Patent Application
20100071418
This application claims the benefit of European Patent Application Serial No. 07300836.9 filed Feb. 28, 2007.
Many glass materials have certain desirable properties, including chemical and biological inertness, mechanical durability (long wearing), mechanical stability or rigidity and transparency. Glasses can also be resistant to thermal shock or large thermal gradients. These properties, taken together, make glass an attractive material for many applications. One of these is microfluidics.
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.
Some of the same properties that make glass attractive for many applications, such as inertness and durability, together with other properties such as hardness and brittleness, make glass difficult to form, however.
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.
Over the course of many years, various hot pressing and hot forming techniques have also been used to shape glass for various applications. Of those techniques capable of formation of fine or very fine features, most are difficult, requiring special equipment, or are expensive, or are environmental liabilities. An economical and simple robust process for forming fine structures in glass is desirable.
Described herein are methods of forming glass, useful for producing small features such as those found in microfluidic devices. The advantages of the materials, methods, and devices described herein will be set forth-in part in the description which follows, or may be learned by practice of the aspects described below. The advantages described below will be realized and attained by means of the elements and combinations particularly pointed out in the appended claims.
In this specification and in the claims that follow, reference will be made to a number of terms that shall be defined to have the following meanings:
Throughout this specification, unless the context requires otherwise, the word “comprise,” or variations such as “comprises” or “comprising,” will be understood to imply the inclusion of a stated feature or step or group of features or steps but not the exclusion of any other feature or step or group of features or steps.
It must be noted that, as used in the specification and the appended claims, the singular forms “a,” “an” and “the” include plural referents unless the context clearly dictates otherwise. Thus, for example, reference to “a glass material” includes mixtures of two or more such materials, and the like.
In one aspect, the method for making a glass-containing microfluidic device comprises:
providing a piece of rigid, non-stick material having a patterned molding surface; providing a first amount of glass-containing material;
contacting the first amount of glass-containing material with the patterned molding surface;
pressing the patterned molding surface and the first amount of glass-containing material together;
heating the piece of rigid non-stick material and the first amount of glass-containing material together sufficiently to soften the amount glass-containing material such that the patterned molding surface is replicated in the first amount of glass-containing material, the first amount of glass-containing material forming a first formed glass-containing article;
stacking the first formed glass-containing article with at least two additional glass-containing articles;
sealing the stacked articles together by heat treatment to create a microfluidic device having at least one fluidic passage therethrough.
The glass-containing material useful herein is any glass-containing material that upon heating can be converted to a viscous material. The glass-containing material may be in the form of a frit, including a filled frit. The glass-containing material may also be in the form of a sheet. The dimensions of the sheet can vary from few hundred square microns up to several decimeters square and have a sheet thickness from several hundred micrometers up to several centimeters. The glass containing material may comprise vitreous glass, glass ceramic, or a glass composite.
The glass composite may comprise a glass frit and a filler. The composite may be prepared, in frit form, by intimately admixing a glass frit and a filler. The resulting frit composite or filled frit may then be used directly as the glass-containing material, in the forming methods of the present invention, or it may first be formed into a glass sheet. In either case, it is desirable that the filler is evenly dispersed or integrated throughout the composite. This helps ensure that the entire glass sheet has reasonably consistent properties (e.g., average thermal conductivity) throughout the entire sheet. Certain glass frit and filler materials useful herein will be described below.
The glass frit is any glass material that upon heating can be converted to a viscous material. A variety of materials can be used herein. In one aspect, the glass frit comprises SiO2 and at least one other alkaline oxide, alkaline earth oxide, a transition metal oxide, a non-metal oxide (e.g., oxides of aluminum or phosphorous), or a combination thereof. In another aspect, the glass frit comprises an alkaline silicate, an alkaline earth silicate, or a combination thereof. Examples of materials useful as glass frits include, but are not limited to, a borosilicate, zirconium-containing borosilicate, or sodium borosilicate.
Turning to the filler, the filler is desirably nearly or completely inert with respect to the glass frit in order to preserve the thermal and mechanical properties of the filler. When the filler is nearly or completely inert with respect to the glass frit, the filler has no or minimal reaction within the filler/frit matrix such that there is essentially no foaming, forming of new phases, cracking and any other processes interfering with consolidation. Under these conditions, it is possible to produce a composite with minimal porosity.
The filler is also generally desirably non-porous or has minimal porosity and possesses low surface area. The filler does not burn out during sintering like organic compounds typically used in the art. The filler can remain rigid, soften, or even melt during thermal processing. In one aspect, the filler has a softening or melting point greater than that of the glass frit. Depending upon the selection of the filler, the filler can form an oxide, which will facilitate its integration into the final composite.
The filler desirably increases the average thermal conductivity of the composite. In one aspect, the filler has an average thermal conductivity greater than or equal to 2 W/m/K, greater than or equal to 3 W/m/K, greater than or equal to 4 W/m/K, or greater than or equal to 5 W/m/K. Examples of fillers useful herein include, but are not limited to, silicon carbide, aluminum nitride, boron carbide, boron nitride, titanium bromide, mullite, alumina, silver, gold, molybdenum, tungsten, carbon, silicon, diamond, nickel, platinum, or any combination thereof.
