Hydration sensor

A hydration sensor is provided that includes a circulator having a plurality of ports, an amplitude-modulated coherent source connected to a first port of the circulator, a rectifier or other power sensor connected to a second port of the circulator followed by an RF baseband low-noise amplifier, a coupling structure connected to the third port of the circulator, and a demodulator connected to the output of the rectifier. The hydration sensor can include an RF low noise amplifier between the circulator and rectifier, and/or a second amplitude modulator between the circulator and the coupling structure. The coupling structure can be either a guided-wave near-field structure or an interfacial capacitive or inductive element. In the former case, the hydration is determined by measuring the reflectivity of the guided-wave radiation, and in the latter case it is determined by measuring the change of reflectivity (through change of impedance) of the interfacial element.

BACKGROUND

By most medical standards, the skin is considered an organ of the human body and carries out many vital functions such as thermal regulation and protection against external biological microorganisms. The dermis and epidermis vary in thickness greatly depending on location on the body. The epidermis varies from roughly 50-micron thick on “thin-skin” areas such as the eyelids, to over 1-mm thick on heavy-use areas such as the palms of the hands or soles of the feet. The average value around the entire body is roughly 100 micron. And the epidermis is a stratified squamous epithelial tissue, meaning that does not have its own blood supply. Instead, it relies solely on the blood supply from the dermis, which is its primary source of water and nourishment.

Independent of location on the body, the skin displays a characteristic variation of hydration with depth as illustrated in the hydration curve100inFIG. 1. Hydration is defined as the fraction of water in the skin tissue per unit mass, following the convention established for all “soft” tissue. More specifically, it is water in the aqueous state, not the “bound” state whereby water is chemically bonded to biomolecules (e.g., proteins) or other biological structures. The outermost sublayer of the epidermis, the stratum corneum, typically has about 20% hydration. This abruptly increases to approximately 60% in the innermost sublayer, the basale corneum. Once into the dermis, the hydration levels off to approximately 65% in a typical, healthy person, which is roughly the hydration level in soft tissue of all sorts.

Significant variations, however, can occur because of skin maladies or because of underlying disease. For example, skin burns introduce a lateral variation in hydration depending on the severity of the burn, be it 1stdegree (which damages only the epidermis), 2nddegree (which extends into the dermis, either partially or fully), and 3rddegree (damage into the subcutaneous). Carcinomas generally involve only the epidermis, and melanomas usually start in the epidermis but then grow vertically into the dermis where they can readily metastasize through the blood supply there (which is why melanomas are more deadly than carcinomas). Skin hydration can also be affected by internal disease, which affects the skin tissue through edema—the abnormality associated with leaking of blood vessels into surrounding soft tissue, which usually raises the fluid and hydration levels and leads to overall swelling. A common internal disease that causes this is congestive heart failure. Another is clot- or tumor-blockage of major blood vessels.

SUMMARY

The following presents a simplified summary in order to provide a basic understanding of some aspects of the innovation. This summary is not an extensive overview of the innovation. It is not intended to identify key/critical elements or to delineate the scope of the innovation. Its sole purpose is to present some concepts of the innovation in a simplified form as a prelude to the more detailed description that is presented later.

In one aspect of the innovation, a sensor is provided that includes a circulator having a plurality of ports, a coherent source coupled to a first port of the plurality of ports of the circulator, a power sensor or rectifier coupled to a second port of the plurality of ports of the circulator, an optional first low-noise amplifier between the circulator and the rectifier, a demodulator connected to the rectifier, and a coupling structure coupled to the a third port of the circulator, the coupling structure configured to couple radiation to a sample under test as a guided wave.

DETAILED DESCRIPTION

While specific characteristics are described herein (e.g., thickness, orientation, configuration, etc.), it is to be understood that the features, functions and benefits of the innovation can employ characteristics that vary from those described herein. These alternatives are to be included within the scope of the innovation and claims appended hereto.

Given this simple picture of skin physiology and pathology, a logical method of skin malady detection is one that can measure hydration through the epidermis and some distance into the dermis. The hydration can then be used to determine the presence of skin maladies through models, such as a stratified-medium dielectric-constant model. If hydration is measured for penetration well into the dermis, the presence of melanoma and other maladies can be determined.

