Table of Contents

Special Section: Bioprinting of 3D Functional Tissue Constructs

Regular Section

Review article

by Monisha Monisha, Shweta Agarwala

Biodegradable materials are designed to degrade in a desired time either through the action of microorganisms or under certain physical conditions. The driving force behind the rise of biodegradable materials is the growing problem of electronic waste (e-waste), low recyclability, and toxicity of electronic materials. Transient response of biodegradable materials has found application in next-generation health-care and biomedical devices. Advances in material science and manufacturing technique have pushed the envelope of innovation further. This review discusses different biodegradable material classes that have emerged to replace the traditional non-biodegradable materials in electronics. Focus has been given to conversion of biodegradable materials to inks and pastes that find use in printed electronics to create flexible, bendable, soft, and degradable devices. Material degradation behavior and dissolution chemistries have been illustrated to understand their impact on electrical performance of devices. Finally, some short-term and long-term challenges are pointed out to overcome the commercialization barrier.

Original research article

by Peng Chen, Zhaoqing Li, Sheng Liu, Jin Su, Haoze Wang, Lei Yang, Chunze Yan, Yusheng Shi

Laser powder bed fusion (LPBF) additive manufacturing is an effective method to prepare three-dimensional ordered network titanium dioxide (TiO2) photocatalytic materials, therefore enhancing the absorption intensity of incident light and improving the photocatalytic efficiency. However, TiO2 is difficult to be directly sintered by LPBF due to the high melting point and brittleness. In this study, we prepared a polyamide 6 (PA6)-coated TiO2 photocatalytic composite powder for LPBF based on the dissolution precipitation polymer coating (DPPC) method and evaluated its LPBF processability. In the precipitation process of PA6, there was a significant crystallization exotherm with temperature recovery. Effective temperature control of this precipitation process had a significant effect on the morphology and particle size distribution of the precipitated powder. The increase of the dissolved concentration of PA6 to 150 g/L produced an obvious temperature gradient of the reactor, resulting in a wide particle size distribution and a powder with a characteristic porous surface. The prepared PA6/TiO2 composite powder presents a near-spherical porous-surfaced morphology, a high specific surface area of 240.5 m2/kg, an appropriate Dv(50) of 48.8 μm, and a wide sintering window of 26.6°C, indicating a good LPBF processability and potential of the photocatalytic application.

Original research article

by Mingyang Li, Yiwei Weng, Zhixin Liu, Dong Zhang, Teck Neng Wong

Printability of 3D printable cementitious materials is related to material rheological properties, and is affected and controlled by modern concrete chemical admixtures. In this work, the influence of several chemical admixtures including superplasticizer, retarder, and accelerator on the rheological characteristics of printable materials was investigated using central composite design (CCD). Twenty test points with varying dosages of chemical admixtures were performed to evaluate the primary effects of chemical admixtures and their combined interactive effects on the rheological properties. The results indicate that with the increase of retarder or superplasticizer dosage, all rheological parameters decrease while accelerator possesses an opposite impact. The rheological properties are negatively proportional to the combined interactive effect of retarder and accelerator. The combined interactive effect of retarder and superplasticizer positively affects dynamic yield stress, plastic viscosity, and thixotropy, while it negatively impacts static yield stress. The combined interactive effect of accelerator and retarder positively affects the yield stress, whereas it negatively influences the plastic viscosity and thixotropy. The results indicate that the CCD is an efficient method to find the desirable formulation within a given boundary.

Original research article

by Shujie Tan, Xi Zhang, Ziyu Wang, Liping Ding, Wenliang Chen, Yicha Zhang

Triply periodic minimal surface (TPMS) cellular structures of Ti6Al4V with theoretically calculated relative densities ranging from 4% to 22.6% were designed using a toolpath-based construction method and fabricated by laser powder bed fusion, and their macrostructure, microstructure, and compression performance were investigated. The results indicated that the macrostructure was the same as that of TPMS structures designed using the traditional method. In contrast, the microstructures of the as-built samples and the samples after stress-relief annealing were slightly different from those of the traditional ones. Moreover, compression test results of the Schwarz-P structures showed that the compressive modulus was positively related to the calculated relative density, and a Gibson-Ashby model was established to quantitatively describe the relationship between the compressive modulus and theoretical relative density. The findings of this work show that the mechanical performance of a TPMS structure obtained using a toolpath-based construction design can be accurately predicted using geometric parameters or printing toolpaths. This will be helpful during the design stage.

