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Paper 2: A Biomimetic/Bionic Study of Arctium Minus (Burdock) – Imaging Small (~mm) Hooks of Cellulose and Chitin
Copyright 2016 Bruce E Saunders
Also, now complete with images!!
Now complete with images!!
Copyright 2016 Bruce E Saunders
Believe it or not I have been arrogant enough to have entered my 3 papers into the Buckminster Fuller Institute competition this year!
What the hell, huh? You only live once and you are dead for a loooong time.
Wish me luck!
I’m doing my best to get some images up. Please be patient while I struggle with file compatabilities etc.
The span of the hook is about 200 microns. This image is composed of a stack of .tif images that slice through the hook. It is the cellulose in the plant cells hat is fluorescing. Look closely at the shoulder of the rotating hook and you can see the “stepped” outline where the voxels of one plane meet the voxels of the next image section/plane. See Experimentation: Part (II)
A Biomimetic Study of Long Shaft Cellulose Hooks after Arctium minus (Burdock) Part III
It is necessary to consolidate information before proceeding with the design. The morphology of the A. minus hook has been accurately recorded and its performance under tension has been tested and compared with those of four other species . A. minus is supposed to be the biological inspiration behind Velcro  and it is interesting to compare their relative functionalities. It will become apparent that functionally, the Velcro hook resembles that of C. lutetiana more than it resembles the hook of A. minus. This shall lead to product description based upon long-shaft cellulose hooks and the opening elements of the design process for a product closely based upon the shape, functionality and material of these hooks.
In considering the hooks of A. minus, C. lutetiana, A. eupatoria, G aperine, and G. urbanum the hooks of each of these plant species is associated with plant dispersal, for attaching the plant fruit to passing hosts but each has a different hook arrangement in terms of number of hooks per fruit and their arrangement on the fruit. Gorb’s purpose in studying C. lutetiana, A. eupatoria, G aperine, and G. urbanum  was to find an optimum hook (or burr) morphology, investigate scale effects of the burrs per fruit and to assess the contact force of a single burr compared to the mass of the fruit. By adding a study of A. minus to his work and concentrating on burr morphology and material properties and assessing the structural information available it will be shown that the biological indicators do indeed supply information that can lead to the generation of a product or a family of products that is different from conventional Velcro, by following a biomimetic process. (Note: At no stage in this work has a patent database been consulted. This was a purposeful omission so as not to influence the biomimetic process.)
In all five species the burrs in their various formations and numbers form probabilistic fasteners. By definition from Nachtigall  a probablistic fastener is a random hooking mechanism that is a releasable attachment mechanism by one structure, i.e. there is no single matching structure required for attachment to take place. This definition does not specify that the structure should be part of a field of structures nor that interaction between component structures is a necessary part of attachment. Considering Gorb’s work on frictional parabolic fasteners  his model specifically includes the interaction of neighbouring elements as contributing directly and not indirectly, to the attachment. All the hooks of the five species in this paper are a part of fields of structures of different numbers but the hooks do not interact to increase their individual attachment force. They all act individually and their collective action serves to arithmetically increase the overall attachment of the fruit to the host.
Further, it is stated by Gorb  that he has an interest in measuring the scaling effects of the detachment force of the fruit from their supporting structure on the parent plant. If one considers that the burrs have a direct purpose of hooking the fruit to a host then it would seem that with respect to his study of morphological variables and their respective influence on burr strength  there are finite possible responses when a hook comes into contact with a host. They are:
- Fruit detachment from the parent plant.
- Hook flexure if fruit detachment does not take place, to release the host and keep the hook intact for the next host.
- Removal of host fibres.
Any other response such as hook fracture and shaft fracture would result from the fruit being held in place on the parent plant and would render the hook useless for its intended purpose which would be contrary to an evolutionary strategy.
2 Hook structure
Three of the species (C. lutetiana, G. aperine, A. eupatoria) are the weakest in tension and they originate from trichomal structures. From Devlin  trichome is a collective noun for all types of outgrowths supported by the cell wall of the epidermal layer. The epidermis is the outermost layer of cells in a plant and its functions include manufacturing the structural material of the plant. A trichome is a plant hair and can be glandular and non-glandular, cellular, multi-cellular, branched or unbranched. Of these three species, G. aperine exhibits a multi-degree of freedom due to, from Gorb , a hollow base. The other two species derive from the surface of the mericarp.
Of the other species, A. minus and G. urbanum, their hooks originate from the bract and carpel of the fruit and exhibit respectively the strongest contact separation forces. Both have a single degree of freedom due to their origination from a flattened structure.
From this information it is possible to derive space charts of biological hook design criteria for cellulose hooks:
Fixed base Single Multiple
Degrees of Freedom
All of these are based upon hooks made of cellulose. Introducing materials suitable for manufacture with their own properties will produce different relationships.
3.1 Plant hooks
The following is compiled from both the research of Parts I and II and from the papers of Gorb et al  .
- In the plant kingdom, with regards to plant growth and evolution, only energy efficiency can be assumed with regards to structure. All structures are governed by the same basic physical and chemical laws. This implies a direct link between material composition, morphology and function but does not necessarily indicate a best overall solution to a prescribed problem from an engineering perspective since engineers have a wide variety of materials at their disposal.
- The tensile testing upon Arctium minus confirmed that S N Gorb’s conclusion with regards to hook strength, that shaft strength and then hook radius was of primary importance morphologically speaking were true to a degree but that the material resistance to shear induced by bending was more important than the influence of bending moment due to the length of the lever arm.
- It was noted that functionally burdock hooks are single use only and that the originating bracts, from which the hooks form act as a single degree of freedom hinges thereby aiding attachment.
- It was noted that the diameter range of known natural host hair fell between 20 and 100 microns in diameter, all consisting of the material a-keratin.
- It was observed that burdock hooks have a radius or curvature of between 50 and 150 microns and a thickness of approximately 200 microns and that cellulose microfibril alignment is all parallel to the direction of the hook.
- It was observed that unlike Velcro, burdock hooks have a pointed tip and their hooks are thin enough in silhouette to combine a hooking function with an action of piercing the fibres.
- It was noted that hooks originating from trichomes (plant hairs) appear to be weaker that hooks originating from carpels or bracts. This could be due to the fact that trichomes are simpler structures and less developed.
As a product, Velcro is purported to derive from burdock. It is of interest therefore to make a comparison between the two before attempting to develop a further product biomimetically from these plant hooks.
- It was noted that the nylon from which Velcro is manufactured is flexible and resilient enough to withstand multiple attachments and re-attachments. The artificial substrate however can be damaged over time.
- It was observed Velcro hooks which are assembled in parallel fields which is similar to the spherical fields of both the A. minus and the G. urbanum.
- It was observed that confocal microscopy, although memory intensive, was a viable means of non-destructive indirect measurement of a micron-sized cellulose hook with natural fluorescent properties. Further it is surmised that the product of a confocal microscope, a stack of .tif images, could be converted directly for virtual reality applications, rapid prototyping devices and could have application in the field of producing nano- and micro-structures.
4 Design brief
Unlike conventional design briefs where a required function is specified with parameters to place constraints on the design, here it is the shape and functionality that is specified for which a suitable material and application is sought.
Figure 1 shows the burdock hook shape that shall be used in the design. It shall be taken as prescribed. This image is repeated from Part I. This shape shall be central to the full design.
