Bubble based micropump

A micro-pump pumps either electrically conductive or non-conductive liquids through channels of the micro-pump and/or micro-devices. 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.

DESCRIPTION
 Background of the Invention
 1. Field of the Invention
 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.
 2. Background of the Invention
 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.
 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.
 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.
 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.
 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.
 SUMMARY OF THE INVENTION
 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.
 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.
 It is still a further object of the present invention to provide a
 micro-pump that does not utilize any mechanical moving parts.
 It is also another object of the present invention to provide a micro-pump
 that offers simplicity of construction and operation.
 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.
 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.
 A method of using the micro-pump of the present invention is also
 contemplated for use herein.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION
 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.
 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.
 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.
 Electrical Conducting Micro-Pump
 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.
 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.
 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.
 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.
 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 .
 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.
 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).
 Non-Electrical Conducting Micro-Pump
 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.
 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.
 Moreover, in the preferred embodiment, the conical chamber 10 provides an
 asymmetry of the micro-pump of the present invention as described above.
 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.
 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.
 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).
 Multi-Heater Liquid Micro-pump
 FIG. 3a is a top view of the multi-heater liquid micro-pump and FIG. 3b is
 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. 3a and 3b is 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. 3a and 3b provided non-electric heaters are used or,
 with electric heaters, provided they are covered by a thin film of
 electrically insulating material.
 The embodiment of FIGS. 3a and 3b include 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 36.sub.1, 36.sub.2, 36.sub.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.
 In use, each of the heaters 36.sub.1, 36.sub.2, 36.sub.n is briefly powered
 in succession (electrically or otherwise), such that vapor bubbles form at
 each heater 36.sub.n, 36.sub.2, 36.sub.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 36.sub.2, the previous bubble associated with heater 36.sub.n,
 is just starting to collapse. Thus, as the bubble grows on heater
 36.sub.2, the bubble on 36.sub.n, effectively blocks the channel and
 prevents the liquid from being pushed backward.
 With this arrangement, in order to accommodate the growth of the bubble on
 heater 36.sub.2, liquid can only move forward in the direction of heater
 36.sub.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.sub.n has collapsed, the cycle repeats,
 starting again from 36.sub.n, and so on.
 As a variant of the basic design of FIGS. 3a and 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.
 Two Reservoir Liquid Micro-Pump
 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.
 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.
 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.
 Application of Use
 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.
 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.
 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.
 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.
 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.