## 1 Introduction  
Bone tissue engineering includes the combination of scaffolds, cells, and other bioactive factors to repair critical size bone defects with the ultimate scope of the formation of new bone. Due to the limitations of bone autografts, allografts, or alloplastic implants, new fabrication technologies have been developed. Three-dimensional printing techniques have opened opportunities in bone tissue engineering by enabling the creation of customized implants through the additive manufacturing process with a predefined shape, size, and microarchitecture [1]. After the first introduction of this technique by [PERSON], many 3D printing technologies have been developed [2]. The main method used for manufacturing 3D scaffolds for tissue engineering applications is based on the extrusion of an ink material, the subsequent creation of the 3D structure, and, finally, stabilization of the structure through crosslinking. During the design and fabrication of a scaffold for bone tissue engineering applications, many parameters have to be considered. The biocompatibility of the raw materials, the mechanical properties together with the micro- and macroarchitecture, and structural integrity play a key role in the final performance. All these parameters affect the cell attachment, proliferation, and differentiation, and, finally, the host tissue remodeling [3]. The selection of materials used as inks is crucial, and today, a variety of hydrogels are used in 3D printing. The term hydrogel refers to 3D structures of hydrophilic polymers that are joined together by crosslinking and have the ability to retain large quantities of water in their structure. The most commonly used hydrogels based on natural polymers are alignes, agarose, gelatin, fibrin, collagen, and chitosan. Alginic acid is an anionic polysaccharide biopolymer found in nature with many applications in biotechnology and pharmacology. Alignates are used in the form of hydrogels for wound and burn healing, drug delivery, as impression materials in dentistry, tissue engineering, etc. Alginic acid is mainly extracted from brown algae, which is the main polysaccharide and makes up at least 40% of their dry weight. Regarding its chemical structure, alginic acid is composed of linear, non-branched polysaccharides containing variable amounts of (1,4)-linked \(\beta\)-D-mannuronate and \(\alpha\)-\(L\)-galuronate residues [4]. Alginate-based hydrogel inks are commonly used in printing due to their biocompatibility, biodegradation, and cell adhesion. However, due to their low mechanical properties and their poor structural stability, they are not suitable for load-bearing bone tissue engineering applications. To overcome this drawback, alginate composites have been manufactured. Alginate blending with other biopolymers or reinforcement with particles or fibers results in the formation of alginate composites with higher mechanical properties and better stability [5]. Single sheets of graphene oxide dispersed into alginate/polyacrylamideamide gels enhanced their mechanical properties and increased their tensile strength and toughness [6]. [PERSON] et al. added magnesium silicate clay laponite into an alginate/MC hydrogel loaded with immortalized human mesenchymal stem cells. It was found that the addition of laponite improved printability and favored the release of growth factors. [7]. Hydroxyapatite, due to its excellent bioactivity, has been widely used as a filler material in alginate-based 3D-printed scaffolds. It has been reported that the dispersion of HAp in alginate gels increased the survival and proliferation of chondrocytes and the level of calcified cartilage markers [8]. Alginate/nano-HAp composites were fabricated by the 3D printing technique. Surface mineralization of the scaffolds improved their mechanical properties and also enhanced the response of human bone marrow-derived mesenchymal stem cells [9]. Bioactive glasses (BG) have been used in recent years in alginate matrices to produce 3D-printed scaffolds for bone tissue engineering applications.