Abstract:
A method to adjust a temperature of a fluid is disclosed, the method comprising the steps of providing a heat sink taking effect on the fluid, heating the fluid by Joule heating, and adjusting the heat provided by Joule heating in order to reach a desired temperature of the fluid. Furthermore, an apparatus to carry out the method is also disclosed. Therewith, a very efficient temperature change of the fluid can very quickly be obtained since the temperature of the surrounding is not substantially changed by heat diffusion. While a temperature increase is carried out by Joule heating, a temperature decrease is carried out by simply deactivating Joule heating. As soon as Joule heating is deactivated, the heat is extracted from the fluid by the heat sink, thereby initiating a high speed temperature change towards lower temperatures.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention is directed to a method to adjust a temperature of a fluid, an apparatus and a micro fabricated system for doing the same.  
       DESCRIPTION OF RELATED ART  
       [0002]     PCR (Polymerase Chain Reaction) and other cyclic polymerase-mediated reactions are standard tools of modern biological research, and are also commonly used for numerous applications, such as medical diagnostic procedures and forensic applications.  
         [0003]     Recently, attempts have been made to shorten the time required for each cycle of a PCR. Such methods often reduce the time by, for example, performing the PCR in devices that allow rapid temperature changes, thereby eliminating much of the time previously required for PCR to “ramp” the temperature of the solution from one stage of the PCR to the next. In addition, it has been recognized that the use of apparatus that allow greater heat transfer, e.g. thin-walled tubes, turbulent air-based machines, also allow the use of shorter cycle times.  
         [0004]     It is well known that thermo cycling PCR and other thermo cyclic applications can be conducted in micro fluidic systems using joule heating. In brief, energy is provided to heat fluids by application of electric current to fluids in micro channels. A power source is used to pass electrical current through the fluid in a channel for heating purposes as well as for material transport (WO 00/60 108).  
         [0005]     Furthermore, a micro fabricated reactor is described in U.S. Pat. No. 5,639,423. This known teaching comprises a reactor with electrodes that are not in contact with the fluid to be heated.  
         [0006]     EP-0 963 250 B1 discloses an apparatus for performing the polymerase chain reaction in a plurality of liquid reaction mixtures. Thereby, a Peltier electro thermal device is used to adjust a desired temperature of the liquid. The Peltier electro thermal device is, on one side, in close contact with the liquid and, on the other side, in close contact with a heat sink. In order to adjust the temperature of the liquid, heat must be transferred from or to the Peltier electro thermal device.  
         [0007]     The speed of the known PCR technology is limited by the temperature ramping speed of the surrounding vessel or instrument, respectively, and the speed of the heat transfer, given by the heat conductivity of the used material and of the fluid, from the heated wall of the vessel to the center of the fluid.  
       SUMMARY OF INVENTION  
       [0008]     It is an objective of the present invention to provide a method that allows an increased speed for the heat transfer compared to known methods.  
         [0009]     This objective is achieved by providing a heat sink ( 4 ) taking effect on the fluid ( 8 ), by heating the fluid ( 8 ) by Joule heating, and, finally, by adjusting the heat provided by Joule heating in order to reach a desired temperature of the fluid ( 8 ). In this manner, a very efficient temperature change of the fluid can very quickly be obtained since the temperature of the surrounding medium is not substantially changed by heat diffusion. While a temperature increase is carried out by Joule heating, a temperature decrease is carried out by simply deactivating Joule heating. As soon as Joule heating is deactivated, the heat is extracted from the fluid by the heat sink, thereby initiating a high speed temperature change towards lower temperatures.  
         [0010]     The present invention will be further explained in the following by referring to exemplified embodiments shown in drawings outlined below.  
     
    
     BRIEF DESCRIPTION OF THE FIGURES  
       [0011]      FIG. 1  is a schematic diagram of an apparatus according to the present invention comprising a vessel, a heat sink and electrodes.  
         [0012]      FIG. 2  is a schematic diagram of a specific embodiment of an electrode used in the apparatus according to the present invention.  
         [0013]      FIG. 3  is a schematic diagram of a partial view of embedded electrodes in the bottom of the vessel.  
         [0014]      FIG. 4  is a schematic diagram of a partial view of electrodes mounted at the side of the vessel in a yet another embodiment of the present invention.  
