Marcos A. Sabino

Study of the hydrolytic degradation of polydioxanone PPDX

The hydrolytic degradation of bioabsorbable polydioxanone (PPDX) was studied in a phosphate buffer solution, pH=7.4, 37°C. The degradation was evaluated by analyzing the changes in the elastic modulus and morphological changes. After 10 weeks, the weight loss, pH and molecular weight changes suggested diffusion of low molecular weight chain segments into the reaction medium as a consequence of the breaking of ester bonds in the material. The crystallinity (Xc) increased with hydrolysis time, indicating the first step of the degradation process. Modulus was found to decrease, indicating that chain scission proceeded in two steps: the first occurring in the amorphous regions of microfibrils and intermicrofibrillar space; the second in the crystalline regions, resulting in a lower stress value for the chain segments when submitted to external forces. Surface morphological changes suggest a heterogeneous degradation mechanism by layers.

Heterogeneous nucleation and self-nucleation of poly(p-dioxanone)

The changes in nucleation behaviour upon addition of Boron Nitride (BN), Talc and Hydroxyapatite (HA) to poly(p-dioxanone) (PPDX) were monitored by DSC and Polarised Optical Microscopy (PM). Self-nucleation DSC studies evidenced the existence of the usual three self-nucleation domains depending on the self-nucleation temperature (Ts) employed. By far the best nucleation agents for PPDX were its own self-nuclei and this result was independent of the presence or absence of any of the other nucleating agents employed; once Domain II was reached, self-nucleation dominated the nucleation process. BN and Talc were able to nucleate PPDX, thereby increasing its nucleation density, its dynamic crystallisation temperature upon cooling from the melt (Tc) and its enthalpy of crystallisation (ΔHc). BN was a better nucleating agent than talc. HA on the other hand caused an “antinucleation” effect on PPDX characterised by a decrease in its nucleation density, a decrease in its Tc and in ΔHc. Isothermally crystallised PPDX exhibited large banded spherulites whose morphology changed as a function of crystallisation temperature from single banded structures with a very clear Maltese cross to double banded spherulites. PPDX also shows a change in growth regime upon increasing crystallisation temperature (from Regime III to Regime II) according to the kinetic interpretation of growth rate data. BN did not cause any significant modification of the spherulitic growth kinetics (in Regime II) except for a small decrease in surface free energy of PPDX crystals (σe). On the other hand HA was found to increase the spherulitic growth rate and the overall crystallisation rate of PPDX, this increase was caused by a degradation process experienced by the polymer during the treatments involved in isothermal crystallisation that was only present in the samples with HA. It is postulated that the interaction between the phosphate groups on the surface of HA and the ester groups of PPDX are responsible for both the antinucleation effect and the catalysis of the hydrolytic degradation of PPDX.

Crystallisation and morphology of neat and degraded poly(p-dioxanone)

The present work compares the morphology and crystallisation kinetics of neat and hydrolytically degraded poly(p-dioxanone), PPDX. The hydrolytic degradation was performed in a phosphate buffered saline solution (0.2 M, pH 7.4) at 37 °C. The spherulitic morphology and the isothermal spherulitic growth rate were studied by optical microscopy while the overall crystallisation rate was determined by DSC. A thermal fractionation procedure was applied to the samples using the successive self-nucleation and annealing (SSA) protocol. The results indicate that the peculiar spherulitic morphologies exhibited by neat PPDX change as a function of molecular weight loss, but this change can be qualitatively rationalised as a function of the apparent supercooling applied to the crystallising sample. A change in growth regime with supercooling (from regime II to regime III) was detected using Hoffmans kinetic crystallisation theory for neat and degraded PPDX. It was found that both the overall crystallisation rate and the spherulitic growth rate were substantially increased as the molecular weight was decreased by hydrolytic degradation. The SSA results indicate that degradation depletes the longest chains within the molecular weight distribution of neat PPDX first, a fact consistent with an initial attack of the amorphous regions of the sample.

