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Investigations on the heating effect of PE-LD induced by high-intensity focused ultrasound
Ultrasonics 70 (2016) 204–210
Contents lists available at ScienceDirect
Ultrasonics journal homepage: www.elsevier.com/locate/ultras
Investigations on the heating effect of PE-LD induced by high-intensity focused ultrasound Lukas Oehm ⇑, Sascha Bach, Jens-Peter Majschak Dresden University of Technology, Chair of Processing Machines/Processing Technology, Bergstraße 120, 01069 Dresden, Germany
a r t i c l e
i n f o
Article history: Received 22 October 2015 Received in revised form 29 April 2016 Accepted 2 May 2016 Available online 3 May 2016 Keywords: High-intensity focused ultrasound (HIFU) Infrared thermography Polymer material Heating effect
a b s t r a c t High-intensity focused ultrasound is widely applied in tissue treatment as well as for heating of solid polymer materials. Previous studies investigating the heating effect in polymer materials utilized sound transmission through water or other fluids at low HIFU power. In this study, the ultrasonic transducer possesses a solid sound conductor made of aluminum and a high HIFU power of above 100 W was applied to heat solid PE-LD samples. Temperature measurements were performed by calibrated non-invasive infrared thermal imaging. A strong heating effect with heating above melting temperature and evaporation temperature within less than 1 s of irradiation was observed. Furthermore, the acoustic coupling defined by the force applied by the ultrasonic applicator to the polymer material was found to be fundamental to induce the heating effect. This investigation reveals HIFU for new applications in the field of polymer processing. Ó 2016 Elsevier B.V. All rights reserved.
1. Introduction High-intensity focused ultrasound (HIFU) is an advanced medical technology based on the deposition of a large amount of acoustic energy in a small spot of tissue, which is achieved by focusing of ultrasonic waves [1,2]. HIFU treatment is associated with ultrasound-induced heating caused by conversion of mechanical into thermal energy [3]. Temperatures above 80 °C can be obtained within a few seconds of irradiation at acoustic intensities of up to 10 000 W cm 2 [4]. Thus, HIFU is used for in situ non-invasive or minimally invasive treatment of cancer, e.g. in prostate, liver, breast, kidney, bone and pancreas, as well as in glaucoma treatment and for inducing hemostasis [3,5–7]. The ultrasonic waves are usually generated by a piezoelectric transducer that is spherically shaped or coupled to an acoustic lens for focusing [6]. Depending on the application, frequencies from 0.5 to 7 MHz are used [6]. In addition to tissue treatment, HIFU has also been utilized to locally heat polymer materials thereby triggering drug release or shape memory [8,9]. In general, absorption of ultrasound due to internal friction and relaxation results in a heating effect in visco-elastic polymer materials. The HIFU-induced heating effect in different polymer materials (PE-LLD, PE-LD, PE-HD, PA-6, PP,
⇑ Corresponding author. E-mail addresses: [email protected] (L. Oehm), sascha.bach@ tu-dresden.de (S. Bach), [email protected] (J.-P. Majschak). http://dx.doi.org/10.1016/j.ultras.2016.05.002 0041-624X/Ó 2016 Elsevier B.V. All rights reserved.
PS, PC and PMMA) was investigated by Liu et al. [10]. They placed a 1.1 MHz focusing piezoceramic ultrasonic transducer in a waterfilled tank and irradiated solid polymer samples located at the water surface. The HIFU power was 8 W at maximum to avoid cavitation. Non-invasive temperature measurement was done by infrared thermal imaging at the back surface of the samples. A correlation between the HIFU-induced thermal effect and the type of polymer materials was found, which was explained by different absorption coefficients of the different polymer materials due to differences in inner friction behaviors of macromolecular chains. The maximum equilibrium temperature was found for PMMA with 180 °C after 25 s of irradiation with 4 W HIFU power. PE-LD was heated up to 80 °C after 20 s at 4 W. For higher HIFU power a material-dependent threshold of 6–8 W for sample damage was found. Furthermore, the authors described different initial temperature rises for the different polymer materials, the highest one for PP. Both the equilibrium temperature and the initial temperature rise were found to be dependent on the sample thickness in case of PE-LD. Increasing sample thickness results in increasing equilibrium temperature and decreasing initial temperature rises. They explained the increasing equilibrium temperature with the enlarged ‘‘focused volume, which makes the sample absorb more ultrasound energy” [10]. Similar results were obtained by Li et al. for shape memory polymer P(MMA-BA) [9]. Investigations with higher HIFU power on the same shape memory polymer material were done by Li et al. [8]. They reported a polymer heating of 140 °C in the samples after 10 min of irradiation at 50 W HIFU
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power, which was measured by embedded thermocouples [8]. Further reports on HIFU-induced thermal heating have been published by Hallez et al. [11] and Huamao et al. [12]. However, in all known investigations water has been used for sound transmission and the HIFU power was limited to 50 W to avoid cavitation. As a result, the temperature rise of the samples was limited and the irradiation times for reaching the equilibrium temperature were several seconds. Rapid local heating of solid polymers is of broad interest for several areas of application in the field of polymer processing, such as forming or welding. However, the implementation of HIFU in these applications requires an understanding of the heating effect at high HIFU power. To circumvent cavitation, it is necessary to substitute water as the sound-transmitting medium. A number of medical devices possess solid coupling cones build from e.g. aluminum for sound transmission [13]. In this study, this ultrasonic applicator design is adapted by using an ultrasonic applicator with solid aluminum sound conductor. To the best of our knowledge the HIFU-induced heating effect in solid polymer materials has not yet been investigated using a solid sound conductor. Hence, the aim of the present work is to investigate the heating effect of solid polymer samples using high HIFU power and an ultrasonic applicator with solid sound conductor. First, the ultrasonic applicator is characterized by impedance measurements to find the frequency for maximum output power generation. A setup for non-invasive temperature measurements with infrared thermal imaging is described and calibrated. Both, the temperature values and the temperature distribution in the irradiated volume are measured. Furthermore, the influence of the acoustic coupling of the sound conductor to the samples on the heating effect is investigated.
