Wavelength Selective Multi-Material 3D Printing of Soft Active Devices Using Orthogonal Photoreactions.

Orthogonal photoreactions provide a unique way to locally and independently control (thermo)mechanical properties and functionality of polymer networks simply by choice of the wavelength. Herein, a library of acrylate functional coumarin monomers is synthesized, which are cured by sequence-dependent wavelength orthogonality. In the presence of a long wavelength absorbing photoinitiator, the monomers undergo rapid curing by visible light induced radical chain growth polymerization. Subsequent irradiation with light in the UV-A region selectively initiates the [2+2] photocycloaddition of the coumarin chromophores, which is confirmed by FTIR and UV-vis experiments. Through a well-targeted design, acrylate-based and thiol-acrylate resin formulations are prepared, whose fast curing rate, low viscosity, and prolonged storage stability enable the one-step fabrication of multi-material structures by digital light processing (DLP) 3D printing. By using a dual-wavelength printer, which operates at two different wavelengths (405 and 365 nm), objects comprising soft (ε = 22%, σ = 7.5 MPa) and stiff (ε = 2%, σ = 8.3 MPa) domains are printed with a single resin vat. Along with tensile properties, the wavelength selective change in the network structure features a local control of the glass transition temperature (ΔTg  = 17 °C) in the 3D-printed objects. Soft active devices are fabricated by dual-wavelength DLP 3D printing, with distinct domains having a higher Tg and the local programming of multi shapes is demonstrated.

Blocked Phosphates as Photolatent Catalysts for Dynamic Photopolymer Networks.

While latent catalysts are a well-established strategy for initiating and controlling the rate of polymerization reactions, their use in dynamic polymer networks is still in its infancy. The ideal latent catalyst should be thermally stable and release a highly active species in response to an external trigger. Here, we have synthesized a temperature resistant (>200 °C) organic phosphate with a photolabile o-nitrobenzyl protecting group that can be cleaved by UV light. Introduced in a visible light curable thiol-click photopolymer, the sequence-dependent λ-orthogonality of the curing and cleavage enables an efficient network formation at 451 nm, without premature release of the catalyst. Once cured, irradiation at 372 nm spatiotemporally activates the phosphate, which catalyzes transesterifications at elevated temperature. The formed catalyst has no effect on the thermal stability of the polymeric network and allows the activation of bond exchange reactions in selected domains of printed 3D objects.

On-Demand Activation of Transesterification by Chemical Amplification in Dynamic Thiol-Ene Photopolymers.

Chemical amplification is a well-established concept in photoresist technology, wherein one photochemical event leads to a cascade of follow-up reactions that facilitate a controlled change in the solubility of a polymer. Herein, we transfer this concept to dynamic polymer networks to liberate both catalyst and functional groups required for bond exchange reactions under UV irradiation. For this, we exploit a photochemically generated acid to catalyse a deprotection reaction of an acid-labile tert-butoxycarbonyl group, which is employed to mask the hydroxy groups of a vinyl monomer. At the same time, the released acid serves as a catalyst for thermo-activated transesterifications between the deprotected hydroxy and ester moieties. Introduced in an orthogonally cured (450 nm) thiol-click photopolymer, this approach allows for a spatio-temporally controlled activation of bond exchange reactions, which is crucial in light of the creep resistance versus reflow ability trade-off of dynamic polymer networks.

Photopatternable and Rewritable Epoxy-Anhydride Vitrimers.

