Bubble based micropump

[0002] This application claims benefit to U.S. provisional application 60/117627 filed Jan. 28, 1999.

[0004] 1. Field of the Invention

[0005] The present invention generally relates to a liquid pump and, more particularly, to a liquid pump which forms vapor bubbles in order to transport either electrically conductive or non-conductive liquid through channels and/or micro-devices.

[0006] 2. Background of the Invention

[0007] Micro-pumps have considerable applications, for example in existing and prospective micro-fluid-handling systems such as “laboratory-on-a-chip” devices increasingly used in biomedicine, pharmaceuticals, environmental monitoring, and other applications. Other applications, actual or under consideration, include, for example, miniature polymerase chain reactors, electronic cooling systems, micro-mixing apparatuses, etc. In all of these applications, micro-pumps increase the pressure of the fluid and/or cause the motion of liquid for the transport of chemicals, heat transfer, or other known purposes.

[0008] Many micro-pumps use mechanical moving parts in order to provide pumping action. By way of example known actuation mechanisms include (i) piezoelectric micro-pumps and (ii) thermo-pneumatic micro-pumps.

[0009] As a general background, a piezoelectric micro-pump uses piezoelectric disks to drive valves (e.g. check valves) that, opening and closing at opportune times during the cycle, promote the motion of the fluid in one direction only. In a thermo-pneumatic micro-pump the same action is achieved by means of a small amount of gas (or a gas/liquid mixture) contained in a cavity separated by a suitable membrane from the liquid. By alternatively heating and cooling the gas (or the mixture), the gas (or the mixture) pressure rises and falls and actuates the membrane. This motion of the membrane then displaces the liquid within the cavity of the thermo-pneumatically driven micro-pump, much as in the piezoelectric system previously described.

[0010] Many of these micro-pumps have known drawbacks which contribute to their inefficiency. For example, a drawback of the piezoelectric micro-pump is the size of the piezoelectric disks (about 10 mm) that prevents a true miniaturization of the device. In addition, these systems require high voltages (with attendant high costs), and only provide small displacements, of the order of a few microns. Due to the relative slowness of heat transport in the existing devices, thermo-pneumatically driven micro-pumps suffer from a low frequency of operation which severely limits the liquid flow rate achievable with these systems. Moreover, since all the above devices (and pumps in general) contain moving mechanical parts, they are subject to mechanical failure due to imperfection of construction or materials, stress, fatigue, and other mechanical factors.

[0011] Of course other micro-pumps also exist that are based on non-mechanical moving parts, for example (i) ultrasonically driven micro-pumps, (ii) evaporation/condensation systems, and (iii) valveless micro-pumps. By way of example, ultrasonically driven micro-pumps induce fluid motion by the peristaltic action of traveling flexural waves. Similar to the piezoelectric pumps described above, these systems cannot be made very small due to the intrinsic size of the ultrasonic source and vibrating membranes. Evaporation/condensation systems do provide transport of liquid by causing evaporation in one place and condensation in another one (e.g., micro-heat pipes) but, again, their smallest size is limited to the centimeter scale and requires that the entire amount of liquid achieve a high temperature, which may cause undesirable degradation and would not be applicable to the transport, e.g., of liquid with dissolved proteins or other biological material. Some arrangements have been proposed in which ordinary valves are not required (hence the denomination “valveless”), but again one needs an actuation mechanism—piezoelectric or thermo-pneumatic—with all the above described drawbacks. What is thus needed is a micro-pump that does not rely on any mechanical moving parts in order to provide proper transport of fluid. What is further needed is a micro-pump that offers greater simplicity of construction and operation and the ability to work “on demand” with great flexibility of operation in terms of pumping rates and faster flow rates than those presently known.

[0012] It is therefore an object of the present invention to provide a micro-pump which forms vapor bubbles in order to transport either electrically conductive or non-conductive liquid through channels of the micro-pump and/or micro-devices.

[0013] It is a further object of the present invention to provide a micro-pump capable of pumping liquid in very small channels by exploiting bubbles properties.

[0014] It is still a further object of the present invention to provide a micro-pump that does not utilize any mechanical moving parts.

[0015] It is also another object of the present invention to provide a micro-pump that offers simplicity of construction and operation.

