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5/24/2018 Structural Design and Mechanical Behavior of Alligator (Alligator Mississippiensis) Osteoderms
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Structural design and mechanical behavior of alligator (Alligator
mississippiensis) osteoderms
Chang-Yu Sun, Po-Yu Chen
Department of Materials Science and Engineering, National Tsing Hua University, 101, Sec. 2, Kuang-Fu Rd, Hsinchu 30013, Taiwan
a r t i c l e i n f o
Article history:Received 9 April 2013
Received in revised form 12 July 2013
Accepted 16 July 2013
Available online 24 July 2013
Keywords:
Osteoderm
Armor
Composites
Mechanical property
Toughening mechanisms
a b s t r a c t
Alligator is a well-adapted living fossil covered with dorsal armor. This dermal shield consists of bonyplates, called osteoderms, interconnected by sutures and non-mineralized collagen fibers, providing a
dual function of protection and flexibility. Osteoderm features a sandwich structure, combining an inner
porous core and an outer dense cortex, to offer enhancements for stiffness and energy absorbance. In this
study, we investigated the multi-scale structure and mechanical behaviors of the American alligator (Alli
gator mississippiensis) osteoderm. Microcomputed tomography was applied to reveal the complex neuro-
vascular network. Through the observation under optical and scanning electron microscopes, the
osteoderm was found to consist of woven bone in the dorsal region and lamellar-zonal bone in the ven-
tral region. Nanoindentation and compressive tests were performed to evaluate the mechanical proper-
ties of osteoderms. The varying mineral contents and porosity result in a graded mechanical property: a
hard and stiff dorsal cortex gradually transform to a more compliant ventral base. Three protective mech-
anisms optimized for alligator osteoderms were proposed and elucidated.
2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved
1. Introduction
Many structural biological materials have been extensively
investigated in recent years due to their superior mechanical prop-
erties, considering the weak building blocks of which they are
composed[13]. Currently, flexible and lightweight dermal armors
have aroused increasing interest due to their intriguing designs for
protection[4,5], including fish scales[612],turtle shells[1316]
and armadillo carapaces [17,18]. Fish scales, such as P. senegalus
[7],A. gigas[8]and A. spatula[10,12], have been widely studied la-
tely. Despite the differences in material compositions, mineral con-
tent and thicknesses, they all applied a similar strategy of
combining a stiffer and harder external region with a softer inter-
nal base. The scales of these marine species exhibit flexibility
through interlocking and overlapping[4,5].On the other hand, armadillo carapace [17,18] and turtle shell
[1316]utilize rather distinct strategies from fish scales. These ar-
mors share many similar structural features: (1) the main constit-
uents of these mineralized tissues are bone, consisting of collagen
fibers and hydroxyapatite minerals; (2) the bony plates are con-
nected by soft tissues or joints; (3) they are covered by keratinous
layers on the outer surface; (4) they are both sandwich composites
with a dense cortex and a porous core. Chen et al.[17]found that
the non-mineralized collagen fibers are responsible for the
macroscopic mechanical responses of the armadillo carapace. The
stretching of these connective fibers between hexagonal plates is
the major contribution to tensile and shear strengths[17]. In turtle
shells, the bony segments are juxtaposed with zigzag joints inter-
locking in between, called sutures. The sutures are three-dimen-
sional (3-D) and complicated structures with organic tissues
giving rise to effortless deformation under small loads and trans-
ferring to stiffer responses after locking under higher degrees o
movement [15]. Rhee et al. [13]reported that the porous core o
the turtle shell is made of closed-cell foam, causing the sandwich
structure to undergo a nonlinear deformation, which leads to a
higher specific energy absorption compared with the dense cortex
alone. Recent investigations conducted by Achrai and Wagner[14
revealed that the dorsal and ventral cortices of the sandwich struc
ture own various mechanical properties as a result of different fi-ber arrangements. The randomly oriented fibrillar network in the
dorsal cortex can sustain sharp impact isotropically, while the ply-
wood arrangement of fibers in the ventral cortex possesses aniso-
tropic mechanical properties and is beneficial for structura
support[14]. The turtle carapace also appears to be a functionally
graded material (FGM) in terms of composition, porosity and
mechanical properties.
Crocodilian osteoderm is another interesting topic in natura
flexible dermal armors. These ancient reptiles have long been con-
sidered as fierce carnivorous tetrapods with heavily armored skins
Although they seldom encounter predators, territorial fight
among the same species can often be deadly because of thei
1742-7061/$ - see front matter 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.actbio.2013.07.016
Corresponding author. Tel.: +886 3 571 5131x33889.
E-mail address: poyuchen@mx.nthu.edu.tw(P.-Y. Chen).
Acta Biomaterialia 9 (2013) 90499064
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extremely high bite force, reaching 10 kN, the highest value that
has been reported for living animals to date[19]. Thus, well-devel-
oped armor designs for excellent mechanical performances are de-
manded, along with some flexibility for speedy and agile
movements in order to capture preys. As a matter of fact, the dorsal
sheaths of crocodilians have been used as armor suits for ancient
warriors since they are found to repel knives and arrows, and are
even bulletproof under certain conditions, as discovered recently[20]. However, the microstructure, mechanical properties and
deformation mechanisms have not been thoroughly investigated.
In this study, we investigated the osteoderm of American alliga-
tor (Alligator mississippiensis) by multi-scale structural character-
izations using materials science approaches. Mechanical
behaviors were evaluated and related to the structure features at
varying length scales, and the deformation as well as toughening
mechanisms of this biological armor when subjected to external
forces were proposed. We hope this study can provide further
understanding of biological defensive designs, and offer inspiration
for novel synthetic armors and advanced composites.
2. Background
Reptiles are cold-blooded animals featuring scales that cover
their whole body. Among them, crocodilians, including crocodiles,
alligators and caimans, are amazing living fossils which appeared
150 million years ago and have evolved into one of the most adap-
tive modern animals on the planet. These large tetrapods possess
not only keratinous scales on their external surfaces, but also un-
ique bony plates underneath the keratinous scales for reinforced
protection, called osteoderms. Crocodilian osteoderms are found
mainly on the dorsal dermis (also on the abdomen for some spe-
cies, such as most caimans), sheltering areas from the nuchal to
the caudal region. These natural armors are composed of mineral-
ized bony plates which are connected by fibrous tissues, similar to
armadillo and turtle carapaces. The hierarchical structure of the
dermal armor of Alligator mississippiensis is schematically pre-sented inFig. 1. The whole armor includes about 70 pieces of bony
plates. Each plate has a longitudinal keel in the middle. Through a
transverse cross-section, various structural features are demon-
strated. The external surface of the bony plates is covered by a thin
layer of keratinous scutes. These scutes, or scales, cover the dorsal
armor of alligator as well as all other parts of its body, and may
vary in shape, composition and formation mechanism. They result
from morphological transitions through differentiation and
keratinization of the crocodilians epidermis [21]. Harder and
tougherb-keratin outer layer coats the osteoderms to provide wear
resistance, while thea-keratin forms mainly the matrix and hingeregions, acting as a barrier to water and electrolyte exchange
[21,22]. In addition, connective fibers are found at the junction of
laterally neighboring bony plates.
