Photoacoustic devices and systems including surface wave sensing components

Abstract

An apparatus may include a substrate, a light source system, and a receiver system. The light source system may be configured to emit light through a region of the substrate from a first side of the substrate toward a target object in contact with an opposite second side of the substrate. The receiver system may include one or more receivers present in, on, or near the substrate. The receiver system may be configured to detect surface acoustic waves propagating in the substrate corresponding to a photoacoustic response of the target object to the light emitted by the light source system. In some examples, an apparatus may include a control system. The control system may be configured to receive a surface acoustic wave signal from the receiver system corresponding to the detected surface acoustic wave, and detect at least one structure within the target object based on the surface acoustic wave signal.

Description

[0001] Cross - Reference to Related Applications

[0002] This application claims priority to U.S. Patent Application No. 18 / 319,414, entitled "PHOTOACOUSTIC DEVICES AND SYSTEMS INCLUDING SURFACE WAVE SENSING COMPONENTS", filed on May 17, 2023, and U.S. Provisional Patent Application No. 63 / 476,346, entitled "PHOTOACOUSTIC DEVICES AND SYSTEMS INCLUDING SURFACE WAVE SENSING COMPONENTS", filed on December 20, 2022. Both of the above - mentioned applications are hereby incorporated by reference in their entireties and for all purposes. Technical Field

[0003] This disclosure generally relates to photoacoustic devices and systems. Background Art

[0004] A variety of different sensing technologies and algorithms are being implemented in devices for various biometric and biomedical applications, including health and wellness monitoring. This push is in part due to the limited availability of traditional measurement devices for continuous, non - invasive, and ambulatory monitoring. Some such devices are, or include, photoacoustic devices. Although some previously deployed photoacoustic devices and systems can provide acceptable results, improved photoacoustic devices and systems would be desirable. Summary of the Invention

[0005] The systems, methods, and devices of this disclosure each have several aspects, none of which alone is responsible for the desirable attributes disclosed herein.

[0006] One innovative aspect of the subject matter described in this disclosure can be implemented in a device. In some embodiments, a mobile device (such as a wearable device, a cellular phone, etc.) can be, or can include, at least a part of the device.

[0007] According to some examples, the device may include a substrate, a light source system, and a receiver system. In some examples, the light source system may be configured to emit light through a region of the substrate from a first side of the substrate toward a target object in contact with a second side of the substrate opposite the first side. In some examples, the receiver system may include one or more receivers that reside in, on, or near the substrate. According to some examples, the receiver system may be configured to detect surface acoustic waves propagating in the substrate that correspond to a photoacoustic response of the target object to the light emitted by the light source system.

[0008] In some embodiments, the device may include a control system. The control system may include one or more general single-chip or multi-chip processors, digital signal processors (DSPs), application specific integrated circuits (ASICs), field programmable gate arrays (FPGAs), or other programmable logic devices, discrete gate or transistor logic, discrete hardware components, or combinations thereof. According to some examples, the control system may be configured to receive a surface acoustic wave signal corresponding to the detected surface acoustic waves from the receiver system and to detect at least one structure within the target object based on the surface acoustic wave signal.

[0009] According to some examples, the at least one structure may be a vascular structure. In some examples, the control system may also be configured to estimate one or more heart-related characteristics based at least in part on the at least one structure.

[0010] In some examples, the light source system may be configured to emit laser pulses. In some such examples, the light source system may be configured to emit laser pulses with a pulse width in the range of 3 nanoseconds to 1000 nanoseconds. In some examples, the laser pulses may be in the wavelength range of 500 nm to 1000 nm.

[0011] According to some examples, the substrate may be transparent. In some examples, the substrate may include a piezoelectric material.

[0012] In some examples, the combined thickness of the substrate and the light source system may be in the range of 2 mm to 5 mm. According to some examples, the thickness of the substrate may be in the range of 0.5 mm to 1.0 mm. In some examples, the total area of the substrate may be in the range of 0.5 cm 2 to 2.0 cm 2 range.

[0013] According to some examples, the receiver system may include a piezoelectric material. In some examples, the receiver system may include at least one receiver element located on a first side of the substrate. According to some examples, the receiver system may include at least one receiver element that is laterally offset from a region of the substrate in a first direction, and at least one receiver element that is laterally offset from a region of the substrate in an opposite second direction. In some examples, the receiver system may include at least one interdigital transducer.

[0014] In some examples, the control system may also be configured to detect blood within a blood vessel based on the surface acoustic wave signal. According to some examples, the blood vessel may be an artery.

[0015] According to some examples, a photoacoustic response may generate object acoustic waves within a target object. In some such examples, at least a portion of the object acoustic waves may be converted into surface acoustic waves that propagate in the substrate. In some examples, the object acoustic waves may include longitudinal ultrasonic waves.

[0016] In some examples, the device may include a substrate, a light source system, and a receiver system. In some examples, the light source system may be configured to emit light through a region of the substrate from a first side of the substrate toward a target object that contacts an opposite second side of the substrate. In some examples, the receiver system may include one or more receivers present in, on, or near the substrate. According to some examples, the receiver system may be configured to selectively detect one or more specific types of surface acoustic waves that propagate in the substrate and correspond to the photoacoustic response of the target object to the light emitted by the light source system.

[0017] According to some examples, the piezoelectric material may include a type of piezoelectric crystal that enhances the sensitivity of the receiver system to one or more specific types of surface acoustic waves. In some such examples, the crystal may have a piezoelectric crystal cut that enhances the sensitivity of the receiver system to one or more specific types of surface acoustic waves.

[0018] In some examples, the receiver system may include at least one interdigital transducer. According to some examples, the receiver system may include a piezoelectric material.

[0019] Other innovative aspects of the subject matter described in this disclosure can be implemented in methods. In some examples, a method can include: controlling, by a control system, a light source system to emit light through a region of a substrate from a first side of the substrate toward a target object in contact with a second, opposite side of the substrate. In some examples, a method can include: receiving, by the control system, from a receiver system, a surface acoustic wave signal corresponding to a surface acoustic wave propagating in the substrate, the receiver system including one or more receivers present in, on, or near the substrate. The surface acoustic wave can correspond to a photoacoustic response of the target object to the light emitted by the light source system. In some examples, a method can include: detecting, by the control system, at least one structure within the target object based on the surface acoustic wave signal.

[0020] In some examples, a method can include: estimating, at least in part, one or more heart-related characteristics based on the at least one structure. According to some examples, controlling the light source system can include: controlling the light source system to emit laser pulses. In some examples, the at least one structure can be a vascular structure.

[0021] Some or all of the methods described herein can be performed by one or more devices according to instructions (e.g., software) stored on a non-transitory medium. Such a non-transitory medium can include memory devices such as those described herein, including but not limited to random access memory (RAM) devices, read only memory (ROM) devices, and the like. Thus, some innovative aspects of the subject matter described in this disclosure can be implemented in one or more non-transitory media having software stored thereon. The software can include instructions for controlling one or more devices to perform one or more of the disclosed methods.

[0022] In some examples, a method can include: controlling, by a control system, a light source system to emit light through a region of a substrate from a first side of the substrate toward a target object in contact with a second, opposite side of the substrate. In some examples, a method can include: receiving, by the control system, from a receiver system, a surface acoustic wave signal corresponding to a surface acoustic wave propagating in the substrate, the receiver system including one or more receivers present in, on, or near the substrate. The surface acoustic wave can correspond to a photoacoustic response of the target object to the light emitted by the light source system. In some examples, a method can include: detecting, by the control system, at least one structure within the target object based on the surface acoustic wave signal.

[0023] In some examples, a method can include: estimating, at least in part, one or more heart-related characteristics based on the at least one structure. According to some examples, controlling the light source system can include controlling the light source system to emit laser pulses. In some examples, the at least one structure can be a vascular structure.

[0024] Details of one or more implementations of the subject matter described in this disclosure are set forth in the accompanying drawings and the description below. Other features, aspects, and advantages will become apparent from the specification, the drawings, and the claims. Note that the relative dimensions of the following figures may not be drawn to scale. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 is a block diagram showing example components of a device according to some disclosed implementations.

[0026] Figure 2 Shows example components of a device according to some disclosed implementations.

[0027] Figure 3 Shows example components of a device according to some alternative implementations.

[0028] Figure 4 Shows a bottom view of example components of a device according to some disclosed implementations.

[0029] Figure 5A , 5B , 5C, and 5D show examples of simulated photoacoustic waves and corresponding simulated surface acoustic waves.

[0030] Figure 6A is a flowchart showing examples of some disclosed operations.

[0031] Figure 6B is a flowchart showing examples of some additional disclosed operations.

[0032] Figure 7 Shows that according to Figure 6A method or Figure 6B Examples of heart rate waveform (HRW) features that can be extracted according to some implementations of the method.

[0033] Figure 8 Shows an example of a device that can be used in a system for estimating blood pressure based at least in part on pulse transit time (PTT).

[0034] Figure 9 Shows a cross-sectional side view of a graphical representation of a portion of an artery through which a pulse propagates.

[0035] Figure 10A Shows an example of a wearable dynamic monitoring device designed to be worn on the wrist according to some implementations.

[0036] Figure 10B Shows an example of a wearable dynamic monitoring device designed to be worn on a finger according to some implementations.

[0037] Figure 10CIllustrated is an example dynamic monitoring device designed to be present in or on an earbud, according to some embodiments.

