POLYMERIC COMPOUND OF GLUCURONIC ACID WITH PHENOLIC GROUPS, GEL-FORMING COMPOSITION COMPRISING SUCH A COMPOUND AND METHOD FOR PRODUCING THE SAME TECHNICAL FIELD The present invention relates generally to polymeric compounds of glucuronic acid and a method of hydrogelation of such compounds. The present invention also relates to the use of the hydrogel structures (particles, films or 3D-strucutres) obtained by this method, notably as three-dimensional cell culture material to support biological molecules used as active ingredients, cell colonisation or for tissue regeneration. BACKGROUND Polyglucuronic acids (PGU) also called glucuronan is a homopolymer of glucuronic acid composed of [→4)-β-D-GlcpA-(1→] residues partially acetylated at the C-3 and/or the C-2 position produced by the strain Sinorhizobium meliloti M5N1CS
[1] . First described in cell walls of Mucor rouxii
[2] , these polyuronides have since been isolated from other sources such as in the cell walls of green algae
[3] . However the most described polysaccharide was obtained by the Rhizobia strains. However recent progresses in the oxidation of primary hydroxyls groups by 2,2,6,6- tetramethylpiperidine-1- oxyl radical (TEMPO) reagents permits to obtained PGU mimick derivatives from cellulose, xanthan, curdlan, scleroglucan, chitosan, starch, fungal α-(1,3)-glucan, etc. on a large scale-up and concomitantly new polysaccharide lyase family able to degrade these polyglucuronic acids have been identified
[4],[5] . In the field of poly-and oligo-glucuronic acids, different applications of these compounds, and in particular the French patent FR2781673
[6] and the international application WO 1993/18174
[7] teach the biocompatibility of PGU and its use in food products, farming, pharmaceutics, cosmetics or water purification, particularly as a gelling, thickening, hydrating, stabilizing, chelating or flocculating agent. Another application concerned the immunostimulating properties on human blood monocytes, low molecular weight PGU enhanced the production of cytokines IL-1, IL-6 and TNF-α
[8] . Cosmetics application of PGU have been claimed by Lintner (1999)
[9] in association with an algae extracted from Haematococcus pluvialis or in WO2010/067327
[10] for oligo-PGU stimulating of elasticity of the dermis and epidermis. Biological activities of these low molecular weight glucuronans modified by sulphonation were also investigated on a model of injured extensor digitorum longus (EDL) muscles on rats and demonstrated that the regeneration activity is not induced only by the presence of sulfate groups, but also by acetyl groups. The renewal process of cells is regulated by specific signals (or communication peptides such as growth factors) of the extracellular matrix. These signals are stored, protected, and positioned on a family of large polysaccharides called Heparan Sulfates (HS). In cases of injury, specific enzymes destroy HS, that no longer protect the specific signals. Other enzymes called proteases then destroy specific signals along with other structural proteins of the extracellular matrix. Due to their resistance against natural enzymes from the extracellular matrix, the biological effect of these modified bacterial polysaccharides could be explained
[11] . In that context, the Applicant has recently discovered the hydrogelation property of PGU or PGU derivatives having phenolic hydroxyl moieties (herein after designated by PGU-Ph) and their potential as components of bioinks for bioprinting. Thus the Applicant has further developed a method for hydrogelating these PGU-Ph compounds in which the phenolic hydroxyl moieties allow a rapid formation of stable hydrogels through horseradish peroxidase (HRP)-catalyzed crosslinking. HRP-assisted hydrogelation is already known by the man of the art as an effective method for obtaining cell-laden hydrogels from a variety of derivatives of natural and synthetic polymers such as alginate
[12] , hyaluronic acid
[13] , gelatin
[14] , dextran
[15] , and poly(vinyl alcohol)
[16] . Recently, HRP-assisted hydrogelation was applied to 3D bioprinting
[17],[18] , in which rapid curation of inks ejected from needles are required for fabricating 3D constructs with higher fidelity to blueprints. 3D bioprinting is a known technique of fabrication of cell-laden constructs based on digital blueprints. The resultant cell-laden constructs are fabricated for the sake of wound dressing and tissue engineering for drug screening and regenerative medicine
