Fig. 2: Design and sensing mechanism of the proposed tactile/touchless flexible bimodal smart skin (FBSS).

a Soft sensor design with different functional layers stacked together. These layers include a flexible dielectric layer (cyan), a flexible electrode layer (gray), a stimulation layer (pale yellow), the liquid metal material (black), and a package layer (orange). b Electron microscope image of the micro-pyramid structures on the top side of the flexible dielectric layer. c An optical microscope image of the liquid metal material printed on the silicone material layer. d A bending photograph of the FBSS prototype demonstrates its flexibility. e A stretching photograph (maximum stretching rate is 58.4%) of the prototype demonstrates its stretching ability. f The tactile/touchless sensing mechanism of the prototype: (i) The equal density of negative and positive charges was generated on the flexible dielectric layer (gray) and external object (red) due to the different electron affinities after a few contacts. (ii) The free electrons were driven to flow from the ground to the flexible electrode as the external object approached the flexible dielectric layer. (iii) The external object (red) starts to contact the FBSS, increasing the transfer of electrons, and the liquid metal resistance increases. (iv) The external object (red) is entirely in contact with the FBSS; charge neutralization occurs and the free electrons stop moving and the resistance of the liquid metal reaches the maximum. (v) As the external pressure was released, the electrons flowed back from the flexible electrode (gray) to the ground, and the resistance of the liquid metal decreased as the channel recovered to its intial state. (vi) As the external object (red) was separated from the FBSS, the backflow electrons increased and the liquid metal’s resistance remains stable.