Ver no Mapa Semântico Ver no Índice A-Z
Número de visualizações:
3565

Ultrafast Lasers for Nano- and Microsurgery

Biswajit Mishra

Aditya Roy

Berk Camli

Adela Ben-Yakar

Mechanical Engineering Department, The University of Texas at Austin, USA

Lasers emitting pulses of durations shorter than 10 ps are known as ultrafast lasers. Delivery of pulses in very short durations allows ultrafast lasers to generate high peak intensities at the focal volume while keeping pulse energies low. High peak intensities lead to non-linear light-matter interactions, which induce plasma-mediated damage. However, the major distinguishing characteristic of ultrafast lasers is the spatial and thermal confinement that enables very high precision ablations without damaging the out-of-focus matter. Owing to the high three-dimensional spatial and thermal confinement, ultrafast lasers became valuable tools for high precision surgery of tissues, cells, and even subcellular structures. Applications ranging from mitochondrial dissection for fundamental biology studies (Shen et al., 2005) to capsulotomy of the eye in the clinic (Friedman et al., 2011) have been shown in the literature. Ultrafast lasers are actively used today for ophthalmic procedures such as in LASIK surgeries (Slade, 2007), while other clinical applications are currently in development.

Cells and tissues are modeled as wide bandgap dielectrics when studying ultrafast laser-tissue interactions (Stuart et al., 1995). The mechanisms of ultrafast laser ablation can include photochemical, thermoelastic stress generation, and plasma-mediated ablation, as illustrated in Table 1, depending on the peak intensity and laser repetition rate (Ben-Yakar and Bourgeois, 2009). Nano- and microsurgery applications primarily rely on plasma-mediated ablation. At the high peak intensities that ultrafast laser pulses can generate, plasma is formed as a result of nonlinear absorption via multiphoton ionization followed by impact ionization events, creating an exponential growth of free electrons (Noack and Vogel, 1999). When the free electron density reaches a critical value, typically around 1021 /cm3, optical breakdown occurs (Niemz, 2019). Since very high peak intensities are achieved only within the focal volume, plasma-mediated ablation is highly confined. Two factors contribute to this confinement even further. First is plasma having high reflectance, shielding the plasma in the focal volume from absorbing energies beyond those necessary for optical breakdown. Second is the non-linear absorption, limiting the free-electron distribution in a region narrower than focal volume at irradiances close to the threshold to achieve nano-scale precision (Vogel and Venugopalan, 2003). Resulting high confinement of ablation allows the ability to perform sub-surface ablation with a unique precision that is crucial in various biomedical applications such as ophthalmic procedures and treatment of scarred vocal folds (Friedman et al., 2011; Gabay et al., 2022).

TABLE 1: Major categories of ultrafast laser-tissue interaction

Damage Mechanism Photochemical Damage Thermoelastic Stress Confinement Optical Breakdown
Intensity threshold 0.26 × 1012 W/cm2 5.1 × 1012 W/cm2 6.54 × 1012 W/cm2
Electron density at threshold 2.1 × 1013 cm–3
One free electron in the focal volume
0.24 × 1021 cm–3
Induced thermal stress overcomes the tensile strength of water
1.0 × 1021 cm–3
Critical electron density for optical breakdown
Description Free electrons participate in chemical reactions to form destructive reactive oxygen species and lead to breaking of chemical bonds. Thermalization of plasma occurs faster than acoustic relaxation time. Confinement of thermal stresses leads to formation of nano-scale transient bubbles. Damage is created by high pressure and high temperature plasma and by the accompanying shock wave and cavitation bubble.
Pulse repetition rate > 1 MHz
A large number of pulses are required. For practical reasons, high repetition rate lasers are preferable.
< 1 MHz
Bubble lifetime is 100-500ns. At higher repetition rates and higher irradiances, heat accumulation and long-lasting bubble formation can become significant.