2.2. Experimental set-up for temperature measurements The temperature measurements were performed using an experimental set-up initially introduced for the in-situ investigation of 20 kHz ultrasonic sealing systems by infrared thermography [14]. According to this set-up the ultrasonic applicator is placed on two polymer sample sheets, which are positioned on a sapphire glass (Fig. 2). This set-up allows high resolution temperature measurement in the xy-plane of the transparent polymer samples by infrared thermography. Therefore, the infrared thermography system is placed orthogonally to the polymer samples on a tripod for maximum stability. A mirror reflects the infrared radiation emitted from the polymer samples through to the infrared thermography system. The measurement range of the IR thermography system is 0–380 °C with a temperature resolution of 1 K at 800 Hz measuring frequency. The picture format of the lens is 160 128 pixel, which for a resolution of 70 70 lm results in an image field of 11.2 9 mm. This ensures the capturing of the whole coupling surface of the sound conductor, which is 5 mm in diameter. To ensure constant test conditions reproducible sound transmission from the sound conductor to the polymer samples is necessary. Therefore, the ultrasonic applicator is placed parallel to the sapphire glass and a force of 80 N is applied. This jacking force causes the sound conductor to sink into the polymer samples and ensures an acoustic coupling at the whole coupling surface. The applied pressure on the polymer samples in relation to the jacking force and the diameter of the sound conductor coupling surface is about 4 N mm 2. Therefore, changes of the acoustic properties of the polymer material like acoustic velocity and acoustic impedance due to the compression of the samples can be neglected.
2. Experimental section 2.1. HIFU system
2.3. Polymer samples
For this investigation the HIFU system shown in Fig. 1 has been used. The HIFU ultrasonic applicator consists of an ultrasonic transducer, which contains a spherically curved piezoceramic and an aluminum sound conductor. The diameter of the piezoceramic is 64 mm and the geometric focus length is 49 mm. The sound conductor length is 48.9 mm, thus the focus of the sound beam is close to the sound conductor surface. The sound conductor coupling surface is 5 mm in diameter. The nominal resonance frequency of the thickness mode is 1.0 MHz. Both parts of the ultrasonic applicator, the piezoceramic and the sound conductor, are mounted with epoxy. The cw-signal is generated by a signal function generator and amplified by a power amplifier. The output power is measured with a wattmeter. Since the maximum electrical output power of the amplifier is generated at an output impedance of 50 X, a matching network for impedance matching is used.
For the investigation the low-density polyethylene Lupolen 2420 F from LyondellBasell Industries was used. The transparent samples have a thickness of 0.32 mm. At 23 °C the density is 0.92 g cm 3 [15] and the Poisson’s ratio is 0.45 [16]. The Young’s Modulus was measured by tensile test with 187 N mm 2. Hence, at 1 MHz the acoustic velocity is 876.16 m s 1 and the wavelength is 0.88 mm. The characteristic acoustic impedance is 0.81 MRayl. The melting temperature is 111 °C [15] and the decomposition temperature is above 400 °C [16]
z a b
x y
f
a
c
b
e
g
c d Fig. 1. HIFU system: (a) function generator, (b) power amplifier, (c) wattmeter, (d) impedance matching network, (e) HIFU ultrasonic applicator consisting of a (f) spherical curved piezoceramic transducer and an (g) aluminum sound conductor.
d
e
Fig. 2. Experimental set-up (schematic): (a) sound conductor of the ultrasonic transducer, (b) polymer samples, (c) sapphire glass, (d) mirror, (e) infrared thermography system.
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3. Results and discussion 3.1. Applicator characterization Fig. 3 shows the impedance curves and the phase angle curves of the ultrasonic applicator both with and without the impedance matching network as a function of frequency measured by an impedance analyzer. For maximum sound radiation the ultrasonic applicator should be operated at the thickness natural-mode of the piezoceramic transducer, called resonance frequency [17]. The resonance frequency is characterized by the minimum of the absolute impedance value and a phase shift at the same frequency and can be determined in the curves without matching network [17]. The resonance frequency was found to be 1.0150 MHz for our transducer. The phase angle at this frequency was 25°. The matching network raised the absolute value of the impedance at the same frequency to 14.4 X, while the phase angle was almost unchanged at 25.0°. The generated output power of the amplifier while irradiating the polymer samples was measured at different frequencies for detecting the highest output power. Therefore, different frequency values were adjusted at the function generator and varied with the increment of 0.1 kHz. Fig. 4(a) shows one representative output power curve of each frequency set. The range of 1.0120–1.0150 MHz represents the frequencies with the highest output power. The frequency of 1.0140 MHz was found to generate the highest output power (Fig. 4(a)) and the highest electric energy output within 1.0 s of irradiation (Fig. 4(b)). The lower output generation at the thickness natural-mode frequency can be explained with the lower impedance of the circuit of ultrasonic applicator and the matching network. The impedance value at 1.0140 MHz was at least 30.9 X. So it was closer to the optimal impedance value of 50 X. The same trend was found for the phase angle. For 1.0150 MHz the deviation from the optimal phase angle of 0° was 25° while for 1.0140 MHz it was only 10°. Therefore, for the further investigation the frequency of 1.0140 MHz had been used as working frequency. Fig. 5 shows the output power at different input voltages at 1.0140 MHz while irradiating the polymer samples. As expected, the output raised with increasing input voltage. As the curves were not constant during the irradiation time, it was not acceptable to name them with an output power value. Therefore, we used the output voltage of the signal generator to identify the measurement sets. 3.2. Calibration of the IR temperature measurement system Due to the influence of the environment and the transmission and reflection coefficients of the sapphire glass and the mirror on
