Patent Publication Number: US-2021178384-A1

Title: Improved Device For The Analysis Of Nucleic Acid Molecules

Description:
TECHNICAL FIELD OF THE INVENTION 
     The invention relates to a device and a process for analyzing molecules. The invention applies in particular to the analysis of nucleic acid molecules such as DNA or RNA. 
     BACKGROUND ART 
     If nucleic acid molecules are attached to micro-scale beads, information about the nucleic acid structure can be inferred by manipulating the beads and tracking their position with high resolution. Under particular experimental conditions, tracking the location of the bead in real time can be used to generate useful information about the structure of the DNA or RNA molecule to which it is attached. 
     This can, in turn, be used to determine the molecule&#39;s gross organization, base sequence, the presence of biochemical modifications to the nucleic acid bases, and the interactions of the molecule with proteins such as polymerases, helicases, topoisomerases, etc. 
     A device for performing such analysis of nucleic acid molecules has been described in document US 2003/0027187. This device comprises optical means to determine the position of the beads attached to the molecule. 
     More specifically, beads are illuminated from above and viewed through a microscope objective via an image captured on an HD video camera. The position of a bead is followed in real time by a tracking algorithm measuring x and y coordinates from the bead image, whereas the determination of the z-position, that is, the height of the bead with reference to the surface to which the molecule is attached, is determined thanks to light diffraction patterns surrounding the image of the bead. 
     Indeed, the light illuminating the bead is scattered from the bead and creates a diffraction pattern by interfering with the direct illumination light. In order to use this diffraction pattern to infer the z-position of the bead, a set of images of the diffraction pattern is acquired at various distances from the focal point of the objective by keeping the bead in a fixed location while moving the objective in precise steps. These images are used to calibrate the system for each bead independently. 
     Thereafter the objective is maintained very precisely in a fixed location, and the ring pattern can then be used to track the distance of the bead from the focal point along the optical z-axis, via cross reference to the calibration set. 
     This optical approach to bead position determination is effective. However, it suffers from a number of drawbacks that affect the cost and scalability of the approach. 
     Indeed, to perform an analysis of nucleic acid molecules, it is needed to achieve nearly single base resolution, which is in the range of 1 nanometre of bead movement, as 1 nanometre equals the distance increase each time one dsDNA base pair opens in an unzipping configuration. This distance is the sum of the extension of the ssDNA single nucleotide of the two complementary bases that have opened. The situation is analogous for a RNA molecule. 
     This level of resolution is achievable with the optical approach disclosed hereinabove. However, such a system requires a quite complex mechano-optical setup, and with a relatively small field of view, which limits the number of beads that the system can analyze simultaneously. For instance, a 30× objective can have a field of view of about 300 by 300 microns, which allows analyzing only about 1000 beads. 
     By contrast, it is preferable for particular applications to perform analysis of many more molecules simultaneously; such as for instance up to 10 9  molecules. 
     Moreover, even though the optical approach allows analyzing about 1000 beads simultaneously, it requires to generate a set of calibration images for each bead, which is time consuming and computationally intensive. Thus, making higher throughput with the same optical approach is quite complicated. 
     SUMMARY OF THE INVENTION 
     One aim of the invention is to overcome the aforementioned disadvantages of the prior art by providing a system and process for analyzing nucleic acids molecules, which allow the simultaneous analysis of a higher number of molecules than the prior art. 
     Another aim of the invention is to keep at least the same resolution than the prior art to perform precise analysis of the molecules. 
     To this end, a device for performing analysis of nucleic acid molecules is disclosed, comprising:
         a bead, on which one molecule can be anchored at one end,   a surface, on which the molecule can be anchored at the other end,   an actuator, adapted to cause the bead to move relative to said surface in one direction of motion,   a sensor, adapted to measure a distance between the bead and the surface, the device being characterized in that it further comprises a well, having an axis extending along the direction of motion of the bead and a bottom which is formed by said surface, said well being filled with electrically conductive solution, and the bead being received in the well, and in that the sensor is adapted to measure an impedance of the well, said impedance depending on a distance between the bead and the surface, to determine, from the measured impedance, the distance between the bead and the surface.       

