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the laser, where the square of the optical intensity exceeds the threshold needed to initiate polymerization. In this way, very small feature sizes of less than 100 nm can be achieved. [3,4] Microstructures pre pared via DLW have for example been used in microfluidic chips, [5,6] metama terials, [7] or as 3D microscaffolds for cell and tissue engineering. [8] Commercial resist mixtures are available that are well suited for the rapid generation of stable structures. However, when using standard resists, the resulting microstructures are static. In order to further expand the rep ertoire of applications of DLW, new photo resists have to be designed that directly impart functionality onto the structures emerging from the writing process. [9] Our group has been pursuing this goal, pro ducing resists that, e.g., lead to conduc tive, [10] stimuliresponsive, [11] and erasable structures. [12] The mechanical properties of microstructures are critical for their application, especially when used as cell scaf folds. [13] The mechanical properties of microstructures created via DLW can certainly be tuned by careful adjustment of the writing parameters (i.e., laser power and scan speed), but only to a limited extent. [14] Herein, we present the first photoresist for DLW that is capable of generating microstructures whose mechanical properties can be adjusted postwriting, solely The photochemistry of anthracene, a new class of photoresist for direct laser writing, is used to enable visible-light-gated control over the mechanical properties of 3D microstructures post-manufacturing. The mechanical and viscoelastic properties (hardness, complex elastic modulus, and loss factor) of the microstructures are measured over the course of irradiation via dynamic mechanical analysis on the nanoscale. Irradiation of the microstructures leads to a strong hardening and stiffening effect due to the generation of additional crosslinks through the photodimerization of the anthracene functionalities. A relationship between the loss of fluorescence-a consequence of the photodimerization-and changes in the mechanical properties isestablished. The fluorescence thus serves as a proxy read-out for the mechanical properties. These photoresponsive microstructures can potentially be used as "mechanical blank slates": their mechanical properties can be readily adjusted using visible light to serve the demands of different applications and read out using their fluorescence. 3D MicrostructuresDirect laser writing (DLW) is the only manufacturing tech nique capable of producing truly 3D structures on the micro scopic scale. [1] What differentiates DLW from conventional 3Dprinting techniques is the fact that it exploits twophoton absorption (TPA) to initiate polymerizations. While single photon absorption (SPA) is proportional to the optical inten sity, TPA is proportional to the square of the optical intensity. [2] Thus, polymerization will only take place in the focal volume of

and morphologies, are highly promising materials for water collection, smart textiles for moisture management, dual drug release, actuation, etc.Electrospinning is a versatile method for the production of thin fibers with a high surface-to-volume-ratio from a multitude of materials in a simple and straightforward way. [4] Various fiber morphologies have been realized by either controlling the spinning parameters or by using special spinning nozzles, e.g., homogenous fibers, particles, or particles immobilized on fibers (also called beaded fibers) as well as fibers with bead-on-string or pearl-necklace morphology. [5] Beaded fibers, in which beads of different shapes (i.e., spherical, spindle shape) and the joints (fiber segment between the two beads) are made up of the same polymer, already gained an edge over conventional nanofibers in providing superhydrophobicity as well as encapsulation and release of microparticle drugs from the beads. [6] Complex fiber morphologies with precisely defined composition and properties are accessible employing special spinning nozzles, which combine two or more components in either s-b-s or core-shell (c-s) fashion. [7] This method also allows to combine bioinspired structures with appropriate material properties to achieve complex functional materials with unique properties. For example, wool-like crimps were generated by spinning polymers with different mechanical properties in an s-b-s fashion, both by melt and solution electrospinning. [7b,8] Bicomponent fibers with core-shell-type beads (coaxial beadon-string morphology) are another example. The formation of these fibers is not trivial and can be realized by using a polymer solution, which contains swollen crosslinked polymeric colloids. Applying a simple spinning nozzle produced beaded fibers, in which a row of colloidal particles is embedded in the matrix polymer. [9] Alternatively, bicomponent fibers with coaxial bead-on-string structure can be obtained by combining electrospraying with electrospinning in a coaxial electrospinning setup. [10] Similar fibers have also been produced employing a coating method. [11] In this approach, a polymer solution is coated onto a supporting fiber, which further on breaks up into droplets driven by Rayleigh instability. Fibers with coaxial beadon-string morphology are very promising materials for water harvesting applications, which demands for the development of efficient large-scale production methods. [12] Bioinspired Electrospun Bicomponent Fibers Nature is an intriguing inspiration for designing a myriad of functional materials. However, artificial mimicking of bioinspired structures usually requires different specialized procedures and setups. In this study, a new upscalable concept is presented that allows to produce two bioinspired, bicomponent fiber morphologies (side-by-side and coaxial bead-on-string) using the same electrospinning setup, just by changing the employed spinning solvent. The generated fiber morphologies are highly attractive for thermoresponsive actuat...

The [4+4] photocycloaddition of anthracene is one of most relevant photoreactions and is widely applied in materials science, as it allows to remote‐control soft matter material properties by irradiation. However, highly energetic UV irradiation is commonly applied, which limits its application. Herein, the wavelength dependence of the photodimerization of anthracene is assessed for the first time, revealing that the reaction is induced just as effectively with mild visible light (410 nm). To fully establish [4+4] cycloadditions within defined chemical environments, a conceptual framework for the solution kinetics of the photo‐dimerization up to long reaction times is established by developing a novel photoreaction rate law that is dependent on individual rate coefficients of the key reaction steps. These coefficients can be determined based on low conversion photochemical experiments. Both differential and integral quantum yields can subsequently be predicted that are strongly time‐dependent, highlighting the need for a detailed reaction pathway analysis. The presented approach simplifies a complex photochemical scenario, making the photochemical anthracene dimerization, or potentially any other photochemical dimerization, amenable to a time‐dependent understanding at the elementary reaction level.

To advance the applications of direct laser writing (DLW), adaptability of the printed structure is critical, prompting a shift toward printing structures that are comprised of different materials, and/or can be partially or fully erased on demand. However, most structures that contain these features are often printed by complex processes or require harsh developing techniques. Herein, a unique photoresist for DLW is introduced that is capable of printing 3D microstructures that can be erased by exposure to darkness. Specifically, microstructures based on light‐stabilized dynamic materials are fabricated that remain stable when continously irradiated with green light, but degrade once the light source is switched off. The degradation and light stabilization properties of the printed materials are analyzed in‐depth by time‐lapse scanning electron microscopy. It is demonstrated that these resists can be used to impart responsive behavior onto the printed structure, and –critically– as a temporary locking mechanism to control the release of moving structural features.