Interaction of fibroblast with poly(p-dioxanone) and its degradation products

In vitro techniques were used to evaluate the interactions between Fibroblastic cells and Poly(p-Dioxanone) PPDX and compared its performance with that of other polymeric substrates. In vitro biocompatibility was assessed by studying cell adhesion and cell growth of cells on the polymer films themselves as well as in media enriched with the degradation products of PPDX and Poly(glycolic)/Poly(L-lactide 90:10 copolymer (PGLA-910). Our results show that althought all polymers tested were suitable for initial attachment, PPDX proved to be the most favorable surface for cell growth; as cell density after 48 h of culture was similar to that obtained on tissue culture Polystyrene TCPS (control). No signs of cell damage were detected using scanning electron microscopy (SEM) where after 48 h. of cell seeding on PPDX, fibroblast exhibited a confluent cell multilayer similar to TCPS. In addition, the products of the hydrolytic degradation of PPDX had no citotoxic effect on the adherence and proliferation of fibroblastic cell on TCPS. The hydrolytic degradation starts in the amorphous regions, as the tie-chain segments in these regions degrade into fragments causing the pH decrease in buffer solution and weight loss of degradable polymers. The in vitro evaluation suggests that PPDX may be candidate biomaterial for the construction a cell-polymer matrix.

Evaluation of the potential of novel PCL–PPDX biodegradable scaffolds as support materials for cartilage tissue engineering

Cartilage is a specialized tissue represented by a group of particular cells (the chondrocytes) and an abundant extracellular matrix. Because of the reduced regenerative capacity of this tissue, cartilage injuries are often difficult to handle. Nowadays tissue engineering has emerged as a very promising discipline, and biodegradable polymeric scaffolds are widely used as tissue supports. In cartilage injuries, the use of autologous chondrocyte implantation from non‐affected cartilage zones has emerged as a very interesting technique, where chondrocytes are expanded in order to obtain a greater number of cells. Nevertheless, it has been reported that chondrocytes in bidimensional cultures suffer a dedifferentiation process. The present study sought, in the first place, to standardize a novel protocol in order to obtain primary cultures of chondrocytes from newborn rabbit hyaline cartilage from the xiphoid process. Second, the potential of porous three‐dimensional (3D) biodegradable polymeric matrices as support materials for chondrocytes was evaluated: a novel poly(ε‐caprolactone)–poly(p‐dioxanone) (PCL–PPDX) blend in a 90:10 w:w ratio and poly(ε‐caprolactone) (PCL). After achieving the standardization, a typical round‐shaped chondrocyte morphology and the expression of collagen type II and aggrecan, evaluated by RT–PCR, were observed. Second‐passage chondrocytes adhered effectively to these scaffolds, although cell growth at 7 days in culture was significantly less in the PCL–PPDX blend. After 3 weeks of culture on PCL–PPDX or PCL, the cells expressed collagen type II. The present study demonstrates the potential, unknown until now, of PCL–PPDX blend scaffolds in the field of cartilage tissue engineering. Copyright

Starch and chitosan oligosaccharides as interpenetrating phases in poly(N-isopropylacrylamide) injectable gels

Thermosensitive interpenetrating gels were prepared by physically blending poly(N-isopropylacrylamide) (PNIPA) as the matrix and the following polysaccharides as interpenetrating phases: chitosan oligosaccharides (identified as QNAD and QNED) and soluble starch (STARCH). The molecular weight of the dispersed phase, the free water/bound water ratio and the thermosensitivity (transition temperature: LCST) of the gels were determined. It was found that these gels are pseudoplastic and that their viscosity depends on the molecular weight of the dispersed phase. LCST transition occurred around 35-37°C. The morphology of the porosity of the freeze-dried samples was studied by Scanning Electron Microscopy (SEM). An in vitro test of cell hemolysis on blood agar showed that these gels are noncytotoxic. According to the results obtained, these interpenetrating gels show characteristics of an injectable material, and have a transition LCST at body temperature, which reinforces their potential to be used in the surgical field and as scaffolds for tissue engineering.

