Scaffolds are implanted in a living body, and as such, they need to adhere to a series of principles for proper functioning. The following topics are listed as the main properties required for these scaffolds.
The three main biological properties that scaffolds need are biocompatibility, biodegradability, bioreabsorption, and bioactivity. Biocompatibility refers to the capability of the material not to be rejected by the biological environment in which it is implanted. In other words, the material should induce minimal biological response [1]. Scaffolds and their byproducts generated by degradation need to be biocompatible to avoid immunological damaging responses, such as infections, which can impair their functioning and the biological system in which they are implanted. Biodegradability can be understood as the capacity of the material to degrade in the environment where it is placed, through bacteria, fungi, or other means, reducing it to non-toxic substances. In the context of implants, this means that the material must be able to deteriorate through biological means inherent to the implanted site without causing damage elsewhere. Due to the fact that scaffolds are non-permanent transplants in bones, intended to restructure them, their biodegradability is fundamental so that surgical removal of the implanted material is not necessary. In addition to being non-toxic, the byproducts must be properly absorbed by the environment through a process called bioreabsorption. Bioactivity refers to a beneficial reaction of the material with the biological system. In the case of osseous implants, this is achieved through osseointegration, which is defined as a structural bond between the implant and the bone, involving tissue growth in the region, resulting in a firm, direct, and lasting connection between the bone and the scaffold [2]. This aspect is necessary for the bone to regenerate itself through times that it does not need scaffold presence permanently, in addition to creating a structure that favors the mechanical properties of the injured site.
During the implantation period, the scaffold must withstand the loads imposed by the body under normal conditions, thus requiring proper mechanical resistance. If the mechanical resistance is too low, several problems may occur, such as mechanical failure during use or even during surgery. Conversely, if the mechanical strength is too high, the cells adhered to the material may not be subjected to the necessary conditions for healthy development, as exposure to loads is essential for their growth. A material with high mechanical strength acts as a barrier preventing loads from reaching the cells, but this scenario is rare due to the high porosity of the produced scaffolds [3].
For complete integration of the bone and scaffold, precise control of scaffold pores is necessary, as they serve as pathways for distributing nutrients to the surrounding bone tissue. Therefore, the porosity should closely match that of the bone structure. The ideal porosity for a scaffold is around 90%; however, samples with porosity ranging between 55% and 74% are generally used due to the lower mechanical resistance of the material compared to bone. Additionally, the pores must be interconnected to facilitate better diffusion of substances throughout the organ [3, 4].
Regarding pore size, the requirement is closely related to the type of bone being replaced. In the present study, trabecular bones were examined, where the pore size typically ranges from 75 to 200 µm [5] that therefore will be the range of ideal size of the scaffolding pores to be modeled.
In addition to the basic structural requirements, producing scaffolds for bone implants that are increasingly efficient requires understanding the tissue and its components. By comprehending their functions, processes, and components, it becomes possible to mimic or stimulate them using the scaffold's structure or components. The ultimate goal is to accelerate the regeneration and calcification of bone tissue [6].
Understanding the requirements of a scaffold is fundamental to the present work because in the development of new models, it is crucial to consider everything necessary or unnecessary for the project. This ensures there are no unnecessary problems of incompatibility with the main objectives of the application.
Used in the last couple of decades and significantly boosted in recent years, additive manufacturing is defined by ASTM as a process of joining materials to create objects through 3D model data, typically layer by layer. It stands in contrast to subtractive manufacturing methodologies, as it involves adding material to a structure instead of removing material from it. This definition, solely focused on the process, applies to any material [7].
Material extrusion is currently the most common additive manufacturing process used in universities and industries due to its relatively low cost and simple process reproducibility. The machine consists of a head that is fed by a raw material yarn, that yarn is melted at its tip and then the material is deposited at the designated location. This is a constant process where the yarn is consumed and feeds the head regularly. As with the previous processes, the process also occurs in layers divided by the machine software, in each layer the head passes the marked path depositing molten material, which adheres to the previous layers and is rapidly solidified, forming the base layer for the next ones [8].
Among the various limitations of material extrusion process, one is the necessity to use only thermoplastic polymers due to the requirement of melting the material. Additionally, because the process is gravity-dependent, the range of possible geometries is relatively limited. Parts that increase in height horizontally, akin to oblique prisms, must have angles greater than 45° to allow proper structuring of the material during deposition. Parts with smaller angles require temporary supports to prevent material leakage or collapse during the process [9]. Another issue stemming from the characteristics of the molten material is the potential formation of unwanted small wires between material spans. This occurs due to either the lack of adhesion between the molten material and the base or when dealing with very small spans.
For the present work, understanding additive manufacturing processes is crucial because it determines how the scaffold will be produced, which significantly influences its properties. Additionally, it helps identify limitations in part geometry, making the choice of the manufacturing process very important.
Scaffold research has grown exponentially in recent decades, the number of results for the word "scaffold" in 2023 was 16,460, almost ten times greater than in 2003, which was only 1,631 according Web of Science® database. This size growth demonstrates that this is still a developing area, with a large open margin for evolution.
Regarding the pore geometry, the honeycomb format is used in several works, with square or hexagonal pores with 400 to 800 µm in size, ideally ranging from 50 to 710 µm, and total porosity ranging from 50 to 70%, ideally greater than 90% [10, 11]. Thavornyutikarn et al. [12] did a modeling using several types of pores in the form of repetitive cells, Fig. 1(a) and concluded that the format that generated better mechanical properties was the diamond format. The porosity obtained was 60%, with pore diameter ranging from 700 to 400 µm and with compressive strength of 3.5 and 6.7 MPa, respectively. There is also research using, as in the present work, the Voronoi function of Grasshopper to make a scaffold model, but in cubic form, obtaining comparative graphs of elastic modulus, porosity and specific surface area for scaffolds with different amounts of pores, which can be changed freely using the program. The development using the Grasshopper program can be seen in Fig. 1(b) [13].
Understanding the global panorama and previous research is essential for the present work to establish a foundation for the research and provide valuable comparison data. Additionally, it helps determine the relevance of the current work to the existing needs.