However, many of these skin maladies occur on a size scale smaller than the wavelength of common electromagnetic radiation. A good example is tumors of melanoma in its early stages. So it is important that the sensor have sub-wavelength spatial resolution and can provide imagery by manual scanning or some other method.

Skin is an organ of the human body with a varying degree of reflectivity depending on hydration and other biological composition, and its location relative to soft tissue or the bone below it. The medical field has known that hydration is an important factor of human health, perhaps even a “vital sign.” The innovation provides systems and methods for determining the hydration level (through skin models) on a whole body or specific locations of a body. The systems and methods can achieve an accuracy of approximately 1%. The systems and methods sample the epidermis and dermis through the introduction of low-level, non-ionizing electromagnetic energy either as guided-wave radiation incident perpendicular to the skin layers, or as electric or magnetic fields from interfacial (capacitive or inductive) couplers. The systems and methods determine the hydration level depending on the form of the energy: (1) through a more accurate measurement of the electromagnetic reflectance in the millimeter-wave region, for example in W band (75-110 GHz) or Ka band (26-40 GHz), and/or (2) through a more accurate measurement of the electromagnetic impedance change of interfacial couplers. The reflectance of the radiation is measured and mapped back the hydration level through electromagnetic modeling of the skin tissue or calibration (“look-up”) tables. In some embodiments, in the millimeter-wave frequency region, the reflectance is sensitive to the hydration level because of the high magnitude of the dielectric constant of liquid water, be it in a pure or physiological form.

In some embodiments, a guided-wave coupling includes near-field capability, which results in an improved spatial resolution (e.g., approximately 1 mm). The technological method is to utilize either a “Bulls-eye” structure, or a fundamental-mode metal waveguide to direct coherent radiation onto the skin-under-test.

In some embodiments, an interfacial coupling provides comparable spatial resolution depending on the frequency and the proximity. For mm-wave frequencies and close proximity to the skin, an interfacial coupler may provide finer spatial resolution than 1 mm. A significant advantage of the interfacial coupling over the guided-wave coupling is the use of coaxial transmission line described in detail below. In some embodiments, the coaxial line works well up to roughly 100 GHz.

In some embodiments, for both types of coupling, the reflected energy is measured by placing a circulator or directional coupler between the source and the skin-under-test. The circulator can be a three-port, non-reciprocal device. The circulator can facilitate distinguishing between incident radiation and reflected radiation based on port locations. In some embodiments, the radiation is introduced to first port, a human skin sample to a second port, and the receiver (or detector) is coupled through a third port. The power detected in the third port is proportional to the reflectance in second port. The power coupling in the second port can occur either through a horn antenna, an open-ended waveguide, or an interfacial coupler. The horn antenna facilitates contact-free coupling to the skin having a diffraction-limited resolution of approximately several millimeters to one centimeter. The open ended waveguide facilitates contact coupling to the skin with “near-field”, sub-wavelength resolution of approximately 1 mm. The interfacial coupler facilitates contact or proximity-contact sub-wavelength resolution of 1 mm or less.

The described coupling methods can facilitate whole-body hydration assessment or measurement for skin-malady diagnoses and vital-sign sensory applications. These facilitate, for example, body hydration mapping as applied to physical examinations, burn and skin cancer imaging, and other human malady sensing.

The depth-dependent hydration illustrated inFIG. 1has been studied with respect to electric current flow and electromagnetic propagation (i.e., radiation). Incident radiation can come from outside the body and propagate perpendicular to the skin. Minimizing incident radiation such that there is little or no radiation transmitted through the body at frequencies below x-ray facilitates improves the accuracy of measures. Minimizing incident radiation can be achieved due to strong absorptive attenuation by liquid water, and/or strong scattering from soft and hard tissue.

FIG. 2Aillustrates an absorption coefficient α plot200of liquid water in the frequency range of interest using a double-Debye model. The model is determined to be accurate at radio frequencies up to at least 600 GHz. For example, at 600 GHz, the absorption coefficient α≈140 cm−1, which means that the penetration depth δ≡1/α≈70 micron. Soft tissue is about 65% hydrated and therefore can support greater penetration depth; the penetration depth is not high enough to get through the centimeter-scale or greater paths offered by a body. Thus, the innovation provides systems and methods to detect hydration level through reflected radiation.

