A wearable flexible near-infrared transparent ultrasonic transducer and its preparation method

Abstract

The present invention discloses a wearable, flexible, near-infrared, transparent ultrasonic transducer and its preparation method. The transducer comprises a PDMS flexible substrate, multiple rigid micro-CMUT elements, serpentine silver nanowire electrodes, and electrodes for external connections. The multiple rigid micro-CMUT elements are arranged in parallel and interconnected via the silver nanowire electrodes to form a serpentine interconnection circuit. The micro-CMUT elements, serpentine silver nanowire electrodes, and electrodes for external connections are encapsulated within the PDMS flexible substrate. The wearable, flexible, near-infrared, transparent ultrasonic transducer allows near-infrared laser beams to penetrate, effectively integrating the light source and ultrasonic transducer.

Description

Technical Field

[0001] The present invention relates to the technical field of wearable flexible near-infrared transparent ultrasonic transducers, and more particularly to a wearable flexible near-infrared transparent ultrasonic transducer and a preparation method thereof. Background Art

[0002] An ultrasonic transducer is a device that converts ultrasonic signals and electrical signals into each other and is commonly used in the field of ultrasonic imaging and photoacoustic imaging systems. As the core component of the imaging system, it can be used as both an ultrasonic transmitting device and an ultrasonic receiving and transmitting device. At present, the ultrasonic transducers widely used in clinical practice are usually traditional rigid probes. The mismatch between their rigid surface and the irregular surface of the target tissue will produce an irregular thick coupling layer, resulting in distortion and loss of sensitivity, which may limit their application in certain scenarios. For example, it cannot fit closely to the curved, complex and dynamic surfaces of the body and industrial samples, and there will be discomfort at the contact interface with the skin, which cannot meet the clinical needs of continuous detection and diagnosis. In order to overcome these shortcomings, researchers have tried to transform rigid ultrasonic transducers into flexible and wearable ones to further expand the application range of imaging systems.

[0003] Flexible ultrasonic transducers are thin and compact, allowing them to conform tightly to surfaces of any shape. They possess the advantage of self-alignment with complex surfaces, maximizing the energy of transmitted ultrasound waves and enabling high-quality detection. Therefore, the high sensitivity and continuous, real-time, dynamic detection capabilities of flexible wearable devices could play a key role in remotely monitoring vital signs and cardiovascular health.

[0004] In recent years, capacitive micromachined ultrasonic transducer (CMUT) arrays based on microelectromechanical systems (MEMS) technology have become increasingly attractive for use in imaging systems. Typical approaches to designing flexible CMUT arrays involve integrating multiple miniaturized rigid CMUT cells onto a flexible substrate or fabricating the entire CMUT array from a flexible material. CMUTs utilize integrated circuit manufacturing techniques to achieve high electromechanical coupling coefficients, providing improved load matching, wider bandwidth, and higher sensitivity.

[0005] In addition, in order to solve the position problem between the light source and the ultrasound transducer in the photoacoustic imaging system, researchers are trying to create a flexible CMUT array ultrasound transducer that is transparent to visible light / near-infrared light to solve the illumination problem of the sample tissue target. Therefore, a new type of wearable flexible near-infrared transparent ultrasound transducer is developed to allow the near-infrared laser beam to penetrate for photoacoustic / ultrasound imaging, so as to achieve a wider range of clinical application scenarios. Summary of the Invention

[0006] The purpose of the present invention is to provide a wearable flexible near-infrared transparent ultrasonic transducer and a preparation method thereof, in order to solve the technical problems in the background technology.

[0007] In order to achieve the above object, the present invention adopts the following technical solutions:

[0008] A wearable flexible near-infrared transparent ultrasonic transducer comprising:

[0009] A polydimethylsiloxane (PDMS) flexible substrate, multiple micro-rigid CMUT elements, serpentine silver nanowire electrodes and electrodes connected to the outside, multiple micro-rigid CMUT elements are arranged in parallel, and the silver nanowire electrodes are interconnected to form a serpentine interconnection circuit. The micro-rigid CMUT elements, serpentine silver nanowire electrodes and electrodes connected to the outside are jointly encapsulated in the PDMS flexible substrate.

[0010] In some embodiments, the micro-rigid CMUT element is composed of an indium tin oxide (ITO) top electrode, a vibrating membrane, a capacitor plate structure with a vacuum cavity, an ITO bottom electrode, and a substrate; wherein the top electrode is fixed to the upper surface of the vibrating membrane, and the capacitor plate structure with a vacuum cavity is adhered to the lower surface of the vibrating membrane; the ITO bottom electrode is adhered to the lower part of the capacitor plate structure with a vacuum cavity, and the substrate is adhered to the lower part of the ITO bottom electrode; the vibrating membrane is a silicon nitride vibrating membrane, and the substrate is a silicon crystal substrate.

