Wide-area scanner for high-speed atomic force microscopy

High-Speed Atomic Force Microscopy for Filming Protein Molecules in Dynamic Action

Structural biology is currently undergoing a transformation into dynamic structural biology, which reveals the dynamic structure of proteins during their functional activity to better elucidate how they function. Among the various approaches in dynamic structural biology, high-speed atomic force microscopy (HS-AFM) is unique in the ability to film individual molecules in dynamic action, although only topographical information is acquirable. This review provides a guide to the use of HS-AFM for biomolecular imaging and showcases several examples, as well as providing information on up-to-date progress in HS-AFM technology. Finally, we discuss the future prospects of HS-AFM in the context of dynamic structural biology in the upcoming era.

Technical advances in high-speed atomic force microscopy

It has been 30 years since the outset of developing high-speed atomic force microscopy (HS-AFM), and 15 years have passed since its establishment in 2008. This advanced microscopy is capable of directly visualizing individual biological macromolecules in dynamic action and has been widely used to answer important questions that are inaccessible by other approaches. The number of publications on the bioapplications of HS-AFM has rapidly increased in recent years and has already exceeded 350. Although less visible than these biological studies, efforts have been made for further technical developments aimed at enhancing the fundamental performance of HS-AFM, such as imaging speed, low sample disturbance, and scan size, as well as expanding its functionalities, such as correlative microscopy, temperature control, buffer exchange, and sample manipulations. These techniques can expand the range of HS-AFM applications. After summarizing the key technologies underlying HS-AFM, this article focuses on recent technical advances and discusses next-generation HS-AFM.

Morphometric and Nanomechanical Screening of Peripheral Blood Cells with Atomic Force Microscopy for Label-Free Assessment of Alzheimer's Disease, Parkinson's Disease, and Amyotrophic Lateral Sclerosis

Neurodegenerative disorders (NDDs) are complex, multifactorial disorders with significant social and economic impact in today's society. NDDs are predicted to become the second-most common cause of death in the next few decades due to an increase in life expectancy but also to a lack of early diagnosis and mainly symptomatic treatment. Despite recent advances in diagnostic and therapeutic methods, there are yet no reliable biomarkers identifying the complex pathways contributing to these pathologies. The development of new approaches for early diagnosis and new therapies, together with the identification of non-invasive and more cost-effective diagnostic biomarkers, is one of the main trends in NDD biomedical research. Here we summarize data on peripheral biomarkers, biofluids (cerebrospinal fluid and blood plasma), and peripheral blood cells (platelets (PLTs) and red blood cells (RBCs)), reported so far for the three most common NDDs-Alzheimer's disease (AD), Parkinson's disease (PD), and amyotrophic lateral sclerosis (ALS). PLTs and RBCs, beyond their primary physiological functions, are increasingly recognized as valuable sources of biomarkers for NDDs. Special attention is given to the morphological and nanomechanical signatures of PLTs and RBCs as biophysical markers for the three pathologies. Modifications of the surface nanostructure and morphometric and nanomechanical signatures of PLTs and RBCs from patients with AD, PD, and ALS have been revealed by atomic force microscopy (AFM). AFM is currently experiencing rapid and widespread adoption in biomedicine and clinical medicine, in particular for early diagnostics of various medical conditions. AFM is a unique instrument without an analog, allowing the generation of three-dimensional cell images with extremely high spatial resolution at near-atomic scale, which are complemented by insights into the mechanical properties of cells and subcellular structures. Data demonstrate that AFM can distinguish between the three pathologies and the normal, healthy state. The specific PLT and RBC signatures can serve as biomarkers in combination with the currently used diagnostic tools. We highlight the strong correlation of the morphological and nanomechanical signatures between RBCs and PLTs in PD, ALS, and AD.

A high-bandwidth voltage amplifier for driving piezoelectric actuators in high-speed atomic force microscopy

High-speed atomic force microscopy (HS-AFM) is a technique capable of revealing the dynamics of biomolecules and living organisms at the nanoscale with a remarkable temporal resolution. The phase delay in the feedback loop dictates the achievable speed of HS-AFM instruments that rely on fast nanopositioners operated predominantly in conjunction with piezoelectric actuators (PEAs). The high capacitance and high operating voltage of PEAs make them difficult to drive. The limited bandwidth of associated high-voltage piezo-amplifiers is one of the bottlenecks to higher scan speeds. In this study, we report a high-voltage, wideband voltage amplifier comprised of a separate amplification and novel voltage-follower power stage, requiring no global feedback. The reported amplifier can deliver a current over ±2 amps, offers a small-signal bandwidth of 1 MHz, and exhibits an exceptionally low phase lag, making it particularly well suited for the needs of next-generation HS-AFMs. We demonstrate its capabilities by reporting its achievable bandwidth under various PEA loads and showcasing its merit for HS-AFM by imaging tubulin protofilament dynamics at sub-second frame rates.

