Patent Publication Number: US-2017370883-A1

Title: Multi-spot contaminant detection with magnetostrictive sensors

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority to U.S. Provisional Application Ser. No. 62/353,935, filed Jun. 23, 2016, the entire disclosure of which is hereby incorporated by reference. 
    
    
     BACKGROUND 
     Foodborne illnesses are primarily caused by food contaminated with pathogenic microorganisms in the field or during food processing under unsanitary conditions. Hence, surveillance of bacterial contamination of fresh produce through the food supply chain is of great importance to the food industry. However, such surveillance is a challenge since the food supply chain is a lengthy trail with many opportunities to cause food contamination. Food products may be cleaned at the harvesting site, transported to a warehouse, re-cleaned, and repackaged several times before reaching retail outlets. 
     Typical microbiological methods for pathogen detection, such as colony counting, immunoassay, and polymerase chain reaction (PCR), offer very high sensitivities. However, they require pre-analytical sample preparation, which generally includes sample collecting, separating target pathogen cells from food, increasing cell concentration, and achieving analysis volume from bulk samples before detection. These processes are time consuming, resulting in delays in obtaining the screening results. Also, only small samples (for example, 1 mL samples) may be evaluated for pathogens. More importantly, food samples have to be delivered to laboratories for culture preparation and analysis. Label-free biosensors are available in today&#39;s market. However, they also require sample preparation prior to the actual testing (i.e. sampling from fresh produce, filtration and purification of the collected samples, and injection of the filtered/purified samples into a flow system where a biosensor resides). Due to the complexity of these test procedures and the requirements of expensive equipment and highly trained personnel, current food safety controls mainly rely on control of worker/environment hygiene in the food processing industry, rather than the direct pathogen detection. 
     Free-standing phage-based magnetoelastic biosensors have been investigated as a label-free wireless biosensor system for real-time pathogen detection. The magnetoelastic biosensor is typically composed of a magnetoelastic resonator that is coated with a bio-molecular recognition element that binds specifically with a target pathogen. Once the biosensor comes into contact with the target pathogen, binding occurs, causing an increase in the mass of the resonator resulting in a decrease in the resonant frequency of the sensor (as well as other characteristic frequencies of the sensor). 
     SUMMARY 
     According to one aspect, a method for contaminant detection includes distributing a plurality of magnetostrictive sensors on a top surface of a nonmagnetic index plate, wherein the index plate has an array of wells formed in the top surface, wherein each well is sized to receive a magnetostrictive sensor; placing a magnetic backing plate below the index plate in response to distributing the plurality of magnetostrictive sensors; inverting the index plate and the magnetic backing plate in response to placing the magnetic backing plate below the index plate; and placing the index plate on a sample surface in response to inverting the index plate and the magnetic backing plate. 
     In some embodiments, the sample surface may include two-dimensional food. In some embodiments, the method may further include pressing the index plate downward on the sample surface. In some embodiments, the method may further include applying a uniform magnetic field to the magnetostrictive sensors in response to distributing the plurality of magnetostrictive sensors on the index plate; and vibrating the index plate in response to applying the uniform magnetic field. 
     In some embodiments, the method may further include placing the index plate and the magnetic backing plate on a nonmagnetic cover plate in response to placing the index plate on the sample surface, wherein the cover plate is positioned above a first sensor coil; removing the magnetic backing plate from the index plate in response to placing the index plate on the nonmagnetic cover plate; removing the index plate from the nonmagnetic cover plate in response to removing the magnetic backing plate; applying a varying magnetic field, using the first sensor coil, to a first magnetostrictive sensor positioned on the nonmagnetic cover plate in response to removing the index plate; and detecting a frequency response of the first magnetostrictive sensor using the first sensor coil while applying the varying magnetic field. In some embodiments, the method may further include determining whether a microorganism is present based on the frequency response of the first magnetostrictive sensor, wherein the first magnetostrictive sensor comprises a biorecognition element. 
     In some embodiments, the method may further include positioning the first sensor coil beneath the first magnetostrictive sensor, wherein applying the varying magnetic field comprises applying the varying magnetic field in response to positioning the first sensor coil. In some embodiments, the method may further include positioning the first sensor coil beneath a second magnetostrictive sensor positioned on the nonmagnetic cover plate in response to detecting a frequency response of the first magnetostrictive sensor; applying a varying magnetic field, using the first sensor coil, to the second magnetostrictive sensor in response to positioning the first sensor coil; and detecting a frequency response of the second magnetostrictive sensor using the first sensor coil while applying the varying magnetic field. 
