Composite Hydrogels for the Replacement of the Nucleus Pulposus

The intervertebral disc (IVD), which provides flexibility to the spine and facilitates damping and motion, is composed of two distinct structures: the annulus fibrosus (AF) and the nucleus pulposus (NP). The AF, which is fibrous and elastic, confines the NP, a gel-like material, containing 65 to 90% of water. The NP mediates load bearing in the spine by maintaining a suitable disc height and adequate loading of the AF. The effectiveness of this mechanism depends on the swelling capacity of the NP and the intra-discal pressure, and it becomes severely limited once the NP has degenerated. Up to now, the main strategy for relieving back pain has been vertebral fusion or total disc replacement. However, there is currently a shift in emphasis towards preservation techniques. NP replacement in particular has attracted considerable interest and various types of synthetic NP have been investigated as a means to restore IVD functions. The present study has focused on injectable, UV curing composite hydrogels for NP replacement. The objectives have been to: (a) synthesize methacrylated monomers based on Tween 20 surfactant precursors, and to use these as a crosslinker in combination with linear hydrophilic molecules in order to obtain hydrogels, (b) investigate the UV curing kinetics, swelling and mechanical properties of the hydrogels and their variation with crosslinker and photoinitiator concentration, (c) evaluate their biocompatibility and (d) study the influence of the filler on the performance of composite hydrogels based on these materials, and hence assess their potential as NP implants. The synthesis of new methacrylated monomers based on Tween 20, yielded reproducible conversions between 75 to 85 %. Tween 20 trimethacrylate (T3), a branched molecule, was then used as a crosslinker in hydrogel systems derived from either N-vinyl-2-pyrrolidone (NVP) or 2-hydroxyethyl methacrylate (HEMA). The study of the curing kinetics by photorheology and photo differential scanning calorimetry of both systems showed that the concentration of T3 controled the crosslinking density whereas the amount of initiator and of UV intensity controled the time of curing and the formation of dangling chains and cyclization. The build-up of the network prior to gelation of the two blends was analyzed using existing growth models and particularly complex patterns of growth were identified for T3/NVP. From the study of the curing kinetics, optimum conditions for processing could be determined. The mechanical behavior of T3/NVP hydrogels was strongly dependent on the concentration of T3 and on the water content of the network. Swelling ratios varied from 1.6 to 5.8 with decreasing T3 content, which is comparable with the behaviour of the human NP, which shows swelling ratios from 1.8 to 9. The elastic modulus varied from about 4400 to 3 kPa for dried and fully hydrated samples, respectively, which is close to the modulus of 3 to 6 kPa observed in the native NP at swelling equilibrium. Preliminary biocompatibility studies showed cell viability and the hydrogels exhibited a highly interconnected porous structure at swelling equilibrium. They could also be further reinforced with nano-fibrillated cellulose (NFC), a hydrophilic material, with a 3 to 8-fold increase in elastic modulus being observed with respect to the neat hydrogel depending on the NFC content, i.e. greater stiffness than the native NP, as required. However, the swelling ratio was decreased compared to that of the neat hydrogel. To overcome this without compromising the mechanical properties, the NFC fibres were chemically modified to increase their hydrophilicity. The swelling ratio of these new composite hydrogels was increased by 1 to 20 % depending on the NFC content. The possibility of tailoring both the swelling behaviour and mechanical performance suggests these materials to be ideal for NP replacement.

Curing Kinetics and Mechanical Properties of a Composite Hydrogel for the Replacement of the Nucleus Pulposus

Ana Borges de Couraça, Pierre-Etienne Bourban, Jan-Anders Månson, Dominique Pioletti

A polymer material system has been developed to propose an injectable, UV and in situ curable hydrogel with properties similar to the native nucleus pulposus of intervertebral disc. Neat hydrogels based on Tween® 20 trimethacrylates (T3) and N-vinyl-2-pyrrolidone (NVP) and composite hydrogels of same composition reinforced by nano-fibrillated cellulose were synthesized with different T3 concentrations and their curing kinetics was investigated by photorheology using UV light. The T3 concentration has an influence on the time of curing and final shear stiffness of the material. NFC does not alter the time of curing but increases the final mechanical performance of the hydrogels for a same chemical composition. Hydrogel samples, neat and composite, were then tested in unconfined compression at different hydration stages and in confined compression and their elastic modulus was determined. The amount of fluid present in the network is mostly responsible for the mechanical properties and NFC fibres proved to be an efficient reinforcement. The elastic modulus ranged from 0.02 to 8 MPa. Biocompatibility studies showed that cells are confluent at 90% and do not show any morphology change when in contact with the hydrogel. The present hydrogel can therefore be considered for NP replacement.