The present work highlights a new approach to write, erase, and rewrite micropatterns into the same region of covalent adaptable polymer networks. Thermal curing of an epoxy-terminated o-nitrobenzyl ester (o-NBE) derivative with hexahydrophthalic anhydride in the presence of 1,5,7-triazabicyclo[4.4.0]dec-5-ene yields a dynamic covalent network, whose solubility is locally controlled by irradiation with ultraviolet (UV) light. The photolysis of the o-NBE chromophores enables a well-defined cleavage of the epoxy-anhydride network, and the formation of soluble photolysis products is confirmed by sol-gel analysis. The photo-induced change in solubility is exploited to inscribe micropatterns by photolithographic techniques and after development in an organic solvent positive tone structures with a feature size of 20 µm are obtained. Due to the thermo-activated exchange reactions of the hydroxyl ester links and the related macroscopic reflow, the polymer patterns are fully erased at temperatures well above the topological freezing transition of the vitrimer network. The regenerated film has a smooth surface topology and can be reused to inscribe new micropatterns via mask lithography.

Introduction of Photolatent Bases for Locally Controlling Dynamic Exchange Reactions in Thermo-Activated Vitrimers.

Vitrimers exhibit a covalently crosslinked network structure, as is characteristic of classic thermosetting polymers. However, they are capable of rearranging their network topology by thermo-activated associative exchange reactions when the topology freezing transition temperature (Tv ) is exceeded. Despite the vast number of developed vitrimers, there is a serious lack of methods that enable a (spatially) controlled onset of these rearrangement reactions above Tv . Herein, we highlight the localized release of the efficient transesterification catalyst 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD) by the UV-induced cleavage of a photolatent base within a covalently crosslinked thiol-epoxy network. Demonstrated with stress relaxation measurements conducted well above the network's Tv , only the controlled release of TBD facilitates the immediate onset of transesterification in terms of a viscoelastic flow. Moreover, the spatially resolved UV-mediated photoactivation of vitrimeric properties is confirmed by permanent shape changes induced locally in the material.

Design and characterisation of vitrimer-like elastomeric composites from HXNBR rubber.

The present study aims at the incorporation of vitrimer-like properties into elastomeric composites as a promising approach towards the sustainable production of rubber-based materials. In particular, hydrogenated carboxylated nitrile butadiene rubber (HXNBR), as a technically relevant high-performance rubber, is covalently cross-linked with epoxy group-functionalised calcium silicate (Esilicate) across its pending carboxylic acid moieties. Reaction with the reactive functions attached on the filler surface results in the formation of ß-hydroxyl ester linkages at the HXNBR-Esilicate interface, which undergo thermo-activated transesterifications in the presence of a suitable catalyst. Topology rearrangements in the composites are confirmed by stress relaxation measurements at elevated temperatures. Comparison with an unfilled reference network reveals that the extent of stress relaxation can be mostly maintained upon the addition of the reactive filler even at large quantities. The Esilicate serves as both cross-linker and reinforcing filler, leading to a significant enhancement of the mechanical properties.

Laser-Triggered Writing and Biofunctionalization of Thiol-Ene Networks.

The light responsivity of ortho-nitrobenzyl esters (o-NBE) is exploited to inscribe µ-scale 2.5D patterns in thiol-ene networks by direct laser writing. For this purpose, a multifunctional thiol and a photosensitive alkene with an o-NBE chromophore are cured upon visible light exposure without inducing a premature photocleavage of the o-NBE links. Once the network is formed, a laser beam source with a wavelength of 375 nm is used for selectively inducing the photocleavage reaction of the o-NBE groups. Positive tone patterns are directly inscribed onto the sample surface without the requirement of a subsequent development step (removing soluble species in an appropriate organic solvent). Along with the realization of dry-developable micropatterns, the chemical surface composition of the exposed areas can be conveniently adjusted since different domains with a tailored content of carboxylic groups are obtained simply by modulating the laser energy dose. In a following step, those are activated and exploited as anchor points for attaching an Alexa-546 conjugated Protein A. Thus, the laser writable thiol-ene networks do not only provide a convenient method for the fabrication of positive tone patterns but also open future prospectives for a wide range of biosensing applications.

Stress relaxation and thermally adaptable properties in vitrimer-like elastomers from HXNBR rubber with covalent bonds.