- 3-d reconstruction from SolidWorks of a burdock hook and shaft
4.2 Functionality and structural description
In considering the range of attachment types shown in the five species considered, they indicate that a certain range of hooks could be considered (see Figure 1 to Figure 4) varying in structure, flexibility, size and strength. If structure and size are considered to be “given” constraints, flexibility and strength are material and manufacturing properties. Because of the asymptotic approach to the design all possibilities can be explored and confirmed or eliminated as the design progresses. Considering the reproduction of a hook as shown in Figure 5 the following functions can be defined:
- To hook and to flex to release substrate or not to flex and not release.
- To pierce such that the hook can be inserted with a forward action like a barbed spike or simply to thread between a fibrous substrate.
- To flex in a single plane to aid attachment through a flexible base, not to flex in any plane or to flex in multiple planes.
The structure of the burdock hook indicates shows that the shaft emerges from a flattened supporting bract. The burdock bract itself is rectangular with the hook emerging from one of the narrow edges. This bract has its own permanent attachment to the fruit pedicle at the base. Replicating this bract opens a number of possibilities in terms of hook configurations. See Section 4.2.3 below.
A flexible base could be produced by mimicking the hook of the G. aperine fruit. This would rely upon introducing a variable material thickness in the shaft to allow buckling in different directions.
A solid inflexible base is the third possibility and will produce increased strength and directionality, perhaps at the cost of efficiency of attachment.
The substrate shall be defined by testing for suitability and attachment strength and shall be dependent upon the finished size, shape and material of the hook. The natural substrate for cellulose hooks is fibrous. Similarly this could be replicated for a product but this is not a necessity. It will depend upon the selected material and the abilities and application of the finished design.
For instance, in considering biomedical applications such as sutures it would seem that a barbed spike is a better solution for piercing flesh than a piercing hook but this may not necessarily be the case since the success of such an implement must depend upon the amount of trauma induced, the recovery and the behaviour in situ post recovery. These effects cannot be assumed.
4.3.2 Hook field structure
It has been noted that hooks in nature are assembled in different configurations and numbers. Gorb compared the mass of the fruit with the contact separation force of single hooks in order to assess the hook performance for each species and the number of hooks required to support the fruit which could be viewed as a measure of design efficiency.
With the design progressing with a development of a modular hook, a hook with supporting “bract”, the opportunity exists to experiment with:
- Field configurations, densities and numbers.
- Bract shapes i.e. square, rectangular, circular, octagonal etc.
- Bract attachment mechanisms for both attaching bracts to each other to form composite fields as well as for attachment to a structure needing an attachment mechanism.
- Bract shapes also offer the opportunity to manufacture the hooks in flattened rows.
In fact modularising the hooks appears to offer many permutations which eventually will have to be modified during the process of material selection and manufacturing process selection and refined by application.
4.3.3 Scaling Effects
The ability to reproduce scaling effects shall be constrained by the mode of manufacture and material selection. It is clear that scaling effects are maximized when a material or structure is acting at its physical limits as occurs in optimized and very small structures. Since a burdock hook is made of cellulose, scaling effects combine with material properties to produce the resultant effect. It follows therefore that an artificial material, stronger than cellulose, will demonstrate reduced scaling effects.
4.4 Product Development
The image overleaf shows the result of extending the digitized burdock hook through a lofted feature into a flange similar to the flattened bract from which the natural bract originates. This image represents a modular hook from which the product design can be developed.
A Biomimetic Study of Long Shaft Cellulose Hooks after Arctium minus (Burdock) Part II
Hooks associated with plant seed and fruit dispersal, with relatively long-shafts and short spans have been identified in five species and have already been the source of engineering design inspiration for George de Mestral and Velcro. However there are marked differences in the shape and functionality of natural hooks and the probabilistic fastener that he designed and developed. This paper continues the work of Part I and morphological study of the A. minus hook with an examination of the behaviour of A. minus under tensile load, the material behaviour and properties and the results are compared with results for four other species with similar hooks after the work by S. N. Gorb. It is concluded that there are four general indicators for product design apart from morphological variables already concluded by Gorb and these are flexible versus fixed bases to the shafts and degrees of resilience of the component material. The natural substrate properties are noted as being relevant but not necessarily governing.
Keywords: Fibre dimensions, tensile testing, fracture, composites, bending and axial loading, biological design indicators
The behaviour of the Artium minus hook under loading is studied here. The hook is composed of the bio-composite cellulose which is comprised of cellulose fibrils bound together in a matrix of hemi-cellulose and lignin. Full descriptions of the chemical composition and formation of these can be found in appropriate texts such as  and .
This paper forms parts two and three of the biomimetic process as described by Gorb , namely material study and mathematical analysis of loading. Note that Gorb includes duties for chemists in attachment structures that include secretions but this is not applicable in the case of burdock.
Part I of this study described the capturing of the morphology of the hook. Studying the functional ecology of the burdock hook must include its interaction and relationship with the substrate. In this paper this is limited to a cursory study since it will be relevant to the prototyping stage, further into this study. It was decided that the prototyping stage was the point at which this study would be performed in earnest since, as will be seen in the following papers, there is sufficient information already available from previous research.
This paper includes an investigation of the possible scaling effects associated with the burdock hook as per S N Gorb’s paper on the contact separation force of fruit burrs . The results are related to those of that paper and the methodology follows that of Gorb’s paper to a degree. The conclusions of this paper provide an indication of the direction which the design will take, following biological design indicators provided from A. minus, A. eupatoria, G. aperine and G. urbanum, the hooks that fractured in the test.
2 The substrate
In terms of the host substrate being fur or feathers, the qualities of these substrates have been studied elsewhere for other purposes. Table 1: the diameters of some common natural fibres.
- Natural fibre diameters 
|Type of hair||Diameter (microns)|
Hair is made of keratin. Keratins form a group of varied proteins which contain significant amounts of sulphur cross-linking and stabilizing in the material. It is found in horn, hair, hoof, feather, skin, claws etc. The types of keratin include mammalian, avian and others such as reptilian. 
In mammals, keratin occurs in hair, hoof and horn. Human hair is a composite consisting of a fibre/matrix mix. In a relaxed state the hair is mainly a-form and when heated and pulled straight it turns into a b-form. The a-form is the a-helix and the b-form is the anti-parallel b-sheet which results from pulling the a-helices beyond their yield point. Bird feathers are also made of keratins as is silk.
This can be related to the diameter of the hook, that is, the inner radius in order to seek design indicators of an optimum ratio. Such experimentation can be left until later in the product design but for the moment it should be mentioned that the inner diameter of the burdock hook, from outer tip to inner shaft surface, is approximately 250 microns.
The most important aspect to note with reference to the functionality of A. minus is not a critical geometrical relationship between the hook and a particular fibre but the fact that the hook accepts all fibre diameters for the purposes of mechanical interlock.
3 Tensile Testing of the A. minus Hook
To investigate the fracture force and mode of fracture of hooks from the plant genus Arctium minus or common burdock using an Instron tensile testing machine to observe any telling differences that could be observed from the conclusion of Gorb that the span of the hook was the significant factor in contact separation force by testing specimens of different radius of hook collected from burdock pods of varying diameters. Further, to observe the nature of fracture of the composite biomaterial and to use this information in developing a design for an attachment mechanism based upon the burdock hook and its functionality.