[0011] In some embodiments, the capacitor plate structure with a vacuum cavity includes several insulator BCBs, wherein two insulator benzocyclobutene (BCB) distributed on the sides and an insulator BCB distributed on the bottom form a sealed vacuum cavity with a vibration membrane; the capacitor plate structure with a vacuum cavity is composed of several insulator BCBs, a vibration membrane and a vacuum cavity.

[0012] The present application also provides a method for preparing a wearable flexible near-infrared transparent ultrasonic transducer, comprising the following steps:

[0013] Fabrication of serpentine silver nanowire electrodes: Silver nanowires were mixed with silver flake ink, and the mixed silver nanowire / silver composite ink was then screen-printed on a PDMS flexible substrate at room temperature using a screen printer;

[0014] Fabrication of CMUT rigid components: The CMUT was fabricated using an adhesive wafer bonding process, with styrene as the adhesive and sidewall layer of the CMUT.

[0015] The fabrication of the serpentine silver nanowire electrode comprises: mixing silver nanowires with silver flake ink, and then screen printing the mixed silver nanowire / silver composite ink on a PDMS flexible substrate at room temperature using a screen printer; and comprising: using a customized screen with an open area of ​​a serpentine pattern as a printing template, and adding the silver nanowire / silver composite ink to the customized screen.

[0016] The manufacturing of the CMUT rigid element: the CMUT is prepared by an adhesive wafer bonding process, and styrene is used as the adhesive and sidewall layer of the CMUT, including:

[0017] Prepare two wafers, a silicon wafer containing silicon nitride and a silicon substrate wafer; place both wafers in a mixture of hydrogen peroxide and concentrated sulfuric acid for oxidation and dissolution to clean metal impurities;

[0018] Depositing a silicon nitride layer on a silicon nitride wafer using a low-pressure chemical vapor deposition process to construct a low-stress vibration membrane;

[0019] A 250-260 nm layer of indium tin oxide (ITO), a transparent conductive material, is sputtered onto the second silicon substrate wafer to form an ITO bottom electrode. This is then cleaned in a mixed solution of ammonium hydroxide, hydrogen peroxide, and deionized water to remove organic contaminants.

[0020] Spin-coat a layer of AP3000 adhesive on silicon substrate wafer 1 and silicon substrate wafer 2 at a speed of 3000-3200 rpm for 25-40 seconds, and then soft-bake at 140-160° C. for 50-70 seconds;

[0021] Spin-coat BCB on a second silicon substrate wafer at a speed of 6500-7000 rpm for 40-60 seconds, soft-bake at 50-70°C for 90-100 seconds, wherein the BCB layer is exposed to ultraviolet light, and then bake at 40-60°C for curing;

[0022] The cavities were defined by rinsing with DS2100 developer and spin drying;

[0023] Using a wafer bonding machine to bond the silicon nitride wafer 1 and the silicon substrate wafer 2 with an adhesive;

[0024] A 200-210nm layer of transparent conductive material ITO is sputtered on the silicon nitride vibration membrane as the top electrode. The top ITO layer is then patterned using positive photoresist and wet etching to define the top electrode of the CMUT element. A cutting tool is used to cut the wafer in predefined horizontal and vertical directions to cut the wafer into chip islands with individual CMUT elements. A PDMS flexible layer is quickly spin-coated on the back of the wafer and cured.

[0025] The serpentine silver nanowire interconnected electrodes and the CMUT array are simultaneously encapsulated in a PDMS flexible substrate to form a flexible near-infrared transparent CMUT array.

[0026] The method uses a wafer bonding machine to bond a silicon nitride wafer (1) and a silicon substrate wafer (2) with an adhesive. The method includes: separating the wafers with a gasket to ensure a vacuum-sealed gap; after loading the wafers, lowering the chamber pump to 0.5 mTorr, removing the gasket, allowing the two wafers to contact each other, and forming a vacuum-sealed chamber; then applying a compression pressure of 0.5 MPa to the wafers and maintaining it for 1 hour, and removing the wafers from the chamber after cooling.

[0027] Compared with the prior art, the present invention has the following beneficial effects:

[0028] (1) The wearable flexible near-infrared transparent ultrasound transducer is thin and soft in structure, can adapt to any shape of the contact interface, and can automatically align with the target interface to meet the clinical needs of continuous detection.

