Improvement of toughness by stereocomplex crystal formation in optically pure polylactides of high molecular weight.

A solution casting method followed by thermal homogenization was performed for the preparation of 1:1 blends and non-blended films from poly(d-lactide) (PDLA) and poly(l-lactide) (PLLA) of three different molecular weights, and their thermal and mechanical properties were determined via differential scanning calorimetry (DSC) and tensile tests. According to the literature, when Mw is below 1.0×10(5)g/mol only stereocomplex crystallization takes place, and when it is higher, both homocrystallites and stereocomplex crystallites co-exist. In order to promote crystallization as a homocrystal in neat polylactides and to promote the stereoselective crystallization as stereocomplex in the case of non-blended films, and in turn, to achieve different degrees of crystallinity, several thermal treatments of annealing were carried out in this work. Highly stereocomplexed blends were found by the stereospecific thermal treatments. As a consequence, the toughness of 1:1 blends was found significantly enhanced over those of non-blended films, irrespective of molecular weight. For instance, in B2-5050 stereocomplexed blend having poly(l-lactide) and poly(d-lactide) of Mw=1.2×10(5)g/mol, tensile strength increased from 44.0±2.1MPa to 65.1±6.1MPa, and the elongation at break from 10.8±2.5% to 33.1±8.1% with respect to its non-blended poly(l-lactide) counterpart crystallized as homocrystal. This improvement in mechanical properties in stereocomplexed blends is not attributed to the inherent properties of the type of crystal polymorph but to the presence of a higher density of intercrystalline connections through a mobile amorphous phase, i.e. tie chains in the stereocomplexed supramolecular spherulitic entities that provide in the stereocomplexed samples enhanced strength and elongation at break at the same time.

Crystallinity assessment and in vitro cytotoxicity of polylactide scaffolds for biomedical applications.

Bioresorbable polylactides are one of the most important materials for tissue engineering applications. In this work we have prepared scaffolds based on the two optically pure stereoisomers: poly(L: -lactide) (PLLA) and poly(D: -lactide) (PDLA). The crystalline structure and morphology were evaluated by DSC, AFM and X-ray diffraction. PLLA and PDLA crystallized in the α form and the equimolar PLLA/PDLA blend, crystallized in the stereocomplex form, were analyzed by a proliferation assay in contact with mouse L-929 and human fibroblasts and neonatal keratinocytes for in vitro cytotoxicity evaluation. SEM analysis was conducted to determine the cell morphology, spreading and adhesion when in contact with the different polymer surfaces. The preserved proliferation rate showed in MTT tests and the high colonization on the surface of polylactides observed by SEM denote that PLLA, PDLA and the equimolar PLLA/PDLA are useful biodegradable materials in which the crystalline characteristics can be tuned for specific biomedical applications.

[Night blindness and xerophthalmia after surgery for morbid obesity]. / Ceguera nocturna y xeroftalmía tras cirugía de obesidad mórbida.

CASE REPORT: We describe ocular findings due to a vitamin A deficiency in a 50-year-old man. The patient had undergone intestinal bypass surgery two years before. After therapy with oral vitamin A the symptoms improved. DISCUSSION: The incidence of morbid obesity is increasing throughout much of the developed world, with intestinal bypass surgery the treatment of choice for most people with the condition. This type of surgery can lead to a vitamin A deficiency, with remarkable ophthalmological consequences, which without correct treatment may ultimately cause blindness. For this reason, the pathology, even though rare at present, must considered seriously.

Ceguera nocturna y xeroftalmía tras cirugía de obesidad mórbida / Night blindness and xerophthalmia after surgery for morbid obesity

Caso clínico: Se describen los hallazgos oculares debidos a un déficit de vitamina A en un varón de 50 años. Fue sometido a una cirugía de bypass intestinal dos años antes. Después de tratamiento con vitamina A oral, los síntomas mejoraron. Discusión: La incidencia de obesidad mórbida sigue aumentando alarmantemente en todo el mundo, siendo actualmente el tratamiento mas adoptado en la mayoría de los pacientes la cirugía de bypass intestinal. Este tipo de cirugía puede provocar un déficit de vitamina A, con importantes consecuencias oftalmológicas, que podrían incluso llegar a la ceguera sin un correcto tratamiento. Por consiguiente, se debería considerar seriamente esta patología hasta ahora poco frecuente en países desarrollados

Case report: We describe ocular findings due to a vitamin A deficiency in a 50-year-old man. The patient had undergone intestinal bypass surgery two years before. After therapy with oral vitamin A the symptoms improved. Discussion: The incidence of morbid obesity is increasing throughout much of the developed world, with intestinal bypass surgery the treatment of choice for most people with the condition. This type of surgery can lead to a vitamin A deficiency, with remarkable ophthalmological consequences, which without correct treatment may ultimately cause blindness. For this reason, the pathology, even though rare at present, must considered seriously

Conformational behavior of poly(L-lactide) studied by infrared spectroscopy.

This paper reports the analysis of the C=O stretching region of poly(L-lactide). This spectral band splits into up to four components, a phenomenon that a priori can be explained in terms of carbonyl-carbonyl coupling or specific interactions (such as C-H...O hydrogen bonding or dipole-dipole). Hydrogen bonding can be discarded from the analysis of the C-H stretching spectral region. In addition, low molecular weight dicarbonyl compounds of chemical structure similar to that of PLLA, such as diacyl peroxides, show a remarkable splitting of the carbonyl band attributed to intramolecular carbonyl-carbonyl coupling. Several mechanisms can be responsible for this behavior, such as mechanical coupling, electronic effects, or through-space intramolecular TDC (transition dipole coupling) interactions. Intermolecular dipole-dipole interactions (possible in the form of interchain TDC interactions) are proven to be of minor relevance taking into account the spatial structure of the PLLA conformers. The Simply Coupled Oscilator (SCO) model, which only accounts for mechanical coupling, has been found to predict adequately the relative intensity of the symmetric and asymmetric bands of dicarbonyl compounds. The dispersion curves predicted for PLLA by the SCO model also match those given by more general treatments, such as Miyazawa's first-order perturbation theory. Hence, the SCO model is adopted here as an adequate yet simple tool for the interpretation of band splitting caused by intramolecular coupling of polylactide. The four components observed in the C=O stretching band of semicrystalline PLLA are attributed to the four possible conformers: gt, gg, tt, and tg. The narrow bands observed for the interlamellar material are attributed to highly ordered chains, indicating the absence of a truly amorphous phase in the crystalline polymer. The interphase seems to extend over the whole interlamellar region, showing the features of a semiordered metastable phase. In amorphous PLLA, bands corresponding to gt, gg, and tt conformers also can be resolved by second derivative techniques, and curve-fitting results provide information about the conformational population at different temperatures.