Welcome back to CrAMmed, our additive manufacturing research digest. Here you’ll find a selection of papers and developments in 3D printing from the academic world. The studies below surround topics such as microneedles, material parameters, cancer treatment, and metal droplet deposition.
Advanced algorithm for removing self-intersections in 3D mesh data
In a study titled, ‘A Robust Algorithm to Remove the Self-intersection of 3D Mesh Data without Changing the Original Shape,’ researchers Jiang Zhu, Yurio Hosaka, and Hayato Yoshioka propose an algorithm for removing the self-intersections of 3D mesh models.
Published in the Journal of Physics: Conference Series, the research team explains that various non-manifold parts in 3D meshes are often created due to the limits of the 3D scanning devices or triangulation algorithms used. These range from holes, areas with zero thickness, and self-intersections. Such non-manifold meshes often require manual tweaking from the operator to remove them, which can be a time-consuming process. The authors write: “In order to realize the automation of 3D sensing and 3D printing, automatic detection and correction of the non-manifold mesh became an essential task to solve. In the past decades, there are many researchers proposed different algorithms to fix the non-manifold mesh. However, there are few algorithms developed to remove the self-intersection of the 3D mesh data.”
The research paper presents a robust algorithm that can automatically detect and remove the self-intersection, without changing the original shape of the mesh. It is composed of four steps: it detects the intersected facets in the mesh model, divides the intersected facets along the crossing lines, locates the outside facets, and deletes the inner facets and burying facets. To test the algorithm, the researchers created different 3D mesh models, including a pyramid-in-cube, demonstrating the successful removal of intersections using the method.
Elsewhere, researchers from the U.S. have developed a novel technique for 3D printing a mesh reinforcement on electrospun scaffolds to improve their mechanical properties. Titled ‘3D printed mesh reinforcements enhance the mechanical properties of electrospun scaffolds,’ the paper explains that there is significant interest in the usage of electrospun scaffolds as substrates for tissue regeneration and repair due to their fibrous, extracellular matrix-like composition. However, a limitation of the scaffolds surrounds their inherently low mechanical strength and stiffness, restricting their use in some clinical applications.
To resolve this limitation, the researchers 3D printed a PLA mesh directly onto electrospun scaffolds composed of a 40:60 ratio of poly(ε-caprolactone) (PCL) to gelatin, respectively. The PLA grids were 3D printed onto the scaffolds with a 6 mm or 8 mm distance between the struts. To determine whether the 3D printing process affected the architecture of the electrospun scaffold, the research team used scanning electron microscopy, tensile testing, and an in vivo bone graft model.
From the test, the paper explains that the tensile strength and elastic modulus of the electrospun scaffolds were markedly increased, and ductility reduced, by the addition of the 3D printed PLA meshes. They also retained their matrix-like structure. The study concludes that “3D printed mesh reinforcements offer a new tool for enhancing the mechanical strength of electrospun scaffolds while preserving the advantageous extracellular matrix-like architecture.” It is written by Nicholas W. Pensa, Andrew S. Curry, Paul P. Bonvallet, Nathan F. Bellis, Kayla M. Rettig, Michael S. Reddy, Alan W. Eberhardt & Susan L. Bellis, and published in Biomaterials Research.
The latest 3D printing research in materials
Halmstad University researchers in Sweden have completed an investigation into the use of thermoplastic elastomer (TPE) filament in the FDM 3D printing process. The ‘Investigation on Filament Extrusion of Thermoplastic Elastomer (TPE) for Fused Deposition Modeling’ paper, written by Wang Zicheng and Mohammad Nouri, aims to optimize the quality of the filament in order to make the TPE material compatible with FDM manufacturing.
To carry out the investigation, Zicheng and Nouri conducted optimization experiments to find out key parameters in the extrusion process that determine the quality of the filament. After determining the optimal parameters, further investigation into the additive content of the TPE granulate was made to solve the current limitations of the material in practical 3D printing, caused by its high surface friction that has a significant impact on the FDM process. The filaments were manufactured using a desktop filament extruder from 3devo and the surface friction tests were performed on a tribotester. Applications for TPE in 3D printing include electronics, home appliances, the automotive industry, and packing materials.
In other advanced material related research, a team from Xi’an Jiaotong University have produced a study focused on improving the 3D printing parameters of continuous carbon fiber/epoxy composites (CCF/EPCs).
