Fig. 4. Examples of bioprinted and molded biorobotic systems.

Extrusion printing of muscle cell laden bioink and PDMS pillars for cell alignment (a). Microscopic image of the print (b). Myotube alignment in the printing direction and in parallel to the passive force from the pillars (c). Fold increment of the achieved force of the cells before (F^D6) and after 4 days of stimulation (F^D9) with differing stiffness of the pillars (d)¹⁶ (© 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) Schematic manufacturing process of muscle strips and rings with stereolithographic 3D printed molds (e), injection of the bioink and following muscle compaction to achieve muscle strips and rings for bioreactors (f). Actuation of the genetic modified muscle cells via optical stimulation (g) and electrical stimulation (h). Active tension changes with the stimulation method and the muscle shape h)³⁷. Biohybrid robot powered by an antagonistic pair of skeletal muscle tissues (i). Example movement of the biorobot grabbing a ring (j). Contractile force is increased with a sufficient maturation and therefore striped structure of the muscle cells (k) (from ref. ². Reprinted with permission from AAAS.) Biofabrication of a free floating biorobotic swimmer driven by on-board neuromuscular unit (o) with the steps of fabrication and assembly (l), muscle strip formation (m), co-culture of muscle cells and the neurosphere in a continuous ECM-gel (n) (from ref. ⁵⁵ Biofabrication timeline and free swimming driven by neuromuscular units by Aydin et al. (CC BY NC ND 4.0)).