## 1 Introduction  
Natural polymers possess biocompatibility and biodegradability. Therefore, they can be used as feedstock for biomaterials [1, 2]. In particular, polysaccharides are abundant in nature and have been used as materials for paper and cloth since ancient times. In recent years, polysaccharides have been applied to biomaterials such as drug delivery systems (DDS) [3, 4, 5, 6], tissue engineering [7, 8], and wound dressings [9, 10]. These biomaterials are used in various structural forms such as bulks [11], nanocomposites [12], gels [13], films [14], porous materials [15], and fibers [16]. Thus, the moldability of raw materials is essential. Cellulose and chitin, which are naturally abundant polysaccharides, are insoluble in water and difficult to mold. On the other hand, polysaccharides that have ionic functional groups in their molecules, such as chitosan (CHI), sodium alginate (ALG), and chondroitin sulfate C sodium salts (CS) are soluble in water and easy to mold. These polysaccharides can interact with inorganic ions, proteins, and oppositely charged polysaccharides, through electrostatic interactions [17, 18, 19]. This property enables the fabrication of gel-like, film-like, and fiber-like structural materials. For example, ALG, which is a block copolymer composed of two uronic acids, \(\beta\)-D-mannuronic acid and \(\alpha\)-L-galuronic acid (Figure 1) [20], forms a cross-linked water-insoluble gel with Ca\({}^{2+}\), i.e., calcium alginate (ALG-Ca) [20, 21]. CHI is a polysaccharide consisting of glucosamines, which has an amino group. Therefore, CHI is one of the basic polymers among bio-derived polysaccharides which has positive charges in an acidic solution [22; 23] and also has potential for fast gelation in acidic solutions [24]. CHI can form polyion complexes (PICs), a water-insoluble gel, by electrostatic interactions with negatively charged acidic polymers such as hyaluronic acid (HYA), CS, and ALG in aqueous solutions [25; 26]. We have reported on the fabrication of polysaccharide composite materials made from polysaccharide PICs [27; 28; 29; 30; 31; 32; 33]. For example, polysaccharide composite films consisting of CS or HYA were successfully prepared using hot-press techniques. Composite films with charged polysaccharides have also been fabricated by layer-by-layer (LbL) assemblies [34; 35; 36]. However, the process generally provides thinner films than those obtained by hot-press techniques. Further, hot-press techniques have the advantage that these processes are simple and can provide free-standing films. The obtained films showed swelling properties depending on the solution condition [27; 28; 32], molecular permeability [29; 31], and drug-loading and sustained-release abilities [30]. We also reported the fabrication of polysaccharide composite films with mechanical anisotropy using roll-press (heat-stretching) techniques [33]. These polysaccharide composite films are expected to be applied in biomaterials such as drug carriers and wound dressings. Fibers are one candidate of the many forms of polysaccharide composite materials. Biocompatible fibers are expected to be used as absorbable surgical sutures and sustained drug release carriers. Fibers derived from polysaccharides can also be used as raw materials in more complex structures such as woven fabrics, non-woven fabrics, and hierarchical three-dimensional structures. The fabrication of polysaccharide microfibers such as those derived from CHI [37; 38] and ALG-Ca [39; 40] by solution spinning and polysaccharide nanofibers made of CHI [41; 42] by electrospinning have been reported. In addition, fabrication of composite fibers, which are composed of two or more raw materials, have also attracted attention [43; 44; 45; 46]. In particular, [PERSON] and coworkers reported on the fabrication of PIC fibers composed of polysaccharides [43; 44].