Membrane integrity testing is important in in-vitro percutaneous absorption studies. OECD Guidelines recommend that barrier integrity be tested before permeation experiments are carried out [1, 2]. TEWL measurements are recognised as a valid alternative approach to tritiated water flux measurements.

 

TEWL methods have the unique advantage of testing membranes under conditions that resemble normal in-vivo skin:- dry on top, wet underneath. Recent studies have shown that TEWL is a good alternative to tritiated water flux and that the AquaFlux™ was particularly sensitive in such tests [3, 4].

 

You perform an AquaFlux™ membrane integrity test by sealing the measurement head to the donor chamber via an appropriate push-fit coupling, as illustrated on the right. This has the following advantages:-
1.	Reproducible measurement conditions.
2.	Calibrated against in-vivo skin barrier [5].
3.	Testing the whole membrane without contact.
4.	In-situ testing, in the same set-up as the subsequent permeation experiment.

 

In practice, assemble the Franz cell with the membrane and acceptor liquid in place. Then dry out the donor chamber and acclimatise the membrane to ensure that it mimics in-vivo skin, ie dry on top, wet underneath. Use a fan to to accelerate drying. When ready, couple to the AquaFlux™ probe and initiate a TEWL measurement in the normal way. Note that you may need to allow for longer settling times in the software because of the increased distance between the measurement head and the membrane compared with normal in-vivo measurements.

 

References
1.	OECD test guideline 428: Skin absorption: In-vitro method: 2004. Click here to download the pdf.
2.	OECD series on testing and assessment, No. 28: Guidance document for the conduct of skin absorption studies: 2004. Click here to download the pdf.
3.	Correlation of Transepidermal Water Loss with Skin Barrier Properties in-vitro: Comparison of three Evaporimeters R. Elkeeb, X. Hui, H. Chan, L. Tien & H.I. Maibach. Skin Res Tech, 16(1), 9–15, 2010.
4.	Transepidermal Water Loss for Probing Full-thickness Skin Barrier Function: Correlation with Tritiated Water Flux, Sensitivity to Punctures and Diverse Surfactant Exposures E. Elmahjoubi, Y. Frum, G.M. Eccleston, S.C. Wilkinson & V.M. Meidan. Toxicology in Vitro, 23(7), 1429-35, 2009.
5.	Calibration of Franz Cell Membrane Integrity Test by the TEWL Method R.E. Imhof, L.I. Ciortea & P. Xiao. 13th International PPP Conference, La Grande Motte, France 2012. Click here to download the pdf.