A nanometric cushion for enhancing scratch and wear resistance of hard films.

Scratch resistance and friction are core properties which define the tribological characteristics of materials. Attempts to optimize these quantities at solid surfaces are the subject of intense technological interest. The capability to modulate these surface properties while preserving both the bulk properties of the materials and a well-defined, constant chemical composition of the surface is particularly attractive. We report herein the use of a soft, flexible underlayer to control the scratch resistance of oxide surfaces. Titania films of several nm thickness are coated onto substrates of silicon, kapton, polycarbonate, and polydimethylsiloxane (PDMS). The scratch resistance measured by scanning force microscopy is found to be substrate dependent, diminishing in the order PDMS, kapton/polycarbonate, Si/SiO2. Furthermore, when PDMS is applied as an intermediate layer between a harder substrate and titania, marked improvement in the scratch resistance is achieved. This is shown by quantitative wear tests for silicon or kapton, by coating these substrates with PDMS which is subsequently capped by a titania layer, resulting in enhanced scratch/wear resistance. The physical basis of this effect is explored by means of Finite Element Analysis, and we suggest a model for friction reduction based on the "cushioning effect" of a soft intermediate layer.

A stiffness switch in human immunodeficiency virus.

After budding from the cell, human immunodeficiency virus (HIV) and other retrovirus particles undergo a maturation process that is required for their infectivity. During maturation, HIV particles undergo a significant internal morphological reorganization, changing from a roughly spherically symmetric immature particle with a thick protein shell to a mature particle with a thin protein shell and conical core. However, the physical principles underlying viral particle production, maturation, and entry into cells remain poorly understood. Here, using nanoindentation experiments conducted by an atomic force microscope (AFM), we report the mechanical measurements of HIV particles. We find that immature particles are more than 14-fold stiffer than mature particles and that this large difference is primarily mediated by the HIV envelope cytoplasmic tail domain. Finite element simulation shows that for immature virions the average Young's modulus drops more than eightfold when the cytoplasmic tail domain is deleted (930 vs. 115 MPa). We also find a striking correlation between the softening of viruses during maturation and their ability to enter cells, providing the first evidence, to our knowledge, for a prominent role for virus mechanical properties in the infection process. These results show that HIV regulates its mechanical properties at different stages of its life cycle (i.e., stiff during viral budding versus soft during entry) and that this regulation may be important for efficient infectivity. Our report of this maturation-induced "stiffness switch" in HIV establishes the groundwork for mechanistic studies of how retroviral particles can regulate their mechanical properties to affect biological function.

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 Young's 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.

Mechanical properties of murine leukemia virus particles: effect of maturation.

After budding from the host cell, retroviruses undergo a process of internal reorganization called maturation, which is prerequisite to infectivity. Viral maturation is accompanied by dramatic morphological changes, which are poorly understood in physical/mechanistic terms. Here, we study the mechanical properties of live mature and immature murine leukemia virus particles by indentation-type experiments conducted with an atomic force microscope tip. We find that both mature and immature particles have an elastic shell. Strikingly, the virus shell is twofold stiffer in the immature (0.68 N/m) than the mature (0.31 N/m) form. However, finite-element simulation shows that the average Young's modulus of the immature form is more than fourfold lower than that of the mature form. This finding suggests that per length unit, the protein-protein interactions in the mature shell are stronger than those in the immature shell. We also show that the mature virus shell is brittle, since it can be broken by application of large loading forces, by firm attachment to a substrate, or by repeated application of force. Our results are the first analysis of the mechanical properties of an animal virus, and demonstrate a linkage between virus morphology and mechanical properties.

Self-assembled peptide nanotubes are uniquely rigid bioinspired supramolecular structures.

We recently presented a novel class of self-assembled diphenylalanine-based peptide nanotubes. Here, for the first time, we present their mechanical properties, which we directly measured through indentation type experiments using atomic force microscopy. We find that the averaged point stiffness of the nanotubes is 160 N/m, and that they have a correspondingly high Young's modulus of approximately 19 GPa, as calculated by finite element analysis. This high value places these peptide nanotubes among the stiffest biological materials presently known, making them attractive building blocks for the design and assembly of biocompatible nanodevices.