42nd AIAA Aerospace Sciences Meeting & Exhibit Reno, Nevada, 5-8 Jan 2004.

1 American Institute of Aeronautics and Astronautics

ON THE CHARACTERISTICS OF MICRO-FLUID JETS GENERATED BY HO:YAG PULSED LASER FOCUSING

Viren M*, Hashimoto T*, Ohki T**, Sun M#, Takayama K## Shock Wave Research Center, Institute of Fluid Science, Tohoku University, Sendai 980-8577,Japan.

Nakagawa A$ Department of Neurosurgery, Tohoku University Grad. School of Medicine, Sendai 980-8577, Japan.

And

Jagadeesh G$$ Department of Aerospace Engineering, Indian Institute of Science, Bangalore 560-012, India.

ABSTRACT

A micro-water jet device that can be used for precise dissection of soft tissue in surgical procedures has been developed. The water jet is generated by irradiating a pulsed Ho:YAG laser, through an optical fiber, into a thin tube/catheter filled with water or a physiological saline solution. A micro-metallic nozzle of 100~200µm exit diameter has been connected to the end of the tube. Pulse laser irradiation within the tube evaporates water generating a vapor bubble, whose rapid expansion drives a micro-water jet through the tube exit. This laser induced micro-water jet can readily penetrate into soft tissue. The jet is applicable to neuroendoscopic surgery, treatment of cerebral thrombosis, intravascular drug delivery etc. The Ho:YAG laser has wavelength and pulse duration of 2.1µm and 350µs respectively. The laser energy is variable from 250 to 800mJ/pulse. The thermal effect and controllability of the jet were first investigated on a 10% gelatin layer which simulates soft tissue, and then the jet was used to dissect the ventricular wall of a cadaver rabbit. The result indicated that the incision was well controlled, preserving capillary blood vessels thicker than 0.2 mm in diameter, and the collateral damage to the tissue was negligible. ______________________________________ *Postdoctoral Fellow, **Graduate Student, #Researcher, ##Professor, $Research Student, $$Assistant Professor.

INTRODUCTION

Micro-jets are of interest in a variety of engineering applications such as combustion or heat transfer systems, micro-thrusters for miniature satellites, divert and attitude reaction control systems in the case of space vehicles to perform maneuvers during flight1 etc. On the other hand we have been developing applications of laser driven micro-water jets to medical treatment such as intravascular drug delivery, and also as a substitute to lasers for dissecting soft tissues. Lasers have widely been used for precise incision of soft tissue containing water, but if irradiated in such a soft tissue, its absorption of laser energy results in an explosive evaporation of water, forming a water vapor bubble that could cause serious damage to the neighboring tissues/blood vessels during its growth and collapse2. Laser induced micro-water jets, in which the energy is deposited mainly in the direction of flow can be used for precise incision of soft tissue without any thermal effects and collateral damage to the tissues. Continuous liquid jets have successfully been used for tissue dissection, but when applied in liquid media it is difficult to control the penetration depth with a continuous flow of jet fluid, and also the splash of blood caused by this method is no longer desirable. Hence for micro-surgical applications, a pulsed water jet is a unique method that can achieve moderate penetrations into soft tissue by preserving capillary blood vessels of diameter thicker than 0.2mm.

We developed a Ho:YAG laser induced pulsed water jet for soft tissue incision in neuroendoscopic surgery3. Following are the objectives of the present study: (1) To analyze the dynamics of the jet by measuring its pressure, velocity and temperature. (2) To assess the feasibility of using this jet for some medical applications such as soft tissue dissection and intravascular drug delivery.

42nd AIAA Aerospace Sciences Meeting and Exhibit5 - 8 January 2004, Reno, Nevada

AIAA 2004-927

Copyright © 2004 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved.

2 American Institute of Aeronautics and Astronautics

(3) To study the dynamics of laser induced micro bubbles inside a thin tube with double exposure holographic interferometry. (4) Comparison of experimental results with the numerical results.

The micro-water jet device, extracted mechanical information on the jet and the bubbles, and physical interpretation of soft tissue dissection are briefly discussed.