         [0015]      FIG. 5  is a schematic diagram of a further embodiment of an apparatus according to the present invention comprising a vessel with a transparent cover. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0016]     In  FIG. 1 , an apparatus according to the present invention is depicted in a cross-sectional view, the apparatus comprising a vessel  1  containing a fluid  8 , electrodes  2 , a supply unit  3 , a heat sink  4  and instruments, which can be, for example, a fluorescence excitation unit  6  and a fluorescence detection unit  7 .  
         [0017]     The vessel  1  is formed by sidewalls  1   a  and  1   b , and a bottom layer, which consists of a material with a higher thermal conductivity than the material of the sidewalls  1   a ,  1   b . The bottom layer is therefore also referred to as heat transfer layer  5 . Furthermore, the vessel  1  has, for example, a flat shape to hold a volume of 50 μl, for example. The term flat shape means that the height of the vessel  1  is smaller than the width of the vessel  1 . In one embodiment of the present invention, the ratio of height and width is larger than one over five. The small fluid volume guarantees rapid heat diffusion. In another embodiment of the present invention, the vessel  1  is a cylinder, again with a heat transfer layer as bottom layer. In yet another embodiment, the vessel is disposable.  
         [0018]     As it is depicted in  FIG. 1 , the vessel  1  has an opening, also referred to as window, through which access is provided for the above-mentioned instruments  6  and  7  or any other device, such as a pipette, for example, to introduce the fluid  8  to be analyzed or to introduce an additional chemical substance into the fluid  8 .  
         [0019]     In a further embodiment, the opening of the vessel is covered by a transparent cover made of a transparent material, such as polycarbonate, polypropylene or PMMA (plexiglass or acrylic plastic) having a thickness of 1 mm, for example. In general, a transparent polymer is used for the cover. In such an embodiment, the fluid cannot be introduced through the opening at the top of the vessel but must be introduced in another manner, for example through one or several fluid channels leading into the vessel  1  (not shown in  FIG. 1 ). In such an embodiment at least one input and preferably one output channel are provided.  
         [0020]     The heat sink  4  is arranged in close thermal contact to the heat transfer layer  5  and possibly to the sidewalls of the vessel  1 , as it is the case for the embodiment depicted in  FIG. 1 . The heat sink  4  is designed to have a large thermal capacity compared to the one of the other components, in particular the fluid  8 , the electrodes  2  and the vessel  1 . For example, the thermal capacity of the heat sink  4 , maybe for example, ten times larger than one of the other components. Furthermore, a large surface area is provided for the heat sink  4  in order to establish equal temperature with its surrounding very quickly. In order to achieve a large surface area, the heat sink  4  is built in a conventional manner, as it is depicted in  FIG. 1 . In one embodiment, the above-described heat sink  4  is a passive cooling element.  
         [0021]     In a further embodiment of the present invention, the heat sink  4  is an active cooling element, which is able to adjust the temperature independently of the ambient temperature. A possible active element is, for example, a Peltier Thermal Electrical Device (TED) that is well known in the state of the art (EP-0 963 250 B1).  
         [0022]     The electrodes  2 , which, when in operation, are connected to the supply unit  3 , are in direct and close contact to the fluid  8  as well as to the heat transfer layer  5 . Conveniently, the heat transfer layer  5  is an integral part of the vessel  1 . Therewith, it is assured that heat transfer between the fluid  8 , the heat transfer layer  5 , the electrodes  2  and the heat sink  4  can take place very easily and very quickly.  
         [0023]     It is though pointed out that it is not absolutely necessary that a heat transfer layer  5  is present. The invention can very well be realized without heat transfer layer  5 . In such an embodiment of the present invention, the electrodes  2  are directly mounted on the heat sink  4 , which must consist of an electrically non conducting material.  
         [0024]     As has already been mentioned, the supply unit  3  is removeably connected to the electrodes by wires. To avoid redox reactions or accumulation of ions at the electrodes  2   a ,  2   b  an AC-(alternating current) source is proposed in the supply unit  3  providing an alternating field in the fluid  8 , the frequency of the alternating field being greater than 15 kHz, for example.  
         [0025]     Alternatively, the supply unit  3  can also be a DC-(direct current) source.  
         [0026]     When using the present invention for thermo cyclic applications, such as PCR, for example, very short cycles can be obtained since the energy supply to the fluid  8  by the electrodes  2 , in case of a temperature increase, and the energy transfer from the electrodes  2  or the fluid  8 , respectively, and the heat sink  4  (through the heat transfer layer  5 ), in case of a temperature decrease, is very fast. In other words, while a so-called Joule heating is used to increase the temperature of the fluid  8 , heat diffusion occurs to decrease the temperature of the fluid  8 . In case a heat transfer layer  5  is used, as it is depicted in  FIG. 1 , the material for the heat transfer layer  5  must have a low electrical conductivity, while a high thermal conductivity must be guaranteed.  
