## 1 Introduction  
Due to demographic change, the average age of the population will continue to rise. According to the Federal Statistical Office of Germany, demographic change in Germany is becoming increasingly acute. Every second person in Germany today is over the age of 45 and every fifth person over the age of 66 [1]. The inhabitants of the EU are also not younger, here one of five inhabitants is older than 65 years [2]. Coupled with this, the number of cases of bone diseases and defects, such as osteoporosis, will also increase [3]. In 2019 alone, 239,204 arthroplasties were implanted in the hip and 190,427 in the knee [4]. According to a Eurostat study for 2019, Switzerland, Austria, Belgium, and Finland are at the same high level with 275 to 300 hip replacements per 100,000 inhabitants as Germany with 311 [5]. Open reductions of multiple fragment fractures of long tubular bones in the joint region with subsequent osteosynthesis also represent a large proportion of total operations (225,974 operations; 16,974,415; as of 2019 in Germany) [4]. The mentioned methods for the treatment of bone defects, however, have some weaknesses. In the case of implants in the knee and hip area, the phenomenon of stress shielding occurs again and again. It describes that due to a too high modulus of elasticity of the implant, the load that is normally distributed to the bone is now mostly transferred to the implant and the bone loses density due to lack of load. This greatly compromises the long-term stability of the implants [6]. Another disadvantage exists with the previous osteosynthesis material. Metal implants are often used in surgeries to fix fractures. Since these do not degrade on their own, they are removed in another operation. In 2018, the number of such cases was 176,768 [4]. This is one of the main reasons why interest in biodegradable replacements for metal implants continues to grow [7]. Another treatment method for bone defects is autogenous bone grafting. In this procedure, bone is harvested from a healthy part of the patient's own body and placed in the defective site. This treatment has been the gold standard for bone replacement for many years. In addition, there is the option of an allogeneic bone graft in which the tissue is transplanted from one person to another [8,9]. While these treatments have been revolutionary and life-saving, complications can come with both allogeneic and autogenous bone grafting. Autogenous graft harvesting is expensive, painful, limited by anatomic constraints, and associated with donor site morbidity due to infection and hematoma. Similarly, allogeneic bone grafts have serious limitations because access to sufficient tissue is problematic for affected patients and there is a risk of rejection by the immune system. In addition, there is a possibility here of transmitting infection or disease from the donor to the patient [10]. A solution to these problems may lie in tissue engineering and 3D printing. [PERSON] and [PERSON] define tissue engineering as follows: \"Tissue engineering is an interdisciplinary field of research that applies the principles of engineering and the life science toward the development of biological substitutes that restore, maintain, or improve tissue function\" [11]. Scaffolds are used in tissue engineering. These are three-dimensional constructs with defined properties (e.g., porosity, mechanical strength, degradation behavior). Three-dimensional printing has become an important tool in the field of tissue engineering. The ability to print scaffolds of biomaterials with controllable properties, such as internal structure, porosity, and interconnectivity (all pores are interconnected), makes three-dimensional printing so superior to conventional techniques [12,13]. For example, custom constructs can be made to fit the exact needs of the patient and their injury [12,14]. The scaffolds provide a matrix for the cells to build new tissue [15]. In addition, the interconnectivity of the pores provides nutrient and metabolic waste transport.