the infrared intensity a calibration of the infrared thermography system was necessary. Therefore, the temperature of the polymer sample, which represents the real temperature in the center of the samples, is measured with the IR thermography system and a thermocouple simultaneously. The thermocouple is positioned between the two polymer sample sheets in the xy-plane. However, the sound beam has an effect on the thermocouple measurements causing artefacts while irradiation, e.g. due to localized viscous friction between the thermocouples and the sample surfaces which also is described in [18]. To avoid this effects on the measurements, only the decay of the temperature while cooling was analyzed. The temperatures were measured immediately after switching off the ultrasound, such that no heating occurs any longer and the ultrasound has no effect on the thermocouple. The calibration curve in Fig. 6 shows the temperature measured by the thermocouple plotted against the temperature measured by infrared thermal imaging system. The calibration curve up to 250 °C IR temperature was determined by multiple measurements and the confidence interval was evaluated. As expected, the thermocouple temperature is higher than the IR temperature. The initial slope above 100 °C measured by thermocouple can be attributed to crystallization and a resulting increase of emissivity of the polymer. The confidence interval increases at higher temperatures. Both value and trend are similar to the calibration by Thürling [14], who used a similar experimental set-up and calibration method. To extend the calibration curve to higher temperatures, the trend of the curve was linearly extrapolated. For extrapolation, we calculated the slope of the measured calibration curve between 150 °C and 200 °C IR temperature and extrapolated the calibration curve up to the high end of the range of the IR thermal imaging system of 380 °C. 3.3. Heating effect Fig. 7 shows infrared thermal images of heating in the polymer samples at different irradiation times. For this set of experiments an output voltage of 0.95 V was used. The irradiated volume is not heated homogenously. Local temperature hot spots with higher temperatures than in the surrounding volumes were found. Most of the hot spots emerged in the center of the irradiated volume. The amount of hot spots increased with longer irradiation and neighboring hot spots fused to larger ones, as it can be seen in between 0.3 s, 0.4 s and 0.6 s. In general, the heating profile of the irradiated area in the xy-plane is fundamentally different to the one characterized by Liu [10]. While Liu described a temperature maximum in the
Fig. 3. (a) Impedance and (b) phase angle for both applicator with and without matching network versus frequency.
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Fig. 4. (a) Output power versus time at different frequencies and (b) electric energy output within 1.0 s of irradiation at constant AC input voltage of the amplifier.
Fig. 5. Output power of the amplifier versus time at different AC input voltages of the amplifier for 1.0140 MHz.
Fig. 6. Calibration curve for the infrared temperature measurement system.
center of the irradiated area, we observed additional heating at the edge of the sound conductor coupling surface (Fig. 7(a)). The heated area in the xy-plane corresponds to the diameter of the sound conductor surface at the beginning of the irradiation. During irradiation a magnification of the heated area occurred. This effect may be associated with thermal conductivity and the applied force that causes subsidence of the cone-shaped sound conductor into the polymer sample due to softening the polymer at increasing temperatures.
Multiple measurements for several output power values were performed for investigating the output power dependence of the heating effect. Fig. 8 shows one representative temperature curve of each output power set and represents the maximum temperature of the irradiated volume. The manual operation of the function generator caused the different irradiation times and the different length of the temperature curves. However, the minimum irradiation time for all measurements was 2.0 s. As expected the heating increased by raising output power. This is shown in the irradiation time for heating the samples to a temperature of 100 °C, which dropped with higher power values (Fig. 9). The heating time at 0.95 V was found to be 116 ms, leading to a heating rate of 681 K s 1, which is 170 times higher than the reported heating rate by Liu [10]. For output power values greater or equal than 0.8 V the temperature curves of every single measurement had a temperature shift up to higher temperature values. At 0.75 V the temperature shifts occurred in four out of five measurements and there was no temperature shift within the irradiation time up to 2.0 s for lower output power sets. We found a correlation between the shift in the temperature curve and the occurrence of the hot spots in the infrared thermal images including the temperature maximum. Because the initial temperature of the shift was found to be equal for all measurements at 140 °C, we believe that the temperature jump is based on the phase change of the polymer. In general, ultrasonic absorption increases in liquid phase compared to solid phase [1]. Because the temperature maximum was located in the hot spots and the temperature was close to the melting temperature of the PE-LD, we believe that the higher absorption in the liquid phase induced the temperature shift. The output power curves of the sets of 0.8–0.95 V show characteristic discontinuities in each measurements (Fig. 5). These discontinuities are characterized by a turning point in the curves from decreasing to increasing progression as it can be seen in the measurement with 0.9 V in Fig. 5 at 0.55 s for example. However, there were no discontinuities in the output power curves at measurements without shifts in the temperature curve within 1.0 s irradiation. We found that the temperature shift took place at the same time of the discontinuity in the output power curve. We believe that the phase change of the polymer material effects the discontinuities in the output power curve, because only the temperature in the samples of the sets with discontinuities reached the melting point of the polymer material. The change from solid to liquid phase decreases the acoustic impedance of the polymer material rapidly and the acoustic absorption increases. This causes decreasing sound transition from the coupling surface of the sound conductor to the samples and the higher acoustic absorption in the
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Fig. 7. IR thermal images of the heating effect of polymer samples by irradiation with 0.95 V input voltage of the amplifier.
Fig. 8. Maximum temperature in the polymer samples versus time at different output voltage.
