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Mechanics of snake biting: Experiments and modelling
Abstract
Among all the vertebrates, snakes possess the most sophisticated venom delivering system using their fangs. Fangs of many animals are well adapted to the mechanical loads experienced during the functions such as breaking the diet and puncturing the skin of the prey. Thus, investigation and modelling of puncturing mechanics of snakes is of importance to understand the form-function relationship of the fangs and tissue-fang interactions in detail. We have thus chosen fangs of two snake species i.e. viper (Bitis arietans) and burrowing snake (Atractaspis aterrima), with different shape and size, and performed insertion experiments using tissue phantoms. Our results showed that the fangs of both species have similar mechanical properties but there was a difference in the insertion forces owing to the difference in shape of the fang. Also, our modelling of the fang-tissue interactions predicted some material parameters close to the experimental values. Thus, our study can help in the development of bioinspired needles that can potentially have reduced insertion forces and less damage to the tissue.
... Given the length scale and focus of our puncture experiments on deep penetration of layered materials, we adopted conical geometry as a simplified representation of natural puncture tools. It provides a solid mathematical basis and facilitates comparison with previous studies adopting similar geometry on the biomechanics of puncture [3,[55][56][57][58][59]. The choice of the cusp angle corresponds approximately to the average value of a range of tool angles found in nature [19]. ...
... Previous studies on the mechanics of living puncture systems often focus on the form-function relationship associated with morphology of the puncture tool [19,43,55,57,[76][77][78][79][80]. Our findings highlight the necessity of incorporating bio-relevant dynamic material response of layered skin-like material structures for a comprehensive biomechanical analysis. ...
... However, existing puncture models often lack predictive power for such puncture behaviours due to their focus on low-rate and quasi-static puncture mechanics related to needle insertion or deep indentation in polymeric materials (e.g. [57,62,64,69,[83][84][85]). To our knowledge, the mathematical framework for puncture energetics developed by Zhang & Anderson [13] is a notable exception that addresses ductile failure behaviours for dynamic biological puncture. ...
The integumentary system in animals serves as an important line of defence against physiological and mechanical external forces. Over time, integuments have evolved layered structures (scales, cuticle and skin) with high toughness and strength to resist damage and prevent wound expansion. While previous studies have examined their defensive performance under low-rate conditions, the failure response and damage resistance of these thin layers under dynamic biological puncture remain underexplored. Here, we utilize a novel experimental framework to investigate the mechanics of dynamic puncture in both bilayer structures of synthetic tissue-mimicking composite materials and natural skin tissues. Our findings reveal the remarkable efficiency of a thin outer skin layer in reducing the overall extent of dynamic puncture damage. This enhanced damage resistance is governed by interlayer properties through puncture energetics and diminishes in strength at higher puncture rates due to rate-dependent effects in silicone tissue simulants. In addition, natural skin tissues exhibit unique material properties and failure behaviours, leading to superior damage reduction capability compared with synthetic counterparts. These findings contribute to a deeper understanding of the inherent biomechanical complexity of biological puncture systems with layered composite material structures. They lay the groundwork for future comparative studies and bio-inspired applications.
... Biological puncture can be broken down into different stages: initiation, deformation, fracture, penetration and withdrawal (Anderson, 2018;Kundanati et al., 2020;Zhang and Anderson, 2022). Organisms using puncture to perform different functions may emphasize different stages, requiring different types of puncture experiments to test the relationship between morphology and performance. ...
... These force displacement data were used to calculate the work required to achieve each step ( Fig. 2; Fig. S2): (i) the work to initiate fracture (WI), (ii) the work to penetrate to the leading edge of the pore (WPD), (iii) the work to penetrate to the trailing edge of the pore (WPP) and (iv) the work to penetrate to the arbitrary measure of 15% of the total fang length (W15). The work measured here is only one portion of the overall energy involved during puncture (Kundanati et al., 2020;Zhang and Anderson, 2022). Snake strikes in particular will also have energy expenditures related to the ballistic head strike, venom production, venom injection, etc. ...
... Even the '15% of fang length' standard depth (W15) is useful if the question is less about the function of specific taxa and more about tooth form in general. Several recent studies concerning the shape of puncture tools have treated tool taper as an important metric (Bar-On, 2019; Kundanati et al., 2020;Evans et al., 2021;Zhang and Anderson, 2022). For such comparative studies, a 15% depth standard may be appropriate as it removes details of the different systems and allows for a comparison purely based on the shape across systems. ...
When designing experimental studies, it is important to understand the biological context of the question being asked. For example, many biological puncture experiments embed the puncture tool to a standardized depth based on a percentage of the total tool length, to compare the performance between tools. However, this may not always be biologically relevant to the question being asked. To understand how definitions of penetration depth may influence comparative results, we performed puncture experiments on a series of venomous snake fangs using the venom pore location as a functionally relevant depth standard. After exploring variation in pore placement across snake phylogeny, we compared the work expended during puncture experiments across a set of snake fangs using various depth standards: puncture initiation, penetration to a series of depths defined by the venom pore and penetration to 15% of fang length. Contrary to our hypothesis, we found almost no pattern in pore placement between clades, dietary groups or venom toxicity. Rank correlation statistics of our experimental energetics results showed no difference in the broad comparison of fangs when different puncture depth standards were used. However, pairwise comparisons between fangs showed major shifts in significance patterns between the different depth standards used. These results imply that the interpretation of experimental puncture data will heavily depend upon which depth standard is used during the experiments. Our results illustrate the importance of understanding the biological context of the question being addressed when designing comparative experiments.