     In some embodiments, the device may comprise at least one of the following features:
         the sensor may comprise:
           a main electrode, positioned on top of the well, in contact with the electrically conductive solution, the electrode being submitted to a known potential,   a secondary electrode at the bottom of the well, carrying the surface to which the molecule can be anchored, and   an electronic circuit, adapted to measure a current flowing between the electrodes.   
           The electronic circuit may comprise:
           a current to voltage amplifier connected to the secondary electrode,   a voltmeter adapted to measure an output voltage of the current to voltage amplifier, and   a computing circuit adapted to compute an impedance of the well from the measured voltage.   
           The area of a cross-section of the well transversal to its axis may be strictly increasing from the bottom of the well to the top.   The area of the cross-section at the bottom of the well may be greater than the area of the largest cross-section of the bead.   The well may have a shape of a truncated cone.   The area of the cross-section of the well may grow linearly with the distance from the bottom of the well.   The cross-section of the well may for instance be circular at its bottom and grow linearly in a direction orthogonal to the axis of the well with the distance from the bottom of the well.   The actuator may comprise at least one magnet mounted to be displaceable along the direction of the axis of the well, and the bead is made in a paramagnetic material, and is interposed between the bottom of the well and the magnet.   The device may comprise a plurality of identical beads and a plurality of identical wells, each adapted to receive one bead. Furthermore, in an embodiment in which the sensor comprises a main electrode, a secondary electrode, and an electronic circuit, adapted to measure a current flowing between the electrodes, said sensor may comprise a plurality of secondary electrodes, each disposed at the bottom of a respective well and forming a surface to which is anchored a respective bead, and the electronic circuit may then a plurality of current to voltage amplifiers, each connected to a respective secondary electrode, with the computing circuit further adapted to simultaneously measure output voltages of the current to voltage amplifiers and compute the impedances of the respective wells.   The device may comprise a plate of electrically insulating material in which the wells are formed, all wells opening out at a top surface thereof, and the actuator further comprising a plurality of bars made of magnetic material, each bar being disposed on the top surface and extending between two adjacent wells.   Each bar may have a length inferior to 10 μm, and each bead may have a diameter inferior or equal to 1 μm.       

     An analysis process of nucleic acid molecules is also disclosed, said process being implemented by a device according to the above description, and comprising at least one step of measuring a distance between the bead and the bottom of the well, each measuring step comprising measuring an impedance of the well in view of determining a position of the bead in the well. 
     In an embodiment, the analysis process may also comprise a preliminary step of anchoring at least one molecule to a bead and the bottom of a well, said step comprising:
         positioning at least one bead on which a molecule is anchored in the solution filling the well,   applying a first potential difference between main and secondary electrodes of the sensor to drive the bead in contact with the bottom of the well, and   reversing the potential difference between the electrodes.       

     The device according to the invention allows analyzing a molecule attached at one end to a micro-bead and at the other to the bottom of a well. By monitoring the impedance changes of the well according to the position of the bead in the well one can measure its position with great precision. 
     Indeed, the overall conductance of the well corresponds to the conductance of the solution that fills the well. When the bead moves inside the well and occupies a portion of the latter, it reduces the cross-section of the well that is filled with the electrically conductive the solution and thus changes the conductance of the well. 
     Therefore, by determining an exact shape of the well, and in particular when the well has a cross-section area increasing with the distance to the bottom of the well, a position of the bead in the well can be easily determined. 
     This device can be multiplexed to allow simultaneous analysis of a great number of nucleic acid molecules, without reducing the resolution. Indeed, when the device comprises a plurality of wells and respective beads, all the beads can be monitored by a sensor comprising, for each well, a secondary electrode and a current to voltage amplifier allowing precise monitoring of the position of each bead in its respective well. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The features and advantages of the invention will be apparent from the following more detailed description of certain embodiments of the invention and as illustrated in the accompanying drawings, in which: 
         FIG. 1 a    shows an exemplary embodiment of a device for analysing nucleic acid molecules, 
         FIGS. 1 b  and 1 c    respectively show a cross-sectional view and a perspective view of possible shapes of a well of a device, 
         FIG. 2  shows the resistance of the well according to the position of the bead, in the embodiment of  FIG. 1   a.    
         FIG. 3  schematically shows the electric equivalent circuit of a device according to an embodiment of the invention. 
         FIGS. 4 a  and 4 b    are schematic drawings of an analysis device according to two embodiments, 
         FIGS. 5 a  and 5 b    show the gradient of magnetic field exerted on a bead in an exemplary analysis device, 
         FIG. 6  shows the main step of an analysis process according to one embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION 
     Device for the Analysis of Molecules 
     Overall Description of the Device 
     With reference to  FIG. 1 a   , a device  1  for analysing nucleic acids molecules is shown. These molecules may in particular be DNA or RNA molecules. Preferably, as shown in the figure, the molecule may be a double-stranded molecule of the hairpin type. 
     Hairpin means a double helix wherein the  5 ′ end of one strand is physically linked to the  3 ′ end of the other strand through an unpaired loop. This physical link can be either covalent or non-covalent, but is preferably a covalent bond. 
     Thus a hairpin consists of a double-stranded stem and an unpaired single-stranded loop. 
     The device  1  comprises a plate  10  of electrically insulating material. For instance the plate can be made of silicon, glass, a non-conducting polymer or resin. 
     At least one well  11  is made within the plate, each well extending along a main axis X-X which is orthogonal to the plane in which the plate extends. Each well opens out at a top surface  101  of the plate. 
     Moreover, each well  11  has a bottom  110 , preferably extending orthogonally to said axis X-X. 
     The wells are so-called microwells, because the order of magnitude of their dimensions (depth, largest length of a cross-section) is about 1 μm or about 0.1 μm. For instance, a well can have a depth along the X-X axis of a few micrometres, for instance comprised between 1 and 10 micrometres, for instance equal to 8 μm. 
     A largest length of a cross-section of the well in a plane orthogonal to the X-X axis can range from a few hundreds nanometres to a few micrometres, for instance about 4 or 5 μm as shown in  FIG. 1 . 
     Existing techniques allow producing such wells, such as for instance a technique called track etching technique, comprising a step or irradiation with heavy ions to form latent tracks and a step of chemical etching. For more details about techniques for producing those wells, one may refer for instance to the works of the Siwy Research Lab (website of the lab: http://www.physics.uci.edu/˜zsiwy/fab.html) or the publication by M. Davenport, K. Healy, M. Pevarnik, N. Teslich, S. Cabrini, A. P. Morrison, Z. S. Siwy and S. E. Létant, “The Role of Pore Geometry in Single Nanoparticle Detection”, in ACSNANO, vol. 6, no. 9, 8366-8380, 2012. 
     The device furthermore comprises at least one bead  20 . Preferably, the device comprises a plurality of beads  20 , in equal number to the number of wells. 
     The number of wells and beads may preferably be greater than 1000, for instance greater than 10000, for instance about 100000 or 1000000. 
     Each bead is spherical and has a diameter not greater than 5 μm. For instance, the bead  20  may have a diameter of about 1.5 μm or 1 μm. Preferably, the bead may be even smaller and have a diameter of less than 1 μm, for instance of 0.3 μm. 
     As non-limiting examples, the following references of beads can be used:
         MyOne, produced by Invitrogen, having a diameter of 1.04 μm   M270, produced by Invitrogen, of 2.8 μm diameter,   M450, produced by Invitrogen, of 5.5 μm diameter,   Ademtech 500, produced by Ademtech, of 0.5 μm diameter,   Ademtech 300, produced by Ademtech, of 0.3 μm diameter.       

