Taehwa Lee

Micro-ultrasonic cleaving of cell clusters by laser-generated focused ultrasound and its mechanisms

Laser-generated focused ultrasound (LGFU) is a unique modality that can produce single-pulsed cavitation and strong local disturbances on a tight focal spot (<100 μm). We utilize LGFU as a non-contact, non-thermal, high-precision tool to fractionate and cleave cell clusters cultured on glass substrates. Fractionation processes are investigated in detail, which confirms distinct cell behaviors in the focal center and the periphery of LGFU spot. For better understanding of local disturbances under LGFU, we use a high-speed laser-flash shadowgraphy technique and then fully visualize instantaneous microscopic processes from the ultrasound wave focusing to the micro-bubble collapse. Based on these visual evidences, we discuss possible mechanisms responsible for the focal and peripheral disruptions, such as a liquid jet-induced wall shear stress and shock emissions due to bubble collapse. The ultrasonic micro-fractionation is readily available for in vitro cell patterning and harvesting. Moreover, it is significant as a preliminary step towards high-precision surgery applications in future.

Optimization of polymer photovoltaic cells with bulk heterojunction layers hundreds of nanometers thick: modifying the morphology and cathode interface

In polymer photovoltaic (PV) cell, it is desirable to use a relatively thick polymer semiconductor film in order to maximize the light absorption, and to achieve better controllability and reproducibility of the film in manufacturing processes. However, the low fill factor due to restricted charge transport and extraction at large film thickness serially limits the performance of the polymer PV cell. In this work, we investigate the factors that can impact the device performances as film thickness is increased. We also introduce ways to help alleviate these problems in thick BHJ PVs. Our measurement results, based on the space-charge limited-current (SCLC) model and the photo-induced carrier extraction by linearly increasing voltage (photo-CELIV) method, show that the thicker BHJ devices have relatively low electron mobility compared with hole mobility, which directly correlates with high contact resistance at the top cathode interface that prevents efficient transport of photo-generated electrons. Specifically, we found that the newly introduced ESSENCIAL fabrication process helps improve the blend donor and acceptor domain morphologies; and adding an ultrathin C60 layer at the cathode interface helps improve the surface morphology and significantly reduce the contact resistance. The effects of the added thin C60 layer on PV cells were further studied by examining several important diode characteristics. Our results proved that this layer not only decreases the contact resistance at the cathode but also improves the hole-blocking, thereby providing significantly suppressed recombination at the cathode interface. Consequently, the fabricated PV devices optimized in morphology and interface show significantly improved internal quantum efficiency (IQE) as compared with the thermally annealed conventional PV cells, leading to 5.11% PCE from a P3HT:PCBM blend system. The modifications to the fabrication of BHJ PV cells described in this work allow for photoactive layers to be hundreds of nanometers thick for efficient light absorption and better controllability.

Multi-film roll transferring (MRT) process using highly conductive and solution-processed silver solution for fully solution-processed polymer solar cells

To produce practical large area polymer solar cells (PSCs), it is highly desirable that the Ag (silver) top electrodes be made by a printing process rather than by vacuum evaporation. However, directly printing electrodes using highly conductive metal inks, such as organometallic and nanoparticle inks, has risks which can cause the infiltration and contamination of the underlying polymer layers during the printing and annealing processes. Moreover, the metal inks usually require high sintering temperatures to achieve high-conductivity electrodes. To overcome these limitations, we introduce a multi-layer roll transferring (MRT) approach, in which a high performance solution processed Ag electrode is prepared separately from the rest of the organic layers, and the device is completed by a final transferring process. By optimizing the processing conditions of the reductive organometallic Ag solution, the resulting metal electrode has an excellent resistivity (3.4 μΩ cm−1) and a morphology comparable to that of a thermally evaporated silver film. The performances of the devices fabricated by the MRT process were comparable to those of metal evaporated devices. Furthermore we achieved fully solution processed devices fabricated by integrating the roll-to-roll coating of the polymer cathode, polymer semiconductor and charge extraction layer and the MRT process.

Nano-structural characteristics of carbon nanotube-polymer composite films for high-amplitude optoacoustic generation.

We demonstrate nano-structural characteristics of carbon nanotube (CNT)-polydimethylsiloxane (PDMS) composite films that can be used as highly efficient and robust ultrasound transmitters for diagnostic and therapeutic applications. An inherent architecture of the nano-composite provides unique thermal, optical, and mechanical properties that are accommodated not just for efficient energy conversion but also for extraordinary robustness against pulsed laser ablation. First, we explain a thermoacoustic transfer mechanism within the nano-composite. CNT morphologies are examined to determine a suitable arrangement for heat transfer to the surrounding PDMS. Next, we introduce an approach to enhance optical extinction of the composite films, which uses shadowed deposition of a thin Au layer through an as-grown CNT network. Finally, the transmitter robustness is quantified in terms of laser-induced damage threshold. This reveals that the CNT-PDMS films can withstand an order-of-magnitude higher optical fluence (and extinction) than a Cr film used as a reference. Such robustness is crucial to increase the maximum-available optical energy for optoacoustic excitation and pressure generation. All of these structure-originated characteristics manifest the CNT-PDMS composite films as excellent optoacoustic transmitters for high-amplitude and high-frequency ultrasound generation.

