Laser structuring of materials for biomedical applications

Laser processing methods have become very appealing for the fabrication of micro/nano structures. To fabricate 3D structures with high resolution andarbitrary complexity, several material deposition processes are in use. By using appropriate moulding techniques, these structures can be fabricated out of a variety of materials such as polymers, ceramics and composites. In this work different lasers have been investigated regarding their suitability for additive and subtractive patterning of small features for biomedical applications. The main focus is on a technique based on two-photon polymerisation of photosensitive materials; this is a nonlinear optical stereo lithography which allows direct-writing of high-resolution three dimensional structures. During the two-photon absorption process, temporal and spatial overlap of photons leads to nonlinear absorption in a highly localized volume. Absorbed photons induce chemical reactions which cause a polymer to form. Due to the quadratic intensity dependence of the process, resolutions of less than 100nm in polymerized structures can potentially be achieved because of the well-defined polymerization threshold. Here, we have emphasised another regime whereby deep structures (~300µm) can be generated in a single pass. This allows rapid fabrication of structures suitable for cell scaffolds where the length scales are small (~10µm) and are required over long ranges (~cm). A Ti: sapphire femtosecond laser at 800nm wavelength with 150fs pulse duration and 1kHz repetition rate was used to determine the two-photon absorption cross section of photoinitiators. This approach was used to initiate two-photon polymerization of resin allowing the fabrication of cell scaffolds suitable for biomedical applications. Diffraction calculations for the imaging optics employed, show that spherical aberration plays a significant role in determining the feature sizes achieved.For subtractive patterning of materials, a femtosecond laser system and an ArF excimer laser have been used. Using ablative techniques keratin films were processed to investigate physical realisation of the commonly used theoretical bricks-and-mortar description of skin. This structure was successfully fabricated and is being used for skin cream research. Also the threshold fluence for ablation of Polyimide Kapton (HN) foils has been measured at oblique angles as an analogue for corneal sculpturing based on beam scanning