Fig. 9. Irradiation time to 100 °C versus output voltage.
polymer material. Both the sound transition and the acoustic absorption affect the impedance value of the HIFU system, such that the output power of the amplifier is chanced. The maximum temperature value was limited to 750 °C because of the limited temperature range of the infrared thermal imaging system. Because of the trend of the temperature curves of 0.8–0.95 V it can be expected that the absolute temperature
maximum in the polymer samples was higher than the measured 750 °C. However, the temperature in the hot spots exceeded the melting temperature as well as the decomposition temperature of PE-LD. Microscope images obtained after cooling the samples show holes in the center of the irradiated volume (Fig. 10(b)). These were located at the same position as the temperature hot spots. This suggests that the melted polymer was vaporized and gas bubbles were embedded in the inside of the sample during cooling. Holes remained inside the samples at the same place where the hot spots were located. Fig. 10(a) shows an optical image of the irradiated area and Fig. 10(b) an enlarged one obtained by light microscopy. The applied force was significantly reduced for investigations of the acoustic coupling between the sound conductor and the polymer samples. Fig. 11 shows the heating of the polymer samples at 500 ms of irradiation. The circles in the infrared thermal images indicate the edge of the sound conductor coupling surface for dimension reference of the heated area. For detecting initial temperatures a lower temperature scale set up in the IR thermography system of 50–150 °C was used. Because the calibration curve was not valid for that temperature scale set up, the temperatures in Fig. 11 have only qualitative significance. We found that there was no heating effect in the polymer samples at a force of 7 N. At 10 N a small volume at the edge of the sound conductor coupling surface was heated. Heating in the whole area of the sound conductor coupling surface was measured at 13 N. For improving the acoustic coupling between the sound conductor and the upper polymer sample, we placed water-based coupling gel between each other. As a result, heating already occurred at a force of 7 N. We found that there was no heating effect in the polymer samples at low forces. The roughness and none-flatness of the sound conductor coupling surface leads to small air-filled gaps between the surface and the upper polymer sample. Additionally, the roughness of polymer samples leads to air inclusions between the two polymer sample sheets. Air results in total reflection, which means that there is no or lower sound transmission from the aluminum sound conductor into the polymer samples or between both polymer sample sheets. Due to the applied force the polymer samples were deformed and the coupling surface was pressed in the upper polymer sample sheet. As a result, the sound conductor surface was completely in contact with the polymer sample and the gaps were closed. This leads to a better acoustic coupling and sound transmission into the polymer samples, where absorption leads
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Fig. 10. (a) Optical image and (b) microscope image of the cooled down irradiated polymer sample.
Fig. 11. Not calibrated IR thermal images of the heating effect at different applied forces.
to heating. The small heated volume at 10 N was a result of the uneven and not perfectly round sound conductor coupling surface and a deviation of the parallelism to the sapphire glass. That means that just a small area at the edge of the sound conductor coupling surface was in contact to the upper polymer sample sheet. Therefore, acoustic coupling and sound transmissions occurs only at that small area and heating took place only in that volume. The coupling gel coupled the whole sound conductor coupling surface to the upper polymer sample sheet independent on the applied force. The heated volume beside the sound conductor coupling surface was a result of displaced coupling gel that wetted the coneshaped edge of the sound conductor. Thereby, the coupling area and the irradiated area were increased so that an enlarged volume was irradiated and heated. That implies that for generating heating effect in polymer samples using a solid sound conductor a sufficient acoustic coupling is necessary, which can be reached due to an adequate force or coupling gel.
4. Conclusion The HIFU-induced heating of solid PE-LD using a solid sound conductor for sound transmission from a piezoceramic transducer to the samples was investigated. Our investigations showed that high ultrasonic energy can be coupled into solid polymer samples for strong heat generation using a HIFU ultrasonic applicator with solid sound conductor. For inducing the heating effect an acoustic coupling of the sound conductor to the sample is required, which can be performed by applying a jacking force. We have found that the heating profile in the xy-plane is quite different to profiles obtained from coupling with water published by other researchers [9,10]. The irradiated volume was characterized by hot spots. The temperature in the hot spots reached the melting temperature and even the evaporation temperature within less than 1 s irradiation time. An indicator for temperature rise was found in the form of a discontinuity in the output power curve. These findings are
relevant for processes in which solid polymers need to be heated quickly and locally such as in welding or forming. Future work will investigate the heating effect in other thermoplastic polymer materials, different sample thickness and the influence of the material and thickness of the counterholder that was limited to sapphire glass in this study.
References [1] O.V. Abramov, High-intensity ultrasonics. Theory and Industrial Applications, Gordon and Breach Science Publ., Amsterdam, 1998. [2] J.R. Frederick, Ultrasonic Engineering, Wiley, New York, 1965. [3] J.E. Kennedy, High-intensity focused ultrasound in the treatment of solid tumours, Nat. Rev. Cancer 5 (2005) 321–327. [4] M.R. Bailey, V.A. Khokhlova, O.A. Sapozhnikov, S.G. Kargl, L.A. Crum, Physical mechanisms of the therapeutic effect of ultrasound (a review), Acoust. Phys. 49 (2003) 369–388. [5] J.E. Kennedy, G.R. ter Haar, D. Cranston, High intensity focused ultrasound: surgery of the future?, Br J. Radiol. 76 (2003) 590–599. [6] D.L. Miller, N.B. Smith, M.R. Bailey, G.J. Czarnota, K. Hynynen, I.R.S. Makin, Overview of therapeutic ultrasound applications and safety considerations, J. Ultrasound Med. 31 (2012) 623–634. [7] G. ter Haar, Therapeutic applications of ultrasound, Prog. Biophys. Mol. Biol. 93 (2007) 111–129. [8] G. Li, G. Fei, H. Xia, J. Han, Y. Zhao, Spatial and temporal control of shape memory polymers and simultaneous drug release using high intensity focused ultrasound, J. Mater. Chem. 22 (2012) 7692–7696. [9] G. Li, G.X. Fei, B. Liu, H.S. Xia, Y. Zhao, Shape recovery characteristics for shape memory polymers subjected to high intensity focused ultrasound, RSC Adv. 4 (2014) 32701–32709. [10] B. Liu, H.S. Xia, G.X. Fei, G. Li, W.R. Fan, High-intensity focused ultrasoundinduced thermal effect for solid polymer materials, Macromol. Chem. Phys. 214 (2013) 2519–2527. [11] L. Hallez, F. Touyeras, J.Y. Hihn, Y. Bailly, Interactions H.I.F.U./polymer films, Phys. Procedia 3 (2010) 179–184. [12] L. Huamao, L. Yuhua, L. Zhouhua, The heating phenomenon produced by an ultrasonic fountain, Ultrason. Sonochem. 4 (1997) 217–218. [13] M.D. Brentnall, R.W. Martin, S. Vaezy, P. Kaczkowski, F. Forster, L. Crum, A new high intensity focused ultrasound applicator for surgical applications, IEEE Trans. Ultrason. Ferroelectr. Freq. Control 48 (2001) 53–63. [14] K. Thürling, B. Stephan, S. Bach, J.-P. Majschak, ‘‘In situ” temperature measurement at ultrasonic sealing using IR-thermography, in: 26th IAPRI Symposium on Packaging 2013, VTT, Espoo, 2013, pp. 549–554.
