EP1611160A2

EP1611160A2 - Cross-linking of low and high molecular weight polysaccharides preparation of injectable monophase hydrogels and polysaccharides and hydrogels thus obtained

Info

Publication number: EP1611160A2
Application number: EP04742458A
Authority: EP
Inventors: Pierre Lebreton
Original assignee: Corneal Industrie SA
Current assignee: Allergan Industrie SAS
Priority date: 2003-04-10
Filing date: 2004-04-08
Publication date: 2006-01-04
Also published as: US8338388B2; US20060194758A1; US11045490B2; FR2861734B1; US20120295870A1; EP2289945A1; US7741476B2; US10653716B2; US20210322460A1; JP4827727B2; CA2521961C; US20190022130A1; US9062130B2; US20150065701A1; FR2861734A1; US20100226988A1; WO2004092222A3; EP2289945B1; US10080767B2; JP2006522851A

Abstract

The invention relates to a method for cross-linking at least one polymer, selected from the polysaccharides and the derivatives thereof, application thereof in an aqueous solvent, by the action of an effective and non-excessive quantity of at least one cross-linking agent, characterised in being applied to a mixture including at least one polymer of low molecular weight and at least one polymer of high molecular weight, a method for preparation of an injectable monophase hydrogel with at least one cross-linked polymer selected from polysaccharides and the derivatives thereof and the cross-linked polymers and injectable monophase hydrogels which may be obtained respectively by the methods above.

Description

Crosslinking of low and high molecular weight polysaccharides: preparation of injectable monophasic hydroqels; polysaccharides and hydroαels obtained.

The subject of the present invention is:

- an original crosslinking process of at least one polymer chosen from polysaccharides and their derivatives,

- A process for preparing an injectable monophasic hydrogel of at least one such polymer, - crosslinked polymers and injectable monophasic hydrogels, capable of being obtained respectively by each of said processes.

The hydrogels in question, based on said crosslinked polymers, have numerous outlets, in particular as filling material in restorative, aesthetic, dental surgeries, in ophthalmology, in orthopedics ... as a product preventing tissue adhesions, in general surgery, urology ... Said hydrogels are particularly suitable for repairing the vocal cords. The outlets indicated above, in a nonlimiting manner, for this type of product, are familiar to those skilled in the art.

The invention results from a real effort to optimize the implementation of the crosslinking of the polymers in question, with a view to obtaining injectable monophasic hydrogels, which are particularly advantageous with reference to the compromise below: mechanical properties and persistence, on the one hand, injectability (with injection forces and acceptable injection needle diameters), on the other hand.

It is specified here that the injectable qualifier used in the present text, both with reference to the hydrogels of the prior art and to the hydrogels of the invention, means manually injectable by means of syringes fitted with conventional needles (of a diameter between 0.1 and 0.5 mm). In the context of the present invention, it is in particular possible to formulate hydrogels injectable through hypodermic needles of 30 G ^1. , 27 G ¹ /., 26 G ¹ /., 25 G.

According to the prior art, preparations have already been made from polysaccharides and their derivatives - in particular from acid salts. hyaluronic - no, weakly or strongly crosslinked, hydrogels, especially injectable hydrogels.

With reference to the specific problem of injectability, two-phase compositions have been proposed, the continuous phase of which in particular is based on such hydrogels. Said continuous phase serves as a plasticizer, as an injection vehicle for a dispersed phase. This dispersed phase is more or less solid, more or less differentiated from the continuous phase. So :

- In patent application EP-A-0 466 300, the biphasic compositions described consist of two phases - continuous, dispersed - bioresorbable and are in the form of sludge. Said two phases are advantageously prepared from Hylan fibers (natural hyaluronic acid chemically modified in situ in order to facilitate its extraction from the tissues), - in application WO-A-96 337 51, the biphasic compositions described also have two bioresorbable phases, better separated, the dispersed phase consisting of insoluble fragments of a polymer hydrogel (chosen from hyaluronic acid and its salts), highly crosslinked; - In application WO-A-00 014 28, the biphasic compositions described contain a dispersed phase, not bioresorbable (particles of at least one hydrogel of a (co) polymer obtained by polymerization and crosslinking of acrylic acid and / or methacrylic acid and / or at least one derivative of said acids), in suspension in an aqueous solution of a polymer chosen from proteins, polysaccharides and their derivatives, crosslinked or not.

