Auxetic materials for MEMS: modeling, optimization and additive manufacturing

Auxetic materials are of interest because of enhanced material properties related to negative Poisson’s ratio, such as increased shear modulus, indentation resistance, fracture toughness, energy absorption, porosity/permeability variation with strain and synclastic curvature. In addition, the Poissons ratio does not depend on scale: deformation can take place at the nano- (molecular), [1], micro-[2], or even at the macro-level, [3]; the only requirement is the right combination of the geometry and the deformation mechanism. The most popular feature of auxetic structures is that it can expand in the direction perpendicular to an externally exerted tension. This property makes auxetic structures strongly appealing for MEMS applications (i.e. motion conversion and resonators) [4]. Here we present a full study of a re-entrant honeycomb structure which can be used in MEMS devices as motion conversion spring. The design of the proposed structure is obtained from a Matlab optimization procedure targeted to reach a Poisson’s ratio equal to -1. An additional nonlinear numerical simulation (see [5]) is done to verify the behavior of the structure under large deformations regime, which is the one required by the MEMS applications and then, a 3D-printed model (see [6]) of the optimized structure is obtained. Finally, a topology optimization is performed to obtain an auxetic structure that, in principle, can amplify the motion in the direction othogonal to the driving one. With this procedure there is no need to create an input structure from which the optimization procedure can start. Also for one of these optimized auxetic structures, an additional nonlinear numerical simulation is done.