Therapeutic ultrasound has been in use for over 70 1 years but has primarily been a thermal modality. Sonoporation, 2 the use of ultrasound and stable gas microbubbles in the size 3 range of 2-10 µm to form transient pores in cell membranes, has 4 been of great interest in the past 15 years. This technique could 5 be used to improve the delivery of current drugs in very localised 6 regions. There are several phenomena behind sonoporation 7 that all occur non-exclusively: push, pull, jetting, inertial 8 cavitation, shear and, translation. Pre-clinical work has shown 9 that sonoporation can be used to reduce primary tumour burden 10 and inhibit metastatic development. Our clinical trial showed that 11 ultrasound in combination with microbubbles and chemotherapy 12 can effectively double the number of chemotherapy cycles 13 patients can undergo, meaning that the patients were healthier 14 for a longer period of time. Nevertheless, sonoporation is still in 15 its infancy and there is vast room for improvement in both the 16 areas of microbubbles and ultrasound. resulting in 9 doses of 0.5 mL each over 31.5 minutes. 220 To evaluate the effect of the treatment, we compared the 221 number of chemotherapy cycles our patients could undergo 222 versus 80 historical patients treated at the same hospital. The 223 healthier the patients are, the more treatment cycles they 224 can undergo. The historical group was able to receive 9±6 225 treatment cycles. The patients treated with sonoporation were 226 able to undergo almost double the amount of treatment with 227 16±7 cycles of chemotherapy. It is important to note that 228 several patients were removed from the study due to the 229 efficacy of the treatment. These patients would be able to go 230 substantially more cycles, hence the treated cycles number is 231 artificially low. 232 Maximum tumour diameters were measured using CT 233 imaging from independent radiologists. The primary tumour 234 growth was inhibited and in some cases, even reduced. 235 In conclusion, the clinical trials showed that there are 236 no immediate adverse side effects to sonoporation, and that 237 sonoporation can help increase the efficacy of the treatment. 238 VI. FUTURE WORK 239 Whilst sonoporation has shown success in the lab and in 240 the clinic, there is still a lot of room for improvement. To 241 date, sonoporation has primarily been performed by combining 242 existing materials and technologies aimed for other tasks, for 243 example the ultrasound contrast agents and clinical diagnostic 244 machines are designed for imaging not treatment. Both these 245 aspects need to be drastically improved. 246 A. Microbubbles 247 In the area of microbubbles, new agents need to be 248 developed that can carry a therapeutic load to the target region.

249 These bubbles need to be highly acoustically sensitive, and be 250 able to release their load on demand. Current concepts allow 251 loading inside the shell, or on the surface of the shell, hence 252 only small loads can be carried. New methods and concepts are 253 needed that allow for larger volumes of therapeutic loads and 254 specific release mechanisms. Such a method would only treat a 255 localised region, and would have minimal affect on metastasis, 256 for this reason, it may be ideal to combine sonoporation with 257 traditional chemotherapy, ensuring maximum affect on both 258 primary tumour and metastasis. 259 B. Ultrasound 260 Clinical diagnostic machines are highly specialised and have 261 inverse acoustic properties than those needed for sonoporation. 262 Sonoporation requires a large area to be treated, so a large 263 footprint probe, that can treat a whole tumour is necessary. In 264 addition, longer duty cycles are required to continuously excite 265 the microbubbles in the target region increasing the cellular 266 permeability.

I. INTRODUCTION 18

Therapeutic ultrasound has been in use since the early 1940s 19 [START_REF] Robertson | A review of therapeutic ultrasound: 288 effectiveness studies[END_REF], primarily as method to heat deep tissue to promote healing.

39

II. MICROBUBBLES IN A SAFE CLINICAL SETTING

40

The first clinical use of microbubbles was reported in 1969 41 [START_REF] Gramiak | Echocardiography of the aortic root[END_REF]. Microbubbles in the blood pool are used to improve 42 the signal-to-noise ratio or contrast of blood when imaged using B-mode sonography. Hence, these microbubbles are 44 also known as ultrasound contrast agents. Their sizes range 45 between 2-5 µm, typically with over 95% below 10 µm so 46 that the microbubbles can pass through the lung capillaries. 47 Microbubbles, like most physical objects, have a resonant 48 frequency; a frequency where its oscillation amplitude is 49 maximal. Similar to a bell, when a force strikes it, the 50 microbubble will volumetrically oscillate and generate sound 51 at its resonance frequency. For a free gas bubble this frequency 52 can be approximated by [START_REF] Postema | Ultrasound-induced gas release from contrast agent microbubbles[END_REF]:

53 f r ≈ 6.5 D , (1)
where f r is the resonant frequency and D is the diameter 54 of the bubble. From this we can see that a 2.5 µm gas 55 bubble will resonate at approximately 2.6 MHz, a typical 56 diagnostic ultrasound frequency. Free gas microbubbles are 57 inherently unstable, and dissolve almost instantly. For this 58 reason clinical diagnostic ultrasound contrast agents typically 59 have a lipid or albumin shell with a slowly diffusing gas core, 60 increasing stability, allowing for efficient clinical imaging. 61 Table I summarises clinically approved ultrasound contrast 62 agents and their contents.

MI = p - √ f c , (2)
where p -is the peak-negative acoustic pressure in-situ As can be expected, injecting a gas bubble into the 103 blood stream, that generates a detectable acoustic force, may 104 have some inherent side-effects. In 1997, upon studying 105 such side-effects [START_REF] Bao | Transfection of a reporter 309 plasmid into cultured cells by sonoporation in vitro[END_REF] and [START_REF] Unger | SonoPoration and Gene Delivery with Acoustically 312 Active Carriers[END_REF] showed that ultrasound in 106 combination with microbubbles could increase the uptake of 107 fluorescent genes. The MI for both frequencies was ≤ 0.2 and acoustic 133 intensity (I SPTA ) was 1 W/cm 2 . Imaging was performed 134 using a high-magnification, high-numerical aperture (NA) 135 setup using a 60×/0.90-NA objective or using a confocal 136 fluorescence microscope with a 40×/1.30-NA objective. 137 Following sonication, a range of phenomena was observed, 138 including bubbles hammering back and forth into cells [START_REF] Marmottant | Controlled vesicle deformation and 314 lysis by single oscillating bubbles[END_REF], 139 to complete bubble translation into a cell. To validate if the 140 microbubble had truly translated into the cell, 3D confocal 141 fluorescence imaging was performed. Results showed that the 142 lipid bonded fluorescent probe was then within the cell. To 143 exclude the chance of endocytosis, experiments were re-done 144 with endocytosis inhibitors where no change in localised 145 microbubble translation was seen.

146

In conclusion, to date there are several non-exclusive 147 phenomena that can be directly related to this increased up 148 take [START_REF] Postema | Sonoporation: Why microbubbles create pores[END_REF]: Push and pull, where the microbubble near, or in 149 contact with the cell, stretches the bilayer membrane opening 150 pores. Jetting, when a bubble collapses asymmetrically, 151 making a needle-like point towards the cell. Streaming, 152 where the fluid flow around the bubble near a cell is 153 strong enough to shear the membrane apart. Translation, 154 where a microbubble with a therapeutic load can be forced 155 inside a cell. Inertial cavitation, where the violent formation 156 After only two treatments, i.e., 5 weeks after xenografting, 188 a statistical significant difference in tumour volume was seen 189 between the sonoporated group and the chemotherapy alone 190 group. Figure 3 shows the bioluminescent images and tumour 191 volumes 10 weeks after xenografting (after six treatments). 192 By week 10, the primary tumour in the sonoporation group 193 was nearly 4 times smaller than the control and chemotherapy 194 alone tumour. In addition the metastatic spread was limited 195 to the liver in the sonoporation group whereas the Gemzar 196 and control group showed whole body metastatic spread. 197 In conclusion, translating the identical acoustic conditions 198 seen to force microbubbles into cells in vitro resulted in 199 primary tumour inhibition and delayed metastatic spread when 200 combining a chemotherapeutic.

201

V. CLINICAL TRIALS 202

In many cases pre-clinical work does not translate well into 203 the clinic. In our work here we perform, to our knowledge, 204 the world first clinical trial on sonoporation [START_REF] Kotopoulis | Treatment of human pancreatic cancer using combined ultrasound, 328 microbubbles, and gemcitabine: A clinical case study[END_REF].

205

A total of five patients with PDAC were treated using 206 Gemzar, SonoVue R and ultrasound. The chemotherapeutic 207 was administered in accordance to the manufacturer's 208 guidelines; infusion over 30 minutes. At the completion 209 of the infusion, the concentration of the chemotherapeutic 210 was maximal in the blood. At this point sonoporation was 211 initiated. Ultrasound was generated using a clinical diagnostic 212 scanner (GE LOGIQ 9 + 4C probe, GE Healthcare, Waukesha, 213 WI), with optimised acoustic output conditions. The clinical 214 diagnostic machine allowed for simultaneous imaging and 215 treatment of the primary tumour. The output frequency was 216 1.8 MHz with a duty cycle of 1%. To compensate for the 217 reduced duty cycle the ultrasound treatment lasted 31.5 min.