The layered nature of the epidermis and upper dermis can be used to construct a stratified-media model to thereby predict the reflectivity in the skin tissue. From the stratified-media model in conjunction with the Bruggeman effective-medium model and the physiological hydration curve100illustrated inFIG. 1, a penetration depth vs frequency curve202can be obtained as illustrated inFIG. 2B. At frequencies around 500 GHz, the penetration depth is not great enough to reach the dermis on many parts of the human body. The curve202, however, also shows that deeper penetration can be achieved by lowering the frequency to approximately 100 GHz or less. The trade-off between these two frequencies is illumination area and spatial resolution. Using ordinary antennas or quasi-optics, and free-space coupling, the illumination area on the skin from a transmitter is determined by the diffraction limit d˜2.44 f·λ, where d is the diameter of an illumination spot, f is the f-number of the antenna or optics, and λ is the free-space wavelength. For standard antennas, such as pyramidal or diagonal horns, and standard free-space optics, the lowest practical f is approximately 1.0. In some embodiments, for 600 GHz radiation (λ=0.5 mm), the minimum spot diameter is ≈1.2 mm. In other embodiments, for 100 GHz radiation (λ=3.0 mm), the minimum spot diameter is ≈7.3 mm.

A related issue is magnitude of reflectivity. The larger the magnitude of reflectivity, the easier it is to measure. In particular, a larger magnitude of reflectivity eases the effects of physical noise. A stratified-media model can be used to compute the magnitude of reflectivity as a function of frequency. A plot204is illustrated inFIG. 2C. The plot204is parameterized by mean hydration through the epidermis; i.e., the mean value over approximately the first 75 microns inFIG. 1. From the plot204, operation of the hydration sensor at 100 GHz enjoys three times the reflectivity as 600 GHz. The signal-to-noise ratio of a reflective sensor can be three times more sensitive.FIG. 2Cfurther depicts a rapid increase in reflectivity with decreasing frequency to approximately 20 GHz, below which rate of increase decreases to values approximately 0.5.

FIGS. 3A and 3Billustrate two example circuit embodiments of hydration sensors300,302in accordance with an aspect of the innovation. The hydration sensors300,302achieve measurements approximately 1% more accurate or better (in measuring hydration). The hydration sensors300,302can mechanically scan quickly (e.g. seconds) to measure large areas of the human body and be compatible within a clinical (medical) setting. In some embodiments, the hydration sensors300,302can be a waveguide design.

The hydration sensors300,302include an oscillator304that is utilized as a coherent source. The oscillator304can be amplitude modulated and coupled to a metal waveguide of having a dimension and a bandwidth. The oscillator304is coupled directly to a waveguide based circulator306. In some embodiments, the circulator306can have a first port308, a second port310, and a third port312.

In some embodiments, the oscillator304is an electromagnetic non-reciprocal device in which radiation propagates in one helicity (e.g., clockwise or counter-clockwise). For example, the example hydration sensors300,302utilize a counter-clockwise propagating helicity. This facilitates that radiation entering the first port308will propagate to the second port310but not the third port312. Similarly, radiation entering the second port310will propagate to the third port312but not the first port308. In embodiments, each port of the circulator306is extractive, e.g. radiation propagating with the correct helicity will exit the next port it encounters after entering the circulator306. Once through the port and into a connected waveguide or transmission line, the radiation will not enter the circulator306again unless it encounters an impedance mismatch. Under an impedance mismatch, some of the radiation reflects from the mismatch back into the same port and enter the circulator306a second time with the allowed helicity.