Osteoderm is not an original element in evolutionary for croc-odilians. This type of integumentary skeleton is a plesiomorphic
trait for tetrapods [23,24], and has been well demonstrated in
many dinosaurs, such as the renowned stegosaurs [25], ankylo-
saurs [20], and other extinct relatives such as squamates [26]
and archosaurs [27]. The osteoderms in various species differ in
size, shape, ornamentation and functions. In addition to protection
from claws and teeth of predators, other functions of osteoderms,
including heat transfer [25,28], mineral storage[29]and locomo-
tion aid[30], have been suggested. Since these integumentary skel-
etons of reptiles are not subjected to external forces and do not
likely undergo bone remodeling, the ages of these animals can be
estimated by counting the growth marks in the osteoderms corre-
sponding to the seasonal changes in growth rate[31], which is an
equally valid yet much more convenient method to obtain ages
than counting or observing the growth marks in the interior bones
(e.g. long bones) from living or preserved species [32].
3. Materials and methods
3.1. Sample preparation
A complete dorsal armor of an American alligator was obtained
from Jernigans Taxidermy (Waco, Texas, USA). The alligator armor
was prepared using a relatively harmless and natural method
without using strong chemicals, which may alter the natural state
of the samples. The longitudinal length of the armor is 0.85 m,
indicating the animal may have had a body length of1.61.8 m
since the armored part, excluding the head and about half the tail,accounts for 50% of the length of the entire animal. It it is likely to
be a mature alligator, 810 years old, depending on the gender. It
has been reported that male alligators in Texas can be 8 ft long
at the age of 10 and female alligators 67 ft long at the same age
[33].
Keratin coverings on the dorsal surface and dried dermis on the
ventral surface were removed in order to observe the structural
Fig. 1. Hierarchical structure of alligator osteoderm from macro-, meso- and micro-, to nanometer-length scales.
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features of individual osteoderms. All samples prepared for micro-
structural characterization and mechanical testing were taken
from the central and caudal parts of the whole armor to maintain
consistency, since the cervical and transverse terminal osteoderms
possess rather irregular shapes and non-uniform keel heights. It
should be noted that the experimental samples are taken from a
single alligator, and may not be representative of the entire
species.
3.2. Elemental analysis
3.2.1. Ash content measurement
17 samples sectioned from five mid-dorsal osteoderms (34
samples from each) with regions varied from the keel to the edge
were used to determine the average mineral content of the osteo-
derm by the ash-weight method [17]. Since the samples for the
measurement were all from the mid-dorsal osteoderms, the results
may stand for the major portion of the alligator armor. However, it
should be noted that the mineral content may change for osteo-
derms at different locations on the body. Samples were dried on
a hot plate at 105C for 12 h and the dry weights were measured
using an electronic balance. Samples were then ashed at 600C
for 24 h and ash weights were measured. The water content and
ash content (in wt.%) were calculated.
3.2.2. X-ray diffraction
X-ray diffraction (XRD) was carried out on powders of ground
alligator osteoderms utilizing a powder X-ray diffractometer
(XRD-6000, Shimadzu Co., Kyoto, Japan). A continuous scan using
Cu Ka1 (k= 0.154 nm) as the radiation source was performed in a
h2h mode scanning from 2h= 20 to 60, with a step size of
0.02at a rate of 2min1.
3.2.3. Electron probe microanalysis
The localized elemental compositions were analyzed by field-
emission electron probe microanalysis (FE-EPMA) with a JEOL
JXA-8500F EPMA (JEOL Ltd., Tokyo, Japan). Three cross-sectional
samples (10 5 5 mm3) with both dorsal and ventral regions
of the osteoderm were sectioned and embedded in epoxy followed
by grinding and polishing. The samples were coated with a thin
layer of carbon instead of other common conductive coatings such
as gold or platinum because these heavy metal coatings can se-
verely suppress the emission of X-rays induced by the incident
electron, serving as a barrier layer for the signals to come out
and be detected. Five quantitative measurements were taken from
the dorsal and ventral region, respectively, for each specimen, and
the results were then averaged to compare the compositional dif-
ferences between the two regions.
3.2.4. Energy-dispersive spectroscopy
Elemental mapping at interfacial regions was achieved by an
energy-dispersive spectroscope (EDS) within a field-emission scan-
ning electron microscope (FE-SEM) (JSM-7600F, JEOL Ltd.). Sam-
ples were sectioned and ground from the edge of an osteoderm
and coated with a thin layer of carbon (10 nm) to prevent elec-
tron charging. X-rays were produced from electron bombardments
under an accelerating voltage of 10 kV and a working distance of
15 mm. A silicon-drift detector (SDD) (X-Max SDD, Oxford Instru-
ments, Abingdon, Oxfordshire, UK) was used to collect the charac-
teristic X-rays from the sample, while the software AZtec (Oxford
Instruments) was applied to analyze and map the elementaldistributions.
3.3. Structural characterization
3.3.1. Macroscopic observation
External shape and morphology of osteoderms were taken from
central and edge regions of the whole armor. For cross-sectiona
observation, a sample was cut by a hand saw and through the kee
region followed by grinding and polishing. Photographs of top, bot-
tom and cross-sectional views were taken by a digital camera.
3.3.2. Microcomputed tomography (l-CT)l-CT was accomplished by unmonochromatized synchrotron
hard X-rays with energy ranging from 5 keV to 35 keV at the Na
tional Synchrotron Radiation Research Center (NSRRC) in Hsinchu
Taiwan[34]. l-CT images were obtained from a CCD camera (model 211, Diagnostic instruments, 1600 1200 pixels) after convert
ing the X-rays into visible lights by a scintillator. The resolution o
the corresponding images (in pixel size) of a 2 lens was
2.8 lm 2.8 lm. In order to reconstruct 3-D tomographic mod-els, images were collected with a regular step size of 0.3over a to-
tal 180 rotation of the specimen stage. Xradia software (Xradia
Inc., Pleasanton, CA, USA) was applied to reconstruct the raw data
which was then visualized by Amira software (Visualization Sci-
ence Group, a FEI company, Burlington, MA, USA).