Although sonoporation was discovered only in 1997 and 269 clinical trials were first done from 2013, we have shown 270 that there is great potential in sonoporation. In-vitro results 271 have shown that microbubbles can be forced into cells, 272 whereas pre-clinical mice studies under identical conditions 273 have shown to reduce primary tumour burden and metastatic 274 development. Our clinical trial showed that combining 275 sonoporation and chemotherapy can help the patients to 276 undergo more chemotherapy cycles, presumably prolonging 277 survival.

20

  Nevertheless, in the past 15 years, due to advancements 21 in technology, new ultrasound-based therapeutic modalities 22 have surfaced; specifically High Intensity Focused Ultrasound 23 (HIFU) and sonoporation. HIFU is used to heat targeted tissue 24 in very specific regions to such a high temperature that thermal 25 necrosis occurs. This is often done with the aid of Magnetic 26 Resonance Imaging to visualise the target tissue and measure 27 the thermal deposition [2]. Sonoporation on the other hand 28 aims to solve the problem of time and space controlled drug 29 delivery [3]. One of the major barriers in effective drug 30 delivery is the cell membrane; it acts as wall selectively 31 choosing what to allow in and out of the cell [4]. Sonoporation 32 can be defined as the the transient formation of pores in a cell 33 membrane due to ultrasound or a combination of ultrasound 34 and microbubbles. By forming pores in the cell membranes we 35 can effectively bypass this barrier that is the cell membrane, 36 resulting in highly efficient targeted drug delivery. In our work 37 we explore the phenomenon of sonoporation, from the lab 38 bench all the way to the World's first clinical trials.

Fig. 1 .

 1 Fig. 1. Photo micrographs depicting clusters of Definity R microbubbles attracting and merging in under 2 seconds.

68

  measured in MPa, and f c is the centre frequency in MHz. 69 An MI<0.3 is considered safe, an MI between 0.3 and 0.7 70 is considered moderately safe as there is a risk of damage 71 to neonatal lung tissue or intestine, and an MI>0.7 there is 72 a high risk of inertial cavitation of the ultrasound contrast 73 agents, and a theoretical risk of cavitation formation without 74 ultrasound contrast agents present [7]. For this reason, in 75 all our work, we attempt to work at MI≤0.2, ensuring no 76 destructive and currently uncontrollable inertial cavitation is 77 present and allowing for faster transition from lab to clinic. 78 The first step in being able to induce localised sonoporation 79 was to attempt to control the location of the microbubbles [8]. 80 A custom made experimental setup incorporating a 2.2 MHz 81 ultrasound transducer, a synthetic 200-µm capillary and a 82 high-speed camera was built on top of an inverted microscope. 83 Continuous-wave ultrasound with centre frequencies of 2-MHz 84 and 7-MHz with peak-to-peak acoustic pressures of 20 kPa 85 were used in conjunction with Definity R ultrasound contrast 86 agent. Upon insonation, the microbubbles were seen to attract 87 each other forming small spherical clusters of microbubbles. 88 These clusters of 1-2 thousand microbubbles would form 89 within several seconds, and were spaced 1 /4λ apart. As 90 the attracted microbubbles were oscillating in phase, when 91 continuing sonication, the clusters were attracted to each other, 92 forming even larger clusters (c.f. Fig. 1). When increasing 93 the acoustic pressure the clusters could be radiated towards 94 the membrane wall. These results indicated that if necessary, 95 microbubbles could be accumulated to specific regions and 96 radiated toward a vascular wall. 97 III. In-vitro WORK 98 In-vitro work has been the hallmark for evaluating cellular 99 effects for years. In our work, we attempted to understand why 100 specific acoustic conditions were optimal for increased drug 101 delivery.102

108

  Following this exciting discovery, several research groups, 109 both experimental and theoretical rushed to understand the 110 physical behaviours behind this increased uptake. It accepted 111 that the oscillatory behaviour of the microbubbles under 112 sonication forms small transient pores in the cell membrane, 113 yet the question of how still remained. 114 Hence, the next step was to attempt to understand the 115 physical mechanisms of microbubble-cell interaction using 116 optimised acoustic conditions. Whilst a vast majority of 117 high-speed imaging is performed at several million frames 118 per second, we attempted to record at slower frame rates 119 giving us longer imaging duration but with sufficient temporal 120 resolution to track microbubble motion. For this work we 121 used immortalised cervical cancer cells (HeLa cells). 1.6 × 122 10 6 cells were seeded into an OptiCell TM and incubated for 123 24 hours. Following incubation, low concentrations of DiD 124 lipophilic fluorescent probe (Vybrant TM Molecular Probes) 125 tagged lipid shelled microbubbles, Definity R , SonoVue R or 126 MicroMarker TM (Bracco) were injected into the OptiCell TM . 127 Concentrations of 1 bubble to 1 cell were aimed for. The 128 OptiCell TM was then placed in a water bath cell-side up, 129 ensuring the floating bubbles were in contact with the cells. 130 Two ultrasound frequencies were evaluated: 1.0 MHz and 131 6.6 MHz with duty cycles of 40% and 50%, respectively. 132

Fig. 2 . 1 ×

 21 Fig. 2. Illustration depicting five of the sonopration mechanisms.

179

  Mice were imaged and treated once weekly using Gemzar 180 (Eli Lilly and Company, Indianapolis, IN) the current 181 best chemotherapeutic for treating PDAC, or Gemzar in 182 combination with ultrasound and SonoVue R microbubbles. 183 Acoustic conditions remained the same as in the in-vitro work, 184 where bubbles were forced into the cells, i.e., 1-MHz, 40

Fig. 3 .

 3 Fig. 3. Bioluminesence imaging and tumour volume of mice with orthotopic pancreatic cancer treated with Gemzar alone or Gemzar with microbubbles and ultrasound.

  187

In clinical diagnostic ultrasound imaging, the Mechanical

Index (MI) gives an indication of the mechanical damage

possible due to inertial cavitation when an ultrasound contrast

agent is present. It is defined by:

ACKNOWLEDGEMENTS 279 This study has been supported by MedViz 280 (http://medviz.uib.no/), an interdisciplinary research cluster 281 from Haukeland University Hospital, University of Bergen, 282 and Christian Michelsen Research AS. The authors would like 283 to thank The Michelsen Centre for Industrial Measurement 284 Science and Technology for their support throughout this 285 project.

SonoVue R were injected intravenously every 3.5 minutes


Torstein Yddal

Odd Spiros Kotopoulis

Odd Helge Gilja

Sandy Cochran

Michiel Postema

Transparent glass-windowed ultrasound transducers

published or not. The documents may come

In our work here we compare four different designs 5 for transparent glass windowed ultrasound transducers. Each distance, performing chemical analysis, and more [START_REF] Amann | Laser 281 ranging: a critical review of usual techniques for distance measurement[END_REF], [START_REF] López-Higuera | Fiber optic sensors in structural health monitoring[END_REF].

44

These optically transparent transducers need to be able to 45 handle high isostatic pressures, as in oil and gas pipelines 46 the pressures may reach 1000 psi [START_REF] Miesner | Oil & gas pipelines in nontechnical language[END_REF]. The transducers need to 47 be able to withstand acidic corrosion, e.g. in harsh chemical 48 environments, and they need to be powerful enough to induce 49 inertial cavitation.

50

We evaluated several geometric designs to see the effect on 51 the acoustic output and evaluate the ability to handle isostatic 52 pressures via finite element modelling simulations.

53

II. MATERIALS AND METHODS

54

A. Design 55 Typical ultrasound transducers are fabricated from a single 56 piezoelectric disc to generate acoustic pressure waves, or use 57 a bolt-clamped langevin type design to generate mechanical 58 motion [START_REF] Adachi | Construction of Torsional-Vibration Systems 291 with a Hollow Cylindrical Bolt-Clamped Langevin-Type Transducer and 292 Their Application to Ultrasonic Plastic Welding[END_REF]. The transparent glass windowed ultrasound 59 transducers took advantages from both types of transducers 60 designs. A low frequency (300-kHz) piezoelectric ring was 61 used in place of the disc. A low frequency was used as it is 62 easier to generate cavitation at lower frequencies. The hole 63 in the middle of the ring was sealed, then a glass disc was 64 bonded to the surface of the piezoelectric ring. A glass disc 65 was used as it is low cost, has the best optical clarity compared 66 to plastics, has a higher yield strength and resists erosion 67 (chemical and cavitation induced) well. In addition the glass 68 could be replaced with stronger equivalent materials such as 69 sapphire.

