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(11) | EP 1 614 469 A2 |
| (12) | EUROPEAN PATENT APPLICATION |
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| (54) | Method and apparatus for dispensing a liquid with a pipetting needle |
| (57) A method and a micropipetting apparatus for dispensing a liquid volume into a vessel
by means of a pipetting needle and without any contact between said needle and a liquid
contained in said vessel. The method comprises forming a drop (17) at the delivery
tip (22) of the pipetting needle (11), said drop being retained at the tip by adhesion
forces, and ejecting said drop (17) from said tip (22) by focusing at the tip of the
pipetting needle (11) a mechanical excitation wave applied to an excitation point
at some distance from said needle. The apparatus comprises a pipetting needle (11),
an electromechanical transducer (32) mechanically connected with said pipetting needle
(11), electrical signal generating means (33) for generating an excitation pulse (26)
signal and for applying this signal to said electromechanical transducer (32) for
mechanically exciting said pipetting needle (11) with a pulse of mechanical waves
that propagate through said needle.
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[0001] The invention concerns a method for dispensing a liquid volume into a vessel by means of a pipetting needle and without any contact between said needle and a liquid contained in said vessel.
[0002] The invention further concerns a micropipetting apparatus for dispensing a liquid volume into a vessel by means of a pipetting needle and without any contact between said needle and a liquid contained in said vessel.
[0003] Pipetting of liquids is an important function of automatized analysis of samples examined for the purposes of medical diagnosis. Mastering of the pipetting operations is a basic condition for performing analysis which are correct, fast, cheap and ecological. There is a need for a pipetting apparatus which is able to pipette with the required accuracy liquid volumes in the nanoliter and microliter range.
[0004] Dispensing of very small liquid volumes requires contact of the pipetting needle with a solid surface or with another liquid to which the dispensed volume is added. This is so because the adhesion forces which retain the small volume to be dispensed to the pipetting needle are larger than the weight of that small liquid volume. This weight alone is thus not sufficient for releasing a drop attached by adhesion forces to the tip of a pipetting needle. In prior art automatic pipetting apparatus of analyzers used for medical diagnosis a drop of a liquid to be dispensed is therefore brought into contact with and thereby delivered into another liquid, which can be a sample or a reagent. In order to avoid erroneous analysis results, it is necessary to clean the pipetting needle after each such contact with liquid in a container and this requires a lot of time.
[0005] According to prior art delivery of a liquid to be dispensed can only be achieved by contact of the tip of the pipetting needle with a liquid contained in a container which receives the dispensed liquid. In some applications it is however desirable to dispense a liquid without any contact between the tip of the pipetting needle and a liquid contained in a container that receives the dispensed liquid, since in this case cleaning of the needle would not be any more necessary after each dispensing operation. This is the case e.g. when aliquots of a liquid sample are have to be distributed to liquids contained in a plurality of containers. In this case the time for distributing the aliquots to the plurality of containers would be considerably reduced, because it would not be necessary to clean the pipetting needle after dispensing each aliquot.
[0006] Fig. 1 shows a prior art dispensing of very small aliquots of a liquid to a plurality of different vessels. As shown in Fig. 1, a pipetting needle 11 is used for taking a sample of liquid contained in a vessel 12 and for successively dispensing aliquots of that sample to different vessels 13 and 14. For each such dispensing the tip needle 11 has to contact a liquid contained in the vessel 13, 14 which receives the aliquot. For the reasons mentioned above the needle 11 has to be cleaned after each such dispensing, before dispensing an aliquot in a different vessel. Fig. 1 shows cleaning positions 15 and 16 of the pipetting needle. In Fig. 1 arrows represent the sense of motion of the pipetting needle during the above-mentioned dispensing operations.
[0007] Drops can be dispensed for instance like in inkjet printers, wherein a pressure pulse is generated within a liquid and this pulse propagates towards a nozzle which closes one end of a container which contains the liquid to be dispensed. Due to the reduction of the cross-section at the transition from the interior of the container to the nozzle, a small liquid volume is strongly accelerated and this allows to release through the nozzle one drop of liquid from the container. The size of a drop generated by the inkjet principle lies in a range going from 5 to 500 picoliter and depends from the properties of the liquid and from the size of the nozzle. Drops generated only by the weight of the drop to be dispensed are much larger. When a pipetting needle having a cross-section with an external diameter of 10 micrometer is used for dispensing drops only by means of gravitational force (i.e. the weight of the drop) the size of each drop would be of 30 nanoliter if the liquid dispensed is an aqueous solution.