... Despite their richness and complexity in shape (Vaeth et al., 1985;Young & Kardong, 1996), studies on snake tooth morphology are scarce and either lack a quantitative approach or are phylogenetically limited (Berkovitz & Shellis, 2017;Britt et al., 2009;Evans et al., 2019;Rajabizadeh et al., 2020;Ryerson & Van Valkenburgh, 2021). Fangs, and mostly front fangs, have recently attracted some scientific attention (Broeckhoven & du Plessis, 2017;Cleuren, Parker, et al., 2021;Crofts et al., 2019;Kundanati et al., 2020;Palci et al., 2021;Plessis et al., 2018). Yet, fangs are phylogenetically and functionally limited; their only purpose is to puncture the prey to deliver venom and consequently, fangs are not representative of snake tooth diversity. ...
... Future investigations of the biomechanics of snake teeth may help establish the link between their morphological and behavioral variability and would enrich our understanding of tooth evolution and function in vertebrates. Experimental designs (Kundanati et al., 2020), simulations (Bar-On, 2019; Rajabizadeh et al., 2020), and analytical tools (Huie et al., 2022) have recently been developed and can be used to better understand the dental biomechanics of snakes using the shapes highlighted in the present study, in a functionally relevant context. ...
The structure, composition, and shape of teeth have been related to dietary specialization in many vertebrate species, but comparative studies on snakes' teeth are lacking. Yet, snakes have diverse dietary habits that may impact the shape of their teeth. We hypothesize that prey properties, such as hardness and shape, as well as feeding behavior, such as aquatic or arboreal predation, or holding vigorous prey, impose constraints on the evolution of tooth shape in snakes. We compared the morphology of the dentary teeth of 63 species that cover the phylogenetic and dietary diversity of snakes, using 3D geometric morphometrics and linear measurements. Our results show that prey hardness, foraging substrate, and the main feeding mechanical challenge are important drivers of tooth shape, size, and curvature. Overall, long, slender, curved teeth with a thin layer of hard tissue are observed in species that need to maintain a grip on their prey. Short, stout, less curved teeth are associated with species that undergo high or repeated loads. Our study demonstrates the diversity of tooth morphology in snakes and the need to investigate its underlying functional implications to better understand the evolution of teeth in vertebrates.
... Despite their richness and complexity in shape (Vaeth et al., 1985;Young & Kardong, 1996), studies on snake tooth morphology are scarce and either lack of a quantitative approach or are phylogenetically limited (Berkovitz & Shellis, 2017;Britt et al., 2009;Evans et al., 2019;Rajabizadeh et al., 2020;Ryerson & Van Valkenburgh, 2021). Fangs, and mostly front fangs, have recently attracted some scientific attention (Broeckhoven & du Plessis, 2017;Cleuren, Parker, et al., 2021;Crofts et al., 2019;du Plessis et al., 2018;Kundanati et al., 2020;Palci et al., 2021). Yet, fangs are phylogenetically and functionally limited; their only purpose is to puncture the prey to deliver venom and consequently, fangs are not representative of snake tooth diversity. ...
... Future investigations of the biomechanics of snake teeth may help establish the link between their morphological and behavioral variability and would enrich our understanding of tooth evolution and function in vertebrates. Experimental designs (Kundanati et al., 2020), simulations (Bar-On, 2019; Rajabizadeh et al., 2020) and analytical tools (Huie et al., 2022) have recently been developed and can be used to better understand the dental biomechanics of snakes using the shapes highlighted in the present study, in a functionally relevant context. ...
1. The structure, composition, and shape of teeth have been related to dietary specialization in many vertebrate species, except snakes. Yet, snakes have diverse dietary habits that may impact the shape of their teeth. We hypothesize that prey properties, such as hardness and shape, as well as feeding behavior, such as aquatic or arboreal predation, or holding vigorous prey, impose constraints on the evolution of tooth shape in snakes. 2. We compared the morphology of the dentary teeth of 63 species that cover the phylogenetic and dietary diversity of snakes, using 3D geometric morphometrics and linear measurements. 3. Our results show that prey hardness, foraging substrate and the main mechanical challenge are important drivers of tooth shape, size, and curvature. 4. Overall, long, slender, curved teeth with a thin layer of hard tissue are observed in species that need to maintain a grip on their prey. Short, stout, less curved teeth are associated with species that undergo high or repeated loads. 5. Our study demonstrates the diversity of tooth morphology in snakes and the need to investigate its underlying functional implications to better understand the evolution of teeth in vertebrates.
... These various mechanical challenges may have driven the evolution of tooth shape in snakes. Despite their richness and complexity in shape [15,16], studies on snake tooth morphology are scarce and either lack of a quantitative approach or are phylogenetically limited [11,[17][18][19][20]. Fangs, and mostly front fangs, have recently attracted some scientific attention [21][22][23][24][25][26]. Yet, fangs are phylogenetically and functionally limited as their only purpose is to puncture the prey to deliver venom under the skin, and they are two highly derived teeth of over a hundred (for some species) that are involved in the whole feeding sequence. ...
