This disclosure relates to measuring electrochemical impedance spectra (EIS).
Electrochemical impedance spectrogram (EIS) is a very sensitive and widely used characterization method of a material and its surface properties in an electrolyte. EIS is a diagnostic that is foundational to studies involving electrochemistry and all the applications thereof. It is used in a wide range of applications encompassing biosensors, implantable devices and packages, food monitoring, fuel cells, energy storage devices, etc. Current EIS using an electrochemical workstation typically takes several seconds to multiple minutes depending on the bandwidth of frequencies over which the impedance is being measured. Reliable electrochemical impedance measurements at low frequencies are particularly time-consuming. For instance, obtaining a reliable electrochemical impedance at 1 hertz (Hz) or <1 Hz frequency requires a wait of several seconds to obtain multiple repeats of sinusoidal waves at that frequency.
There are many applications where a rapid EIS would allow tracking impedance changes in real time. EIS changes reflect underlying physiological processes, or material surface changes, or more generally changes in the electrolyte that can often be rapid (e.g., less than one second to a few seconds). However, some of the current advanced electrochemical workstations (such as the one from ZAHNER-elektrik GmbH & Co. KG) use a sum-of-sine wave approach to determine the electrochemical impedance simultaneously at multiple frequencies, which can take several seconds or longer to obtain results.
Systems and methods to determine an electrochemical impedance spectrogram (EIS) rapidly in real time are provided. Embodiments described herein provide an approach to determine an EIS rapidly (e.g., much less than one second) over a wide band of frequencies. Since the EIS is a foundational diagnostic that is used in a wide variety of applications involving electrochemical events, the proposed approach will significantly impact a wide range of applications. Novel waveforms and systems analysis techniques are used to determine the electrochemical impedance over a wide range of frequencies simultaneously.
Current techniques to determine an EIS typically (depending on the start frequency and number of samples along the frequency axis) take several seconds to minutes to measure the EIS over a similar range of frequencies. Some more advanced existing techniques use a sum-of-sines approach to determine the EIS over a wide range of frequencies simultaneously, but still take at least several seconds to achieve a result. The approach described herein is distinct from the sum-of-sines approach in that embodiments use a novel signals and systems approach to sample the frequency axis at multiple points and determine the impedance spectra simultaneously.
Embodiments can be readily integrated into conventional electrochemical workstations and embedded devices. Other embodiments can be instantiated into custom-made hardware depending on the needs of a given application. The speed and resolution of the EIS obtained using this approach can be tailored to a wide variety of applications. This approach provides an EIS having a time-frequency distribution of electrochemical impedance at unprecedented resolution in the time and frequency axis that are determined at unprecedented speeds.
An exemplary embodiment provides a method for measuring an EIS in real time. The method includes applying a triangular excitation signal to a subject; obtaining electrical parameter measurements in response to the triangular excitation signal; and measuring an EIS of the subject based on the electrical parameter measurements.
Another exemplary embodiment provides an EIS measurement system. The EIS system includes a working electrode configured to provide a triangular excitation signal to a subject; a counter electrode configured to measure an electrical parameter in response to the excitation signal; and a processing device configured to obtain an EIS of the subject based on the triangular excitation signal and the measured electrical parameter.
Another exemplary embodiment provides a non-transitory computer-readable medium which, when executed by a processing device, causes the processing device to: cause a triangular excitation signal to be applied to a subject; obtain electrical parameter measurements in response to the triangular excitation signal; and measure an EIS of the subject based on the electrical parameter measurements.
Those skilled in the art will appreciate the scope of the present disclosure and realize additional aspects thereof after reading the following detailed description of the preferred embodiments in association with the accompanying drawing figures.
The accompanying drawing figures incorporated in and forming a part of this specification illustrate several aspects of the disclosure, and together with the description serve to explain the principles of the disclosure.
The embodiments set forth below represent the necessary information to enable those skilled in the art to practice the embodiments and illustrate the best mode of practicing the embodiments. Upon reading the following description in light of the accompanying drawing figures, those skilled in the art will understand the concepts of the disclosure and will recognize applications of these concepts not particularly addressed herein. It should be understood that these concepts and applications fall within the scope of the disclosure and the accompanying claims.
