Biomimetic Growth of Hydroxyapatite in Hybrid Polycaprolactone/Graphene Oxide Ultra-Porous Scaffolds.

This paper reports the preparation and characterization of hybrid scaffolds composed of polycaprolactone (PCL) and different graphene oxide (GO) amounts, intending to incorporate the intrinsic characteristics of their constituents, such as bioactivity and biocidal effect. These materials were fabricated by a solvent-casting/particulate leaching technique showing a bimodal porosity (macro and micro) that was around 90%. The highly interconnected scaffolds were immersed in a simulated body fluid, promoting the growth of a hydroxyapatite (HAp) layer, making them ideal candidates for bone tissue engineering. The growth kinetics of the HAp layer was influenced by the GO content, a remarkable result. Furthermore, as expected, the addition of GO neither significantly improves nor reduces the compressive modulus of PCL scaffolds. The thermal behavior of composites was investigated by differential scanning calorimetry, showing an increase in crystallinity as the addition of GO raised, which implies that GO nanosheets can act as seeds to induce the crystallization of PCL. The improved bioactivity was demonstrated by the deposition of an HAp layer on the surface of the scaffold with GO, especially with a 0.1% GO content.

Kinetic study of thermal degradation of chitosan as a function of deacetylation degree.

Thermal degradation of chitosan with varying deacetylation degree (DD) ranging between 50 and 85% was analyzed by dynamic thermogravimetric analysis at different heating rates. The present study focused on the temperature range between 500 and 800K, above water evaporation. Thermal degradation showed a main degradation stage in this temperature interval with a second stage that appeared in the weight derivative curves as a shoulder in the high temperature side of the main peak with increasing intensity as the DD decreased. The Kissinger and isoconversional Ozawa-Flynn-Wall models were employed to evaluate the Ea of both thermal degradation processes. Different kinetic models were tested to computer simulate the thermogravimetric traces calculating the model parameters with a non-linear least squares fitting routine. The Sestack-Berggren model allowed reproducing accurately the overlapping of the two degradation mechanisms and calculating the mass fraction lost in each of them revealing the coupling between the two degradation mechanisms.

Determining the influence of N-acetylation on water sorption in chitosan films.

Water absorption in chitosan rapidly increases when the deacetylation degree decreases between 85 and 45%. This seems to contradict the fact that water absorption in chitin is much lower than that of chitosan. The aim of this paper is to understand this feature by measuring the main parameters affecting equilibrium water content. Since swelling capacity depends on the water-polymer interaction, the Flory Huggins interaction parameter was evaluated, finding small or null dependence on the deacetylation degree. Other factor influencing elastic energy is chain stiffness related to the elastic modulus that was measured as a function of deacetylation degree. Besides, crystalline structure was measured by X-ray diffraction patterns as a characteristic of cross-linking density. These observations led us to conclude that the instability of crystals during the swelling process increases with decreasing deacetylation degree, explaining the high equilibrium water content of low deacetylation chitosans.

Biomimetic hydroxyapatite coating on pore walls improves osteointegration of poly(L-lactic acid) scaffolds.

Polymer-ceramic composites obtained as the result of a mineralization process hold great promise for the future of tissue engineering. Simulated body fluids (SBFs) are widely used for the mineralization of polymer scaffolds. In this work an exhaustive study with the aim of optimizing the mineralization process on a poly(L-lactic acid) (PLLA) macroporous scaffold has been performed. We observed that when an air plasma treatment is applied to the PLLA scaffold its hydroxyapatite nucleation ability is considerably improved. However, plasma treatment only allows apatite deposition on the surface of the scaffold but not in its interior. When a 5 wt % of synthetic hydroxyapatite (HAp) nanoparticles is mixed with PLLA a more abundant biomimetic hydroxyapatite layer grows inside the scaffold in SBF. The morphology, amount, and composition of the generated biomimetic hydroxyapatite layer on the pores' surface have been analyzed. Large mineralization times are harmful to pure PLLA as it rapidly degrades and its elastic compression modulus significantly decreases. Degradation is retarded in the composite scaffolds because of the faster and extensive biomimetic apatite deposition and the role of HAp to control the pH. Mineralized scaffolds, covered by an apatite layer in SBF, were implanted in osteochondral lesions performed in the medial femoral condyle of healthy sheep. We observed that the presence of biomimetic hydroxyapatite on the pore's surface of the composite scaffold produces a better integration in the subchondral bone, in comparison to bare PLLA scaffolds.

Alkaline degradation study of linear and network poly(ε-caprolactone).

Alkaline hydrolysis of a polycaprolactone (PCL) network obtained by photopolymerization of a PCL macromer was investigated. The PCL macromer was obtained by the reaction of PCL diol with methacrylic anhydride. Degradation of PCL network is much faster than linear PCL; the weight loss rate is approximately constant until it reaches around 70%, which happens after approximately 60 h in PCL network and 600 h in linear PCL. Calorimetric results show no changes in crystallinity throughout degradation, suggesting that it takes place in the crystalline and amorphous phases simultaneously. Scanning electron microscopy microphotographs indicate that degradation is produced by a different erosion mechanism in both kinds of samples. The more hydrophilic network PCL would follow a bulk-erosion mechanism, whereas linear PCL would follow a surface-erosion mechanism. Mechanical testing of degraded samples shows a decline in mechanical properties due to changes in sample porosity as a consequence of the degradation process.