The first port308of the circulator306is connected to the oscillator304via a transmission line. The second port310is coupled to a sample under test (SUT), e.g. skin via a coupling structure314. In some embodiments, the second port310is coupled to the SUT either through a waveguide or coaxial transmission line. The SUT creates reflected radiation that propagates back into the circulator through the second port310. The reflected radiation enters the circulator306through the second port310and is subsequently extracted out of the third port312. The radiation exits the third312and into detector electronics that create a detector output signal (Xs) that is proportional to the reflected power. In some embodiments, the reflected power can be a function of the hydration (H) such that Xs=F(H). Given a calibration of the reflection described in detail below, the inverse of the function can be used to find H, which is H=G(Xs) where G=F−1. The functions are dependent on the SUT. In some embodiments, the calibration depends on the manner by which radiation is coupled to the SUT.

Different coupler structures314(or couplers) may be used for different roles.FIGS. 4 and 5illustrate guide-wave couplers.FIGS. 6 and 7illustrates interfacial couplers. The guided-wave couplers ofFIGS. 4 and 5are differentiated from the interfacial couplers ofFIGS. 6 and 7. The guided-wave couplers irradiate the skin with propagation electromagnetic radiation and then collect the reflected radiation from the skin in proportion to its reflectivity. In contrast, the interfacial couplers are sensitive to the skin through its effect on the impedance. The change of impedance then changes the reflected energy incident on the coupler from the second port310in the hydration sensors300,302.

Interfacial couplers, described in detail below, provide an additional benefit of providing higher spatial resolution sensing of the skin tissue than the guided-wave couplers. In some embodiments, an inductive interfacial coupler is used having impedance sensitive to changes in the magnetic permeability in the surrounding region. Magnetic fields will penetrate deeper into the skin than electric fields, which are governed by the electric permittivity. The electric permittivity is large in magnitude and has significant imaginary part for water and soft tissue of all sorts, which are why the absorption coefficient is relatively large and the penetration depth relatively small in the plots inFIGS. 2A and 2B.

In some embodiments, an important concern for a sensor is sensitivity. In some embodiments, sensitivity can be quantified as a minimum-detectable reflectivity (MDR) difference. The MDR can be affected by physical noise in detection electronics. The detection electronics can include a Schottky rectifier316(or other power sensor) followed by a baseband low-noise amplifier (LNA)318. With continuing reference toFIGS. 3A and 3B, sensitivity can be affected by thermal noise and shot noise in the Schottky rectifier316and/or in the baseband LNA318. The combination of the Schottky rectifier316and baseband LNA318creates an overall detector noise factor.

In some embodiments, the MDR is expected to be finer with guided-wave couplers than interfacial couplers. In particular, it is much finer than an inductive interfacial coupler as illustrated inFIG. 7. Hence, there can be an engineering trade-off presented by the coupling method chosen, the trade-off being between MDR, spatial resolution, and depth-of-penetration.

In the example hydration sensor302illustrated inFIG. 3B, the hydration sensor302includes a receiver having a radio frequency (RF) LNA320followed by the Schottky rectifier316. In some embodiments, the RF LNA320has been developed to W band and beyond using monolithic microwave integrated circuit (MMIC) technology. A secondary benefit of a MIMIC LNA is superior impedance matching. Schottky rectifiers can be reactive at high frequencies, so they reflect some of the incident power from the third port312back into the circulator306.

Independent of how radiation propagates out of the circulator306, there is a function of effectively coupling it to the SUT. The manner in which this is done depends on the chosen spatial resolution, depth of resolution, and the measurement time. In some embodiments, a directive antenna such as a waveguide horn400shown inFIG. 4Acan be used to achieve a spatial resolution of many wavelengths with relatively short measurement time. Ideally the antenna400has a high directivity in only one direction and the antenna400is oriented so that this direction is perpendicular to the surface of the SUT (i.e., “normal incidence”). Under this condition, most of the radiation reflected from the SUT will be retrodirective and therefore efficiently coupled back into the antenna400. In some embodiments, the horn antenna400can be made more directive (i.e., collimated) by filling its opening402with a lens404, as illustrated inFIG. 4B. In some embodiments, a plano-convex lens can be used as the lens404. In some embodiments, the lens404may be made of a plastic such as Teflon; the planar side of the lens404mates well to flat SUTs and is biochemically inert.

