Polymer based MEMS and microfluidic devices have the advantages of mechanical flexibility, lower fabrication cost and faster processing over silicon based ones. Also, many polymer materials are considered biocompatible and can be used in biological applications. A valuable class of polymers for microfabricated devices is piezoelectric functional polymers. In addition to the normal advantages of polymers, piezoelectric polymers can be directly used as an active material in different transduction applications. This paper gives an overview of piezoelectric polymers based on their operating principle. This includes three main categories: bulk piezoelectric polymers, piezocomposites and voided charged polymers. State-of-the-art piezopolymers of each category are presented with a focus on fabrication techniques and material properties. A comparison between the different piezoelectric polymers and common inorganic piezoelectric materials (PZT, ZnO, AlN and PMN?PT) is also provided in terms of piezoelectric properties. The use of piezopolymers in different electromechanical devices is also presented. This includes tactile sensors, energy harvesters, acoustic transducers and inertial sensors.

https://iopscience.iop.org/article/10.1088/0964-1726/23/3/033001/pdf

A review of piezoelectric polymers as functional materials for electromechanical transducers

A simple physical model of 1-3 composite piezoelectrics is advanced for the material properties that are relevant to thickness-mode oscillations. This model is valid when the lateral spatial scale of the composite is sufficiently fine that the composite can be treated as an effective homogeneous medium. Expressions for the composite's material parameters in terms of the volume fraction of piezoelectric ceramic and the properties of the constituent piezoelectric ceramic and passive polymer are derived. A number of examples illustrate the implications of using piezocomposites in medical ultrasonic imaging transducers. While most material properties of the composite roughly interpolate between their values for pure polymer and pure ceramic, the composite's thickness-mode electromechanical coupling can exceed that of the component ceramic. This enhanced electromechanical coupling stems from partially freeing the lateral clamping of the ceramic in the composite structure. Their higher coupling and lower acoustic impedance recommend composites for medical ultrasonic imaging transducers. The model also reveals that the composite's material properties cannot be optimized simultaneously; tradeoffs must be made. Of most significance is the tradeoff between the desired lower acoustic impedance and the undesired smaller electromechanical coupling that occurs as the volume fraction of piezoceramic is reduced. >

Modeling 1-3 composite piezoelectrics: thickness-mode oscillations

Piezoelectric materials have dominated the ultrasonic transducer technology. Recently, capacitive micromachined ultrasonic transducers (CMUTs) have emerged as an alternative technology offering advantages such as wide bandwidth, ease of fabricating large arrays, and potential for integration with electronics. The aim of this paper is to demonstrate the viability of CMUTs for ultrasound imaging. We present the first pulse-echo phased array B-scan sector images using a 128-element, one-dimensional (1-D) linear CMUT array. We fabricated 64- and 128-element 1-D CMUT arrays with 100% yield and uniform element response across the arrays. These arrays have been operated in immersion with no failure or degradation in performance over the time. For imaging experiments, we built a resolution test phantom roughly mimicking the attenuation properties of soft tissue. We used a PC-based experimental system, including custom-designed electronic circuits to acquire the complete set of 128/spl times/128 RF A-scans from all transmit-receive element combinations. We obtained the pulse-echo frequency response by analyzing the echo signals from wire targets. These echo signals presented an 80% fractional bandwidth around 3 MHz, including the effect of attenuation in the propagating medium. We reconstructed the B-scan images with a sector angle of 90 degrees and an image depth of 210 mm through offline processing by using RF beamforming and synthetic phased array approaches. The measured 6-dB lateral and axial resolutions at 135 mm depth were 0.0144 radians and 0.3 mm, respectively. The electronic noise floor of the image was more than 50 dB below the maximum mainlobe magnitude. We also performed preliminary investigations on the effects of crosstalk among array elements on the image quality. In the near field, some artifacts were observable extending out from the array to a depth of 2 cm. A tail also was observed in the point spread function (PSF) in the axial direction, indicating the existence of crosstalk. The relative amplitude of this tail with respect to the mainlobe was less than -20 dB.

Capacitive micromachined ultrasonic transducers: next-generation arrays for acoustic imaging?

A comprehensive review on the state-of-the-art of piezoelectric energy harvesting

INTRODUCTION History Role of Ultrasound in Medical Imaging Purpose of the Book Reference Further Reading FUNDAMENTALS OF ACOUSTIC PROPAGATION Stress and Strain Relationships Acoustic Wave Equation Characteristic Impedance Intensity Radiation Force Reflection and Refraction Attenuation, Absorption, and Scattering Nonlinearity Parameter B/A Doppler Effect References ULTRASONIC TRANSDUCERS AND ARRAYS Piezoelectric Effect Piezoelectric Constitutive Equation Ultrasonic Transducers Mechanical Matching Electrical Matching Transducer Beam Characteristics Arrays References GRAY-SCALE ULTRASONIC IMAGING A (Amplitude)-Mode and B (Brightness)-Mode Imaging M-Mode and C-Mode Ultrasound Computed Tomography (CT) Coded Excitation Imaging Compound Imaging Synthetic Aperture Imaging References DOPPLER FLOW MEASUREMENTS Nondirectional CW Flow Meters Directional Doppler Flow Meters Pulsed Doppler Flow Meters Clinical Applications and Doppler Indices Potential Problems in Doppler Measurements Tissue Doppler and Multigate Doppler References FLOW AND DISPLACEMENT IMAGING Color Doppler Flow Imaging Color Doppler Power Imaging Time-Domain Flow Estimation Elasticity Imaging B-Flow Imaging References CONTRAST MEDIA AND HARMONIC IMAGING Contrast Agents Nonlinear Interactions Between Ultrasound and Bubbles Modified Rayleigh-Plesset Equation for Encapsulated Gas Bubbles Solutions to Rayleigh-Plesset Equation Harmonic Imaging Native Tissue Harmonic Imaging Clinical Applications of Contrast Agents and Harmonic Imaging References INTRACAVITY AND HIGH-FREQUENCY (HF) IMAGING Imaging Intravascular Imaging High-Frequency Imaging Acoustic Microscopes References MULTIDIMENSIONAL IMAGING Parallel Processing Multidimensional Arrays Three-Dimensional Imaging References BIOLOGICAL EFFECTS OF ULTRASOUND Acoustic Phenomena at High-Intensity Levels Ultrasound Bioeffects Mechanical Effects and Index References METHODS FOR MEASURING SPEED, ATTENUATION, ABSORPTION, AND SCATTERING Velocity Attenuation Scattering References INDEX

Diagnostic Ultrasound: Imaging and Blood Flow Measurements

Combining a piezoelectric ceramic and a passive polymer to form a piezocomposite allows the transducer engineer to design new piezoelectrics that offer substantial advantages over the conventional piezoelectric ceramics and polymers. The rod composite geometry provides materials with enhanced electromechanical coupling and with acoustic impedance close to that of tissue; these factors yield transducers for medical ultrasonic imaging with high sensitivity and compact impulse response. The dice-and-fill technique produces piezocomposites that can be readily formed into complex shapes to facilitate focusing the ultrasonic beam. Proper design of the rod spacing yields materials that exhibit low crosstalk between array elements formed by patterning the electrode alone, without cutting between the elements. In this way, curved annular arrays have been made that provide high-quality clinical images of substantial diagnostic value to physicians. Included is an extensive bibliography of papers documenting the role of piezocomposites in ultrasonic imaging transducers. >

W.A. Smith

Papers

The role of piezocomposites in ultrasonic transducers