[0029] (2) The wearable flexible near-infrared transparent ultrasonic transducer allows the near-infrared laser beam to penetrate, which can effectively realize the integration of light source and ultrasonic transducer. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 Schematic diagram of the structure of the pen laser in the embodiment.

[0031] Figure 2 Schematic diagram of the structure of a wearable flexible near-infrared transparent ultrasonic transducer in an embodiment.

[0032] Figure 3 FIG. 4 is a schematic structural diagram of a rigid CMUT in an embodiment.

[0033] Figure 4 In the embodiment, a schematic diagram of the working principle of the photoacoustic / ultrasound dual-modality imaging system is shown.

[0034] Figure 5 It is a schematic diagram of the structure of the photoacoustic / ultrasound dual-modality imaging system for detecting the central blood vessels of the chest. DETAILED DESCRIPTION

[0035] In order to make the purpose, technical solutions and advantages of the present application clearer, the technical solutions in the embodiments of the present application will be described in more detail below in conjunction with the drawings in the preferred embodiments of the present application. In the drawings, the same or similar reference numerals throughout represent the same or similar parts or parts with the same or similar functions. The described embodiments are part of the embodiments of the present application, not all of the embodiments. The embodiments described below with reference to the drawings are exemplary and are intended to be used to explain the present application, and should not be understood as limitations on the present application. Based on the embodiments in the present application, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of this application.

[0036] The embodiments of the present application are described in detail below with reference to the accompanying drawings.

[0037] In the description of this application, it should be noted that, unless otherwise expressly specified or limited, the terms "mounted," "connected," and "connected" should be understood in a broad sense. For example, they may refer to a fixed connection, an indirect connection via an intermediate medium, internal communication between two components, or an interaction between two components. Those skilled in the art will understand the specific meanings of the above terms in this application based on the specific circumstances.

[0038] In the description of this application, it should be understood that the terms "upper", "lower", "front", "back", "vertical", "horizontal", "top", "bottom", "inside", "outside", etc., indicating orientations or positional relationships, are orientations or positional relationships based on the accompanying drawings, and are only for the convenience of describing this application and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore should not be understood as a limitation on this application.

[0039] In addition, the terms "comprises" and "having" and any variations thereof are intended to cover a non-exclusive inclusion; for example, a process, method, system, product, or display that comprises a series of steps or elements is not necessarily limited to those steps or elements expressly listed but may include other steps or elements not expressly listed or inherent to such process, method, product, or display.

[0040] The following will be combined Figure 1-5 , a wearable flexible near-infrared transparent ultrasonic transducer and a preparation method thereof involved in the embodiments of the present application are described in detail. It is worth noting that the following embodiments are only used to explain the present application and do not constitute a limitation of the present application. In order to better illustrate a wearable flexible near-infrared transparent ultrasonic transducer and a preparation method thereof of the present application, it is applied to a wearable flexible near-infrared transparent photoacoustic / ultrasound dual-modality imaging system for illustration.

[0041] Embodiment 1:

[0042] like Figure 1-5 As shown, the wearable flexible near-infrared transparent photoacoustic / ultrasound dual-modality imaging system includes: a pen-type laser 1 and a wearable flexible near-infrared transparent ultrasonic transducer 2. The pen-type laser is used to generate a laser light source; the wearable flexible near-infrared transparent ultrasonic transducer is used to transmit ultrasonic signals or to receive and transmit photoacoustic signals generated when the pen-type laser irradiates a blood vessel 3.

[0043] The pen laser can freely control the emission of laser light through an external trigger. When integrated with an ultrasonic transducer and applied to a photoacoustic imaging system, this ensures the synchronization of near-infrared light irradiation and photoacoustic signal reception, thereby achieving continuous, real-time and dynamic detection of deep tissues.

[0044] The pen laser can activate a laser driver 12 by controlling an external trigger 11, which in turn drives a high-efficiency diode array 13 to emit a laser beam. This allows for controlled near-infrared laser irradiation of deep tissue. When the pen laser is integrated with an ultrasound transducer and applied to a photoacoustic imaging system, this is crucial for the synchronization of laser irradiation and photoacoustic signal reception.