Original research article

by Cory Groden, Victor Champagne, Susmita Bose, Amit Bandyopadhyay

Bimetallic structures and coatings through additive manufacturing (AM) have demonstrated a high degree of freedom for tailoring properties depending on the application. In this study, Inconel 718 and CoCrMo were used as both are common alloys and exhibit unique properties, such as high-temperature oxidation, wear, and fatigue resistance. Using directed energy deposition-based metal AM, bimetallic structures containing these two alloys were manufactured, and the resulting structures exhibited no intermetallic phase formation, cracking, or porosity. Scanning electron microscopy and energy dispersive spectroscopy revealed a smooth elemental transition between the two compositions. Hardness testing showed a linear transition in the interfacial zone, validating no brittle intermetallic phase formation. Compression testing and fracture surface analysis revealed that the failures were not dependent on the interface properties. High-temperature oxidation showed no distinct effect on the interface, a firmly attached chromium oxide layer on the Inconel 718 side and a loosely attached chromium oxide layer on the CoCrMo side. There was also evidence of pit formation on the Inconel 718 surface, but not on the CoCrMo. These findings confirm a stable bimetallic system in which one of the two alloys can be used on the other material to improve the structure’s high-temperature oxidation or wear/corrosion resistance.

Original research article

by Shaddin AlZaid, Noofa Hammad, Hamed I. Albalawi, Zainab N. Khan, Eter Othman, Charlotte A. E. Hauser

This research highlights the development of a two-dimensional (2D) and three-dimensional (3D) preview software for additive manufacturing (AM). The presented software can produce a virtual representation of an actuator’s path movements by reading and parsing the orders of the desired geometric code (G-code) file. It then simulates the coded sections into separate 2D layers and colored 3D objects in a graphical model. This allows users to validate the shapes before the 3D printing process. G-code is an operation language which is based on command lines of code written in an alphanumeric format. Each line of these commands controls one machining operation; this instructs the machine’s motion to move in an arc, a circle, or a straight line to perform a specific shape after compiling all code lines. AM technology is widely used in most manufacturing fields (e.g., medical, chemical, and research laboratories) as a prototyping technology due to its ability to produce rapid prototyping models. 3D printing creates physical 3D models by extruding material layer by layer as 2D layers. At present, the most critical challenges in AM technology are drastically reducing prototyping materials’ consumption and time spent. To address these challenges, the proposed software allows for visualization of G-code files and predicting the overall layers’ shapes, allowing both structure prediction and subsequent printing error reduction.

Original research article

by Ana Paula Clares, Yawei Gao, Ryan Stebbins, Adri C.T. van Duin, Guha Manogharan
Binder jetting is an additive manufacturing (AM) technology that has gained popularity and attention in recent years for production applications in tooling, biomedical, energy, and defense sectors. When compared to other powder bed fusion-based AM methods, binder jetting processes powder feedstock without the need of an energy source during printing. This avoids defects associated with melting, residual stresses, and rapid solidification within the parts. However, one of the challenges of this process is the relatively lower densities which impacts part density, and subsequently, sintering and mechanical properties. In this study, we investigated the influence of bimodal powder size distributions (a mixture of coarse to fine particles) as a method for increasing part density and mechanical strength, and used stainless steel (SS) 316L bimodal mixtures in this case. Four unimodal and two bimodal groups were evaluated under similar AM processing conditions for sintered density measurements and flexural strengths. Our results demonstrated that bimodal size distributions showed a statistically significant increase in density by 20% and ultimate flexural strength by 170% when compared to the highest performing unimodal group. In addition to experimental findings, reactive molecular dynamics simulations showed that the presence of finer powders along with coarser particles in the bimodal particle mixture contribute to additional bonds that are stronger across the particle interfaces. Findings from this study can be used to design bimodal particle size distributions to achieve higher density and better mechanical properties in binder jetting AM process.