4-Axis 3D Printing Enables Tubular Implants with Controllable Mechanical Properties

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Disease and other trauma can cause hollow, tubular human tissues, like the trachea, intestine, bone, and blood vessels, to be negatively affected by long-segmental defects. Autologous grafts can help fix these, but there are limitations, such as not enough tissue availability and an extra surgery, which comes with a potential site morbidity, so a synthetic version is ideal. However, even that isn’t without its issues, as tubular implant grafts made with conventional methods, like melt electrowriting with a rotational collector and electrospinning, can cause calcification in the long run.

According to a team of researchers from the University Politehnica of Bucharest (UPB) and Maastricht University, using 3D printing to create a scaffold for tissue regeneration purposes could be a very useful solution to a slew of problems. The researchers recently published a paper on their work, “Controllable four axis extrusion-based additive manufacturing system for the fabrication of tubular scaffolds with tailorable mechanical properties,” in Materials Science and Engineering.

“Current additive manufacturing (AM) systems do not commonly implement the use of a rotational axis, which limits their application for the fabrication of tubular scaffolds,” the researchers wrote in the abstract. “In this study, a four axis extrusion-based AM system similar to fused deposition modeling (FDM) has been developed to create tubular hollow scaffolds.”

Extrusion-based additive manufacturing processes, like FDM, can help increase a scaffold’s mechanical integrity, as the technology is capable of fabricating fibers with hundreds of micrometers, and adding a fourth axis can make even stronger tubular scaffolds with more complex designs. But, if the rotational axis does not communicate with the 3D printer, you can only get helical designs. So, the team wanted to show how extrusion-based AM, with the addition of a synchronized fourth rotational axis, can be used to create complex tubular geometries for tissue regeneration scaffolds. They also studied “the role of geometry in scaffold design,” as well as how it can influence mechanical properties like three-point bending, radial compression, and tensile strength.

Overview of the printer and indicated parts.

The researchers created their AM system by taking a Roland EGX-360, with a controllable fourth axis, and replacing its engraving head with a custom, heated pressure-driven dispensing cartridge. The polymer they used was poly (ε-caprolactone), or PCL, which was extruded in a molten form through a 260 μm nozzle. Deposition was controlled by an electromagnetic pressure valve, and the PCL was deposited on to a rotating mandrel of stainless steel, which was controlled by the 3D printer and attached to the fourth axis. The team said that their four-axis extrusion-based system could achieve full control over the design and geometry of the scaffolds, “which has not been reported with other techniques.”

Fourth axis extrusion-based system. (A) Schematic overview of the fabrication process. (B) Examples of possible designs that can be manufactured with the fourth axis FDM system. (C) Rectangular pore design with rings and struts that can be introduced. (D) Diamond pore design with the amount of helices and rotations that can be varied in the design.

“All scaffolds were manufactured with a travel speed of the extruder set at 2 mm/s and the distance between nozzle and the stainless steel mandrel was set at 200 μm. The total scaffold length in all tested cases was kept constant at 18 mm with an inner diameter of 2 mm,” the researchers explained.

This system was used to fabricate two scaffold patterns: a diamond, which was labeled “according to the amount of helices (H) and the pitch (P) of the diamond,” and a rectangle, labeled “according to the amount of rings (R) and struts (S) in the scaffolds.”

Overview of the tested designs. (A) The rectangular pore design with varying amount of rings and struts in the scaffold. (B) The diamond design with varying pitch and amount of helices in the scaffold. Scale bar represents 1 mm.

“One additional script was written for the Hilbert curve as an example to show the complexity of the fabricated structures that can be achieved. The mandrel for this case had a diameter of 12 mm and the scaffold followed a 4th generation Hilbert Curve,” they wrote.

Fourth generation Hilbert curve scaffold printed on a 12 mm mandrel. Scale bar represents 1 mm.

The scaffold structure was visualized with the help of scanning electron microscopy (SEM) and stereomicroscope, and SEM micrographs were used to measure the diameter of the filaments. The researchers investigated the 3D structure with MicroCT, and various software programs were used to reconstruct the micrographs, visualize pore shape, and analyze scaffold volume. Then, they performed compression tests and tensile tests to determine mechanical characterization of the structures, simulated a 3D model of the scaffolds with finite element modeling, and completed a three-point bending test to test the scaffolds’ flexibility.

Radial compression test with a strain rate of 1% per second.

Longitudinal tensile tests with a strain rate of 1% per second. (A) Young’s modulus in the rectangular pore design with increasing amount of rings and the same amount of struts. (B) Young’s modulus in the rectangular pore design with increasing amount of struts and the same amount rings. (C) Young’s modulus in the diamond design with the same amount of helices and increasing amount of complete helix turns. (D) Young’s modulus in the diamond design with increasing amount of helices and the same amount of complete helix turns.

First, they found that the fibers in all of the 3D printed samples showed proper fusion, which can increase mechanical properties. Testing found that the diamond pore design had a higher Young’s modulus (19,8 ± 0,7 MPa) in radial compression mode, but while in the longitudinal tensile mode, the rectangular pore design was higher (5,8 ± 0,2 MPa). Additionally, by increasing the weight of the diamond pore scaffold, available surface area increases and porosity decreases.

“The weight of the scaffold correlated with the porosity, by increasing the weight of the scaffold, the porosity decreases and the available surface area increases. A variation in fiber size was observed between the rectangular pore designs, ranging from 341 ± 24 μm in the 5R9S sample to 370 ± 28 μm in the 6R7S sample,” the team wrote. “A certain level of thickness heterogeneity was observed in micro-CT, which was in good correlation with similar features noticed in the diamond pore designs.”

Micro-CT in combination with longitudinal tensile test data on the diamond pore design. Images were taken at 0, 2, 4, 6 and 8 mm of strain. Scale bar = 500 μm.

According to the results of the three-point bending analyses, the diamond design is more resistant to luminal collapse, though both designs “are more than capable of preventing luminal collapse.”

“This flexibility allow the manufacturing of scaffolds for diverse tubular tissue regeneration applications by designing suitable deposition patterns to match their mechanical pre-requisites,” the team wrote.

All of these tests demonstrated that both the diamond and rectangular pore designs were suitable to use for 3D printing vascular scaffolds. The researchers were able to determine that they can make systematic changes in the design of the tubular scaffolds in order to adjust their mechanical properties for whichever organ or tissue is needed.

Three-point bending at 40% deformation on the 5R9S design. (A) Three-point bending at 40% deformation on the 5R9S design when the probe is centred on top of the ring. (B) Three-point bending at 40% deformation on the 5R9S design when the probe is centred between two rings. (C) Difference in luminal diameter at 40% deformation during three-point bending on the 5R9S design.

“Previous works have investigated the implementation of a fourth axis in AM for creating helical structures [[25][26][27]]. The advantage of our method is that the fourth rotational axis is synchronized with the X-Y and Z axis and, therefore, the toolpath can be directly coded, which allows high precision in scaffold design and the ability to fabricate more complex designs,” the researchers explained. “Future optimization studies will aim at decreasing the fiber size below 100 μm [28].”

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