A Versatile Bonding Method for PDMS and SU-8 and Its Application towards a Multifunctional Microfluidic Device

Abstract

This paper reports a versatile and irreversible bonding method for poly(dimethylsiloxane) (PDMS) and SU-8. The method is based on epoxide opening and dehydration reactions between surface-modified PDMS and SU-8. A PDMS replica is first activated via the low-cost lab equipment, i.e., the oxygen plasma cleaner or the corona treater. Then both SU-8 and plasma-treated PDMS samples are functionalized using hydrolyzed (3-aminopropyl)triethoxysilane (APTES). Ultimately, the samples are simply brought into contact and heated to enable covalent bonding. The molecular coupling and chemical reactions behind the bonding occurring at the surfaces were characterized by water contact angle measurement and X-ray photoelectron spectroscopy (XPS) analysis. The reliability of bonded PDMS-SU-8 samples was examined by using tensile strength and leakage tests, which revealed a bonding strength of over 1.4 MPa. The presented bonding method was also applied to create a metal-SU-8-PDMS hybrid device, which integrated SU-8 microfluidic structures and microelectrodes. This hybrid system was used for the effective trapping of microparticles on-chip, and the selective releasing and identification of predefined trapped microparticles. The hybrid fabrication approach presented here, based on the PDMS-SU-8 bonding, enables multifunctional integration in complex microfluidic devices.

Keywords: PDMS; SU-8; bonding; cell trapping; impedance measurement; microfluidics; multifunctional integration; negative dielectrophoretic (nDEP).

Figure 1

Surface modification and reaction in the poly(dimethylsiloxane) (PDMS)-SU-8 bonding process: (a) hydrolysis of APTES reagent in water; (b) surface reaction of plasma-activated PDMS in the aqueous (3-aminopropyl)triethoxysilane (APTES) solution. The hydrolysate of APTES molecules is coupled to the silanol groups on the PDMS surface; (c) surface reaction of SU-8 in the aqueous APTES solution. The amino groups of APTES molecules react with the epoxy groups on the SU-8 surface (SN₂ reaction); (d) further epoxide opening reaction (SN₂) and condensation (reticulation, –Si–O–Si– formation) occurred at the sample interfaces when the APTES-functionalized PDMS and SU-8 were contacted and heated.

Figure 2

Setup of the tensile strength test. (a) Setup of the tensile tester. It has an immobile jaw at the lower position and a movable jaw connected to a force sensor at the upper position. The sample is fixed on the jaws through screws; (b) assembled sample in the tester. The sample holders were first glued with two bare glass slides. The bonded PDMS-SU-8 sample was then sandwiched between the two glass slides using silicone glue.

Figure 3

Water contact angles measured on the pristine PDMS and SU-8, corona- and oxygen plasma-treated PDMS, APTES-modified PDMS and SU-8, as well as 30-min- and 1-h-aged PDMS and SU-8 after APTES functionalization.

Figure 4

XPS of PDMS and SU-8: (a) pristine, plasma-activated, and APTES-modified PDMS; and (b) pristine and APTES-modified SU-8.

Figure 5

Tensile strength test for the PDMS-SU-8 bonding. (a) Photo of a broken sample after the tensile test; (b) bonding strength of samples treated under different conditions: corona-activated PDMS, oxygen plasma-activated PDMS and 30-min-aged PDMS and SU-8 after APTES silanization. Error bars represent the standard deviation of seven measured samples under each condition.

Figure 6

Liquid leakage test for the bonded PDMS-SU-8 channels. (a) Single-ended channel infused with a red dye solution. The flow rate was increased from 50 µL/min to 800 µL/min, at which point the red dye leaked out of the inlet; (b) double-ended channel infused with a red dye solution. The flow rate was increased up to 1500 µL/min when the red dye leaked out of the inlet. The hydraulic pressure from the channel inlet to the outlet, Δp, was calculated by using Hagen-Poiseuille’s equation, according to the corresponding flow rate and the geometric parameters of the channel.

Figure 7

A metal-SU-8-PDMS hybrid device for multifunctional application. (a) Photo of the device indicated with the fluidic connections; (b) close-up of the device showing five orifices (01–05) and respective electrodes. The fluidic structures, patterned in SU-8, were precisely aligned with Pt electrodes. Blue arrows indicate the flow directions during the microparticle trapping; (c) close-up of a unit for microparticle trapping, nDEP releasing and local impedance measurement. The tip of the tip electrode is right situated under the orifice; (d) micrographs of one trapped microparticle and two vertically stacked microparticles. The diameter of microparticles used in this work is 6 µm; (e–i) identification of single and two vertically stacked microparticle(s) at the orifices 01 to 05 through impedance measurements. $\left|Z\right|$ and $\left|Z_e\right|$ are the impedance magnitudes from measuring the trapped microparticle(s) and the empty orifice, respectively.