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Passive Cushiony Biomechanics of Head Protection in Falling Geckos

Passive Cushiony Biomechanics of Head Protection in Falling Geckos Hindawi Applied Bionics and Biomechanics Volume 2018, Article ID 9857894, 7 pages https://doi.org/10.1155/2018/9857894 Research Article Passive Cushiony Biomechanics of Head Protection in Falling Geckos 1 1 2 1 1 Hao Wang , Wenbo Wang, Yi Song, Lei Cai , and Zhendong Dai College of Astronautics, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, China College of Aerospace Engineering, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, China Correspondence should be addressed to Hao Wang; haowang@nuaa.edu.cn Received 16 October 2017; Revised 23 December 2017; Accepted 15 January 2018; Published 19 February 2018 Academic Editor: Qi Shen Copyright © 2018 Hao Wang et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Gekko geckos are capable to crawl on the steep even on upside-down surfaces. Such movement, especially at great altitude, puts them at high risks of incidentally dropping down and inevitable body or head impactions, though they may trigger air-righting reaction (ARR) to attenuate the landing shocks. However, the air-righting ability (ARA) in Gekko geckos is not fully developed. The implementation of ARR in some geckos is quite slow; and for those without tails, the ARR is even unobservable. Since ARA is compromised in Gekko geckos, there must be some other mechanisms responsible for protecting them from head injuries during falls. In this study, we looked into a Gekko gecko’s brain to study its internal environment and structure, using the magnetic resonance imaging (MRI) technique. The results showed that the brain parenchyma was fully surrounded by the cerebrospinal fluid (CSF) in the skull. A succulent characteristic was presented, which meant the intracalvarium was significantly occupied by the CSF, up to 45% in volume. Then a simplified three-dimensional finite element model was built, and a dynamic simulation was conducted to evaluate the mechanical property of this succulent characteristic during the head impactions. These implied the succulent characteristic may play certain roles on the self-protection in case of head impaction, which is adaptable to the Gekko gecko’s locomotion and behavior. not always performed perfectly in geckos. It was showed that 1. Introduction the geckos without tails could not perform air-righting reac- Many animals in nature are able to crawl, climb, or run on tion (ARR) at all, and even for the geckos with tails, almost different inclined surfaces, such as walls, ceilings, branches, one-tenth could not perform ARR well [5]. Actually, the and leaves, at a certain height above the earth. Falling down, ARA might be weaker in Gekko geckos, because the relevant air-righting reflex did not mature, developmentally speaking, as a common experience, is somehow unavoidable. To avoid the body being injured when falling and impacting with the into a complete central program. The published data [9] pro- ground, animals have developed various abilities, such as vided evidence that the ARA probably starts developmentally air-righting abilities (ARA). Cats try to make their feet to as a reflex and within days/weeks mature into a central pat- attach on the ground at first to reduce the impact forces act- tern generator (CPG) by showing that the completion of ing on the body by turning up the body upside-down through the ARR maturation process had no dependency on loads the vertebra and tail [1–4]. Gekko geckos turn up by rotating attached to different parts of the body. In our behavior exper- their tail [5]. Preventing the brain injury from the falling and iments, not all geckos could turn around successfully in impacting to the ground, a sort of self-protection is one of the abdomen-up falling down. Especially when the falling height important survival skills gained from natural selection and is not so high, the time for free fall is not sufficient for this evolution in animals. Aerial maneuverability or air-righting performance. Even though, it rarely causes any injury in the performance is the most important mechanism of self- brain. The head impaction is cushioned somehow. It seems protection in insects [6, 7], cats [1–4], rats [8, 9], rabbits that Gekko geckos may possess certain characteristics to pre- [10], frogs [11], and geckos [5]. However, the air-righting is vent themselves especially their head from injury caused by 2 Applied Bionics and Biomechanics (a) 60 mm (b) Brain parenchyma Cerebrospinal fluid Figure 1: (a) A Gekko gecko lizard and (b) the observation on the intracalvarium of gecko brain using a digital microscope (VHX-600, Keyence, Japan) after craniotomy (the skull was opened and the dura was removed while the arachnoid was intact). impacting the ground while falling down. To disclose the 2.2. The