This invention pertains to a pump for a microelectromechanical system, and, more specifically, to a pump exploiting the principles of fluid mechanics to draw fluid from a reservoir and project it along a channel.
The earliest computers were huge labyrinths of wires and vacuum tubes, perhaps best characterized by the dream of a “computer that will fit in a room” immortalized in the movie Apollo 13. The development of the transistor enabled an immediate miniaturization of electronic components, and researchers have continued to develop smaller and smaller semiconductor devices. As ever-smaller devices were developed to perform more and more functions at faster rates, devices have shrunk from room-sized behemoths to portable personal computers to handheld personal digital assistants (PDA's) that are quickly replacing pocket calendars and personal organizers.
As electronic circuitry becomes smaller and smaller, the techniques for fabricating these electronic devices are also being exploited to produce lilliputian mechanical devices. Miniature accelerometers control the inflation of airbags in automobiles. Techniques for fabricating microelectromechanical systems (“MEMS”) have also been used to produce microscopic gears and actuators. MEMS including arrays of tiny mirrors, each rotated individually in response to a miniature control circuit, are used to digitally project movies onto theater screens. However, most MEMS have tiny moving parts that are easily broken but not so easily repaired. Furthermore, moving parts in MEMS devices often stick to each other, preventing further motion and rendering the device useless. As a result, it is desirable to fabricate a MEMS device that is more robust.
The invention is a microelectromechanical (MEM) device for controlled movement of a fluid. The device includes a chamber having a heating element, an inlet, and a constricted egress channel.
The invention is described with reference to the several figures of the drawing, in which,
The invention includes a microelectromechanical (MEM) device for pumping a fluid. The device comprises a chamber having a heating element and a channel providing egress from the chamber. The channel includes a constriction. The device may have a series of chambers and channels in fluidic communication. In addition, the invention includes a method for coordinating activation of the heating elements in subsets of the chambers. For example, the chambers may be divided into groups of three, four, or more chambers within which the heating elements are activated sequentially.
The device may include a fluid reservoir and an inlet that provides fluidic communication between the reservoir and one or more of the constrictions. When the heating element in one of the chambers is activated, it vaporizes a portion of the fluid in the chamber, causing fluid to flow through the egress channel and into a downstream chamber. Activation of the downstream heating element continues projection of the fluid through the channels. A chamber may include two or more egress channels leading to downstream chambers instead of or in addition to a heating element. Heating elements in the downstream chambers may be electrically controlled to permit fluid to flow through specific downstream paths. In addition, the invention includes a method for pumping a fluid utilizing a series of chambers and channels.
The invention exploits physical principles such as resistive heating and Bernoulli's principle to create a pump for a microelectromechanical system (MEMS). The pump 10 includes a series of chambers, for example, chambers 12-17 (FIG. 1A). Each chamber includes a heating element, for example, sheet resistors 20-25. The chambers are connected in series via channels, for example, channels 30-34. Channels 30-34 each have a constriction, for example, constriction 30a in FIG. 1B.
To operate the pump, a voltage is applied to sheet resistor 20. The voltage is applied in a step profile with a period on the order of microseconds and generates enough heat to vaporize a portion of a fluid disposed in chamber 12. The resulting explosive vaporization displaces the remaining fluid in chamber 12 into channel 30 and from thence into chamber 13. Fluid that is already in channel 30, chamber 13, channel 31, etc., will also be displaced towards the left in
As the vaporized fluid cools, the resulting vacuum causes fluid in channel 30 to progress into chamber 13. Fluid in chamber 12 moves into channel 30, and chamber 12 is refilled from channel 29. Furthermore, fluid in channel 31 exerts pressure on fluid in chamber 14, channel 32, etc., causing it to proceed through the pump. The process is repeated for sheet resistors 22, 23, etc. Because the chambers are so small (approximately 10-50 μm or greater on a side), the chambers are not only refilled by vacuum, but by capillary action of the fluid along the walls of the chambers.