[0038] Like reference numerals and names in the various figures indicate like elements. Detailed Description

[0039] For purposes of describing aspects of the present disclosure, the following description is directed to certain embodiments. However, those of ordinary skill in the art will readily recognize that the teachings herein can be applied in many different ways. Some of the concepts and examples provided in this disclosure are particularly applicable to blood pressure monitoring applications. However, some embodiments may also be applicable to other types of biosensing applications, as well as other fluid flow systems. The described embodiments can be implemented in any device, apparatus, or system that includes the devices disclosed herein. Additionally, it is contemplated that the described embodiments can be included in or associated with various electronic devices, such as, but not limited to: mobile phones, cellular phones supporting multimedia Internet, mobile TV receivers, wireless devices, smart phones, smart cards, wearable devices (such as bracelets, armbands, wristbands, rings, headbands, patches, etc.), Bluetooth® devices, personal data assistants (PDAs), wireless email receivers, handheld or portable computers, netbooks, notebooks, smartbooks, tablets, printers, copiers, scanners, fax devices, global positioning system (GPS) receivers / navigators, cameras, digital media players, game consoles, watches, clocks, calculators, TV monitors, flat panel displays, electronic reading devices (e.g., e-readers), mobile health devices, computer monitors, automotive displays (including odometer and speedometer displays, etc.), cockpit controls and / or displays, camera view displays (such as the display of a vehicle rearview camera), building structures, microwave ovens, refrigerators, stereo systems, cassette recorders or players, DVD players, CD players, VCRs (video cassette recorders), radios, portable storage chips, washing machines, dryers, washer / dryers, parking meters, vehicle doors, autonomous or semi-autonomous vehicles, drones, Internet of Things (IoT) devices, etc. Thus, these teachings are not intended to be limited to the specific embodiments depicted and described with reference to the figures; rather, these teachings have broad applicability, as will be apparent to those of ordinary skill in the art.

[0040] Non-invasive health monitoring devices, including but not limited to devices configured for photoacoustic plethysmography (PAPG), have various potential advantages compared to more invasive health monitoring devices such as cuff-based or catheter-based blood pressure measurement devices. However, it has proven very difficult to design satisfactory PAPG-capable health monitoring devices. For example, some PAPG-capable devices recently developed by the present assignee are advantageously configured to mitigate artifact signals such as electromagnetic interference (EMI) signals, signals from reflected light, and signals from reflected sound waves. However, some such devices may be too large to be conveniently deployed in wearable devices such as watches, patches, or earbuds.

[0041] Some disclosed devices include a substrate, a light source system, and a receiver system. The receiver system can be or can include a surface acoustic wave receiver system. According to some embodiments, the light source system can be configured to emit light toward a target object such as a finger or a wrist. The emitted light can include laser pulses. The receiver system can be configured to detect surface acoustic waves propagating in the substrate that correspond to the photoacoustic response of the target object to the emitted light. According to some examples, the device can include a control system. The control system can be configured to receive a surface acoustic wave signal corresponding to the detected surface acoustic waves from the receiver system. The control system can be configured to detect at least one structure within the target object based on the surface acoustic wave signal. The at least one structure can be a vascular structure such as an arterial structure. In some examples, the control system can be configured to detect changes in the artery corresponding to the phases of the cardiac cycle, such as changes in artery diameter, changes in artery distension, etc. The detected structure(s) or their changes can be used for various applications, such as for estimating heart-related characteristics such as blood pressure. Thus, some disclosed devices can be PAPG-capable.

[0042] Specific implementations of the subject matter described in this disclosure can be implemented to achieve one or more of the following potential advantages. Various disclosed configurations include PAPG-capable devices that are compact enough to be present in wearable devices. At least in part because the disclosed devices are configured to detect surface acoustic waves, the corresponding arrangements of the light source system, the substrate, and the receiver system allow for the implementation of very thin, compact, and lightweight devices, for example, with a total thickness between 2 mm and 5 mm.

[0043] Figure 1FIG. is a block diagram showing example components of an apparatus according to some disclosed embodiments. In this example, apparatus 100 includes substrate 101, receiver system 102, and light source system 104. Some embodiments of apparatus 100 may include control system 106, interface system 108, or both.

[0044] Substrate 101 may include various materials such as glass, acrylic, polycarbonate, etc. At least a portion of the substrate (through which the light source system provides light) should include a transparent material such as polycarbonate, glass, etc. According to some embodiments, at least a portion of the substrate may include a piezoelectric material such as lithium niobate (LiNO3), lead magnesium niobate-lead titanate (PMM-PT), quartz, etc.

[0045] At least a portion of the outer surface of substrate 101 may be configured, for example, to receive a target object such as a human finger. (As used herein, the terms "finger" and "digit" may be used interchangeably, and thus a thumb is an example of a finger.)

[0046] In some examples, substrate 101 may have a thickness in the range of 0.1 mm to 2 mm. According to some examples, at least the outer surface of substrate 101 may have an acoustic impedance configured to contrast with the acoustic impedance of human skin. For example, the acoustic impedance of substrate 101 may be configured to be substantially greater than the acoustic impedance of human skin. The conventional range of human skin acoustic impedance is 1.53–1.680 MRayls (1,530–1,680 x 10 3 kg / (sec m 2 ). In some examples, substrate 101 may include a material having an acoustic impedance in the range of 2.5 - 16.0 MRayls (2,500 - 16,000×10 3 kg / (sec m 2 ), such as polycarbonate, glass, quartz, or other piezoelectric crystals, etc.

[0047] In some examples, the light source system 104 may be configured to emit light through a region of the substrate 101 from a first side of the substrate 101 toward a target object in contact with a second, opposite side of the substrate 101. In some examples, the light source system 104 may be configured to emit laser pulses. In some examples, the light source system 104 may include one or more light emitting diodes. In some embodiments, the light source system 104 may include one or more laser diodes. According to some embodiments, the light source system 104 may include one or more vertical cavity surface emitting lasers (VCSELs). In some embodiments, the light source system 104 may include one or more edge emitting lasers. In some embodiments, the light source system may include one or more neodymium-doped yttrium aluminum garnet (Nd:YAG) lasers. In some such examples, the light source system 104 may be configured to emit laser pulses with a pulse width in the range of 3 nanoseconds to 1000 nanoseconds.

[0048] In some examples, the light source system 104 can be configured to emit laser pulses in a wavelength range of 500 nm to 1000 nm. In some examples, the light source system 104 can be configured to send light in one or more wavelength ranges. In some examples, the light source system 104 can be configured to send light in a wavelength range of 500 to 600 nanometers (nm). According to some examples, the light source system 104 can be configured to send light in a wavelength range of 800 to 950 nm. Considering factors such as skin reflectivity, fluence, absorption coefficients of blood and various tissues, and skin safety limits, one or both of these wavelength ranges can be suitable for various usage scenarios. For example, both the wavelength range of 500 nm to 600 nm and the wavelength range of 800 to 950 nm can be suitable for obtaining photoacoustic responses from relatively small, shallow blood vessels (such as blood vessels with a diameter of approximately 0.5 mm and a depth in the range of 0.5 mm to 1.5 mm, such as blood vessels that can be found in a finger). For example, the wavelength range of 800 to 950 nm can be suitable for obtaining photoacoustic responses from relatively large, deep blood vessels (such as blood vessels with a diameter of approximately 2.0 mm and a depth in the range of 2 mm to 3 mm, such as blood vessels that can be found in an adult wrist). In some embodiments, the light source system 104 can be configured to emit light of various wavelengths, which can be selectable to trigger acoustic wave emission primarily from a specific type of material. For example, because hemoglobin in blood strongly absorbs near-infrared light, in some embodiments, the light source system 104 can be configured to emit one or more wavelengths of light in the near-infrared range to trigger acoustic wave emission from hemoglobin. However, in some examples, the control system 106 can control the (one or more) wavelengths of the light emitted by the light source system 104 to preferentially induce acoustic waves in blood vessels, other soft tissues, and / or bones. For example, an infrared (IR) light-emitting diode (LED) can be selected and short pulses of IR light can be emitted to illuminate a portion of the target object and generate acoustic wave emission, which is then detected by the receiver system 102. In another example, an IR LED and a red LED or other colors, such as green, blue, white, or ultraviolet (UV), can be selected, and short pulses of light can be emitted sequentially from each light source, and an ultrasonic image can be obtained after light is emitted from each light source. In other embodiments, one or more light sources of different wavelengths can be activated sequentially or simultaneously to generate acoustic emissions that can be detected by a surface acoustic wave receiver. Image data from the surface acoustic wave receiver obtained at different depths in the target object using light sources of different wavelengths (e.g., obtained by changing the range gate delay (RGD)) can be combined to determine the location and type of materials in the target object.Since materials in the body typically absorb light of different wavelengths differently, image contrast may occur. When materials in the body absorb light of a specific wavelength, they may generate different amounts of heat and acoustic emissions of short enough light pulses with sufficient intensity. Light of different wavelengths and / or intensities at each selected wavelength can be used to obtain depth contrast. That is, continuous images can be obtained by varying the light intensity and wavelength at a fixed RGD (which can correspond to a fixed depth of the target object) to detect materials and their locations within the target object. For example, hemoglobin, blood glucose, or blood oxygen in blood vessels in a target object such as a finger can be detected photoacoustically.

[0049] According to some examples, the receiver system 102 can include one or more receivers present in, on, or near the substrate. In some examples, the receiver system 102 can include at least one receiver element located on one side of the substrate 101 where the light-emitting portion of the light source system 104 is present (this side may sometimes be referred to herein as the "first side" of the substrate 101). According to some examples, the receiver system 102 can include at least one receiver element that is laterally offset from a region of the substrate (the region through which the light source system transmits light) in a first direction and at least one receiver element that is laterally offset from the region of the substrate in an opposite second direction. In some examples, the receiver system 102 can include at least one interdigital transducer.

[0050] In some examples, the receiver system 102 can be configured to detect surface acoustic waves propagating in the substrate that correspond to the photoacoustic response of the target object to light emitted by the light source system. According to some examples, the photoacoustic response of the target object can include the photoacoustic response of blood vessels within the target object, blood within the blood vessels, or a combination thereof. In some cases, the blood vessels can be arteries. According to some examples, the photoacoustic response of the target object can generate target object acoustic waves within the target object. The target object acoustic waves can include longitudinal ultrasonic waves. At least some of the target object acoustic waves can be converted into surface acoustic waves propagating in the substrate.

[0051] In some examples, the receiver system 102 can include piezoelectric materials such as lithium niobate (LiNO3), lithium tantalate, lead magnesium niobate-lead titanate (PMM-PT), lithium tantalate, quartz, polyvinylidene fluoride (PVDF) polymer, polyvinylidene fluoride-trifluoroethylene (PVDF-TrFE) copolymer, piezoelectric composites, etc. In some embodiments, a single piezoelectric layer can be used as a surface acoustic wave receiver. In some embodiments, other piezoelectric materials can be used in the piezoelectric layer, such as aluminum nitride (AlN) or lead zirconate titanate (PZT). In some examples, a type of piezoelectric crystal, a piezoelectric crystal cut, or both can be selected to enhance the sensitivity of the receiver system 102 to one or more specific types of surface acoustic waves.

