Photoacoustic transducer array and methods of making the same

Abstract

Embodiments relate to a transducer. The transducer can include a substrate having a front surface, a back surface, and a via extending through the substrate from the front surface to the back surface. The transducer can include a via conductive layer disposed on: a surface of the substrate defined by the via; a front surface region adjacent the via; and a back surface region adjacent the via. The transducer can include a via-fill material disposed within the via and occupying a volume of space formed by the via. The transducer can include an anisotropic layer disposed on the front surface region and the via-fill material located at or near the front surface. The transducer can include a piezoelectric film disposed on the anisotropic layer. The transducer can include a conductive layer disposed on the piezoelectric film.

Description

Photoacoustic Transducer Array And Methods Of Making The SameCROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application is related to and claims the benefit of U.S. provisional patent application 63 / 602,848, filed on November 27, 2023, the entire contents of which is incorporate by reference herein.STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH AND DEVELOPMENT

[0002] This invention was made with government support under Grant No. EB030370 awarded by the National Institutes of Health. The Government has certain rights in the invention.FIELD OF THE INVENTION

[0003] Embodiments relate to multidimensional transducer arrays and method of making the same. The method of making the transducer can facilitate bulk manufacturing processes at low- cost, without using laborious ultrasound transducer fabrication techniques such as dicing and without advanced cleanroom technologies. Transducer and transducer arrays made using the disclosed method provide for scalable aperture size, element count, and frequency of operation.BACKGROUND OF THE INVENTION

[0004] Photoacoustic (PA) imaging is a non-ionizing molecular imaging modality delivering rich optical absorption-based contrast from various chromophores, such as hemoglobin molecules present inside blood vasculature. It offers deep tissue imaging capabilities with spatial resolution scalable with ultrasound (US) transducer parameters; typically, l / 200th to 1 / 100th of the imaging depth (e.g., a resolution of 0.3 mm at 3 cm depth). Thus, PA imaging devices are emerging as a preferred choice for several pre-clinical and clinical applications covering cancer, neurological and vascular diseases. PA images are obtained using US transducers functioning in receive only mode. A single element US transducer provides depth encoded A-line information at a givenpoint in space. A linear or curvilinear one-dimensional (ID) ultrasound transducer array provides 2D (B-mode) images at given position in space. Volumetric (3D) PA images of deep tissue are obtained by scanning a US transducer in the following ways: (i) 2D raster scanning a single-element US transducer in the imaging plane; (ii) ID linear or rotational scanning of a linear or curvilinear US transducer array; and (iii) depth scanning using a ring transducer.

[0005] While the PA imaging speed, measured as frames per second (FPS), is proportional to the pulse repetition frequency (PRF) of the laser, it is limited by scan time and scanning space. Because single-element and ID array US transducers both require scanning for 3D PA imaging, 2D US arrays provide an advantage of scan-less volumetric PA imaging in real-time. Towards this goal, a 2D capacitive micromachined ultrasonic transducer (CMUT) array was demonstrated for deep-tissue PA imaging - see https: / / ieeexplore.ieee.org / abstract / document / 6129397. The need for tight bonding of 2D integrated electronics with the CMUT arrays for high-fidelity parallel receiving of data from all channels further increases the complexity and cost of the micromachined ultrasound transducer arrays. Commercial 2D array US devices made of conventional piezoelectric materials are also being investigated for a scan-less volumetric PA imaging. These 2D arrays are typically manufactured using complex fabrication tools involving dicing and filling, incorporation of dedicated matching and backing layers, resulting in increased cost, manufacturing time and limited availability. In contrast, a hemispherical US transducer array fabricated on an undiced dielectric preform has been demonstrated for volumetric PA imaging. This array uses polyvinylidene fluoride-co-trifluoroethylene (PVDF - TrFe) piezoelectric material which exhibits a high piezoelectric constant and wideband receive sensitivities for high frequency PA imaging. The choice of PVDF - TrFe as piezo layer removed the need of dicing the piezo layer and the preform. Z-axis tape was used to ensure conduction of electrical signal from the PVDF -TrFe to the substrate through the tape while ensuring no cross- connection between the bottom electrodes. However, device geometry and sideways light coupling necessitates significant acoustic coupling, which introduces challenges for in vivo longitudinal imaging of living subjects. The curvature of the array substrate, or preform, limits scalability with respect to aperture size or element count.

[0006] Others have demonstrated printed circuit boards (PCB) as a low-cost, scalable, easy-to- manufacture substrate for fabricating 2D planar array US transducers using conventional leadzirconate titanate (PZT) material. However, both the PZT material and the top of the PCB preform, need to be diced along the kerf and filled with non-conducting kerf-filler for element wise electrical isolation. The 2D dicing required for the 2D array transducer, and the need for additional matching and backing layers, significantly complicates and lengthens the fabrication process.

[0007] To overcome the above limitations, the inventors developed a method that combines a PCB-based 2D ultrasound array fabrication that is dicing-free and does not require advanced cleanroom technologies, with added advantages such as low-cost and ease of manufacturing 2D transducer arrays with scalable aperture size, element count, and frequency of operation.SUMMARY OF THE INVENTION

[0008] In exemplary embodiment can relate to a transducer. The transducer can include a substrate having a front surface, a back surface, and a via extending through the substrate from the front surface to the back surface. The transducer can include a via conductive layer disposed on: a surface of the substrate defined by the via; a front surface region adjacent the via; and a back surface region adjacent the via. The transducer can include a via-fill material disposed within the via and occupying a volume of space formed by the via. The transducer can include an anisotropic layer disposed on the front surface region and the via-fill material located at or near the front surface. The transducer can include a piezoelectric film disposed on the anisotropic layer. The transducer can include a conductive layer disposed on the piezoelectric film.

[0009] In some embodiments, the transducer can be an ultrasonic transducer.

[0010] In some embodiments, the substrate can be a printed circuit board (PCB) substrate.

[0011] In some embodiments, the via conductive layer can include immersion gold.

[0012] In some embodiments, the via-fill material can include conductive or non-conductive epoxies such as silver epoxy or type of siloxane (e.g., polydimethylsiloxane (PDMS)).

[0013] In some embodiments, the via-fill material can occupy the entire volume of space formed by the via.

[0014] In some embodiments, the anisotropic material can be electrically conductive in a vertical direction, the vertical direction being a direction running from the front surface to the back surface.

[0015] In some embodiments, the piezoelectric film can include polyvinylidene fluoride (PVDF) or polyvinyl difluoride trifluoro ethylene (PVDF-TrFE).

[0016] In some embodiments, the conductive layer can include metal.

[0017] In some embodiments, the conductive layer can have a thickness of 200 nm.

[0018] In some embodiments, a protective layer can be disposed on the conductive layer and on the substrate.

[0019] In some embodiments, the protective layer can include parylene or polymers with suitable acoustic impedance.

[0020] In some embodiments, the protective layer can have a thickness of 3 pm.

[0021] An exemplary embodiment can relate to a transducer array. The transducer array can include a substrate having a front surface, a back surface, and plural vias, each via extending through the substrate from the front surface to the back surface. The transducer array can include a via conductive layer disposed on: for each via, a surface of the substrate defined by the via; for each via, a front surface region adjacent the via; and for each via, a back surface region adjacent the via. The transducer array can include a via-fill material, for each via, disposed within the via and occupying a volume of space formed by the via. The transducer array can include an anisotropic layer disposed on the front surface region and the via-fill material located at or near the front surface. The transducer array can include a piezoelectric film disposed on the anisotropic layer. The transducer array can include a conductive layer disposed on the piezoelectric film.

[0022] An exemplary embodiment can relate to a method producing a transducer having a substrate with a front surface, a back surface, and a via extending through the substrate from the front surface to the back surface. The method can involve disposing a via conductive layer on: a surface of the substrate defined by the via; a front surface region adjacent the via; and a back surface region adjacent the via. The method can involve disposing a via-fill material within the via so that the via-fill material occupies a volume of space formed by the via. The method can involve disposing an anisotropic layer on the front surface region and the via-fill material locatedat or near the front surface. The method can involve disposing a piezoelectric film on the anisotropic layer. The method can involve disposing a conductive layer on the piezoelectric fdm.

[0023] In some embodiments, the method can involve disposing a protective layer on the conductive layer and on the substrate.

[0024] An exempalry embodiment can relate to a photoacoustic imaging device. The device can include a transducer array. The transducer array can include a substrate having a front surface, a back surface, and plural vias, each via extending through the substrate from the front surface to the back surface. The transducer array can include a via conductive layer disposed on: for each via, a surface of the substrate defined by the via; for each via, a front surface region adjacent the via; and for each via, a back surface region adjacent the via. The transducer array can include a via-fill material, for each via, disposed within the via and occupying a volume of space formed by the via. The transducer array can include an anisotropic layer disposed on the front surface region and the via-fill material located at or near the front surface. The transducer array can include a piezoelectric film disposed on the anisotropic layer. The transducer array can include a conductive layer disposed on the piezoelectric film. The device can include a light guide coupled to the tranduscer array.

[0025] In some embodiments, the transducer array can be configured as a multichannel 2D planar matrix array. The light guide can be a coaxial optical tube.

[0026] Further features, aspects, objects, advantages, and possible applications of the present invention will become apparent from a study of the exemplary embodiments and examples described below, in combination with the Figures, and the appended claims.BRIEF DESCRIPTION OF THE DRAWINGS

[0027] The above and other objects, aspects, features, advantages and possible applications of the present innovation will be more apparent from the following more particular description thereof, presented in conjunction with the following drawings. Like reference numbers used in the drawings may identify like components.

[0028] FIGS. 1 A and IB show an exemplary low-cost Printed Circuit Board (PCB) backed Polyvinylidene fluoride-co-trifluoroethylene (PVDF-TrFE) receiver for PA imaging application. FIG. 1 A shows a 3D schematic of an exemplary PVDF-on-PCB PA receiver and an exemplarymethod of producing the same. FIG. IB shows the physical appearance of the PVDF-TrFE-on- PCB substrate.

