|Our group does both modeling and mechanical testing on a wide
range of cellular solids. Our group has contributed to the
understanding of the mechanics of cellular solids, as well as to their
use in many of the above applications. Currently we are working on the
structure and mechanics of bamboo, with a view to the development of
structural bamboo products, and of balsa, with a view towards guiding
the design of engineering materials inspired by balsa.
Structure and Mechanics of Balsa for Sandwich Panels
||Balsa is widely used in the cores of
structural sandwich panels, due to its low density and high specific
mechanical properties. The density and mechanical properties along the
grain vary by a factor of about 6, however. In this project we are
studying the structure and mechanical properties of balsa wood, with a
view to developing engineering materials inspired by balsa. Our group
is modeling balsa wood from the cell wall level up to the macroscale,
in collaboration with Prof. Markus Buehler’s group, who doing
molecular and coarse-grained modeling of the constituents of the cell
wall. The input from the models is guiding the development of 3D
printed materials in Prof. Jennifer Lewis’ group at Harvard.
Structural Bamboo Products (SBP)
||Bamboo has great potential as
sustainable construction material. It grows rapidly and is common
throughout the developing world. Its mechanical properties are
comparable to woods used for structural purposes. In this project, we
are working on the development structural bamboo products, analogous to
wood products like plywood, oriented strand board, and glue-laminated
wood. At MIT, we are characterizing the microstructure and mechanical
properties of culms, of bamboo elements used to make structural
products, and SBP; we are focusing our efforts on Moso bamboo, widely
used in China. Our collaborators at the University of British Columbia
are processing structural bamboo products while those at the University
of Cambridge are developing building codes for SBP, and large scale
||Unlike birds and bats, insects have
no musculature in their wings, so that the only factor influencing the
shape of the wing in flight is the stiffness of the wing itself.
Previous studies, using small scale tensile tests, have shown that the
Young’s modulus of the wing varies with position in the wing. In
this project, we performed nanoindentation tests both on the surface of
the wing and on the cross section, to get a better understanding of the
variation in modulus both across the surface of the wing and through
||Silica aerogels are known for their
remarkably low thermal conductivities. However, the microstructural
features that give rise to their low thermal conductivity also give
poor mechanical properties. In this project, we developed sandwich
panels with 3D truss cores, filled with compacted silica aerogel in the
voids. The panels offer substantial stiffness and strength along with
improved thermal insulation, conserving energy.
|Modeling the microstructure and mechanics of engineering honeycombs and foams forms the basis for all our work
on cellular materials. In the past, we have studied polymer, metal and ceramic
honeycombs and foams as well as their applications in lightweight structural
sandwich panels and in energy absorption devices.
Currently, we are interested in cellular materials for sustainability,
with an emphasis on building materials. We recently studied composite aerogel
panels for building insulation that makes use of the low thermal conductivity
of silica aerogels. Our current projects include structural bamboo products,
analogous to wood products like plywood and oriented strandboard, and
engineered materials inspired by balsa.
Examples of engineering cellular solids: left, an aluminum honeycomb;
center, an open-cell polyurethane foam; right, a closed-cell polyethylene foam
(Gibson, 2005 J. Biomechanics).
|Cellular solids appear in the body as trabecular bone and lung alveoli. Highly porous, foam-like engineering scaffolds are designed to regenerate tissues such as skin, bone and cartilage. The microstructure and mechanical properties of tissue engineering scaffolds affects the attachment, migration and contractile response of cells. Our group has studied the structure and mechanics of trabecular bone, for instance, modeling the residual stiffness and strength as a result of bone loss in osteoporosis. We have also developed, in conjunction with Professor Ioannis Yannas at MIT and Professor Bill Bonfield at Cambridge University, a bilayer osteochondral scaffold designed for the regeneration of cartilage as well as the underlying bone. We have also studied the mechanics of cell-scaffold interactions such as cell adhesion, contraction and migration on collagen based scaffolds.
Osteochondral scaffold with acollagen-GAG layer on top and a calcium
phosphate mineralized collagen-GAG layer below (Harley et al. 2010, Journal of
Biomedical Materials Research).
|Many plant materials, including wood, cork, plant parenchyma, leaves and stems, as well as animal tissues, such as trabecular bone and lung alveoli, have a cellular structure. Their mechanical behaviour can be understood in terms of the models for engineering honeycombs and foams. Current projects include: structural bamboo composites, similar to wood products such as oriented strand board and glue laminated beams, and engineered materials inspired by balsa wood.
Robert Hooke's drawing of the cells in cork, from his book, Micrographia (1665).