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[15] LyondellBasell, Product Data and Technical Information Lupolen 2420 F, in,. 2015. [16] H. Domininghaus, P. Elsner, P. Eyerer, T. Hirth, Kunststoffe. Eigenschaften und Anwendungen, eighth ed., Springer, Heidelberg, 2012.
[17] M.S. Vijaya, Piezoelectric Materials and Devices. Applications in Engineering and Medical Sciences, CRC Press, Boca Raton, 2013. [18] K.J. Parker, Effects of heat conduction and sample size on ultrasonic absorption measurements, J. Acoust. Soc. Am. 77 (1985) 719–725.
Contents lists available at ScienceDirect
Ultrasonics journal homepage: www.elsevier.com/locate/ultras
Investigations on the heating effect of PE-LD induced by high-intensity focused ultrasound Lukas Oehm ⇑, Sascha Bach, Jens-Peter Majschak Dresden University of Technology, Chair of Processing Machines/Processing Technology, Bergstraße 120, 01069 Dresden, Germany
a r t i c l e
i n f o
Article history: Received 22 October 2015 Received in revised form 29 April 2016 Accepted 2 May 2016 Available online 3 May 2016 Keywords: High-intensity focused ultrasound (HIFU) Infrared thermography Polymer material Heating effect
a b s t r a c t High-intensity focused ultrasound is widely applied in tissue treatment as well as for heating of solid polymer materials. Previous studies investigating the heating effect in polymer materials utilized sound transmission through water or other fluids at low HIFU power. In this study, the ultrasonic transducer possesses a solid sound conductor made of aluminum and a high HIFU power of above 100 W was applied to heat solid PE-LD samples. Temperature measurements were performed by calibrated non-invasive infrared thermal imaging. A strong heating effect with heating above melting temperature and evaporation temperature within less than 1 s of irradiation was observed. Furthermore, the acoustic coupling defined by the force applied by the ultrasonic applicator to the polymer material was found to be fundamental to induce the heating effect. This investigation reveals HIFU for new applications in the field of polymer processing. Ó 2016 Elsevier B.V. All rights reserved.
1. Introduction High-intensity focused ultrasound (HIFU) is an advanced medical technology based on the deposition of a large amount of acoustic energy in a small spot of tissue, which is achieved by focusing of ultrasonic waves [1,2]. HIFU treatment is associated with ultrasound-induced heating caused by conversion of mechanical into thermal energy [3]. Temperatures above 80 °C can be obtained within a few seconds of irradiation at acoustic intensities of up to 10 000 W cm 2 [4]. Thus, HIFU is used for in situ non-invasive or minimally invasive treatment of cancer, e.g. in prostate, liver, breast, kidney, bone and pancreas, as well as in glaucoma treatment and for inducing hemostasis [3,5–7]. The ultrasonic waves are usually generated by a piezoelectric transducer that is spherically shaped or coupled to an acoustic lens for focusing [6]. Depending on the application, frequencies from 0.5 to 7 MHz are used [6]. In addition to tissue treatment, HIFU has also been utilized to locally heat polymer materials thereby triggering drug release or shape memory [8,9]. In general, absorption of ultrasound due to internal friction and relaxation results in a heating effect in visco-elastic polymer materials. The HIFU-induced heating effect in different polymer materials (PE-LLD, PE-LD, PE-HD, PA-6, PP,
⇑ Corresponding author. E-mail addresses: [email protected] (L. Oehm), sascha.bach@ tu-dresden.de (S. Bach), [email protected] (J.-P. Majschak). http://dx.doi.org/10.1016/j.ultras.2016.05.002 0041-624X/Ó 2016 Elsevier B.V. All rights reserved.
