Variable-focus optoacoustic lens with wide dynamic range and long focal length by using a flexible polymer nano-composite membrane

Muhammad Awais Abbasi; Muhammad Faraz; Min Gyu Joo; Donghee Son; Sang Min Won; Jong G Ok; Hui Joon Park; Hyoung Won Baac

doi:10.1016/j.ultras.2021.106545

Variable-focus optoacoustic lens with wide dynamic range and long focal length by using a flexible polymer nano-composite membrane

Ultrasonics. 2021 Dec:117:106545. doi: 10.1016/j.ultras.2021.106545. Epub 2021 Jul 29.

Authors

Muhammad Awais Abbasi¹, Muhammad Faraz¹, Min Gyu Joo¹, Donghee Son¹, Sang Min Won¹, Jong G Ok², Hui Joon Park³, Hyoung Won Baac⁴

Affiliations

¹ Department of Electrical and Computer Engineering, Sungkyunkwan University, 2066 Seobu-ro, Jangan-gu, Suwon 16419, Republic of Korea.
² Department of Mechanical and Automotive Engineering, Seoul National University of Science and Technology, 232 Gongneung-ro, Nowon-gu, Seoul 01811, Republic of Korea. Electronic address: jgok@seoultech.ac.kr.
³ Department of Organic and Nano Engineering, Hanyang University, Seoul 04763, Republic of Korea. Electronic address: huijoon@hanyang.ac.kr.
⁴ Department of Electrical and Computer Engineering, Sungkyunkwan University, 2066 Seobu-ro, Jangan-gu, Suwon 16419, Republic of Korea. Electronic address: hwbaac@skku.edu.

PMID: 34343758
DOI: 10.1016/j.ultras.2021.106545

Abstract

We demonstrate a variable-focus optoacoustic lens (VFOL) by pneumatically controlling a flexible polymer nano-composite membrane, which can produce laser-generated focused ultrasound (LGFU) with a high peak amplitude (>30 MPa) and a tight focal dimension (several hundred μm) over a wide dynamic range of focus variation (>20 mm) together with a long focal length up to 60 mm, each of which is widest and longest among optoacoustic lenses developed so far. Two different designs in lens dimension have been fabricated and characterized: VFOL-L with a 40-mm diameter and VFOL-S with 10 mm. VFOL-L exhibits a wide dynamic range of focal length variation from 38.52 to 60.39 mm with a center frequency near ~ 10 MHz, which is proper for practical long-range applications with several-cm depth. In comparison, VFOL-S covers a focal variation range from 6.75 to 11.1 mm with ~ 14 MHz, producing a relatively higher-pressure amplitude, which allows the inception of acoustic cavitation at an impedance-mismatched boundary. The nano-composite membrane of VFOL is actuated from a planar to deeply curved shape by externally injecting liquid into the VFOL, resulting in a focal gain up to 255 and a positive peak pressure of > 30 MPa in the VFOL-L case. The minimum-geometrical f-number as low as 0.963 is achieved at the shortest focal length (38.52 mm) with 6-dB lateral and axial spot dimensions of 304 μm and 2.86 mm, respectively. We expect that the proposed VFOL-based LGFU with a high peak pressure, a wide dynamic axial range, and a tight focal dimension are suitably applied for depth-dependent characterization of tissues and shockwave treatment, taking advantages of optoacoustic pulses as input with inherent broadband high-frequency characteristics.

Keywords: Laser-generated focused ultrasound; Laser-ultrasonic transmitter; Liquid-filled lens; Optoacoustic lens; Variable focus.