[0008] For dispensing drops by the inkjet principle a very strong acceleration of the liquid volume in the nozzle is necessary (accelerations of up to 105 g). The energy required for releasing a 500 picoliter drop is of about 10-8 Joule. A nozzle of the type used in inkjet printers cannot be a part of a pipetting needle of an analyzer for analyzing samples for the purposes of medical analysis, because the structure of the pipetting needle should allow the required sufficient cleaning of the pipetting needle as often as required the presence of a nozzle in the structure of the pipetting needle would render this impossible. Other requirements the pipetting needle should fulfill are:
- it should be suitable for piercing a closure of a liquid container, and
- it should have an elongated shape and should be long enough to penetrate deep enough in a liquid container.
[0009] All these required features of the pipetting needle show that important features of the needle are dictated by the intended use of the pipetting needle and cannot be modified.
[0010] An aim of the invention is to provide a method and an apparatus of the above mentioned kind which enable a contact-free dispensing of liquid drops from the tip of a pipetting needle, and which in particular allow the use of a pipetting needle which can be properly cleaned by washing it with conventional washing means and which is suitable for piercing a closure of a vessel.
[0011] According to a first aspect of the invention the above mentioned first aim is achieved with a method according to claim 1. Preferred embodiments are defined by claims 2 and 3.
[0012] According to a second aspect of the invention the above mentioned first aim is achieved with a micropipetting apparatus according to claim 4. Preferred embodiments are defined by claims 5 to 10.
[0013] The methods and apparatus according to the invention have the following advantages:
- The method allows a contact-free dispensing of drops without including a nozzle in the structure of the pipetting needle. A thorough cleaning of the pipetting needle is therefore possible.
- The achievement of the technical effects attained with the method, in particular the focusing of the mechanical waves at the delivery tip of the pipetting needle, does not require or depend on any specific geometrical features of the pipetting needle. This method thus makes it possible to achieve those effects using pipetting needles having various shapes and dimensions.
- Release of drops from a pipetting needle is achieved by use of a piezoelectric actor and does not require use of any movable part.
- A complete system including the piezoelectric transducer and liquid contained in the pipetting needle can be simulated by means of an FDM-Code.
[0014] The subject invention will now be described in terms of its preferred embodiments with reference to the accompanying drawings. These embodiments are set forth to aid the understanding of the invention, but are not to be construed as limiting.
- Fig. 1
- shows schematically a prior art method for dispensing aliquots of a liquid sample taken with a pipetting needle 11 from a vessel 12 to a plurality of vessels 13, 14.
- Fig. 2
- shows schematically the principle of a method according to the invention for dispensing aliquots of a liquid sample taken with a pipetting needle 11 from a vessel 12 to another vessel 13.
- Fig. 3
- shows schematically successive dispensing of drops 18, 19 to different vessels 13, 14 respectively, by the method represented in Fig. 2.
- Fig. 4
- shows schematically a simulation step wherein a desired focused mechanical pulse 21 is applied as excitation pulse at a point 22 of a pipetting needle 11 wherein focused pulse 21 should result in a real experiment. Point 22 is the drop delivery tip of needle 11.
- Fig. 5
- shows schematically a further simulation step corresponding to the propagation 23 of the mechanical pulse applied at point 22 in Fig. 4 towards a desired excitation point 24 in a real experiment. Point 24 is the point of needle 11 where a mechanical excitation pulse is applied in a real experiment.
- Fig. 6
- shows schematically a further simulation step wherein a signal 25 is recorded which corresponds to a mechanical pulse which reaches point 24 in Fig. 5 as a result of the propagation 23 represented in Fig. 5.
- Fig. 7
- shows schematically time inversion of recorded signal 25 for generating an excitation pulse 26.
- Fig. 8
- shows schematically application of excitation pulse 26 at excitation point 24 of needle 11 and propagation 27 of that pulse towards point 24 of needle 11.
- Fig. 9
- shows schematically a desired focused mechanical pulse 21 which reaches point 22 as a result of the propagation 27 represented in Fig. 8.