Referring toFIG. 4C, in some embodiments, a bull's-eye coupler406can achieve spatial resolutions of approximately one wavelength or less. The bull's-eye coupler406can be placed flush with the flat side of the convex lens404. The bull's-eye coupler406has a pattern of concentric circular grooves408. In some embodiments, the entire (or substantially the entire) grooved side of the bull's-eye coupler406is coated with a metal film except in a small central disc. The grooves408act to concentrate the incident radiation into the disc by constructive interference of surface currents excited on the metal film. In some embodiments, the diameter of the disc should be less than a wavelength, but not so small as to make the coupling efficiency impractically low. An approximate range of the disc diameter d is λ/10<d<λ/4. In some embodiments, an approximate number of concentric grooves408is 5 to 10.

In other embodiments, a sub-wavelength antenna can achieve spatial resolutions of approximately one wavelength or less when placed in close proximity to the SUT such that the SUT is illuminated with near-field radiation. In some embodiments, the sub-wavelength antenna is a Hertzian dipole.

The effective use of the bull's-eye coupler406requires that it be placed with the central disc in close proximity to the SUT to achieve sub-wavelength spatial resolutions. This is also “near-field” coupling. In some embodiments, the central disc can susceptible to moisture, oil, or other organic material. In this embodiment, a thin plastic film can be used on the grooved side to protect it against contamination of the central disc. This also prevents the metal film from tarnishing when the SUT is human skin, for example.

FIG. 5Aillustrates an open-ended metal waveguide500. The open-ended waveguide500is capable of achieving sub-wavelength resolutions. The open-ended waveguide500can be a type of “near-field” coupling that is used in close proximity to the SUT, as shown inFIG. 5B. The close proximity is made possible by a dielectric membrane502thin enough to provide efficient coupling without losses, but thick enough to chemically isolate the waveguide from the SUT. The spatial resolution for this approach can depend on waveguide dimensions. For example, for a “W band” (75-110 GHz) coupler, the standard waveguide is WR-10 having dimensions 0.05 inch×0.10 inch (height×width). Using an operating frequency of 100 GHz, the wavelength is λ=3 mm, for which the WR-10 dimension are ≈0.4·λ×0.8·λ=2.9 mm2. In some embodiments, the membrane can be made of one of several different elastic plastics. The plastics can be highly transparent at RF frequencies (including the W band), and many of which are chemically inert. For example, the membrane can be made of Latex plastic.

The reflected signal level can be calibrated to determine the reflectivity of the SUT accurately. This can be achieved by replacing the SUT with a sample where the reflectivity is known to be unity, or close thereto, and is called a calibration sample (CAS). The form of the CAS depends on the method used. In some embodiments, the CAS will provide greatest accuracy if it acts as a “short circuit” element in the transmission line or waveguide. In some embodiments, a flat metal plate can accomplish the calibration when replacing the SUT inFIGS. 4B and 5Bfor the low- and high-resolution coupling methods, respectively. In some embodiments, the CAS can be made of aluminum. In other embodiments, alternative metals having high electrical conductivity can be used.

During calibration, the CAS produces a signal Xc at the detector that should always be larger in magnitude than the signal Xs from the SUT because it reflects essentially all of the radiation impinging on it from the second port310. The reflectivity of the SUT can be estimated as R≈Xs/Xc. The isolation structures, e.g. lens404or membrane502, will leak a small amount of the radiation laterally so that Xc is a bit smaller than ideal. However, this is also true for Xs when the SUT is measured, so the error is canceled to first order.

In some embodiments, for radar, an antenna314is connected to the second port310which is a transmit port such that radiation entering the second port310from the oscillator304encounters the first significant reflection at the SUT in free-space. The reflected radiation from the SUT comes back to the second port310and re-enters the circulator306where it is subsequently extracted out by the third port312and detected by a receiver that includes the Schottky rectifier316and baseband LNA318. In some embodiments, the oscillator304is modulated such that it radiates short pulses with a low duty cycle. The modulation can be performed by a modulator at frequency f1322. The modulator facilitates determining the reflectivity of the SUT and its distance (i.e., “range”) from the sensor, by measuring the amplitude and time-of-flight using sensitive time-domain instruments (e.g., an oscilloscope) in the third port312. After the Schottky rectifier316and the baseband LNA318, the output is detected by a synchronous demodulator324. The synchronous demodulator324is locked to the modulator322frequency to facilitate accurate and noise-free detection.