     In some embodiments, the cover plate may be positioned above an array of sensor coils that includes the first sensor coil, and wherein each sensor coil of the array of sensor coils is aligned with a magnetostrictive sensor. In some embodiments, the method may further include selecting the first sensor coil from the array of sensor coils with a low-loss radio-frequency switch, wherein applying the varying magnetic field comprises applying the varying magnetic field in response to selecting the first sensor coil. In some embodiments, the radio-frequency switch may include a solid-state semiconductor switch. In some embodiments, the array of sensor coils may be a two-dimensional array, and selecting the first sensor coil may further include selecting the first sensor coil with a second low-loss radio frequency switch. 
     According to another aspect, a system for contaminant detection may include a nonmagnetic cover plate and an array of sensor coils positioned below the nonmagnetic cover plate. The system may further include a controller coupled to the array of sensor coils. The controller is to select a first sensor coil of the array of sensor coils, wherein the first sensor coil is positioned below a first magnetostrictive sensor that is positioned on top of the nonmagnetic cover plate; apply a varying magnetic field with the first sensor coil to the first mangetostrictive sensor in response to selection of the first sensor coil; and detect a frequency response of the first magnetostrictive sensor with the first sensor coil during application of the varying magnetic field. In some embodiments, the controller may be further to determine whether a microorganism is present based on the frequency response of the first magnetostrictive sensor, wherein the first magnetostrictive sensor comprises a biorecognition element. 
     In some embodiments, the controller may be further to select a second sensor coil of the array of sensor coils in response to detection of the frequency response of the first magnetostrictive sensor, wherein the second sensor coil is positioned below a second magnetostrictive sensor that is positioned on top of the nonmagnetic cover plate; apply a varying magnetic field with the second sensor coil to the second mangetostrictive sensor in response to selection of the second sensor coil; and detect a frequency response of the second magnetostrictive sensor with the second sensor coil during application of the varying magnetic field. 
     In some embodiments, the array of sensor coils may be a linear array. In some embodiments, the array of sensor coils may be a two-dimensional array. In some embodiments, to select the first sensor coil may include to select the first sensor coil with a low-loss radio-frequency switch. In some embodiments, the radio-frequency switch may include a solid-state semiconductor switch. 
     In some embodiments, the array of sensor coils may be a two-dimensional array, and to select the first sensor coil may include to select the first sensor coil with a first low-loss radio-frequency switch and a second low-loss radio-frequency switch. In some embodiments, each of the first switch and the second switch may include a single-pole, multiple throw switch, each sensor coil of the array of sensor coils may include a first terminal and a second terminal. The two-dimensional array of sensor coils may include a plurality of rows and a plurality of columns, wherein each first terminal of the sensor coils in a row is coupled to a corresponding output terminal of the first switch, and wherein each second terminal of the sensor coils in a column is coupled to a corresponding output terminal of the second switch. To select the first sensor coil may include to select a row that includes the first sensor coil with the first switch and a column that includes the first sensor coil with the second switch. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The concepts described in the present disclosure are illustrated by way of example and not by way of limitation in the accompanying figures. For simplicity and clarity of illustration, elements illustrated in the figures are not necessarily drawn to scale. For example, the dimensions of some elements may be exaggerated relative to other elements for clarity. Further, where considered appropriate, reference labels have been repeated among the figures to indicate corresponding or analogous elements. The detailed description particularly refers to the accompanying figures in which: 
         FIG. 1  is a simplified schematic diagram of a magnetostrictive sensor assembly; 
         FIG. 2  is another simplified schematic diagram of the magnetostrictive sensor assembly of  FIG. 1 ; 
         FIG. 3  is a simplified cross-sectional diagram of a magnetostrictive sensor of the magnetostrictive sensor assembly of  FIGS. 1 and 2 ; 
         FIG. 4  is a simplified schematic diagram of a system for detecting pathogens that includes the magnetostrictive sensor assembly of  FIGS. 1-3 ; 
         FIG. 5  is a simplified top view of the system of  FIG. 4 ; 
         FIG. 6  is a simplified top view of another embodiment of the system of  FIG. 4 ; 
         FIG. 7  is a simplified schematic diagram of a sensor coil array of the system of  FIGS. 4-6 ; and 
         FIGS. 8A and 8B  are a simplified flow diagram of one embodiment of a method for pathogen detection that may be performed using the magnetostrictive sensor assembly and system of  FIGS. 1-7 . 
     