- Fig. 10
- shows an example of a wave shape of a desired focused mechanical pulse 21 which should reach point 22 of needle 11 in a simplified structure not taking into account any liquid contained in needle 11 and without any electromechanical transducer mechanically connected with needle 11. Such a pulse 21 is used in a simulation of the type represented in Fig. 4.
- Fig. 11
- shows an example of a wave shape of a signal 25 which is recorded at point 24 when a mechanical pulse 21 is applied at point 22 in a simulation step of the type represented in Fig. 5 and under the conditions mentioned in the above description of Fig. 10.
- Fig. 12
- shows an example of a wave shape of a desired focused mechanical pulse 21 which reaches point 22 of needle 10 when a signal 26 is applied to point 24 of needle 11 and said signal 26 is obtained from a signal 25 shown in Fig. 11 by the method step represented in Fig. 7 and under the conditions mentioned in the above description of Fig. 10.
- Fig. 13
- shows a block diagram of the structure of a first embodiment of micropipetting apparatus according to the invention.
- Fig. 14
- shows an enlarged view of a part of the block diagram shown by Fig. 13.
- Fig.15
- shows schematically the shape and dimensions of a first embodiment of pipetting needle 12 in Fig. 1.
- Fig.16
- shows schematically the shape and dimensions of a second embodiment of pipetting needle 12 in Fig. 1.
- Fig. 17
- shows an example of the wave shape of excitation pulse signal 26.
- Fig. 18
- shows the dispersion characeristics of a Timoshenko beam.
- Fig. 19
- represents a desired transverse displacement pulse at a given point of a Timoshenko beam.
- Fig. 20
- represents a mechanical excitation pulse applied at an excitation point of a Timoshenko beam in order to obtain the transverse displacement pulse shown by Fig. 19.
| REFERENCE NUMERALS IN DRAWINGS | |
| 11 | pipetting needle |
| 12 | vessel |
| 13 | vessel |
| 14 | vessel |
| 15 | cleaning position of pipetting needle 11 |
| 16 | cleaning position of pipetting needle 11 |
| 17 | drop / volume of drop held by tip of needle 11 |
| 18 | drop |
| 19 | drop |
| 21 | focused pulse at tip of needle 11 |
| 22 | drop delivery tip of pipetting needle / spot of pipetting needle where mechanical wave is focused |
| 23 | wave propagation through the pipetting needle |
| 24 | excitation point of pipetting needle |
| 25 | recorded signal at excitation point 24 |
| 26 | time inverted signal derived from signal 25 / excitation pulse / excitation pulse signal |
| 27 | wave propagation through the pipetting needle |
| 31 | needle holder |
| 32 | electromechanical transducer / piezoelectric transducer |
| 33 | generator of electrical excitation signal |
| 34 | connecting piece |
| 35 | conduit |
| 36 | transport system |
| 37 | control unit |
| 38 | arm of transport system |
| 41 | vessel |
| 47 | free surface of liquid 48 in vessel 41 |
| 48 | liquid in vessel |
EMBODIMENT OF A METHOD ACCORDING TO THE INVENTION FOR CONTACT-FREE DISPENSING OF A LIQUID
[0015] A method according to the invention is described hereinafter with reference to Figures 2 to 12. This method is suitable for dispensing a liquid volume into a vessel by means of a pipetting needle and without any contact between said needle and a liquid contained in said vessel.
[0016] As shown in Fig. 2 a volume of liquid 17 corresponding to the volume of a drop to be dispensed is formed at the tip 22 of a pipetting needle 11 by exerting pressure on the liquid contained in the interior of the pipetting needle. Adhesion forces retain the drop so formed attached to the tip 22 of the pipetting needle 11. By performing a method described hereinafter the volume 17 is ejected from the tip of needle 11 as a drop 18 which is delivered to and thereby added to a liquid contained in a vessel 13. It should be noted that drop 18 is delivered without any contact between needle 11 and a liquid contained in vessel 13.
[0017] Fig. 3 illustrates a successive dispensing of drops 18, 19 to different vessels 13, 14 respectively, by the method represented in Fig. 2.
[0018] The invention makes use of the fact that the tubular wall of the pipetting needle 11 is a dispersive medium for the transmission of mechanical waves. Due to the dispersion characteristics of the pipetting needle a mechanical wave which is applied at a first point (excitation point) of the pippeting needle and propagates through the tubular wall of the needle travels towards the tip of the needle at a speed which depends from the frequency of that wave.
