© 2002 WIT Press, Ashurst Lodge, Southampton, SO40 7AA, UK. All rights reserved. Web: www.witpress.com Email [email protected] Paper from: Design and Nature, CA Brebbia, L Sucharov & P Pascola (Editors). ISBN 1-85312-901-1

Miniature attachment systems: exploring biological design principles

S.N. Gorb Biological Microtribology Group, Division 14 MPI of Developmental Biolop, Tuebingen, Germany.

Abstract

One of the greatest challenges for engineering science today is miniaturisation. and other have solved many problems correlated with small size during their evolution. A variety of biomechanical systems of insects, adapted for attachment of parts of the body to each other or attaching the organism to a substrate, are the main topic of the present paper, There are eight fimdamental classes of attachment principles: clamp, spacer, sucker, expansion anchor, hooks, lock or snap, adhesive secretions, and friction. Different combinations of these principles occur in the majority of biological attachment structures. Friction-based probabilistic fasteners provide precise reversible coupling of surfaces with a minimum expenditure of force, Patent databases contain a huge number of ideas dealing with applications of existing fasteners. However, most of these applications use the same types of available hook-like tapes. Biological systems provide a variety of microscale surface patterns, which may serve as a source for Mure prototyping of novel types of releasable fasteners and micro-fasteners. An engineering approach is to copy the surface profile using available technologies. As an initial stage of prototyping diverse surface microsculpture, the low-viscosity wax cast technique is applied to produce surface casts, Since forces in the contact areas of most biological systems have not been previously measured, the fust step in this direction is taken toward identification of the interesting properties of systems. This approach combines the knowledge of biologists and the measuring techniques used in material science. There are three main areas, in which Nature’s solutions of attachment problems may be applied: (1) precise mechanics, (2) gluing and joining technology, and (3) material science of surface-active composite materials. © 2002 WIT Press, Ashurst Lodge, Southampton, SO40 7AA, UK. All rights reserved. Web: www.witpress.com Email [email protected] Paper from: Design and Nature, CA Brebbia, L Sucharov & P Pascola (Editors). ISBN 1-85312-901-1

124 DesignandNature 1 Why study biological attachment?

Throughout evolution, Nature has constantly been called upon to act as an engineer in solving technical problems. Organisms have evolved an immense variety of shapes and structures. Although often intricate and Ii-agile, they can, nonetheless, deal with extreme mechanical loads. Many tlmctional solutions are based on a variety of ingenious structural solutions, Biologists have collected a huge amount of information about the variety of biomechanical systems adapted for attachment of parts of the body to each other, or attaching the organism to a substrate. Understanding these is of great scientific interest, since we can learn about their use as structural elements and their biological role and function, This knowledge is also highly relevant for technical applications. Information on biological prototypes can be utilised to mimic them for industrial applications. There are three main areas where Nature’s solutions of attachment problems may potentially be applied: (1) precise mechanics, (2) gluing technology, and (3) material science of surface-active composite materials. Possible innovations may also appear at the boundaries of the named areas. Also, knowledge about the properties of biological systems might be useful for pest control, by modifying plant surfaces, In the present paper, we give a short overview of the functional design of attachment devices occurring in insects and how Nature’s design may be used as a basis for biomimetics in various technological areas,

2 Biological attachment devices

Biological attachment devices are fi.mctional systems, the purpose of which is either temporary or permanent attachment of an organism to the substrate, to another organism, or temporary interconnection of body parts within an organism. Their design varies enormously and is subject to different functional loads [1], Design and functional principle depend on the biology of the concrete species. The evolutionary background and the habits influence the specific composition of attachment systems in each particular species, In insects and other arthropods, cuticle and its derivatives play a crucial role in the design of the devices, among which eight fimdamental classes of attachment principles have been recognised: (1) hooks, (2) lock or snap, (3) clamp, (4) spacer, (5) sucker, (6) expansion anchor, (7) glue, and (8) friction [2]. Some of them, together with a biological example, are illustrated in Figure 1. Additionally, different combinations of these principles occur in existing attachment structures, Most attachment devices are composed of microscopical structures and driven by muscular force, However, many systems have involved surfaces with particular frictional and adhesive properties. Generally, any movement involving contact between two surfaces or between a surface and a medium deals with the resistance of the surfaces or medium, This resistance is called fi-iction, a phenomenon which has a great influence on the design of biomechanical © 2002 WIT Press, Ashurst Lodge, Southampton, SO40 7AA, UK. All rights reserved. Web: www.witpress.com Email [email protected] Paper from: Design and Nature, CA Brebbia, L Sucharov & P Pascola (Editors). ISBN 1-85312-901-1

Design and Nature 125 structures. Living creatures posses specialised surfaces enabling the minimisation of contact forces (anti-friction systems) or their maximisation (friction systems) (Fig. 2). It is interesting that in both cases the resulting task of such a system is to save muscular energy, One always needs friction to generate force for overcoming the drag caused by friction in other parts of the system. Optimisation then becomes the exercise of minimizing friction at one end of the systeu while maximizing it at the other [3], For example, in the case of terrestrial locomotion, for effective propulsive movements, a high ffiction is necessary for contact of the limbs with the substratum and a lower friction – within the joints of the limbs.