- “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 , and
- “Contact Separation Force of the Fruit Burrs in Four Plant Species Adapted to Dispersal by Mechanical Interlocking” E Gorb, S Gorb 
These two papers yield fundamental descriptions and conclusions upon which the following experiment is based. Their investigation into scaling effects in small hooks led to the following assertions: the four main attributes that influence burr performance are span, structure, size and material flexibility. All four species tested (were found to exhibit behaviour within the known bounds of cellulose performance. (E = 7 – 15GPa). The significant difference detected was an unforeseen weakness in strength displayed by C Lutetiana. This was associated with an increase in material flexibility; the hooks didn’t fracture, they flexed to release the loop and the hooks remained intact. This form of response is very similar to that of commercial Velcro (see Part III of this study).
The four main attributes described are repeated here:
- Structure: stomatal structures are smaller, rely upon the thickness of the cell wall and have stress concentrations at their base.
- Size: the longer the shaft length the greater the strength. This is probably due to a shaft diameter to length ratio, the longer the shaft the thicker the shaft.
- Span: the smaller the span of the hook the greater the strength due to a reduction in lever arm and hence bending moment.
- Flexibility: the greater the material flexibility the greater the chance of the test loop slipping from the hook.
Gorb suggests that it would be of interest to investigate the required force for detachment of the fruit from the stems but this was not considered relevant to this method for designing a biomimetic fastener.
- Specimens were collected from four separate burdock plants that grow behind the University of Bath accommodation blocks. The plants all stand in a line next to a sandy path that passes between the University grounds and the golf course.
- Note was maintained of the conditions of collection and the regions of the individual plants from which specimens were collected. These specimens were collected late in October 2003 and tested in December. It was observed that the plants themselves were brown and dry with the leafy vegetation of early season growth disappeared and the seedpod fully developed.
- Specimen hooks were collected in the form of whole fruits. The specimens were stored in paper packets until it was possible to conduct experiments upon them. (~30 days).
- It was judged that the effect of the delay between the collection and testing of the hooks would have little effect on the relative performance of the hooks and probably little effect on the absolute performance of the individual hooks given that they were collected in a naturally desiccated state and maintained in a dry condition until ready for testing, thereby preventing/inhibiting decomposition. Their desiccated state also made them ready for SEM work.
- Five individual fruit specimens (each consisting of an array of approximately 100 hooked bracts) were collected from each plant giving 20 specimens in total. A selection of these specimens was then tested.
- At the commencement of each test the burdock fruit was sectioned in half and one half labelled and stored. These are still available.
Each fruit was sectioned into halves under a dissecting microscope. One of these halves was returned to the specimen packet in case more hooks from the same specimen would be required. The other hemisphere of bracts/ovary/seeds was separated to “free up” the hooked bracts that surround the ovary.
Ten individual hooks were taken from the dispersed hemisphere. These were mounted in preparation for testing in the Instron machine by gluing each separate bract to a plastic mounting with 5mm of the bract shaft with its hook extended and exposed for interaction with a testing substrate (in this case a loop of silk thread as will be discussed). The extension was set at 1mm/sec using a 1N load cell.
Each specimen was mounted with 5mm +/- 0.3mm of shaft exposed, with the idea that all shaft lengths would be equal thereby reducing their influence upon the results. Gorb states that the shaft length is the principle main morphological variable influencing a hook’s strength. Thereafter, hook span and material flexibility are most important, the theory being that the larger the span the greater the lever arm of the bending moment and the stress at the shaft. In this case material composition is constant for all hooks.
- A rack of 5 hooks ready for testing
- Hook testing in the Instron Tensile tester. Arrow indicates looped thread.
Images were retained of all the stages of the experimentation.
- Fracture forces of burdock hooks [Height and Diameter of fruit refers to the entire fruit]. Specimens 1-3.
|Fracture Force (N)||Fracture Force (N)||Fracture Force (N)|
|Average fracture force||0.001004||0.000983333||0.00085375|
|Std dev (s)||0.031685959||0.003401055||0.000150867|
|Fruit Height (mm)||17.5||10||14|
|Fruit Diameter (mm)||23.5||15||14|
- Fracture forces of burdock hooks [Height and Diameter of fruit refers to the entire fruit] Specimens 4-6.
|Fracture Force (N)||Fracture Force (N)||Fracture Force (N)|
|Average fracture force||0.001163333||0.001168||0.00117875|
|Std dev (s)||0.002073291||0.000250516||0.000108428|
Figure 3 below shows the detachment/fracture force of each set of specimen hooks. The mean value of these results for each specimen is plotted versus the diameter of the fruit.
- Average fracture force of hooks of specimens 1-6
- The contact separation of the burrs from Gorb et al 
Figure 4: shows the results of Gorb et al’s experiments. Note how the results of C. lutetiana show the lowest contact separation force as these hooks flexed due to material resilience. Comparing these with the force results below shows that the results for A.minus fall between those of A. eupatoria and G. urbanum but significantly higher than the other species with a flexible base, G. aperine.
|Mean Hook Fracture Force (N)||Fruit height (mm)||Fruit diameter (mm)|
The SEM images of the fractured hooks in Figure 5: overleaf clearly show the fibrous nature of the hook material. The fracture surface indicates that the inner fibres failed in tension. Thereafter the outer fibres too failed in tension. The composite nature of the material is demonstrated and noting how the fracture surface fractures unevenly, there is substantial fibre “pull-out”. From Prof J F V Vincent’s notes on the mode of fracture of fibrous composites from  the fibre “pull-out” indicates that the fibre/matrix interface of the plant material consisting of cellulose microfibrils, hemi-cellulose and lignin is tightly bound. It can be seen from Figure 5 that all 4 specimens (taken to be typical examples) experienced failure on the inner curvature of the hook as would be expected for a hook of material experiencing a bending moment with the inner fibres under tension while the external fibres are in a state of relative tension about a point of rotation. The surface is typical of a fibrous composite break in bending. All hooks fractured at the region of the join between hook and shaft which is the region of the entire hook structure (both shaft and arced tip) that experiences the stress when the hook is placed in tension.
- SEM’s showing sample fracture surfaces of cellulose microfibrils
The specimen fruit size ranged from 14mm to 26mm in diameter. It was reasonably assumed that the sizes of hooks increased proportionally from fruit to fruit, including shaft length and span.
With reference to Table 4: (the graph of fracture forces) it can be seen that the hook fracture forces are all of the same order of magnitude but that there is a levelling of the slope in the region of the diameter equalling 20mm.
The separation force levels out at 1.2 mN which can be taken to indicate the ultimate tensile strength of the cellulose microfibrils and the point at which the crack accelerates across the fracture plane. More on this follows in the next section.
All hooks fractured in the region just beneath or at the commencement of curvature. This is in line with bending theory since the material at this point will experience the highest bending moment and the maximum distortion, particularly at the innermost fibres which will be in tension. These fibres will experience the highest strain as the hook tries to straighten under loading. The outermost fibres will be in compression as the hook distorts and “straightens” under loading. Once the inner fibres have failed the full load is then transferred to the outer fibres which immediately fail in tension under the significantly higher stress.
- Scheme of morphological burr variables from Gorb et al 
- Images of burr separation for each of the four species from Gorb et al  and lastly an image of a burdock hook prepared for testing with silk thread.
