3D printing has changed the fabrication of advanced materials as it can provide customized and on-demand 3D networks. However, 3D printing of polymer materials with the capacity to be transformed after printing remains a great challenge for engineers, material, and polymer scientists. Radical polymerization has been conventionally used in photopolymerization-based 3D printing, as in the broader context of crosslinked polymer networks. Although this reaction pathway has shown great promise, it offers limited control over chain growth, chain architecture, and thus the final properties of the polymer networks. More fundamentally, radical polymerization produces dead polymer chains incapable of postpolymerization transformations. Alternatively, the application of reversible deactivation radical polymerization (RDRP) to polymer networks allows the tuning of network homogeneity and more importantly, enables the production of advanced materials containing dormant reactivatable species that can be used for subsequent processes in a postsynthetic stage. Consequently, the opportunities that (photoactivated) RDRP-based networks offer have been leveraged through the novel concepts of structurally tailored and engineered macromolecular gels, living additive manufacturing and photoexpandable/transformable-polymer networks. Herein, the advantages of RDRP-based networks over irreversibly formed conventional networks are discussed.

/pdf/reversible-deactivation-radical-polymerization-from-polymer-4n4ti8lmd8.pdf

Reversible Deactivation Radical Polymerization: From Polymer Network Synthesis to 3D Printing.

RAFT facilitated digital light projection 3D printing of polymeric materials provides a convenient and facile route for inducing post-fabrication transformations via reactivation of dormant chain transfer agents. In this work, we report the use of a Norrish type I photoinitiator in conjunction with a RAFT agent to produce a variety of open-air 3D printable resins that rapidly cure under visible light irradiation. The photoinitiator-RAFT system polymerizes extremely quickly and provides high 3D printing build rates of up to 9.1 cm h-1 , representing a 7-fold increase compared to previous RAFT mediated 3D printing systems. 3D printed materials containing thiocarbonylthio groups can be also produced using low concentrations of divinyl comonomers in the initial resins, which has not been successfully achieved using other photocontrolled RAFT polymerization techniques. Interestingly, the inclusion of RAFT agents significantly improves 3D printing resolution compared to formulations without RAFT agent, allowing the fabrication of intricate and complex objects. Spatiotemporally controlled surface modifications of the 3D printed objects from the dormant RAFT agent groups on the material surfaces were also performed under one and two-pass configurations, inducing multiple successive post-printing transformations on the same object.

https://onlinelibrary.wiley.com/doi/pdf/10.1002/ange.202181662

Rapid High-Resolution 3D Printing and Surface Functionalization via Type I Photoinitiated RAFT Polymerization

Recent years have seen major advances in the developments of both additive manufacturing concepts and responsive materials. When combined as 4D printing, the process can lead to functional materials and devices for use in health, energy generation, sensing, and soft robots. Among responsive materials, liquid crystals, which can deliver programmed, reversible, rapid responses in both air and underwater, are a prime contender for additive manufacturing, given their ease of use and adaptability to many different applications. In this paper, selected works are compared and analyzed to come to a didactical overview of the liquid crystal-additive manufacturing junction. Reading from front to back gives the reader a comprehensive understanding of the options and challenges in the field, while researchers already experienced in either liquid crystals or additive manufacturing are encouraged to scan through the text to see how they can incorporate additive manufacturing or liquid crystals into their own work. The educational text is closed with proposals for future research in this crossover field.

https://onlinelibrary.wiley.com/doi/pdf/10.1002/adma.202104390

4D Printing of Liquid Crystals: What's Right for Me?

Three-dimensional (3D) printing has exploded in interest as new technologies have opened up a multitude of applications1-6, with stereolithography a particularly successful approach4,7-9. However, owing to the linear absorption of light, this technique requires photopolymerization to occur at the surface of the printing volume, imparting fundamental limitations on resin choice and shape gamut. One promising way to circumvent this interfacial paradigm is to move beyond linear processes, with many groups using two-photon absorption to print in a truly volumetric fashion3,7-9. Using two-photon absorption, many groups and companies have been able to create remarkable nanoscale structures4,5, but the laser power required to drive this process has limited print size and speed, preventing widespread application beyond the nanoscale. Here we use triplet fusion upconversion10-13 to print volumetrically with less than 4 milliwatt continuous-wave excitation. Upconversion is introduced to the resin by means of encapsulation with a silica shell and solubilizing ligands. We further introduce an excitonic strategy to systematically control the upconversion threshold to support either monovoxel or parallelized printing schemes, printing at power densities several orders of magnitude lower than the power densities required for two-photon-based 3D printing. 

/pdf/triplet-fusion-upconversion-nanocapsules-for-volumetric-3d-3mctxccr.pdf

Triplet fusion upconversion nanocapsules for volumetric 3D printing

This work was supported by Marie Sklodowska-Curie Research

and Innovation StaffExchanges (RISE) under Grant No.
823989 “IONBIKE”. EU Horizon 2020 FETOPEN-2018−2020 Programme “LION-HEARTED”, Grant No. 828984.
N.A. has received funding from the European Union’s Horizon
2020 research and innovation program under the Marie

Sklodowska-Curie Grant No. 753293, acronym NanoBEAT.

Additive Manufacturing of Conducting Polymers: Recent Advances, Challenges, and Opportunities

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.

