Two Beam Initiation Threshold Measurements of Photo-Initiators for Laser Writing of Biocompatible 3D Structures

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We present our latest work on the investigation of new photo-initiator materials using two beam initiation threshold (2-BIT) [1] for Direct Laser Writing (DLW). The technique used for fabricating, often complex, 3D structures is multiphoton polymerisation (MPP) of photopolymers [2]. These have found uses in such diverse fields as optical sensors, metamaterials, and biomedical scaffolding. At the heart of this technique is the localised polymerization/cross linking of photosensitive materials through nonlinear optical absorption of ultrafast femtosecond laser pulses. This photosensitive system is mediated through a photo­initiator (PI) in order to start the polymerisation/cross linking of the multi-photon absorption. The criteria for a successful PI are high multi-photon absorption cross section, good solubility in the solvent medium, and high radical generation yield. 2-BIT is a useful addition to screen promising PIs for their suitability, unambiguously gives the order of the nonlinear absorption and can used with equipment readily available in a nonlinear lithography lab. This technique was used to determine the order of the effective nonlinear absorption of three new PIs [3]. This was further compared with the usual technique of z-scan where the order of the nonlinear absorption is often ambiguous. To ensure the overall viability for biocompatible DLW, the UV-Viz absorption and emission spectra were taken, along with test structures written using different laser intensities and velocities. Further biocompatibility was checked through adhesion, viability, and proliferation of dental Stem Cells.