[19],[20] . The applicant has discovered that the addition of Ph clusters to PGU or its derivatives allows the use of a particular (enzymatic) cross-linking pathway which dispenses with using inorganic cross-linkers which are slower and blur the structure formed. The hydrogel structures thus obtained form a transparent network and with such a good retention of the printed forms. SUMMARY OF THE INVENTION Consequently, the Applicant has developed a method for producing a polymeric compound of glucuronic acid PGU-Ph (hereinafter designated by the acronym PGU-Ph) which comprises the following steps: - providing a polymeric compound of glucuronic acid (PGU); - reacting said polymeric compound of glucuronic acid (PGU) with a phenolic compound, so as to form a polymeric compound of glucuronic acid comprising at least one phenolic group (PGU-Ph) either by radical polymerisation or by chemical region-selective grafting of phenol group onto carboxylic group of glucuronic acid. According to a first embodiment of the invention (hereinafter designated by first method for producing a PGU-Ph, by chemical region-selective grafting of phenol group onto carboxylic group of glucuronic acid), the method comprises the following steps: - providing a polymeric compound of glucuronic acid (herein after designated by the acronym PGU) of formula (A) or (B) as defined above; - dissolving the PGU in an acid buffered solution for forming a solution of PGU, preferably in a 2-(N- morpholino)ethanesulfonic acid (MES) buffered solution (pH6.0, 100 mM) at 1 w/v%; - sequentially adding, in said solution of PGU, a X-(Y- amino-Z-hydroxyalkyl)-n-hydroxyl-phenol or a salt derivative thereof (such as hydrochlorides), N- Hydroxysuccinimide (NHS) and ethyl-3-(3- dimethylaminopropyl)carbodiimide (WSCD), for forming the final composition of the reaction medium; - stirring said final composition (preferably for at least 4 hours (and preferably 20 hours) at room temperature (i.e. between 20°C and 25°C) until obtaining a resultant polymer consisting in a polymeric compound of glucuronic acid comprising at least one phenolic hydroxyl moieties. The last step of stirring may be followed by a step of precipitating said resultant polymer in acetone and then washing it with an 90% ethanol + 10% water until the absorbance at 275 nm attributed to the existence of X-(Y-amino-Z-hydroxyalkyl)-n- hydroxyl-phenol became undetectable in the washing solution, as shown in example 1. Preferably, in the method for producing a polymeric compound of glucuronic acid PGU-Ph according the first embodiment of the invention, the X-(Y-amino-Z-hydroxyalkyl)-n-hydroxyl-phenol may be selected from the group constituted by tyramine, dopamine and octopamine, and even more preferably tyramine. The Applicant has also developed another embodiment of the method for producing a polymeric compound of glucuronic acid PGU-Ph according to the invention (hereinafter designated by second method for producing a PGU-Ph, by radical polymerisation), which comprises the following steps: - providing a radical of a polymeric compound of glucuronic acid of formula (C) - reaction of said radical of formula (C) with a phenolic compound of formula (I) (see above) or(D) by radical coupling so as to obtain a polymeric compound of glucuronic acid of formula (E)or (F) respectively Preferably, in the method for producing a polymeric compound of glucuronic acid PGU-Ph according to the second embodiment of the invention, the phenolic compound of formula (D) may be selected from the group constituted by tyramine, dopamine and octopamine, and preferably tyramine. But it is possible to use any phenolic compound. Another object of the invention is an aqueous solution comprising a polymeric compound of glucuronic acid of the invention (atypically in an amount of 0.5 to 8 w/v%) obtainable by any of the methods for producing such a polymeric compound according the first and second embodiments. The drying of such a solution once coated on a flat surface leads to a water soluble ungelled films of PGU-Ph (which are just dried, but not cross- linked). A cross-link process could be produce on dried film using classical cross-linker solution composed of divalent ion (M