FIGS. 6 and 7illustrate interfacial couplers.FIGS. 6A and 6Bdisplay a micro-capacitor600and a coaxial transmission line602which connects the micro-capacitor to the second port310of the circulator306. The coaxial transmission line602includes an outer conductor and a center conductor. The micro-capacitor600includes parallel metal plates separated by a dielectric layer604. A ground plate606is connected to the outer conductor of the coaxial transmission line602. A signal plate608is connected to the center conductor of the coaxial transmission line602.

The micro-capacitor600has a thickness-to-width (T/W) ratio depicted inFIG. 6B. In some embodiments, the T/W ratio can be approximately 1.0 or greater. The micro-capacitor600creates fringing electric fields610between the signal plate608and the ground plate606. The fringing electric fields610make the capacitance of the micro-capacitor600sensitive to the surrounding medium. In particular, the capacitance is sensitive to a SUT when the micro-capacitor is brought into close proximity. The hydration state of the SUT affects the fringing effect of the fringing fields610and the absolute value of the RF capacitance accordingly. By measuring the impedance or reflectivity of the capacitor in the circuits300or302, the hydration level can be derived. In some embodiments, the dielectric layer604can be chosen to have RF electrical properties that support the fringing effect and therefore the change in capacitance per unit change in hydration. The dielectric property of concern is the complex dielectric constant, ε=ε′+jε″, where ε′ is the real part and ε″ is the imaginary part. In some embodiments, the dielectric material has low absorptive losses (i.e., ε″<<ε′) and a small real part (ε′ just over 1.0) to enhance the fringing effect. In some embodiments, the dielectric material is an artificial (e.g., humanmade) dielectric material, such as a plastic.

FIGS. 7A and 7Bincludes a micro-inductor700and a coaxial transmission line702which connects the micro-inductor700to the second port310of the circulator306. The micro-inductor700includes a planar spiral inductor704mounted on a dielectric substrate706contained inside a ground ring708. The outer termination of the planar spiral inductor704is electrically connected to the ground ring708which in turn is connected to a ground conductor of the coaxial transmission line702. The inner termination of the planar spiral inductor704is connected to a center conductor of the coaxial transmission line702. The micro-inductor700creates a magnetic field710having magnetic field lines as inFIG. 7B. The micro-inductor700radiates a higher magnetic-to-electric field ratio than the micro-capacitor600. Soft tissue such as skin is generally non-magnetic in nature (except for a small effect from blood). The magnetic field710is not attenuated as strongly in skin tissue as the electric field610. The overall electromagnetic radiation from the micro-inductor700can penetrate deeper than that of the micro-capacitor600. Hydration can be determined by measuring the change of impedance or reflectivity via the electrical inductance. In some embodiments, the micro-inductor700may be less sensitive to hydration changes than the micro-capacitor600; however, the greater penetration depth of the micro-inductor700will offer a trade-off between the micro-inductor700and the micro-capacitor600. Magnetic fields are generally not affected by electrical permittivity (i.e., dielectric constant). The material type of the dielectric substrate706is not as critical as for the dielectric layer604of the micro-capacitor600inFIG. 6. In some embodiments, a dielectric substrate706having a moderate ε′ and low ε″ can be used.

The micro-capacitor600and the micro-inductor700provide high spatial resolutions compared to the “near-field” techniques based on waveguide or Bull-eye-lens coupling ofFIGS. 4 and 5. This is because the resolution is determined by the physical width W, and W can be made relatively small compared to a wavelength, rendering deep-subwavelength spatial resolutions possible. In some embodiments, spatial resolutions in the range λ/100<W<λ/10 can be achieved for practical free-space wavelengths of excitation, λ.

Referring toFIGS. 8A and 8B, and continuing reference toFIGS. 3A and 3B, the hydration sensors300,302can be address leakage radiation issues to better improve the accuracy of the hydration sensors300,302. In some embodiments, the hydration sensors300,302can be more demanding in some respects than the typical radar because the SUT is much closer to the circulator306than a typical radar target. The proximity makes it impractical to form short pulses. Furthermore, the hydration sensors300,302can be susceptible to leakage radiation that propagates around the circulator306opposite to the preferred helicity, meaning propagation from the first port308to the third port312in addition to the desired second port310. Generally the leakage power is just a few percent or less of the preferred power; however the leakage can compromise the accuracy.