[0045] A three-dimensional view of the pen laser reveals a cylindrical structure with a length of 12 cm and a base diameter of 3 cm. The main component located in the center of the pen laser is a high-efficiency laser diode array, with the laser driver and cylindrical lens located at its top and bottom, respectively. The laser driver, high-efficiency laser diode array, cylindrical lens 14, and diffractive optical element 15 are connected via a substrate. The top of the laser driver is connected to one end of the trigger, and the other end of the trigger is connected to the rear of the cylindrical housing. This allows for free control of laser emission via an external switch button, meeting the basic requirements for photoacoustic imaging. A circular glass 17 forms the head of the cylindrical housing, and the outer shell 16 is made of aluminum and has a cooling function to limit heat gain.

[0046] By pressing the trigger connected to the tail of the pen laser, the laser driver connected to the other end of the trigger can be activated, thereby driving the high-efficiency laser diode array connected by the substrate to emit a near-infrared laser beam. At this time, the quality of the emitted laser beam is poor and the divergence is very obvious, so an optical system is required to collimate and reshape the laser beam to minimize energy loss and ensure that the area of ​​interest in deep tissue is illuminated with the required laser beam profile. The emitted laser beam is first collimated by a cylindrical lens placed in front of the high-efficiency laser diode array to minimize the divergence of the beam and achieve beam shaping. The collimated laser beam is reshaped by a diffraction optical element to make the laser beam uniform and irradiated to the area of ​​interest in a rectangular form through the circular glass at the bottom of the pen laser.

[0047] The portable pen laser provided in this application has a compact structure, miniaturizing the illumination system and making it easy to integrate into a photoacoustic imaging system. The near-infrared laser beam emitted by the pen laser has a greater penetration depth, facilitating imaging of deep tissues.

[0048] The wearable flexible near-infrared transparent ultrasound transducer provided in this application allows near-infrared laser beams to penetrate for photoacoustic / ultrasound imaging, thereby realizing a wider range of clinical application scenarios.

[0049] The wearable, flexible, near-infrared, transparent ultrasound transducer of this invention consists of a series of rigid micro-CMUT elements interconnected by serpentine silver nanowires, encapsulated in flexible polypropylene (PDMS). Its compact structure allows near-infrared light to penetrate, conforming comfortably to the target tissue surface. It can transmit and / or receive ultrasonic signals, producing continuous, real-time, high-quality images, and is suitable for use in ultrasound and photoacoustic imaging systems.

[0050] The wearable flexible near-infrared transparent ultrasonic transducer includes: a PDMS flexible substrate 21, multiple miniature rigid CMUT elements 22, a serpentine silver nanowire electrode 23, and an electrode 24 connected to the outside. The multiple miniature rigid CMUT elements are arranged in parallel and interconnected based on the silver nanowire electrodes to form a serpentine interconnection circuit. The rigid CMUT elements, the serpentine silver nanowire electrodes, and the electrodes connected to the outside are jointly encapsulated in the PDMS flexible substrate.

[0051] The micro-rigid CMUT element consists of an ITO top electrode 25, a vibrating membrane 26, a capacitor plate structure with a vacuum cavity, an ITO bottom electrode and a substrate; wherein the top electrode is fixed to the upper surface of the vibrating membrane, and the capacitor plate structure with a vacuum cavity is attached to the lower surface of the vibrating membrane; the ITO bottom electrode is attached to the lower part of the capacitor plate structure with a vacuum cavity, and the substrate is attached to the lower part of the ITO bottom electrode 29; the vibrating membrane is a silicon nitride vibrating membrane, and the substrate is a silicon crystal substrate 30.

[0052] The capacitor plate structure with a vacuum cavity includes several insulators BCB, wherein two insulators BCB distributed on the sides and one insulator BCB distributed on the bottom form a sealed vacuum cavity 27 with a vibration membrane; the capacitor plate structure with a vacuum cavity is composed of several insulators BCB28, a vibration membrane and a vacuum cavity.

[0053] When a CMUT is operating, a DC bias voltage is typically applied to sink the diaphragm into the vacuum chamber. This stress on the diaphragm increases the transducer's sensitivity. When transmitting or emitting ultrasonic waves, an AC signal is superimposed on the applied DC bias voltage. The DC voltage brings the top and bottom electrodes closer together, driving the diaphragm to generate an ultrasonic signal. The driving frequency of the signal is the transmission frequency of the ultrasonic wave. When receiving an ultrasonic signal, only a DC voltage is applied to maintain a fixed potential difference. The incident sound wave modulates the height of the vacuum chamber gap according to the wave's frequency, causing the capacitance of the diaphragm to change, generating an output current. This output current is converted to a voltage signal and amplified by a transimpedance amplifier to enable signal reception.

