Mechanisms of Pulsed Laser Ablation of Biological Tissues
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Chem. Rev. 2003, 103, 577−644 577 Mechanisms of Pulsed Laser Ablation of Biological Tissues Alfred Vogel*,† and Vasan Venugopalan‡ Medical Laser Center Lu¨beck, Peter-Monnik-Weg 4, D-23562 Lu¨beck, Germany, and Department of Chemical Engineering and Materials Science and Laser Microbeam and Medical Program, Beckman Laser Institute, University of California, Irvine, California 92697 Received September 12, 2002 Contents 2. Plume Dynamics for Microsecond Pulses 602 3. Postpulse Ablation 604 I. Introduction 578 II. Properties of Soft Biological Tissues 579 B. Recoil Stress 604 A. Tissue Composition and Morphology 579 1. Temporal Shape and Amplitude 604 B. Mechanical Properties 582 2. Recoil-Induced Material Expulsion 605 C. Thermal Denaturation 583 3. Collateral Tissue Effects Induced by 606 Recoil Stress III. Energy Deposition and Transport 584 C. Flow-Induced Material Redeposition 607 A. Optical Absorption Properties of Tissue 584 D. Shielding by the Ablation Plume 608 1. Ultraviolet Radiation (λ ) 180−400 nm) 584 2. Visible Radiation (λ ) 400−780 nm) 585 VII. Ablation Models and Metrics 609 3. Near−Far-Infrared Radiation 585 A. Heuristic Models 609 (λ ) 780 nm−15 µm) 1. Blow-off Model 609 B. Optical Scattering Properties of Tissue 585 2. Steady-State Models 610 C. Dynamic Optical Properties in Tissue 586 3. Comparison of Blow-off and Steady-State 611 IV. Linear Thermomechanical Response of Tissue to 588 Models Pulsed Irradiation 4. Applicability of Blow-off and Steady-State 611 A. Temperature Rise and Thermal Diffusion 588 Models B. Thermoelastic Stress Generation and 588 5. Unification of Blow-off and Steady-State 612 Propagation Models C. Implications for Precise Tissue Ablation 590 B. Ablation Metrics and Their Relationship to 612 V. Thermodynamics and Kinetics of Phase 590 Heuristic Model Predictions Transitions 1. Ablation Threshold 612 A. Phase Diagrams 590 2. Ablation Enthalpy 613 B. Surface Vaporization 591 3. Ablation Efficiency 613 C. Normal Boiling 592 C. Mechanistic Models 613 D. Phase Explosions: Bubble Nucleation and 592 1. Steady-State Vaporization Models 614 Spinodal Decomposition 2. Thermomechanical Models 614 E. Confined Boiling 595 D. Molecular Dynamics Simulations 615 F. Effects of the Tissue Matrix on the Interplay 596 of Phase Explosions and Confined Boiling VIII. UV and IR Ablation 615 G. Effect of Stress Confinement on the Ablation 596 A. Tissue Absorption Coefficients 615 Process B. Ablation without Stress Confinement 617 Downloaded via UNIV OF CALIFORNIA IRVINE on October 31, 2018 at 20:59:34 (UTC). 1. Stress Confinement and Phase 597 1. Temperature Rise and Thermal Diffusion 617 See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles. Transitions 2. Kinetics of Tissue Decomposition 618 2. Stress Confinement and Tissue Fracture 597 3. Material Ejection 618 3. Stress Confinement and Ablation 598 C. Ablation with Stress Confinement 620 Threshold 1. Reduction of the Ablation Threshold 621 H. Photochemical Decomposition 598 2. Kinetics of Tissue Decomposition and 621 I. Interplay of Photochemical Decomposition 600 Material Ejection and Confined Boiling in Tissue 3. Precision Achieved by Ablation under 622 VI. Ablation Plume Dynamics 600 Stress-Confined Conditions A. Plume Formation and Expansion 600 D. Overall Picture 623 1. Plume Dynamics for Nanosecond Pulses 601 IX. Ablation in a Liquid Environment 623 A. Bubble Formation 623 * To whom correspondence should be addressed. Phone: B. Amplification of Mechanical Effects by Liquid 625 +49-451-500-6504. Fax: +49-451-505-486. E-mail: vogel@ Confinement mll.mu-luebeck.de. † Medical Laser Center Lu¨ beck. C. Influence of the Bubble Dynamics on Ablation 626 ‡ University of California, Irvine. E-mail: [email protected]. Efficiency 10.1021/cr010379n CCC: $44.00 © 2003 American Chemical Society Published on Web 02/12/2003 578 Chemical Reviews, 2003, Vol. 103, No. 2 Vogel and Venugopalan X. Plasma-Mediated Ablation 627 A. Kinetics of Plasma Formation in Biological 627 Tissues B. Threshold for Plasma Formation 629 C. Plasma Formation above the Breakdown 630 Threshold D. Plasma Absorption 631 E. Plasma Energy Density 632 F. Thermomechanical and Chemical Effects 632 G. Implications for Tissue Ablation 634 XI. Control of Precision, Thermal and Mechanical 636 Damage, and Ablated Mass A. Control of Precision 636 B. Control of Thermal Side Effects 636 C. Control of Mechanical Side Effects 637 Alfred Vogel studied physics and sociology, receiving the University degree D. Maximizing the Ablated Mass 637 for high school teaching and the Ph.D. degree in physics from the Georg- August University of Go¨ttingen, Germany. Later, he earned the degree of E. Selective Ablation 637 Habilitated