    
    
     DETAILED DESCRIPTION OF THE DRAWINGS 
     While the concepts of the present disclosure are susceptible to various modifications and alternative forms, specific exemplary embodiments thereof have been shown by way of example in the drawings and will herein be described in detail. It should be understood, however, that there is no intent to limit the concepts of the present disclosure to the particular forms disclosed, but on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the present disclosure. 
     References in the specification to “one embodiment,” “an embodiment,” “an example embodiment,” etcetera, indicate that the embodiment described may include a particular feature, structure, or characteristic, but every embodiment may not necessarily include the particular feature, structure, or characteristic. Moreover, such phrases are not necessarily referring to the same embodiment. Further, when a particular feature, structure, or characteristic is described in connection with an embodiment, it is submitted that it is within the knowledge of one skilled in the art to effect such feature, structure, or characteristic in connection with other embodiments whether or not explicitly described. 
     Referring now to  FIG. 1 , a simplified schematic diagram of one illustrative embodiment of a magnetostrictive sensor assembly  10  is shown. The magnetostrictive sensor assembly  10  includes an index plate  12 . The index plate  12  may be embodied as a non-magnetic material such as silicon or glass. The index plate  12  includes multiple wells  14  formed in the top surface of the index plate  12 . The wells  14  may be rectangular in shape and may be arranged on the index plate  12  in a regular pattern, such as a rectangular grid array. The wells  14  may be etched into the top surface of the index plate  12 . Each of the wells  14  is sized to receive a magnetostrictive sensor  16 . Because the magnetostrictive sensors  16  are also rectangular, the wells  14  hold the magnetostrictive sensors  16  in a predetermined orientation, which may improve wireless detection of microorganisms or other pathogens, as described further below. As described further below, the magnetostrictive sensor assembly  10  may be used to measure many spots on a sample surface for contamination at once. Additionally, the spacing of the wells  14  may ensure that placement of the sensors  16  may be reliably replicated. 
     The magnetostrictive sensors  16  are small devices made of a magnetostrictive and/or magnetoelastic material that may be coated with a biorecognition element that binds to a particular target particle, such as a pathogen. For example, the biorecognition element may include antibodies or genetically engineered phages that bind to particular bacteria, such as  Salmonella Typhimurium . The magnetostrictive material converts magnetic energy to mechanical energy and vice versa. In other words, magnetostrictive materials generate mechanical strain when the magnetic energy is applied and generate magnetic energy in response to mechanical strain. Throughout this disclosure, the terms magnetostrictive material and magnetoelastic material may be used interchangeably. In the illustrative embodiment, the magnetostrictive sensors  16  are embodied as thin strips of material that may be actuated into resonance by application of a varying magnetic field. The magnetostrictive sensors  16  are illustratively rectangular in shape; however, in other embodiments, any elongated shape may be used. Upon contact with the specific target pathogen, the pathogen binds with the biorecognition element and increases the mass of the magnetostrictive sensor  16 . This additional mass causes the characteristic frequency of the magnetostrictive sensors  16  to decrease. As described further below, the characteristic frequency may be measured by a sensor coil, allowing quantitative detection and characterization of the pathogen. One embodiment of a magnetostrictive sensor  16  is further described below in connection with  FIG. 3 . 
     The magnetostrictive sensor assembly  10  further includes a magnetic backing plate  18  positioned below the index plate  12 . The magnetic backing plate  18  may be embodied as a magnetic glass sheet, a permanent magnet, or other magnetic material. The magnetic backing plate  18  generates a magnetic field that holds the magnetostrictive sensors  16  against the index plate  12 , within the wells  14 . 
     Referring now to  FIG. 2 , the magnetostrictive sensor assembly  10  may be inverted so that the magnetic backing plate  18  is positioned above the index plate  12 . While inverted, the magnetic field generated by the magnetic backing plate  18  continues to hold the sensors  16  against the index plate  12  within the wells  14 , overcoming the force of gravity pulling the sensors  16  downward. 
     As shown in  FIG. 2 , the magnetostrictive sensor assembly  10  may be placed onto a surface to be tested for contaminants, which is illustratively a two-dimensional food  20 . The two-dimensional food  20  may be embodied as fresh leaves (e.g., lettuce leaves or other fresh vegetable leaves) or any other substantially planar food item. When placed on the two-dimensional food  20 , the magnetostrictive sensor assembly  10  may cause the two-dimensional food  20  to flatten and contact the sensors  16 . The magnetostrictive sensor assembly  10  may also be pressed down against the two-dimensional food  20  with an external force to cause the sensors  16  to physically contact the surface of the two-dimensional food  20 . Of course, although illustrated as two-dimensional food  20 , it should be understood that the magnetostrictive sensor assembly  10  may be used to simultaneously test multiple spot locations on other types of sample surfaces. 
     Referring now to  FIG. 3 , a simplified schematic diagram of one illustrative embodiment of a magnetostrictive sensor  16  of the magnetostrictive sensor assembly  10  is shown. The sensor  16  includes a body  22  coated with an immobilized biorecognition element  24 . The body  22  is made from a magnetoelastic material, such as a magnetostrictive alloy. In one illustrative embodiment, the body  22  may be mechanically polished and cut (diced) from a strip of METGLAS™ 2826MB, which is commercially available from Honeywell Inc., of Conway, S.C. Additionally, although illustrated as including a single side of the body  22  coated with the biorecognition element  24 , it should be understood that in some embodiments two or more sides of the body  22  may be coated with the biorecognition element  24 . 
     In the illustrative embodiment, each magnetostrictive sensor  16  has a length L, a thickness t, and a width w (not shown). For example, in some embodiments the magnetostrictive sensors  16  may be one millimeter in length, four millimeters in length, or another length. The magnetostrictive sensor  16  is in the shape of a thin strip, meaning that the length L is larger than the width w and much larger than the thickness t (i.e., L&gt;w&gt;&gt;t). Upon application of a varying magnetic field, the dimensions of the magnetostrictive sensor  16  change. Accordingly, the magnetostrictive sensor  16  mechanically vibrates in response to the varying magnetic field. In particular, due to its thin strip shape, the magnetostrictive sensor  16  vibrates mainly longitudinally; in other words, when an oscillating external magnetic field is applied, the magnetostrictive sensor  16  vibrates between the length L and a length L′. The fundamental resonant frequency of this longitudinal oscillation is given as: 
     
