The application of photochemistry to polymer and material science has led to the development of complex yet efficient systems for polymerization, polymer post-functionalization, and advanced materials production. Using light to activate chemical reaction pathways in these systems not only leads to exquisite control over reaction dynamics, but also allows complex synthetic protocols to be easily achieved. Compared to polymerization systems mediated by thermal, chemical, or electrochemical means, photoinduced polymerization systems can potentially offer more versatile methods for macromolecular synthesis. We highlight the utility of light as an energy source for mediating photopolymerization, and present some promising examples of systems which are advancing materials production through their exploitation of photochemistry.

/pdf/seeing-the-light-advancing-materials-chemistry-through-4jb2o1rrgq.pdf

Seeing the Light: Advancing Materials Chemistry through Photopolymerization.

Production of objects with varied mechanical properties is challenging for current manufacturing methods. Additive manufacturing could make these multimaterial objects possible, but methods able to achieve multimaterial control along all three axes of printing are limited. Here we report a multi-wavelength method of vat photopolymerization that provides chemoselective wavelength-control over material composition utilizing multimaterial actinic spatial control (MASC) during additive manufacturing. The multicomponent photoresins include acrylate- and epoxide-based monomers with corresponding radical and cationic initiators. Under long wavelength (visible) irradiation, preferential curing of acrylate components is observed. Under short wavelength (UV) irradiation, a combination of acrylate and epoxide components are incorporated. This enables production of multimaterial parts containing stiff epoxide networks contrasted against soft hydrogels and organogels. Variation in MASC formulation drastically changes the mechanical properties of printed samples. Samples printed using different MASC formulations have spatially-controlled chemical heterogeneity, mechanical anisotropy, and spatially-controlled swelling that facilitates 4D printing. Objects with varied mechanical properties can be produced by additive manufacturing, but multimaterial control along all three axes of printing is still limited. Here the authors use wavelength control during vat polymerization and demonstrate printing of objects with spatial control of the composition and stiffness.

/pdf/multimaterial-actinic-spatial-control-3d-and-4d-printing-4t8nja3hoi.pdf

Multimaterial actinic spatial control 3D and 4D printing

Soft robotics is a growing field of research, focusing on constructing motor-less robots from highly compliant materials, some are similar to those found in living organisms. Soft robotics has a high potential for applications in various fields such as soft grippers, actuators, and biomedical devices. 3D printing of soft robotics presents a novel and promising approach to form objects with complex structures, directly from a digital design. Here, recent developments in the field of materials for 3D printing of soft robotics are summarized, including high-performance flexible and stretchable materials, hydrogels, self-healing materials, and shape memory polymers, as well as fabrication of all-printed robots (multi-material printing, embedded electronics, untethered and autonomous robotics). The current challenges in the fabrication of 3D printed soft robotics, including the materials available and printing abilities, are presented and the recent activities addressing these challenges are also surveyed.

3D Printing Materials for Soft Robotics.

Light-driven 3D printing to convert liquid resins into solid objects (i.e., photocuring) has traditionally been dominated by engineering disciplines, yielding the fastest build speeds and highest r...

Rapid High-Resolution Visible Light 3D Printing.

Multi-material additive manufacturing (MMAM) enables rapid design and direct fabrication of three-dimensional (3D) objects consisting of multiple materials without the need for complex manufacturing process and expensive tooling. Through concurrent integration of multiple materials having their own unique properties, MMAM allows for facile production of multi-functional devices and systems. Various MMAM techniques have recently been developed and a wide range of new applications have been demonstrated using MMAM. This paper reviews recent advances in MMAM methods and key applications enabled by MMAM in three areas; biomedical engineering, soft robotics, and electronics.

Recent advances in multi-material additive manufacturing: methods and applications

A novel methodology for printing 3D objects with spatially resolved mechanical and chemical properties is reported. Photochromic molecules are used to control polymerization through coherent bleaching fronts, providing large depths of cure and rapid build rates without the need for moving parts. The coupling of these photoswitches with resin mixtures containing orthogonal photo-crosslinking systems allows simultaneous and selective curing of multiple networks, providing access to 3D objects with chemically and mechanically distinct domains. The power of this approach is showcased through the one-step fabrication of bioinspired soft joints and mechanically reinforced "brick-and-mortar" structures.

Solution Mask Liquid Lithography (SMaLL) for One-Step, Multimaterial 3D Printing

Strong and well-engineered interfaces between dissimilar materials are a hallmark of natural systems but have proven difficult to emulate in synthetic materials, where interfaces often act as points of failure. In this work, curing reactions that are triggered by exposure to different wavelengths of visible light are used to produce multimaterial objects with tough, well-defined interfaces between chemically distinct domains. Longer-wavelength (green) light selectively initiates acrylate-based radical polymerization, while shorter-wavelength (blue) light results in the simultaneous formation of epoxy and acrylate networks through orthogonal cationic and radical processes. The improved mechanical strength of these interfaces is hypothesized to arise from a continuous acrylate network that bridges domains. Using printed test structures, interfaces were characterized through spatial resolution of their chemical composition, localized mechanical properties, and bulk fracture strength. This wavelength-selective photocuring of interpenetrating polymer networks is a promising strategy for increasing the mechanical performance of 3D-printed objects and expanding light-based additive manufacturing technologies.

Tough Multimaterial Interfaces through Wavelength-Selective 3D Printing.

Multimaterial additive manufacturing is an enabling tool for exploring difficult to access structure–property relationships. In this work, a recently developed multimaterial printing approach, solution mask liquid lithography, is used to produce porous polymer–polymer composites inspired by tough, hierarchical structures found in nature. The results demonstrate that varying the size and packing of pores in the core structure leads to significant enhancement in crack deflection. Finite element analysis reveals that this enhancement is linked to geometry-dependent stress distribution.

Engineering crack tortuosity in printed polymer–polymer composites through ordered pores

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Solution Mask Liquid Lithography (SMaLL) for One-Step, Multimaterial 3D Printing

Tough Multimaterial Interfaces through Wavelength-Selective 3D Printing.

Engineering crack tortuosity in printed polymer–polymer composites through ordered pores