Micro-electromechanical-system (MEMS) based actuators, which transduce certain domains of energy into mechanical movements in the microscopic scale, are increasingly contributing to the areas of biomedical engineering and healthcare applications. They are enabling new functionalities in biomedical devices through their unique miniaturized features. An effective selection of a particular actuator, among a wide range of actuator types available in the MEMS field, needs to be made through the assessment of many factors involved in both the actuator itself and the target application. This paper presents an overview of the state-of-the-art MEMS actuators that have been developed for biomedical applications. The actuation methods, working principle, and imperative features of these actuators are discussed along with their specific applications. An emphasis of this review is placed on temperature-responsive, electromagnetic, piezoelectric, and fluid-driven actuators towards various application areas including lab-on-a-chip, drug delivery systems, cardiac devices and surgical tools. It also highlights the key issues of MEMS actuators in light of biomedical applications.

This paper presents a monolithic novel polydimethylsiloxane-based dual-channel bellows-structured pneumatic actuator, fabricated through a sacrificial molding technique. A finite element analysis was performed to find the optimal structure and analyze the bending performance of the square-bellows actuator. The actuator was designed with an overall cross-sectional area of 5 × 5 mm ² , while its structural parameters were characterized in terms of 8, 12, 16 and 20 number of bellows. The actuator models were fabricated using acrylonitrile butadiene styrene-based sacrificial molds to form the channel and bellows structures. The experimental validation has revealed that the actuator having highest number of bellows with a size of 5 × 5 × 68.4 mm ³ attained a smooth bi-directional bending motion, with maximum angles of −65° and 75°, and force of 0.166 and 0.221 N under left and right channel actuation, respectively, at 100 kPa pressure. Hence, the state-of-the-art dual-channel square-bellows actuator was able to achieve an optimal bi-directional bending with slight variations to change in temperature, which would push the boundaries of soft robotics toward the development of safer and more flexible robotic surgical tools.

In this paper, metallic thermoelectric generators were studied and fabricated using copper (Cu) and cobalt (Co) as their respective positive and negative thermoelements. Thus, the chosen Cu-clad polyimide substrate alleviated the deposition of Cu and eased the microfabrication. A lateral device structure might assist in generating larger output power through its longer thermoleg length. Hence, the fabricated thick-film devices had planar and lateral structures with lateral heat flow and lateral thermopile layout. The strong correlations of electrical and thermal conductivities in metal thermoelements have resulted in lower Seebeck coefficient along with reduced thermoelectric power-generating performances. Alternatively, a thermoleg cross-sectional area (A) optimization approach may optimize these disrupting correlations and improve their power-generating effectiveness, whereas a sandwiched planar structure can allow more thermopiles to be integrated without influencing the generator's size. Both A optimization and sandwiched planar structure have rarely been applied and studied in the past works and have never been implemented using metal thermoelements. Hereafter, a Cu-Co device was enhanced through A optimization by increasing the thickness of Co over 3.86 times the Cu thickness, and the implementations of a sandwiched planar structure. Herein, a flexible sandwiched planar thermoelectric generator was fabricated for the first time, using simpler microfabrication. This enhanced Cu-Co generator achieved a thermoelectric efficiency factor of 6.6 x 10⁻³ μWcm⁻²K⁻² (12.89 μWcm⁻²) at a temperature difference of 44.2 K. It remarked 3.1 times of improvement (by stacking three sets of thermopile) than its similar single Cu-Co thermopile of ten thermocouples.

This paper presents a novel polydimethylsiloxane-based dual-channel bellows-structured pneumatic actuator, fabricated through sacrificial molding technique. A finite element analysis was performed to find the optimum structure and analyze the bending performance of the square-bellows actuator. The actuator was fabricated using acrylonitrile butadiene-styrene-based sacrificial mold to form the channel and bellow structures with an overall actuator size of 5 × 5 × 27.6 mm ³ . The experimental validation has revealed that the actuator attained a smooth bi-directional bending motion with maximum angles of -25° and 35° and force of -0.168 and 0.212 N under left and right channel actuation, respectively, at 100 kPa pressure. Hence, the state-of-the-art dual channels square-bellows actuator was able to achieve an optimum bi-directional bending with low input pressure, which would push the boundaries of soft robotics towards the development of more safe and flexible robotic surgical tools.

Piezo-actuated micropositioning stages consist of a piezoelectric actuator that operates a positioning system. Hysteresis nonlinearity is one of the significant variables limiting the positioning precision of these stages. This paper introduces a technique of developing a hybrid controller for a precise positioning tracking of a piezoelectric micropositioning system. Bouc-Wen nonlinear hysteresis model is utilized to denote the hysteresis nonlinear phenomenon of the piezo-actuated system. A hysteresis observer-based feedforward controller is designed based on Luenberger observer. This feedforward controller is then coupled with a particle swarm optimization (PSO)-based proportional-integral-derivative (PID) feedback controller to form a hybrid controller. A new fitness function is used to compute the optimal PID gains. This fitness function is intended to reduce the overshoot, steady-state error, and the rise and settling times. The findings of this work indicate that using the developed controller structure can significantly decrease the hysteresis effect. In addition, the proposed structure shows the ability to reduce the error is to 0.046% of the maximum displacement range. Such performance demonstrates that the proposed hybrid control structure is efficient for precise micropositioning applications.