[0016] The present invention is directed to a micro-pump for pumping either electrically conductive or non-conductive liquids through channels of the micro-pump and/or micro-devices. In order to accomplish the above objectives, a conductive or non-conductive liquid, depending on the specific application of the present invention, is disposed within a liquid chamber and/or channel of the micro-pump. An energy source is then applied to the micro-pump of the present invention in order to form one or more vapor bubbles within the chamber and/or channel. Thereafter the vapor bubble(s) is collapsed, and the process of forming and collapsing the vapor bubble may thereafter be repeated. By the formation and collapsing cycle of the vapor bubble, a pumping action of the liquid is effectuated thereby transporting the liquid within the micro-pump of the present invention and/or micro-devices.

[0017] In use, the underlying concepts of the present invention may be utilized in several known embodiments, all of which form and collapse vapor bubbles in order to transport liquids. For example, in one embodiment, an electrically conductive liquid is disposed within opposing electrically conductive channels of different diameters. Electrical current in then provided to the conductive channels (thereby completing a conductive path between the conductive channels and the conductive liquid) in order to form the vapor bubble in a conical section disposed between the opposing conductive channels. In other embodiments, a heat source is applied to the liquid disposed within a channel in order to create the vapor bubbles therein. In some of these other embodiments, liquid disposed within a conical section of the channel is contemplated for use by the present invention, such that the heater is placed in the conical section (partially or fully surrounding it) and the vapor bubble is formed therein. It is important to note that the pumping action is due to the asymmetrical properties of the micro-pump of the present invention, whether it be the asymmetrical properties presented by the conical section or the asymmetrical properties created by the placement of the energy source along the chamber and/or channel.

[0018] A method of using the micro-pump of the present invention is also contemplated for use herein.

[0019] The foregoing and other objects, aspects and advantages will be better understood from the following detailed description of a preferred embodiment of the invention with reference to the drawings, in which:

[0020] FIG. 1 shows a liquid pump for electrically conducting liquid of the present invention using tube electrodes;

[0021] FIG. 2 shows a liquid pump for electrically conducting or non-conducting liquid of the present invention;

[0022] FIG. 3ashows a top view of the multi-heater liquid micro-pump of the present invention;

[0023] FIG. 3bshows a side view of the multi-heater liquid micro-pump of the present invention; and

[0024] FIG. 4 shows a two reservoir liquid pump of the present invention.

[0025] The present invention is directed to a micro-pump for pumping electrically non-conductive or conductive liquids through a channel, and more specifically to a micro-pump that transports the non-conductive or conductive liquids through the channel without any moving mechanical parts. The micro-pump of the present invention further offers simplicity of construction and operation and the ability to have flexibility of operation in terms of pumping and flow rates.

[0026] In order to accomplish the objectives of the present invention, the micro- pump of all embodiments of the present invention form vapor bubbles within a channel containing a liquid, such as, for example, water with dissolved NaCl or other conductive or non-conductive liquid. The channel may be any known shape, such as, for example, rectangular, triangular, circular and the like. The formation of the vapor bubble in the channel causes a motion of the liquid which is capable of overcoming a pressure difference equivalent to the hydrostatic head (of the liquid) of several centimeters. In the embodiments of the present invention, the formation of the vapor bubble is provided by either an electric current or a heater, depending on the liquid within the micro-pump or other variables.

[0027] In order to better understand the micro-pump of the present invention and the specific application thereof, it is important to note that vapor bubbles in liquids possess mechanical properties that are extremely useful and beneficial to the application of micro-devices and more specifically to the application of the micro-pump of the present invention with micro-devices. In particular, vapor bubbles have intrinsic time scales which are very short thus making them suitable for operation at tens or perhaps hundreds of kHz. Moreover, the energy density of vapor bubbles is on the order of tens of MW/m3, which offers advantages for effective pumping actuation. It is further understood that other important features of the present invention include (i) direct and efficient electrical-to-mechanical power conversion, (ii) absence of mechanical moving parts and (iii) the absence of solid-solid friction.

[0028] Electrical Conducting Micro-Pump

[0029] The micro-pump of the present invention is suitable for pumping liquids in channels, preferably with diameters on the order of several microns to approximately five millimeters. However, it is well understood that the present invention is not limited to the above range of diameters, and that other diameters may equally be used with the present invention. Thus, the specific numbers and dimensions specified herein (for all embodiments) are not to be construed as limitations on the scope of the present invention unless otherwise noted herein, but are meant to be merely illustrative of one particular application of the present invention.

