# Nanomaterials in Bone Regeneration
## 1 Introduction  
Nanomaterials are currently in the spotlight of bone regeneration and tissue engineering in general. Due to their size on a nanoscale, the nanoparticles express higher availability in biological systems. Rapid progress in the field of biomaterials nanotechnologies contributes to the solution to growing demands on the number of functional bone grafts and implants. To date, the biomedical uses of nanomaterials have been extensively reviewed [1; 2; 3]. Tissue engineering is a multidisciplinary field that applies the principles of mechanical engineering, material science, and life sciences with the aim of developing biological substitutes that restore, maintain, and improve tissue function by preparing porous three-dimensional scaffolds [4]. With the ever-increasing age of the population, the need for various biomedical implants and scaffolds is becoming a significant economic burden to the health care system [5; 6]. Conventional tissue replacements (such as autografts and allografts), although there are revolutionary methods, encounter many problems that must be addressed for individual patients. Considering that cells interact with tissues that are nanometer in size and form a nanostructured matrix, nanomaterials play a key role in stimulating cell migration and proliferation, which is very important for the regeneration of bone tissue in particular. Bones can be thought of as the scaffold of the body supporting organ systems and are accountable for protection, load-bearing, and locomotion. The bone tissue is a dynamic, macro-, micro-, and nanostructured system that consists of both inorganic and organic material [7; 8]. A bone has the ability to be constantly remodeled and repaired during its lifetime [5]. Unfortunately, natural repair and reconstruction of hard bone tissue lead todefects, and bone damage as a result of resection of tumors, trauma, infection, or congenital diseases still needs external conventions [9; 10]. Of course, the healing of the bone itself is also influenced by many other factors related to the patients, such as their age, gender, and other illnesses [11]. Therefore, bone regeneration requires a material that is biocompatible, osteoconductive, osteoinductive, and tolerated by the patient's immune system. Cells mediating the process of osteogenesis, osteoblasts, are derived from mesenchymal stem cells (MSCs) that differentiate through pre-osteoblasts and finally mature into secretive osteoblasts producing bone matrix. The bone matrix becomes hard due to mineralization by insoluble inorganic compounds, mostly calcium phosphate hydroxide (hydroxyapatite) [12]. Nanotechnologies have received a large amount of attention in bone tissue engineering and are connected to many disciplines such as physics, chemistry, engineering, life sciences, and medicine, representing a real combination of different fields. Nanomaterials are currently defined as materials with basic structural units smaller than 100 nm in at least one dimension and exhibit unique properties different from their bulkier counterparts that have made them highly effective in numerous biomedical applications (Figure 1) [13; 14; 15]. There are included not only nanoparticles but also nanoclusters, nanocrystals, nanotubes, nanofibers, nanowires, nanorods, nanofilms, etc. The decrease in material size into the nanoscale radically increases surface area, roughness, and the ratio of surface area to volume leading to superior physico-chemical properties. These nanoparticles affect cell signaling, proliferation, cell viability, and integration [8]. Nanoparticles do not only support the cells; they also regulate the osteoblast function, proliferation, differentiation, and migration [16]. A variety of materials can be used to prepare nanoparticles, such as metals, ceramics, polymers, and organic materials [17; 18; 19], but usually, the nanoparticles are part of a different matrix, so nanocomposite scaffolds are created [20; 21].