Doctor of Physics from the University of Lu¨beck. In 1988, he XII. Outlook and Challenges 638 joined the laser laboratory of the Eye Hospital of the Ludwig-Maximilians XIII. Acknowledgment 639 University Munich, and in 1992, he moved to the Medical Laser Center XIV. References 639 Lu¨beck, where he has been Vice-Chairman since 1999. His research interests include laser−tissue interactions in biomedical applications of photodisruption, pulsed laser ablation, and photocoagulation as well as I. Introduction laser-induced plasma formation, cavitation, and stress waves. Soon after the invention of the pulsed ruby laser by Maiman in 1960, investigators were eager to examine the potential of pulsed laser radiation for medical applications. It was greatly anticipated that lasers would enable manipulation and destruction of biological tissue with unprecedented precision and selectivity.1,2 Within a few years, reports of the use of pulsed lasers for precise tissue coagulation in patients along with the development of laser systems designed for clinical use appeared in the literature.3-6 However, a clinically viable application of pulsed laser ablation was not reported until the early 1970s.7 It was only at the beginning of the 1980s that lasers were routinely used for ophthalmic dissection and ablation procedures. In other medical subspecialties, routine laser use did not occur until the mid-1980s.8 Vasan Venugopalan received his B.S. degree in mechanical engineering The delay between the invention of the laser and its from the University of California, Berkeley, and S.M. and Sc.D. degrees successful clinical application was due largely to a in mechanical engineering from MIT. He held postdoctoral positions at lack of understanding of the fundamental mecha- the Wellman Laboratories of Photomedicine, Harvard Medical School, and - the Departments of Molecular Biology and Physics, Princeton University. nisms that govern laser tissue interactions. Much He joined the University of California, Irvine, in 1996, where he is currently progress has been made in this regard, and now, at Assistant Professor of Chemical Engineering, Biomedical Engineering, and the beginning of the 21st century, medical procedures Surgery. His research focuses on applications of laser-induced thermal, that employ pulsed lasers are present in nearly every mechanical, and radiative transport processes in medical diagnostics, medical subspecialty; for many ophthalmologists and therapeutics, biotechnology, and micro-electromechanical systems. dermatologists, lasers are considered essential tools for medical practice. on general aspects of tissue ablation include those of As the understanding of laser-tissue interactions Mu¨ ller,24 Oraevsky,25 van Leeuwen,26 and Walsh.27 matured in the 1990s, three books became available Specific aspects of laser ablation processes have also and serve as a valuable resource for the field,9-11 been considered in comprehensive studies. Medical together with the books by Ready12 and by Ba¨uerle13 applications of laser-induced plasmas were reviewed that address general aspects of laser ablation. Previ- by Gitomer and Jones,28 while ablation processes ous survey papers include the reviews on laser- resulting from nonlinear absorption have been con- tissue interactions by Hillenkamp14 and by Jacques;15 sidered in both a review paper29 and a monograph30 ultraviolet laser ablation of polymers and biological by Vogel. tissues by Srinivasan,16 Srinivasan and Braren,17 and Despite the increasing medical use of lasers, a Pettit;18 photophysics of laser-tissue interactions by comprehensive presentation of the fundamental mech- Boulnois;19 thermal processes in laser-tissue inter- anisms involved in pulsed laser tissue ablation has actions by McKenzie;20 medical laser applications by not appeared in the scientific literature. This is not Verdaasdonk;21 and laser applications in the cornea surprising, as the elements required for an under- and in ophthalmology by Marshall,22 Krauss and standing of the relevant processes range from non- Puliafito,8 and by Krueger and co-workers.23 Reviews equilibrium thermodynamics to photochemistry to Pulsed Laser Ablation of Biological Tissues Chemical Reviews, 2003, Vol. 103, No. 2 579 plasma physics to tissue biomechanics. Consequently, ing the amount of energy reaching the tissue (section researchers who investigate the mechanisms of pulsed VI). Once the various steps constituting the ablation laser tissue ablation originate from several disci- process have been analyzed, we survey the theoretical plines. Moreover, the resulting scientific reports are models that have been developed to describe this often narrow in scope and scattered in journals whose process as well as the use of metrics such as ablation foci range from the medical