ABSTRACT
The commercial polydimethysiloxane elastomer Sylgard(®) 184 with mixing ratio 10:1 is in wide use for biomedical research or fundamental studies of mechanobiology. In this paper, a comprehensive study of the large strain mechanical behavior of this material under multiaxial monotonic and cyclic loads, and its change during the first 26 days after preparation is reported. The equibiaxial stress response studied in inflation experiments reveals a much stiffer and more nonlinear response compared to the uniaxial and pure shear characteristics. The polymer revealed remarkably elastic behavior, in particular, very little dependence on strain rates between 0.3%/s and 11%/s, and on the strain history in cyclic experiments. On the other hand, both the small-strain and large strain nonlinear mechanical characteristics of the elastomer are changing with sample age and the results suggest that this process has not ceased after 26 days. A recent re-interpretation of the well-known Ogden model for incompressible rubber-like materials was applied to rationalize the results and accurate agreement was obtained with the experimental data over all testing configurations and testing times. The change of a single parameter in this model is shown to govern the evolution of the nonlinear material characteristics with sample age, attributed to a continuation of the cross-linking process. Based on a kinetic relation to account for this process over time, the model provided successful predictions of the material behavior even after more than one year.
Subject(s)
Silicone Elastomers/chemistry , Rubber , Stress, MechanicalABSTRACT
A randomized controlled trial was performed to compare two techniques of bilateral subtotal thyroidectomy for non-toxic nodular goitre with regard to postoperative parathyroid function. The 50 patients in group 1 underwent ligation of the trunks of the inferior thyroid arteries. In group 2 (50 patients) the branches of these arteries were suture-ligated at the thyroid capsule. Total calcium, ionized calcium and parathyroid hormone levels were determined before operation, and 6, 24 and 72 h after surgery. Ninety-one patients were seen at follow-up 5-10 months after operation. Ten patients in group 1 and 12 in group 2 required calcium and/or vitamin D supplementation for symptomatic hypocalcaemia in the immediate postoperative period. At follow-up only one patient in each group had mild hypoparathyroidism. No statistically significant differences were found between groups regarding total calcium, ionized calcium and parathyroid hormone levels. Truncal ligation of the inferior thyroid arteries during bilateral subtotal thyroidectomy does not cause hypoparathyroidism or hypocalcaemia.
