# A Feasibility Study of a Novel Piezo MEMS Tweezer for Soft Materials Characterization
## 1 Introduction  
Early diagnosis is crucial to prevent disease progressions [1]. The correlation between the mechanical characteristics of a soft tissue and its status has been highlighted by several studies in many cell types like cancer cells, epithelial cells and laminopathies associated with diseases of the nuclear membrane [2]. In cancer cells, for example, the malignant transformation influences the mechanical properties by disruption and/or reorganization of cytoskeleton [3]. The stiffness, the elastic modulus, and the dynamic viscosity are among the most commonly used mechanical quantities to evaluate the state of a soft tissue. However the possibility to identify a diseased soft tissue in situ, at the microscale, can increase significantly the possibility of the cure. Different techniques are used to perform such measurements as micropipette aspiration [4], magnetic bead twisting [5], atomic force microscopy [6], optical tweezers [7] or mechanical tweezers [8]. In recent years, researchers focused on the developing of micro-electro-mechanical systems (MEMS) tweezers because of their efficiency and relative simplicity with respect to other systems [9]. New gripping tweezers have been built taking cue from the kinematics of articulated mechanisms [10] as the property of parallelograms [11; 12; 13; 14; 15]. More complex motion, with an increasing of the number of degrees of freedom, can be obtained adopting the concept of lumped compliance. In fact, concentrated flexibility has been used in flexure hinges in several investigations [16; 17; 18; 19; 20]. In order to improve the displacement accuracy and to lower the stress levels in the flexible elements a new conjugate surfaces flexure hinge (CSFH) flexure has been proposed [21; 22; 23; 24; 25] and optimized [26; 27]. The system has shown promising results in terms of of versatility and applicability to different types of tissue [28]. Thanks to their high level of miniaturization, MEMS-Technology based tweezers can be employed also in minimal invasive [29; 30; 31] or gastrointestinal surgery [32], and more generally speaking, in endoluminal surgery, for example, in TEM [33; 34; 35]. The actuation of the tweezer can be obtained by different systems, such as linear electrostatic actuators [36; 37], rotatory electrostatic actuators [38; 39] or electrothermal actuation [40; 41; 42; 43; 44; 45; 46]. However the advent of smart materials in the last decades has greatly increased the possibilities of development of new smart structures [47; 48; 49] with the ability, not possible for the traditional systems, to adapt to external conditions variations. Because of their speed of response, low power consumption and high operating bandwidth the piezoelectric materials are, among the smart materials, the most promising ones for active vibration control [50; 51; 52; 53] and MEMS applications [54]. However the research in the latter field mainly focuses on the energy harvesting [55; 56; 57; 58] or MEMS tweezers for manipulating micro-objects and microassembly [59; 60; 61]. This paper presents the design of a novel piezoMEMS tweezer for the analysis and characterization of soft materials. Each jaw can be built as a sandwich composite beam and activated by an electric field. The tweezer is supposed to be force controlled because the piezoelectric materials produce a stress proportional to the applied electrical field. Furthermore, a sensor is supposed to be integrated into the structure for displacement control. This gives rise to improve the analysis of the cell properties. In fact, the displacement-controlled actuator is able to identify the beginning of a rupture during straining or softening, while a force-controlled system maintains a constant force regardless of the required displacement [62]. The action of the piezoelectric material has been modeled by the Pin Force Model [63]. By applying symmetric electric fields to the composite jaws, they will bend in opposite directions allowing the gripping of the soft tissue.