[0052] The control system 106 can include one or more general-purpose single-chip or multi-chip processors, digital signal processors (DSPs), application-specific integrated circuits (ASICs), field-programmable gate arrays (FPGAs), or other programmable logic devices, discrete gate or transistor logic, discrete hardware components, or combinations thereof. The control system 106 can also include (and / or be configured to communicate with) one or more memory devices, such as one or more random access memory (RAM) devices, read-only memory (ROM) devices, etc. Thus, the device 100 can have a memory system that includes one or more memory devices, although Figure 1 the memory system is not shown. The control system 106 can be configured to receive and process data from the receiver system 102, for example, as described below. If the device 100 includes an ultrasonic transmitter, the control system 106 can be configured to control the ultrasonic transmitter. In some embodiments, the functions of the control system 106 can be divided among one or more controllers or processors, such as a dedicated sensor controller and an application processor of a mobile device.

[0053] In some examples, the control system 106 can be configured to control the light source system 104. For example, the control system 106 can be configured to control one or more light-emitting portions of the light source system 104 to emit laser pulses. In some examples, the laser pulses can be in the wavelength range of 500 nm to 1000 nm. In some examples, the laser pulses can have a pulse width in the range of 3 nanoseconds to 1000 nanoseconds.

[0054] In some examples, the control system 106 can be configured to receive signals from the receiver system 102 that correspond to target ultrasonic waves generated by a target object in response to light from the light source system 104 and surface acoustic waves generated by the target ultrasonic waves. According to some examples, the control system 106 can be configured to receive surface acoustic wave signals corresponding to detected surface acoustic waves from the receiver system and to detect at least one structure within the target object based on the surface acoustic wave signals. In some examples, the at least one structure can be a vascular structure, such as an arterial structure. In some cases, the surface acoustic wave signals can correspond to a photoacoustic response of blood vessels within the target object, blood within the blood vessels, or a combination thereof. In some examples, the control system 106 can be configured to estimate one or more heart-related characteristics at least in part based on the surface acoustic wave signals. In some such examples, the control system 106 can be configured to estimate one or more heart-related characteristics at least in part based on at least one structure detected within the target object according to the surface acoustic wave signals. In some such examples, the control system 106 can be configured to estimate one or more heart-related characteristics at least in part based on changes in arterial dilation caused by phases of the cardiac cycle and detected using the surface acoustic wave signals. According to some examples, the heart characteristic can be or can include blood pressure.

[0055] Some embodiments of the apparatus 100 can include an interface system 108. In some examples, the interface system 108 can include a wireless interface system. In some embodiments, the interface system 108 can include a user interface system, one or more network interfaces, one or more interfaces between the control system 106 and the memory system, and / or one or more interfaces between the control system 106 and one or more external device interfaces (e.g., ports or application processors), or a combination thereof. According to some examples in which there is an interface system 108 and it includes a user interface system, the user interface system can include a microphone system, a speaker system, a haptic feedback system, a voice command system, one or more displays, or a combination thereof. According to some examples, the interface system 108 can include a touch sensor system, a gesture sensor system, or a combination thereof. The touch sensor system (if present) can be or can include a resistive touch sensor system, a surface capacitive touch sensor system, a projected capacitive touch sensor system, a surface acoustic wave touch sensor system, an infrared touch sensor system, any other suitable type of touch sensor system, or a combination thereof.

[0056] In some examples, the interface system 108 may include a force sensor system. The force sensor system, if present, may be or may include a piezoresistive sensor, a capacitive sensor, a thin film sensor (e.g., a polymer-based thin film sensor), other types of suitable force sensors, or combinations thereof. If the force sensor system includes a piezoresistive sensor, the piezoresistive sensor may include silicon, metal, polysilicon, glass, or combinations thereof. In some examples, the interface system 108 may include an optical sensor system, one or more cameras, or combinations thereof.

[0057] Device 100 can be used in a variety of different environments, many examples of which are disclosed herein. For example, in some embodiments, a mobile device may include device 100. In some such examples, the mobile device may be a smart phone. In some embodiments, a wearable device may include device 100. For example, the wearable device may be a bracelet, an armband, a wristband, a watch, a ring, a headband, or a patch.

[0058] Figure 2 An example component of a device according to some disclosed embodiments is shown. Similar to other figures provided herein, Figure 1 likewise, Figure 2 the number, type, and arrangement of the elements shown are presented by way of example only. In this example, device 100 is Figure 1 an instance of the device 100 shown in the figure. According to this example, device 100 includes a substrate 101, a receiver system 102, and a light source system 104.

[0059] In this example, the outer surface 208b of the substrate 101 is configured to receive a target object, such as a finger 255, a wrist, etc. In Figure 2 the figure, only a portion of the finger 255 is shown. In this example, the substrate 101 includes substrate regions 201a, 201b, and 201c. According to this example, at least substrate region 201a is transparent, allowing light 250 from the light source system 104 to be transmitted from the first side of the substrate adjacent to the light source system 104 towards (and in this example, into) the target object in contact with the opposite second side of the substrate 101. In Figure 2 the figure, surface 208a is an example of the first side of the substrate 101, and the outer surface 208b is an example of the second side of the substrate 101. Thus, at least region 201a of the substrate 101 includes a transparent material, such as glass, acrylic resin, polycarbonate, etc.

[0060] In some examples, the substrate 101 may have a thickness in the range of 0.1 mm to 2 mm. According to some examples, the substrate 101 may have an acoustic impedance configured to contrast with the acoustic impedance of human skin. For example, the acoustic impedance of the substrate 101 may be configured to be substantially greater than the acoustic impedance of human skin. The conventional range of the acoustic impedance of human skin is 1.53–1.680 MRayls. In some examples, the substrate 101 may have an acoustic impedance in the range of 2.5 - 16.0 MRAyls, such as polycarbonate, glass, quartz, or other piezoelectric crystals.

[0061] According to this example, the receiver system 102 is, or includes, a surface acoustic wave receiver system configured to detect surface acoustic waves 222. In this example, the receiver system 102 includes surface acoustic wave receiver elements 102a and 102b, and piezoelectric film layers 202a and 202b. According to this example, the surface acoustic wave receiver element 102a is present on the piezoelectric film layer 202a in the substrate region 201b. In this example, the surface acoustic wave receiver element 102b is present on the piezoelectric layer 202b in the substrate region 201c. Thus, in this example, the receiver system 102 includes at least one surface acoustic wave receiver element that is laterally offset from the substrate region 201a in a first direction, and at least one surface acoustic wave receiver element that is laterally offset from the substrate region 201a in an opposite second direction. In some examples, the surface acoustic wave receiver elements 102a and 102b may include interdigital transducers. Some examples of suitable interdigital transducers are described herein with reference to Figure 4 Some examples of suitable interdigital transducers are described.

[0062] The piezoelectric layers 202a and 202b may include, for example, polyvinylidene fluoride (PVDF) polymers, polyvinylidene fluoride-trifluoroethylene (PVDF-TrFE) copolymers, aluminum nitride (AlN), lead zirconate titanate (PZT), piezoelectric composites (such as 1-3 composites, 2-2 composites, 3-3 composites, etc.), or combinations thereof. In some examples, the piezoelectric layers 202a and 202b may include one or more piezoelectric crystals, such as lithium niobate (LiNO3), lithium tantalate, lead magnesium niobate-lead titanate (PMM-PT), lithium tantalate, quartz, etc.

[0063] According to some examples, the receiver system 102 can be configured to selectively detect surface acoustic waves propagating in the substrate that correspond to the photoacoustic response of a target object to light emitted by the light source system. In some such examples, a type of piezoelectric crystal, a piezoelectric crystal cut, or both can be selected to enhance the sensitivity of the receiver system 102 to one or more specific types of surface acoustic waves. For example, 36-degree YX-cut lithium tantalate is suitable for generating or receiving shear horizontal waves. 128-degree YX-cut lithium niobate is suitable for generating or receiving Rayleigh waves. Thus, the receiver system 102 can be configured to be relatively more sensitive to one or more specific types of surface acoustic waves than to other types of surface acoustic waves.

[0064] Alternatively or additionally, in some examples, the receiver system 102 can be configured to selectively detect a specific type of surface acoustic wave propagating in the substrate. In some examples, the selective detection can be performed according to a selected range gate delay (RGD), which can be applied by the control system 106. The RGD can correspond to the time interval for detecting a specific type of surface acoustic wave propagating in the substrate that corresponds to the photoacoustic response of a target object to light emitted by the light source system. Longitudinal acoustic waves, shear acoustic waves, and Rayleigh acoustic waves generally have different speeds within the same medium. Thus, the receiver system 102 configured with a specific RGD can selectively detect, for example, a shear horizontal wave that corresponds to the photoacoustic response of a target object to light from the light source system 104 and that corresponds to the travel time based on the speed of the shear horizontal wave in the medium through which it travels. The receiver system 102 configured with another RGD can selectively detect a Rayleigh wave that corresponds to the photoacoustic response of a target object. The RGD can correspond to the expected travel time of the Rayleigh wave based on the speed of the Rayleigh wave in the medium through which it travels. Alternatively or additionally, in some examples, the selective detection can be performed according to characteristic features of the shear horizontal wave, characteristic features of the Rayleigh wave, etc. (such as wave propagation type, waveform, etc.). For example, the control system can be configured to detect a Rayleigh wave based at least in part on the retrograde elliptical motion that is characteristic of a near-surface Rayleigh wave.

[0065] According to this example, the light source system 104 includes at least a first light-emitting component (in this example, the light-emitting component 235a) and the light source system circuit 245a. For example, the light-emitting component 235a can include one or more light-emitting diodes, one or more laser diodes, one or more VCSELs, one or more edge-emitting lasers, one or more neodymium-doped yttrium aluminum garnet (Nd:YAG) lasers, or a combination thereof.

[0066] In this example, the light source system 104 is configured to emit light through the substrate region 201a. According to this example, the light source system 104 is configured to send light 250 to the finger 255 in contact with the substrate region 201a, and the light 250 passes through the substrate region 201a. In this example, the blood vessel 207, the blood within the blood vessel 207, or both generate photoacoustic waves 211 in response to the light 250. According to some examples, the blood vessel 207 can be an artery. In this example, the photoacoustic waves 211 are converted into surface acoustic waves 222 in the substrate 101.