In some embodiments, with reference toFIGS. 8A and 8B, the hydration sensors300,302can be augmented with a second modulator802to amplitude modulate the power in the second port310with a different frequency f2, in addition to amplitude modulating the oscillator304with the first modulator322having a frequency f1. In some embodiments, one or both of f1and f2can be “baseband” frequencies, i.e., much less than the oscillator304frequency. In this embodiment, the hydration sensors300,302include a pin diode switch804. The pin-diode switch804modulates the RF power between the circulator port310and the coupling structure314. Thus,FIGS. 8A and 8Billustrate additional example embodiments of a hydration sensor300with an RF LNA, and a hydration sensor without an RF LNA followed by the Schottky rectifier (or other power sensor). The reflected power from the SUT will be double-modulated, and the power that goes from the first port308to the third port312(i.e., the “leakage” power) will be single-modulated. Using standard techniques of signal processing, analog or digital, the double-modulated power received in the third port312via the SUT port310can be discriminated from the single-modulated leakage power from the first port308, and the effect of leakage can be corrected with post processing.

It is to be understood that all aspects of the invention described above are amenable to operation in other regions of the electromagnetic spectrum besides the millimeter-wave region. From the tissue-interaction radiative phenomenology described inFIG. 2, there are two advantages to operate the hydration sensor at different frequencies. In some embodiments, THz-frequency components can attain better spatial resolutions than W-band component in proportion to the operating wavelength due to higher operating frequencies, e.g. 300 GHz and above or the THz region of the spectrum. For example, a 550-GHz centered sensor system has approximately 5.7 times the spatial resolution as a 94-GHz system. In some embodiments, such an operating frequency can utilized for corneal hydration sensing and/or other applications.

For many applications such as the whole-body hydration mapping described below, operating frequencies below the millimeter-wave region may become attractive for two reasons. First, there is greater penetration into the skin as seen in the plot ofFIG. 2B. For example, at operation around 10 GHz, the penetration depth becomes several millimeters. In regions on the body where the skin is relatively thick, this means that the hydration can be measured deep into the dermis. This is important because the dermis is the layer where skin maladies are often critical. For example, the treatment of skin burns is simple if the burn damage goes partially into the dermis, but may require surgical intervention (i.e., grafting) if the damage goes deep into the dermis or into the subcutaneous layer. Similarly, other skin disorders such as edema, a frequent problem in patients subject to long hospitalization, involve significant increases in hydration levels in the dermis. A second reason for lower frequency operation is cost and complexity. As a general rule in RF electronics, the power and overall performance of devices and components improve significantly as the frequency goes down.

The simplicity of the sensor schematic designs inFIGS. 4A and 4BorFIGS. 8A and 8B, along with the compactness of the near-field waveguide coupling inFIG. 5, lend themselves to multiple-frequency operation. For example, two-frequency operation could be carried out as shown inFIGS. 9A and 9B. The near-field coupling is done by two adjoined waveguides902and904. In some embodiments, the waveguides902,904are rectangular or substantially rectangular in shape. The first waveguide902can propagate the frequency f3and the second waveguide904can propagate the frequency f4where it was assumed f3<f4(note: these are the frequencies of the oscillators304, not the modulation frequencies). In some embodiments, the contact between both waveguides902,904and the SUT can be through a membrane502approach as inFIG. 5B. The electronic components for frequencies f3and f4, whether the design ofFIGS. 3A and 3BorFIGS. 8A and 8B, can be integrated compactly because they are all packaged in a waveguide, which has very low radiation cross-coupling. In some embodiments, the two frequencies can be complementary in the sense that the f3radiation will have greater depth of penetration but lower spatial resolution than the f4radiation. This facilitates estimation of the size of a hydrated target, such as a tumor or cyst.

