Mapping nanomechanical properties of live cells using multi-harmonic atomic force microscopy

The nanomechanical properties of living cells, such as their surface elastic response and adhesion, have important roles in cellular processes such as morphogenesis, mechano-transduction, focal adhesion, motility, metastasis and drug delivery. Techniques based on quasi-static atomic force microscopy techniques can map these properties, but they lack the spatial and temporal resolution that is needed to observe many of the relevant details. Here, we present a dynamic atomic force microscopy method to map quantitatively the nanomechanical properties of live cells with a throughput (measured in pixels/minute) that is ～10-1,000 times higher than that achieved with quasi-static atomic force microscopy techniques. The local properties of a cell are derived from the 0th, 1st and 2nd harmonic components of the Fourier spectrum of the AFM cantilevers interacting with the cell surface. Local stiffness, stiffness gradient and the viscoelastic dissipation of live Escherichia coli bacteria, rat fibroblasts and human red blood cells were all mapped in buffer solutions. Our method is compatible with commercial atomic force microscopes and could be used to analyse mechanical changes in tumours, cells and biofilm formation with sub-10?nm detail.     

An atomic force microscope tip designed to measure time-varying nanomechanical forces

Tapping-mode atomic force microscopy (AFM), in which the vibrating tip periodically approaches, interacts and retracts from the sample surface, is the most common AFM imaging method. The tip experiences attractive and repulsive forces that depend on the chemical and mechanical properties of the sample, yet conventional AFM tips are limited in their ability to resolve these time-varying forces. We have created a specially designed cantilever tip that allows these interaction forces to be measured with good (sub-microsecond) temporal resolution and material properties to be determined and mapped in detail with nanoscale spatial resolution. Mechanical measurements based on these force waveforms are provided at a rate of 4 kHz. The forces and contact areas encountered in these measurements are orders of magnitude smaller than conventional indentation and AFM-based indentation techniques that typically provide data rates around 1 Hz. We use this tool to quantify and map nanomechanical changes in a binary polymer blend in the vicinity of its glass transition.     

Characterization of materials' nanomechanical properties by force modulation and phase imaging atomic force microscopy with soft cantilevers

Soft cantilevers, although having good force sensitivity, have found limited use for investigating materials' nanomechanical properties by conventional force modulation (FM) and intermittent contact (IC) atomic force microscopy. This is due to the low forces and small indentations that these cantilevers are able to exert on the surface, and to the high amplitudes required to overcome adhesion to the surface. In this paper, it is shown that imaging of local elastic properties of surface and subsurface layers can be carried out by employing electrostatic forcing of the cantilever. In addition, by mechanically exciting the higher vibration modes in contact with the surface and monitoring the phase of vibrations, the contrast due to local surface elasticity is obtained.     

Atomic force microscopy for the measurement of flexibility of single softwood pulp fibres

A new method based on the atomic force microscope has been developed to measure the lateral flexibility of single wood pulp fibres. In this method, individual wet pulp fibres from earlywood and latewood of Pinus radiata were placed on a newly designed two-point support, and the load and the deflection of fibres were measured under three-point bending test using a modified cantilever probe. The lateral flexibility values of the fibres were then calculated using propped cantilever beam theory. The results obtained indicate that earlywood fibres are substantially more flexible, and have a greater range of flexibility values than latewood fibres.     

Atomic force microscopy of electrochemical nanoelectrodes

Nanometer-sized electrodes have recently been used to investigate important chemical and biological systems on the nanoscale. Although nanoelectrodes offer a number of advantages over macroscopic electrochemical probes, visualization of their surfaces remains challenging. Thus, the interpretation of the electrochemical response relies on assumptions about the electrode shape and size prior to the experiment and the changes induced by surface reactions (e.g., electrodeposition). In this paper, we present first AFM images of nanoelectrodes, which provide detailed and unambiguous information about the electrode geometry. The effects of polishing and cleaning nanoelectrodes are investigated, and AFM results are compared to those obtained by voltammetry and SEM. In situ AFM is potentially useful for monitoring surface reactions at nanoelectrodes.     

Atomic force microscopy using plastic replicas

Scanning probe microscopy is now an accepted tool in both industrial and research efforts. Its development parallels the advances in technology and imaging applications found in the history of progress of both transmission electron microscopy and scanning electron microscopy. All three forms of microscopy ultimately suffer a fundamental application problem—situations arise where it is either unreasonable or impossible to observe a particular sample within the sample stage of the microscope. For the transmission and electron and scanning electron microscopies, this problem has been resolved by resorting to making a replica of the area of interest on the actual sample and preparing the replica so that it may be imaged directly by the desired microscopy technique. This work attempts to ascertain the suitability of observing replicas using a scanned probe microscope; specifically, employing the techniques of atomic force microscopy to image plastic surface replicas.     

An atomic force microscopy tip model for investigating the mechanical properties of materials at the nanoscale

Investigation of the mechanical properties of materials at the nanoscale is often performed by atomic force microscopy nanoindentation. However, substrates with large surface roughness and heterogeneity demand careful data analysis. This requirement is even more stringent when surface indentations with a typical depth of a few nanometers are produced to test material hardness. Accordingly, we developed a geometrical model of the nanoindenter, which was first validated by measurements on a reference gold sample. Then we used this technique to investigate the mechanical properties of a coating layer made of Balinit C, a commercially available alloy with superior anti-wear features deposited on steel. The reported results support the feasibility of reliable hardness measurements with truly nanosized indents.     

