Mohammad Maroufi

The biomechanics of peripheral nerves are determined by the blood-nerve barrier (BNB), together with the epineural barrier, extracellular matrix, and axonal composition, which maintain structural and functional stability. These elements are often ignored in the fabrication of penetrating devices, and the implant process is traumatic due to the mechanical distress, compromising the function of neuroprosthesis for sensory-motor restoration in amputees. Miniaturization of penetrating interfaces offers the unique opportunity of decoding individual nerve fibers associated to specific functions, however, a main issue for their implant is the lack of high-precision standardization of insertion forces. Current automatized electromechanical force sensors are available; however, their sensitivity and range amplitude are limited (i.e. mN), and have been tested only in-vitro . We previously developed a high-precision bi-directional micro-electromechanical force sensor, with a closed-loop mechanism (MEMS-CLFS), that while measuring with high-precision (−211.7μN to 211.5μN with a resolution of 4.74nN), can be used in alive animal. Our technology has an on-chip electrothermal displacement sensor with a shuttle beam displacement amplification mechanism, for large range and high-frequency resolution (dynamic range of 92.9 dB), which eliminates the adverse effect of flexural nonlinearity measurements, observed with other systems, and reduces the mechanical impact on delicate biological tissue. In this work, we use the MEMS-CLFS for in-vivo bidirectional measurement of biomechanics in somatic and autonomic nerves. Furthermore we define the mechanical implications of irrigation and collagen VI in the BNB, which is different for both autonomic and somatic nerves (∼ 8.5-8.6 fold density of collagen VI and vasculature CD31+ in the VN vs ScN). This study allowed us to create a mathematical approach to predict insertion forces. Our data highlights the necessity of nerve-customization forces to prevent injury when implanting interfaces, and describes a high precision MEMS technology and mathematical model for their measurements.

We present a control scheme for video-rate atomic force microscopy with rosette pattern. The controller structure involves a feedback internal-model-based controller and a feedforward iterative learning controller. The iterative learning controller is designed to improve tracking performance of the feedback-controlled scanner by rejecting the repetitive disturbances arising from the system nonlinearities. We investigate the performance of two inversion techniques for constructing the learning filter. We conduct tracking experiments using a two-degree-of-freedom microelectromechanical system (MEMS) nanopositioner at frame rates ranging from 5 to 20 frames per second. The results reveal that the algorithm converges rapidly and the iterative learning controller significantly reduces both the transient and steady-state tracking errors. We acquire and report a series of high-resolution time-lapsed video-rate AFM images with the rosette pattern.

We present a microelectromechanical system (MEMS)-based closed-loop force sensor, capable of measuring bidirectional forces along one axis of motion. The device comprises an on-chip electrothermal displacement sensor and electrostatic actuators. It features a voltage-controlled stiffness-adjustment mechanism, providing the user with an additional means of tuning the sensor's characteristics. This silicon-on-insulator device is an improvement on our previous design, offering a better dynamic range and a less complex calibration process. The sensor is fabricated based on a standard SOI MEMS process. It is then fully characterized and instrumented with a feedback controller. The closed-loop characterization reveals a 1σ-resolution of 4.74 nN and a force sensing range of -211.7μN to 211.5μN, leading to a dynamic range of 92.9 dB. [2019-0248]

Performance of several readout circuits, when applied to a MEMS-based bulk piezoresistive displacement sensor is compared in this study. The sensor comprises a pair of tilted clamped-guided silicon beams whose bulk piezoresistivity is used for displacement sensing. Wheatstone half-bridge, constant-voltage, and constant-current circuits are implemented to measure resistance changes of this sensor. We report full characterization of the sensor with these circuits and explore the sensor's important characteristics such as linearity, bandwidth, and noise. The results reveal that the constant-current circuit provides the highest sensitivity at equal-power bias condition. We find that the resolution of the sensor is degraded from 1.6 nm with the Wheatstone half-bridge and constant-voltage circuits to about 4.2 nm with the constant-current configuration. In the frequency domain, the sensor captures the full dynamics of the MEMS nanopositioner up to 20 kHz with all readout circuits. [2019-0168]

We present an analysis and a systematic design methodology for a novel nonraster scan method based on a rosette pattern and demonstrate its application in video-rate atomic force microscopy. This pattern is traced when the lateral axes of a parallel kinematic scanner are commanded to follow a combination of two sinusoids with identical amplitudes and different frequencies. We design an internal-model-based controller to enhance the tracking performance of this pattern and implement the scheme on a microelectromechanical system scanner. The results reveal high-precision tracking of the rosette pattern in order to acquire time-lapsed atomic force microscope images at the rate of 10 frames/s.