       
         
           
             
               
                 
                   
                     
                       f 
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                   ( 
                   1 
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     where V is the acoustic velocity of the material along its length L. Addition of a small mass (Δm&lt;&lt;M) on the magnetostrictive sensor  16  surface causes a change in the resonant frequency (Δf). This resonant frequency change is proportional to the initial frequency f 0  and the mass added (Δm) and is inversely proportional to the initial sensor mass M. Assuming the added mass is uniformly distributed on the surface of the magnetostrictive sensor  16 , the resonant frequency change may be approximated as: 
     
       
         
           
             
               
                 
                   
                     
                       Δ 
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                             f 
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                           m 
                         
                         
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                           M 
                         
                       
                     
                   
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                     . 
                   
                 
               
               
                 
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     The negative sign in Equation (2) means that the resonant frequency of the magnetostrictive sensor  16  decreases with the increase of the mass load. The additional mass load on the magnetostrictive sensor  16  can be obtained by measuring the shift in the resonant frequency (or another characteristic frequency related to the resonant frequency). 
     When the magnetostrictive sensor  16  comes into contact with a target pathogen, the biorecognition element  24  immobilized on the magnetostrictive sensor  16  surface will bind/capture the target pathogen. This adds an additional mass load on the magnetostrictive sensor  16 . As described above, this additional mass causes a drop in a characteristic frequency of the magnetostrictive sensor  16 . Therefore, the presence of any target pathogens can be identified by monitoring for a shift in the characteristic frequency of the magnetostrictive sensor  16 . Additionally or alternatively, rather than a biorecognition element  24 , the magnetostrictive sensor  16  may include a chemical layer that similarly binds with one or more contaminants such as mercury or heavy metals. 
     The simple strip-shaped configuration of the illustrative magnetostrictive sensor  16  described above may make fabrication relatively easy and/or inexpensive. Additionally, the magnetostrictive sensors  16  are passive sensors that do not require on-board power. As described above, the magnetostrictive sensor  16  may be fabricated by mechanical methods (e.g., polish and dice) or by microelectronics fabrication methods (e.g., sputter deposit, thermal deposit, or electrochemical deposit). These methods can mass-produce fabricated magnetostrictive sensors  16  with very low cost. Additional details of illustrative magnetoelastic ligand detectors are described in U.S. Pat. No. 7,759,134 (“Magnetostrictive Ligand Sensor”), the entire disclosure of which is incorporated herein by reference. 
     As described above, the biorecognition element  24  may be immobilized on the surface of each magnetostrictive sensor  16  to bind a specific target pathogen. In some embodiments, the biorecognition element  24  may be embodied as a chemical binding element or an interaction layer immobilized on the body  22  of the magnetostrictive sensor  16 . For example, the biorecognition element  24  may be a traditional antibody. Additionally or alternatively, in some embodiments, the biorecognition element  24  may be a genetically engineered bacteriophage (“phage”). The use of phages as a substitute for antibodies offers a stable, reproducible, and inexpensive alternative. In particular, phages have high affinity for binding with target pathogen cells, the phage structure is robust and stable, and phages may bind target pathogens in air with certain humidity. Additionally or alternatively, the biorecognition element  24  may be embodied as DNA, RNA, proteins, aptamers, or other biorecognition elements. Specific ligand recognition devices that may be illustratively used as the biorecognition element  24 , as well as illustrative application methods, are discussed in U.S. Pat. No. 7,138,238 (“Ligand Sensor Devices and Uses Thereof”), U.S. Pat. No. 7,267,993 (“Phage Ligand Sensor Devices and Uses Thereof”), and U.S. Pat. No. 7,670,765 (“Method of Forming Monolayers of Phage-Derived Products and Used Thereof”), the entire disclosures of which are incorporated herein by reference. 