[0029] FIG. 2 (steps A-F) shows an exemplary fabrication process to make an embodiment of the PVDF-TrFE-on-PCB substrate.

[0030] FIG. 3 shows impedance spectra of an embodiment of an embodiment of the PVDF- TrFE-on-PCB substrate.

[0031] FIG. 4 shows an ultrasound pulse echo characterization, wherein image A shows a schematic block diagram of the pulse echo setup and image B is a time and frequency domain pulse echo response from an exemplary PVDF-TrFE-on-PCB substrate.

[0032] FIG. 5 shows two sets of PCBs for the parametric study experiment. Image A shows PCBs for studying the effect of variation in aperture size to pulse echo characteristics. Image B shows PCBs for studying the effect of variation of hole size to pulse echo characteristics.

[0033] FIGS. 6A and 6B show parametric study results of transducer characteristics on pulse echo response. FIG. 6A shows the effect of the variation in aperture dimension on the pulse echo response i. 4mm aperture transducer time and frequency domain response in comparison to ii. 1 mm aperture transducer time and frequency domain response, at a constant via size of 100 pm. FIG. 6B shows the effect of the variation in via hole dimension on the pulse echo response i. 0.5 mm via transducer time and frequency domain response in comparison to ii. 1.5 mm via transducer time and frequency domain response, at a constant aperture size of 5 mm.

[0034] FIG. 7 shows the effect of zero backing on the transducer characteristics. Graph A is a time domain comparison of air backed and epoxy backed transducer. Graph B is a frequency domain comparison of air backed and epoxy backed transducer.

[0035] FIG. 8 shows a schematic of the experimental setup for PA characterization and imaging.

[0036] FIG. 9 shows a PA A-line signal in time and frequency domain depicting a fractional bandwidth of 148%.

[0037] FIGS. 10A, 10B, and 10C show PA Imaging using PVDF-TrFE-on-PCB substrate, wherein FIG, 10A is a picture showing PA imaging experimental setup, FIG, 10B is a B-mode PA image obtained using synthetic aperture technique, and FIG. 10C is a 3D plot to characterize the quality of the B-mode image.

[0038] FIG. 11 (images a, b, c, d, e, and f) shows comparative photoacoustic (PA) imaging simulations to study volumetric imaging capabilities of 2D arrays. Schematic of imaging for image (a) 4x4. Volumetric PA image from 4x4 array with image (b) fc = 10 MHz and image (c) fc = 25 MHz. Schematic of imaging for image (d) 8x8. Volumetric PA image from 8x8 array with image (e) fc = 10 MHz and image (f) f = 25 MHz. Depth of light absorbing PA targets (0.4 mm diameter) from 2D PCB Arrays: Tl : 1.5 mm T2: 3 mm. Lateral distance between targets: 3 mm.

[0039] FIG. 12 (images a and b) shows a PCB-based 2D ultrasound transducer array fabrication: image (a) Cross-sectional view highlighting different layers of a single element of the PCB transducer array; image (b) Fabrication steps; (1) PCB with unfilled vias (2) Silver Epoxy filling (3) Z-Axis tape put over all elements (4) Undiced PVDF-TrFe bonded to Z-Axis tape (5) Front electrode connection as required (6) Insulating Parylene coating.

[0040] FIG. 13 (images a, b, c, and d) shows PCB-Based 2D Matrix Arrays. Image (a) Front and back side schematic of 4x4 PCB array, with element size 1 mm2with pitch of 1.2 mm, with header pins for elements and ground vias circled in red and black, respectively. Image (b) Fabricated 4x4 Array with backend interfacing connections for real time volumetric PA data acquisition. Image (c) Front and back side schematic of 8x8 PCB array, with element size 1 mm2 with pitch of 1.5 mm, with peripheral solder pads for elements (connected by routing traces as shown by red arrows) and ground vias circled in red and black, respectively. Image (d) Fabricated 8x8 Array with backend interfacing connections.

[0041] FIG. 14 shows (images a, b, c, d, e, f, g, and h) show typical characterization results of 2D PCB-based ultrasound transducer arrays. Electrical Impedance (red: \z\, black: zz) for image (a) 4x4 and image (b) 8x8 arrays. Pulse Echo US A-line and 6dB FWHM US BW for image (c) 4x4 and image (d) 8x8 arrays. PA A-line and 6dB FWHM PA BW for image (e) 4x4 and image (f) 8x8 2D arrays. Pulse-echo US peak-to-peak amplitude of individual elements (black: peak- to-peak mean, red: single standard deviation above and below peak-to-peak mean) for image (g) 4x4 array using 5073 PR and image (h) 8x8 array using Verasonics.

[0042] FIG. 15 (images a and b) shows an experimental setup for volumetric PA data acquisition using 2D PCB matrix arrays. Image (a) Schematic of the experimental setup for acquiring PA data from 1.5% intralipid phantom embedded with two pencil lead targets and imageReconstruction process flow. Image (b) Picture of experimental setup for 1.5% intralipid phantom imaging using PCB Matrix Arrays and oblique angle of laser firing bundle.

[0043] FIGS. 16 (images a, b, c, d, e, f, g, and h) shows volumetric photoacoustic imaging of two pencil lead targets (T1 and T2) inside 1.5% intralipid phantom using 4x4 and 8x8 2D PCB Arrays. Image (a) 4x4 Volumetric PA Image and corresponding image (b) XZ and image (c) YZ maximum intensity projection (MZP) images. Image (d) 8x8 Volumetric PA Image and corresponding image (e) XZ and image (f) YZ MIP images. Regions highlighted by white arrows denote regions of non-uniform receive sensitivity in the 8x8 array. Image (g) 4x4 and image (h) 8x8 FWHM Axial and Lateral Spatial Resolution profiles of T1 and T2 highlighting improved axial and lateral resolution observed with 8x8 PCB Array. Depth of light absorbing PA targets (0.4 mm diameter) from surface of 4x4 and 8x8 2D PCB arrays: T1 : 7 mm. T2: 9 mm. Lateral distance between T1 and T2 is 2 mm.

[0044] FIG. 17 (images a, b, c, d, and e) shows an exemplary photoacoustic imaging device using an embodiment of the transducer array. Image (a) shows a front side schematic of a 240 element PCB array with a central aperture to enable coaxial laser illumination of targets for PAI. Image (b) shows a back side PCB array schematic highlighting peripheral IPEX connector solder pads wherein every 60 sensing elements are routed to one row of pads. Image (c) shows a midfabrication array front-view with hole-punched PVDF. Image (d) shows a back-view with 4 IPEX receptacles soldered for interconnection. Image (e) shows an interconnect schematic containing the matrix array PCB, a 3D-printed casing, and an interconnect PCB wire bonded to UTA for a DAQ connection.DETAILED DESCRIPTION OF THE INVENTION

[0045] The following description is of exemplary embodiments that are presently contemplated for carrying out the present invention. This description is not to be taken in a limiting sense, but is made merely for the purpose of describing the general principles and features of the present invention. The scope of the present invention is not limited by this description.

[0046] Referring to FIGS. 1-2, embodiment can relate to single element of a multidimensional transducer 100 (e.g., a device that transforms one form of energy to another). It is contemplated for the transducer 100 to be configured as an acoustic (e.g., ultrasound) transducer so as totransform ultrasound energy into electrical signals. The transducer 100 can be used for ultrasound imaging and / or photoacoustic imaging, used as a component in an ultrasound-based sensor. It should be noted that the schematic showing the piezoelectric stack represents one element in a multidimensional transducer array, as demonstrated here for a planar 2D matrix array. As will be explained herein, embodiments of the single element transducer 100 can made into a transducer array 100’ (e.g., a Printed Circuit Board (PCB) based ultrasound transducer array). The transducer array 100’ can be used to provide multimodal ultrasound and photoacoustic imaging, for example.

[0047] The transducer 100 can include a substrate 102. The substrate 102 is a printed circuit board (PCB). Using a PCB substrate 102 makes the transducer 100 manufacturable in bulk and does away the need to laboriously dice multidimensional arrays to ensure element wise electrical isolation. PCBs of various thicknesses, sizes, curvatures and geometries can be designed using standard PCB design software and manufactured at low-cost in bulk.

[0048] The substrate 102 can have a front surface 104 and a back surface 106. It is contemplated for the front surface 104 to be the working end of the transducer 100 (e.g., the end that receives the acoustic energy). It is contemplated for the back surface 106 to be in connection with or at least partially form an electronic module (e.g., an electronic assembly, an integrated circuit, another printed circuit board, etc.). As will be explained herein, the transducer 100 can be formed by performing processing steps on the substrate 102. This can involve deposition of layers, (e.g., chemical vapor deposition, physical vapor deposition, atomic layer deposition, anion exchange deposition, plasma deposition, sputtering, etc.), creating formations or components via lithography / etching, forming electrical connections via soldering, etc. Such processing on substrates 102 for the creation of electronic modules is well known.

[0049] The substrate 102 can have one or more vias 108. Avia 108 is a hole (e.g., 10 to 500 microns) that is used to make an electrical connection between different layers, components, electrical contacts, etc. of the electronic module. The hole 108 diameters can range in size from 10 microns to covering 3 / 4 the area of the element pad. The lower limit is restricted by standard PCB processing techniques and upper limit is decided by how much acoustic backing material needs to be fdled into the via 108. The element pad size can change from array to array. The via(s) 108 can be formed by drilling a hole(s) in the substrate 102, wherein each via 108 is a voiddefined by a volume of space within the substrate 102. One or more of the vias 108 can extend through the substrate 102 from the front surface 104 to the back surface 106, which can include extending all the way through the substrate 102.

