Calcium-assisted glass-to-glass bonding for fabrication of glass microfluidic devices

Abstract

Glass is a desired material for many microfluidics applications. It is chemically resistant and has desirable characteristics for capillary electrophoresis. The process to make a glass chip, however, is lengthy and inconvenient, with the most difficult step often being the bonding of two planar glass substrates. Here we describe a new glass bonding technique, which requires only washing of the glass surfaces with a calcium solution and 1-2 h of bonding at 115 degrees C. We found calcium uniquely allows for this simple and efficient low-temperature bonding to occur, and none of the other cations we tried (e.g., Na (+), Mg (2+), Mn (3+)) resulted in satisfactory bonding. We determined this bond is able to withstand high applied field strengths of at least up to 4 kV x cm (-1). When intense pressure was applied to a fluid inlet, a circular portion of the coverslip beneath the well exploded outward but very little of the glass-glass interface debonded. In combination with the directed hydrofluoric acid etching of a glass substrate using a poly(dimethylsiloxane) (PDMS) etch guide, we were able to make glass chips with better than 90% yield within 6 h. This technique is compatible with inexpensive unpolished glass and is limited in resolution by the PDMS etch guide used and the intrinsic properties of isotropic etching.

Figure 1

(A) Sequence from top to bottom showing two masks (I and II) and the combined pattern (I + II) utilized for the photolithographic generation of the master for the molded PDMS etch guide. We implemented this design in two layers to avoid problematical low-aspect-ratio structures in PDMS. (B) A schematic diagram of the major stages of etching and bonding shows how the PDMS etch guide directs HF onto the glass substrate to produce channels. After drilling, the glass is cleaned and rinsed with detergent containing calcium. The glass and its cover are rinsed with water and then assembled wet. The assembly was dried and then baked at 115 °C to produce the final bond.

Figure 2

(A) Images of the final products showing the beveled edges of the wide channels. (B) The effects of broadening on small and dense (2 μm separated by 4 μm) channels due to the isotropic nature of HF etching. (C) The strategy for the use of this design. The labeled arrows show the inlets for buffer (b), sample (s), and cleaning solution (c) as well as the outlets for waste. The electrical connections are also indicated. (D) The side profile of the wide side channels (top) and the more narrow separation channels (bottom). To the left are SEM images of the channel cross sections; light microscope images of the corresponding channels are at right.

Figure 3

(A–E) Sample injection shown as a sequence of events with fluorescence micrographs at left and a corresponding diagram at right. Arrows in the diagram indicate applied pressure. The sequence starts with (A) an empty channel with pressure directed into the buffer inlet. Pressure is then directed into both the buffer and the sample inlet and sample begins to flow (B) into the channel. Pressure to the sample is removed (C) and the sample flow recedes from the wide channel as buffer flushes it out of the channel. As a result, a small plug of sample is injected into the small separation channel (D). Finally, voltage is initiated (E) and the plug of sample is moved down the separation channel to the right. The electropherogram in (F) shows four peaks, which are labeled as follows: Rhod B for the rhodamine B peak, which also acts as a marker of EOF, FITC for the unreacted dye peak, Gly for labeled glycine, and Glu for labeled glutamate. In the example, the channel was 1 cm long and the applied voltage was 400 V. (G) We performed a series of measurements of EOF based on the retention time of zwitterionic Rhodamine B injected as in (A–F). The results are plotted with their corresponding best-fit line. The error bars represent the relative standard deviation between runs.