Widening the scope of skeletons in the chemistry of vitrimer(-like) high molecular weight rubbers, the present study highlights the preparation of vitrimer-like elastomers based on a technically relevant rubber that is characterised by high thermal and oxidation stability. In particular, we prepared covalently crosslinked hydrogenated carboxylated nitrile butadiene rubber (HXNBR) networks that can rearrange their topology due to the exchangeable nature of the crosslinks. By crosslinking with a di-functional epoxide, ß-hydroxyl ester linkages are incorporated into the rubber, enabling thermo-activated transesterifications in the presence of the catalyst triazabicyclodecene. At moderate temperatures, the covalent linkages ensure good mechanical properties as well as chemical and thermal stability of the rubber, which is essential for most applications. In addition, bond exchange reactions allow for fast and distinctive stress relaxation at elevated temperatures. Due to the enhanced network mobility above the vitrification transition temperature, the materials exhibit thermally adaptable properties. A comparative study throughout all experiments with catalyst-free samples serving as a reference is made. Shape change experiments reveal a certain malleability of the HXNBR elastomers and improved adhesion properties are shown by means of lap shear tests. In the presence of catalyst, the failure mechanism changes from adhesive to cohesive failure proving the weldability of the material. Furthermore, the samples exhibit thermally triggered repair capabilities as demonstrated by stress-rupture tests. In general, it is shown that already low quantities of exchangeable crosslinks of associative nature impart a promising thermal adaptability into high molecular weight HXNBR rubber.

Spatially Resolved Cross-Linking Characterization by Imaging Low-Coherence Interferometry.

A method to characterize cross-linking differences in polymers such as waveguide polymers has been developed. The method is based on the scan-free information acquisition utilizing a low-coherence interferometer in conjunction with an imaging spectrometer. By the introduction of a novel analyzing algorithm, the recorded spectral-phase data was interpreted as wavelength-dependent optical thickness which is matchable with the refractive index and therefore with the degree of cross-linking. In the course of this work, the method was described in its hardware and algorithmic implementation as well as in its accuracy. Comparative measurements and error estimations showed an accuracy in the range of 10-6 in terms of the refractive index. Finally, photo-lithographically produced samples with laterally defined cross-linking differences have been characterized. It could be shown, that differences in the optical thickness of ±1.5 µm are distinguishable.

Exploring Network Formation of Tough and Biocompatible Thiol-yne Based Photopolymers.

This work deals with the in-depth investigation of thiol-yne based network formation and its effect on thermomechanical properties and impact strength. The results show that the bifunctional alkyne monomer di(but-1-yne-4-yl)carbonate (DBC) provides significantly lower cytotoxicity than the comparable acrylate, 1,4-butanediol diacrylate (BDA). Real-time near infrared photorheology measurements reveal that gel formation is shifted to higher conversions for DBC/thiol resins leading to lower shrinkage stress and higher overall monomer conversion than BDA. Glass transition temperature (Tg ), shrinkage stress, as well as network density determined by double quantum solid state NMR, increase proportionally with the thiol functionality. Most importantly, highly cross-linked DBC/dipentaerythritol hexa(3-mercaptopropionate) networks (Tg ≈ 61 °C) provide a 5.3 times higher impact strength than BDA, which is explained by the unique network homogeneity of thiol-yne photopolymers.

Synthesis of Bio-Based Polyester Resins for Vat Photopolymerization 3D Printing.

Driven by environmental considerations, the scientific community has directed great effort towards the synthesis of new materials derived from renewable resources. However, for photocurable resins, most commercially available building blocks still rely on petroleum-based precursors. Herein, we present a simple synthesis route for bio-based acrylate-modified polyester resins, whose viscosity is sufficiently low for processing them with vat photopolymerization 3D printing. The established synthesis route enables the gradual substitution of fossil-based raw materials with bio-based alternatives. The acid number, color and viscosity of the bio-based acrylic resins are characterized and photocurable formulations are prepared by adding a radical photoinitiator. The photopolymerization kinetics, and thermomechanical and mechanical properties of the photopolymers are investigated as a function of the resin structure and benchmarked against a commercially available petroleum-based counterpart. Finally, the processability of the new bio-based resins via digital light processing 3D printing is demonstrated and test specimens are successfully 3D printed with a resolution in the millimeter range.