[0050] The transducer 100 can include a via-conductive layer 110 (e.g., electrically conductive). The conductive layer thickness can range between 200 nm to microns, depending on the application. It is contemplated for this via-conductive layer 110 to cover at least a portion of the surface of the substrate 102 where the via(s) 108 are formed. The via-conductive layer 110 may also cover a portion(s) of the front surface 104 and / or back surface 106. In an exemplary embodiment, the transducer 100 can include a via-conductive layer 110 disposed on: a surface of the substrate 102 defined by the via 108; a front surface region adjacent the via 108; and a back surface region adjacent the via 108. The via-conductive layer 110 can be a conductive material (e.g., silver, copper, gold, immersion gold, etc.). As will be explained herein, it is contemplated for the transducer 100 to be made into a transducer array 100’ (e.g., a substrate 102 having several vias 108 so as to form an array of transducers 100). It is contemplated for each via 108 to have the same type of material, same thickness, same area of coverage, etc. for the via- conductive layer 110 disposed thereon, but it is understood that any one via 108 can have a via- conductive layer 110 material, thickness, area of coverage, etc. that differs from a via-conductive layer 110 material, thickness, area of coverage, etc. of another via 108.

[0051] The transducer 100 can include a via-fill material 112. The via-fill material 112 helps electrical conductivity and provides acoustic backing as needed, since different via-fill material 112 can be used. Also, all vias 112 can be fdled in parallel which saves time. This also obviates use of cleanroom technologies or laborious practices like dicing which is common in conventional ultrasound transducer fabrication. The via-fill material 112 can be disposed within the via 108 and occupy a volume of space formed by the via 108. This can include occupying the entire volume of space formed by the via 108. The via-fdl material 112 can be conductive or non-conductive epoxies (e.g., silver epoxy, polymer-based epoxy (e.g., silicone, siloxane (e.g., polydimethylsiloxane (PDMS)), optical epoxy, etc. The via-fill material 112 can either be conducting or non-conducting depending on how much electrical conductivity is desired through the via 108. Additionally, the via-fill material 112 also serves as acoustic backing, therefore, different materials with varying acoustic backing properties may be fdled in the vias 108 asdesired. As will be explained herein, it is contemplated for the transducer 100 to be made into a transducer array 100’ (e.g., a substrate 102 having several vias 108 so as to form an array of transducers 100). It is contemplated for each via 108 to have the same type of material, occupy the same amount of volume, etc. for the via-fill material 112, but it is understood that any one via 108 can have a via-fdl material 112 material, occupy a volume of space, etc. that differs from a via-fill material 112 material, occupy a volume of space, etc. of another via 108.

[0052] The transducer can include an anisotropic layer 114 (e.g., 50 to 200 microns thick). The anisotropic layer 114 can be disposed on the front surface region (e.g., the portion of the front surface 104 that is adjacent the via 108 and has via-conductive layer 110 formed thereon) and the via-fill material 112 located at or near the front surface. In other words, the anisotropic layer 114 can cover the via 108 and an area of the front surface 104 that is defined by the front surface region. The anisotropic layer 114 can be any electrically conducting material with low resistivity (e.g., z-axis tape). When disposed on the transducer 100, the anisotropic material can be electrically conductive in a vertical direction - the vertical direction being a direction running from the front surface 104 to the back surface 106 but not electrically conductive in non-vertical directions. The anisotropic layer 114 can be disposed on the entire front surface region or a portion thereof, on the entire area of he via-fill material or a portion thereof, etc. As will be explained herein, it is contemplated for the transducer 100 to be made into a transducer array 100’ (e.g., a substrate 102 having several vias 108 so as to form an array of transducers 100). It is contemplated for each via 108 to have the same type of material, same thickness, same area of coverage, etc. for the anisotropic layer 114, but it is understood that any one via 108 can have an anisotropic layer 114 material, thickness, area of coverage, etc. that differs from an anisotropic layer 114 material, thickness, area of coverage, etc. of another via 108.

[0053] The transducer can include a piezoelectric film 116 disposed on the anisotropic layer 114. The piezoelectric film 116 can be polyvinylidene fluoride (PVDF), polyvinyl difluoride trifluoro ethylene (PVDF-TrFE), PZT composite, PMN-PT, etc. The piezoelectric film 116 can be disposed on the entire anisotropic layer 114 or a portion thereof, etc. The piezoelectric film 116 can have a thickness ranging from 5 to 500 microns. The thickness of the piezoelectric film 116 is dependent on the frequency of imaging being targeted. Both a high frequency (corresponding to a thinner piezo layer thickness) and a low frequency (corresponding to athicker piezo layer thickness) were tested with the PCB-based substrate 102. Different thicknesses, going beyond 500 micron may be used, depending on the piezo layer being used and the frequency of imaging being targeted. One of the advantages of the present invention is that ultrasound transducers of wide frequency range can be made by merely selecting appropriate thickness of the piezoelectric film 116. The lower the thickness of the piezoelectric film 116, the higher is the frequency of the transducer. As will be explained herein, it is contemplated for the transducer 100 to be made into a transducer array 100’ (e.g., a substrate 102 having several vias 108 so as to form an array of transducers 100). It is contemplated for each via 108 to have the same type of material, same thickness, same area of coverage, etc. for the piezoelectric film 116, but it is understood that any one via 108 can have a piezoelectric film 116 material, thickness, area of coverage, etc. that differs from a piezoelectric film 116 material, thickness, area of coverage, etc. of another via 108.

[0054] The transducer 100 can include a conductive layer 118 (e.g., electrically conductive) disposed on the piezoelectric film 116. The conductive layer 118 can be metal (e.g., copper, chromium-gold, etc.), silver epoxy or any other electrically conductive material that ensures conduction of electrical signal from the front face, or top electrode, of the ultrasound transducer array). This can include any metal film deposited using chemical vapor deposition, physical vapor deposition, atomic layer deposition, anion exchange deposition, plasma deposition, sputtering, etc. The thickness of the conductive layer 118 can depend upon the process used for the deposition and its parameter. The thickness can range from a few nanometers to several micrometers. The conductive layer 118 can be 200 nm thick, for example. The conductive layer 118 can be disposed on the entire piezoelectric film 116 or a portion thereof, etc. As will be explained herein, it is contemplated for the transducer 100 to be made into a transducer array 100’ (e.g., a substrate 102 having several vias 108 so as to form an array of transducers 100). It is contemplated for each via 108 to have the same type of material, same thickness, same area of coverage, etc. for the conductive layer 118, but it is understood that any one via 108 can have a conductive layer 118 material, thickness, area of coverage, etc. that differs from a conductive layer 118 material, thickness, area of coverage, etc. of another via 108.

[0055] The transducer 100 can include a protective layer 120 (e.g., to prevent or impede corrosion, oxidation, wear, cracking, etc.) disposed on the transducer 100. This can includedisposing the protective layer 120 on the conductive layer 118 and on the substrate 102 (e.g., the front surface 104 that has not had any other deposition layer). The conductive layer 118 can be parylene, PDMS, etc. The thickness of protective layer 120 can depend on the amount of precursor used. It is noted that the protective layer 120 can also be used as matching layer.Thus, the thickness may vary depending on the piezo layer 116 being used. For a piezo layer 116 that requires significant matching layer, the thickness of the insulating / matching layer is as per calculated based on the acoustic impedance of the piezo layer 116. In case of PVDF-TrFe which is acoustically impedance matched to coupling media by itself, the layer primarily serves as insulation and can be chosen to be kept thick enough depending expected duration of usage.

[0056] The thickness can range from a few microns to several tens or hundreds of microns. The protective layer 120 can be 3 pm thick, for example. The protective layer 120 can be disposed on the entire piezoelectric film 116 or a portion thereof, etc. As will be explained herein, it is contemplated for the transducer 100 to be made into a transducer array 100’ (e.g., a substrate 102 having several vias 108 so as to form an array of transducers 100). It is contemplated for each via 108 to have the same type of material, same thickness, same area of coverage, etc. for the protective layer 120, but it is understood that any one via 108 can have a protective layer 120 material, thickness, area of coverage, etc. that differs from a protective layer 120 material, thickness, area of coverage, etc. of another via 108.

[0057] An exemplary embodiment can relate to a transducer array 100’. The transducer array 100’ can include plural transducers 100. The plural transducer 100 can be arranged in an array (e.g., a 4 x 4 grid, a 8 x 8 grid, a 1 x 8 strip, a 2 x 8 strip, etc.). The transducer array 100’ can be formed form a single substrate 102, multiple substrates 102, etc. Again, use of a PCB substrate 102 makes the transducer 100, 100’ manufacturable in bulk and does away the need to laboriously dice multidimensional arrays to ensure element wise electrical isolation. PCBs of various thicknesses, sizes, curvatures, and geometries can be designed using standard PCB design software, and can be manufactured at low-cost in bulk.

[0058] In an exemplary embodiment, plural vias 108 are formed in a single substrate 102 and then subjected to the processing steps discussed herein to form the transducer array 100’. For instance, the transducer array 100’ can include a substrate 102 having a front surface 104, a back surface 106, and plural vias 108. One or more of the vias 108 can be formed to extend throughthe substrate 102 from the front surface 104 to the back surface 106. The transducer array 100’ can include a via-conductive layer 110 disposed on: for one or more vias 108, a surface of the substrate defined by the via 108; for one or more vias 108, a front surface region adjacent the via 108; and for one or more vias 108, a back surface region adjacent the via 108. The transducer array 100’ can include a via-fill material 112, for one or more vias 108, disposed within the via 108 and occupying a volume of space formed by the via 108. The transducer array 100’ can include an anisotropic layer(s) 114 disposed on the front surface region(s) and the via-fill material(s) 112 located at or near the front surface 104. The transducer array 100’ can include a piezoelectric film(s) 116 disposed on the anisotropic layer(s) 114. The transducer array 100’ can include a conductive layer(s) 118 disposed on the piezoelectric fdm(s) 116. The conductive layer 118 does not form electrical connection between vias 108. Rather, it ensures the top surface of the piezoelectric layer 116 has a conformal electrical connection.. The transducer array 100’ can include a protective layer(s) 120 disposed on the conductive layer(s) 118 and substrate 102.