With 3D printing, the desire is to be "limited only by imagination," and although remarkable advancements have been made in recent years, the scope of printable materials remains narrow compared to other forms of manufacturing. Light-driven polymerization methods for 3D printing are particularly attractive due to unparalleled speed and resolution, yet the reliance on high-energy UV/violet light in contemporary processes limits the number of compatible materials due to pervasive absorption, scattering, and degradation at these short wavelengths. Such issues can be addressed with visible-light photopolymerizations. However, these lower-energy methods often suffer from slow reaction times and sensitivity to oxygen, precluding their utility in 3D printing processes that require rapid hardening (curing) to maximize build speed and resolution. Herein, multifunctional thiols are identified as simple additives to enable rapid high-resolution visible-light 3D printing under ambient (atmospheric O2 ) conditions that rival modern UV/violet-based technology. The present process is universal, providing access to commercially relevant acrylic resins with a range of disparate mechanical responses from strong and stiff to soft and extensible. Pushing forward, the insight presented within this study will inform the development of next-generation 3D-printing materials, such as multicomponent hydrogels and composites.

Additives for Ambient 3D Printing with Visible Light.

The ability to 3D print structures with low-intensity, long-wavelength light will broaden the materials scope to facilitate inclusion of biological components and nanoparticles. Current materials limitations arise from the pervasive absorption, scattering, and/or degradation that occurs upon exposure to high-intensity, short-wavelength (ultraviolet) light, which is the present-day standard used in light-based 3D printers. State-of-the-art techniques have recently extended printability to orange/red light. However, as the wavelength of light increases, so do the inherent challenges to match the speed and resolution of traditional UV light-induced solidification processes (i.e., photocuring). Herein, a photosystem is demonstrated to enable low-intensity (<5 mW/cm2), long-wavelength (∼850 nm) near-infrared (NIR) light-driven 3D printing, "invisible" to the human eye. The combination of a NIR absorbing cyanine dye with electron-rich and -deficient redox pairs was required for rapid photocuring in a catalytic manner. The rate of polymerization and time to solidification upon exposure to NIR light were characterized via in situ spectroscopic and rheological monitoring. Translation to NIR digital light processing (projection-based) 3D printing was accomplished through rigorous optimization of resin composition and printing parameters to balance the speed (<60 s/layer) and resolution (<300 μm features). As a proof-of-concept, composite 3D printing with nanoparticle-infused resins was accomplished. Preliminary analysis showed improved feature fidelity for structures produced with NIR relative to UV light. The present report provides key insight that will inform next-generation light-based photocuring technology, such as wavelength-selective multimaterial 3D bio- and composite-printing.

"Invisible" Digital Light Processing 3D Printing with Near Infrared Light.

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 resolution of any additive manufacturing process. However, the
reliance on high energy UV/violet light derived from decades of
photolithography research, limits the materials scope due to degradation and
attenuation (e.g., absorption and/or scattering). Chemical innovation to shift
the spectrum into more mild and tunable visible wavelengths promises to improve
compatibility and expand the repertoire of accessible objects, including those
containing biological compounds and multi-material structures. Photochemistry
at these longer wavelengths currently suffers from slow reaction times precluding
its utility. Herein, novel panchromatic photopolymer resins were developed and
applied for the first time to realize rapid high resolution visible light 3D
printing. The combination of electron deficient iodonium and rich borate
co-initiators were critical to overcoming the speed-limited photocuring with
visible light. Furthermore, azo-dyes were identified as vital resin components
to confine curing to irradiation zones, improving spatial resolution. A unique
screening method was used to streamline optimization (e.g., exposure time and
azo-dye loading) and correlate resin composition to resolution, cure rate, and
mechanical performance. Ultimately, a versatile and general visible light-based
printing method was shown to afford 1) stiff and soft objects with feature
sizes < 100 μm, 2) build speeds up to 45 mm/h, and 3) mechanical isotropy,
rivaling modern UV-based 3D printing technology and providing a foundation from
which bio- and composite-printing can emerge.

/pdf/rapid-high-resolution-visible-light-3d-printing-b7o6ua4jkv.pdf

Rapid High Resolution Visible Light 3D Printing

The utility of visible light for 3D printing has increased in recent years owing to its accessibility and reduced materials interactions, such as scattering and absorption/degradation, relative to traditional UV light-based processes. However, photosystems that react efficiently with visible light often require multiple molecular components and have strong and diverse absorption profiles, increasing the complexity of formulation and printing optimization. Herein, a streamlined method to select and optimize visible light 3D printing conditions is described. First, green light liquid crystal display (LCD) 3D printing using a novel resin is optimized through traditional empirical methods, which involves resin component selection, spectroscopic characterization, time-intensive 3D printing under several different conditions, and measurements of dimensional accuracy for each printed object. Subsequent analytical quantification of dynamic photon absorption during green light polymerizations unveils relationships to cure depth that enables facile resin and 3D printing optimization using a model that is a modification to the Jacob's equation traditionally used for stereolithographic 3D printing. The approach and model are then validated using a distinct green light-activated resin for two types of projection-based 3D printing. 

Lynn M. Stevens

Papers

Rapid High-Resolution Visible Light 3D Printing.

Additives for Ambient 3D Printing with Visible Light.

"Invisible" Digital Light Processing 3D Printing with Near Infrared Light.

Rapid High Resolution Visible Light 3D Printing

Counting All Photons: Efficient Optimization of Visible Light 3D Printing