Investigating the morphological dynamics of the plasma membrane by high-speed atomic force microscopy

Despite numerous recent developments in bioimaging techniques, nanoscale and live-cell imaging of the plasma membrane has been challenging because of the insufficient z-resolution of optical microscopes, as well as the lack of fluorescent probes to specifically label small membrane structures. High-speed atomic force microscopy (HS-AFM) is a powerful tool for visualising the dynamics of a specimen surface and is therefore suitable for observing plasma membrane dynamics. Recent developments in HS-AFM for live-cell imaging have enabled the visualisation of the plasma membrane and the network of cortical actin underneath the membrane in a living cell. Furthermore, correlative imaging with fluorescence microscopy allows for the direct visualisation of morphological changes of the plasma membrane together with the dynamic assembly or disassembly of proteins during the entire course of endocytosis in a living cell. Here, we review these recent advances in HS-AFM in order to analyse various cellular events occurring at the cell surface.

Development of a flexure-based nano-actuator for high-frequency high-resolution directional sensing with atomic force microscopy

Scanning probe microscopies typically rely on the high-precision positioning of a nanoscale probe in order to gain local information about the properties of a sample. At a given location, the probe is used to interrogate a minute region of the sample, often relying on dynamical sensing for improved accuracy. This is the case for most force-based measurements in atomic force microscopy (AFM) where sensing occurs with a tip oscillating vertically, typically in the kHz to MHz frequency regime. While this approach is ideal for many applications, restricting dynamical sensing to only one direction (vertical) can become a serious limitation when aiming to quantify the properties of inherently three-dimensional systems, such as a liquid near a wall. Here, we present the design, fabrication, and calibration of a miniature high-speed scanner able to apply controlled fast and directional in-plane vibrations with sub-nanometer precision. The scanner has a resonance frequency of ∼35 kHz and is used in conjunction with a traditional AFM to augment the measurement capabilities. We illustrate its capabilities at a solid-liquid interface where we use it to quantify the preferred lateral flow direction of the liquid around every sample location. The AFM can simultaneously acquire high-resolution images of the interface, which can be superimposed with the directional measurements. Examples of sub-nanometer measurements conducted with the new scanner are also presented.

An ultra-wide scanner for large-area high-speed atomic force microscopy with megapixel resolution

High-speed atomic force microscopy (HS-AFM) is a powerful tool for visualizing the dynamics of individual biomolecules. However, in single-molecule HS-AFM imaging applications, x,y-scanner ranges are typically restricted to a few hundred nanometers, preventing overview observation of larger molecular assemblies, such as 2-dimensional protein crystal growth or fibrillar aggregation. Previous advances in scanner design using mechanical amplification of the piezo-driven x,y-positioning system have extended the size of HS-AFM image frames to several tens of micrometer, but these large scanners may suffer from mechanical instabilities at high scan speeds and only record images with limited pixel numbers and comparatively low lateral resolutions (> 20–100 nm/pixel), complicating single-molecule analysis. Thus, AFM systems able to image large sample areas at high speeds and with nanometer resolution have still been missing. Here, we describe a HS-AFM sample-scanner system able to record large topographic images (≤ 36 × 36 µm²) containing up to 16 megapixels, providing molecular resolution throughout the image frame. Despite its large size, the flexure-based scanner features a high resonance frequency (> 2 kHz) and delivers stable operation even at high scans speeds of up to 7.2 mm/s, minimizing the time required for recording megapixel scans. We furthermore demonstrate that operating this high-speed scanner in time-lapse mode can simultaneously identify areas of spontaneous 2-dimensional Annexin A5 crystal growth, resolve the angular orientation of large crystalline domains, and even detect rare crystal lattice defects, all without changing scan frame size or resolution. Dynamic processes first identified from overview scans can then be further imaged at increased frame rates in reduced scan areas after switching to conventional HS-AFM scanning. The added ability to collect large-area, high-resolution images of complex samples within biological-relevant time frames extends the capabilities of HS-AFM from single-molecule imaging to the study of large dynamic molecular arrays. Moreover, large-area HS-AFM scanning can generate detailed structural data sets from a single scan, aiding the quantitative analysis of structurally heterogenous samples, including cellular surfaces.

High-Speed Atomic Force Microscopy to Study Myosin Motility

High-speed atomic force microscopy (HS-AFM) is a unique tool that enables imaging of protein molecules during their functional activity at sub-100 ms temporal and submolecular spatial resolution. HS-AFM is suited for the study of highly dynamic proteins, including myosin motors. HS-AFM images of myosin V walking on actin filaments provide irrefutable evidence for the swinging lever arm motion propelling the molecule forward. Moreover, molecular behaviors that have not been noticed before are also displayed on the AFM movies. This chapter describes the principle, underlying techniques and performance of HS-AFM, filmed images of myosin V, and mechanistic insights into myosin motility provided from the filmed images.