[0067] According to some examples, the control system 106 ( Figure 2 not shown) can be configured to estimate one or more heart-related characteristics based at least in part on the surface acoustic wave signals received from the receiver system 102 that correspond to the photoacoustic response of the blood vessel 207, the blood within the blood vessel 207, or a combination thereof. In some such examples, the control system 106 can be configured to detect a change in the diameter of the blood vessel 207 based on the surface acoustic wave signals. According to some examples, the blood vessel 207 can be an artery. In some such examples, the control system 106 can be configured to detect a change in the arterial dilation caused by the phases of the cardiac cycle based at least in part on the surface acoustic wave signals. In some examples, the control system is capable of estimating the pulse wave velocity (PWV) of the propagating pulse within the artery based at least in part on the surface acoustic wave signals. Some examples are described below with reference to Figures 8 - 10C description.

[0068] In Figure 2 the example shown, the thickness (along the z-axis) of the device 101 from the top of the substrate 101 to the bottom of the light source system circuit 245a is T. In some examples, T can be in the range of 2 mm to 5 mm. According to some examples, the substrate 101 can have a thickness in the range of 0.1 mm to 1.5 mm. In some examples, the light source system 104 can have a thickness in the range of 0.5 mm to 1.5 mm. According to some examples, the distance between the light source system 104 and the surface 208a can be less than 1 mm.

[0069] Figure 3 An example component of a device according to some alternative embodiments is shown. Like other attachments provided herein, Figure 1 the number, type, and arrangement of the elements shown Figure 3 are presented by way of example only. In this example, the device 100 is Figure 1 an instance of the device 100 shown. According to this example, the device 100 includes a substrate 101, a receiver system 102, and a light source system 104.

[0070] Figure 3 The device 101 shown in Figure 2The devices shown in [reference] are very similar. Therefore, all details will not be repeated here. Instead, Figure 3 the description will mainly focus on Figure 2 the differences between Figure 3 and

[0071] According to this example, the receiver system 102 is or includes a surface acoustic wave receiver system configured to detect surface acoustic waves 222. In this example, the receiver system 102 includes surface acoustic wave receiver elements 102a and 102b. However, different from the example shown in Figure 2 , in this example, the receiver system 102 does not include piezoelectric thin film layers 202a and 202b. According to this example, the substrate 101 itself is partially or completely formed of a piezoelectric material. In such an example, the substrate 101 can be considered as part of the receiver system 102.

[0072] In some examples, the substrate 101 may include one or more piezoelectric crystals, such as lithium niobate (LiNO3), lithium tantalate, lead magnesium niobate-lead titanate (PMM-PT), lithium tantalate, quartz, etc. In some such examples, the receiver system 102 can be configured to selectively detect surface acoustic waves 222, for example, as described above with reference to Figure 2 . In some examples, a type of piezoelectric crystal, a piezoelectric crystal cut, or both can be selected to enhance the sensitivity of the receiver system 102 to one or more specific types of surface acoustic waves. In one such example, the substrate 101 may include 128-degree YX-cut lithium niobate. As pointed out elsewhere herein, 128-degree YX-cut lithium niobate is suitable for generating or receiving Rayleigh waves. In other embodiments, the substrate 101 may include different types of piezoelectric crystals, different types of piezoelectric crystal cuts, or a combination thereof. Therefore, the receiver system 102 can be configured to be more sensitive to one or more specific types of surface acoustic waves at least partially based on the piezoelectric material of the substrate 101.

[0073] Figure 4 Shows a bottom view of an example component of a device according to some disclosed embodiments. Similar to other attachments provided herein, Figure 1 the number, type, and arrangement of the elements shown in Figure 4 are presented only by way of example. In this example, the device 100 is Figure 1 an instance of the device 100 shown in

[0074] Because Figure 4 what is shown is a bottom view, the viewing direction is towards the surface 208a of the substrate 101. As Figure 2 and Figure 3As shown, surface 208a is adjacent to light source system 104, thus Figure 4 shows the bottom of the light source system circuit. In this example, substrate 101 includes a piezoelectric material, which in some examples can be substantially as described above with reference to Figure 2 . In other examples, substrate 101 may not include a piezoelectric material. In some such examples, Figure 2 the piezoelectric thin film layers 202a and 202b of Figure 4 would be visible in a bottom view similar to

[0075] According to this example, receiver system 102 is, or includes, a surface acoustic wave receiver system configured to detect surface acoustic waves 222. In this example, receiver system 102 includes surface acoustic wave receiver elements 102a and 102b. According to this example, surface acoustic wave receiver elements 102a and 102b include interdigital transducers (IDTs), the IDT fingers 405 of which are shown in Figure 4 . In some examples, the IDT can be a thin (such as less than 2 microns) metal electrode deposited on substrate 101, patterned and etched. In this example, surface acoustic wave receiver elements 102a and 102b include IDT fingers 405 having a dual electrode or "split" electrode configuration. Other examples of receiver system 102 may include other types of surface acoustic wave receiver elements, such as surface acoustic wave receiver elements having IDT fingers 405 with a single electrode configuration.

[0076] The size and layout of the IDT fingers 405 can depend at least in part on the type of IDT, the speed of the surface acoustic waves in substrate 101, and one or more frequencies of interest. In some embodiments of substrate 101, the speed of the surface acoustic waves in substrate 101 can be about 4000 m / s. In some examples, the frequency of interest can be in the range of 2 MHz to 10 Mhz. For a dual electrode configuration, it is advantageous for the width W of each of the dual fingers to be λ / 8 and the center-to-center (C2C) distance to be λ / 4, where λ is the wavelength of the received surface acoustic wave. For example, if the speed of the surface acoustic waves in substrate 101 is 4000 m / s and the frequency of interest is 10 MHz, then λ is 0.4 mm.

[0077] Figure 4Also shown are example components of the receiver system circuitry 410. In this example, the receiver system circuitry 410 includes amplifiers 415a and 415b and a receiver system control system 406, where amplifiers 415a and 415b are radio frequency amplifiers in this example. According to this example, amplifier 415a is configured to amplify the output signal “Output of 102a” from receiver element 102a and provide a corresponding amplified signal to the receiver system control system 406. Similarly, in this example, amplifier 415b is configured to amplify the output signal “Output of 102b” from receiver element 102b and provide a corresponding amplified signal to the receiver system control system 406. For example, the control system 406 can be a component of the control system 106 described herein with reference to Figure 1 the control system 106 described herein.

[0078] Figure 5A , Figure 5B , Figure 5C and Figure 5D show examples of an analog photoacoustic wave and a corresponding analog surface acoustic wave. In these examples, Figure 5A represents one time instance of the simulation, and Figures 5B - 5D represents a subsequent time instance of the simulation. According to these examples, the tissue 505 is human tissue. In these examples, the substrate 101 is a piezoelectric substrate, which includes lithium niobate in this example and has a thickness of 700 micrometers.

[0079] In Figure 5A the example shown, the blood vessel 207 within the tissue 505 has been irradiated with a laser pulse and is generating a photoacoustic wave 211. By the time corresponding to Figure 5B , the photoacoustic wave 211 has propagated further within the tissue 505. Some of the photoacoustic wave 211 has reached the substrate 101 and has generated a surface acoustic wave 222 within the substrate.

[0080] By the time corresponding to Figure 5C , the photoacoustic wave 211 has been reflected from the substrate 101, generating a reflected photoacoustic wave 211a. Additionally, the surface acoustic wave 222 has propagated within the substrate, substantially along the x-axis in this view. The direction of propagation of the surface acoustic wave (SAW) is indicated by the marked arrow. By the time corresponding to Figure 5D , the photoacoustic wave 211, the reflected wave, and the surface acoustic wave 222 have propagated further: some of the surface acoustic wave 222 has propagated to the outer edge of the substrate 101.

[0081] Figure 6A is a flowchart showing an example of some disclosed operations. Figure 6A The blocks of (and the blocks of other flowcharts provided herein) can be performed, for example, by Figure 1 the apparatus 100 of or a similar apparatus. As with other methods disclosed herein,Figure 6A The methods outlined in Figure 6A may include more or fewer blocks than those indicated. Additionally, the blocks of the methods disclosed herein are not necessarily executed in the order indicated. In some cases,

[0082] In this example, block 605 involves a control system controlling a light source system (which may be an example of the light source system 104 and the control system 106 of Figure 1 ) to emit light through a region of a substrate from a first side of the substrate towards a target object in contact with a second, opposite side of the substrate. In some examples, block 605 may include controlling the light source system to emit one or more laser pulses. Depending on the specific example, the target object may be a finger, a wrist, etc.

[0083] According to this example, block 610 includes a control system receiving a surface acoustic wave signal corresponding to surface acoustic waves propagating in the substrate from a receiver system, the receiver system including one or more receivers present in, on, or near the substrate. The receiver system may be an example of the receiver system 102 of Figure 1 , Figure 2 or Figure 3 . Thus, in some examples, block 610 may include receiving the surface acoustic wave signal from the surface acoustic wave receiver elements 102a and 102b of Figure 2 , Figure 3 or Figure 4 . In this example, the surface acoustic waves correspond to a photoacoustic response of the target object to the light emitted by the light source system. Figure 2 , Figure 3 and Figures 5A - 5D The surface acoustic waves 222 of

[0084] are examples of such surface acoustic waves.

[0085] Figure 6B is a flowchart showing an example of some additional disclosed operations. Figure 6B The blocks of Figure 1 (and the blocks of other flowcharts provided herein) may be performed, for example, by the device 100 of Figure 6B or a similar device. As with other methods disclosed herein, the methods outlined in Figure 6BOne or more of the blocks shown may be executed simultaneously.

