[show abstract][hide abstract] ABSTRACT: We examine particle production from spherical bodies collapsing into extremal Reissner-Nordstr\"om black holes. Kruskal coordinates become ill-defined in the extremal case, but we are able to find a simple generalization of them that is good in this limit. The extension allows us to calculate the late-time worldline of the center of the collapsing star, thus establishing a correspondence with a uniformly accelerated mirror in Minkowski spacetime. The spectrum of created particles associated with such uniform acceleration is nonthermal, indicating that a temperature is not defined. Moreover, the spectrum contains a constant that depends on the history of the collapsing object. At first sight this points to a violation of the no-hair theorems; however, the expectation value of the stress-energy-momentum tensor is zero and its variance vanishes as a power law at late times. Hence, both the no-hair theorems and the cosmic censorship conjecture are preserved. The power-law decay of the variance is in distinction to the exponential fall-off of a nonextremal black hole. Therefore, although the vanishing of the stress tensor's expectation value is consistent with a thermal state at zero temperature, the incipient black hole does not behave as a thermal object at any time and cannot be regarded as the thermodynamic limit of a nonextremal black hole, regardless of the fact that the final product of collapse is quiescent. Comment: 13 pages, 2 epsf figures, RevTeX 3. Minor changes, version published in PRD
[show abstract][hide abstract] ABSTRACT: The poly(lactide-co-glycolide) (PLGA) sponge fabricated by a gelatin porogen leaching method was filled with fibrin gel to obtain a hybrid scaffold
for chondrocytes culture in vitro. The fibrin gel evenly distributed in the hybrid scaffold with visible fibrinogen fibers after drying. In vitro culture it was found that in the hybrid scaffold the chondrocytes distributed more evenly and kept a round morphology as
that in the normal cartilage. Although the chondrocytes seeded in the control PLGA sponges showed similar proliferation behavior
with that in the hybrid scaffolds, they were remarkably elongated, forming a fibroblast-like morphology. Moreover, a larger
amount of glycosaminoglycans was secreted in the hybrid scaffolds than that in the PLGA sponges after in vitro culture of chondrocytes for 4 weeks. The results suggest that the fibrin/PLGA hybrid scaffold may be favorably applied for
cartilage tissue engineering.
KeywordsCartilage tissue engineering–PLGA sponge–Fibrin gel–Chondrocytes
Chinese Journal of Polymer Science 05/2012; 29(2):233-240.