... Future investigations of the biomechanics of snake teeth may help draw the link between their morphological and behavioral variability and would enrich our understanding of tooth evolution and function in vertebrates. Experimental designs [21], simulations [20,50] and analytical tools [60] have recently been developed and can be used to better understand the dental biomechanics of snakes using the shapes highlighted in the present study, in a functionally relevant context. ...
Teeth are one of the most studied hard tissues in vertebrates. Their structure, composition and shape are related to dietary specialization in many species. At first glance, snake teeth all look similar; conical, sharp, curved. Yet, snakes, like other vertebrates, have very diverse diets that may have affected their shape. We compared the morphology of the teeth of 63 species that cover both the phylogenetic and dietary diversity of snakes. We predicted that prey properties play a role in shaping snakes teeth along with their feeding behavior. Limblessness combined to the peculiar feeding behavior of snakes impose strong functional constraints on their teeth, especially during arboreal or aquatic feeding. Our results show that prey hardness, foraging substrate and the main feeding constraint are drivers of tooth shape, size, and curvature. We highlight two main morphotypes: long, slender, curved with a thin layer of hard tissue for snakes that need a good grip on their prey and short, stout, less curved teeth in snakes that eat hard or long prey. Our study demonstrates the diversity of tooth morphology in snakes and the need to investigate the underlying functional implications to better understand the evolution of teeth in vertebrates.
... This interplay is ultimately responsible for the initiation and propagation of fracture inside the material substrate during puncture. Previously established energy-based models for puncture have demonstrated their effectiveness in mathematically describing how initial energy investment for puncture failure is divided into energy contributions for fracture, elastic deformation, and friction dissipation [7,18,21,27,[38][39][40][41][42][43]. However, there exists a knowledge gap between the observed unique morphology of puncture fracture surfaces and the underlying fracture mechanics. ...
... 21 Scientists are taking inspiration by puncturing systems found in nature to develop new bio-inspired solutions. 22,23 In addition, biological tissuemimicking phantoms obtained by additive manufacturing techniques are employed in medical training, [24][25][26] thanks to their capability to replicate the mechanical conditions encountered in surgical procedures involving cutting and puncturing. 27 A central part in the mechanics of puncturing is played by fracture mechanics. ...
The integrity of soft materials against puncturing is of great relevance for their performance because of the high sensitivity to local rupture caused by rigid sharp objects. In this work, the mechanics of puncturing is studied with respect to a sharp-tipped rigid needle with a circular cross section, penetrating a soft target solid. The failure mode associated with puncturing is identified as a mode-I crack propagation, which is analytically described by a two-dimensional model of the target solid, taking place in a plane normal to the penetration axis. It is shown that the force required for the onset of needle penetration is dependent on two energy contributions, that are, the strain energy stored in the target solid and the energy consumed in crack propagation. More specifically, the force is found to be dependent on the fracture toughness of the material, its stiffness and the sharpness of the penetrating tool. The reference case within the framework of small strain elasticity is first investigated, leading to closed-form toughness parameters related to classical linear elastic fracture mechanics. Then, nonlinear finite element analyses for an Ogden hyperelastic material are presented. Supporting the proposed theoretical framework, a series of puncturing experiments on two commercial silicones is presented. The combined experimental-theoretical findings suggest a simple, yet reliable tool to easily handle and assess safety against puncturing of soft materials.
... The analysis and experimental measurements of the penetration forces are of great interest for scientific speculations as well as engineering applications, e.g. concerning food industry, robotic surgical operations, experimental testing and biological puncture systems (Frick et al., 2001;DiMaio and Salcudean, 2003;McGorry et al., 2003;Goh et al., 2005;Takabi and Tai, 2017;Anderson, 2018;Kundanati et al., 2020;Terzano et al., 2020Terzano et al., , 2021. Puncture testing can also specifically used for the characterization of tissue-mimicking composites formed by additive manufacturing (e.g. ...
In the present paper, the mechanics of puncturing is studied with refer to a foreign tool penetrating a soft (nearly incompressible) target solid. The penetrating tool is here described by a sharp tipped rigid needle with a circular cross section. Puncturing can be characterised as a Mode I fracture process, which is here analytically described by a two-dimensional model related to the plane normal to the penetration axis. It is shown that the force required for the onset of needle penetration is dependent on two energy contributions, that is, the strain energy stored in the target solid and the energy consumed in Mode I crack propagation. Such a penetration force is analytically demonstrated to be dependent on the fracture toughness of the material, its elastic modulus, and the sharpness of the penetrating tool.
... Of importance in these studies is the assignment of correct material properties to the structures 522 being tested. A very recent study demonstrates more accurate data on the material properties of 523 venomous snake fangs and a detailed methodology of how to test them (Kundanati et al., 2020). 524 Future studies are now able to run similar computer simulations to test the penetrative ability of 525 different fang shapes into multiple tissue types. ...