70

Four designs variations were fabricated and evaluated (c.f. 71 Figure 1). A minimum of two of each design were made 72 to evaluate consistency between devices. Table I shows the 73 dimensions of these designs.

74

B. Fabrication 75

The transducers were fabricated using an established 76 process with minor variations [START_REF] Kotopoulis | Lithium niobate 295 ultrasound transducers for MRI-guided ultrasonic microsurgery[END_REF]. The piezoelectric material 77 of choice was Pz26 with silk screen printed silver electrodes 78 (Meggit, PLC, Christchurch, United Kingdom) due to its low 79 losses and high mechanical quality factor. Designs 1, 3 and 80 4 were fabricating by first aligning and bonding the glass 81 disc (H. Baumbach & Co Ltd, Suffolk, United Kingdom) to 82 the Pz26 ring using epoxy (EpoFix, Struers A/S, Ballerup, 83 Denmark) and left to dry for 24 hours under a 2-kg weight. The 84 central hole in the Pz26 ring was filled with modelling putty to 85 ensure no epoxy seeped into the optical window. Excess epoxy 86 Field scans were performed to visualise the acoustic 113 propagation pattern. The acoustic power was measured using 114 a radiation force balance and frequency characterizations were 115 performed to measure the bandwidth of the devices. 116 1) Field scan: The ultrasound field was inspected using 117 a closed loop, custom made 3D-scanning system using a 118 200-µm hydrophone. The z-axis was denoted as the acoustic 119 propagation direction, and the xand y-axis are the lateral 120 and elevation directions, respectively. For each transducer 3 121 different scans were performed. Two in the xy-plane, one in 122 the focus of the transducer, and one, < 1-mm away from the 123 front surface of the transducer. The third scan was performed 124 in the yz-plane including the entire near field of the transducer 125 in addition to the focus. 2) Frequency Characterization: The frequency response 127 of the transducers were characterized using a pulse-response 128 method.

129

The transducers were characterised between 100 kHz and 130 1 MHz with a frequency step size of 1 kHz. The bandwidth in 131 percentage was then calculated by

132 BW % = f c f 6dB+ -f 6dB- • 100%, (1)
where f c is the centre frequency, f 6 dB+ and f 6 dB-are the 133 upper and lower frequency cut-off points; -6 dB left and right 134 of f c , the frequency with the maximum sensitivity. The + and 135 sign indicate that the frequency is higher and lower than 136 the centre frequency, respectively. 3) Power and Pressure Characterization: The power output 138 of the transducer were measured using an absolute calibration 139 technique with a radiation force balance, based on the standard 140 IEC 61161 [START_REF]Ultrasonics -Power 299 measurement -Radiation force balances and performance requirements[END_REF]. The setup used was based on a "weigh-all" 141 approach, which uses an acoustic absorber (Aptflex F28, 142 Precision Acoustics Ltd, Dorset, United Kingdom) attached 143 to a precision scale. The acoustic power can be calulated by 144

P = c • F = c • m • g, ( 2
)
where P is the acoustic power, F is the acoustic force exerted 145 on the absorber, c is the speed of sound in water, m is the 146 weighed "acoustic mass", and g is the gravitational constant. 147

The powers were measured for multiple input electrical 148 powers, and were subsequently plotted against the output 149 voltage from the waveform generator.

150

The pressure was calculated by

p = F A = P c • A , ( 3
)
where p is the calculated pressure, and the area A of the 177

Figure 3 shows a typical response from the 3D field scan.

178

The field scans showed that the acoustic propagation pattern The location of these bonding-defects can clearly be seen in 188 Fig. 3B where there are several breaks in the high pressure 189 areas (red) in the concentric rings. A single high pressure 190 point can be seen at the centre of the glass disc/PZT ring. This 191 may indicate that the glass behaves as some sort of oscillating 192 diaphragm, a desired effect. This would ensure pressure peak 193 at the centre of the ultrasound transducer on the front face 194 increasing the amount of cavitation based cleaning in the 195 desired location. The acoustic focus was perfectly spherical 196 with a pseudo-Gaussian pressure distribution (c.f. Fig. 3C). 197 Only minor side lobes were visible.

198

Table II summarises the acoustic outputs of the four designs. 199 The two larger devices had a lower centre frequencies of 200 358 kHz and 465 kHz with mean bandwidths of 74%. The 201 power efficiency of the devices was at 13%, with a theoretical 202 maximum of 50%. The location of the acoustic focus, and 203 size, between the two devices was significantly different, 204 with Design 1 having a long, distant focus, and Design 2 205 having a short focus, extremely close to the front face of the 206 device. Designs 3 and 4 were much more similar with only 207 15 kHz difference in centre frequency, a bandwidth of 36% 208 and efficiencies of 42%. The significant difference between 209 these two designs was the size of the acoustic focus. Design 210 3 has a focal length of 153 mm; 50% larger than Design 2. 211 The low power output may be due to the mismatch of acoustic 212 impedances between PZT, glass, and water. 5) Future Perspectives: A primary issue during fabrication 243 was ensuring a perfect bond. Air pockets (c.f. Fig. 2) can get 244 trapped between the glass and PZT. This adhering process 245 needs to be optimised to extract all possible air pockets. 246 Placing the device in a vacuum may help extract the trapped 247 air.

248

The glass used did not have an optimised thickness, hence 249 destructive interference may be occurring in the glass. Using 250 quarter wavelength thick glass, or odd multiple of would 251 greatly improve the efficacy of the devices. In addition 252 electrical impedance matching would greatly help reduce 253 electrical reflections.

254

A major issue with the current design was thermal 255 degradation. When running continuously in excess of 100 W 256 input power, the PZT would overheat and de-polarise. Adding 257 active cooling would resolve such issues.

258

As both simulations show promise, in future, such designs 259 will be simulated and virtually optimised to match all the 260 necessary criteria prior to fabrication.

69

  design was characterised using field scanning, radiation force 7 measurements, frequency sensitivity measurement and FEM 8 simulations. Field scans showed that small variations in design can greatly 10 affect the size and location of the acoustic focus. The results 11 coincided with those seen in the simulations. Radiation force 12 measurements showed that the devices were able to easily exceed 13 acoustic powers of 10 W, with efficiencies of up to 40%. Isostatic 14 simulations shows that the design also affects the physical 15 strength of the devices. Current designs were able to withstand 16 between 300 and 700 psi on the front surface.

  17

  The devices were cost effective due to the minimal amount 18 of materials necessary and the simple fabrication process. More work needs to be done to improve the power output and stress 20 handling capabilities. 21 I. INTRODUCTION 22 Optical measurements are a cornerstone of today's 23 technology, they are widely used for measuring speed and 24

25

  Optical equipment draws great advantages from modern 26 micro-electronics knowledge, making it extremely compact.27Nevertheless, optical equipment tends to be very susceptible 28 to physical damage hence needs to be shielded and requires an 29 optically clear path, e.g., a glass window. Some environments 30 makes this difficult to obtain, for example measurements 31 in oil pipes where crude oil will attach to the optical 32 measurement window. Ultrasound is also used in several 33 chemical production techniques to homogenise emulsions, and 34 optical spectroscopy is required to evaluate the quality of the 35 emulsion. Currently such problems are solved by measuring 36 after emulsification and evaluating the quality of the finished 37 product [3]. 38 In our work here, we aim to solve such problems by creating 39 an optically transparent transducer. The cavitational effects of 40 the ultrasound can be used to clean the optical window or 41 homogenise emulsions, whilst the optically clear path allows a 42 vast range of optical based measurements, from spectroscopy, 43 to high-frame rate imaging cameras, or optical microscopy.

Fig. 1 .

 1 Fig. 1. Graphical representation showing the construction of (A.) a traditional transducer and (B. four different transparent transducer designs.)

  126

  137

Fig. 2 .

 2 Fig. 2. Photograph showing final construction of prototype glass windowed ultrasound transducer based on Design 1. Red stars indicate bonding defects.

(Figure 2 Design 1 .

 21 Figure 2 shows an example of a prototype transparent glass

179Fig. 3 .

 3 Fig. 3. 2D scans of the acoustic field of the transducer with Design 1.

  213

Figure 4

 4 Figure4shows the acoustic power output as a function 214 of waveform generator voltage. Design 3 and 4 shows a 215 significantly higher output power when compared to Designs 216

Fig. 4 .

 4 Fig. 4. Acoustic power output of various designs when driven at their resonance frequencies measured via a radiation force balance.

