Abstract:
A method for shielding an electrical signal without substantially degrading the system speed or substantially increasing the bulk of the system is provided. The method includes applying a first signal to a conductor coupled to the electrode, applying a second signal to a shield substantially surrounding the conductor, blocking electrical interference to the first signal, and increasing an effective impedance on the electrode coupled to the conductor. The second signal may be a buffered and compensated version of the first signal.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of U.S. Provisional Application No. 61/863,400, filed on Aug. 7, 2013, which is hereby incorporated herein by reference in its entirety. 
     
    
     BACKGROUND  
       [0002]    The reference electrode on a potentiostat system is highly susceptible to electrical interference from external sources. Preventing electrical interference from external sources is especially important when the size of the reference electrode is reduced and/or when miniaturization is desired, for example, in medical diagnostic devices. Various designs use an external faraday cage and/or shielded coaxial cables to decrease interference on the reference signal. Because the reference electrode has a very high impedance, the capacitance added when shielding an electrical signal from external interference has the side effect of slowing down the potentiostat system, and thus the performance of the diagnostic device. Thus, alternative systems and methods for reducing external influences on an electrical signal and increasing impedance on a reference electrode in a potentiostat system could be beneficial for diagnostic devices. 
         [0003]    Biomarker analysis based on electronic readout has long been cited as a promising approach that would enable a new family of chip-based devices with appropriate cost and sensitivity for medical diagnostic devices (Drummond et al.,  Nat. Biotechnol.  21:1192, Katz et al.,  Electroanalysis    15:913). The sensitivity of electronic readout is in principle sufficient to allow direct detection of small numbers of analyte molecules with simple instrumentation. However, despite tremendous advances in this area as well as related fields working towards new diagnostics (Clack et al.,   Nat. Biotechnol.  26:825, Geiss et al., Nat. Biotechnol. 26:317, Hahm et al., Nano Lett. 4:51, Munge et al, Anal. Chem. 77:4662, Nicewarner-Pena et al.,  Science  294:137, Park et al,  Science  295:1503, Sinensky et al., Nat. Nano. 2:653, Steemers et al.,  Nat. Biotechnol.  18:91, Xiao et al., J Am. Chem. Soc. 129:11896, Zhang et al.,  Nat. Nano.  1:214, Zhang et al.,  Anal. Chem.  76:4093, Yi et al.,  Biosens. Bioelectron.  20:1320, Ke et al.,  Science  319:180, Armani et al., Science 317:783), current multiplexed chips have yet to achieve direct electronic detection of biomarkers in cellular and clinical samples. The challenges that have limited the implementation of such devices primarily stem from the difficulty of obtaining very low detection limits in the presence of high background noise levels present when complex biological samples are assayed, and the challenge of generating multiplexed systems that are highly sensitive and specific. Therefore, systems, methods, and devices that improve the signal to noise ratio of such detection devices is desirable. 
       SUMMARY  
       [0004]    Disclosed herein are systems, devices and methods for shielding an electrical signal without substantially degrading the system speed, or substantially increasing the bulk of the system. In one aspect, a method for transmitting a signal on a transmission line includes applying a first signal to a conductor, applying a second signal to a shield substantially surrounding the conductor, blocking electrical interference to the first signal, and increasing the effective impedance seen by an electrode coupled to the conductor, while decreasing the effects of capacitive loading. In certain implementations, the second signal is a buffered and compensated version of the first signal. The second signal may be created using compensation circuitry by measuring the first signal on the conductor, amplifying the first signal, removing at least one high frequency component from the first signal, and phase shifting the first signal. In certain implementations, the method includes substantially reducing or eliminating a potential difference between the conductor and the shield. In certain implementations, the second signal is applied by a low impedance source. 
         [0005]    In another aspect, a method for detecting a target in a sample using a point-of care diagnostic device is provided wherein the diagnostic device includes a potentiostat that provides active shielding for one or more reference electrodes in the potentiostat. In certain implmentations, the signal path comprises a reference electrode in a potentiostat. In some implementations, the signal path comprises a high impedance transducer interface. In certain implementations the device uses the methods disclosed herein (and variations thereof). 
