Cellular Solids:Structure, Properties and Applications

Many materials have a cellular structure, with either a two-dimensional array of prismatic cells, as in a honeycomb, or a three-dimensional array of polyhedral cells, as in a foam. Engineering honeycombs and foams can now be made from nearly any material: polymers, metals, ceramics, glasses and composites, with pore sizes ranging from nanometers to millimeters. Their cellular structure gives rise to a unique combination of properties which are exploited in engineering design: their low weights make them attractive for structural sandwich panels, their ability to undergo large deformations at relatively low stresses makes them ideal for absorbing the energy of impacts, their low thermal conductivity makes them excellent insulators, and their high specific surface areas make them attractive for substrates for catalysts for chemical reactions. Cellular materials are increasingly used in biomedical applications. Open-cell titantium foams are being studied for coatings for orthopaedic implants. Porous scaffolds for regeneration of damaged or diseased tissues often resemble an open-cell foam. Cellular materials are also widespread in nature in plant and animal tissues: examples include wood, cork, plant parenchyma, trabecular bone and lung alveoli.

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.

See the course video for 3.054x, Cellular Solids 1: Structures, Properties and Engineering Applications.

(a) Aluminum and (b) paper-phenolic honeycombs. From Gibson LJ and Ashby MF (1997) Cellular Solids. Second Edition, Cambridge University Press.

(a) Open-cell polyurethane (b) closed-cell polyethylene (c ) nickel (d) copper (e) zirconia (f) mullite (g) glass foams and (h) a polyether foam with both open and closed cells. From Gibson LJ and Ashby MF (1997) Cellular Solids. Second Edition, Cambridge University Press.

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 developing processes for making structural bamboo products while those at the University of Cambridge are doing large scale structural testing and developing building codes for SBP.

Bamboo Oriented Strand Board

MIT News : Building up Bamboo

Engineering honeycombs and foams

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.

In-plane deformation of elastomeric honeycombs. (a) undeformed (b) undeformed (c ) uniaxial compression in the horizontal direction (d) uniaxial compression in the vertical direction (e) shear (f) buckling from uniaxial compression in the vertical direction. From Gibson LJ and Ashby MF (1997) Cellular Solids. Second Edition, Cambridge University Press.

Cellular materials in medicine

Trabecular bone is a porous type of bone found at the ends of the long bones and within the vertebrae and shell-like bones, such as the skull and pelvis. In patients with osteoporosis, loss of trabecular bone can lead to increased risk of fracture of vertebrae and the proximal femur. In collaboration with the Orthopaedics Biomechanics Laboratory at the Beth Israel Deaconess Hospital, our group has studied the structure and mechanics of trabecular bone. For instance, we have developed finite element models of the residual stiffness and strength of trabecular bone following bone loss.

Micrographs of cross sections of lumbar vertebrae of a 55 year old woman (top left) and an 86 year old woman (top right). Graph: The reduction in the Young's modulus and compressive strength of a random Voronoi model of trabecular bone, with resorption of trabeculae. (a,b) From Vajjhala S, Kraynik AM and Gibson LJ (2000) A cellular solid model for modulus reduction due to respiration of trabecular in bone. J Biomech. Eng. 122, 511-15. (c ) From Guo XE and Kim CH (1999) Effects of age-related bone loss. ASME 1999 Bioengineering Conference, ASME BED 42, 327-8.

Tissue engineering scaffolds designed to regenerate tissues such as skin, bone and cartilage usually have a highly porous, foam-like structure. We have developed, in conjunction with the groups of 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.

A mineralized collagen-GAG scaffold for regenerating bone. Microscopy by Biraja Kanungo. From Gibson LJ, Ashby MF and Harley BA (2010) Cellular Materials in Nature and Medicine. Cambridge University Press.

The behavior of cells within a tissue engineering scaffold depends on the microstructure and mechanical properties of the scaffold. We have studied the mechanics of cell-scaffold interactions such as cell adhesion, contraction and migration on collagen-based scaffolds.

Cellular materials in nature

Many plant materials, including wood, cork, plant parenchyma, leaves and stems have a cellular structure. We have used our models for cellular solids to understand the mechanical behavior of selected plant systems. For instance, wood and cork can be modeled as honeycombs while plant parenchyma can be modeled as a foam. The leaves of many monocotyledon plants, such as irises, are sandwich panels, with stiff, fiber-reinforced faces separated by a foam-like core of parenchyma cells; the sandwich structure reduces the weight of the leaf for a its required stiffness. Plant stems often have a "core-rind" structure, with a dense outer cylindrical shell supported by a honeycomb-like or foam-like core; the cellular core increases the resistance of the stem to failure by local kinking.

Cellular materials in nature: cedar (top left), cork (top right), trabecular bone (bottom left) and carrot parenchyma (bottom right). From Gibson LJ, Ashby MF and Harley BA (2010) Cellular Materials in Nature and Medicine. Cambridge University Press.

Cellular structures in plants: cross sections of an iris leaf (top) and milkweed stem (bottom). Microscopy by Don Galler.