Titled ‘A Sensitivity Analysis-Based Parameter Optimization Framework for 3D Printing of Continuous Carbon Fiber/Epoxy Composites,’ the paper states that “the 3D printing parameters [of CCF/EPCs] and their relationship with the mechanical properties of the final printed samples have not been fully investigated in a computational and quantifiable way.” The research paper, therefore, presents a sensitivity analysis (SA)-based parameter optimization framework for the 3D printing of CCF/EPCs.
Fiber-reinforced polymer composites (FRPCs) are equipped with properties such as low density and high strength, making them suitable for applications in industries such as construction, transportation, and aerospace. When manufactured using 3D printing, the material can be processed with a number of benefits including higher speeds, more design freedom, lower costs and more.
To carry out their study, the researchers analyzed experimental data and created a surrogate model for process parameters. SA was implemented to determine the parameters and mechanical properties and was subsequently used for additional testing to verify the SA-based framework. From the results, the researchers concluded that ‘critical’ parameters, such as printing speed, thickness, and space were difficult to control. The study was conducted by Hong Xiao, Wei Han, Yueke Ming, Zhongqiu Ding, and Yugang Duan, and published in Materials.
Environmental conditions can have a significant effect on the mechanical properties of various materials. Such environmental parameters include temperature and humidity, which can have a major impact on mechanical properties of materials such as compressive strength, tensile strength, bending strength and impact strength. Suresh Thota, a researcher at South Dakota State University, has produced a paper investigating these effects titled ‘A Study of the Effect of Heat Treatment on 3D Printed PLA Impact Strength.’
Thota’s study focuses on the effect of temperature and humidity on the impact strength of 3D printed PLA plastic. Different 3D printed PLA specimens were subjected to various experimental testings surrounding their impact strength, which denotes the material’s ability to absorb energy caused by an impact load. These tests revolved around the placement of 6 pairs of 3D printed PLA parts in a heated water bath to reach the desired temperature. For each different test, the temperature in the water bath was incremented by 10⁰ C to reach a maximum of 95⁰C, starting from 25⁰ C, totaling to eight different temperature experiments. For temperature effects, the six 3D printed PLA specimens were placed in a nonvacuum oven, at eight different temperatures as well. These parts were then impact tested following ASTM D256 standards test methods for Determining Izod Pendulum Impact Toughness of Plastic Materials, showing that the impact strength of the PLA increased with an increase in temperature treatment. However, when impact tested after aging at room temperature post-heat treatment, they demonstrated a considerably low impact strength. “This concludes,” writes Thota, “the impact strength of PLA is not sustaining with aging of samples.” Heat treatment can, therefore, change the strength of PLA, however, the strength can not be sustained over time.
Metal droplet deposition 3D printing
Researchers from Northwestern Polytechnical University have proposed a method for suppressing the effects of gravity on droplet-based metal 3D printing, using an anti-gravity electric field. Titled “Suppression of gravity effects on metal droplet deposition manufacturing by an anti-gravity electric field,” the research paper focuses on the application of droplet-based 3D printing for metal forming in space, as it does not require large-scale energy equipment or customized materials, which can prove advantageous in outer-space applications. However, the conditions of space, for example microgravity, has a significant impact on the mechanisms of droplet-based 3D printing, which hinders its use for such applications. The researchers explain that “To develop a droplet-based 3D printing technique suitable for space manufacturing, the droplet deposition behavior under microgravity should be physically simulated in a normal gravity environment.”
To achieve this, the research team set about developing a novel experimental system comprising an anti-gravity electric field to suppress the gravity effects on droplets deposition. They manipulated the droplets to deposit horizontally on a vertical substrate, and charged them before ejection to eliminate the inertia. When the droplets travel into the deflection electric field, the trajectories could be changed by regulating the charging and deflection voltage. The researchers conclude that under the manipulation of the anti-gravity electric field, droplets can accurately deposit on the objective vertical substrate and solidify into regular morphologies. From the results of the study, the authors suggest that usage of the anti-gravity electric field is effective in suppressing the gravity effects on droplet deposition 3D printing, paving the way for an “applicable additive manufacturing approach in space.” The paper was written by Jieguang Huang, Lehua Qi, Jun Luo, Lei Zhao, and Hao Yi, and published in International Journal of Machine Tools and Manufacture.
Advances in medical 3D printing
In a recently published study titled ‘A 3D-printed microfluidic-enabled hollow microneedle architecture for transdermal drug delivery,’ a group of researchers investigated different methods for drug delivery using a 3D printed microneedle.
