Tetracaine, surface activity

This ester-type anesthetic is a combination of tetracaine HCl (2%), butyl aminobenzoate (2%), and benzocaine (14%). Benzalkonium chloride and cetyl dimethylammonium bromide are included as surface-active agents to facilitate the passage of benzocaine into the mucosal tissues. 

Salts of mineral acids (particularly polybasic acids), phenols, and tannins will coagulate solutions of methylcellulose, although this can be prevented by the addition of ethanol (95%) or glycol diacetate. Complexation of methylcellulose occurs with highly surface-active compounds such as tetracaine and dibutoline sulfate. 

Active substances such as antazoline and tetracaine reduce the surface tension. Eye drops with a surface tension below 35 mN/m (normal surface tension of lachrymal fluid is 40-46 mN/m) are painful and uncomfortable [30]. 

Since absorption of local anesthetics through the skin is enhanced by heat, a topical patch with an intrinsic heating system was developed [4], This incorporates a controlled heat-assisted delivery system that is physically separated from a eutectic mixture of 70 mg of lidocaine and 70 mg of tetracaine. The surface area of the entire Synera patch is approximately 50 cm, 10 cm of which is active. 

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