The amount of filler can vary depending upon, among other things, the type of glass frit selected and the desired average thermal conductivity. In one aspect, the amount of filler is greater than or equal to 5% by volume of the composite. In another aspect, the amount of filler is from 15% to 60% by volume of the composite.
With respect to the material used to make the mold, the porosity and chemical stability of the mold are to be considered in addition to the CTE/Young's modulus of the mold material relative to the glass. With respect to porosity, the mold most desirably possesses a certain degree of porosity so that gases produced during thermal processing can escape the molten glass through the porous mold and not be entrapped in the glass. In one aspect, the mold has an open porosity greater than 5%, that is, greater than 5% of the volume of the mold is open. In another aspect, the mold has an open porosity of at least 10%.
Another consideration when selecting the mold material is that the mold should be chemically stable at elevated temperatures, particularly those required to adequately soften the glass composite. The term “chemically stable” as used herein with respect to the mold material is defined as the resistance of the mold material to be converted from an inert material to a material that can interact with the molten glass. For example, while boron nitride could be used, boron nitride can be converted to boron oxide at temperatures greater than 700° C. Boron oxide can chemically interact with glass, which results in the glass sticking to the mold. Thus according to one aspect of the present invention, boron nitride may be used but is not preferred.
More desirably, the mold material comprises carbon, most desirably porous carbon such as grade 2450 PT graphite manufactured by Carbone Lorraine. This grade of graphite has a CTE of 25×10−7/° C. at 300° C. and open porosity level of about 10%. Techniques such as CNC machining, diamond ultra high speed machining, electro discharge machining, or a combination thereof can be used to make specific molding surfaces. The molding surface design can vary depending upon the desired features. As will be discussed in detail below, the methods described herein permit the use of molding surfaces with high aspect ratios (height/width greater than 3) and absolute heights from few microns up to several millimeters. Absolute heights and aspect ratios are not restricted to single values and can vary from one area of the molding surface to another. The molding surface can possess a variety of different three-dimensional (3D) grooved structures (e.g., channels, cavities) and raised structures (e.g., walls, pillars), which are desirable in microfluidic devices. Moreover, a release angle of 90° is possible with the grooved or raised structures on the mold, the relevance of which will be described in more detail below.
One embodiment for producing formed glass-containing articles will now be described with reference to
A release agent may optionally be used. The release agent can be applied to any of the second surface 14, the glass-containing composition 2, and the first surface 12 as desired. The amount of release agent that may be applied can vary. It is desirable that the material of the second surface 14 and release agent have similar properties or that they are composed of similar materials. For example, when the second surface or molding surface 14 is composed of graphite, the release agent is desirably carbon soot.
Pressure is desirably applied to the interface between the glass-containing composition 2 and the second surface 14. This may be achieved by a load 4 placed on top of the second structure 3 to facilitate penetration of the second surface or molding surface 14 into the glass-containing composition 2 during heating. The first structure 1, the glass-containing composition 2, the second structure 3 and the load 4 together form a stacked system 10. The load can be prepared from any material that can withstand elevated temperatures (i.e., temperatures required to adequately soften the glass-containing composition 2). The weight of the load can vary depending upon the amount or thickness of the glass-containing composition 2 and the desired amount of penetration of the second surface or molding surface 14 into the composition.
Once the stacked system 10 composed of the first structure, the glass-containing composition, the second structure, and the optional load is prepared, the stacked system 10 is heated to a temperature sufficient to result in viscous flow of the glass-containing composition 2. To perform this heating, the stacked system 10 can be placed in an oven. Prior to heating, air in the oven is desirably removed by vacuum, and an inert gas such as nitrogen is introduced into the oven. It is contemplated that one or more stacked systems can be introduced into the oven.
A series of stacked systems can be introduced into the oven by way of a conveyor belt, and the stacked systems can include more than one amount of glass-containing composition. This aspect is depicted in
As shown in
The temperature and duration of thermal processing of the stacked system 10 or 20 can vary among several parameters including, but not limited to, the viscosity of the glass-containing composition, the aspect ratio of the surface 14, and the complexity of the surface 14. Typical techniques for making glass molding surfaces are limited to short heating times in order to avoid sticking of the molten glass to the surface. This results in the formation of simple molding surfaces. The methods described herein avoid sticking of the molten glass to the molding surface during processing. Thus, longer heating times are possible with the methods described herein, which permit the softened glass-containing composition to penetrate each opening of an intricate molding surface. This ultimately results in the formation of more intricate formed glass-containing articles. Thus, the stacked system can be heated from one minute to one hour or even more, which is a much broader range than current hot forming techniques.