The paper explains that 3D printing is a fabrication technique capable of simultaneously creating and integrating “complex millimeter/centimeter-long microfluidic structures and micrometer-scale microneedle features.” This, therefore, means that microfluidic architectures can be embedded within microneedles to enable fluid management capabilities that present new degrees of freedom for transdermal drug delivery. However, the researchers also explain that 3D printing has previously been limited by a lack of versatility and high cost to print both features in a single step and the throughput to render components within distinct length-scales.
The research team has devised a new method allowing for the creation of hollow microneedles interfaced with microfluidic structures in a single step, leveraging SLA 3D printing technology. They have been able to produce complex architectures with lower cost and higher print speed and throughput than previously reported methods. To achieve this, the group devised an elaborate hollow microneedle design and a refined print setup to produce needle tips substantially finer than the SLA printer’s laser spot size and with high yield, using a Formlabs Form 2 3D printer.
Potential applications of the microfluidic-enabled microneedle architecture include implementation in future biomedical devices to facilitate new modes of operations for transdermal drug delivery applications. Published in Biomicrofluidics, authors of the study include Christopher Yeung, Shawnus Chen, Brian King, Haisong Lin, Kimber King, Farooq Akhtar, Gustavo Diaz, Bo Wang, Jixiang Zhu, Wujin Sun, Ali Khademhosseini, and Sam Emaminejad.
An additional 3D printing study relating to microneedles, titled ‘Biocompatible 3D Printed Microneedles for Transdermal, Intradermal, and Percutaneous Applications,’ was also recently published in Advanced Engineering Materials by researchers Khalil Moussi, Abdullah Bukhamsin, Tania Hidalgo, and Jurgen Kosel from King Abdullah University of Science and Technology (KAUST). The authors investigate the use of 3D printed microneedles for a variety of biomedical applications.
Microneedles are mostly used in “minimally invasive methods […] that require imperceptible tissue penetration and drug delivery,” as explained in the paper. Despite their advantages, microneedles are yet to see wide adoption in clinical settings, due to the “complexities associated with the microfabrication techniques used for the development of MNs, which are often multistep, labor-intensive, and require expensive cleanroom equipment.”
However, the researchers explain that an alternative option for producing microneedles lies in 3D printing, which can improve their integration within microelectromechanical devices, using methods such as SLA, FDM and two‐photon polymerization (TPP). Focusing on TPP, the paper concludes that high‐resolution TPP 3D printing allows for the robust and seamless integration of MNs with a delivery system for biomedical applications while preventing the need for laborious and complex fabrication techniques. The researchers 3D printed two hollow MNs with inner diameter and height ranging from 80 to 120 μm and from 200 to 400 μm, respectively. To corroborate the devices, they were used in a successful penetration test into both a skin‐like material and mouse skin. Potential applications of the 3D printed microneedles include direct tissue interfacing or implants.
A collaborative project between researchers from the US and China has investigated the combination of virtual surgery and 3D printing for the postoperative treatment of cancer, particularly for the head and neck. This part of the body features complex anatomy and therefore presents a challenge when it comes to surgical reconstruction of the head and neck. Functional reconstruction, however, plays a significant role in the quality of life of patients undergoing head and neck surgery, as precision medical treatment can greatly improve the prognosis of patients with head and neck tumors.
To improve the restoration and reconstruction of head and neck surgical defects, the researchers have employed CAD/CAM, 3D printing and VR technology in five different cases of head and neck surgery. “Digital surgical technology has great potential applications in the clinical treatment of head and neck cancer because of its advantages of personalization and accuracy,” explain the researchers. From the results, the authors conclude that 3D models, both in printed and CAD form, can provide important information such as tumor location, scope of invasion, blood supply, and also provide the potential for preoperative simulation for complex defect repair. Utilizing VR technology for simulating surgery for surgeons was deemed a “more complex and time-intensive preoperative assessment,” however it still provides relevant morphological and functional information. Although modern digital technologies such as 3D printing and VR can help provide precise surgical treatment of head and neck cancer patients, its application scope is still limited as more systematic results are needed to confirm the overall and reliable clinical value.
The research paper, “Combined application of virtual surgery and 3D printing technology in postoperative reconstruction of head and neck cancers,” is published in BMC Surgery. It is co-authored by Chao Li, Yongchong Cai, Wei Wang, Yan Sun, Guojun Li, Amy L. Dimachkieh, Weidong Tian and Ronghao Sun.
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Featured image shows CrAMmed logo on the image of 3D-printing of microfluidic-enabled hollow microneedle devices. Original image via Biomicrofluidics.