  In nature interactions between biopolymers and motor proteins give rise to
biologically essential emergent behaviours. Besides cytoskeleton mechanics,
active nematics arise from such interactions. Here we present a study on 3D
active nematics made of microtubules, kinesin motors and depleting agent. It
shows a rich behaviour evolving from a nematically ordered space-filling
distribution of microtubule bundles toward a flattened and contracted 2D ribbon
that undergoes a wrinkling instability and subsequently transitions into a 3D
active turbulent state. The wrinkle wavelength is independent of the ATP
concentration and our theoretical model describes its relation with the
appearance time. We compare the experimental results with a numerical
simulation that confirms the key role of kinesin motors in cross-linking and
sliding the microtubules. Our results on the active contraction of the network
and the independence of wrinkle wavelength on ATP concentration are important
steps forward for the understanding of these 3D systems.
