Spicules and the effect of rigid rods on enclosing membrane tubes

Membrane tubes (spicules) arise in cells, or artificial membranes, in the nonlinear deformation regime due to, e.g. the growth of microtubules, actin filaments or sickle hemoglobin fibers towards a membrane. We calculate the axial force exerted by the cylindrical membrane tube, and its average radius, by taking into account steric interactions between the fluctuating membrane and the enclosed rod. The force required to confine a fluctuating membrane near the surface of the enclosed rod diverges as the separation approaches zero. This results in a smooth crossover of the axial force between a square root and a linear dependence on the membrane tension as the tension increases and the tube radius shrinks. This crossover can occur at the most physiologically relevant membrane tensions. Our work may be important in (i) interpreting experiments in which axial force is related to the tube radius or membrane tension (ii) dynamical theories for biopolymer growth in narrow tubes where these fluctuation effects control the tube radius.