3.3.3. Optical microscopy and stereoscopy
Cross-sectional samples through the keel region were ground
and polished before observing under an optical microscope
(BX51M, Olympus Co., Tokyo, Japan) equipped with a 0.8 mega-
pixel digital camera (DP12, Olympus Co., Tokyo, Japan). An inte-
grated view of the keel cross-section was achieved by combining
17 micrographs of consecutive regions.
Stereoscopic images were taken from an Olympus SZX7 Zoom
Stereomicroscope (Olympus Co., Tokyo, Japan) with a 2.0 mega-
pixel CCD camera (Infinity 1, Lumenera Co., Ontario, Canada). The
magnification of the stereoscope ranged from 8to 56.
3.3.4. Scanning electron microscopy
Microstructural characterization of the fracture surfaces were
observed by a FE-SEM (JSM-7600F, JEOL Ltd.). Fracture surfaces
were created by exerting a bending force through a clamp and a
wrench. The specimens were coated with a thin layer of platinum
to enhance electron conductivity on the surface. Secondary elec
tron images (SEIs) were taken with an accelerating voltage o
10 kV and a working distance of 10 mm.
3.4. Mechanical testing
Schematic representations of specimens prepared for mechani
cal testing are shown in Fig. 2. The system of coordinates we
adopted throughout the text is illustrated inFig. 2a. Longitudina
is defined as the direction along the keel long axis, transverse is re-ferred to the direction perpendicular to the keel and vertical is the
direction through the thickness of the osteoderm. Longitudinal and
transverse are both included when the term horizontal i
referred.
3.4.1. Nanoindentation
A sample for nanoindentation was taken from the keel region o
a caudal osteoderm. The sample was mounted with the longitudi-
nal cross-sectional area (Fig. 2a) revealed, followed by grinding and
fine polishing with Al2O3 suspensions from particle size of 1 lm0.3 lm and finally 0.05 lm. The average surface roughness of thefinal sample was 20 nm, measured with atomic force microscopy
(Dimension Icon, Bruker Corp., Billerica, MA, USA). Nanoindenta-
tion tests were conducted by using a Hysitron TI900 TriboIndenter(Hysitron Inc., Eden Prairie, MN, USA) with a Hysitron TI-0039
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Berkovich diamond tip under a load-controlled mode. The radius of
the tip was 100 nm. The load function can be separated into three
stages: linear loading, holding and linear unloading, and the dura-tion for each stage was 5 s, with the peak load set to be 1000 lN.The area function of the tip was calibrated before conducting the
tests with a fused quartz bulk specimen as a standard material
due to its low elastic-modulus-to-hardness ratio [35]. A series of
indentations was performed vertically across the cross-section
(Fig. 2b) with an interval of 300 lm and a total number of 53groups. Each group contained eight indent points forming a 2 4
rectangular region with a lateral space of 15 lm between neigh-boring indents, which is a significantly large distance compared
with the indent size to avoid effects of adjacent indents. The hard-
ness and reduced modulus values of each group were then aver-
aged. The area function was calculated again after the test and
showed no significant variations (0.3%) on the tip geometry, sug-
gesting that the results were reliable even after a large amount ofindentations were performed.
3.4.2. Compressive testing
Samples for compressive testing were cut into 2.3
2.3 4.5 mm3 rectangular pieces by a rotating diamond blade.
The dimensions were chosen to prevent buckling by the Eulers cri-
teria. Each facet of the samples was then ground carefully using a
clamp to ensure that the two surfaces in contact with the upper
and lower load cells are parallel to each other, while keeping the
side surfaces perpendicular to the ends as precise as possible to
eliminate eccentric loading. 80 vertical and 80 horizontal samples
were prepared, as shown in Fig. 2c. The horizontal samples were
taken from flat (non-keel) regions, since the large deviation on
the vertical direction of the keel cross-section may have significantinfluences on experimental results. However, for vertical samples,
the height 4.5 mm cannot be satisfied at locations far away from
the keel due to the decrease in thickness from the keel toward
the edge. Therefore, vertical samples were taken from non-keel re-gions near the keels. The difference in regional distribution be-
tween the horizontal and vertical samples led to different
portions of dorsal and ventral regions, which is clearly demon-
strated inFig. 2c: vertical samples contain more dorsal region than
the horizontal samples. 40 samples from each group (longitudinal
and vertical directions) were immersed in Hanks balanced saline
solution (HBSS) (H2387, SigmaAldrich Co., St Louis, MO) for
24 h before mechanical testing. The rehydrated samples were
tested immediately after taking out of HBSS in order to prevent fur-
ther drying. The other set of samples (40 each direction) were
tested in ambient dry condition. Compressive tests were conducted
by using a universal testing machine (Instron 3343 Single Column
Testing System, Norwood, MA, USA) with a 1 kN load cell at a strain
rate of
1
10
3
s
1
.
3.4.3. Flexibility demonstration
Two adjacent osteoderms were taken from the mid-dorsal re-
gion of the alligator armor to demonstrate the flexibility of the
joints. The bony plates were sectioned transversely to better dis-
play the angles bent by bare hands.
3.4.4. Whole osteoderm compression
A transversely cross-sectioned large osteoderm sample with
keel height of15 mm was used to demonstrate the deformation
of the sandwich structure under compression. The sample was
ground at the bottom to create a flat contact surface, followed by
immersion in HBSS for 24 h before the test. The large-scalecompressive tests were carried out by a universal testing machine
Fig. 2. Schematic illustrations of sample preparations for mechanical tests. The shades of the color in (b) and (c) denote different regions, i.e. dorsal and ventral region. (a) The
headtail direction along which the keel is oriented is defined as the longitudinal (or parasagittal) direction. The direction along the lateral row of osteoderm is defined as the
transverse direction. Both directions are included when horizontal direction is referred. The horizontal direction is perpendicular to the vertical direction. (b) An illustration
of the direction where nanoindentation tests were performed. (c) An illustration of the locations where compression samples of different orientations were taken. Due to the
geometrical limitations, vertical samples are taken near the keel and possess more dorsal region, while the horizontal samples are taken mainly within ventral regions.
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(Instron 4468, Double Column Testing System, Norwood, MA, USA)
with a 50 kN load cell at a strain rate of1 103 s1.
