Fabrication Of PDMS Microfluidic Chips Integrated With Cu Microelectrodes

Polydimethylsiloxane (PDMS) is a high-elasticity organosilicon compound, and has been widely used to fabricate microfluidic chips due to its low cost, biocompatibility and ease of fabrication. Electrochemical detection is a simple and universal method, and the detecting electrode can be miniaturized and integrated on the chip, which makes it one of the most popular methods for microfluidic chips. Hence, the fabrication of PDMS microfluidic chips integrated with microelectrodes for electrochemical detection has recently attracted attentions.The fabrication of PDMS microfluidic chips integrated with microelectrodes includes microchannel fabricating, microelectrode fabricating and chip bonding. First, in this paper, a method for fabricating SU-8moulds for replicating PDMS microchannels was presented. SU-8is a thick and epoxy-based photoresist. SU-8can be fabricated into microstructures on the silicon substrate by photolithography processes, and used as a mould to replicate PDMS microchannels. However, due to the poor adhesion of SU-8on silicon, SU-8is prone to detaching from the silicon substrate during the replication process, and one SU-8mould can only be used to replicate about10PDMS devices. Therefore, we used a thin negative photoresist (BN308) as an adhesive layer and spin-coated on the glass substrate, and then SU-8was patterned on the BN308layer, which improved the lifetime of a SU-8mould up to over50cycles. According to the work of adhesion, the effects of the adhesion behavior of the negative photoresist and SU-8on substrates on the durability of the SU-8mould were investigated. The work of adhesion of BN308on glass was51.2mJ/m2, which is22.5%higher than that of SU-8on silicon and32.3%higher than that of SU-8on glass. This indicates that the increase of the work of adhesion can improve the adhesion strength between SU-8and the substrate, and then improves the lifetime of the SU-8mould. Moreover, the abilities of the method for replicating high-aspect-ratio microstructures were also tested. One SU-8mould having60×60array micropillars with aspect ratios lower than3could be used to replicate at least20polydimethylsiloxane (PDMS) devices.Second, the fabrication of copper microelectrodes on PDMS substrates was studied. Due to the high expansion coefficient of PDMS (310×10-6/℃), the copper layer sputtered on the PDMS substrate suffered a large thermal stress, which caused copper microelectrodes to wrinkle. The characteristics of the copper microelectrode were tested. Under natural conditions, the variation of the electrical resistance of the copper microelectrode was about 1%after144hours. When an electrical current of hundreds of microampere was applied, the variation of the electrical resistance was lower than1%after30min. A Scotch tape test was conducted, and the result indicated that the copper microelectrode had strong adhesion strength with the PDMS substrate. Moreover, three-hundred folding and straightening tests were also applied, and the microelectrode still kept structural integrity. All above tests demonstrate that the wrinkled copper microelectrode can meet the requirements of electrochemical detection.Finally, a PDMS microfluidic chip with dual contactless conductivity/amperometric detection system was fabricated. This chip was composed of a glass slide with Pt microelectrodes, a PDMS film with Cu microelectrodes and a PDMS cover plate with microchannels. A plasma surface treatment method was used to assemble this chip.