Enhanced Femtosecond Laser Micromachining of Biological Hard Tissue and Industrial Materials

Abstract

Ultrafast laser micromachining with pulse widths < 10 ps have been shown to be a promising tool that can fabricate microstructures on almost any material with high resolutions and minimal collateral damage. The ultrashort pulse widths allow for nonlinear absorption, allowing machining of a wide range of materials regardless of their absorption characteristic. Meanwhile, the ultrashort pulse widths enable “cold cutting” with little or no damage to the surrounding area. Several novel applications of lasers in the medical field have been explored such as dentistry, optometry and cosmetics. However, lasers have yet to be implemented clinically for the removal of hard tissue such as bone. Previous experiments using continuous wave (CW) or long pulses (≥nanosecond) lasers have been explored but have shown extensive detrimental heat affected zones (HAZ) around the irradiation site, making them unsuitable for clinical use. The ability for ultrashort pulsed lasers to remove material using their “cold cutting” abilities make them a prime candidate for machining of biological hard tissue. Despite this advantage, the material removal rates for ultrashort pulsed lasers remain too slow for widespread use in industry and medicine. Whilst significant research has been made to improve processing speeds of ultrafast laser micromachining by increasing average powers and repetition rates, an alternative to this approach is to change the spatial profile of the incident beam to increase machining efficiency. Femtosecond laser micromachining with λ = 800 nm, τ = 230 fs and 1 kHz repetition rates with a traditional Gaussian beam was used to measure the standard metrics of laser micromachining on bone tissue – ablation threshold (φth), incubation effects, ablation rates (μm pulse-1) and feature assessment. This served as a baseline assessment for femtosecond lasers as a high precision orthopaedic tool. We determined the ablation threshold of both bovine and ovine cortical and skull bone to be 0.91 ± 0.03 J/cm2. An incubation coefficient of 1.02 ± 0.05 indicates that, unlike with most other materials, there is virtually no change in the ablation threshold with successive number of applied pulses. Maximum ablation rates of bovine and ovine cortical bone were found to be ≈ 0.90 μm pulse-1 at a fluence of 5.3 J/cm2. The removal of bone material was found to be relatively insensitive to the position of the focal point of the beam below the sample surface. A linear increase in feature depth as the focal point was moved deeper into the material terminated in a maximum feature depth of approximately 100 μm at a focal depth of 2.1 – 2.4 mm (fluence range: 2.5 - 5.6 J/cm2). No structural damage or heat affected zones (discoloration, charring, thermal shockwave cracking, and molten debris) were observed. Collagen tufts, canaliculi and hydroxyapatite crystals are identified at high magnifications verifying that energy deposition does not cause melting of the target tissue. Despite the several advantages shown using femtosecond lasers, the main drawback was the maximum drilling rate corresponding to 54 mm min-1 at a pulse repetition rate of 1 kHz. This is extremely low when compared to conventional drilling tools which can reach up to 5.53 m min-1. Using a liquid crystal on silicon spatial light modulator (LCOS-SLM), the standard metrics of micromachining for freshly harvested ovine and bovine cortical bone were obtained when laser micromachining was performed using a zero-order Bessel beam with a cone angle of 4.56o. Results showed a significant decrease in the ablation threshold of up to 7X lower than for Gaussian beams whilst also showing increased maximum ablation depths and ablation rates 14X greater than Gaussian beams. In order to better understand the reason why Bessel beams showed lower ablation thresholds and enhanced ablation efficiency in bone, further studies were performed on well characterised materials: silicon and quartz. Using the adapted diagonal-scan technique for Bessel and vortex beams of varying orders generated using the LCOS-SLM, ablation thresholds and incubation effects were studied. We observed that the laser ablation threshold does indeed depend on the beam shape and reinforces the fact that femtosecond laser micromachining and the ablation of materials occurs through complex, non-linear light-matter interactions and subsequent cascade events. Our results showed increased efficiency during the ablation of silicon and quartz with Bessel beams, similar to results seen for bovine and ovine cortical bone. Throughout our experiments, we observed a common feature in the diameter regression data for all materials, that being the dislocations of the trendline as pulse energies increased as well as discrepancies between the theoretical and experimentally derived beam waists that are crucial for determining the ablation threshold. Several strange physical features were also observed in the ablation features generated. We attempt to link these features and deviations to changes in the Keldysh parameter from the multiphoton ionisation regime to tunnelling ionisation as well as the atmospheric conditions where the ablation process takes place. We identified the ablation threshold for silicon, stainless steel and sapphire by performing the diameter regression technique in ambient and low vacuum conditions. We have identified up to four different ablation regimes by analysing the inner and outer diameter features which are evident at varying fluences and pulse numbers. We show that the theoretical beam waist described by classical Gaussian optics is only true for the lower ablation regime where pulse energies are less than ~90 μJ. The dislocations in the diameter regression trendlines can be explained by changes in the Keldysh parameter. Meanwhile, we attempt to explain the multi-diameter features seen during ablation using previous studies and simulations that take in to account the Kerr effect and plasma distortion of the beam profile before it reaches the sample to perform ablation.