Intracalvarium Morphology Investigation. The intra- potential mechanism underlying the self-protection in head calvarium morphology was investigated by two ways, the impaction, it is necessary to look into the intracalvarium qualitative observation after the surgical anatomy and the structure and material of the animal’s head. quantitative measurement using MRI. Not like a woodpecker’s head, which has been studied After the surgical anatomy, we found that the gecko’s thoroughly for decades [12–15], a Gekko gecko’s head has brain parenchyma (Figure 1) is surrounded by the cerebro- been rarely investigated, since its significance of antishock spinal fluid (CSF). characteristic is much lower than the former. The wood- For MRI investigation, the animal was anaesthetized by pecker’s head can stand high-frequency shocks that are intro- the intraperitoneal injection of 0.4% sodium pentobarbital duced from its drumming beak during the daily forage, while in a 0.75% NaCl solution. A dose of 0.75 ml/100 g body the gecko’s head is only in the risk of one sudden head shock weight was administered. After the pain reflex had disap- caused by an incident dropping. The underlying mechanism peared, the gecko was fixed to a custom-designed fixture of head protection should be different. Here, we took a close (manuscript in preparation, see Figure 2) and then placed look at the intracranial structures by the magnetic resonance into the MRI instrument (BioSpec 7T/20 cm, Bruker, Ger- imaging (MRI) technique. Then a simplified mathematical many). The whole brain was scanned in three orthogonal model was built to qualitatively evaluate the corresponding (sagittal, coronal, and horizontal) planes (Figure 3), and the mechanical property. corresponding spatial interval of the scan was all 0.30 mm. Based on the MRI image sequence, the distribution and the volume of the CSF in the skull were evaluated with the 2. Materials and Methods help of the open source software ImageJ (http://rsbweb.nih .gov/ij/). It is not difficult to distinguish the brain paren- 2.1. Experimental Animals. The Gekko gecko lizards were chyma and the CSF by gray level of the MRI image. Then, brought from Nanning, Guangxi Province, China, and habit- the surface integral and the volume integral were conducted uated to the study colony for two months before the experi- for the brain parenchyma and the CSF, respectively. ments. The mean temperature and relative humidity were 25 C and 65%, respectively, which were close to the values 3. Results and Discussion for the natural ambiance of Gekko geckos. Adult Gekko geckos weighted 40–70 g were selected for the MRI study. 3.1. Distribution of the CSF in the Gecko Skull. The percent- The entire study was carried out in accordance with the age distribution of the CSF in the gecko skull is shown in Guide of Laboratory Animal Management Ordinance of Figure 4. Since the dimensions of the brain along the sagittal, China and approved by the Jiangsu Association for Labora- coronal, and vertical directions are different, the number of tory Animal Science (Jiangsu, China). slices is variable. It provided relatively more detail along the Applied Bionics and Biomechanics 3 (a) (b) Figure 2: An anaesthetized gecko is placed into the MRI instrument, whose head and body are fixed to a custom-designed fixture (a). The close view of the fixed gecko in the fixture without cover (b). (a) (b) (c) Figure 3: The MRI scanning of a gecko head in the sagittal plane (a), horizontal plane (b), and coronal plane (c). The dark area (marked by “1”) indicates the brain parenchyma while the bright area (marked by “2”) the CSF. sagittal direction (coronal plane, Figure 4(c)), so the corre- is clearly symmetrical along the coronal direction (sagittal sponding image sequence was employed to calculate the vol- plane, Figure 4(a)) due to the morphologically bilateral sym- ume of the brain parenchyma and the CSF. The distribution metry of the brain. The MRI images and the diagrams show 4 Applied Bionics and Biomechanics 90 70 100 100 80 80 60 60 40 40 20 20 0 0 0 1 6 11 16 21 159 13 MRI slice index (from le to right) MRI slice index (from top to bottom) Percentage of cerebrospinal fluid Percentage of cerebrospinal fluid Cerebrospinal fluid Cerebrospinal fluid Brain parenchyma Brain parenchyma (a) (b) 35 100 1 11213141 MRI slice index (from head to tail) Percentage of cerebrospinal fluid Cerebrospinal fluid Brain parenchyma (c) Figure 4: The percentage distribution of the CSF in the gecko skull along the coronal direction ((a), sagittal plane), vertical direction ((b), horizontal plane), and sagittal direction ((c), coronal plane). The outer envelope of the CSF indicates the full sectional area of the inner skull. that the brain parenchyma is fully surrounded by the CSF. of CSF was also measured around 989 kg/m . And the viscos- The minimum percentage of the CSF is still above 20%. ity coefficient (μ) of the CSF [18] is around 0.85 mPa·s. Based on the data shown in Figure 4(c), the volume of the The plan model was introduced to present the brain parenchyma and the CSF was integrated as 159.0 mm mechanics of the head impact cushioning. The contact tar- and 127.4 mm , respectively. Thus, the CSF accounts for get surface (ground) is defined as a rigid body in the about 45% of the entire brain volume. model. The elasticity modulus and passion ratio of skull and brain parenchyma are set to be 15 GPa and 0.3 GPa, 3.2. The Finite Element Model of a Simplified Gecko Brain. respectively. The Young’s modulus for the CSF was set The gecko brain was simplified into concentric spheres filled at 2.436 GPa, similar processing as the biomechanical with liquid interval (Figure 5(a)). The parenchyma density of study on the hemolymph in insects [19]. the gecko brain was around 1060 kg/m , which was estimated The finite element model was built and simulated using the ABAQUS/Explicit (ABAQUS 6.6, ABAQUS, Inc., USA), by the ratio of mass to volume. Considering the irregular geo- metrical shape, the parenchyma volume was decided using which is always chosen to solve nonlinear dynamical the fluid volume measuring method. The density of the brain problems such as impact and explosion. Considering the parenchyma [16, 17] is quite close to 1036 kg/m . The density symmetrical structure, the finite element model was built as Area (mm ) Area (mm ) Percentage (%) Area (mm ) Percentage (%) Percentage (%) Applied Bionics and Biomechanics 5 Skull Pressure (MPa) 1077.780 953.083 828.385 703.687 Brain 578.989 parenchyma 454.291 329.593 204.895 80.198 Cerebrospinal −44.500 fluid −169.198 −293.896 −418.594 (a) (b) Figure 5: (a) The simple finite element model of a gecko brain. (b) The pressure distribution around the brain, indicated by a red curve. The black circle denotes the brain surface, and the color-coded mesh represents the values of pressure. Table 1: The comparison of pressure on the brain parenchyma for between the brain and the skull’s vascular tissue, called a sub- different proportion of the CSF at different falling heights. arachnoid cavity. The cavity houses the CSF. It is said that CSF can only provide cushioning from minor bumps and jos- CSF 45% CSF 22% tling. In the instances of strong vibrations or blows, CSF will Height Positive Negative Positive Negative allow excessive movement of the brain, potentially resulting (m) pressure pressure pressure pressure in bruising and concussions. One factor of the antishock (MPa) (MPa) (MPa) (MPa) mechanism in the woodpecker is that it has relatively little 1.0 1077.78 428.92 1285.06 396.25 CSF [12, 20], thereby reducing the transmission of the 0.5 925.93 337.26 1064.86 302.90 mechanical excitations into the brain through the CSF [21, 22]. This is contradictory to what we found in a gecko’s head. However, from a physical point of view, a half-plane strain model. The initial contact between the the mode of head shock in a woodpecker is different from ground and the skull, between the CSF and the skull, and that in the gecko. The former is a bilateral vibration in a the brain and the CSF were established using the contact- certain frequency, by which the brain parenchyma may pair tool in ABAQUS/Explicit. All the structures were be restrained near the equilibrium position relative to the meshed into a quadrilateral, and the CPE4R elements were skull. The latter is an instant unidirectional impaction, employed throughout. The model was built based on the by which the brain parenchyma may overshoot far from fundamental principles of continuum mechanics, and the its equilibrium position. This might be the reason why simplification on the fluid of CSF also had precedents [19], the amounts of CSF in the woodpecker and in gecko are which guaranteed the model as secure as possible. so different. The simulation showed the pressure distribution According to the comparison in Table 1, the high propor- around the brain was varying during the interaction tion of the CSF helps to reduce the maximal positive pres- between the head and the ground. When the two objects sure, around 16% and 13%, at the falling height 1 m and got stuck, strain built up. Along the vertical direction, 0.5 m, respectively. But it also causes an increase in the max- the compressive stress was increasing that caused positive imal negative pressure, around 8% and 11%, at the falling pressure on the brain, while along the lateral direction, height 1 m and 0.5 m, respectively. Both positive and negative the tensile stress was increasing that caused negative pres- pressures in the intracalvarium are the reasons for brain sure on the brain (Figure 5(b)). injury [23]. Decreased positive pressure and increased nega- The CSF accounts for around 45% of a gecko brain. Dif- tive pressure are the two opposites of the self-protection in ferent proportion of the fluid may affect the pressure limits the head impaction but which affects the brain more is still caused by falling and head impact. Additionally, the height unknown. Speculatively, especially for the high falling height of falling could be another factor. Here, we simulated two (1.0 m), for the high proportion of the CSF, the benefitof proportions of the CSF, 45% and 22%, given two different decreasing positive pressure (16%) surpasses the perils of falling heights, 1 m and 0.5 m, respectively. The comparison increasing negative pressure (8%). This may be one of the is shown in Table 1. advantages of natural selection and evolution for animals who can move at high altitude. 