4. Results and discussion
4.1. Macroscopic observation
An osteoderm from the mid-dorsal region appears in a quadrate
shape 5 cm in length and width with a parasagittally aligned keelof 1.5 cm in height (Fig. 3a). Osteoderms taken from different
locations show distinct appearances, in both shape and keel height
An osteoderm from the transverse terminal is shown inFig. 3b for
comparison. The large concave regions on the external surface con-
tain small cavities that connect to the vascular channels, which
were proposed to be the evidence for the thermoregulation func
tion of alligator osteoderms[30].The neurovascular foramina ente
the bony plates from the ventral surface, as shown inFig. 3c, where
the grooves are traces of bifurcated dorsal median arteries circulat-
ing across the surface [30]. The transverse edge of an alligato
osteoderm contains connective collagenous fibers (Fig. 3d) be-tween two adjacent plates, similar to the armadillo carapac
Fig. 3. Photographs showing the top views of mid-dorsal (a) and transverse terminal (b) osteoderms show different shapes and features. The ventral surface (c) of a mid-
dorsal osteoderm shows that arterial grooves were used to hold vessel branches. The arrows indicate pits where the artery bifurcates into nutrient foramina entering the
osteoderm. The edge of the osteoderm between laterally neighboring plates is shown in (d), which was covered by non-mineralized connective fibers. By removing the non-
mineralized connective fibers, the sutures can be observed in (e). A SEM image in (f) shows the 3-D feature of the sutures, which contain pits that are connected to the
neurovascular system within the plates.
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[17]. These fibers account for connectivity and flexibility enhance-
ments. By removing the connective soft fibers, serrated sutures
(Fig. 3e) are revealed, which share the same functional design for
interlocking as the turtle shell[15]. From SEM, we can clearly ob-
serve the 3-D characteristic of the sutures (Fig. 3f). Numerous tiny
pits are also shown inFig. 3f, which are connected to the neurovas-
cular foramina, presumably for vascularization, sensing and nutri-
ent transportation.
4.2. Mineral content measurement and elemental analysis
From the ash-weight measurement, the water content is
10.70 0.58 wt.% and the mineral content of the dried specimen
is 65.77 2.26 wt.%, giving 34.23 2.26 wt.% of the dried alligator
osteoderm to be organic components. The osteoderm of American
alligator possesses a similar mineral content to that of armadillo
carapace (65 wt.%)[17]and bovine femur (67 wt.%)[36], which
is higher than other natural armors such as tortoise shell
(53 wt.%) [37] and fish scale (46 wt.%) [6], as well as that of
some mammalian compact bones, for instance, elk antler
(57 wt.%)[38]. It is also found that the mineral content decreased
gradually from the keel (67.13 0.58 wt.%) through the transitionregion (65.54 0.56 wt.%) to the edge (62.89 1.60 wt.%) as shown
inFig. 4a, which is functionally graded since the edge with sutures
and connective fibers serve for flexibility enhancements, and may
be a result of evolutionary convergence with the turtle shell[14].
To confirm the mineral constituents, the crystalline phase was
detected by XRD. The resulting pattern inFig. 4b can be indexed
to JCPDS 09-0432, revealing hydroxyapatite as the main compo-
nent, which is the same as the minerals in bone and other bony tis-
sues[6,17,38]. Furthermore, localized elemental analysis measuredby FE-EPMA shows that the dorsal region contains more calcium
(29.58 1.15 wt.%) and phosphorus (13.13 0.68 wt.%) than the
ventral region (Ca: 24.53 1.62 wt.%, P: 10.38 0.75 wt.%). The re-
sults indicate that the dorsal region contained more minerals,
which are primarily non-stoichiometric hydroxyapatite, while the
ventral region appeared to be less-mineralized and had more or-
ganic constituents.
Elemental mapping by EDS under FE-SEM was conducted to
analyze the compositional difference between the bony plate
and the connective fibers (Fig. 5). As the secondary electron im-
age (SEI) in Fig. 5a shows, the area of interest was the interface
between sutures, which is still a part of the bony plate, and the
connective fibers at the edge of an osteoderm. Carbon mapping
in Fig. 5b indicated that both regions contain organic contents,
which is collagen [24], and that the amount of organic contents
is obviously much richer in the connective fibers. On the other
hand, distribution of Ca (Fig. 5c) and P (Fig. 5d) clearly illus-
trated that hydroxyapatite (Ca10(PO4)6(OH)2), as the major min-
eral content, is confined in the suture. Therefore, it was
confirmed that the fibers connecting bony plates together are
not mineralized.
4.3.l-CT imaging
A cross-sectional view in the longitudinal direction of an osteo-
derm keel reveals a sandwich structure (Fig. 6a), where the porous
interior is surrounded by compact cortex. A sandwich structure
also appears in various light-weight designs, such as leaves [39],bird beaks [40] and feathers [41], as well as many defensive de-
vices against impact and bending, including human skull[39], tur-
tle shell [14], armadillo carapace [17], fish armor [11] and
horseshoe crab exoskeleton[1]. The main advantage of sandwich
structures is to provide high bending stiffness with minimum
weight. Defensive designs applying this principle also serve the
function of energy absorbance under impact loads by deformations
of the cellular core through elastic bending, brittle fracture or plas-
tic buckling of the walls or trabeculae before undergoing densifica-
tion[39]. The reduced burden of these lightweight armors can thus
enhance locomotion along with improvements in mechanical
properties.
Since cross-sections can only show porosities and incomplete
channels, a computed tomographic technique was applied to gain3-D information of the complex neurovascular network in the
bony plates. By collecting the transmitted X-ray signals and the
corresponding intensities, we can identify the porosity since the
absorbance along the X-ray path is different for materials with
porosity and without porosity. Through rotation during scanning,
the information of the whole specimen can be obtained and 3-D
models can be further reconstructed. A sectioned image of the
reconstructed 3-D model from l-CT scans is shown in Fig. 6b.The neurovascular channels form an intricate 3-D network, where
the major cavity in the center branches out toward the dorsal and
ventral regions with much smaller pipes. The bifurcation within
the dorsal region tends to be more complex compared with the
ventral region, whereas both regions contain evidences of sea-
sonal and/or annual growth, indicated by the white arrows inFig. 6b.
Fig. 4. Minerals in alligator osteoderm. (a) The amount of mineral content withinthe osteoderm was measured by ash-weight method, where the results showed
decrease from the keel to the edge (keel: 67.13 0.58 wt.%, transition:
65.54 0.56 wt.%, edge: 62.89 1.60 wt.%). (b) X-ray diffraction (XRD) pattern of
the alligator osteoderm confirmed that hydroxyapatite is the main mineral
component.