These two-phase systems are not fully satisfactory insofar as there is reason to fear jerks at injection and especially after injection, a more rapid disappearance of the continuous phase (not or weakly crosslinked) and therefore a loss at less partial of the effect, in particular of filling, sought.

We have therefore also proposed, in parallel, monophasic hydrogels, produced from the same types of polymers.

In applications WO-A-98 356 39 and WO-A-98 356 40, the product in question is not an injectable hydrogel but a product of solid consistency. In said applications, eye implants are in fact described, used to temporarily fill a vacuum created surgically. In US-A-4,716,154, the hydrogel produced is proposed as a substitute for the vitreous body. The polymer in question (sodium hyaluronate) is very little crosslinked, to obtain an injectable hydrogel. In application WO-A-02 057 53, the monophasic hydrogel described is charged with an antiseptic, effective to protect it, after implantation, against free radicals. In application WO-A-02 063 50, a process is described, capable of generating this type of hydrogel, very homogeneous in its mass.

All these monophasic hydrogels were obtained from polymers of high molecular mass, crosslinked with the intervention of an effective and not excessive amount of at least one crosslinking agent, in aqueous solvent. In view of this prior art, the inventors wished to improve the efficiency of the crosslinking of the polymer in question so as in particular to improve the resistance to degradation (the persistence) of the implanted hydrogel, while retaining the possibility of injecting said hydrogel under acceptable conditions. To improve the efficiency of crosslinking, the inventors first thought of involving more crosslinking agent. This approach was quickly discarded insofar as it inevitably leads to denaturation of the polymer in question and chemical pollution of the crosslinked product obtained. Said inventors then thought of increasing the concentration of polymer in the reaction mixture. This second approach must, a priori, also be excluded from the consideration of the polymers conventionally used to date: polymers of high molecular mass. Thus, for sodium hyaluronate, it is always used at high masses (Mw> 10 ⁶ Da, ≈ 2.10 ⁶ Da, 3.10 ⁶ Da), at concentrations close to the maximum concentration which is approximately 105-110 mg / g. Using it at a higher concentration is difficult (the viscosity of the reaction mixture becomes too high) and inevitably leads to problems of solubility, poor homogeneity ... Concentrating the reaction medium, however, is possible with polymers of low molecular mass (sodium hyaluronate with molecular mass 300,000 Da, having an intrinsic viscosity of 600 ml / g (those skilled in the art are fully aware of the relationship between these two parameters: molecular mass (M) and intrinsic viscosity (η), relation given by the Mark-Houwin formula: M = k η ^α , k and α having values which depend on the nature of the polymer in question) can be concentrated from 110 to 200 mg / g). Unfortunately, the crosslinked polymer obtained generates under these conditions a non-homogeneous, biphasic injectable hydrogel.

In such a context, the inventors have surprisingly established that by combining low molecular weight polymer (s) and high molecular weight polymer (s), an excellent compromise is obtained, namely the possibility of generating, for a non-excessive crosslinking rate (equivalent to that of the prior art) a monophasic, injectable hydrogel with improved mechanical properties and remanence. This low mass / high mass association makes it possible to obtain a hydrogel which more than satisfactorily meets the following specifications: - monophasic,

- better mechanical properties and remanence than those of equivalent products of the prior art,

- injectability not affected, even improved, remaining possible with conventional injection forces by means of conventional injection devices.

The key factor of the crosslinking process of the invention therefore resides in the concentration of the reagents (which is increased compared to that of the reaction mixtures of the prior art, due to the intervention of low molecular weight polymer (s). ), the crosslinking of said concentrated reagents being however "controlled" by the intervention of polymer (s) of high molecular weight, which guarantee the homogeneity of the crosslinked product and then of the hydrogel obtained.