         [0006]    In yet another aspect, a signal transmission system is provided, the signal transmission system including a transmission line including a conductor and a shield substantially surrounding the conductor, first and second compensation circuits coupled between the conductor and the shield, and a unity gain buffer coupled between the first and second compensation circuits. In sonic implementations, the transmission system includes an electrode coupled to the conductor. in certain implementations, the first compensation circuit comprises a first resistor and a first capacitor capable of removing gain at high frequencies from a first signal. In some implementations, the second compensation circuit comprises a second resistor and a second capacitor capable of phase shifting the first signal, In certain implementations, the transmission line is a coaxial cable. In certain implementations, the shield comprises a braided cylinder. Ire certain implementations, the shield comprises a faraday cage. In some implementations, a point-of-care diagnostic device is provided that includes a potentiostat that includes the signal transmission system described above, thereby providing active shielding for one or more reference electrodes in the potentiostat. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]    The foregoing and other objects and advantages will be apparent upon consideration of the following detailed description, taken in conjunction with the accompanying drawings, in which like reference characters refer to like parts throughout, and in which: 
           [0008]      FIG. 1  depicts a block diagram of an illustrative 3-electrode potentiostat system; 
           [0009]      FIG. 2  depicts a block diagram of an illustrative transmission line system; 
           [0010]      FIG. 3  depicts a circuit diagram of an illustrative compensation circuit; 
           [0011]      FIG. 4  depicts an illustrative cartridge system for receiving, preparing, and analyzing a biological sample; 
           [0012]      FIG. 5  depicts an illustrative cartridge for an analytical detection system; 
           [0013]      FIG. 6  depicts an illustrative automated testing system; 
           [0014]      FIG. 7  depicts a voltage-current curve measured without active shielding; 
           [0015]      FIG. 8  depicts a voltage-current curve measured with active shielding; 
           [0016]      FIG. 9  depicts representative electrocatalytic detection signals; 
           [0017]      FIG. 10  depicts an analysis chamber with a pathogen sensor and a host sensor; 
           [0018]      FIG. 11  depicts an analysis chamber with a pathogen sensor and a host sensor; and 
           [0019]      FIG. 12  depicts an additional embodiment of an analysis chamber. 
       
    
    
     DETAILED DESCRIPTION 
       [0020]    To provide an overall understanding of the systems, devices, and methods described herein, certain illustrative embodiments will be described. It is to be understood that the systems, devices, and methods disclosed herein, while shown for use in diagnostic systems for bacterial diseases such as Chlamydia, may be applied in other applications including, but not limited to, detection of other bacteria, viruses, fungi, prions, plant matter, animal matter, protein, RNA sequences, DNA sequences, as well as cancer screening and genetic testing, including screening for genetic traits and disorders. 
         [0021]      FIG. 1  depicts a block diagram of a 3-electrode potentiostat system  100 . The potentiostat system  100  may be used, for example, in a diagnostic device. The potentiostat system  100  comprises a voltage source  102  coupled between an auxiliary electrode  104  and a working electrode  106 , and a control circuit  108  coupled between a reference electrode  110  and the working electrode  106 . During operation, the potentiostat system may function to maintain the potential of the working electrode  106  at a pre-determined level with respect to the reference electrode  110  by adjusting the controlled voltage source  102  at the auxiliary electrode  104 . The transmission line  112  connected to the reference electrode  110  (highlighted with a dashed box  114  in  FIG. 1 ) carries a reference signal and may be shielded to prevent interference with the reference signal by outside influences. For example, the transmission line  112  may include a coaxial cable with an inner conductor substantially surrounded by an outer shield. In some embodiments, the outer shield may be braided or may include a faraday cage. The outer shield may be used to block interference with the reference signal carried on the inner conductor of the coaxial cable. In conventional designs, the outer shield may be grounded. However, when the outer shield is grounded, the capacitance created between the outer shield and the inner conductor loads the reference electrode output and may slow down detection by the diagnostic device. 
         [0022]    In this disclosure, slowing of the diagnostic system may be prevented by inhibiting a capacitance between the inner conductor and the outer shield from being charged or discharged. A suitably buffered and compensated version of the reference signal is applied to the outer shield instead of connecting the outer shield to ground potential. As a result, substantially no potential difference exists between the reference signal on the inner conductor and the outer shield. Furthermore, the impedance on the outer shield is made very low so that external influences couple to the outer shield and do not make their way to the inner conductor. Reducing the potential difference between the outer shield and the inner conductor can increase the effective impedance on the reference electrode and improve performance of the associated diagnostic device. 
