What happens in laser surgery, when a laser pulse hits tissue?
Surgery plays a crucial role in treating numerous health conditions but can result in scarring, affecting both cosmetic appearance and the functional healing of tissues. Traditional surgical procedures using scalpels apply shear stress to tissues, leading to increased damage and scarring in surrounding areas. Laser surgery, on the other hand, alleviates mechanical stress concerns and, in theory, allows for precision at the single-cell level. Yet, in practice, it faces challenges such as shock waves and thermal damage, which can similarly lead to scarring.
The Atomically Resolved Dynamics research group at the Max Planck Institute for the Structure and Dynamics of Matter (MPSD) has been developing a laser system (PIRL) to address the issues of shockwaves and thermal damage commonly associated with conventional laser surgery. Unlike traditional systems that indiscriminately heat tissue with long laser pulses (~10 nanoseconds), their approach involves a laser designed to selectively excite the O-H bond water molecules with short laser pulses (~10 picoseconds). This technique leads to the rapid vaporization of tissue, with water being near-instantaneously eliminated and tissue molecules ejected into air, thereby minimizing in-tissue interactions and substantially reducing the damage zone. The laser ablation process also occurs at a significantly faster rate compared to thermal energy transfer and shockwave generation, effectively preventing processes that lead to scarring, such as plasma formation and ionizing radiation. The employment of shorter laser pulses, which possess lower energy, further enhances the pros of this method. Finally, the employment of shorter laser pulses, which possess lower energy, underscores the method's advantage in reducing scarring.
Enter my work. I spent 8 months in an internship with the group, building a system capable of capturing the dynamics of laser ablation using the proposed laser. My objectives were to a) construct an imaging system that would allow us to visualize the rapid dynamics of this process and thus gain insights into the energy transfers involved, b) devise a method to quantify the amount of material removed per ablation, and c) investigate whether bonds other than the O-H bond were being excited by the laser, exploring the implications for energy transfers and ablation dynamics. Overall, these objectives were all directed towards characterizing the residual damage inflicted on tissue through energy deposits and shockwaves potentially caused by the proposed laser.
On a more personal level, this experience was extraordinary—undoubtedly one of the most enjoyable research ventures I've undertaken so far. Sascha Epp is among the most delightful mentors I've had the pleasure of working with. His curiosity for science and research is infectious, and I credit him with showing me how fun a career in research could be. Beyond Sascha, the support and expertise of the MPSD staff engineers were invaluable. They were not just instrumental in bringing my designs to life but also took the extra step to impart design principles to me. The environment was further enriched by other group leaders and researchers, who were always approachable and open to discussion. The graduate students and post docs at the time, including Frederik Busse, Andrey Krutilin, Pedram Mehrabi, and others, were exceptionally welcoming, always including me in their weekly social outings. The workplace culture was incredible, fostering a sense of community. In summary, the research environment and the people at MPSD made my experience truly memorable.
Anyways 😊 The remainder of this article will focus on the projects I undertook in my internship, organized by the internship objectives mentioned previously:
Objective: what is the damage (in volume/area) done by laser ablation?
CAD model of interferometer.
A conventional laser surgery ablation system may result in a damage zone exceeding 800 micrometers, in contrast to the proposed laser system, which limits the damage zone to less than 10 micrometers. With this in mind, we sought to estimate the volume of material removed in a single pulse, which would allow us to deduce information about the depth of the pulse's impact.
The length scales we are dealing with here is less than 10 micrometers, which is quite small but falls within an ideal range for utilizing wave optics, specifically interferometry for measurements. We eventually converged upon the idea of suspending thin films (as proxy for tissue) in the path of an interferometer, and measuring it’s thickness with the interferometer before and after laser ablation. This section will detail the construction of said thin films and interferometer.
<aside> <img src="/icons/directional-sign-right_gray.svg" alt="/icons/directional-sign-right_gray.svg" width="40px" /> TLDR: We designed a Mach-Zehnder interferometer capable of measuring free-standing thin film thicknesses (200 nm - 200 μm) and refraction indices (1.3 - 1.8) simultaneously.
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Thin water-based films are amazing tools for suspending proteins in atmosphere for laser ablation at different angles. Mixed with appropriate proteins, the surface tension of water can be increased to create a free standing water film that has thickness on the order of nanometers.
Also, a couple of my Engineering Physics seniours (Yuqing Du, Daniel Schultz, et al) had come up with a Lagrangian mechanics formulation for the wall thickness of a bubble. It was nice piggy-backing off of their work to derive something similar for a thin film.
There are two unknowns that my interferometer needed to measure simultaneously: the sample thickness and index of refraction. Here’s some design processes I went through.