Copyright B E Saunders 2016
The Design of a Silent Re-useable Hooked Attachment Mechanism
A Proposal for the Design of an Autonomous Silent Re-usable Hooked Attachment Device Suitable as an Appendage as a Robotic Attachment Device Based Upon an Insect Tarsus
This proposal draws upon the work of my Phd so far. It outlines the development of a silent releasable hooked attachment mechanism based upon an insect tarsus including structural mimicry, static modeling and finite element analysis. It is suggested that the previous paper “The Functional Ecology and Mechanical properties of Biological Hooks: A Biomimetic study of Burdock (Arctium Minus”) be consulted prior to reading this. Further, direct reference is made through this to the author’s paper entitled the “Functional Ecology and Mechanical Properties of Hooks in Nature” and appendices.
Insect morphologies are characterized by structures supporting reduced energy expenditure; moving parts are simple in terms of musculature and augmented by physical structures to reduce energy requirements. Frictional attachment devices are a prime example such as the head arresting mechanism of the dragonfly which is a frictional device made up of two opposing surfaces of parabolic structures that interlock and release through the insect’s cycle of common activities. Once the head is retracted and locked into position on the anterior face of the thorax the neck muscles can relax. (See “Evolution of the Dragonfly Head Arresting System” by S N Gorb,  as described in Appendix 4 Section 6) and (“Attachment Devices of Insect Cuticle” by S N Gorb  as described in Appendix 1)
Gorb concludes his description of the functions of hooks in insect species with the following description (p50):
“The hook mechanism is usually comprised of two complementary surfaces. These surfaces are not necessarily mirrored copies of each other but some dependence on the corresponding surface does exist. If both surfaces bear hooks (wing-interlock) their dimensions are usually predefined in order to optimise attachment and the probability of attachment as well. When only one surface bears real hooks, they could only attach efficiently to a particular range of textures (tarsal claws, hooks of phoretic and parasitic animals). The hook design can range from unicellular acanthi and multicellular setae to spines and cuticular folds.”
In the final sentence of the above quote we have an example of the deceptiveness of the human language; a hook design with spikes? This derives from the verb “to hook” where a spike can be used “to hook onto” something.
But it is emphasised that for the purposes of this research topic, the structure being studied has been rigorously restricted to hooked shapes, pointed tapered shafts with a demonstrable curvature.
1.1 Silent Velcro
The term “Silent Vecro” is quite non-specific in the light of the above. Velcro as a generic term means different things to different people.
Conventional Velcro as patented by George de Mestral (made of polyester resin) could be described as a releasable, reusable, probablistic hooked mechanical interlocking fastener.
The term “Velcro” is also loosely applied to describe the form of metallic joining method that was recently developed to replace tacking when welding metallic plates together, which is a permanent join.
So the word “Velcro” has come to mean a manner of joining two opposing surfaces together without adhesive using matching surface structures that generally speaking can be separated again without damage to the surfaces although this is not the case with the metallic Velcro described above.
Further, it would seem that the term Velcro is no longer limited to hooked structures. On the contrary, fastener structures have developed and a new kind of fastener using mushroom shaped structures with a matching receiving surface is now commonly seen on the packets of rolling tobacco for example, but this is not a probabilistic fastener because the matching surfaces require alignment.
In functional terms, silent Velcro could be defined to mean:
No audible energy upon release.
Number of parts (two – structure and matching substrate)
Attachment structure morphology
Strength to attach
Strength to release
Frictional component and other scaling factors that contribute to attachment
Each of these points should be expanded upon as the absence or misinterpretation of any of them changes the resulting attachment mechanism.
A reusable attachment mechanism can be static such as a Velcro strap on a bag or it can be moveable. A foot is a reusable attachment mechanism. A shoe is a reusable attachment mechanism. Reusable implies that the principles or structures that form the attachment are either undamaged by separation or are replenished or repaired. Appendix 1 – Library of Zoological Examples of Attachment Mechanisms Classified According to Shape and Function of the authors transfer report details the various relationships and types of attachment.
A Velcro strap has a corresponding predefined surface. According to Nachtigall  it is a releasable attachment device by two structures. A shoe does not need a predefined surface. According to Nachtigall it is a releasable attachment device by one structure.
In the case of insect tarsi, Appendix 4 Sections 10 and 11, “Performance and Adaptive Value of Tarsal Morphology in Rove Beetles of the Genus Stenus (Coleoptera, Staphylinidae)” by O Betz  as described in Appendix 4 Section 8 and “Roughness-dependent friction force of the tarsal claw system in the beetle Pachnoda marginata (Coleoptera, Scarabaeidae)” by Z Dai, S N Gorb, U Schwarz  as described in Appendix 4 Section 9, provides a sufficient insight into the relative performance of the tarsal structures, the claws and the setae.
The structure of the insect attachment mechanism generally consists of a number of claws, commonly two, and setae or hairs that can vary in length, density and number. Scaling effects when copying these structures means that the significance of each and their contribution to attachment will vary. The setae can also secrete an adhesive that contributes to the attachment strength.
