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Soft Materials and Rheology Group Research Natural and synthetic papillae This research is supported by the Air Force Office of Scientific Research (AFOSR). Biomechanics
of Cephalopod Adaptive Camouflage: Papillae and skin texturing Cephalopods
(which include squid, octopus, and cuttlefish) are well-known for their
adaptive skin coloration, which allows them to blend into a variety of
undersea environments (Fig 1). In addition to coloration, many species are
also able to rapidly and reversibly alter the texture of their skin from
smooth to bumpy, rippled, or leafy (Fig 2). The aim of our research is to
probe the mechanical properties of cephalopod skin which enable this remarkable transition.
We utilize a variety of measurement techniques to characterize local and
global skin behavior, including membrane inflation, indentation, cavitation, and creep experiments (Fig 3). These
measurements are interpreted in context with histological analyses performed
by our marine biologist collaborators in order to formulate a more complete
picture of the mechanism by which the reversible skin texturing occurs. This
research is supported by the Air Force Office of Scientific Research, and is
performed in collaboration with Dr.
Roger Hanlon, Marine Biological Laboratory, Woods Hole, MA.
Fig. 1: Image of a
cuttlefish blending into its surroundings using both color and textural
patterning. (courtesy Roger Hanlon)
Fig. 2: a) Australian
giant cuttlefish (Sepia Apama) expressing bumpy
(arrows) and rippled (circle) papillae. b) Sequence depicting the retraction
of expressed leaf-like papillae in sepia apama. The
skin transitions from highly expressed to completely flat in roughly
one second. See
video here. (Images courtesy of Roger Hanlon).
Fig. 3: Skin
inflation experiments using octopus skin. a) O-rings are glued to the skin of
the octopus before removal to preserve residual skin stress. b) The skin is
clamped and then inflated into a cylindrical hole to characterize its biaxial
mechanical behavior. Synthetic
papillae and reversibly texturing surfaces In addition to elucidating the fundamental mechanisms of
papillae extension, we seek to implement bioinspired
analogs of papillae – surfaces that will undergo reversible change in
texture at the ‘flip of a switch’. Some of the reversible changes
in skin textures expressed by cephalopods may be anaolgous
to an old and well known problem in mechanics, the buckling instability.
Diverse phenomena, such as the wrinkling of skin, or the growth of thin films
for optoelectronic applications is governed by the buckling of rigid thin
films attached to a compressible soft substrate. Such buckling phenomena may
be harnessed to devise synthetic surfaces that show reversible texture.
Mimicking the cephalopod's skin texturing requires the ability to spatially
and temporally control of the buckling process. In order to achieve this, We
use composites based on a shape memory alloy, a smart material, whose shape
can be controlled simply by the flip of a switch. Using the Shape memory alloy as an "artificial
muscle", we have been able to show the formation of wrinkles (Fig. 4).
The wrinkles are sinusoidal undulations, with lengthscales
of the order of millimeter, similar to what is observed in the cephalopods.
The buckling process is reversible, and the film returns back to its previous
configuration, on the relaxation of the shape memory effect. Observe that the
critical strains are relatively small (of the order of 3%),
nevertheless, significant surface texture can be developed. In effect, by
using the strain to drive a mechanical instabilies,
it is possible to generate large displacements even with small strains.
Current studies are focusing on modeling the basic mechanics and dynamics of the actuation
and deactuation of these synthetic papillae. Additionally, we are also studying delaminations,
where the buckling causes the debonding of the film
from the substrate (Fig. 5), resulting in undulations of even larger
magnitude. The delaminations, too are reversible, as seen in the movie below. Finally, while such surface texturing on the flip of a switch
is neat, it is far short of what a cuttlefish can do (Fig. 6 or Fig. 2b)! We
still have a long way to go.
Fig. 4: Wrinkling of a thin polyester film adhering to a
silicone substrate. The wrinkling is caused due to the contraction of the
shape memory wire embedded in the silicone. The transformation from the top
to the bottom picture takes about 0.1 seconds and is reversible.
Fig. 3: Delamination of a polyester film attached to a
silicone surface.
Fig. 6: Papillae arising on the skin of a cuttlefish Sepia Apama.
This sequence image from Allen, J. J.; Mathger, L.
M.; Barbosa, A.; Hanlon, R. T. "Cuttlefish use
visual cues to control three-dimensional skin papillae for camouflage",
J. Comparative Physiology A 2009, 195, 547. Download full paper. Questions, Suggestions,
Comments? Send e-mail to velankar@pitt.edu
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Current projects Interfacially-active
particles Natural and
synthetic papillae
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