Multi-Material 3D Printing of Biobased Epoxy Resins.

Additive manufacturing (AM) has revolutionised the manufacturing industry, offering versatile capabilities for creating complex geometries directly from a digital design. Among the various 3D printing methods for polymers, vat photopolymerisation combines photochemistry and 3D printing. Despite the fact that single-epoxy 3D printing has been explored, the fabrication of multi-material bioderived epoxy thermosets remains unexplored. This study introduces the feasibility and potential of multi-material 3D printing by means of a dual-vat Digital Light Processing (DLP) technology, focusing on bioderived epoxy resins such as ELO (epoxidized linseed oil) and DGEVA (vanillin alcohol diglycidyl ether). By integrating different materials with different mechanical properties into one sample, this approach enhances sustainability and offers versatility for different applications. Through experimental characterisation, including mechanical and thermal analysis, the study demonstrates the ability to produce structures composed of different materials with tailored mechanical properties and shapes that change on demand. The findings underscore the promising technology of dual-vat DLP technology applied to sustainable bioderived epoxy monomers, allowing sustainable material production and complex structure fabrication.

Redox cationic frontal polymerization: a new strategy towards fast and efficient curing of defect-free fiber reinforced polymer composites.

Frontal polymerization of epoxy-based thermosets is a promising curing technique for the production of carbon fiber reinforced composites (CFRCs). It exploits the exothermicity of polymerization reactions to convert liquid monomers to a solid 3D network. A self-sustaining curing reaction is triggered by heat or UV-radiation, resulting in a localized thermal reaction zone that propagates through the resin formulation. To date, frontal polymerization is limited to CFRCs with a low fiber volume percent as heat losses compromise on the propagation of the heat front, which is crucial for this autocatalytic curing mechanism. In addition, the choice of suitable epoxy monomers and thermal radical initiators is limited, as highly reactive cycloaliphatic epoxies as well as peroxides decarboxylate during radical induced cationic frontal polymerization. The resulting networks suffer from high defect rates and inferior mechanical properties. Herein, we overcome these shortcomings by introducing redox cationic frontal polymerization (RCFP) as a new frontal curing concept. In the first part of this study, the influence of stannous octoate (reducing agent) was studied on a frontally cured bisphenol A diglycidyl ether resin and mechanical and thermal properties were compared to a conventional anhydride cured counterpart. In a subsequent step, a quasi-isotropic CFRC with a fiber volume of >50 vol%, was successfully cured via RCFP. The composite exhibited a glass transition temperature > 100 °C and a low number of defects. Finally, it was demonstrated that the redox agent effectively prevents decarboxylation during frontal polymerization of a cycloaliphatic epoxy, demonstrating the versatility of RCFP in future applications.

Sustainable Bio-Based UV-Cured Epoxy Vitrimer from Castor Oil.

Vitrimers brought new properties in thermosets by allowing their reshaping, self-healing, reprocessing, and network rearrangement without changing structural integrity. In this study, epoxidized castor oil (ECO) was successfully used for the straightforward synthesis of a bio-based solvent-free vitrimer. The synthesis was based on a UV-curing process, which proceeded at low temperatures in the absence of any solvents, and within a short time. Real time Fourier-transformed infrared spectroscopy and photo-DSC were exploited to monitor the cationic photocurable process. The UV-cured polymer networks were able to efficiently undergo thermo-activated bond exchange reactions due to the presence of dibutyl phosphate as a transesterification catalyst. Mechanical properties, thermal resistance, glass transition temperature, and stress relaxation were investigated as a function of the amount of transesterification catalyst. Mechanical properties were determined by both DMTA and tensile tests. Glass transition temperature (Tg) was evaluated by DMTA. Thermal stability was assessed by thermogravimetric analysis, whilst vitrimeric properties were studied by stress relaxation experiments. Overall, the ECO-based vitrimer showed high thermal resistance (up to 200 °C) and good mechanical properties (elastic modulus of about 10 MPa) and can therefore be considered as a promising starting point for obtaining more sustainable vitrimers.