     To perform an analysis of a nucleic acid molecule, one molecule M is anchored to the bead  20  at one end thereof, and to the surface of the bottom of the well  110  on its other end. 
     To achieve anchoring of the molecule on the bead and on the bottom surface of the well, the bead and the surface may be coated with a specific material adapted to bind with an end of the molecule. 
     For instance, the DNA or RNA molecules may be labelled with biotin at one end, digoxigenin at another end, and the beads may be coated with streptavidin to bind with a labelled (for example biotin) end of a DNA/RNA hairpin molecule, and the bottom of the well  110  may further be coated with anti-Dig antibodies to bind a Dig-labelled end of the DNA/RNA molecule, see for instance Hunter M M, Margolies M N, Ju A, Haber E, “High-affinity monoclonal antibodies to the cardiac glycoside, digoxin, Journal of Immunology, 1982 September; 129(3): 1165-1172. 
     Therefore the bead is attached to the bottom  110  of the well  11  via the molecule. 
     Moreover, the bead  20  is free to move relative to the bottom  110  of the well  11 . In particular, the bead  20  can move along the direction of the X-X axis. In order to control the motion of the bead  20  along this axis, the device  1  further comprises an actuator  30  adapted to cause the bead  20  to move in translation along said axis. 
     According to a preferred embodiment, the control of the motion of the bead may rely on a magnetic force applied by the actuator  30  on the bead  20 . In that case, the bead is made of a paramagnetic material, such as a superparamagnetic material. For instance, the bead may be made in latex with incorporated ferrites, and coated with streptavidin for anchoring the molecule M. 
     The actuator  30  may comprise at least one permanent magnet  31 , which can be controlled to move in translation along the X-X axis. Preferably, as shown in  FIG. 1 , the actuator  30  can comprise two permanent magnets  31 , positioned at equal distance of the X-X axis and having their magnetic poles aligned perpendicular to the X-X axis, the North pole of a magnet facing the South pole of the other. 
     The bead  20  is positioned between the bottom  110  of a well  11  and the magnets  31 . 
     These magnets allow one to apply a force on the bead and consequently on the molecule to which it is anchored. By moving the magnets closer to or further from the bead  20  in the direction of the X-X axis, one changes the magnetic field and thus controls the magnitude of the force applied to the bead, thus controlling the stretching of the sample in the direction of the X-X axis. 
     Another embodiment of the actuator  30  may comprise a permanent magnet and a strip covered with a magnetisable material positioned at a fixed distance relative to the well  11 , of about a few micrometres. By bringing the permanent magnet closer or further from the strip covered with magnetisable material the field applied by said strip on the bead can be varied (see also embodiment disclosed in the section Disposition with a plurality of wells below). 
     Other ways of controlling the motion of the bead  20  can be used, such as optical or acoustic tweezers, the latter implying application of acoustic waves on the bead, see for instance G. Sitters, D. Kamsma, G. Thalhammer, M. Ritsch-Marte, E. J. G. Peterman and G. L. J. Wuite, “Acoustic Force spectroscopy”, in Nature Methods, Vol. 12, No 1, January 2015, or X. Ding, Z. S. Lin, B. Kiraly, H. Yue, S. Li, I. Chiang, J. Shi, S. J. Benkovic and T. J. Huang, “On-Chip Manipulation of single microparticles, cells, and organisms using surface acoustic waves”, PNAS, Jul. 10, 2012, vol. 109, no. 28, 11105-11109. 
     Change in Well Impedance with the Position of the Bead 
     Last, the device  1  allows determining the distance between the bead  20  and the bottom  110  of the respective well  11 , corresponding to the length of the molecule anchored to the bead and the bottom  110  of the well, by monitoring the impedance, in particular the resistance (or conductance) of the well. 
     To this end, each well  10  is filled by an electrically conductive solution  40 . 
     The electrically conductive solution  40  preferably has a conductivity of between 10 −7  S/cm and 10 1 , preferably between 10 −3  and 10 −2  S/cm. 
     For instance, the solution  40  may be an aqueous solution of sodium chloride at a concentration of 100 mmol/m 3  (100 mM). The solution  40  may alternatively comprise a buffer compatible with the preservation of DNA molecules, such a buffered aqueous solution containing 10 mM Tris HCl and 0.1 mM EDTA and sodium Chloride at 100 mM. The buffer may also contain divalent cations compatible with enzymatic activities, such a 10 mM MgCl 2 . In some embodiments, the buffer should support electrophoresis (e.g. Tris Borate EDTA buffer). 
     As can be seen in  FIG. 1 a   , the electrically conductive solution  40  completely fills each well  11  and covers the top surface  101  of the plate. 
     The bead  20  must have a different conductivity than the conductivity of the solution. The bead preferentially is electrically insulating. 
     Moreover, the well  11  has a cross-section that varies with z, z being the distance along the X-X axis from the bottom of the well. This distance can be measured from the closest point of the bead to the bottom of the well, or from the centre of the bead. 
     Preferably, the area of a cross-section of the well  11  transversal to the X-X axis strictly increases with the distance z from the bottom of the well (the z axis is shown on  FIGS. 1 a  to 1 c    with the origin of the axis at the centre of the bottom of the well). 
     Therefore, as the bead  20  is of constant size, it occupies a varying proportion of the internal volume of the well. 
     For instance, if the bead is very close to the bottom of the well, it occupies a major proportion of the portion P of the well extending around the bead. The portion P is defined as the volume of the well extending between the cross-sections occupied by the lowest and highest points of the bead. The resistance of this portion is therefore increased dramatically because there is very little space left for the conductive solution. As the overall resistance of the well is the integral of the resistance along the whole depth of the well (along the X-X axis), the overall resistance is also increased dramatically. 