PS, PC and PMMA) was investigated by Liu et al. [10]. They placed a 1.1 MHz focusing piezoceramic ultrasonic transducer in a waterfilled tank and irradiated solid polymer samples located at the water surface. The HIFU power was 8 W at maximum to avoid cavitation. Non-invasive temperature measurement was done by infrared thermal imaging at the back surface of the samples. A correlation between the HIFU-induced thermal effect and the type of polymer materials was found, which was explained by different absorption coefficients of the different polymer materials due to differences in inner friction behaviors of macromolecular chains. The maximum equilibrium temperature was found for PMMA with 180 °C after 25 s of irradiation with 4 W HIFU power. PE-LD was heated up to 80 °C after 20 s at 4 W. For higher HIFU power a material-dependent threshold of 6–8 W for sample damage was found. Furthermore, the authors described different initial temperature rises for the different polymer materials, the highest one for PP. Both the equilibrium temperature and the initial temperature rise were found to be dependent on the sample thickness in case of PE-LD. Increasing sample thickness results in increasing equilibrium temperature and decreasing initial temperature rises. They explained the increasing equilibrium temperature with the enlarged ‘‘focused volume, which makes the sample absorb more ultrasound energy” [10]. Similar results were obtained by Li et al. for shape memory polymer P(MMA-BA) [9]. Investigations with higher HIFU power on the same shape memory polymer material were done by Li et al. [8]. They reported a polymer heating of 140 °C in the samples after 10 min of irradiation at 50 W HIFU
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power, which was measured by embedded thermocouples [8]. Further reports on HIFU-induced thermal heating have been published by Hallez et al. [11] and Huamao et al. [12]. However, in all known investigations water has been used for sound transmission and the HIFU power was limited to 50 W to avoid cavitation. As a result, the temperature rise of the samples was limited and the irradiation times for reaching the equilibrium temperature were several seconds. Rapid local heating of solid polymers is of broad interest for several areas of application in the field of polymer processing, such as forming or welding. However, the implementation of HIFU in these applications requires an understanding of the heating effect at high HIFU power. To circumvent cavitation, it is necessary to substitute water as the sound-transmitting medium. A number of medical devices possess solid coupling cones build from e.g. aluminum for sound transmission [13]. In this study, this ultrasonic applicator design is adapted by using an ultrasonic applicator with solid aluminum sound conductor. To the best of our knowledge the HIFU-induced heating effect in solid polymer materials has not yet been investigated using a solid sound conductor. Hence, the aim of the present work is to investigate the heating effect of solid polymer samples using high HIFU power and an ultrasonic applicator with solid sound conductor. First, the ultrasonic applicator is characterized by impedance measurements to find the frequency for maximum output power generation. A setup for non-invasive temperature measurements with infrared thermal imaging is described and calibrated. Both, the temperature values and the temperature distribution in the irradiated volume are measured. Furthermore, the influence of the acoustic coupling of the sound conductor to the samples on the heating effect is investigated.
2.2. Experimental set-up for temperature measurements The temperature measurements were performed using an experimental set-up initially introduced for the in-situ investigation of 20 kHz ultrasonic sealing systems by infrared thermography [14]. According to this set-up the ultrasonic applicator is placed on two polymer sample sheets, which are positioned on a sapphire glass (Fig. 2). This set-up allows high resolution temperature measurement in the xy-plane of the transparent polymer samples by infrared thermography. Therefore, the infrared thermography system is placed orthogonally to the polymer samples on a tripod for maximum stability. A mirror reflects the infrared radiation emitted from the polymer samples through to the infrared thermography system. The measurement range of the IR thermography system is 0–380 °C with a temperature resolution of 1 K at 800 Hz measuring frequency. The picture format of the lens is 160 128 pixel, which for a resolution of 70 70 lm results in an image field of 11.2 9 mm. This ensures the capturing of the whole coupling surface of the sound conductor, which is 5 mm in diameter. To ensure constant test conditions reproducible sound transmission from the sound conductor to the polymer samples is necessary. Therefore, the ultrasonic applicator is placed parallel to the sapphire glass and a force of 80 N is applied. This jacking force causes the sound conductor to sink into the polymer samples and ensures an acoustic coupling at the whole coupling surface. The applied pressure on the polymer samples in relation to the jacking force and the diameter of the sound conductor coupling surface is about 4 N mm 2. Therefore, changes of the acoustic properties of the polymer material like acoustic velocity and acoustic impedance due to the compression of the samples can be neglected.
2. Experimental section 2.1. HIFU system
2.3. Polymer samples
For this investigation the HIFU system shown in Fig. 1 has been used. The HIFU ultrasonic applicator consists of an ultrasonic transducer, which contains a spherically curved piezoceramic and an aluminum sound conductor. The diameter of the piezoceramic is 64 mm and the geometric focus length is 49 mm. The sound conductor length is 48.9 mm, thus the focus of the sound beam is close to the sound conductor surface. The sound conductor coupling surface is 5 mm in diameter. The nominal resonance frequency of the thickness mode is 1.0 MHz. Both parts of the ultrasonic applicator, the piezoceramic and the sound conductor, are mounted with epoxy. The cw-signal is generated by a signal function generator and amplified by a power amplifier. The output power is measured with a wattmeter. Since the maximum electrical output power of the amplifier is generated at an output impedance of 50 X, a matching network for impedance matching is used.
For the investigation the low-density polyethylene Lupolen 2420 F from LyondellBasell Industries was used. The transparent samples have a thickness of 0.32 mm. At 23 °C the density is 0.92 g cm 3 [15] and the Poisson’s ratio is 0.45 [16]. The Young’s Modulus was measured by tensile test with 187 N mm 2. Hence, at 1 MHz the acoustic velocity is 876.16 m s 1 and the wavelength is 0.88 mm. The characteristic acoustic impedance is 0.81 MRayl. The melting temperature is 111 °C [15] and the decomposition temperature is above 400 °C [16]
z a b
x y
f
a
c
b
e
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c d Fig. 1. HIFU system: (a) function generator, (b) power amplifier, (c) wattmeter, (d) impedance matching network, (e) HIFU ultrasonic applicator consisting of a (f) spherical curved piezoceramic transducer and an (g) aluminum sound conductor.
d
e
Fig. 2. Experimental set-up (schematic): (a) sound conductor of the ultrasonic transducer, (b) polymer samples, (c) sapphire glass, (d) mirror, (e) infrared thermography system.