After the heating step, the stacked system is allowed to slowly cool down to at least 100° C., and desirably all the way to room temperature over time. The methods described herein not only prevent the softened glass-containing composition from sticking to the molding surface or surfaces, the methods described herein permit slow cooling of the glass-containing composition and the molding surface together, without the glass freezing (i.e., sticking) to the molding surface. By cooling slowly, the formation of cracks in the second structure and the molding surface can be prevented, such that the second structure and its molding surface may be re-used. Moreover, because the molding surface does not stick to the formed article, the second structure and its molding surface can be removed from the formed article by hand, and not by techniques commonly used in the art such as etching. This has a dramatic effect on production cost and the overall quality of the formed article.
As described above, the methods described herein permit the production of formed glass-containing articles with intricate and detailed features. For example, the molding surface can possess a plurality of areas that can penetrate the glass-containing composition at a depth of greater than 100 μm and a width greater than 100 μm. In another aspect, the depth can be from of 100 μm to 10 mm and the widths can be from 100 μm to 10 mm. In another aspect, the molding surface has an aspect ratio greater than three, where the aspect ratio is the height of the area or feature of the surface 14 (in the vertical direction in the Figures) over the width of the area or feature. Referring to
Although the first surface 12 of the first structure in
In another aspect, two or more first or second structures may be disposed on the same surface of the glass-containing composition, wherein the structures comprise identical or different patterned surfaces. In
The techniques described above are also useful in making a plurality (i.e., two or more) formed glass-containing articles simultaneously. In one aspect, the method comprises:
providing a first structure having a first surface;
providing a second structure having a second surface and a surface opposite the second surface, said second surface being patterned and porous;
disposing between said first surface and said second surface a first amount of a composition comprising a glass;
providing a third structure having a third surface, disposing between said third surface and the surface opposite said second surface a second amount of a composition comprising a glass, one of the opposite surface and the third surface being patterned;
heating together the first, second, and third structures and the first and second amounts of a composition comprising a glass sufficiently to soften the first and second amounts of a composition comprising a glass such that the first and second structures, and the second and third structures, under gravity or an otherwise applied force, move toward each other, such that the first amount of the composition forms a first formed article and the second amount of the composition forms a second formed article patterned by the respective patterned surfaces.
Referring to
The formed glass-containing articles produced by the methods described herein are useful in the production of microfluidic devices such as microreactors. Multiple formed articles having cooperating facing structures can be stacked and sealed. In one aspect, the stacked formed articles can be sealed at elevated temperature in air. The temperature and duration of heating will vary depending upon the material used to make the formed articles. The duration of heating is long enough to ensure that a complete seal is formed between each of the contacting formed articles. In the case of microreactors, this is important so that no reactants leak from the system as well as to maintain internal pressure within the microreactor.
Because both sides of the formed articles can be structured, and structured to some degree independently of the other, this method minimizes the number of glass components needed to make a glass microfluidic device or microreactor, particularly a glass microreactor with multiple layers.
In other aspects, it may be desirable to attach a formed glass-containing article to a substrate that is not glass. For example, a formed glass-containing sheet sealed to a high thermal conductivity substrate can improve heat transfer of the resulting microreactor. In one aspect, the material used for the substrate has a CTE similar to that of the glass-containing composition to be formed and can withstand the processing temperature. Examples of substrates useful herein include, but are not limited to, silicon, silicon carbide, alumina, metals, and the like. In one aspect, the method for attaching a glass mold on a substrate, comprises:
providing a first structure having a first surface;
providing a second structure having a second surface, said second surface being patterned and porous;
disposing between said first surface and said second surface a first amount of a composition comprising a glass;
heating together the first and second structures and the first amount of the composition sufficiently to soften the first amount of the composition such that the first and second structures, under gravity or an otherwise applied force, move toward each other, such that the pattern of the second surface is formed into the first amount of the composition;
wherein the step of heating includes fusing said first amount of the composition comprising glass to said first surface, resulting in the first amount of a composition comprising a glass forming, together with the first structure, a formed glass-containing article.
Fabrication of a molding surface 14 such as that shown in
Referring to
The stacked assembly 10 is loaded into an oven and heated under nitrogen flowing. Prior to introducing nitrogen, air in the oven was removed by vacuum. The temperature of the furnace was increased up to 900° C. over two hours to induce viscous deformation of the glass sheet into the recesses of the surface 14. There was a one-hour dwell followed by cooling down to room temperature over five hours. The first and second structures and the formed glass sheet were disassembled by hand.
In order to make a microfluidic component 57, two formed glass sheets produced by the procedure above were sealed together at 800° C. in air. Referring to
One particular value for microfluidic devices is found in assembling three or more of the formed articles produced according to the steps herein particularly if all through-holes are formed as a part of the initial forming process. For example, formed structures 91, 93, 95, 97, and 99 can be stacked and sealed together to form a multiple layer microfluidic device.
| Number | Date | Country | Kind |
|---|---|---|---|
| 07300836.9 | Feb 2007 | EP | regional |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/US08/02585 | 2/27/2008 | WO | 00 | 8/12/2009 |