     On the contrary, if the bead is close to the top surface of the plate, it occupies a minor proportion of the portion P of the well extending around the bead. The resistance of this portion is thus less increased than in the previous example, and the overall resistance of the well is smaller than that of the previous example. 
     Therefore as shown for instance in  FIG. 2 , a curve showing the resistance of the well as a function of the distance of the bead from the well&#39;s bottom can be generated, in which each resistance value corresponds to a single distance value between the bead and the bottom of the well. By measuring the resistance value of the well it is therefore possible to infer with good precision the corresponding distance of the bead from the bottom of the well. 
     In order to obtain a good precision in determining the position of the bead  20  within the well  11 , the smallest cross-section of the well is preferably larger than the largest cross-section of the bead. In particular, in the preferred case where the area of the cross-section of the well transversal to its axis is strictly increasing with the distance from the bottom, the smallest cross-section is located at the bottom  110  of the well. The fact that this cross-section is larger than the bead allows the bead to reach the bottom of the well, and hence allows measuring all positions of the bead in the well, including a position in which the distance between the bead and the well is null. 
     The well can have various different shapes. First, the well may be rotationally symmetric around the X-X axis. 
     For instance, as shown on  FIG. 1 b   , the well may have the shape of a truncated cone, the smallest section of which corresponds to its bottom  110 . 
     Alternatively, on  FIG. 1 a   , the radius r of the wall of the well is defined according to the distance z from the bottom by the following equation: 
         z=I   0  tan  h 2( r−r   0 ) 
     Where I 0  is the height of the well and r 0  is the bottom radius of the well. This well shape gives the resistance curve of  FIG. 2 . 
     According to still a different embodiment, the well preferably has a cross section which area increases linearly with the distance from the bottom of the well. Therefore, the difference between the area of the cross-section of the well and that of the bead is proportional to the distance of the bead to the bottom of the well. 
     Thus, the resistance of the well decreases linearly with the increase of the distance between the bead and the bottom of the well. 
     For instance, as shown in  FIG. 1 c   , the well may have an oblong cross section, formed, at the bottom  110  of the well, by a circular cross-section, which increases linearly with z along a direction orthogonal to the X-X axis of the well. 
     It is thus even easier to infer the position of the bead from a measure of the resistance of the well. In particular, the curve shown in  FIG. 2  showing the resistance of the well according to the position of the bead for a well which geometry is shown in  FIG. 1 a    becomes a straight line. 
     Impedance Sensor 
     With reference to  FIG. 3 , the device  1  further comprises a sensor  50  which is adapted to measure an impedance of the well  11  and infer, from this measurement, a distance z between the bead  20  and the bottom surface  110  of the well  11 . 
     The sensor  50  comprises a main electrode  51 , which is able to set the electrically conductive solution  40  to a reference voltage V 0 . As shown in  FIG. 1 a   , the main electrode  51  is in contact with the electrically conductive solution  40 . Preferably, the main electrode  51  can lie on the top surface of the plate  10 . As the conductive solution extends over said top surface, it is in contact with the main electrode  51 . The main electrode  51  is of course connected to a voltage source (not shown), preferably a DC voltage source. 
     The voltage V 0  preferably is a constant voltage. It is preferably below 0.5 V to avoid electrolysis phenomenon inside the well. Preferably, the voltage V 0  is comprised between 10 and 500 mV, even preferably comprised between 50 and 300 mV. For instance, the voltage V 0  may be equal to 0.25V. 
     Moreover, the sensor  50  comprises at least one secondary electrode  52 . More specifically, the sensor  50  comprises secondary electrodes  52  in equal number to the number of wells  11 , being also the number of beads  20 . Each secondary electrode  52  may be a gold or platinum electrode. 
     The secondary electrode  52  forms the bottom  110  of the well and has a surface  520  forming the surface to which the molecule is anchored. 
     On  FIG. 3 , the columns of conductive solution  40  with variable positions of the respective beads form resistances which are equivalent to resistors R 1 , R 2 , R 3 . Each resistor has one pole in contact with the main electrode  51 , and the other pole in contact with the secondary electrode  52  at the bottom of the well. 
     The sensor  50  further comprises an electronic circuit  53  which measures the resistance of the well by measuring the current flowing through the well. To this end, according to one embodiment, the electronic may comprise a determined resistance connected in series with the secondary impedance. A potential difference at the poles of the resistance may be measured to infer the current flowing through the resistance. 
     However this embodiment may not be precise because the currents flowing in each well are very low. The electronic circuit  53  therefore preferably comprises an amplification of the current flowing in the wells. 
     To this end, the electronic circuit  53  preferably comprises one current to voltage amplifier  530  for each secondary electrode. 
     This current to voltage amplifier comprises an operational amplifier which inverting input is connected to the secondary electrode  52 , non-inverting input is connected to the ground, and output is connected to the inverting input through a feedback resistance of known value. 
     The current i 1  flowing between the main electrode  51  and one secondary electrode  52  is given by the following equation: 
     