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3. Results and discussion 3.1. Applicator characterization Fig. 3 shows the impedance curves and the phase angle curves of the ultrasonic applicator both with and without the impedance matching network as a function of frequency measured by an impedance analyzer. For maximum sound radiation the ultrasonic applicator should be operated at the thickness natural-mode of the piezoceramic transducer, called resonance frequency [17]. The resonance frequency is characterized by the minimum of the absolute impedance value and a phase shift at the same frequency and can be determined in the curves without matching network [17]. The resonance frequency was found to be 1.0150 MHz for our transducer. The phase angle at this frequency was 25°. The matching network raised the absolute value of the impedance at the same frequency to 14.4 X, while the phase angle was almost unchanged at 25.0°. The generated output power of the amplifier while irradiating the polymer samples was measured at different frequencies for detecting the highest output power. Therefore, different frequency values were adjusted at the function generator and varied with the increment of 0.1 kHz. Fig. 4(a) shows one representative output power curve of each frequency set. The range of 1.0120–1.0150 MHz represents the frequencies with the highest output power. The frequency of 1.0140 MHz was found to generate the highest output power (Fig. 4(a)) and the highest electric energy output within 1.0 s of irradiation (Fig. 4(b)). The lower output generation at the thickness natural-mode frequency can be explained with the lower impedance of the circuit of ultrasonic applicator and the matching network. The impedance value at 1.0140 MHz was at least 30.9 X. So it was closer to the optimal impedance value of 50 X. The same trend was found for the phase angle. For 1.0150 MHz the deviation from the optimal phase angle of 0° was 25° while for 1.0140 MHz it was only 10°. Therefore, for the further investigation the frequency of 1.0140 MHz had been used as working frequency. Fig. 5 shows the output power at different input voltages at 1.0140 MHz while irradiating the polymer samples. As expected, the output raised with increasing input voltage. As the curves were not constant during the irradiation time, it was not acceptable to name them with an output power value. Therefore, we used the output voltage of the signal generator to identify the measurement sets. 3.2. Calibration of the IR temperature measurement system Due to the influence of the environment and the transmission and reflection coefficients of the sapphire glass and the mirror on
the infrared intensity a calibration of the infrared thermography system was necessary. Therefore, the temperature of the polymer sample, which represents the real temperature in the center of the samples, is measured with the IR thermography system and a thermocouple simultaneously. The thermocouple is positioned between the two polymer sample sheets in the xy-plane. However, the sound beam has an effect on the thermocouple measurements causing artefacts while irradiation, e.g. due to localized viscous friction between the thermocouples and the sample surfaces which also is described in [18]. To avoid this effects on the measurements, only the decay of the temperature while cooling was analyzed. The temperatures were measured immediately after switching off the ultrasound, such that no heating occurs any longer and the ultrasound has no effect on the thermocouple. The calibration curve in Fig. 6 shows the temperature measured by the thermocouple plotted against the temperature measured by infrared thermal imaging system. The calibration curve up to 250 °C IR temperature was determined by multiple measurements and the confidence interval was evaluated. As expected, the thermocouple temperature is higher than the IR temperature. The initial slope above 100 °C measured by thermocouple can be attributed to crystallization and a resulting increase of emissivity of the polymer. The confidence interval increases at higher temperatures. Both value and trend are similar to the calibration by Thürling [14], who used a similar experimental set-up and calibration method. To extend the calibration curve to higher temperatures, the trend of the curve was linearly extrapolated. For extrapolation, we calculated the slope of the measured calibration curve between 150 °C and 200 °C IR temperature and extrapolated the calibration curve up to the high end of the range of the IR thermal imaging system of 380 °C. 3.3. Heating effect Fig. 7 shows infrared thermal images of heating in the polymer samples at different irradiation times. For this set of experiments an output voltage of 0.95 V was used. The irradiated volume is not heated homogenously. Local temperature hot spots with higher temperatures than in the surrounding volumes were found. Most of the hot spots emerged in the center of the irradiated volume. The amount of hot spots increased with longer irradiation and neighboring hot spots fused to larger ones, as it can be seen in between 0.3 s, 0.4 s and 0.6 s. In general, the heating profile of the irradiated area in the xy-plane is fundamentally different to the one characterized by Liu [10]. While Liu described a temperature maximum in the
Fig. 3. (a) Impedance and (b) phase angle for both applicator with and without matching network versus frequency.
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Fig. 4. (a) Output power versus time at different frequencies and (b) electric energy output within 1.0 s of irradiation at constant AC input voltage of the amplifier.
Fig. 5. Output power of the amplifier versus time at different AC input voltages of the amplifier for 1.0140 MHz.
Fig. 6. Calibration curve for the infrared temperature measurement system.
center of the irradiated area, we observed additional heating at the edge of the sound conductor coupling surface (Fig. 7(a)). The heated area in the xy-plane corresponds to the diameter of the sound conductor surface at the beginning of the irradiation. During irradiation a magnification of the heated area occurred. This effect may be associated with thermal conductivity and the applied force that causes subsidence of the cone-shaped sound conductor into the polymer sample due to softening the polymer at increasing temperatures.