         [0023]      FIG. 2  depicts a block diagram of a transmission line system  200 . The transmission line system  200  of  FIG. 2  includes a transmission line  202 . The transmission line  202  may be the same transmission line  112  highlighted by the dashed box  114  in  FIG. 1 . Transmission line  202  includes conductor  210  and shield  212 . The shield  212  substantially surrounds the conductor  210 . The transmission line system  200  of  FIG. 2  also includes a first compensation circuit  204 , a unity gain buffer  206  and a second compensation circuit  208  coupled between conductor  210  and shield  212 . In certain embodiments, the transmission line system  200  of  FIG. 2  may include a High-Z buffer (not shown) coupled between the “signal out to instrument” node and the first compensation circuit  204 . To provide a suitably buffered and compensated version of the reference signal from the conductor  210  to the outer shield  212 , the reference signal from the conductor  210  is measured. The signal is then passed through the High-Z buffer (not shown in  FIG. 2 ). As an example, the High-Z buffer may include a collection of circuit components as shown in  FIG. 3  as U 204 . Next, the first compensation circuit  204  removes the high frequency components of the signal. This prevents the circuit from oscillating due to coupling between the outer shield  212  and the inner conductor  210 . The unity gain buffer  206  amplifies the signal, and the second compensation circuit  208  shifts the phase of the signal. Phase-shifting the signal provides an additional stability margin between the second signal (applied to the shield  212 ) and the first signal (carried by the conductor  210 ). The modified signal is then applied to the shield  212 . 
         [0024]      FIG. 3  depicts a circuit diagram of an active shield circuit  300 . The REF node is the reference line coupled to the reference electrode. The reference line may be routed to the reference electrode using a coaxial cable where the outer braid of the coaxial cable is connected to the REF Shield node, As previously indicated, in a conventional design, the outer coaxial braid would be grounded. The capacitance created between the grounded braid and center conductor loads the electrode output and slows down detection. 
         [0025]    Active shield circuit  300  includes a compensation circuit that may be used to prevent the capacitance between the outer braid of the coaxial cable and the inner conductor from being charged or discharged. Resistor R 205  and capacitor C 209  may, for example, make up a first compensation circuit, such as first compensation circuit  204  of  FIG. 2  which can provide the compensation needed for stability in the system. Unity gain buffer U 202  of the circuit in  FIG. 3  may function as the unity gain buffer  206  of the transmission system of  FIG. 2 . Capacitor C 214  and resistor R 212  make up a second compensation circuit, similar to second compensation circuit  208  of  FIG. 2 . The second compensation circuit shifts the phase of the output of buffer U 204  to prevent positive feedback due to capacitive coupling between the signal on the transmission line (REF) and the signal on the shield (REF Shield). The system of compensation circuits, buffers and other circuit components of  FIG. 3  may function to modify a reference signal on a conductor in a transmission line before applying it to the shield of the transmission line. This active shield circuit  300  may have feedback paths (e.g., through the cable capacitance) that could pose the risk of causing instabilities. To prevent such instabilities, components R 205 , C 209 , C 214  and R 212  provide compensation that prevents or reduces oscillations of the circuit. In addition to making the cable capacitance invisible to the electrode, the circuit of  FIG. 3  greatly increases electromagnetic compatibility (EMC) performance by driving the outer braid of the coaxial cable from a low impedance source. As a result, external electric field interference is unable to substantially change the potential of that node. 
         [0026]      FIG. 7  depicts a voltage-current curve measured without active shielding, and  FIG. 8  depicts a voltage-current curve measured with active shielding.  FIG. 7  shows that the noise measured when an active shielding system (e.g., active shield circuit  300 ) is not in use has a RMS of over  600  femtoAmps in the voltage range of 0 to −600V.  FIG. 8  shows that the noise measured when an active shielding system (e.g., active shield circuit  300 ) is used is has a RMS of less than 30 femtoAmps in the voltage range of 0 to −600 mV. Thus, active shielding of a transmission line may significantly reduce the noise in a current signal transmitted on the transmission line. 