With reference to Figure 7: the specimen sequence is, left to right, A. Eupatoria, C. Lutetiana, G. aperine, G. urbanum, A. minus. Note the difference in diameter of wire used by Gorb and thread. Both the wire loop and the silk thread represent an artificial substrate, replacing natural fibre. Silk thread is a composite of natural silk which is much finer than fur fibres. The wire loop appears much finer than a natural fibre and the question should be asked if it contributed to severing the hook tips.
The analysis that follows derives from undergraduate engineering structural analysis. Note that although cellulose is a biocomposite, the density of the fibres in the fibre/matrix composite is such that for the purposes of analysis the hook material shall be treated as an anisotropic solid. The basic morphology of the hook shall follow the sketch in Figure 6. Dimensions for the hook are taken from the SEM below. An SEM of a fractured hook has been placed alongside.
- An SEM of an minus hook reproduced from Part I and an SEM of a fractured hook which appears again later in the Results.
Let the hook be placed under tension. The stresses in the material will comprise two components due to the tensile loading:
- Free Body Diagram and stress diagrams for the bending stress and axial stress characteristic of a hook under tensile load from Fenner 
From Part I it is noted that the A. minus hook does not taper in diameter from the top of the shaft. Instead there is an increase in material on the shoulder of the hook before it tapers to a point. This must have an influence on the hook’s behaviour in tension, increasing its resistance to flexing, increasing its attachment to its host and its disposition to fracture.
The analysis of a hook under loading is completely presented in Fenner .
Vincent  presents standard figures for the elastic modulus of cellulose as 7-15 Gpa and notes that for biomaterials the Poisson ratio is generally taken to 0.5 (though not always).
By using these figures as a maximum and minimum value and calculating the stress due the loading it should be possible to evaluate the percentage strain during the test. All dimensions are taken from Figure 8. Calculations are based upon the sketches in Figure 9.
This calculation shall be based upon the average fracture force of specimen 3 i. e.
f = 0.001168 N
E = elastic modulus = s/e = stress/strain (1)
st = tensile stress = force/area = f/A (2)
where the area is the cross-section of the hook.
A = (p *d**2)/4 (3)
Where d = 200 x 10-6
L = lever arm = d/2 + span/2 (4)
Span = 100 microns, therefore
L = 200 microns
st = (1.17 x 10-3 x 4)/(p x (120 x 10-6)2) = 1.035MPa
sbm = (100 x 10-6 x 200 x 10-6 x 1.17 x 10-3 x 4)/(p x (1202 x 10-6)2)
= 2.069 x 10-3 Pa
stotal = 1.035MPa
Now by substituting each of the range values of E = (7-15GPa) for cellulose we find emn and emax.
emax = 0.000148, emin = 0.000069
These results suggest that:
- Since the strains are so low, there must be some other reason for failure.
- The bending moment has a negligible impact upon the failure in direct stress.
If we consider shear stress, ss = Fs/A
Where A is the angled shear plane at angle q and Fs is the shear component of the applied load, then
Fcosq /Asecq = ts (5)
Let q = 30o
Then ts = ((1.17 x 10-3)cos q)/((p x d**2) x secq/4)
If G = E/(2 x (1 + u)) (6)
Then G = E/3 = (2.33Mpa, 7.5Mpa)
And g, the shear strain, where g = t/G (7)
= (13.7, 4.23)
This suggests that the hooks fail in shear due to the bending moment.
This experiment confirms that the separation force of a natural hooked structure is indirectly dependent upon the span or radius of a hook and directly dependent upon the component material’s resistance to shear.
It could be assumed in mechanical design of a hook that the smaller the span of the hook the higher the separation force since this would reduce the tensile stress due to the bending moment. But in composite biomaterials this is not the full story. The bending moment does not add substantially to the direct stress the hook experiences. The true weakness can be found in the cellulose microfibril’s low resistance to failure in shear.
From Vincent  biomaterials have two types of natural resistance to crack retardation, the Cook-Gordon model where a weak matrix/fibre interface intercepts the crack propagation and absorbs energy of crack propagation and the second is a toughening coating on the exterior of a material. In this test which was performed on an Instron tensile tester which applies load through displacement, the load is not removed or eased after the first crack appears. Loading continues at the same rate (1 mm/sec in this case). This means that there is no chance for any natural resistance to appear as the hook is steadily ripped apart.
There can be reasonable argument put that using a Poisson’s ratio of 0.5 is not accurate and this may be true. However even so, the results for strain (e) are very low. From observation of the SEM’s of fracture there isn’t any sign of delamination in compression which means that the matrix fibril interface remained intact.
This experiment helps illustrate the types of properties that shall be important in the specification of a product. Further it demonstrates (as a biological indicator) that it would be better to design a single degree of freedom hook from a composite than a multiple degree of freedom hook which would result in a decrease in strength. It must be born in mind that all the hooks in this experiment as well as in Gorb’s will demonstrate energy efficiency and therefore material and stress optimization in their structure.
- Vincent J. F. V., Structural Biomaterials, The Macmillan Press, 1982
- Devlin R. M., Witham F. H., Plant Physiology, Fourth Ed., Devlin and Witham, PWS, 1983
- Gorb S. N., Miniature attachment systems: Exploring biological design principles. Design and Nature, DN02/40799, 2002
- Gorb E, Gorb S, Contact Separation Force of the Fruit Burrs in Four Plant Species Adapted to Dispersal by Mechanical Interlocking. Plant Physiology and Biochemistry, 40, pp. 373-381, 2002
- Stamburg G., Wilson D., Veterinary Medicine http://www.llamapaedia.com/wool/
- Popov E. V., Popov V. L., Gorb S. N., Natural hook-and-loop fasteners: anatomy, mechanical properties, and attachment force of the jointed hooks of the Galium aparine fruit. Design and Nature Review Paper DN02/40800 2002
- Fenner R. T., Mechanics of Solids, Blackwell Scientific Publications, pp. 296-297, 1993
Part I: The Biomimetic study of Cellulose Hooks: Burdock (Arctium minus) OR How to make a cellulose hook that’s small and efficient.
An updated version
APPENDIX – ZOOLOGICAL BACKGROUND AND ARRANGING THE SPECIMENS AS A FUNCTION OF MATERIAL
Keywords in the Section:
systematics, paleontology, cladistics, functionality, rigid and permanent attachments,
Section 2.1 contains a description of related fields of research and a glossary of terms used within this report.
1. GLOSSARY AND GENERAL BIOLOGICAL BACKGROUND MATERIAL
All definitions related to insect morphologies are derived from Evans  and Nachtigall . Similarly for plant morphology Bell  was the main source.
These are very general background definitions of terms I encountered in my background reading. Whilst perhaps not strictly relevant to engineering, they are of scientific interest to me and so I have included them here.
Paleontology: The study of fossilized remains of plants and animals to learn about life through the geologic past.
Systematic biology: Biological information organised in a taxonomic or phylogenic manner.
Taxonomy: The science of organising living things into groups.
Phylogeny: The natural, evolutionary ancestor/descendant relationships between groups of living things. Such groups are called taxa.