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4.4. Microstructural characterization
4.4.1. Optical microscopy
Four different regions with different microstructural morpholo-
gies of bone can be distinguished from the optical micrograph, as
shown inFig. 7a. At the outer sheath, randomly oriented wovenbone can be observed along with extensive vascularization. The
vascular channels connect to small pits on the external surface,
mainly located within the large concave regions, as previously
shown inFig. 3a and b. These pits and vascular systems are sug-
gested to be the major evidence of the role of osteoderm in ther-
moregulation of the body [30]. The second region beneath the
outer sheath is composed of dense woven bone and scattered
lamellar rings deposited around the neurovascular channels. An
SEM image taken from this region is shown inFig. 7b. The patterns
on the surface indicate collagen fiber bundles being ruptured and
pulled out, where no preferred orientation can be observed, illus-
trating bundles entangled in a randomly woven manner. Scattered
lamellar rings can be recognized in this region. These concentric
lamellar structures are not likely to be secondary osteons sinceno canaliculi and well-developed vascular systems (Haversian
and Volkmanns channels) are observed. Moreover, osteoderms
are not constantly subjected to external loading and bone remod-
eling may be restricted and limited. The third region contains por-
ous woven bone with large neurovascular channels. The woven
bone in this region is similar to that of the outer sheath. The major
neurovascular foramen and branches mainly locate in this region
and the large cavities can reach up to hundreds of micrometers
or several millimeters in diameter. Lamellar bone is also found
around the channels, forming circular rings. At the bottom, the ba-
sal region consists of lamellar-zonal bone[42]. This type of bone is
commonly seen in reptiles, and is related to the poor vasculariza-
tion, which derives from low metabolic rate[42]. The lamellar-zo-
nal bones correspond to seasonal or annual growth, where bone
growth stops or slows down in winters, leaving lines of arrested
growth (LAGs). The lamellae in this region are not the same a
the lamellar bone, but rather a parallel-fibered bone, which is con-
structed by woven collagen fibers with a preferred orientation
(Fig. 7c). Furthermore, Sharpey-fibered bone is also found mostly
in the ventral region and sometimes on the edge of dorsal regionextending in oblique directions from the margin to the core with
wavy or zigzag structures[20,23,24]. This type of bone is derived
from non-mineralized Sharpeys fibers functioning for connection
which fuse into the osteoderm and anchor the bony plates to the
epidermis. The Sharpey-fibered bone in the ventral region indi-
cated locations where the osteoderm contact the epidermis, while
those in the dorsal region implied that the genesis of osteoderm
took place within the skin. It is thus discovered that the alligato
osteoderm consists of various types of bone, resulting in a hetero
geneous composition. The differences in fiber orientation at differ-
ent locations are schematically presented in Fig. 1. It should be
noted that the structural variations between adjacent regions un
dergo a gradual change.
Thin sections of the osteoderm were observed under the stereoscope (Fig. 8). LAGs are clearly observed in the ventral region, while
only vague annuli can be recognized at the dorsal region because
no complete halt occurred during growth. The growth rate is also
different in the two regions. Although this is not a precise quanti-
tative evaluation due to the limited resolution, we can still distin
guish that the interval between annual growth marks in the ventra
region (0.5 mm) is much narrower compared to those in the dor
sal region (1.1 mm). Hence, we proposed a growth model, as
shown in the schematic illustration (Fig. 8). The growth rate in
the keel region is higher than the basal region, leading to a unique
shape of a ridged keel. In addition, according to Vickaryous and
Hall [24], the mineralization process of the osteoderm initiate
from the keel, and then extends radially across the whole plate
Therefore, combining the non-simultaneity in calcification and
Fig. 5. Elemental mapping by energy-dispersive spectroscopy (EDS) at the edge of the osteoderm. (a) An SEI image of the area being analyzed, which shows no significant
distinguishing structural features of the two regions. (b) Carbon mapping indicates that the carbon content is much richer in the connective fibers. (c, d) Calcium and
phosphate mapping clearly demonstrates the lack of minerals, which is mainly hydroxyapatite, within the connective fibers.
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the anisotropy in growth rate, it is implied that the degree of min-
eralization may not be uniform within the entire osteoderm, espe-
cially along the vertical direction.
4.4.2. Scanning electron microscopy
Microstructural characterization of a fractured osteoderm keel
is shown inFig. 9,where comparisons between the dorsal, ventral
regions and the non-mineralized collagen fibers are made. At a
lower magnification, the dorsal region (Fig. 9a) shows a densely
packed and relatively flat fracture surface, indicating a direct fail-
ure and rather brittle behavior upon fracture. The mineralized col-
lagen fiber bundles are fused together and the patterns are
obscured. Under a more detailed view (Fig. 9b), granular morphol-ogies of mineral aggregates on the surface are observed. Individual
fibers can hardly be recognized in this region, implying that the fi-
bers are highly mineralized and form bundles which cannot be eas-
ily separated. In comparison, the fracture surface taken from the
ventral region (Fig. 9c) exhibits a fibrous feature. These collagen fi-
bers were stretched and twisted upon breakage, revealing a more
ductile behavior. Individual fibrils can be easily distinguished at
higher magnification (Fig. 9d), where the surfaces of these fibrils
appear to be smoother compared with those inFig. 9b. It is there-fore suggested that these collagenous fibers should possess a lower
degree of mineralization, in contrast to those in the dorsal region,
which corresponds to the growth rate and mineralization process
as previously proposed in Fig. 8. The SEM observation is also in
good agreement with the previous EPMA results, which suggested
mineral content difference in dorsal and ventral regions.
Fig. 9e shows the microstructural features of the non-mineral-
ized connective fibers between neighboring bony plates to provide
flexibility for the dorsal shield. These organic bundles are ran-
domly oriented, with smooth surface morphologies as shown in
the higher magnification (Fig. 9f). Characteristic patterns of
67 nm periodicity in collagen fibrils, which derives from the stag-
gered molecular arrangement, can also be observed inFig. 9f.
In summary, the alligator osteoderm is a complex, heteroge-
neous, hierarchically structured bio-composite with varying de-
grees of mineralization and porosity at different locations.
Through a thorough compositional and structural characterization
at multiple levels with XRD, EPMA, EDS,l-CT, OM (optical micros-copy) and SEM, the whole dermal armor is revealed to be a hybrid
system of mineralized bony plates and non-mineralized connective
collagen fibers, whereas each bony plate is also a combination of a
heavily-mineralized interwoven dorsal cortex and a less-mineral-
ized parallel-fibered ventral base, with complex 3-D neurovascular
channels branching from the core of the osteoderm. These features
are integrated and organized in the schematic illustrations ofFig. 1.