[0059] An exemplary embodiment can relate to a method producing a transducer 100 having a substrate 102 with a front surface 104, a back surface 106, and a via 108 extending through the substrate 102 from the front surface 104 to the back surface 106. The method can involve disposing a via-conductive layer 110 on: a surface of the substrate 102 defined by the via 108; a front surface region adjacent the via 108; and a back surface region adjacent the via 108. The method can involve disposing a via-fill material 112 within the via 108 so that the via-fill material 112 occupies a volume of space formed by the via 108. The method can involve disposing an anisotropic layer 114 on the front surface region and the via-fill material 112 located at or near the front surface 104. The method can involve disposing a piezoelectric film 116 on the anisotropic layer 114. The method can involve disposing a conductive layer 118 on the piezoelectric film 116. The method can involve disposing a protective coating (e.g., protective layer 120) on the conductive layer 118 and on the substrate 102.

[0060] Referring to FIG. 17, an exempalry embodiment can relate to a photoacoustic imaging device 200. The device 200 can include an embodiment of the transducer array 100’. The transducer array 100’ can be configured as a multichannel 2D planar matrix array. The transducer array 100’ can include a PCB substrate 102 having a front surface 104, a back surface 106, and plural vias 108, each via extending through the PCB substrate 102 from the frontsurface 104 to the back surface 106. The transducer array 100’ can include a via conductive layer 110 disposed on: for each via 108, a surface of the substrate 102 defined by the via 108; for each via 108, a front surface region adjacent the via 108; and for each via 108, a back surface region adjacent the via 108. The transducer array 100’ can include a via-fill material 112, for each via 108, disposed within the via 108 and occupying a volume of space formed by the via 108. The transducer array 100’ can include an anisotropic layer 114 disposed on the front surface region and the via-fill material 112 located at or near the front surface. The transducer array 100’ can include a piezoelectric fdm 116 disposed on the anisotropic layer 114. The transducer array 100’ can include a conductive layer 118 disposed on the piezoelectric film 116. The device 200 can include a light guide 202 coupled to the tranduscer array 100’. The light guide 202 can be a coaxial optical tube, for example. In the exemplary embodiment shown, the device 200 is configured as a 240 element planar matrix array with a coaxial laser aperture. Image (a) shows a front side schematic of the 240 element PCB array 100’ with a central aperture to enable coaxial laser illumination of targets for PAI. Image (b) shows a back side PCB array 100’ schematic highlighting peripheral IPEX connector solder pads wherein every 60 sensing elements are routed to one row of pads. Image (c) shows a mid-fabrication array front-view with hole- punched PVDF. Image (d) shows a back-view with 4 IPEX receptacles soldered for interconnection. Image (e) shows an interconnect schematic containing the matrix array PCB 100’, a 3D-printed casing 204, and an interconnect PCB wire bonded to UTA for a DAQ connection. The interconnect board is an inverted version of the transducer PCB. Four 60- element IPEX connectors connect the two PCBs through the 3D-printed casing 204. The casing 204 also contains a coaxial optical tube 202 for light delivery.

[0061] EXAMPLES

[0062] The following disclosure discusses exemplary transducers, transducer arrays, methods of producing the same, and test results.

[0063] EXAMPLE 1

[0064] Test results demonstrate that the methods disclosed herein facilitate development of a cost effective and high- performance ultrasound transducer with several advantages, such as high throughput, easy manufacturing, and scalability. Through an investigation of the impact of different PCB backings- on transducer performance, the inventors have identified thesignificance of element size and via size in achieving optimal imaging outcomes. Detailed iterations have highlighted the substantial influence of adjusting these parameters on overall transducer performance, emphasizing the need for careful design and optimization during manufacturing. Additionally, the inventors have demonstrated the importance of filling the PCB via with conductive, non-conductive epoxy compared to leaving it air backed. The transducer exhibits an impressive photoacoustic fractional bandwidth of 148% with conductive epoxy backing, enabling the acquisition of high quality- images from custom targets. Furthermore, the inventors have successfully obtained synthetic aperture-based B— mode photoacoustic images using the PVDF- PCB receiver, showcasing the proof of concept, and validating the potential of this low- cost transducer architecture for high-quality photoacoustic imaging. Overall, this study demonstrates- the successful development and capabilities of the low cost- ultrasound transducer.

[0065] Photoacoustic imaging (PAI) is a non-invasive hybrid imaging technique which fuses the benefits of optical and ultrasound imaging. The involvement of transmitted light allows functional imaging through the distinct spectroscopic selectivity of endogenous chromophores in vivo by probing various wavelength specific molecular mechanisms, whereas the involvement of sound enhances the range for which such functional information is carried from the target to the detector. Thus, in short it combines the best of light and sound. PAI has shown an extraordinary capability for preclinical research in small-animal whole- body imaging, e.g., mapping the macro and micro vasculature network and studying the dormant state functional activity of the mouse brain. Lately, it has even been applied for several rather crucial medical conditions such as cancer stage detection, diagnosis of cardiovascular diseases, functional human brain imaging, and image-guided surgery.

[0066] Although the field has matured from its initial days in terms of low- cost optical instrumentation capabilities, by implementing pulsed laser diodes and light emitting diodes as cost- efficient measures to retrieve PA signals, an equal effort for making low-cost and high throughput bulk ultrasound transducers is largely missing in the photoacoustic literature. Mostly the effort towards developing non-conventional ultrasound transducers for photoacoustic imaging have been made by incorporating capacitive and piezoelectric MEMS or even silicon photonics-based ultrasound transducers, the making of which requires large investments in termsof capital and time. The scenario is even more unfavorable while making non -conventional arrays such as the 2D arrays and annular arrays for real-time tomographic applications.

[0067] Others have made initial efforts towards this direction by constructing a spherical 3D array on a home-built dielectric preform and a metallized PVDF sandwich. The array contained provision for 512 circular shaped elements in the form of hollow non-conductive through-perform vias to facilitate front-end connections. Additional efforts lead to the construction of 2D arrays on a printed circuit board (PCB) with a metallized PZT sandwich following a dice-and-fill approach. However, this construction was applied to ultrasound imaging rather than PAI. Some efforts in this direction were to highlight the importance of 2D array geometry developed in spiral fashion.

[0068] Although these above-mentioned approaches can reduce the overall cost of ultrasound transducer production, they have some pivotal limitations. The former firstly uses a 3D printed preform as the substrate, which decreases element uniformity owing to the minute variations in the 3D printing technique. Secondly, the electrode connections to each one of the elements need to be manually created by filling the holes accurately, so as to ensure perfect connectivity, which reduces the overall throughput of production, and makes manufacturing costlier. Thirdly, drawing of wires have to be directly made from the through preform vias by inserting the signal cables inside the vias which again makes the manufacturing complicated and adds a significant amount of noise in the system, thereby requiring a separate noise reduction scheme for each one of the channels. Prior work focusing on a PCB is a better solution than constructing a preform, but it also uses a piezoelectric solid ceramic which needs dicing and filling with an absorbing filler material to reduce the effects of shear-wave coupling. Also, the process involves the usage of a precision dicer, which further increases the production cost. Furthermore, in either of the contributions, through substrate vias have been used to establish electrical connections only without any mention of the effect of via filling with conductive epoxy on the electroacoustic response of the device.

[0069] In this report, the inventors intend to fill in the missing gaps in the above available literature and further propose on developing a low- cost PCB backed single element ultrasound transducer with the added advantage of: (a) high throughput; (b) ease of manufacturing; and (d) scalability. Furthermore, the inventors also study the effect of the PCB backing under variousconditions of through substrate via filling and document our reasoned observations in this report. Two detailed iterations with altering the element size (at a constant via size) and altering the via size at (a constant element size) have been carried out. It has been found that both of these two parameters have a significant impact on the overall transducer performance with crucial imaging implications. In order to verify and test their hypothesis, the inventors have progressed a step ahead to obtain photoacoustic B- mode images from an embodiment of a transducer, thereby demonstrating the capability of our cheap transducer solution to create next generation PA receiver arrays.

[0070] FIGS. 1 A and IB show a low-cost Printed Circuit Board (PCB) backed Polyvinylidene fluoride (PVDF) receiver for PA imaging application. FIG. 1 A shows a 3D schematic of the PVDF-on-PCB PA receiver and a method of producing the same. FIG. IB shows the physical appearance of the PVDF-on-PCB receiver. FIG. 2 (steps A-F) shows an exemplary fabrication process to make the PVDF-on-PCB PA receiver. FIG. 3 shows impedance spectra of an embodiment of a PVDF-on-PCB receiver.

[0071] Transducer Architecture

[0072] Simply put, the transducer is PVDF adhered to a metal patterned PCB having a through substrate via to enable for the provision of the backing layer and to effectively carry the signal and ground connections from the transducer front to its back. This facilitates establishing a hassle-free connection to the backend. Each transducer contains a square elemental aperture having an edge dimension a and a circular via having diameter <£> respectively as two of the significant variables. FIG. 1 A depicts the 3D cross-section schematic of the transducer and FIG. IB shows the real look of a typical transducer having a: 5 mm and : 1 mm.

[0073] Transducer Fabrication

[0074] The fabrication process flow of the single element transducer begins with the PCB as described above in FIG. 2 (step A). One of the faces (top face in the figure) is then attached to a glass slide using a double-sided tape (step B). The elemental via is then completely filled with H20E from EPO-TEK Inc. and also with E-solder 3022 by Van Roll Inc. from the bottom side, while ensuring absolute flatness on the top side (the side attached to the glass) and then partially cured at 120°C for 5 minutes to achieve a uniformly planar filling without formation of any meniscus on the working front side of the transducer. The filling process is repeated again toensure the complete filling of the via and thereby completely cure at 120°C for an hour. The glass slide is next detached from the transducer’s front surface (step C). Backend micro coaxial wires are next soldered to the bond pads available on the bottom side of the PCB to eliminate post heating issues of the piezoelectric polymer film. The elemental pad is subsequently adhered to a z-axis conductive tape from 3M Inc. which is responsible for conducting only in the vertical direction, while eliminating any electrical connectivity in the lateral direction (step D). This enables signal to pass from the elemental electrode pad to the piezoelectric polymer film and vice-versa. A premetallized polyvinyl difluoride (PVDF) or poly vinyledine fluoride-co- trifluoro ethylene (PVDF) from PolyK Inc. thin sheet having a thickness of 28 pm is next bonded to the -zaxis- tape (step E). The entire assembly is subsequently selectively copper sputtered to connect the top metallization of PVDF to the other PCB via, thereby establishing the ground connection as shown in step F. All the transducers used in this contribution have been fabricated at the Centre for Nano Science and Engineering, Indian Institute of Science.