Thermally Latent Bases in Dynamic Covalent Polymer Networks and their Emerging Applications.

A novel strategy allowing temporal control of dynamic bond exchange in covalently crosslinked polymer networks via latent transesterification catalysts is introduced. Obtained by a straightforward air- and water-tolerant synthesis, the latent catalyst is designed for an irreversible temperature-mediated release of a strong organic base. Its long-term inactivity at temperatures below 50 °C provides the unique opportunity to equip dynamic covalent networks with creep resistance and high bond-exchange rates, once activated. The presented thermally latent base catalyst is conveniently introducible in readily available building blocks and, as proof of concept, applied in a radically polymerized thiol-ene network. Light-mediated curing is used for 3D-printing functional objects, on which the possibility of spatially controlled reshaping and welding based on dynamic transesterification is illustrated. Since the catalyst is thermally activated, limitations regarding sample geometry and optical transparency do not apply, which facilitates a transfer to well-established industrial technologies. Consequently, fiber-reinforced and highly filled magneto-active thiol-ene polymer composites are fabricated by a thermal curing approach. The on-demand activation of dynamic transesterification is demonstrated by (magneto-assisted) reshaping experiments, highlighting a wide range of potential future applications offered by the presented concept.

Digital light processing 3D printing of dynamic magneto-responsive thiol-acrylate composites.

Additive manufacturing is one of the most promising processing techniques for fabricating customized 3D objects. For the 3D printing of functional and stimuli-triggered devices, interest is steadily growing in processing materials with magnetic properties. Synthesis routes for magneto-responsive soft materials typically involve the dispersion of (nano)particles into a non-magnetic polymer matrix. Above their glass transition temperature, the shape of such composites can be conveniently adjusted by applying an external magnetic field. With their rapid response time, facile controllability, and reversible actuation, magnetically responsive soft materials can be used in the biomedical field (e.g. drug delivery, minimally invasive surgery), soft robotics or in electronic applications. Herein, we combine the magnetic response with thermo-activated healability by introducing magnetic Fe3O4 nanoparticles into a dynamic photopolymer network, which undergoes thermo-activated bond exchange reactions. The resin is based on a radically curable thiol-acrylate system, whose composition is optimized towards processability via digital light processing 3D printing. A mono-functional methacrylate phosphate is applied as a stabilizer to increase the resins' shelf life by preventing thiol-Michael reactions. Once photocured, the organic phosphate further acts as a transesterification catalyst and activates bond exchange reactions at elevated temperature, which render the magneto-active composites mendable and malleable. The healing performance is demonstrated by recovering magnetic and mechanical properties after the thermally triggered mending of 3D-printed structures. We further demonstrate the magnetically driven movement of 3D-printed samples, which gives rise to the potential use of these materials in healable soft devices activated by external magnetic fields.

Vitrimeric shape memory polymer-based fingertips for adaptive grasping.

The variability in the shapes and sizes of objects presents a significant challenge for two-finger robotic grippers when it comes to manipulating them. Based on the chemistry of vitrimers (a new class of polymer materials that have dynamic covalent bonds, which allow them to reversibly change their mechanical properties under specific conditions), we present two designs as 3D-printed shape memory polymer-based shape-adaptive fingertips (SMP-SAF). The fingertips have two main properties needed for an effective grasping. First, the ability to adapt their shape to different objects. Second, exhibiting variable rigidity, to lock and retain this new shape without the need for any continuous external triggering system. Our two design strategies are: 1) A curved part, which is suitable for grasping delicate and fragile objects. In this mode and prior to gripping, the SMP-SAFs are straightened by the force of the parallel gripper and are adapted to the object by shape memory activation. 2) A straight part that takes on the form of the objects by contact force with them. This mode is better suited for gripping hard bodies and provides a more straightforward shape programming process. The SMP-SAFs can be programmed by heating them up above glass transition temperature (54°C) via Joule-effect of the integrated electrically conductive wire or by using a heat gun, followed by reshaping by the external forces (without human intervention), and subsequently fixing the new shape upon cooling. As the shape programming process is time-consuming, this technique suits adaptive sorting lines where the variety of objects is not changed from grasp to grasp, but from batch to batch.

