Mechanical tension contributes to clustering of neurotransmitter vesicles at presynaptic terminals. 

Fascinating paper. Congrats to Wei, Xuanhe, and Zhigang. Nice to see a simple and an elegant model together with an intuitively appealing physical interpretation of the bifurcation phenomenon in gels. It woud be interesting to see the time evolution of the drying process and the orientation (theta) of the nano wires.  

The Center for Cellular Mechanics at U of Illinois has recently hosted a week long summer workshop on Cell Mechano Sensitivity (July 30-Aug 3, 2007. The workshop had lectures in the morning an hands on-labs in the aternoons. All the lectures are on the web (with slides and video). They include a large collection of references. The web also has the laboratory protocols for cell culture, cell fixing and staining, single molecule detection, florescence microscopy, and much more. Visit the website:

www.ccm.uiuc.edu  

Apply for NSF fellowships to attend 1-week long workshop on cell mechanics, July 31-Aug 3, 2007.

Check out www.ccm.uiuc.edu

Apply for NSF fellowships to attend 1-week long workshop on cell mechanics, July 31-Aug 3, 2007.

Check out www.ccm.uiuc.edu  

Science 30 March 2007:
Vol. 315. no. 5820, pp. 1831 - 1834
DOI: 10.1126/science.1137580
Jagannathan Rajagopalan, Jong H. Han, M. Taher A. Saif*
In nanocrystalline metals, lack of intragranular dislocation sources leads to plastic deformation mechanisms that substantially differ from those in coarse-grained metals. However, irrespective of grain size, plastic deformation is considered irrecoverable. We show experimentally that plastically deformed nanocrystalline aluminum and gold films with grain sizes of 65 nanometers and 50 nanometers, respectively, recovered a substantial fraction (50 to 100%) of plastic strain after unloading. This recoverywas time dependent and was expedited at higher temperatures. Furthermore, the stress-strain characteristics during the next loading remained almost unchanged when strain recovery was complete.These observations in two dissimilar face-centered cubic metals suggest that strain recovery might be characteristic of other metals with similar grain sizes and crystalline packing.

Department of Mechanical Science and Engineering, University of Illinois at Urbana-Champaign, Urbana, IL 61801, USA.

We report the loading and unloading force response of single living adherent fibroblasts due to large lateral indentation obtained by a two-component microelectromechanical systems (MEMS) force sensor. Strong hysteretic force response is observed for all the tested cells. For the loading process, the force response is linear (often with small initial non-linearity) to a deformation scale comparable to the undeformed cell size, followed by plastic yielding. In situ visualization of actin fibers (GFP) reveals that during the indentation process, actin network depolymerizes irreversibly at discrete locations to form well-defined circular actin agglomerates all over the cell, which explains the irreversibility of the force response. Similar agglomeration is observed when the cell is compressed laterally by a micro plate. The distribution pattern of the agglomerates strongly correlates with the arrangement of the actin fibers of the pre-indented cell. The size of the agglomerates increases with time as ta  with a= 2~3 initially,   followed by a=.5~1. The higher growth rate suggests influx of actin into the agglomerates. The slower rate suggests a diffusive spreading, but the diffusion constant is two orders of magnitude lower than that of an actin monomer through the cytoplasm. Actin agglomeration has previously been observed due to biochemical treatment, gamma-radiation, and ischemic injury, and has been identified as a precursor to cell death. We believe, this is the first evidence of actin agglomeration due to mechanical stimuli. The study demonstrates that living cells may initiate similar functionalities in response to dissimilar mechanical and biochemical stimuli.