Multi-actuation and PI control: a simple recipe for high-speed and large-range atomic force microscopy

Review: Advanced Atomic Force Microscopy Modes for Biomedical Research

Visualization of biomedical samples in their native environments at the microscopic scale is crucial for studying fundamental principles and discovering biomedical systems with complex interaction. The study of dynamic biological processes requires a microscope system with multiple modalities, high spatial/temporal resolution, large imaging ranges, versatile imaging environments and ideally in-situ manipulation capabilities. Recent development of new Atomic Force Microscopy (AFM) capabilities has made it such a powerful tool for biological and biomedical research. This review introduces novel AFM functionalities including high-speed imaging for dynamic process visualization, mechanobiology with force spectroscopy, molecular species characterization, and AFM nano-manipulation. These capabilities enable many new possibilities for novel scientific research and allow scientists to observe and explore processes at the nanoscale like never before. Selected application examples from recent studies are provided to demonstrate the effectiveness of these AFM techniques.

An ultrafast piezoelectric Z-scanner with a resonance frequency above 1.1 MHz for high-speed atomic force microscopy

The Z-scanner is the major component limiting the speed performance of all current high-speed atomic force microscopy systems. Here, we present an ultrafast piezoelectric Z-scanner with a resonance frequency above 1.1 MHz, achieving a record response time of ∼0.14 µs, approximately twice as fast as conventional piezoelectric-based Z-scanners. In the mechanical design, a small piezo-stack is supported at its bottom four vertices on a cone-like hollow, allowing the resonance frequency of the Z-scanner to remain as high as that of the piezo in free vibration. Its maximum displacement, ∼190 nm at 50 V, is large enough for imaging bio-molecules. For imaging bio-molecules in a buffer solution, the upper half of the Z-scanner is wrapped in a thin film resistant to water and chemicals, providing an excellent waterproof and mechanical durability without lowering the resonance frequency. We demonstrate that this Z-scanner can observe actin filaments, fragile biological polymers, for more than five times longer than the conventional Z-scanner at a tip velocity of 800 µm/s.

Observer Design for Topography Estimation in Atomic Force Microscopy Using Neural and Fuzzy Networks

In this study, a novel artificial intelligence-based approach is presented to directly estimate the surface topography. To this aim, performance of different artificial intelligence-based techniques, including the multi-layer perceptron neural, radial basis function neural, and adaptive neural fuzzy inference system networks, in estimation of the sample topography is investigated. The results demonstrate that among the designed observers, the multi-layer perceptron method can estimate surface characteristics with higher accuracy than the other methods. In the classical imaging techniques, the scanning speed of atomic force microscope is restricted due to the time required by the oscillating tip to reach the steady state motion while the closed-loop controller tries to maintain the tip vibration amplitude at a set-point value. To address this issue, we have proposed an innovative imaging technique that not only eliminates the need to a closed-loop controller but also estimates the surface topography very quick and accurate compared to the conventional imaging method. Also, the proposed technique is capable of simultaneous estimation of the topography, Hamaker parameter, and the tip-sample interaction force.

360° multiparametric imaging atomic force microscopy: A method for three-dimensional nanomechanical mapping

Atomic Force Microscopy (AFM) has been intensively used for imaging, characterization and manipulation at the micro- and nanoscale. Taking into account that the material is usually anisotropic, it needs to be characterized in various regions and orientations. Although recent advances of AFM techniques have allowed for large area scan of the sample on a two-dimensional plane, mapping a three-dimensional (3D) sample at a full orientation of 360° remains challenge. This paper reports a multiparametric imaging atomic force microscope via robot technique for 360° mapping and 3D reconstruction of the sample's topography and nanomechanical properties. The system is developed by integrating a three degrees of freedom (DoFs) high-precision rotation stage and a home positioning approach is proposed to compensate for the eccentric distance between the cross-section center of the sample and the ration center of the stage. With this method, the sample surface can be fully mapped by the force-distance-based AFM via rotating the sample with a complete orientation. 360° multiparametric mapping and 3D reconstruction results (e.g., topography, adhesion, modulus, energy dissipation) of a human hair demonstrate practicability and reliability of the proposed method.

Real-time scan speed control of the atomic force microscopy for reducing imaging time based on sample topography

Here, a novel method, real-time scan speed control for raster scan amplitude modulation atomic force microscopes (AM-AFMs), is proposed. In general, the imaging rate is set to a fixed value before the experiment, which is determined by the feedback control calculations on each imaging point. Many efforts have been made to increase the AFM imaging rate, including using the cantilever with high eigenfrequency, employing new scan methods, and optimizing other mechanical components. The proposed real-time control method adjusts the scan speed linearly according to the error of every imaging point, which is mainly determined by the sample topography. Through setting residence time on each imaging point reasonably, the performance of AM-AFMs can be fully exploited while the scanner vibration is avoided when scan speed changes. Experiments and simulations are performed to demonstrate this control algorithm. This method would increase the imaging rate for samples with strongly fluctuant topography up to about 3 times without sacrificing any image quality, especially in large-scale and high-resolution imaging, in the meanwhile, it reduces the professional requirements for AM-AFM operators. Since the control strategy employs a linear algorithm to calculate the scanning speed based on the error signal, the proposed method avoids the frequent switching of the scanning speed between the high speed and the low speed. And it is easier to implement because there is no need to modify the original hardware of the AFM for its application.

Design and control of multi-actuated atomic force microscope for large-range and high-speed imaging

This paper presents the design and control of a high-speed and large-range atomic force microscopy (AFM). A multi-actuation scheme is proposed where several nano-positioners cooperate to achieve the range and speed requirements. A simple data-based control design methodology is presented to effectively operate the AFM scanner components. The proposed controllers compensate for the coupled dynamics and divide the positioning responsibilities between the scanner components. As a result, the multi-actuated scanner behavior is equivalent to that of a single X-Y-Z positioner with large range and high speed. The scanner of the designed AFM is composed of five nano-positioners, features 6 μm out-of-plane and 120 μm lateral ranges and is capable of high-speed operation. The presented AFM has a modular design with laser spot size of 3.5 μm suitable for small cantilever, an optical view of the sample and probe, a conveniently large waterproof sample stage and a 20 MHz data throughput for high resolution image acquisition at high imaging speeds. This AFM is used to visualize etching of calcite in a solution of sulfuric acid. Layer-by-layer dissolution and pit formation along the crystalline lines in a low pH environment is observed in real time.

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.