Linnaeus classification: Carl von Linne used an organism’s morphology to categorise it and thereby to establish a classification hierarchy with five levels (Kingdom, Class, Order, Genus, Species). Phylum and Family were later added:
Kingdom (i.e. Plant or Animal)
Phylum (called the Division in the Plant Kingdom)
The Linnaeus system has since been superceded by a system of biological classification introduced by Carl Woese in the ‘70’s. In his new system, all living organisms are grouped into three domains:
Archaea – this group was considered to be bacteria until they were found to be different in cell compositions and metabolisms and so were given their own grouping. Many of them live in extreme environments. It is unlikely that any of these organisms will have hooks.
Bacteria – microscopic, mostly single-celled with simple cell structure, without nucleus and organelles of more complex life forms. Again, these organisms are unlikely to have hooks.
Eukaryotes – the domain of Eukaryotes contains the kingdoms of Animalia, Plantae, Fungi and Protista.
Binomial System: Organisms are identified by their genus and species name.
Arthropoda (Arthropoda = jointed foot): These are the most successful life-form on Earth in terms of variety. This phylum includes over 1 million species.
• They have a versatile exo-skeleton that is highly protective and mobile. This prevents dehydration.
• They exhibit segmentation and tagmosis (see glossary below): for more efficient locomotion, feeding and sexual reproduction.
• In land organisms the cells are directly ventilated through a tracheal system.
• They have highly developed sensory organs
• They exhibit complex behaviour and may have evolved social systems
• There may be an ecological separation of life stages thereby reducing competition by metamorphosis.
• They are able to exploit a wide variety of ecological niches.
Insects belong to the phylum Arthropoda, which also includes spiders, mites, and centipedes. The class Insecta is divided into two sub-classes, Apterygota (wingless) and Pterygota (winged).
Sub-class Pterygota is divided into two infra-classes: Paleoptera and Neoptera. Paleoptera cannot fold their wings back (dragonflies, mayflies) and Neoptera can hold their wings back against their body.
Elytra: fore-wings that have been modified such as in Coleoptera (beetles) for instance, into hard protective covers.
Cladistics: a system of phylogenic classification that uses certain features of organisms, called shared derived characters, to establish evolutionary relationships
Phylogenic tree: used to show the evolutionary relationship thought to exist among groups of organisms. A phylogenic tree represents a hypothesis, generally based upon a fossil record, morphology, embryological patterns of development, and chromosome and macromolecules.
Homologous features: similar features that originate from a shared ancestor.
Tagmosis: the evolutionary process of fusing and modifying segments.
Two Methods of Classifying Hooks
In the sections immediately following I have directly summarised from Nachtigall  and Gorb .
The world wide web was also an important source of immediate definitions and images, from the less scientific to formal research papers.
The first reference for specimens with hook attachments  was published in 1973 and has the subtitle “The Comparative Morphology and Bioengineering of Organs for Linkage, Suction and Adhesion”. The book contains a subsection devoted solely to organisms that have features that use a hooking action for attachment. In this text Nachtigall includes descriptive mechanical analogues for the biological attachment mechanisms he has studied.
My second main hook reference  was published in 2001 and concentrates on a subset of biological attachment mechanisms, those of entomology and therefore made of insect cuticle (chitin). These are generally relatively small attachment mechanisms and the text has the added feature of redressing the classification of hooks, that is, the hooks are defined not according to shape but according to functionality.
The definition of a hook according to functionality requires a direct reference to the substrate in which the hook is engaged. The environment can thus be considered to be a biological design parameter because it has a direct impact on the shape and strength of the hook.
Sections 2.3.1, 2.3.2, and 2.3.3 are lists and images of biological examples extracted from . Section 2.4 is a summary from .
Images of attachment mechanisms have been included where possible.
Classification by Morphology – Nachtigall
Rigid and Permanent Attachments
In Chapter One Nachtigall begins by defining the set of mechanical joining techniques as laid out in the table below:
Rigid Electrical plug and socket Riveted plates of a ship’s hull
Flexible Tailor’s hook and eye The hinge of a door
Table 1 – Mechanical mechanisms (Nachtigall p1)
But he groups his biological attachment mechanisms (based on morphology) into two main subsets:
a) Rigid and permanent, or
He does this because, he says, it is difficult to categorise biological attachment mechanisms according to the strict categories of the mechanical connections as per Table 1.
His set of biological rigid and permanent attachments includes:
a) Special devices: In engineering these are screws, pins or some other third device that is present only to facilitate attachment. Such types of attachment devices (with these separate, dedicated structures) do not exist in nature.
b) Amorphous bonding material: Organisms secrete some form of “glue” or “cement” that can utilise hard foreign bodies such as grains of sand to form a composite material. Examples include the gelatinous matrix that sometimes surrounds cells that form a colony in unicellular marine algae (e.g. Prymnesiophyta, Stephanosphaera, Gonium, Eudorina, Pandorina, Volvox), caddis-fly larvae which make their own matrix body sheath (e.g. Limnophilus, Phryganea) and polychaete worms that build free-standing tubes above the sand of their burrows by combining their body wall secretions with foreign bodies such as sand grains and shells debris (e.g. Arenicloa, Terebellomorphs, Sabellariids, Sabellids).
c) Softening and re-hardening of material: This definition is used to describe the growing together or fusion of one or more organisms. The quoted example is the formation of a callus when bone is healing. Nachtigall compares this process to welding together two separate parts in conventional engineering.
d) Connection by anchoring and interlacing: The roots of higher plants grow into a substrate, interweave and branch for anchorage. The secondary processes that emerge such as enlargements or branching take on a secondary anchoring function.
Fish parasites of the Phylum Arthropoda such as Copepods, Branchurians, Isopods, mites and bi-valves can have outgrowths that penetrate into the flesh of the fish. Commonly they will inhabit the gills, the mouth and the outer skin of a fish. Figure 2 below is two SEM images of Argulus, a sea louse that is an ectoparasite with modified maxillae that grow “downwards” between the scales of a fish and into the underlying flesh for anchorage. In some species these can penetrate through the flesh to the heart of the fish. (Figure 2)
Figure 2 – i) and ii) Argulus, branchurian parasite of fish. The maxillae are modified for attachment (indicated by arrow A) [I].
e) Interlocking joints and mitre interlocks: These are found between the skeletal plates of barnacles where two plates have their edges shaped so that they fit snugly together. Beetle elytra (wing covers) have bevelled edges that fit together and secondary interlocking is provided by the fitting of tooth-like projections of one plate into corresponding holes on the other.
There can be simultaneous coarse and fine interlocking where ridges and grooves have a double interlock because the ridge is slightly over-sized. If the ridge and groove are not straight it gives additional interlock strength. The Balanus improvisus barnacle has a rabbet joint with tooth-like secondary interlock. (Figure 3, Figure 4 and 5)
Figure 3 A – Miter joints between basal and lateral plates of balanids,
B – Halved joint in a balanid, C – Compressed aggregation of balanids (Nachtigall p15)
Figure 4 – i) Giant Acorn barnacle Balanus nubilis ii) Acorn barnacle structure [II] and [IIIIIIII]
RIGID AND PERMANENT – SPECIES FOR FURTHER STUDY
a) Hydroporos ferrugineus (water beetle) – the elytra have a rabbet joint with a double tongue and groove (Figure 5).
