Thermal And Mechanical Analysis Of An Su8 Polymeric Actuator Using Infrared Thermography

Introduction

Microgrippers have been one of the typical applications of MEMS technology. Microgrippers were developed for systems that can handle microparts or manipulate cells. For the latter application, biocompatibility and gentle handling are often required. Most of the microgrippers reported in the past are based on silicon micromachining. Thus, most of these components are made of silicon or glass (Solano1 Rolt Wood 23-34). Using alternative microtechnologies such as LIGA or laser cutting, metals can also be used as structural materials for microgrippers. Grippers were made of nickel , steel or titanium-nickel alloy (Martin Mitchell 67-71). All the above grippers are made of relatively stiff materials, which require a large actuation force and may cause harsh handling conditions such as higher operating temperatures and large forces. Furthermore, incompatibility with the biological samples could be another drawback of the above microgrippers. Polymers offer a much lower Young's modulus, and thus much lower actuation force and handling force. Recently SU-8, a thick-film negative resist, was used as the structural material for microgrippers . Different actuators such as electrostatic comb-drives , piezoelectric stacks , shape memory alloy (SMA) , or thermal actuators were used in microgrippers. Since polymers such as SU-8 are not electrically conducting, only external actuators such as a pneumatic actuator or SMA actuator have been used with the passive SU-8 parts(Brawner Camacho Long Lord 2009 ). The thermal actuating concept originally proposed by Guckel et al is an alternative for actuating microgrippers. This so-called heatuator consists of two electrically conducting and connected arms. Due to different widths and consequently different resistance and heat dissipating properties, the thinner arm has a higher temperature than the thicker one. The two arms are joined at the free end. The thicker arm has a thin flexure acting as a compliant joint. As a result of thermal expansion, the joined tip moves laterally in an arc motion towards the thick-arm side. Until now polymers have not been used for this actuator type, because most of them are electrically non-conducting. Furthermore, the low glass temperature of polymer restricts the use of a thermal actuator. 

Research Question

The thick-film resist SU-8 has a unique characteristic: it does not soften at elevated temperatures. Higher temperatures cause better cross-links and make SU-8 even harder.

Literature Review

Importance and Role of Mechanical Engineers

Traditionally, Mechanical engineers are concerned with the principles of motion, energy and force. It is the responsibility of mechanical engineers to design machines whose parts and assemblies function in a safe and reliable, efficient and predictable fashion. (Ramasubramanian, 2001) The mechanical engineer applies scientific method to the solution of mechanical problems. Indeed, the numerous laws and principles developed by scientists are the everyday tools of the mechanical engineer. Anything the engineer does that is related to the motion and dynamics of heat, gas, air, fluids and solid materials requires the understanding and application of science.

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