## 1 Introduction  
In the last few decades, several innovative therapeutic modalities have been designed to enhance bone regeneration [1]. Alloplastic bone substitutes, including beta-tricalcium phosphate (\(\beta\)-TCP), have osteeconductive features, meaning that they act as a scaffold for the influx of osteogenic cells, although they themselves do not have any osteoinductive or osteogenic ability [2]. Emerging bone tissue engineering strategies are based on seeding osteogenic cells onto calcium phosphate ceramic scaffolds in order to foster new bone tissue formation [3].  
Among the various types of progenitor cells, mesenchymal stem cells (MSCs) represent the most frequently used population in different fields of regenerative medicine,including bone regeneration [4; 5]. Dental-derived MSCs bear similar multipotent potential and immunoregulatory capacities in comparison to bone marrow-derived MSCs, which have been traditionally applied to seed biomaterials in numerous pre-clinical and clinical studies [6; 7; 8]. The advantages of periodontal ligament stem cells (PDLSCs) over other MSCs obtained from dental tissue include easier availability, lower donor site morbidity, and lower requirements for invasive surgery [9]. It has been reported that PDLSCs have osteogenic potential and show better regeneration capability and multipotency in comparison to other subpopulations of stem cells of dental origin, which makes them a suitable alternative to BM-MSCs in terms of using cells in bone repair [9; 10].  
Recent research has shown that biomaterials, depending on their physicochemical properties, can modify the microenvironment of cells and trigger regulatory signals that induce the differentiation of these cells into specific cell lines. The surface characteristics of materials, such as the chemical composition, hydrophilicity, surface energy, and topography, are key factors that control the behavior of cells on the surface of biomaterials [11; 12]. Various studies in recent years have shown that all these properties can be very easily modified via treatment with cold atmospheric plasma (CAP) [13; 14; 15; 16].  
Along with well-established processing using cold plasmas established at low pressures [17], cold plasmas at atmospheric pressure have paved the way for the efficient processing of different materials. CAP provides an active chemical environment close to room temperature, enabling physical and chemical interaction with the surface of materials--such as etching, deposition and activation [18]. Due to generating a diversity of reactive oxygen and nitrogen species (RONS), such as radicals, ions, excited atoms and molecules (O, OH, H\({}_{2}\)O\({}_{2}\), O\({}_{3}\), N\({}_{2}\)*, O\({}_{2}^{-}\), etc.), CAP is used as a prosperous technology in the medical field, with applications ranging from sterilization of medical equipment, surface modification of biomaterials, cancer treatment, to treatment of wounds and tissues, etc. [19; 20; 21; 22]. Most of these plasma sources belong to the dielectric barrier discharge (DBD) type [23], whose main asset is a possibility to provide a large effective surface for treatment in comparison to the other plasma sources operating at atmospheric pressure. However, applications related to plasma treatments of (bio)material surfaces, i.e., bone grafting material, are infrequent and in most cases performed using low-pressure plasma systems [13; 24]. Bearing in mind the potential of CAP to induce changes in the treated biomaterials [25; 26], it is reasonable to believe that it can be used to enhance the regenerative potential of grafting materials, in particular when combined with MSCs.  
Therefore, the aims of this in vivo study were to assess the regenerative potential of human PDLSCs seeded onto a \(\beta\)-TCP scaffold and to investigate if bone regeneration can be further improved via CAP pretreatment of the \(\beta\)-TCP.