Figure 5 – Hydroporous ferrugineus (Nachtigall p16)
76 – interlocking of abdomen and elytra
77 – interlocking of scutellum and elytra
78 – double tongue and groove joint between the elytra
b) Lamellicorn beetles have 15 clasps on their elytra.
c) Stephanolepas (a type of barnacle) has a mitre joint with deep tongue and groove connection.
d) Carabidae ground and tiger beetles are large beetles with a permanent mortis joint between elytra.
e) Cnemidotus water beetles have elytra with a locking mechanism.
f) Gygrinidae (whirligig beetles), hydrobiidae (spiral snails), haliplidae (crawling water beetles), dryopidae (water beetles) all have elytra that are watertight.
g) Siliphidae (carrion beetle) have sealed elytra to prevent moisture loss in their arid environments.
Releasable attachments of two matchng structures
Nachtigall defines releasable attachment mechanisms as those mechanisms allowing two different structural components to be quickly coupled and decoupled. The two parts are held together firmly as long as the connection is maintained such as key-in-lock or plug-in-socket type mechanisms with an exact morphological correspondence between mating parts.
For example, the sex organs of copulating mosquitos, dragonflies and mayflies he describes as resembling an electrical socket in functionality. They are able to maintain the connection once it has been made against the vibrations and disturbances of in-flight copulation because their sex organs have structures that match and interlock.
He groups both rigid and flexible releasable attachments together. By his classification system, the group of releasable attachments (both rigid and flexible) is made up of two main groups:
a) Connection by two complementary parts, and
b) Attachment by one specialised device.
• snap fasteners and
• multiple connections
as mechanical analogues to some of the complentary-part devices. These are described in the next four sub-sections.
A rod-like component is introduced axially into a corresponding tube-like component. The connection is secured against 3 kinds of displacement by the use of:
a) A tongue and groove against rotation about the longitudinal axis.
b) Guide channels with matching surfaces to prevent tilting with respect to the longitudinal axis.
c) External clamping and internal anchoring to prevent displacement along the longitudinal axis. (Figure 6)
Figure 6 – Plug and socket analogue in copulation of the midge Limnophyes pusillus (Nachtigall p29)
Some examples of organisms that have plug and socket-type joints are:
a) copulating insects such as the dragonfly,
b) crocodiles, with teeth that fit into holes in the opposing jaw,
c) the undulating ridges of large clams Tridacna,
d) the midge Tanytarsus sylvaticus, and
e) copulating yellow fever mosquito’s Aedes aegypti. (Figure 7)
Stage One Stage Two Fully-docked!
Figure 7 – Egyptian mosquitos Aedes aegypte approaching the copulatory position with the final engagement of two hook mechanisms (Evans p23).
These connections are accomplished by engaging the hook of one body part with the eye of another and they are good for tension but not for tilt or rotation.
The presence of guide grooves can prevent tilt in some species and rotation may be prevented by the presence of two hooks of the same type next to each other but with the greatest possible distance of separation between them to increase the opposing rotating moment. For example, bird mites have genitalia that lock together, with a flange-capsule structure on the female dorsal surface matching capstan-like protuberances on the posterior ventral surface. In this case the female flange guides the male capstan into the correct “docking” position for insemination (see Figure 8 – (Nachtigall p35)).
Figure 8 – (Nachtigall p35)
A – Bird mite Dermanyssus [IV]
B – In copulation the male slides over the top of the female to engage sex organs
C – Capstan and flange of engaged bird mite sex organs (Pterophagus strictus)
a) As per common engineering terminology, the male is the peg that is expanded at the end and the female is a socket of the same diameter as the peg.
b) In squid, the mantle is joined to the body by two snap fasteners. The female socket (mantle) will have an inner rim reduced slightly by some spring arrangement (such as cartilage) to maintain a seal when the muscular mantle contracts and seals to expel water under pressure down the funnel, for propulsion through the water. e.g. squid mantles Symplectoteuthis and Grimalditeuthis, Cranchidae, Oegopsidae. (See Figure 9 – Snap-type connection in Sepia officinalis (Nachtigall p38)
Figure 9 – Snap-type connection in Sepia officinalis (Nachtigall p38)
c) The tentacle suckers on the two long tentacles of some squid species have a modified system of suckers, with multiple hooks for holding prey. A system of studs and sockets on the tentacle surfaces above and below the hooked sections of each tentacle interact and connect when the prey is clasped by an opposing tentacle, thus aiding the clasping effort. See squid Onychoteuthis, Abralia, Galiteuthis.(see Figure 10 through to Figure 14)
Figure 10 – Tentacular fasteners (Nachtigall p38)
Figure 11 – Squid Abralia [VVIII]
Figure 12 – Squid Onychoteuthis [VI]
Figure 13 – Squid Galiteuthis glacialis [VI]
Figure 14 – Squid Galiteuthis glacialis (drawing) [1VVI]
A mechanical example of multiple connections is a zipper, where a separate slider draws two edges of complementary morphologies together. Such sliders do not exist in nature and from the notes in Nachtigall it would seem that multiple connectors are mostly modified systems of alternate teeth, that vary greatly in modifications from simple interlocking rows of a few teeth to the interlocking of the two hemielytra of the north American water bug Plea striola where a great degree of interlocking is required to maintain the water tight seal when the bug is submerged.
Radiolaria are protista, uni-cellular organisms known for their geometric form and their symmetry because they produce a skeleton of crystal silica. They date back to the Cambrian age and some use delicate interlocking shell margins of hooks and eyes for attachment.
Figure 15 – Radolaria (Nachtigall p42)
Releasable attachements by one structure
c) hooks for special substrates,
d) multiplehook devices,
e) probabilistic fasteners, and
f) expansion fastenings.
CLAMPS – (BIOLOGICAL ANALOG: THE STRUCURES ON PREHENSILE LEGS OF INSECTS)
• Vices – e.g pincer wasps, family Dryinidae, mantids.
• Split-sleeve clamps: consider the notched antenna cleaner in foreleg of honey bee (see Figure 16 and Figure 17).
Figure 16 – Antenna cleaning apparatus of honey bee (Apis mellifera) (Nachtigall p38)
Figure 17, 1-4: SEM’s of the vice mechanism of the praying mantis, zooming in on a single tooth to show surface morphology (Saunders, 2002).
• Forceps and medical equipment: consider the beaks of many birds – they have been copied in many surgical instruments that perform similar tasks. (see Figure 18)
Figure 18 – Clockwise from top left: An eagle, a vulture, Anarhynchus frontalis, a spoonbill (x2) (Nachtigall p52)
• Nutcracker type
• 4 Jaw grippers
• Antennae of some insects have prehensile joints that can be used for gripping the mate during copulation.
HOOKS FOR SPECIAL SUBSTRATES:
Hooks are used to:
• Connect insect wings reversibly with one another
• To attach the body of an animal to some substrate
• To manipulate particles of food and other objects.
In the case of wing connectors (see Figure 19), Lepidopterans (butterflies and moths) have 2 types of hook mechanisms, jungate (the hooking mechanism extends from the forewing) and frenate (the hooking mechanism extends from the hindwing).
Figure 19 – Wing connections jungate (A) and frenate (B) (Nachtigall p61)
Hooks are also used to join the forewings and hindwings of:
• Hymenopterans (bees, wasps, ants, sawflies), particularly insects with 2 pairs of wings that beat with a high frequency such as bees (Figure 20), shield moths (which both have finely matched hook mechanisms) and hawkmoths (Manduca sexta) (Figure 21 – Manduca sexta (Hawkmoth) in flight). The honeybee Apis mellifera has a single row of fine hooks (hamuli) on the costal vein of the hindwing which catch upon the undercut ridge of the posterior margin of the forewing.