It will be understood that, although the terms first, second, etc. may be used herein to describe various elements, these elements should not be limited by these terms. These terms are only used to distinguish one element from another. For example, a first element could be termed a second element, and, similarly, a second element could be termed a first element, without departing from the scope of the present disclosure. As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items.
It will be understood that when an element such as a layer, region, or substrate is referred to as being “on” or extending “onto” another element, it can be directly on or extend directly onto the other element or intervening elements may also be present. In contrast, when an element is referred to as being “directly on” or extending “directly onto” another element, there are no intervening elements present. Likewise, it will be understood that when an element such as a layer, region, or substrate is referred to as being “over” or extending “over” another element, it can be directly over or extend directly over the other element or intervening elements may also be present. In contrast, when an element is referred to as being “directly over” or extending “directly over” another element, there are no intervening elements present. It will also be understood that when an element is referred to as being “connected” or “coupled” to another element, it can be directly connected or coupled to the other element or intervening elements may be present. In contrast, when an element is referred to as being “directly connected” or “directly coupled” to another element, there are no intervening elements present.
Relative terms such as “below” or “above” or “upper” or “lower” or “horizontal” or “vertical” may be used herein to describe a relationship of one element, layer, or region to another element, layer, or region as illustrated in the Figures. It will be understood that these terms and those discussed above are intended to encompass different orientations of the device in addition to the orientation depicted in the Figures.
The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the disclosure. As used herein, the singular forms “a,” “an,” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises,” “comprising,” “includes,” and/or “including” when used herein specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof.
Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this disclosure belongs. It will be further understood that terms used herein should be interpreted as having a meaning that is consistent with their meaning in the context of this specification and the relevant art and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein.
Systems and methods to determine an electrochemical impedance spectrogram (EIS) rapidly in real time are provided. Embodiments described herein provide an approach to determine an EIS rapidly (e.g., much less than one second) over a wide band of frequencies. Since the EIS is a foundational diagnostic that is used in a wide variety of applications involving electrochemical events, the proposed approach will significantly impact a wide range of applications. Novel waveforms and systems analysis techniques are used to determine the electrochemical impedance over a wide range of frequencies simultaneously.
Current techniques to determine an EIS typically (depending on the start frequency and number of samples along the frequency axis) take several seconds to minutes to measure the EIS over a similar range of frequencies. Some more advanced existing techniques use a sum-of-sines approach to determine the EIS over a wide range of frequencies simultaneously, but still take at least several seconds to achieve a result. The approach described herein is distinct from the sum-of-sines approach in that embodiments use a novel signals and systems approach to sample the frequency axis at multiple points and determine the impedance spectra simultaneously.
Embodiments can be readily integrated into conventional electrochemical workstations and embedded devices. Other embodiments can be instantiated into custom-made hardware depending on the needs of a given application. The speed and resolution of the EIS obtained using this approach can be tailored to a wide variety of applications. This approach provides an EIS having a time-frequency distribution of electrochemical impedance at unprecedented resolution in the time and frequency axis that are determined at unprecedented speeds.
I. Introduction
Electrochemical systems are inherently non-linear and therefore linear systems analysis techniques are not directly applicable. However, under small signal (<50 millivolts (mV)) voltage excitation, electrochemical systems are typically assumed to be linear. Embodiments described herein provide a small amplitude triangular excitation signal and make measurements in response to the excitation signal. For example, a triangular excitation voltage signal can be provided, with an amplitude between −50 mV and 50 mV, can be applied to a subject, and a current response can be measured. In another example, a triangular excitation current signal can be provided, with an amplitude of less than 5 milliamps (mA), can be applied to a subject, and a voltage response can be measured.