According to its first object, the present invention therefore relates to a process for crosslinking at least one polymer chosen from polysaccharides and their derivatives, used in aqueous solvent by action of an effective and not excessive amount of at least one crosslinking agent, improved in that it is used on a mixture containing at least one low molecular weight polymer and at least one high molecular weight polymer. Said mixture naturally includes the said (s) polymer (s) of low molecular weight in sufficient quantity to guarantee a relatively high concentration of the reaction medium in polymer (s) and the said (s) polymer (s) ) of high molecular mass in an amount sufficient to guarantee a homogeneous consistency to said crosslinked polymer obtained. The crosslinking method of the invention is a method of crosslinking polymers chosen from polysaccharides and their derivatives. The polymer (s) in question can therefore not be natural or synthetic. Examples of natural polymers are hyaluronic acid and its salts, other glycosaminoglycans, such as chondroitin sulfates, keratan sulfate, heparin, heparan sulfate, alginic acid and its biologically acceptable salts, starch, amylose, dextran, xanthan, pullulan ... Examples of synthetic derivatives of natural polysaccharides are carboxyoelulose, carboxymethylcellutose, alkylcelluloses, such as hydroxyethylcellulose, hydroxypropylmethylcellulose (HPMC) , oxidized starch ...

The process of the invention is likely to be suitable for the crosslinking of any one of these polymers, insofar as said polymer intervenes at low (s) and high (s) molecular weights.

The process of the invention is likely to be suitable for the crosslinking of mixtures of such polymers, said mixtures containing at least one low molecular weight polymer and at least one high molecular weight mixture.

The qualifiers "weak" and "strong" of the molecular masses in question obviously cannot be defined more precisely, at this stage of the description of the invention, insofar as they depend on the mixture in question, on the nature of the ) polymer (s) present. Similarly, it is not generally possible to indicate relative proportions of intervention of the polymer (s) present. Those skilled in the art, however, fully understood the spirit of the invention. This involves concentrating the reaction medium with the low mass polymer (s) molecular but to however involve at least one high molecular weight polymer to moderate, control the crosslinking involved.

We want to obtain a coherent crosslinked product, precursor of a monophasic hydrogel. We want to avoid the formation of lumps, possibly coherent at the end of crosslinking but liable to lose their consistency during the preparation of the injectable hydrogel.

The explanations given above are a posteriori. The result obtained was in no way predictable.

In the context of an advantageous variant, the reaction medium contains a single polymer, which is involved in at least two differentiated molecular weights: at least one weak and at least one strong.

In the context of this advantageous variant, the same polymer is preferably used at a single low molecular weight and at a single high molecular weight. The polymer in question is advantageously a salt of hyaluronic acid. It is very advantageously chosen from the sodium salt, the potassium salt and their mixtures. It preferably consists of the sodium salt (NaHA).

In the context of crosslinking of this type of polymer, a person skilled in the art understands that said crosslinking is carried out in a basic aqueous solvent. In general, said crosslinking is obviously carried out under pH conditions favorable to the dissolution of the polymer in question.

In the context of crosslinking of this type of polymer (salt (s) of hyaluronic acid), according to a preferred variant of implementation of crosslinking, the reaction mixture contains:

at least one salt of hyaluronic acid of low molecular mass m, with m <9.9.10 ⁵ Da, advantageously

10 ⁴ Da <m <9.9.10 ⁵ Da; and - at least one salt of hyaluronic acid of high molecular mass M, with M> 10 ⁶ Da, advantageously 10 ⁶ Da <M <10 ⁸ Da, and very advantageously 1.1 × 10 ⁶ Da <M <5.10 ⁶ Da; said low and high molecular weight salts being advantageously of the same nature, very advantageously consisting of sodium hyaluronate (NaHA). In such a context, said reaction mixture advantageously has an intrinsic viscosity of less than 1,900 ml / g, ie

∑ω _. Ililo <1900 ml / g, with coi: mass fraction of fraction i of polymer, having an intrinsic viscosity _r \ {\ _{Q /} in the reaction mixture. Those skilled in the art know the intrinsic viscosity parameter and are aware of the additivity laws of said parameter.