3.3. The Succulent Characteristic of a Gecko’s Head. The MRI The finite element model and simulation developed in here is quite simple, which need further investigation using investigation has disclosed the succulent characteristic of a gecko’s head. The intracalvarium of the head is full of the more fidelity models, such as considering the complex corti- cal bone property [24, 25]. However, the simple model has CSF, up to 45% in volume. Animal skulls contain a space 6 Applied Bionics and Biomechanics implied the succulent characteristics of head impact in Gekko Conflicts of Interest geckos qualitatively from several aspects. First, the fluid The authors declare that there are no conflicts of interest spreads the impulse across a wide area, allowing the material regarding the publication of this paper. to absorb more of the impact. Second, the hydrodynamic drag gradually slows down the motion of the brain paren- chyma caused by inertia after head impact. Thirdly, the good Acknowledgments liquidity of the CSF attenuates the positive pressure in the The authors thank Dr. Xuxia Wang at Wuhan Institute of intracalvarium. All those imply that the succulent character- Physics and Mathematics, CAS, for her help in MRI scan- istics of a gecko’s head may play an important role in the self- ning. This research was supported by NSFC (61375096) protection in the head impaction, which is worth further and 863 Program (2015AA042304). studying and supplements to the behavioral and bionic appli- cation studies in Gekko geckos [26]. ARA is one of the important abilities for animals who References move on high. It has been shown that ARA probably starts [1] T. R. Kane and M. P. Scher, “A dynamical explanation of the developmentally as a reflex and within days/weeks matures falling cat phenomenon,” International Journal of Solids and into CPG [9]. From an evolutionary point of view, this is a Structures, vol. 5, no. 7, pp. 663–670, 1969. relatively later evolutionary state. The succulent characteris- [2] E. J. Marey, “The movements that certain animals execute to tic of a gecko’s head may provide a case in which the ARA fall on their feet when they are tossed from an elevated place starts in an earlier evolutionary state. 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Passive Cushiony Biomechanics of Head Protection in Falling Geckos

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Hindawi Applied Bionics and Biomechanics Volume 2018, Article ID 9857894, 7 pages https://doi.org/10.1155/2018/9857894 Research Article Passive Cushiony Biomechanics of Head Protection in Falling Geckos 1 1 2 1 1 Hao Wang , Wenbo Wang, Yi Song, Lei Cai , and Zhendong Dai College of Astronautics, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, China College of Aerospace Engineering, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, China Correspondence should be addressed to Hao Wang; haowang@nuaa.edu.cn Received 16 October 2017; Revised 23 December 2017; Accepted 15 January 2018; Published 19 February 2018 Academic Editor: Qi Shen Copyright © 2018 Hao Wang et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Gekko geckos are capable to crawl on the steep even on upside-down surfaces. Such movement, especially at great altitude, puts them at high risks of incidentally dropping down and inevitable body or head impactions, though they may trigger air-righting reaction (ARR) to attenuate the landing shocks. However, the air-righting ability (ARA) in Gekko geckos is not fully developed. The implementation of ARR in some geckos is quite slow; and for those without tails, the ARR is even unobservable. Since ARA is compromised in Gekko geckos, there must be some other mechanisms responsible for protecting them from head injuries during falls. In this study, we looked into a Gekko gecko’s brain to study its internal environment and structure, using the magnetic resonance imaging (MRI) technique. The results showed that the brain parenchyma was fully surrounded by the cerebrospinal fluid (CSF) in the skull. A succulent characteristic was presented, which meant the intracalvarium was significantly occupied by the CSF, up to 45% in volume. Then a simplified three-dimensional finite element model was built, and a