Microtubules and vesicles under controlled tension

Microtubules trapped inside cell-sized vesicles ({~}10-{μ}m diameter) define shapes reminiscent of living cells. In this work, three configurations are studied: an individual microtubule, a linear bundle, and a circular band. Mechanical properties are probed by varying the vesicle membrane tension using micropipet aspiration. Quantitative results include measurements of the persistence length of a single microtubule, 6.3±{}2.4 mm, and the stretching modulus of the vesicle membrane, kₛ {}140 erg/cm² . The number of microtubules in a typical bundle is {~}100, and the total length in a typical band is {~}0.5 cm. A tension-induced first-order transition from linear bundle to circular band is observed and modeled. The loss of stability of the bent bundle is associated with a saddle-node bifurcation.