Comparative evaluation of the biocompatible and physical-chemical properties of poly(lactide-co-glycolide) and polydopamine as coating materials for bacterial cellulose.

The aim of this study was to investigate the influence of poly(lactide-co-glycolide) (PLGA) and polydopamine (PDA) as coating materials on the tensile strength, surface performance, in vitro cell behavior and the in vivo material-tissue reaction of bacterial cellulose (BC) membranes. The coated membranes were characterized by Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM) and scanning electron microscopy (SEM), demonstrating that the PDA forms the dispersion phase and penetrates into the BC substrates while PLGA only adheres to the BC surface. Mechanical tests and fractured surface morphology reveal that penetration of PDA into BC membranes enhances the mechanical strength by strongly bonding the nanofibers. The PLGA coated BC membrane obtained by adhesion debonds from the BC substrate under stress, leading to a decrease in the mechanical strength of the membrane. The in vitro cell culture experiments were established to assess cell attachment and spreading by SEM and DAPI (4',6-diamidino-2-phenylindole) staining and expression of collagen I, which showed a better performance on the PDA-BC than on the PLGA-BC and bare BC membranes. However, the in vivo results of the rabbit back implantation indicated that BC membrane surface modification by PDA is not effective for cell proliferation and collagen accumulation when compared to bare and PLGA coated BC, whereas PLGC-BC were surrounded by a thicker layer of connective tissues with slight neovascularization demonstrating superior tissue integration. PDA based materials still have a long way to go before clinical applications. However, PLGA coating has excellent biocompatibility in clinical as well as in experimental use.

High corrosion resistance and weak corrosion anisotropy of an as-rolled Mg-3Al-1Zn (in wt.%) alloy with strong crystallographic texture.

Combining with electrochemical corrosion measurements, immersion and hydrogen evolution testing performed in 0.9 wt.% NaCl solution at 37 °C, the corrosion resistance of an as-rolled Mg-3%Al-1%Zn alloy before and after a 3% compressive strain along the rolling direction was investigated. Results revealed that the corrosion behavior of differently oriented surfaces of the as-rolled samples with a strong basal texture was obviously different. Among them, the corrosion rate of sample surface with the orientation parallel to the normal direction (ND) of the plate was the fastest, the corrosion rate of sample surface with the orientation parallel to the rolling direction (RD) of the plate took the second place and the corrosion rate of sample surface with the orientation parallel to the transverse direction (TD) was the slowest. After being pre-strained, the activation of a high density of {10-12} twins could remarkably reduce the corrosion rate of surrounding α-Mg matrix and simultaneously weaken the corrosion anisotropy between differently oriented samples. The main reason was that similar to grain boundaries, twin boundaries acted as physical barriers to the corrosion attack. Moreover, the activated twins increased the protectiveness of surface films and then suppressed the micro corrosion couples occurred in twinned grains.

Natural ageing responses of duplex structured Mg-Li based alloys.

Natural ageing responses of duplex structured Mg-6%Li and Mg-6%Li-6%Zn-1.2%Y alloys have been investigated. Microstructural analyses revealed that the precipitation and coarsening process of α-Mg particles could occur in ß-Li phases of both two alloys during ageing process. Since a certain amount of Mg atoms in ß-Li phases were consumed for the precipitation of abundant tiny MgLiZn particles, the size of α-Mg precipitates in Mg-6%Li-6%Zn-1.2%Y alloy was relatively smaller than that in Mg-6%Li alloy. Micro hardness measurements demonstrated that with the ageing time increasing, the α-Mg phases in Mg-6%Li alloy could have a constant hardness value of 41 HV, but the contained ß-Li phases exhibited a slight age-softening response. Compared with the Mg-6%Li alloy, the age-softening response of ß-Li phases in Mg-6%Li-6%Zn-1.2%Y alloy was much more profound. Meanwhile, a normal age-hardening response of α-Mg phases was maintained. Tensile results indicated that obvious ageing-softening phenomenon in terms of macro tensile strength occurred in both two alloys. Failure analysis demonstrated that for the Mg-6%Li alloy, cracks were preferentially initiated at α-Mg/ß-Li interfaces. For the Mg-6%Li-6%Zn-1.2%Y alloy, cracks occurred at both α-Mg/ß-Li interfaces and slip bands in α-Mg and ß-Li phases.

Effect of solution treatment on stress corrosion cracking behavior of an as-forged Mg-Zn-Y-Zr alloy.

Effect of solid solution treatment (T4) on stress corrosion cracking (SCC) behavior of an as-forged Mg-6.7%Zn-1.3%Y-0.6%Zr (in wt.%) alloy has been investigated using slow strain rate tensile (SSRT) testing in 3.5 wt.% NaCl solution. The results demonstrated that the SCC susceptibility index (ISCC) of as-forged samples was 0.95 and its elongation-to-failure (εf) was only 1.1%. After T4 treatment, the SCC resistance was remarkably improved. The ISCC and εf values of T4 samples were 0.86 and 3.4%, respectively. Fractography and surface observation indicated that the stress corrosion cracking mode for as-forged samples was dominated by transgranular and partially intergranular morphology, whereas the cracking mode for T4 samples was transgranular. In both cases, the main cracking mechanism was associated with hydrogen embrittlement (HE). Through alleviating the corrosion attack of Mg matrix, the influence of HE on the SCC resistance of T4 samples can be greatly suppressed.

The relationship between microstructure and in vivo degradation of modified bacterial cellulose sponges.

Bacterial cellulose (BC) and hydroxyapatite (HA) possess unique structures and excellent biocompatibility. Considerable work has been performed to develop composites that promote bone repair. However, the use of BC/HA composites is limited because the lack of corresponding enzymes makes them non-degradable in vivo. In the present study, C6-carboxylated bacterial cellulose (TBC) was prepared in a bromide-free system. Several composite methods of TBC and HA are compared, including in situ formation, physical mixing and biomineralization. Composite sponges prepared by different methods were characterized by tensile testing, X-ray diffraction, Fourier transform infrared spectroscopy, scanning electron microscopy and in vivo degradation. The structural anisotropy of various sponges was analyzed to quantitatively evaluate their microstructure. The results suggest that the interaction between HA and TBC nanofibers has a large influence on microstructure and macroscopic properties. Moreover, the structural anisotropy and the speed of granulation ingrowth were strongly interdependent. This improved understanding of slowly degrading BC-based materials suggests that modified cellulose-based materials can be made degradable by altering their microstructure.