[0086] In this example, block 655 includes controlling, by a control system, a light source system to emit light toward a target object on or near an outer surface of a platen. The light source system and the control system may be Figure 1 instances of the light source system 104 and the control system 106. Depending on the particular example, the target object may be a finger, a wrist, etc. According to this example, block 660 includes receiving, by the control system, a signal from a surface acoustic wave receiver system (which may be an Figure 1 instance of the receiver system 102), the signal corresponding to ultrasonic waves generated by the target object in response to light emitted by the light source system. Figure 2 , Figure 3 and Figures 5A - 5D The photoacoustic wave 211 is an example of such ultrasonic waves. In some examples, block 660 may include receiving the signal from an Figure 2 , Figure 3 or Figure 4 surface acoustic wave receiver element 102a and 102b of the surface acoustic wave receiver system.

[0087] According to this example, block 665 includes identifying, by the control system, a vascular signal from the surface acoustic wave receiver system, the vascular signal corresponding to ultrasonic waves generated by blood within a blood vessel of the target object, by one or more blood vessel walls, or a combination thereof. According to some examples, block 665 may include identifying, by the control system, an arterial signal from the surface acoustic wave receiver system, the arterial signal corresponding to ultrasonic waves generated by blood within an artery of the target object passing through one or more arterial walls, or a combination thereof. For example, the vascular signal may be identified by implementing a range gate delay (RGD) corresponding to an expected depth of the blood vessel. Alternatively or additionally, the arterial signal may be identified based on one or more characteristics of the photoacoustic response of the blood vessel wall, blood, or a combination thereof. According to some examples, the arterial signal and the venous signal may be distinguished at least in part based on an estimated concentration of oxyhemoglobin (HbO2) in the blood within the blood vessel.

[0088] In this example, block 670 includes estimating, by the control system, one or more cardiac characteristics at least in part based on the vascular signal. In some examples, block 670 may include estimating blood pressure at least in part based on the vascular signal. In some such examples, block 670 may include estimating blood pressure at least in part based on the arterial signal. According to some examples, another aspect of block 670 or method 650 may include extracting and evaluating heart rate waveform (HRW) features.

[0089] Figure 7 illustrates examples of heart rate waveform (HRW) features that may be extracted according to some implementations of the Figure 6A method or the Figure 6B method.Figure 7 The horizontal axis represents time, and the vertical axis represents signal amplitude. The cardiac cycle is indicated by the time between adjacent peaks of the HRW. Systolic and diastolic time intervals are indicated below the horizontal axis. During the systolic phase of the cardiac cycle, as the pulse propagates along the artery through a specific location, the arterial wall expands according to the pulse waveform and the elastic properties of the arterial wall. Associated with the expansion is a corresponding increase in the blood volume at the specific location or region, and as the blood volume increases, one or more properties in that region change accordingly. Conversely, during the diastolic phase of the cardiac cycle, the arterial blood pressure drops, and the arterial wall contracts. Associated with the contraction is a corresponding decrease in the blood volume at the specific location, and as the blood volume decreases, one or more properties in that region change accordingly.

[0090] Figure 7 The HRW features shown in involve the widths of the systolic and / or diastolic portions of the HRW curve at different "heights", which are indicated as a percentage of the maximum amplitude. For example, the SW50 feature is the width of the systolic portion of the HRW curve at the "height" of 50% of the maximum amplitude. In some embodiments, the HRW features used for blood pressure estimation may include some or all of the HRW features such as SW10, SW25, SW33, SW50, SW66, SW75, DW10, DW25, DW33, DW50, DW66, and DW75. In other embodiments, additional HRW features may be used for blood pressure estimation. In some cases, such additional HRW features may include the sum and ratio of SW and DW at one or more "heights", such as (DW75 + SW75), DW75 / SW75, (DW66 + SW66), DW66 / SW66, (DW50 + SW50), DW50 / SW50, (DW33 + SW33), DW33 / SW33, (DW25 + SW25), DW25 / SW25, and / or (DW10 + SW10), DW10 / SW10. Other embodiments may use additional HRW features for blood pressure estimation. In some cases, such additional HRW features may include sums, differences, ratios, and / or other operations based on more than one "height", such as (DW75 + SW75) / (DW50 + SW50), (DW50 + SW50 / (DW10 + SW10), etc.

[0091] Figure 8 Shows an example of a device that can be used in a system for estimating blood pressure based at least in part on pulse transit time (PTT). As with other appendices provided herein Figure 1Likewise, the number, type, and arrangement of the components are presented only by way of example. According to this example, system 800 includes at least two sensors. In this example, system 800 includes at least an electrocardiogram sensor 805 and a device 810 configured to be mounted on a finger of a person 801. In this example, device 810 is, or includes, a device configured to perform at least some of the PAPG methods disclosed herein. For example, device 810 may be, or may include Figure 1 device 100 or a similar device.

[0092] As shown in chart 820, the pulse arrival time (PAT) includes two parts: the pre-ejection period (PEP, the time required to convert an electrical signal into a mechanical pumping force and isovolumetric contraction to open the aortic valve) and PTT. The start time of PAT can be estimated based on the QRS complex (the electrical signal characteristics of ventricular electrical stimulation). As shown in chart 820, in this example, the start of PAT can be calculated based on the peak of the R wave measured by the electrocardiogram sensor 805, and the end of PAT can be detected via analysis of the signal provided by device 810. In this example, it is assumed that the end of PAT corresponds to the intersection point between the tangent of the local minimum detected by device 810 and the tangent of the maximum slope / first derivative of the sensor signal after the minimum time.

[0093] There are many known blood pressure estimation algorithms based on PTT and / or PAT, some of which are summarized in Table 1 and described in the corresponding text on pages 5-10 of Sharma, M. et al., Cuff-Less and Continuous Blood Pressure Monitoring: a Methodological Review (“Sharma”), in Multidisciplinary Digital Publishing Institute (MDPI) Technologies 2017, 5, 21, both of which are incorporated herein by reference.

[0094] Some previously disclosed methods have involved calculating blood pressure based on one or more equations shown in Table 1 of Sharma or other known equations, based on PTT and / or PAT measured by a sensor system including a PPG sensor. As described above, some of the disclosed PAPG-based embodiments are configured to distinguish arterial HRW from other HRW. Such embodiments can provide more accurate PTT and / or PAT measurements relative to PTT and / or PAT measured by a PPG sensor. Thus, the disclosed PAPG-based embodiments can provide more accurate blood pressure estimates, even when the blood pressure estimate is based on previously known formulas.

[0095] Other embodiments of system 800 may not include electrocardiogram sensor 805. In some such embodiments, a device 815 configured to be mounted on the wrist of a person 801 may be, or may include, means configured to perform at least some of the PAPG methods disclosed herein. For example, device 815 may be or may include Figure 2 device 200 or a similar device. According to some such examples, device 815 may include a light source system and two or more surface acoustic wave receivers. An example is described below with reference to Figure 10A In some examples, device 815 may include an array of surface acoustic wave receivers.

[0096] In some embodiments of system 800 that do not include electrocardiogram sensor 805, device 810 may include a light source system and two or more surface acoustic wave receivers. An example is described below with reference to Figure 10B An example.

[0097] Figure 9 A cross-sectional side view showing a graphical representation of a portion of an artery 900 (through which a pulse 902 propagates) is shown. Figure 9 The boxed arrows in show the direction of blood flow and pulse propagation. As shown, the propagating pulse 902 causes strain in the arterial wall 904, which manifests as an expansion of the diameter (and thus cross-sectional area) of the arterial wall - referred to as "dilation". The spatial length L of the actual propagating pulse along the artery (in the direction of blood flow) is typically comparable to the length of a limb, such as the distance from a subject's shoulder to the subject's wrist or finger, and is typically less than one meter (m). However, the length L of the propagating pulse can vary significantly between different subjects and, for a given subject, can vary significantly over time depending on various factors. The spatial length L of the pulse generally decreases as the distance from the heart increases until the pulse reaches the capillaries.

[0098] As described above, some embodiments relate to devices, systems, and methods for estimating blood pressure or other cardiovascular characteristics based on an estimate of an arterial dilation waveform. Unless otherwise specified, the terms "estimate," "measure," "calculate," "infer," "derive," "evaluate," "determine," and "monitor" may be used interchangeably herein where appropriate. Similarly, derivatives based on the roots of these terms may also be used interchangeably where appropriate; for example, the terms "estimate," "measure," "calculate," "infer," and "determine" may also be used interchangeably herein. In some embodiments, the pulse wave velocity (PWV) of a propagating pulse may be estimated by measuring the pulse transit time (PTT) when a pulse propagates from a first physical location along an artery to another, more distal second physical location along the artery. It should be understood that this PTT is different from the PTT described above. However, either version of the PTT may be used for the purpose of blood pressure estimation. Assuming that the physical distance ΔD between the first and second physical locations is determinable, the PWV may be estimated as the quotient of the physical space distance ΔD traveled by the pulse divided by the time (PTT) it takes for the pulse to travel that physical space distance ΔD. Generally, a first sensor positioned at the first physical location is used to determine the start time (also referred to herein as the "first time location") at which a pulse arrives at or propagates through the first physical location. A second sensor at the second physical location is used to determine the end time (also referred to herein as the "second time location") at which the pulse arrives at or propagates through the second physical location and continues through the remainder of the arterial branch. In such an example, the PTT represents the time distance (or time difference) between the first time location and the second time location (start time and end time).

[0099] The fact that the arterial dilation waveform is measured at two different physical locations means that the estimated PWV inevitably represents an average of the entire path distance ΔD traveled by the pulse between the first and second physical locations. More specifically, the PWV generally depends on a number of factors, including blood density ρ, the stiffness E (or conversely, elasticity) of the arterial wall, arterial diameter, arterial wall thickness, and blood pressure. Since arterial wall elasticity and the reference resting diameter (e.g., the diameter at the end of ventricular diastole) vary widely throughout the arterial system, the PWV estimate obtained from PTT measurements is inherently an average (averaged over the entire path length ΔD between the two locations where the measurements are made).

[0100] In traditional methods for obtaining PWV, the start time of the pulse is obtained from the heart by detecting the electrical signals of the heart using an electrocardiogram (ECG) sensor. For example, the start time can be estimated based on the QRS complex (the electrical signal characteristics of ventricular electrical stimulation). In such methods, the end time of the pulse is typically obtained using a different sensor located at a second location (e.g., a finger). As will be understood by those of ordinary skill in the art, there are many arterial discontinuities, branches, and variations along the entire path length from the heart to the finger. Along each stretch of the entire path length from the heart to the finger, the variation in PWV can reach or exceed an order of magnitude. Therefore, PWV estimation based on such a long path length is unreliable.