[0075] Electrical Impedance Characterization

[0076] One of the important parameters often used to characterize any piezoelectric transducer electrically is the electrical impedance. Different transducers having various V and a were developed and electrically characterized using Impedance Analyzer (4294A from Agilent Inc.) FIG. 3 depicts a typical impedance spectra as observed from a transducer having a: 1 mm and 100 pm respectively. The resonant and antiresonant frequencies have been determined to be39.4 MHz and 42.3 MHz respectively, with an impedance magnitude of 1.6 kQ at resonance and 2.3 kQ at antiresonance respectively. The phase change at resonance was observed to be 25°.

[0077] Ultrasound Pulse-Echo Characterization

[0078] The next important parameter to characterize the transducer electromechanically is the pulse-echo (pitch catch) from the transducers. FIG. 4 shows an ultrasound pulse echo characterization, wherein image A shows a schematic block diagram of the pulse echo setup and image B is a time and frequency domain pulse echo response from an exemplary PVDF-on-PCB transducer.

[0079] Pulse-Echo Experimental Setup

[0080] In order to evaluate all the transducers’ performance, a pulse-echo testing setup was established which comprised of a fish tank filled with water in the bottom of which a mild steelplate was attached. Each one of the transducers was coated with Parylene- C to electrically isolate the transducer from any external interference and then housed onto a 3D printed holder to facilitate attachability to a 3- axis translation stage from Thor Labs Inc. Each transducer was then connected to an ultrasonic pulser receiver 5073 from Olympus Inc. which was responsible for sending nanosecond pulses to the transducers and receiving echoes from them. Transducers were tested one after the other to ensure better uniformity of experimental conditions. The distance between the transducer element and metal plate was kept constant. Each of the transducers was actuated with a 50 V peak- to- peak (Vpp) pulse and the receive echo was amplified by 39 dB before sending the signal to the oscilloscope. A 2D block diagram of the pulse echo setup is depicted in FIG. 4.

[0081] Pulse-Echo Response

[0082] FIG. 5 shows two sets of PCB for a parametric study experiment that was conducted. Image A shows PCBs for studying the effect of variation in aperture size to pulse echo characteristics. Image B shows PCBs for studying the effect of variation of hole size to pulse echo characteristics. Pulse echo response from a collection of transducers having various elemental aperture and via dimensions were obtained and one of the typical plots as obtained from a transducer having 1 mm elemental aperture with 100 pm via diameter is being depicted in FIG. 4. The peak-to-peak transceiver voltage after a gain of 39 dB was observed to be 259 mV with a -6dB fractional bandwidth of 43%.

[0083] Parametric Study of the Transducers Characteristics

[0084] In order to further investigate the influence of the two specific transducer parameter - (a) active aperture elemental dimension and (b) hole- to- aperture fraction for enabling baking layer provision, a parametric experimental study was carried out. Two sets of PCBs were made - (i) consisting of two different PCBs of 4 mm and 1mm apertures and a constant hole size of 100 pm as shown in FIG. 5 (image A) and (ii) consisting of two different PCBs of 1.5 mm and 0.5 mm hole size and a constant element size of 5 mm as shown in FIG. 5 (image B). Both the PCBs were further processed as described in the transducer fabrication section in order to fabricate the transducers. The measured observables were the peak-to-peak transceive voltage, the received echo’s pulse length and the -6 dB fractional bandwidth respectively when the transducer parameters such as the aperture and the hole-to-aperture fraction were varied.

[0085] FIGS. 6A and 6B shows parametric study of the transducer characteristics on pulse echo response. FIG. 6A shows the effect of the variation in aperture dimension on the pulse echo response i. 4mm aperture transducer time and frequency domain response in comparison to ii. 1 mm aperture transducer time and frequency domain response, at a constant via size of 100 pm. FIG. 6B shows the effect of the variation in via hole dimension on the pulse echo response i. 0.5 mm via transducer time and frequency domain response in comparison to ii. 1.5 mm via transducer time and frequency domain response, at a constant aperture size of 5 mm.

[0086] The pulse-echo response from the first set of transducers reveals the following two hypotheses (Figure 6A): with increase in the element aperture dimension from 1 mm (FIG. 6Aii) to 4mm (FIG. 6Ai), (a) the transceive voltage increases from 259 mV to 820 mV which is nearly 3 times. The increase is reasoned to be caused due to the increase in effective area responsible for generating charge upon insonification from the reflected echo; and (b) the pulse length which is a direct measure of the transducer bandwidth increases 1.67 times from 210 ns to 350 ns which is indicative of lower loss of energy owing to a higher fraction of solid PCB backing. Thus, there is a tradeoff between the signal to noise ratio (SNR) and the bandwidth as the aperture dimension of the transducer is varied. Bigger transducers will have higher SNR but lower bandwidth. The pulse-echo response from the second set of transducers reveals the importance of the PCB hole filled with epoxy (FIG. 6B). With increase in the epoxy filling fraction from 10% (Figure 6Bi) (500 pm in 5 mm aperture) to 30% (FIG. 6Bii) (1.5 mm in 5 mm aperture), the pulse length was significantly observed to decrease, thereby increasing the -6 dB fractional bandwidth from 8% to 28%. The increase in bandwidth for a higher hole size is due to the enhancement in the net fraction of epoxy filling which serves to absorb a considerable portion of energy from the vibrating PVDF film upon insonification from the reflected echo. This depicts the importance of the presence of the hole filled with absorbing material in fabricating low-cost PCB based transducers, and suggests a possibility to further engineer and tune such transducers’ imaging response. It may also be possible to further improve the fractional bandwidth upon using several different combinations of material and hole geometry.

[0087] Effect of Zero Backing on the Transducer Characteristics

[0088] FIG. 7 shows the effect of zero backing on the transducer characteristics. Graph A is a time domain comparison of air backed and epoxy backed transducer. Graph B is a frequency domain comparison of air backed and epoxy backed transducer.

[0089] In order to further investigate the effect of presence of the via filled epoxy backing on the transducer ultrasound properties, two similar additional transducers were fabricated having (a, <P) (1 mm, 0.5 mm) and were used to obtain the pulse-echo ultrasound signal. As shown in FIG. 7, in time domain, the air backed transducer demonstrated a ringing time of 320 ns in comparison to 200 ns as demonstrated by the epoxy backed transducer. In the frequency domain, these ringing values translate to a fractional bandwidth of 23% and 52% for the air backed and epoxy backed transducers respectively. These results further emphasize the importance of the presence of the via filled epoxy as an active damper, thereby directing to the possibility of enhancing the spatial resolution.

[0090] The next important parameter which has direct relevance to the aims of this contribution is the photoacoustic characterization and imaging which helps to ascertain the capability of the low-cost PCB backed PVDF transducers in functional imaging applications.

[0091] Experimental Setup for PAI

[0092] FIG. 8 shows a schematic of the experimental setup for PA characterization and imaging.

[0093] A plastic tank measuring 32 cm x 18 cm * 12 cm was filled with deionized water and utilized for PAI experiments. The tank submerged the entire PAI device, including the PVDF -PCB receiver, with the receiver and light output directed towards the imaging phantom placed at the bottom of the tank. To ensure proper alignment, the rear end of the PAI device was securely attached to a motorized three-axis linear stage. During the experiments, light pulses emitted from the wavelength tunable OPO laser fiber head (Phocus Mobile, Opotek Inc., Carlsbad, CA, USA) passed through the imaging phantom, where they were absorbed by the chromophores. This absorption resulted in the generation of broadband photoacoustic pressure waves caused by transient thermoelastic expansion. The PVDF-PCB receiver detected these photoacoustic pressures. For the experiments, the laser provided a maximum fluence of around 10 mJ / cm2at 800 nm, which was within the safety limits defined by the ANSI. The voltage output from an individual PCB array element was connected to the Olympus 5073, which offereda gain of 39 dB. The preamplified signal was then digitized using the oscilloscope at 1 GSPS (giga-samples per second) and synchronized with each laser pulse.

[0094] Photoacoustic Characterization and Phantom Imaging

[0095] FIG. 9 shows a PA A-line signal in time and frequency domain depicting a fractional bandwidth of 148%. The PVDF-PCB receiver was characterized by illuminating the flat metal target with the nanosecond laser pulse. A single A-line signal was captured for the maximum photoacoustic output as is shown in FIG. 9. The PA pulse was received ~ 32.6 ps after the laser excitation, which matches the 50 mm distance from the face of the receiver. The maximum peak-to peak voltage amplitude observed was 2.7 V at an amplified gain of 39 dB. The time domain A-line was then subsequently transformed into the frequency domain by using the in-built oscilloscope math function and is also demonstrated in the FIG. 9. The center frequency is found to be ~ 27 MHz with a -6dB fractional bandwidth of 148%. The lowering of frequency might be attributed to the broadening of the frequency response. These numbers are indicative of the inherent capability of the PVDF-PCB receiver based PAI imager for diverse PAI applications.

