The medical field strives to provide patients with the best quality of life through the use of state-of-the-art technology. However, the application of technology in the health sector is not a modern innovation; orthotics, prosthetics, splints, and braces have been utilized since the Stone Age [1]– [3]. While the devices are similar, the materials used to construct them have progressed from plants, rawhides, and leather to the familiar plasters, polymers, and alloys used in modern medical devices and aids. Many of these advancements in manufacturing processes were necessitated by the devastation caused by the two world wars and various epidemics in the 20th century, such as polio [4], [5]. The design and manufacturing of prosthetics and orthotics today have grown into a multidisciplinary research field actively improving the quality of life by continuously searching for advancements that increase comfort and customization for the patients. Additive manufacturing is therefore a natural next step in medical device innovation.
Using additive techniques for the rapid production of orthotics and prosthetics is attractive due to the expanded design paradigm. New and customizable geometries and material transitions can potentially solve many classic problems still present in the medical field. One such problem is the natural geometric variability from patient to patient; a “one-size-fits-all” methodology may be cost-effective but does not lend itself to great comfort and can potentially cause long-term health implications [6]. By leveraging 3D scanning technology to accurately reproduce the dimensional anatomy of individual patients, medical devices and implants can be customized and tailored to the patient via additive techniques. Additionally, there are new geometries and structures which demonstrate flexible, functionally graded mechanical behaviors that cannot be manufactured through traditional, subtractive means. Concerning implant design, these possible complex shapes can be used to limit the phenomena known as stress shielding, where the transfer of load from bone onto an implant decreases bone density. Finally, faster turnaround times for patients are also possible using additive techniques. This is critical for time-sensitive surgeries, such as dental implants, that can reduce the overall treatment period if early placement is performed [7].
While additive manufacturing is deemed as the gateway for the rapid manufacturing of complex shapes, the physics behind the behavior of additively manufactured surfaces when in contact with similar or dissimilar surfaces is often pretermitted. In the past decade, there has been an emphasis within the tribological field to study the role of friction and wear for such surface contacts [8]. The study of the interacting surfaces while in relative motion to each other is called tribology, i.e., the study of friction, lubrication, and wear. In general, a tribological system is comprised of a few elements such as the collective stress/operational inputs, the system structure, and the loss outputs. The operational inputs include the technical and physical load parameters such as sliding speed, duration, normal load along with the movement direction, and the temperature conditions stressing the system structure. On the other hand, the properties of the substantial elements, i.e., the base, counter-body, environmental conditions, and the intermediate medium, determines the system structure. Tribology is a multidisciplinary endeavor, and the application of tribology extends to sub-fields such as biotribology, nanotribology, and space tribology, to name a few. Due to the wide range of applications, tribological studies can have a broad impact upon global concerns such as the economy and the environment. For example, tribological analyses could reduce primary energy consumption by 24%, saving $160 billion and globally reducing the manufacturing industry’s carbon footprint [9–11].
Overall, tribological studies can be considered as key components for evaluating the performance and life cycle of manufactured parts. The performance of 3D printed parts can be adjusted by changing the print parameters. The lifespan of 3D printed parts is increased when the layers lie perpendicular to an applied normal force during mechanical test; it is easier to pull apart 3D prints layer by layer than trying to break multiple layers at a time. This is because the multiple layers act as reinforcement, and the areas between each layer are a weak point and similar mechanism can affect the tribological properties. On the other hand, the exact wear properties of additively manufactured parts remain unknown. To shed more light on the wear properties of 3D printed parts, the effect of friction on these parts should be analyzed, as friction is a crucial factor in dictating how fast a product wears down during constant use. In recent times, researchers have taken interest in the build orientation and its impact on the functionality and found print orientation to have noticeable effects on parts performance [12–19]. Analyzing tribological properties in conjunction with build orientation allows the wear properties of 3D printed parts to be characterized and print parameters optimized for better performance.
Considering the sculpted shape of orthotics and prosthetics, not to mention the dynamic loading, how a part is printed is just as important as what material it is printed with. If that is not complicated enough, different areas of the same part may need different wear and strength properties. For example, there must be high strength and high wear resistance where a foot meets the ground on a prosthetic leg but lower wear resistance and greater flexibility at the knee connection for functionality and mobility [17]. Research is currently focusing on exploring the effect of print orientation on the mechanical and tribological properties of various polymers. PLA and ABS are two of the most common thermoplastic materials used in additive prototyping and have shown promising aspects of their usage in 3D printed scaffolds, implants, and polymeric screws for orthopedic surgeries. Investigations of how the print parameters and build orientation affect the wear rate and coefficient of friction of 3D printed PLA and ABS has also been reported in some recent research and found layer thickness and normal load to have a visible effect, but the effect of layer orientation is found to be inconclusive [18]–[26]. On the other hand, Hanon et al. found that the vertical and horizontal print orientation has a significant effect on the dynamic coefficient of friction and wear properties of 3D printed PLA. However, these studies do not cover the effect the different print orientations may have on the material’s tribological properties when tested for a prolonged period. Also, literature reviews revealed a gap in knowledge regarding the effect of sliding speed on the wear behavior of additively manufactured plastics during dry sliding tests for a significantly longer (~ 3500 m) duration, which this study aims to meet. The material used in the print also influences the lifespan of the part. By determining the optimal print orientation for any specific polymer, the tribological performance of any 3D-printed parts can be tailored and enhanced. In this work, we compare the effect of three print orientations (X, Y & Z) on the tribological properties of two commonly used 3D printing filaments: PLA and ABS. Samples were fabricated using fused deposition modeling (FDM).