2+ such as preferentially Ca
2+ or Mg
2+ ). The reticulation of PGU-Ph solutions could be performed using:
(1) a cross-link catalyst consisting in divalent cation (M
2+ ) such as Ca2+, Mg2+ etc., or (2) a photo-gelification process under visible and/or UV light using photoinitiator such as riboflavin, ruthenium II trisbipyridyl chloride ([Ru(bpy)
3 ]
2+ ), or their derivatives. Still another object of the invention is a gel-forming composition comprising: - a polymeric compound of glucuronic acid according to the invention or obtainable by the methods according to the first and second embodiments of the invention, and - a cross-linking catalyst consisting in an oxidase or a compound (in liquid or gaseous form) containing H
2 O
2 or capable of generating H
2 O
2 in situ (for instance reducing molecules such as glucose, fructose or galactose (monosaccharide), or lactose and maltose (disaccharides)). In this gel-forming composition, if the compound containing H
2 O
2 or capable of generating H
2 O
2 is glucose (or fructose, or galactose) the cross-linking catalyst may further contain glucose oxidase (or fructose oxidase, or galactose oxidase respectfully), H
2 O
2 is generated more rapidly. According to a first embodiment of the gel-forming composition of the invention, the polymeric compound of glucuronic acid PGU-Ph may be associated to an oxidase as cross- linking catalyst, said oxidase being chosen from the group consisting of oxido-reductase, peroxidase, catalase, laccase, tyrosinase and/or monosaccharide oxidase, and the mixtures thereof. Preferably, the oxidase may be a horseradish peroxidase (hereinafter designated by the acronym HRP) containing enzyme in an amount of at least 0.1 U/mL, preferably comprised between 0.1 U/mL and 20 U/mL in the composition for an efficient and fast cross-linking, and even better in the order of 5 U/mL. Below 0.1 U/mL, the gelling is very slow and above 200 U/mL, the gelling is too fast and difficult to be controlled. The enzyme unit (U) is defined as follows: one pyrogallol unit will form 1.0 mg purpurogallin from pyrogallol in 20 sec at pH 6.0 at 20 C. According to a second embodiment of the gel-forming composition of the invention, the polymeric compound of glucuronic acid PGU-Ph may be associated to a compound containing H
2 O
2 in an amount comprised between 0.05 mmol/L and 1 mmol/L, which leads, in presence of HRP to a well-controlled gelling for a gelling time of 2s to 5 s. Outside this range, notably from 0.01-0.05 mmol/L and higher than 1 mmol/L, the gelling is much more slower(gelling time 15 s order of magnitude); but such an amount of a compound containing H
2 O
2 could be interesting for applications requiring handling time e.g. hydrogel film making and/or injectable hydrogel). For either the first or the second embodiment of the gel- forming composition of the invention, the amount of polymeric compound of glucuronic acid PGU-Ph may be comprised between 0.01 w/v % to 8 w/v %, and preferably 0.1 w/v % to 2 w/v %. If the amount of polymeric compound of glucuronic acid PGU-Ph is 0.01 w/v %, the gelling is feasible but very slow. Above 8 w/v % of polymeric compound of glucuronic acid, the gel-forming composition of the invention is very viscous and difficult to use. Furthermore, it is difficult for a crosslinker to penetrate the composition. According to a first variant of the gel-formingcomposition of the invention which is applicable to both embodiments, the gel-forming composition of the invention, may further comprise another active biodegradable polymers. Advantageously, the biodegradable polymers may be a polysaccharide such as glycosaminoglycan, or a protein selected from the group consisting of collagen, adhesin, gelatin, and the mixtures thereof. Preferably, the biodegradable polymer may be a gelatin derivative comprising phenolic hydroxyl moieties (Gelatin-Ph). According to a second variant of the gel-formingcomposition of the invention which is also applicable to both embodiments, the gel-forming composition of the invention, may further comprise suspended cells of animal, bacterial or plant origin. These may be 10T1/2 cells or HepG2 cells, present in said composition at a concentration in the range of 3.10
