The application of additive manufacturing (AM) in the field of manufacturing biocompatible materials has really been beneficial in increasing the biomechanical performance of end products, as was developed in new research in the journal MDPI Polymers.
Specimen designs, sized according to the corresponding ASTM standards for each characterization test. From left to right: tensile test (ASTM D638), bending test (ASTM D790), shear test (ASTM D5379), fatigue and impact test (ASTM D7774 and ISO 179-1).
AM now allows the replacement or strengthening of bones as well as prosthetic limbs by synthetic geometric imitation. The sequential multi-layered carbonate deposition during the process ensures structural integrity, superior mechanical properties, increased service life as well as mimicry of natural bone porosity, making it ideally suited for the biomechanical field.
Polycarbonate and its properties
ISO 10993 certified polycarbonate (PC) is a thermoplastic material with high geometric complexity conveniently sterilized by gamma rays and ethylene oxide. The high quality engineering polymer is robust and has high fatigue resistance as well as heat and chemical resistance.
Cross sections of X-Flat, X-Edge and Z-Flat samples. Image credit: Gras, G. G, Polymers.
Thus, it is widely used for various medical applications such as sterilization, kidney dialysis, surgical instrument manufacturing, and cardiac surgery products. These distinct beneficial properties have motivated researchers to focus on their intensive use in the medical field, and the latest study by Marco A. Perez and his team successfully analyzed the properties of 3D printed PCs for biomedical functionality.
Tests and methods
The latest study involves extensive testing such as impact testing, shear testing, fatigue testing, tensile testing, and bending testing to accurately assess all relevant mechanical properties. Since the printing mechanism is essential for the orientation of fibers in the polymeric building resin, two separate printing orientations, Z-Flat and Z-Edge, were used and Z-Flat (ZX configurations) investigated. .
Three hundred and twenty-two samples were tested with well documented standard test procedures and test equipment such as the use of Zwick 30kN equipment. The tests scrupulously followed the standardized test protocol ASTM D638. Such a consistent approach to testing and evaluation is essential for accurate results and performance analysis.
Analysis of tensile and bending performance
The performance of the polymer has been carefully evaluated, keeping in mind the results of each test individually to get a clear idea of the properties and performance of the polymer. The performance of samples fabricated on the Z axis was found to be the worst. These samples take the longest time to print.
However, the overall tensile modulus results are very close to the modulus reference value (2000 MPa), showing orthotropic stiffness properties.
The lower strength values of polymers printed on the Z axis represent the weak point in vertical polymer manufacturing. Bending performance was not affected by variation in orientation or printing direction and is well within the reference value of 2100 MPa. However, the Z-axis samples suffered fractures occurring between different layers.
Experimental setup for ASTM D5379 shear test standard with digital image correlation equipment. Image credit: Gras, G. G, Polymers.
The strength performance of the X-axis samples was lowest for 90 degree filament orientation. The performance analysis clearly showed that if the filament is in the direction of the stretched fiber (the angle is 0 degrees), the maximum strength and resilience is displayed by the polymer.
Assessment of impact and fatigue performance
The Charpy test was selected for the evaluation of impact performance, and like the tensile test, the Z-axis samples had the lowest toughness property.
This is because higher stress values lead to the formation of cracks, which acts as a point of failure. Therefore, it is quite evident that a 0 degree screen angle provides polymers with the highest fatigue strength, while specimens oriented at 90 degrees had the lowest fatigue strength value. Again, the vertical polymers on the Z axis exhibited the worst bending fatigue strength limit as their layers suffered disintegration.
These results raise a serious question as to the properties and mechanical performance of polymers oriented on the Z axis.
The detailed performance analysis published in Polymers was able to identify key points regarding the orientation of the raster. Z-axis straight samples, having the greatest number of layers in their geometric structure, also take a lot of time and resources to synthesize.
The bleeding of the layer tips is also a critical factor for this extended 3D printing time.
Although the test results for different 3D printed samples are almost proportional, the performance of the Z axis and mechanical analysis identifies it as the weakest sample. The sudden separation of the carbonate layers leads to brittle fractures in these vertically oriented samples, illustrating its lower resistance properties to exerted stresses.
Experimental set-up for ASTM D7774 three-point fatigue tests. Image credit: Gras, G. G, Polymers.
Fatigue properties are largely affected by the screen angle and axis orientation of the polymer 3D printing. The overall analysis of mechanical performance and test results indicate that the X-flat orientation is the most reliable mechanically and has the highest strength.
However, an essential point to note is that these properties are still a far cry from the well-known properties of human bone. Therefore, although the PC polymer has the certifications for doping biocompatibility with other elements or compounds, it is necessary for improved mechanical performance and increased reliability.
Gras, GG, Abad, MD, & Pérez, MA (2021, October 25). Mechanical performance of biocompatible 3D printed polycarbonate for biomechanical applications. Polymers, 13(21). https://www.mdpi.com/2073-4360/13/21/3669