Figure 20 – The connections between the wings of Apis mellifera seen from above and a honey bee in flight. (Nachtigall p61) [VIIIVI]
Figure 21 – Manduca sexta (Hawkmoth) in flight [VII]
• Pentatomid beetles of the genus Palomena have a snap mechanism against tensile stress worth looking at. Also Graphosoma italicum. (Figure 22 and Figure 23)
Figure 22 – Shield bug Palomena [XVIII]
Figure 23 – mating shield bugs Graphosoma [IX]
• Pyrrhocoridae (red bugs, Figure 24), winged heteropteran bugs Gerridae (water striders, Figure 25), and Homoptera (cicadas, leafhoppers, aphids, scale insects and mealy bugs etc) similarly, all having hooking mechanisms clasping their elytra together.
Figure 24 –pale cotton stainer bug Pyrrhocridae: Dysdercus sidae [X]
Figure 25 – water strider Gerridae [XI]
• Similaly, european Ciccada Triecphora vulnerata (Cercopidae, Figure 26) and Snakeflies (Raphidioptera, Trichoptera, Mecoptera, Figure 27) and sand flies Rhyacophila dorsalis have wing connectors.
Figure 26 – female cicada T.pruinosa [XII]
Figure 27 – Scorpion fly (Mecoptera panorpidae) XVXIII]
Sessile animals have special clinging or fixation organs:
• The oncomiracidium (Phylum Platyhelminthes, Class Monogenea) (Figure 28) are generally ectoparasites in the gills or body surface of fish and typically dorso-ventrally flattened, acoelomate with no anus and bilaterally symmetrical, or endoparasitic in the buccal cavity, cloaca or bladder. They have a large posterior sucker or opisthaptor as an attachment mechanism which is a disc with a double circle of hooks, each hook like a “halberd”with a shaft, sharp cutting edge and a terminal process on one side.
• Branchiuran crustacean or carp lice have suckers and angled hooks at the base of the first pair of antenna (Figure 29).
Figure 28 – Oncomiracidium (drawing) [XVIXIV]
Figure 29 – fishlouse Branchiuran crustacean (also see Figure 2) [XV]
• Blood-sucking isopods: Gnathia live on fish and have boat-hooks and harpoon hooks to grasp host fish and attach themselves to them (Figure 30).
Figure 30 – Gnathia maxilaris [XVI]
• Kalyptorhynchia (turbellarian worms) have conical pincers which have a pair of hooks at the tip which they dig into the flesh of prey.
• There are also instances of two rows of hooks acting in opposition to each other, for example, Sea stars (Pectinate pedicellariae).
• Peacock, heron, dipper and woodpecker tongues are horny with edges with keratinised teeth. These teeth are saw-like and serve to prevent the prey working loose when they are speared by the tongue. (Figure 31)
Figure 31 – Woodpecker tongue showing keratinised teeth [XVII]
There are many climbing plants of which the most successful make use of spines or hooks to attach themselves to the host plants.
• Gleichenia linearis is a tropical rain forest fern and palms of Plectocomia (Figure 32).
Figure 32 – Plectocomia himilayana showing climbing spines [XXXVIII]
• Uncaria have climbing hooks on their stems that anchor into rough or moldering substrates (Figure 33).
Figure 33 – Uncaria with the red arrow indicating typical hook position [XXIXIX]
• The stalk of the hop plant has longitudinal climbing hooks and the runner bean has climbing hairs in the shape of crampons.
• Trypanorhyncha (tapeworms of the cestode order) (Figure 34) have arrays of hooks, macrohooks, microhooks, microhooklets and hook chains.
Figure 34 – tapeworm trypanorhyncha lascisthynchus [XXIIXX]
PROBABLISTIC FASTENERS (RANDOM HOOKING OR “BURR”-TYPE DEVICES)
Nachtigall has divided this group into devices obeying
a) The Burr Principle (having hooks),
b) The Comb Principle (no hooks),
c) The Feather Principle (barbs and barbules) or
d) The Microhook Principle (fields of tiny hooks)
These devices are described below.
a) Burr principle
• Species Arctium lappa (burdock) (Figure 35)
• Agrimonia eupatoria The fruits of both of these consist of round fruits with rings of barbs having long shafts and single barbs. (Figure 36)
Figure 35 – Burdock Arctium lappa [XXIIIXXI]
Figure 36 – A. eupatoria [XXII]
• Cynoglossum spine tips have a double anchor.
• Galium (bedstraw) has thousands of fine barbs on its climbing stem.(Figure 37)
Figure 37 – Bedstraw Galium [XXIII]
• Bellostoma stouti (a type of fish) lays eggs that have bundles of hornlike threads at each end with “buttons” at the tip. In water these anchor filaments extend to catch the hooks of nearby eggs.
• Acanthocephalan worms have a proboscis that can invert so that the ring of hooks points inwards. When internal pressure is increased the teeth appear “at the margin” and are turned to the outside where they catch into the villi of the host’s gut. It uses a peristaltic action to manoeuvre through the gut. (Figure 38)
Figure 38 – Echinorhynchus salmoides acanthocephalan worm [XXIV]
• Kinorhyncha (of the class Aschelminthes) have a crest of hooks on their heads. They use this as a “mud” anchor which anchors them to or helps them to move about, on the sea floor. See also echiuroid worms.
• Cat and cow tongues as a brush and curry-comb since the tongue is covered in fields of “re-curved, horny teeth”.
b) The Comb Principle
Consider rough hair being combed with a very fine tooth comb. The comb snags because hairs running at angles to one another become entwined. This principle is used in ecto-parasitic insects living in the fur of vertebrates. The combs are called ctenidia and are one or more rows of closely and evenly spaced bristles.
• Siphonaptera (fleas)
• Nycteribiidae (batflies)
• Polyctenidae (bat bugs)
• Platypsyllus castoris (beaver beetle)
c) Feather Principle
The vane of a feather consists of a number of side branches called barbs. Each barb has “second order side branches” called barbules.
A feather vane can be broken up into segments by pulling in a direction perpendicular to the barb longitudinal axis.
Figure 39 – a) hook barbule b) bow barbule c) probabilistic fasteners on hook and bow barbules interlocked (Nachtigall p75)
One side of the barbule has hook barbules and the other has rows of bow barbules. It is a probabilistic fastening with no particular matching of elements required. The vane is allowed to separate under strong local pressures into segments because otherwise the vane would be destroyed. (Figure 39)
The hook barbule has small “inward curving spurs at its distal end” to prevent the attachment from separating by axial sliding.
d) Micro hook Principal
Geckonidae and some Iguanidae (Anolis) have “leaf-like broadenings of fingers and toes called digital pads or discs” (p78) (even on the tip of the tail sometimes). The surfaces of these pads have transverse lamellae that are covered in dense rows or brushes of tiny bristles. Increased blood pressure in the capillary system of the pads causes the brushes to be pressed firmly against the substrate. Each bristle in turn has a tuft of hundreds of minute processes with tips bent like hooks. These hooked tips are thought to interact with the most microscopic of surface irregularities and to utilise forces of a electrical (surface atomic and molecular charges) or capillary (gluing with water) nature.