Equally and as important is the substrate. For instance, in the case of the pitcher plant, directional hairs help to guide the insect in the direction of the pitcher. In the case of a manufactured attachment mechanism this effect can correspond to patterned surfaces corresponding to the claw configurations.
All the structures consist of insect cuticle and are shaped accordingly. To make use of their shape optimization properties it must be born in mind that the final material chosen for manufacture must have similar properties or the shape must be adjusted according to the new material properties.
1.1.2 Attachment type and strength
Attachment can be permanent or temporary. The strength depends upon material and shape. This proposal is set within the bounds of hook-shaped attachment mechanisms and the possibility of looking at parabolic fasteners is therefore artificially restricted. Hooks are modeled by Gorb as tapered shafts and material qualities are studied in his paper “Natural Hook and Loop Fasteners: Anatomy, Mechanical properties and Attachment Force of the Jointed Hooks of the Galium Aparine Fruit” E V Gorb, V L Popov, S N Gorb  as described in Appendix 4 Section 3.
This work can be adapted to model a dual hook combination. Further Gorb defines attachment mechanism characteristics such as the ratio between attachment and detachment forces and their variation.
For the purpose of this research it is suggested that it is possible to produce a variable detachment force based upon the angle of the attachment device. This would mean a low force of attachment with a resulting high detachment force which decreases with rotation to a low detachment force, much like the placement of a foot during walking. Heel-down produces attachment, toe-down provides release.
Gorb discusses probabilistic fasteners and he states that it is parabolic fasteners and not hooked fasteners that yield the best possibilities for investigation in nature. (See “Probablistic Fasteners with Parabolic Elements: Biological System, Artificial Model and Theoretical Considerations” S N Gorb, V L Popov,  as described in Appendix 4 Section 6) In this case what is being discussed is the interface between mechanical interlocking and friction. The interface is the dependence of the mechanical interlocking upon the frictional force between surfaces.
Hooks are shape optimized for an optimum angle of force application. The relationship between the hook tip and the substrate will defined the strength of attachment depending on the degree of mechanical interlock and the frictional force. Appendix 3 details typical organs of plant surfaces that are the natural substrates of insect tarsi. Modeling the surfaces as a matching substrate to tarsi configurations would optimize the attachment.
1.1.4 No driven moving parts
In this proposal it is being considered to study a mechanism that has no external forces other than those derived indirectly from the placement of the mechanism. S N Gorb has done work on the definition of attachment mechanisms including static analysis and the mathematical modeling of the relationships between attachment and detachment forces that define the effectiveness of an attachment mechanism (See “Miniature Attachment Systems: Exploring Biological Design Principles” by S N Gorb  as described in Appendix 4 Section 4). Also see Experiment 3 in the main body of the report.
This means that an assembly of hooks and structures needs to be designed that is hinged and reactive to positioning.
This mechanism need not be limited to simply two hooks neither need it be limited to hooks that directly resemble insect claws. For instance a tarsal mechanism using hooks that are the shape of burdock hooks would equally be valid.
1.1.5 No audible energy upon release
As mentioned previously there are variations as to the interpretation of the word noise. The environmental agencies will have one version in terms of suburban noise, for instance. There is also the relationship of audible noise to audible frequencies. What is certain is that any vestige of attaching force that is present will imply a loss of energy when it is released.
This describes the ability to attach randomly without precise matching of surfaces. Velcro is a probabilistic fastener as is a robot foot that can attach to different surfaces and release with the minimum of noise and hence energy release.
1.2 The design process
The steps to record and reproduce the morphology of a specimen tarsus using Solid Modeling are as follows:
1) Digitize structure profiles to obtain the sweep path. (see Experiment 1 and 2 in the main body of the report and Appendix 5.) (Paper 3)
2) Section the structures with a microtome and digitize the sections to gain the shapes.
3) Preserve the sectioned material for analysis for signs of trace materials and other non-homogeneities in the material.
4) Reconstruct the structures in Solid Modeling and assemble them together. Also see Appendix 6.
1.3 Finite element analysis
Using Solid Modelling the direction of optimum force application can be found by looking for the optimum stress profiles. This must be done after first assembling the component structures into their composite structure and examining the freedom of movement and the angles of engagement with a substrate.
Thereafter a static force model can be developed which can be done in terms of physical variables for the range of movement of the attachment structure.
1.4 Rapid prototyping
The data file for the assembly can be saved in a .stl format and exported to a rapid prototyping device. As is discussed above, scaling effects will influence the significance of the setae, their length and number and whether they should even be included in the design.
The size of the prototype device should be judged according the form of test to be conducted:
A larger device can be used to measure and test the range of movement and test the engaging and disengaging action. Thereafter a range of sizes should be produced and the relative attachment forces measured.
