Blake W. Erickson

Harnessing the damping properties of materials for high-speed atomic force microscopy

The success of high-speed atomic force microscopy in imaging molecular motors, enzymes and microbes in liquid environments suggests that the technique could be of significant value in a variety of areas of nanotechnology. However, the majority of atomic force microscopy experiments are performed in air, and the tapping-mode detection speed of current high-speed cantilevers is an order of magnitude lower in air than in liquids. Traditional approaches to increasing the imaging rate of atomic force microscopy have involved reducing the size of the cantilever, but further reductions in size will require a fundamental change in the detection method of the microscope. Here, we show that high-speed imaging in air can instead be achieved by changing the cantilever material. We use cantilevers fabricated from polymers, which can mimic the high damping environment of liquids. With this approach, SU-8 polymer cantilevers are developed that have an imaging-in-air detection bandwidth that is 19 times faster than those of conventional cantilevers of similar size, resonance frequency and spring constant.

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.

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.

Large-scale analysis of high-speed atomic force microscopy data sets using adaptive image processing

Summary Modern high-speed atomic force microscopes generate significant quantities of data in a short amount of time. Each image in the sequence has to be processed quickly and accurately in order to obtain a true representation of the sample and its changes over time. This paper presents an automated, adaptive algorithm for the required processing of AFM images. The algorithm adaptively corrects for both common one-dimensional distortions as well as the most common two-dimensional distortions. This method uses an iterative thresholded processing algorithm for rapid and accurate separation of background and surface topography. This separation prevents artificial bias from topographic features and ensures the best possible coherence between the different images in a sequence. This method is equally applicable to all channels of AFM data, and can process images in seconds.

Single-Cycle-PLL Detection for Real-Time FM-AFM Applications

In this paper we present a novel architecture for phase-locked loop (PLL) based high-speed demodulation of frequency-modulated (FM) atomic force microscopy (AFM) signals. In our approach, we use single-sideband (SSB) frequency upconversion to translate the AFM signal from the position sensitive detector to a fixed intermediate frequency (IF) of 10 MHz. In this way, we fully benefit from the excellent noise performance of PLL-based FM demodulators still avoiding the intrinsic bandwidth limitation of such systems. In addition, the upconversion to a fixed IF renders the PLL demodulator independent of the cantilevers resonance frequency, allowing the system to work with a large range of cantilever frequencies. To investigate if the additional noise introduced by the SSB upconverter degrades the system noise figure we present a model of the AM-to-FM noise conversion in PLLs incorporating a phase-frequency detector. Using this model, we can predict an upper corner frequency for the demodulation bandwidth above which the converted noise from the single-sideband upconverter becomes the dominant noise source and therefore begins to deteriorate the overall system performance. The approach is validated by both electrical and AFM measurements obtained with a PCB-based prototype implementing the proposed demodulator architecture.

Outstanding meeting paper: high-speed photography of the development of microdamage in trabecular bone during compression

The mechanical properties of healthy and diseased bone tissue are extensively studied in mechanical tests. Most of this research is motivated by the immense costs of health care and social impacts due to osteoporosis in post-menopausal women and the aged. Osteoporosis results in bone loss and change of trabecular architecture, causing a decrease in bone strength. To address the problem of assessing local failure behavior of bone, we combined mechanical compression testing of trabecular bone samples with high-speed photography. In this exploratory study, we investigated healthy, osteoarthritic, and osteoporotic human vertebral trabecular bone compressed at high strain rates. Apparent strains were found to transfer into to a broad range of local strains. Strained trabeculae were seen to whiten with increasing strain. Comparison of whitened regions seen in high-speed photography sequences with scanning electron micrographs showed that the observed whitening was due to the formation of microcracks. From the results of a motion energy filter applied to the recorded movies, we saw that the whitened areas are, presumably, also areas of high deformation. In summary, high-speed photography allows the detection of microdamage in real time, leading toward a better understanding of the local processes involved in bone failure.

Reducing uncertainties in energy dissipation measurements in atomic force spectroscopy of molecular networks and cell-adhesion studies

Atomic force microscope (AFM) based single molecule force spectroscopy (SMFS) is a valuable tool in biophysics to investigate the ligand-receptor interactions, cell adhesion and cell mechanics. However, the force spectroscopy data analysis needs to be done carefully to extract the required quantitative parameters correctly. Especially the large number of molecules, commonly involved in complex networks formation; leads to very complicated force spectroscopy curves. One therefore, generally characterizes the total dissipated energy over a whole pulling cycle, as it is difficult to decompose the complex force curves into individual single molecule events. However, calculating the energy dissipation directly from the transformed force spectroscopy curves can lead to a significant over-estimation of the dissipated energy during a pulling experiment. The over-estimation of dissipated energy arises from the finite stiffness of the cantilever used for AFM based SMFS. Although this error can be significant, it is generally not compensated for. This can lead to significant misinterpretation of the energy dissipation (up to the order of 30%). In this paper, we show how in complex SMFS the excess dissipated energy caused by the stiffness of the cantilever can be identified and corrected using a high throughput algorithm. This algorithm is then applied to experimental results from molecular networks and cell-adhesion measurements to quantify the improvement in the estimation of the total energy dissipation.

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

Scientific Reports 5: Article number: 11987; 10.1038/srep11987 published online July142015; updated: March102016 The authors neglected to cite previous studies related to the control concepts of high speed atomic force microscopy in the Introduction section of this Article. These additional references are listed below as references 1 and 2, and should appear in the text as below. “We propose a solution for a large range, high—speed system through simple modifications of a commercial system to move the sample with two different actuators in mechanical series and extending the control loop with model-based control for this scanner.” should read: “In line with the controls concepts introduced by Bozchalooi et al.1,2, we present a solution for a large range, high–speed system through simple modifications of a commercial system to move the sample with two different actuators in mechanical series and extending the control loop with model based control for this scanner.”