4.5. Mechanical behavior
4.5.1. Nano-mechanical evaluation
Nanoindentation tests along the cross-section of an alligator
osteoderm reveal the difference in mechanical properties for dorsal
and ventral regions. Hardness and reduced modulus of each indent
are calculated from the loaddepth curves according to the Oliver
Pharr method [43]. Fig. 10a shows the hardness variation across
the keel while the photo above the plot corresponds to the inden-
tation position. The average hardness values are 367 94 MPa for
the ventral region and 690 170 MPa for the dorsal region.
Fig. 10b shows the change in reduced modulus through the keel
cross-section, where the trend is the same as the hardness values.
The ventral region was found to possess an average reduced mod-
ulus value of 13.9 2.1 GPa, gradually increasing to 20.3 3.4 GPa
in the dorsal region. Drops at the exterior region for both hardness
and reduced modulus are observed. The relatively high standarddeviation for both hardness and reduced modulus across the whole
sample can be related to the non-uniform porosity within the
osteoderm. The projected area of contact of nanoindentation on
the specimen is 15 lm2, and if we consider the deformed sur-face profiles around the indents, the area influenced by the inden-
tation can reach up to 10 lm2. On the other hand, the size of theporosity in the osteoderm spreads over a wide range from sub-
micrometer to millimeter, where those smaller than tens of
micrometers may cause serious effects on the indentation results.
We have avoided the larger pores at the scales from millimeters
down to several tens of micrometers under the optical microscope
while choosing the indent positions. However, smaller voids be-
yond the limit of light microscopy can still exist, which we were
unable to identify and avoid. Therefore, it is possible that we haveset our indents on or nearby the porosity and affected the results,
Fig. 6. (a) The sandwich structure of an osteoderm keel cross-section. The porosity
is caused by the foramina branching system. (b) 3-D image reconstructed froml-CTscans by synchrotron X-rays. The white arrows indicate annual growth of the
branches.
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giving rise to the high standard deviation. Furthermore, since the
porosity is three-dimensional, it is very likely that there are voidsunderneath the indentation surface at any depth, which we cannot
observe but may have significant effects on the measured values. In
addition, the osteoderm is composed mainly of woven bone, whichis a loosely structured mineralized tissue. Thus, it is also possible
(a)
Woven & lamellar bone
(b)
Lamellar-zonal bone
(c)
Fig. 7. (a) A combined optical micrograph of an osteoderm keel cross-section. Four regions can be distinguished as marked in the Fig.: outer sheath, woven bone and lamella
bone, woven bone with neurovascular channels and lamellar-zonal bone. (b) An SEM image of the dorsal region showing tangled woven collagen fiber bundles. (c) An SEM
image from the ventral region showing parallel-fibered bone (woven bone with a preferred orientation).
Fig. 8. A schematic drawing demonstrating the proposed growth mechanism of an alligator osteoderm. The insets are stereoscopic images of thin-sectioned samples from the
dorsal (left) and ventral regions (right). The black arrows indicate growth marks (LAGs or annuli).
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that we have indented onto the spacing between the collagen fiber
bundles, resulting in large deviations.
The distribution of localized mechanical properties is in good
agreement with the microstructural features in different regions
previously described in Fig. 7a. The second region (II) from the
external surface contains highly mineralized, relatively dense wo-
ven bone and has the highest hardness and reduced modulus;
whereas less-mineralized lamellar-zonal bone in the basal region
(region IV) possesses the lowest values in both properties. The
exterior region (I) and the porous core (III) both seem to have an
intermediate mechanical property between the dense dorsal re-
gion and the ventral base, corresponding to their similar microcon-
stituents (woven bone with high porosity). Fig. 10c shows the
typical loaddepth curves of the dorsal and ventral regions, illus-
trating the distinct difference in mechanical properties. Under a
constant maximum load, it is apparent that the dorsal region
showed a higher stiffness and hardness with smaller indentation
depth and larger slope of the unloading path, whereas the ventral
region showed an advantage of higher energy dissipation esti-
mated by the area under the curves. The different mechanical
properties between the dorsal and ventral region may relate to
the compositional difference in mineral content and variation in
porosity, as demonstrated in elemental analysis and microstruc-
tural characterization.
The combination of hard external region and soft internal re-
gion is analogous to many biological composites, such as arthropod
Dorsal Region
(a) (b)
Ventral Region
(c) (d)
Non-mineralized
Collagen Fibers
(e) (f)
Fig. 9. SEM fractographs at low and high magnifications of the dorsal (a, b) and ventral (c, d) region of an alligator osteoderm keel and the connective fibers (e, f). Different
morphologies indicate the varied amount of minerals on the collagen fibers: the dorsal region is highly mineralized, the ventral region is less mineralized and the connective
fibers are non-mineralized.
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exoskeleton[44]and fish scales[7,8]. This is beneficial for defen-
sive armors since the stiff and hard exterior can resist penetrations
by sharp claws or teeth, whereas the compliant interior can arrest
crack propagation and provide toughness, acting as a cushion by
absorbing impact energy. However, most of them reported a dis-
continuity (or extremely steep gradations) in mechanical re-
sponses between the stiff and compliant layers. In this study,
hardness and elastic modulus change gradually rather than there
being discrete interfaces, as found in the scales of alligator gar
(Atractosteus spatula), which are composed of two distinct materi-
als (ganoin and bone) [10,12,45]. The alligator osteoderm contains
only bone but undergoes gradual transitions in microstructuresat different locations due to their growth, leading to mechanical
functions. The advantages of comprising a homogeneous material
with mechanical property gradient (i.e. FGM) instead of abrupt
changes include better stress redistribution and enhanced resis-
tance to interfacial failure[7].