Effect of plasma CVD operating temperature on nanomechanical properties of TiC nanostructured coating investigated by atomic force microscopy

The structure, composition, and mechanical properties of nanostructured titanium carbide (TiC) coatings deposited on H₁₁ hot-working tool steel by pulsed-DC plasma assisted chemical vapor deposition at three different temperatures are investigated. Nanoindentation and nanoscratch tests are carried out by atomic force microscopy to determine the mechanical properties such as hardness, elastic modulus, surface roughness, and friction coefficient. The nanostructured TiC coatings prepared at 490 °C exhibit lower friction coefficient (0.23) than the ones deposited at 470 and 510 °C. Increasing the deposition temperature reduces the Young's modulus and hardness. The overall superior mechanical properties such as higher hardness and lower friction coefficient render the coatings deposited at 490 °C suitable for wear resistant applications.     

Atomic force microscopy of scratch damage in polypropylene

Abstract

Atomic force microscopy (AFM) in tapping mode has been used to characterise surface damage on deformed polypropylenes induced during a scratch test. Atomic force micrographs revealed differences in microstructures that could be used to predict the deformation resistance of two different types of polypropylene. The undeformed surface of the two types of polypropylene (identified as polypropylene-L and polypropylene-R) was characterised by differences in arrangement (regular or irregular) of fibrils depending on their melt flow conditions. Polypropylene-L is a polymer with longer chains and with restricted flow, whereas polypropylene-R has shorter chains obtained by controlled rheology. The microfibrils in undeformed polypropylene-L bend, forming raised surface features of height in the region of 10- 50 nm. In comparison to polypropylene-L, the microfibrils in undeformed polypropylene-R exhibited surface features of relatively lower height (10 - 20 nm). 30 × 30 nm scan AFM images provided details of microfibrils containing chains of molecules of ~0.5 nm wide. Surface deformation induced by the scratch resulted in the formation of scratch tracks characterised by regions of quasi-periodic (consecutive) cracking. This type of deformation is attributed to higher applied loads or to higher contact strains. This is particularly important in semicrystalline polymers, where there is partial reorganisation of microstructure on the application of surface stresses because of their viscoelastic properties. Atomic force micrographs of mechanically deformed polypropylene-L and polypropylene-R at a scan size of 1 × 1 μm indicated a lesser amount of reorganisation of microstructure in polypropylene-L as compared with polypropylene-R. Surface profiles and section analysis of the AFM micrographs suggested that polypropylene-R is more scratch resistant in comparison to polypropylene-L under identical scratch test conditions, consistent with Raman spectroscopy observations of tensile deformed polypropylene.     

High-resolution and large dynamic range nanomechanical mapping in tapping-mode atomic force microscopy

High spatial resolution imaging of material properties is an important task for the continued development of nanomaterials and studies of biological systems. Time-varying interaction forces between the vibrating tip and the sample in a tapping-mode atomic force microscope contain detailed information about the elastic, adhesive, and dissipative response of the sample. We report real-time measurement and analysis of the time-varying tip-sample interaction forces with recently introduced torsional harmonic cantilevers. With these measurements, high-resolution maps of elastic modulus, adhesion force, energy dissipation, and topography are generated simultaneously in a single scan. With peak tapping forces as low as 0.6?nN, we demonstrate measurements on blended polymers and self-assembled molecular architectures with feature sizes at 1, 10, and 500?nm. We also observed an elastic modulus measurement range of four orders of magnitude (1?MPa to 10?GPa) for a single cantilever under identical feedback conditions, which can be particularly useful for analyzing heterogeneous samples with largely different material components.     

Atomic force microscopy of electrospun organic-inorganic lipid nanofibers

An organic-inorganic hybridization strategy has been proposed to synthesize polymerizable lipid-based materials for the creation of highly stable lipid-mimetic nanostructures. We employ atomic force microscopy (AFM) to analyze the surface morphology and mechanical property of electrospun cholesteryl-succinyl silane (CSS) nanofibers. The AFM nanoindentation of the CSS nanofibers reveals elastic moduli of 55.3?±?27.6 to 70.8?±?35 MPa, which is significantly higher than the moduli of natural phospholipids and cholesterols. The study shows that organic-inorganic hybridization is useful in the design of highly stable lipid-based materials.     

Atomic force microscopy study of surface-modified carbon fibers

Structural carbon fibers surface-modified with titanium carbide have been studied by atomic force microscopy (AFM). Using statistical analysis of AFM images, quantitative characteristics of fiber surfaces have been determined. The results demonstrate that surface modification has a significant effect on the surface topography of the fibers: their surface becomes smoother and more uniform, as evidenced by the decrease in roughness value and average height and the narrower height distribution.     

Atomic force microscopy study of silica nanopowder compacts

Nanometer silica powders compacted at different pressures have been studied by atomic force microscopy (AFM). Local elastic moduli measurements made on the powder compacts yield values smaller than that of bulk silica. Loading force-distance curves measured show break points at some critical pressures. AFM images obtained at constant contact forces above and below the critical force at which a break point occured show the break point was a result of AFM tip plowing into the nanometer powder compacts. The applied force required for break points to occur increases with sample density. Such a behavior has been qualitatively explained in terms of adhesion force between nanoparticle and sample surface morphology.     

Atomic force microscopy and the mechanism of fatigue crack growth

Fatigue crack test was performed using a grain-orientated 3% silicon iron under constant amplitude loading. Growth behaviour of the fatigue crack and slip deformation behaviour near the crack tip were observed in detail by using an atomic force microscope. In the lower K region, only one preferential slip system of this material operated and the fatigue crack grew along that slip plane. It was found that constraint of slip deformation due to cyclic strain hardening resulted in crack arrest and crack branching. The fatigue crack grew in a zigzag manner as a result of such successive crack branching and deflection. In the high K region, two preferential slip systems operated simultaneously to an almost identical extent and the fatigue crack grew in a direction perpendicular to the loading axis. The slipping distance in one loading cycle was measured quantitatively by using the image processing technique and the crack growth mechanism is discussed.