5 cells/mL. Another object of the invention is a method for manufacturing a hydrogel structure (hereinafter designated by first method for manufacturing a hydrogel structure), comprising the following steps: - providing a gel-formingcomposition according to the first variant, - hydrogelating said gel-formingcomposition: o either by contact with a fluid or gaseous medium containing H
2 O
2 if the gel-forming composition comprises an oxidase; o or by contact with an oxidase if the gel-forming composition comprises a compound containing H
2 O
2 or capable of generating H
2 O
2 in situ. Still another object of the invention is another method for manufacturing a hydrogel structure (hereinafter designated by second method for manufacturing a hydrogel structure), comprising the following steps: - providing a gel-formingcomposition according to the second variant, - hydrogelating said gel-forming composition: o either by contact with a fluid or gaseous medium containing H
2 O
2 if the gel-forming composition comprises an oxidase; o or by contact with an oxidase if the gel-forming composition comprises a compound containing H
2 O
2 or capable of generating H
2 O
2 in situ. Both methods may consist in a bioprinting process, for manufacturing: - one dimensional hydrogel structures such as particles (micro- and nanoparticles): for instance, by using various processes (micellar solutions, drop by drop in a cross-linking solution; or - two-dimensional hydrogel structures such as lattices/films: for instance, by using classic shaping processes (spin coating, tape casting, etc.; or - three-dimensional hydrogel structures for instance by using three-dimensional bioprinting, preferably a process selected from the group consisting of inkjet bioprinting, extrusion bioprinting, stereolithography bioprinting, and laser-assisted bioprinting. These examples are by no means exhaustive and are given by way of example only. Another object of the invention is a hydrogel obtainable by the first method for manufacturing a hydrogel structure, regardless of the bioprinting process used. The t obtained hydrogel structure may be used as cell culture material to support cell colonisation (3D application) or as a patch or a plaster (2D application). Still another object of the invention is a hydrogel structure obtainable by the second method for manufacturing a hydrogel structure, regardless of the bioprinting process used. The thus obtained hydrogel structure may be used for tissue regeneration. BRIEF DESCRIPTION OF THE FIGURES Other innovative features and advantages of the invention will emerge from a reading of the following description followed by way of indication and in no way imitatively, with reference to the examples and corresponding figures. The figures are presented below: - FIG.1 shows the UV-Vis absorbance spectra of PGU and PGU-Ph at 0.1 w/w% (see example 1); - FIG.2 shows the shear rate-viscose profiles of PGU and PGU- HPh at 1 and 2 w/v% (see example 1); - FIG.3 shows the effect on gelation of a) PGU-Ph at 5 U/mL HRP and 0.1 mM H
2 O
2 , and the effect of b) HRP at 1 w/v% PGU- HPh and 0.1 mM H
2 O
2 , as well as the effect of c) H
2 O
2 at 1 w/v% PGU-HPh and 5 U/mL HRP (see example 2); - FIG. 4 shows the morphologies of cells and mitochondrial activity of 10T1/2 cells at 20 hours of culture in the mixture solutions of medium (50 vol%) and PBS (50 vol%) containing PGU or PGU-Ph after 0.5 w/v% (see example 3); - FIG.5 are microphotographies of a) 10T1/2 cells at 1, and 4 days, and b) HepG2 cells at 1, and 3 days of seeding on cell culture dish (Dish), PGU-Ph hydrogel, and PGU-Ph + Gelatin- Ph hydrogel (Bars: 100 µm) (see example 4); - FIG.6 are merged microphotographies of a) 10T1/2 cells and b) HepG2 cells enclosed in PGU-Ph hydrogels through bioprinting at 1, 4 and 8 days of printing. The cells were stained using calcein-AM (Green) and PI (red) (Bars: 200 µm) (see example 5); - FIG.7 are examples of 3D-constructions of PGU-Ph hydrogel using blue print 3D CAD-Model (see example 6); - FIG.8 are PGU-Ph beads obtained by dropping PGU-Ph+HRP solution in H
2 0
2 (0.5 M)(see example 7); - FIG. 9. is antioxidant PGU-Ph film obtained by drying a solution of PGU-Ph (1 w/v %)(see example 8). Figures 1 to 9 are described in more detail in the examples which follows, given by way of indication, and which illustrates the invention, but without limiting the scope thereof. EXAMPLES MATERIALS AND TESTS Materials - Tyramine hydrochloride purchased from Combi-Blocks (San Diego, CA); - water-soluble carbodiimide (WSCD) purchased from Peptide Institute (Osaka, Japan); - N-Hydroxysuccinimide (NHS), HRP (210 units/mg), and H