[0019] The physical facts which cause the above mentioned dispersion properties of the pipetting needle can be explained as follows:
[0020] In the case of a simple beam (e.g. a steel beam having a radius of 0.005 m) that obeys Timoshenko Beam theory (see Graff, K. F., Wave Motion in Elastic Solids, Ohio State University Press, 1975), due to the dispersion characteristics of such a beam the phase speed of a mechanical wave that propagates through the beam is a function of the frequency of the mechanical wave. Fig. 18 shows the dispersion characteristics of the above-mentioned Timoshenko beam, i.e. how the phase speed varies in function of the frequency in such a case. According to Fig. 18 mechanical waves which have higher frequencies propagate through the beam with higher phase speeds and have also higher group velocities than mechanical waves which have lower frequencies. This means that mechanical waves which have higher frequencies travel faster than mechanical waves which have lower frequencies.
[0021] Fig. 19 represents a transverse displacement pulse corresponding to one period of a sine signal with an Amplitude = 1 at a location x = 1 m, i.e. at a distance of 1 m from the point of the beam where a mechanical excitation is applied.
[0022] In order to obtain the transverse displacement pulse shown by Fig. 19 it is necessary to apply at the excitation point of the beam a mechanical excitation which corresponds to the signal represented by Fig. 20. Such a signal is computed using the dispersion characteristics shown by Fig. 18 and in the frequency space using well known methods based on the Fourier Transform described e.g. by Doyle, J.F., Wave Propagation in Structures, Springer, New York, 1989.
[0023] Fig. 20 shows that the low frequency components of the excitation signal have to be sent off first, because their wave speed is smaller. This is the basic principle used according to the invention for focusing a pulse using the dispersion characteristics of a mechanical structure: a relatively long pulse is transformed by the dispersion characteristics of the beam into a short pulse that can be used e.g. for releasing a drop as proposed by the instant invention.
[0024] If a tube is used instead of a beam as medium for transmitting a mechanical wave, the dispersion characteristics necessary for computing the excitation signal can be taken from any book of wave propagation, e.g. Graff, K. F., Wave Motion in Elastic Solids, Ohio State University Press, 1975. In a tube there are several propagation modes and each mode has its own dispersion characteristics. This property can be used in addition for obtaining the desired focusing effect. An additional focusing effect is obtainable by effecting suitable time delays of pulses of the various modes with respect to each other and thereby obtaining an overlap of pulses corresponding to the various modes at a desired location.
[0025] According to a preferred embodiment of the method according to the invention pulses of several modes of propagation having each their individual dispersion characteristics are focused and superimposed at the tip of the pipetting needle (11). This is achieved for instance by effecting suitable time delays of pulses of the various modes with respect to each other and thereby obtain an overlap of pulses corresponding to the various modes and a focusing of the energy of those pulses at the tip of the pipetting needle (11) .
[0026] The portion of the pipetting needle used according to the invention for the above mentioned transmission of mechanical waves is shown in Figures 4 to 9. This portion has the shape of a capillary tube.
[0027] According to the invention a suitable composite mechanical excitation pulse is applied at a point 24 of the pipetting needle which lies at some distance from the needle delivery tip 22 from which the drops are ejected. The latter excitation pulse and the mechanical system comprising the needle 11, the liquid in the needle and the means for generating the excitation pulse, e.g. a piezoelectric transducer connected to the needle, are so configured that the excitation pulse has frequency components which arrive simultaneously to the drop delivery tip 22 of the needle and thereby provide a maximum of mechanical energy at that tip. In other words the latter configuration is such that transmission of the excitation pulse by the mechanical system mentioned above focuses the mechanical wave at the tip 22 of the pipetting needle and reflection of the focused wave at that tip causes ejection of a drop which was held there by adhesion forces.
[0028] According to the invention a drop 17 is formed at the delivery tip 22 of the pipetting needle 11 by pressing a predetermined liquid volume out of the needle and thereby forming a liquid meniscus at the delivery tip 22 of the pipetting needle. After that the above mentioned transmission of mechanical waves can take place, e.g. in one of the following ways:
1) A mechanical excitation pulse is applied to the needle at point 24. This pulse causes displacements in the tubular wall of the needle 11 mainly in axial direction and is focused at the delivery tip 22 of the needle. The reflection of this pulse at the delivery tip 22 of the pipetting needle releases drop 17 from that tip.
