Nozzle-Free Printing of CNT Electronics Using Laser-Generated Focused Ultrasound.

Printed electronics have made remarkable progress in recent years and inkjet printing (IJP) has emerged as one of the leading methods for fabricating printed electronic devices. However, challenges such as nozzle clogging, and strict ink formulation constraints have limited their widespread use. To address this issue, a novel nozzle-free printing technology is explored, which is enabled by laser-generated focused ultrasound, as a potential alternative printing modality called Shock-wave Jet Printing (SJP). Specifically, the performance of SJP-printed and IJP-printed bottom-gated carbon nanotube (CNT) thin film transistors (TFTs) is compared. While IJP required ten print passes to achieve fully functional devices with channel dimensions ranging from tens to hundreds of micrometers, SJP achieved comparable performance with just a single pass. For optimized devices, SJP demonstrated six times higher maximum mobility than IJP-printed devices. Furthermore, the advantages of nozzle-free printing are evident, as SJP successfully printed stored and unsonicated inks, delivering moderate electrical performance, whereas IJP suffered from nozzle clogging due to CNT agglomeration. Moreover, SJP can print significantly longer CNTs, spanning the entire range of tube lengths of commercially available CNT ink. The findings from this study contribute to the advancement of nanomaterial printing, ink formulation, and the development of cost-effective printable electronics.

Micro-ultrasonic Assessment of Early Stage Clot Formation and Whole Blood Coagulation Using an All-Optical Ultrasound Transducer and Adaptive Signal Processing Algorithm.

Abnormal formation of solid thrombus inside a blood vessel can cause thrombotic morbidity and mortality. This necessitates early stage diagnosis, which requires quantitative assessment with a small volume, for effective therapy with low risk to unwanted development of various diseases. We propose a micro-ultrasonic diagnosis using an all-optical ultrasound-based spectral sensing (AOUSS) technique for sensitive and quantitative characterization of early stage and whole blood coagulation. The AOUSS technique detects and analyzes minute viscoelastic variations of blood at a micro-ultrasonic spot (<100 µm) defined by laser-generated focused ultrasound (LGFU). This utilizes (1) a uniquely designed optical transducer configuration for frequency-spectral matching and wideband operation (6 dB widths: 7-32 MHz and d.c. ∼ 46 MHz, respectively) and (2) an empirical mode decomposition (EMD)-based signal process particularly adapted to nonstationary LGFU signals backscattered from the spot. An EMD-derived spectral analysis enables one to assess viscoelastic variations during the initiation of fibrin formation, which occurs at a very early stage of blood coagulation (1 min) with high sensitivity (frequency transition per storage modulus increment = 8.81 MHz/MPa). Our results exhibit strong agreement with those obtained by conventional rheometry (Pearson's R > 0.95), which are also confirmed by optical microscopy. The micro-ultrasonic and high-sensitivity detection of AOUSS poses a potential clinical significance, serving as a screening modality to diagnose early stage clot formation (e.g., as an indicator for hypercoagulation of blood) and stages of blood-to-clot transition to check a potential risk for development into thrombotic diseases.

Experimental Demonstration of a Stacked Hybrid Optoacoustic-Piezoelectric Transducer for Localized Heating and Enhanced Cavitation.

Laser-generated focused ultrasound (LGFU) is an emerging modality for cavitation-based therapy. However, focal pressure amplitudes by LGFU alone to achieve pulsed cavitation are often lacking as a treatment depth increases. This requires a higher pressure from a transmitter surface and more laser energies that even approach to a damage threshold of transmitter. To mitigate the requirement for LGFU-induced cavitation, we propose LGFU configurations with a locally heated focal zone using an additional high-intensity focused ultrasound (HIFU) transmitter. After confirming heat-induced cavitation enhancement using two separate transmitters, we then developed a stacked hybrid optoacoustic-piezoelectric transmitter, which is a unique configuration made by coating an optoacoustic layer directly onto a piezoelectric substrate. This shared curvature design has great practical advantage without requiring the complex alignment of two focal zones. Moreover, this enabled the amplification of cavitation bubble density by 18.5-fold compared to the LGFU operation alone. Finally, the feasibility of tissue fragmentation was confirmed through a tissue-mimicking gel, using the combination of LGFU and HIFU (not via a stacked structure). We expect that the stacked transmitter can be effectively used for stronger and faster tissue fragmentation than the LGFU transmitter alone.

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.

Localized micro-scale disruption of cells using laser-generated focused ultrasound.

We utilize laser-generated focused ultrasound (LGFU) to create targeted mechanical disturbance on a few cells. The LGFU is transmitted through an optoacoustic lens that converts laser pulses into focused ultrasound. The tight focusing (<100 µm) and high peak pressure of the LGFU produces cavitational disturbances at a localized spot with micro-jetting and secondary shock-waves arising from micro-bubble collapse. We demonstrate that LGFU can be used as a non-contact, non-ionizing, high-precision tool to selectively detach a single cell from its culture substrate. Furthermore, we explore the possibility of biomolecule delivery in a small population of cells targeted by LGFU at pressure amplitudes below and above the cavitation threshold. We experimentally confirm that cavitational disruption is required for delivery of propidium iodide, a membrane-impermeable nucleic acid-binding dye, into cells.