Patent Abstract:
the present invention relates to reinforced composite hydrogel based on a polymer blend and comprising a network of fibres , said polymer blend comprising uv sensitive molecules . it also relates to a process for preparing the reinforced composite hydrogel according to the invention .

Detailed Description:
the present invention relates to a composite hydrogel made of a polymer matrix composed of one or more polymers reinforced with nanofibres that create an interpenetrating network . the fibres are disposed in the polymer matrix , creating unique 3 - dimensional microstructure and characteristics . the mechanical properties of the reinforced hydrogel , e . g . elastic modulus can be varied as a function of the fibre content thereby permitting control of the stiffness of the structure . the swelling capacity of the hydrogel can also be tuned by the fibre content and the type of fibres used . thus , the aim consists of producing composite hydrogels that can withstand compressive and hydrostatic loads when hydrated . fig1 shows the curing behavior of a hydrogel composed of two monomers sensitive to uv light , photoinitiator and deionised water . the reaction mechanism is described by a radical polymerization . the curing profile determined by photorheology varies with the concentration of branched monomer and the curing time decreases as the branched monomer is increased . the curing profile has three different phases : first , the very steep increase in the storage modulus g ′ indicates the creation of new chemical bonds and the formation of the network ; secondly , the deceleration step where the curing becomes diffusion - controlled . in this step , after the consumption of the active radicals and the formation of the network , the rate of curing decreases due to the lack of radicals and also to the fact that the remaining radicals are trapped in the newly formed network and cannot diffuse through the latter . finally , the last step is characterized by a plateau , indication that the reaction is complete . the amount of monomers , photoinitiator and water are generally expressed in volume fraction . the created hydrogel is a porous structure as observed in fig2 . porosity is defined in terms of relative volume of pores . the pores can be closed or open when the pores are interconnected as it is the case in this invention . an open porosity is important for fluid flow through the structure and for transport of nutriments if living cells are for example introduced in the porous material . hydrogels are considered to be weak structures , with a low stiffness of the network . therefore , to increase the mechanical properties of such hydrated structures , reinforcement is needed . the choice of fillers is of paramount importance . the difference in stiffness of the matrix and the filler should not be important to avoid the creation of stresses at the interfaces . fibres can be of different aspect ratios between their lengths and diameters and should form a network . the distribution of the fibres can be random or oriented through the structure . the fibres can be used in their dry form or in the form of a gel composed of a certain amount of fibres dispersed in water . the fibres or gel of fibres are mixed with the monomers using a high - shear mixer and are then cured under uv light . the amount of fibres is relative to the amount of polymer matrix and is generally expressed in mass fraction . the fibres should also be hydrophilic to insure water uptake of the structure . fig4 shows the volume increase of hydrogel samples at swelling equilibrium with increasing cellulose nanofibril content . the sample on the left is the hydrogel sample after polymerization , i . e . not hydrated . the porosity being defined above as the relative volume of pores , when adding the fibres to the structure , the volume of pores will decrease and subsequently the water absorption will follow the same trend , as observed on fig4 . chemically modified fibres with increased hydrophilicity can be used in order to avoid this limitation . in the case of a composite hydrogel , as proposed in this invention , mechanical properties and swelling capacity are interdependent . the elastic modulus , i . e . the slope of the linear part of the stress - strain curves of fig3 , increases with the cellulose nanofibril content . the swelling capacity , however , decreases with increasing fibril content as observed in fig4 . the ideal composite hydrogel designed for a specific application should therefore be a compromise between mechanical performance and swelling ability . the method of this invention to process the mentioned composite hydrogels is described in detail below . monomers , aqueous solution of photoinitiator and deionised water are mixed manually to obtain a homogeneous precursor solution . fibres , in their dry form or in the gel form , are added to the precursor solution and stirred with a high - shear mixer during 20 minutes to obtain a good dispersion of the fibres . the precursor solution with the fibres is then degassed for about 15 minutes under a vacuum of 10 mbar to remove bubbles . this solution is then casted in cylindrical silicon moulds resistant to uv light and exposed to uv light during 30 minutes . the uv intensity can be as high as 145 mw / cm 2 . the hydrogel samples are then removed from the moulds and stored in phosphate buffered saline ( pbs ) to allow swelling equilibrium to be reached . the time needed to reach equilibrium can vary from 24 to 48 hours . testing can be performed when the samples are at swelling equilibrium . special care should be taken with the evaporation of the fluid during testing and adapted set - ups should be developed to obtain reliable measurements . fig1 shows an example of humidity chamber