2) A mechanical excitation pulse is applied to the needle 11. This pulse causes displacements in the tubular wall of the needle mainly in radial direction. Due to mechanical coupling of the tubular wall of the needle 11 and liquid contained in the needle, liquid within the needle is also displaced and accelerated towards the delivery tip 22 of the needle by the propagation of the excitation pulse. This displacement of fluid and the focusing and reflection of the excitation pulse at the delivery tip 22 of the pipetting needle releases drop 17 from that tip.
[0029] The above described method for dispensing a liquid thus essentially comprises
(a) forming a drop 17 at the delivery tip 22 of a pipetting needle 11, said drop being retained at the tip by adhesion forces, and
(b) ejecting the drop 17 from the tip 22 by focusing a mechanical wave at the tip 22 of the pipetting needle 11.
[0030] In a preferred embodiment the ejection of the drop is achieved by mechanically exciting the needle by means of an excitation pulse having a composition that focuses a pulsed wave at the tip of said pipetting needle. A superposition of a focused incident wave with a reflected wave at the delivery tip of the pipetting needle causes ejection of the drop from the tip. As described hereinafter a suitable composition of the excitation pulse is obtained by a simulation process.
EMBODIMENT OF A METHOD FOR GENERATING AN EXCITATION PULSE HAVING A COMPOSITION SUITABLE FOR PERFORMING A METHOD ACCORDING TO THE INVENTION
[0031] In order to generate an excitation pulse which has a composition or structure suitable for performing a method according to the invention, the wave propagation of a desired focused pulse in a system comprising a needle filled with a liquid and a piezoelectric actuator mechanically connected with the needle is simulated by means of finite difference method (FDM). After that an excitation pulse with a suitable structure for obtaining the desired focused pulse is calculated by a time reversal method.
[0032] In order to simulate the above mentioned wave propagation a FDM code is written for the case of an axial symmetric tube, filled with a liquid and mechanically connected to a piezoelectric transducer. The latter FDM code is based on a code described in the publication: Thesis of Tobias F. Leutenegger entitled "Detection of defects in cylindrical structures using a time reverse numerical simulation method", thesis submitted at the Swiss Federal Institute of Technology, Zurich (ETH Zürich), Switzerland, No. 14833, 2002.
[0033] The FDM code is programmed with second order central differences, a so called staggered grid being used for discretization in space and time. The liquid is modeled as an acoustical fluid. In this way the behavior of the complete system can be simulated. By means of the FDM code the excitation pulses necessary for the desired energy focusing are computed making use of a time reversal method.
[0034] The principle of this method is described as follows with reference to Figures 4 to 12, wherein a portion of pipetting needle is shown, and this portion has the shape of a capillary tube:
- A real experiment with the pipetting needle is described by / simulated with a FDM code. The spot 24, where the mechanical excitation pulse is applied to the needle, and the spot 22, where the mechanical pulses should be focused, are defined.
- In the FDM code a desired focused pulse 21 is applied as excitation pulse at the spot 22, where the mechanical pulses should be focused in the real experiment.
- The propagation of the mechanical wave is simulated over a time interval which extends until the excitation pulse passes through the spot 24, where the mechanical excitation pulse is applied to the needle in the real experiment.
- The displacements in all directions are recorded as a signal 25 over a time interval at the spot 24, where the mechanical excitation pulse is applied to the needle in the real experiment.
- The recorded signal 25 is reversed in time for generating a signal 26 which is the excitation signal that has to be applied at spot 24, where the mechanical excitation pulse is applied to the needle in the real experiment, in order to obtain a desired focused pulse 21 at the spot 22, where the mechanical pulses should be focused in the real experiment (see Figures 7 to 9).
[0035] The result of the above mentioned calculation is the electrical signal 26 to be applied to the piezoelectric transducer. This signal is generated by a function generator. A piezoelectric tube with electrodes on its major surfaces (radial electric field) and radial polarization generates mainly radial displacements. A piezoelectric tube with electrodes on its major surfaces (radial electric field) and axial polarization generates mainly axial displacements. Generation of the desired displacements in the pipetting needle can thus be obtained by election of a suitable piezoelectric transducer.
