Strain-Dependent Photoacoustic Characteristics of Free-Standing Carbon-Nanocomposite Transmitters.

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.

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

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.

Stretchable and Robust Candle-Soot Nanoparticle-Polydimethylsiloxane Composite Films for Laser-Ultrasound Transmitters.

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.