Category: Formulation and Quality
Purpose: Petrolatum is a purified mixture of semisolid hydrocarbons obtained from petroleum. Commonly used as an ointment base or skin protectant, petrolatum can vary in color, composition, and consistency depending on the material source and refinement process. We sought to create a hydrophobic ointment base that could be used as an alternative to petrolatum with enhanced performance attributes and an improved sensory profile.
Methods: To develop an anhydrous substitute for petrolatum, microscopy, rheology, and sensory studies were conducted. A light microscope was utilized to identify microstructural differences (crystals, etc.). Samples were imaged in both bright field and polarized light modes. A TA Instruments Discovery HR-1 Hybrid rheometer was used to quantify viscosity over a range of shear rates. The shear sweep was conducted with a 40 mm/2º cone at 25ºC. Lastly, a sensory study was conducted on a panel of fifteen trained subjects to discriminate twelve different parameters. The anhydrous substitute was compared against the reference, white petrolatum, USP, in a pair-wise fashion. The degree of comparative differences was reported on a -1, -0.5, 0, 0.5, 1 scale. The ointment base was formulated with the following excipients: Kollisolv® MCT 70 (medium-chain triglycerides), Kollicream® 3C (cocoyl caprylocaprate), Kollicream® CP 15 (cetyl palmitate 15), Kolliwax® GMS II (mono- and di-glycerides), Novata® BC PH (hard fat), and white wax (beeswax).
Results: The three brands of commercially available white petrolatum, USP were found to demonstrate various degrees of crystalline structure and viscosity based on microscopic imaging and rheologic measurements, respectively. To develop an anhydrous substitute for petrolatum, a blend of high and low melting point excipients was identified and processed to form an ointment base that could offer improved lipophilic drug solubilization and enhanced skin penetration capabilities. Composed of mild emollients to minimize irritation potential, specifically cocoyl caprylocaprate and cetyl palmitate 15, the anhydrous formulation was found offer a comparable or improved sensory profile. The hydrophobic ointment base demonstrated statistically significant improvements in the following parameters: distribution, absorption after one minute, tackiness, waxiness, powdery feeling, smoothness, and overall patient acceptance. While not statistically significant, the hydrophobic formulation was also associated with a slightly oilier feeling than petrolatum. Rheological studies found that the ointment base was more viscous than petrolatum when stationary but offered a faster rub-in when spread on the skin as demonstrated by its pseudoplastic behavior when sheared at 100 s-1.
Conclusion: Petrolatum microstructure and viscosity varies depending on the source and refinement process of petroleum. An anhydrous alternative to petrolatum would allow for the minimization of variation in color, composition, and consistency during topical formulation development and production. A customized substitution for petrolatum can be formulated to optimize performance attributes, rheological behavior, and sensory properties.
Norman Richardson– Technical Sales Manager - North America - Skin Delivery, BASF Corporation, Tarrytown, New York