[0101] In various embodiments described herein, PTT estimation is obtained based on measurements associated with arterial dilation signals (also referred to as "arterial dilation data" or more generally as "sensor data") obtained by each of a first arterial dilation sensor 906 and a second arterial dilation sensor 908 along an artery of interest, respectively, proximate first and second physical locations. In some particular embodiments, the first arterial dilation sensor 906 and the second arterial dilation sensor 908 are advantageously positioned near the first and second physical locations where arterial characteristics (such as wall elasticity and diameter) of the artery of interest between the first and second physical locations can be considered or assumed to be relatively constant. In this way, the PWV calculated based on the PTT estimation is more representative of the actual PWV along a specific segment of the artery. Subsequently, the blood pressure P estimated based on the PWV is more representative of the true blood pressure. In some embodiments, the magnitude of the spacing distance ΔD between the first arterial dilation sensor 906 and the second arterial dilation sensor 908 (and thus the distance between the first and second positions along the artery) can range from about 1 centimeter (cm) to several tens of centimeters - long enough to distinguish the arrival of the pulse at the first physical location and the arrival of the pulse at the second physical location, yet close enough to provide sufficient assurance of arterial consistency. In some particular embodiments, the distance ΔD between the first arterial dilation sensor 906 and the second arterial dilation sensor 908 can range from about 1 cm to about 30 cm, and in some embodiments, is less than or equal to about 20 cm, and in some embodiments, is less than or equal to about 10 cm, and in some particular embodiments, is less than or equal to about 5 cm. In some other embodiments, the distance ΔD between the first arterial dilation sensor 906 and the second arterial dilation sensor 908 can be less than or equal to 1 cm, such as about 0.1 cm, about 0.25 cm, about 0.5 cm, or about 0.75 cm. As a reference, a conventional PWV can be about 15 meters per second (m / s). Using a dynamic monitoring device in which the first arterial dilation sensor 906 and the second arterial dilation sensor 908 are spaced a distance of about 5 cm apart, and assuming a PWV of about 15 m / s implies a PTT of about 3.3 milliseconds (ms).

[0102] The value of the magnitude of the distance ΔD between the first arterial dilation sensor 906 and the second arterial dilation sensor 908 can be pre-programmed, respectively, into a memory within a monitoring device associated with the sensors (e.g., such as the memory of the control system 106 described above with reference to Figure 1 or a memory configured to communicate therewith). As will be understood by those of ordinary skill in the art, in such an embodiment, the spatial length L of the pulse can be greater than the distance ΔD from the first arterial dilation sensor 906 to the second arterial dilation sensor 908. Thus, although Figure 9The illustrated graphical pulse 902 is shown as having a spatial length L equivalent to the distance between the first arterial dilation sensor 906 and the second arterial dilation sensor 908. However, in reality, each pulse can typically have a spatial length L that is greater than, and even much greater than (e.g., approximately an order of magnitude or more), the distance ΔD between the first arterial dilation sensor 906 and the second arterial dilation sensor 908.

[0103] Sensing Architecture and Topology

[0104] In some embodiments of the dynamic monitoring device disclosed herein, both the first arterial dilation sensor 906 and the second arterial dilation sensor 908 are sensors of the same sensor type. In some such embodiments, the first arterial dilation sensor 906 and the second arterial dilation sensor 908 are the same sensor. In such embodiments, each of the first arterial dilation sensor 906 and the second arterial dilation sensor 908 utilizes the same sensor technology that has the same sensitivity to arterial dilation signals caused by propagating pulses and has the same time delay and sampling characteristics. In some embodiments, each of the first arterial dilation sensor 906 and the second arterial dilation sensor 908 is configured for photoacoustic plethysmography (PAPG) sensing, e.g., as disclosed elsewhere herein. Some such embodiments include a light source system and two or more surface acoustic wave receivers, which can be Figure 1 instances of the light source system 104 and the receiver system 102. In some embodiments, each of the first arterial dilation sensor 906 and the second arterial dilation sensor 908 is configured for ultrasonic sensing via the transmission of ultrasonic signals and the reception of corresponding reflections. In some alternative embodiments, each of the first arterial dilation sensor 906 and the second arterial dilation sensor 908 can be configured for impedance plethysmography (IPG) sensing, also known as bioimpedance sensing in a biomedical context. In various embodiments, regardless of the type of sensor used, each of the first arterial dilation sensor 906 and the second arterial dilation sensor 908 is widely used to capture and provide arterial dilation data that indicates the arterial dilation signal generated by a pulse propagating through a portion of the artery near which the corresponding sensor is disposed. For example, the arterial dilation data can be provided by the sensor to the processor in the form of a voltage signal that is generated or received based on the ultrasonic signal or impedance signal sensed by the corresponding sensor.

[0105] As described above, during the systolic phase of the cardiac cycle, when the pulse propagates along the artery through a specific location, the arterial wall expands according to the pulse waveform and the elastic properties of the arterial wall. Associated with the expansion is a corresponding increase in the blood volume at the specific location or region, and as the blood volume increases, one or more properties in that region change accordingly. Conversely, during the diastolic phase of the cardiac cycle, the arterial blood pressure decreases and the arterial wall contracts. Associated with the contraction is a corresponding decrease in the blood volume at the specific location, and as the blood volume decreases, one or more properties in that region change accordingly.

[0106] In the context of bioimpedance sensing (or impedance plethysmography), the blood in the artery has a greater electrical conductivity than the surrounding or adjacent skin, muscle, fat, tendon, ligament, bone, lymph, or other tissues. The susceptance (and thus the dielectric constant) of the blood also differs from that of other types of surrounding or nearby tissues. When the pulse propagates through a specific location, the corresponding increase in the blood volume results in an increase in the electrical conductivity at the specific location (and more generally, an increase in admittance, or equivalently a decrease in impedance). Conversely, during the diastolic phase of the cardiac cycle, the corresponding decrease in the blood volume results in an increase in the resistivity at the specific location (and more generally, an increase in impedance, or equivalently a decrease in admittance).

[0107] Bioimpedance sensors typically operate by applying an electrical excitation signal at an excitation carrier frequency to the region of interest via two or more input electrodes and detecting the output signal (or signals) via two or more output electrodes. In some more specific embodiments, the electrical excitation signal is a current signal injected into the region of interest via the input electrodes. In some such embodiments, the output signal is a voltage signal representing the voltage response of the tissue in the region of interest to the applied excitation signal. The detected voltage response signal is affected by the different, and in some cases time-varying, electrical properties of the various tissues through which the injected excitation current signal passes. In some embodiments in which the bioimpedance sensor is operable to monitor blood pressure, heart rate, or other cardiovascular characteristics, the detected voltage response signal is amplitude and phase modulated by the time-varying impedance (or conversely, admittance) of the underlying artery, which fluctuates in synchrony with the user's heartbeat as described above. To determine various biological properties, information in the detected voltage response signal is typically demodulated from the excitation carrier frequency component using various analog or digital signal processing circuits, which may include both passive and active components.

[0108] In some examples incorporating an ultrasound sensor, the measurement of arterial dilation can include, for example, directing ultrasonic waves toward an artery in a limb via one or more ultrasound transducers. Such an ultrasound sensor is also configured to receive reflected waves that are at least partially based on the directed waves. The reflected waves can include scattered waves, specularly reflected waves, or both scattered waves and specularly reflected waves. The reflected waves provide information about the arterial wall and thus information about arterial dilation.

[0109] In some embodiments, regardless of the type of sensors used for the first arterial dilation sensor 906 and the second arterial dilation sensor 908, both the first arterial dilation sensor 906 and the second arterial dilation sensor 908 can be arranged, assembled, or otherwise included within a single housing of a single dynamic monitoring device. As described above, the housing and other components of the monitoring device can be configured such that when the monitoring device is secured or otherwise physically coupled to a subject, both the first arterial dilation sensor 906 and the second arterial dilation sensor 908 are in contact with the subject, or are adjacent to the skin of the user at a first location and a second location, respectively, at an interval distance ∆D, and in some embodiments, along a segment of an artery, it can be assumed that various arterial characteristics between the segments are relatively constant. In various embodiments, the housing of the dynamic monitoring device is a wearable housing or is incorporated into or integrated with a wearable housing. In some specific embodiments, the wearable housing includes (or is connected to) a physical coupling mechanism for removably and non-invasively attaching to a user. The housing can be formed using any of a variety of suitable manufacturing processes, including injection molding and vacuum molding, among others. Additionally, the housing can be made of any of a variety of suitable materials, including but not limited to plastics, metals, glass, rubber, and ceramics, or combinations of these or other materials. In a specific embodiment, the housing and the coupling mechanism enable full dynamic use. In other words, some embodiments of the wearable monitoring device described herein are non-invasive, non-physically inhibiting, and generally do not restrict the unrestrained movement of the subject's arm or leg, thereby enabling continuous or periodic monitoring of cardiovascular characteristics such as blood pressure, even when the subject is moving or otherwise physically active. Thus, the dynamic monitoring device facilitates and enables long-term wear and monitoring of one or more biological characteristics of interest (e.g., for days, weeks, or a month or longer without interruption) to better obtain information about such characteristics over an extended period of time and generally to better understand the health status of the user.

[0110] In some embodiments, the dynamic monitoring device can be positioned around a user's wrist with a strap or band, similar to a watch or a fitness / activity tracker. Figure 10AAn exemplary ambulatory monitoring device 1000 designed to be worn on a wrist is shown according to some embodiments. In the example shown, the monitoring device 1000 includes a housing 1002 integrally formed, coupled, or otherwise integrated with a wristband 1004. In some cases, the first arterial dilation sensor 1006 and the second arterial dilation sensor 1008 can each include the above-referenced Figures 1 - 4 1000 is an example of a surface acoustic wave receiver and light source system 104 as described. In this example, the dynamic monitoring device 1000 is coupled around a wrist such that a first arterial dilation sensor 1006 and a second arterial dilation sensor 1008 within the housing 1002 are each positioned along a segment of a radial artery 1010 (note that the sensors are typically hidden from view from the exterior or outer surface of the housing facing the subject when the monitoring device is coupled to the subject, but are exposed on the inner surface of the housing to enable the sensors to obtain measurements from the underlying artery through the subject's skin). Also as shown, the first arterial dilation sensor 1006 and the second arterial dilation sensor 1008 are spaced a fixed distance ∆D. In some other embodiments, the dynamic monitoring device 1000 can be similarly designed or adapted to be positioned around a forearm, upper arm, ankle, calf, thigh, or finger (all of which are hereinafter referred to as "extremities") using a strap or band.

