Advances in cellular nanoscale force detection and manipulation

The mechanical properties of cells reflect the state of macro-organism. While many approaches are available for performing mechanical property investigations on cells, to characterize dynamic mechanical property of a single cell is challenging. We measure the mechanical properties of a single cell using an Atomic Force Microscopy and develop a novel method to characterize dynamic mechanical property of a single cell based on the theory of system identification. We evaluated the new method by comparing its performance with the other methods in classifying different kinds of cells. The results show that the method we proposed outperformed other methods by achieving average classification accuracy of more than 90%. This work provides a new attempt for characterization of a single cell system, which is of great significance for studying the dynamic changes of the mechanical properties of single cells and analyzing or controlling the state of cells.

The silicon surface of commercial atomic force microscopy (AFM) probes loses its hydrophilicity by adsorption of airborne and package-released hydrophobic organic contaminants. Cleaning of the probes by acid piranha solution or discharge plasma removes the contaminants and renders very hydrophilic probe surfaces. Time-of-flight secondary-ion mass spectroscopy and X-ray photoelectron spectroscopy investigations showed that the native silicon oxide films on the AFM probe surfaces are completely covered by organic contaminants for the as-received AFM probes, while the cleaning methods effectively remove much of the hydrocarbons and silicon oils to reveal the underlying oxidized silicon of the probes. Cleaning procedures drastically affect the results of adhesive force measurements in water and air. Thus, cleaning of silicon surfaces of the AFM probe and sample cancelled the adhesive force in deionized water. The significant adhesive force values observed before cleaning can be attributed to formation of a bridge of hydrophobic material at the AFM tip-sample contact in water. On the other hand, cleaning of the AFM tip and sample surfaces results in a significant increase of the adhesive force in air. The presence of water soluble contaminants at the tip-sample contact lowers the capillary pressure in the water bridge formed by capillary condensation at the AFM tip-sample contact, and this consequently lowers the adhesive force.

A hierarchic set of gradually complicated models is suggested. The models describe the contact condition of the mechanical interaction between the atomic force microscope (AFM) probe and the investigated surface while allowing for various factors. A variant of the AFM probe-surface mechanical interaction model is developed which takes into account the simultaneous effect of the probe geometry and the elastic deformations of the AFM cantilever that take place when there is a small or moderate curvature of surfaces under study. A variant of the model of interaction between the AFM probe and the surface of the investigated specimen is discussed, taking into consideration the simultaneous effect of the probe geometry and elastic deformations of the AFM cantilever and the probe contact with the surface under study within the framework of Derjaguin-Muller-Toropov model. Algorithms to determine the AFM probe position are created to optimize computations.

Carbon nanotubes (CNTs) can be used as atomic force microscope (AFM) probes due to their robust mechanical properties, high aspect ratio and small diameter. In this study, a model of CNTs clamped in an elastic medium is proposed as CNT AFM probes. The buckling instability of the CNT probe clamped in elastic medium is analyzed based on the nonlocal Euler–Bernoulli beam model and the Whitney–Riley model. The clamped length of CNTs, and the stiffness of elastic medium affect largely on the stability of CNT AFM probe, especially at high buckling mode. The result shows that the buckling stability of the CNT AFM probe can be largely enhanced by increasing the stiffness of elastic medium. Moreover, the nonlocal effects of buckling instability are investigated and found to be lager for high buckling mode. The theoretical investigation on the buckling stability would give a useful reference for designing CNT as AFM probes.

This paper presents the design and evaluation of an Atomic Force Microscope (AFM) probe with exchangeable tips. It can be employed to replace tips worn out during imaging, or to exchange tips during multi-step nano-metrology and manipulation tasks. The probe employs a liquid meniscus-based micro-gripper at its end to enable exchange of tips. The design of the gripper and tip-supply station are discussed and the stiffness of the gripper is analyzed. Subsequently, the system is fabricated and evaluated by performing contact mode imaging of a calibration grating. The resulting topographical image is shown to be free of artifacts, thereby underscoring the fact that the micro-gripper can grip the AFM tip adequately well to resist the effect of lateral forces in contact mode AFM. It also shows that probes with exchangeable tips can be used in applications similar to that of a conventional AFM probe.