Venomous snakes are among the world's most specialised predators. During feeding, they use fangs to penetrate the body tissues of their prey, but the success of this penetration depends on the shape of these highly specialised teeth. Here, we examined the evolution of fang shape in a wide range of snakes using 3D geometric morphometrics (3DGM) and cross‐sectional tooth sharpness measurements. We investigated the relationship of these variables with six diet categories based on the prey's biomechanical properties, and tested for evolutionary convergence using two methods. Our results show that slender elongate fangs with sharp tips are used by snakes that target soft‐skinned prey (e.g. mammals), while fangs become more robust and blunter as the target's skin becomes scaly (e.g. fish, reptiles) and eventually hard‐shelled (e.g. crustaceans), both with and without correction for evolutionary allometry. Convergence in fang shape is present, indicating that fangs of snakes with the same diet are more similar than those of closely related species with different diets. Establishing the relationship between fang morphology and diet helps to explain how snakes became adapted to different lifestyles, while also providing a proxy to infer diet in lesser‐known species or extinct snakes from the fossil record.
This article is protected by copyright. All rights reserved
Living organisms have evolved various biological puncture tools, such as fangs, stingers, and claws, for prey capture, defense, and other critical biological functions. These tools exhibit diverse morphologies, including a wide range of structural curvatures, from straight cactus spines to crescent-shaped talons found in raptors. While the influence of such curvature on the strength of the tool has been explored, its biomechanical role in puncture performance remains untested. Here, we investigate the effect of curvature on puncture mechanics by integrating experiments with finite element simulations. Our findings reveal that within a wide biologically relevant range, structural curvature has a minimal impact on key metrics of damage initiation or the energies required for deep penetration in isotropic and homogeneous target materials. This unexpected result improves our understanding of the biomechanical pressures driving the morphological diversity of curved puncture tools and provides fundamental insights into the crucial roles of curvature in the biomechanical functions of living puncture systems.
Indentation measurement has emerged as a crucial technique for elucidating the mechanical properties of soft hydrated materials. These materials, encompassing gels, cells, and biological tissues, possess pivotal mechanical characteristics crucial for a myriad of applications across engineering and biological realms. From engineering endeavors to biological processes linked to both normal physiological activity and pathological conditions, understanding the mechanical behavior of soft hydrated materials is paramount. The indentation method is particularly suitable for accessing the mechanical properties of these materials as it offers the ability to conduct assessments in liquid environment across diverse length and time scales with minimal sample preparation. Nonetheless, understanding the physical principles underpinning indentation testing and the corresponding contact mechanics theories, making judicious choices regarding indentation testing methods and associated experimental parameters, and accurately interpreting the experimental results are challenging tasks. In this review, we delve into the methodology and applications of indentation in assessing the mechanical properties of soft hydrated materials, spanning elastic, viscoelastic, poroelastic, coupled viscoporoelastic, adhesion properties, and fracture toughness. Each category is accomplished by the theoretical models elucidating underlying physics, followed by ensuring discussions on experimental setup requirements. Furthermore, we consolidate recent advancements in indentation measurements for soft hydrated materials highlighting its multifaceted applications. Looking forward we offer insights into the future trajectory of the indentation method on soft hydrated materials and the potential applications. This comprehensive review aims to furnish readers with a profound understanding of indentation techniques and a pragmatic roadmap of characterizing the mechanical properties of soft hydrated materials.
Years of research show that the Trans-dermal drug delivery (TDD) route showed promising results due to good immunogenic responses. In this paper, we have proposed a bio-inspired micro-needle suggested by a snake belonging to the family of Elapids, since they inject venom with high pressures during the bite. The proposed micro-needle is strong enough to puncture the skin and withstand different kinds of loads during the insertion. The proposed micro-needle is of
length, and the maximum compressive, buckling, bending, load it can handle are 0.27N, 0.16N, 0.024N respectively. The proposed micro-needle (MN) has an inner channel diameter of 44
and it gives a flow rate of
/s. In our work, we have modeled a substrate of epidermis and dermis as a porous medium with porosity and permeability as 0.74,
respectively. The porosity and permeability are calculated using an SEM image of the human dermis consisting of only collagen fibers and empty pores. We have applied Darcy’s law to the modeled substrate and obtained the velocity field of the drug administrated. The diffusion study of Doxorubicin (
mol/l) is carried out using Darcy velocity field and concentration gradient.
Nature is a substantial repository for various kinds of aquatic, terrestrial, and wind‐borne plants and animals adapting extensively to sustain life on earth’s different environmental conditions. The organisms or structures of these natural creatures possess extraordinary properties or functionalities through well‐organizing usually weak biopolymers and bioinorganic structures, which provide good examples to emulate their mechanical and structural characteristics in materials innovations and discoveries. This review collectively investigated studies of the structures and mechanical properties of some representative fauna and flora with robust mechanical properties and the corresponding bioinspired materials with mimicking structures and mechanical properties. By learning from the natural structures with robust mechanical properties, bioinspired materials and composites with superior mechanical performance to the constituent materials have been designed and fabricated. Via this study, we hope to draw some principles on designing innovative materials with extraordinary properties from existing common materials by learning from nature. It is expected that the understandings on the extraordinary natural mechanical properties and the robust bioinspired materials can provide some insights into the design of novel materials and composites. This article is protected by copyright. All rights reserved.