[0054] Fabrication of serpentine silver nanowire electrodes: Silver nanowires are mixed with silver flake ink, and the mixed silver nanowire / silver composite ink is then screen-printed on a PDMS flexible substrate at room temperature using a screen printer. During this process, a custom screen with open areas for a serpentine pattern is used as a printing template, and the silver nanowire / silver composite ink is applied to the custom screen. As a scraper is scraped across the screen surface, the silver nanowire / silver composite ink passes through the open areas in the screen and remains on the flexible PDMS substrate, creating a serpentine pattern. All interconnecting electrode lines connecting the ultrasonic transducer and the electrode lines for external connections are constructed using this screen-printing technique.

[0055] Fabrication of CMUT rigid components: CMUTs are fabricated using an adhesive wafer bonding process. BCB, a photosensitive polymer, serves as the CMUT's adhesive and sidewall layer. The CMUT's enclosed vacuum cavity structure offers advantages, including better control over cavity size, shape, and membrane material uniformity.

[0056] First, two wafers are prepared: a silicon wafer containing silicon nitride (1) and a silicon substrate wafer (2). Both wafers are placed in a mixture of hydrogen peroxide and concentrated sulfuric acid for oxidation and dissolution to clean metal impurities.

[0057] A silicon nitride layer is deposited on a silicon nitride wafer using a low-pressure chemical vapor deposition process to create a low-stress diaphragm. Photolithography and reactive ion etching are used to remove the silicon nitride on the back side of the diaphragm while the photoresist protects the front side. The photoresist on the front side is then stripped off to form the silicon nitride diaphragm of the CMUT element.

[0058] To improve the optical transmittance of silicon substrates, three key manufacturing steps are required: (1) grinding and thinning the silicon substrate wafer to reduce the thickness of the silicon substrate and reduce light absorption. (2) using a chemical mechanical polishing process to achieve a mirror-like finish, minimizing the scattering of infrared light during illumination. (3) adding an anti-reflective coating to the surface of the silicon substrate to achieve higher optical transmittance.

[0059] A 250nm layer of transparent conductive material, ITO, is sputtered onto the silicon substrate wafer to form the ITO bottom electrode. The wafer is then cleaned in a mixture of ammonium hydroxide, hydrogen peroxide, and deionized water to remove any organic contaminants. The ammonia and hydrogen peroxide remove particles adhering to the silicon wafer surface and improve its cleanliness, which directly affects the bonding process.

[0060] To ensure proper bonding of the BCB to the ITO bottom electrode, a thin layer of AP3000 adhesive (Dow Chemical) is first spin-coated on silicon substrate wafer two at 3000 rpm for 30 seconds and then soft-baked at 150°C for 60 seconds. This step also needs to be performed on silicon nitride wafer one to improve adhesion during the adhesive bonding step later in the process. Subsequently, the BCB is spin-coated on silicon substrate wafer two at 6500 rpm for 45 seconds and soft-baked at 60°C for 90 seconds. The BCB layer needs to be exposed to UV light and then cured at 50°C for 60 seconds. Subsequently, it is rinsed with DS2100 developer for 2 minutes and spin-dried for 2 minutes to define the cavity. The thickness of the BCB layer determines the gap distance of the CMUT element.

[0061] A wafer bonder is used to bond silicon nitride wafer (1) and silicon substrate wafer (2) together using an adhesive. The wafers are first separated by a spacer to ensure a vacuum-tight gap. After loading the wafers, the chamber pump is lowered to 0.5 mTorr, and the spacer is removed, allowing the two wafers to contact each other and forming a vacuum-tight chamber. A compressive pressure of 0.5 MPa is then applied to the wafers for one hour. After cooling, the wafers are removed from the chamber. A combination of dry and wet etching processes is used to accelerate the removal of the wafer's silicon handle.

[0062] A 200nm layer of transparent conductive material, ITO, was sputtered onto the silicon nitride diaphragm as the top electrode. The top ITO layer was then patterned using positive photoresist and wet etching to define the top electrode of the CMUT element. To reduce the resistivity of the top electrode and maintain the transparency of the device, it was patterned in the form of thin strips using a lift-off technique. Subsequently, a cutting tool was used to cut the wafer in predefined horizontal and vertical directions, dividing it into chip islands with individual CMUT elements. A flexible layer of PDMS was then rapidly spin-coated on the back of the wafer and cured. Finally, the serpentine silver nanowire interconnect electrodes and the CMUT array were simultaneously encapsulated in a flexible PDMS substrate to form a flexible near-infrared transparent CMUT array.