[0096] FIGS. 10A, 10B, and 10C show PA Imaging using PVDF-on-PCB transducer, wherein FIG, 10A is a picture showing PA imaging experimental setup, FIG, 10B is a B-mode PA image obtained using synthetic aperture technique, and FIG. 10C is a 3D plot to characterize the quality of the B-mode image. Next, the PVDF-PCB receiver was used to image 3 individual graphite targets having cross-sectional diameter of 400 pm submerged in deionized water (see FIG. 10A for the experimental setup). The optical fiber head was positioned accurately in order to allow for uniform illumination of the targets. The PVDF-PCB receiver was then raster scanned for 64 iterations following a synthetic aperture imaging scenario thereby quasi-imitating a 64 channel PVDF-PCB transducer. A-line signals obtained at various positions were then normalized, thresholded, and log compressed before feeding it to the delay multiply and sum + coherence factor (DMAS + CF) based beamformer. The B-mode image obtained is depicted in a 2.8 cm x 1.6 cm window, in which 3 targets are distinctly visible as shown in FIG. 10B. Further analysis of the image was carried out in the form of a 3D plot to determine its quality (see FIG. 10C). The PA amplitude was observed to fall off by 60% over a distance of ~12 mm. The lateral andthe axial full width at half maximum (FWHM) was also evaluated and was found to be ~ 900 pm for lateral and -200 pm for axial.

[0097] Thus, in this contribution the inventors have demonstrated successful development of a low-cost, high-performance ultrasound transducer that offers a range of benefits, including high throughput, ease of manufacturing, and scalability. By studying the effect of PCB backing on the transducer’s performance, the project has identified the importance of both the element size and via size in achieving optimal imaging results. The two detailed iterations carried out as part of the project have demonstrated that altering either the element size or the via size can have a significant impact on the overall transducer performance, highlighting the importance of careful design and optimization in the manufacturing process. Furthermore, the importance of filling the PCB via with epoxy has been also demonstrated in contrast to an un-filled air backed scenario. An unparallel fractional bandwidth of 148% is also reported which helps in achieving good quality images from custom targets. In a final note, the PVDF-PCB receiver has been applied to obtain synthetic aperture-based B-mode PA image to demonstrate the proof of concept, thereby proving the capability of such a low-cost transducer architecture in obtaining high quality PA image.

[0098] EXAMPLE 2

[0099] Photoacoustic (PA) imaging provides deep tissue molecular imaging of chromophores with optical absorption contrast and ultrasonic resolution. Present PA imaging techniques are predominantly limited to one 2D plane per acquisition. 2D ultrasound transducers, required for real-time 3D PA imaging, are high-cost, complex to fabricate and have limited scalability in design. We present novel PCB-based 2D matrix ultrasound transducer arrays that are capable of being bulk manufactured at low-cost without using laborious ultrasound fabrication tools. The 2D ultrasound array specifications are easily scalable with respect to widely available PCB design and fabrication tools at low cost. To demonstrate scalability, we fabricated low (11 MHz) frequency 8x8 matrix array and high (40 MHz) frequency 4x4 matrix array by directly bonding an undiced polyvinylidene fluoride (PVDF) piezoelectric material of desired thickness to the custom designed PCB substrate. Characterization results demonstrate wideband PA receive sensitivity for both low (87%) and high (188%) frequency arrays. Volumetric PA imaging resultsof light absorbing targets inside optical scattering medium demonstrate improved spatial resolution and field of view with increase in aperture size.

[0100] PA imaging is a non-ionizing molecular imaging modality delivering rich optical absorption-based contrast from various chromophores, such as hemoglobin molecules present inside blood vasculature. It offers deep tissue imaging capabilities with spatial resolution scalable with ultrasound (US) transducer parameters; typically, l / 200thto 1 / 100thof the imaging depth (i.e., a resolution of 0.3 mm at 3 cm depth). Thus, a PA imaging device is an emerging choice for several pre-clinical and clinical applications covering cancer, neurological and vascular diseases.

[0101] PA images are obtained using US transducers functioning in receive only mode; (i) a single element US transducer provides depth encoded A-line information at a given point in space; (ii) a linear or curvilinear array provides 2D (B-mode) images. And volumetric (3D) PA images of deep tissue are obtained by scanning a US transducer in the following ways: (i) 2D raster scanning a single-element US transducer in the imaging plane (ii) ID raster or rotational scanning of a linear or curvilinear US transducer array and, (iii) depth scanning using a ring transducer.

[0102] While the PA imaging speed, measured as frames per second (FPS), is proportional to the pulse repetition frequency (PRF) of the laser, it is limited by scan time and scanning space. Because single-element and ID array US transducers both require scanning for 3D PA imaging, 2D US arrays provide advantage of scan-less volumetric PA imaging in realtime. Towards this goal, a 2D capacitive micromachined ultrasonic transducer (CMUT) array was demonstrated for deep-tissue PA imaging. The integrated electronics of the device, however, prevented parallel receiving of data from all channels. Commercial 2D array US devices made of conventional piezoelectric materials are also being investigated for scan-less volumetric PA imaging. These 2D arrays are typically manufactured using complex fabrication tools involving dicing and filling, resulting in increased cost, manufacturing time and limited availability. In contrast, a hemispherical US transducer array fabricated on an undiced dielectric preform has been demonstrated for volumetric PA imaging. This array uses poly vinylidene difluoride (PVDF) piezoelectric material which exhibits a high piezoelectric receiving constant and wideband receive sensitivities for high frequency PA imaging. The choice of PVDF as piezolayer removed the need of dicing the piezo layer and the preform. Z-axis tape was used to ensure conduction of electrical signal from the PVDF to the substrate through the tape while ensuring no cross-talk between elements. However, the device geometry and sideways light coupling necessitates significant acoustic coupling, which introduces challenges for in-vivo longitudinal imaging of living subjects. The curvature of the array substrate, or preform, limits scalability with respect to aperture size or element count.

[0103] Fuller et al. demonstrated printed circuit boards (PCB) as a low-cost, scalable, easy-to-manufacture substrate for fabricating 2D planar array US transducers using conventional lead zirconate titanate (PZT) material. However, both the PZT material and the top of the PCB preform, need to be diced along the kerf and filled with non-conducting kerf-filler for element- wise electrical isolation. The 2D dicing required for the 2D array transducer significantly complicates and lengthens the fabrication process.

[0104] To overcome the above limitations, we propose combining a PCB-based 2D ultrasound array fabrication that is dicing-free and does not require advanced cleanroom technologies, with added advantages such as low-cost and ease of manufacturing 2D transducer arrays with scalable aperture size, element count and frequency of operation. In the following sections, we first present PA imaging simulations for our choice of planar array architectures. Next, we outline the transducer fabrication process flow of the PCB matrix arrays demonstrating adynamic fabrication process flow of the PCB matrix arrays of two different frequencies and aperture size. This is followed by characterization results of electrical impedance, pulse echo US and PA A-line signals from a flat metal target.

[0105] Photoacoustic Imaging Simulations

[0106] A key advantage of the PCB matrix arrays presented in this work is their scalability in dimension and element count. In this section, we use PA imaging simulations to study and optimize the 2D transducer array specifications such as aperture size, element size, kerf, and pitch of the array. Higher element counts and aperture size increase the field of view (FOV) and lateral resolution of volumetric PA imaging.

[0107] Comparative PA imaging simulations, in an optically and acoustically homogenous medium consisting of PA targets, are performed for a 4x4 array of 16 elements and an 8x8 array of 64 elements, both with an element size of 1 mm X 1 mm and pitch of 1.2 mm.

[0108] The image reconstruction simulations are carried out using the MATLAB based k-Wave toolbox, A \cm X \cm X 0.5cm water mimicking homogenous medium, consisted of two pencil lead targets (T1 and T2) of 400 / m diameter arranged with a lateral offset of 3 mm and axial offset of 1 ,5mm between them and the first target being 1.5 mm from the array. These are considered as the source of PA waves under the assumption of uniform light illumination throughout the medium. Further, to study imaging performance with respect to frequency, simulations are performed for two frequency configurations: Center frequencies (fc) of 10 MHz and 25 MHz with 6dB bandwidth (BW) of 70%. The grid size is kept as dx = 20 [im, as per to ensure measurement of acoustic waves of up to 39 MHz;minis the minimumwavelength theoretically detectable by the transducer defined by its fcand BW. Speed of sound is chosen as 1540 m / s. The computation time required for the simulations for each array was ~9 hours on an i 5 processor with 16 GB RAM.

[0109] FIG. 11 depicts the schematic of the 4x4 and 8x8 PCB-based 2D ultra sound arrays respectively along with the light-absorbing pencil lead targets in the homogenous medium. FIG. 11 demonstrates the effect of aperture size and element count on the volumetric PA images obtained from both 8x8 and 4x4 arrays considering a / cof 10 MHz; FIG 11 (images f and g) respectively consider 4x4 and 8x8 arrays with afcof 25 MHz. The YZ maximum intensity projection (MIP) images were used to measure the longitudinal FOV, characterized as the reconstructed length of Tl; the 4x4 array has a YZ FOV of 4 mm while the 8x8 array YZ FOV is approximately double at 8.25 mm. The reconstructed lateral diameter of Tl is used to characterize the spatial resolution in the different cases. The spatial resolution improves from 8x8 to 4x4 in both cases; from 0.78 mm to 0.44 mm for 10 MHz, and from 0.64 mm to 0.42 mm for 25 MHz. Additionally, comparison between images obtained by the same array at different fcdemonstrates that higher fcimproves spatial resolution, although this is more pronounced for the 4x4 case. Simulations were also performed for studying effect of varying pitch by changing the kerf from 200 / jm to 500 / jm, keeping^ at 10 MHz. The coarser pitch degraded the spatial resolution slightly from 0.78 mm to 0.88 mm in the 4x4 case, and from 0.44 mm to 0.6 mm in the 8x8 case.

[0110] PCB-Based 2D Array Transducer Fabrication

[0111] In this section, we discuss fabrication process for two PCB based 2D planar transducer arrays: a 4x4 matrix array and an 8x8 matrix array. The arrays are fabricated on PCBs manufactured by PC Process (Bengaluru, India). To demonstrate scalability with array pitch, the 4x4 PCB has 1.2 mm pitch while the 8x8 PCB has 1.5 mm pitch.