This paper aims to uncover if there is a significant difference in the strike location of snake species that have different values of LD50% venom. It is thought that most snakes strike their prey in the anterior (head) area in order for their venom to work quicker in killing them. Venom toxicity is measured by its LD50% value, which is the amount of venom, in mg/kg, to kill 50% of a test population. The Copperhead has an LD50% value of 10.9 mg/kg, and the Timber Rattlesnake has an LD50% value of 1.64 mg/kg. The hypothesis was that if venom toxicity had an impact on strike location, then the Copperhead species would strike at a higher rate towards the anterior area of their prey compared to the Timber Rattlesnake. Three Copperheads and Three Timber Rattlesnakes were used in the study. Strike location data was collected over the course of a seven-week period. The snakes were placed into an eight-foot wide by three-foot deep arena filled with materials to simulate their natural habitat. The snakes were fed rats or mice depending on their size, and strikes were recorded on an iPhone XR and a GoPro. Strikes were designated as either on the anterior or posterior of the prey, and a t-test was conducted to determine if there was a significant difference present. A t-test was conducted on the number of strikes on the anterior of prey of both the Timber Rattlesnake and the Copperhead, as well as the total number of strikes on the anterior vs. the total number of strikes on the posterior. It was determined that there was not a significant difference in any of the data meaning that there is not a clear connection between strike location and venom toxicity in Timber Rattlesnakes and Copperheads.
Mechanical characterization of hydrogels is a challenging task because they are much softer than metals, ceramics or polymers. The elastic modulus of hydrogels is within 100-102kPa range. Because they easily break and slump under their own weight, tensile and bending tests are not suitable configurations to assess elastic modulus. This work reports on the determination of elastic modulus of a gelatin gel by indentation experiments. Indentation is very simple configuration, it is of technological importance and it can be applied at different length scales with high accuracy. The gelatin hydrogel behavior is first calibrated by uniaxial compression and low strain rheological measurements. It behaves as a hyperelastic solid with strain hardening capability at large strains and shows no dependence with frequency in the linear viscoelastic range. It can be properly characterized by the First order Ogden material model. Indentation experiments are carried out at macro and nanoscales using spherical and flat-ended cylindrical punches. Elastic contact solutions and inverse analysis accounting for hyperelasticity are used to extract the elastic modulus from experimental force-depth curves. Adhesion between punch and hydrogel influences the indentation response and affects the accuracy of elastic modulus determination in a larger extent than the assumption of linear elasticity. Adhesion leads to overestimation of elastic modulus values. The influence of adhesive forces increases with decreasing the length scale. A markedly decay of elastic modulus with increasing maximum load is observed at nanoscale. A hybrid model based on Hertz elastic contact solution and Johnson-Kendal-Roberts model for adhesion is used to determine elastic modulus. This model yields an elastic modulus in good agreement with that obtained from uniaxial compression test.
Through natural selection, many animal organs with similar functions have evolved different macroscopic morphologies and microscopic structures. Here, we comparatively investigate the structures, properties, and functions of honey bee stings and paper wasp stings. Their elegant structures were systematically observed. To examine their behaviors of penetrating into different materials, we performed penetration-extraction tests and slow motion analyses of their insertion process. In comparison, the barbed stings of honey bees are relatively difficult to be withdrawn from fibrous tissues (e.g., skin), while the removal of paper wasp stings is easier due to their different structures and insertion skills. The similarities and differences of the two kinds of stings are summarized on the basis of the experiments and observations.
© 2015. Published by The Company of Biologists Ltd.
Micromechanical models are developed for the deep penetration of a soft
solid by a flat-bottomed and by a sharp-tipped cylindrical punch. The
soft solid is taken to represent mammalian skin and silicone rubbers,
and is treated as an incompressible, hyperelastic, isotropic solid
described by a one-term Ogden strain energy function. Penetration of the
soft solid by a flat-bottomed punch is by the formation of a mode-II
ring crack that propagates ahead of the penetrator tip. The sharp-tipped
punch penetrates by the formation of a planar mode-I crack at the punch
tip, followed by wedging open of the crack by the advancing punch. For
both modes of punch advance the steady-state penetration load is
calculated by equating the work done in advancing the punch to the sum
of the fracture work and the strain energy stored in the solid. For the
case of a sharp penetrator, this calculation is performed by considering
the opening of a plane-strain crack by a wedge using a finite-element
approach. Analytical methods suffice for the flat-bottomed punch. In
both models the crack dimensions are such that the load on the punch is
minimized. For both geometries of punch tip, the predicted penetration
pressure increases with diminishing punch radius, and with increasing
toughness and strain-hardening capacity of solid. The penetration
pressure for a flat-bottomed punch is two to three times greater than
that for a sharp-tipped punch (assuming that the mode-I and mode-II
toughnesses are equal), in agreement with experimental observations
reported in a companion paper.
Spider silk is extraordinarily strong, mollusk shells and bone are tough, and porcupine quills and feathers resist buckling.