Nano-scientific Application of Atomic Force Microscopy in Pathology: from Molecules to Tissues

The advantages of atomic force microscopy (AFM) in biological research are its high imaging resolution, sensitivity, and ability to operate in physiological conditions. Over the past decades, rigorous studies have been performed to determine the potential applications of AFM techniques in disease diagnosis and prognosis. Many pathological conditions are accompanied by alterations in the morphology, adhesion properties, mechanical compliances, and molecular composition of cells and tissues. The accurate determination of such alterations can be utilized as a diagnostic and prognostic marker. Alteration in cell morphology represents changes in cell structure and membrane proteins induced by pathologic progression of diseases. Mechanical compliances are also modulated by the active rearrangements of cytoskeleton or extracellular matrix triggered by disease pathogenesis. In addition, adhesion is a critical step in the progression of many diseases including infectious and neurodegenerative diseases. Recent advances in AFM techniques have demonstrated their ability to obtain molecular composition as well as topographic information. The quantitative characterization of molecular alteration in biological specimens in terms of disease progression provides a new avenue to understand the underlying mechanisms of disease onset and progression. In this review, we have highlighted the application of diverse AFM techniques in pathological investigations.

High-speed atomic force microscope with a combined tip-sample scanning architecture

A high-speed atomic force microscope (HS-AFM) based on a tip-sample combined scanning architecture is presented. In this system, the X-scanner, which is separated from the AFM head, carries the sample and scans along the fast-axis. The Y and Z scanners integrated in the AFM head oscillate an ultrashort cantilever probe and scan in the other two dimensions. The optical beam deflection method is improved to enable the laser to track the probe over a wide scan range. A novel probe holder realizes easy exchange and alignment of the probe. Due to the separation of the X and Y scanners, both appear with better dynamic performance and carrying capacity. Experiments show that the HS-AFM established in this work can achieve a line rate of up to 100 Hz with the basic proportional-integral-derivative control algorithm and linear driving. The permissible sample size and mass can be as large as several centimeters and above 40 g.

High-speed near-field fluorescence microscopy combined with high-speed atomic force microscopy for biological studies

BACKGROUND

High-speed atomic force microscopy (HS-AFM) has successfully visualized a variety of protein molecules during their functional activity. However, it cannot visualize small molecules interacting with proteins and even protein molecules when they are encapsulated. Thus, it has been desired to achieve techniques enabling simultaneous optical/AFM imaging at high spatiotemporal resolution with high correlation accuracy.

METHODS

Scanning near-field optical microscopy (SNOM) is a candidate for the combination with HS-AFM. However, the imaging rate of SNOM has been far below that of HS-AFM. We here developed HS-SNOM and metal tip-enhanced total internal reflection fluorescence microscopy (TIRFM) by exploiting tip-scan HS-AFM and exploring methods to fabricate a metallic tip on a tiny HS-AFM cantilever.

RESULTS

In tip-enhanced TIRFM/HS-AFM, simultaneous video recording of the two modalities of images was demonstrated in the presence of fluorescent molecules in the bulk solution at relatively high concentration. By using fabricated metal-tip cantilevers together with our tip-scan HS-AFM setup equipped with SNOM optics, we could perform simultaneous HS-SNOM/HS-AFM imaging, with correlation analysis between the two overlaid images being facilitated.

CONCLUSIONS

This study materialized simultaneous tip-enhanced TIRFM/HS-AFM and HS-SNOM/HS-AFM imaging at high spatiotemporal resolution. Although some issues remain to be solved in the future, these correlative microscopy methods have a potential to increase the versatility of HS-AFM in biological research.

GENERAL SIGNIFICANCE

We achieved an imaging rate of ~3 s/frame for SNOM imaging, more than 100-times higher than the typical SNOM imaging rate. We also demonstrated ~39 nm resolution in HS-SNOM imaging of fluorescently labeled DNA in solution.

Application of atomic force microscopy in cancer research

Atomic force microscopy (AFM) allows for nanometer-scale investigation of cells and molecules. Recent advances have enabled its application in cancer research and diagnosis. The physicochemical properties of live cells undergo changes when their physiological conditions are altered. These physicochemical properties can therefore reflect complex physiological processes occurring in cells. When cells are in the process of carcinogenesis and stimulated by external stimuli, their morphology, elasticity, and adhesion properties may change. AFM can perform surface imaging and ultrastructural observation of live cells with atomic resolution under near-physiological conditions, collecting force spectroscopy information which allows for the study of the mechanical properties of cells. For this reason, AFM has potential to be used as a tool for high resolution research into the ultrastructure and mechanical properties of tumor cells. This review describes the working principle, working mode, and technical points of atomic force microscopy, and reviews the applications and prospects of atomic force microscopy in cancer research.

High-speed large area atomic force microscopy using a quartz resonator

A high-speed atomic force microscope for scanning large areas, utilizing a quartz bar driven close to resonance to provide the motion in the fast scan axis is presented. Images up to 170 × 170 μm² have been obtained on a polydimethylsiloxane (PDMS) grating in 1 s. This is provided through an average tip-sample velocity of 28 cm s^-1 at a line rate of 830 Hz. Scan areas up to 80 × 80 μm² have been obtained in 0.42 s with a line rate of 1410 Hz. To demonstrate the capability of the scanner the spherulitic crystallization of a semicrystalline polymer was imaged in situ at high speed.

