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This will be a repeat presentation for those who missed the previous talk.

Cracks are the major vehicle for material failure and often exhibit complex dynamics. In spite of the fact that the laws that govern their motion have been intensively investigated for nearly a century, several fundamental issues in dynamic fracture remain poorly understood. A major stumbling block in making progress in this problem is that it involves the coupling between widely separated scales; fast fracture, which is ultimately driven by the release of (linear) elastic energy slowly stored on large scales, is affected by the rapid, non-linear and dissipative dynamics taking place in the very small scales near the front of a moving crack. In this talk, I will describe some of the major challenges in this field and review recent experimental and theoretical advances, highlighting basic properties of the recently developed “Weakly Nonlinear Theory of Dynamic Fracture”.

Cells respond to external signals by modifying their internal state and their environment. In multicellular organisms in particular, cellular differentiation and intra-cellular signaling are essential for the coordinated development of the organism. While some of the major players of these complex interaction networks have been identified, much less is known of the quantitative rules that govern their interaction with one another and with other cellular components (affinities, rate constants, strength of non linearities such as feedback or feedforward loops, etc.). To investigate these interactions (a prerequisite before understanding or modeling them), one needs to develop means to control or interfere spatially and temporally with these processes.
In the preceding context, we have retained the principle of a small lipophilic molecule to photo-activate several properly engineered proteins in vivo. We have adopted a steroid-related inducer as various proteins (e.g. Engrailed, Otx2, Gal4, p53, kinases such as Raf-1, Cre and Flp recombinases) fused to a steroid receptor were shown to be activated by binding of an appropriate ligand.[1] In its absence, the receptor forms a cytoplasmic assembly with a chaperone complex: the fusion-protein is inactivated. Its function is restored in the presence of the steroid ligand which binds to the receptor and disrupts the complex.
The present non-invasive optical method has been implemented for the fast control of protein activity down to the single cell level in a live zebrafish embryo.[2] In particular, we labeled single cells transiently (by activating a fluorescent protein) or irreversibly (by activating a Cre recombinase in an appropriate transgenic animal). The present method could be used more generally to investigate important physiological processes (for example in embryogenesis, organ regeneration and carcinogenesis) with high spatio-temporal resolution (single cell and faster than minute scales).
References
1 D. Picard, Curr. Op. Biotech.,1994, 5, 511-515.
2 D. K. Sinha et al., ChemBioChem. 2010, 11, 653-663 ;Zebrafish, 2010