dynamic simulation was conducted to evaluate the mechanical property of this succulent characteristic during the head impactions. These implied the succulent characteristic may play certain roles on the self-protection in case of head impaction, which is adaptable to the Gekko gecko’s locomotion and behavior. not always performed perfectly in geckos. It was showed that 1. Introduction the geckos without tails could not perform air-righting reac- Many animals in nature are able to crawl, climb, or run on tion (ARR) at all, and even for the geckos with tails, almost different inclined surfaces, such as walls, ceilings, branches, one-tenth could not perform ARR well [5]. Actually, the and leaves, at a certain height above the earth. Falling down, ARA might be weaker in Gekko geckos, because the relevant air-righting reflex did not mature, developmentally speaking, as a common experience, is somehow unavoidable. To avoid the body being injured when falling and impacting with the into a complete central program. The published data [9] pro- ground, animals have developed various abilities, such as vided evidence that the ARA probably starts developmentally air-righting abilities (ARA). Cats try to make their feet to as a reflex and within days/weeks mature into a central pat- attach on the ground at first to reduce the impact forces act- tern generator (CPG) by showing that the completion of ing on the body by turning up the body upside-down through the ARR maturation process had no dependency on loads the vertebra and tail [1–4]. Gekko geckos turn up by rotating attached to different parts of the body. In our behavior exper- their tail [5]. Preventing the brain injury from the falling and iments, not all geckos could turn around successfully in impacting to the ground, a sort of self-protection is one of the abdomen-up falling down. Especially when the falling height important survival skills gained from natural selection and is not so high, the time for free fall is not sufficient for this evolution in animals. Aerial maneuverability or air-righting performance. Even though, it rarely causes any injury in the performance is the most important mechanism of self- brain. The head impaction is cushioned somehow. It seems protection in insects [6, 7], cats [1–4], rats [8, 9], rabbits that Gekko geckos may possess certain characteristics to pre- [10], frogs [11], and geckos [5]. However, the air-righting is vent themselves especially their head from injury caused by 2 Applied Bionics and Biomechanics (a) 60 mm (b) Brain parenchyma Cerebrospinal fluid Figure 1: (a) A Gekko gecko lizard and (b) the observation on the intracalvarium of gecko brain using a digital microscope (VHX-600, Keyence, Japan) after craniotomy (the skull was opened and the dura was removed while the arachnoid was intact). impacting the ground while falling down. To disclose the 2.2. The Intracalvarium Morphology Investigation. The intra- potential mechanism underlying the self-protection in head calvarium morphology was investigated by two ways, the impaction, it is necessary to look into the intracalvarium qualitative observation after the surgical anatomy and the structure and material of the animal’s head. quantitative measurement using MRI. Not like a woodpecker’s head, which has been studied After the surgical anatomy, we found that the gecko’s thoroughly for decades [12–15], a Gekko gecko’s head has brain parenchyma (Figure 1) is surrounded by the cerebro- been rarely investigated, since its significance of antishock spinal fluid (CSF). characteristic is much lower than the former. The wood- For MRI investigation, the animal was anaesthetized by pecker’s head can stand high-frequency shocks that are intro- the intraperitoneal injection of 0.4% sodium pentobarbital duced from its drumming beak during the daily forage, while in a 0.75% NaCl solution. A dose of 0.75 ml/100 g body the gecko’s head is only in the risk of one sudden head shock weight was administered. After the pain reflex had disap- caused by an incident dropping. The underlying mechanism peared, the gecko was fixed to a custom-designed fixture of head protection should be different. Here, we took a close (manuscript in preparation, see Figure 2) and then placed look at the intracranial structures by the magnetic resonance into the MRI instrument (BioSpec 7T/20 cm, Bruker, Ger- imaging (MRI) technique. Then a simplified mathematical many). The whole brain was scanned in three orthogonal model was built to qualitatively evaluate the corresponding (sagittal, coronal, and horizontal) planes (Figure 3), and the mechanical property. corresponding spatial interval of the scan was all 0.30 mm. Based on the MRI image sequence, the distribution and the volume of the CSF in the skull were evaluated with the 2. Materials and Methods help of the open source software ImageJ (http://rsbweb.nih .gov/ij/). It is not difficult to distinguish the brain paren- 2.1. Experimental Animals. The Gekko gecko lizards were chyma and the CSF by gray level of the MRI image. Then, brought from Nanning, Guangxi Province, China, and habit- the surface integral and the volume integral were conducted uated to the study colony for two months before the experi- for the brain parenchyma and the CSF, respectively. ments. The mean temperature and relative humidity were 25 C and 65%, respectively, which were close to the values 3. Results and Discussion for the natural ambiance of Gekko geckos. Adult Gekko geckos weighted 40–70 g were selected for the MRI study. 