  242

  261IV. CONCLUSION 262 Fabricating high-power transparent windowed ultrasound 263 transducers is feasible at a low cost. Early prototypes have 264 shown that the design can drastically vary the electric and 265 acoustic profile of the devices. Simulations shows that it is 266 possible to predict the acoustic profile and that the designs 267 have potential for use in high-pressure environments. Such 268 devices may have many applications in the field of both 269 research and industry. Nevertheless, these devices are only 270 prototypes and there is vast room for improvement.

TABLE I TRANSPARENT

 I Table I shows the dimensions of the GLASS WINDOWED ULTRASOUND TRANSDUCERS DIMENSIONS. ALL UNITS ARE IN MM. (KEY: ID -INNER DIAMETER, OD -OUTER DIAMETER, TH -THICKNESS, H -HEIGHT)

	Design	ID	PZT OD	TH	Glass OD TH	ID	Case OD	H
	1	20	40	5	40	4	44.5	51	50
	2	20	40	5	50	2	44.5	51	50
	3	10	25	4	25	3	31.5	38	50
	4	10	25	4	30	3	31.5	38	50
	C. Characterization						112

110 different designs.

111

TABLE II SUMMARY

 II OF OUTPUT PROPERTIES FROM DIFFERENT GLASS WINDOWED ULTRASOUND TRANSDUCERS.

	Design	fc (kHz)	BW (%)	Power efficiency (%)	Pressure output per W (kPa/W)	Focal distance (mm)	Focal Length (mm)
	1	465	71	13	92	81	111
	2	358	77	12	143	11	26
	3	615	36	40	312	34	153
	4	630	37	43	478	39	100

ACKNOWLEDGEMENTS 272 This study has been supported by MedViz 273 (http://medviz.uib.no/), an interdisciplinary research cluster 274 from Haukeland University Hospital, University of Bergen, 275 and Christian Michelsen Research AS. The authors would like 276 to thank The Michelsen Centre for Industrial Measurement 277 Science and Technology for their support throughout this 278 project.

as Design 2 failed at 300 psi. This is due to the thinness In oil and gas pipelines, the pressure may vary between 300


Kristoffer Johansen

Torstein Yddal

Spiros Kotopoulis
email: spiros.kotopoulis@uib.no

Michiel Postema

Acoustic filtering of particles in a flow regime

come

I. INTRODUCTION

Hydroelectric power plants are a highly efficient source of clean energy, to date providing more than 20% of the World's total electric power, and up to 99% in Norway [START_REF] Wagner | Introduction to Hydro Energy Systems: Basics, Technology and Operation[END_REF].

Nevertheless, all power plants are plagued with a common problem: silt. Silt consists of fine particles of sand or clay carried by running water from natural sources. The damage done by these particles is twofold, 1) high-velocity impact creates micron-sized pits on turbine blades, and 2) these particles also act as nuclei for cavitation [START_REF] Wagner | Introduction to Hydro Energy Systems: Basics, Technology and Operation[END_REF]. Such damage to the blades result in reduced efficiency [START_REF] Padhy | A review on silt erosion in hydro turbines[END_REF] or even complete turbine failure requiring shut down and/or replacement [START_REF] Singh | Silt Damage Control Measures for Underwater Parts -Nathpa Jhakri Hydro Power Station Case Study of a Success Story[END_REF]. Current solutions employ filters and meshes resulting in reduced water flow and in consequence reduced power output. Unfortunately, such filters cannot filter particles in the micron and sub-micron range. Removing silt from water, would increase the long-term efficiency of hydroelectric power plants, reduce the maintenance costs, and increase the productivity of renewable energy.

Ultrasound is known for its benefits in non-destructive testing and clinical imaging. Modern day research has been taking advantage of ultrasound to manipulate micron-sized particles [START_REF] Bernassau | Interactive manipulation of microparticles in an octagonal sonotweezer[END_REF]. Our previous work showed that it was possible to cluster and push micron-sized particles towards a boundary using low power ultrasound (<1W) [START_REF] Kotopoulis | Microfoam formation in a capillary[END_REF]. Whilst these experiments were performed in micron-sized capillaries and at low flow rates, in this study we attempt to take advantage of this phenomenon, and up-scale it, to force particles to separate from the water into a separate "waste" stream. The benefits of such a methodology is that the resultant product could be a bolt-on solution to existing power plants requiring little to no modification of the existing configuration. In our work here, we perform simulations and experiments to evaluate the efficacy of using ultrasound at low frequencies to radiate particles in flowing regime.

II. MATERIALS AND METHODS

A. Simulations

Two types of simulations were performed; 1D acoustic radiation force simulations and fluid flow simulations. Knowing the mean primary radiation force working on a spherical incompressible particle in a viscous fluid, assuming a plane progressive wave incident. A second order differential equation was derived to compute the distance a particle can be pushed towards the waste gate as it passes through the acoustic field. We study the case λ δ. Where λ is the wavelength, and δ is the viscous depth of penetration. Equation 1 was used to compute the radiation distance [START_REF] Settnes | Forces acting on a small particle in an acoustical field in a viscous fluid[END_REF]:

m ∂ 2 x ∂t 2 = πka 3 f 2 k 0 P 2 a F rad - FStokes 6πµa ∂x ∂t , (1)
where m is the effective mass of the particle, k is the wave number, a is the radius of the particle, f 2 is the dipole scattering coefficient, k 0 is the compressibility of the particle, P a is the amplitude of the acoustic pressure, µ is the dynamic viscosity of water, F rad is the mean acoustic radiation force, and F Stokes is the Stokes drag force acting on the particle. Equation 1 was solved for x, the distance travelled in direction of the acoustic field. Computations were done in MATLAB R2013a (The MathWorks, Inc., Natick, MA).

Fluid flow simulations were performed in order to evaluate the turbulence of the flow chamber and the amount of fluid that flow into the waste port (Output I) in comparison the primary port (Output II). Simulations were performed in Solidworks Professional 2014 using FlowXpress (Dassault Systèmes SolidWorks Corporation, Waltham, MA). Simulations were performed using water as the flow media at a flow-rate of 9 L/min. This flow-rate was chosen as it matched the maximum flow-rate possible from the experimental setup.

B. Experimental setup

Figure 1 shows a schematic illustration of the experimental setup and the flow chamber. Tap water was contaminated using natural silt filtered through a 200-µm metal mesh at a ratio of 2 g/L. A 60 L bucket was filled with 50 L of particle contaminated water. A Wolfcraft 2207 Super Pump (Wolfcraft, Kempenich, Germany) driven by a RS 14.4V Li-ion drill (RS Components Ltd, Corby, United Kingdom) was used to pump water from the contaminated tank through the flow chamber via a 1" pipe. One-inch diameter, 3-55 L/min variable-area, in-line, waste-water, flow meters (Universal Flow Monitors, Inc., Hazel Park, MI) were placed at the input and output channels of the flow chamber to characterise the flow rates during operation. The processed water from Output I and II were collected in separate containers.

A custom made flow chamber was designed in Solidworks Professional 2014 and 3D printed using a Projet 1500 (3D systems, Rock Hill, SC). Figure 1B shows a graphical rendering of the flow chamber. The main channel (Input I -Output II) had an inner diameter of 1" and a total length of 130 mm. This diameter was chosen to match the piping and flow meter size. Having equal diameters ensurers maximum accuracy of the flow meters and helps reduce turbulence. The ultrasound transducer was positioned 58 mm down-flow from the Input port. Output I had an internal diameter of 15 mm and was position 1" down-flow from the ultrasound transducer at a 145 • angle respective to the Input port.

C. Ultrasound generation & characterisation

A custom-made ultrasound transducer was made base on 25 mm, 10 mm thick PZ26 piezoelectic elements (Meggitt A/S, Kvistgård, Denmark). The transducer was fabricated using a previously established technique [START_REF] Kotopoulis | Lithium niobate ultrasound transducers for MRI-guided ultrasonic microsurgery[END_REF]. The centre frequency of the transducer was measured via impedance measurements at 195 kHz. The transducer was electrically impedance-matched to 50 Ω using a 9-to-1 transmission line transformer to reduce the real resistance.

The ultrasound transducer was driven by 195 kHz continuous sine wave, generated by a 33511B waveform generator (Agilent Technologies, Santa Clara, CA,) and amplified by a 2200L linear RF amplifier (Precision Acoustics, Dorchester, United Kingdom).

The power output of the transducer were measured using an absolute calibration technique with a radiation force balance, based on the standard IEC 61161 [START_REF]Radiation force balances and performance requirements (IEC 61161)[END_REF].

D. Particle concentration measurements

Particle concentration was measured acquiring a minimum of 3 samples from each water reservoir per experimental condition. Samples were stored in 10 mL sealed test tubes for evaluation. From each sample, a minimum of 5 sub-samples were analysed. A 1-mL droplet from each sub-sample was placed on an optical test slide with a 200-µm spacer, and covered with a glass cover slip, generating a precise volume. Sample volumes were imaged using an inverted microscope with a 5×/0.4-NA objective lens (Nikon Corporation, Tokyo, Japan) and a DCC1645C CMOS camera (Thorlabs, Inc., Newton, NJ). This imaging configuration had a maximum resolution of 0.8 µm. The number of particles in the imaging volume was counted using ImageJ. The images were processed by using a rolling ball background subtraction, thresholding, and counting particles. Identical threshold and counting parameters were used for all measurements.