         [0027]      FIG. 2  shows a top view of electrodes  2   a ,  2   b  as they can be used, for example, in the embodiment depicted in  FIG. 1 . Basically, each of the two electrodes  2   a ,  2   b  is realized as a comb in such a manner that the two electrodes  2   a ,  2   b  can intermesh without touching one another. At the same time, a constant distance is provided between the electrodes. Both electrodes  2   a ,  2   b  are operationally connected to the supply unit  3 .  
         [0028]     The electrodes  2   a ,  2   b  can be formed by removing a track out of a gold coated polycarbonate film of a thickness of 200 μm, for example.  
         [0029]     In order to adjust the temperature in the vessel  1 , a temperature measurement unit is provided (not shown in FIGS.  1  to  4 ) to measure the temperature of the fluid  8 . To fully control the temperature of the fluid  8 , a closed loop is obtained by operationally connecting the output of the temperature measurement unit back to the supply unit  3 , in which the output signals for the electrodes  2   a ,  2   b  is adjusted accordingly. For example, the temperature measurement unit could comprise a ring electrode incorporated into the vessel  1 . The ring electrode changes conductivity as a function of the momentary temperature, and is isolated from the fluid by a thin polymer-layer that is smaller than 50 μm, for example.  
         [0030]     In a specific embodiment of the present invention, the ring electrode is incorporated into the transparent cover in such a manner that the optical path of the light coming from the excitation unit  6 , for example, is not or only slightly disturbed. In one embodiment, the ring electrode is arranged at a constant distance from the sidewalls  1   a ,  1   b  of the vessel  1 . The distance is in the range of 1 mm, for example. It is pointed out that the ring electrode to measure the temperature of the fluid  8  can either be arranged on the upper side of the cover or on the lower side of the cover. In the latter case, where the ring electrode is in close contact with the fluid  8 , its temperature can be determined very quickly, which allows a higher control over the actual temperature of the fluid  8 .  
         [0031]      FIG. 3  schematically shows, in a partial view, a further embodiment of an inventive arrangement of electrodes  2   a ,  2   b  in the heat transfer layer  5 . In contrast to the embodiment of  FIG. 1 , the electrodes  2   a ,  2   b  are now embedded into the heat transfer layer  5  in that the upper surface of the electrodes  2   a ,  2   b  is flush with the upper surface of the heat transfer layer  5 . Additionally, the electric field lines are also depicted as broken lines in  FIG. 3 , for illustration purposes only.  
         [0032]      FIG. 4  schematically shows, again in a partial view, a yet another embodiment of an arrangement of electrodes  2   a ,  2   b  according to the present invention. In contrast to the embodiments described along with FIGS.  1  to  3 , this embodiment has no electrodes in the bottom of the vessel  1  ( FIG. 1 ) but the electrodes form at least a part of the sidewalls of the vessel receiving the fluid being analyzed. Preferably, the heat transfer layer  5  encompasses the electrodes  2   a ,  2   b  or they are incorporated into the heat transfer layer  5 , respectively, being incorporated or forming the side walls of the vessel.  
         [0033]     In general, the embodiment of  FIG. 4  opens up numerous variations with electrodes in the side walls. For example, one such embodiment may have a similar structure of electrodes in the sidewalls of the vessel as is provided according to  FIG. 3  in the bottom of the vessel. Accordingly, the electrodes are positioned circumferentially in such an embodiment.  
         [0034]     As in  FIG. 1 , the heat sink  4  is also provided for the embodiment of  FIGS. 3 and 4  at the bottom of the vessel. Alternatively, as it is indicated by the heat transfer layer  5  in  FIG. 4 , the heat sink  4  may also encompass the vessel, i.e. the vessel may completely be embedded into the heat sink  4  with the exception of the window, which is either provided as upper opening of the vessel, or which is covered by a transparent cover as has already been described.  
         [0035]     The heat transfer layer  5  preferably consists of a polymer film, for example, while the heat sink  4  is preferably made of a metal, e.g. aluminum, or of ceramic. The electrodes  2   a ,  2   b  as well as the ring electrode for temperature measurement, if present, are also made of a metal, e.g. aluminum, which might be coated by gold or platinum.  