The micro-Wilhelmy method is a well-established method of determining surface tension by measuring the force of withdrawing a tens of microns to millimeters in diameter cylindrical wire or fiber from a liquid. A comparison of insertion force to retraction force can also be used to determine the contact angle with the fiber. Given the limited availability of atomic force microscope (AFM) probes that have long constant diameter tips, force-distance (F-D) curves using probes with standard tapered tips have been difficult to relate to surface tension. In this report, constant diameter metal alloy nanowires (referred to as "nanoneedles") between 7.2 and 67 microm in length and 108 and 1006 nm in diameter were grown on AFM probes. F-D and Q damping AFM measurements of wetting and drag forces made with the probes were compared against standard macroscopic models of these forces on slender cylinders to estimate surface tension, contact angle, meniscus height, evaporation rate, and viscosity. The surface tensions for several low molecular weight liquids that were measured with these probes were between -4.2% and +8.3% of standard reported values. Also, the F-D curves show well-defined stair-step events on insertion and retraction from partial wetting liquids, compared to the continuously growing attractive force of standard tapered AFM probe tips. In the AFM used, the stair-step feature in F-D curves was repeatably monitored for at least 0.5 h (depending on the volatility of the liquid), and this feature was then used to evaluate evaporation rates (as low as 0.30 nm/s) through changes in the surface height of the liquid. A nanoneedle with a step change in diameter at a known distance from its end produced two steps in the F-D curve from which the meniscus height was determined. The step features enable meniscus height to be determined from distance between the steps, as an alternative to calculating the height corresponding to the AFM measured values of surface tension and contact angle. All but one of the eight measurements agreed to within 13%. The constant diameter of the nanoneedle also is used to relate viscous damping of the vibrating cantilever to a macroscopic model of Stokes drag on a long cylinder. Expected increases in drag force with insertion depth and viscosity are observed for several glycerol-water solutions. However, an additional damping term (associated with drag of the meniscus on the sidewalls of the nanoneedle) limits the sensitivity of the measurement of drag force for low-viscosity solutions, while low values of Q limit the sensitivity for high-viscosity solutions. Overall, reasonable correspondence is found between the macroscopic models and the measurements with the nanoneedle-tipped probes. Tighter environmental control of the AFM and treatments of needles to give them more ideal surfaces are expected to improve repeatability and make more evident subtle features that currently appear to be present on the F-D and Q damping curves.

The atomic force microscope (AFM) is a probe-based microscope that uses nanoscale and structural imaging where high resolution is desired. AFM has also been used in mechanical, electrical, and thermal engineering applications. This unique technique provides vital local material properties like the modulus of elasticity, hardness, surface potential, Hamaker constant, and the surface charge density from force versus displacement curve. Therefore, AFM was used to measure both the diameter and mechanical properties of the collagen nanostraws in human costal cartilage. Human costal cartilage forms a bridge between the sternum and bony ribs. The chest wall of some humans is deformed due to defective costal cartilage. However, costal cartilage is less studied compared to load bearing cartilage. Results show that there is a difference between chemical fixation and non-chemical fixation treatments. Our findings imply that the patients' chest wall is mechanically weak and protein deposition is abnormal. This may impact the nanostraws' ability to facilitate fluid flow between the ribs and the sternum. At present, AFM is the only tool for imaging cells' ultra-structure at the nanometer scale because cells are not homogeneous. The first layer of the cell is called the cell membrane, and the layer under it is made of the cytoskeleton. Cancerous cells are different from normal cells in term of cell growth, mechanical properties, and ultra-structure. Here, force is measured with very high sensitivity and this is accomplished with highly sensitive probes such as a nano-probe. We performed experiments to determine ultra-structural differences that emerge when such cancerous cells are subject to treatments such as with drugs and electric pulses. Jurkat cells are cancerous cells. These cells were pulsed at different conditions. Pulsed and non-pulsed Jurkat cell ultra-structures were investigated at the nano meter scale using AFM. Jurkat cell mechanical properties were measured under different conditions. In addition, AFM was used to measure the charge density of cell surface in physiological conditions. We found that the treatments changed the cancer cells' ultra-structural and mechanical properties at the nanometer scale. Finally, we used AFM to characterize many non-biological materials with relevance to biomedical science. Various metals, polymers, and semi-conducting materials were characterized in air and multiple liquid media through AFM - techniques from which a plethora of industries can benefit. This applies especially to the fledging solar industry which has found much promise in nanoscopic insights. Independent of the material being examined, a reliable method to measure the surface force between a nano probe and a sample surface in a variety of ionic concentrations was also found in the process of procuring these measurements. The key findings were that the charge density increases with the increase of the medium's ionic concentration.