         [0027]    The systems, circuits, devices, and methods described above may be incorporated in a diagnostic system for detecting the presence or absence of a target marker using electrocatalytic techniques. The active shielding disclosed herein can be used to shield the reference electrode of a potentiostat that applies a voltage to an electrode to detect the presence of a target marker in a solution. Electrochemical techniques including, but not limited to cyclic voltammetry, amperometry, chronoamperometry, differential pulse voltammetry, calorimetry, and potentiometry may be used for detecting a target marker. A brief description of one of these techniques, as applied to the current system, is provided below, it being understood that the electrocatalytic techniques are illustrative and non-limiting and that other techniques can be envisaged for use with the other systems, devices and methods of the current system. Applications of electrocatalytic techniques are described in further detail in U.S. Pat. Nos. 7,361,470 and 7,741,033, and PCT Application No. PCT/US12/024015, which are hereby incorporated by reference herein in their entireties. 
         [0028]    Chart  200  of  FIG. 9  depicts representative electrocatalytic detection signals generated using a potentiostat having the active shielding circuit described above. The potentiostat, is used to apply a voltage signal at an electrode. The potentiostat may cycle or ramp the applied voltage between two points, such as from 0 mV to −300 mV and back to 0 mV, while the resultant current is measured. Accordingly, chart  200  depicts the current along the vertical axis at corresponding potentials between 0 mV and −300 mV, along the horizontal axis. Data graph  202  represents a signal measured at an electrode in the absence of a target marker. Data graph  204  represents a signal measured at an electrode in the presence of a target marker. As can be seen on data graph  204 , the signal recorded in the presence of the target molecule provides a higher amplitude current signal, particularly when comparing peak  208  with peak  206  located at approximately −100 mV. Accordingly, the presence and absence of the marker can be differentiated. However, the applied voltages are relative to the potential of the reference electrode. Therefore, if electromagnetic interference alters the voltage measured at the reference electrode, the potential applied to the electrodes, and therefore the overall measurement, could be disturbed. 
         [0029]    In certain applications, a single electrode or sensor is configured with two or more probes, arranged next to each other, or on top of or in close proximity within the chamber so as to provide target and control marker detection in an even smaller point-of-care size configuration. For example, a single electrode sensor may be coupled to two types of probes, which are configured to hybridize with two different markers. In certain approaches, a single probe is configured to hybridize and detect two markers. In certain approaches, two types of probes may be coupled to an electrode in different ratios. For example, a first probe may be present on the electrode sensor at a ratio of 2:1 to the second probe. Accordingly, the sensor is capable of providing discrete detection of multiple analytes. For example, if the first marker is present, a first discrete signal (e.g., current) magnitude would be generated, if the second marker is present, a second discrete signal magnitude would be generated, if both the first and second marker are present, a third discrete signal magnitude would be generated, and if neither marker is present, a fourth discrete signal magnitude would be generated. Similarly, additional probes could also be implemented for increased numbers of multi-target detection. 
         [0030]    In certain aspects, the sensors and electrodes described herein are integrated into a sensing or analysis chamber, for example in a point-of-care device, to analyze a sample from a biological host.  FIG. 10  depicts an analysis chamber  400  with a pathogen sensor  406  and a host sensor  410 . The chamber  400  includes walls  402  and  404  that form a space with which a sample is retained and analyzed at sensors  406  and  410 . Pathogen sensor  406  includes a conductive trace  408  to connect the sensor  406  to controlling instrumentation such as a potentiostat. Host sensor  410  is also connected to external or controlling instrumentation with a conductive trace  412 . Pathogen sensor  406  and host sensor  410  are separated by a distance X. 
         [0031]    In certain aspects, the systems, methods, and devices described herein are integrated into a sensing or analysis chamber, for example in a point-of-care device, to analyze a sample from a biological host.  FIG. 11  depicts an analysis chamber  400  with a pathogen sensor  406  and a host sensor  410 . The chamber  400  includes walls  402  and  404  that form a space with which a sample is retained and analyzed at sensors  406  and  410 . Pathogen sensor  406  includes a conductive trace  408  to connect the sensor  406  to controlling instrumentation such as a potentiostat. Host sensor  410  is also connected to external or controlling instrumentation with a conductive trace  412 . Pathogen sensor  406  and host sensor  410  are separated by a distance X 1 . 