Pilot-scale synthesis and rheological assessment of poly(methyl methacrylate) polymers: Perspectives for medical application

This work presents the rheological assessment of poly(methyl methacrylate) (PMMA) polymers synthesized in a dedicated pilot-scale plant. This material is to be used for the construction of scaffolds via Rapid Prototyping (RP). The polymers were prepared to match the physical and biological properties required for medical applications. Differential Scanning Calorimetry (DSC) and Size Exclusion Chromatography (SEC) measurements verified that the synthesized polymers were atactic, amorphous and linear in chains. Rheological properties such as viscosity, storage and loss modulus, beyond the loss factor, and creep and recovery were measured in a plate-plate sensor within the viscoelastic linear region. The results showed the relevant influence of the molecular weight on the viscosity and elasticity of the material, and how, as the molecular weight increases, the viscoelastic properties are getting closer to those of human bone. This article demonstrates that by using the implemented methodology it is possible to synthesize a polymer, with properties comparable to commercially-available PMMA.

Physicomechanical characterization and biological evaluation of bulk-fill composite resin

The aim of this study was to evaluate the cytotoxic effect, degree of conversion (% DC), Vickers hardness (VH), and surface morphology of composite resins. Eleven resins, nine bulk-fill resins, and two conventional resins were evaluated. Each material was sampled to evaluate DC (using FTIR), VH, cytotoxicity (using MTT and Neutral Red - NR test), surface morphology (using SEM and AFM), and organic filler (using EDS). All statistical tests were performed with SPSS and the level of significance was set at 0.05. MTT revealed that the materials presented low or no cytotoxic potential in relation to the control. Opus was the resin with the lowest cell viability at a 1:2 concentration at 72 h (32%) and at 7 days (43%), but that significantly increased when the NR test was applied at a 1:2 concentration after 7 days. Thickness and surface subjected to polymerization had no influence on DC, and differences were observed only between the materials. In the microhardness test, statistical differences were observed between the evaluated thicknesses. The bulk-fill resins analyzed in this study exhibited low and/or no cytotoxicity to L929 cells, except for Opus, which showed moderate cytotoxicity according to the MTT assay. When the NR test was used, results were not satisfactory for all composites, indicating the need for different methodologies to evaluate the properties of these materials. The assessed resins demonstrated acceptable physicomechanical properties.

Análisis, Diseño y Construcción de una Nueva Alternativa de Fijación Interna para el Quinto Metacarpiano Empleando un Polímero Biodegradable

Intramedullary devices for treatment of bone fractures are an effective available method. A new intramedullary locked nail for the fifth metacarpal bone, using a degradable and biadsorbable polyester Poly(p-dioxanone (PPDX) reforzed with boron nitride (BN), has been designed in order to reduce the immobilization time of the injured zone, enabling faster bone tissue healing and avoiding further surgery. Therefore, this nail allows for earlier functional ability of the bone. The name of the selected polymer is polydyoxanone PPDX o PDS. When it is nucleated with Boron Nitride BN, it’s mechanical and biomedical properties improve, which fulfil design requirements. PPDX belongs to the aliphatic polyesters family approved by the FDA (Food and Drug Administration) and it is bio inert. Intramedullary nail design were 3Dgeometricly modelled through Computer Aided Design (CAD), which allowed the analysis of the prototypes when submitted to static charges by element finite method (EFM). These loads like bending, tension, and compression simulate habitual hand movement forces, reaching solution values approximate to the real system. The EMF results show that both devices fulfil design requirements and the necessary strength provides a total fracture stability BN-reinforced PPDX but not reduce the immobilization time as pretend. However, PPDX/BN guaranty clinical observation during biodegradation and bioadsortion time. Finally, the new intramedullary locked nail mold matrix for the fifth metacarpal bone was manufactured with 1020 steel and injection transference mold was made, by polyurethane for laboratory scale. Then proceed to inject the melted biopolymer into the mold and obtained the intramedullary device.