     Referring now to  FIG. 4 , a simplified schematic diagram of one illustrative embodiment of a system  100  for detecting pathogens is shown. The system  100  includes a magnetostrictive sensor assembly  10  that has been inverted as shown in  FIG. 2 . The magnetostrictive sensor assembly  10  is positioned on top of a non-magnetic cover plate  102 . For example, as described further below in connection with  FIGS. 8A and 8B , the magnetostrictive sensor assembly  10  may be placed on the cover plate  102  after being pressed onto a sample surface such as the two-dimensional food  20 . The cover plate  102  may be embodied as a nonmagnetic material such as glass. 
     The system  100  also includes a controller  104  coupled to one or more sensor coils  112 . The sensor coils  112  may be arranged in an array  114 , which may be embodied as a linear array, a two-dimensional array, a three-dimensional array, or another regular arrangement of sensor coils  112 . Each of the sensor coils  112  may be positioned beneath a well  14  of the index plate  12  and therefore also beneath a magnetostrictive sensor  16 . The sensor coils  112  and/or the index plate  12  may be translatable (e.g., in one, two, or three dimensions) to position a sensor coil  112  beneath a particular well  14 , as shown by the arrows  116  of  FIGS. 4 and 5 . As described further below, the controller  104  causes the sensor coil  112  to apply a varying magnetic field  118  to a magnetostrictive sensor  16 . In some embodiments, the controller  104  may also select or otherwise activate a particular sensor coil  112  of the array  114 . The controller  104  measures a magnetic field produced by the magnetostrictive sensor  16  in response to the varying magnetic field  118  using the sensor coil  112 . The controller  104  determines a frequency response of the magnetostrictive sensor  16  based on the measured magnetic field. This frequency response is related to the resonant frequency of the magnetostrictive sensor  16 , as well as other material properties (e.g., the magnetoelastic coupling coefficient) and the environment (e.g., friction forces or damping effects). As described further below, microorganisms or other pathogens bound to the magnetostrictive sensors  16  may cause an increase in the mass of the magnetostrictive sensors  16  and a corresponding decrease in the frequency response. The system  100  may determine whether microorganisms or other pathogens are present by determining whether the frequency response shifts. Multiple types of pathogens may be detected simultaneously by using separate groups of magnetostrictive sensors  16 , with each group of magnetostrictive sensors  16  binding to a different type of microorganism or other pathogen. 
     As described briefly above, the system  100  includes the controller  104 . The controller  104  is responsible for activating or energizing electronically-controlled components of the system  100 , including the sensor coil  112 . The controller  104  is also responsible for interpreting electrical signals received from components of the system  100 , including the sensor coil  112 . To do so, the controller  104  may include a number of electronic components commonly associated with units utilized in the control of electronic and electromechanical systems. For example, the controller  104  may include, amongst other components customarily included in such devices, a processor  106  and a memory device  108 . The processor  106  may be any type of device capable of executing software or firmware, such as a microcontroller, microprocessor, digital signal processor, or the like. The memory device  108  may be embodied as one or more non-transitory, machine-readable media. The memory device  108  is provided to store, amongst other things, instructions in the form of, for example, a software routine (or routines) which, when executed by the processor  106 , allows the controller  104  to perform sensor interrogation and pathogen detection using the other components of the system  100 . 
     The controller  104  also includes an analog interface circuit  110 , which may be embodied as any electrical circuit(s), component, or collection of components capable of performing the functions described herein. The analog interface circuit  110  converts output signals (e.g., from the sensor coil  112 ) into signals which are suitable for presentation to an input of the processor  106 . In particular, the analog interface circuit  110 , by use of a network analyzer, an analog-to-digital (A/D) converter, or the like, converts analog signals into digital signals for use by the processor  106 . Similarly, the analog interface circuit  110  converts signals from the processor  106  into output signals which are suitable for presentation to the electrically-controlled components associated with system  100  (e.g., the sensor coil  112 ). In particular, the analog interface circuit  110 , by use of a variable-frequency signal generator, digital-to-analog (D/A) converter, or the like, converts digital signals generated by the processor  106  into analog signals for use by the electronically-controlled components associated with the system  100 . It is contemplated that, in some embodiments, the analog interface circuit  110  (or portions thereof) may be integrated into the processor  106 . 