[0030] Referring now to FIG. 1, a first embodiment of the present invention is shown. The embodiment of FIG. 1 is used for the formation of vapor bubbles with the use of a conductive liquid such as, for example, salt water, liquid metals and the like. The first embodiment shown in FIG. 1 includes a conical chamber 10 having a constricted throat 12. In the preferred implementation of the micro-pump of the present invention, the conical chamber 10 is preferably a non-conductive material, such as, plastic, Plexiglas and the like; but may equally be other materials depending on the specific application of the micro-pump of the present invention. Moreover, in the preferred embodiment, the conical chamber 10 provides an asymmetry within the micro-pump of the present invention and more specifically, in embodiments, between two suitably-sized conductive electrode channels 14 and 16 as described more fully below.

[0031] Still referring to FIG. 1, two suitably-sized conductive electrode channels 14 and 16 are disposed at opposing ends of the conical chamber 10. In the preferred embodiment, the conductive electrode channel 14 is larger than the conductive electrode channel 16, and even more preferably, the diameter of the conductive electrode channel 14 is approximately 1.5 to twice the diameter of the conductive electrode channel 16. It is important to note that each of these conductive electrode channels 14 and 16 are capable of conducting electricity, and that the conductive electrode channels 14 and 16 provide a liquid tight connection at the ends of the chamber 10. It is further important to note that the conductive electrode channels 14 and 16 are not limited to any specific conductive material and may include any conductive material that is appropriate for the specific application of the micro-pump of the present invention.

[0032] In one embodiment of the micro-pump of the present invention, the chamber 10 may be formed in a Plexiglas plate by drilling, laser ablation and the like, and the conductive electrode channels 14 and 16 may be needles communicating with the chamber 10 and embedded within the Plexiglas plate. However, and as discussed above, it is well understood that the chamber 10 and the conductive electrode channels 14 and 16 may equally be other types of materials. Moreover, the exact dimensions of the present invention are not critical to the understanding of the present invention, and the only limitation on the dimensions of the present invention is that (i) the constricted throat 12 has a smaller diameter than the channel 16, (ii) the diameter of the conductive electrode channel 16 is smaller than the diameter of the conductive electrode channel 14, (iii) the conical chamber 10 provides an asymmetry within the micro-pump and (iv) the diameters of the conductive electrode channels 14 and 16 are preferably in the range between approximately several microns to five millimeters.

[0033] To begin pumping, current is passed through the respective conductive electrode channels 14 and 16, and a conductive liquid filling the channels 14 and 16. The current is preferably placed upstream and downstream of the constricted throat 12, and provides current pulses to the conductive electrode channels 14 and 16. The conductive liquid in combination with the conductive electrode channels 14 and 16 completes an electric circuit such that the electric current passing through the liquid is “squeezed” in the constricted throat 12. This “squeezing” action results in an intense localized heating which causes the formation and rapid growth of a vapor bubble 15 (i.e., local boiling). In the preferred embodiments, the vapor bubble 15 grows in a direction of the wider channel 14 due to effects of surface tension of the asymmetry presented by the conical chamber 10 .

[0034] As discussed, the action of the surface tension of the conductive liquid causes the vapor bubble 15 to grow preferentially in the expanding portion of the conical chamber 10 (e.g., above the constricted throat 12 and in the direction of the wider channel 14). As the vapor bubble 15 expands and forms, it exerts force on the column of liquid in the wider channel 14 . This application of force on the column of liquid thus pushes the liquid along the conductive electrode channel 14. That is, the formation and expansion of the vapor bubble 15 within the conical chamber 10 displaces a certain volume of the conductive liquid preferably within wider channel 14. When the electrical current is stopped, the bubble 15 collapses and the conical chamber 10 refills with liquid. It is important to the understanding of the present invention that during bubble growth the liquid is passed preferably in the direction of the wider channel 14, while the liquid enters into the conical chamber 10 approximately in equal amounts from both channels 14 and 16 when the bubble collapses. Thus, by applying and stopping the current, an overall net displacement of liquid is effectively provided.

[0035] It is well understood that more than one pulse frequency and current may be applied to the micro-pump of the present invention. However, as can be readily appreciated by one of ordinary skill in the art, the pulse frequency and the current is dependent on the diameter of the conductive electrode channels 14 and 16 as well as the specific materials and conductive liquids used with the present invention. For example, larger diameter conductive electrode channels 14 and 16 would result in the need for a larger current and/or longer pulses so as to provide the beneficial effects of the present invention (i.e., to adequately grow the bubble 15 in order to provide an adequate pumping force of the liquid).

[0036] Non-Electrical Conducting Micro-Pump

[0037] Similar to the electrical conducting micro-pump described above, the non-electrical conducting micro-pump of the present invention is suitable for pumping liquids in various sized channels, and offers simplicity of construction and operation and the ability to have great flexibility of operation in terms of pumping and flow rates. The non-electrical conducting micro-pump is especially adapted for use with non-conductive liquid; however, it is well understood that conductive liquids may equally be used with this embodiment.

