Design and characterization of a polymer electrothermal microgripper with a polynomial flexure for efficient operation and studies of moisture effect on negative deflection

Microgrippers are important devices for micro-manipulation in various applications in automation, microrobotics, material science, and biological studies. There has been a significant interest in improving the performance and reliability of microgrippers by developing efficient designs and actuation methods, and in reducing the cost of fabrication by utilizing polymer materials. This paper presents the design and characterization of an SU-8 based electrothermal microgripper with embedded Au thin film heaters. The design combines a Z-shape chevron actuator and a polynomial-shape flexure to convert the driving force into rotational movement of the microgripper arm. The results of electro-thermo-mechanical simulation using COMSOL Multiphysics software tool have shown that the optimized polynomial design is 5–10% more efficient in operation as compared to the straight beam design due to better stress distribution. The microgripper was capable of producing a gripping stroke of ~ 80 µm at 0.4 V driving voltage, the corresponding heater temperature was 120 °C. The characteristic behavior of negative deflection in SU-8 based microgrippers was observed at low actuation temperatures, resulting in several microns of displacement in the direction opposite to the expected movement. This effect was found to be associated with moisture absorption from the ambient environment by the polymer structure. Further tests under controlled humidity conditions were carried out and showed that the negative deflection is due to the moisture absorption effect in the SU-8 structures. It is shown that the microgripper is capable of performing pick-and-place of micro-objects using spherical glass micro-beads, therefore demonstrating the potential for applications in microassembly.