David Barlam

Measurement of the mechanical properties of isolated tectorial membrane using atomic force microscopy

The tectorial membrane (TM) is an extracellular matrix situated over the sensory cells of the cochlea. Its strategic location, together with the results of recent TM-specific mutation studies, suggests that it has an important role in the mechanism by which the cochlea transduces mechanical energy into neural excitation. A detailed characterization of TM mechanical properties is fundamental to understanding its role in cochlear mechanics. In this work, the mechanical properties of the TM are characterized in the radial and longitudinal directions using nano- and microindentation experiments conducted by using atomic force spectroscopy. We find that the stiffness in the main body region and in the spiral limbus attachment zone does not change significantly along the length of the cochlea. The main body of the TM is the softest region, whereas the spiral limbus attachment zone is stiffer, with the two areas having averaged Youngs modulus values of 37 ± 3 and 135 ± 14 kPa, respectively. By contrast, we find that the stiffness of the TM in the region above the outer hair cells (OHCs) increases by one order of magnitude in the longitudinal direction, from 24 ± 4 kPa in the apical region to 210 ± 15 kPa at the basilar end of the TM. Scanning electron microscopy analysis shows differences in the collagen fiber arrangements in the OHC zone of the TM that correspond to the observed variations in mechanical properties. The longitudinal increase in TM stiffness is similar to that found for the OHC stereocilia, which supports the existence of mechanical coupling between these two structures.

Sound-Evoked Deflections of Outer Hair Cell Stereocilia Arise from Tectorial Membrane Anisotropy

The exceptional performance of mammalian hearing is due to the cochleas amplification of sound-induced mechanical stimuli. During acoustic stimulation, the vertical motion of the outer hair cells relative to the tectorial membrane (TM) is converted into the lateral motion of their stereocilia. The actual mode of this conversion, which represents a fundamental step in hearing, remains enigmatic, as it is unclear why the stereocilia are deflected when pressed against the TM, rather than penetrating it. In this study we show that deflection of the stereocilia is a direct outcome of the anisotropic material properties of the TM. Using force spectroscopy, we find that the vertical stiffness of the TM is significantly larger than its lateral stiffness. As a result, the TM is more resistant to the vertical motion of stereocilia than to their lateral motion, and so they are deflected laterally when pushed against the TM. Our findings are confirmed by finite element simulations of the mechanical interaction between the TM and stereocilia, which show that the vertical outer hair cells motion is converted into lateral stereocilia motion when the experimentally determined stiffness values are incorporated into the model. Our results thus show that the material properties of the TM play a central and previously unknown role in mammalian hearing.

Radial compression studies of WS2 nanotubes in the elastic regimea)

Multiwalled nanotubes and nanoparticles of metal dichalcogenides express unique mechanical and tribological characteristics. A widely studied member of this class of materials is the WS2 nanotube whose structure consists of layers of covalent W-S bonds joined by the van der Waals interactions between the sulfur layers which mediate any interlayer sliding or compression. One of the intriguing aspects of these structures is the response of these layers under mechanical stress. Such internal degrees of freedom can profoundly impact on the overall mechanical response. The fact that the internal structure of these nanotubes is well characterized enables a full treatment of the problem. Here, the authors report an experimental and modeling study of the radial mode of deformation. Three independent atomic force microscope experiments were employed to measure the nanomechanical response using both large (radius=100 nm) and small (radius=3–15 nm) probe tips. Two different analytical models were applied to analyze ...

Experimental, finite element, and density-functional theory study of inorganic nanotube compression

Interactions between the walls in multiwalled nanotubes are key to determining their mechanical properties. Here, we report studies of radial deformation of multiwalled WS2 nanotubes in an atomic force microscope. The experimental results were fitted to a finite element model to determine the radial modulus. These results are compared with density-functional tight-binding calculations of a double-walled tube. Good agreement was obtained between experiment and calculations. The results indicate the importance of the sliding between layers in moderating the radial modulus. A plateau in the deformation curves is seen to have atomistic origins.

Generating an inhomogeneous stress field as a technique to study cell mechanoresponse

Over the last decade, it has been shown that cells can sense and respond to mechanical perturbations in their underlying substrate (or extra-cellular matrix, ECM). However, the effect of an inhomogeneous stress on cell response has been scarcely studied, mainly due to technical difficulty to create a well-controlled stress (or strain) state in the ECM, even though stress gradients are of critical importance in—and likely induce—wound healing, for example, through local cell growth and tissue formation. Here, we present a technique which can be easily used to study the response behavior of cells to a well-defined inhomogeneous stress field.

AFM Investigation of Mechanical Properties of Dentin

Mechanical properties of peritubular dentin were investigated using scanning probe microscopy techniques, namely Nanoindentation and Band Excitation. Particular attention was directed to the possible existence of a gradient in these properties moving outward from the tubular lumen to the junction with the intertubular dentin. Finite element analysis showed that the influence of the boundaries is small relative to the effects observed. Thus, these results strongly support the concept of a lowering of modulus and hardness from the tubular exterior to its periphery, which appear to correlate with graded changes in the mineral content.