How are these notable properties achieved? The building blocks of the materials listed above are primarily minerals and biopolymers,
mostly in combination; the first weak in tension and the second weak in compression. The intricate and ingenious hierarchical
structures are responsible for the outstanding performance of each material. Toughness is conferred by the presence of controlled
interfacial features (friction, hydrogen bonds, chain straightening and stretching); buckling resistance can be achieved by
filling a slender column with a lightweight foam. Here, we present and interpret selected examples of these and other biological
materials. Structural bio-inspired materials design makes use of the biological structures by inserting synthetic materials
and processes that augment the structures' capability while retaining their essential features. In this Review, we explain
this idea through some unusual concepts.
The Colubroidea represents approximately 2300 of the 2700 species of living snakes and includes all venomous taxa. Although many morphological studies of colubroid snakes have been carried over the last hundred years, the phylogenetic relationships within this group are poorly known. In this study, components of the venom-delivery system (VDS) were examined within the context of two conflicting phylogenetic hypotheses proposed in 1988 by Cadle and in 1998 by Kraus & Braun. The results suggest that several major morphological changes occurred early in colubroid evolution: a Duvernoy's gland evolved, the posterior maxillary teeth became specialized relative to the anterior maxillary teeth, and the attachment of the pterygoideus muscle moved forward to a position associated with the posterior maxillary teeth. These innovations may have allowed the great radiation of colubroid snakes that led to the Colubroidea representing such a large percentage of living snakes. More recently, three separate lineages of colubroids have independently evolved highly specialized front-fanged VDSs with large and complex venom glands, venom gland compressor muscles, and tubular fangs. © 2003 The Linnean Society of London, Zoological Journal of the Linnean Society, 2003, 137, 337−354.
Two experiments were performed to determine the effects of friction and needle geometry during robotic needle insertion into soft tissues. In Experiment I, friction forces along the instrument axis were characterized during needle insertion into bovine liver under CT fluoroscopic imaging. Because the relative velocity of the tissue and needle affect viscous and Coulomb friction, the needle insertion process was segmented into several phases of relative motion: none, partial and complete. During the complete relative motion phase, it was found that Coulomb friction accounts for the majority of needle force. In Experiment II, insertion forces along and orthogonal to the needle axis were measured during insertion into a silicone rubber phantom with a consistency similar to liver. The effects of needle diameter and tip type (bevel, cone, and triangle) on insertion force were characterized. A bevel tip causes more needle bending and is more easily affected by tissue density variations. Forces for larger diameter needles are higher due to increased cutting and friction forces. These results may be used in the control of needle insertion for robot-assisted percutaneous therapies.
During needle-based procedures, transitions between tissue layers often lead to rupture events that involve large forces and tissue deformations and produce uncontrollable crack extensions. In this paper, the mechanics of these rupture events is described, and the effect of insertion velocity on needle force, tissue deformation, and needle work is analyzed. Using the J integral method from fracture mechanics, rupture events are modeled as sudden crack extensions that occur when the release rate J of strain energy concentrated at the tip of the crack exceeds the fracture toughness of the material. It is shown that increasing the velocity of needle insertion will reduce the force of the rupture event when it increases the energy release rate. A nonlinear viscoelastic Kelvin model is then used to predict the relationship between the deformation of tissue and the rupture force at different velocities. The model predicts that rupture deformation and work asymptotically approach minimum values as needle velocity increases. Consequently, most of the benefit of using a higher needle velocity can be achieved using a finite velocity that is inversely proportional to the relaxation time of the tissue. Experiments confirm the analytical predictions with multilayered porcine cardiac tissue.
The fangs of vipers are extremely long, rotating, hollow teeth. Analysis of video records of more than 750 strikes recorded at 60 or 250 frames per second for 285 individuals representing 86 species in 31 genera shows that vipers reposition fangs after initial contact with prey in more than a third of the strikes. Repositioning resulted when fangs missed prey entirely or hit prey regions that did not permit adequate penetration. The prevalence of repositioning, even among species that normally release prey, suggests strong selective pressure for rapid neuromotor response to fang placement error. The rapidity of repositioning suggests the existence of (a) fine-scale sensory detection of fang penetration depth, (b) rapid modulation of contraction of antagonistic muscles, and (c) possibly neurological modifications to shorten transmission time between sensory input and motor output. Extreme fang length has apparently coevolved with extreme functions.
Elapids, viperids, and some other groups of colubroid snakes have tubular fangs for the conduction of venom into their prey. The literature describing the development of venom-conducting fangs provides two contradictory accounts of fang development. Some studies claim that the venom canal forms by the infolding of a deep groove along the surface of the tooth to produce an enclosed canal. In other works the tubular fang is said to form by the deposition of material from tip to base, so that the canal develops without any folding. This study was undertaken to examine fang development and to account for the disagreement in the literature by determining whether fang formation varies among groups of venomous snakes and whether it differs between embryos and adults. Adult and embryonic representatives of elapids and viperids were examined. All fangs examined, elapid and viperid, embryos and adults, were found to develop into their tubular shape by the addition of material to the basal end of the tooth rather than by the folding inward of an ungrooved tooth to form a tubular fang. In some cases, the first fang that develops in embryonic snakes differs morphologically from all those formed subsequently.