         [0032]    The pathogen sensor  406  is used to determine whether or not the marker is present in the sample. Although not depicted in  FIG. 11 , pathogen sensor  406  includes a probe configured to couple to a target marker from a pathogen. In certain approaches, the probe is a peptide nucleic acid probe. For example, the probe coupled to the pathogen sensor  406  may include a nucleotide sequence that is complementary to a nucleotide sequence from a pathogen which is unique to that pathogen. 
         [0033]    The host sensor  410  includes a probe configured to couple to a host marker. The host marker is an endogenous element from a biological host, such as a DNA sequence, RNA sequence, or peptide. For example, the probe coupled to host sensor  410  may be configured with a nucleotide sequence that hybridizes with a nucleotide sequence unique to the human genome. In certain approaches, the probe for the host marker is a peptide nucleic acid probe. Preferably, the host marker is present in every biological sample taken from a human patient, and therefore can serve as a positive, internal control for the analysis process, Accordingly, detection of the host marker at host sensor  410  serves as a control for the assay. Specifically, detection of the host marker confirms that the sample was taken correctly from the host (e.g., a patient), that the sample was processed correctly, and that hybridization of the probe and marker in the analysis chamber has taken place successfully. If any part of the assay fails, and the host marker is not detected at host sensor  410 , the assay is considered indeterminate. 
         [0034]    The pathogen sensor  406  and host sensor  410  operate using the electrocatalytic methods described in detail in U.S. Pat. Nos. 7,361,470 and 7,741,033, and PCT Application No. PCT/US12/024015 (although such sensors and the internal control techniques discussed herein could also be applied in other diagnostic methods).  FIG. 11  depicts only two sensors, but any number of sensors may be used For example, chamber  400  may include a plurality of pathogen sensors  406  and a plurality of host sensors  410 . When a plurality of sensors is used, each sensor may optionally be configured to sense a different target marker in order to detect the presence or absence of different pathogens, different hosts, or different parts of the same pathogen or the same host. In alternative approaches, a plurality of pathogen sensors  406  is used, but each pathogen sensor is configured to sense the same target marker in order to provide additional verification of the presence or absence of that target marker. Similarly, a plurality of host sensors  410  may also be used with each sensor being configured to detect the presence or absence of the same host target marker to provide additional verification of the measurement. 
         [0035]      FIG. 12  depicts an additional embodiment of an analysis chamber. Chamber  500  is similar to chamber  400  in that it includes walls  402  and  404 , pathogen sensor  406  and host sensor  410 . Chamber  500  additionally includes a non-sense sensor  414 . Similar to pathogen sensor  406  and host sensor  410 , non-sense sensor  414  is electrically coupled to controlling instrumentation, such as a potentiostat, with a conductive trace  416 . The non-sense sensor  414  may also include an electrode, such as a nanostructured microelectrode. Non-sense sensor  414  includes a probe, such as probe  106 . In certain approaches, the non-sense probe is a peptide nucleic acid probe. The non-sense probe, however, is not configured to mate with a marker from the pathogen or the biological host. Instead, the probe coupled to non-sense sensor  414  has a structure, such as a nucleotide sequence, which is not found in either the pathogen or the biological host. The non-sense sensor serves as an additional control to verify that the conditions within analysis chamber  500  can provide accurate sensing results. Non-sense sensor  414  tests fir nonspecific binding. Nonspecific binding of a nucleotide sequence may occur under inappropriate hybridization conditions in chamber  500 . For example, nonspecific binding may occur when the pH, ionic strength, or temperature are not appropriate for accurate testing. If binding occurs at non-sense sensor  414 , then other nonspecific binding may take place at pathogen sensor  406  and the host sensor  410 , and therefore the assay would be inaccurate. The non-sense sensor  414  is thereby able to act as an additional control for testing conditions. The non-sense sensor  414  may also function using electrocatalytic techniques as previously described. Although  FIG. 12  depicts three sensors, any number of sensors could be used. Sensors  406 ,  410 , and  414  are arranged in chamber  500  in a linear arrangement. However, sensors  406 ,  410 , and  414  may also be arranged in other patterns. 
