Tough Multimaterial Interfaces through Wavelength-Selective 3D Printing

Light-Based 3D Multi-Material Printing of Micro-Structured Bio-Shaped, Conducting and Dry Adhesive Electrodes for Bioelectronics

In this work, a new method of multi-material printing in one-go using a commercially available 3D printer is presented. The approach is simple and versatile, allowing the manufacturing of multi-material layered or multi-material printing in the same layer. To the best of the knowledge, it is the first time that 3D printed Poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) micro-patterns combining different materials are reported, overcoming mechanical stability issues. Moreover, the conducting ink is engineered to obtain stable in-time materials while retaining sub-100 µm resolution. Micro-structured bio-shaped protuberances are designed and 3D printed as electrodes for electrophysiology. Moreover, these microstructures are combined with polymerizable deep eutectic solvents (polyDES) as functional additives, gaining adhesion and ionic conductivity. As a result of the novel electrodes, low skin impedance values showed suitable performance for electromyography recording on the forearm. Finally, this concluded that the use of polyDES conferred stability over time, allowing the usability of the electrode 90 days after fabrication without losing its performance. All in all, this demonstrated a very easy-to-make procedure that allows printing PEDOT:PSS on soft, hard, and/or flexible functional substrates, opening up a new paradigm in the manufacturing of conducting multi-functional materials for the field of bioelectronics and wearables.

Accessing pluripotent materials through tempering of dynamic covalent polymer networks

Pluripotency, which is defined as a system not fixed as to its developmental potentialities, is typically associated with biology and stem cells. Inspired by this concept, we report synthetic polymers that act as a single "pluripotent" feedstock and can be differentiated into a range of materials that exhibit different mechanical properties, from hard and brittle to soft and extensible. To achieve this, we have exploited dynamic covalent networks that contain labile, dynamic thia-Michael bonds, whose extent of bonding can be thermally modulated and retained through tempering, akin to the process used in metallurgy. In addition, we show that the shape memory behavior of these materials can be tailored through tempering and that these materials can be patterned to spatially control mechanical properties.

Colorful 3D Printing: A Critical Feasibility Analysis of Multi-Wavelength Additive Manufacturing

Employing two colors of light to 3D print objects holds potential for accessing advanced printing modes, such as the generation of multi-material objects from a single print. Thus, dual-wavelength-driven photoreactive systems (reactions that require or utilize two wavelengths) and their exploitation as chemo-technological solutions for additive manufacturing technologies have experienced considerable development over the last few years. Such systems saw an increase in printing speeds, a decrease in resolution thresholds, and─perhaps most importantly─the actual generation of multi-material objects. However, the pace at which such reactive systems are developed is moderate and varies significantly depending on the fashion in which the two colors of light are employed. Herein, we address for the first time the varying logic conjugations of light-activated chemical compounds in dual-wavelength photochemical processes in a systematic manner and consider their implications from a photochemical point of view. To date, four dual-wavelength reaction types have been reported, termed synergistic (λ1 AND λ2), antagonistic (reversed λ1 AND λ2), orthogonal (λ1 OR λ2), and─most recently─cooperative (λ1 AND λ2 or λ1 OR λ2). The progress of their implementation in additive manufacturing is assessed individually, and their concurrent and individual chemical challenges are identified. These challenges need to be addressed for future dual-wavelength photochemical systems to progress multi-wavelength additive manufacturing technologies beyond their current limitations.

Cooperative Network Formation via Two-Colour Light-Activated λ-Orthogonal Chromophores

Independently addressing photoreactive sites within one molecule with two colours of light is a formidable challenge. Here, we combine two sequence independent λ-orthogonal chromophores in one heterotelechelic dilinker molecule, to exploit their disparate reactivity utilizing the same reaction partner, a maleimide-containing polymer. We demonstrate that polymer network formation only proceeds if two colours of light are employed. Upon single colour irradiation, linker-decorated post-functionalized polymers are generated at either wavelength and in either sequence. Network formation, however, is only achieved by sequential or simultaneous two colour irradiation. The herein introduced photoreactive system demonstrates the power of wavelength orthogonal chemistry in macromolecular synthesis.

Triplet Fusion Upconversion for Photocuring 3D-Printed Particle-Reinforced Composite Networks

High energy photons (λ < 400 nm) are frequently used to initiate free radical polymerizations to form polymer networks, but are only effective for transparent objects. This phenomenon poses a major challenge to additive manufacturing of particle-reinforced composite networks since deep light penetration of short-wavelength photons limits the homogeneous modification of physicochemical and mechanical properties. Herein, the unconventional, yet versatile, multiexciton process of triplet-triplet annihilation upconversion (TTA-UC) is employed for curing opaque hydrogel composites created by direct-ink-write (DIW) 3D printing. TTA-UC converts low energy red light (λ_max  = 660 nm) for deep penetration into higher-energy blue light to initiate free radical polymerizations within opaque objects. As proof-of-principle, hydrogels containing up to 15 wt.% TiO₂ filler particles and doped with TTA-UC chromophores are readily cured with red light, while composites without the chromophores and TiO₂ loadings as little as 1-2 wt.% remain uncured. Importantly, this method has wide potential to modify the chemical and mechanical properties of complex DIW 3D-printed composite polymer networks.

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 (ΔT_g  = 17 °C) in the 3D-printed objects. Soft active devices are fabricated by dual-wavelength DLP 3D printing, with distinct domains having a higher T_g and the local programming of multi shapes is demonstrated.