Classification by Function – Gorb
Gorb follows a classification definition based on functionality.
For the class of insect cuticle attachment devices he defines the functions:
a) Hooking to the substratum
b) Animal associations: phoresy (the behaviour of animal dispersal using other animals), parasitism, predation
c) Hooking within biological tissues
d) Attachment during copulation
e) Interlocking of body parts
Figure 40 below illustrates 8 fundamental classes of fixation principles including hooks, lock or snap, clamp, spacer, sucker, expansion anchor, adhesive secretions, friction.
Figure 40 – Eight fundamental classes of fixation principles: hooks (A), lock or snap (B), clamp (C), spacer (D), sucker (E), expansion anchor (F), adhesive secretions (G), friction (H) (from Gorb p38)
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.”
HOOKING TO THE SUBSTRATUM
This function is viewed in terms of claws to aid locomotion or to provide anchorage. For example in terrestrial locomotion the tarsal claw/s interlock with the surface textures to generate friction. Orb-web spiders have claws with a comb-like serrate bristle edge. This plays an important part in interlocking on the silk thread. Some butterfly pupae (e.g. Thyridopteryx ephemeraeformis, Lepidoptera, Psychidae) have hooks at their posterior end to anchor them inside their cocoon. Hooks are also used to attach material to an animal body (the crab Loxorhynchus crispatus decorates its shell by hooking with specialised setae).
ANIMAL ASSOCIATIONS: PHORESY, PARASITISM, PREDATION
Bird mites (Order Acari genus Michaelicus) parasite onto bird feathers. They have an asymmetric design of their legs with only one leg having a tarsus with a claw, which they feed through the barbules of the feathers to find anchorage. Hook-like devices are found only on mites parasitizing onto the stiff parts of the feather whereas mites that live on the soft parts of the feather use a clamping device.
Copepoda parasitica are parasitic copepod crustaceans with hooked appendages for attachment to host and some species of fish lice use hook-like appendages to attach themselves to their hosts.
HOOKING INTO BIOLOGICAL TISSUES
Gorb lists copepod crustaceans, ixodid ticks and dipteran insects as having mouthparts with hook-like structures for attachment to hosts. Also parasitic copepods such as Hatschekia pseudohippoglossi and Trebius clidodermi.
ATTACHMENT DURING COPULATION
Gorb adds the attachment structures of Harpocera thoracica to the organisms mentioned previously in section 2.2.2.
18.104.22.168 INTERLOCKING OF BODY PARTS
This category mainly includes wing-connectors. (Figure 41)
Figure 41 – Wing inter-lock devices in Heteroptera and Auchenorrhyncha (Gorb p45)
MECHANICAL PROPERTIES OF BIOLOGICAL MATERIALS: A BRIEF DISCUSSION OF THEIR ORIGINS IN BIOLOGICAL ATTACHMENT SYSTEMS
The mechanical properties of a biological material have their origins at molecular level.
Self-assembly is an area of study all of its own. From what I have gathered from the literature it is the process (or a portion of the process) by which biological materials grow. Molecules come together to form the required structures which develop into the forms and structures that make up the organism.
Currently, research into self-assembly has developed to the point where nacre can be manufactured (it is being developed for use in rocket exhaust nozzles for its insulating properties as a ceramic) but it has not led to the harvesting of free-standing structures possessing biomimetic properties See Benyus .
I discuss growth in relation to biological hooks in the final discussion and conclusions of this report.
The behaviour of a structure does not depend solely upon its shape and material strength; some structures in nature are actually made up of a single, small structure that is repeated many times. This has been described as a field of structures such as those used in probabilistic fasteners (see Section 22.214.171.124). These fields of structures can possess further behavioural properties deriving from their proximity to one another and their order of size magnitude which brings into play intra-molecular forces such as friction, charge and capillary action (surface tension) due to the presence of moisture.
The dragonfly head-arrestor mechanism as will be discussed in Case Study 2 is itself not a single structure that performs the attachment but instead a field of the structures that intermesh in a somewhat “unplanned” fashion. Its structures are not hooked, in fact they are flattened projections that use friction as a retaining force.
An overview of the biological material properties presents us with the conclusion that the overall behaviour of a structure is a summation of the properties of the component structures. And further, with regard to the small (micro and smaller) structures that many biological attachment mechanisms are, the resultant attachment force is the sum of component forces, sometimes with contributions from unexpected sources.
1. “Insect Biology A Textbook of Entomology”, H E Evans, 1984, Addison-Wesley, ISBN 0-201-11981-1
2. “Plant form An Ilustrated guide to Flowering Plant Morphology”, A E Bell, 1991, Oxford University Press, ISBN 0-19-854219-4
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. “Biomimicry Innovation Inspired by Nature” J M Benyus 1997, William Morrow and Company, ISBN 0-688-13691-5
I. http://www.aquaculturemag.com/siteenglish/printed/archives/issues03/03articles/HeckmanFeatureForWeb.pdf Argulus figure 2
II. http://www.mov.vic.gov.au/crust/barnbiol.html Acorn barnacle figure 3
III. http://www.enature.com/fieldguide/showSpecies_LI.asp?imageID=19348 Giant Acorn barnacle figure 4
IV. http://medent.usyd.edu.au/fact/birdmite.html figure 9
V. http://tolweb.org/tree?group=Abralia&contgroup=Enoploteuthidae figure 11
VI. http://tolweb.org/tree?group=Galiteuthis&contgroup=Cranchiidae figure 13
VIII. http://www.floridanature.org/species.asp?species=Apis_mellifera figure 20
IX. http://www.bioimages.org.uk/HTML/T1457.HTM shield bug figure 22
X. http://www.geocities.com/pelionature/Graphosoma_italicum2.htm figure 23
XI. http://www.insectimages.org/browse/family.cfm?id=Gerridae figure 25
XII. http://eny3005.ifas.ufl.edu/lab1/Homoptera/Homoptera.htm figure 26
XIII. http://www.lib.ncsu.edu/agnic/sys_entomology/taxon/raphidiodea/#order figure 27
XIV. http://bioweb.uwlax.edu/zoolab/Table_of_Contents/Lab-4a/Class_Monogenea/class_monogenea.htm figure 28
XV. http://www-biol.paisley.ac.uk/courses/Tatner/biomedia/pictures/fishl.htm figure 29
XVI. http://www.konig- photo.com/english/galerie/zoom.asp?pre=8206&NumPhoto=8207&suiv=8208&Rub=483 figure 30
XVII. http://www.hiltonpond.org/ThisWeek030308.html woodpecker tongue figure 31
XVIII. http://www.palmsoftheworld.com/plec.htm plectocomia himalayana figure 32
XIX. http://www.pharmakobotanik.de/systematik/7_bilder/yamasaki/Uncaria.jpg figure 33
XXI. http://botanical.com/botanical/mgmh/b/burdoc87.html burdock figure 35
XXII. http://www.bioimages.org.uk/HTML/P146161.HTM agrimonia eupatoria figure 36
XXIII. http://plants.usda.gov/cgi_bin/large_image_rpt.cgi?imageID=gaap2_002_avp.tif galium figure 37
XXIV. http://www.inhs.uiuc.edu/chf/pub/surveyreports/nov-dec96/acanth.html acanthocephalan worm from rainbow trout figure 38