E. Experimental procedure

A total of 9 L of contaminated water was pumped through the flow chamber. To prevent dry running and damage to the ultrasound transducer it was turned on several seconds after water flowed through the flow chamber. The processed water was then sampled and then re-mixed and homogenised and used as the input source. This helped prevent large changes in input particle concentrations.

F. Statistics

Statistics were performed using Prism v5.0a (GraphPad Software, Inc., La Jolla, CA). An unpaired Student's t-test was used to compare particle concentration differences. The results were considered statistically significant when P≤0.05. In all results values indicate mean±SEM.

III. RESULTS AND DISCUSSION

A. Simulations

Fluid flow simulations showed a relatively laminar flow through the primary channel with peak velocities of 0.3 m/s (c.f., Fig 2). The peak velocity into Output II was very low (<0.05 m/s) indicating that the majority of the flow would go through the primary channel. This was a desired result, as we did not want to reduce the velocity of the fluid flow for energy production. Minimal flow was seen also into the transducer housing indicating that there may not be complete water contact with the front face of the ultrasound transducer, resulting in reduced power output. These side chambers (waste channel and transducer housing) induce flow resulting in turbulence. Whilst in this case, it is minimal, the design should be optimised to reduce the turbulence and ensure the transducer is in continuous contact with water.

Figure 3 shows radiation distance, in the acoustic propagation direction, as a function of time and acoustic pressure. Simulations indicated that it was possible to radiate a 200-µm particle over total distance of 2 cm within 80 ms, when using an acoustic pressure of 12 MPa. In general, simulations showed an exponential-like increase in radiation distance with acoustic pressure duration of stay in the acoustic field.

B. Acoustics

Figure 4 shows the acoustic power output as a function of waveform generator voltage. The custom-made ultrasound transducer was capable of producing up to 8W of acoustic power in a stable regime. At higher powers, a large deviation between measurements was observed, indicating that the transducer was unstable, either due to cavitation, or due to reaching the maximum excursion possible. For this reason, the results above 8W may have a large deviation and higher uncertainty. Better electrical and acoustic impedance matching may improve the output power. From these resultant powers, estimations to the acoustic pressure were performed. These were used to estimate the radiation force on the particles.

C. Particle concentration

At maximum flow rate, the flow meters indicated a rate of ≈9 L/min, 3 L/min and 6 L/min for the Input, Output I and Output II, respectively. No change in flow was seen when the ultrasound was turned on. Figure 5 shows sample microscope images from the input and output channels. A total of 434 samples were measured. The silt particles can be clearly distinguished as the darker spots within the image. In these sample images we can see that Output II had a three fold lower particle concentration when compared to the Input. Output I had a higher concentration as the flow rate into this channel was approximately half of Output II.

The lower photographs show the water vesicles used. A large amount of silt has sunk to the bottom in the input channel. This was a drawback, as the particle concentration would be constantly varying during water treatment. When comparing Output I and Output II, the amount of sunken particles in Output I is substantially higher, as desired.

Figure 6 shows the effect of increasing the acoustic power on particle concentration. The horizontal bars indicate the mean particle concentrations when no ultrasound was present. The thickness of the bars indicated the SEM error in measurement. The control experiments (no ultrasound) show that Output II had a higher particle concentration than Output I, indicating that this was the preferred path, consistent with the fluid flow simulations.

Upon applying ultrasound an immediate decrease in particle concentration can be seen in Output II and an increase in Output I. The particle concentration continues to decrease as the acoustic power increases. Output II becomes statistically significantly cleaner at 2.6 W and continues to 6.4 W. From 8 W onwards, the transducer was considered unstable, and this can be seen in the filtration results. At 8.9 and 11.9 W the SEM is much larger. Nevertheless, at 11.9 W, a statistical significant increase in particle concentration is seen in Output I.

D. Future Perspectives

Whilst these preliminary results have shown it is possible to filter particles from water in flow using ultrasound, there are several areas of improvement that should be addressed in order to increase the efficacy of the system. The flow chamber should be redesigned to decrease fluid flow turbulence. Having laminar flow would ensure good coverage of the acoustic propagation face, and would reduce potential particle back-flow into the primary channel. Positioning the transducer closer to the primary channel would also align the acoustic focus, the area of highest acoustic pressure, with the centre of the flow chamber, hopefully forcing more particle into the waste port. Fabricating a more stable acoustic source, capable of higher pressures would also improve the acoustic filtering.

In the work presented here, we used continuous-wave ultrasound, this may generate standing waves with nodes and anti-nodes in the wrong locations. Using pulsed ultrasound would prevent standing waves, but would reduce the radiation time. The ideal acoustic conditions need to be evaluated.

In these experiments, particles <200µm were used. Other particle size ranges and concentrations should be evaluated.

IV. CONCLUSION Our setup has shown that filtering micron-sized particles using low frequency ultrasound is possible. Nevertheless, our configuration is far from optimal, resulting in low efficiencies. Major challenges are the overabundance of particles, the turbulent flow in the chamber, the high velocity of the water, and the unstable acoustic output at high powers. Improving these factors should greatly improve the efficacy of this system.

Fig. 1 .

 1 Fig. 1. Schematic illustration depicting experimental configuration used evaluate acoustic filtering of particles in a flow regime (A). Graphical rendering of flow chamber depicting position of ultrasound transducer and input and output channels (B).

Fig. 2 .

 2 Fig. 2. FloXpress simulation of fluid flow at 9 L per min. Laminar flow can be seen at the center of the chamber. The transducer port and lateral exit port show very low flows.

Fig. 3 .

 3 Fig. 3. Results of mathematical simulation. Comparison of radiation distance as a function of acoustic pressure and duration in acoustic field.

Fig. 4 .

 4 Fig. 4. Power output as a function of peak-to-peak waveform generator (WFG) voltage of the 195-kHz ultrasound transducer measured using a radiation force balance.

Fig. 5 .

 5 Fig. 5. Micrographs (top row) and photographs (bottom row) of contaminated water before and after treament with ultrasound.

Fig. 6 .

 6 Fig. 6. Paticle concentration as a function acoustic power for flow chamber outputs. Horizontal bars indicate mean particle concentrations with no ultrasound. Stars indicate statistically significant differences.

ACKNOWLEDGEMENTS

This study has been supported by MedViz (http://medviz.uib.no/), an interdisciplinary research cluster from Haukeland University Hospital, University of Bergen, and Christian Michelsen Research AS. The authors would like to thank The Michelsen Centre for Industrial Measurement Science and Technology for their support throughout this project. We would also like to thank Bergenshalvens Kommunale Kraftselskap (BKK) for financially supporting this research.


Michiel Postema
email: michiel.postema@wits.ac.za

Ryunosuke Matsumoto

Ri-Ichiro Shimizu

Albert Thijs Poortinga

Nobuki Kudo

Ryonosuke Matsumoto

High-speed footage shows transient ultrasonic nucleation of different hydrophobic particles in suspension

Ultrasonic technology is commonly used for medical imaging, 1) but it is also used to manipulate, 2) deform, 3) and disrupt particles with microscopic sizes. 4) Straightforward applications of particle manipulation are in separation processes such as wastewater treatment 2) and battery recycling. 5) The opposite of separation, i.e., mixing, has also been done with the aide of ultrasound. It was recently found, that ultrasound can even rapidly disperse nanoparticles, a process which has been associated with inertial cavitation and acoustic streaming. 6) Hand-held ultrasonic devices mix small quantities whilst requiring the ultrasonic tip to be dipped in the liquid to be mixed, which may be undesirable. 7) Ultrasound cleaning baths, commonly used for cleaning metal objects, 8) have been proposed as convenient replacement of acoustic shakers for bulk mixing. 9) Hydrophobic particles inherently resist being suspended in water. Hydrophobic particles might be regarded as tiny solid particles surrounded by a thin gaseous shell. 10) It has been hypothesised that hydrophobic particles act as cavitation nuclei. 10) This cavitation behaviour would explain the translation speeds observed when hydrophobic polystyrene microspheres were driven through a liquid medium by means of ultrasound. 11) These translation speeds corresponded to those observed with gas microbubbles of similar sizes. 11) If hydrophobic particles do have a thin gaseous layer surrounding the solid cores, a sound field of sufficient pressure amplitude must force the gas layer to form and inertial cavity which must subsequently fragment during the collapse phase. Such behaviour has been observed form other cavitation nuclei, using high-speed photography. 4,12,13) The effect of an ultrasonic cleaner on suspensions with hydrophobic particles was recently tested on two materials. 14) The first, carbon black, is a common particle in lithium ion battery cathodes, but also in tattoo ink. Hydrophobic carbon black has been observed to generate a harmonic response to a low-amplitude, i.e., non-destructive, ultrasound field. 5) The second material, hydrophobically modified zinc oxide, builds the endeskeleon of acoustically active antibubbles. 15) Vials with both materials before and after 1-minute sonication in an ultrasonic bath operating at 45 kHz have been shown in Ref. 14. The particles of both materials were suspended optically homogeneously.

