Dynamics of gelatin ablation due to free-electron-laser irradiation

Abstract

We have carried out simultaneous, time-dependent measurements of the free-electron-laser ͑FEL͒-induced stress transients and ablation plume in gelatin, which serves as a model system for collagenous tissues. The Mark-III FEL is tunable in the mid-IR ͑2-10 m͒ and produces macropulses of microsecond duration comprised of picosecond micropulses separated by 350 ps. The macropulse duration was shortened with a broadband, IR Pockels cell, producing pulse durations as short as 60 ns and energies in the range of 0.1-1 mJ. The IR beam was focused to a diameter of 112-210 m, depending on the wavelength, and measurements were made at 3.0, 3.36, and 6.45 m. For fluences below the ablation threshold, stress transients were measured and accounted for with a standard thermoelastic mechanism. Of particular interest were the measurements with fluences above the ablation threshold, where two classes of dynamics were observed. A cw HeNe beam monitors the plume: at 3.0 m a single maximum of the ''shadow'' is observed, while at 3.36 m and 6.45 m a second maximum also was resolved at later times. In addition, at 3.36 m and 6.45 m the duration of the momentum recoil is about twice as long as that observed for comparable exposure parameters at 3.0 m.

Submicron foaming in gelatine by nanosecond and femtosecond pulsed laser irradiation

“…Collagen and gelatine have served in previous laser ablation studies as model substances for soft biological tissue since their light absorption spectra and dielectric properties are similar to those of biological tissue. For gelatine, laser irradiation studies [11][12][13][14] have focussed on the mid infrared wavelengths where this material absorbs efficiently using Er:YAG laser and freeelectron lasers (FEL). Gelatine with 80% water was efficiently ablated by FEL irradiation at 6.1 mm, where the primary absorber is water.…”

Mechanisms for soft‐tissue ablation and the development of alternative medical lasers based on investigations with mid‐infrared free‐electron lasers

“…Damage patterns indicative of pressure wave propagation deep to the ablation cavity have been observed in Mark-III FEL investigations of tissue ablation [54]. Furthermore, an FEL wavelength dependence for stress waves propagating from the ablation cavity deep into the tissue was observed in gelatin ablation [39]. One of the key features for mid-infrared tissue ablation is the explosive vaporization of tissue water, i.e.,thermodynamics under extreme conditions as the water is heated to the spinodal limit in tens of nanoseconds or less [5,6].…”

“…The initial proposal to account for the wavelength dependence of Mark-III FEL soft tissue ablation was based on the denaturation of structural proteins via resonant absorption of the amide bonds in addition to explosive vaporization of water via the resonant absorption of the bending mode [1,4,12,39]. Neither the optical breakdown model nor the thermal confinement model accounted for the experimental results.…”

Time-resolved, light scattering measurements of cartilage and cornea denaturation due to free electron laser radiation

“…Tissue denaturation by IR lasers is a thermally and mechanically mediated process that sets an upper bound for low-energy procedures, such as reshaping of cartilage, and determines the thermal damage for high-energy laser processes, such as tissue ablation and welding. [1][2][3] Laserinduced alteration of tissue structure changes the number and size of light-scattering centers in the tissue. It allows us to consider denaturation as an alteration in organization of tissue native structure and to study the process by means of optical techniques.…”

“…Denaturation delay time t* (in seconds) versus the pulse radiant exposure F (in J/cm 2 ) for cartilage: (1) ϭ8.05 m,(2) ϭ6.00 m, and (3) ϭ2.50 m. The solid curves are calculated in accordance with the function t*ϭt 0 ϩB/(FϪF th ) 2 . The denaturation threshold F th is determined as an asymptotic pulse radiant exposure when denaturation delay tends to infinity.Time-Resolved, Light Scattering Measurements .…”