Flexible sensors have the potential to be seamlessly applied to soft and irregularly shaped surfaces such as the human skin or textile fabrics. This benefits conformability dependant applications including smart tattoos, artificial skins and soft robotics. Consequently, materials and structures for innovative flexible sensors, as well as their integration into systems, continue to be in the spotlight of research. This review outlines the current state of flexible sensor technologies and the impact of material developments on this field. Special attention is given to strain, temperature, chemical, light and electropotential sensors, as well as their respective applications.

Flexible Sensors—From Materials to Applications

Ultrasound detection is one of the major components of photoacoustic imaging systems. Advancement in ultrasound transducer technology has a significant impact on the translation of photoacoustic imaging to the clinic. Here, we present an overview on various ultrasound transducer technologies including conventional piezoelectric and micromachined transducers, as well as optical ultrasound detection technology. We explain the core components of each technology, their working principle, and describe their manufacturing process. We then quantitatively compare their performance when they are used in the receive mode of a photoacoustic imaging system.

Overview of Ultrasound Detection Technologies for Photoacoustic Imaging

The ultrasonic transducer industry is dominated by piezoelectric materials. As an emerging alternative, capacitive micromachined ultrasound transducers (CMUTs) offer wider bandwidth, better integration with electronics, and ease of fabricating large arrays. CMUTs have a sealed cavity between a fixed electrode and a suspended metalized membrane. Manufacturing cost and sensitivity are limiting factors in current CMUTs that depend on the fabrication equipment and, especially, on the materials used. For widespread use of CMUTs, a much lower fabrication cost that uses inexpensive materials, which maintain or improve upon existing sensitivity, is needed. Herein, a new fabrication process is described for polymer-based CMUTs (polyCMUTs) using the photopolymer SU-8 and Omnicoat. The first ultrasound B-mode image of a wire phantom created with a 64-element linear array using synthetic aperture beamforming techniques is presented. A 12 VAC signal superimposed on a 10 VDC signal was used on the transmission side, and only a bias-tee, with no amplifiers, was used on the receiving side. The low operational voltage and high sensitivity of this device can be partially attributed to a pre-biasing condition on the membrane. By using a novel sacrificial layer combined with a top electrode embedded inside the membrane, we demonstrated that SU-8 can be used to manufacture CMUTs inexpensively. Moreover, the fabrication used relatively simple equipment, and the number of fabrication steps was reduced compared to traditional CMUT fabrication. This new fabrication process has the potential to increase the use of CMUTs in the ultrasound market, including the market for wearable transducers.

/pdf/fabrication-and-testing-of-polymer-based-capacitive-4ud8pd1asi.pdf

Fabrication and testing of polymer-based capacitive micromachined ultrasound transducers for medical imaging

Wireless power transfer—the transmission of electromagnetic energy without physical connectors such as wires or waveguides—typically exploits electromagnetic field control methods that were first proposed decades ago and requires some essential parameters (such as efficiency) to be sacrificed in favour of others (such as stability). In recent years, novel approaches to electromagnetic field manipulation have been developed that can be used to create advanced forms of wireless power transfer. Here we review the development of novel physical effects and materials for wireless power transfer. We explore techniques based on coherent perfect absorption, parity–time symmetry and exceptional points, and on-site power generation. We also explore the use of metamaterials and metasurfaces in wireless power transfer, and the use of acoustic power transfer. Finally, we highlight potential routes for the further development of wireless power transfer technology. This Review examines the development of novel physical effects and materials for wireless power transfer, considering techniques based on coherent perfect absorption, parity–time symmetry and exceptional points, and on-site power generation, as well as the use of metamaterials and metasurfaces, and acoustic power transfer.

Wireless power transfer based on novel physical concepts

As a well-known medical imaging methodology, intravascular ultrasound (IVUS) imaging plays a critical role in diagnosis, treatment guidance and post-treatment assessment of coronary artery diseases. By cannulating a miniature ultrasound transducer mounted catheter into an artery, the vessel lumen opening, vessel wall morphology and other associated blood and vessel properties can be precisely assessed in IVUS imaging. Ultrasound transducer, as the key component of an IVUS system, is critical in determining the IVUS imaging performance. In recent years, a wide range of achievements in ultrasound transducers have been reported for IVUS imaging applications. Herein, a comprehensive review is given on recent advances in ultrasound transducers for IVUS imaging. Firstly, a fundamental understanding of IVUS imaging principle, evaluation parameters and IVUS catheter are summarized. Secondly, three different types of ultrasound transducers (piezoelectric ultrasound transducer, piezoelectric micromachined ultrasound transducer and capacitive micromachined ultrasound transducer) for IVUS imaging are presented. Particularly, the recent advances in piezoelectric ultrasound transducer for IVUS imaging are extensively examined according to their different working mechanisms, configurations and materials adopted. Thirdly, IVUS-based multimodality intravascular imaging of atherosclerotic plaque is discussed. Finally, summary and perspectives on the future studies are highlighted for IVUS imaging applications.

https://www.mdpi.com/1424-8220/21/10/3540/pdf

Recent Advances in Transducers for Intravascular Ultrasound (IVUS) Imaging

https://cyberleninka.org/article/n/923245.pdf

Capacitive Micromachined Ultrasonic Transducers: Technology and Application

The effect of internally resonance at silicon substrate in capacitive micromachined Ultrasonic Transducer presented using Experimental and theoretical calculation. This ringing is clearly observed in immersion transducers with 4.2 MHz. A mathematical analytical model of the attenuation yield by ringing effective and simulation has been introduced. Experimental results are further compared to simulations carried out in time-domain and qualitative agreement is evidence. The insights gained from the simulations and experiments will be use to improve the analytic echoscope. The capacitive micromachined Ultrasonic Transducer whose ringing mode is successfully eliminated with backing layer.

R.B.W. Heng

Papers

Capacitive Micromachined Ultrasonic Transducers: Technology and Application

Internal ringing effect in capacitive micromachined Ultrasonic Transducer