Liquid Metal/Sodium Alginate Composites for Direct-Ink-Writing Functional Soft Components of a Variable-Stiffness Soft Robot

Endowing variable-stiffness soft robots (VSSRs) with sensing capabilities will help to improve their safe interaction capabilities and intelligence. Therefore, this work adopts direct ink writing (DIW) to directly fabricate functional soft components of the VSSR, including heating and sensing. To improve the printing accuracy of DIW, the liquid metal (LM)/sodium alginate (SA) composite ink was prepared by sonicating LM in a SA aqueous solution and centrifuging. Functional soft components were printed on the bottom and top of the soft robot with LM/SA ink by DIW and were used to achieve low-voltage (as low as 1.5 V) Joule heating and to sense the bending deformation of the soft robot with a low response time (about 80 ms), respectively. Then, the photoabsorber Sudan I (0.02 wt %) was added to the commercial photosensitive resin to improve the precision of digital light processing for manufacturing the soft robot body. Through control of the flexible heating circuit to realize the solid–liquid transition of the low-melting-point alloy (47 °C), the stiffness of the soft robot can be changed. This work has great prospects for the design and manufacture of a VSSR with integrated sensing capability.