Additive Manufacturing and 3D Printing

Andrew J. Boydston is the Yamamoto Family Professor of Chemistry at the University of Wisconsin-Madison. As a trained chemist he worked on catalysts for the synthesis of polymers during his postdoc time and started his independent career as an assistant professor of Chemistry in 2010 at the University of Washington. In 2014 he involved in a project with colleagues at the mechanical engineering department at the University of Washington which piqued his interest in additive manufacturing and which remained one of his research lines after moving to the University of Wisconsin-Madison in 2018. He is interested in organocatalysts for polymerization reactions, mechanophores, polymers for controlled release and additive manufacturing.

The limited strength of green parts have been a major hurdle in the Binder Jet Additive Manufacturing. Here the authors apply polyethyleneimine binder to print silica sand structures with double the flexural strength of green parts and 8-fold increase in the strength upon reactive infiltration.

3D printing enables customized manufacturing that is difficult to achieve through traditional material processing but 3D printing with high resolution and high speed is challenging to realize. Here, the authors demonstrate that photooxidation of a ketocoumarin photosensitizer can simultaneously deliver high print speed and high print resolution on a common 3D printer.

Currently UV-based direct ink writing (DIW) is facing a trade-off between required curing intensity and effectiveness range. Here the authors overcome this problem by introducing near-infrared photopolymerization into DIW

Self-healing hydrogels can mimic the damage repair behaviour of living tissues, but such hydrogels have only been processed via extrusion-based additive manufacturing technology. Here, the authors demonstrate a rapidly self-healing hydrogel which can be processed by DLP printing.

Conducting polymers are promising materials for diverse applications but the fabrication of conducting polymers mostly relies on conventional fabrication techniques. Here the authors introduce a high performance 3D printable conducting polymer ink to take full advantage of advanced 3D printing.

Four-dimensional (4D) printing of shape memory polymer (SMP) imparts time responsive properties to 3D structures. Here, the authors explore 4D printing of a SMP in the submicron length scale, extending its applications to nanophononics.

3D-printing tough conductive hydrogels (TCHs) with complex structures is still a challenging task due to their inherent contrasting multinetworks, uncontrollable and slow polymerization. Here the authors show an orthogonal photochemistry-assisted printing strategy to make 3D TCHs in one pot.

3D printing of metals produces elongated columnar grains which are usually detrimental to component performance. Here, the authors combine ultrasound and 3D printing to promote equiaxed and refined microstructures in a titanium alloy and a nickel-based superalloy resulting in improved mechanical properties.

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.

Printing functional inks is attractive for applications in electrochemical energy storage and smart electronics, among others. Here the authors report highly concentrated, additive-free, aqueous and organic MXene-based inks that can be used for high-resolution extrusion and inkjet printing.

Although 3D bioprinting technology has gained much attention in the field of tissue engineering, there are still several significant challenges that need to be overcome. Here, the authors present silk fibroin bioink with printability and biocompatibility suited for digital light processing 3D printing.

Atomically thin black phosphorus shows promise for optoelectronics and photonics, yet its instability under environmental conditions and the lack of well-established large-area synthesis protocols hinder its applications. Here, the authors demonstrate a stable black phosphorus ink suitable for printed ultrafast lasers and photodetectors.

Endowing composite materials with spatially discrete mechanical behaviours is possible by varying the internal concentration and arrangement of particles. Here, the authors demonstrate a 3D magnetic printing technique which enables the fabrication of materials with intricate internal designs.

The application of 3D printing to biomaterials presents interesting possibilities for tissue engineering. Here, the authors show that a printing medium derived from an extracellular matrix can be applied to printing tissue analogues with enhanced cell compatibility.

Robert S. Langer is an Institute Professor at the Massachusetts Institute of Technology. Leading one of the largest biomedical engineering labs in the world his research covers many areas of biotechnology including tissue engineering, drug delivery, biofabrication and the development of medical devices. Mark Tibbitt is an Assistant Professor of Macromolecular Engineering at ETH Zürich. His research focuses on combining polymer engineering, synthetic chemistry, mechanical and bioengineering for biofabrication, drug delivery and mechanobiology applications.

Shape memory scaffolds are needed for minimally invasive tissue repair and void filling. Here the authors report on the development of 4D printed polycarbonate-based scaffolds with surface degradation properties which fill voids without deforming tissue and allow for tissue ingrowth with reduced immune response.

The authors investigate 3D-printed tips, based on controlled microstructural architectured materials, as probes for shear-mode atomic force microscopy. They demonstrate that the tailored stiffness and energy-absorbing behaviour of the material are beneficial for improving image quality.

Here, the authors combine 3D printing and liquid metal filling techniques to fabricate customised probeheads for magnetic resonance experiments. They demonstrate in situ electrochemical nuclear magnetic resonance analysis, reaction monitoring with continues-flow separation and small-sample imaging.