Recent Trends in Advanced Photoinitiators for Vat Photopolymerization 3D Printing

3D printing has revolutionized the way of manufacturing with a huge impact on various fields, in particular biomedicine. Vat photopolymerization-based 3D printing techniques such as stereolithography (SLA) and digital light processing (DLP) attracted considerable attention owing to their superior print resolution, relatively high speed, low cost and flexibility in resin material design. As one key element of the SLA/DLP resin, photoinitiators or photoinitiating systems have experienced significant development in recent years, in parallel with the exploration of 3D printing (macro)monomers. The design of new photoinitiating systems can not only offer faster 3D printing speed and enable low-energy visible light fabrication, but also can bring new functions to the 3D printed products and even generate new printing methods in combination with advanced optics. This review evaluates recent trends in the development and application of advanced photoinitiators and photoinitiating systems for vat photopolymerization 3D printing, with a wide range of small molecules, polymers and nanoassemblies involved. Personal perspectives on the current limitations and future directions are eventually provided. This article is protected by copyright. All rights reserved.

Wavelength Orthogonal Photodynamic Networks

The ability of light to remotely control the properties of soft matter materials in a dynamic fashion has fascinated material scientists and photochemists for decades. However, only recently has our ability to map photochemical reactivity in a finely wavelength resolved fashion allowed for different colors of light to independently control the material properties of polymer networks with high precision, driven by monochromatic irradiation enabling orthogonal reaction control. The current concept article highlights the progress in visible light‐induced photochemistry and explores how it has enabled the design of polymer networks with dynamically adjustable properties. We will explore current applications ranging from dynamic hydrogel design to the light‐driven adaptation of 3D printed structures on the macro‐ and micro‐scale. While the alternation of mechanical properties via remote control is largely reality for soft matter materials, we herein propose the next frontiers for adaptive properties, including remote switching between conductive and non‐conductive properties, hydrophobic and hydrophilic surfaces, fluorescent or non‐fluorescent, and cell adhesive vs. cell repellent properties.

Rational Design of Photocatalysts for Controlled Polymerization: Effect of Structures on Photocatalytic Activities

Over the past decade, the use of photocatalysts (PCs) in controlled polymerization has brought new opportunities in sophisticated macromolecular synthesis. However, the selection of PCs in these systems has been typically based on laborious trial-and-error strategies. To tackle this limitation, computer-guided rational design of PCs based on knowledge of structure-property-performance relationships has emerged. These rational strategies provide rapid and economic methodologies for tuning the performance and functionality of a polymerization system, thus providing further opportunities for polymer science. This review provides an overview of PCs employed in photocontrolled polymerization systems and summarizes their progression from early systems to the current state-of-the-art. Background theories on electronic transitions are also introduced to establish the structure-property-performance relationships from a perspective of quantum chemistry. Typical examples for each type of structure-property relationships are then presented to enlighten future design of PCs for photocontrolled polymerization.

Stiffness control in dual color tomographic volumetric 3D printing

Tomographic volumetric printing (TVP) physically reverses tomography to offer fast and auxiliary-free 3D printing. Here we show that wavelength-sensitive photoresins can be cured using visible ([Formula: see text] nm) and UV ([Formula: see text] nm) sources simultaneously in a TVP setup to generate internal mechanical property gradients with high precision. We develop solutions of mixed acrylate and epoxy monomers and utilize the orthogonal chemistry between free radical and cationic polymerization to realize fully 3D stiffness control. The radial resolution of stiffness control is 300 µm or better and an average modulus gradient of 5 MPa/µm is achieved. We further show that the reactive transport of radical inhibitors defines a workpiece's shape and limits the achievable stiffness contrast to a range from 127 MPa to 201 MPa according to standard tensile tests after post-processing. Our result presents a strategy for controlling the stiffness of material spatially in light-based volumetric additive manufacturing.

Photoswitching Cationic and Radical Polymerizations: Spatiotemporal Control of Thermoset Properties

The ability to fabricate polymeric materials with spatially controlled physical properties has been a challenge in thermoset manufacturing. To address this challenge, this work takes advantage of a photoswitchable polymerization that selectively incorporates different monomers at a growing chain by converting from cationic to radical polymerizations through modulation of the wavelength of irradiation. By regulating the dosage and wavelength of light applied to the system, the mechanical properties of the crosslinked material can be temporally and spatially tuned. Furthermore, photopatterning can be achieved both on the macroscale and the microscale, enabling precise spatial control of crosslink density that results in high-resolution control over mechanical properties.

Chemical and Mechanical Tunability of 3D-Printed Dynamic Covalent Networks Based on Boronate Esters

As the scope of additive manufacturing broadens, interest has developed in 3D-printed objects that are derived from recyclable resins with chemical and mechanical tunability. Dynamic covalent bonds have the potential to not only increase the sustainability of 3D-printed objects, but also serve as reactive sites for postprinting derivatization. In this study, we use boronate esters as a key building block for the development of catalyst-free, 3D-printing resins with the ability to undergo room-temperature exchange at the cross-linking sites. The orthogonality of boronate esters is exploited in fast-curing, oxygen-tolerant thiol-ene resins in which the dynamic character of 3D-printed objects can be modulated by the addition of a static, covalent cross-linker with no room-temperature bond exchange. This allows the mechanical properties of printed parts to be varied between those of a traditional thermoset and a vitrimer. Objects printed with a hybrid dynamic/static resin exhibit a balance of structural stability (residual stress = 18%) and rapid exchange (characteristic relaxation time = 7 s), allowing for interfacial welding and postprinting functionalization. Modulation of the cross-linking density postprinting is enabled by selective hydrolysis of the boronate esters to generate networks with swelling capacities tunable from 1.3 to 3.3.