The condition stated above makes it possible to obtain a monophasic hydrogel optimized with reference to its properties of persistence and injectability. It fixes the relative intervention of low (m) and high (M) molecular weight salts.

In the context currently mentioned (NaHA of molecular masses m and M), the reaction mixture advantageously contains more than 50%, very advantageously more than 70%, by mass of at least one salt of hyaluronic acid of low molecular mass m and therefore, logically, advantageously less than 50%, very advantageously less than 30%, by mass of at least one salt of hyaluronic acid of high molecular mass M.

In general, for obtaining the expected effect, there is at least 5% by mass of at least one salt of hyaluronic acid of high molecular weight M in the reaction mixture.

The crosslinking method of the invention is advantageously implemented with the sodium salt of hyaluronic acid, intervening at a low molecular mass m and at a high molecular mass M. We then very advantageously have: m ≈ 3.10 ⁵ Da and M ≈ 3.10 ⁶ Da. As crosslinking agent, with all types of polymer, any agent known to crosslink polysaccharides and their derivatives can be used, via its hydroxyl functions - crosslinking agent at least bifunctional, to ensure crosslinking - and in particular an epoxy or its derivatives. We recommend the intervention of crosslinking agents, bifunctional, alone or in mixture. Particularly recommended is the intervention of epichlorohydrin, divinylsulfone, 1,4-bis (2,3-epoxypropoxy) butane (or 1,4-bisglycidyloxybutane or 1,4-butanedioldiglycidylether or BDDE), 1,2 -bis (2,3- epoxypropoxy) ethylene, 1- (2,3-epoxypropyl) -2,3-epoxycyclohexane, aldehydes such as formaldehyde, glutaraldehyde and crotonaldehyde, taken alone or as a mixture. We particularly recommend the intervention of 1,4-bis- (2,3-epoxypropoxy) butane or BDDE.

Those skilled in the art will know how to determine the effective and not excessive amount of crosslinking agent (s) to be used. It is recommended to use an effective and not excessive quantity such that it theoretically ensures a crosslinking rate (τ), defined by the ratio

_τ _ Total number of reactive functions of said crosslinking agent _χ ^ QQ Total number of disaccharide units of the polymer molecules between 0.5 and 70%, advantageously between 4 and 50%.

The crosslinking process of the invention is original due to the forms of intervention of the polymers in question. Otherwise, it is implemented in a conventional manner with at least one crosslinking agent. It is noted that said crosslinking agent is generally reacted on the dissolved polymer (s). It is however in no way excluded that it intervenes on the said polymer (s) during hydration, according to the process described in WO-A-02 06 350.

The crosslink obtained at the end of the implementation of the crosslinking process of the invention is generally formulated to generate the desired injectable monophasic hydrogel. It is if necessary neutralized beforehand. We have seen that the crosslinking of the salts of hyaluronic acid is in fact carried out in basic medium. The formulation is implemented in a buffered solution at a pH compatible with the human body (since the hydrogel in question is generally intended for injection into the human body): pH between 6.5 and 7.5 advantageously between 7 and 7.4, very advantageously between 7.1 and 7.3. In said solution, the crosslinked polymer equilibrates. It also acquires an osmolarity compatible with that of the human body ... Surprisingly, at the end of this formulation step, the crosslinked polymers of the invention diluted are monophasic hydrogels.

According to a preferred implementation variant, an injectable hydrogel of the invention is prepared, by crosslinking a mixture of at least one polymer, salt (s) of hyaluronic acid (see above), by neutralizing the reticulate obtained, then by formulating it in a buffered solution at a pH between 7.1 and 7.3, at a concentration between 10 and 40 mg / g, advantageously between 20 and 30 mg / g.

The process for preparing the injectable monophasic hydrogel from the crosslinked polymer (according to the crosslinking process, first object of the present invention) constitutes the second object of the present invention.

We come to the third and fourth objects which consist respectively of the crosslinked polymer capable of being obtained after the implementation of the crosslinking process (first object) and the injectable monophasic hydrogel capable of being obtained by formulation. (second object) of said crosslinked polymer, as specified above.