3.1. Distribution of the CSF in the Gecko Skull. The percent- The entire study was carried out in accordance with the age distribution of the CSF in the gecko skull is shown in Guide of Laboratory Animal Management Ordinance of Figure 4. Since the dimensions of the brain along the sagittal, China and approved by the Jiangsu Association for Labora- coronal, and vertical directions are different, the number of tory Animal Science (Jiangsu, China). slices is variable. It provided relatively more detail along the Applied Bionics and Biomechanics 3 (a) (b) Figure 2: An anaesthetized gecko is placed into the MRI instrument, whose head and body are fixed to a custom-designed fixture (a). The close view of the fixed gecko in the fixture without cover (b). (a) (b) (c) Figure 3: The MRI scanning of a gecko head in the sagittal plane (a), horizontal plane (b), and coronal plane (c). The dark area (marked by “1”) indicates the brain parenchyma while the bright area (marked by “2”) the CSF. sagittal direction (coronal plane, Figure 4(c)), so the corre- is clearly symmetrical along the coronal direction (sagittal sponding image sequence was employed to calculate the vol- plane, Figure 4(a)) due to the morphologically bilateral sym- ume of the brain parenchyma and the CSF. The distribution metry of the brain. The MRI images and the diagrams show 4 Applied Bionics and Biomechanics 90 70 100 100 80 80 60 60 40 40 20 20 0 0 0 1 6 11 16 21 159 13 MRI slice index (from le to right) MRI slice index (from top to bottom) Percentage of cerebrospinal fluid Percentage of cerebrospinal fluid Cerebrospinal fluid Cerebrospinal fluid Brain parenchyma Brain parenchyma (a) (b) 35 100 1 11213141 MRI slice index (from head to tail) Percentage of cerebrospinal fluid Cerebrospinal fluid Brain parenchyma (c) Figure 4: The percentage distribution of the CSF in the gecko skull along the coronal direction ((a), sagittal plane), vertical direction ((b), horizontal plane), and sagittal direction ((c), coronal plane). The outer envelope of the CSF indicates the full sectional area of the inner skull. that the brain parenchyma is fully surrounded by the CSF. of CSF was also measured around 989 kg/m . And the viscos- The minimum percentage of the CSF is still above 20%. ity coefficient (μ) of the CSF [18] is around 0.85 mPa·s. Based on the data shown in Figure 4(c), the volume of the The plan model was introduced to present the brain parenchyma and the CSF was integrated as 159.0 mm mechanics of the head impact cushioning. The contact tar- and 127.4 mm , respectively. Thus, the CSF accounts for get surface (ground) is defined as a rigid body in the about 45% of the entire brain volume. model. The elasticity modulus and passion ratio of skull and brain parenchyma are set to be 15 GPa and 0.3 GPa, 3.2. The Finite Element Model of a Simplified Gecko Brain. respectively. The Young’s modulus for the CSF was set The gecko brain was simplified into concentric spheres filled at 2.436 GPa, similar processing as the biomechanical with liquid interval (Figure 5(a)). The parenchyma density of study on the hemolymph in insects [19]. the gecko brain was around 1060 kg/m , which was estimated The finite element model was built and simulated using the ABAQUS/Explicit (ABAQUS 6.6, ABAQUS, Inc., USA), by the ratio of mass to volume. Considering the irregular geo- metrical shape, the parenchyma volume was decided using which is always chosen to solve nonlinear dynamical the fluid volume measuring method. The density of the brain problems such as impact and explosion. Considering the parenchyma [16, 17] is quite close to 1036 kg/m . The density symmetrical structure, the finite element model was built as Area (mm ) Area (mm ) Percentage (%) Area (mm ) Percentage (%) Percentage (%) Applied Bionics and Biomechanics 5 Skull Pressure (MPa) 1077.780 953.083 828.385 703.687 Brain 578.989 parenchyma 454.291 329.593 204.895 80.198 Cerebrospinal −44.500 fluid −169.198 −293.896 −418.594 (a) (b) Figure 5: (a) The simple finite element model of a gecko brain. (b) The pressure distribution around the brain, indicated by a red curve. The black circle denotes the brain surface, and the color-coded mesh represents the values of pressure. Table 1: The comparison of pressure on the brain parenchyma for between the brain