Molecular field theory for biaxial nematics formed from liquid crystal dimers and inhibited by the twist-bend nematic.

Liquid crystal dimers with odd spacers are good candidates as materials for biaxial nematic phases (NB). The dimers are flexible molecules sustaining biaxial conformations, and couplings between the conformational and orientational distributions could be expected to stabilise NB. We apply a molecular field theory for flexible molecules developed elsewhere to study a simple system made up of dimers composed of two cylindrically symmetric mesogenic groups. Our model allows for two idealised conformations: one linear and one bent at a tetrahedral angle. For a restricted set of chain lengths, the model predicts a first-order reentrant phase transition from the NB phase into a low temperature uniaxial nematic phase (NU). However the formation of the biaxial nematic could be blocked by the appearance of a twist-bent nematic.

Molecular field theory for biaxial smectic A liquid crystals.

Thermotropic biaxial nematic phases seem to be rare, but biaxial smectic A phases less so. Here we use molecular field theory to study a simple two-parameter model, with one parameter promoting a biaxial phase and the second promoting smecticity. The theory combines the biaxial Maier-Saupe and McMillan models. We use alternatively the Sonnet-Virga-Durand (SVD) and geometric mean approximations (GMA) to characterize molecular biaxiality by a single parameter. For non-zero smecticity and biaxiality, the model always predicts a ground state biaxial smectic A phase. For a low degree of smectic order, the phase diagram is very rich, predicting uniaxial and biaxial nematic and smectic phases, with the addition of a variety of tricritical and tetracritical points. For higher degrees of smecticity, the region of stability of the biaxial nematic phase is restricted and eventually disappears, yielding to the biaxial smectic phase. Phase diagrams from the two alternative approximations for molecular biaxiality are similar, except inasmuch that SVD allows for a first-order isotropic-biaxial nematic transition, whereas GMA predicts a Landau point separating isotropic and biaxial nematic phases. We speculate that the rarity of thermotropic biaxial nematic phases is partly a consequence of the presence of stabler analogous smectic phases.

Biaxiality-induced magnetic field effects in bent-core nematics: molecular-field and Landau theory.

Nematic liquid crystals composed of bent-core molecules exhibit unusual properties, including an enhanced Cotton-Mouton effect and an increasing isotropic (paranematic)-nematic phase transition temperature as a function of magnetic field. These systems are thought to be good candidate biaxial liquid crystals. Prompted by these experiments, we investigate theoretically the effect of molecular biaxiality on magnetic-field-induced phenomena for nematic liquid crystals, using both molecular field and Landau theory. The geometric mean approximation is used in order to specify the degree of molecular biaxiality using a single parameter. We reproduce experimental field-induced phenomena and predict also an experimentally accessible magnetic critical point. The Cotton-Mouton effect and temperature dependence of the paranematic-nematic phase transition are more pronounced with increased molecular biaxiality. We compare our theoretical approaches and make contact with recent relevant experimental results on bent-core molecular systems.

Dexon plus versus Maxon fascial closure in morbid obesity: a prospective randomized comparison.

A monofilament polyglyconate suture (Maxon) was developed as a longer-lasting suture associated with less potential for infection than braided polyglycolic acid suture (Dexon). The authors compared Dexon Plus and Maxon in a prospective randomized study of 84 consecutive morbidly obese patients who underwent vertical banded gastroplasty. Linea alba was closed with continuous running suture, reinforced with a few interrupted, sutures. After more than 2 years' follow-up, one seroma (2.4%), one wound infection (2.4%) and no hernias were found in the Dexon group. In contrast in the Maxon group there were three seromas (7.1%), four wound infections (9.5%) and four incisional hernias (9.5%, p less than 0.05). The hernias were noted between 2 and 10 1/2 months after the operation in two patients who had had wound infections and in two who had had seromas. When Maxon was used, the needle occasionally pulled off the suture and the ends beyond the knot projected rigidly. Dexon Plus was easier to handle in very fatty wounds.

Physical stability of a total nutrient admixture for total parenteral nutrition.

The stability of a total nutrient admixture (TNA) has been postulated to be less than 7 days in refrigerated storage. When a TNA destabilizes, lipid particles coalesce and enlarge. Liposomes larger than 6 microns can obstruct pulmonary capillaries. A TNA containing 1500 ml of 7% Vamin, 1000 ml of 50% dextrose and 500 ml of 10% Intralipid, including the usual electrolytes, minerals and vitamins, was studied. Liposome size was measured in the original Intralipid and the TNA at intervals up to 14 days at 4 degrees C followed by 2 days at 22 degrees C. There was a small increase in liposome size up to 16 days. However, the number of particles larger than 6 microns was insignificant (by light microscopy, 3.9 +/- 2.4 [+/- SD] per 20 high-power fields; by Coulter counter, 99.8% smaller than 1.9 microns, with 0% larger than 6 microns; and by electron microscopy, 100% smaller than 2.0 microns). The osmolality and pH of the TNA were 1472 +/- 31 mOsm/kg and 5.5 +/- 0.1 respectively (mean +/- SD), with no significant change during the study times. The authors concluded that this TNA remains physically stable when refrigerated for 14 days and at room temperature for a further 2 days.

Percutaneous endoscopic gastrostomy by the "pull" and "introducer" methods.

After 28 patients, studied prospectively, underwent percutaneous endoscopic gastrostomy (PEG) by the Ponsky "pull" technique, another 28 patients underwent PEG by the Russell "introducer" method. These two groups were compared retrospectively with 28 patients who had previously undergone Stamm gastrostomy as an independent operation performed by the same group of surgeons. All procedures were done within a 5-year period. The major indication for gastrostomy was the inability to swallow due to neurologic diseases; a similar number of patients in each group also had pharyngeal blockage, cachexia, respiratory failure and inhalation burns. Percutaneous endoscopic gastrostomy could be done most appropriately under local anesthesia, the "introducer" method requiring less time than the "pull" method, which in turn required less time than the Stamm gastrostomy. Feeding was generally instituted successfully 24 hours after PEG compared with 3 days after Stamm gastrostomy. "Introducer" PEG was not associated with peristomal infection, and the authors postulate that the peristomal infections in the "pull" PEG group were due to oropharyngeal bacteria brought through the abdominal wall by that technique.

Major intestinal complications of radiotherapy. Management and nutrition.

Hospitalization was required in 57 patients for intestinal injuries following radiotherapy for carcinoma of the cervix, endometrium, ovary, bladder, rectum, and other primary sites. Intestinal complications included stenosis, perforation, rectal ulcer, and rectovaginal, ileovaginal, and ileovesical fistula; 27 patients had multiple intestinal complications. Operation was necessary in 33 patients, as follows: bowel resections, 18; colostomy alone, five; adhesiolysis, five; ileocolic bypass, three; and Hartmann's procedure for sigmoid perforation, two. Five anastomotic leaks and six postoperative deaths occurred. Causes of death among the remaining patients included residual cancer (ten), de novo bowel cancer (two), radiation injury (four), and unrelated causes (six). Resection to uninvolved bowel, omental wrap of anterior resection anastomosis, avoidance of unnecessary adhesiolysis, and long-tube orientation seemed to contribute to successful operations. Nutritional support was used for repletion, post-operative fistulas, and short-gut syndrome.

Sex hormonal changes accompanying loss of massive excess weight.

With adequate weight loss following bariatric surgery, we previously found correction of the clinical gynecologic abnormalities associated with obesity. In this study, sex hormonal changes offered an explanation for the clinical findings.