A novel single-use SU-8 microvalve for pressure-driven microfluidic applications
Journal of Micromechanics and Microengineering
A novel microfluidic single-use valve for fluid injection and extraction in pressuredriven applications is presented in this paper. The device consists in a thin SU-8 membrane crossed by a resistor that withstands a mechanical stress induced by a pressure difference. When the resistor heats up the membrane, the SU-8 fracture strength drastically decreases causing the valve activation. This device has been designed, fabricated using inexpensive SU-8 and Printed Circuit Board technologies, and
... echnologies, and finally characterized. The hybrid thermalmechanical microvalve operation principle has been demonstrated and experimental results have shown the device characteristics and performance. Specifically, this design was functional at pressures of 0.8 MPa and opened in less than 3.2 seconds with an applied power of 280 mW. The simple fabrication process and the absence of moving mechanical parts have made the valve suitable for large-scale integration in Lab-on-Chip microfluidic platforms. of the fluid. On the other hand, active microvalves require external energy to operate, presenting a structure formed by a closing element and an actuator. Active valves can either be categorized by their initial working state or by their actuation principle. The three initial working states are normally closed, open or bistable, and the main actuation methods are pneumatic, thermopneumatic, thermomechanical, electrostatic, piezoelectric, electromagnetic, electrochemical and phase change. A detailed classification of microvalves has been reported in the literature by several authors [8, 9] . However, high manufacturing costs, fragility and lack of reliability of these devices still present a serious obstacle to integration. Specifically, traditional microvalves are fabricated in silicon, but high costs and limited accessibility of silicon processing has driven interest in exploring other materials and alternative technologies. During the past decade, polymers such as SU-8 and PDMS have emerged as materials of choice for many microfluidic devices and LOC platforms, offering the advantages of inexpensive and simple fabrication processes using rapid prototyping [10, 11] . In this respect, SU-8 technology has been a widely researched and developed area. SU-8 is an epoxy-based negative photoresist, whose properties make it a useful and cheap alternative for many microfluidic applications  . It can be patterned using standard mask processes with very thin films and nearly vertical sidewalls profiles  . Furthermore, the resist is transparent so the fluids can be sensed using an optical approach and inspected during experiments. Biocompatibility and biofouling of SU-8 have been previously evaluated and reported, presenting suitable results for biomedical applications  . Another technology that is becoming important in miniaturized fluid components is Printed Circuit Board Microsystems (PCBMEMS), which combines mechanical, electronic and fluidic devices by adding some new steps on common PCB fabrication process  . The main advantage of these devices lies in high integration with inexpensive materials, making possible to place all the components necessary for sensing, analyzing and controlling the fluids onto the same PCB. In some microfluidic applications it is sufficient to open a flow channel only once in order, for example, to trigger a mixing reaction in a microchamber or to deliver some fluid sample at a certain time. For such one-shot actuations, a disposable microvalve design can minimize reliability problems and manufacturing cost. Single-use microvalves contain a destructible element that is commonly activated by thermal or chemical actuation. The benefits of thermal techniques over chemical ones are simplicity and safety, due to the absence of risky or toxic reagents. Several authors have reported different contributions to this area, developing single-use microvalves with diverse operation principles and materials. Thermally actuated paraffin microvalves  and destructible membranes coated with low melting point alloys  can be found in the literature. Similar studies oriented to high gas pressure applications or about integration of air-bursting detonators are reported in [18, 19] respectively. However, the main disadvantages of thermal actuation are principally related to high energies required for valve activation and slow time responses when compared to many active microvalves. The purpose of this work is to develop a novel single-use microvalve designed to overcome the common limitations presented in traditional microfluidic devices. The presented device is activated combining two simultaneous and different phenomena, thermal and mechanical, minimizing the energy consumption required to activate the device. The valve design has been conceived to control the fluid flow in pressure-driven microfluidic applications, where the fluid motion energy is contained in a pressurized tank. Considerable effort has been directed towards minimizing the device cost, improving its integration and simplifying the fabrication process. Consequently, inexpensive materials like SU-8 and PCB have been selected. This paper is organized as follows. In section 2, device hybrid operation principle is presented and theoretical pressure and temperature effects over the membrane are studied separately. The following section shows the fabrication process and the device finally implemented. Experiments and results are discussed in section 4. Finally, conclusions are presented in section 5.