The pump 10 is fabricated on a substrate, for example, a silicon wafer. The circuitry to control the resistors 20, 21, 22, etc. is deposited on the substrate, as are the resistors themselves. Exemplary resistors include TaA1 thermal ink jet (TIJ) resistors. A photoimagable polymer (photoresist), for example, SU-8 (MicroChem, Newton, Mass.), PARAD™ (DuPont), or VACREL™ (DuPont), is deposited over the circuitry and exposed to light through a mask having a pattern corresponding to the desired pattern of chambers 12, 12, 14, etc., channels 29, 30, 31, etc., and other features of the pump 10. The unexposed portions of the polymer are washed away. Alternately, a polyimide or other film may be deposited on the substrate and laser ablated to form the desired pattern. The tops of the chambers are sealed with a hole-free material such as a polyimide (e.g., KAPTON™ from DuPont, UPILEX™ from UBE Industries/INI America, and APICAL™ from Kaneka High-Tech Materials). The polyimide forms a seal with the polymer upon application of heat. A passivation layer, for example, tantalum, may be applied over the circuitry to prevent generation of a short circuit during operation of the pump.
The size of the chambers 12-17 and the timing of the applied voltage determine the capacity of the pump. The chamber depth is defined by the thickness of the photoimagable polymer, typically 14 or 19 μm. While deeper channels are possible, it is preferable to keep the aspect ratio of the chambers short and wide. Accordingly, the chamber depth is preferably between 10 and 30 μm. While smaller chambers are possible, they may be difficult to manufacture or propel fluid through. The channels should be 2-3 times the side length of the resistor to provide an adequate gap between them. For example, for a 20 μm resistor, the channel should be long enough so that there is about 40-60 μm between the resistors.
The throughput may be increased and the pressure within the pump equalized by applying a voltage to more than one resistor at a time. In one embodiment, the chambers may be divided into groups. A voltage may be applied to the first resistors in each group simultaneously, then to the second resistors, etc. For example, if the resistors in
The voltage should be applied long enough to create sufficient pressure to propel the fluid. For example, a minimum application time of 1 μs is preferred when thermal ink jet-type (TIJ) resistors are used. To increase the efficiency of the system, the time between voltage applications may be optimized to allow the fluid to travel as far as it can under the pressure created by the previous voltage application. The speed with which the fluid is directed through the chambers and channels depends partially on the viscosity of the fluid but can be controlled by adjusting the intervals at which voltage is applied to sheet resistors 20, 21, 22, and 23. For example, if chambers 12, 13, 14, etc., are 30 μm on a side and 19 μm deep, then a flow rate of about 2.7×10−4 cc/s, or 0.016 cc/min, may be achieved for a voltage frequency of 15 kHz.
The required voltage depends in part on the resistance of the sheet resistor. The required energy to vaporize a fluid and create a bubble (“flash vaporization”), called the turn-on energy (TOE), is a constant for a given fluid. The TOE is the product of the power delivered and the time the resistor is on, or
TOE=(V2/R) (Pulse Time)
The resistance R of a square sheet resistor having resistivity ρ depends only on its thickness. Most fluids have turn-on energies between 2 and 6 μJ. In one example, application of a 7 mV pulse to a 36 mΩ resistor for 2 μs delivers 2.7 μJ of energy to the fluid. Almost any aqueous solution may be pumped using the techniques of the invention. As the fraction of water decreases, the TOE increases. For example, a fluid that is about 75% water, such as an ink-jet ink, has a TOE of about 3 μJ. One skilled in the art will recognize that the TOE for a given fluid may be determined without undue experimentation.
The channels 29, 30, 31, and 32 remain constricted as they enter their respective downstream chambers. This minimizes projection of the fluid upstream when the bubble is created. This constriction also requires the fluid to increase in velocity as it travels from one chamber to the next. As the fluid increases in velocity, Bernoulli's principle dictates that the fluid generates a region of lower pressure. As fluid travels through channel 30, the resulting low pressure draws liquid from reservoir 37 via inlet 35. As a result, a second fluid can be mixed with the fluid that is already being directed through the pump. Additional reservoirs may be disposed along the pump to add various fluids to the mixture. To prevent generation of a vacuum in the reservoir as fluid is removed, they may be open to the atmosphere. Alternatively, a flexible chamber, or one sufficiently large to avoid creation of a vacuum, may be employed, or the reservoir may be periodically refilled.