We hypothesised that hydrophobic particles act as cavitation nuclei that lose (part of) their gaseous layer during sonication. In this study, we investigated this hypothesis using high-speed photography. As opposed to Ref. 14, here we present full recordings for events using both carbon black and zinc oxide. In addition, multiple bursts were subjected to the same sample, to investigate if the remainder materials still act as cavitation nuclei. Two-materials were studied using identical protocols. The first material studied was TIM-CAL SUPER C65 Carbon Black (EQ-Lib-SuperC65) conductive additive (MTI Corportation, Richmond, CA, USA), hereafter referred to as C65. The second material studied was hydrophobically modified Zano 10 Plus zinc oxide (Umicore, Brussel, Belgium), hereafter referred to as ZnO. For each material, samples of 1 mg were deposited into a FALCON ® 15 mL High-Clarity Polypropylene Conical Tube (Corning Science México S.A. de C.V., Reynosa, Tamaulipas, Mexico), after which 5 mL of 049-16787 Distilled Water (FUJIFILM Wako Pure Chemical Corporation, Chuo-Ku, Osaka, Japan) was added.

Tubes were either or shaken by hand for 5 minutes, or individually held by hand for 1 minute in a 2510J-MT BRANSONIC ® ULTRASONIC CLEANER (BRANSON ULTRA-SONICS CORPORATION, Danbury, CT, USA) filled with degassed water and operating at a transmitting frequency of 45 kHz. From each tube, 200 µL was pipetted into the observation chamber of a high-speed observation system. 16) The observation chamber was placed under an Eclipse Ti inverted microscope (Nikon Corporation, Minato-ku, Tokyo, Japan) with a Plan Apo LWD 40× WI (N.A. 1.15) objective lens. Attached to the microscope was an HPV-X2 2/9 high-speed camera (Shimadzu, Nakagyo-ku, Kyoto, Japan), operating at 2 or 10 million frames per second. 17) Exposure times corresponded to 500 ns and 100 ns per frame, respectively.

During camera recording, the materials were subjected to ultrasound pulses.

The pulses consisted of either 3 or 100 cycles, each at a centre transmitting frequency of 1 MHz and a peak-negative pressure of 200 kPa, from a laboratory-assembled single-element transducer. 16,17) The time between subsequent pulses was at least 1 minute. The signal fed into the transducer was generated by an AFG320 arbitrary function generator (Sony-Tektronix, Shinagawa-ku, Tokyo, Japan) and amplified by a UOD-WB-1000 wide-band power amplifier (TOKIN Corporation, Shiroishi, Miyagi, Japan).

Figure 1 shows the first 64 frames of a representative example from 56 high-speed video recordings of C65 and ZnO, each consisting of 256 frames. The high-speed video in Figure 1 was recorded at 10 million frames per second, during a 3-cycle burst. The frames shows a manually suspended hydrophobic C65 particle before, during, and after 1-MHz sonication.

During sonication, an inertial cavity can be clearly seen. After sonication, the cavity is seen to be detached and located to the upper right of the deformed or fragmented particle.

In later frames (not shown), the cavity is seen to have dissolved, leaving the deformed or fragmented C65 particle. During subsequent ultrasound pulses, the C65 particle remained unaffected (not shown). For all experiments with manually suspended C65, during the first pulse, the hydrophobic particles were observed to grow into inertial cavities, forcing the gas layer surrounding the particle to be released and dissolved. After the first pulse, no further cavitation activity was observed. This indicates that sonication can remove the thin gas layer surrounding a hydrophobic particle. This is shown in Figure 2, which consists of frames from three subsequent 100-cycle pulses. The high-speed video in Figure 2 was recorded at 2 million frames per second. The frames were selected from the full video, during expansion maxima. Only during the first burst, C65 particles are seen to act as cavitation nuclei. The second burst shows some cavitation activity below the focal plane (second-last frame). No cavitation activity can be observed during the third burst. Only the manually suspended C65 particles show cavitation activity, whereas not a single recording of C65 suspended with the ultrasonic cleaner shows cavitation activity or particle translation. It appears that a single burst of low-amplitude ultrasound is enough to change the hydrophobic nature of the C65 particles.

Figure 3 shows ultrasonically suspended ZnO particles in frames from three subsequent 100-cycle pulses. The high-speed video in Figure 3 was recorded with the same settings as Figure 2. Here, as opposed to C65, the particles keep on generating inertial cavitation bubbles during subsequent pulses. These are seen to have moved in the direction of the sound field. ZnO does not lose its acoustic activity after sonication at low amplitudes.

Forced suspension of hydrophobic particles would be of major interest in ultrasoundguided drug delivery. The ultimate goal might be ultrasound-guided anticancer drugs. 18) If therapeutic compounds could be made hydrophobic, they would not interact with their (fluid) surroundings after injection. Only after turning them hydrophilic, with the aide of ultrasound, would they dissolve and act. Another option might be to let hydrophobic particles interact with cells in a similar way as microbubbles can be forced to interact with cells. 19,20) Hydrophobic particles of the materials carbon black and zinc oxide can be forced to be suspended in water using ultrasound. The high-speed observations shown in this paper confirm that hydrophobic particles can act as cavitation nuclei. The lack of cavitation from carbon black after the first pulse indicates that the gas layer surrounding the hydrophobic particle dissolves after inertial cavitation. Zinc oxide remains acoustically active after sonication. It might be a suitable candidate for an ultrasound contrast agent.

High-speed footage shows transient ultrasonic nucleation of different hydrophobic particles in suspension

Michiel Postema

Netherlands

In this study, we investigated if hydrophobic particles of the materials carbon black and zinc oxide act as cavitation nuclei, using high-speed photography during 1-MHz, low-amplitude sonication. Carbon black particles were observed to lose their gaseous shells and become acoustically inactive, whilst zinc oxide particles were observed to generate bubbles without becoming acoustically inactive. Zinc oxide might be a suitable candidate for an ultrasound contrast agent. Carbon black might be a suitable candidate for ultrasound-guided drug delivery.

Ultrasonic technology is commonly used for medical imaging, 1) but it is also used to manipulate, 2) deform, 3) and disrupt particles with microscopic sizes. 4) Straightforward applications of particle manipulation are in separation processes such as wastewater treatment 2) and battery recycling. 5) The opposite of separation, i.e., mixing, has also been done with the aide of ultrasound. It was recently found, that ultrasound can even rapidly disperse nanoparticles, a process which has been associated with inertial cavitation and acoustic streaming. 6) Hand-held ultrasonic devices mix small quantities whilst requiring the ultrasonic tip to be dipped in the liquid to be mixed, which may be undesirable. 7) Ultrasound cleaning baths, commonly used for cleaning metal objects, 8) have been proposed as convenient replacement of acoustic shakers for bulk mixing. 9) Hydrophobic particles inherently resist being suspended in water. Hydrophobic particles might be regarded as tiny solid particles surrounded by a thin gaseous shell. 10) It has been hypothesised that hydrophobic particles act as cavitation nuclei. 10) This cavitation behaviour would explain the translation speeds observed when hydrophobic polystyrene microspheres * E-mail: michiel.postema@wits.ac.za 1/9

Page 10 of 18 AUTHOR SUBMITTED MANUSCRIPT -JJAP-S1101543.R1

were driven through a liquid medium by means of ultrasound. 11) These translation speeds corresponded to those observed with gas microbubbles of similar sizes. 11) If hydrophobic particles do have a thin gaseous layer surrounding the solid cores, a sound field of sufficient pressure amplitude must force the gas layer to form and inertial cavity which must subsequently fragment during the collapse phase. Such behaviour has been observed form other cavitation nuclei, using high-speed photography. 4,12,13) The effect of an ultrasonic cleaner on suspensions with hydrophobic particles was recently tested on two materials. 14) The first, carbon black, is a common particle in lithium ion battery cathodes, but also in tattoo ink. Hydrophobic carbon black has been observed to generate a harmonic response to a low-amplitude, i.e., non-destructive, ultrasound field. 5) The second material, hydrophobically modified zinc oxide, builds the endeskeleon of acoustically active antibubbles. 15) Vials with both materials before and after 1-minute sonication in an ultrasonic bath operating at 45 kHz have been shown in Ref. 14. The particles of both materials were suspended optically homogeneously.

