Károly Renner

Biocomposite from polylactic acid and lignocellulosic fibers: structure-property correlations

PLA biocomposites were prepared using three corn cob fractions and a wood fiber as reference. The composites were characterized by tensile testing, scanning electron microscopy (SEM) and polarization optical microscopy (POM). Micromechanical deformation processes were followed by acoustic emission measurements. The different strength of the components was proved by direct measurements. Two consecutive micromechanical deformation processes were detected in composites containing the heavy fraction of corncob, which were assigned to the fracture of soft and hard particles, respectively. The fracture of soft particles does not result in the failure of the composites that is initiated either by the fracture of hard particles or by matrix cracking. Very large particles debond easily from the matrix resulting in catastrophic failure at very low stresses. At sufficiently large shear stresses large particles break easily during compounding, thus reinforcement depending on interfacial adhesion was practically the same in all composites irrespectively of initial fiber characteristics.

Thermoplastic starch/wood composites: Interfacial interactions and functional properties

Thermoplastic starch (TPS)/wood composites were prepared from starch plasticized with 36 wt% glycerol. The components were homogenized by dry-blending, extruded and injection molded to tensile bars. Tensile properties, structure, deformation, water adsorption and shrinkage were determined as a function of wood content, which changed between 0 and 40 vol% in 7 steps. The modification of TPS with wood particles improves several properties considerably. Stiffness and strength increases, and the effect is stronger for fibers with larger aspect ratio. Wood fibers reinforce TPS considerably due to poor matrix properties and strong interfacial interactions, the latter resulting in the decreased mobility of starch molecules and in the fracture of large wood particles during deformation. Strong interfacial adhesion leads to smaller water absorption than predicted from additivity, but water uptake remains relatively large even in the presence of wood particles. The shrinkage of injection molded TPS parts is very large, around 10%, and dimensional changes occur on a very long timescale of several hundred hours. Shrinkage decreases to a low level already at 15-20 vol% wood content rendering the composites good dimensional stability.

Adhesion and micromechanical deformation processes in PLA/CaSO4 composites

PLA/CaSO4 composites were prepared from uncoated and stearic acid coated filler particles in a wide composition range. The strength of interfacial adhesion was estimated quantitatively with three independent methods. Structure was characterized by DSC, XRD and SEM measurements, while mechanical properties by tensile and instrumented impact tests. The results proved that adhesion is twice as strong in composites prepared with the uncoated particles than in those containing the coated filler. Coating changes also local deformation processes around the particles. Although debonding is the dominating micromechanical deformation process in all composites, local plastic deformation is larger around coated particles. The extent of this deformation depends very much also on the local distribution of particles. The final properties and performance of the composites depend unambiguously on the micromechanical deformation processes occurring during loading, on debonding and the subsequent plastic deformation. Stearic acid used for the coating of the filler seems to dissolve in the polymer and locally change its properties.

Serum albumin coating of demineralized bone matrix results in stronger new bone formation

Blood serum fractions are hotly debated adjuvants in bone replacement therapies. In the present experiment, we coated demineralized bone matrices (DBM) with serum albumin and investigated stem cell attachment in vitro and bone formation in a rat calvaria defect model. In the in vitro experiments, we observed that significantly more cells adhere to the serum albumin coated DBMs at every time point. In vivo bone formation with albumin coated and uncoated DBM was monitored biweekly by computed tomography until 11 weeks postoperatively while empty defects served as controls. By the seventh week, the bone defect in the albumin group was almost completely closed (remaining defect 3.0 ± 2.3%), while uncoated DBM and unfilled control groups still had significant defects (uncoated: 40.2 ± 9.1%, control: 52.4 ± 8.9%). Higher density values were also observed in the albumin coated DBM group. In addition, the serum albumin enhanced group showed significantly higher volume of newly formed bone in the microCT analysis and produced significantly higher breaking force and stiffness compared to the uncoated grafts (peak breaking force: uncoated: 15.7 ± 4 N, albumin 46.1 ± 11 N). In conclusion, this investigation shows that implanting serum albumin coated DBM significantly reduces healing period in nonhealing defects and results in mechanically stronger bone. These results also support the idea that serum albumin coating provides a convenient milieu for stem cell function, and a much improved bone grafting success can be achieved without the use of exogenous stem cells.

Hydrogen bonding interactions in poly(ethylene-co-vinyl alcohol)/lignin blends

Blends were prepared from lignin and ethylene-vinyl alcohol (EVOH) copolymers to study the effect of hydrogen bonding interactions on compatibility and structure. The vinyl alcohol (VOH) content of the copolymers changed between 52 and 76 mol%, while the lignin content of the blends varied between 0 and 60 vol%. Low density polyethylene with 0 mol% VOH content was used as reference. The components were homogenized in an internal mixer and they were characterized by various methods including Fourier transform infrared spectroscopy (FTIR), dynamic mechanical analysis, differential scanning calorimetry and scanning electron microscopy. The results of the experiments proved that strong hydrogen bonds form between the two components shown by FTIR spectroscopy, a shift in the relaxation temperatures of the matrix polymer and by the decrease of crystallite size and crystallinity with increasing lignin content. In spite of the strong interactions, heterogeneous structure forms in the studied blends since self-interactions within the neat components are also very strong. The size of dispersed lignin particles is determined by competitive interactions in the blends; stronger EVOH/lignin interactions result in smaller particle size. Although hydrogen bonds are strong, miscible polymer/lignin blends can be prepared only by applying other approaches like plasticization or chemical modification.

Absorption and Tensility of Bioactive Sutures Prepared for Cell Transplantation

Biodegradable scaffolds are widely used to transplant stem cells into various tissues. Recent studies showed that living stem cells can be attached to the surface of absorbable sutures in vitro. Soaking the absorbable material polyglactin in a cell culture medium and thereby creating a stem cell biofilm on its surface may initiate the absorption process even before implantation; therefore, the physicochemical properties of the suture may be compromised in vivo. We found that pre-incubation of sutures in cell culture media in vitro results in tensile strength reduction and faster suture absorption in a rat model of muscle injury. Shorter incubation times of up to 48 h do not influence absorption or tensility; therefore, it is advisable to limit incubation times to two days for polyglactin-based cell delivery protocols.