The mechanical properties of single cells have been recently identified as the basis of an emerging approach in medical applications because they are closely related to the biological processes of cells and, ultimately, human health conditions. In this article, we provide a brief review of the intrinsic mechanical properties of single cells related to cancer and aging. The mechanical properties can be used as biomarkers for early cancer diagnosis because cancer cells have a lower Young's modulus, indicating higher elasticity or softness than their counterpart normal cells. The metastatic potential of cancer cells is inversely correlated with their elastic properties. Aging induces stiffness through an increased amount of cytoskeletal fiber. Changes in the mechanical properties also show potential for drug screening. Although there are several challenges to be met before clinical applications can be made, such mechanical properties of single cells may provide new approaches to human diseases.

In atomic force microscopy (AFM), sharp and wear-resistant tips are a critical issue. Regarding scanning electrochemical microscopy (SECM), electrodes are required to be mechanically and chemically stable. Diamond is the perfect candidate for both AFM probes as well as for electrode materials if doped, due to diamond's unrivaled mechanical, chemical, and electrochemical properties. In this study, standard AFM tips were overgrown with typically 300 nm thick nanocrystalline diamond (NCD) layers and modified to obtain ultra sharp diamond nanowire-based AFM probes and probes that were used for combined AFM-SECM measurements based on integrated boron-doped conductive diamond electrodes. Analysis of the resonance properties of the diamond overgrown AFM cantilevers showed increasing resonance frequencies with increasing diamond coating thicknesses (i.e., from 160 to 260 kHz). The measured data were compared to performed simulations and show excellent correlation. A strong enhancement of the quality factor upon overgrowth was also observed (120 to 710). AFM tips with integrated diamond nanowires are shown to have apex radii as small as 5 nm and where fabricated by selectively etching diamond in a plasma etching process using self-organized metal nanomasks. These scanning tips showed superior imaging performance as compared to standard Si-tips or commercially available diamond-coated tips. The high imaging resolution and low tip wear are demonstrated using tapping and contact mode AFM measurements by imaging ultra hard substrates and DNA. Furthermore, AFM probes were coated with conductive boron-doped and insulating diamond layers to achieve bifunctional AFM-SECM probes. For this, focused ion beam (FIB) technology was used to expose the boron-doped diamond as a recessed electrode near the apex of the scanning tip. Such a modified probe was used to perform proof-of-concept AFM-SECM measurements. The results show that high-quality diamond probes can be fabricated, which are suitable for probing, manipulating, sculpting, and sensing at single digit nanoscale.

Tetrapods have the unique property of self-aligning on a flat surface with one arm pointing in the direction perpendicular to the surface plane. The positioning of single tetrapods on the flattened tip of commercial AFM probes (see image) is reported. These tetrapod-functionalized AFM probes are used to image the topography of different samples revealing excellent spatial and vertical resolution. Detailed facts of importance to specialist readers are published as ”Supporting Information”. Such documents are peer-reviewed, but not copy-edited or typeset. They are made available as submitted by the authors. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.