[0038] Now referring to FIG. 2, the non-electrical conducting micro-pump of the present invention is shown which is especially adapted for use with non- conductive liquids. Specifically, FIG. 2 shows a Plexiglas or other non-conductive substrate 20. A non-conductive conical chamber 10 is provided within the non-conductive substrate 20 by drilling, laser ablation or any other well known process. Similar to the embodiment of FIG. 1, the conical chamber 10 may equally be plastic, Plexiglas or any other material depending on the specific application thereof.

[0039] Moreover, in the preferred embodiment, the conical chamber 10 provides an asymmetry of the micro-pump of the present invention as described above.

[0040] Two suitably-sized non-conductive channels 24 and 26 are disposed at opposing ends of the conical chamber 10. Note that conductive channels 24 and 26 may also be used with the embodiment of FIG. 2. In the preferred embodiment, the non-conductive channel 24 is larger than the non-conductive channel 26, and even more preferably, the diameter of the non-conductive channel 24 is approximately 1.5 to twice the diameter of the non-conductive channel 26. A heater 22 is disposed at the conical chamber 10, and provides heat in order to form a bubble at the conical chamber 10. In the embodiments of the present invention, the heater 22 may partially or totally surround the conical chamber 10.

[0041] To begin pumping, the heater 22 is powered which provides localized heat at the conical chamber 10. The heating in the localized area of the conical chamber 10 causes the formation and rapid growth of a vapor bubble 15. As in the embodiment of FIG. 1, the vapor bubble 15 grows in a direction of the wider channel 24 due to the effects of surface tension presented by the asymmetry of the conical chamber 10. As the vapor bubble 15 expands and forms, it exerts force on the column of liquid in the wider channel 24. This application of force on the column of liquid thus pushes the liquid along the conductive channel 24. That is, the formation and expansion of the vapor bubble 15 within the conical chamber 10 displaces a certain volume of the non-conductive liquid preferably within wider channel 24. When the electrical current is stopped, the bubble 15 collapses and the conical chamber 10 refills with liquid. Again, by applying and stopping the localized heating, an overall net displacement of liquid is effectively provided, similar to the embodiment of FIG. 1.

[0042] It is well understood that different heat levels may be applied to the micro-pump of the present invention. However, as can be readily appreciated by one of ordinary skill in the art, the heat is dependent on the diameter of the non-conductive channels 24 and 26 as well as the specific materials and non-conductive liquids used with the present invention. For example, larger diameter non-conductive channels 24 and 26 would result in the need for more heat to provide the beneficial effects of the present invention (i.e., to adequately grow the bubble 15 in order to provide adequate force to pump the liquid).

[0043] Multi-Heater Liquid Micro-pump

[0044] FIG. 3ais a top view of the multi-heater liquid micro-pump and FIG. 3bis a side view of the multi-heater liquid micro-pump of the present invention. Similar to the embodiment of FIG. 2, energy is supplied to the heater in a sequence such that liquid is pushed along the channel in a pumping action. The embodiment of FIGS. 3aand 3bis especially adapted for use with electrically non-conductive liquids; however, it is well understood that conductive liquids may equally well be used with the embodiment of FIGS. 3aand 3bprovided non-electric heaters are used or, with electric heaters, provided they are covered by a thin film of electrically insulating material.

[0045] The embodiment of FIGS. 3aand 3binclude a channel 38 of arbitrary cross section in an electrically non-conducting or conducting solid material 32 such as Plexiglas, plastic, or other well known materials. A series of electric heaters 36₁, 36₂, 36_n, are embedded along the base of the channel 38, as shown in FIG. 3b, or in any other convenient location such as the sides or the top. It is well understood that the present embodiment is not limited to any specific number of heaters, but should preferably include at least three heaters.

[0046] In use, each of the heaters 36₁, 36₂, 36_nis briefly powered in succession (electrically or otherwise), such that vapor bubbles form at each heater 36_n, 36₂, 36_n. Enough power is applied to each heater to generate a bubble, which condenses and collapses when the power is removed. The timing is such that, when a new bubble grows, for example, on heater 36₂, the previous bubble associated with heater 36_n, is just starting to collapse. Thus, as the bubble grows on heater 36₂, the bubble on 36_n, effectively blocks the channel and prevents the liquid from being pushed backward.