Multiple measurements for several output power values were performed for investigating the output power dependence of the heating effect. Fig. 8 shows one representative temperature curve of each output power set and represents the maximum temperature of the irradiated volume. The manual operation of the function generator caused the different irradiation times and the different length of the temperature curves. However, the minimum irradiation time for all measurements was 2.0 s. As expected the heating increased by raising output power. This is shown in the irradiation time for heating the samples to a temperature of 100 °C, which dropped with higher power values (Fig. 9). The heating time at 0.95 V was found to be 116 ms, leading to a heating rate of 681 K s 1, which is 170 times higher than the reported heating rate by Liu [10]. For output power values greater or equal than 0.8 V the temperature curves of every single measurement had a temperature shift up to higher temperature values. At 0.75 V the temperature shifts occurred in four out of five measurements and there was no temperature shift within the irradiation time up to 2.0 s for lower output power sets. We found a correlation between the shift in the temperature curve and the occurrence of the hot spots in the infrared thermal images including the temperature maximum. Because the initial temperature of the shift was found to be equal for all measurements at 140 °C, we believe that the temperature jump is based on the phase change of the polymer. In general, ultrasonic absorption increases in liquid phase compared to solid phase [1]. Because the temperature maximum was located in the hot spots and the temperature was close to the melting temperature of the PE-LD, we believe that the higher absorption in the liquid phase induced the temperature shift. The output power curves of the sets of 0.8–0.95 V show characteristic discontinuities in each measurements (Fig. 5). These discontinuities are characterized by a turning point in the curves from decreasing to increasing progression as it can be seen in the measurement with 0.9 V in Fig. 5 at 0.55 s for example. However, there were no discontinuities in the output power curves at measurements without shifts in the temperature curve within 1.0 s irradiation. We found that the temperature shift took place at the same time of the discontinuity in the output power curve. We believe that the phase change of the polymer material effects the discontinuities in the output power curve, because only the temperature in the samples of the sets with discontinuities reached the melting point of the polymer material. The change from solid to liquid phase decreases the acoustic impedance of the polymer material rapidly and the acoustic absorption increases. This causes decreasing sound transition from the coupling surface of the sound conductor to the samples and the higher acoustic absorption in the
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Fig. 7. IR thermal images of the heating effect of polymer samples by irradiation with 0.95 V input voltage of the amplifier.
Fig. 8. Maximum temperature in the polymer samples versus time at different output voltage.
Fig. 9. Irradiation time to 100 °C versus output voltage.
polymer material. Both the sound transition and the acoustic absorption affect the impedance value of the HIFU system, such that the output power of the amplifier is chanced. The maximum temperature value was limited to 750 °C because of the limited temperature range of the infrared thermal imaging system. Because of the trend of the temperature curves of 0.8–0.95 V it can be expected that the absolute temperature
maximum in the polymer samples was higher than the measured 750 °C. However, the temperature in the hot spots exceeded the melting temperature as well as the decomposition temperature of PE-LD. Microscope images obtained after cooling the samples show holes in the center of the irradiated volume (Fig. 10(b)). These were located at the same position as the temperature hot spots. This suggests that the melted polymer was vaporized and gas bubbles were embedded in the inside of the sample during cooling. Holes remained inside the samples at the same place where the hot spots were located. Fig. 10(a) shows an optical image of the irradiated area and Fig. 10(b) an enlarged one obtained by light microscopy. The applied force was significantly reduced for investigations of the acoustic coupling between the sound conductor and the polymer samples. Fig. 11 shows the heating of the polymer samples at 500 ms of irradiation. The circles in the infrared thermal images indicate the edge of the sound conductor coupling surface for dimension reference of the heated area. For detecting initial temperatures a lower temperature scale set up in the IR thermography system of 50–150 °C was used. Because the calibration curve was not valid for that temperature scale set up, the temperatures in Fig. 11 have only qualitative significance. We found that there was no heating effect in the polymer samples at a force of 7 N. At 10 N a small volume at the edge of the sound conductor coupling surface was heated. Heating in the whole area of the sound conductor coupling surface was measured at 13 N. For improving the acoustic coupling between the sound conductor and the upper polymer sample, we placed water-based coupling gel between each other. As a result, heating already occurred at a force of 7 N. We found that there was no heating effect in the polymer samples at low forces. The roughness and none-flatness of the sound conductor coupling surface leads to small air-filled gaps between the surface and the upper polymer sample. Additionally, the roughness of polymer samples leads to air inclusions between the two polymer sample sheets. Air results in total reflection, which means that there is no or lower sound transmission from the aluminum sound conductor into the polymer samples or between both polymer sample sheets. Due to the applied force the polymer samples were deformed and the coupling surface was pressed in the upper polymer sample sheet. As a result, the sound conductor surface was completely in contact with the polymer sample and the gaps were closed. This leads to a better acoustic coupling and sound transmission into the polymer samples, where absorption leads
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Fig. 10. (a) Optical image and (b) microscope image of the cooled down irradiated polymer sample.
Fig. 11. Not calibrated IR thermal images of the heating effect at different applied forces.
to heating. The small heated volume at 10 N was a result of the uneven and not perfectly round sound conductor coupling surface and a deviation of the parallelism to the sapphire glass. That means that just a small area at the edge of the sound conductor coupling surface was in contact to the upper polymer sample sheet. Therefore, acoustic coupling and sound transmissions occurs only at that small area and heating took place only in that volume. The coupling gel coupled the whole sound conductor coupling surface to the upper polymer sample sheet independent on the applied force. The heated volume beside the sound conductor coupling surface was a result of displaced coupling gel that wetted the coneshaped edge of the sound conductor. Thereby, the coupling area and the irradiated area were increased so that an enlarged volume was irradiated and heated. That implies that for generating heating effect in polymer samples using a solid sound conductor a sufficient acoustic coupling is necessary, which can be reached due to an adequate force or coupling gel.
4. Conclusion The HIFU-induced heating of solid PE-LD using a solid sound conductor for sound transmission from a piezoceramic transducer to the samples was investigated. Our investigations showed that high ultrasonic energy can be coupled into solid polymer samples for strong heat generation using a HIFU ultrasonic applicator with solid sound conductor. For inducing the heating effect an acoustic coupling of the sound conductor to the sample is required, which can be performed by applying a jacking force. We have found that the heating profile in the xy-plane is quite different to profiles obtained from coupling with water published by other researchers [9,10]. The irradiated volume was characterized by hot spots. The temperature in the hot spots reached the melting temperature and even the evaporation temperature within less than 1 s irradiation time. An indicator for temperature rise was found in the form of a discontinuity in the output power curve. These findings are
relevant for processes in which solid polymers need to be heated quickly and locally such as in welding or forming. Future work will investigate the heating effect in other thermoplastic polymer materials, different sample thickness and the influence of the material and thickness of the counterholder that was limited to sapphire glass in this study.
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