[0111] Figure 10B An exemplary ambulatory monitoring device 1000 designed to be worn on a finger is shown in accordance with some embodiments. In some cases, the first arterial dilation sensor 1006 and the second arterial dilation sensor 1008 may each include the above reference Figures 1 - 4 An example of a surface acoustic wave receiver and light source system 104 is described.

[0112] In some other embodiments, the ambulatory monitoring devices disclosed herein can be positioned over a user's area of interest without the use of a strap or band. For example, the first artery dilation sensor 1006 and the second artery dilation sensor 1008 and other components of the monitoring device can be enclosed in a housing that is secured to the user's skin over the area of interest using an adhesive or other suitable attachment mechanism (an example of a "patch" monitoring device).

[0113] Figure 10C An example dynamic monitoring device 1000 designed to be present in or on an earbud according to some embodiments is shown. According to this example, the dynamic monitoring device 1000 is coupled to the housing of the earbud 1020. In some cases, the first arterial dilation sensor 1006 and the second arterial dilation sensor 1008 can each include the above-referenced Figures 1 - 4 An example of a surface acoustic wave receiver and light source system 104 is described.

[0114] Examples of embodiments are described in the following numbered clauses:

[0115] 1. An apparatus, comprising: a substrate; a light source system configured to emit light through a region of the substrate from a first side of the substrate toward a target object in contact with a second side of the substrate opposite to the substrate; a receiver system including one or more receivers present in, on, or near the substrate, the receiver system being configured to detect surface acoustic waves propagating in the substrate corresponding to a photoacoustic response of the target object to the light emitted by the light source system; and a control system configured to: receive a surface acoustic wave signal corresponding to the detected surface acoustic waves from the receiver system; and detect at least one structure within the target object based on the surface acoustic wave signal.

[0116] 2. The apparatus according to clause 1, wherein the at least one structure is a vascular structure.

[0117] 3. The apparatus according to clause 1 or clause 2, wherein the control system is further configured to estimate one or more heart-related characteristics at least in part based on the at least one structure.

[0118] 4. The apparatus according to any one of clauses 1-3, wherein the light source system is configured to emit laser pulses.

[0119] 5. The apparatus according to clause 4, wherein the laser pulses are in a wavelength range of 500 nm to 1000 nm.

[0120] 6. The apparatus according to clause 4 or clause 5, wherein the light source system is configured to emit laser pulses with a pulse width in a range from 3 nanoseconds to 1000 nanoseconds.

[0121] 7. The apparatus according to any one of clauses 1-6, wherein the substrate is transparent.

[0122] 8. The apparatus according to any one of clauses 1-7, wherein the substrate comprises a piezoelectric material.

[0123] 9. The apparatus according to any one of clauses 1-8, wherein a combined thickness of the substrate and the light source system is in a range of 2 mm to 5 mm.

[0124] 10. The apparatus according to any one of clauses 1-9, wherein a thickness of the substrate is in a range of 0.5 mm to 1.0 mm.

[0125] 11. The apparatus according to any one of clauses 1-10, wherein a total area of the substrate is in 0.5 cm 2 to 2.0 cm 2 range.

[0126] 12. The device according to any one of clauses 1 - 11, wherein the receiver system comprises a piezoelectric material.

[0127] 13. The device according to any one of clauses 1 - 12, wherein the receiver system comprises at least one receiver element on a first side of the substrate.

[0128] 14. The device according to any one of clauses 1 - 13, wherein the receiver system comprises at least one receiver element that is laterally offset from a region of the substrate in a first direction and at least one receiver element that is laterally offset from the region of the substrate in an opposite second direction.

[0129] 15. The device according to any one of clauses 1 - 14, wherein the receiver system comprises at least one interdigital transducer.

[0130] 16. The device according to any one of clauses 1 - 15, wherein the control system is further configured to detect blood within a blood vessel based on the surface acoustic wave signal.

[0131] 17. The device according to clause 16, wherein the blood vessel is an artery.

[0132] 18. The device according to any one of clauses 1 - 17, wherein a photoacoustic response generates a target object acoustic wave within the target object, and wherein at least a portion of the target object acoustic wave is converted into a surface acoustic wave that propagates in the substrate.

[0133] 19. The device according to clause 18, wherein the target object acoustic wave comprises longitudinal ultrasonic waves.

[0134] 20. A device comprising: a substrate; a light source component configured to emit light through a region of the substrate from a first side of the substrate to a target object in contact with an opposite second side of the substrate; a receiver component configured to detect a surface acoustic wave propagating in the substrate that corresponds to a photoacoustic response of the target object to the light emitted by the light source component; and a control component configured to: receive a surface acoustic wave signal corresponding to the detected surface acoustic wave from the receiving component; and detect at least one structure within the target object based on the surface acoustic wave signal.

[0135] 21. The device according to clause 20, wherein the at least one structure is a vascular structure.

[0136] 22. The device according to clause 20 or clause 21, wherein the control component comprises a component configured to estimate one or more heart - related characteristics based at least in part on the at least one structure.

[0137] 23. The device according to any one of clauses 20 - 22, wherein the light source component is configured to emit laser pulses.

[0138] 24. The device according to any one of clauses 20 - 23, wherein the substrate comprises a piezoelectric material.

[0139] 25. The device according to any one of clauses 20 - 24, wherein the combined thickness of the substrate and the light source component is in the range of 2 mm to 5 mm.

[0140] 26. The device according to any one of clauses 20 - 25, wherein the receiver component comprises at least one receiver element located on a first side of the substrate.

[0141] 27. The device according to any one of clauses 20 - 26, wherein the receiver component comprises at least one receiver element that is laterally offset from a region of the substrate in a first direction and at least one receiver element that is laterally offset from the region of the substrate in an opposite second direction.

[0142] 28. The device according to any one of clauses 20 - 27, wherein the receiver component comprises at least one interdigital transducer.

[0143] 29. A method, comprising: controlling, by a control system, a light source system to emit light through a region of a substrate from a first side of the substrate toward a target object in contact with a second side of the substrate opposite to the first side; receiving, by the control system, from a receiver system a surface acoustic wave signal corresponding to a surface acoustic wave propagating in the substrate, the receiver system comprising one or more receivers present in, on, or near the substrate, the surface acoustic wave signal corresponding to a photoacoustic response of the target object to the light emitted by the light source system; and detecting, by the control system, at least one structure within the target object based on the surface acoustic wave signal.

[0144] 30. The method according to clause 29, wherein the at least one structure is a vascular structure.

[0145] 31. The method according to clause 29 or clause 30, further comprising estimating one or more heart - related characteristics based at least in part on the at least one structure.

[0146] 32. The method according to any one of clauses 29 - 31, wherein controlling the light source system comprises controlling the light source system to emit laser pulses.

[0147] 33. A device, comprising: a substrate; a light source system configured to emit light through a region of the substrate from a first side of the substrate toward a target object in contact with a second side of the substrate opposite to the first side; and a receiver system comprising one or more receivers present in, on, or near the substrate, the receiver system being configured to selectively detect one or more specific types of surface acoustic waves propagating in the substrate and corresponding to a photoacoustic response of the target object to the light emitted by the light source system.

[0148] 34. The device according to clause 33, wherein the receiver system includes at least one interdigital transducer.

[0149] 35. The device according to clause 33 or clause 34, wherein the receiver system includes a piezoelectric material.

[0150] 36. The device according to clause 35, wherein the piezoelectric material includes a type of piezoelectric crystal that enhances the sensitivity of the receiver system to one or more specific types of surface acoustic waves.

[0151] 37. The device according to clause 36, wherein the crystal has a piezoelectric crystal cut that enhances the sensitivity of the receiver system to one or more specific types of surface acoustic waves.

[0152] As used herein, the phrase referring to a list of items “at least one” means any combination of those items, including a single member. By way of example, “at least one of a, b, or c” is intended to cover: a, b, c, a - b, a - c, b - c, and a - b - c.

[0153] The various illustrative logical, logical block, modules, circuits, and algorithmic processes described in connection with the embodiments disclosed herein can be implemented as electronic hardware, computer software, or a combination of both. The interchangeability of hardware and software has been generally described in terms of functionality and is illustrated in the various illustrative components, blocks, modules, circuits, and processes described above. Whether such functionality is implemented as hardware or software depends upon the particular application and design constraints imposed on the overall system.

[0154] The hardware and data processing apparatus for implementing the various illustrative logical, logical block, modules, and circuits described in connection with the aspects disclosed herein can be implemented or performed with a general - purpose single - chip or multi - chip processor, a digital signal processor (DSP), an application - specific integrated circuit (ASIC), a field - programmable gate array (FPGA), or other programmable logic device, discrete gate or transistor logic, discrete hardware components, or any combination thereof designed to perform the functions described herein. The general - purpose processor can be a microprocessor, or any conventional processor, controller, microcontroller, or state machine. The processor can also be implemented as a combination of computing devices, e.g., a combination of a DSP and a microprocessor, multiple microprocessors, one or more microprocessors in conjunction with a DSP core, or any other such configuration. In some embodiments, particular processes and methods may be performed by circuitry specific to a given function.

[0155] In one or more aspects, the described functionality may be implemented in hardware, digital electronic circuitry, computer software, firmware (including the structures disclosed in this specification and structural equivalents thereof), or any combination thereof. Implementations of the subject matter described in this specification may also be implemented as one or more computer programs, i.e., one or more modules of computer program instructions encoded on a computer storage medium for execution by, or to control the operation of, a data processing apparatus.