A

Figure 1, Attachment rnicrostructures of body based on different principles, Left panels show diagrams explaining principles of attachment in structures shown in right panels, A, Hooks of the wing locking mechanism of the femoratus, B, Lock of the head in the damselfly Pyrrhosoma nymphula. C. Friction-active structures of the head- arresting mechanism of the dragonfly Aeshna mixta, D. Microsuckers of the fust legs in the males of the beetle Dytiscus inarginatus. E, Soft pads of the grasshopper Tettigonia viridissima. F. Hairy pads with anisotropic terminal elements on the legs of the beetle Rhagorzycha fulva, Dark-gray is an body; light-gray indicates a substrate.

Among various cases of contact pairs in biology, anti-friction systems always have a predefine pair of surfaces, whereas, among friction systems, there are some that deal with predefine surfaces, and others, in which one surface remains unpredictable, The first type of friction system occurs, for example, in wing-locking devices and head-arresting systems and is called probabilistic fasteners [4-6], The second type is mainly represented by insect attachment pads © 2002 WIT Press, Ashurst Lodge, Southampton, SO40 7AA, UK. All rights reserved. Web: www.witpress.com Email [email protected] Paper from: Design and Nature, CA Brebbia, L Sucharov & P Pascola (Editors). ISBN 1-85312-901-1

126 of two alternative designs: hairy and smooth [7], The relationship between surface patterns and/or mechanical properties of materials of contact pairs results in two main working principles of the frictional devices: mechanical interlocking and maximisation of the contact area (Fig. 2). Since biological surfaces are a part of the physical world, most of the friction and adhesion phenomena in biomechanical systems can be explained by mechanical interlocking andlor area of contact between surfaces, independent of the basic physical forces involved in the particular attachment mechanism, This indicates that the geometry of the surface, load forces at which the system operates, and mechanical properties of material will play essential roles in the design of the actual system [8-10], In addition, chemistry of surfaces, the presence and nature of secretory fluids additionally mediate surface forces. Since friction and adhesion are very complex physical phenomena, the biggest challenge in studying them in biological systems is to collect maximum information about gross morphology, ultrastructure, chemistry, and mechanics of surfaces to explain the functional principles of particular attachment systems (Fig, 3).

friction anti-friction systems systems / -’% 4 surfaces are one sulfaca is swfaces are pmt%fined mpredictabie pmdofined J/ --m My wnooth

w~ mm,

Figure 2. Functional significance and working principles of contacting surfaces in biological objects. Living creatures posses specialised surfaces enabling the minimisation of contact forces (anti-friction systems) or their maximisation (friction systems), Among such systems, there are some composed of two co-opted (yredefmed) surfaces, whereas in others one surface remains unpredictable, The relationship between surface patterns and/or mechanical properties of the material of contact pairs results in three main working principles: (1) mechanical interlocking, (2) maximisation of the contact area, and (3) minimisation of the contact area, © 2002 WIT Press, Ashurst Lodge, Southampton, SO40 7AA, UK. All rights reserved. Web: www.witpress.com Email [email protected] Paper from: Design and Nature, CA Brebbia, L Sucharov & P Pascola (Editors). ISBN 1-85312-901-1

Designand Nature 127 3 Precise mechanics and fastening technology

Biological systems may provide many innovations for material design and their surface structuring. Since many electro-mechanical systems (MEMS) become smaller and smaller, many systems, known from the existing macroscale devices, have to be miniaturised in different ways. Attachment of parts of the MEMS can be achieved by gluing parts together, but sometimes releasable attachment fasteners are also required. Natural attachment systems may also be a rich source for surface patterning on a macro scale as, for example, in the tyre industry.

-wGeneral design of the system - classical morpho!qy Architecture of material - Ultrastructoraf .wAies Frictional and adhesive properties m Tti@Iwy Mechanical properties of material - Material wiemx Nature of secretions * Ghemistw Motion analysis of living systems * Somwhani@ n

Figure 3. Interdisciplinary character of studies of biological attachment systems. Data integration fi-om several disciplines and on both global and local scales are required.