       
         
           
             
               i 
               i 
             
             = 
             
               
                 
                   V 
                   - 
                 
                 - 
                 
                   V 
                   0 
                 
               
               
                 R 
                 i 
               
             
           
         
       
     
     Where V− is the potential of the inverting input of the operational amplifier, and R i  the resistance value of the i th  well. 
     As the two inputs are theoretically at the same potential: V − =0V and hence: 
     
       
         
           
             
               i 
               i 
             
             = 
             
               
                 - 
                 
                   V 
                   0 
                 
               
               
                 R 
                 i 
               
             
           
         
       
     
     Moreover, in the feedback loop, the current i i  can be expressed by the following equation: 
     
       
         
           
             
               i 
               l 
             
             = 
             
               
                 
                   
                     V 
                     - 
                   
                   - 
                   
                     V 
                     i 
                   
                 
                 
                   R 
                   l 
                 
               
               = 
               
                 
                   - 
                   
                     V 
                     i 
                   
                 
                 
                   R 
                   l 
                 
               
             
           
         
       
     
     Where R i  is the value of the resistance of the feedback loop and V i  the output potential of the current to voltage amplifier  530 . 
     Hence, by measuring the potential V i , one can obtain the value of the current ii flowing inside the i th  well by this equation: 
     
       
         
           
             
               i 
               i 
             
             = 
             
               
                 - 
                 
                   i 
                   l 
                 
               
               = 
               
                 
                   V 
                   i 
                 
                 
                   R 
                   l 
                 
               
             
           
         
       
     
     Moreover, one can then obtain the value of the resistance of the well: 
     
       
         
           
             
               R 
               i 
             
             = 
             
               
                 - 
                 
                   
                     V 
                     0 
                   
                   
                     V 
                     i 
                   
                 
               
                
               
                 R 
                 l 
               
             
           
         
       