and the skull’s vascular tissue, called a sub- different proportion of the CSF at different falling heights. arachnoid cavity. The cavity houses the CSF. It is said that CSF can only provide cushioning from minor bumps and jos- CSF 45% CSF 22% tling. In the instances of strong vibrations or blows, CSF will Height Positive Negative Positive Negative allow excessive movement of the brain, potentially resulting (m) pressure pressure pressure pressure in bruising and concussions. One factor of the antishock (MPa) (MPa) (MPa) (MPa) mechanism in the woodpecker is that it has relatively little 1.0 1077.78 428.92 1285.06 396.25 CSF [12, 20], thereby reducing the transmission of the 0.5 925.93 337.26 1064.86 302.90 mechanical excitations into the brain through the CSF [21, 22]. This is contradictory to what we found in a gecko’s head. However, from a physical point of view, a half-plane strain model. The initial contact between the the mode of head shock in a woodpecker is different from ground and the skull, between the CSF and the skull, and that in the gecko. The former is a bilateral vibration in a the brain and the CSF were established using the contact- certain frequency, by which the brain parenchyma may pair tool in ABAQUS/Explicit. All the structures were be restrained near the equilibrium position relative to the meshed into a quadrilateral, and the CPE4R elements were skull. The latter is an instant unidirectional impaction, employed throughout. The model was built based on the by which the brain parenchyma may overshoot far from fundamental principles of continuum mechanics, and the its equilibrium position. This might be the reason why simplification on the fluid of CSF also had precedents [19], the amounts of CSF in the woodpecker and in gecko are which guaranteed the model as secure as possible. so different. The simulation showed the pressure distribution According to the comparison in Table 1, the high propor- around the brain was varying during the interaction tion of the CSF helps to reduce the maximal positive pres- between the head and the ground. When the two objects sure, around 16% and 13%, at the falling height 1 m and got stuck, strain built up. Along the vertical direction, 0.5 m, respectively. But it also causes an increase in the max- the compressive stress was increasing that caused positive imal negative pressure, around 8% and 11%, at the falling pressure on the brain, while along the lateral direction, height 1 m and 0.5 m, respectively. Both positive and negative the tensile stress was increasing that caused negative pres- pressures in the intracalvarium are the reasons for brain sure on the brain (Figure 5(b)). injury [23]. Decreased positive pressure and increased nega- The CSF accounts for around 45% of a gecko brain. Dif- tive pressure are the two opposites of the self-protection in ferent proportion of the fluid may affect the pressure limits the head impaction but which affects the brain more is still caused by falling and head impact. Additionally, the height unknown. Speculatively, especially for the high falling height of falling could be another factor. Here, we simulated two (1.0 m), for the high proportion of the CSF, the benefitof proportions of the CSF, 45% and 22%, given two different decreasing positive pressure (16%) surpasses the perils of falling heights, 1 m and 0.5 m, respectively. The comparison increasing negative pressure (8%). This may be one of the is shown in Table 1. advantages of natural selection and evolution for animals who can move at high altitude. 3.3. The Succulent Characteristic of a Gecko’s Head. The MRI The finite element model and simulation developed in here is quite simple, which need further investigation using investigation has disclosed the succulent characteristic of a gecko’s head. The intracalvarium of the head is full of the more fidelity models, such as considering the complex corti- cal bone property [24, 25]. However, the simple model has CSF, up to 45% in volume. Animal skulls contain a space 6 Applied Bionics and Biomechanics implied the succulent characteristics of head impact in Gekko Conflicts of Interest geckos qualitatively from several aspects. First, the fluid The authors declare that there are no conflicts of interest spreads the impulse across a wide area, allowing the material regarding the publication of this paper. to absorb more of the impact. Second, the hydrodynamic drag gradually slows down the motion of the brain paren- chyma caused by inertia after head impact. Thirdly, the good Acknowledgments liquidity of the CSF attenuates the positive pressure in the The authors thank Dr. Xuxia Wang at Wuhan Institute of intracalvarium. All those imply that the succulent character- Physics and Mathematics, CAS, for her help in MRI scan- istics of a gecko’s head may play an important role in the self- ning. 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