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Introduction to Paper Four

Title: The micro-design of hooked biological attachment mechanisms and soft robotics – a Biomimetic approach.


Hooked attachment mechanisms are a subset of all Biological Attachment Mechanisms and a useful starting position for experiments on the imaging of all biological attachment mechanisms such that they can be adopted in the engineering domain. A hook has an overhang which makes the imaging and transfer to .stl format a challenge, a test that once passed, allows for the further imaging of attachment mechanisms of all shapes and of differing materials. Confocal microscopy seems to have solved the issue so that it is now possible to move from the attachment mechanism directly to the finished model without user interference [1]. Here, the work to-date is summarised, imaging cellulose and chitin hooks so that the process can move forward to other attachment devices of interest such as the mating parts of sexual organs in insects or other biological sub-structures that are not hooked. Progress is reported to have been made into the development of chitin nano-tubules so clearly there is hope that this work will yield a standard for mechanical attachment mechanisms of soft tissues or materials that can interact safely with human flesh with medical applications.

Keywords: hooks, probability, scaling effects, biomaterials.


This is a review article of the three papers published in the Springer-Open journal, “The Journal of Robotics and Biomimetics” in a special issue on nano-/micro-robotics under the following titles:

1. A biomimetic study of natural attachment mechanisms— Arctium minus part 1 [2]

2. A biomimetic study of natural attachment mechanisms: imaging cellulose and chitin part 2 [3]

3. Micro-design using frictional, hooked, attachment mechanisms: a biomimetic study of natural attachment mechanisms—Part 3 [1]

The title of part 3 above displays the underlying motive behind the exploration of the detail of papers 1 and 2. It accepts the viability of using cladistic methods to arrive at a scenario where a structure that has survived the “evolutionary sieve” is selected, to quote Nicklaus et al [4], over the use of Linnaeus or other classification methods which can be seen as insignificantly better when it comes to evolutionary manifestations of properties and/or structures. [5] goes some way to describing this technology transfer.

The first view was that it was unsuitable to study with available technology. The decision was made to proceed with the use of a confocal microscope instead of light microscopy. Subsequently it has become possible only through the work of Hirt et al [6], by their work on a layered manufacturing device that can accept .tiff files as input and produce form. Now a hook can be manufactured at a 1:1 scale to the specimen that is to be reverse engineered and that means that designers are on the brink of being able to make things that are of use, in the micro-realm (of the order of 10-100 microns in size). It all began with the discovery that it was possible to image one of the hooked probabilistic fasteners under laser light, namely the cellulose hook of burdock (Arctium minus). Therefore the work continued with the chitinous growths of the bee and the grasshopper (Apis mellifera and Omocestus viridulus) tarsii [3]. This encounter with luck was able to make true the theory that the use of the microscope could be for the imaging of a specimen and then the transfer of data directly to a layered manufacture device that was suitable, namely the work of Hirt et al. The point of this imaging was to use it to describe the group of probabilistic fasteners as a number, namely one for the hook, two for the attachment mechanism of the grasshopper O. viridulus with two hooks, and three for the double set of hooks, namely A. mellifera with a separating arolium, irrespective of component material.

The chance of being on top of a specimen structure available without travelling was immense, as these were all available at the University of Bath which is set in the countryside of Western England. Particularly the burdock which is used (apparently) as the basis of Velcro but it is concluded this is without fundament and it seemed better to use it than to use the others (see below), as it will be shown, for the production of a new hook, a multi-use flat structure of multiple hooks that could be used without being entirely known, as per its value and knowledge. i.e. if it is to be the one to be imitated then it needs to be studied more now so that it can be manufactured.


Figure 1: An Arctium minus (commonly known as burdock) fruit showing milli-metric scale. [1]

In Part 1 of the investigation [3], the cellulose hooks of burdock revealed a scaling effect [5] under loading. This is because the hook un-rolls as it is loaded until the radius of curvature is increased in size at fracture, in a similar manner in which a length of iron chain cannot be horizontally loaded until it is pulled straight without failing. The material is simply not as stiff as it would appear in the sketch of the structure for analytical purposes with its Newtonian assumptions and its properties vary under conditions, such as its state of dessication.

The reasons for this have been considered but not concluded as of yet, requiring further inspection of the material properties. All the natural cellulose hooks studied in the literature, Agrimonia eupatoria, Circaea lutetiana, Galium aparine, and Geum urbanum as well as Arctium minus, have been described in terms of their originating structures [2][8].










Table 1: Grouping the cellulose, probabilistic, frictional and long-shafted hooks according to originating structure. [2] and [8][9].

The cellular complexity obviously plays a part and from [2] the micro-fibril strengthening of the structure must play a part too, but this does not satisfy the Newtonian equations of static analysis used for hooks of a larger size. This is an exciting find since it suggests that there may be differing laws governing the behaviour of structures at this level other than standard analysis, rather in the way that the behaviour of fluids differ under different flow conditions [10] governed by the energy equation. Therefore the sense is that it is best to mimic the morphology exactly in order to yield optimal performance and maximum attachment strength when fastened, through fiction and mechanical attachment, bearing in mind that a hook must be paired with a substrate.



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