Publish Date: 01.12.2022
Category: Interdisciplinary research
Sustainable development goals: 3 Good health and well-being, 9 Industry, innovation and infrastructure (Indicators)
The demand for high amplitude and high-frequency ultrasound to achieve high spatial resolution imaging or ablate a target located deep into the bio-tissue has created new challenges and the development of a new class of functional materials, defined as photoacoustic or ‘’piezophotonic’’. Although high-frequency ultrasounds based on a traditional piezoelectric transducer are commonly used, the propagating distance of the wave represents a limiting factor due to the strong attenuation in water and in biological tissue. Moreover, their miniaturization in the MHz frequency range can be a challenging and expensive task. A photoacoustic approach, employing pulsed optical excitation of photoacoustic materials results in ultrasound pulses with high amplitudes as well as high frequency. This minimizes the electrical components and cabling usually used and creates new opportunities.
Researchers at the Faculty of Mechanical Engineering of the University of Ljubljana in cooperation with Jozef Stefan institute have developed a film of graphene-Polydimethylsiloxane (PDMS) composite on a flexible substrate for the photoacoustic generation of ultrasounds. A simple two-step preparation protocol has been developed to guarantee well-dispersed graphene into the polymer matrix and a controllable realization of the film on a large scale. The realized composite is based on two components: graphene which acts as a light-absorbing layer, and PDMS as an expander layer. Two-dimensional graphene was chosen as a candidate due to its low density, high aspect ratio, and stability in many solvents. Particularly, only 1% of the weight of graphene dispersed in the polymer matrix gives rise to light absorption of > 80% in the near-infrared region. The results show an achievable pressure of the ultrasonic plane wave of 11 MPa, and the robustness of the composite against the increasing laser fluence, up to 300mJ/cm2. These values are relatively high if compared with other photoacoustic materials or devices based on different carbon nanostructures. The research work shows that the freestanding composite can be easily embedded onto the surface of an optical lens to create a high-intensity focused ultrasound. This PA lens can launch high amplitude ultrasonic waves (> 40 MPa) with a central frequency of 11 MHz and -6 dB-bandwidth of 21.5 MHz. The monitoring of the PA process at higher pressure strength reveals the formation of cavitation microbubbles in water and in agar phantom which are confined along the axial direction of the PA lens. Hence, depending on the laser energy, hence pressure intensity, this invisible tool could be a scalpel or a pencil and pave the way for novel photoacoustic medical devices and integrated components.
The research was conducted at the Laboratory for Laser Techniques at the Faculty of Mechanical Engineering in Ljubljana (Daniele Vella, Matija Jezeršek) and Jozef Stefan Institute (Aleš Mrzel, Aljaž Drnovšek, Vasyl Shvalya). The results of the research were described in the scientific paper “Ultrasonic photoacoustic emitter of graphene-nanocomposites film on a flexible substrate”, which was published in the journal Photoacoustics. Aleš Mrzel (Complex Matter Department-JSI), Daniele Vella (FS-UL), and Matija Jezeršek (FS-UL) have also filed a patent application.
The paper is freely available at the following links:
https://www.sciencedirect.com/science/article/pii/S2213597922000787
Illustration of the working principle of the ultrasound generation in the composite, based on the thermoelastic effects (top). Bottom panel: a sketch of the PA focusing lens (left), waveform of the ultrasonic wave recorded during the experiment and its Schlieren image in the focus (central), the frequency spectrum of the ultrasonic wave (right).