Biomechanics in Soft Mechanical Sensing: From Natural Case Studies to the Artificial World

Abstract

Living beings use mechanical interaction with the environment to gather essential cues for implementing necessary movements and actions. This process is mediated by biomechanics, primarily of the sensory structures, meaning that, at first, mechanical stimuli are morphologically computed. In the present paper, we select and review cases of specialized sensory organs for mechanical sensing-from both the animal and plant kingdoms-that distribute their intelligence in both structure and materials. A focus is set on biomechanical aspects, such as morphology and material characteristics of the selected sensory organs, and on how their sensing function is affected by them in natural environments. In this route, examples of artificial sensors that implement these principles are provided, and/or ways in which they can be translated artificially are suggested. Following a biomimetic approach, our aim is to make a step towards creating a toolbox with general tailoring principles, based on mechanical aspects tuned repeatedly in nature, such as orientation, shape, distribution, materials, and micromechanics. These should be used for a future methodical design of novel soft sensing systems for soft robotics.

Keywords: bioinspired sensing; biomechanics; biomimetics; flow sensing; mechanical sensing; mechanoreceptor; morphological computation; soft robotics; soft sensors; tactile sensing.

Figure 1

General features of mechanical sensing in nature. (A) The outer stimuli are filtered both biomechanically and neurally, before producing sensory responses in the living organism (adapted from [1] by permission of Oxford University Press). (B) General features of mechanosensory transduction at the cellular level (reprinted by permission from Springer, [8]). Transduction of the stimuli occurs after deformations and/or displacements of structures.

Figure 2

Overview of examples of specialized mechanical sensory organs with significant biomechanical aspects in nature. The different sensor type morphologies are encountered in the plant and animal kingdoms. Plants: (top left) Bryonia dioica Jacq. tendril (A. Moro, Department of Life Sciences, University of Trieste, CC BY-SA 4.0 [37]), (inset) tactile bleps on tendril (reprinted by permission from Springer, [30]); (top right) Drosera rotundifolia, (inset) close up of tentacle tips (photos by Barry Rice, http://www.sarracenia.com); (bottom left) Eccremocarpus scaber (CC BY-NC-ND 2.0 [38]), (inset) tactile papillae on tendril (reprinted from [32] by permission of John Wiley & Sons, Inc.); (bottom right) Dionaea muscipula (reproduced with permission from FlyTrapCare.com), (inset) trigger hair (photo by Martin Brunner, CC BY-SA 2.5 [39]). Animals: (top left) star-nosed mole, (inset) Eimer’s organ on star (adapted from [29], Copyright 2012, with permission from Elsevier); (top right) Drosophila melanogaster (photo by Sanjay Acharya, CC BY-SA 4.0 [37]), (inset) campaniform sensilla on halteres (adapted from [33], Copyright 2017, with permission from Elsevier); (bottom left) Cupiennius salei (reprinted by permission from Springer, [34]), (inset) spider hair sensilla (adapted from [35], Copyright 2004, with permission from Elsevier); (bottom right) Astyanax fasciatus, (inset) cupulae of neuromasts on the lateral line (reproduced from [36] with permission from The Royal Society of Chemistry).

Figure 3

Biomechanical principles of spider sensilla for air flow sensing (trichobothria). (A) The length of trichobothria varies, among the same group and also in different media, following the variation of the boundary layer thickness. Different lengths correspond to different best frequencies’ sensitivity. In longer hairs, the diameter d of and the deflection angle θ are larger than in shorter hairs. (B) Hairs present an angle α of about 90° with respect to the body and microtrichs that might increase sensitivity at low airflows (adapted from [58] with permission from The Royal Society).

Figure 4

Biomechanical principles of spider tactile hairs in tactile sensing. (A) Bending in addition to deflection protects the hair against breaking. The maximum deflection angle of the hair θ is 12°, with a damping element R and a spring element S. (Inset) The hair base is finely tuned, with the presence of a “terminal connecting material” and a “second joint”, described in the text (reprinted by permission from Springer, [81]). (B) Regional heterogeneity along the hair attributes different functions and optimization of each part (reprinted by permission from Springer, [56]). The regions depicted have the following characteristics: (1) plastic region, abrupt decrease of lumen diameter, strong curvature in different directions; (2) approximately one third of hair length, not found in all hairs; (3) rotational symmetry; (4) strong deflections, wall of the hair thicker towards the tarsus; (5) decrease of outer diameter towards the base; (6) morphologically and functionally most complex structure of the hair.

Figure 5

Artificial sensors integrating cantilever shapes. (A) Stiff SU-8 hair sensor (© 2007 IEEE. Reprinted, with permission, from [71]). (B) Synthetic fiber in a hydrogel follicle, using a lipid bilayer as the transduction element (reproduced from [76] with permission from The Royal Society of Chemistry). (C) Polyurethane force sensitive resistor (FSR) allowing bending (© 2006 IEEE. Reprinted, with permission, from [83]). (D) Sensor with array of cilia using magnetoimpedance (reprinted from [84] by permission of John Wiley & Sons, Inc.). (E) Artificial whisker with straight cylindrical shape (reproduced from [108]. The publisher for this copyrighted material is Mary Ann Liebert, Inc. publishers). (F) Artificial whisker with undulated surface compared to a harbor seal whisker (republished with permission of Annual Reviews, from [49]).

Figure 6

General structure and position of main components in the lateral line of fish. (A) hair cell of (B) superficial and (C) canal neuromasts ((A) reprinted from [74] by permission of John Wiley & Sons, Inc.; (B,C) and bottom reprinted by permission from Springer, [112]).

Figure 7

Artificial sensors with shape of cantilevers with domes. (A) Hydrogel cupula on hair sensor (reproduced from [36] with permission from The Royal Society of Chemistry). (B) Piezoresistive sensor with parylene coating (adapted from [123], Copyright 2012, with permission from Elsevier). (C) Hyaluronic acid methacrylic anhydride (HA-MA) based cupula (reproduced from [129], CC BY 4.0 [130]). (D) Artificial lateral line canal system (adapted from [126]. © IOP Publishing. Reproduced with permission. All rights reserved). (E) Piezoelectric sensor using the graded cilia principle (adapted from [124], CC BY 4.0 [130]). LCP: Liquid crystal polymer; PDMS: Polydimethylsiloxane; PVDF: Polyvinylidene fluoride.

Figure 8

Examples of specialized mechanical sensory organs with a dome shape. (A) A diagram of the cross-section of a campaniform sensillum (adapted from [1] by permission of Oxford University Press). (B) Tactile blep architecture (reprinted by permission from Springer, [30]).

Figure 9

Artificial sensors integrating dome shapes. (A) Optical sensor (© 2014 IEEE. Reprinted, with permission, from [159]). (B) Piezoresistive sensor with liquid metals in an elastomeric layer (© 2013 IEEE. Reprinted, with permission, from [164]). (C) Piezoresistive sensor with liquid metals in a polyethylene terephthalate (PET) layer (reprinted from [165] by permission of John Wiley & Sons, Inc.). (D) Capacitive sensor with a multilayered dielectric on a flexible printed circuit board (FPCB) (reprinted by permission from Springer, [166]). (E) Piezoresistive sensor with bioinspired interlocking microstructured layers (adapted with permission from [171]. Copyright 2014 American Chemical Society). PDMS: Polydimethylsiloxane; R_c1, R_c2: Contact resistances; R_f1, R_f2: Film resistances.