The triangular signal waveform (e.g., v(t) or i(t), depending on application) samples the frequency spectra at harmonics of the fundamental frequency (where fundamental frequency=1/a time-period of the triangular waveform) over a wide range of frequencies starting from the fundamental. A Fourier transform of the voltage waveform (v(t), which can be the excitation voltage or the response voltage) is performed to obtain a voltage spectrogram V(w). Similarly, a Fourier transform of the current waveform (i(t), which can be the response current or the excitation current) is performed to obtain a current spectrogram I(w).
A ratio of V(w) to I(w) is determined over the entire range of frequencies (fundamental and the harmonics thereafter) to obtain an impedance spectrogram, Z(w). Thus, an EIS sampled (e.g., sampled at harmonics of the fundamental excitation frequency) over a wide range of frequencies is obtained in 3 rapid steps (that can take a fraction of one second)—(1) generation of a small-amplitude, triangular excitation waveform (voltage/current), (2) measurement of electrical parameter (current/voltage) in response to the excitation waveform, and lastly (3) determining the ratio of the Fourier transforms of the voltages and currents to obtain Z(w).
Some electrochemical systems can take several cycles of the triangular waveforms initially to achieve steady state. But once the system reaches steady state, the time taken to determine EIS over a band of frequencies using this approach depends on the lowest frequency at which the impedance has to be determined (which determines the period of the triangular waveform). For instance, to determine the EIS starting at a fundamental frequency of 1 hertz (Hz) (and at every harmonic thereafter), one would need a triangular waveform with a period of 1 second (s). However, to determine the EIS starting at a fundamental frequency of 10 Hz (and at every harmonic thereafter), one would only need a triangular wave of duration 100 milliseconds (ms). Similarly, to determine the EIS starting at a fundamental frequency of 100 Hz (and at every harmonic thereafter), one would need a triangular waveform of 10 ms and so on.
Since the triangular excitation waveform is pre-determined, the Fourier transform of these waveforms can be generated a priori, further reducing the time to determine the wideband EIS. Numerous techniques (e.g., using software and/or hardware involving the use of specialized conventional digital signal processing microchips) can be used to instantiate embodiments in different configurations suitable to the needs of a given application.
Thus, embodiments result in an EIS that can provide a time-frequency distribution of electrochemical impedance values at unprecedented resolutions in both time and frequency axis at unprecedented speeds (e.g., fractions of one second). Such measurements facilitate fundamental discoveries of electrochemical events at the interface of materials and electrolytes. In particular, embodiments provide rapid EIS measurement in biosensors and implantable devices, laboratory systems studying cells of different types and pathogens, fuel cells and energy storage devices, etc.
II. EIS Measurement System
In a first example, the triangular excitation signal provided by the working electrode 12 is a voltage signal (v(t)), and the electrical parameter measured at the counter electrode 16 is a current response (i(t)). In a second example, the triangular excitation signal provided by the working electrode 12 is a current signal (i(t)), and the electrical parameter measured at the counter electrode 16 is a voltage response (v(t)). In a 3-electrode sensor design, the EIS measurement system 10 further includes a reference electrode 18 which provides a reference voltage (e.g., a ground or other reference level, Vref) for the working electrode 12 and/or the counter electrode 16. In a 2-electrode sensor design, the counter electrode 16 is instead configured to provide a reference voltage.
The subject 14 can be a liquid, solid, gas, gel, or other material of interest. For example, the subject 14 can be an electrolyte solution. The working electrode 12 can be in contact with the subject 14 at an appropriate position (e.g., within the electrolyte solution or on a first surface of the subject 14). The counter electrode 16 can be in contact with the subject 14 at another position (e.g., away from the working electrode 12 and within the electrolyte solution or on a second surface of the subject 14). In some embodiments, the reference electrode 18 is positioned between the working electrode 12 and the counter electrode 16, and in other embodiments the reference electrode 18 is positioned adjacent to or away from the working electrode 12 or the counter electrode 16.