In
The technology of the invention may also be used to fabricate a gate valve (FIG. 3). The gate valve 60 includes an entrance chamber 61 and two gate chambers 62 and 64 that control access to downstream paths 66 and 68. Chambers 62 and 64 contain sheet resistors 70 and 72, respectively. As fluid approaches gate valve 60, it may proceed through either downstream path 66 or downstream path 68 unless one of chambers 62 or 64 is blocked. For example, if voltage is applied to sheet resistor 70, a bubble will form within chamber 62. The bubble will be trapped within chamber 62 by the constrictions on either side of the chamber, thereby blocking access to downstream path 68. Turning off the voltage will permit the bubble to collapse and allow access to downstream path 68. Likewise, application of a voltage to sheet resistor 72 will cause a bubble to form within chamber 64, blocking access to downstream path 66. The gate valve may be used to change the flow path, separate the fluid into two streams or to periodically remove fluid from the pump for analysis or some other application via one of the downstream paths. In one embodiment, entrance chamber 61 for the gate valve 60 has a larger volume than either of chambers 62 or 64. However, this is not necessary. For example, entrance chamber 61 may be the nexus of a T-junction (
Because the fluid is heated for such a short time, many fluids and materials that are ordinarily heat sensitive may be directed through the pump of the invention without damage. For example, even if a protein is sensitive to heat, if it does not denature in the few microseconds of elevated temperatures, its conformation may not be affected by the pumping mechanism.
In addition, the sides of the chambers and channels may also be coated with materials to enhance or prevent interactions of the surface with the pumped fluids. For example, a passivation layer of Ta on the sheet resistor will prevent cognation of ink. Catalysts such as Pt and Pd may be immobilized in the chambers or channels, or the surfaces of the pump may be treated to generate an oxidized layer at the surface of the silicon. Biological molecules or chemical coatings may attract or repel proteins or sugars. Exemplary molecules include extracellular matrix proteins, albumin, amino acid sequences, cell adhesion sequences such as -R-G-D-, synthetic peptides, various proteins and enzymes, and sugars such as lectin binding sugars. Molecules may also be chosen that have specific receptors, such as antibodies and antigens, cell surface receptors and ligands, etc. These molecules may modify a surface, enabling the immobilization of biological molecules, molecular fragments, cells, or cell components. In addition, a variety of biological materials can be used to prevent the attachment of others. For example, intact and fractionated cells and organelles, lipids, simple and complex carbohydrates, and some proteins and nucleic acids have a low affinity for biological molecules and cells.
The fluid in the pump may also be analyzed. In one embodiment, an outlet 48 is disposed in the downstream side of a chamber (FIG. 1). When fluid is propelled from the chamber, a small amount will enter the outlet and flow to a collector 50 or other structure disposed downstream. Alternatively, a sensor may be disposed in a chamber or channel. An electrical circuit may be provided to measure pH, resistance, temperature, or some other characteristic of the fluid. Spectrographic analysis may also be provided if a wall or cover of at least a portion of the pump is sufficiently transparent or if a chamber or channel is fabricated with a fiber optic filament.
These pumps may be used for so-called “lab on a chip” applications, enabling smaller quantities of large numbers of fluids to be mixed and analyzed simultaneously. This would reduce the quantity of material required for such chips and increase the number of reagents that can be used on a single chip.
The invention increases the channel lengths and velocities that can be employed for “lab on a chip” and other applications. Without a pumping action, the fluids can only proceed as far and as fast as they can propel themselves through capillary action or under the direction of an applied voltage through capillary electrophoresis. Pumping enables a greater range of reaction times and higher throughputs.
An exemplary arrangement of channels and chambers is depicted in FIG. 7A. Pump paths 70 and 72 both entrain fluid from reservoir 74. The operation mechanism of the pump prevents backwash into the reservoir 74 that would contaminate the other channel. Pump path 70 entrains a second fluid from reservoir 76, and pump path 72 entrains a second fluid from reservoir 78. The components of these fluids may react with each other and then be pumped to an outlet or an additional chamber where the reaction products can be analyzed. An accumulation chamber 80 may be provided as a reaction vessel for the fluid in the reservoir and the fluid in the pump. A resistive heater 82 and a thermocouple 84 may be provided in the accumulation chamber to control the temperature of the mixed fluids as they react.
Other embodiments of the invention will be apparent to those skilled in the art from a consideration of the specification or practice of the invention disclosed herein. It is intended that the specification and examples be considered as exemplary only, with the true scope and spirit of the invention being indicated by the following claims.
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Number | Date | Country | |
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20030215335 A1 | Nov 2003 | US |