[0112] The linear ultrasound array design rule of pitch < 1.5 , primarily desired to avoid grating lobes in conventional US imaging, is relaxed as the sparse 2D planar arrays fabricated in this work operate solely in receive-only PA imaging, wherein larger individual element size ensures sufficient PA receive sensitivity for each element. Having larger individual element sizes with a sparse 2D architecture also ensures large imaging FOV while requiring parallel data acquisition from a smaller number of elements, thereby ensuring portability without a bulky backend. The kerf of 200 / .im is the minimum required kerf, as dictated by standard PCB designing process, to (i) enable routing of element traces through the PCB substrate for enabling efficient backend connections, and (ii) ensure electrical isolation between elements while ensuring a dicing free fabrication process without complex fabrication techniques. Additionally, receive-only mode volumetric PA imaging has been demonstrated to be achievable without adherence to the linear array design rule while designing the array substrate.

[0113] Scalability of fabrication with respect to the PCB preform itself was demonstrated with variation in terms of the number of PCB layers and total PCB thickness as detailed in Table I below. In the case of both arrays, a dielectric layer exists between each pair of ground and signal layers within the PCB.TABLE IDesign Parameters for PCB Matrix US ArraysParameter Parameter for 4x4 Parameter for 8x8Theoretical / c40 MHz 11.25 MHzElement Count 16 (4x4) 64 (8x8)Pitch 1.2 mm 1.5 mmVia Diameter 0.5 mm 0.5 mmElement Size 1 mm21 mm2Parameter Parameter for 4x4 Parameter for 8x8Aperture Size 21.2 mm2132.3 mm2PCB Layers 4 8PVDF Thickness 28 j m 100 jumPCB Thickness 2.02 mm 1.67 mmFront Electrode 50 / 200 nm Cr / Au 200 nm CuParylene Thickness 3 .m 3 [im

[0114] For both the 4x4 and 8x8 case, each individual ultrasound transducer element is designed as a square gold pad of 1 mm X 1 mm with a through-hole, or via, of 500 jjm diameter. This via provides electrical contact through the PCB. When filled with silver epoxy (E Solder 3022, Von Roll Isola Inc., New Haven, CT, USA), a hybrid backing layer of the PCB material itself and the conductive epoxy is obtained. The acoustic impedance of e-solder is ~5.5 MRayl, and acoustic impedance of PCBs have been reported to be 6-7 MRayl. Thus, the e-solder backing is closely matched in impedance with the PCB substrate while providing electrical connectivity through the PCB. The PCB layout ensures that each individual element is electrically isolated from all others without the need of dicing. The PCB internally routes connections from the elements to solder pads around the periphery of the array to facilitate soldering wired connections without affecting each element itself.

[0115] We demonstrate fabrication of both high and low frequency PCB-based 2D transducer arrays using undiced poly vinylidene fluoride-trifluoroethylene (PVDF-TrFe) (PolyK Technologies, State College, PA, USA) as piezo layer. PVDF-TrFe has a significantly lower clastic modulus of 2-4 GPa compared to that of conventional piezo materials like 60-80 GPa of PZT. This enables higher deformability and lesser shear wave propagation in PVDF-TrFe, thus allowing use of undiced film of PVDF-TrFe. Further, in receive only mode PA imaging with diffused light, intra-element crosstalk is less inherent than conventional ultrasound imaging. The 2250 theoretical fcfor PVDF-TrFe is given as, fc= - - — -MHz where t is the thickness of thePVDF-TrFe film in pnz, and 2250 m / s is the speed of sound in PVDF-TrFe. The thickness of tire PVDF-TrFe used for the 4x4 array is 28 pm, implying ; of 40 MHz. The thickness of the PVDFused for the 8x8 array is 100 pm, implying ; of 11 MHz. The choice of frequencies is supplemented by simulation results that showed an appreciable improvement in spatial resolution for 4x4 array with higher fc, whereas this change is comparatively negligible in case of 8x8 array.

[0116] A 3D cross sectional schematic of a single transducer element is shown in FIG. 12 (image a) and the fabrication process is outlined in FIG. 12 (image b): For simplicity, only one element is shown. The process is as follows: (1) We begin with a custom-designed PCB with unfdled vias, (2) Vias of all elements are fdled with e-solder in parallel (3) All array elements are covered with a single piece of Z-axis tape (3M Inc.) of negligible resistance (0.3 Q). (4) An undiced PVDF-TrFe film is then fixed to the z-axis tape as follows: (4a) For the 4x4 PCB, a PVDF TrFe film with a thickness of 28 pm with single side coating of chromium-gold (Cr-Au) is used, with the uncoated side fixed to the Z-axis tape. (4b) For the 8x8 PCB, a double-side uncoated PVDF TrFc film with a thickness of 100 pm is used. Next, a 200 nm layer of copper was sputtered on the front side of the PVDF film. (5) The sputtered copper layer enables front electrode connection for the 8x8 PCB, while for the 4x4 PCB, electrical connection is made using conductive epoxy from a corner on tire front side of the metallized PVDF TrFe film, coated with Cr-Au electrode, to the ground pads of the PCB. (6) A 3 / jm insulating layer of parylene is deposited on front surface of the array.

[0117] Because PVDF-TrFe film does not require dicing, double-side uncoated and single-side coated films are equally preferred for front electrode connection, enabling greater flexibility to the transducer array design and fabrication process.

[0118] FIG. 13 shows the schematic and a picture of fabricated 16-channel 4x4 PCB planar array respectively, as well as the schematic and a picture of fabricated 64 channel 8x8 PCB planar array respectively.

[0119] Array Characterization & Interfacing

[0120] Both 4x4 and 8x8 matrix ultrasound arrays were characterized for their electrical impedance response using a Key sight Technologies E4990A Impedance Analyzer. FIG. 14 shows typical impedance response plots for the 4x4 and 8x8 arrays respectively, demonstrating resonance and anti -resonance peaks around 33 MHz and 17 MHz respectively. Following electrical impedance testing, the individual elements of both the arrays were tested for their pulse echo US and PA A-line responses. The transducer was kept above a flat metal target which wasimmersed in deionized water. An Olympus 5073 pulser-receiver (PR) was used to excite each element using a 2 / / / ultrasound pulse. The received pulse-echo from a flat metal target is bandpass filtered between 5-20 MHz for the 11 MHz 8x8 array, and between 20-70 MHz for the 40 MHz 4x4 array, before amplification by a gain of 39 dB. Typical pulse echo US signal and corresponding bandwidth (BW) from one of the elements for tire 4x4 and 8x8 arrays is also shown.

[0121] For acquiring PA excitation, laser light (750 nm, pulse width 5-7 ns, PRF 10 Hz) from an internally triggered optical parametric oscillator (OPO) laser source (Phocus Mobile, Opotek Inc., Carlsbad, CA., USA) illuminated a flat metal target at an oblique angle of illumination. The laser energy was approximately 65 mJ / pulse and illuminated the metal plate at 15 mJ cm-2, well within the American National Standards Institute (ANSI) laser safety limits at 750 nm. The received PA A-line signal from each element is processed by the Olympus 5073 PR in the same manner as the US pulse echo signal. FIG. 14 also shows PA A-line signal and corresponding bandwidth (BW) from one of the elements for the 4x4 and 8x8 arrays respectively. The analysis of US and PA fcand full width at half maxima (FWHM) BW is highlighted in Table II. The high PABW observed in both the arrays can be attributed to the high receive sensitivity of PVDF material.Table IIUS and PA A-Line Characterization ResultsParameter Result for 4x4 Result for 8x8US f 43.5 MHz 9.8 MHzUS BW 53% 60%PA / c 25 MHz 11.9 MHzPABW 188% 87%

[0122] We further studied element to element variations of both the arrays by analyzing the recorded peak-to-peak amplitude variations from the flat metal target. In FIG. 14 we plot the peak-to-peak pulse-echo amplitude for all 16 elements in the 40 MHz 4x4 array, measured using the Olympus 5073 PR due to its high bandwidth. The black line denotes the mean, and the red lines denote the level of one standard deviation above and below the mean. To allow faster acquisition of pulse-echo US amplitudes of 64 element 8x8 array, the array was interfaced using a 70-pin interface board to the signal wires of a 64-element universal transducer adapter (UTA) that plugs into a Verasonics Vantage256 (Vantage 256, Verasonics Inc., Kirkland, WA, USA) ultrasound data acquisition (DAQ) system. Verasonics supplied a 10 V excitation pulse, and the received US pulse-echo from the flat metal target was acquired for all elements. A plot of peak- to-peak amplitude of pulse echo US signal for all 64 elements of the 8x8 array is also shown in FIG. 14 (image h); the black line denotes the mean, and the red lines denote the level of one standard deviation above and below tire mean.

[0123] Comparison of FIG 14 images g and h shows that the 4x4 array exhibit higher uniformity than the 8x8 array. We hypothesize that this is due to variations resulting from manual fdling of silver epoxy into PCB vias to supplement PCB backing. The smaller area, and lesser number of elements in the 4x4 case result in a more uniform fabrication process for each element and subsequently, more uniform receive sensitivities across elements. In future, when working with large arrays, we plan to mitigate these problems using less viscous backing materials which allow for more uniform filling in the PCB vias.

[0124] PA Experimental Setup, Data Acquisition and Image Reconstruction

[0125] FIG. 15 (image a) is a schematic of the PA data acquisition and imaging process using the matrix arrays. Both 4x4 and 8x8 arrays are interfaced to the Verasonics DAQ, programmed in receive only mode and synchronized with laser pulse emission, for real time parallel PA data acquisition from all elements. The DAQ and the Phocus OPO laser are controlled with a function generator (hat acts as master. The function generator syncs the emission of the Q-switched laser pulse having pulse width of 5-7 ns with the acquisition of one frame of PA data on the Verasonics DAQ. A fixed wavelength of 750 tun of the OPO laser pulse was used for all acquisitions. FIG. 15 (image b) shows the experimental setup for acquiring PA data from an intralipid phantom setup embedded with four pencil lead targets. The light fromlaser fiber bundle illuminates the phantom from an oblique angle with respect to the vertically positioned US matrix array. The energy of the laser emission at the phantom surface was measured at 15-20 mJ cm-2, within ANSI safety limits at 750 nm.