[0036] Fig. 17 shows an example of the wave shape of excitation pulse signal 26 obtained as described above by simulation of the wave propagation in the tubular body of the needle and by time reversal of the signal recorded in that simulation. The obtention of the latter excitation pulse signal 26 thus take into account the complete wave propagation behavior of that tubular body, all wave propagation modes in the frequency range used and their frequency dependent propagation speeds.
[0037] A method for generating an excitation pulse signal 26 that has a composition suitable for focusing the energy applied by that pulse to the pipetting needle is described as follows. To simplify the description and make it easier to understand the method is described for the simplified case of a pipetting needle which has the shape of simple empty tube, which does not contain any liquid and which is not mechanically coupled with any electromechanical transducer. A FDM code of the above mentioned case is written for this case and is used for the simulation.
Three Hanning pulses comprising each five periods of their central frequencies of 0.5 MHz, 1.2 MHz and 2.7 MHz are symmetrically superposed in order to form a desired pulse 21 shown by Fig. 10 which should be the pulse resulting of focusing the excitation pulse applied to the needle. In the simulation this desired pulse 21 is applied at the spot 22, where the mechanical pulses should be focused in the real experiment, and a signal 25 shown by Fig. 11 is recorded at the spot 24, where the mechanical excitation pulse is applied to the needle in the real experiment. Time reversal of recorded signal 25 and selection of a portion of this signal with a suitable time window provides the excitation pulse 26. The time window is so chosen that only the first arriving pulses are considered, but not those already reflected. An excitation pulse 26 obtained in the latter way is applied at spot 24 of the needle and this provides the desired focused pulse 21 shown in Fig. 12.
[0038] The meaning of n and m in the labels of Figures 11 and 12 is as follows:
n is the azimuthal wave number and describes wave modes with regard to their azimuthal characteristic.
n = 0 means that the wave mode has an axial symmetry.
n = 1 means that the displacements have one maximum and one minimum over the circumference.
n = 2 means that the displacements have two maxima and two minima over the circumference.
[0039] Modes of a given wave number n are numbered in the order of their appearance with m= 1, m=2, etc.
[0040] In the case of a capillary tube, e.g. the portion of a pipetting needle shown by Figures 4 to 9, and at low frequencies, there are only a first mode with n = 0, m =1 and a second mode with n = 0 and m =2 which have both an axial symmetry. The first mode is a torsional mode. At sufficiently low frequencies the second mode is a longitudinal mode. A third mode with n = 0, m = 3 arises at a frequency of 2 MHz.
[0041] Displacements in radial and axial direction at the complete front surface of the left end of the capillary are recorded. In Fig. 11 only axial displacements at the inner diameter of the left end of the capillary are represented.
[0042] In order to obtain a most suitable excitation signal, it would be advantageous to record displacements over the entire front surface of the left end of the capillary, but recording of displacements at a few point of that surface would suffice.
[0043] Fig. 11 shows four pulses of different modes which reach spot 24 in Fig. 5 and which are the result of applying a pulse 21 at spot 22 in Fig. 5. The latter four pulses have different group velocities. Therefore, as shown by Fig. 11 these pulses reach the front surface of the left end of the capillary at different points of time. The latest pulse which reaches the latter front surface is already reflected once by the left and once by the right end of the capillary tube. This pulse should not be taken into consideration when forming the excitation pulse on the basis of the pulses shown by Fig. 11. Therefore, the time window used for forming this pulse should extend from t = 0 s to about t = 0.9 10-4 s. Reversal in time of the portion of the pulses shown in Fig. 11 which are within the latter window provide an excitation pulse 26 (shown in Fig. 8) propagation of which results in a pulse 21 at spot 22 in Fig. 9. In the generation of excitation pulse 26 it is important to take into account both the axial and the radial displacements obtained by simulation at the front surface of the left end of the capillary. In the attached drawings only the axial displacements are represented.
[0044] The above mentioned excitation pulse 26 consists of four pulses. Fig. 12 shows six pulses that arise at the right end 22 of the capillary tube when a mechanical excitation corresponding to excitation pulse 26 is applied at spot 24. The latter six pulses result from the four pulses of the excitation pulse 26, because above 2 MHz there are 2 propagation modes.
[0045] Four of the above mentioned six pulses form focused pulse 21 shown in the gray area of Fig. 12. Two other modes outside of this gray area are also represented in Fig. 12. These two modes, which are not desirable, but are also generated by the excitation with pulse 26, reach the right end 22 of the capillary tube at other points of time. The latter modes do not interfere with the release of a drop from the tip of the pipetting needle, because they lie outside of the time window of the desired pulse 21 which is used for releasing the drop.
