**5. Laser-induced acoustic effects**

Laser-induced hydrodynamics processes in water-saturated bio-tissues causes generation of intense acoustic waves. We have studied the peculiarities of generation of such acoustic waves in water and water-saturated biotissue (intervertebral disc, bone, et al.) in the vicinity of blackened optical fiber tip using acoustic hydrophone (Brul and Kier 8100, Denmark). The hydrophone with 0 – 200 KHz band was placed in water or biotissue at 1cm distance from optical fiber tip. Fig. 15 demonstrates typical example of acoustic response to laser irradiation for two different cases: in the bath of free water (Fig. 15a) and in the case of water

Typical micron-scale circular holes on the film surface (Fig. 13a) can be caused by cavitation collapse of single bubbles. It is well known that cavitation collapse of bubbles in liquid in the vicinity of the solid surface gives rise to the high-speed cumulative microjets which can destroy the solid surface (Suslick, 1994). Apparently, this effect leads to multiple cracks on the film and the formation of the porous structure (Fig. 13a), since the cumulative microjets

Collapse of cavitation bubble apart from high pressure generation (up to106 MPa) can cause overheating of gas up to temperatures as high as 104К. Such high values of water pressure and temperature can result in formation of supercritical water (critical pressure of water is Рc=218 atm, critical temperature - Tc =374ºС), which can dissolve silica fiber (Bagratashvili et

Fig. 14 shows Raman spectra of some areas of laser irradiated fiber tip surface (curves 3-5) compared with that of graphite (1) and diamond (2). Raman bands at 1590 cm-1 and 1590 cm-

can punch holes, cause cracks in the film, and destroy the structure of silica fiber tip.

1 to diamond and graphite nano-phases correspondingly (Yusupov et al., 2011a).

Fig. 14. Raman spectra from different areas of laser fiber tip surface (curves 3, 4 and 5)

Formation of diamond nanophase at a fiber tip surface in this case is rationalized by extremely high pressures and temperatures caused by cavitation processes stimulated by

Laser-induced hydrodynamics processes in water-saturated bio-tissues causes generation of intense acoustic waves. We have studied the peculiarities of generation of such acoustic waves in water and water-saturated biotissue (intervertebral disc, bone, et al.) in the vicinity of blackened optical fiber tip using acoustic hydrophone (Brul and Kier 8100, Denmark). The hydrophone with 0 – 200 KHz band was placed in water or biotissue at 1cm distance from optical fiber tip. Fig. 15 demonstrates typical example of acoustic response to laser irradiation for two different cases: in the bath of free water (Fig. 15a) and in the case of water

compared with that of graphite (1) and diamond (2) (Yusupov et al., 2011a).

laser irradiation (Yusupov et al., 2011a).

**5. Laser-induced acoustic effects** 

al., 2009).

The fiber tip surface is blackened before laser irradiation with 0.97 µm wavelength.

Fig. 15. Fragments of acoustic response to 3 W laser irradiation of water for two different cases: in a bath of free water (**a**) and in a water-filled capillary (**b**).

filled capillary (Fig. 15b). In the case of the bath with free water, the short random laserinduced acoustic spikes take place. At the same time, the acoustic response to laser irradiation in the case of water-filled capillary (which imitates situation in real water-filled biotissue channel) is different (Fig. 15b). Acoustic signal is amplitude-modulated by its feature, and low-frequency modulation period is about 2 s.

Fig. 16 demonstrates acoustic response to laser irradiation of nucleus pulposus *in vivo* when optical fiber was moved forward (regime of channels formation in the course of laser healing of degenerated disc). The acoustic signal is non-stationary by its nature. The shortpulse intense acoustic spikes take place and the signal itself is amplitude modulated (similarly to that in water-filled capillary) with a modulation period of about 3 s.

Arrows show the moments of laser on and laser off.

Fig. 16. Acoustic response to 3 W laser irradiation with 0.97 µm wavelength of nucleus pulposus *in vivo,* when optical fiber was moved forward in the intervertebral disc.

Laser-Induced Hydrodynamics in Water and Biotissues Nearby Optical Fiber Tip 111

 In this division we will show that existence of strongly absorbed agents (in a form of Ag nanoparticles, in particular) in laser irradiated water nearby optical fiber tip can result in appearance of filamentary structures of these agents (Yusupov et al., 2011b). Medium power (0.3 – 8.0 W) 0.97 µm in wavelength laser irradiation of water with added Ag nanoparticles (in the form of Ag-albumin complexes) through 400 µm optical fiber stimulates selforganization of filaments of Ag nanoparticles for a few minutes. These filaments represent themselves long (up to 14 cm) liquid gradient fibers with unexpectedly thin (10 – 80 μm) core diameter. They are stable in the course of laser irradiation, being destroyed after laser radiation off. Such effect of filaments of Ag nanoparticles self-organization is rationalized by the peculiarities of laser-induced hydrodynamic processes developed in water in presence of