Said polymer and hydrogel advantageously contain low molecular weight sodium hyaluronate and high molecular weight sodium hyaluronate; said low molecular weight sodium hyaluronate very advantageously occurring at more than 50% by mass.

The structure of the injectable monophasic hydrogel - fourth object of the present invention - is original. Its consistency resists degradation. This resistance of the hydrogel is much higher than that of equivalent products of the prior art.

Those skilled in the art are aware that one of the methods for estimating the consistency of a hydrogel, in particular of this type, is the measurement of the parameter: tan.delta = - _r = f (frequency of stress). G

The hydrogels of the invention have the outlets indicated in the introduction to this text. They are particularly effective there. It is now proposed to illustrate the invention in its various aspects, by the examples below. More precisely, we have:

- Example 1 which illustrates the prior art (crosslinking of a high molecular weight polymer), - Example 2 which illustrates the remarks made in the introduction to this text (crosslinking of the same low molecular weight polymer),

- Examples 3 and 4 which illustrate the invention (crosslinking of the same polymer occurring in different relative amounts at low and high molecular weight).

A few measurement methods are used beforehand, used to characterize the products in question.

Measurement of intrinsic viscosity

The intrinsic viscosity of sodium hyaluronate (NaHA) (in ml / g) is determined in accordance with the European Pharmacopoeia of NaHA (2.2.9), by a capillary viscometer of Ubelhode type.

Ejection force measurement (no specific standard on this test)

The injectability of the NaHA-based gel is determined by measuring the force required (in Newton, N) to eject the gel contained in a standard syringe, through a 27 GVz needle, at a speed of 12.5 mm / min. The tests were carried out on a Verstatet ^® traction device sold by the company Mecmésin.

Remanence measurement

The consistency of the gel is characterized at 25 ° C. by a rheological measurement of the elastic (G ¹ ) and viscous (G ") modules as a function of the frequency (from 0.05 to 10 Hz) and in the fields of constant deformation, at using a constrained constraint rheometer (Carrimed CSL 500 from TA Instruments) and a cone / plane geometry 4 cm 2 ° This rheometer is checked and calibrated regularly The degradation of the crosslinked gel results in an evolution of its consistency , which is measured by increasing the tangent delta parameter (tan.delta = G "/ G ') as a function of time, at the frequency of 1 Hz. The gels are degraded by being brought to the temperature of 93 °. vs. At this temperature, the time after which the tan.delta reaches the value 0.65 (value which corresponds to a degraded gel state) is measured. Arbitrarily, a remanence index of 1 (corresponding to said time) was fixed for the gel of example 1. The remanence index values, indicated for the other gels, are relative values. Aspect of the Monophasic hydrogel Microscopic aspect: no apparent liquid phase - fine fragmentation of the gel in facets

Macroscopic aspect: flexible and continuous

biphasic

Microscopic appearance: fragments of gels which bathe in a low viscous liquid medium.

Macroscopic aspect: "puree" which fragments very easily - no cohesion of the gel nor of shooting aspect.

EXAMPLE 1: High Mass Fibers 3.5 g of sodium hyaluronate fibers (NaHA), of intrinsic viscosity = 2800 ml / g, are weighed with a moisture content of 8.7%, to which 25 are added, 6 g of NaOH at 0 _f 25 N. The hydration of the fibers lasts 2 h, with regular manual homogenization with a spatula. 0.96 g of a solution of 1,4-butanedioldiglycidylether (BDDE) diluted 1/5 ^th in 0.25 N sodium hydroxide is added to the reaction medium followed by mechanical homogenization for 15 min before immersion in a bath thermostatically controlled at 50 ° C ± 1 ° C. R = [BDDE) o / [NaHA] ₀ = 6%; [NaHA] i = 105 mg / g

The reaction lasts 2 h. The crosslink is neutralized at pH = 7.2 in a phosphate buffer solution, then is dialyzed. The hydrogel obtained is then adjusted in concentration ([NaHA] _f = 26 mg / g) and mechanically homogenized before being packaged in syringes and sterilized in an autoclave by moist heat. Injection force after sterilization: 25 N Hydrogel remanence index: 1.0 Monophasic hydrogel