The EIS measurement system 10 further includes a processing device 20 which obtains the EIS of the subject 14. As described above, the EIS produced by the processing device 20 can be a sampled spectrogram based on the triangular excitation signal and the measured electrical parameter. The processing device 20 can include or be coupled to additional circuitry 22, such as an excitation source 24, a reference source 26, and measuring circuitry 28. For example, where the triangular excitation signal is a voltage signal, the measuring circuitry 28 can measure the response current (e.g., using a voltage divider or other circuitry) through the counter electrode 16 and provide an indication (e.g., a voltage signal representative of the response current) to the processing device 20. In other examples, measurements may be performed directly by the processing device 20.
The processing device 20 represents one or more commercially available or proprietary general-purpose processing devices, such as a microprocessor, central processing unit (CPU), or the like. The processing device 20 may be a microprocessor, field programmable gate array (FPGA), a digital signal processor (DSP), an application-specific integrated circuit (ASIC), or other programmable logic device, a discrete gate or transistor logic, discrete hardware components, or any combination thereof designed to perform the functions described herein. Furthermore, the processing device 20 may be a microprocessor, or may be any conventional processor, controller, microcontroller, or state machine. The processing device 20 may also be implemented as a combination of computing devices (e.g., a combination of a DSP and a microprocessor, a plurality of microprocessors, one or more microprocessors in conjunction with a DSP core, or any other such configuration).
The processing device 20 can cause the triangular excitation signal to be generated in an appropriate manner. For example, the triangular excitation signal is readily generated in current conventional electrochemical workstations using a cyclic-voltammogram feature. In embedded systems, the processing device 20 includes or is connected to the excitation source 24, which can provide a triangular excitation signal having a desired shape, period (e.g., resolution) and amplitude (e.g., according to material properties of the subject 14). In further examples, the excitation source 24 may be programmable to provide triangular excitation signals with different shapes (e.g., a ramp signal, a sawtooth signal, a combination signal), different periods (e.g., a constant or varying period), and different amplitudes (e.g., an amplitude which is constant over periods or varying across periods).
In an exemplary aspect, the processing device 20 causes the working electrode 12 to generate the triangular excitation signal (v(t) or i(t)) and obtains an electrical parameter measurement (i(t) or v(t)) at the counter electrode 16 in response to the triangular excitation signal. The processing device 20 further obtains a Fourier transform of the triangular excitation signal (e.g., by performing a fast Fourier transform (FFT) or retrieving a stored Fourier transform from memory) and a Fourier transform (e.g., FFT) of the measured electrical parameter. In this manner, the processing device 20 obtains a voltage spectrogram V(w) and a current spectrogram I(w), and uses a ratio of V(w) to I(w) to obtain a corresponding impedance spectrogram Z(w). From the impedance spectrogram Z(w), the processing device 20 determines the EIS, as described further below.
In some embodiments, the processing device 20 includes or is coupled to a memory storing one or more measurement profiles. The measurement profile(s) may include excitation signal parameters (e.g., whether a voltage or current waveform is to be generated, a shape, an amplitude, a period), a Fourier transform of the excitation signal, an expected range of response signal, and so on. The measurement profile(s) may improve performance of the EIS measurement system 10 and/or provide flexibility to measure multiple types of subjects 14.
A. Example Triangular Excitation Signals
B. Example Measurements
In the example of
III. Flow Diagram
The process continues at operation 804, with measuring an EIS of the subject based on the electrical parameter measurements. Measuring the EIS of the subject can include determining a ratio of Fourier transforms (e.g., FFTs) of voltages of the triangular excitation signal and corresponding current measurements. The process optionally continues at operation 806, with producing a sampling of the EIS at a plurality of frequencies.
Although the operations of
IV. Computer System
The exemplary computer system 900 in this embodiment includes a processing device 20 or processor, a system memory 902, and a system bus 904. The system memory 902 may include non-volatile memory 906 and volatile memory 908. The non-volatile memory 906 may include read-only memory (ROM), erasable programmable read-only memory (EPROM), electrically erasable programmable read-only memory (EEPROM), and the like. The volatile memory 908 generally includes random-access memory (RAM) (e.g., dynamic random access memory (DRAM), such as synchronous DRAM (SDRAM)). A basic input/output system (BIOS) 910 may be stored in the non-volatile memory 906 and can include the basic routines that help to transfer information between elements within the computer system 900.
