抗原变异可使得多种致病微生物易于逃避宿主免疫应答。表达在感染红细胞表面的恶性疟原虫红细胞表面蛋白1（PfPMP1）与感染红细胞、内皮细胞、树突状细胞以及胎盘的单个或多个受体作用，在黏附及免疫逃避中起关键的作用。每个单倍体基因组var基因家族编码约60种成员，通过启动转录不同的var基因变异体为抗原变异提供了分子基础。

疟原虫var基因转换速率变化导致抗原变异[英]／Paul H, Robert P, Christodoulou Z, et al//Proc Natl Acad Sci U S A

This article reviews the use of electron microscopy in liquids and its application in biology and materials science.

Electron microscopy of specimens in liquid

Piezo-actuated stages have become more and more promising in nanopositioning applications due to the excellent advantages of the fast response time, large mechanical force, and extremely fine resolution. Modeling and control are critical to achieve objectives for high-precision motion. However, piezo-actuated stages themselves suffer from the inherent drawbacks produced by the inherent creep and hysteresis nonlinearities and vibration caused by the lightly damped resonant dynamics, which make modeling and control of such systems challenging. To address these challenges, various techniques have been reported in the literature. This paper surveys and discusses the progresses of different modeling and control approaches for piezo-actuated nanopositioning stages and highlights new opportunities for the extended studies.

Modeling and Control of Piezo-Actuated Nanopositioning Stages: A Survey

Recent interest in high-speed scanning probe microscopy for high-throughput applications including video-rate atomic force microscopy and probe-based nanofabrication has sparked attention on the development of high-bandwidth flexure-guided nanopositioning systems (nanopositioners). Such nanopositioners are designed to move samples with sub-nanometer resolution with positioning bandwidth in the kilohertz range. State-of-the-art designs incorporate uniquely designed flexure mechanisms driven by compact and stiff piezoelectric actuators. This paper surveys key advances in mechanical design and control of dynamic effects and nonlinearities, in the context of high-speed nanopositioning. Future challenges and research topics are also discussed.

/pdf/invited-review-article-high-speed-flexure-guided-4qdbjhhogg.pdf

Invited review article: high-speed flexure-guided nanopositioning: mechanical design and control issues.

Position sensors with nanometer resolution are a key component of many precision imaging and fabrication machines. Since the sensor characteristics can define the linearity, resolution and speed of the machine, the sensor performance is a foremost consideration. The first goal of this article is to define concise performance metrics and to provide exact and approximate expressions for error sources including non-linearity, drift and noise. The second goal is to review current position sensor technologies and to compare their performance. The sensors considered include: resistive, piezoelectric and piezoresistive strain sensors; capacitive sensors; electrothermal sensors; eddy current sensors; linear variable displacement transformers; interferometers; and linear encoders.

/pdf/a-review-of-nanometer-resolution-position-sensors-operation-52os024r58.pdf

A review of nanometer resolution position sensors: Operation and performance

The development of a high-performance three-axis serial-kinematic nanopositioning stage is presented. The stage is designed for high-bandwidth applications that include video-rate scanning probe microscopy and high-throughput probe-based nanofabrication. Specifically, the positioner employs vertically stiff, double-hinged serial flexures for guiding the motion of the sample platform to minimize parasitic motion (runout) and off-axis effects compared to previous designs. Finite element analysis (FEA) predicts the dominant resonances along the fast ( x-axis) and slow (y-axis) scanning axes at 25.9 and 6.0 kHz, respectively. The measured dominant resonances of the prototype stage in the fast and slow scanning directions are 24.2 and 6.0 kHz, respectively, which are in good agreement with the FEA predictions. In the z-direction, the measured dominant resonance is approximately 70 kHz. The lateral and vertical positioning ranges are approximately 9 μm × 9 μm and 1 μm, respectively. Four approaches to control the lateral motion of the stage are evaluated for precision tracking at high-scan rates: (1) open-loop smooth inputs; (2) PID feedback; (3) discrete-time repetitive control implemented using field-programmable gate array (FPGA) hardware; and (4) model-based feed forward control. The stage is integrated with a commercial scan-by-probe atomic force microscope (AFM) and imaging and tracking results up to a line rate of 7 kHz are presented. At this line rate, 70 frames/s atomic force microscope video (100 × 100 pixels resolution) can be achieved.

Design and Control of a Three-Axis Serial-Kinematic High-Bandwidth Nanopositioner

http://biophys.w3.kanazawa-u.ac.jp/References/High-speed_AFM/Fleming_Ultramicroscopy_2010.pdf

Bridging the gap between conventional and video-speed scanning probe microscopes.

The mechanical design of a high-bandwidth, short-range vertical positioning stage is described for integration with a commercial scanning probe microscope (SPM) for dual-stage actuation to significantly improve scanning performance. The vertical motion of the sample platform is driven by a stiff and compact piezo-stack actuator and guided by a novel circular flexure to minimize undesirable mechanical resonances that can limit the performance of the vertical feedback control loop. Finite element analysis is performed to study the key issues that affect performance. To relax the need for properly securing the stage to a working surface, such as a laboratory workbench, an inertial cancellation scheme is utilized. The measured dominant unloaded mechanical resonance of a prototype stage is above 150 kHz and the travel range is approximately 1.56 μm. The high-bandwidth stage is experimentally evaluated with a basic commercial SPM, and results show over 25-times improvement in the scanning performance.

/pdf/compact-ultra-fast-vertical-nanopositioner-for-improving-44fe4qsd58.pdf

Compact ultra-fast vertical nanopositioner for improving scanning probe microscope scan speed.

A new three-axis serial-kinematic nanopositioning stage developed for high-bandwidth applications such as video-rate scanning probe microscopy (SPM) is presented. The stage employs uniquely designed compliant flexures for guiding the motion of the sample platform and to minimize parasitic motion (runout) and off-axis effects compared to previous designs. Finite element analysis (FEA) predicts the dominant resonances along the fast (x-axis) and slow (y-axis) scanning axes at 25.9 and 5.96 kHz, respectively. The performance of the nanopositioning stage is evaluated and the measured dominant resonances in the fast and slow scanning directions are 24.2 and 6.0 kHz, respectively, which are in good agreement with the FEA predictions. The lateral and vertical positioning range of the prototype stage is approximately 9×9 µm2 and 1 µm, respectively. Experimental atomic force microscope imaging and tracking results for closed- and open-loop feedforward control are presented to demonstrate the performance of the stage.

/pdf/design-characterization-and-control-of-a-monolithic-three-2b4tyy01oc.pdf

Brian J. Kenton

Papers

Invited review article: high-speed flexure-guided nanopositioning: mechanical design and control issues.

Design and Control of a Three-Axis Serial-Kinematic High-Bandwidth Nanopositioner

Bridging the gap between conventional and video-speed scanning probe microscopes.

Compact ultra-fast vertical nanopositioner for improving scanning probe microscope scan speed.

Design, characterization, and control of a monolithic three-axis high-bandwidth nanopositioning stage