EXPERIMENTAL SETUP

Figure 1 shows a Ho:YAG laser induced pulsed water jet device in which the laser beams are irradiated in a catheter or a thin metallic tube of 800µm~1mm internal diameter. The catheter is filled with a physiological saline solution or water at room temperature, and an optical fiber connected to the laser is inserted into it. The end of the catheter is fixed with a metallic nozzle with an exit diameter of 100~200µm. A standoff distance (gap) is maintained between the optical fiber end and the nozzle exit, in which sufficient volume of water is accommodated such that a portion of it is discharged as a water jet. Explosive bulging of the water vapor bubble, on laser irradiation, displaces water out of the nozzle exit. The tip of the optical fiber is covered with a hollow metallic tube of around 800µm in order to prevent ablative damage to the catheter inner wall. The Ho:YAG laser beam has wavelength and pulse duration of 2.1µm and 350µs respectively. The laser energy is variable from 250 to 800mJ/pulse, depending on the core diameter of the optical fiber. In the present study, a 400µm core diameter fiber has been used and the laser output voltage has been fixed at 1.5kV, which is equivalent to an energy level of 433mJ/pulse.

MECHANICAL ANALYSIS OF THE MICRO LIQUID JET

The ejection of water jet from the micro-nozzle has been visualized using a high-speed camera (ISIS Prototype CCD Camera: Shimadzu Co. Ltd., Kyoto, Japan) at a frame speed of 31,250frames/s. The experiment was performed in a stainless steel chamber of dimensions 110mm × 110mm × 130mm, with observation windows on its sidewalls. A commercial strobe flash with pulse duration of 3.3ms has been used as a light source. In this study, the evolution of the micro-water jet released in air in the test chamber has been visualized. Typical time resolved images of the micro-water jet discharged in air at atmospheric pressure are shown in Fig.2. The catheter nozzle exit

diameter and the standoff distance have been 200µm and 45mm respectively. The peak average velocity of the jet measured from the initial frames of these time resolved images is around 50m/s. A polyvinylidene di-fluoride (PVDF) needle hydrophone (Imotec Messtechnik, 0.5mm diameter sensing element with a sensitivity of 0.00144 Volt/bar) located axially opposite to the catheter nozzle, at a distance of 1 to 2mm, is used to record the stagnation pressure of the impinging micro-jet. Figure 3 shows the typical stagnation pressure signal of the micro-jet for a standoff distance and laser energy of 45mm and 433mJ/pulse respectively, which is corresponding to the peak jet velocity of 50m/s. The maximum-recorded stagnation pressure of the jet is about 12 atmospheres.

The micro liquid jet is generated by depositing the laser energy. Though the jet is micro and the time duration of energy deposition is quite small (350µs), while using the pulsed jet successively, there could be an increase in the temperature at the location of jet impingement. In order to precisely know the rise in temperature, we measured the temperature of the impinging jet using a k-type thermocouple of Yokogawa make (MX100). The typical temperature signal is shown in Fig. 4. A temperature rise of around 200C is noticed at the point of jet impingement for the maximum jet velocity. The temperature change did not exceed 200C even after continuously using the jet for a few minutes.

It has also been attempted to look into the generation of micro bubbles, due to laser energy deposition, that drive the micro liquid jet out of the catheter nozzle. Visualization of bubble growth is done using the same high-speed camera (Shimadzu) in transparent glass tubes of 8mm and 1mm internal diameters. Figure 5 shows such a bubble growth on laser energy deposition into water in a tube of 8mm internal diameter. The growth and collapse of the vapor bubble on absorption of the laser energy can be seen in this picture. Since the tube inner diameter is large in comparison with the bubble size, the bubble is confined to the center of the tube and grows spherical in shape. Figure 6 shows the growth and collapse of the micro vapor bubble in a glass tube of 1mm internal diameter. In this case, the bubble occupies the entire inner diameter of the glass tube and grows longitudinally. This quick longitudinal growth of the vapor bubble pushes the liquid ahead of it, forming a high-speed jet through the catheter nozzle.

3 American Institute of Aeronautics and Astronautics

RESULTS ON MEDICAL APPLICATIONS

The thermal effect and controllability of the jet were first investigated on a 10% thin gelatin slab of 1mm thickness, which is similar to a soft body tissue. Figure 7a shows the liquid jet penetrating the 1mm thick gelatin slab held (moulded) in a thin plastic frame. It was noticed that there was no distortion of the thin membrane of the slab during the penetration procedure. There were no occurrences of the bubbles during the penetration, disturbing the endoscopic view, and the shape and extent of gelatin enucleation could be controlled well. Figure 7b shows the gelatin slab after enucleation. There was no melting of the gelatin indicating absence of the thermal effects.