The modeling of forces during needle insertion into soft tissue is important for accurate surgical simulation, preoperative planning, and intelligent robotic assistance for percutaneous therapies. We present a force model for needle insertion and experimental procedures for acquiring data from ex vivo tissue to populate that model. Data were collected from bovine livers using a one-degree-of-freedom robot equipped with a load cell and needle attachment. computed tomography imaging was used to segment the needle insertion process into phases identifying different relative velocities between the needle and tissue. The data were measured and modeled in three parts: 1) capsule stiffness, a nonlinear spring model; 2) friction, a modified Karnopp model; and 3) cutting, a constant for a given tissue. In addition, we characterized the effects of needle diameter and tip type on insertion force using a silicone rubber phantom. In comparison to triangular and diamond tips, a bevel tip causes more needle bending and is more easily affected by tissue density variations. Forces for larger diameter needles are higher due to increased cutting and friction forces.
Fangs are specialised long teeth that contain either a superficial groove (Gila monster, Beaded lizard, some colubrid snakes), along which the venom runs, or an enclosed canal (viperid, elapid and atractaspid), down which the venom flows inside the tooth. The fangs of viperid snakes are the most effective venom-delivery structures among vertebrates and have been the focus of scientific interests for more than 200 years. Despite this interest the questions of how the canal at the centre of the fang forms remains unresolved. Two different hypotheses have been suggested. The mainstream hypothesis claims that the venom-conducting canal develops by the invagination of the epithelial wall of the developing tooth germ. The sides of this invagination make contact and finally fuse to form the enclosed canal. The second hypothesis, known as the "brick chimney", claims the venom-conducting canal develops directly by successive dentine deposition as the tooth develops. The fang is thus built up from the tip to the base, without any folding of the tooth surface. In an attempt to cast further light on this subject the early development of the fangs was followed in a pit viper, Trimeresurus albolabris, using the expression of Sonic hedgehog (Shh). We demonstrate that the canal is indeed formed by an early folding event, resulting from an invagination of epithelial cells into the dental mesenchyme. The epithelial cells proliferate to enlarge the canal and then the cells die by apoptosis, forming an empty tube through which the poison runs. The entrance and discharge orifices at either end of the canal develop by a similar invagination but the initial width of the invagination is very different from that in the middle of the tooth, and is associated with higher proliferation. The two sides of the invaginating epithelium never come into contact, leaving the orifice open. The mechanism by which the orifices form can be likened to that observed in reptiles with an open groove along their fangs, such as the boomslang. It is thus tempting to speculate that the process of orifice formation in viperids represents the ancestral pleisomorphic state, and that enclosed canals developed by a change in the shape and size of the initial invagination.
The composition of dental tissues and their interaction determines its mechanical properties. The mechanical properties and chemical composition of the teeth of extant reptiles are still poorly studied areas. As a preliminary study the fangs of four species of snakes and a human tooth were investigated through nanoindentation and Raman spectroscopy. The average elastic modulus values for the main body of the fangs ranged from 15.3 GPa to 24.6 GPa, and 19.1 GPa for the human dentine. Raman spectroscopy and principal component analysis (PCA) showed that snake fangs are similar in composition to human dentine, both of which comprised of hydroxyapatite and an organic matrix. The elastic modulus and hardness data were correlated to the Raman spectra using partial least squares regression (PLS). The spectral features which correlated with the elastic modulus would suggest that elastic modulus is dependent on the relative protein to mineral amounts in the tooth. The form of the phosphate and the relative levels of phosphate to organic components also appear to be governing factors for elastic modulus. The PLS of Raman spectra against the hardness gave very similar results. The small differences between snake fangs and human dentine appeared to be because of carbonate content, with higher levels of carbonate in the human tooth than the snake fangs.
Snake fangs should be able to withstand large lateral forces. Human dentine aids in dissipating imposed loads. This similarity in the chemical composition of the snake fangs and human dentine supported the findings of the similarities in mechanical properties, which may be attributed to the similar functional demands of these biocomposites. Copyright © 2016 John Wiley & Sons, Ltd.
Changes leading to the evolution of the viperid fang involved lengthening of rear maxillary teeth and have usually been attributed to the adaptive value derived by providing for more effective venom injection. As an alternative interpretation, it is argued here that an initial factor favoring elongation was not venom injection but instead increased swallowing effectiveness. During swallowing, rear maxillary teeth are the first teeth of the maxilla to engage the prey upon each swallowing cycle. Further, they are geometrically positioned more favorably than other maxillary teeth so as to swing through a longer retraction arc and hence move the prey farther. Consequently, rear maxillary teeth might be expected to be the ones most likely modified in any dental adaptations that served swallowing. Elongation of a few teeth permits deeper penetration into the tissues and hence increases purchase of the jaws on the prey. These factors, it is argued, all favored the initial elongation of posterior maxillary teeth. However, once elongated to enhance swallowing effectiveness, they became a key feature preadapted for subsequent modification along several evolutionary pathways. One direction led toward the Xenodon and Heterodon-like snakes in which the manipulation function of the elongated teeth during swallowing was further enhanced. These snakes thus represent not 'protovipers' but simply a specialized form of snake in which the teeth aid prey manipulation during swallowing. A second line of evolution led to viperid snakes in which the emphasis shifts so that the elongated teeth now play a greater role in venom injection. Although the superficial impression would be that of two parallel lines of evolution involving rear maxillary tooth elongation, the selective forces acting in each line would be quite different. In one, the adaptations would serve to improve prey manipulation during swallowing while in the other to improve envenomation. Just as the mechanics of swallowing could have been the initial factor favoring tooth elongation, one might expect that oral gland secretions also served initially not a toxic function but as an aid in prey handling or swallowing.