for testing hydrated materials such as hydrogels . the method can also be used to create composite hydrogels with very specific properties . the matrix can be reinforced by different types of fibres and the degree of shear deformation can be influenced by judicious rearrangement of fibres that could maximize the shear and therefore enhance toughness and impact resistance . this method was used to produce non - degradable composite hydrogels but it could also be used for the production of degradable composite hydrogels given the use of adequate material systems . mechanical properties such as elastic modulus as well as swelling capacity can vary on a large range depending on the fibril content and type . examples will provide values for specific material systems . in all examples presented in the following sections , the hydrogel matrix was composed of tween 20 ® trimethacrylate ( t3 ), n - vinyl - 2 - pyrrolidone ( nvp ), photoinitiator irgacure 2959 as aqueous solution of 0 . 05 wt % of irgacure 2959 and deionised water . the t3 concentrations varied from 1 to 15 vol % and the concentrations of nvp from 35 to 49 vol %. the concentration of the irgacure solution was kept constant at 10 vol % and the amount of water was invariably 40 vol %. cellulose nanofibrils were used in the upcoming examples . the fibril content varied from 0 . 2 to 1 . 6 wt %. any molecule that is uv sensitive and polymerizes through a free - radical pathway to produce hydrogels can be used . these include poly ( ethylene ) dimethacrylate ( pegdma ), hydroxyethyl methacrylate ( hema ) and all acrylic molecules that are able to produce a 3d network . concerning the fillers , fibres and mesh of fibres randomly distributed in the matrix or oriented can be used . fibres are preferably hydrophilic or chemically modifiable to increase their hydrophilicity and have to be deformable with the hydrogel matrix . some suitable examples can be natural fibres such as silk and flax , wood fibres , cellulose fibres and nanofibres of cellulose and polymer fibres . this example is to illustrate a method for the preparation of a composite hydrogel reinforced with cellulose nanofibrils . in addition , the range of swelling and the mechanical properties are indicated . 20 g of tween 20 was dissolved in 100 ml of tetrahydrofuran ( thf ), to which 6 . 2 g of 4 -( n , n - dimethylamino ) pyridine ( dmap ) was introduced under argon . after cooling to 0 ° c ., 4 . 9 ml of methacryloyl chloride ( meocl ) in 30 ml of thf was added dropwise to the mixture over 30 minutes under stirring . the mixture was then protected from light and let stirred overnight at room temperature . the resulting precipitate was then filtered off , washed with thf and dried avoiding exposure to light . the crude product was then purified by column chromatography . synthesis of t3 / nvp hydrogels reinforced with cellulose nanofibrils ( t3 concentration 4 . 5 vol %): a batch of precursor solution of 6 . 4 ml was prepared as follows : t3 , nvp and photoinitiator were added to a tube . the density of the cellulose nanofibrils gel was assumed to be 1 ( gel contains 98 % of water ). samples containing 0 . 2 , 0 . 4 , 0 . 8 and 1 . 6 wt % of cellulose nanofibrils were prepared by first mixing manually the components and then dispersing the fibrils for 20 minutes using a high shear mixer . the precursor solution was then degassed under vacuum at 10 mbar and finally cast in silicon moulds and uv - cured for 30 minutes at 145 mw / cm 2 . the surface of the composite hydrogels showed rough regions and clusters , possibly originating from fibrils acting as nucleation points for the matrix ( fig6 ). the swelling ratio of hydrogels was determined gravimetrically in dependence of time . fig5 shows the swelling ratios at equilibrium for hydrogels with varying cellulose nanofibrils contents . with increasing content of cellulose nanofibrils , the swelling ratio of the composite hydrogels decreased due to stronger crosslinked network . the mechanical properties in compression of the hydrogels were determined using a universal testing machine . the stiffness of the composite hydrogels was increased with increasing content of cellulose nanofibrils , as shown in fig3 . an increasing content of cellulose nanofibrils therefore increases the stiffness of the composite hydrogel and decreases its swelling ratio at equilibrium . a broad range of properties can therefore be achieved with these composite hydrogels . the objective of the present example is to demonstrate the feasibility of producing composite hydrogels reinforced with chemically modified cellulose nanofibrils and its effect of the swelling and mechanical properties . carboxymethylated cellulose nanofibrils with three different degrees of substitution ( ds ) were prepared : 0 . 074 , 0 . 176 and 0 . 225 . with increasing ds the hydrophilicity of the carboxymethylated cellulose nanofibrils increases . the carboxymethylated cellulose nanofibrils were prepared in powder form [ 29 ]. the powders were added to the precursor solution and the mixture was homogenized using a high shear mixer . concentrations of modified fibrils of 0 . 2 , 0 . 4 , 0 . 8 and 1 . 6 wt % are used . the hydrogel samples were produced as described in the previous example . for the same fibril content , the swelling capacity of the composite hydrogels was increased by 1 to 20 % with increasing ds due to the hydrophilic functions of the carboxymethylated cellulose nanofibrils ( fig9 ). by increasing the amount of liquid phase in the composite structure , the stiffness of the network was decreased with increasing ds for the same carboxymethylated cellulose nanofibrils contents ( fig7 and 8 ) but it is still above the results obtained for the non - 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