[0063] When manufacturing a CMUT, there are several key design factors to consider, including diameter, material type, and vacuum gap height, which can affect the performance of the CMUT. The CMUT is modeled as a second-order system to obtain important parameters such as resonant frequency and pull-in voltage.

[0064] The resonant frequency (ω0) is a key parameter that determines the image resolution.

[0065]

[0066] t represents the thickness of the vibrating membrane, a is the radius of the membrane, E represents the Young's modulus of the membrane material, v is the Poisson's ratio of the membrane, ρ m is the density of the medium and ρ is the density of the membrane.

[0067] Another key parameter is the pull-in voltage (V pull in The pull-in voltage is the point where the electrostatic and mechanical forces are equal, causing the top electrode to snap onto the substrate. Therefore, it is very important to operate the CMUT below the pull-in voltage.

[0068]

[0069] g eff Represents the effective gap height, calculated as

[0070] g0 is the original gap height, t m is the thickness of the film, t i is the thickness of the insulator, ε r is the relative dielectric constant of the insulator and membrane material, ε0 is the dielectric constant of free space, k is the spring constant, and A is the electrode area.

[0071] The wearable, flexible, near-infrared transparent ultrasound transducer is thin and flexible, adapting to any shape of the contact interface and capable of self-aligning to the target interface, meeting the clinical needs of continuous detection. The wearable, flexible, near-infrared transparent ultrasound transducer allows near-infrared laser beams to penetrate, effectively integrating the light source and ultrasound transducer.

[0072] This embodiment also provides a wearable flexible near-infrared transparent photoacoustic / ultrasound dual-modality imaging method, which is implemented based on the above system and includes the following steps:

[0073] A pencil laser is used to emit a near-infrared laser beam to illuminate the central blood vessels of the chest to generate photoacoustic signals;

[0074] Adjust the ultrasound scanner parameters and switch to photoacoustic imaging mode;

[0075] A wearable flexible near-infrared transparent ultrasound transducer attached to the chest receives the photoacoustic signal;

[0076] The image reconstruction algorithm is used to construct the light absorption distribution map of hemoglobin, and the blood oxygen saturation is quantified according to the absorption coefficient of hemoglobin;

[0077] Switch the ultrasound scanner to ultrasound imaging mode;

[0078] A wearable flexible near-infrared transparent ultrasound transducer is used to transmit pulsed ultrasound waves to detect the central blood vessels of the chest and simultaneously receive echo signals from the reflector.

[0079] An image reconstruction algorithm is used to display a B-mode grayscale image of the central thoracic vessels. Blood pressure within the vessels is calculated based on this B-mode grayscale image. The Doppler effect is then used to detect blood velocity and spatial distribution. A pen laser is driven by a custom laser driver. This driver, triggered by an external trigger (i.e., a switch button), emits a near-infrared laser beam. At this point, a commercial ultrasound scanner (4) is synchronously adjusted to prevent ultrasound transmission, allowing the system to switch to photoacoustic imaging mode. This ensures that the laser pulse irradiation and echo signal reception proceed simultaneously. When the laser beam irradiates the central thoracic vessels, the local tissue undergoes thermoelastic expansion, generating a photoacoustic signal that propagates outward to the skin surface and is received by a wearable, flexible, near-infrared, transparent ultrasound transducer attached to the chest. An image reconstruction algorithm is used to construct a hemoglobin light absorption profile, which is then used to quantify blood oxygen saturation based on the hemoglobin absorption coefficient.

[0080] In the near-infrared spectral range used in this system, the changes of carboxyhemoglobin and methemoglobin are not considered. Therefore, the absorption of light mainly reflects the total concentration of deoxyhemoglobin (Hb) and oxyhemoglobin (HbO2).

[0081] In some embodiments, the wavelength of the near-infrared laser beam is 900-1100 nm. This imaging system uses near-infrared wavelengths as a suitable spectral range. Near-infrared lasers can penetrate biological tissues to a depth of several centimeters, and hemoglobin has a strong absorption advantage over other endogenous chromophores in the near-infrared wavelength range. In the wavelength range of 900-1100 nm, the difference in absorption rate between deoxyhemoglobin and oxyhemoglobin is most significant. In addition, based on the low absorption rate of silicon to near-infrared wavelengths, near-infrared light can pass through the CMUT array without strong attenuation, so that the generated photoacoustic signal is successfully received by the wearable flexible near-infrared transparent ultrasound transducer.

