Prof. Keiichi Nakagawa’s bio-medical precision engineering laboratory at the University of Tokyo

by Michelle Haung

My experience in Japan over the past 6 weeks is one that I will never forget. I had the amazing opportunity to work in Professor Keiichi Nakagawa’s bio-medical precision engineering laboratory at the University of Tokyo. Currently, there are limitations to brain imaging and therapy due to the absorption and scattering effects of biological tissue, which hinder the focus and delivery of light to the site. Along with my mentor, Maosen, I worked on studying the use of acoustic waves as a light-guiding technique for bioimaging of deep tissue sights. This method could also have potential as a laser skincare treatment due to deeper penetration of the laser into the skin. Previous studies have found that photoacoustic guiding, also known as PAG, can increase the transmittance of light by one order of magnitude. Therefore, my main goal was to expand upon these findings by determining the change in transmittance for media with varying scattering coefficients.

To study this, I worked with hydrogels of varying scattering coefficients that ranged from zero all the way to roughly that of the cortex. The first experiment determined the relationship between the scattering coefficient and relative transmittance. By directing a laser through the scattering medium into a photon detector, we were able to confirm the expected outcome, with the higher scattering media having less transmittance of the laser. This is due to larger scattering effects with the increased amount of particles in the hydrogel.

The second experiment we performed was to verify the presence of acoustic waves in our optical setup. For this, we used a pump laser, a probe laser, and a camera. These components were all connected to a digital delay generator to capture images of the acoustic wave being produced. The high energy single pulse from the pump laser produced a high electric field that turned the distilled water into plasma. This created a high temperature, leading to the production of an acoustic wave. Using the camera, we could align the acoustic waves at a singular point, which is the PAG. Therefore, we were able to confirm the presence of an acoustic wave, as well as use this setup to align the laser and PAG for our following experiment.

Figure 1: Propagation of acoustic wave viewed through high speed camera

In the final experiment, I had the opportunity to test the transmittance of light through media of varying scattering coefficients both with and without PAG. We found that the transmittance of the laser displayed an increase for the lower scattering media along with just distilled water. We faced an issue with the photon detector not being sensitive enough to detect the change in transmittance for the higher scattering media. Therefore, next steps in this study would be to use a different photon detector that is more sensitive in order to detect the remaining signals in addition to decreasing the noise found in our results.

Outside of the laboratory, I had the most amazing experience exploring Japanese culture through eating yummy food, meeting many friendly people, and immersing myself in several activities. Alongside several of my program-mates and peers, I have been able to travel all around Japan to places such as Nagoya, Osaka, Nara, Kyoto, Hakone, and Okinawa to name a few. Some of my favorite dishes that I ate were hitsumabushi in Nagoya and okonomiyaki in Osaka, but there were many delicious restaurants around every corner. As much as I loved traveling around, I thoroughly enjoyed exploring Tokyo in the hours outside of the lab through activities like shopping, karaoke, and sightseeing. I also had the opportunity to participate in a wadaiko workshop that became one of the highlights of my trip. Overall, the experiences I had, both large and small, made my time in Japan truly unforgettable.

This experience has been so valuable and life changing, as I have made forever friends, connections, and memories. I have grown as a student, individual, and person by trying new things and putting myself out there. I am so grateful to Friends of UTokyo for supporting me throughout my research abroad in Japan. I also want to thank my wonderful laboratory for their guidance throughout my 6 weeks here, especially my mentor, Maosen. Lastly, I want to thank Professor Nakagawa, UCEAP, and ESEP for this amazing opportunity. I will take the skills and knowledge that I learned from this research and cultural experience into the remainder of my schooling, career, and beyond.