The system bus 904 provides an interface for system components including, but not limited to, the system memory 902 and the processing device 20. The system bus 904 may be any of several types of bus structures that may further interconnect to a memory bus (with or without a memory controller), a peripheral bus, and/or a local bus using any of a variety of commercially available bus architectures.
As described above, the processing device 20 represents one or more commercially available or proprietary general-purpose processing devices, such as a microprocessor, CPU, or the like. More particularly, the processing device 20 may be a complex instruction set computing (CISC) microprocessor, a reduced instruction set computing (RISC) microprocessor, a very long instruction word (VLIW) microprocessor, a processor implementing other instruction sets, or other processors implementing a combination of instruction sets. The processing device 20 is configured to execute processing logic instructions for performing the operations and steps discussed herein. In this regard, the various illustrative logical blocks, modules, and circuits described in connection with the embodiments disclosed herein may be implemented or performed with the processing device 20.
The computer system 900 may further include or be coupled to a non-transitory computer-readable storage medium, such as a storage device 912, which may represent an internal or external hard disk drive (HDD), flash memory, or the like. The storage device 912 and other drives associated with computer-readable media and computer-usable media may provide non-volatile storage of data, data structures, computer-executable instructions, and the like. Although the description of computer-readable media above refers to an HDD, it should be appreciated that other types of media that are readable by a computer, such as optical disks, magnetic cassettes, flash memory cards, cartridges, and the like, may also be used in the operating environment, and, further, that any such media may contain computer-executable instructions for performing novel methods of the disclosed embodiments.
An operating system 914 and any number of program modules 916 or other applications can be stored in the volatile memory 908, wherein the program modules 916 represent a wide array of computer-executable instructions corresponding to programs, applications, functions, and the like that may implement the functionality described herein in whole or in part, such as through instructions 918 on the processing device 20. The program modules 916 may also reside on the storage mechanism provided by the storage device 912. As such, all or a portion of the functionality described herein may be implemented as a computer program product stored on a transitory or non-transitory computer-usable or computer-readable storage medium, such as the storage device 912, non-volatile memory 906, volatile memory 908, instructions 918, and the like. The computer program product includes complex programming instructions, such as complex computer-readable program code, to cause the processing device 20 to carry out the steps necessary to implement the functions described herein.
An operator, such as the user, may also be able to enter one or more configuration commands to the computer system 900 through a keyboard, a pointing device such as a mouse, or a touch-sensitive surface, such as the display device, via an input device interface 920 or remotely through a web interface, terminal program, or the like via a communication interface 922. The communication interface 922 may be wired or wireless and facilitate communications with any number of devices via a communications network in a direct or indirect fashion. An output device, such as a display device, can be coupled to the system bus 904 and driven by a video port 924. Additional inputs and outputs to the computer system 900 may be provided through the system bus 904 as appropriate to implement embodiments described herein.
The operational steps described in any of the exemplary embodiments herein are described to provide examples and discussion. The operations described may be performed in numerous different sequences other than the illustrated sequences. Furthermore, operations described in a single operational step may actually be performed in a number of different steps. Additionally, one or more operational steps discussed in the exemplary embodiments may be combined.
Those skilled in the art will recognize improvements and modifications to the preferred embodiments of the present disclosure. All such improvements and modifications are considered within the scope of the concepts disclosed herein and the claims that follow.
This application claims the benefit of provisional patent application Ser. No. 62/987,090, filed Mar. 9, 2020, the disclosure of which is hereby incorporated herein by reference in its entirety. The present application is related to concurrently filed U.S. patent application Ser. No. ______ filed on ______ entitled “Rapid Assessment of Microcirculation in Patients to Realize Closed-Loop Systems,” the disclosure of which is hereby incorporated herein by reference in its entirety.
This invention was made with government support under 1650566 awarded by The National Science Foundation. The government has certain rights in the invention.
Number | Date | Country | |
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62987090 | Mar 2020 | US |