Elsevier2010

Dark curing kinetics of UV printing inks by real-time ATR-FTIR spectroscopy

Eric Jacques Nouzille

In UV-curable printing technology, an ink or a varnish is exposed to the UV-light source within a fraction of a second. Subsequent polymerization therefore proceeds in the dark, referred to as dark curing. Because dark curing can be critical and contributes to a large part of the overall conversion and to the final properties of the printed ink, the aim of this work was to get a better understanding of the dark curing process. Because an ink is a complex mixture of components, simpler systems representative of free radical and cationic chemistry were studied. Special attention was given to free radical polymerization with a detailed kinetic modeling. For cationic systems, the study was more application oriented. Attenuated-Total-Reflection Fourier-Transform Infrared (ATR-FTIR) spectroscopy was found to be the method of choice to follow in real-time the progress of the reaction under UV irradiation and in the dark. This technique allowed the study of curing conditions close to those encountered in industry. Dark curing of free radical was examined in systems consisting of a monomer (multifunctional acrylate) and a photoinitiator. The reactivities of different monomers were investigated and it was found that the chemistry of the spacer between the acrylate functional groups had an influence on the extent of dark curing. For example, TPGDA (tripropylene glycol diacrylate) led to higher overall conversion than HDDA (hexanediol diacrylate) although both are diacrylate monomers. Several explanations were put forward, such as the effect of dissolved oxygen, primary cyclization and initiation efficiency. The efficiency of photoinitiators (PIs) was examined and different behaviours were highlighted. For example, it was observed that DMPA (dimethoxyphenyl acetophenone) has a better quantum yield for α-cleavage compared to HCPK (1-hydroxycyclohexylphenyl ketone), whereas HCPK is more efficient in initiating the reaction, resulting in a better overall conversion in the dark. It is suggested that the two primary radicals resulting from the cleavage of the PI behave differently for DMPA and HCPK. One radical effectively initiates the reaction and the second is mainly involved in the termination process. These two different behaviours were observed under specific experimental conditions: (i) by covering the sample with a quartz plate and taking into account dissolved molecular oxygen and (ii) by curing under inert conditions with nitrogen. Photo-crosslinking polymerization reactions are strongly influenced by diffusion effects due to the formation of an infinite network. Therefore, it was proposed that a redistribution of reactivity within the system occurs, such as the diffusion of unreacted photoinitiators towards concentrations of unreacted monomers. This diffusion process has two positive effects: (i) for the same UV dose, but different intensities, a higher extent of conversion was reached with longer irradiation times under the condition investigated; (ii) multiple exposures led to a higher overall conversion than for a single exposure, for a fixed overall UV dose. Other parameters were explored, such as temperature and diffusing oxygen from air. With temperature, different trends were observed depending on the photoinitiator and the start of dark curing, either starting before the maximum rate of polymerization (Rp,max), or after Rp,max. The development of a kinetic model to describe dark curing of free radical polymerization was proposed. Existing models based on physical parameters, such as those incorporating diffusion-controlled phenomena and free-volume are relatively complex and require extensive information on the system studied. Therefore, a semi empirical approach was chosen in order to study the variation of fitting parameters when changing curing parameters, such as the photoinitiator concentration and type, the irradiation time, the UV light intensity and the temperature. The model was derived from kinetic data and it was assumed that rate coefficients are time-dependent and that termination, which was considered to be a first-order process, is propagation dependent. The kinetic equation has the form of a stretched exponential function and incorporates four fitting parameters. The model was evaluated for a large number of experimental data and very good fits were obtained with a random distribution of residuals. The exponent β was found to be sensitive to the termination process and reflected different reactivities in the dark as in the case of DMPA and HCPK. Dark curing of cationic systems continues until complete monomer conversion but at a slower rate. Thus systems with high reaction rates, both during UV irradiation and during the first seconds of the dark period, are of most importance for UV-curable printing technology. The most widely used diepoxide monomer in cationic ink formulation is 3,4-epoxycyclohexylmethyl-3,4-epoxycyclohexane carboxylate (DECC). In order to improve its reactivity during the first seconds of dark curing, the influence of several parameters was examined, such as the addition of an oxetane co-monomer (TMPO), the influence of relative humidity, UV dose, light intensity, temperature, and film thickness. The enhanced reactivity of DECC with TMPO was confirmed. During the cationic curing process, two reaction mechanisms compete; an activated chain end (ACE) mechanism and an activated monomer (AM) mechanism. It was observed that these two processes depend on the presence of hydroxyl containing compounds such as TMPO monomer, polyol compounds, and, most important, water. When the concentration of these components is increased the AM mechanism is enhanced, leading to higher conversion but at a slower curing rate, and a degradation of the mechanical properties. The presence of water is unavoidable and its concentration in the ink varies in presence of hydrophilic compounds, such as polyol. Furthermore, the water content increases with increasing relative humidity in air. It was shown that the reactivity of the system decreased as the percent of relative humidity increased. Moreover, although a higher overall conversion is achieved, the crosslink density of the film was severely reduced. This lower crosslink density was the result of the predominance of chain transfer reactions, leading to more pendant and short chains which were hydroxyl terminated. Moreover, the extent of conversion was thickness dependent since less water diffuses into the deeper layers occurred in thicker films.