     As also mentioned above, the controller  104  is coupled to the sensor coil  112 . In the illustrative embodiment, the sensor coil  112  is used as an energizing excitation source for the magnetostrictive sensors  16  and as a detector of signals received from the magnetostrictive sensors  16 . In some embodiments, the sensor coil  112  may be a solenoid with loops having a generally rectangular cross-section. In some embodiments, the sensor coil  112  may be embodied as a flat coil as described in U.S. Patent Publication No. 2014/0120524 (“In-Situ Pathogen Detection Using Magnetoelastic Sensors”), the entire disclosure of which is incorporated herein by reference. To improve performance of the system  100 , the sensor coil  112  may be impedance-matched to the electrical circuitry of the controller  104 . Additionally, although illustrated as a single sensor coil  112 , it should be understood that in some embodiments the system  100  may include a separate drive coil and one or more pickup coils to perform the functions of the sensor coil  112 . 
     The system  100  may further include a magnetic field generator configured to generate a constant, uniform magnetic field. The uniform magnetic field extends through the wells  14 . The uniform magnetic field may bias the magnetostrictive sensors  16  during application of the varying magnetic field  118 , increasing the magnitude of the frequency response. The magnetic field generator may be embodied as any component capable of generating the uniform magnetic field, for example, a pair of permanent magnet arrays or a Helmholtz coil. 
     Referring now to  FIG. 5 , a top-view schematic diagram of the system  100  is shown. In the illustrative view of  FIG. 5 , the magnetic backing plate  18  and the index plate  12  have been removed from the cover plate  102 . Because the magnetic field generated by the magnetic backing plate  18  was removed, the sensors  16  have fallen out of the wells  14  onto the cover plate  102 . Thus, the sensors  16  remain positioned on the cover plate  102  in the same arrangement as the wells  14  of the index plate  12 , which is illustratively a rectangular grid array as shown in  FIG. 5 . As further shown in  FIG. 5 , in some embodiments, a single sensor coil  112  may be positionable below each of the sensors  16  by translating the sensor coil  112  and/or the cover plate  102 , as illustrated by the arrows  116 . 
     Referring now to  FIG. 6 , a top-view schematic diagram of another embodiment of the system  100  is shown. In the embodiment of  FIG. 6 , the controller  104  is coupled to an array  114  of sensor coils  112 . Illustratively, the array  114  is embodied as a rectangular grid array of sensor coils  112 ; however, in other embodiments the array  114  may be embodied as a linear array, a three-dimensional array, or another regular arrangement of sensor coils  112 . In some embodiments, the controller  104  may select or otherwise activate a particular sensor coil  112  using a switched array  114 . One embodiment of a switched array  114  is described further below in connection with  FIG. 7 . 
     Referring now to  FIG. 7 , a schematic diagram of an array  114  of sensor coils  112  is shown. The array  114  is illustratively a rectangular grid array of nine sensor coils  112  including three rows  120  and three columns  122 . Each of the sensor coils  112  includes two terminals  124 ,  126 . The array  114  further includes two single-pole, multi-throw switches  128 ,  130 . Each of the switches  128 ,  130  may be embodied as a low-loss, bidirectional radio frequency (RF) switch, such as a solid-state semiconductor switch. As shown, for each row  120  of sensor coils  112 , the terminal  124  of each sensor coil  112  is connected to the same output terminal of the RF switch  128 . For example, the terminal  124  of each sensor coil  112  in the row  120   a  is connected to one output terminal, the terminal  124  of each sensor coil  112  in the row  120   b  is connected to another output terminal, and so on. Similarly, for each column  122  of sensor coils  112 , the terminal  126  of each sensor coil  112  is connected to the same output terminal of the RF switch  130 . For example, the terminal  126  of each sensor coil  112  in the column  122   a  is connected to one output terminal, the terminal  126  of each sensor coil  112  in the row  122   b  is connected to another output terminal, and so on. As described further below, a particular sensor coil  112  may be selected or otherwise activated by configuring the switches  128 ,  130  to complete a circuit to the corresponding row  120  and column  122  that include the particular sensor coil  112 . Additionally, although illustrated as a two-dimensional array  114 , it should be understood that in other embodiments the array  114  may have a different number of dimensions and a corresponding number of switches (e.g., one switch for a linear array, three switches for a three-dimensional array, and so on). 