4.5.2. Compressive mechanical behavior
Fig. 11a and b shows the representative stressstrain curves of
the compressive tests for vertical and horizontal specimens, respec-
tively, under dry and rehydrated conditions. In the dry condition,
vertical samples can sustain higher stresses (142.1 21.4 MPa)
but eventually underwent a direct fracture in a relatively brittle
manner. On the contrary, the horizontal samples showed lower
strengths (117.5 15.9 MPa) with several stages of deformation be-fore ultimate failure, fractured in a more ductile mode. It should be
noticed that for both orientations, the ultimate strengths occurred
at about the same amount of strain (30%), meaning that both
directions are able to suffer certain amounts of plastic deforma
tions. However, the horizontal samples yielded a much higher com
pressive strain (>40%) before totally breaking down due to th
impedance to direct failure, providing a greater toughness. Further
more, the osteoderm samples of both orientations were also tested
under rehydrated conditions in HBSS to simulate the actual envi
ronments of the biological system, where both vertica
(124.2 21.8 MPa) and horizontal samples (88.6 12.5 MPa
showed a decrease in compressive strength. It was discovered that
in both dry and rehydrated conditions, the vertical samples exhibited higher strengths than the horizontal samples (dry vertical
142.1 MPa vs. dry horizontal: 117.5 MPa, rehydrated vertical
124.2 MPa vs. rehydrated horizontal: 88.6 MPa); moreover, the ver
tical samples in both conditions failed similarly in a direct and
rather brittle way, whereas the horizontal samples in both condi
tions underwent multiple toughening mechanisms upon breakage
(Fig. 11a and b). This implies that the anisotropic mechanical re-
sponse is actually employed by the armor within biological sys-
tems. The compressive mechanical properties are summarized in
Table 1.
Typical fracture samples after compressive tests in both direc
tions are examined under the stereoscope, as demonstrated in
Fig. 11c and d. Direct crack propagation for vertical samples can
be found in Fig. 11c, which is in accordance with the relativelybrittle failure shown in the ss curve. Horizontal samples, on
Fig. 10. Nanoindentation tests along the cross-section of an osteoderm show difference in (a) hardness and (b) reduced modulus in various regions. Note the gradien
transition between the two regions. (c) Representative loaddepth curves for dorsal and ventral regions show distinct difference in mechanical properties at the maximumload of 1000lN.
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the other hand, exhibit wavy and complex fracture paths and re-
veal evidence of lamellae buckling and fiber bridging, as shown inFig. 11d. These toughening mechanisms can prevent catastrophic
collapse of the whole structure and correspond to the resisting
steps before ultimate fracture observed in the ss curve
(Fig. 11b). The observation further illustrates the anisotropy in
mechanical properties for alligator osteoderm.
To establish a relationship connecting the macroscopic com-
pressive behaviors and the localized nanoindentation results, the
elastic modulus from the two tests were compared. To begin with,
the elastic modulus was first calculated from the reduced modulus
reported from the nanoindentation test. The relationship between
reduced modulus and elastic modulus can be presented as
1
Er1 m2s
Es1 m2i
Ei
where Eris the reduced modulus, Es is the elastic modulus of the
specimen andEiis the elastic modulus of the indenter tip.msrepre-sents the Poissons ratio of specimen andmimeans the Poissons ra-tio of the indenter tip. For a Berkovich diamond tip, Ei is 1140 GPa
andm i is 0.07[35]. Since we are unable to determine the Poissonsratio of the osteoderm, the Poissons ratio of a typical compact bone,
which is 0.18 for bovine femur [46], is used to calculate the elastic
modulus. From the above equation, the elastic moduli of alligator
osteoderm from nanoindentation tests are 13.6 GPa in the ventral
region and 20.0 GPa in the dorsal region (using the average reduced
Table 2
Elastic modulus measured from nanoindentation on dorsal and ventral regions of alligator osteoderm, with comparison to bovine and human femurs, and the porosity effect on
the elastic modulus calculated by the Bonfield and Clark equation.
Nanoindentation modulus (GPa) 10% porosity (GPa) 20% porosity (GPa) 30% porosity (GPa) 40% porosity (GPa)
Alligator Osteoderm (Dorsal) 20.0 16.4 10.7 5.5 2.1
Alligator Osteoderm (Ventral) 13.6 11.2 7.3 3.7 1.4
Human Femur 20.1[47]
Bovine Femur 23.1[48]
Fig. 11. Representative compressive stressstrain curves for (a) vertical and (b) horizontal samples under dry and rehydrated conditions. Stereoscopic images showing typical
failure mechanisms in (c) vertical and (d) horizontal samples after compressive deformation.
Table 1
Mechanical properties of dry and rehydrated alligator osteoderm samples in
horizontal and vertical directions (40 samples for each condition).
Direction Hydration
state
Ultimate compressive
strength ravg(MPa)Compressive elastic
modulus E(GPa)
Vertical Dry 142.1 21.4 1.04 0.11
Vertical Rehydrated 124.2 21.8 0.94 0.15
Horizontal Dry 117.5 15.9 1.19 0.22
Horizontal Rehydrated 88.6 12.5 1.07 0.17
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Fig. 12. Experimental demonstrations of two proposed deformation mechanisms: (a) Flexibility provided by sutures and connective fibers allows limited bending. The
neighboring plates could be bent upward to 10 and downward to 20. (b) The major porosity in the center core of the osteoderm is able to absorb some energy a
deformations lower than 10% before cracks start to propagate in the cortex. Large channels and small voids (circled) were found to be squeezed and distorted from 0% and4% to 7% deformation.
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moduli), which showed comparable values with the elastic moduli
of human[47]and bovine[48] femur. However, it is significantly
higher and cannot relate to the compressive modulus. We sug-
gested that it should be a result of porosity effect for macroscopic
mechanical tests. By applying the Bonfield and Clark[49]equation,
which is a modified version of the Mackenzie [50]equation, we can
estimate the contribution of porosity to the elastic modulus:
E E01 1:9p 0:9p2
whereEis the elastic modulus with porosity, E0 is the elastic mod-
ulus of the solid phase only and p represents porosity. Table 2
gives the elastic modulus of bulk material at different porosities
where the elastic modulus from the nanoindentation test is usedas E0. As the result showed, the elastic modulus of compressive
tests clearly cannot reach the calculated value, even when an over-
estimated 40% porosity (the average porosity was 13% as esti-
mated from the l-CT scans) is taken into account, implying thatthe alligator osteoderm is a complex composite and may incorpo-
rate factors other than porosity upon mechanical responses at the
macroscale.
4.6. Deformation mechanisms
Apart from the localized and global mechanical responses of the
mineralized bony tissues within the osteoderm, two additional
deformation mechanisms were proposed: flexibility and sandwich
structure. Thus, simple tests were conducted to demonstrate these
mechanisms, as shown inFig. 12. The sutures and non-mineralizedconnective fibers at the lateral edge of osteoderms provide a
Fig. 13. Schematic illustrations of three deformation mechanisms of alligator dermal armor: (a) Sutures and non-mineralized collagen fibers provide flexibility; (b) sandwich
structure absorbs energy; (c) graded mechanical property from dorsal to ventral regions offers optimization in load redistribution and energy dissipation.
