Considerable attention has been devoted to the development of nanomaterial-based photoacoustic transmitters for ultrasound therapy and diagnosis applications. Here, we fabricate and characterize candle-soot nanoparticles (CSNPs) and polydimethylsiloxane (PDMS) composite-based photoacoustic transmitters, based on a solution process, not just to achieve high-frequency and high-amplitude pressure outputs, but also to develop physically stretchable ultrasound transmitters. Owing to its non-porous and non-agglomerative characteristics, the composite exhibits unique photo-thermal and mechanical properties. The output pressure amplitudes from CSNPs–PDMS composites were 20–26 dB stronger than those of Cr film, used as a reference. The proposed transmitters also offered a center frequency of 2.44–13.34 MHz and 6-dB bandwidths of 5.80–13.62 MHz. Importantly, we characterize the mechanical robustness of CSNPs–PDMS quantitatively, by measuring laser-damage thresholds, to evaluate the upper limit of laser energy that can be ultimately used as an input, i.e., proportional to the maximum-available pressure output. The transmitters could endure an input laser fluence of 54.3–108.6 mJ·cm−2. This is 1.65–3.30 times higher than the Cr film, and is significantly higher than the values of other CSNPs–PDMS transmitters reported elsewhere (22–81 mJ·cm−2). Moreover, we characterized the strain-dependent photoacoustic output of a stretchable nanocomposite film, obtained by delaminating it from the glass substrate. The transmitter could be elongated elastically up to a longitudinal strain of 0.59. Under this condition, it maintained a center frequency of 6.72–9.44 MHz, and 6-dB bandwidth ranges from 12.05 to 14.02 MHz. We believe that the stretchable CSNPs–PDMS composites would be useful in developing patch-type ultrasound devices conformally adhered on skin for diagnostic and therapeutic applications.

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In this paper we demonstrate strain-dependent photoacoustic (PA) characteristics of free-standing nanocomposite transmitters that are made of carbon nanotubes (CNT) and candle soot nanoparticles (CSNP) with an elastomeric polymer matrix. We analyzed and compared PA output performances of these transmitters which are prepared first on glass substrates and then in a delaminated free-standing form for strain-dependent characterization. This confirms that the nanocomposite transmitters with lower concentration of nanoparticles exhibit more flexible and stretchable property in terms of Young’s modulus in a range of 4.08–10.57 kPa. Then, a dynamic endurance test was performed revealing that both types of transmitters are reliable with pressure amplitude variation as low as 8–15% over 100–800 stretching cycles for a strain level of 5–28% with dynamic endurance in range of 0.28–2.8%. Then, after 2000 cycles, the transmitters showed pressure amplitude variation of 6–29% (dynamic endurance range of 0.21–1.03%) at a fixed strain level of 28%. This suggests that the free-standing nanocomposite transmitters can be used as a strain sensor under a variety of environments providing robustness under repeated stretching cycles.

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We demonstrate a multi-mode ultrasonic non-destructive evaluation (NDE) system based on a hybrid transducer including an optoacoustic (OA) prism. In this system, the OA prism with a flexible curved transmitter is capable of not only generating acoustic signals with high OA conversion efficiency but also covering a wide angular range to initiate various acoustic wave modes into a specimen whose interface is in contact with the OA prism. The latter angular manipulation provides great flexibility for NDE, which is uniquely enabled by developing and employing the curved OA transmitter coated on the prism. Using finiteelement-based simulation, we validate acoustic signal generation and propagation inside an aluminum specimen. Then, excitation, propagation, and acquisition of shear and Rayleigh wave modes are confirmed experimentally. As feasibility demonstration, our hybrid transducer system, consisting of the OA prism and a piezoelectric receiver, is utilized for NDE of an aluminum specimen which includes air void discontinuity of 5-mm diameter. This is realized by multi-mode acoustic excitation from the OA prism with incident angles of 33°, 42°, 47°, and 60°. This exhibits detection accuracy less than 2% as confirmed by comparing calculated and measured time-of-flight values. Such OA prism-based realization of all range of angular wave modes allows our system to be potentially useful for NDE over broad metallic materials as long as their sound speed is faster than that of the OA prism, without causing additional reverberation noise commonly observed in conventional piezoelectric counterparts.