[Progress in the applications of high-speed atomic force microscopy in cell biology]

Without losing its high resolution, high-speed atomic force microscope (HS-AFM) represents a perfect combinationof scanning speed and precision and allows real-time and in situ observation of the dynamic processes in a biological system atboth the cellular and molecular levels. By combining the extremely high temporal resolution with the spatial resolution andcoupling with other advanced technologies, HS-AFM shows promising prospects for applications in life sciences such as cellbiology. In this review, we summarize the latest progress of HS-AFM in the field of cell biology, and discuss the impact ofenvironmental factors on conformation dynamics of DNA, the binding processes between DNA and protein, the domainchanges of membrane proteins, motility of myosin, and surface structure changes of living cells.

Time-Resolved Imaging of Bacterial Surfaces Using Atomic Force Microscopy

Time-resolved atomic force microscopy (AFM) offers countless new modes by which to study bacterial cell physiology on relevant time scales, from mere milliseconds to hours and days on end. In addition, time-lapse AFM acts as a complementary tool to optical fluorescence microscopy (OFM), for which the combination offers a correlative link between the physical manifestation of bacterial phenotypes and molecular mechanisms obeying those principles. Herein we describe the essential materials and methods necessary for conducting time-resolved AFM and dual AFM/OFM experiments on bacteria.

Scanning speed phenomenon in contact-resonance atomic force microscopy

This work presents data confirming the existence of a scan speed related phenomenon in contact-mode atomic force microscopy (AFM). Specifically, contact-resonance spectroscopy is used to interrogate this phenomenon. Above a critical scan speed, a monotonic decrease in the recorded contact-resonance frequency is observed with increasing scan speed. Proper characterization and understanding of this phenomenon is necessary to conduct accurate quantitative imaging using contact-resonance AFM, and other contact-mode AFM techniques, at higher scan speeds. A squeeze film hydrodynamic theory is proposed to explain this phenomenon, and model predictions are compared against the experimental data.

Fast Specimen Boundary Tracking and Local Imaging with Scanning Probe Microscopy

An efficient and adaptive boundary tracking method is developed to confine area of interest for high-efficiency local scanning. By using a boundary point determination criterion, the scanning tip is steered with a sinusoidal waveform while estimating azimuth angle and radius ratio of each boundary point to accurately track the boundary of targets. A local scan region and path are subsequently planned based on the prior knowledge of boundary tracking to reduce the scan time. Boundary tracking and local scanning methods have great potential not only for fast dimension measurement but also for sample surface topography and physical characterization, with only scanning region of interest. The performance of the proposed methods was verified by using the alternate current mode scanning ion-conductance microscopy, tapping, and PeakForce modulation atomic force microscopy. Experimental results of single/multitarget boundary tracking and local scanning of target structures with complex boundaries demonstrate the flexibility and validity of the proposed method.

High-speed atomic force microscopy imaging of live mammalian cells

Direct imaging of morphological dynamics of live mammalian cells with nanometer resolution under physiological conditions is highly expected, but yet challenging. High-speed atomic force microscopy (HS-AFM) is a unique technique for capturing biomolecules at work under near physiological conditions. However, application of HS-AFM for imaging of live mammalian cells was hard to be accomplished because of collision between a huge mammalian cell and a cantilever during AFM scanning. Here, we review our recent improvements of HS-AFM for imaging of activities of live mammalian cells without significant damage to the cell. The improvement of an extremely long (~3 μm) AFM tip attached to a cantilever enables us to reduce severe damage to soft mammalian cells. In addition, a combination of HS-AFM with simple fluorescence microscopy allows us to quickly locate the cell in the AFM scanning area. After these improvements, we demonstrate that developed HS-AFM for live mammalian cells is possible to image morphogenesis of filopodia, membrane ruffles, pits open-close formations, and endocytosis in COS-7, HeLa cells as well as hippocampal neurons.

Directly watching biomolecules in action by high-speed atomic force microscopy

Proteins are dynamic in nature and work at the single molecule level. Therefore, directly watching protein molecules in dynamic action at high spatiotemporal resolution must be the most straightforward approach to understanding how they function. To make this observation possible, high-speed atomic force microscopy (HS-AFM) has been developed. Its current performance allows us to film biological molecules at 10-16 frames/s, without disturbing their function. In fact, dynamic structures and processes of various proteins have been successfully visualized, including bacteriorhodopsin responding to light, myosin V walking on actin filaments, and even intrinsically disordered proteins undergoing order/disorder transitions. The molecular movies have provided insights that could not have been reached in other ways. Moreover, the cantilever tip can be used to manipulate molecules during successive imaging. This capability allows us to observe changes in molecules resulting from dissection or perturbation. This mode of imaging has been successfully applied to myosin V, peroxiredoxin and doublet microtubules, leading to new discoveries. Since HS-AFM can be combined with other techniques, such as super-resolution optical microscopy and optical tweezers, the usefulness of HS-AFM will be further expanded in the near future.

