Transcranial Doppler screening of Medicaid-insured children with sickle cell disease.

Transcranial Doppler screening reduces the risk of stroke in children with sickle cell disease. We tested the effect of informational letters sent to parents and doctors of Medicaid-insured children on improving screening efficiency. The letters did not improve the low baseline screening rates, suggesting the need for more aggressive outreach. Hematologist visits were correlated with increased screening rates.

Dielectric and other properties of polyimide aerogels containing fluorinated blocks.

The dielectric and other properties of a series of low-density polyimide block copolymer aerogels have been characterized. Two different anhydride-capped polyimide oligomers were synthesized: one from 2,2-bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride (6FDA) and 4,4'-oxidianiline (ODA) and the other from biphenyl-3,3',4,4'-tetracarboxylic dianhydride and ODA. The oligomers were combined with 1,3,5-triaminophenoxybenzene to form a block copolymer networked structure that gelled in under 1 h. The polyimide gels were supercritically dried to give aerogels with relative dielectric constants as low as 1.08. Increasing the amount of 6FDA blocks by up to 50% of the total dianhydride decreased the density of the aerogels, presumably by increasing the free volume and also by decreasing the amount of shrinkage seen upon processing, resulting in a concomitant decrease in the dielectric properties. In this study, we have also altered the density independent of fluorine substitution by changing the polymer concentration in the gelation reactions and showed that the change in dielectric due to density is the same with and without fluorine substitution. The aerogels with the lowest dielectric properties and lowest densities still had compressive moduli of 4-8 MPa (40 times higher than silica aerogels at the same density), making them suitable as low dielectric substrates for lightweight antennas for aeronautic and space applications.

Low dielectric polyimide aerogels as substrates for lightweight patch antennas.

The dielectric properties and loss tangents of low-density polyimide aerogels have been characterized at various frequencies. Relative dielectric constants as low as 1.16 were measured for polyimide aerogels made from 2,2'-dimethylbenzidine (DMBZ) and biphenyl 3,3',4,4'-tetracarbozylic dianhydride (BPDA) cross-linked with 1,3,5-triaminophenoxybenzene (TAB). This formulation was used as the substrate to fabricate and test prototype microstrip patch antennas and benchmark against state of practice commercial antenna substrates. The polyimide aerogel antennas exhibited broader bandwidth, higher gain, and lower mass than the antennas made using commercial substrates. These are very encouraging results, which support the potential advantages of the polyimide aerogel-based antennas for aerospace applications.