4D Printing at the Microscale Spiegel, Christoph A.; Hippler, Marc; Münchinger, Alexander ...
Advanced functional materials,
06/2020, Volume:
30, Issue:
26
Journal Article
Peer reviewed
Open access
3D printing of adaptive and dynamic structures, also known as 4D printing, is one of the key challenges in contemporary materials science. The additional dimension refers to the ability of 3D printed ...structures to change their properties—for example, shape—over time in a controlled fashion as the result of external stimulation. Within the last years, significant efforts have been undertaken in the development of new responsive materials for printing at the macroscale. However, 4D printing at the microscale is still in its early stages. Thus, this progress report will focus on emerging materials for 4D printing at the microscale as well as their challenges and potential applications. Hydrogels and liquid crystalline and composite materials have been identified as the main classes of materials representing the state of the art of the growing field. For each type of material, the challenges and critical barriers in the material design and their performance in 4D microprinting are discussed. Importantly, further necessary strategies are proposed to overcome the limitations of the current approaches and move toward their application in fields such as biomedicine, microrobotics, or optics.
Toward dynamic microstructures: This progress report focuses on emerging materials for 4D printing at the microscale, with special emphasis on hydrogels and liquid crystalline and composite materials, as well as their challenges and potential applications.
Nanoporous materials relying on supramolecular liquid crystals (LCs) are excellent candidates for size- and charge-selective membranes. However, whether they can be manufactured using printing ...technologies remained unexplored so far. In this work, we develop a new approach for the fabrication of ordered nanoporous microstructures based on supramolecular LCs using two-photon laser printing. In particular, we employ photo-cross-linkable hydrogen-bonded complexes, that self-assemble into columnar hexagonal (Colh) mesophases, as the base of our printable photoresist. The presence of photopolymerizable groups in the periphery of the molecules enables the printability using a laser. We demonstrate the conservation of the Colh arrangement and of the adsorptive properties of the materials after laser microprinting, which highlights the potential of the approach for the fabrication of functional nanoporous structures with a defined geometry. This first example of printable Colh LC should open new opportunities for the fabrication of functional porous microdevices with potential application in catalysis, filtration, separation, or molecular recognition.
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Ideally, many materials should have a “knob” that allows for changing its properties at will, including the possibility to flip the sign of its behavior. This “knob” could be used to ...continuously tune the properties or in the sense of a digital switch. Such extreme level of stimulus–responsiveness has come into reach with recently increased possibilities of manufacturing complex rationally designed artificial materials called metamaterials on the micrometer scale. Here, we present mechanical metamaterials composed of liquid–crystal elastomers, whose director field is arranged into a designed complex three-dimensional (3D) pattern during the 3D laser printing process. External light from a blue LED, with intensities in the range of 10–30 W/cm2, serves as the stimulus. In the first example, we repeatedly flip the sign of the Poisson’s ratio of an achiral architecture within classical elasticity. In the second example, we flip the sign of the twist per strain in a chiral metamaterial beyond classical elasticity. The presented examples overcome major limitations in responsive mechanical metamaterials and we foresee many possible three-dimensional responsive micro-architectures manufactured along these lines.
The two‐photon Schwarzschild effect in photoresists suitable for 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.
The two‐photon Schwarzschild effect in photoresists suitable for 3D laser lithography is revisited. For very short exposure times (regime I) and low photoinitiator concentrations, deviations from the simple scaling expected for two‐photon absorption are found. For very large exposure times (regime III) and liquid monomers, strong deviations from the Schwarzschild behavior of intermediate exposure times (regime II) are found.
An approach is presented to align the direction of liquid crystal networks or elastomers in situ during multi‐photon laser printing for each voxel in three dimensions by applying a quasi‐static ...electric field with variable orientation. This approach enables the making of 3D micro‐heterostructures operating under ambient conditions that show large‐amplitude elastic actuation, with temperature serving as the stimulus (“4D microstructures”). The approach involves two novelties. First, a dedicated sample cell with a variable height suitable for laser printing is introduced. It is based on optically transparent electrodes and allows to apply arbitrary electric field vectors in three dimensions, for example, parallel or normal to the substrate plane. Second, a variable optical phase plate combined with a pivotable half‐wave plate warrants a single well‐defined laser focus for nearly all possible quasi‐static electric field vectors. Without the latter, one generally obtains two spatially separated laser foci, an ordinary and an extraordinary one, due to the optical birefringence of the medium induced by the alignment of the liquid crystal director via the applied quasi‐static electric field. The versatility of the approach is illustrated by manufacturing and characterizing several exemplary architectures.
A 4D printing approach is presented to structure liquid crystal networks or elastomers via two‐photon absorption lithography. The director is controlled in situ by a freely adjustable electric field. Combining a pivotable half‐wave plate and a variable spiral waveplate allows to focus the printing laser inside the birefringent ink. Several exemplary architectures illustrate the versatility of the approach.
Liquid crystal elastomers (LCEs) are highly suitable materials for the fabrication of flexible photonic elements due to their ability for directional actuation induced by external stimuli. 3D laser ...printing (3DLP) is a well-established method to realize complex photonic architectures. In this paper, we present the technological adaptations necessary to combine the actuation-controlled flexibility of LCE with the design options inherent to 3DLP to realize a platform for tunable photonics. The role of birefringence of the LCE in the 3DLP fabrication is addressed and theoretically modelled. We demonstrate how LCEs can be used both as a flexible substrate for arrays of rigid photonic elements and as a material for tunable photonic structures itself. Flexible coupling of two optical whispering gallery mode cavities and full spectral tunability of a single cavity are presented as exemplary applications.
For microscale 4D photoresponsive actuators, light is crucial in two ways. First, the underlying additive manufacturing techniques rely on photopolymerization processes triggered by the absorption of ...light. Second, the absorption of light serves as the actuation stimulus. The two absorptions can be conflicting. While the microstructure requires strong absorption at the actuation wavelength(s), this absorption should not interfere with that of the manufacturing process. Herein, a simple strategy is proposed to overcome these limitations and allow for the fabrication of multi‐photoresponsive 3D microstructures that can be actuated at different wavelengths of light. Two‐photon 3D laser printing is selected as the fabrication technique and liquid crystalline (LC) elastomers as the functional materials. In a first step, 3D microstructures are fabricated using an aligned LC ink formulation. Thereafter, up to five different dyes exhibiting absorptions that extend over the entire visible regime (400–700 nm) are successfully incorporated into the LC microstructures by an exchange process enabling a programmable actuation by irradiating with the suitable wavelength. Furthermore, by combining dyes exhibiting orthogonal absorptions, wavelength‐selective actuations are demonstrated.
A simple strategy allowing for the fabrication of multi‐phototoresponsive 4D microstructures using two‐photon laser writing that can be actuated at different wavelengths of light, is presented. In particular, three classes of dyes (azobenzenes, donor‐acceptor Stenhouse Adducts (DASAs), and anthraquinone) enabling actuation using blue, green or red light, respectively, are exploited.
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