Cell biology of microbes and pharmacology of antimicrobial drugs explored by Atomic Force Microscopy

Antimicrobial molecules have been used for more than 50 years now and are the basis of modern medicine. No surgery can nowdays be imagined to be performed without antibiotics; dreadful diseases like tuberculosis, leprosis, siphilys, and more broadly all microbial induced diseases, can be cured only through the use of antimicrobial treatments. However, the situation is becoming more and more complex because of the ability of microbes to adapt, develop, acquire, and share mechanisms of resistance to antimicrobial agents. We choose to introduce this review by briefly drawing the panorama of antimicrobial discovery and development, but also of the emergence of microbial resistance. Then we describe how Atomic Force Microscopy (AFM) can be used to provide a better understanding of the mechanisms of action of these drugs at the nanoscale level on microbial interfaces. In this section, we will address these questions: (1) how does drug treatment affect the morphology of single microbes?; (2) do antimicrobial molecules modify the nanomechanical properties of microbes, or do the nanomechanical properties of microbes play a role in antimicrobial activity and efficiency?; and (3) how are the adhesive abilitites of microbes affected by antimicrobial drugs treatment? Finally, in a second part of this review we focus on recent studies aimed at changing the paradigm of the single molecule/cell technology that AFM typically represents. Recent work dealing with the creation of a microbe array which can be explored by AFM will be presented, as these developments constitute the first steps toward transforming AFM into a higher throughput technology. We also discuss papers using AFM as NanoMechnanicalSensors (NEMS), and demonstrate the interest of such approaches in clinical microbiology to detect quickly and with high accuracy microbial resistance.

Atomic Force Microscopy in Characterizing Cell Mechanics for Biomedical Applications: A Review

Cell mechanics is a novel label-free biomarker for indicating cell states and pathological changes. The advent of atomic force microscopy (AFM) provides a powerful tool for quantifying the mechanical properties of single living cells in aqueous conditions. The wide use of AFM in characterizing cell mechanics in the past two decades has yielded remarkable novel insights in understanding the development and progression of certain diseases, such as cancer, showing the huge potential of cell mechanics for practical applications in the field of biomedicine. In this paper, we reviewed the utilization of AFM to characterize cell mechanics. First, the principle and method of AFM single-cell mechanical analysis was presented, along with the mechanical responses of cells to representative external stimuli measured by AFM. Next, the unique changes of cell mechanics in two types of physiological processes (stem cell differentiation, cancer metastasis) revealed by AFM were summarized. After that, the molecular mechanisms guiding cell mechanics were analyzed. Finally the challenges and future directions were discussed.

Rotational scanning atomic force microscopy

A non-raster scanning technique for atomic force microscopy (AFM) imaging which combines rotational and translational motion is presented. The use of rotational motion for the fast scan axis allows us to significantly increase the scanning speed while imaging a large area (diameter > 30 μm). An image reconstruction algorithm and the factors influencing the resolution of the technique are discussed. The experimental results show the potential of the rotational scanning technique for high-throughput large area AFM investigation.

Imaging and Force Recognition of Single Molecular Behaviors Using Atomic Force Microscopy

The advent of atomic force microscopy (AFM) has provided a powerful tool for investigating the behaviors of single native biological molecules under physiological conditions. AFM can not only image the conformational changes of single biological molecules at work with sub-nanometer resolution, but also sense the specific interactions of individual molecular pair with piconewton force sensitivity. In the past decade, the performance of AFM has been greatly improved, which makes it widely used in biology to address diverse biomedical issues. Characterizing the behaviors of single molecules by AFM provides considerable novel insights into the underlying mechanisms guiding life activities, contributing much to cell and molecular biology. In this article, we review the recent developments of AFM studies in single-molecule assay. The related techniques involved in AFM single-molecule assay were firstly presented, and then the progress in several aspects (including molecular imaging, molecular mechanics, molecular recognition, and molecular activities on cell surface) was summarized. The challenges and future directions were also discussed.

Watching cellular machinery in action, one molecule at a time

Monachino et al. review recent developments in single-molecule biophysical approaches and the cell biological advances they allow.

Single-molecule manipulation and imaging techniques have become important elements of the biologist’s toolkit to gain mechanistic insights into cellular processes. By removing ensemble averaging, single-molecule methods provide unique access to the dynamic behavior of biomolecules. Recently, the use of these approaches has expanded to the study of complex multiprotein systems and has enabled detailed characterization of the behavior of individual molecules inside living cells. In this review, we provide an overview of the various force- and fluorescence-based single-molecule methods with applications both in vitro and in vivo, highlighting these advances by describing their applications in studies on cytoskeletal motors and DNA replication. We also discuss how single-molecule approaches have increased our understanding of the dynamic behavior of complex multiprotein systems. These methods have shown that the behavior of multicomponent protein complexes is highly stochastic and less linear and deterministic than previously thought. Further development of single-molecule tools will help to elucidate the molecular dynamics of these complex systems both inside the cell and in solutions with purified components.