Thermal effects can be understood in terms of plasma energy density, which is directly related to the absorbed fraction of incident energy (Noack and Vogel, 1999). For pulse durations longer than 10 ps, a large part of the incident energy results in the heating of atomic, molecular, and plasma constituents through energy transfer mechanisms such as inelastic collisions and non-radiative recombination events. Long pulses are, therefore, associated with high plasma energy densities (Noack and Vogel, 1999), which result in a large spread of heat from the focal volume. In addition, out of focus linear absorption by cells and tissues causes generation of excessive heat and significantly higher thermal damage. On the other hand, the plasma energy density remains low when pulse durations are shorter than the timescale of said energy transfer mechanisms. Experimental findings show that the plasma energy density varies little with pulse durations shorter than 10 ps (Noack and Vogel, 1999). Owing to the low plasma energy density, ultrashort laser pulses offer high thermal confinement, which leads to high thermoelastic stress buildup. Emergent thermoelastic stress can induce fracture in material or generate transient cavitation bubbles assisting in material removal processes. Lifetimes and sizes of these transient cavitation bubbles enable intracellular dissections without cell damage (Noack and Vogel, 1999; Vogel and Venugopalan, 2003).

The mechanism of ultrafast laser ablation from a pulse train depends on the repetition rate. Overlapping laser pulses, independent of their repetition rate, reduce the damage threshold of the material through a phenomenon known as incubation effect (Rosenfeld, 1999). The reduction in damage threshold enables ablation even at low pulse energies when compared to single pulse ablation (Ben-Yakar and Byer, 2004). While operating at MHz repetition rates, ablation is achieved through accumulative chemical effects. The pulse energies used in the MHz regime are kept low, ensuring that no transient cavitation bubbles with sub-microsecond lifetimes are produced to avoid pulse-to-pulse interactions (Vogel et al., 2005). Ultrafast lasers with repetition rates in the MHz regime have been used in the ablation of nanometer-sized regions within the nucleus of a cell or create transition nano-scale pores on cellular membranes (Koenig, 1999). At repetition rates in the order of tens of kHz, temporal separation of laser pulses limit accumulative effects. Therefore, pulse energies can be high enough to generate transient cavitation bubbles with microsecond lifetimes, which decay before the subsequent laser pulse arrives. Repetition rates up to several kHz have been used in mitochondrial dissection (Shen et al., 2005) and dissection of axons of C. elegans (Fatih Yanik, 2004).

Ultrafast laser nano- and microsurgery applications often rely on use of tabletop systems where the ablation target is easily accessible (Fatih Yanik, 2004). However, many clinical applications require flexible delivery of ultrafast laser pulses to hard-to-reach targets using optical fibers (Hoy et al., 2014). Since peak intensities required for ultrafast laser ablation can easily damage common optical fiber core materials, hollow core optical fibers should be used. In such fibers, laser pulses travel in the hollow core in air, which reduces damage risk as well as dispersion effects distorting the pulse duration (Hoy et al., 2014). In laser surgeries, ablating over a large region of interest requires beam scanning mechanisms to steer the laser focal region. While operating at high pulse repetition rates, one needs to consider the pulse overlap rates, in context of material removal rate (MRR), incubation effect, and potential accumulative thermal damages.

There has been considerable effort for the development of fiber-coupled ultrafast laser surgical probes for clinical applications in hard-to-reach areas in the body. Over the years, our group has developed three generations of such surgical probes, as shown in Figure 1. The first-generation probes utilized MEMS mirrors for beam steering and an air-core photonic bandgap fiber for laser delivery. A combination of miniaturized aspheric and graded-index lenses was used to focus laser pulses. A second, high numeric aperture large-core fiber was incorporated for multiphoton fluorescence and second harmonic generation imaging, demonstrating potential for image-guided ultrafast laser surgery (Hoy et al., 2008, 2011). The second-generation probes switched to an in-line architecture utilizing a piezoelectric tube actuator for fiber scanning that resulted in a considerable reduction in the probe size (Ferhanoglu et al., 2014). Furthermore, the use of high repetition rate laser pulses that were delivered using a Kagome fiber and custom miniaturized lenses enabled achieving higher speed ablation. The probe was tested ex-vivo on bovine ribs and a maximum MRR of 0.13 mm3/min was achieved (Subramanian et al., 2021). The main distinctive characteristic of the third-generation probe was the high-resolution side-firing architecture. Side-firing is required by the specific application of scarred vocal fold surgery. The probe was shown to be able to create sub-surface voids in ex-vivo porcine and in-vivo canine samples, becoming the first ultrafast laser ablation probe to be successfully used in a pre-clinical setting (Andrus et al., 2022).