2 O
2 aqueous solution (31 w/w%) purchased from Fujifilm Wako Pure Chemical Industries (Osaka, Japan); - Mouse fibroblast 10T1/2 cells and human hepatoma HepG2 cells obtained from the Riken Cell Bank (Ibaraki Japan), which were grown in Dulbecco’s modified Eagle’s medium (DMEM, Nissui, Tokyo, Japan) supplemented with 10 v/v% feal bovine serum in a 5 % CO
2 incubator: - Sinorhizobium meliloti M5N1CS (or S. meliloti M5N1CS strain). PGU production and extraction - The S. meliloti M5N1CS strain was grown at 30°C in a 20 L bioreactor (SGI) with 15 L of Rhizobium complete medium, supplemented with sucrose 1% (w/v) (RCS medium); - The inoculum was a 1.5 L of RCS medium inoculated with S. meliloti M5N1CS, and was incubated during 20 hours at 30°C on a rotary shaker (120 rpm); - After 72 h of incubation, the obtained broth was centrifuged at 33,900 x g during 40 min at 20°C. The supernatant was purified by tangential ultrafiltration on a 100,000 normal-molecular-weight cutoff (NMWCO) polyethersulfone membrane from Sartorius (Goettingen, Germany) against distilled water; - Finally, the retentate solution was freeze-dried to obtain a PGU of formula (B) having a β-(1,4)-D- polyglucuronic acid chainwith O-acetylation at carbon 2 and carbon 3 of β-D-glucuronic acid units. TESTS Shear rate-viscosity profile Shear rate-viscosity profiles of solutions were measured using a rheometer (HAAKE MARS III, Thermo Fisher Scientific, Waltham, MA) equipped with a parallel plate of a 25-mm radius with a 0.5- mm gap at 20˚C. Hydrogelation time - The gelation time was measured for a phosphate-buffered saline solution (50-100 mM, pH 7.4) containing PGU-Ph at room temperature; - This PGU-Ph solution was poured into a 24-well plate at 0.2 mL/well; - Then, 0.1 mL of HRP and 0.1 mL H
2 O
2 solutions were sequentially added into well and stirred using a magnetic stirrer bar (10 mm long); - The gelation was confirmed when magnetic stirring was hindered and the surface of the solution swelled. Cytocompatibility - 10T1/2 cells were seeded in the wells of 96-well cell culture plate at 4 × 10
3 cells/well and incubated in medium for 20 hours in a humidified 5% CO
2 incubator at 37°C; - Subsequently, the medium was changed to the medium (0.2 mL) containing PGU or PGU-Ph at 0.5 w/v % and incubated for an additional 24 hours; - Then, the medium containing the polymers were changed to the medium (0.2 mL) containing 1/20 vol of the reagent from a colorimetric mitochondrial activity assay kit (purchased under the commercial name Cell Counting Kit- 8, by the Japanese firm Dojindo, Kumamoto, Japan); - After 2 hours of incubation, the absorbance at 450 nm was measured using a spectrophotometer; - Sodium alginate (Alg) and alginate possessing Ph moieties (Alg-Ph) were used as controls. Cell behavior on hydrogels - A solution containing 1 w/v% PGU-Ph, or 1 w/v% PGU-Ph + 1 w/v% Gelatin-Ph, and 5 U/mL HRP was poured into the wells of 12-well cell culture dish at 0.5 mL/well; - Subsequently, the dish was put in a plastic container; - Air containing 8 ppm H
2 O
2 , obtained by bubbling air in a 0.5 M aqueous H
2 O
2 solution, flows into a plastic container at 10 L/min ; - After 15 min of the exposure to the air containing H2O2, the wells coated with hydrogels were rinsed sequentially with PBS and medium ; - 10T1/2 cells and HepG2 cells were suspended in medium containing 0.3 mg/mL of catalase, and poured into each well at 6 × 10
4 cells/well. PRINTING PROCESS AND SYSTEM - An extrusion 3D printing system that has been developed by modifying a commercial 3D printing system (purchased under the commercial name Anycubic i3 Mega by the firm Anycubic, Guangdong, China) is used for 3D bioprinting. - This extrusion 3D printing system consisted of a syringe pump for flowing ink, a 27-gauge stainless steel needle for extruding the ink, a bubbling system for supplying air containing 8 ppm H
2 O