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limited flexibility that allows bending to some extent. As illus-
trated inFig. 12a, the connection device of alligator armor is able
to be bent upward to 10, while the bending downward can go
up to 20. This implied that the osteoderm is designed to suffer
more flexure downward, corresponding to the main function of
load dissipation for protection. On the other hand, a whole piece
of osteoderm with a cross-sectional surface revealed was subjected
to a large-scale compressive test to demonstrate the function of thesandwich structure. We focused on the deformation of the major
channel in the center core and the surrounding small voids before
cracks started to propagate in the dense cortex, which is 10%
deformation. The three successive images at the bottom half of
Fig. 12b showed the cross-section at 0%, 4% and 7% strain, where
the major channel was found to be squeezed and distorted, and
were thus able to absorb additional energy. Also, the void at the
lower right became smaller as the bony plate is being compressed,
possibly resulting from plastic buckling and wall collapsing of the
cellular foam. These simple demonstrations illustrated that flexi-
bility and sandwich structure are both incorporated in the protec-
tion mechanisms of the dorsal shield of alligator.
Based on the experimental results, we summarized three defor-
mation mechanisms of the alligator armor under external loads
(Fig. 13). Whenan externalcompressiveforce actson theosteoderm,
sutures and non-mineralized collagenous fibers connecting adja-
cent bony plates can dissipate loads by providing a limited amount
of flexibility (Fig. 13a)[15]. The movements between neighboring
osteoderms can avoid further deformation under small loads. As
the load increases, the sandwich structure of a bony plate functions
as a preliminary route for energy absorbance (Fig. 13b). The porous
interior acts as a cellular foam, undergoing deformations such as
elastic bending, plastic bucklingor wall breakageto absorb a certain
amount of energy. This mechanism may prevent cracking and thus
maintainstructural integrityof thecortex at small strains. The third
mechanism incorporates the structural and mechanical properties
of the material itself by combining the dorsal and ventral cortex
(Fig. 13c).The hard andhighly mineralized dorsal regioncan sustain
higher vertical stresses as the mechanical testing indicates, and be-cause of the unique shape of the osteoderm, the loads then transfer
down to the less-mineralized, more compliant ventral region. The
ventral region thus suffers from stresses oriented preferentially in
thehorizontal direction. Thebasal regionconsists mainlyof parallel
oriented collagen fibrils (lamellar-zonal bone) and possesses better
capability to absorb energy horizontally through various toughen-
ing mechanisms. In addition, the dorsal and ventral region is joined
through a mechanical property gradient, where interfacial tough-
ness is enhanced and stress is better redistributed. The integration
of these three deformation mechanisms mayleadto a synergistic ef-
fect and therefore an optimized dermal armor for alligator.
5. Conclusions
Dermal armors developed in reptiles as well as some mammals
and fish are considered to be optimized for both protection and
flexibility through millions of years of evolution. In the present
study, we investigated the structure and mechanical behaviors of
the American alligator (Alligator mississippiensis) osteoderms at
multiple length scales. The dermal armor of alligator is a hierarchi-
cally structured composite consisting of mineralized rigid bony
plates connected by non-mineralized collagen fibers. Through the
experiments, we established the structuralmechanical property
relationships and deformation mechanisms of the osteoderm, and
eventually proposed how the dorsal shield of alligator protects
against external threats from the whole armor system to individual
plates. The major discoveries of this study are concluded asfollows:
(1) Complex 3-D sutures and the non-mineralized connective
fibers between neighboring osteoderms provide flexibility
for the whole armor. Strategies such as bridging and stretch
ing of the collagen fibers and interlocking of the sutures are
utilized, which are similar to those observed in armadillo
carapace and turtle shell.
(2) The sandwich structure of the osteoderm shows a compact
cortex surrounding the porous core, enhancing bending stiffness and energy absorption ability with reduced weight. The
intricate 3-D network of the neurovascular system is respon
sible for the spongy interior.
(3) The osteoderm is composed of four different bone morphol
ogies, vertically from the outmost surface to the ventra
region: outer sheath (woven bone with porosity), woven
and lamellar bone, woven bone with major neurovascular
cavities and lamellar-zonal bone. The varying mineral con-
tents and porosity result in different localized hardnes
and reduced modulus values across the osteoderm: from a
hard and stiff dorsal cortex gradually transform to a more
compliant ventral base. Similar design strategies have been
applied in various natural armors as well, implying evolu-
tionary convergence for defensive functionality.
(4) Cross-sectional fracture surfaces of osteoderm through kee
region indicate various degrees of mineralization and thus
different microstructures in the dorsal and the ventra
regions: the dorsal region is highly mineralized, showing
granular morphology and flat fracture surface while the ven-
tral region is less mineralized, showing flexible and twisted
fibrils. Incorporating the two regions with mild mechanica
gradient leads to the anisotropy in compressive behaviors
the vertical orientation is able to bear higher loads, while
the horizontal orientation can absorb more energy through
multiple toughening mechanisms including lamellae buck-
ling and fiber bridging.
(5) Three deformation mechanisms are proposed for the derma
armor of alligator: (1) the flexibility provided by sutures and
non-mineralized collagen fibers can dissipate energy undersmall loads; (2) deformations of the cellular foam interior
absorb impact energy without cortex cracking; (3) a combi
nation of the hard dorsal region and the compliant ventra
region with graded mechanical properties offers optimiza-
tion in load re-distribution and energy absorbance.
Acknowledgements
The authors gratefully thankYu-ChenChan, Hsien-WeiChen, Su
Yueh Tsai and Prof. Jenq-Gong Duh (MSE Department, Nationa
Tsing Hua University) for their support and assistant with the FE
SEM, FE-EPMA and ash-content measurements. We acknowledgeChia-Chi Chien, Bai-Hong Ke, Tsung-Tse Lee and Prof. Yeu-Kuang
Hwu (Institute of Physics, Academia Sinica) forhelp with thetechni
calwork andadvices onl-CT scans and3-D image reconstruction athe National Synchrotron Radiation Research Center (NSRRC). We
would also thank Hsi-Ming Yang, Li-Chi Hsu and Prof. Jyh-Wei Lee
(MSE Department, Ming ChiUniversity of Technology) for their help
with nanoindentation measurements. This research is supported by
National Science Council, Taiwan (NSC100-2218-E-007-016-MY3
and NSC101-2628-E-007-017-MY3).
Appendix A. Figures with essential colour discrimination
Certain figures in this article, particularly Figs. 113, are diffi-cult to interpret in black and white. The full colour images can
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be found in the on-line version, at doi:http://dx.doi.org/10.1016/
j.actbio.2013.07.016.
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