EPFL2007

Nano Fibrillated Cellulose Based Composite Hydrogels for an Injectable Intervertebral Disc Implant

Azadeh Khoushabi

Replacing the gel-like central part of the intervertebral disc (IVD), so-called nucleus pulposus (NP), via a minimally invasive surgery is one of the most recent approaches in the lower back pain treatments. An appropriate biomaterial for NP replacement (NPR) preferably possesses the mechanical and swelling properties similar to those of the native tissue. Furthermore, it should be injectable, biocompatible and stable in the IVD environment. From the surgical point of view, employing a photopolymerizable hydrogel for NPR is beneficial as it can be injected in the liquid form to the IVD and subsequently cured in situ. Such an approach provides a better control for the surgeon over the operation and minimizes the invasion of the surrounding tissue. The application of hydrogels as load-bearing biomedical components is, however, limited by their mechanical properties. Conventional approaches to address such an issue often favors one mechanical property while compromising the others. The present study aims to develop a novel injectable, photopolymerizable hydrogel for NPR with concurrently enhanced mechanical properties and optimal processability. Hydrogels based on photopolymerization of a monomer, i.e. N-vinyl-2-pyrrolidone (NVP), and cross-linking of a preformed functionalized polymer, i.e. poly(ethylene glycol)dimethacrylate (PEGDM), are synthesized. The hydrogels are thoroughly characterized as a function of cross-link density and initial water content in order to examine their applicability for NPR. While a wide range of properties is obtained by variation of the aforementioned parameters, an enhancement of stiffness is associated with brittleness or loss of water content. In addition, homogenous volumetric photopolymerization of such hydrogels is found to be limited especially in case of local illumination. To resolve such issues, a composite material design approach is adopted and the abovementioned polymer networks of hydrogels are combined with nano fibrillated cellulose (NFC). It is demonstrated that the NFC significantly increases the hydrogelsâ stiffness (up to 275%) without compromising the failure strength or reducing the water content. Moreover, the NFC fibers effectively prevent formation of the swelling induced cracks in the NVP based hydrogels. The results of rheology experiments indicate that the dispersed NFC fibers form a network-like structure in the hydrogels precursor solution. Such a structure, as confirmed through the photorheology tests, serves as a light-scattering source when the solution is illuminated by UV light. The latter accelerates the kinetics of photopolymerization and consequently allows reducing the required concentration of photoinitiator. The enhanced and tunable properties of the PEGDM-NFC composite hydrogel, developed in this work, make it a promising implant material for NPR. The applicability and reliability of such a material design approach for NPR is examined using a bovine organ model. In order to meet the properties of the native tissue, the mechanical properties of the composite hydrogel are tuned. The performance of the optimized composite hydrogel versus the native NP tissue is evaluated in several functional tests such as extrusion, confined compression and swelling pressure...