Permeability frequency effect

X (p in Q cm) as in Table 30.1 /= frequency of the system in Hz p = effective permeability of the medium in which the field exists (aluminium in the present case), and will depend upon the electric field induced in the enclosure... 

Ramundo-Orlando, A., Mattia, E, Palombo, A., and D Inzeo, G. (2000). Effect of low frequency, low amplitude magnetic fields on the permeability of cationic liposomes entrapping carbonic anhydrase, part II. Bioelectromagnetics, 21,499-507. 

Tezel A, Sens A, Tuchscherer J, Mitragotri S. Synergistic effect of low-frequency ultrasound and surfactants on skin permeability. J Pharm Sci 2002 91 91-100. 

As frequency increases, the current is forced out of the center of the conductor toward its periphery, a phenomenon known as the skin effect . A measure of the depth of penetration of the current into the conductor is the skin depth, defined as 8 = V(p/ir/p,), where / is the frequency and x is the conductor permeability (1.26 X 10 6 H/m for nonmagnetic conductors). For copper, the skin depth is 2 p,m at 1 GHz. When the skin depth is less than the conductor thickness, the line resistance becomes greater than the dc resistance. 

PORE ROUGHNESS EFFECTS ON HIGH-FREQUENCY PERMEABILITY... 

Ultrasound cleaning has also been used to remove fouling from UF polysulphone and MF cellulose membranes used to treat peptone and milk aqueous solutions, respectively. The US employed had 28, 45 and 100 kHz frequency with 23 W/cm output power. With 28-kHz US, water was found to be effective for recovery from a deteriorating condition due to fouling US-enhanced permeability of membranes was also observed. It is worth noting... 

Ultrasound may enhance transdermal transport by inducing skin alteration and active transport (forced convention) in the skin. Various other means of transport enhancement, including chemicals, iontophoresis and electroporation, may enhance transport synergis-tically with US. Thus, the evaluation of the synergistic effect of low-frequency US with chemical enhancers and surfactants for permeation of mannitol revealed that application of US or sodium lauryl sulfate (SLS) alone, both for 90 min, increased skin permeability about 8 and 3 times, respectively. However, the combined use of US and a 1% SLS solution increased the skin permeability 200 times to mannitol [129]. 

This paper reports an investigation of the effects of porous solid structures on their electrical behaviour at different frequencies (from 100 Hz to 100 kHz). For that, we study different parameters such as formation resistivity factor, cementation factor, chargeability, resistivity index and saturation exponent. Different porous solid structures are quantified from the petrographic image analysis and Hg-injection technique. Then, by using different models we obtain the permeability prediction from the electrical behaviour and structure parameters. 

Additional work, however, has addressed mechanistic aspects of the effects of low-frequency US. Cavitation and thermal effects have been postulated and, to a certain extent, characterized, but further work is clearly needed to define exactly how US interacts with the skin barrier to increase its permeability. 

In a recent systematic study of the dependence of 20 kHz sonophoresis on ultrasound parameters, Mitragotri et al. showed that the enhancement of skin permeability varies linearly with ultrasound intensity and ultrasound on-time (for pulsed ultrasound, ultrasound on-time equals the product of total ultrasound application time and duty cycle), while is independent of the ultrasound duty cycle. Based on those findings, fhe authors reported that there is a threshold energy dose for ultrasound induced transdermal drug transport. Once the threshold value is crossed, the enhancement of skin permeability varies linearly with the ultrasound energy dose (J/cm ), which is calculated as the product of ultrasound intensity and ultrasound on-time. This result indicates that ultrasound energy dose can be used as a predictor of the effect of 20 kHz sonophoresis. The authors also indicated that it is important to determine the threshold energy dose for each individual sonophoresis system, for example, the real in vivo situation, because it may vary from system to system. Specifically, it may vary between different skin models, as well as with the ultrasound frequency and the distance of the transducer from the skin surface, etc. 

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