[0046] The method just described above is just a simplified example of a method for focusing mechanical pulses. For the purpose of releasing drops from the delivery tip of a pipetting needle not only the behavior of a capillary tube (pipetting needle), but also the behavior of a piezoelectric transducer used for applying the mechanical pulses is simulated. Simulation of the behavior of a liquid in the interior of the needle is less important than simulation of the behavior of the capillary tube and the piezoelectric transducer, because the liquid in the needle has less influence on the process for releasing a drop by the above described method.
[0047] A suitable FDM code of the above mentioned kind is also available for performing a simulation of the behavior of the capillary tube and the piezoelectric transducer for the propagation of a mechanical pulse applied by the transducer to the capillary tube. If the simulation includes simulation of the behavior of the piezoelectric transducer, a voltage would be recorded that corresponds to the displacements shown in Fig. 11.
[0048] The above described method for generating an excitation pulse suitable for performing the above mentioned dispensing method and in particular for exciting an electromechanical transducer in a micropipetting apparatus of the kind described hereinafter essentially comprises:
(a) simulating by means of a finite difference method propagation of a mechanical pulse through the wall of a portion of a pipetting needle that has the shape of a capillary tube, said pulse being applied in the simulation at the spot where in the real experiment a focused pulse is to be generated for thereby ejecting a drop formed at the delivery tip of the needle and attached thereto by adhesion forces,
(b) recording an electrical pulse signal which correspond to mechanical pulses which in the simulation arise at the spot where the mechanical excitation pulse is to be applied to said pipetting needle in reality, and
(c) calculating an excitation pulse signal to be applied in reality to said piezoelectric transducer, the latter excitation pulse signal being calculated by time reversal of said recorded signal obtained by step (b).
FIRST EMBODIMENT OF AN APPARATUS ACCORDING TO THE INVENTION FOR CONTACT-FREE DISPENSING OF A LIQUID
[0049] A first embodiment of a micropipetting apparatus according to the invention is described hereinafter with reference to Figures 13 and 14. This micropipetting apparatus is suitable for dispensing a liquid volume into a vessel by means of a pipetting needle and without any contact between said needle and a liquid contained in said vessel.
[0050] As shown by Fig. 13 a micropipetting apparatus according to the invention comprises a pipetting needle 11, a needle holder 31, an electromechanical transducer 32, a generator 33 for generating electrical signals, a connecting piece 34 which fluidically connects needle 11 with a conduit 35 which connects needle 11 with a source of positive or negative pressure, a transport system 36 for transporting needle holder 31 and a control unit 37 for controlling the operation of the entire system.
[0051] Needle 11 has a substantially constant cross-section over the portion thereof that ends in a delivery tip 22 and that portion extends over more than one half of the total length of needle 11.
[0052] Electromechanical transducer 32 is e.g. a piezoelectric transducer mechanically connected pipetting needle 11. This piezoelectric transducer comprises one or more piezoelectric elements.
[0054] Fig. 14 shows a cross-sectional view of arm 38, needle holder 31, electromechanical transducer 32 and of a portion of needle 11.
[0055] Signal generator 33 generates an excitation pulse signal and applies this signal to piezoelectric transducer 32 for mechanically exciting pipetting needle 11 at an excitation point 24 with an excitation pulse 26 that propagates through needle 11 and is focused at the end tip 22 thereof (as shown in Figures 8 and 9). The latter mechanical excitation thereby causes release of a drop from tip 22 of needle 11.
[0056] In a preferred embodiment the composition of the excitation pulse 26 is adapted to the length and the wave propagation characteristic of the portion of needle 11 that has a substantially constant cross-section.
[0059] In a preferred embodiment, the composition of excitation pulse 26 signal applied to piezoelectric transducer 32 is such that it causes a mainly radial displacement of liquid within said needle.
[0060] In another preferred embodiment, the composition of excitation pulse 26 signal applied to piezoelectric transducer 32 is such that it causes a mainly axial displacement of liquid within said needle.
[0061] In all above described embodiments of the micropipetting apparatus described with reference to Figures 13 and 14, the excitation signal 26 applied at the excitation point 24 of needle 11 is generated by a method as described above with reference to Figures 4-12.