         [0036]      FIG. 13  depicts an additional embodiment of an analysis chamber  600  which is similar to chambers  400  and  500  previously described.  FIG. 13  also depicts a reference electrode  418  and a counter electrode  422 . The reference electrode  418  and counter electrode  422  are connected to the controlling instrumentation (e.g., a potentiostat having shielding circuit  300 ) by conductive traces  420  and  424 , respectively. All or part of the conductive trace  420  may be shielded using the active shielding systems, methods, and devices described above. The reference electrode  418  and counter electrode  422  are used in the electrocatalytic measurements. The reference electrode  418  serves as a reference for applying a voltage at any of the sensors  406 ,  410 , and  414 . When a voltage is applied at a sensor (e.g., sensors  406 ,  410  and  414 ), the current generated flows through a sensor (e.g., sensors  406 ,  410 , and  414 ), through the hybridized complex of the probe and target, through the sample, and through the counter electrode  422 . 
         [0037]    The systems, devices, methods, and all embodiments described above may be incorporated into a cartridge to prepare a sample for analysis and perform a detection analysis.  FIG. 4  depicts a cartridge system  1600  for receiving, preparing, and analyzing a biological sample. For example, cartridge system  1600  may be configured to remove a portion of a biological sample from a sample collector or swab, transport the sample to a lysis zone where a lysis and fragmentation procedure are performed, and transport the sample to an analysis chamber for determining the presence of various markers and to determine a disease state of a biological host, 
         [0038]    Cartridges may use any appropriate formats, materials, and size scales for sample preparation and sample analysis. In certain approaches, cartridges use microfluidic channels and chambers. In certain approaches, the cartridges use macrofluidic channels and chambers. Cartridges may be single layer devices or multilayer devices. Methods of fabrication include, but are not limited to, photolithography, machining, micromachining, molding, and embossing. 
         [0039]      FIG. 6  depicts an automated testing system to provide ease of processing and analyzing a sample. System  1800  may include a cartridge receiver  1802  for receiving a cartridge, such as cartridge  1700 . System  1800  may include other buttons, controls, and indicators. For example, indicator  1804  is a patient ID indicator, which may be typed in manually by a user, or read automatically from cartridge  1700  or cartridge container  1704 . System  1800  may include a “Records” button  1812  to allow a user to access or record relevant patient record information, “Print” button  1814  to print results, “Run Next Assay” button  1818  to start processing an assay, “Selector” button  1818  to select process steps or otherwise control system  1800 , and “Power” button  1822  to turn the system on or off. Other buttons and controls may also be provided to assist in using system  1800 . System  1800  may include process indicators  1810  to provide instructions or to indicate progress of the sample analysis. System  1800  includes a test type indicator  1806  and results indicator  1808 . For example, system  1800  is currently testing for Chlamydia as shown by indicator  1806 , and the test has resulted in a positive result, as shown by indicator  1808 . System  1800  may include other indicators as appropriate, such as time and date indicator  1820  to improve system functionality. 
         [0040]    The foregoing is merely illustrative of the principles of the disclosure, and the systems, devices, and methods can be practiced by other than the described embodiments, which are presented for the purposes of illustration and not of limitation. It is to be understood that the systems, devices, and methods disclosed herein, while shown for use in detection systems for bacteria, and specifically, for Chlamydia Trachomatis, may be applied to systems, devices, and methods to be used in other applications including, but not limited to, detection of other bacteria, viruses, fungi, prions, plant matter, animal matter, protein, RNA sequences, DNA sequences, as well as cancer screening and genetic testing, including screening for genetic disorders. 
         [0041]    Variations and modifications will occur to those of skill in the art after reviewing this disclosure. The disclosed features may be implemented, in any combination and subcombination (including multiple dependent combinations and subcombinations), with one or more other features described herein. The various features described or illustrated above, including any components thereof, may be combined or integrated in other systems. Moreover, certain features may be omitted or not implemented. 
         [0042]    Examples of changes, substitutions, and alterations are ascertainable by one skilled in the art and could be made without departing from the scope of the information disclosed herein. All references cited are hereby incorporated by reference herein in their entireties and made part of this application.