Additive Manufacturing of Microcantilevers of Varying Stiffness for Sensing Applications

Abstract

Introduction Microcantilevers have numerous applications in physical, biological and chemical sensing. They have been employed for determination of the fluid viscosity and density [1], detection of biomolecular interaction [2], and part per billion (ppb) level of ammonia in ambient atmosphere [3]. The silicon-based microcantilevers are the most common type of these microdevices, however the biological and physical applications of the polymeric and polymer-composite microcantilevers have been reported in the literature [4], [5]. The polymeric cantilevers have lower Young’s modulus compared to the ones made of silicon and are more sensitive for static deflection measurement [6]. The polydimethylsiloxane (PDMS) and SU-8 are examples of polymers used for fabricating cantilevers through the soft-lithography process. Although the soft-lithography is a matured process for fabrication of the micro-electromechanical systems (MEMS), it is time consuming and costly in comparison with the additive manufacturing process. This may justify further investigations on employing Additive Manufacturing (AM) technologies in fabrication of the MEMS devices. In this study, we report additive manufacturing of the polymeric microcantilevers with different dimensions using SLA 3D printing technology for employing in micro-sensing applications. Method Four 300 μm width microcantilevers with different lengths and thicknesses, were 3D printed using the Form 2 SLA 3D printer. Table 1 presents the dimensions of the cantilevers. Figure 1 shows the images of the green parts on the building platform and the final parts after post processing. 50 μm was chosen as a thickness of each layer for printing. The building material was Flexible resin (Formlabs, USA). Table 2 presents the characteristics of the printing material. The printing process of all cantilevers took 2.5 hours. To investigate the mechanical property of printed cantilevers, the linear stiffness of each part was determined by the static load-deflection tests. The tests were performed by imposing the deflection to the tip of cantilever and recording the corresponding load. The maximum deflection applied to cantilevers was 300 μm. The FemtoTools FT-RS1002 Microrobotic Measurement System was employed to perform the measurements using the mounted sensing probe with the needle tip section size of (50 × 50 μm), force range of ±100000 μN, and resolution of 5 μN. Figure 2 illustrates the front and side views of the sensing probe in contact with the one of cantilevers at the beginning of the measurement test. Results and Conclusions The length and thickness of the microcantilever affect its sensitivity. Figure 3 displays the force-deflection graphs of microcantilevers with different length-thickness ratios obtained from the tests explained in methodology. Due to the small deflection imposed to all plastic microcantilevers with respect to their dimensions, the results show the bilinear response of the cantilevers. The linear relation between the force and the deflection can be observed when the samples deflects beyond the 80 μm. Despite various dimensions of the fabricated microcantilevers, the graph confirms that the length-thickness ratio affects the stiffness of the beams which is determined by the slope of each graphs. Table 3 presents the stiffness of fabricated samples. The graphs show increasing the length-ratio of the microbeams results in increasing the stiffness of the beam. The mechanical stiffnesses of the low length-thickness ratio microcantilevers, samples 1 and 2, are comparable with the PDMS cantilever’s stiffness reported in reference [7] . 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