Design of a high-bandwidth tripod scanner for high speed atomic force microscopy

Tip-scanning high-speed atomic force microscopes (HS-AFMs) have several advantages over their sample-scanning counterparts. Firstly, they can be used on samples of almost arbitrary size since the high imaging bandwidth of the system is immune to the added mass of the sample and its holder. Depending on their layouts, they also enable the use of several tip-scanning HS-AFMs in combination. However, the need for tracking the cantilever with the readout laser makes designing tip-scanning HS-AFMs difficult. This often results in a reduced resonance frequency of the HS-AFM scanner, or a complex and large set of precision flexures. Here, we present a compact, simple HS-AFM designed for integrating the self-sensing cantilever into the tip-scanning configuration, so that the difficulty of tracking small cantilever by laser beam is avoided. The position of cantilever is placed to the end of whole structure, hence making the optical viewing of the cantilever possible. As the core component of proposed system, a high bandwidth tripod scanner is designed, with a scan size of 5.8 µm × 5.8 µm and a vertical travel range of 5.9 µm. The hysteresis of the piezoactuators in X- and Y-axes are linearized using input shaping technique. To reduce in-plane crosstalk and vibration-related dynamics, we implement both filters and compensators on a field programmable analog array. Based on these, images with 512 × 256 pixels are successfully obtained at scan rates up to 1024 lines/s, corresponding to a 4 mm/stip velocity. SCANNING 38:889-900, 2016. © 2016 Wiley Periodicals, Inc.

High speed single- and dual-stage vertical positioners

This article presents a high-speed single- and dual-stage vertical positioners for applications in optical systems. Each positioner employs a unique end-constraint method with orthogonal flexures to preload a piezoelectric stack actuator. This end-constraint method also significantly increases the first mechanical resonance frequency. The single-stage positioner has a displacement range of 7.6 μm and a first resonance frequency of 46.8 kHz. The dual-stage design consists of a long-range slow-stage and a short-range fast-stage. An inertial counterbalance technique was implemented on the fast-stage to cancel inertial forces resulting from high-speed motion. The dual-stage positioner has a combined travel range of approximately 10 μm and a first evident resonance frequency of 130 kHz.

Development of a detachable high speed miniature scanning probe microscope for large area substrates inspection

We have developed a high speed, miniature scanning probe microscope (MSPM) integrated with a Positioning Unit (PU) for accurately positioning the MSPM on a large substrate. This combination enables simultaneous, parallel operation of many units on a large sample for high throughput measurements. The size of the MSPM is 19 × 45 × 70 mm(3). It contains a one-dimensional flexure stage with counter-balanced actuation for vertical scanning with a bandwidth of 50 kHz and a z-travel range of more than 2 μm. This stage is mechanically decoupled from the rest of the MSPM by suspending it on specific dynamically determined points. The motion of the probe, which is mounted on top of the flexure stage is measured by a very compact optical beam deflection (OBD). Thermal noise spectrum measurements of short cantilevers show a bandwidth of 2 MHz and a noise of less than 15 fm/Hz(1/2). A fast approach and engagement of the probe to the substrate surface have been achieved by integrating a small stepper actuator and direct monitoring of the cantilever response to the approaching surface. The PU has the same width as the MSPM, 45 mm and can position the MSPM to a pre-chosen position within an area of 275×30 mm(2) to within 100 nm accuracy within a few seconds. During scanning, the MSPM is detached from the PU which is essential to eliminate mechanical vibration and drift from the relatively low-resonance frequency and low-stiffness structure of the PU. Although the specific implementation of the MSPM we describe here has been developed as an atomic force microscope, the general architecture is applicable to any form of SPM. This high speed MSPM is now being used in a parallel SPM architecture for inspection and metrology of large samples such as semiconductor wafers and masks.

Studying biological membranes with extended range high-speed atomic force microscopy

High—speed atomic force microscopy has proven to be a valuable tool for the study of biomolecular systems at the nanoscale. Expanding its application to larger biological specimens such as membranes or cells has, however, proven difficult, often requiring fundamental changes in the AFM instrument. Here we show a way to utilize conventional AFM instrumentation with minor alterations to perform high-speed AFM imaging with a large scan range. Using a two—actuator design with adapted control systems, a 130 × 130 × 5 μm scanner with nearly 100 kHz open—loop small-signal Z—bandwidth is implemented. This allows for high-speed imaging of biologically relevant samples as well as high-speed measurements of nanomechanical surface properties. We demonstrate the system performance by real-time imaging of the effect of charged polymer nanoparticles on the integrity of lipid membranes at high imaging speeds and peak force tapping measurements at 32 kHz peak force rate.