Miniaturized ultrafast laser surgery probes in increasing order of ease to clinical translation

Figure 1. Miniaturized ultrafast laser surgery probes in increasing order of ease to clinical translation. (a) MEMS-based miniaturized surgery probes in the order of decreasing in size. (b) In-line architecture surgery probes with diameters < 5 mm with increased surgery speeds. (c) High-resolution, side firing miniaturized probe for sub-epithelial ablation for treatment of scarred vocal folds. (d) An example of a sub-surface cut of the canine vocal fold in a pre-clinical setting performed in-vivo using the probe shown in (c). Red dye injected under the epithelial layer marks the extent of the sub-surface void created ultrafast laser ablation.

One of the major aims in ultrafast laser micro-surgeries for clinical applications is maximizing the MRR, while keeping thermal damage negligible. Although operating at low repetition rates can prevent thermal damage, the MRR remains low. On the other hand, high repetition rates create accumulative effects as discussed. Therefore, the designer faces an optimization problem, keeping the repetition rate on the order of hundreds of kHz, while working with pulse durations < 10 ps. Numerical and subsequent experimental studies demonstrated that very high MRR, comparable to conventional surgical tools, can be achieved by using high average power laser pulses that are scanned over a large area. However, realization of such a system entails the design of involved optical and opto-mechanical systems. Further studies are needed to improve our understanding of laser-tissue interactions in the hundreds of kHz repetition rate range, since this range offers a sweet spot in achieving high MRR while avoiding detrimental nonlinear effects and minimizing thermal damage. Furthermore, there is a lot of room for design improvements in the realm of fibers, probes, and scanning mechanisms, which could potentially offer a smoother translation to the clinical settings. The precision of ultrafast laser surgery holds great promise to multiple biomedical applications in the clinic, ranging from spine surgeries to scarred vocal fold treatments. The recent efforts in the development of miniaturized ultrafast laser surgery probes bring us a step closer to clinical translation.

REFERENCES

Shen, N., Datta, D., Schaffer, C.B., Leduc, P., Ingber, D.E., and Mazur, E., Ablation of cytoskeletal filaments and mitochondria in live cells using a femtosecond laser nanoscissor, 2005.

Friedman, N.J. et al., Femtosecond laser capsulotomy, J. Cataract. Refract. Surg., vol. 37, no. 7, pp. 1189–1198, Jul. 2011. DOI: 10.1016/j.jcrs.2011.04.022

Slade, S.G., The use of the femtosecond laser in the customization of corneal flaps in laser in situ keratomileusis, Current Opinion in Opthalmology, vol. 18, pp. 314–317, 2007.

Stuart, B.C., Feit, M.D., Rubenchik, A.M., Shore, B.W., and Perry, M.D., Laser-induced damage in dielectrics with nanosecond to subpicosecond pulses, Phys. Rev. Lett., vol. 74, no. 12, pp. 2248–2251, 1995. DOI: 10.1103/PhysRevLett.74.2248

Ben-Yakar, A. and Bourgeois, F., Ultrafast laser nanosurgery in microfluidics for genome-wide screenings, Curr. Opin. Biotechnol., vol. 20, no. 1, pp. 100–105, 2009. DOI: 10.1016/j.copbio.2009.01.008

Noack, J. and Vogel, A., Laser-induced plasma formation in water at nanosecond to femtosecond time scales: calculation of thresholds, absorption coefficients, and energy density, IEEE J. Quantum Electron., vol. 35, no. 8, pp. 1156–1167, 1999. DOI: 10.1109/3.777215

Niemz, M.H., Laser-Tissue Interactions, 4th ed. Springer, 2019.

