Beauty has been an everlasting human pursuit. As people become increasingly demanding about their bodily image, skin surgery techniques require further refinements. Presently, the skin surgery primary methods used for the removal of lesioned or necrotic tissues include scalpel, mechanical, autolysis, chemical, and biological methods 1. However, these methods require that the surgeon cuts the tissue manually, which might often entail operation errors. It has been reported that an experienced surgeon can debride over 40% of normal tissue when removing necrotic skin tissue 2, which is detrimental to skin healing. Therefore, it is necessary to set up a novel skin surgery technique that while being precise is not affected by doctors' skills. To this aim, scientists have turned their attention to laser surgery techniques.
Since 1970s, laser has been used in clinical surgery 3. Currently, CO2 laser, Er-Cr:YSGG laser, He-Ne laser, Nd:YAG laser, Er:YAG laser, and excimer laser are widely used in clinical applications 4. Nowadays, laser surgery mainly works via the laser-evoked thermal effects. That is, the tissues' local temperature rises due to the absorption of the laser light. This leads to protein denaturation, solidification, even carbonization and gasification, thereby achieving the purpose of removing the targeted skin tissue 5. Studies have shown that the thermal injury's thickness of the fractional laser can exceed 200 µm 6. This is particularly unfavorable for skin surgery, because an injury depth of about 200 µm entails that the probability of after surgery scarring will increase significantly 7. Therefore, it is necessary to use a safer and more effective laser technique.
In the 1980s, with the appearance of chirped pulse amplification (CPA) technology 8, the time scale of laser pulse could be further compressed. In 1991, the first fs-laser device was successfully developed in Austria 9, ushering in a new era of fs-laser application. When the fs-laser interacts with any material, the entire process takes a very short time: the material's temperature reaches its peak instantaneously, being directly converted into a plasma state before melting. Hence, the material can be removed without any obvious slag and debris 10. Since the interaction time is extremely short, the laser-induced electron heat cannot diffuse to the surrounding materials. Therefore, the size of the material's heat-affected zone is very small, which translates into a laser-evoked non-thermal melting, so that the fs-laser processing is also called "cold-ablation" 11. Currently, fs-lasers have been widely used for precision machining and modification of various materials such as polymers 12, glasses 13, semiconductors 14 and metals 15. In bio-medical field, fs-lasers have been used in a variety of researches 16, such as stomatology 17, urology 18, otolaryngology 19 and orthopedics 20, which demonstrated minimal collateral damage of fs-laser to biological tissues, revealing its potential as a surgical tool. However, despite the reported utilization of femtosecond lasers in scenarios that do not demand meticulous control, such as photodynamic therapy (PDT) 21–23, drug delivery enhancement 24, and skin suturing assistance 25, their application as a highly precise surgical tool in dermatology and plastic surgery remains unattained. The main reason is the challenges posed by uneven skin surfaces, making it difficult to achieve accurate control of fs-laser during the cutting and removal of necrotic skin tissue, which is essential in skin surgery. Without an accurate calibration and focusing system, unpredictable damage to the skin is likely to occur, and it is impossible to further explore the effect of different parameters of fs-laser on skin surgery, which is a prerequisite for the application of femtosecond laser in dermatology and plastic surgery.
To achieve precise focusing and ablation of femtosecond laser on uneven skin surfaces, our team has introduced a three-axis coordinated motion micrometer-level precision displacement stage to facilitate relative motion between the sample and the laser focal spot 26. This is a mature laser processing system that does not require additional hardware. By utilizing software function modules designed and developed by our research team, compensation and calibration of the relative distance between the laser focal spot and the sample surface can be achieved. The specific methodology involves initially measuring the height of selected sampling points on the sample surface, then reconstructing the surface using two-dimensional interpolation. Finally, the planar processing path is projected onto the reconstructed surface, enabling processing along this three-dimensional path.
In this study, a focus-corrected fs-laser ablation system for skin surgery was built by our team using a correcting method based on two-dimensional interpolation. The purpose of this study was to explore the feasibility and safety of the focus-corrected fs-laser ablation system for skin surgery, and to analyze the dose-effect relationship of key parameters by using fs-lasers with different key parameters to debride normal and burned pig skin. These approaches aimed at defining an implementation scheme for the precise debridement of two-dimensional and three-dimensional shapes by using an fs-laser, and at laying the foundations for the application of fs-lasers in ultra-precision skin surgery.