Fast, multi-frequency, and quantitative nanomechanical mapping of live cells using the atomic force microscope

A longstanding goal in cellular mechanobiology has been to link dynamic biomolecular processes underpinning disease or morphogenesis to spatio-temporal changes in nanoscale mechanical properties such as viscoelasticity, surface tension, and adhesion. This requires the development of quantitative mechanical microscopy methods with high spatio-temporal resolution within a single cell. The Atomic Force Microscope (AFM) can map the heterogeneous mechanical properties of cells with high spatial resolution, however, the image acquisition time is 1-2 orders of magnitude longer than that required to study dynamic cellular processes. We present a technique that allows commercial AFM systems to map quantitatively the dynamically changing viscoelastic properties of live eukaryotic cells at widely separated frequencies over large areas (several 10's of microns) with spatial resolution equal to amplitude-modulation (AM-AFM) and with image acquisition times (tens of seconds) approaching those of speckle fluorescence methods. This represents a ~20 fold improvement in nanomechanical imaging throughput compared to AM-AFM and is fully compatible with emerging high speed AFM systems. This method is used to study the spatio-temporal mechanical response of MDA-MB-231 breast carcinoma cells to the inhibition of Syk protein tyrosine kinase giving insight into the signaling pathways by which Syk negatively regulates motility of highly invasive cancer cells.

Method of mechanical holding of cantilever chip for tip-scan high-speed atomic force microscope

In tip-scan atomic force microscopy (AFM) that scans a cantilever chip in the three dimensions, the chip body is held on the Z-scanner with a holder. However, this holding is not easy for high-speed (HS) AFM because the holder that should have a small mass has to be able to clamp the cantilever chip firmly without deteriorating the Z-scanner's fast performance, and because repeated exchange of cantilever chips should not damage the Z-scanner. This is one of the reasons that tip-scan HS-AFM has not been established, despite its advantages over sample stage-scan HS-AFM. Here, we present a novel method of cantilever chip holding which meets all conditions required for tip-scan HS-AFM. The superior performance of this novel chip holding mechanism is demonstrated by imaging of the α3β3 subcomplex of F1-ATPase in dynamic action at ∼7 frames/s.

Long-tip high-speed atomic force microscopy for nanometer-scale imaging in live cells

Visualization of morphological dynamics of live cells with nanometer resolution under physiological conditions is highly desired, but challenging. It has been demonstrated that high-speed atomic force microscopy is a powerful technique for visualizing dynamics of biomolecules under physiological conditions. However, application of high-speed atomic force microscopy for imaging larger objects such as live mammalian cells has been complicated because of the collision between the cantilever and samples. Here, we demonstrate that attaching an extremely long (~3 μm) and thin (~5 nm) tip by amorphous carbon to the cantilever allows us to image the surface structure of live cells with the spatiotemporal resolution of nanometers and seconds. We demonstrate that long-tip high-speed atomic force microscopy is capable of imaging morphogenesis of filopodia, membrane ruffles, pit formation, and endocytosis in COS-7, HeLa cells and hippocampal neurons.

Real-time deflection and friction force imaging by bimorph-based resonance-type high-speed scanning force microscopy in the contact mode

We report herein an alternative high-speed scanning force microscopy method in the contact mode based on a resonance-type piezoelectric bimorph scanner. The experimental setup, the modified optical beam deflection scheme suitable for smaller cantilevers, and a high-speed control program for simultaneous data capture are described in detail. The feature of the method is that the deflection and friction force images of the sample surface can be obtained simultaneously in real time. Images of various samples (e.g., a test grating, a thin gold film, and fluorine-doped tin oxide-coated glass slides) are acquired successfully. The imaging rate is 25 frames per second, and the average scan speed reaches a value of approximately 2.5 cm/s. The method combines the advantages of both observing the dynamic processes of the sample surface and monitoring the frictional properties on the nanometer scale.

PACS

07.79.Lh; 07.79.Sp; 68.37.Ps.

Probing in vivo dynamics of mitochondria and cortical actin networks using high-speed atomic force/fluorescence microscopy

The dynamics of the cell membrane and submembrane structures are closely linked, facilitating various cellular activities. Although cell surface research and cortical actin studies have shown independent mechanisms for the cell membrane and the actin network, it has been difficult to obtain a comprehensive understanding of the dynamics of these structures in live cells. Here, we used a combined atomic force/optical microscope system to analyze membrane-based cellular events at nanometer-scale resolution in live cells. Imaging the COS-7 cell surface showed detailed structural properties of membrane invagination events corresponding to endocytosis and exocytosis. In addition, the movement of mitochondria and the spatiotemporal dynamics of the cortical F-actin network were directly visualized in vivo. Cortical actin microdomains with sizes ranging from 1.7×10(4) to 1.4×10(5) nm2 were dynamically rearranged by newly appearing actin filaments, which sometimes accompanied membrane invaginations, suggesting that these events are integrated with the dynamic regulation of submembrane organizations maintained by actin turnovers. These results provide novel insights into the structural aspects of the entire cell membrane machinery which can be visualized with high temporal and spatial resolution.