     Referring now to  FIGS. 8A and 8B , one illustrative embodiment of a method  200  that may be used for pathogen detection with the magnetostrictive sensor assembly  10  and the system  100  is shown as a simplified flow diagram. The method  200  is illustrated as a series of blocks  202 - 232 , some of which may be optionally performed in some embodiments (and, thus, are shown in dashed lines). It will be appreciated by those of skill in the art that some embodiments of the method  200  may include additional or different processes and sub-processes. 
     The method  200  begins with block  202 , in which multiple magnetostrictive sensors  16  are scattered or otherwise distributed on the index plate  12 . In block  204 , a uniform magnetic field is applied to align the magnetic sensors  16 . The magnetic field aligns with the length dimension of the wells  14  of the index plate  12 , causing the sensors  16  to align in the same direction. In block  206 , the index plate  12  is vibrated to cause the sensors  16  to fall into the wells  14 . Thus, after being vibrated, the sensors  16  are aligned in the same orientation and are positioned in a predetermined pattern by the wells  14 . For example, the sensors  16  may be positioned in a rectangular grid array. 
     In block  208 , the magnetic backing plate  18  is added to the back side of the index plate  12 . The magnetic backing plate  18  creates a magnetic field that attracts the sensors  16  and holds the sensors  16  against the index plate  12  within the wells  14 . After adding the backing plate  18 , the magnetostrictive sensor assembly  10  may be complete as illustrated in  FIG. 1 . It should be understood that in some embodiments, the magnetic backing plate  18  and the index plate  12  may have been pre-assembled prior to the distribution of the sensors  16 . 
     In block  210 , the magnetostrictive sensor assembly  10  (including the index plate  12  and the magnetic backing plate  18 ) is inverted. As described above, the magnetic field generated by the magnetic backing plate  18  holds the sensors  16  against the index plate  12  within the wells  14 , even when the magnetostrictive sensor assembly  10  is inverted. After being inverted, the openings of the wells  14  are oriented downwards, allowing access from below to the sensors  16 . 
     In block  212 , the magnetostrictive sensor assembly  10  (including the index plate  12  and the magnetic backing plate  18 ) is pressed down onto two-dimensional food  20  or another sample surface to be tested for contamination. The magnetostrictive sensor assembly  10  may be pressed against the two-dimensional food  20  as illustrated in  FIG. 2 . The magnetostrictive sensor assembly  10  may be pressed down with the force of gravity or with additional, external force. As described above, when pressed against the two-dimensional food  20 , the two-dimensional food  20  may flatten and come into contact with the magnetostrictive sensors  16 . Thus, the magnetostrictive sensors  16  may simultaneously contact multiple spots on the two-dimensional food  20  to test for contamination. 
     In block  214 , the magnetostrictive sensor assembly  10  (including the index plate  12  and the magnetic backing plate  18 ) is removed from the two-dimensional food  20  or other sample surface. In block  216 , the magnetostrictive sensor assembly  10  (including the index plate  12  and the magnetic backing plate  18 ) is placed on the non-magnetic cover plate  102  of the system  100 . As shown in  FIG. 4 , the magnetostrictive sensor assembly  10  remains in the inverted orientation when placed on the cover plate  102 . 
     In block  218 , the magnetic backing plate  18  is removed from the index plate  12 . After removing the magnetic backing plate  18 , the magnetic field is removed from the magnetostrictive sensors  16 . Thus, the magnetostrictive sensors  16  may fall onto the cover plate  102 . In block  220 , the index plate  12  is removed from the cover plate  102 . After removal of the index plate  12 , the magnetostrictive sensors  16  remain arranged on the cover plate  102  in the same arrangement as the wells  14  of the index plate  12 . For example, as shown in  FIGS. 5 and 6 , the sensors  16  may be arranged in a rectangular grid array. 
     Referring now to  FIG. 8B , in block  222  the controller  104  may select or otherwise activate a sensor coil  112  beneath a magnetostrictive sensor  16 . In some embodiments, in block  224  the controller  104  may cause the sensor coil  112  and/or the cover plate  102  to translate to position the magnetostrictive sensor  16  above the active sensor coil  112 . The controller  104  may control a translation stage or other actuator to translate the sensor coil  112  and/or the cover plate  102 . For example, in embodiments with a single sensor coil  112 , the controller  104  may sequentially position the sensor coil  112  under each of the magnetostrictive sensors  16 . As another example, in embodiments with an array  114  of multiple sensor coils  112 , the controller  104  may align the array  114  so that the sensor coils  112  are each positioned beneath a magnetostrictive sensor  16 . 