[0082] The quantitative determination of blood oxygen saturation based on the absorption coefficient of hemoglobin includes:

[0083] When the wavelengths λ1 and λ2 meet the preset conditions, the absorption coefficient μ λ It can be calculated by formula (1) and (2):

[0084]

[0085]

[0086] Among them, C Hb and C HbO2 Respectively represent the content of deoxyhemoglobin and oxyhemoglobin; ε λ1 Hb , ε λ2 Hb and ε λ1 HbO2， ε λ2 HbO2 represent the extinction coefficients of deoxyhemoglobin and oxyhemoglobin at wavelengths λ1 and λ2, respectively;

[0087] It is known that the intensity of the photoacoustic signal is proportional to the absorption coefficient of the tissue. After simplifying the equation, the blood oxygen saturation SaO2 can be calculated using formulas (3), (4) and (5):

[0088]

[0089]

[0090]

[0091] Subsequently, the commercial ultrasound scanner was restarted and switched to ultrasound imaging mode. Ultrasound imaging of the central thoracic vessels was performed using a wearable, flexible, near-infrared, transparent ultrasound transducer. The wearable, flexible, near-infrared, transparent ultrasound transducer was attached to the skin surface corresponding to the central thoracic vessels. The CMUT array continuously emitted short ultrasound pulses at a fixed frequency, with all elements in the phased array participating in the transmission of each acoustic beam. The CMUT array emitted pulsed ultrasound along the propagation path of each acoustic beam line and then received and processed the echo signals from each reflector along the beam path, from shallow to deep. An image reconstruction algorithm was used to display a B-mode grayscale image of the central thoracic vessels.

[0092] It is known that the measured artery meets the following conditions: rotational symmetry, certain elasticity and very small viscoelasticity. Then the blood pressure waveform (pr(t)) can be calculated according to the blood vessel diameter waveform using formula (6):

[0093] The blood pressure waveform (pr(t)) can be calculated based on the blood vessel diameter waveform using formula (6):

[0094]

[0095] pr d is the brachial artery diastolic pressure measured by a cuff sphygmomanometer; d is a cross-section of the artery during diastole;

[0096] α is the stiffness coefficient of the blood vessel, which can be calculated using formula (7):

[0097]

[0098] Pr s is the brachial artery systolic blood pressure measured by a commercial cuff sphygmomanometer; s It is a cross-section of the artery during systole;

[0099] ar(t) can be calculated using formula (8):

[0100]

[0101] D(t) is the blood vessel diameter waveform measured by a wearable flexible near-infrared transparent ultrasound transducer.

[0102] In some embodiments, detecting blood flow velocity information and spatial distribution by the Doppler effect includes:

[0103] The Doppler effect is used to detect blood flow information. Multiple acoustic beams are used for rapid sampling. The obtained Doppler information is subjected to phase detection, autocorrelation processing, and color coding. Different colors are used to identify the direction of blood flow. The color brightness indicates the speed. The information is then superimposed on the B-mode grayscale ultrasound image to obtain the spatial distribution and flow velocity information of blood flow in the blood vessels.

[0104] The blood flow velocity (v) of the arterial blood vessels can be calculated using the Doppler principle using formula (9):

[0105]

[0106] C is the speed of sound, that is, the propagation speed of ultrasound in soft tissue; f0 is the transmission frequency; f d is the Doppler shift, that is, the difference between the receiving frequency and the transmitting frequency; θ is the Doppler angle, that is, the angle between the ultrasound beam and the direction of blood flow.

[0107] Therefore, the photoacoustic / ultrasound dual-modality imaging system of the present invention can realize real-time, continuous and dynamic detection of blood oxygen saturation and hemodynamic information of the central blood vessels of the chest.

