Functional Materials for Two‐Photon Polymerization in Microfabrication

Abstract

The creation of 3D micro and nanostructures is of particular interest in the fields of photonics, [3] microelectromechanical systems (MEMS), [4] metamaterials, [5] micro/ nanofluidics, [6] cellular proliferation, and differentiation. [7] To create 3D objects of arbitrary shape, 3D printing techniques offer many versatile solutions. [8] Among these, direct laser writing (DLW) allows for the simple preparation of (sub)micrometric objects and patterns with a resolution that can go below 100 nm. [9][10][11] DLW is an additive manufacturing technique based on twophoton polymerization (2PP). To produce a structure, a fs-pulsed long-wavelength laser is focused, by means of optical elements, in a photosensitive resin which is able to crosslink (negative photoresist) or decompose (positive photoresist) under an energetic radiation but that is also transparent at the laser wavelength: if the intensity of the excitation radiation is high enough-as it is the case in the focus-the resin can undergo two (or more)-photon absorption and polymerize (or decompose). [12] Such multiphotons absorption phenomena are nonlinear and the cross-section of the different materials scales with the square (or higher) of the intensity of the radiation. For these reasons, the laser beam can enter the photoresist without being absorbed and trigger the photochemical process(es) only in a small volume around the focus. This volume is named "voxel" being the 3D analogue of a 2D pixel. [13] The absorption probability outside of the voxel is small, thus suppressing the propagation of the reaction and resulting in fine resolution. By moving around the voxel, one can obtain complex 3D structures that can be isolated after developing. [14,15] More than two decades have passed since DLW was proposed [16] and much progress has been made in terms of improving feature size and resolution, [9,[17][18][19][20][21] enlarging the library of materials that can be used, [22][23][24][25] and the possible applications of these finely controlled structures. [6,[25][26][27][28] The research on functional materials-here intended as materials that can be employed in 2PP and show peculiar features that encourage their use in particular applications-has propelled the field of micro/nanostructuring forward by promoting the interdisciplinarity between chemistry, physics, biology, and material science. [26] In this review, we will summarize and critically discuss the different functional materials proposed in the literature, dividing them and confronting them according to their preparation and their application (Figure 1; Table 1). We start the main body discussing the properties and applications of nanocomposite resists used in 2PP. In the second part, Direct laser writing methods based on two-photon polymerization (2PP) are powerful tools for the on-demand printing of precise and complex 3D architectures at the micro and nanometer scale. While much progress was made to increase the resolution and the feature size throughout the years, by carefully designing a material, one...

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