Structures for biomimetic, fluidic, and biological applications
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Introduction to ultrafast laser fabrication of biomimetic structures The study and simulation of biological systems is popularly known as biomimetics. Nature offers a diverse wealth of functional surfaces that are unmatched by today’s artificial materials. Based on the ideas, concepts, and underlying principles developed by nature, a highly interdisciplinary field has emerged to design, synthesize, and fabricate biomimetic structures.1 As a result, several methodologies have been developed to form bioinspired constructs, exhibiting hierarchical structuring at length scales ranging from hundreds of nanometers to several micrometers. The two main fabrication routes are based on top-down and bottom-up processing schemes, respectively. In the top-down approach, a material is produced in bulk and then shaped into a finished part through a variety of processes. In contrast, bottom-up techniques construct the desired features from fundamental building blocks, without the need for patterning. Laser processing is a maskless top-down approach that allows localized modifications with a large degree of control over the shape and size of the features that are formed.2 It is generally less expensive than e-beam texturing and more flexible, since it does not require complex vacuum facilities. While the minimal achievable structure size is limited
by diffraction to the order of wavelength (microscale) in the far-optical field, the optimal interplay between the laser and material parameters may allow the fabrication of features with dimensions beyond this diffraction limit (nanoscale). This has been accomplished via the application of femtosecond (fs) lasers in materials processing, which has increasingly proved to be a powerful approach to overcome the diffraction limit of longer pulse and continuous laser processes and has improved the lateral resolution in the fabricated structures.3 The most useful property of fs laser-induced modification is the limited size of the affected volume. Material structuring with laser pulses is induced by optical breakdown, which generates a plasma at the focal point of the laser. Ablation of the substrate is initially confined to a small volume due to setin of shock wave propagation and cavitation, arising from plasma recombination before thermal diffusion. Although the intensity required to initiate breakdown is fairly high, the short pulse duration allows achieving the threshold intensity with a modest fluence. The combination of localized excitation and low-threshold fluence can greatly reduce the extent of collateral damage to surrounding areas, such that the size of the affected material can be below the diffraction-limited focusing volume. Another key advantage of using fs lasers is that nonlinear absorption effects can be driven due to the
Emmanuel Stratakis, Institute of Electronic Structure and Laser, Foundation for Research and Technology–Hellas, Greece; [email protected] Hojeong Jeon, Center for Biomaterials, Biomedical Research Institute, Korea Institute of Science and Technology, South Kore
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