{"status":"ok","message-type":"work","message-version":"1.0.0","message":{"indexed":{"date-parts":[[2025,11,15]],"date-time":"2025-11-15T19:08:51Z","timestamp":1763233731245,"version":"build-2065373602"},"reference-count":29,"publisher":"MDPI AG","issue":"8","license":[{"start":{"date-parts":[[2016,8,6]],"date-time":"2016-08-06T00:00:00Z","timestamp":1470441600000},"content-version":"vor","delay-in-days":0,"URL":"https:\/\/creativecommons.org\/licenses\/by\/4.0\/"}],"content-domain":{"domain":[],"crossmark-restriction":false},"short-container-title":["Sensors"],"abstract":"<jats:p>Beauty treatment for skin requires a high-intensity focused ultrasound (HIFU) transducer to generate coagulative necrosis in a small focal volume (e.g., 1 mm3) placed at a shallow depth (3\u20134.5 mm from the skin surface). For this, it is desirable to make the F-number as small as possible under the largest possible aperture in order to generate ultrasound energy high enough to induce tissue coagulation in such a small focal volume. However, satisfying both conditions at the same time is demanding. To meet the requirements, this paper, therefore, proposes a double-focusing technique, in which the aperture of an ultrasound transducer is spherically shaped for initial focusing and an acoustic lens is used to finally focus ultrasound on a target depth of treatment; it is possible to achieve the F-number of unity or less while keeping the aperture of a transducer as large as possible. In accordance with the proposed method, we designed and fabricated a 7-MHz double-focused ultrasound transducer. The experimental results demonstrated that the fabricated double-focused transducer had a focal length of 10.2 mm reduced from an initial focal length of 15.2 mm and, thus, the F-number changed from 1.52 to 1.02. Based on the results, we concluded that the proposed double-focusing method is suitable to decrease F-number while maintaining a large aperture size.<\/jats:p>","DOI":"10.3390\/s16081248","type":"journal-article","created":{"date-parts":[[2016,8,8]],"date-time":"2016-08-08T10:14:38Z","timestamp":1470651278000},"page":"1248","update-policy":"https:\/\/doi.org\/10.3390\/mdpi_crossmark_policy","source":"Crossref","is-referenced-by-count":31,"title":["Design and Fabrication of Double-Focused Ultrasound Transducers to Achieve Tight Focusing"],"prefix":"10.3390","volume":"16","author":[{"ORCID":"https:\/\/orcid.org\/0000-0002-8179-9410","authenticated-orcid":false,"given":"Jihun","family":"Jang","sequence":"first","affiliation":[{"name":"Department of Electronic Engineering, Sogang University, Seoul 04107, Korea"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-8323-4202","authenticated-orcid":false,"given":"Jin","family":"Chang","sequence":"additional","affiliation":[{"name":"Department of Electronic Engineering, Sogang University, Seoul 04107, Korea"},{"name":"Department of Biomedical Engineering, Sogang University, Seoul 04107, Korea"},{"name":"Sogang Institutes of Advanced Technology, Sogang University, Seoul 04107, Korea"}]}],"member":"1968","published-online":{"date-parts":[[2016,8,6]]},"reference":[{"key":"ref_1","doi-asserted-by":"crossref","first-page":"93","DOI":"10.1002\/lsm.20877","article-title":"Intense pulsed light (IPL): A review","volume":"42","author":"Babilas","year":"2010","journal-title":"Lasers Surg. 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