2 , and a stage for layering the extruded ink. The flow rates of inks in the needle and the moving speed of the stage were fixed at 22 mm/s. - The printing of cell-laden 3D hydrogel constructs was performed in a biological safety cabinet. - Inks containing 1 w/v% PGU-Ph, or 1 w/v% PGU-Ph and 1 w/v% Gelatin-Ph, and 5 U/mL HRP were used. - The effect of the extrusion with the inks on cells was determined by measuring the viabilities of 10T1/2 cells and HepG2 cells suspended in the inks at 3× 105 cells/mL. - The inks containing cells were collected at the tip of the needle and the cells were stained with trypan blue dye for the measurement using a hemocytometer. - The viabilities of the cells enclosed in the hydrogels obtained through the printing process were determined by staining the cells with fluorescence dyes, calcein-AM and propidium iodide (PI). EXAMPLE 1: PGU-PH SYNTHESIS REALIZED ACCORDING TO THE FIRST METHOD FOR PRODUCING A PGU-PH This synthesis is realized according to the first method for producing a PGU-Ph, as follows: - This resulting was dissolved in 2-(N- morpholino)ethanesulfonic acid (MES) buffered solution (pH6.0) at 1 w/v%. - Tyramine hydrochloride, NHS, and WSCD were sequentially added at 45 mM, 10 mM, and 20 mM, respectively, and stirred for 20 h at room temperature; - The resultant polymer was precipitated in acetone and then washed with 90% ethanol and 10% water until the absorbance at 275 nm attributed to the existence of tyramine became undetectable in washing solution; - The resulting phenolized PGU is a PGU-Ph according the first embodiment of the invention. Fig. 1 shows the UV-vis spectrum of a 0.1 w/w% PGU-Ph solution as described above (PGU-Ph solution), which is compared to that of 0.1 w/w% PGU (PGU solution). FIG.2 notably shows that the PGU solution does not have a peak around 275 nm, whereas PGU-Ph solution had a peak around 275 nm attributed to Ph moieties. The contents of Ph moieties calculated on the basis of a calibration curve obtained from a known percentage of tyramine solution is 3.7× 10
-4 mol- Ph/g. Fig. 2 shows the shear rate-viscosity profiles of 1 and 2 w/v% PGU-Ph solutions, which are compared to the shear rate-viscosity profiles of 1 and 2 w/v% PGU solutions. Fig.2 notably shows that the viscosities of the PGU-Ph solutions are higher than those of the PGU solutions, and the viscosity of the 2 w/v% PGU-Ph solution is higher than that of the 1 w/v% PGU-Ph solution. EXAMPLE 2: Hydrogelation of PGU-Ph solutions Hydrogels were produced in accordance with the first method for manufacturing a hydrogel structure according to the invention, using the PGU-Ph solutions obtained in Example 1 by HRP-catalysed reaction in the presence of H
2 O
2 . Fig. 3a shows the effect of PGU-Ph concentration on hydrogelation time at 5 U/mL HRP and 0.1 mM H
2 O
2 . The gelation time of a PGU-Ph solution at 0.5 w/v% is 6.0 s. Fig. 3b and 3c show the effects of HRP and H
2 O
2 concentrations on gelation time measured for 1.0 w/v% PGU-Ph solutions. Gelation time decreases as HRP concentration increases from 71 s at 0.1 U/mL to 2 s at 20 U/mL (Fig. 3b). Gelation time decreases as H
2 O
2 concentration increases from 0.05 mM to 1 mM, but increases as H
2 O
2 concentration further increases. Note to mention that higher concentration of H
2 O
2 will lead to depolymerisation of PGU-Ph and then reduce considerably the gelification. EXAMPLE 3: Cytocompatibility of PGU-Ph For evaluating the cytocompatibility of the PGU-Ph obtained at example 1, 10T1/2 cells were incubated in a solution containing PGU-Ph. Solutions containing PGU, Alg, or Alg-Ph were used as controls. Fig. 4a,b,c show the morphologies and mitochondrial activities of cells at 20 h of culture in the mixture solutions of medium (50 vol%) and PBS (50 vol%) containing either the PGU-Ph of example at 0.5 w/v% or tis corresponding PGU. There are no remarkable differences in cell morphology specific to the exposure to PGU-Ph. In addition, there was no significant decrease in the mitochondrial activity of cells incubated in the mixture solutions caused by Ph moieties introduced in PGU (p = 0.45), as the same with the cells incubated in the mixture solutions containing 0.5 w/v% Alg and Alg- Ph (p = 0.28, Fig. 4c). The mitochondrial activities of the cells incubated in the solutions containing PGU and PGU-Ph were about 20% higher than those incubated in the solutions containing Alg and Alg- Ph (p < 0.03). EXAMPLE 4: Cell behavior on PGU-Ph hydrogels Hydrogels containing PGU-Ph alone (as obtained in example 1) and Hydrogels containing both PGU-Ph (as obtained in example 1) and Gelatin-Ph were used for evaluating cytocompatibility and cell adhesiveness