Thermoplastics such as cyclic olefin copolymer (COC) and polymethylmethacrylate (PMMA) have been increasingly used in fabricating microfluidic devices. However, the state-of-the-art microvalve technology is a polydimethylsiloxane (PDMS)-based three-layer structure. In order to integrate such a valve with a thermoplastics-based microfluidic device, a bonding method for thermoplastics/PDMS must be developed. We report here a method to bond COC with PDMS through surface activation by corona discharge, surface modification using 3-(trimethoxysilyl)propyl methacrylate (TMSPMA), and thermal annealing. The method is also applicable to PMMA. The bonding strength between thermoplastics and PDMS was represented by the peeling force, which was measured using a method established by the International Organization for Standardization (ISO). The bonding strength measurement offered an objective and quantitative indicator for protocol optimization, as well as comparison with other PDMSassociated bonding methods. Using optimized bonding conditions, two valve arrays were fabricated in a COC/PDMS/COC device and cyclic operations of valve closing/opening were successfully demonstrated. The valve-containing devices withstood 100 psi (~689 KPa) without delamination. Further, we integrated such valve arrays in a device for protein separation and demonstrated isoelectric focusing in the presence of valves.Thermoplastics have been increasingly employed for microfluidics applications. 1 Compared with glass and silicon, thermoplastics offer several advantages including manufacturability, low cost, and biocompatibility. [1][2][3][4] Among several thermoplastics, cyclic olefin copolymers (COC) and polymethylmethacrylate (PMMA) are frequently exploited for making microfluidic devices. These devices were often fabricated by bonding a cover sheet with a substrate containing microchannels and other microfeatures using various bonding methods including thermal fusion, 3-7 solvent bonding, [8][9][10][11][12][13][14] surface treatment, [15][16][17][18][19][20] and adhesives. 21,22 Each of these methods has advantages and disadvantages as reviewed in the literature. 1,2 A microfluidic system consists of a number of necessary building blocks. One of them is microvalves that may regulate flows, contain fluids, and isolate one region from the other. 23,24 Microvalves can be actuated using a variety of mechanisms, including electric, pneumatic, and thermal methods. 23 The state-of-the-art microvalve technique for lab-on-achip systems is elastic membrane-based polydimethylsiloxane (PDMS) valves. Quake's * To whom correspondence should be addressed. hfan@ufl.edu; Fax: 352-392-7303. . 25 followed by several studies using thousands of integrated microvalves for high-throughput applications. [26][27][28][29] The elastomer valves consists of three layers of PDMS. Control channels in the top layer are used to regulate the fluid channels in the bottom layer through the elastic property of the middle PDMS layer. The elastomer valve has been adopted by...