Session S5 - Laser Ablation of Biological and Polymeric Materials.
INVITED session, Wednesday afternoon, March 24
524AB, Palais des Congres

[S5.001] Twenty Years of Ablative Photodecomposition (Industrial Applications of Physics Prize Lecture)

It was a little over twenty years ago that two groups, one in the U.S. and another in Hull, U.K. working independently, reported that ultraviolet laser pulses of nanosecond duration were capable of etching organic polymer films without any need for further development.In 1982, our group at the IBM Research Center which studied this phenomenon at 193nm named the process "Ablative Photodecomposition". Dyer and his group in Hull reported similar results at 248nm and 308nm in 1983. We also showed that the process worked well in the etching of tissue and had the potential to be of use in surgery since there was no detectable heat damage around the exposed area. These reports led to a burst of activity from numerous groups all over the world which has not slowed down significantly to the present day. The driving force behind the activity was and continues to be the potential for applications in several areas of computer hardware technology as well as in corneal refractive surgery. Today these applications can be said to be firmly in place and well-accepted. Strangely, the science behind the process is still a subject for serious discussion. The reason for this is the mix of physicists, chemists, medical doctors and engineers who have stepped into the debate. The result is a true 'Babel' which will be pointed out.

[S5.002] Photochemical Ablation of Organic Solids

As discovered by Srinivasan in 1982, irradiation of materials by far UV laser light can lead to photochemical ablation, a process distinct from normal thermal ablation in which the laser primarily heats the material. A versatile mesoscopic model for molecular dynamics simulations of the laser ablation phenomena is presented. The model incorporates both the thermal and photochemical events, that is, both heating of the system and UV induced bond-cleavage followed by abstraction and radical-radical recombination reactions. The results from the simulations are compared to experimental data and the basic physics and chemistry for each irradiation regime are discussed. Initial results from polymer ablation simulations will be presented.

L. V. Zhigilei, P. B. S. Kodali and B. J. Garrison, J. Phys. Chem. B, 102, 2845-2853 (1998); L. V. Zhigilei and B. J. Garrison, Journal of Applied Physics, 88, 1281-1298 (2000).

Y. G. Yingling, L. V. Zhigilei and B. J. Garrison, J. Photochemistry and Photobiology A: Chemistry, 145, 173-181 (2001); Y. G. Yingling and B. J. Garrison, Chem. Phys. Lett., 364, 237-243 (2002).

[S5.003] Chemical and Spectroscopic Aspects of Polymer Ablation-Special Features and Novel Directions-

Laser ablation of polymers has become an established technique in the electronic industry and the large number of studies published annually indicates that this is still an attractive area of research. Several new approaches with new techniques and materials have given new insights in the ablation process. One of these approaches is the development of polymers designed specifically for laser ablation which are a unique tool for probing the ablation mechanisms as well as for improving ablation properties. These novel polymers exhibit very low thresholds of ablation, with high ablation rates (even at low fluences), and excellent ablation quality. New commercial applications will require improved ablation rates and control of undesirable surface effects, such as debris. The complexity of the interactions between polymers and laser photons are illustrated by the various processes associated with different irradiation conditions. i) Photothermal-photochemical laser ablation under excimer laser irradiation. ii) Dopant-induced laser ablation. iii) Photo-oxidative etching with lamps in an oxidizing atmosphere. iv) VUV etching in the absence of oxidizing conditions. v) Photokinetic etching with CW UV lasers. vi) Ultrafast laser ablation, affected by pulse length, wavelength, and possibly shock waves. vii) Shock assisted photothermal ablation on picosecond time scales. viii) VUV laser ablation: purely photochemical? ix) Synchrotron structuring. x) Mid-IR ablation (FEL and CO2 laser), the influence of exciting various functional groups. Several of these new approaches and processes will be discussed to emphasize the importance of different approaches but also to review some fundamental processes. The combination of various experimental techniques (new approaches and ‘well-known’) with materials made to measure has given new insights in the ablation mechanisms, but has also shown new possible future directions of laser polymer ablation.

[S5.004] Cell-by-Cell Fabrication of Biological Systems by Laser Forward Transfer

Through a series of experiments performed at the US Naval Research Laboratory, we have demonstrated the ability to fabricate novel 3-D tissue constructs using a unique laser transfer process. At the heart of this technology is the ability to rapidly build, by a CAD/CAM process (i.e., Rapidly Prototype), engineered tissue constructs cell-by-cell, layer-by-layer, and unit-by-unit in order to simulate or facilitate native structured tissue. Powered by this breakthrough in biomaterial processing, we can now enhance understanding, development, and exploitation of the field of tissue engineering by the ability to group and order specific, defined populations of cells and bioscaffolding with precision. The eventual goal is to demonstrate specific biological function by engineering tissue constructs consisting of defined mammalian cell populations. This presentation will then summarize the contribution our laser transfer approach makes to rapid prototyping as it applies to tissue engineering.

[S5.005] Laser Ablation of Polymer Microfluidic Devices

Microfluidic technology is ideal for processing precious samples of limited volumes. Some of the most important classes of biological samples are both high in sample complexity and low in concentration. Combining the elements of sample pre-concentration, chemical separation and high sensitivity detection with chemical identification is essential for realizing a functional microfluidic based analysis system.

Direct write UV laser ablation has been used to rapidly fabricate microfluidic devices capable of high performance liquid chromatography (HPLC)-MS. These chip-LC/MS devices use bio-compatible, solvent resistant and flexible polymer materials such as polyimide. A novel microfluidic to rotary valve interface enables, leak free, high pressure fluid switching between multiple ports of the microfluidic chip-LC/MS device. Electrospray tips with outer dimension of 50 um and inner of 15 um are formed by ablating the polymer material concentrically around a multilayer laminated channel structure.

Biological samples of digested proteins were used to evaluate the performance of these microfluidic devices. Liquid chromatography separation and similar sample pretreatments have been performed using polymeric microfluidic devices with on-chip separation channels. Mass spectrometry was performed using an Agilent Technologies 1100 series ion trap mass spectrometer. Low fmol amounts of protein samples were positively and routinely identified by searching the MS/MS spectral data against protein databases. The sensitivity and separation performance of the chip-LC devices has been found to be comparable to state of the art nano-electrospray systems.

Part S of program listing