Previous research showed that mid-infrared free-electron lasers could reproducibly ablate soft tissue with little collateral damage. The potential for surgical applications motivated searches for alternative tabletop lasers providing thermally confined pulses in the 6- to-7-μm wavelength range with sufficient pulse energy, stability, and reliability. Here, we evaluate a prototype Raman-shifted alexandrite laser. We measure ablation thresholds, etch rates, and collateral damage in gelatin and cornea as a function of laser wavelength (6.09, 6.27, or 6.43 μm), pulse energy (up to 3 mJ/pulse), and spot diameter (100 to 600 μm). We find modest wavelength dependence for ablation thresholds and collateral damage, with the lowest thresholds and least damage for 6.09 μm. We find a strong spot-size dependence for all metrics. When the beam is tightly focused (∼100-μm diameter), ablation requires more energy, is highly variable and less efficient, and can yield large zones of mechanical damage (for pulse energies >1 mJ). When the beam is softly focused (∼300-μm diameter), ablation proceeded at surgically relevant etch rates, with reasonable reproducibility (5% to 12% within a single sample), and little collateral damage. With improvements in pulse-energy stability, this prototype laser may have significant potential for soft-tissue surgical applications.
Researchers have previously observed that tissue ablation with a free electron laser tuned to wavelengths between 6-7
μm is accompanied by remarkably little collateral damage. Attempts to explain these observations have invoked a
wavelength-dependent loss of protein structural integrity; however, the molecular nature of this structural failure has
been heretofore ill-defined. In this report, we evaluate several candidates for the relevant transition by analyzing the
non-volatile debris ejected during ablation. Porcine corneas were ablated with a free electron laser tuned to either 2.77
or 6.45 μm - wavelengths that are equally well absorbed by hydrated corneas, but that respectively target water or
protein as the primary chromophore. The ejected debris was characterized via gel electrophoresis, as well as FTIR,
micro-Raman and <sup>13</sup>C-NMR spectroscopy. We find that high-fluence (240 J/cm<sup>2</sup>) ablation at 6.45 μm, but not at 2.77
μm, leads to protein fragmentation. This fragmentation is accompanied by the accumulation of nitrile and alkyne
species. Although these initial experiments did not detect significant protein unfolding, the loss of collagen triple-helix
structure was evident using UV and vibrational circular dichroism. The candidate transition most consistent with all
these observations is scission of the collagen protein backbone at N-alkylamide bonds. Identifying this transition is a
key step towards understanding the observed wavelength-dependence of collateral damage.
We present in some detail a theoretical model that provides a dynamical account for the experimentally observed ablative properties of an FEL tuned near 6.45 microns. The model is based on thermal diffusion and chemical kinetics in a system of alternating layers of protein and saline as heated by an infrared Mark-III FEL. We compare exposure at 3.0 microns, where water is the sole absorber, to that at 6.45 microns, where both protein and water absorb. The picosecond pulses of the Mark-III superpulse are treated as a train of impulses. We consider the onset of both the helix-coil transition and chemical bond breaking in terms of the thermal, chemical, and mechanical properties of the system as well as laser wavelength and pulse structure.
We have investigated the experimental consequences of two picosecond infrared lasers, both tuned to 6.45micrometers and focused on ocular tissue. The exposure conditions were comparable, other than pulse repetition rate, where an optical parametric oscillator/amplifier laser (OPA) system operates at a kilohertz and the Mark-III FEL at 3 gigahertz. In both cases, the peak intensity was near 2x10<SUP>14</SUP> W/m<SUP>2</SUP> and the total delivered energy was approximately 125 mJ. The Mark-III consistently ablates tissue, while the OPA fails to ablate or to damage corneal tissue. In particular, there is no experimental evidence for protein denaturation due to OPA irradiation. WE account for these observations in terms of a theoretical model based on thermal diffusion and threshold conditions for superheating and chemical kinetics. We comment on the relevance of tissue geometry.
We describe the commissioning of a novel two-color beamline at the Duke Free Electron Laser Laboratory, designed to perform time-resolved FTIR spectroscopy in a pump-probe scheme with sub-nanosecond resolution to measure dynamical processes with durations as long as ten nanoseconds. The UV pump pulses are produced by the tunable (193 to 700 nm) output of the OK-4 Storage-Ring FEL. The broadband, infrared probe pulses are generated as synchrotron radiation in a bending magnet downstream of the OK-4 wiggler. The repetition rate of the light source (2.79 MHz) is ideal for operating the interferometer in the rapid-scan, asynchronous sampling mode.
The Duke FEL Laboratory is a national and international users facility. We describe the current light source capabilities in the infrared, visible, ultraviolet, and Gamma rays. Plans are summarized for the development of two novel beamlines, one for UV-resonant Raman spectroscopy and the other an essentially jitter-free UV-pump, IR-probe `two- color' source with rapid-scan FTIR time-resolved detection of the broadband infrared. Current applications research is summarized, with a more detailed description of research in corneal wound healing.