High-speed imaging upgrade for a standard sample scanning atomic force microscope using small cantilevers

We present an atomic force microscope (AFM) head for optical beam deflection on small cantilevers. Our AFM head is designed to be small in size, easily integrated into a commercial AFM system, and has a modular architecture facilitating exchange of the optical and electronic assemblies. We present two different designs for both the optical beam deflection and the electronic readout systems, and evaluate their performance. Using small cantilevers with our AFM head on an otherwise unmodified commercial AFM system, we are able to take tapping mode images approximately 5-10 times faster compared to the same AFM system using large cantilevers. By using additional scanner turnaround resonance compensation and a controller designed for high-speed AFM imaging, we show tapping mode imaging of lipid bilayers at line scan rates of 100-500 Hz for scan areas of several micrometers in size.

High-speed force mapping on living cells with a small cantilever atomic force microscope

The imaging speed of the wide-spread force mapping mode for quantitative mechanical measurements on soft samples in liquid with the atomic force microscope (AFM) is limited by the bandwidth of the z-scanner and viscous drag forces on the cantilever. Here, we applied high-speed, large scan-range atomic force microscopy and small cantilevers to increase the speed of force mapping by ≈10-100 times. This allowed resolving dynamic processes on living mouse embryonic fibroblasts. Cytoskeleton reorganization during cell locomotion, growth of individual cytoskeleton fibers, cell blebbing, and the formation of endocytic pits in the cell membrane were observed. Increasing the force curve rate from 2 to 300 Hz increased the measured apparent Young's modulus of the cells by about 10 times, which facilitated force mapping measurements at high speed.

The path to visualization of walking myosin V by high-speed atomic force microscopy

The quest for understanding the mechanism of myosin-based motility started with studies on muscle contraction. From numerous studies, the basic frameworks for this mechanism were constructed and brilliant hypotheses were put forward. However, the argument about the most crucial issue of how the actin-myosin interaction generates contractile force and shortening has not been definitive. To increase the "directness of measurement", in vitro motility assays and single-molecule optical techniques were created and used. Consequently, detailed knowledge of the motility of muscle myosin evolved, which resulted in provoking more arguments to a higher level. In parallel with technical progress, advances in cell biology led to the discovery of many classes of myosins. Myosin V was discovered to be a processive motor, unlike myosin II. The processivity reduced experimental difficulties because it allowed continuous tracing of the motor action of single myosin V molecules. Extensive studies of myosin V were expected to resolve arguments and build a consensus but did not necessarily do so. The directness of measurement was further enhanced by the recent advent of high-speed atomic force microscopy capable of directly visualizing biological molecules in action at high spatiotemporal resolution. This microscopy clearly visualized myosin V molecules walking on actin filaments and at last provided irrefutable evidence for the swinging lever-arm motion propelling the molecules. However, a peculiar foot stomp behavior also appeared in the AFM movie, raising new questions of the chemo-mechanical coupling in this motor and myosin motors in general. This article reviews these changes in the research of myosin motility and proposes new ideas to resolve the newly raised questions.

Design, analysis and testing of a parallel-kinematic high-bandwidth XY nanopositioning stage

This paper presents the design, analysis, and testing of a parallel-kinematic high-bandwidth XY nanopositioning stage driven by piezoelectric stack actuators. The stage is designed with two kinematic chains. In each kinematic chain, the end-effector of the stage is connected to the base by two symmetrically distributed flexure modules, respectively. Each flexure module comprises a fixed-fixed beam and a parallelogram flexure serving as two orthogonal prismatic joints. With the purpose to achieve high resonance frequencies of the stage, a novel center-thickened beam which has large stiffness is proposed to act as the fixed-fixed beam. The center-thickened beam also contributes to reducing cross-coupling and restricting parasitic motion. To decouple the motion in two axes totally, a symmetric configuration is adopted for the parallelogram flexures. Based on the analytical models established in static and dynamic analysis, the dimensions of the stage are optimized in order to maximize the first resonance frequency. Then finite element analysis is utilized to validate the design and a prototype of the stage is fabricated for performance tests. According to the results of static and dynamic tests, the resonance frequencies of the developed stage are over 13.6 kHz and the workspace is 11.2 μm × 11.6 μm with the cross-coupling between two axes less than 0.52%. It is clearly demonstrated that the developed stage has high resonance frequencies, a relatively large travel range, and nearly decoupled performance between two axes. For high-speed tracking performance tests, an inversion-based feedforward controller is implemented for the stage to compensate for the positioning errors caused by mechanical vibration. The experimental results show that good tracking performance at high speed is achieved, which validates the effectiveness of the developed stage.

High-speed atomic force microscope combined with single-molecule fluorescence microscope

High-speed atomic force microscopy (HS-AFM) and total internal reflection fluorescence microscopy (TIRFM) have mutually complementary capabilities. Here, we report techniques to combine these microscopy systems so that both microscopy capabilities can be simultaneously used in the full extent. To combine the two systems, we have developed a tip-scan type HS-AFM instrument equipped with a device by which the laser beam from the optical lever detector can track the cantilever motion in the X- and Y-directions. This stand-alone HS-AFM system is mounted on an inverted optical microscope stage with a wide-area scanner. The capability of this combined system is demonstrated by simultaneous HS-AFM∕TIRFM imaging of chitinase A moving on a chitin crystalline fiber and myosin V walking on an actin filament.