Vogel, A. and Venugopalan, V., Mechanisms of pulsed laser ablation of biological tissues, Chem. Rev., vol. 103, no. 2, pp. 577–644, 2003. DOI: 10.1021/cr010379n

Gabay, I., Subramanian, K., Andrus, L., DuPlissis, A., Yildirim, M., and Ben-Yakar, A., In vivo hamster cheek pouch subepithelial ablation, biomaterial injection, and localization: pilot study, J. Biomed. Opt., vol. 27, no. 08, Aug. 2022. DOI: 10.1117/1.jbo.27.8.080501

Rosenfeld, A., Lorenz, M., Stoian, R., and Ashkenasi, D., Ultrashort-laser-pulse damage threshold of transparent materials and the role of incubation, Appl. Phys. A, vol. 69, pp. S373–S376, 1999. DOI: 10.1007/s003399900237

Ben-Yakar, A. and Byer, R.L., Femtosecond laser ablation properties of borosilicate glass, J. Appl. Phys., vol. 96, no. 9, pp. 5316–5323, 2004. DOI: 10.1063/1.1787145

Vogel, A., Noack, J., Hüttman, G., and Paltauf, G., Mechanisms of femtosecond laser nanosurgery of cells and tissues, Appl. Phys. B, vol. 81, no. 8, pp. 1015–1047, 2005. DOI: 10.1007/s00340-005-2036-6

Koenig, K., Intracellular nanosurgery with near infrared femtosecond laser pulses, Cell. Mol. Biol., vol. 45, no. 2, pp. 195–201, 1999.

Fatih Yanik, M., Cinar, H., Nese Cinar, H., Chisholm, A.D., Jin, Y., and Ben-Yakar, A., Functional regeneration after laser axotomy, Nature, vol. 432, p. 822, 2004.

Hoy, C.L. et al., Clinical ultrafast laser surgery: Recent advances and future directions, IEEE Journal on Selected Topics in Quantum Electronics, vol. 20, no. 2, 2014. DOI: 10.1109/JSTQE.2013.2287098

Hoy, C.L. et al., Miniaturized probe for femtosecond laser microsurgery and two-photon imaging, Opt. Express., vol. 16, no. 13, p. 9996, 2008. DOI: 10.1364/oe.16.009996

Hoy, C.L. et al., Optical design and imaging performance testing of a 9.6-mm diameter femtosecond laser microsurgery probe, Opt. Express., vol. 19, no. 11, p. 10536, 2011. DOI: 10.1364/oe.19.010536

Ferhanoglu, O., Yildirim, M., Subramanian, K., and Ben-Yakar, A., A 5-mm piezo-scanning fiber device for high speed ultrafast laser microsurgery, Biomed. Opt. Express., vol. 5, no. 7, p. 2023, Jul. 2014. DOI: 10.1364/boe.5.002023

Subramanian, K., Andrus, L., Pawlowski, M., Wang, Y., Tkaczyk, T., and Ben-Yakar, A., Ultrafast laser surgery probe with a calcium fluoride miniaturized objective for bone ablation, Biomed. Opt. Express., vol. 12, no. 8, p. 4779, Aug. 2021. DOI: 10.1364/boe.426149

Andrus, L. et al., Ultrafast laser surgery probe for sub-surface ablation to enable biomaterial injection in vocal folds, Sci. Rep., vol. 12, no. 1, p. 20554, Nov. 2022. DOI: 10.1038/s41598-022-24446-5

