Characterising the effects of power ultrasonic devices on surrogate tissue materials

Wang, Dong (2014) Characterising the effects of power ultrasonic devices on surrogate tissue materials. PhD thesis, University of Glasgow.

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Power ultrasonic surgical devices operating at low ultrasonic frequencies (20-100 kHz) have been shown to provide some advantages over reciprocating or manual devices in removal of tissue. Despite opportunities for widespread applications in orthopaedic, dental, ophthalmic and general surgical procedures, the mechanisms of interaction between high power ultrasound and human tissue is not well understood and little is known about the effects and safety implications for power ultrasonics in human tissue. Therefore, the effects of power ultrasonic devices on human tissue are investigated in this thesis.
From the limited literature on this topic, the basic effects caused by high power ultrasound on human tissue can be summarized into three main categories: mechanical response, thermal effects and acoustic cavitation. However, the relative contribution of each of these remains unclear. Thus, this study aims to analyse these responses and effects of tissue mimic materials subject to power ultrasonic excitation and hence investigate the potential to quantitively characterise the power ultrasonic damage on tissue. Due to the complexity of the interaction between cutting surface with tissue, the damage caused by cutting process was not covered. But the investigation of the damage by accumulated ultrasound energy in tissue was the main research topic in this thesis.
To substitute for human tissue, representative materials such as E-glass filled epoxy resin (ER), polyurethane foam (PUF) and transparent silicone elastomer (SE) were used in experimental studies to simulate the behaviour of cortical bone, cancellous bone and soft tissue respectively. Their mechanical properties were characterised using uni-axial compression and tensile tests to measure elastic modulus and dynamic mechanical analysis (DMA) to study the viscoelastic behaviour. Frequency and temperature dependent behaviour of tissue surrogates under power ultrasonic excitation were determined using the DMA data based on the time/frequency temperature superposition principle (TTS). The parameters relevant to thermal properties, including thermal conductivity and specific heat capacity were measured using the heat flow method and differential scanning calorimetry (DSC).
Ultrasonic horns were designed using the finite element (FE) method. The performance of the power ultrasonic system was then examined using experimental modal analysis (EMA) with a laser Doppler vibrometer (LDV) to ensure the system met the experimental requirements.
To characterise the responses and effects of tissue mimics subject to power ultrasonic excitation, non-invasive field measurements have been developed for the fast and reproducible experimental assessment of ultrasonic displacement, strain, stress and temperature fields. An ultra-high speed camera and an infrared (IR) camera were used simultaneously for ER and PUF plate samples to obtain the imaging data which provided the displacement and strain fields with digital image correlation (DIC) technique and the steady-state temperature distribution with thermal imaging. The stress field in a transparent rectangular cubiod SE sample during power ultrasonic loading was mapped using a laser interferometer with acousto-optic effects. Due to the absorption of IR light by transparent SE, the temperature distribution of SE was not recorded by IR camera.
Numerical and analytical models were developed to simulate the ultrasonic wave propagation using ABAQUS FE software package and Mathematica respectively. These models incorporated frequency dependent mechanical properties of the mimic materials to verify experimental results. The results of the models matched well with the experimental findings of ultrasonic displacement, strain and stress fields. To assess the thermal effects of power ultrasound on the viscoelastic tissue mimics, thermo-mechanical FE models were created using the PZFlex FE software package. Furthermore, FE models for thermal analysis were parameterized in terms of dynamic modulus and acoustic damping coefficient with frequency and temperature dependency for determination of the heat generation and thermal conductivity and specific heat capacity for characterisation of the heat transfer. The FE results have close correlation with measurement results by an IR camera.
Based on the experimental and numerical studies, the damage of tissue mimicking materials under power ultrasonic excitation is related to accumulations of cyclic deformation and heating. Non-invasive full-field surface displacement, strain, stress and temperature measurements have the potential to be used to predict the damage of tissue samples interacting with the power ultrasonic devices. This study has provided confidence that the methodology can be applied to study tissue samples subject to excitations typical of ultrasonic surgical devices, including those for orthopaedic bone cutting.

Item Type: Thesis (PhD)
Qualification Level: Doctoral
Keywords: Power Ultrasonics, Material Characterisation, Finite Element Analysis, ABAQUS, PZFlex, Experimental Modal Analysis, Ultra-high Speed Camera, Digital Image Correlation, Displacement/Strain Field, Power Ultrasonic Vibration, Thermal Imaging, Acousto-optic Effect, Laser Doppler Vibrometer
Subjects: T Technology > TJ Mechanical engineering and machinery
Colleges/Schools: College of Science and Engineering > School of Engineering
Supervisor's Name: Lucas, Professor Margaret and Tanner, Professor Elizabeth
Date of Award: 2014
Depositing User: Mr D WANG
Unique ID: glathesis:2014-5924
Copyright: Copyright of this thesis is held by the author.
Date Deposited: 20 Jan 2015 11:20
Last Modified: 20 Jan 2015 11:42

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