Electrostatic microactuator control system for force spectroscopy
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Single molecule force spectroscopy is an important technique to determine the interaction forces between biomolecules. Atomic force microscopy (AFM) is one of the tools used for this purpose. So far, AFMs usually use cantilevers as the force sensors and piezoelectrics as the actuators which may have some drawbacks in terms of speed and noise. In this research, a micromachined membrane actuator was used in two important types of experiments, namely the single molecule pulling and force-clamp based force spectroscopy. These two methods permit a more direct way of probing the forces of biomolecules, giving a detailed insight into binding potentials, and allowing the detection of discrete unbinding forces. To improve the quality of the experiments there is a need for high force resolution, high time resolution and increase in the throughput. This research focuses on using the combination of AFM and membrane based probe structures that have electrostatic actuation capability. The membrane actuators are characterized for range, dynamics, and noise to illustrate their adequacy for these experiments and to show that the complexity they introduce does not affect the noise level in the system. The control system described in this thesis utilizes the novel membrane actuator structures and integrates it into the current AFM setup. This is a very useful tool which can be implemented on any AFM without changing its mechanical architecture. To perform an experiment, all that is needed is to place the membrane actuator on the AFM stage, under the imagining head, and run the control system, which was implemented using LabVIEW. The system allows the user to maintain a precise and continuous control of the force. This was demonstrated by performing a life time experiment using biomolecules. Moreover, by slightly modifying the control scheme, the system allows us to linearize the membrane motion, which is inherently non-linear. The feasibility of using this control system for a variety of loading rate experiments are also demonstrated.