Referências

  1. Shen, N., Datta, D., Schaffer, C.B., Leduc, P., Ingber, D.E., and Mazur, E., Ablation of cytoskeletal filaments and mitochondria in live cells using a femtosecond laser nanoscissor, 2005.
  2. Friedman, N.J. et al., Femtosecond laser capsulotomy, J. Cataract. Refract. Surg., vol. 37, no. 7, pp. 1189–1198, Jul. 2011. DOI: 10.1016/j.jcrs.2011.04.022
  3. Slade, S.G., The use of the femtosecond laser in the customization of corneal flaps in laser in situ keratomileusis, Current Opinion in Opthalmology, vol. 18, pp. 314–317, 2007.
  4. Stuart, B.C., Feit, M.D., Rubenchik, A.M., Shore, B.W., and Perry, M.D., Laser-induced damage in dielectrics with nanosecond to subpicosecond pulses, Phys. Rev. Lett., vol. 74, no. 12, pp. 2248–2251, 1995. DOI: 10.1103/PhysRevLett.74.2248
  5. Ben-Yakar, A. and Bourgeois, F., Ultrafast laser nanosurgery in microfluidics for genome-wide screenings, Curr. Opin. Biotechnol., vol. 20, no. 1, pp. 100–105, 2009. DOI: 10.1016/j.copbio.2009.01.008
  6. Noack, J. and Vogel, A., Laser-induced plasma formation in water at nanosecond to femtosecond time scales: calculation of thresholds, absorption coefficients, and energy density, IEEE J. Quantum Electron., vol. 35, no. 8, pp. 1156–1167, 1999. DOI: 10.1109/3.777215
  7. Niemz, M.H., Laser-Tissue Interactions, 4th ed. Springer, 2019.
  8. Vogel, A. and Venugopalan, V., Mechanisms of pulsed laser ablation of biological tissues, Chem. Rev., vol. 103, no. 2, pp. 577–644, 2003. DOI: 10.1021/cr010379n
  9. Gabay, I., Subramanian, K., Andrus, L., DuPlissis, A., Yildirim, M., and Ben-Yakar, A., In vivo hamster cheek pouch subepithelial ablation, biomaterial injection, and localization: pilot study, J. Biomed. Opt., vol. 27, no. 08, Aug. 2022. DOI: 10.1117/1.jbo.27.8.080501
  10. Rosenfeld, A., Lorenz, M., Stoian, R., and Ashkenasi, D., Ultrashort-laser-pulse damage threshold of transparent materials and the role of incubation, Appl. Phys. A, vol. 69, pp. S373–S376, 1999. DOI: 10.1007/s003399900237
  11. Ben-Yakar, A. and Byer, R.L., Femtosecond laser ablation properties of borosilicate glass, J. Appl. Phys., vol. 96, no. 9, pp. 5316–5323, 2004. DOI: 10.1063/1.1787145
  12. Vogel, A., Noack, J., Hüttman, G., and Paltauf, G., Mechanisms of femtosecond laser nanosurgery of cells and tissues, Appl. Phys. B, vol. 81, no. 8, pp. 1015–1047, 2005. DOI: 10.1007/s00340-005-2036-6
  13. Koenig, K., Intracellular nanosurgery with near infrared femtosecond laser pulses, Cell. Mol. Biol., vol. 45, no. 2, pp. 195–201, 1999.
  14. Fatih Yanik, M., Cinar, H., Nese Cinar, H., Chisholm, A.D., Jin, Y., and Ben-Yakar, A., Functional regeneration after laser axotomy, Nature, vol. 432, p. 822, 2004.
  15. Hoy, C.L. et al., Clinical ultrafast laser surgery: Recent advances and future directions, IEEE Journal on Selected Topics in Quantum Electronics, vol. 20, no. 2, 2014. DOI: 10.1109/JSTQE.2013.2287098
  16. Hoy, C.L. et al., Miniaturized probe for femtosecond laser microsurgery and two-photon imaging, Opt. Express., vol. 16, no. 13, p. 9996, 2008. DOI: 10.1364/oe.16.009996
  17. Hoy, C.L. et al., Optical design and imaging performance testing of a 9.6-mm diameter femtosecond laser microsurgery probe, Opt. Express., vol. 19, no. 11, p. 10536, 2011. DOI: 10.1364/oe.19.010536
  18. Ferhanoglu, O., Yildirim, M., Subramanian, K., and Ben-Yakar, A., A 5-mm piezo-scanning fiber device for high speed ultrafast laser microsurgery, Biomed. Opt. Express., vol. 5, no. 7, p. 2023, Jul. 2014. DOI: 10.1364/boe.5.002023
  19. Subramanian, K., Andrus, L., Pawlowski, M., Wang, Y., Tkaczyk, T., and Ben-Yakar, A., Ultrafast laser surgery probe with a calcium fluoride miniaturized objective for bone ablation, Biomed. Opt. Express., vol. 12, no. 8, p. 4779, Aug. 2021. DOI: 10.1364/boe.426149
  20. Andrus, L. et al., Ultrafast laser surgery probe for sub-surface ablation to enable biomaterial injection in vocal folds, Sci. Rep., vol. 12, no. 1, p. 20554, Nov. 2022. DOI: 10.1038/s41598-022-24446-5
Voltar para o topo © Copyright 2008-2024