polymers Article High-Frequency Acoustic Imaging Using Adhesive-Free Polymer Transducer Abhishek Ranjan 1 , Chengxiang Peng 1 , Sanat Wagle 2 , Frank Melandsø 1 and Anowarul Habib 1, *   Citation: Ranjan, A.; Peng, C.; Wagle, S.; Melandsø, F.; Habib, A. High-Frequency Acoustic Imaging Using Adhesive-Free Polymer Transducer. Polymers 2021, 13, 1462. https://doi.org/10.3390/ polym13091462 Academic Editor: Roman A. Surmenev Received: 15 February 2021 Accepted: 27 April 2021 Published: 30 April 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1 Department of Physics and Technology, UiT The Arctic University of Norway, 9019 Tromsø, Norway; abhishek.ranjan@uit.no (A.R.); chengxiang_peng@sina.com (C.P.); frank.melandso@uit.no (F.M.) 2 Elop AS, Nordvikvegen 50, 2316 Hamar, Norway; Sanat@elop.no * Correspondence: anowarul.habib@uit.no Abstract: The piezoelectric polymer PVDF and its copolymers have a long history as transducer materials for medical and biological applications. An efficient use of these polymers can potentially both lower the production cost and offer an environment-friendly alternative for medical transducers which today is dominated by piezoelectric ceramics containing lead. The main goal of the current work has been to compare the image quality of a low-cost in-house transducers made from the copolymer P(VDF-TrFE) to a commercial PVDF transducer. Several test objects were explored with the transducers used in a scanning acoustic microscope, including a human articular cartilage sample, a coin surface, and an etched metal film with fine line structures. To evaluate the image quality, C- and B-scan images were obtained from the recorded time series, and compared in terms of resolution, SNR, point-spread function, and depth imaging capability. The investigation is believed to provide useful information about both the strengths and limitations of low-cost polymer transducers. Keywords: cartilage; acoustic microscopy; ultrasound; P(VDF-TrFE); transducers 1. Introduction Scanning acoustic microscopy (SAM) is a widefield non-destructive and non-invasive technique that has been widely used over several decades for surface and subsurface microscopic imaging especially for industrial and biological specimens [1–3]. SAM has the potential to employ high-frequency waves which are transmitted via a coupling medium into the sample and become reflected based on the stiffness of the sample [4]. SAM can for instance be used to observe internal structures, subsurface features, structural characterization of materials, and detect changes in the elastic properties of solids [5,6]. The typical commercial transducers used in SAM are made up of ceramic, single crystals, or thin films of piezoelectric materials. Polyvinylidene difluoride (PVDF) and its copolymer with trifluoroethylene P(VDF-TrFE) are ferromagnetic materials that inherently possesses several benefits for an acoustic transducer. These are flexible materials which allow a high degree of physical focusing without lenses [7]. Polymer transducers typically offer wide bandwidth or short impulse response [8], and much a better acoustic impedance match to biological tissue than ceramic-based transducers. The copolymer P(VDF-TrFE) is commercially available in different mass mixing ratios where a ratio around 70 to 30 between PVDF and TrFE is known to produce the highest piezoelectric activity [9]. Ceramic-based transducers, on the other hand, normally have advantages in terms of a higher dielectric permittivity, higher electromechanical coupling, and lower loss factors. Most commercial high-frequency transducers are produced by depositing a thin layer of the piezoelectric material on the flat side of the buffer rod. A concave spherical sapphire lens rod is typically used to focus acoustic energy through a coupling medium (i.e., water) onto the sample plane. The pronounced impedance mismatch leads to reduced sound transmissivity, significant bandwidth reduction, and geometrical aberration of the focusing beam [5]. For PVDF and P(VDF-TrFE), on the other hand, focusing can be archived by Polymers 2021, 13, 1462. https://doi.org/10.3390/polym13091462 https://www.mdpi.com/journal/polymers