[0126] The RF signal sampling rate of the Vcrasonics DAQ is set at 42 MHz for the low frequency 8x8 array and is set to the highest possible rate of 62.5 MHz for the high frequency 4x4 array. For the 4x4 array, a low pass anti-aliasing filter was applied to preserve PA RF data below 30 MHz. Owing to the wideband PA receive sensitivity and fcof 25 MHz, this antialiasing filter ensures Nyquist sampling of the RF data while also preserving most of its frequency components. The overall acquisition rate is limited by the PRF of the laser, which is 10 Hz, leading to the capture of one PA frame every 100ms. A total of 20 PA frames are captured within each singular PA acquisition loop. Data acquisition on Verasonics is followed by offline image reconstruction of the filtered and sampled PARF data as per FIG. 15 (image a). The averaged PA data matrix was passed through a singular value decomposition (SYD) filter to remove system noise, such as stray PA reflections or jitter in laser triggering. The PA images are reconstructed using delay-multiply-and-sum beamforming algorithm one row at a time giving rise io one beamformed XZ plane B-mode image for each row in the array. In addition, coherent beamforming is performed to improve spatial resolution. A power-law compression was performed during beamforming g to further suppress noise. The processed XZ plane images for each row in the array are then stitched together to generate the 3D volumetric PA image of the phantom. The kerf region between two rows is filled by linear interpolation.

[0127] FIG. 16 compares volumetric PA imaging results of 8x8 and 4x4 arrays for the 1.5% intralipid phantom embedded with two pencil lead targets as shown in FIG> 15 (image a) schematic. The first target (Tl) is at a depth of ~7 mm with the second target (T2) offset laterally and axially by 2 mm. FIG. 16 (images a and d) show volumetric PA images obtained from 4x4 array and 8x8 array respectively. FIG. 16 (images b and e) compare the XZ MIP images obtained from the corresponding volumetric PA images of 4x4 and 8x8 arrays respectively; FIG. 16 (images c and f) compare the YZ MIP images obtained from the corresponding volumetric PA images of 4x4 and 8x8 arrays respectively.

[0128] FIG. 16 (images g and h) show quantified FWHM lateral and axial resolutions, obtained from the XZ MIP images for 4x4 and 8x8 arrays respectively. The resolutions arederived as the FWHM spread of the point spread functions of each target in both the lateral and axial directions in the XZ MIP image. For 4x4 array, axial resolutions were 0.38 mm and 0.35 mm while lateral resolution were 0.7 mm and 1 mm for T1 and T2 respectively. Similarly, for 8x8 array, axial resolutions were 0.20 mm and 0.24 mm while lateral resolution were 0.53 mm and 0.82 mm for T1 and T2 respectively. The spatial resolutions obtained from the 4x4 array are lower than those from the 8x8 array, despite higher fc. This is attributed to the smaller FOV of the array, the fewer number of sensing elements, and the limited sampling rate of the DAQ in case of the 4x4 array. The difference in uniform PA receive sensitivity as observed in the 8x8 array gives rise to some ‘gap’ regions and artifacts in the volumetric image. The artifacts are mitigated upon taking the XZ and YZ MIPs. Further, owing to the greater element count and aperture size of the 8x8 array, the FOV and spatial resolutions are improved over the 4x4 array. Conversely, the 4x4 array benefits from greater uniform sensitivity of elements as observed by the lack of any gaps.

[0129] Our study validates the volumetric photoacoustic imaging capabilities of low-cost, scalable, and easily manufacturable PCB-based 2D matrix ultrasound arrays fabricated with wideband sensitive PVDF-TrFe piezoelectric material. Scalability was demonstrated by fabricating both high and low frequency 2D arrays with two different aperture sizes. The fabricated arrays were characterized using electrical impedance as well as pulse-echo US and PA A-line responses from a flat metal target. These results demonstrated up to 188% PA bandwidth. Volumetric PA imaging of targets kept inside a light scattering intra lipid phantom was demonstrated. These results showed that spatial resolution and FOV improved with the aperture size and element count of the array. However, the larger arrays need care fdl fabrication to avoid element to element variations in the sensitivity, arising mainly due to non-uniform epoxy filling in the PCB vias. As discussed in the characterization section, this is proposed to be mitigated by filling the vias with less viscous epoxy to serve as hybrid backing layer, thereby ensuring greater uniformity in receive sensitivity and BW across elements.

[0130] Based on the findings of this study, future scope of work will involve(i) fabrication of denser, higher element count 2D ultrasound arrays with uniform sensitivity for all elements, (ii) backend analog filtering and amplifier electronics for robust signal receiving capabilities along with integration of higher sampling rate DAQs, (iii) co-axial light delivery toensure uniform illumination across the field of view of the 2D array, and (iv) in-vivo P A imaging of animal models.

[0131] It should be understood that the disclosure of a range of values is a disclosure of every numerical value within that range, including the end points. It should also be appreciated that some components, features, and / or configurations may be described in connection with only one particular embodiment, but these same components, features, and / or configurations can be applied or used with many other embodiments and should be considered applicable to the other embodiments, unless stated otherwise or unless such a component, feature, and / or configuration is technically impossible to use with the other embodiment. Thus, the components, features, and / or configurations of the various embodiments can be combined together in any manner and such combinations are expressly contemplated and disclosed by this statement.

[0132] It will be apparent to those skilled in the art that numerous modifications and variations of the described examples and embodiments are possible considering the above teachings of the disclosure. The disclosed examples and embodiments are presented for purposes of illustration only. Other alternate embodiments may include some or all of the features disclosed herein. Therefore, it is the intent to cover all such modifications and alternate embodiments as may come within the true scope of this invention, which is to be given the full breadth thereof.

[0133] It should be understood that modifications to the embodiments disclosed herein can be made to meet a particular set of design criteria. Therefore, while certain exemplary embodiments of the devices, systems, circuits, and methods of using and making the same disclosed herein have been discussed and illustrated, it is to be distinctly understood that the invention is not limited thereto but may be otherwise variously embodied and practiced within the scope of the following claims.

[0134] References

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Claims

WHAT IS CLAIMED IS:

1. A transducer, comprising: a substrate having a front surface, a back surface, and a via extending through the substrate from the front surface to the back surface; a via conductive layer disposed on: a surface of the substrate defined by the via; a front surface region adjacent the via; and a back surface region adjacent the via; a via-fill material disposed within the via and occupying a volume of space formed by the via; an anisotropic layer disposed on the front surface region and the via-fill material located at or near the front surface; a piezoelectric film disposed on the anisotropic layer; and a conductive layer disposed on the piezoelectric film.

2. The transducer of claim 1, wherein: the transducer is an ultrasonic transducer.

3. The transducer of claim 1, wherein: the substrate is a printed circuit board (PCB) substrate.

4. The transducer of claim 1, wherein: the via conductive layer includes immersion gold.

5. The transducer of claim 1, wherein: the via-fill material includes a conductive epoxy and / or a non-conductive epoxy.

6. The transducer of claim 1, wherein: the via-fill material occupies the entire volume of space formed by the via.

7. The transducer of claim 1, wherein: the anisotropic material is electrically conductive in a vertical direction, the vertical direction being a direction running from the front surface to the back surface.

8. The transducer of claim 1, wherein: the piezoelectric film includes polyvinylidene fluoride (PVDF) or polyvinyl difluoride trifluoro ethylene (PVDF-TrFE) or other such piezoelectric polymers or materials.

9. The transducer of claim 1, wherein: the conductive layer includes metal.

10. The transducer of claim 1, wherein: the conductive layer has a thickness of 200 nm.

11. The transducer of claim 1, further comprising: a protective layer disposed on the conductive layer and on the substrate.

12. The transducer of claim 1, wherein: the protective layer includes parylene or other polymers with suitable acoustic impedance.

13. The transducer of claim 1, wherein: the protective layer has a thickness of is 3 pm.

14. A transducer array, comprising: a substrate having a front surface, a back surface, and plural vias, each via extending through the substrate from the front surface to the back surface; a via conductive layer disposed on: for each via, a surface of the substrate defined by the via;for each via, a front surface region adjacent the via; and for each via, a back surface region adjacent the via; a via-fill material, for each via, disposed within the via and occupying a volume of space formed by the via; an anisotropic layer disposed on the front surface region and the via-fdl material located at or near the front surface; a piezoelectric fdm disposed on the anisotropic layer; and a conductive layer disposed on the piezoelectric fdm.

15. A method producing a transducer having a substrate with a front surface, a back surface, and a via extending through the substrate from the front surface to the back surface, the method comprising: disposing a via conductive layer on: a surface of the substrate defined by the via; a front surface region adjacent the via; and a back surface region adjacent the via; disposing a via-fill material within the via so that the via-fill material occupies a volume of space formed by the via; disposing an anisotropic layer on the front surface region and the via-fill material located at or near the front surface; disposing a piezoelectric film on the anisotropic layer; and disposing a conductive layer on the piezoelectric film.

16. The method of claim 15, further comprising: disposing a protective layer on the conductive layer and on the substrate.

17. The method of claim 15, wherein: the via-fill material includes a conductive epoxy and / or a non-conductive epoxy.

18. The method of claim 15, wherein:the piezoelectric film includes poly vinylidene fluoride (PVDF) or polyvinyl difluoride trifluoro ethylene (PVDF-TrFE).

19. A photoacoustic imaging device, comprising: a transducer array, comprising: a substrate having a front surface, a back surface, and plural vias, each via extending through the substrate from the front surface to the back surface; a via conductive layer disposed on: for each via, a surface of the substrate defined by the via; for each via, a front surface region adjacent the via; and for each via, a back surface region adjacent the via; a via-fill material, for each via, disposed within the via and occupying a volume of space formed by the via; an anisotropic layer disposed on the front surface region and the via-fill material located at or near the front surface; a piezoelectric film disposed on the anisotropic layer; and a conductive layer disposed on the piezoelectric film; and a light guide coupled to the tranduscer array.

20. The photoacoustic imaging device of claim 19, wherein: the transducer array is configured as a multichannel 2D planar matrix array; and the light guide is a coaxial optical tube.

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