A transverse dynamic force microscope, more commonly known as shear force microscope, has been used to investigate confined water films under shear. A cylindrically tapered glass probe was mounted perpendicularly to the sample surface. Pure water was confined between the probe and a freshly cleaved mica surface and a sinusoidal shear strain was applied by setting the probe into transverse oscillation. Repeated measurements of the probe oscillation amplitude and relative phase lag, at different tip-sample separations, exhibited a clear step-like behavior. The periodicity, recorded over several curves, ranged between 2.4 and 2.9 Å, which is similar to the diameter of the water molecule. The in-phase (elastic) and the out-of-phase (viscous) stress response of the confined water film was evaluated (from the experimental data) by assuming a linear viscoelastic behavior. Finally, by modeling the water film with the Maxwell mechanical model, the values for the shear viscosity and shear rigidity were obtained.

An atomic force microscope capable of obtaining images in less than 20 ms is presented. By utilizing a microresonator as a scan stage, and through the implementation of a passive mechanical feedback loop with a bandwidth of more than 2 MHz, a 1000-fold increase in image acquisition rate relative to a conventional atomic force microscope is obtained. This has allowed images of soft crystalline and molten polymer surfaces to be collected in 14.3 ms, with a tip velocity of 22.4 cm s −1 while maintaining nanometer resolution.

Conformational changes of proteins adsorbing on biomaterial surfaces affect biocompatibility. Titanium is among the most successful biomaterials; however, the mechanisms leading to its biocompatibility are not yet understood. The primary objective of this study was to investigate the conformation of human plasma fibrinogen (HPF) adsorbed on titanium oxide surfaces. A method for preparing ultraflat titanium substrates was developed. This allowed high-resolution investigation of both the titanium oxide surfaces and the adsorbed state of HPF. The titanium oxide surfaces were first imaged with an atomic force microscope in air and subsequently incubated in a solution of HPF in phosphate buffer and imaged in fluid with tapping mode AFM. The titanium oxide surfaces exhibited a root-mean-squared (RMS) roughness of (0.29 ± 0.03) nm over (1.00 × 1.00) μm2 areas. Different degrees of molecular order were found on the titanium oxide surface. In crystalline surface areas square lattices with parameters a 0 = b 0 ≈ 0.5 nm were resolved, which is consistent with the (001) planes of the titanium dioxide TiO2 rutile. Repetitive scanning of TiO2 surface areas resulted in the abrasion of the outermost TiO2 layers by the AFM tip to a depth varying between 0.3 and 1.0 nm. When imaging adsorbed HPF, individual molecules and aggregates, often joined through their D domains, were resolved, and the typical HPF multiglobular structure was observed. The mean length and height of single molecules were (46 ± 3) nm and (1.4 ± 0.2) nm, respectively. Additional features (chain segments) adjacent to the D and E domains were resolved and attributed to the α chains and their C-termini. The combination of AFM and the ultraflat titanium preparation method has proven successful for the high-resolution study of both the TiO2 surface and the adsorbed HPF.

In insects and vertebrates alike, hearing is assisted by the motility of mechanosensory cells. Much like pushing a swing augments its swing, this cellular motility is thought to actively augment vibrations inside the ear, thus amplifying the ear's mechanical input. Power gain is the hallmark of such active amplification, yet whether and how much energy motile mechanosensory cells contribute within intact auditory systems has remained uncertain. Here, we assess the mechanical energy provided by motile mechanosensory neurons in the antennal hearing organs of Drosophila melanogaster by analyzing the fluctuations of the sound receiver to which these neurons connect. By using dead WT flies and live mutants (tilB 2 , btv 5P1 , and nompA 2 ) with defective neurons as a background, we show that the intact, motile neurons do exhibit power gain. In WT flies, the neurons lift the receiver's mean total energy by 19 zJ, which corresponds to 4.6 times the energy of the receiver's Brownian motion. Larger energy contributions (200 zJ) associate with self-sustained oscillations, suggesting that the neurons adjust their energy expenditure to optimize the receiver's sensitivity to sound. We conclude that motile mechanosensory cells provide active amplification; in Drosophila, mechanical energy contributed by these cells boosts the vibrations that enter the ear.cochlear amplifier ͉ hearing ͉ auditory mechanics ͉ cell mobility ͉ hair cell T he cochlear amplifier is the dominant unifying concept in cochlear mechanics (1). The concept assumes that the cochlea is endowed with a biological energy source that amplifies the ear's input by pumping mechanical energy into the vibrations inside the ear (1-6). The validity of the concept is supported by the mechanics of the cochlea and its mechanosensory cells. Hair cells, the cochlear mechanosensory cells, provide a source of mechanical energy. In addition to transducing mechanical vibrations into electrical responses, some hair cells are equipped with molecular motors that convert metabolic or electrical energy into mechanical energy, resulting in active movements of the cells (1, 3-7). These cellular movements, in turn, exert positive feedback on the cochlear mechanics. By nonlinearly undamping the cochlear resonances as the stimulus intensity declines, this feedback selectively improves the ear's sensitivity to small vibrations induced by faint sound (1, 3-6). Notably, this hair cell-based feedback occasionally becomes unstable, leading to self-sustained feedback oscillations within the cochlear duct. Such self-sustained feedback oscillations may account for the ear's ability to generate spontaneous otoacoustic emissions, i.e., to spontaneously emit sound (8, 9).Collectively, the hair cells' motility, the cochlea's nonlinearity, and the ear's spontaneous otoacoustic emissions document the presence of hair cell-based mechanical feedback inside the cochlear duct. Yet, whether this feedback brings about power gain by expending biological energy, as assumed by the concept of the cochlear ampli...

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Molecular processes are both dynamic and energetic in nature and thus exhibit both conservative (elastic) and dissipative (viscous) forces. We present a method suitable for application to a conventional atomic force microscope (AFM) that is capable of measuring this complex quantity and use it to study the forceextension response of a single molecule. The dynamic force sensitivity of the AFM cantilever was increased with a positive feedback system producing a stable effective quality factor of over 300 in a liquid environment compared to the nominal value of ∼1. This facilitated tracking of the resonant frequency and the separation of conservative (elastic) and dissipative (viscous) forces. With access to this extra information, not available by conventional static force spectroscopy, it is possible to estimate the effective viscosity of a single dextran molecule to be 6000 Pa s.

In this work, a cantilever in a fluid is driven by a mixed signal composed of a standard driving signal and a feedback signal consisting of the amplified and phase shifted oscillation signal. This mimics the oscillation of a cantilever with a quality factor up to three orders of magnitude higher than its natural Q (∼1). This technique allows the identification of the resonance frequency of the cantilever by mechanical excitation of the fluid. The improved sensitivity has been checked by imaging a very soft sample of 1% agarose gel in the dynamic mode. A force smaller than 50 pN could be applied to the sample, improving the spatial resolution and the phase contrast significantly. This technique provides a major improvement in atomic force microscopy/spectroscopy in liquids.

We present a new dynamic force microscopy technique for imaging in liquids in the piconewton regime. The low quality factor (Q) of the cantilever is increased up to three orders of magnitude by the implementation of a positive feedback control. The technique also includes a phase-locked loop unit to track the resonance of the cantilever. Experiments and computer simulations indicate that the tip-sample forces are below 100 pN, about two orders of magnitude lower than in conventional tapping mode atomic force microscopy. Furthermore, the spectroscopic ability is greatly enhanced. Either the phase shift or the resonant frequency shows a high sensitivity to variations in either the energy dissipation or conservative interactions between the tip and the sample, respectively. The potential of this technique is demonstrated by imaging living cells.