We hypothesised that hydrophobic particles act as cavitation nuclei that lose (part of) their gaseous layer during sonication. In this study, we investigated this hypothesis using high-speed photography. As opposed to Ref. 14, here we present full recordings for events using both carbon black and zinc oxide. In addition, multiple bursts were subjected to the same sample, to investigate if the remainder materials still act as cavitation nuclei. Two-materials were studied using identical protocols. The first material studied was TIM-CAL SUPER C65 Carbon Black (EQ-Lib-SuperC65) conductive additive (MTI Corportation, Richmond, CA, USA), hereafter referred to as C65. The second material studied was hydrophobically modified Zano 10 Plus zinc oxide (Umicore, Brussel, Belgium), hereafter referred to as ZnO. For each material, samples of 1 mg were deposited into a FALCON ® 15 mL High-Clarity Polypropylene Conical Tube (Corning Science México S.A. de C.V., Reynosa, Tamaulipas, Mexico), after which 5 mL of 049-16787 Distilled Water (FUJIFILM Wako Pure Chemical Corporation, Chuo-Ku, Osaka, Japan) was added.

Tubes were either or shaken by hand for 5 minutes, or individually held by hand for 1 minute in a 2510J-MT BRANSONIC ® ULTRASONIC CLEANER (BRANSON ULTRA-SONICS CORPORATION, Danbury, CT, USA) filled with degassed water and operating at a transmitting frequency of 45 kHz. From each tube, 200 µL was pipetted into the observation chamber of a high-speed observation system. 16) The observation chamber was placed under an Eclipse Ti inverted microscope (Nikon Corporation, Minato-ku, Tokyo, Japan) with a Plan Apo LWD 40× WI (N.A. 1.15) objective lens. Attached to the microscope was an HPV-X2 2/9 high-speed camera (Shimadzu, Nakagyo-ku, Kyoto, Japan), operating at 2 or 10 million frames per second. 17) Exposure times corresponded to 500 ns and 100 ns per frame, respectively.

During camera recording, the materials were subjected to ultrasound pulses.

The pulses consisted of either 3 or 100 cycles, each at a centre transmitting frequency of 1 MHz and a peak-negative pressure of 200 kPa, from a laboratory-assembled single-element transducer. 16,17) The time between subsequent pulses was at least 1 minute. The signal fed into the transducer was generated by an AFG320 arbitrary function generator (Sony-Tektronix, Shinagawa-ku, Tokyo, Japan) and amplified by a UOD-WB-1000 wide-band power amplifier (TOKIN Corporation, Shiroishi, Miyagi, Japan).

Figure 1 shows the first 64 frames of a representative example from 56 high-speed video recordings of C65 and ZnO, each consisting of 256 frames. The high-speed video in Figure 1 was recorded at 10 million frames per second, during a 3-cycle burst. The frames shows a manually suspended hydrophobic C65 particle before, during, and after 1-MHz sonication.

During sonication, an inertial cavity can be clearly seen. After sonication, the cavity is seen to be detached and located to the upper right of the deformed or fragmented particle.

In later frames (not shown), the cavity is seen to have dissolved, leaving the deformed or fragmented C65 particle. During subsequent ultrasound pulses, the C65 particle remained unaffected (not shown). For all experiments with manually suspended C65, during the first pulse, the hydrophobic particles were observed to grow into inertial cavities, forcing the gas layer surrounding the particle to be released and dissolved. After the first pulse, no further cavitation activity was observed. This indicates that sonication can remove the thin gas layer surrounding a hydrophobic particle. This is shown in Figure 2, which consists of frames from three subsequent 100-cycle pulses. The high-speed video in Figure 2 was recorded at 2 million frames per second. The frames were selected from the full video, during expansion maxima. Only during the first burst, C65 particles are seen to act as cavitation nuclei. The second burst shows some cavitation activity below the focal plane (second-last frame). No cavitation activity can be observed during the third burst. Only the manually suspended C65 particles show cavitation activity, whereas not a single recording of C65 suspended with the ultrasonic cleaner shows cavitation activity or particle translation. It appears that a single burst of low-amplitude ultrasound is enough to change the hydrophobic nature of the C65 particles.

Figure 3 shows ultrasonically suspended ZnO particles in frames from three subsequent 100-cycle pulses. The high-speed video in Figure 3 was recorded with the same settings as Figure 2. Here, as opposed to C65, the particles keep on generating inertial cavitation bubbles during subsequent pulses. These are seen to have moved in the direction of the sound field. ZnO does not lose its acoustic activity after sonication at low amplitudes.

Forced suspension of hydrophobic particles would be of major interest in ultrasoundguided drug delivery. The ultimate goal might be ultrasound-guided anticancer drugs. 18) If therapeutic compounds could be made hydrophobic, they would not interact with their (fluid) surroundings after injection. Only after turning them hydrophilic, with the aide of ultrasound, would they dissolve and act. Another option might be to let hydrophobic particles interact with cells in a similar way as microbubbles can be forced to interact with cells. 19,20) Hydrophobic particles of the materials carbon black and zinc oxide can be forced to be suspended in water using ultrasound. The high-speed observations shown in this paper confirm that hydrophobic particles can act as cavitation nuclei. The lack of cavitation from carbon black after the first pulse indicates that the gas layer surrounding the hydrophobic particle dissolves after inertial cavitation. Zinc oxide remains acoustically active after sonication. It might be a suitable candidate for an ultrasound contrast agent.

3 / 9 Fig. 1 .

 391 Fig. 1. C65 particle during a 3-cycle burst. Frames are ordered top-down, left-right. Time between frames is 100 ns. Frame width is 145 µm.

4 / 9 Fig. 2 .

 492 Fig. 2. C65 during three separate 100-cycle bursts (left-middle-right column). Frames are ordered top-down, left-right. Time between frames is 16 µs. Frame width is 145 µm.

5 / 9 Fig. 3 .

 593 Fig. 3. ZnO during three separate 100-cycle bursts (left-middle-right column). Frames are ordered top-down, left-right. Time between frames is 16 µs. Frame width is 145 µm.

3 / 9 Fig. 1 .

 391 Fig. 1. C65 particle during a 3-cycle burst. Frames are ordered top-down, left-right. Time between frames is 100 ns. Frame width is 145 µm.

4 / 9 Fig. 2 .

 492 Fig. 2. C65 during three separate 100-cycle bursts (left-middle-right column). Frames are ordered top-down, left-right. Time between frames is 16 µs. Frame width is 145 µm.

5 / 9 Fig. 3 .

 593 Fig. 3. ZnO during three separate 100-cycle bursts (left-middle-right column). Frames are ordered top-down, left-right. Time between frames is 16 µs. Frame width is 145 µm.

  1 * , Ryonosuke Matsumoto 2 , Ri-ichiro Shimizu 2 , Albert T. Poortinga 3 , and School of Electrical and Information Engineering, University of the Witwatersrand, Johannesburg, 1 Jan Smuts Laan, 2001 Braamfontein, South Africa 2 Faculty of Information Science and Technology, Hokkaido University, Kita 9 Chome, Nishi 14 Jo, Kita-ku, Department of Mechanical Engineering, Eindhoven University of Technology, De Zaale, 5600 MB Eindhoven,

	Nobuki Kudo 2
	Sapporo, Hokkaido 060-0814, Japan

1 3

Acknowledgement

This work was supported by JSPS KAKENHI Grant Number 17H00864.

7/9

Acknowledgement

This work was supported by JSPS KAKENHI Grant Number 17H00864.

7/9


Maria Mushtaq

David Novo

Florent Bruguier

Pascal Benoit

Muhammad Khurram Bhatti

Transit-Guard: An OS-based Defense Mechanism Against Transient Execution Attacks

come

Transit-Guard: An OS-based Defense Mechanism

Against Transient Execution Attacks Maria Mushtaq, David Novo, Florent Bruguier, Pascal Benoit LIRMM-CNRS, Univ. Montpellier Montpellier, France {maria.mushtaq, david.novo, florent.bruguier, pascal.benoit}@lirmm.fr Muhammad Khurram Bhatti Information Technology University (ITU)

Lahore, Pakistan khurram.bhatti@itu.edu.pk Abstract-Transient attacks manipulate speculative execution to alter the control flow path in an application program and modify microarchitectural state. These state changes are not captured by the existing Instruction Set Architectures (ISAs). In this paper, we propose a novel OS-level detection-based mitigation mechanism, called Transit-Guard, that uses machine learning and real-time behavioral data of concurrent processes to detect and subsequently mitigate these attacks at run-time.

Index Terms-Secure Systems, Microarchitecture, Transient execution, Spectre, Meltdown, Mitigation, OS, Machine learning.