The hook-and-loop fastening principle was initially transferred to industrial applications from biological systems, namely from the hooks of a plant’s fruits and seeds interlocking in animal hairs and fur. The hooks compose the rigid part of the fastener. The corresponding surface is made of a textile tape consisting of thin, flexible loops. Presently, there are two main types of such Velcro fasteners: (1) classical hook-and-loop type, and (2) mushroom-and-loop type, in which hooks are replaced with mushroom-shaped structures, Patent databases contain a huge number of ideas dealing with applications of existing fasteners. They are almost universally used in textile industry, medicine, sports, transport, etc. However, all of these applications use the same types of available hook-like tapes. Natural systems provide a variety of microscale surface patterns, which may serve as a source for future prototyping of novel types of releasable fasteners, and microfasteners. Since forces in the contact areas of most natural systems have not been previously measured, the first step for biologists in this direction should be identification of interesting properties of natural systems, This requires a knowledge about biology of the systems and measuring techniques used in material science. It is necessary to study the mechanical properties of natural material, of which systems are made, and the range of forces holding contacting surfaces together (Fig, 3). © 2002 WIT Press, Ashurst Lodge, Southampton, SO40 7AA, UK. All rights reserved. Web: www.witpress.com Email [email protected] Paper from: Design and Nature, CA Brebbia, L Sucharov & P Pascola (Editors). ISBN 1-85312-901-1

128 Design and Nature Walking machines usually use suckers to hold onto vertical surfaces, and hold under a surface. A primary disadvantage of this attachment principle is that a very smooth substrate surface is required, The future goal should be walking robots, able to attach to a variety of surfaces. Insects can walk rather well on smooth and structured substrata, on inclines, vertical surfaces, and some of them even on the ceiling. Hairy and smooth leg attachment pads [11] are promising candidates to mimic into robot soles adapted for locomotion. Similar principles can be applied for design of microgripper mechanisms, with an ability to adapt to a variety of surface profiles [9, 12], One would hardly expect a real innovation to improve, for example, existing tyre profiles. However, Continental@ has developed a winter tyre with honeycomb profiles similar to those existing on the attachment pads of the grasshopper Tettigonia viridissima [13]. Such a pattern provides less aquaplaning, better braking on wet roads, substantially improved lateral guidance, better grip, and more traction on ice. If one is going to create attachment systems based on principles similar to biological systems, it is necessary to evaluate which materials would be the best candidates (1) to mimic such a wide range of properties of natural materials, and (2) to allow similar possibilities of combining subunits with different material properties into a structural continuum with local mechanical differences, Among thousands of existing polymers, polyurethane are possibly the most appropriate class of materials for this purpose. Polyurethane have a wide range of material properties depending on their chemical composition and fiwther foaming, Furthermore, technology of local foaming provides the possibility of varying material properties in the same piece of material, Presumably, polyurethane foams can be used to mimic biological attachment devices with visco-elastic properties,

4 Microreplication technique

An engineering approach, applied after detailed studies on the natural system, would be most promising, However, engineers can also simply copy the surface shape of a variety of scales and materials using available technologies of chemistry and processing. Both approaches may run parallel for some time and possibly converge later, An overview of modem technologies, which may be applied for prototyping of diverse surface rnicrosculpture, is given elsewhere [14]. A simple technique, using a two-component fluid silicon, can be applied to produce negative surface casts of a living surface at room temperature. Positive casts can then be obtained in an epoxy resin. Fluid silicon, applied on a biological surface, spreads very quickly over the sample and fills surface irregularities (Fig. 4 A-B). After hardening (2-5 rein), the biological sample can be easily removed without any damage, because of a high elasticity of the hardened wax (Fig. 4 C-D). Then the wax casts are filled with Spurr’s resin [15], or other low viscosity epoxy resin, which, after polymerisation at 60-70”C for 24 h, becomes relatively stiff and again can be removed from the {{negative>)without damage (Fig. 4 E-F). The {megative)) © 2002 WIT Press, Ashurst Lodge, Southampton, SO40 7AA, UK. All rights reserved. Web: www.witpress.com Email [email protected] Paper from: Design and Nature, CA Brebbia, L Sucharov & P Pascola (Editors). ISBN 1-85312-901-1