Living cells can precisely assemble to build 3D functional architectures. Here the authors produce an extrudable microbial ink entirely from the engineered cells, which can be further programmed to 3D print functional living materials.

Mammalian cell-based cultured meat has mostly been unstructured, leaving a demand for artificial steak-like meat. Here the authors present an assembled steak-like tissue of bovine skeletal muscle, adipose tissue, and blood capillary tissue fabricated by tendon-gel integrated printing technology.

Designing efficient sensors for smart tires for autonomous vehicles remains a challenge. Here, the authors present a tire-integrated system that combines direct mask-less 3D printed strain gauges, flexible piezoelectric energy harvester for powering the sensors and secure wireless data transfer electronics, and machine learning for predictive data analysis.

Fabrication of dynamic, reversible and biocompatible scaffolds with non-invasive external triggers has so far been limited. Here, the authors report on the creation of 3D printed scaffolds with Janus structure that produce nanovibrations when exposed to ultrasound, promoting bone regeneration.

Cellular models are needed to study disease in vitro and to screen drugs for toxicity and efficacy. Here the authors develop a bioprinting approach to transfer spheroids into self-healing support hydrogels at high resolution, which enables their patterning and fusion into high-cell density microtissues of prescribed spatial organization.

Injury of corpus cavernosa results in erectile dysfunction, and repair leading to restoration of function is difficult. Here the authors construct 3D printed hydrogel constructs seeded with HIF-1α-expressing muscle derived stem cells to restore corpus function in a rabbit model.

3D bioprinting of skeletal muscle using primary human muscle progenitor cells results in correct muscle architecture, but functional restoration in rodent models is limited. Here the authors include human neural stem cells into bioprinted skeletal muscle and observe improved architecture and function in vivo.

The development of novel low-cost fabrication schemes for realizing stretchable transistor arrays with applicability in wearable electronics remains a challenge. Here, the authors report skin-like electronics with stretchable active materials and devices processed exclusively from ink-jet printing.

Mechanical computing based on the logic devices that utilize mechanical energy can be an alternative to conventional electronic computing. Here, Song et al. show a micromechanical logic gate, fabricated using multi-stable buckling flexures, which is capable of realizing all digital logic operations.

The ability to print arbitrary colors and shapes in all three dimensions at microscopic length scales is still lacking. Here, the authors introduce a means to produce three-dimensionally-printed photonic crystals with a periodicity as small as 280 nm, achieving sub-100-nm features with a full range of colors.

3D-printed soft actuators have limited motion and are far from reaching the level of complexity found in biological systems. Here the authors present a multimaterial 3D printing platform for the fabrication of soft actuators displaying a wide range of motions that are programmable.

There is a clinical need to develop a bioengineering system to support ovary transplantation. Here, the authors generate a bioprosthetic ovary using 3D printed scaffolds of varying pore architectures to support follicle survival and ovarian function in sterilized mice.

Nanoparticles capable of selectively binding target chemicals have potential for detoxification processes, but can lead to accumulation in the liver. Here the authors show a 3D-printed device containing functional nanoparticles, allowing the detox potential to be realised while holding the particles in place.

Professor Julia R Greer is a materials scientist at the California Institute of Technology. Her group focuses on designing, fabricating and characterising micro- and nano-architected materials using 3D lithography, nanofabrication, and additive manufacturing (AM) techniques for a multitude of applications ranging from biological devices to damage-tolerant fabrics.

Tomographic volumetric printing (TVP) is considered as a fast and auxiliary-free 3D printing technique, but control of internal mechanical properties remains less investigated for TVP. Here, the authors show that wavelength-sensitive photoresins can be cured using visible and UV sources simultaneously in a TVP setup to generate internal mechanical property gradients with high precision.

Metal 3D printing is a very promising technology to revolutionize catalytic systems. Here the authors show that metal 3D printing products themselves can simultaneously serve as chemical reactors and catalysts for conversion of C1 molecules into high value-added chemicals.

3D printing offers flexibility in fabrication of polymer objects but fabrication of large polymer structures with micrometer-sized geometrical features are challenging. Here, the authors introduce a method combining advantages of 3D printing and polymerization-induced phase separation, which enables formation of 3D polymer structures with controllable inherent porosity.

Responsive soft materials which can exhibit various three-dimensional (3D) shapes under the same stimulus are desirable for applications in adaptive and reconfigurable soft robots. Here, the authors report a laser rewritable magnetic composite film, whose responsive shape-morphing behaviors induced by a magnetic field can be digitally and repeatedly reprogrammed by a facile method of direct laser writing.