     
     The electronic circuit  53  therefore further comprises a device  531  adapted to measure the potential V i , for instance a voltmeter, as well as a computing circuit  532 , comprising for instance a processor, controlling the acquisition measurements by the device  531 , and inferring the value of the resistance from the measured potential. The processor is preferably adapted to run a dedicated program comprising a set of instructions for controlling the sensor (in particular the measurement device  531 ), inferring the value of the resistance of each well as well as the corresponding distance of each bead from the bottom of the respective well. 
     Preferably, the measurements are achieved over the whole device at a measuring sampling rate of about 50 Hz (measurement performed in parallel at the sampling rate over all the wells of the device). The measured potential should be integrated or averaged between two measurements to reduce noise or suppress parasite signal. In that respect the sampling rate may be a multiple of the main power frequency. 
     Detailed Implementation Example 
     An implementation example is detailed with reference to the attached drawings.  FIG. 1 a    illustrates the case of a single microwell  11  whose profile is described by the equation: 
         z=I   0  tan  h 2( r−r   0 ) 
     With I 0 =8 μm being the height of the well and r 0 =2 μm being the bottom radius of the well. The well is partially obstructed by a bead of diameter d=1.5 μm. 
     This well is filled with an aqueous solution of sodium chloride at a concentration of 100 mol/m3, corresponding to a conductivity σ=10 −2  S/cm. 
     For a voltage across the well V, the current in the well is I=V/R, with R the equivalent resistance of the well filled with solution  40  and partially obstructed by the bead. Said resistance is shown in  FIG. 2  as a function of the distance of the bead&#39;s centre from the bottom of the well. One can see that by measuring the resistance of the well, the position of the bead can be inferred by reporting the resistance value on this curve and determining the corresponding distance. 
     The electronic current noise measured over a bandwidth Δf can be due to two sources:
         Johnson noise, also called thermal noise, being the electronic noise generated by the thermal agitation of the electrons, this noise being expressed by the following equation:       

         inJ =√{square root over (4 k   B   TΔfR )}
         Where T is the ambient temperature, kB is the Boltzmann constant, and   Shot-noise (associated with the stochastic flow of elementary electron charges, which is expressed by the following equation:       

         inS =√{square root over (2 IeΔf )}
 
     Where I is the signal current in the well and e the elementary charge carried by an electron (in absolute value). 
     One can see that, for a voltage V superior to 2k b T/e, that is about 50 mV at ambient temperature of about 293K, the Johnson noise is dominated by the shot noise associated to the signal current. Moreover, said shot noise increases with the resistance of the well. Thus in order to keep the signal to noise as high as possible the size of the well may not be too small; for instance a well having dimensions of the order of magnitude of 1 nm (“nanowell”) would provide results too noisy to be exploited. 
     Back to this example, the voltage applied by the main electrode  51  equals 0.25V. According to  FIG. 2 , the maximal resistance of the well for the electrolyte solution of this example is about 2500 kΩ. For such a voltage the current is I=V/R=100 nA and the current noise is inS≅2 pA (for a bandwidth Δf=100 Hz). 
     The change of impedance ΔR over a distance δI=5 μm is about 300 kΩ, which means that to resolve a change of distance of about 1 nm (which corresponds to a single base-pair in a double stranded DNA molecule of 2500 base pairs), one has to be able to resolve a change in impedance of δR=0.06 kΩ. This corresponds to a change in current δI=I. δR/R=2.5 pA, which corresponds to a signal/noise ratio of about 1. 
     Averaging over Δf=1 Hz allows yielding single base pair detection with a comfortable signal/noise ratio of 10. 
     Disposition with a Plurality of Wells 
     As indicated above, the device  1  disclosed hereinabove preferably comprises a plurality of wells  11  and an equal number of beads  20 , the wells being packed at high density on a chip, for instance at a density comprised between 10 5  and 10 8  wells/cm 2 , for instance if each microwell device has a largest cross-section (at the top surface of the plate) of 5×5 microns, about 4·10 6  wells per square centimeter should be obtained. 
     In that case the sensor  50  is easily multiplexed by providing an equal number of secondary electrodes  52 , and current to voltage amplifiers  530 , in order to allow parallel measurement of the conductivity of the wells, in a similar manner to a CMOS camera comprising a plurality of pixels, and measuring the photo-current of each pixel. 
     The computing unit  532  is however common to all the wells  11 . 
     To keep the computational requirements of the computing unit  532  as low as possible, all the beads and all the wells are preferably identical, with the tolerance given by the manufacturing processes. Furthermore, a single main electrode  51  is common to all the wells  11  and applies the same voltage V 0  to all the wells, since it is in contact with the electrically conductive solution  40  which fills all the wells and extends over the top surface  101  of the plate  10 . 
     Furthermore, in the case of many wells  11  arranged in a common plate  10 , the actuator  30  may preferably comprise a pair of main macroscopic permanent magnets  31 , which is displaceable in translation along the X-X axis, and covers substantially all the surface of the plate  10  in order to generate a homogenous magnetic field. 
     The actuator  30  further comprises a plurality of bars  32  made of magnetic material, such as for instance an alloy of iron and nickel known under the trade name of Permalloy®, that are fixed on the plate  10 . 
     As shown on  FIGS. 4 a  and 4 b   , each bar  32  is positioned on the plate  10 , on the top surface  101  thereof. Each bar  32  extends between two adjacent wells in order for a polar end  320  of a bar  32  to be flush with an edge of the well. Therefore the bars  32  are very close to the beads  20 . One understands that when wells are packed at high-density in a plate  10 , the distance between two adjacent wells is of the order of magnitudes of some μm or some tens of μm. Therefore the magnetic bars  32  have lengths of the same order of magnitude. 
     According to the example shown in  FIG. 5 a   , the magnetic bars  32  for instance have a length of 8 μm, a width of 1 μm and are spaced apart by a distance of 2 μm. 
     Each bar  32  of magnetic material is magnetized under the influence of the magnetic field produced by the main magnets  31 . Therefore the displacement of the main magnets  31  to bring them closer or further from the plate  10  increases or reduces the magnetic field density between the bars, which in turn increases or reduces the force exerted on the beads. 
     This embodiment is particularly preferred because the magnetized bars  32  are smaller than the main magnets  31  and are closer to the beads  20 . By diminishing the size of the magnets, the gradient of magnetic field in the vicinity of the beads  20  increases, and hence the magnetic force applied on the bead increases. 
     Indeed, simulations reproduced in  FIGS. 5 a  and 5 b    have shown that, when the bars  32  of magnetic material are exposed to a homogeneous magnetic field of 0.088 T generated by the main magnets  31 , the field is enhanced between the bars  32 , which produce a significant field gradient along the X-X axis. As visible in  FIG. 5 b   , the maximum value of this field gradient is about 0.09 T/μm, that is to say 90·10 3  T/m. 
     The magnetic force applied of the beads depends on this field gradient. In particular, the maximum magnetic force (in N) applicable on a bead can be expressed as follows: 
     