     In some embodiments, in block  226  the controller  104  may select the active sensor coil  112  in an array  114  of sensor coils  112  using one or more RF switches. For example, as shown in  FIG. 7 , the controller  104  may configure the RF switch  128  to select the row  120  that includes the selected sensor coil  112  and configure the RF switch  130  to select the column  122  that includes the selected sensor coil  112 , completing a circuit with the selected sensor coil  112 . By using RF switches to select among multiple sensor coils  112 , the system  100  may support rapid measurement of multiple sensors  16  with a single controller  104 , network analyzer, or other RF signal analyzer. 
     In block  228 , the controller  104  activates the sensor coil  112  to generate the varying magnetic field  118 . As described above, the varying magnetic field  118  causes the magnetostrictive sensors  16  to oscillate. Because the magnetostrictive sensors  16  were aligned by the wells  14  in a predetermined orientation, the longitudinal oscillation of all (or, at least, most) of the sensors  16  may be in the same direction. Thus, the magnetic flux picked up by the sensor coil  112  may thus contain frequency response information for all (or, at least, most) of the sensors  16 . The frequency of the varying magnetic field  118  may be varied through a range of frequencies. The range of frequencies may include a resonant frequency of the magnetostrictive sensors  16  when a microorganism has not been bound (i.e., when the sensors  16  are unloaded). For example, in some embodiments the range of frequencies applied by the sensor coil  112  may cover from 50% of unloaded resonant frequency to slightly more than the unloaded resonant frequency. Binding of microorganisms on the magnetostrictive sensor  16  surface is typically a small mass change, and the decrease in the characteristic frequency of the magnetostrictive sensors  16  due to this small mass change is normally less than 50% of the unloaded resonant frequency of the magnetostrictive sensor  16 . 
     In block  230 , the controller  104  measures the frequency response of the magnetostrictive sensors  16  using the sensor coil  112 , and any shift in resonant frequency of the magnetostrictive sensors  16  is determined. The controller  104  may monitor the characteristic frequency in real time or record data for later analysis. The measurement of each magnetostrictive sensor  16  may require about 2 seconds. As described above, the magnetostrictive sensors  16  include the biorecognition element  24  that will bind with microorganisms upon contact. Binding increases the mass of the magnetostrictive sensor  16 , which causes a resonant frequency of the magnetostrictive sensor  16  to decrease. Thus, a measured shift in the resonant frequency indicates that microorganisms are present on the magnetostrictive sensor  16 . In some embodiments, the sensors  16  may be preselected with the same resonant frequency, and thus the system  100  may only measure the final frequency response of the sensors  16  to determine the shift in resonant frequency. Additionally or alternatively, in some embodiments, the shift in resonant frequency may be determined by taking an initial measurement of the resonant frequency of the magnetic sensors  16  before they are put into contact with the two-dimensional food  20  as described above in connection with block  212  of  FIG. 8A  and then comparing the initial measurement to the current measurement of the frequency response. 
     In block  232 , the controller  104  determines whether additional magnetostrictive sensors  16  should be measured. For example, the controller  104  may determine whether additional magnetostrictive sensors  16  remain in a pre-programmed pattern corresponding to the wells  14  of the index plate  12 . If additional magnetostrictive sensors  16  remain to be measured, the method  200  loops back to block  222 , in which a sensor coil  112  may be selected beneath another magnetostrictive sensor  16 , for example by translating the sensor coil and/or cover plate  102  or by configuring the switched array  114 . If no further magnetostrictive sensors  16  remain to be measured, the method  200  may loop back to block  202  to perform measurement of another sample. 
     While certain illustrative embodiments have been described in detail in the figures and the foregoing description, such an illustration and description is to be considered as exemplary and not restrictive in character, it being understood that only illustrative embodiments have been shown and described and that all changes and modifications that come within the spirit of the disclosure are desired to be protected. There are a plurality of advantages of the present disclosure arising from the various features of the apparatus, systems, and methods described herein. It will be noted that alternative embodiments of the apparatus, systems, and methods of the present disclosure may not include all of the features described yet still benefit from at least some of the advantages of such features. Those of ordinary skill in the art may readily devise their own implementations of the apparatus, systems, and methods that incorporate one or more of the features of the present disclosure.