EXAMPLE 2: low mass fibers

Weigh 1.56 g of sodium hyaluronate fibers (NaHA), of intrinsic viscosity = 600 ml / g, with a humidity rate of 5.5%, to which 7.15 g of 0.25 N NaOH is added. The hydration of the fibers lasts

2 h, with regular manual homogenization with a spatula. 0.31 g of a solution of 1,4-butanediol diglycidyl ether (BDDE) diluted l / 5 ^th in 0.25N sodium hydroxide is added to the reaction mixture followed by mechanical homogenization for 15 min before immersion in a bath thermostatically at 50 ° C ± 1 ° C.

R = [BDDE] ₀ / [NaHA] o = 6.8%; [NaHAjj = 174 mg / g

The reaction lasts 2 h. The crosslink is neutralized at pH = 7.2 in a phosphate solution, then is dialyzed. The hydrogel obtained is then adjusted in concentration ([NaHA] _f = 26 mg / g) and mechanically homogenized before being packaged in syringes and sterilized in an autoclave.

Injection force after sterilization: 24 N

Hydrogel retention index: 6.0 Biphasic hydrogel

EXAMPLE 3 mixture of fibers

We weigh 0.763 g of sodium hyaluronate fibers (NaHA) of intrinsic viscosity = 600 ml / g with a moisture content of 5.5% and 0.237 g of sodium hyaluronate fibers of intrinsic viscosity

2800 ml / g, with a humidity rate of 9.3%. Mass proportion in the mixture: 600/2 800: 77/23 (w / w).

The operating mode remains identical to that of Example 2.

R = [DBBE] ₀ / [NaHA] ₀ = 7%; [NaHA] i = 140 mg / g; [NaHA] _f = 26 mg / g Injection force after sterilization: 15 N

Hydrogel retention index: 3.6

Monophasic hydrogel

EXAMPLE 4 Mixing of fibers The experiment of Example 3 is reproduced by modifying the mass proportion. Mass proportion in the mixture: 600/2 800: 90/10 (w / w)

The procedure is identical to that of Example 2. R = [BDDE] ₀ / [NaHA] ₀ = 6.5%; [NaHA] i = 140 mg / g; [NaHA] _f = 26 mg / g Injection force after sterilization: 14 N Hydrogel remanence index: 7.7 Monophasic hydrogel

These examples are summarized in the table below.

BOARD

[NaHA] o = NaHA concentration in the reaction medium at

[NaHA] f = NaHA concentration in the final hydrogel after reaction and dilution with a quantity qs of phosphate buffer

G ': elastic modulus of the final hydrogel (Pa.s)

G ": viscous module of the final hydrogel (Pa.s) Carrimed CSL 500 rheometer

Tan.delta = G "/ G 'ηint.: Intrinsic viscosity of the NaHA fiber / Ubelhode F viscometer: gel ejection force in N through a needle 27G ^1/2 / Dynamometer 1Q0N n ° ηm (ml / g ) R = m _B DDi- / m _Na HA [NaHA] ₀ [NaHA] _f Aspect * G ', G ", tan.delta' ap ster Index of

% = mass proportion in mg / g in gel (1 Hz) 27 GVz afterglow final mixture mg / g

1 (100%) 2,800 6% 105 26 M 143/65 / 0.40 25 1

2 (100%) 600 6.8% 174 26 B 1,300 / 100 / 0.08 24 6

3 (77%) 600 + (23%) 2,800 7 140 26 M 262/27 / 0.10 15 3.6

4 (90%) 600 + (10%) 2,800 6.5 140 26 M 571/41 / 0.07 14 7.7

* M = monophasic B = biphasic

In the appended figure, the curves for each of the four hydrogels, prepared according to examples 1 to 4, have been shown: Tan.delta = f (frequency of stress).

The hydrogels of the invention (examples 3 and 4) have a rheological behavior different from that of the hydrogel of the prior art (example 1).

They are also monophasic and in this very different from the hydrogel of Example 2 (biphasic).

