3D Printing Research CrAMmed: DTU, TU Delft, and Imperial College London

In this edition of CrAMed, our academic additive manufacturing digest, we bring you the latest literature on metamaterials, 4D printing, regenerative medicine, multimaterial 3D printing and more from leading institutions around the globe.

3D printing better bones

In recent years, 3D printing has helped make important breakthroughs in regenerative medicine and tissue engineering. Continuing this trend, in a recently published paper, Mexican researchers found a way to enhance the mechanical properties of 3D printable alginate hydrogels.

Reinforced hydrogels are created by the team by adding titanium dioxide (TiO2) and β-tricalcium phosphate (β-TCP). With these additions, they were able to increase the material’s elastic modulus up to 20 MPa. The added elasticity gives the material more support to cell scaffolds, particularly those for bone regrowth due to the β-TCP (hydroxyapatite) content.

According to the authors, “The study provides new insights into the possibilities of alginate/gelatin added with TiO2 nanoparticles and β-TCP composite hydrogel 3D printing for tissue regeneration, where further assays are being made to prove cytotoxicity, cell proliferation, and bioactivity of the material.”

The procedure for creating reinforced hydrogel is described in the paper “Alginate/Gelatin Hydrogels Reinforced with TiO2 and β-TCP Fabricated by Microextrusion-based Printing for Tissue Regeneration” published in Polymers journal.

Various scaffold designs 3D printed in the study. Image via MDPI.
Various scaffold designs 3D printed in the study. Image via MDPI.

Hole-free multi-material DLP

At the University of Washington, J. J. Schwartz & A. J. Boydston have shown how multi-material 3D printing is possible with a DLP printer. The paper titled “Multimaterial actinic spatial control 3D and 4D printing” details the findings of the study.

Multi-material 3D printing using a resin-based machine has been accomplished before, but according to the researchers, previous works had limiting factors. For example, in earlier studies using DLP, it was only possible to 3D print a multi-material object on the z-axis but not on x and y-axes.

Such methods also required changing the material in the vat during the printing process, which leaves holes in the final model.

To counter these problems the researchers manipulated the wavelength of the light projected to cure the resins. Each material used in the printing process is cured at a different wavelength.

The researchers state, “Inspired by the potential for fully integrated chemical synthesis and AM technologies, we explore the use of digital light processing AM (DLP-AM) to control chemical composition along all three axes of an object by simultaneously projecting more than one light source into a vat of photoresin.”

Furthermore, using the same method the researchers were able to create 4D objects capable of changing shape as a result of external stimuli.

“The disparate materials combinations have manifested heterogeneous imagery, mechanical anisotropy, and 4D printing through spatially controlled swelling, each from single resin vats simply by inputting controlled combinations of light.”

Multi-material DLP printed objects capable of changing shape. Image via Nature Communications.
Multi-material DLP printed objects capable of changing shape. Image via Nature Communications.

Shock absorbing metamaterials

Furthering their investigation into materials for 4D printing, researchers from Rutgers University and Korea University have created metamaterials.

Howon Lee, co-author of the paper and an assistant professor in the Department of Mechanical and Aerospace Engineering, said, “We believe this unprecedented interplay of materials science, mechanics and 3D printing will create a new pathway to a wide range of exciting applications that will improve technology, health, safety and quality of life.”

The findings are published in a paper called “4D Printing Reconfigurable, Deployable and Mechanically Tunable Metamaterials“.

The study concluded, “Our lightweight SMP microlattices have unprecedented capability of mechanical adaptation to unpredictable circumstances such as varying external loading and geometrically complex environment.”

“Reconfigurable and tunable mechanical metamaterials may find a broad range of applications such as tunable shock absorbing interfaces, morphing aerospace structures, and minimally invasive biomedical devices.”

Bespoke photoactive resins

By tweaking SLA resins UK scientists have found a way to make those materials photoactive, i.e. materials which respond physically or chemically to light. In the research titled ‘Additive manufacturing of photoactive polymers for visible light harvesting,” the authors made photoreactive resins to 3D print continuous flow reactors, which carry materials as a continuous stream.

Previously, such devices were made using traditional methods such as injection molding. In the latest study, a photoactive monomer was added to Clear FLGPCL02 by Formlabs. The Form 1+ SLA printer was used to print the photoactive resin, St-BTZ.

The authors stated, “This work provided an insight into how the development of bespoke photoactive resins can enable the application of SLA in the fabrication of continuous flow photoreactors, where a photo-active unit is directly incorporated within the polymer matrix.”

“Future work will involve design and fabrication of larger volume photoreactors with intricate flow features and development of SLA resins to produce organic solvent resistant structures.”

A continuous flow photoreactor made with using a photoreactive resin and SLA printer. Image via Elsevier.
A continuous flow photoreactor made with using a photoreactive resin and SLA printer. Image via Elsevier.

Topology optimization 

Polymer parts can be coated with metal lamination to provide extra strength to the object. One of the methods used to do this is via electroplating. The plastic part is made porous allowing the coating material to bond firmly with the plastic.

Mechanical engineers at TU Delft and Technical University of Denmark (DTU) have suggested a new method for topology optimization of such coated materials. By defining specific microstructures as part of the infill of a 3D model stronger parts can be obtained.

In “Homogenization-based stiffness optimization and projection of 2D coated structures with orthotropic infill“, it is argued that “Performing homogenization-based topology optimization allows for the modeling of designs with complex microstructures on a relatively coarse mesh, thus resulting in low computational cost. Furthermore, the double smoothing and projection (DSP) approach ensures in almost all cases a clear distinction between coating, infill and void.”

“This overall promising approach allows for extension of the method to 3D or to more complex loading situations. The main challenge here will lie in finding a parameterization that allows for smoothly varying microstructures through the domain. We are confident that such a parameterization can and will be found”

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Featured image shows CrAMmed logo over 4D printed structures. Image via Nature Communications.