       
         
           
             
               F 
               M 
             
             = 
             
               
                 m 
                 s 
               
               . 
               
                 
                   d 
                    
                   B 
                 
                 
                   d 
                    
                   z 
                 
               
             
           
         
       
     
     Where m s  is the saturation value of the magnetic moment of the bead, in A·m 2  and dB/dz is the gradient of the magnetic field, in T/m. 
     Here are the saturation values of the magnetic moments of the beads according to the examples given above:
         MyOne by Invitrogen: 13.2·10 −15  A·m 2      M270 by Invitrogen: 57.5·10 −15  A·m 2      M450 by Invitrogen: 238.5·10 −15  A·m 2      Ademtech 500: 4.7·10 −15  A·m 2      Ademtech 300: 1.017·10 −15  A·m 2          

     Therefore the magnetic force applied on the bead can for instance exceed 1000 pN for a MyOne type bead with the configuration shown in  FIG. 5 . 
     By contrast, without the magnetized bars  32  interposed between the plate  10  and the main magnets  31 , the maximum field gradient resulting from the magnets is about 1.66·10 3  T/m. This maximum field gradient gives a maximum force exerted on a bead of about 22 pN for a MyOne type bead. Thus the presence of the magnetized bars allows obtaining a magnetic force on the beads which is about 55 times greater than without the magnetized bars  32 . 
     This increase of magnetic force applied to the beads  20  makes it possible to reduce the size of the beads, while maintaining a force at least equivalent, or even greater to, that which was applied on the beads by the magnets shown in  FIG. 1   a.    
     The reduction of the size of the beads in turn allows reducing the size of the wells, and hence increasing the density of the wells and the device throughput. Therefore, the number of molecules which can be simultaneously analysed can reach 5×10 7  molecules/cm 2  (assuming a bead diameter of 300 nanometres). 
     As shown in  FIGS. 4 a  and 4 b   , wells can be packed in the plate  10  according to various configurations to reach various densities. For instance in  FIG. 4 a   , a lower density of wells is shown than in  FIG. 4 b   . In the latter, the wells  11  extends in one dimension orthogonal to the X-X axis, which allows a more linear conductivity with the position of the bead and also allows reducing the overall unit size, since the cross-section of a well only grows in one direction. 
     Process for Analysing Nucleic Acid Molecules 
     With reference to  FIG. 6 , the main steps of a process for analysing nucleic acid molecules such as DNA or RNA molecules are shown. The analysis for example comprises the determination of a nucleic acid sequence, i.e. the deciphering of the actual succession of bases in a nucleic acid, but also the determination of other pieces of information on the nucleic acid sequence, such as the detection of a particular sequence in a nucleic acid molecule, the detection of a difference between the sequences of two different nucleic acid molecules, or the binding of a protein to a specific sequence, see e.g. WO 2011/147931; WO 2011/147929; WO 2013/093005; WO 2014/114687. 
     As previously indicated, the analysed molecules M are of hairpin type. In a hairpin molecule, the ends of the two strands which are not engaged in the loop are attached respectively to the bead and the bottom of the well and can thus be pulled apart upon motion of the bead. It is even possible to completely open a hairpin double-stranded nucleic acid molecule by pulling on each end of the molecule which a determined force. 
     The process will be described for a single bead in a single well but is applicable to any number of wells. 
     This process comprises a first step  100  of anchoring a bead to the bottom of a well via a nucleic acid molecule M, that is to say obtaining a first end of the molecule M anchored to a bead  20  and the second end of the molecule anchored to the bottom of a well  11 . 
     The above-disclosed structure of the device allows performing this step according to a preferred embodiment, especially when the device comprises a large number of wells packed at high density in a plate. According to this embodiment, the step  100  comprises a first sub-step  110  of positioning the beads, with nucleic acid molecules attached thereto, are placed in the device, in the solution  40  above the wells  11 . 
     A second sub-step  120  then comprises applying a potential difference between the main electrode  51  and the secondary electrodes to drive the beads towards the electrode  52  at the bottom of the well by electrophoresis. The potential difference applied between the electrodes is about a few V/cm. For more information about electrophoretic mobility of beads, see B. Xiong, A. Pallandre, I. le Potier, P. Audebert, E. Fattal, N. Tsapis, G. Barratt and M. Taverna, “Electrophoretic mobility measurement by laser Doppler velocimetry and capillary electrophoresis of micrometric fluorescent polystyrene beads”, in Analytical. Methods, 2012, 4, 183. 
