The Use of Percutanteous Absorption Analysis
Regardless of how promising a botanically derived compound may seem to be from screening studies using cell culture models, there is no guarantee that this compound will be beneficial when applied topically. Such potential problems as compatibility with formulation ingredients, inability to penetrate through the stratum corneum, inherent instability, etc. can prevent a potential candidate from ever being developed into a product. One of the first tests that is conducted on any potential "candidate" is to incorporate it into a topical formulation and test its ability to move into the stratum corneum. To conduct this test, a technique called Percutaneous Absorption Analysis (commonly called Franz Cell Testing) is used.
What is Franz Cell Analysis?
A Franz cell is a specially designed apparatus that allows for the determination of the precise amount of any compound that can penetrate through either human skin (the preferred method), or through a synthetic membrane. To obtain the most useful and predictive data, human skin is used . A diagram of a Franz cell unit is here.
As shown in the diagram, a section of human skin is sandwiched between an upper donor chamber and a lower receptor chamber. The lower receptor chamber is filled with fluid and is equilibrated to a temperature of 37 C (normal body temperature) by the circulating water jacket. The test formulation is applied to surface of the skin through the upper chamber and samples of fluid in the lower receptor chamber are taken at specific time intervals (ex. 4, 8, 12 and 24 hours). The samples from the receptor chamber are then analyzed by HPLC to quantify the amount of any ingredient (bioactive) that has penetrated through the skin and entered the receptor chamber.