical properties and biological functions of many biological materials, e.g., climbing tendrils and twisted leaves, which exhibit chiral growth. However, the mechanisms underlying the chiral growth of biological materials remain unclear. In this paper, we investigate how the Towel Gourd tendrils achieve their chiral growth. Our experiments reveal that the tendrils have a hierarchy of chirality, which transfers from the lower levels to the higher. The change in the helical angle of cellulose fibrils at the subcellular level induces an intrinsic torsion of tendrils, leading to the formation of the helical morphology of tendril filaments. A chirality transfer model is presented to elucidate the chiral growth of tendrils. This present study may help understand various chiral phenomena observed in biological materials. It also suggests that chirality transfer can be utilized in the development of hierarchically chiral materials having unique properties.

Elasticity-driven droplet movement on a microbeam with gradient stiffness: a biomimetic self-propelling mechanism.

Directional movement of liquid droplets is of significance not only for certain physiological processes in nature but also for design of some microfluidic devices. In this study, we report a novel way to drive directional movement of liquid droplets on a microbeam with a varying or gradient stiffness. We use the energy method to theoretically analyze the interaction between a droplet and the elastic microbeam. The system tends to have the minimum potential energy when the droplet moves to the softer end of the beam. Therefore, a gradient change of the bending stiffness may be utilized to help the directional motion of droplets. Similarly, one can also drive droplets to move in a designed direction by varying the cross sectional geometry of the beam. Finally, some possible applications of this self-propelling mechanism are suggested.

Molecular dynamics simulations of deformation and rupture of super carbon nanotubes under tension.

The mechanical properties of super carbon tubes, a recently developed network material made by conjoined single-walled carbon nanotubes, are studied via molecular dynamics simulations. It is found that such tubes have some unusual properties distinctly different from both individual and bundled carbon nanotubes. The rupture strains of super carbon tubes can reach up to 31-47%, several times higher than that of single-walled carbon nanotubes. Their mechanical behavior is sensitively dependent on both the chiral vectors of the first- and the second-level structures, as well as the inter-junction distance. The hierarchical structure plays a dominant role in the deformation and rupture behavior of super carbon tubes. Owing to their unique and superior properties, super carbon tubes might be used as novel materials with extremely low mass density, high strength and high flexibility, which are of extensive interest in a broad range of technologically important applications.