Poster 120 - Non-invasive soft material characterization - Investigation of transverse wave generation and measurement of wave velocity

Saturday, October 26, 2024

10:00 AM - 12:00 PM EST

Location: Exhibit Hall E, F & G

Presenting Author(s)

CM

Claire Mao

Student
International Academy East, United States

Primary Investigator(s)

Jinjun Xia, PhD (he/him/his)

Assistant Professor
Lawrence Technological University
Southfield, Michigan, United States

Introduction: Laboratory-grown soft biological tissues, such as vessels and heart muscles, are critical for repair and surgery. Hence, extensive studies have been conducted to investigate biochemical and biological processes to grow soft tissues [e.g. 1]. It remains critical to evaluate the mechanical properties of the grown soft tissues to ensure that these tissues can match with their counterparts such as arteries, vessels, or ventricle tissues. When measuring the mechanical properties, a non-invasive approach conducted in situ is preferred over conventional testing methods, i.e. tension, compression, or shear, which could either induce damage to the tested tissues or introduce mechanical stresses that could alter the growth process [2]. A Sagnac interferometer can determine the wave speed of a transverse wave on a medium by detecting the phase shift between two measured waves [3]. The objective of this study is to develop methods that can induce transverse wave propagation in a soft polymer-based phantom mimicking biological soft tissue, and measure wave velocity for shear modulus calculation.

Materials and

Methods: Transverse Wave Generation
The sample used in this study was a copolymer-based phantom. A 10% block copolymer-based phantom sample was prepared using 10g of SEBS (Styrene Ethylene Butylene Styrene) in 90g of mineral oil, with 0.1% TiO2 added to enhance scattering properties for optical measurement.
We investigated three designs to generate transverse waves in the soft phantom (Figure 1a). In Design 1, we used a commercial speaker cone and attached it to a function generator. A PMMA rod, connected to the surface of the speaker cone, touched the phantom surface. In Design 2, a piezoelectric transducer replaced the speaker cone. In both Design 1 and Design 2, the maximum frequency for wave excitation was limited to 5 kHz due to the limitations of the piezo materials. In Design 3, we used a transducer with a resonance frequency of 700KHz connected to an aluminum rod for wave excitation.

Shear Wave Measurement
A Sagnac interferometer system developed in-house was used as the transverse wave receiver to detect the excited waves. The Sagnac receiver head can be moved to scan different positions on the sample surface to collect traveling transverse waves. The wave excitation probe was fixed touching the sample surface.

Shear Modulus Calculation
We used the classical transverse wave speed equation (Figure 1c) to estimate the shear modulus. The transverse wave velocity was calculated by dividing the laser movement distance (1 mm) over the time of the transverse wave phase shift. In total, 2 measurements were conducted using Design 3.

Results, Conclusions, and Discussions: Wave Generation
In both Design 1 and 2, although there were waves being picked up by the Sagnac system (Figure 2a), it was analyzed that these waves were not traveling transverse waves. The change of laser location didn’t yield any phase shift of waves. The detected waves were deemed as either vibrational waves or standing waves. Design 3 successfully generated transverse waves on the phantom surface (Figure 2b). These waves demonstrated a frequency between 1-2 kHz, consistent with the property of transverse waves in soft materials. A clear phase shift was seen when moving the detecting location of the laser beam.

Shear Modulus
The velocity of the transverse wave was calculated at 2.94 m/s and 4.35 m/s at two different points. The shear moduli were roughly estimated to be 8.64 kPa and 18.9 kPa.

Discussions
We developed a non-invasive approach that generated surface transverse waves in a soft phantom. We also developed a non-invasive laser Sagnac system that measured surface transverse waves with a frequency between 1 to 2 kHz. No visible deformation of vibrations of the soft phantom was observed during the experiments. Compared to traditional tension, compression, or shearing testing approaches, our methods could be used to evaluate the lab-grown soft biological tissues without damaging or introducing extra mechanical stresses that could affect the growth of these tissues. Moreover, the mechanical properties of these tissues could be continuously evaluated to monitor changes over time.
One limitation is that the aluminum rod must physically touch the material to transduce longitudinal waves to the phantom and excite transverse waves. Ideally, an air-coupled device, such as an ultrasound generator, can be used to generate waves without touching specimens. Nevertheless, the gentle touching in our experiment didn’t induce any visible deformations before, during, and after experiments.

Conclusions
We developed a novel non-invasive approach to generate transverse waves on a soft phantom surface and then measure transverse waves using a laser-based Sagnac method. Using this non-invasive approach, we approximated the shear modulus of a soft phantom at around 8.64 kPa and 18.9 kPa.

Acknowledgements (Optional):

References (Optional): 1. Dark et al., Generation of left ventricle-like cardiomyocytes with improved structural, functional, and metabolic maturity from human pluripotent stem cells, Cell Reports Methods, vol. 3, no. 4, 2023.
2. Hoefer et al., Biomechanical factors as triggers of vascular growth, Cardiovascular Research, vol. 99, no. 2, pp. 276-283, 2013.
3. Jang et al., Noncontact detection of ultrasonic waves using fiber optic Sagnac interferometer, IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control, vol. 49, no. 6, pp. 767-775, 2002.