Classification and Evaluation of Octopus-Inspired Suction Cups for Soft Continuum Robots

Abstract

The emergence of the field of soft robotics has led to an interest in suction cups as auxiliary structures on soft continuum arms to support the execution of manipulation tasks. This application poses demanding requirements on suction cups with respect to sensorization, adhesion under non-ideal contact conditions, and integration into fully soft systems. The octopus can serve as an important source of inspiration for addressing these challenges. This review aims to accelerate research in octopus-inspired suction cups by providing a detailed analysis of the octopus sucker, determining meaningful performance metrics for suction cups on the basis of this analysis, and evaluating the state-of-the-art in suction cups according to these performance metrics. In total, 47 records describing suction cups are found, classified according to the deployed actuation method, and evaluated on performance metrics reflecting the level of sensorization, adhesion, and integration. Despite significant advances in recent years, the octopus sucker outperforms all suction cups on all performance metrics. The realization of high resolution tactile sensing in suction cups and the integration of such sensorized suction cups in soft continuum structures are identified as two major hurdles toward the realization of octopus-inspired manipulation strategies in soft continuum robot arms.

Keywords: biomimetics; octopus sucker; soft robotics; suction cup.

Figure 1

Morphology of the octopus sucker and its terminology.

Figure 2

Visual description of the adhesion process of an octopus sucker in five steps. A) Initial contact with the substrate, B) formation of a seal after contracting the infundibular radial muscles, C) pressure reduction through contraction of the acetabular radial muscles, D) interlocking of the protuberance in the orifice through contraction of the meridional muscles, and E) Continued adhesion after relaxing of the radial muscles due to the friction from hairs and ridges, cohesive forces of water and elastic energy in the cross connective tissue fibres.

Figure 3

Visual explanation of how the new adhesion model as proposed by Tramacere et al.^{[ 10 ]} leads to a smaller surface of action.

Figure 4

Electron microscope images of the sucker tissue. In the middle image, the radial grooves are clearly visible, while the right image displays the microdenticles on the infundibular surface.^{[ 13 ]}

Figure 5

Three different types of sensors located on the octopus sucker rim and infundibular surface, as discovered by Graziadei.^{[ 13 ]}

Figure 6

Two cylinders that appear identical to octopuses when sampling the surface with their suckers according to the experiments of Wells.^{[ 17 ]} The reason is that they have similar local surface curvatures.

Figure 7

Hierarchical structure of the neural anatomy in the octopus, as described by Grasso et al.^{[ 16 ]}

Figure 8

Rating of suction cup designs on general performance metrics relating to the main architecture, classified by their actuation principles. The top row displays a comparison to these metrics in the octopus sucker.^{[ 10} , ³² , ^{33 ]}

Figure 9

Visual explanation of the working principles of the actuation methods, elaborated with examples from records.^{[ 26} , ³⁴ , ³⁵ , ³⁶ , ³⁷ , ³⁸ , ³⁹ , ⁴⁰ , ⁴¹ , ⁴² , ⁴³ , ⁴⁴ , ^{45 ]}

Figure 10

Evaluation of suction cup designs on surface adhesion metrics and investigation of strategies for improved adhesion performance, classified by their actuation technologies. The top row displays the outcomes of these metrics in the octopus sucker.

Figure 11

Visual explanation of principles applied for improved adhesion on several types of substrates. A) Mechanical properties, B) surface geometry, C) surface microstructure, D) surface adhesion, and E) stiffness adaptation.

Figure 12

Evaluation of suction cup designs on metrics related to tactile sensing, control & system, integration, classified by their actuation technologies. The top row displays the outcomes of these metrics in the octopus sucker.^{[ 13 ]}

Figure 13

Radar charts representing potential integration of suction cup designs in soft robot arms, elaborated with images from records.^{[ 31} , ³⁸ , ³⁹ , ⁴² , ⁴⁹ , ⁵⁸ , ⁶¹ , ⁶² , ⁶⁶ , ⁶⁸ , ⁷⁰ , ^{73 ]} A more elaborate figure can be found in Section S4 (Supporting Information).