     Upon contacting the bottom of the well, at least one bead binds with the latter. Preferably, each well is designed such as to allow only one bead to bind with its bottom, i.e. the cross-section of the bottom of the well is smaller than twice the cross-section of a bead. 
     Then the anchoring step  100  comprises a third sub-step  130  of reversing the voltage between the electrodes. Thereby, all the unbound beads are driven out of the well. 
     These sub-steps  120  and  130  can be performed repeatedly in order to maximize the overall loading efficiency of the wells. 
     Last, a sub-step  140  of measuring the conductivity of at least one well can be implemented to determine the presence or absence of a bounded bead in the well. 
     The process then comprises at least one step  200  of actuating the bead  20  by the actuator  30  to change a distance between the bead  20  and the bottom  110  of the well  11 , thereby applying a tension on both ends of the molecules, and at least one step  300  of measuring a distance between the bead and the bottom of the well. This distance is measured by measuring the resistance of the well  11 . As already detailed with reference to  FIG. 3 , this measurement is itself carried out by measuring the output voltage of the current to voltage amplifier  530 . 
     The process may be implemented in many different manners, but it preferably comprises a sequence of a plurality of steps  200  of actuating the bead at different distances from the bottom of the well. 
     As shown on  FIG. 6 , the step  300  of measuring the distance between the bead  20  and the bottom  110  of the well  11  is in that case preferably implemented continuously during all the sequence of actuating steps  200 . 
     Alternatively, the measurement step  300  may be carried out simultaneously with each actuating step. 
     As a non-limiting example, the process can for instance be carried out according to the sequence disclosed in document EP2390351, to which one can refer for more details about the implementation of the sequence. This sequence comprises:
         a first couple of steps  200  and  300 , during which the bead is actuated to separate the two strands of the hairpin molecule M, by applying a tension about 15 pN or more on the molecule, for instance equal to 18 pN. The distance between the bead and the bottom  110  of the well  11  is measured, which corresponds to the total length of the opened hairpin molecule.   a step  400  of hybridizing a piece of single-stranded nucleic acid molecule with one of the strands of the molecule M,   a second couple of steps  200  and  300  a step of actuating  200  the bead to release the tension applied to the molecule. The nucleic acid molecule M then rezips to reform a hairpin.
 
However, the presence of a single-stranded nucleic acid molecule hybridized to one of the nucleic acid strands at step  400  leads to a pause in re-hybridization (or rezipping) of the hairpin. The detection of such a pause indicates that the single-stranded nucleic acid molecule comprises a sequence which is complementary to a part of the hairpin molecule M. Moreover the continuing measurement of the length of the molecule during the re-hybridization of the hairpin, including the measurement of the length of the molecule during the pause when the hairpin molecule is partly re-hybridized, allows determining the position of the said sequence in the molecule. Indeed, the comparison between the length of the molecule at the pause and the total length of the molecule allows inferring the exact position of the hybridized nucleic acid molecule, from which the sequence of the molecule M at said position can be deduced.
       

     According to another non-limiting embodiment, the process may be carried out according to a sequence disclosed in document EP 2 390 350, to which one can refer for more implementation details. 
     The above-disclosed device and process show key improvements with regards to the optical detection system known in the prior art. 
     First, the position of each bead is deduced directly from changes in impedance of a simple well structure. This avoids the need for complex optical components. 
     Second, as previously explained, the beads as well as the wells, can be reduced in size. It is therefore possible to pack the wells at high density on a single chip and then read the impedances of the wells in parallel. 
     The above apparatus also have reduced computational requirements, because the relationship between the current in a well and the position of the bead needs little computation to calculate. 
     Last, as shown with reference to  FIGS. 4 a  and 4 b   , it is now possible to directly incorporate onto the plate the magnetic material allowing actuation of the beads. This results in much greater forces exerted on the beads and therefore possible diminution of the size of the beads and wells.