The essence of mechanobiology is, probably, the interrelation between mechanical and biochemical factors.  An exciting example of such phenomenon is signaling associated with the interaction between the cell and extracellular matrix (EM).  While some purely biochemical pathways initiated in the area of contact of the cell and EM are known, there are interesting ideas how the mechanical forces, stresses and strains can be involved too. This view goes back to works of Donald Ingber's group in the 90s that showed how perturbations of the adhesion area as a whole and of an individual integrin result in deformation of the cell nucleus. Interestingly, a distinguished oncologist at Johns Hopkins, Donald Coffey, published similar experimental results about the same time, and he also demonstrated that the observed cytoskeleton/nucleus interaction is different in tumor cells. There are several separate pieces of the puzzle that have been resolved: mechanical forces are generated at focal adhesions, the cytoskeleton is involved, nucleus deforms, gene expression changes as a result of perturbation of the adhesions, however, the whole picture of the interrelated mechanical and biochemical factors has yet to be understood. We recently published some results on this topic in the Journal of Biomechanical Engineering (Jean et al., 2004 and 2005). I was glad to find an interest in the same problem from some participants of this website (e.g., N. Wang, Z. Suo,   Long-distance propagation of forces in a cell, 2005 and P.R. LeDuc and R.M. Bellin, Nanoscale Intracellular Organization and Functional Architecture Mediating Cellular Behavior, 2006). This aspect of mechanotransduction is important for many areas beyond mechanics such as cancer, wound healing, cell adhesion and motility, effect of surface micro- and nanopatterning, etc.

Listed below is a recent publication of mine in Science for your possible interest and critics. This is a review article focusing on the multiscale simulation issues in strucutral composites. I will be more than happy to discuss with those of you who are interested. The following is the abstract.

The difficult challenge of simulating diffuse and complex fracture patterns in tough structural composites is at last beginning to yield to conceptual and computational advances in fracture modeling. Contributing successes include the refinement of cohesive models of fracture and the formulation of hybrid stress-strain and traction-displacement models that combine continuum (spatially averaged) and discrete damage representations in a single calculation. Emerging hierarchical formulations add the potential of tracing the damage mechanisms down through all scales to the atomic. As the models near the fidelity required for their use as virtual experiments, opportunities arise for reducing the number of costly tests needed to certify safety and extending the design space to include material configurations that are too complex to certify by purely empirical methods.

I am at Boston for MRS 2006 Fall meeting this week, where I met a real "spiderman" at the poster session tonight. I'd like to share with you the following videos which were posted at YouTube by the "spiderman" himself, Mr. Jose Berengueres at Tokyo Instititute of Technology.

Mr. "Spiderman" also has posted a video on fasting climbing robot.

A variable core model of a moving crystal dislocation is proposed and used to derive an expression for the Peierls stress. The dislocation width varies periodically as a dislocation moves through the lattice, which leads to an expression for the Peierls stress in terms of the difference of the total energies in the crystal corresponding to stable and unstable equilibrium configurations of the dislocation, rather than the difference in the misfit energies alone. Results for both edge and mixed dislocations are given and proposed to be used in conjunction with ab initio calculations.

Mechanics of Solids and Materials intends to provide a modern and integrated treatment of the foundations of solid mechanics as applied to the mathematical description of material behavior. The book blends both innovative (e.g., large strain, strain rate, temperature, time-dependent deformation and localized plastic deformation in crystalline solids, and deformation of biological networks) and traditional topics (e.g., elastic theory of torsion, elastic beam and plate theories, and contact mechanics) in a coherent theoretical framework. This, and the extensive use of transform methods to generate solutions, makes the book of interest to structural, mechanical, and aerospace engineers.

The German Academic Exchange Service (DAAD) - in cooperation with science organizations in North America and Germany— is to invite undergraduate students from the US and Canada in the fields of biology, chemistry, physics, earth Sciences and engineering to apply for a summer research internship in Germany. RISE summer placements take place with research groups at universities and top research institutions across Germany. The RISE interns are matched with a doctoral student whom they assist and who will also serve as their mentor. This program is funded by the Federal Ministry of Economics and Technology as part of the European Recovery Program (ERP).

More details at http://www.daad.de/rise/en/1.html.

Nature 439, 281 (2006)

The theoretical maximum tensile strain — that is, elongation — of a single-walled carbon nanotube is almost 20%, but in practice only 6% is achieved. Here we show that, at high temperatures, individual single-walled carbon nanotubes can undergo superplastic deformation, becoming nearly 280% longer and 15 times narrower before breaking. This superplastic deformation is the result of the nucleation and motion of kinks in the structure, and could prove useful in helping to strengthen and toughen ceramics and other nanocomposites at high temperatures.

The time-dependent plastic deformation (creep) behaviors of both the as-deposited and annealed plasma-enhanced chemical vapor deposited (PECVD) silicon oxide (SiO_x) films were probed by nanoindentation load relaxation tests at room temperature. Our experiments found a strong size effect in the creep responses of the as-deposited PECVD SiO_x thin films, which was much reduced after rapid thermal annealing (RTA). Based on the experimental results, the deformation mechanism is depicted by the "shear transformation zone" (STZ) based amorphous plasticity theories. The physical origin of the STZ is elucidated and linked with the shear banding dynamics. It is postulated that the high strain gradient at shallow indentation depths may be responsible for the reduction in the stress exponent n=∂log(strain rate)/∂log(stress), characteristic of a more homogenous flow behavior.

Gold films on an elastomeric substrate can be stretched and relaxed reversibly by tens of percents. The films initially form in two different structures, one continuous and the other containing tri-branched microcracks. We have identified the mechanism of elastic stretchability in the films with microcracks. The metal, which is much stiffer than the elastomer, forms a percolating network.