The dissecting ability of the jet was also investigated on the ventricle wall of the brains of a cadaver rabbit. Figure 8a shows the picture of an incision made on a cadaver rabbit ventricle wall using the micro liquid jet under endoscopic vision. A single liquid jet shot was ejected under endoscopic vision, keeping the jet generator nozzle at a distance of 1mm from the ventricle wall. After the incision, the ventricle of the rabbit brain was opened and the picture was taken. Incision is done across a capillary blood vessel and the blood vessel is preserved as shown in the figure. It was observed that the blood vessels over a size of 200µm were preserved by the liquid jet while performing the incision. Figure 8b shows a microscopic photograph of the dissection plane of the cadaver rabbit ventricle wall, cut parallel to the direction of jet penetration. 5µm thick sections were cut and were stained with hematoxylin and eosin, and examined under an optical microscope to evaluate the dissection morphology, dissection depth and vessel preservation. The maximum penetration depth observed in this case was 1.3mm, for laser energy of around 433mJ/pulse.

CONCLUSION

A laser induced pulsed micro liquid jet device has been developed that can be used in neuroendoscopic surgery. The physical properties of the jet have been analyzed for a jet of 200µm diameter and for laser energy of 433mJ/pulse. The dissecting ability of the jet, its controllability and safety have been investigated by carrying out experiments on 10% gelatin and cadaver rabbit ventricle wall. Present results show that the pulsed Laser Induced Liquid Jet (LILJ) has a good scope to become a safe and

reliable dissecting method for endoscopic procedures.

REFERENCES

1 Gimelshein, S. F., Alexeenko, A. A., and Levin, D. A., “Modeling of the Interaction of a Side Jet with a Rarefied Atmosphere,” J. Spacecraft and Rockets, Vol. 39, No. 2, 2002, pp. 168-176. 2 Fletcher, D. A., and Palanker, D. V., “Pulsed Liquid Microjet for Microsurgery,” Applied Phy. Letters, Vol. 78, No. 13, 2001, pp.1933-1935. 3 Hirano, T., Nakagawa, A., Uenohara, H., et al., “Pulsed Liquid Jet Dissector using Holmium: YAG Laser – a Novel Neurosurgical Device for Brain Incision without Impairing Vessels,” Acta Neurochir (Wien) in press, 2003.

4 American Institute of Aeronautics and Astronautics

(a)

(b)

Fig. 1. Schematic of Ho:YAG laser based pulsed micro liquid jet device: (a) the jet knife, (b) experimental setup for inspection of the jet.

436µs 500µs 564µs 628µs

692µs 756µs 820µs 884µs

Fig. 2. Time sequence of the micro jet ejection into atmospheric air from a 200µm nozzle driven by a 433mJ/pulse of Ho:YAG laser energy. Time indicated on the frames is the delay after the laser

energy deposition. Time between two successive frames is 64µs.

micro jet

nozzle

5 American Institute of Aeronautics and Astronautics

-2

0

2

4

6

8

10

12

14

-1 0 1 2 3 4 5 6

Pre

ssu

re (B

ar)

T ime (ms) Fig. 3. Stagnation pressure signal of the micro jet in atmospheric air, corresponding to laser

energy of 433mJ/pulse.

0

20

40

60

80

100

"00:00:10""00:00:21""00:00:32""00:00:43""00:00:54""00:01:05"

T

emp

erat

ure

(0 C)

Time (h:m:s) Fig. 4. Typical temperature signal of the micro jet corresponding to laser energy of 433mJ/pulse,

recorded by a thermocouple.

192µs 384µs 640µs

832µs

Fig. 5. Generation and collapse of a vapor bubble on laser irradiation (433mJ/pulse) in liquid in a glass tube of 8mm internal diameter. Time indicated is the delay from laser energy deposition.

bubble

1.25mm

optical fiber

6 American Institute of Aeronautics and Astronautics

256µs 1024µs 2048µs

3072µs

Fig. 6. Generation and expansion of a micro vapor bubble on laser irradiation (433mJ/pulse) in liquid in a glass tube of 1mm internal diameter. Time indicated is the delay from laser energy deposition.

(a)

(b)

Fig. 7. Penetration of liquid jet into 1mm thick 10% gelatin slab: (a) high-speed photography of jet penetration (interframe 64µs), (b) gelatin after enucleation.

(a) (b)

Fig. 8. Dissection of cadaver rabbit ventricle wall: (a) macroscopic photograph showing dissection under preservation of blood vessel, (b) microscopic picture of dissection plane under

neuroendoscopic view.

micro bubble

jet generator liquid jet

10% gelatin frame

1mm

1mm

cut blood vessel

penetration

0.2mm