Flexible PolyCMUTs: Fabrication and Characterization of a Flexible Polymer‐Based Capacitive Micromachined Ultrasonic Array for Conformal Ultrasonography

Abstract

field of view [2] and the ability to adapt to deformations in the target object over time (e.g., as a muscle deforms during movement). In addition, if a conformal transducer could substantially surround an anatomical target, it could be used to create tomographic images with high spatial resolution. [3] Such advantages may be useful in a variety of applications, such as inspection of complex-shaped parts in nondestructive evaluation of materials, [4] neuromodulation, [5] ultrasound therapy, [6] touch screens, [7] and medical imaging. [8,9] Musculoskeletal conditions would represent a particularly intriguing field for conformal sonography as tomographic ultrasound could supplement computed tomography (CT) or magnetic resonance (MR) imaging, and its higher temporal resolution and lack of ionizing radiation could generate insights currently unavailable through these other imaging modalities (e.g., visualizing relative movements of adjacent tissues in near-real time). The earliest versions of flexible ultrasound arrays were built using piezoceramic transducers. Since these materials were essentially rigid, flexibility was achieved by connecting the rigid transducers with mechanical joints. [4,10] Flexible arrays for biomedical and healthcare applications were also made by embedding piezoelectric components in or on a soft material, [6,[11][12][13][14] but such devices typically had low array densities. The lack of a stiff backing layer in these designs may result in narrower bandwidths compared to conventional rigid Conventional ultrasound transducers are short and rigid, which limits their applications, especially in the area of musculoskeletal imaging where many of the structures to be imaged lie within long curved anatomical structures, such as limbs. In such cases, conformal ultrasound imaging can be advantageous. This paper presents a process for fabricating 1D and 2D flexible polymer-based capacitive micromachined ultrasound transducers (flexible CMUTs). As a proof of concept, all elements of a 32-element linear flexible array are shown to be functional (100% yield) and uniform in fundamental resonant frequency (SD = 1.8%). One-way and two-way acoustic responses during immersion tests in flat, convex, and concave array configurations (radii of curvature = 3 cm) show an average fractional bandwidth of 83% and 75%, respectively, across these bending conditions. The flexible array shows no signal drop after over 100 bending cycles and only 6% variation in pulse amplitude after over 14 h of continuous operation. Finally, the resulting transducers are shown to operate at up to 15 MHz. The findings demonstrate robust operation of flexible CMUT arrays and justify further development targeted at key imaging applications, particularly in the area of diagnosing musculoskeletal conditions.

Design of an ultrasound‐emitting drill guide for freehand pedicle screw navigation

“…The studies outlined above resulted in a nominal tool design with reasonable convergence performance using several of the optimization algorithms. However, since we are developing flexible ultrasound transducers, 21 it is possible for us to design alternative sensor profiles in which one or more of the transducers in each strip could be angled to emit and sense signals in varying directions (see Figure 4). Here, we considered five different array configurations.…”

Flexible large-area ultrasound arrays for medical applications made using embossed polymer structures

“…Where current ultrasound systems require pointing and positioning by a sonographer, patches of flexible and large-sized ultrasound arrays enable hands-free imaging and offer a solution for short and long-term monitoring applications. With some notable exceptions 4 , 5 , most prototypes of ultrasound patches are typically made by micro-assembly of individual piezoelectric transducer materials onto a flexible or stretchable substrate 6 – 10 . The rigid islands contain the functional ultrasound transducers, while thin electrodes in-between provide mechanical flexibility allowing the patch to conform to nonplanar surfaces.…”

Fully Printed Flexible Ultrasound Transducer for Medical Applications

“…[13][14][15] Polymer-based capacitive micromachined US transducers (polyCMUT), for example, are an emerging technology offering wider bandwidth than most ceramics, fast fabrication processes and a high degree of integration. [16,17] Despite their potential advantages, the need for constant DC bias voltage, as well as limited sensitivity and power delivery can pose design and application challenges. [18] Another potential lead-free alternative to conventional piezoelectric ceramics is the use of softer piezoelectric polymers.…”