Heat impact during laser ablation extraction of mineralised tissue micropillars

Samuel Mcphee,Jonathan D Shephard,Alexander Groetsch,Uwe Wolfram

doi:10.1038/s41598-021-89181-9

Abstract

The underlying constraint of ultrashort pulsed laser ablation in both the clinical and micromachining setting is the uncertainty regarding the impact on the composition of material surrounding the ablated region. A heat model representing the laser-tissue interaction was implemented into a finite element suite to assess the cumulative temperature response of bone during ultrashort pulsed laser ablation. As an example, we focus on the extraction of mineralised collagen fibre micropillars. Laser induced heating can cause denaturation of the collagen, resulting in ultrastructural loss which could affect mechanical testing results. Laser parameters were taken from a used micropillar extraction protocol. The laser scanning pattern consisted of 4085 pulses, with a final radial pass being 22 upmu {text {m}} away from the micropillar. The micropillar temperature was elevated to 70.58 ^{circ }{text {C}}, remaining 79.42 ^{circ }{text {C}} lower than that of which we interpret as an onset for denaturation. We verified the results by means of Raman microscopy and Energy Dispersive X-ray Microanalysis and found the laser-material interaction had no effect on the collagen molecules or mineral nanocrystals that constitute the micropillars. We, thus, show that ultrashort pulsed laser ablation is a safe and viable tool to fabricate bone specimens for mechanical testing at the micro- and nanoscale and we provide a computational model to efficiently assess this.

Highlights

The thermal response of bone tissue can be divided into a cellular and an extracellular response
Prior to laser ablation, mineralised turkey leg tendon (MTLT) samples were dissected by scalpel and diamond band saw (Exakt, Norderstedt, Reichert-Jung) before being glued into an aluminium sample holder
We used experimental laser parameters from a micropillar extraction protocol for mineralised collagen fibres reported by Groetsch et al.[33]

Summary

Introduction

The thermal response of bone tissue can be divided into a cellular and an extracellular response. Monitoring and understanding the permissible temperature exposure with respect to time and magnitude will allow for specific laser ablation treatment protocols to be optimised for increased bone removal rates. We deem 150 ◦C , irrespective of any exposure time, as a maximum temperature prior to denaturation as Bozec and Odlyha[26] report this as the temperature at which intramolecular hydrogen bonded water evaporates in dry collagen fibrils. This bonding stabilises the ultrastructure of the collagen helix, and it can be inferred that heat induced loss will allow for fragmentation at lower mechanical loads. The aim of this paper was to (1) implement a heat model representing the laser-tissue interaction into a finite element suite to quantify the temperature elevation during micropillar extraction using picosecond laser ablation; (2) use the model to evaluate the existing ablation process employed by Groetsch et al.[33] for micropillar extraction; (3) use post-production compositional analysis to qualitatively asses denaturation; and (4) use the model to evaluate minimal pillar sizes through laser ablation only

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