I. THE TRANSIT-GUARD MECHANISM

Side-and covert-channel information leakage is a serious threat to modern computer architectures as they exploit microarchitectural features to extract privileged information. Recently, information retrieval attacks have started to target microarchitectural features beyond the memory sub-system. For instance, many of the performance-boosting techniques included in modern processor microarchitectures, such as outof-order and speculative execution, pipelining, and branchprediction [START_REF] Xiong | Survey of transient execution attacks[END_REF], have already been exploited. In our threat model, we assume a co-resident attacker process running in user space and targeting privileged address space without explicit access. We consider the OS does not offer any specific privilege level associated with the attacker process.

Transit-Guard works in two distinct stages. As illustrated in Figure 1, the detection module operates in the Linux user space, while the mitigation module operates in kernel space. Transit-Guard works at runtime, i.e., when the attack is actually happening. The Transit-Guard reuses the detection module proposed in [START_REF] Mushtaq | WHISPER: A tool for run-time detection of side-channel attacks[END_REF]. Both Spectre [START_REF] Kocher | Spectre attacks: Exploiting speculative execution[END_REF] and Meltdown [START_REF] Lipp | Meltdown: Reading kernel memory from user space[END_REF] attacks exploit transient execution to trigger the attack and later retrieve data from caches using covert channels. We have selected appropriate hardware/software performance counters (HPCs/SPCs) that are most affected by these stages of the attack. We have selected Total Branch Instructions, Total Branch Mispredictions, L3 total cache misses and total execution cycles as features for Spectre attack. For Meltdown attack, we have selected Total Page Faults, L3 Total Cache Accesses, L3 Cache Misses and Total Execution Cycles as features. As illustrated in Figure 1, the mitigation module is hosted in the Linux kernel space. Once the detection module reports an attack, mitigation module first evaluates whether the received PIDs from detection module are trusted processes or not. All system processes are considered trusted whereas all the user processes are considered untrusted by default. Transit-Guard does so because, at run-time, it is highly likely that the set of active processes also contain some Fig. 1: Design details of the Transit-Guard Mechanism. Linux's system processes, which are considered as trusted by default. It is, therefore, imperative for the mitigation module to evaluate malicious PIDs. Once it is established that the process under consideration is trusted, the module marks the process as benign/safe. All the untrusted processes are then evaluated further. Since the mechanism associates an instance of each performance counter with processes at the time of their creation, when a process exhibits malicious behaviour the related instance of counter reports abnormality to the detection module, which then acquires the PID of current process. These instances of counters are process-specific and point to the exact process that is exhibiting abnormal behavior at the time of detection. Thus, the mitigation module kills only the malicious process and does not incur any performance overhead other than its own execution time overhead. When a particular model reports a detection, the mitigation module checks the authenticity of the reported detection by waiting for at least 3 consecutive detection reports to further reduce the FPs.

II. EXPERIMENTS, RESULTS AND DISCUSSION

We have performed experiments on Linux Ubuntu LTS 16.04 Kernel version: 4.10.0-28-generic running on Intel's core i7 -4770 CPU at 3.40-GHz with 64KB L1, 256KB L2, 8192KB L3 and 8GB system memory. We have used Performance API (PAPI) [START_REF]Performance application programming interface[END_REF] and Perf [START_REF] Tool | [END_REF] libraries to access SPCs/HPCs.

A. Case Study 1: Detection and Mitigation of Spectre 1) Detection of Spectre: As illustrated in Table I, our results show that LDA, LR and SVM demonstrate 99.01%, 98.61% and 97.10% detection accuracy, respectively, under FL conditions. All ML models provide high detection accuracy for Spectre attack. We have collected the SPCs/HPCs at a (constant) high speed of 10µs. This sampling frequency is adjustable. Results in Table I illustrate that LDA provides 1.20% and 0.79%, LR provides 1.37% and 0.02% and SVM provides 2.87% and 0.03% of FPs and FNs, respectively. Results demonstrate that all models provide a negligible number of FPs and FNs, where most of the time results depict more FPs in ratio. The adaptability and scalability of detection module is highly dependent on performance overhead. Results in Table I illustrate that all ML models report a low performance overhead for detection, i.e., LDA, LR and SVM report only 1.6%, 1.5%, 1.7% overhead, respectively. 2) Mitigation of Spectre: Table II provides results on the measured time at different stages from detection to mitigation, i.e., detection time, PID collection, mitigation and the total time that Transit-Guard takes. The average time Spectre attack takes to execute as stand-alone process on an Intel's Core i7 machine is measured as 256µs. Results in Table II show that Transit-Guard takes 68µs as detection to mitigation time in order to detect an attack process under FL conditions. Once the attack is detected, PIDs of all detected processes are collected within 1.0µs. Once the PIDs of detected processes are collected, the information is relayed to the kernel module through Netlink socket to take mitigation decision, which takes another 18µs to kill the untrusted processes. Therefore, Transit-Guard takes 87µs in total under FL conditions to detect and subsequently mitigate Spectre attack. These results demonstrate that Transit-Guard is able to detect and mitigate Spectre attack in < 27% of attack completion. III shows that LDA, LR and SVM show 98.30%, 96%, 98.35% detection accuracy, respectively, under FL conditions at the sampling granularity of 10µs. Moreover, LDA offers 1.25% and 0.45%, LR offers 3.40% and 0.60% and SVM offers 1.39% and 0.26% of FPs and FNs, respectively. LDA, LR and SVM incur 1.91%, 2.21% and 2.00% overhead, respectively.

2) Mitigation of Meltdown: Table IV shows the detection to mitigation time taken by the Transit-Guard while mitigating Meltdown attack. It takes 70µs to detect the Meltdown attack, 1.0µs to collect PIDs of detected processes and 21µs to mitigate the untrusted processes. Transit-Guard, under FL condition, takes a total of 92µs to detect and mitigate the Meltdown attack. Our experiments show that Meltdown takes 305µs on average to execute the attack on Intel's Core i7 machine. Our results report that Transit-Guard is able to detect and subsequently mitigate Meltdown attack in < 30% of attack's completion time. Table I & III report that most of the performance overhead is coming from detection module (due to the collection of events and binary classification), whereas, the time to collect PIDs and mitigating untrusted process is minimal. Therefore, the overall overhead of Transit-Guard lies between 1.5 -2.2%.

III. CONCLUSION

We propose Transit-Guard, a novel OS-level run-time detection & mitigation mechanism, against transient execution attacks. The Transit-Guard uses multiple machine learning models and profiles concurrent processes using SPCs/HPCs at real-time. Experimental results demonstrate that Transit-Guard is capable of detecting and mitigating Spectre and Meltdown attacks while running under Linux OS. The Transit-Guard is light-weight and resilient to noisy system load conditions.

B. Case Study 2 :

 2 Detection and Mitigation of Meltdown 1) Detection of Meltdown: under FL condition, Table

TABLE I :

 I Results on the detection of Spectre attack

	Model Loads Accuracy	Speed	FP	FN	Overhead
			(%)	(µs)	(%)	(%)	(%)
		NL	99.95	10	0.05	0
	LDA	AL	99.08	10	0.57	0.35	1.6
		FL	98.01	10	1.20	0.79
		NL	99.99	10	0.01	0
	LR	AL	98.51	10	1.17	0.32	1.5
		FL	98.61	10	1.37	0.02
		NL	99.30	10	0.69	0.01
	SVM	AL	98.00	10	1.98	0.02	1.7
		FL	97.10	10	2.87	0.03

TABLE II :

 II Measured time at different stages for mitigation mechanism while detecting Spectre Attack

	Load	Detection	PID Collec-	Mitigation	Total Time
	Type	(µs)	tion (µs)	(µs)	(µs)
	NL	62	0.5	5	67.5
	AL	65	0.5	11	76.5
	FL	68	1.0	18	87

TABLE III :

 III Results on the detection of Meltdown attack

	Model Loads Accuracy	Speed	FP	FN	Overhead
			(%)	µs	(%)	(%)	(%)
		NL	99.99	10	0.01	0
	LDA	AL	99.91	10	0.09	0.00	1.91
		FL	98.30	10	1.25	0.45
		NL	99.41	10	0.59	0
	LR	AL	97.45	10	1.95	0.60	2.21
		FL	96.00	10	3.40	0.60
		NL	99.99	10	0.01	0
	SVM	AL	99.40	10	0.60	0.00	2.00
		FL	98.35	10	1.39	0.26

TABLE IV :

 IV Measured timing at different stages for mitigation mechanism while detecting Meltdown Attack

	Load	Detection	PID Collec-	Mitigation	Total Time
	Type	(µs)	tion (µs)	(µs)	(µs)
	NL	64	0.5	7	71.5
	AL	69	0.5	14	83.5
	FL	70	1.0	21	92