Design and Nature 129 replica can be used again to fabricate a number of resin positives>>. This method delivered good results with relatively long cuticle outgrowths (10-30 pm), as well as with small structures (0.02 pm) (Fig. 4 G-I), Such replicas can be used to test attachment properties of surfaces defined by shape, By varying the resin composition, polymerised material can demonstrate a variety of material properties, from the very hard to quite soft. By using samples with the same shape and different mechanical properties of material, dependence of attachment forces on material properties at a given surface profile can be tested. The main advantage of this method of prototyping is that the area of the replica corresponds exactly to the original area of the biological sample. This could also be a big disadvantage because if an interesting structure pattern is located on a wavy surface, its shape will be repeated exactly in the cast. This may cause difficulties during sample positioning during force testing.

natural hardened

fluid natural fluid hardened silicon rubber surface epoxy resin silicon rubber

E

F © 2002 WIT Press, Ashurst Lodge, Southampton, SO40 7AA, UK. All rights reserved. Web: www.witpress.com Email [email protected] Paper from: Design and Nature, CA Brebbia, L Sucharov & P Pascola (Editors). ISBN 1-85312-901-1

130 Design and Nature However, since there are many difficulties in testing attachment forces with very small pieces of living tissue, this technique may be the fust step for the prototyping of tiny samples for fiu-ther testing, Surfaces of the living tissues, which are cut off the body, dry out very fast, Cast technique allows testing the contribution of shape of surface structures without the influence of material properties specific for biological tissues. For industrial prototypes, larger areas are usually needed,

4 Gluing technology

The industry of adhesives is presently following different goals [16]: (1) an increase in the reliability of glued contact; (2) mimicking of natural, enviromnent-friendly glues; (3) development of mechanisms for the application of a minute amount of glue to the surface, An additional challenge is the use of substances which allow multiple attachment and detachment, and enable attachment to a variety of surfaces. Many biological attachment devices correspond to at least some of these requirements. One such an example is the hairy surface of the leg pulvillus in flies. This system uses a secretion enabling hairs to attach and detach to diverse substrata very quickly, The hair design includes a mechanism that delivers the secretion, in extremely small amounts, directly to the contact area, and only then, when contact to the substrate is achieved [17]. In the case of natural systems, the mechanical and chemical properties of one surface are already predetermined, This can limit possible adhesive substances optimised for this particular surface. However, the goal of biomimetics is not to imitate the entire system but to use interesting principles or design details for new industrial applications, or for improvement of existing ones,

5 Pest control

Most attachment systems cannot attach to surfaces covered by tiny dust particles, This phenomenon was already used to prevent attachment of the parasitic tick Varroa jacobsoni to its host, the honey bee [18]. Attachment abilities of leg pads in many insects are reduced on plant surfaces covered by wax crystals [19, 20] (Fig. 5 A-D), Additionally, in a series of experiments on the chrysomelid beetle Gastrophysa viridula, it was shown that surface roughness strongly influences attachment of insects with a hairy pad system, Minimum attachment ability was observed at a roughness ranging from 0,3 ~m to 3 pm (Fig. 5 E). Knowledge about the structural properties of insect leg attachment devices and plant surfaces together with experimental data on attachment abilities of insect pests on a variety of structured substrata might be usefil for pest control, Directional changes of plant sorts, using genetic technology or a selective process by man, may result in plant surfaces preventing or reducing attachment of particular insect pests. Development of such a method of biological control © 2002 WIT Press, Ashurst Lodge, Southampton, SO40 7AA, UK. All rights reserved. Web: www.witpress.com Email [email protected] Paper from: Design and Nature, CA Brebbia, L Sucharov & P Pascola (Editors). ISBN 1-85312-901-1

Desiw andNature 131 requires, however, not only structural and experimental data on plant hosts and insect pests, but also additional complex studies on the genetic background of surface pattern formation in plants, and on ecology of food chains connected to the particular plant species,

promoting reducing promoting attachment attachment attachment

z E

E 0,01 0.1 1 10 100 surface irregularities, pm

Figure 5. Anti-attachment plant substrata (A-D) and attachment abilities of the beetle Gastrop@,sa viridula (Chrysomelidae) on substrates with different roughness (E) (data ffom Gorb, 2001). A. Elytrigia repens, B. Chelidonium majus. C. Acer negundo. D. Brassica oleracea.

Acknowledgements

Victoria Kastner kindly provided linguistic corrections. This work is supported by the Federal Ministry of Education, Science and Technology, Germany to SNG (ProjectBioFutare031185 1). © 2002 WIT Press, Ashurst Lodge, Southampton, SO40 7AA, UK. All rights reserved. Web: www.witpress.com Email [email protected] Paper from: Design and Nature, CA Brebbia, L Sucharov & P Pascola (Editors). ISBN 1-85312-901-1

132 Design and Nature References

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