[0156] If implemented in software, the functionality may be stored on or transmitted via a computer-readable medium as one or more instructions or code. The processes of the methods or algorithms disclosed herein may be implemented in a processor-executable software module that may reside on a computer-readable medium. Computer-readable media includes both computer storage media and communication media including any medium that can transfer a computer program from one place to another. The storage media may be any available media that can be accessed by a computer. By way of example, and not limitation, non-transitory media may include RAM, ROM, EEPROM, CD-ROM or other optical disk storage, magnetic disk storage or other magnetic storage devices, or any other medium that can be used to store the desired program code in the form of instructions or data structures and that can be accessed by a computer. Also, any connection is properly termed a computer-readable medium. As used herein, disks and disc include compact disc (CD), laser disc, optical disc, digital versatile disc (DVD), floppy disk and Blu-ray disc where disks usually reproduce data magnetically, while discs reproduce data optically with lasers. Combinations of the above should also be included within the scope of computer-readable media. Additionally, the operations of a method or algorithm may exist as code and instructions or any combination or collection thereof on a machine-readable medium and a computer-readable medium, which may be incorporated into a computer program product.

[0157] Various modifications to the embodiments described in this disclosure will be readily apparent to those of ordinary skill in the art, and the general principles defined herein may be applied to other embodiments without departing from the scope of the disclosure. Thus, the disclosure is not intended to be limited to the embodiments shown herein but is to be accorded the widest scope consistent with the claims, principles and novel features disclosed herein. The word “exemplary” is used herein, if at all, specifically to mean “serving as an example, instance, or illustration.” Any embodiment described herein as “exemplary” is not necessarily to be construed as preferred or advantageous over other embodiments.

[0158] Certain features that are described in the context of separate embodiments in this specification can also be implemented in combination in a single embodiment. Conversely, the various features that are described in the context of a single embodiment can also be implemented separately or in any suitable sub-combination in multiple embodiments. Additionally, although features may be described above as acting in certain combinations and even initially claimed as such, in some cases, one or more features from the claimed combination can be deleted from the claimed combination, and the claimed combination can be directed to a sub-combination or a variant of a sub-combination.

[0159] Similarly, although operations are depicted in the figures in a particular order, this should not be construed as requiring that such operations be performed in the particular order shown or in a sequential order, or that all of the illustrated operations be performed, to achieve the desired result. In some cases, multitasking and parallel processing may be advantageous. Additionally, the separation of the various system components in the above embodiments should not be construed as requiring such separation in all embodiments, and it should be understood that the described program components and systems can generally be integrated together in a single software product or packaged into multiple software products. Additionally, other embodiments are within the scope of the appended claims. In some cases, the acts recited in the claims can be performed in a different order and still achieve the desired result.

[0160] It should be understood that, unless the features in any specifically described embodiment are explicitly identified as being incompatible with each other, or the surrounding context implies that they are mutually exclusive and cannot easily be combined in a complementary and / or supportive manner, the entire disclosure contemplates and envisions that the specific features of these complementary embodiments can be selectively combined to provide one or more comprehensive but slightly different technical solutions. Therefore, it should also be understood that the above description is given only by way of example, and modifications in detail can be made within the scope of the present disclosure.

[0161] Various modifications to the embodiments described in this disclosure will be apparent to those skilled in the art, and the general principles defined herein can be applied to other embodiments without departing from the scope of this disclosure. Therefore, the appended claims are not intended to be limited to the embodiments shown herein, but are to be accorded the widest scope consistent with this disclosure, the principles disclosed herein, and novel features.

[0162] Additionally, certain features that are described in the context of separate embodiments in this specification can also be implemented in combination in a single embodiment. Conversely, the various features that are described in the context of a single embodiment can also be implemented separately or in any suitable sub-combination in multiple embodiments. Further, although features may be described as acting in certain combinations and even initially claimed as such, in some cases, one or more features from the claimed combination can be deleted from the combination, and the claimed combination can be directed to a sub-combination or a variation of a sub-combination.

[0163] Similarly, although operations are depicted in the figures in a particular order, this should not be construed as requiring that such operations be performed in the particular order shown or in a sequential order, or that all of the operations shown be performed to achieve a desired result. Additionally, the figures may schematically depict one or more example processes in the form of a flowchart. However, other operations that are not depicted can be incorporated into the example processes that are schematically shown. For example, one or more additional operations can be performed before, after, simultaneously with, or between any of the operations shown. Further, each of the operations that are described and illustrated can itself include and collectively refer to multiple sub-operations. For example, each of the operations described above can itself involve the execution of a process or algorithm. Additionally, in some embodiments, the various operations that are described and illustrated can be combined or performed in parallel. Similarly, the separation of the various system components in the embodiments described above should not be construed as requiring such separation in all embodiments. Accordingly, other embodiments are within the scope of the appended claims. In some cases, the acts recited in the claims can be performed in a different order and still achieve a desired result.

Claims

1. An apparatus, comprising: a substrate; a light source system configured to emit light through a region of the substrate from a first side of the substrate toward a target object in contact with a second side of the substrate opposite the first side; a receiver system including one or more receivers present in, on, or near the substrate, the receiver system configured to detect surface acoustic waves propagating in the substrate corresponding to a photoacoustic response of the target object to the light emitted by the light source system; and a control system configured to: receive a surface acoustic wave signal corresponding to the detected surface acoustic waves from the receiver system; and detect at least one structure within the target object based on the surface acoustic wave signal.

2. The device according to claim 1, wherein The at least one structure is a vascular structure.

3. The device according to claim 1, wherein The control system is further configured to estimate one or more heart-related characteristics at least in part based on the at least one structure.

4. The device according to claim 1, wherein The light source system is configured to emit laser pulses.

5. The device according to claim 4, wherein, The laser pulses are in a wavelength range of 500 nm to 1000 nm.

6. The device according to claim 4, wherein, The light source system is configured to emit the laser pulses with a pulse width in a range from 3 nanoseconds to 1000 nanoseconds.

7. The apparatus according to claim 1, wherein The substrate is transparent.

8. The device according to claim 1, wherein The substrate includes a piezoelectric material.

9. The apparatus according to claim 1, wherein A combined thickness of the substrate and the light source system is in a range from 2 mm to 5 mm.

10. The apparatus according to claim 1, wherein, A thickness of the substrate is in a range from 0.5 mm to 1.0 mm.

11. The device according to claim 1, wherein, The total area of the substrate ranges from 0.5 cm 2 to 2.0 cm 2 .

12. The device according to claim 1, wherein, The receiver system includes a piezoelectric material.

13. The device according to claim 1, wherein The receiver system includes at least one receiver element located on the first side of the substrate.

14. The apparatus according to claim 1, wherein The receiver system includes at least one receiver element laterally offset from the region of the substrate in a first direction and at least one receiver element laterally offset from the region of the substrate in an opposite second direction.

15. The device according to claim 1, wherein, The receiver system includes at least one interdigital transducer.

16. The device according to claim 1, wherein The control system is further configured to detect blood within a blood vessel based on the surface acoustic wave signal.

17. The device according to claim 16, wherein, The blood vessel is an artery.

18. The device according to claim 1, wherein, The photoacoustic response generates target object acoustic waves within the target object, and at least a portion of the target object acoustic waves is converted into the surface acoustic waves propagating in the substrate.

19. The apparatus according to claim 18, wherein, The target object acoustic waves include longitudinal ultrasonic waves.

20. An apparatus, comprising: a substrate; a light source component for emitting light through a region of the substrate from a first side of the substrate toward a target object in contact with a second side of the substrate opposite the first side; a receiver component for detecting surface acoustic waves propagating in the substrate corresponding to a photoacoustic response of the target object to the light emitted by the light source component; and a control component for: receiving a surface acoustic wave signal corresponding to the detected surface acoustic waves from the receiver component; and detecting at least one structure within the target object based on the surface acoustic wave signal.

21. The device according to claim 20, wherein, The at least one structure is a vascular structure.

22. The apparatus according to claim 20, wherein, The control component includes a component for estimating one or more heart-related characteristics at least in part based on the at least one structure.

23. The apparatus according to claim 20, wherein, The light source component is configured to emit laser pulses.

24. The apparatus according to claim 20, wherein, The substrate includes a piezoelectric material.

25. The apparatus according to claim 20, wherein, The combined thickness of the substrate and the light source component is in the range from 2 mm to 5 mm.

26. The apparatus according to claim 20, wherein The receiver component includes at least one receiver element located on the first side of the substrate.

27. The apparatus according to claim 20, wherein, The receiver component includes at least one receiver element that is laterally offset from the region of the substrate in a first direction and at least one receiver element that is laterally offset from the region of the substrate in an opposite second direction.

28. The apparatus according to claim 20, wherein The receiver component includes at least one interdigital transducer.

29. A method, comprising: Controlling, by a control system, a light source system to emit light through a region of a substrate from a first side of the substrate toward a target object in contact with a second side of the substrate opposite the substrate; Receiving, by the control system from a receiver system, a surface acoustic wave signal corresponding to a surface acoustic wave propagating in the substrate, the receiver system including one or more receivers present in, on, or near the substrate, the surface acoustic wave signal corresponding to a photoacoustic response of the target object to the light emitted by the light source system; And Detecting, by the control system, at least one structure within the target object based on the surface acoustic wave signal.

30. The method according to claim 29, wherein the at least one structure is a vascular structure.

31. The method according to claim 29, further comprising estimating one or more heart-related characteristics based at least in part on the at least one structure.

32. The method according to claim 29, wherein, Controlling the light source system includes controlling the light source system to emit laser pulses.

33. An apparatus, comprising: A substrate; A light source system configured to emit light through a region of the substrate from a first side of the substrate toward a target object in contact with a second side of the substrate opposite the substrate; And A receiver system including one or more receivers present in, on, or near the substrate, the receiver system configured to selectively detect one or more specific types of surface acoustic waves propagating in the substrate corresponding to a photoacoustic response of the target object to the light emitted by the light source system.

34. The apparatus according to claim 33, wherein, The receiver system includes at least one interdigital transducer.

35. The apparatus according to claim 33, wherein, The receiver system includes a piezoelectric material.

36. The apparatus according to claim 35, wherein, The piezoelectric material includes a type of piezoelectric crystal that enhances the sensitivity of the receiver system to one or more specific types of surface acoustic waves.

37. The apparatus according to claim 36, wherein The crystal has a piezoelectric crystal cut that enhances the sensitivity of the receiver system to the one or more specific types of surface acoustic waves.

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