         [0036]      FIG. 5  schematically shows a further embodiment of an apparatus according to the present invention. In contrast to the embodiment shown in  FIG. 1 , the embodiment of  FIG. 5  comprises a vessel  1  with a transparent cover  10 . In addition, the vessel  1  is a single piece apart from the transparent cover  10 . As has already been mentioned, the transparent cover  10  is made of a transparent material, such as polycarbonate, polypropylene or PMMA (plexiglass or acrylic plastic) having a thickness of 1 mm, for example. In general, a transparent polymer is used for the cover  10 . The material for the lower part of the vessel  1 —i.e. the sidewalls  1   a ,  1   b  and the heat transfer layer  5 —does not have to be transparent. Although the same materials as for the transparent cover  10  can very well be used for the entire vessel  1 , other non-transparent materials, such as ceramic, are very suitable.  
         [0037]     As in the embodiment of  FIG. 1 , the electrodes  2   a ,  2   b  are integrated into the heat transfer layer  5  forming the bottom of the vessel  1 . The electrodes  2   a ,  2   b  can be of the same type as the one shown in  FIG. 1 . Alternatively, the comb-type electrodes of  FIG. 2  can also be used. In addition to the electrode  2   b  incorporated into the heat transfer layer  5 , a further electrode  2   b ′ is provided in the transparent cover  10 . The electrode  2   b  as well as the further electrode  2   b ′ are also connectable to the supply unit  3  via a switch unit  12 . The switch unit  12  allows to either connect the supply unit  3  to the electrode  2   b  or to the electrode  2   b ′ in order that, on the one hand, an electric field is generated between the electrodes  2   a ,  2   b  in the fluid  8 , or, on the other hand, an electric field is generated between the electrodes  2   a ,  2   b ′ in the fluid  8 .  
         [0038]     While the electric field between the electrodes  2   a ,  2   b  is inhomogeneous in the fluid  8 , the electric field between the electrodes  2   a ,  2   b ′ is substantially homogeneous.  
         [0039]     In connection with the switch unit  12 , the electrodes  2   a ,  2   b ,  2   b ′ allow an optimal temperature control of the fluid  8  in that the combination of the electrodes  2   a ,  2   b ′ is used to reach a desired temperature of the fluid  8 , and the combination of the electrodes  2   a ,  2   b  is used to hold the desired temperature of the fluid  8 . Accordingly, the switch unit  12  is put into the corresponding state.  
         [0040]     As has been mentioned in connection with the temperature control of the fluid  8 , a ring electrode is used to measure or estimate the temperature of the fluid  8 . Such a ring electrode is referred to by the reference sign  11  in  FIG. 5 . In this embodiment, the ring electrode  11  is incorporated into the transparent cover  10 , preferably on its upper side.  
         [0041]     It is expressly pointed out that the electrode  2   b ′ can have any shape, e.g. a comb or a lattice. In order that the excitation unit  6  and the detection unit  7  ( FIG. 1 ) are fully operational, the area of the window of the vessel  1  covered by the electrode  2   b ′ and, if present, of the ring electrode  11  is between 10 and 70% of the full window area. Preferably, 50% of the window area is covered by the electrodes  2   b ′,  11 .  
         [0042]     As has already been pointed out, the electrodes  2   a ,  2   b  respectively  2   b ′ are provided to allow Joule heating. Joule heating is a very efficient heating by application of electric current to the fluid in the vessel. In general, electric current passing through the fluid in the vessel produces heat by dissipating energy through the electrical resistance of the fluid. Power dissipates as the current passes through the fluid and goes into the fluid as energy as a function of time to heat the fluid.  
         [0043]     The apparatus of the present invention is in particular suitable to be implemented as a micro fabricated system. Micro fabricated technology allows the production of electrical, mechanical, electromechanical, optical, chemical and thermal devices including pumps, valves, heaters, mixers and detectors for micro liter to nano liter quantities of solids, liquids and gases. The integration of these devices into a single system allows for the batch production of micro scale reactor-based analytical instruments. Such integrated micro fabricated devices can be manufactured in batch quantities with high precision, yet low cost, thereby making recyclable and/or disposable single-use devices practical.  
         [0044]     While the foregoing invention has been described in some detail for purposes of clarity and understanding, it will be clear to one skilled in the art from a reading of this disclosure that various changes in form and detail can be made without departing from the true scope of the invention. For example, all the techniques and apparatus described above can be used in various combinations. All publications, patents, patent applications, and/or other documents cited in this application are incorporated by reference in their entirety for all purposes to the same extent as if each individual publication, patent, patent application, and/or other document were individually indicated to be incorporated by reference for all purposes