Herein, various 3D additive manufacturing approaches are reviewed in terms of two important figures of merit: maximum voxel printing rate and minimum voxel size. Voxel sizes from several 100 µm down to the 100 nm scale are covered. Original results on multifocus two‐photon printing at around voxel printing rates of 107 voxels s−1 are presented in this context, which significantly surpass previous best values. These advances are illustrated by and applied to the making of microstructured 3D (chiral) mechanical metamaterials that are composed of more than one‐hundred‐thousand unit cells in three dimensions. Previous best values for unit cells of similar complexity are smaller by two orders of magnitude.
The combination of three different photoresists into a single direct laser written 3D microscaffold permits functionalization with two bioactive full-length proteins. The cell-instructive microscaffolds consist of a passivating framework equipped with light activatable constituents featuring distinct protein-binding properties. This allows directed cell attachment of epithelial or fibroblast cells in 3D.
parallel projection technologies, [6-8] computed axial lithography as an inverse tomography approach based on multiple 2D optical one-photon exposures from multiple different directions, [9,10] and different forms of multiphoton-absorption 3D printing, [11-16] mostly based on femtosecond or picosecond pulsed lasers. Twophoton lithography has been pioneered by Maruo et al. in 1997. [17] In a few exceptions, continuous-wave (cw) lasers have been used. [18,19] Going "faster" can mean scanning a single focus faster, [20] adapting multiple foci approaches, [21,22] scanning multiple foci faster, [3] or printing more voxels/ pixels in parallel per unit time in projection-based approaches without scanning in the focal plane, [6,7,9,10,12] yet scanning normal to the focal plane. In any case, increasing the printing rate is inherently connected to either using more laser power and the same photoresist, or to using comparable or less laser power by exploiting optimized more sensitive photoresists. As femtosecond or picosecond laser power is a precious commodity associated with a considerable fraction of the cost of most advanced 3D multiphoton laser printers, we dedicate the main part of this contribution to a screening of sensitive multiphoton-absorption-based photo resists. These photoresists are either taken from the published literature, reproduced and remeasured in our labs, or are newly investigated herein as promising candidates. Driven by recent advances in rapid multiphoton single-focus 3D laser nanoprinting, multifocus variants thereof, and projection-based multiphoton 3D laser nanoprinting, the necessary average total laser powers from femtosecond laser oscillators or even from amplified femtosecond laser systems have exceeded the Watt level. Aiming at ever faster 3D printing, there exist two options: Using yet more powerful lasers or searching for more sensitive photoresists allowing for higher speeds at comparable or lower power levels. Here, altogether more than 70 different photoresists from the literature and a few new candidates are reviewed with regard to effective multiphoton sensitivity. A dimensionless sensitivity figure-of-merit allows to directly compare data taken under sometimes vastly different conditions.
The two-photon Schwarzschild effect in photoresists suitable for three-dimensional (3D) laser lithography is revisited. The study ranges over seven orders of magnitude in exposure time (from 1 µs to 10 s) and investigates a wide variety of different photoresist compositions. For short exposure times ("regime I"), the laser power at the polymerization threshold can scale with the inverse square root of the exposure time, as naively to be expected for two-photon absorption. Substantial deviations occur, however, for low photoinitiator concentrations. For intermediate exposure times ("regime II"), a Schwarzschild-type of behavior is found, as discussed previously. For very long exposure times ("regime III"), an unexpected deviation from regime II is found. By presenting numerical solutions of the coupled 3D reaction-diffusion equations, this behavior is explained in terms of the diffusion of oxygen and photoinitiator molecules, respectively.
need further improvement. First, the total sample fabrication time is presently dominated by sample handling, that is, print setup, sample transport and development. These steps can be automated, for example, by using microfluidics 42 , or by automatically immersing the sample in liquid containers 43 . Second, the photoresin as presented here limits the total exposed volume. After prolonged printing, out-of-focus areas polymerize due to proximity effects. Such accumulation or depletion effects can likely be suppressed by using further improved photoresin formulations. Directions for optimization can be deduced from the rate model analysis presented here. In addition, the in situ replenishment of the used photoresin by using microfluidics, polymerization inhibition by an oxygen-permeable membrane (as used in CLIP 8 ), or volumetric polymerization inhibition patterning 10 can potentially further suppress out-of-focus polymerization. Third, the red solid-state laser can be replaced by inexpensive high-power laser diodes, as is already the case for the blue laser. Compared with the amplified femtosecond pulsed lasers used in FP-TPL, the continuous-wave lasers used in this work are already much less expensive and easier to operate. The development of two-colour two-step photoinitiators improved with respect to sensitivity and tuned to readily available laser wavelengths would be highly desirable for this aspect.The underlying logical-AND-type functionality is not limited to light-sheet 3D printing, but could also prove beneficial in combination with CAL [5][6][7] , where a two-colour projection system was recently demonstrated for multimaterial 3D printing 44 . Related CAL setups could be used to simultaneously expose the photoresin using two-colour two-step absorption, with the two colours impinging from different directions, thereby reducing the proximity effect and voxel size.
A highly efficient strategy for the simultaneous dual surface encoding of 2D and 3D microscaffolds is reported. The combination of an oligo(ethylene glycol)-based network with two novel and readily synthesized monomers with photoreactive side chains yields two new photoresists, which can be used for the fabrication of microstructures (by two-photon polymerization) that exhibit a dual-photoreactive surface. By combining both functional photoresists into one scaffold, a dual functionalization pattern can be obtained by a single irradiation step in the presence of adequate reaction partners based on a self-sorting mechanism. The versatility of the approach is shown by the dual patterning of halogenated and fluorescent markers as well as proteins. Furthermore, we introduce a new ToF-SIMS mode ("delayed extraction") for the characterization of the obtained microstructures that combines high mass resolution with improved lateral resolution.
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