Microfluidic devices as herein understood are devices containing fluidic passages or chambers having typically at least one and generally more dimensions in the sub-millimeter to millimeters range. Partly because of their characteristically low total process fluid volumes and characteristically high surface to volume ratios, microfluidic devices can be useful to perform difficult, dangerous, or even otherwise impossible chemical reactions and processes in a safe, efficient, and environmentally-friendly way, and at throughput rates that are on the order of 100 ml/minute of continuous flow and can be significantly higher.
Microfluidic devices have been made of various materials including metals, ceramics, silicon, and polymers. The shortcomings encountered with these materials are numerous.
For example, devices made of polymers typically cannot withstand temperatures of more than 200° C. to 300° C. over a prolonged period. Moreover, it is often difficult to control surface states effectively within such structures.
Silicon devices are expensive and incompatible with certain chemical or biological fluids. Further, the semiconductive nature of silicon gives rise to problems with implementing certain pumping techniques, such as electro-hydrodynamic pumping and electro-osmotic pumping. Still further, the lithographic techniques used in forming silicon microfluidic devices naturally produce small channels (typically less than 100 μm). Such small channels have high backpressures and have difficulty achieving production throughput requirements.
Devices made of metal are liable to corrode and are typically not compatible with certain chemical or biological fluids.
It is therefore desirable, in numerous contexts, to have microfluidic structures made of glass, or at least having reaction channels lined with glass.
Microfluidic devices made of glass have been obtained by chemical or physical etching. Etching may be used to produce trenches in a glass substrate which trenches may be sealed by a glass lid, for example. Such techniques are not entirely satisfactory, however. Isotropic chemical etching does not enable significant aspect ratios to be obtained, while physical etching is difficult to implement due to its high cost and limited production capacity. To close the open trenches, the technique most often employed to attach or seal a lid is ionic attachment. This technique, however, is expensive and difficult to implement insofar as it is highly sensitive to dust. Moreover, the surface of each layer must be extremely flat in order to provide high quality sealing.
Microfluidic devices formed of structured consolidated frit defining recesses or passages between two or more substrates have been developed in previous work by the present inventors and/or their associates, as disclosed for example in U.S. Pat. No. 6,769,444, “Microfluidic Device and Manufacture Thereof” and related patents or patent publications. Methods disclosed therein include various steps including providing a first substrate, providing a second substrate, forming a first frit structure on a facing surface of said first substrate, forming a second frit structure on a facing surface of said second substrate, and consolidating said first substrate and said second substrate and said first and second frit structures together, with facing surfaces toward each other, so as to form one or more consolidated-frit-defined recesses or passages between said first and second substrates. In devices of this type, because the consolidated frit defines the fluidic passages, the passages can be lined with the glass or glass-ceramic material of the consolidated frit, even if a non-glass substrate is used.
Another approach to making glass microfluidic devices, disclosed for example in International Patent Publication WO 03/086958 involves vapor deposition of the glass on a surface of a temporary substrate that is shaped to serve as a negative mold for the shape to be produced. After glass is formed on the surface by vapor deposition, the temporary substrate is removed from the glass by wet etching. Vapor deposition and etching are relatively slow, expensive and environmentally unfriendly processes.
The present inventors and/or their associates have developed a method of forming a microfluidic device in which a thin sheet of glass is vacuum-formed resulting in an alternating channel structure on opposing sides of the sheet, then closed by fusing with one or more other vacuum-formed or flat sheets, as shown for example in US Patent Publication 2005/0241815. While the method therein disclosed is useful for the purposes described therein, it is desirable to be able to form even finer and more complex structures than is possible with this vacuum-forming technique, including sharp groove angles (e.g., 90°) and a larger variety of channel shapes and sizes.