We demonstrate the application of the internal model principle in tracking a sequential cycloid trajectory to achieve video-rate atomic force microscope (AFM) imaging. To generate a sequential cycloid pattern, one axis of the nanopositioner traces a sinusoidal signal superimposed on a slow triangular wave. Discontinuities at turning points induce large peaks in the steady-state tracking error. To address this issue, a smooth trajectory is designed to reduce the magnitude of error by 60 nm compared with the triangular wave. This trajectory reduces the magnitude of error 10 times. The tracking controller includes the dynamics of the harmonic waveforms and the ramp signal as well as higher order harmonics of the scanning frequency, and an integrator to cope with the system nonlinearities at low frequencies. We perform experiments on a two degree of freedom microelectromechanical system nanopositioner at various scanning frequencies ranging from 500 to 2580 Hz within a scan area of 5 μm × 10 μm. The root-mean-square value of tracking error remains below 6.1 nm with a pitch size of 44.2 nm. We acquire time-lapse AFM images in contact mode at scan rates as high as 20 frames per second.

Microcantilevers featuring separate built-in actuation and displacement sensing capabilities allow effective and simple implementation of control methods, opening a pathway to achieving higher scan speeds in tapping-mode atomic force microscopy. Such active cantilevers are a significant milestone to eventually obtain video-rate on-chip atomic force microscopes (AFMs) that can even surpass the functionality and imaging speed of their macroscale counterparts at a significantly lower cost. In this brief, we present an active AFM cantilever with an on-chip actuator and two built-in displacement sensors, designed to be integrated into on-chip AFMs. The common feedthrough problem present in this type of architecture is addressed by a differential sensing configuration, and the revealed dynamics are used for the system identification. A positive position feedback controller is designed to actively tailor the Q factor of the cantilever. The imaging performance of the microcantilever with and without Q control is compared by attenuating the cantilever's Q factor from 177 to 15 using the feedback loop. A common artifact in high-speed scans, the parachuting effect, is mitigated, rendering higher imaging speeds achievable.

Active microcantilevers with on-chip sensing and actuation capabilities provide significant advantages in tapping-mode atomic force microscopy. The collocated transduction in active cantilevers enables effective control of their dynamics, allowing for the modification of the quality (Q) factor and operation at higher flexural modes to obtain higher scan rates. However, having closely spaced transducers in dynamic applications often results in electrical crosstalk from the actuation signal to the sensor output. As a result, the dynamic response of the cantilever becomes heavily dominated by this feedthrough, making the use of on-chip transduction impractical for atomic force microscope (AFM) imaging without cancelling this undesired effect. In this paper, we propose two on-chip feedthrough cancellation methods based on pseudo-differential actuation and differential sensing concepts. The implementation of the methods is demonstrated by the use of two microfabricated cantilevers with separate piezoelectric sensors and actuators. Following the cancellation of the feedthrough, both cantilevers are successfully employed for AFM imaging using the on-chip transducers for actuation and deflection sensing. [2017-0101].

Nanopositioners are high-precision mechatronic devices that are capable of generating mechanical motion with nanometer or sub-nanometer resolution. With this motion typically being available over mechanical bandwidths of several kilohertz or greater, nanopositioners have become important tools for many areas of nanotechnology. A major development in nanopositioning has been the use of microelectromechanical systems (MEMS) fabrication processes to produce microscale nanopositioners. These MEMS-based devices are conceptually similar to their macroscale counterparts, typically comprising an end effector, actuators, sensors, and suspension structures. However, MEMS nanopositioners potentially offer additional significant advantages as a result of their microfabricated nature, including a much smaller footprint, higher bandwidth, batch manufacturability, and potential for integration with electronic circuits. This paper reviews key concepts regarding the design, actuation, and sensing of MEMS nanopositioners, and explores how they have been implemented in state-of-the-art devices within a range of nanotechnology applications.