  Additive manufacturing strives to combine any combination of materials into
three dimensional functional structures and devices, ultimately opening up the
possibility of 3D printed machines. It remains difficult to actuate such
devices, thus limiting the scope of 3D printed machines to passive devices or
necessitating the incorporation of external actuators that are manufactured
differently. Here we explore 3D printed hybrid thermoplast/conducter bilayers,
that can be actuated by differential heating caused by externally controllable
currents flowing through their conducting faces. We uncover the functionality
of such actuators and show that they allow to 3D print, in one pass, simple
flexible robotic structures that propel forward under step-wise applied
voltages. Moreover, exploiting the thermoplasticity of the non-conducting
plastic parts at elevated temperatures, we show how strong driving leads to
irreversible deformations - a form of 4D printing - which also enlarges the
range of linear response of the actuators. Finally, we show how to leverage
such thermoplastic relaxations to accumulate plastic deformations and obtain
very large deformations by alternatively driving both layers of a bilayer; we
call this ratcheting. Our strategy is scalable and widely applicable, and opens
up a new approach to reversible actuation and irreversible 4D printing of
arbitrary structures and machines.