Claims

1. Method for crosslinking at least one polymer, chosen from polysaccharides and their derivatives, used in aqueous solvent by the action of an effective and not excessive amount of at least one crosslinking agent, characterized in that it is used on a mixture containing at least one low molecular weight polymer and at least one high molecular weight polymer.

2. Method according to claim 1, characterized in that said mixture contains a single polymer, having at least two different molecular weights: at least one weak and at least one strong; advantageously having two different molecular weights: low and high.

3. Method according to one of claims 1 or 2, characterized in that said polymer is a salt of hyaluronic acid.

4. Method according to claim 3, characterized in that said salt of hyaluronic acid is chosen from the sodium salt, the potassium salt and their mixtures; and advantageously consists of the sodium salt.

5. Method according to any one of claims 1 to 4, characterized in that said mixture contains:

10 ⁴ Da <m <9.9.10 ⁵ Da; and - at least one salt of hyaluronic acid of high molecular mass M, with M> 10 ⁶ Da, advantageously 10 ⁶ Da <M <. 10 ⁸ Da, and very advantageously 1.1.10 ⁶ Da <M <5.10 ⁶ Da; said low and high molecular weight salts being advantageously of the same nature, very advantageously consisting of sodium hyaluronate.

6. Method according to claim 5, characterized in that said mixture has an intrinsic viscosity of less than 1900 ml / g.

7. Method according to one of claims 5 or 6, characterized in that said mixture contains more than 50%, advantageously more than 70%, by mass of at least one salt of hyaluronic acid of low molecular mass m and less than 50%, advantageously less than 30% in mass of at least one salt of high molecular weight hyaluronic acid M.

8. Method according to any one of claims 5 to 7, characterized in that said mixture contains at least 5% by mass of at least one salt of high molecular weight hyaluronic acid.

9. Method according to any one of claims 5 to 8, characterized in that said mixture contains approximately 90% by mass of sodium salt of hyaluronic acid having a molecular mass of approximately 3.10 ⁵ Da and approximately 10% mass of sodium salt of hyaluronic acid having a molecular mass of approximately 3.10 ⁶ Da.

10. Method according to any one of claims 1 to 9, characterized in that said crosslinking agent is chosen from bifunctional crosslinking agents and their mixtures; advantageously chosen from epichlorohydrin, divinylsulfone, 1,4-bis (2,3-epoxypropoxy) - butane, 1,2-bis (2,3-epoxypropoxy) ethylene, l- (2,3-epoxypropyl ) -2,3- epoxycyclohex ne, aldehydes such as formaldehyde, glutaraldehyde and crotonaldehyde, and mixtures thereof; very advantageously consists of 1,4-bis (2,3-epoxypropoxy) butane.

11. Method according to any one of claims 1 to 10, characterized in that said effective and not excessive amount of at least one crosslinking agent theoretically ensures a crosslinking rate, defined by the ratio: 100 x (total number of reactive functions of said crosslinking agent / total number of disaccharide units of the polymer molecules present), between 0.5 and 70%, advantageously between 4 and 50%.

12. Method for preparing an injectable monophasic hydrogel of at least one crosslinked polymer chosen from polysaccharides and their derivatives, characterized in that it comprises:

the crosslinking of a mixture according to any one of claims 1 to 11,

- The formulation of said crosslinked mixture, neutralized if necessary, in a buffered solution at a pH between 6.5 and 7.5, advantageously between 7 and 7.4, very advantageously between 7.1 and 7.3.

13. Method according to claim 12, characterized in that it comprises: - crosslinking of a mixture according to any one of claims 3 to 11;

- The formulation of said crosslinked mixture, neutralized, in a buffered solution at a pH between 7.1 and 7.3, at a concentration between 10 and 40 mg / g, advantageously between 20 and 30 mg / g.

14. Crosslinked polymer capable of being obtained at the end of the implementation of a crosslinking process according to any one of claims 1 to 11.

15. injectable monophasic hydrogel capable of being obtained at the end of the implementation of a preparation process according to one of claims 12 or 13.

16. The injectable monophasic hydrogel according to claim 15, containing, crosslinked, low molecular weight sodium hyaluronate and high molecular weight sodium hyaluronate.