# Use of Anionic Polysaccharides in the Development of 3D Bioprinting Technology
## 2 Overview of Natural Anionic Polysaccharides from Plant, Bacteria and Algae
### Polyglucuronic Acid from Bacteria, Fungi and Green Algae  
Polyglucuronic acids are homopolymers of glucuronic acid partially acetylated and initially isolated from microorganisms, such as the bacterial strain _Sinorhizobium meliloti_ M5N1 CS, which produces an extracellular polysaccharide with interesting viscosimetric and gelling properties consisting only of glucuronic acids. Gas chromatography and \({}^{1}\)H nuclear magnetic resonance (NMR) analysis revealed the presence of a single peak that corresponds to \(\beta\)-(1,4)-linked glucuronic acid (Figure 2) [14; 31; 32].  
Gelling properties have also been reported in the presence of monovalent, divalent or trivalent cations as a function of polymer concentration and ionic strength [14; 32]. Glucuronan can form thermo-reversible gels in the presence of monovalent cations in the form of Na\({}^{+}\) high ionic. Thermally stable gels were obtained with divalent cations (Ca\({}^{2+}\), Cu\({}^{2+}\), and Ba\({}^{2+}\)) and the resistance of the gels was modulated by the degree of acetylation of the polymer that disrupts the ionic interaction [14]. The production of glucuronan by the _S. meliloti_ strain M5N1 CS and its use in food products, agriculture, pharmacy, cosmetics or water purification, especially in the form of a gelling agent, thickener, moisturizer, stabilizer, have been claimed in the first patent describing the use of \(\beta\)-(1,4)-p-poly- and oligoglucuronic acids in chelatant or flocculant applications but also in various fields of applications [14; 33; 34].  
Another bacterial strain producing cellulose was isolated from rotten apples. It has been identified as _Gluconacetobacter hansenii_ based on its physiological characteristics and the sequence of the gene coding for 165 rDNA. Specifically named G. _hansenii_ PJK [35; 36], it was used to produce water-soluble polysaccharides composed of glucuronic acid [35; 36; 37; 38]. High-performance liquid chromatography (HPLC) analysis of the polysaccharide hydrolyses produced by this strain revealed the presence of glucuronic acid [39; 40]. Further analysis by NMR determined the structure of this polymer that can be composed of \(\alpha\)-glucuron acid rather than \(\beta\)-glucuron acid [39; 40]. Part of the polysaccharide mixture was composed of unsaturated \(\alpha\)-(1,4)-oligoglucuronic acids [39; 40]. These oligosaccharides have good emulsifying properties and sufficient thermal stability to be commercially developed [39]. Their production from a chemically defined medium was expensive, but the strain produced large amounts (112.65 g/L) of these oligomers on fermentation broth waste from beer [39; 41].  
In fungal species of the genus _Mucor_, the fibrillar structure of the wall is composed of chitin and chitosan [42], which have been incorporated in matrix materials mainly composed of polymers, containing large amounts of glucuronic acid [43]. Two polymers of different acidity were extracted from the mycelial walls of _Mucor rouxii_. The first was an heteropolymer composed of p-glucuron acid, p-mannse, p-galactose and L-fucose. It was designed as mucoran. The other polymer contained large amounts of p-glucuron acid and was designed as mucoric or fungal \(\beta\)-(1,4) glucuronan acid [44].  
Figure 2: Structure of \(\beta\)-(1,4)-p-polylglucuronic acid (PGU) from _Sinorhizobium meliloti_ M5N1 CS. With R=H or COCH\({}_{3}\) (acetyl group).  
Previous studies have also revealed the presence of \(\beta\)-(1,4)-p-polyglucuronic acid in the cell walls of several green algae [45; 46].  
Regarding the very good gelling properties of PGU using divalent ions which gave thermally stable gels [14; 33], it is important to mention that PGU could be efficiently use as the basal hydrogel of bioinks and investigated for new application in 3D bioprinting dedicated to tissue engineering as already done with alginate.