Spiders mainly feed on insects. This means that their fangs, which are used to inject venom into the prey, have to puncture the insect cuticle that is essentially made of the same material, a chitin-protein composite, as the fangs themselves. Here a series of structural modifications in the fangs of the wandering spider Cupiennius salei are reported, including texture variation in chitin orientation and arrangement, gradients in protein composition, and selective incorporation of metal ions (Zn and Ca) and halogens (Cl). These modifications influence the mechanical properties of the fang in a graded manner from tip to base, allowing it to perform as a multi-use injection needle that can break through insect cuticle, which is made of a chitin composite as well.
Wire cutting involves fracture, plastic deformation and surface friction effects so that, in principle, it provides a method for determining parameters for all these properties. This paper describes an analysis in terms of the fracture toughness, G
c, the yield stress, y, and the coefficient of friction, . By measuring the cutting force as a function of wire diameter, G
c and (1 + )y can be found. These values are compared with direct measurements in notched bending for G
c, in simple compression for and y, and in sliding tests for . A comparison of values obtained for a range of cheeses shows encouraging agreement.
A solution of the axisymmetric Boussinesq problem is derived from which are deduced simple formulae for the depth of penetration of the tip of a punch of arbitrary profile and for the total load which must be applied to the punch to achieve this penetration. Simple expressions are also derived for the distribution of pressure under the punch and for the shape of the deformed surface. The results are illustrated by the evaluation of the expressions for several simple punch shapes.RésuméL'auteur a établi une solution du problème axisymétrique de Boussinesq qui lui à permis de déduire des formules simples donnant la profondeur de pénétration d'un pénétrateur de profil arbitraire ainsi que la charge totale nécessaire pour assurer cette pénétration. Il donne également des expressions simples qui définissent la distribution de la pression sous le pénétrateur ainsi que la forme de la surface déformée.Les résultats sont illustrés par l'application de ces expressions aux cas de plusieurs pénétrateurs de forme simple.ZusammenfassungEine Lösung des Boussinesq'schen achsialsymmetrischen Problems wird abgeleitet. Von dieser werden dann weitere einfache Formeln abgeleitet, mit denen sich die Einschlagtiefe einer Stösselspitze mit willkürlichem Profil, und die zur Erzielung diesser Tiefe aufzubringende Gesamtlast bestimmen lässt Ausserdem werden einfache Ausdrücke für die Druckverteilung unter dem Stössel und die Form der deformierten Oberfläche angegeben. Die Ergebnisse werden durch die Auswertung der Ausdrücke für mehrere einfache Stösselformen erleutert.SumàrioSi deriva una soluzione del problema assisimmetrico del Boussinesq dalla quale si deducono semplici formule per la profondità di penetrazione della punta di un punzone di profilo arbitrario e per il carico totale ehe deve venire applicato al punzone per ottenere detta penetrazione. Si derivano inoltre semplici espressioni per la distribuzione della pressione sotto il punzone e per il profilo della superficie deformata. I risultati sono illustrati con la valutazione delle espressioni per vari profili semplici di punzone.РефератДaнo peшeниe aкcиcиммeTpичнoй пpoблeмы Бuccoнeк,a, из кoTopoгo вывeдeны пpocTыe фopмuлы дпя глuбины пoгpuжeния кoнцa пpoбoйникa пpoизвoльнoгo пpoфиля, и для пoлнoй нaгpuзки, кoTopaя дoлжнa быTь пpилoжeнa к пpoбoйиикu, чToбя дaTь эTu глuбинu пoгpuжeния.Taкжe вывeдeны пpocTыe выpaжeния для pacпpeдeлeния дaвлeния пoд чpoбoйникoм и для фopмы дeфopмиpoвaннoй пoвepxнocTи. peзuльTaTы иллюcTpнpoвaны вычиcлeниями Taкиx выpaжeний для нecкoлькиx пpocTыx фopм пpoбoйнкoв.
Damselfly females use their ovipositor valves to saw aquatic plants in order to insert their eggs into the plant tissues. Stiffness of the plant substrata is therefore an important parameter for oviposition substrate choice by females. Using a force transducer combined with a motorised micromanipulator, the bending stiffness of the ovipositor at the axial compressional load was studied in seven European damselfly species and compared to the local stiffness of seven preferred plant substrates. The puncture force of tested plant samples ranged from 105 to 1500 mN, and their local stiffness ranged from 208 to 1776 N/m. The bending stiffness of the ovipositor was estimated as 173-409 N/m depending on the damselfly species. Using original and literature data, a significant positive correlation between mechanical properties of the ovipositor and preferred oviposition substrates was demonstrated. Possible behavioural adaptations to overcome high stiffness of plant tissues during oviposition are discussed.
- M D Abràmoff
- P J Magalhães
Abràmoff, M D, Magalhães, P J, 2004. Image processing with ImageJ. Biophot. Int..