The mechanical properties of cells play important roles in regulating the physiological activities of cells and reflect the state of macro-organisms. Although many approaches are available for investigating the mechanical properties of cells, the fluidity of cytoplasm across cell boundaries makes characterizing the dynamics of mechanical properties of single cells exceedingly difficult. In this study, we present a single cell characterization method by modelling the dynamics of cellular mechanical properties measured with an atomic force microscope (AFM). The mechanical dynamics of a single cell system was described by a linear model with a mechanical stimulus as virtual input and mechanical property parameters as outputs. The dynamic mechanical properties of a single cell were characterized by the system matrix of the single cell system. The method was used to classify different types of cells, and the experimental results show that the proposed method outperformed conventional methods by achieving an average classification accuracy of over 90%. The developed method can be used to classify different cancer types according to the mechanical properties of tumour cells, which is of great significance for clinically assisted pathological diagnosis.

The mechanical properties of cells play important roles in regulating the physiological activities of cells and reflect the state of macro-organisms. Although many approaches are available for investigating the mechanical properties of cells, the fluidity of cytoplasm across cell boundaries makes characterizing the dynamics of mechanical properties of single cells exceedingly difficult. In this study, we present a single cell characterization method by modelling the dynamics of cellular mechanical properties measured with an atomic force microscope (AFM). The mechanical dynamics of a single cell system was described by a linear model with a mechanical stimulus as virtual input and mechanical property parameters as outputs. The dynamic mechanical properties of a single cell were characterized by the system matrix of the single cell system. The method was used to classify different types of cells, and the experimental results show that the proposed method outperformed conventional methods by achieving an average classification accuracy of over 90%. The developed method can be used to classify different cancer types according to the mechanical properties of tumour cells, which is of great significance for clinically assisted pathological diagnosis.

Carbon nanotubes (CNTs) can be used as atomic force microscope (AFM) probes since they are ideal tip materials with a small diameter, high aspect ratio, and stiffness. In this study, a model of CNTs clamped in an elastic medium is proposed as nanoscale force sensing AFM probes. The relationship between vibration frequency and axial force of the CNT probe clamped in an elastic medium is analyzed based on the Euler-Bernoulli beam model and the Whitney-Riley model. The clamped length of CNTs, and the elastic modulus of elastic medium affect largely on the vibration and the buckling stability of a CNT AFM probe. The result showed that the sensitivity to vibration increases as the applied loads increase. The critical load in which the vibration frequency decreases rapidly, moving to large ones with decreasing ratio of length to diameter of CNTs. The theoretical investigation on the vibration frequency of CNT loaded in the axial direction would give a useful reference for designing a CNT used as a nano-force sensor.

The authors report the direct-write growth of hammerhead atomic force microscope (AFM) probes by He+ beam induced deposition of platinum-carbon. In order to grow a thin nanoneedle on top of a conventional AFM probe, the authors move a focused He+ beam during exposure to a PtC precursor gas. In the final growth stage, a perpendicular movement of the beam results in the required three-dimensional (hammerhead) shape. The diameter of the needle depends on the ion beam dose, beam dwell time, and speed of the beam movement. A nanoneedle radius below 10 nm and a hammerhead smaller than 35 nm have been achieved. This fabrication process is robust and enables precise control over the three-dimensions of the hammerhead AFM probe. Finally, the authors test the capabilities of the fabricated AFM probes for two-dimensional metrology of sidewall angles and line-edge roughness of trenches and shark-fins in silicon.

The atomic force microscope (AFM) has high observation resolution and operation precision, which plays an irreplaceable role in the characterization of nanomaterials and the assembly of nanodevices. The selection of AFM scan area depends on an optics microscope, whose resolution is generally no more than 200 nm due to the limitation of wavelength of visible light. Therefore, it is extreme challenge to relocate the AFM probe precisely to the initial scan area when the AFM probe is replaced or the sample is moved. In this paper, we study and present a new label-free method for the relocation of AFM probe. This method can cope with the variation of relative position between the AFM probe tip and target sample, which is caused by the angle rotation and position offset. Two easily-recognized features on the sample substrate are used as the reference points to realize the blind location of AFM probe with micron scale precision based on the principle of coordinate transformation. The high precision relocation of nano targets is obtained through matching AFM images and modifying the scanning parameters. The main advantage of this method is that the special man-made marks at the target sample area are not necessary. The relocation process of AFM probe is simple and rapid, with a wide location range and high accuracy. Experiments demonstrate that this method is very efficient to track the nanospheres, SWCNTs and nanoscratch.