飞秒激光辅助超临界流体进行生物医用高分子材料微加工

The residual pulse energy and accumulation behavior occurred under ultrashort pulsed laser ablation in polymer materials. It is shown that heat affected zone may be involved due to superheating, and damage the biodegradable properties and clean micromachining, and has become the last bottleneck of laser micromachining. A method based on supercritical fluid is an effective technique used to decompose organic polymers. So, we could put the organic polymer into a supercritical fluid, then move laser focus to the interface between the organic polymer and supercritical fluid, and drive the liquid in laser focus into supercritical state by the extreme physical condition in laser focus. The organic polymer in laser focus would be decomposed and removed. And the biodegradable properties in the boundary of the laser ablated zone could be maintained. Because of the ultrashort pulsewidth and very high peak intensity of femtosecond laser, multiphoton absorption is dominant in a transparent materials, and the breakdown threshold becomes precise. Only inside the transparent liquid, the breakdown would be achieved. If we manipulate the laser intensity just above the breakdown threshold, sub-diffraction limited structure would be produced. The modified texture would be avoid, and the biodegradable properties would be maintained. This proposal aim at the physical mechanisms and technologies in the high repetition rate femtosecond laser-assisted supercritical fluid etching of the biomedical polymer materials, and establish the base for the potential applications.

飞秒激光烧蚀高分子材料过程中有剩余热存在和多脉冲热积累效应，所以，加工区边缘（过热）形成热影响区，发生改性，这不但影响加工质量，也影响生物可降解特性，成为激光加工技术应用的瓶颈。超临界流体可以对高分子材料进行氧化分解，因此，可以将激光聚焦在高分子材料与液体交界面上，利用焦点处形成的极端物理条件，使（焦点微区内）液体形成超临界态，以氧化分解方式去除需加工的材料，达到微加工的目的，本并可保持加工区生物可降解特性。由于飞秒激光的脉冲宽度极短、峰值功率极高，在透明液体中，多光子吸收占优势，烧蚀阈值很精确，可以在透明液体内部形成超临界态，因此，将激光强度控制在正好等于或略高于超临界态阈值，可进行低于衍射极限的亚微米加工。从而避免了激光直接加工产生的改性和加工质量问题。本项目旨在系统研究利用高功率高重复率飞秒激光和超临界流体相结合进行生物医用高分子材料微加工的物理过程及其加工工艺。

本课题主要研究了激光加工环境和工艺对生物降解特性影响。发现：低重复率（1 kHz）飞秒激光存在明显的脉冲积累效应，随脉冲数增加，加工区表面粗糙度逐渐减小，可以产生较光滑的加工表面，但是，由于脉冲重复率低，加工效率难以提高；对于高重复率（50 MHz）飞秒激光，虽然提高了加工效率，却产生了强烈的热积累效应，以至于由于热作用形成大量纳米尺度的微泡沫（不能有效去除加工区的材料），在加工区形成凸起和微裂纹。因此，在高重复率下激光加工高分子材料有必要在液体环境下进行，即将激光聚焦在透明超临界流体与有机高分子材料界面上（通过多光子吸收过程，达到超临界态，此时，高分子材料还没有达到改性阈值）形成超临界流体，对焦点处的有机高分子材料进行溶解/分解，实现了加工区的高分子材料的有效去除，但是，选区水解反应较慢，导致高分子材料的去除速率较低。在此基础上，我们通过添加环境友好型化学试剂（包括催化剂和共溶剂等）的方法，大大提高了加工区的高分子材料的去除速率，可产生需要的加工效果，既避免了激光直接加工的热影响区问题，也同时保持了生物可降解特性。在生物医疗微器件加工领域具有工业化应用潜力。