The gait of the insect will naturally play an important part as the activating and deactivating action of the tarsus. The details of this will have to be drawn from standard texts. For instance, in terms of the human gait a foot can placed down heel to toe and at different angles. But this forms only a portion of the full leg motion. Similarly with the gait of an insect only a portion of the gait needs to be considered. This is helpful since clearly the control systems for the accurate mimicking of motion are difficult to reproduce. But the essence of the idea should be clear and it is the fundamental mechanism of attachment and detachment that shall be studied.
1.7 Steps in the mechanism development shall include
Apart from the steps already described above, further development shall include:
1. Establishing the locus of motion of critical points on the mechanism.
2. Cross correlation of material properties of the biomaterial versus some suitable artificial material.
3. Establishing some relation between mechanical interlock/frictional force and angle of attack between mechanism and substrate.
The design of the substrate probably rivals the importance of the development of the mechanism itself.
Patterns, textures and roughness could all play a part. Descriptions of some natural substrates can be found in Appendix 2 of the transfer report.
1.9 Final comment
It is quite likely that a biomimetic attachment device based upon a tarsal mechanism could require properties beyond simple structural detail. For instance, scaling effects will reduce the effect of the setae. It may be possible to introduce analogues to this to increase the effectiveness of attachment. One idea that springs to mind is to secrete an adhesive that could be left behind when the attachment device is lifted from the substrate using capillary action to draw fresh fluid down a hollow structure from a artificial gland within the device. This would act to replenish the adhesive ready for the replacement of the next step.
Bumblebee and grasshopper tarsi
Figure 13 and Figure 14 shows the image sequences through a bee and grasshopper tarsi. In both of these sequences the image starts in the furthest plane and the slices move towards the viewer.
Interest in imaging these two specimens comes from an interest in studying insect/plant surface interactions. (see Section 126.96.36.199).
Insect material doesn’t fluoresce as well as the plant material of the burdock hook but the tarsi as structures are more complex than the hooks. Attempts were made at all 3 wavelengths (red, blue and green) and combinations of the three. It is certain that a 2-photon microscope with its greater focus and depth penetration would improve the image making capability. The laws of physics say that blue light has the highest energy but excitation of molecules is sensitive to the exciting frequency so exciting the molecules with all three colours gives a spread of frequencies that provoke the maximum excitation.
The results to the insect experiments have not been presented as stereograms.
The following sequence of images (Figure 13) shows a progression of the laser through the specimen.
The z-axis of the laser starts furthest away from the viewer and moves “closer” towards the viewer during the progression through the stack, from image 1 – 30.
A combination of all three wavelengths of light (red, green and blue) was used and these are visible in the images. It is interesting to note that different structures in the tarsi fluoresce at different frequencies and that the combination of all three provides an opportunity to look “into” the structure and identify different structures through colour differences. The plain of the laser is identified to the viewer by those parts of the image that reflect white light.
Figure 13 – 1 – 30 confocal microscope image “slices” of hooked bumblebee tarsus (scale bar indicates 200 mm)
See images overleaf.
Figure 14 – 1 – 29 consecutive confocal microscope image “slices” of the hooked grasshopper tarsus (scale bar indicates 200 mm)
It is possible to see the setae (hairs) on the underside of the metatarsus (the structure that supports the tarsi) which aid in adhesion of the structure to plant surfaces through friction and adhesion (setae secrete tarsal fluid which aids with adhesion [<!– [if supportFields]> REF _Ref111113328 \r \h 15 08D0C9EA79F9BACE118C8200AA004BA90B02000000080000000E0000005F005200650066003100310031003100310033003300320038000000 ]).–>
“Evolution of the Dragonfly Head Arresting System” S N Gorb, Proc. R. Soc. Lond. B (1999) 266, p525-535
2. “Attachment Devices of Insect Cuticle” S Gorb, 2001, Kluiwer Academic Publishers, ISBN 0-7923-7153-4
3. “Biological Mechanisms of Attachment, The Comparative Morphology and Bioengineering of Organs for Linkage, Suction and Adhesion”, W Nachtigall, 1974 translated by M A Biederman-Thorson, Springer-Verlag, ISBN 3-540-06550-4
4. “Performance and Adaptive Value of Tarsal Morphology in Rove Beetles of the Genus Stenus (Coleoptera, Staphylinidae)” (2002) O Betz, Journal of Experimental Biology, 205, 1097 – 1113 (2002)
5. “Roughness-dependent friction force of the tarsal claw system in the beetle Pachnoda marginata (Coleoptera, Scarabaeidae)” (2002) Z Dai, S N Gorb, U Schwarz, Journal of Experimental Biology, 205, 2479-2488
6. “Natural Hook and Loop Fasteners: Anatomy, Mechanical properties and Attachment Force of the Jointed Hooks of the Galium Aparine Fruit” E V Gorb, V L Popov, S N Gorb, Design and Nature 2002
7. “Probablistic Fasteners with Parabolic Elements: Biological System, Artificial Model and Theoretical Considerations” S N Gorb, V L Popov, Phil. Trans. R. Soc. London A(2002) 360, 211-225
8. “Miniature Attachment Systems: Exploring Biological Design Principles” S N Gorb, Design and Nature, 2002