One challenge with the two-frequency coupling would be mutual coupling. With enough frequency separation between f3and f4, it would be easy to isolate f3from the second waveguide904by a cutoff mechanism. In some embodiments, rectangular waveguides can have a critical frequency fc below which there is no propagation, e.g. a cutoff. So if f3<fc, low mutual coupling can occur from the second waveguide904to the first waveguide902. However, f4can then propagate in the first waveguide because metal waveguides may have higher-order spatial modes in which higher frequencies than f3can propagate. Hence, isolation can be accomplished using an electromagnetic wall906. In some embodiments, the electromagnetic wall906is a thin layer of a magnetic conductor such as iron or mu-metal.

In some embodiments, three or more frequencies can be used and operated in three or more different waveguides respectively. In other embodiments, separate frequencies within a same waveguide can be accomplished using frequency sweeping or hopping techniques.

With reference toFIG. 10, in some embodiments using interfacial coupling, multiple-frequency (or a pulsed) operation is possible when a micro-capacitor and/or a micro-inductor are designed to employ broadband frequencies. Because a coaxial transmission line is inherently broadband for its fundamental (TEM) mode, two fundamental modes with frequencies f3and f4respectively can be fed to the interfacial couplers (as shown inFIGS. 6 and 7) through a common coaxial transmission line. More than two frequencies are also possible using the interfacial coupler and coaxial transmission line.

In some embodiments, the schematic diagrams ofFIGS. 3A and 3BorFIGS. 8A and 8Bcan be integrated on a printed-circuit board or similar packaging technology. Printed circuit boards can be used for implementations of at least 30 GHz. In other embodiments, as the frequency approaches 100 GHz, monolithic integration can be used to form MMICs. In particular, monolithic integration can be used for RF LNAs are employed as in the designs ofFIG. 4BorFIG. 8B. In some embodiment, MMICs are fabricated from silicon, GaAs, or another semiconductor to obtain different performances and costs. In some embodiments, the circulator306is made of a magnetic material such as ferrite. The circulator306operates with a permanent magnet or electromagnet nearby which is facilitated with printed circuit board implementations.

In one experimental embodiment, the oscillator304was a 94-GHz Gunn diode mounted in a WR-10 waveguide. The circulator306was fabricated in WR-10 and had an isolation of ˜15 dB. The detector316was a WR-10-mounted Schottky rectifier. Experimental tests were conducted on voluntary human subjects measuring the reflectivity at various parts of the human body relative to an aluminum-plate acting as the CAS. The results are illustrated in a bar chart1100inFIG. 11. In another experimental embodiment, the WR-10-mounted Schottky rectifier was replaced with a WR-10 waveguide power sensor and reflectances were measured. The results are illustrated in a bar chart1102inFIG. 11B.

In another experimental embodiment, to demonstrate experimental performance with lower frequency and much greater penetration into skin tissue, a 30 GHz (Ka-band) hydration sensor based on the design ofFIG. 4Aand the high-resolution coupling ofFIG. 5Bwas assembled. The oscillator304was a 30-GHz Gunn diode mounted in WR-28 waveguide. The circulator306was packaged in WR-28 waveguide and had an isolation of ˜20 dB. The detector316was a WR-28-mounted Schottky rectifier. It was demonstrated on voluntary human subjects, measuring the reflectivity at various parts of the body relative to an aluminum-plate acting as the CAS. The results are illustrated in the bar chart1200inFIG. 12.

FIG. 13Ais an example embodiment of a hydration sensor1300that has a pistol-like shape including a battery compartment in a hand-grip.FIG. 13Ashows the hydration sensor1300being coupled to a human forearm. Besides whole-body capability, the unit includes real-time visual display and wireless connection to a computer or smartphone of choice. Many embodiments exist for the layout of the components and electronics inside the pistol form.

The innovation provides unprecedented accuracy of hydration measurement (approximately 1%). Conventional techniques, such as dc skin resistance measurements, are less accurate. In addition, the innovation is non-invasive, whereas dc-based resistance techniques require a more intimate contact between the sensor and the skin. Further, the innovation allows for no contact (e.g., horn-coupled version) or a loose contact with the skin (e.g., near-field waveguide coupler or interfacial-coupler).