of hydrogels containing PGU-Ph. The day after seeding, majority of 10T1/2 cells and HepG2 cells were floating on PGU-Ph hydrogels, and HepG2 cells formed aggregates (Fig. 5a, 5b). During the subsequent incubation period, the cells continued to float on PGU-Ph hydrogels. A small number of cells adhered to the hydrogels, but did not elongated. In contrast, the 10T1/2 cells seeded on PUG-Ph + Gelatin-Ph hydrogels adhered, elongated and proliferated as the same with those on cell culture dish (Fig. 5a). No remarkable morphological difference was found between the 10T1/2 cells on the PUG-Ph + Gelatin-Ph hydrogels and cell culture dish. The HepG2 cells seeded on PGU-Ph + Gelatin-Ph hydrogels also adhered, elongated and proliferated (Fig. 5b). However, their morphologies were obviously different from those on cell culture dish. The HepG2 cells on cell culture dish formed small aggregates with adhering to the substrate the day after seeding. Then, the cells grew as monolayers with increasing the size of aggregates. The HepG2 cells on PGU-Ph + Gelatin-Ph hydrogels did not form obvious aggregates the day after seeding. In the subsequent incubation period, the HepG2 cells grew on the PGU-Ph + Gelatin-Ph hydrogels without forming obvious aggregates, far from the aggregates formed on cell culture dish. EXAMPLE 5: 3D printing of hydrogel as cell culture materials to support cell colonisation. The effects of the 3D printing process and the PGU-Ph hydrogels on cells were evaluated by printing hydrogel constructs enclosing 10T1/2 cells and HepG2 cells. The viabilities of 10T1/2 cells and HepG2 cells the day after bioprinting determined through the staining with calcein-AM and PI were 92.3% and 91.6%, respectively. This result demonstrates the printing process using PGU-Ph solution as ink was not harmful for these cells. Regarding the morphologies of the enclosed cells, 10T1/2 cells kept round shape during 8 days of study without a formation of cell aggregates (Fig. 6a). In contrast, HepG2 cells formed aggregates in the hydrogel constructs and the size of the aggregates increased with increasing culture period (Fig. 6b). There was no obvious increase in dead cells in both the cells. EXAMPLE 6: 3D printing of hydrogel. For evaluating the feasibility of PGU-Ph solution as inks of bioprinting, 1 w/v% PGU-Ph solution containing 5 U/mL HRP was extruded on substrates. As shown in Fig. 7, transparent 3D- hydrogel constructs with good fidelity to blue prints (3D CAD models) were obtained when the solution was extruded in air containing 8 ppm H
2 O
2 . These results demonstrate the feasibility of PGU-Ph solution gellable through HRP-mediated hydrogelation as inks of 3D printing. EXAMPLE 7: Synthesis of particles. For evaluating the feasibility of PGU-Ph particles synthesis such as microbeads, 1 w/v% PGU-Ph solution containing 5 U/mL HRP was dropped in H
2 0
2 (0.5 M) and stirred for 1 minute. Then, the microbeads were washing with water, and was preserved in ethanol/water solution (70/30). As shown in Fig. 8, transparent PGU-Ph beads with good spheric shapes were obtained. EXAMPLE 8: Synthesis of antioxidant PGU-Ph dried films. For evaluating the feasibility of antioxidant PGU-Ph film synthesis, 1 w/v% PGU-Ph solution was pouring in plastic Petri dishes and dried at 50 °C during 24 h. PGU film without Ph moieties was used as control. To estimate the antioxidant effect of PGU-Ph film, the free radical scavenging activity was measured using 1,1-diphenyl-2-picrylhydrazyl (DPPH). Briefly, PGU-Ph film (100 mg) were added into 5.0 ml of DPPH solution (0.1 mM DPPH in ethanol 96°). The solution was left for 24 h under stirring at room temperature in the dark. Then, the absorbance was measured at 517 nm using the Shimadzu UV-1700 spectrophotometer. DPPH radical scavenging activity was calculated as an inhibition percentage based on the following equation (1): DPPH radical inhibition (%)= ((Acontrol- Asample)/Acontrol) ×100 (1) Where A
sample and A
control are the absorbance at 517 nm for the PGU- Ph film and for the control without PGU-Ph film (i.e., the DPPH solution) respectively. As shown in Fig. 9a, transparent PGU-Ph film was obtained. As observed in Fig. 9b, these results demonstrate the feasibility of PGU-Ph solution as antioxidant film for food packaging and biomaterial.
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