## 1 Introduction  
Articular cartilage tissue consists of chondrocytes growing in an extracellular matrix (ECM), which is responsible for the adhesion and differentiation of cells and cell-to-cell communication. The principal components of ECM are certain fibrous proteins; e.g., type II collagen, proteoglycans, hyaluronic acid, and chondro sulfate [1]. Articular cartilage protects the bone ends from friction and compressive loads. The injuries and articular diseases are frequent and may cause pain, arthrodesis, and joint arthroplasty. Due to the limited regenerative capacity of cartilage, tissue repair methods have been sought for. Traditionally, autografts and allografts are used to treat these lesions. However, the donor cartilage tissue is not only available in insufficient amounts but also may induce an immune response, leading to the implant's failure. Tissue engineering (TE) is the promising alternative of cartilage defects treatment. However, its success depends on four key factors such as cells, mechanical environment, growth factors, and scaffolds. An ideal scaffold must fulfill a wide range of criteria, such as biocompatibility, porosity and suitable 3D structure, high water absorption capacity, and high mechanical strength. Furthermore, it has to promote the proliferation of chondrocytes and ensure the diffusion of nutrients, active substances and growth factors. Over the last years, various materials have been studied as potential matrices for TE, being able to mimic the ECM. They are classified as either synthetic (polylactic acid, polyethylene glycol) or natural that are based on proteins (collagen, fibrin, silk) and polysaccharides (hyaluronic acid, chitosan, alginate, cellulose) [2; 3]. Like plant cellulose, bacterial bionanocellulose (BNC) is a linear polymer composed of \(\beta\)-1,4-linked d-glucopyranoside residues. It is produced as an exopolysaccharide having a form of the three-dimensional network of nano-sized fibers by various species of bacteria, such as those of the genera _Komagataebacter_, _Agrobacteriumium_, _Rhizobium_, and _Sarcina_[4]. BNC exhibits many desirable and unique properties such as biocompatibility [5], high mechanical strength, and water holding capacity up to 100 times its dry mass [6] as well as high crystallinity and porosity. Biocompatibility of BNC-based products makes them suitable for numerous biomedical applications, such as wound dressings [7; 8], artificial vessels [9; 10], cornea replacements [11], drug delivery systems [12; 13] and scaffolds for tissue engineering [14]. The interactions of the biomaterial with surrounding cells depend on the proper distribution of functional groups in three-dimensional space [15]. The native BNC-based scaffolds promote neither the excellent cell adhesion nor growth because cellulose is biologically inactive, mainly due to its neutral charge [16; 17]. Therefore, there is a need to modulate the intrinsic properties that may be particularly beneficial in the biomedical application by introducing for instance a negative charge promoting interactions with proteins in the extracellular matrix [18]. For this purpose, we created a composite of bacterial nanocellulose with sulfate groups-rich \(\kappa\)-carrageenan (\(\kappa\)-Car). \(\kappa\)-Car, widely used in the food industry due to its excellent gelling capacity, belongs to the family of linear polysaccharides extracted from red seaweeds, _Rodophyta_. Carrageenan, built from 3-linked \(\beta\)-d-galactopyranose and 4-linked \(\alpha\)-d-galactopyranose or 4-linked 3,6-anhydro-\(\alpha\)-d-galactopyranose units [19], are divided into the iota (-), kappa (\(\kappa\)-) and lambda (\(\lambda\)-) forms, and their mu (\(\mu\)-) and nu (\(\
u\)-) precursors [20; 21]. The classification is based on a number and position of sulfate groups on the repeating disaccharide units. \(\kappa\)-Car, potentially used as a pharmaceutical material due to its good biocompatibility and low toxicity, contains one sulfate group on the C4 of \(\beta\)-d-galactopyranose residue [22].