In Situ Observation of Micro-Patterned Elastomeric Surfaces: The Formation of the Area of Real Contact and the Influence on Its Friction and Deformation Behaviour.

Structured surfaces, which are the basis of the lotus blossom effect, have great potential to serve/operate as functionalised surfaces, i.e., surfaces with specific and/or adjustable properties. In the present study, the aim is to use micro-structured elastomeric surfaces to specifically influence the friction and deformation behaviours on the basis of the shape and arrangement of the structures. Thiol-acrylate-based photopolymers patterned via nanoimprint lithography were investigated by using an in situ tribological measurement set-up. A clear influence of the different structures on the surface's friction behaviour could be shown, and, furthermore, this could be brought into relation with the real area of contact. This finding provides an important contribution to further development steps, namely, to give the structures switchable properties in order to enable the control of friction properties in a targeted manner.

A Review of Multi-Material 3D Printing of Functional Materials via Vat Photopolymerization.

Additive manufacturing or 3D printing of materials is a prominent process technology which involves the fabrication of materials layer-by-layer or point-by-point in a subsequent manner. With recent advancements in additive manufacturing, the technology has excited a great potential for extension of simple designs to complex multi-material geometries. Vat photopolymerization is a subdivision of additive manufacturing which possesses many attractive features, including excellent printing resolution, high dimensional accuracy, low-cost manufacturing, and the ability to spatially control the material properties. However, the technology is currently limited by design strategies, material chemistries, and equipment limitations. This review aims to provide readers with a comprehensive comparison of different additive manufacturing technologies along with detailed knowledge on advances in multi-material vat photopolymerization technologies. Furthermore, we describe popular material chemistries both from the past and more recently, along with future prospects to address the material-related limitations of vat photopolymerization. Examples of the impressive multi-material capabilities inspired by nature which are applicable today in multiple areas of life are briefly presented in the applications section. Finally, we describe our point of view on the future prospects of 3D printed multi-material structures as well as on the way forward towards promising further advancements in vat photopolymerization.

Vat Photopolymerization 3D-Printing of Dynamic Thiol-Acrylate Photopolymers Using Bio-Derived Building Blocks.

As an energy-efficient additive manufacturing process, vat photopolymerization 3D-printing has become a convenient technology to fabricate functional devices with high resolution and freedom in design. However, due to their permanently crosslinked network structure, photopolymers are not easily reprocessed or repaired. To improve the environmental footprint of 3D-printed objects, herein, we combine the dynamic nature of hydroxyl ester links, undergoing a catalyzed transesterification at elevated temperature, with an acrylate monomer derived from renewable resources. As a sustainable building block, we synthesized an acrylated linseed oil and mixed it with selected thiol crosslinkers. By careful selection of the transesterification catalyst, we obtained dynamic thiol-acrylate resins with a high cure rate and decent storage stability, which enabled the digital light processing (DLP) 3D-printing of objects with a structure size of 550 µm. Owing to their dynamic covalent bonds, the thiol-acrylate networks were able to relax 63% of their initial stress within 22 min at 180 °C and showed enhanced toughness after thermal annealing. We exploited the thermo-activated reflow of the dynamic networks to heal and re-shape the 3D-printed objects. The dynamic thiol-acrylate photopolymers also demonstrated promising healing, shape memory, and re-shaping properties, thus offering great potential for various industrial fields such as soft robotics and electronics.