Primena aditivnih tehnologija u izradi anatomski prilagođenih skafolda za rekonstrukciju koštanog tkiva

Abstract

Tissue engineering almost always involves the use of so-called scaffolds, which are expected to perform the role of an artificial (highly porous) extracellular matrix that is to provide a proper and sufficiently rapid cell growth and efficient reconstruction of tissue that has been damaged by injury or disease. In terms of structure, scaffolds are usually artificial lattice-like support structures of biocompatible materials which primarily provide the necessary mechanical support to the cells seeding in the process of tissue reconstruction. The second, equally important function that scaffolds should generate is to provide a smooth reinnervation and revascularization of new tissue and their communication (connection, ingrowth) related to the surrounding tissue. Bearing in mind the great importance of the scaffold in the process of (human bone) tissue reconstruction, the research into the development of an optimal design variant of a scaffold and technologies for scaffold manufacturing are crucial issues that have received much attention in tissue engineering (TE). Although a number of design concepts of scaffolds aimed at bone tissue reconstruction have been developed, there are still challenges to overcome in an effort to optimize the scaffold design. This implies satisfying different groups of requirements that can often be opposite. It is necessary to simultaneously achieve a high permeability of structure and a high level of bio-adhesiveness of the scaffold structure elements. Also, it is preferable for the scaffold to be biodegradable, and to provide the ability of adjusting the mechanical properties of a structure to specific load cases. Finally, scaffolds are expected to be geometrically consistent with patient-specific anatomical forms, while still being easy to fixate and implant, with a relatively simple technological process for their manufacturing. A particular challenge regarding scaffolds is related to finding an appropriate manufacturing process, which is the focus of the research in this dissertation. When it comes to the fabrication of scaffolds, they used to be fabricated by conventional methods for many years. However conventional methods showed several limitations such as the inability to precisely control pore size, pore geometry, pore interconnectivity, spatial distribution of pores and construction of internal channels within the scaffold. A significant problem of conventional manufacturing processes is also the presence of organic solvent residues due to their toxicity and carcinogenity . As an alternative to conventional scaffold fabrication methods, additive manufacturing technologies (AT) have emerged. AT enable the automatic construction of complex scaffold features, layer-by-layer, according to computer-aided design (CAD) data obtained from patient’s medical scans (CT). Advantages of using АТ processes in scaffold manufacturing include customization of the products to meet the individual needs (anatomical custom made scaffolds), ability to create complex geometries and high Примена адитивних технологија у изради анатомски прилагођених скафолда за рeкoнструкцију коштаног ткива 12 accuracy features, and possibility to control pore size and distribution of pores and the entire internal architecture of scaffold. New possibilities of scaffold manufacturing, brought on by AT, have encouraged the emergence of new approaches to the design of the internal architecture of these structures. In an attempt to reach a bone tissue scaffold that meets the TE requirements to the greatest possible extent, the team of the Laboratory for Intelligent Production Systems, engaged in the research on the project III41017, proposed the original concept of the scaffold design (structure) in the form of a solid and maximally permeable 3D latticed support structure - (Anti-Labyrinth) Anatomically Shaped Latticed Scaffold- ASLS, which consists of simple and interconnected struts. This design concept rejects the approach which tries to imitate the trabecular bone structure, the so-called labyrinthine concept. A detailed description of ASLS design concept is shown in the first part of the thesis. The success of the concept was verified by creating the models and manufacturing experimental samples of anatomical custom made scaffolds ASLS for the critical size defect (maximum size 10×8 mm) made on the proximal tibial diaphysis of experimental animals - rabbits. The geometric complexity of the ASLS design and the small size of the experimental samples required the application of AT. In this context, the main objective of this dissertation was to investigate the application of AT in the fabrication of anatomical custom made scaffolds for bone tissue reconstruction on the example of ASLS. After a detailed analysis, three AT were selected (3D bioplotter, DMLS and EBM) that were potentially capable of making such complex forms like ASLS. The first two were used to make permanent ASLS of Ti-alloys (Ti6Al4V and Ti64), while 3D bioploter was chosen as the only commercially available AT developed to plot biodegradable materials and biological cells, for making temporary (biodegradable) ASLS of hydroxyapatite (HA). For this purpose, ASLS samples were fabricated using the selected AT and a detailed analysis of the technological methods of making these samples by using the selected AT was conducted, and it showed the advantages and disadvantages of each of the AT used in the manufacturing of this class of bone scaffolds. These ASLS samples were also used in experiments that explored the features of the design concept ASLS from the point of implantation in vivo. In this pilot experiment, a defect was caused and ASLS was implanted in the tibia of a rabbit in order to examine the biological and mechanical properties ASLS which were characteristic for the bone scaffold. This opens new opportunities for further bio-medical research, which is expected to, in a positive scenario, lead to solutions for improving the recovery of bone tissue. Примена адитивних технологија у изради анатомски прилагођених скафолда за рeкoнструкцију коштаног ткива 13 An important result of the research is the definition of the criterial matrix for the assessment of AT applicability for bone tissue scaffold manufacturing and process applicability calculator. The results showed that EBM is currently the optimal choice for manufacturing of such metallic scaffolds. According to the mentioned comparative analysis of applicability, DMLS is just behind EBM and can support the development of complex forms ASLS, but with slightly lower performance. However, at this point of development, EBM and DMLS cannot be used to create temporary ASLS, i.e. from biodegradable materials. The indisputable advantage of 3D bioplotter, compared to other technologies, is the ability to plot biodegradable materials, and even cells. However, the main drawback of this technology is that it cannot be used (at the current moment of development) to make complex forms ASLS. Also, the criterial matrix for the assessment of AT applicability and process applicability calculator can be applied for comparative analyses of manufacturing processes for making similar shapes with the existing and future AT, changing the relevant parameters and their values defined for a particular case.