Fiber laser radiation (LS-0,97 IRE-Polus, Russia) 0-10 W in output and 0.97 µm in wavelength was delivered into water-filled plastic cell through 400 µm transport silica optical fiber, which was placed horizontally in the cell. Low intensity (up to 1 mW) green pilot beam from the built in diode laser was used to highlight the 0.97 µm laser irradiated zone in the cell. The cell was placed at the sample compartment of optical microscope (MC300, MICROS, Austria) equipped with color digital video-camera (Vision). Spectroscopic studies were performed with fiberoptic spectrum analyzer (USB4000, Ocean Optics) and UV/vis absorption spectrometer (Cary 50, Varian). To measure the refraction index of collargol we have applied the fiber-optic reflectometer FOR-11 (LaserChem, Russia), which provides 10-4 precision of refraction index measurements at 1256 nm wavelength. Cleavage of transport optical fiber has been always produced just before each experiment. Ten minutes later (to provide reasonable attenuation of hydrodynamic motions in the cell) the drop (0.01–1 ml in volume) of brown colored collargol (complex of 25 nm in size Ag nanoparticles with albumin) has been smoothly introduced into

Our in situ optical microscopic studies of laser-induced filament formation were accomplished in two different modes: 1) in transmission mode, using illumination with white light from microscope lamp; 2) in scattering mode, using illumination with green light

Experiments show that 0.97 µm fiber laser irradiation of water in the cell with introduced collargol drop causes (in some period of time from seconds to minutes) formation of thin and long quite homogenous filaments, growing along the axis of 0.97 µm laser beam in water. These filaments are brown colored (that gives the evidence of enhanced Ag

Fig. 18 demonstrates the microscope image (in transmission mode) of one of such filaments. This filament is located along the axis of output laser beam and is about 17 mm in length. The measured profile of optical density of this filament is triangular in its shape with about

Fig. 18. Micro-image (in transmission mode) of filament of Ag nanoparticles fabricated in

nanoparticles concentration in filament) and can be seen even with unaided eye.

the same widths along filament (determined at half-maximum) of ~200 μm.

water nearby optical fiber tip at 2.5 W of laser power (Yusupov et al, 2011b).

**6. Formation of filaments** 

laser light and by formation of liquid fibers.

the water cell 0.5-10 mm aside from the optical fiber tip.

of pilot laser beam through the same transport fiber.

The more detailed studies show that for both *in vivo* and *in vitro* cases laser-induced generation of short-pulse intense quasi-periodic acoustic signals. The fragment of spectrogram of acoustic response given at Fig. 17 clearly demonstrates temporal change of spectral components for acoustic signal generated from laser irradiated nucleus pulposus *in vitro* when optical fiber was moved forward in the intervertebral disc (similar to shown at Fig. 1).

Fig. 17. The fragment of spectrogram (a) ant temporal structure of single pulse (b) of acoustic response generated from laser irradiated nucleus pulposus *in vitro*.

As one can see, the acoustic response in this case has the form of short, intense and broadband (from 0 to 10 kHz) pulses of about 10 ms in duration combined into the series of pulses generated with frequency of 40 Hz. Fig. 17b shows that the amplitude of single pulse is an order of amplitude higher than the background acoustic noise. The most of acoustic power is concentrated in such pulses. The broad spectrum of acoustic pulses and their low duration indicate to shock-type of generated acoustic waves. The acoustic noise has broad spectral maxima in the following spectral intervals: 600 – 700 Hz, 1 - 2 kHz and nearby 10 kHz.

Appearance of these bands are caused by the dynamics of vapor-gas mixture and are associated with acoustic resonances of the system. Notice that laser-induced formation of channels in degenerated spinal discs *in vitro* has been accompanied by 4 Hz in frequency strong visual vibrations of needle with laser fiber.

Generation of such a strong acoustic vibrations is caused in our opinion by contact of overheated (up to >1000 ºС (Yusupov et al., 2011a)) fiber tip with water and water-saturated tissue of spinal disc. Such contact can result in explosive boiling of water solution nearby the fiber tip and, also, in burning of collagen in cartilage tissues. Intense hydrodynamic processes can take place nearby optical fiber tip, which are caused by fast heating of water and tissue, by generation and collapse of vapor-gas bubbles (Chudnovskii et al., 2010a, 2010b; Leighton, 1994). As a result, the free space of disc or bone is filled by liquid saturated by vapor-gas bubbles. Resonance vibrations are excited, since both disc and bone are quite good acoustic resonators. These vibrations give rise to low-frequency modulation of acoustic noise (Fig. 16) and to quasi-periodic generation of short intense pulses (Fig. 17) (Chudnovskii et al., 2010a). The acousto- mechanic shock-type processes in resonance conditions results in mixing and transport of gas-saturated degenerated tissue in the space of defect (Chudnovskii et al., 2010b). These processes destroy hernia and decrease its density (Fig. 2b), thus lowering the pressure to nervous roots. Another important impact of such processes is the regeneration of disc tissues through the effects of mechanobiology (Buschmann et al., 1995; Bagratashvili et al., 2006).