[0047] With this arrangement, in order to accommodate the growth of the bubble on heater 36₂, liquid can only move forward in the direction of heater 36_n, and so forth. In this way, a column of liquid is moved along the channel by the successive growth and collapse of the bubbles. After the bubble on the last heater 36_nhas collapsed, the cycle repeats, starting again from 36_n, and so on.

[0048] As a variant of the basic design of FIGS. 3aand 3b, in order to facilitate the blocking of the channel by a bubble thus preventing the liquid from flowing backward, the channel can be enlarged around each heater so that the bubble can grow larger than the channel cross section and cannot be pushed along the channel by the bubble growing next. Since the blocking action is an effect of the surface tension of the liquid, the channel diameter should not preferably exceed a few millimeters. It is well understood that the precise timing of the bubble generation, the duration and amount of the heating, and other similar operational characteristics will depend on the specific application and conditions of each utilization of the micro-pump.

[0049] FIG. 4 shows a two reservoir liquid pump of the present invention. Similar to the embodiments of FIGS. 1-3, energy is supplied to the system in order to form vapor bubbles which then push the liquid along a channel in order to produce a pumping action. It is further noted, as with the embodiments of FIGS. 1-3, that the micro-pump of the present embodiment can be made of many materials that are suitable for use with either a conductive or non-conductive liquid.

[0050] Referring now to FIG. 4, a channel 40 is disposed between two reservoirs, a first reservoir 42 and a second reservoir 44. A heater 46 is provided at any position along the channel 40, except the middle thereof The asymmetry of the system of the embodiment of FIG. 4 arises due to the fact that the heater 46 is not provided at the mid point of the center channel 40. In the preferred embodiment, the heater 46 is placed at a distance approximately between the range of 20%-40% of the channel 40 length from one of the reservoirs 42, 44.

[0051] Similar to the embodiments of FIGS. 2 and 3, the heater 46 provides a localized heat which forms a vapor bubble. The vapor bubble collapses when the heating is stopped and the localized heat dissipates. Moreover, similar to the embodiments of FIGS. 2 and 3, the reservoirs 42 and 44 as well as the channel 40 may be provided in a substrate, preferably of non-conductive material. However, conductive material is further contemplated for use with the present embodiment of FIG. 4.

[0052] The micro-pump of the present invention may be used in many applications, ranging from micro-electronic devices to printers to medical applications. By way of illustrative examples, a medical application and a printer will be discussed herein. However, it is well understood that the following examples are merely illustrative of the application of the micro-pump of the present invention and is not limited by such illustrative examples.

[0053] By way of example, the micro-pump of the present invention, and specifically the micro-pump of FIG. 3, may be used as a laboratory chip in order to test a small discrete volume of liquid, such as, for example, blood, urine or other bodily fluids. In use, the micro-pump of the present invention would transport the appropriate bodily fluid along channels as described above. At predetermined positions along the channels would be a particular reagent, such as an antibody. If reactive antigens were present in the biological fluid, then such reactive antigen would bind to the antibody thus resulting in a detectable antigen/antibody complex. This complex can then be flushed with other reagents in order to permit detection. This same procedure can be accomplished using the same bodily fluid on the same laboratory chip with different antibodies placed at different channel sites in order to thus detect other antigen/antibody complexes. Thus, by using the micro-pump of the present invention, a laboratory chip can be manufactured and used in an economically viable manner. This same procedure would also help to eliminate waste, and would further eliminate the need to add and/or remove various solutions by mechanical means.

[0054] By way of another example, the micro-pump of the present invention may also be used for sequential organic syntheses, such as, for example, micro- scale peptide syntheses. In general, a first amino acid of the peptide chain may be immobilized in a channel of the micro-pump. A series of coupling reagents may then be “pumped” within the channels of the micro-pump of the present invention in order to react with the immobilized first amino acid. As is known to one of ordinary skill in the art, this bonding between the immobilized first amino acid and the coupling reagents forms a peptide sequence.

[0055] It is also contemplated that the micro-pump of the present invention may be used in printers, such as ink jet printers. In the application of ink jet printers, ink is placed within the channel of the ink-jet printer and the vapor bubble, formed via localized heat, exerts force on the column of ink in the channel. This application of force on the column of ink pushes the ink along the channel and through a nozzle for discharging onto paper.

[0056] While the invention has been described in terms of its preferred embodiments, those skilled in the art will recognize that the invention can be practiced with modification within the spirit and scope of the appended claims. Accordingly, the present invention should not be limited to the embodiments as described above, but should further include all modifications and equivalents thereof within the spirit and scope of the description provided herein.