EXAMPLES OF PIPETTING NEEDLES USED IN THE ABOVE DESCRIBED APPARATUSSES
[0062] The dimensions of the pipetting needle shown in Fig. 15 are as follows:
| Dimension | Size in millimeter |
| A1 | 69 |
| L1 | 86 |
| L2 | 5 |
| L3 | 9 |
| D1 | 0.9 |
| D2 | 1.5 |
| D3 | 3 |
| D4 | 5 |
| L4 | 13.5 |
| L5 | 0.5 |
| D5 | 0.6 |
1. Method for dispensing a liquid volume into a vessel by means of a pipetting needle
and without any contact between said needle and a liquid contained in said vessel,
said method comprising
(a) forming a drop (17) at the delivery tip (22) of the pipetting needle (11), said drop being retained at the tip by adhesion forces, and
(b) ejecting said drop (17) from said tip (22) by focusing a mechanical wave at said tip of the pipetting needle (11), said focusing being achieved by exciting an electromechanical transducer (32) with an electrical excitation pulse signal (26) and by applying this signal to said electromechanical transducer (32) for mechanically exciting said pipetting needle (11) with a pulse of mechanical waves that propagate through said needle, said pulse having a predetermined composition calculated for being focused at the tip thereof by the dispersion characteristics of said pipetting needle, a superposition of a focused incident wave with a reflected wave at the tip of the pipetting needle causing ejection of said drop (17) from said tip.
2. A method according to claim 1 wherein pulses of several modes of propagation having
each their individual dispersion characteristics are focused and superimposed at the
tip of the pipetting needle (11).
3. A method according to claim 1 or 2 wherein said excitation pulse (26) is generated
by
(a) simulating by means of a finite difference method propagation of a mechanical pulse through the wall of a portion of a pipetting needle that has the shape of capillary tube, said pulse being applied in the simulation at the spot (22) where in the real experiment a focused pulse is to be generated for thereby ejecting a drop (17) formed at the delivery tip (22) of said needle (11) and attached thereto by adhesion forces,
(b) recording an electrical pulse signal (25) which corresponds to mechanical pulses which in the simulation arise at the spot (24) where the mechanical excitation pulse is to be applied to said pipetting needle (11) in reality, and
(c) calculating an excitation pulse signal (26) to be applied in reality to said piezoelectric transducer (32), the latter excitation pulse signal being calculated by time reversal of said recorded signal (25) obtained by step (b).
4. Micropipetting apparatus for dispensing a liquid volume into a vessel by means of
a pipetting needle and without any contact between said needle and a liquid contained
in said vessel, said apparatus comprising
(a) a pipetting needle (11) having a first end which comprises a delivery tip and a second end which is connected to a source of positive or negative pressure,
(b) an electromechanical transducer (32) mechanically connected with said pipetting
needle (11)at a first portion thereof which comprises said second end of the pipetting
needle,
a second portion of the needle extending between said first portion and up to said
delivery tip, and
(c) electrical signal generating means (33) for generating an excitation pulse (26) signal and for applying this signal to said electromechanical transducer (32) for mechanically exciting said pipetting needle (11) with a pulse of mechanical waves that propagate through said needle, said pulse having a predetermined composition calculated for being focused at the end tip thereof by the dispersion characteristics of said pipetting needle, said mechanical excitation thereby causing ejection of a drop (17) formed on the delivery tip of said pipetting needle.
5. A micropipetting apparatus according to claim 4, wherein said excitation pulse (26)
signal is adapted to the length and the dispersion characteristics of said pipetting
needle (11).
6. A micropipetting apparatus according to claim 4, wherein said electromechanical transducer
(32) is a piezoelectric transducer.
7. A micropipetting apparatus according to claim 6, wherein said piezoelectric transducer
(32) is radially polarized.
8. A micropipetting apparatus according to claim 6, wherein said piezoelectric transducer
(32) is axially polarized.
9. A micropipetting apparatus according to any of claims 6 to 8, wherein said excitation
pulse (26) signal applied to said piezoelectric transducer (32) is so configured that
it causes a mainly radial displacement of liquid within said needle (11).
10. A micropipetting apparatus according to any of claims 6 to 8, wherein said excitation
pulse (26) signal applied to said piezoelectric transducer (32) is so configured that
it causes a mainly axial displacement of liquid within said needle (11).