[0108] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A method for preparing a wearable flexible near-infrared transparent ultrasonic transducer, characterized in that: Used to prepare a wearable flexible near-infrared transparent ultrasonic transducer, the ultrasonic transducer comprising: A PDMS flexible substrate, a plurality of micro-rigid CMUT elements, serpentine silver nanowire electrodes, and electrodes connected to the outside, wherein the plurality of micro-rigid CMUT elements are arranged in parallel, and the silver nanowire electrodes are interconnected to form a serpentine interconnection circuit, and the micro-rigid CMUT elements, the serpentine silver nanowire electrodes, and the electrodes connected to the outside are encapsulated together in the PDMS flexible substrate; the method comprises the following steps: Fabrication of serpentine silver nanowire electrodes: Silver nanowires were mixed with silver flake ink, and the mixed silver nanowire / silver composite ink was then screen-printed on a PDMS flexible substrate at room temperature using a screen printer; Fabrication of CMUT rigid components: The CMUT was fabricated using an adhesive wafer bonding process, with styrene as the adhesive and sidewall layers of the CMUT. The manufacturing of the CMUT rigid element: the CMUT is prepared by an adhesive wafer bonding process, and styrene is used as the adhesive and sidewall layer of the CMUT, including: Prepare two wafers, a silicon wafer containing silicon nitride and a silicon substrate wafer; place both wafers in a mixture of hydrogen peroxide and concentrated sulfuric acid for oxidation and dissolution to clean metal impurities; Depositing a silicon nitride layer on a silicon nitride wafer using a low-pressure chemical vapor deposition process to construct a low-stress vibration membrane; A 250-260 nm layer of indium tin oxide (ITO), a transparent conductive material, is sputtered onto the second silicon substrate wafer to form an ITO bottom electrode. This is then cleaned in a mixed solution of ammonium hydroxide, hydrogen peroxide, and deionized water to remove organic contaminants. Spin-coat a layer of AP3000 adhesive on silicon substrate wafer 1 and silicon substrate wafer 2 at a speed of 3000-3200 rpm for 25-40 seconds, and then soft-bake at 140-160° C. for 50-70 seconds; Spin-coat BCB on a second silicon substrate wafer at a speed of 6500-7000 rpm for 40-60 seconds, soft-bake at 50-70°C for 90-100 seconds, wherein the BCB layer is exposed to ultraviolet light, and then bake at 40-60°C for curing; The cavities were defined by rinsing with DS2100 developer and spin drying; Using a wafer bonding machine to bond the silicon nitride wafer 1 and the silicon substrate wafer 2 with an adhesive; A 200-210nm layer of transparent conductive material ITO is sputtered on the silicon nitride vibration membrane as the top electrode. The top ITO layer is then patterned using positive photoresist and wet etching to define the top electrode of the CMUT element. A cutting tool is used to cut the wafer in predefined horizontal and vertical directions to cut the wafer into chip islands with individual CMUT elements. A PDMS flexible layer is quickly spin-coated on the back of the wafer and cured. The serpentine silver nanowire interconnected electrodes and the CMUT array are simultaneously encapsulated in a PDMS flexible substrate to form a flexible near-infrared transparent CMUT array.

2. The method for preparing a wearable flexible near-infrared transparent ultrasonic transducer according to claim 1, characterized in that: The micro-rigid CMUT element consists of an ITO top electrode, a vibrating membrane, a capacitor plate structure with a vacuum cavity, an ITO bottom electrode, and a substrate. The top electrode is fixed to the upper surface of the vibrating membrane, and the capacitor plate structure with a vacuum cavity is attached to the lower surface of the vibrating membrane. The ITO bottom electrode is attached to the lower part of the capacitor plate structure with a vacuum cavity, and the substrate is attached to the lower part of the ITO bottom electrode. The vibrating membrane is a silicon nitride vibrating membrane, and the substrate is a silicon crystal substrate.

3. The method for preparing a wearable flexible near-infrared transparent ultrasonic transducer according to claim 2, characterized in that: The capacitor plate structure with a vacuum cavity includes several insulator BCBs, wherein two insulator BCBs distributed on the sides and one insulator BCB distributed on the bottom form a sealed vacuum cavity with a vibration membrane; the capacitor plate structure with a vacuum cavity is composed of several insulator BCBs, a vibration membrane and a vacuum cavity.

4. The method for preparing a wearable flexible near-infrared transparent ultrasonic transducer according to claim 1, characterized in that: The fabrication of the serpentine silver nanowire electrode comprises: mixing silver nanowires with silver flake ink, and then screen printing the mixed silver nanowire / silver composite ink on a PDMS flexible substrate at room temperature using a screen printer; and comprising: using a customized screen with an open area of ​​a serpentine pattern as a printing template, and adding the silver nanowire / silver composite ink to the customized screen.

5. The method for preparing a wearable flexible near-infrared transparent ultrasonic transducer according to claim 4, characterized in that: The method uses a wafer bonding machine to bond a silicon nitride wafer (1) and a silicon substrate wafer (2) with an adhesive. The method includes: separating the wafers with a gasket to ensure a vacuum-sealed gap; after loading the wafers, lowering the chamber pump to 0.5 mTorr, removing the gasket, allowing the two wafers to contact each other, and forming a vacuum-sealed chamber; then applying a compression pressure of 0.5 MPa to the wafers and maintaining it for 1 hour, and removing the wafers from the chamber after cooling.

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