Low f-number photoacoustic lens for tight ultrasonic focusing and free-field micro-cavitation in water

We demonstrate a photoacoustic lens with a low f-number of 0.61 and a high focal gain of 220 at 15-MHz frequency for laser-generated focused ultrasound (LGFU), which enables free-field micro-cavitation in water. Due to tight ultrasonic focusing (90 μm in lateral and 200 μm in longitudinal spot widths at a distance of 9.2 mm), the lens produces a peak pressure of 20 MPa (positive) using an input laser energy of only 1 mJ/pulse (6-ns temporal width). Remarkably, we confirm single-pulsed micro-cavitation in a free-field condition by using this lens, which has not previously been achieved with LGFU. The free-field cavitation was monitored and characterized in terms of a bubble radius, a lifetime, and a probability. Our result demonstrates that LGFU amplitudes can be sufficiently higher than a threshold for free-field cavitation at a microscale spot, which is a crucial step for cavitation-based therapy with high precision.

Dual-frequency focused ultrasound using optoacoustic and piezoelectric transmitters for single-pulsed free-field cavitation in water

Pulsed ultrasonic cavitation is a promising modality for non-contact targeted therapy, enabling mechanical ablation of the tissue. We demonstrate a spatio-temporal superposition approach of two ultrasound pulses (high and low frequencies) producing a tight cavitation zone of 100 μm in water, which is an-order-of-magnitudes smaller than those obtained by the existing high-amplitude transducers. Particularly, laser-generated focused ultrasound (LGFU) was employed for the high-frequency operation (15 MHz). As demonstrated, LGFU plays a primary role to define the cavitation zone. The generation rate of cavitation bubbles could be dramatically increased up to 4.1% (cf. 0.06% without the superposition) with moderated threshold requirement.

Laser-Induced Focused Ultrasound for Cavitation Treatment: Toward High-Precision Invisible Sonic Scalpel

Beyond the implementation of the photoacoustic effect to photoacoustic imaging and laser ultrasonics, this study demonstrates a novel application of the photoacoustic effect for high-precision cavitation treatment of tissue using laser-induced focused ultrasound. The focused ultrasound is generated by pulsed optical excitation of an efficient photoacoustic film coated on a concave surface, and its amplitude is high enough to produce controllable microcavitation within the focal region (lateral focus <100 µm). Such microcavitation is used to cut or ablate soft tissue in a highly precise manner. This work demonstrates precise cutting of tissue-mimicking gels as well as accurate ablation of gels and animal eye tissues.

Laser-generated focused ultrasound for micro-cavitation and its application to high-precision cavitation treatment

Histotripsy is considered a non-invasive therapeutic modality capable of mechanically fractionating tissue using cavitation bubbles induced by high intensity focused ultrasound pulses. Histotripsy-induced cavitation region over relatively large focal volume is effective in treating sizable lesion, while compromising its treatment precision. We demonstrate high-precision cavitation treatment using laser-generated focused ultrasound.

Laser-generated ultrasound for high-precision cutting of tissue-mimicking gels (Conference Presentation)

Laser-generated focused ultrasound has shown great promise in precisely treating cells and tissues by producing controlled micro-cavitation within the acoustic focal volume (<100 um). However, the previous demonstration used cells and tissues cultured on glass substrates. The glass substrates were found to be critical to cavitation, because ultrasound amplitude doubles due to the reflection from the substrate, thus allowing for reaching pressure amplitude to cavitation threshold. In other words, without the sound reflecting substrate, pressure amplitude may not be strong enough to create cavitation, thus limiting its application to only cultured biomaterials on the rigid substrates. By using laser-generated focused ultrasound without relying on sound-reflecting substrates, we demonstrate free-field cavitation in water and its application to high-precision cutting of tissue-mimicking gels. In the absence of a rigid boundary, strong pressure for cavitation was enabled by recently optimized photoacoustic lens with increased focal gain (>30 MPa, negative pressure amplitude). By moving cavitation spots along pre-defined paths through a motorized stage, tissue-mimicking gels of different elastic moduli were cut into different shapes (rectangle, triangle, and circle), leaving behind the same shape of holes, whose sizes are less than 1 mm. The cut line width is estimated to be less than 50 um (corresponding to localized cavitation region), allowing for accurate cutting. This novel approach could open new possibility for in-vivo treatment of diseased tissues in a high-precision manner (i.e., high-precision invisible sonic scalpel).

Air-backed optoacoustic transmitter for high amplitude quasi-monopolar wave

Unipolar acoustic wave is advantageous for high resolution imaging and detection due to its high frequency characteristic. However it is difficult to achieve the unipolar waveform using with traditional piezoelectric transducers. Here, we demonstrate unipolar acoustic wave form using an optoacoustic approach by directly exposing the absorbing layers of photoacoustic transmitters to air (i.e., an air-backed optoacoustic transmitter), which simultaneously increases signal amplitudes by employing an acoustic matching layer on the air-side.