**5. Conclusions**

This paper contributes to the development of miniature mobile robots based on TWs generated by the actuation of symmetrically located piezoelectric patches. Guidelines were proposed for the design of the patches and the driving signals. 3D printed legs were implemented in our devices, which is an approach commonly restricted to standing-wave-based systems. The combination of the optimal patch length and legs resulted in a mobile rigid robot with a speed of 5 BL/s at a voltage of 65 Vpp, with the capability of transporting 40 times its weight.

*Micromachines* **2020**, *11*, 321

**Supplementary Materials:** The following are available online at http://www.mdpi.com/2072-666X/11/3/321/s1, Figure S1: Electrical conductance of a 0.5-mm-legged robot compared with the case without legs. Table S1: Resonant frequency and position of the first zero of the second derivative of the shape of modes (40), (50), (60), and (70) for three cases: experiment, 1D analytical model, and 3D finite element analysis. Video S1: Bidirectional locomotion of a 1-mm-legged structure with no load. Video S2: Bidirectional locomotion of a 1-mm-legged structure with a load of 7.5 g.

**Author Contributions:** Conceptualization, J.L.S.-R.; funding acquisition, J.H.-G. and J.L.S.-R.; investigation, J.L.G.-C.; project administration, J.H.-G. and J.L.S.-R.; software, V.R.-D.; supervision, J.H.-G. and J.L.S.-R.; writing—original draft, J.H.-G.; writing—review and editing, J.H.-G. and J.L.S.-R. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work was supported by the European Regional Development Fund, the Spanish Ministerio de Ciencia, Innovación y Tecnología project (RTI2018-094960-B-100), and the regional governmen<sup>t</sup> (JCCLM) project (SBPLY/17/180501/000139).

**Conflicts of Interest:** The authors declare no conflict of interest.