3D printing has enabled materials, geometries and functional properties to be combined in unique ways but printing multiple materials remains challenging. Here, the authors demonstrate how spatial distribution of different material phases can be modulated by controlling the kinetics of gelation, cross-linking density and material diffusivity in vat polymerization.

Composites which are made up of a single polymer, and yet allow modulation of the mechanical properties of the matrix without stress concentration, are challenging to fabricate. Here, the authors design a selfreinforced homocomposite alginate hydrogel with enhanced mechanical properties incorporating soft dendritic alginate colloids in the matrix and demonstrate its application in extrusion printing.

Additive manufacturing processing requirements pose restrictions on materials and joining chemically dissimilar components. Here the authors use silicone double networks that participate in orthogonal crosslinking mechanisms for independent control of the shape forming process and final mechanical properties.

Stimuli-responsive microstructures are important in soft robotics and biosciences, but water compatible materials to fabricate 3D structures are scarce. Here, the authors demonstrate pNIPAM based hetero-microstructures with substantially different material parameters by variation of the local exposure dose in 3D laser lithography.

Spatially controlled expansion and contraction of soft tissues to achieve complex three dimensional morphologies remains challenging in man-made materials. Here the authors demonstrate encoding of 2D hydrogels with spatially and temporally controlled growth to create dynamic 3D structures.

3D printing of titanium alloys today is based on known alloy compositions that result in anisotropic structural properties. Here, the authors add lanthanum to commercially pure titanium and exploit a solidification path that reduces texture and anisotropy.

Most current methods for additive manufacturing of complex metallic 3D structures are limited to a resolution of 20–50 µm. Here, the authors developed a lithography-based process to produce 3D nanoporous nickel nanolattices with octet geometries and a resolution of 100 nm.

Professor Peter D Lee is a materials scientist at the University College London. His group focuses on X-ray imaging and computational simulation of materials at a microstructural level for materials design and advanced manufacturing.

3D printing can allow for the efficient manufacturing of elaborate structures difficult to realise conventionally without waste, such as the hollow geometries of nickel-based superalloy aeronautic components. To fully exploit this method, we must move towards new alloys and processes.

NEMS devices, nano-electro-mechanical systems, by virtue of their minute size, offer ultra-high sensitivity, though at the expense of manufacturing complexity. Here, Stassi et al succeed in manufacturing high quality factor NEMS devices using high resolution 3D printing.

Thermal metamaterials can be used to manipulate heat flow but experimental fabrication is challenging. Here, the authors report robustly printable freeform thermal metamaterials to tackle this challenge by topology optimization and 3D printing.

Precise patterning of lipid-stabilised aqueous droplets is a key challenge in building synthetic tissue designs. Here, the authors show how the interactions between pairs of droplets direct the packing of droplets within 3D-printed networks, enabling the formation of synthetic tissues with high-resolution features.

Additive Manufacturing has had a large impact on the biomedical field but still lacks tools to generate scaffolds with gradients of physico-chemical properties. Here the authors report on the design of a 3D printer head for continuous gradient printing with an atmospheric pressure plasma jet for simultaneous surface patterning.

Digital light processing and stereolithography have low wet material utilization efficiency. Here the authors propose a one-droplet 3D printing strategy to fabricate 3D structures from a single resin droplet.

Identifying scaling laws in metal 3D printing is key to process optimization and materials development. Here the authors report scaling laws to quantify correlation between process parameters, keyhole stability and pore formation by high-speed synchrotron X-ray imaging and multiphysics modeling.

Understanding metal component microstructure during 3D printing remains a challenge. Here, the authors use local thermal parameters and the solidification microstructure to better understand how the printed microstructure varies with the laser scan strategy.

3D printing pore-free complex metal parts remains a challenge. Here, the authors combine in-situ imaging and simulations to show thermocapillary force can eliminate pores from the melt pool during a laser powder bed fusion process.

Laser-matter interactions during laser powder bed fusion additive manufacturing remain poorly understood. Here, the authors combine in situ X-ray imaging and finite element simulations to show how detrimental pores form under printing conditions and develop a strategy to suppress them.

Understanding what happens to the liquid in melt pools during welding and metal-based additive manufacturing remains a challenge. Here, the authors directly image internal melt pool dynamics using synchrotron radiation to show surface tension affects flow speed, orientation and surface turbulence.

Tuning the mechanical properties of extracellular matrix is of great interest in tissue engineering but spatial control over stiffness in hydrogels has been demonstrated in two dimensions only. Here the authors developed a layer-by-layer printing technique which uses oxygen inhibition to control the heterogeneous stiffness in 3D printed structures.

Subtractive manufacturing of microstructures is important for many applications, yet photoresists for 3D laser lithography allow only removal after development under harsh cleavage conditions. Here, the authors introduce a set of chemoselective cleavable photoresists allowing the orthogonal cleavage of microstructures under mild conditions.