(235b) Carbon Nanotubes As Mechanical Probes of Equilibrium and Non-Equilibrium Biopolymer Networks

A biological cell is a soft material with complex mechanical properties arising from a composite and highly dynamic intracellular protein polymer network (consisting of actin, microtubules and intermediate filaments). Equilibrium soft materials show strain fluctuations due to random thermal stresses; living biological materials exhibit, in addition, non-equilibrium, internal stress fluctuations generated by active force generators (e.g. motor proteins). Understanding the dynamic properties of such heterogeneous and structurally complex networks requires probes that span length-scales from nanometers to micrometers.

We use single-walled carbon nanotubes (SWNTs) as multi-scale micro-probes. SWNTs are nanometer-diameter hollow carbon filaments with micrometer lengths and a tunable bending stiffness. Their persistence length varies between 20 – 100 microns. Therefore they show significant thermal fluctuations on the micron scale.

We study the motion of individual SWNTs in reconstituted actin (equilibrium) and actin-myosin (non-equilibrium) networks by near infrared fluorescence microscopy. At long times, SWNTs reptate in the networks. At short times SWNTs can sample the spectrum of local stresses in both equilibrium and non-equilibrium networks. We can calculate complex shear moduli from recorded fluctuations and observed power-law scaling in equilibrium actin networks. In the non-equilibrium networks we observed strong local shape fluctuations reflecting the activity of the molecular motors.