Skin protects us. It’s not surprising, therefore, that active ingredients (e.g., botanicals) have a hard time getting through. The stratum corneum (a layer of the epidermis) blocks anything and everything but there are some ways around the natural “thickness” of skin. In the spirit of finding ways to push active compounds through the skin, a team of researchers  recently explored using two distinct terpenes.
They started with an active ingredient: curcumin. Curcumin is the key component of turmeric, used for millennia for numerous benefits.  These include antioxidant, anti-cancer, and liver-protective effects in addition to reducing inflammation and cholesterol.  The researchers suggest that curcumin could be helpful for skin disorders, including psoriasis.  But turmeric and curcumin are similar to cannabis in that they have poor oral and dermal absorption. That said, terpenes can boost the penetration of compounds through skin by altering the stratum corneum.
The authors of the study created “curcumin-loaded low energy nanoemulsions” with eucalyptol and pinene. They tested the nanoemulsions in vivo and investigated the properties of the emulsions. One preparation contained medium-chain triglycerides (MCT), another contained an equal ratio of MCT with eucalyptol, and another an equal ratio of MCT with pinene. Polysorbate 80 and soybean lecithin (9:1) were used as stabilizers. The curcumin was solubilized to 3 mg/ml and emulsification performed with magnetic stirring (1000 rpm). The researchers measured average droplet diameter at 102 nanometers to 132 nm.
Interestingly, the formulation with terpenes had a significantly smaller average droplet size. The terpene nanoparticles were also more transparent with smoother, more regular surfaces. The researchers explain, “eucalyptol and pinene express surfactant-like activity (co-stabilizing effect), influencing the surfactant curvature at the interface of low-energy nanoemulsions.” This reduced the amount of surfactants required for stability. The release of curcumin after 6 hours was also greater in the terpene emulsions at roughly 14% compared to 10% for MCT.
Eucalyptol proved the superior terpene in vivo. In this nanoemulsion, curcumin penetrated the stratum corneum at about 35 μg/cm2. The MCT-only formula was next at roughly 30 μg/cm2, and pinene at about 22 μg/cm2. The solubility of curcumin in each emulsion correlated to the success of each: eucalyptol was superior at 4 mg/ml compared to only 0.21 mg/ml for pinene. The solubility parameter is thus paramount when choosing a terpene to enhance transport through skin. Choosing the right terpene for a given formulation is also essential. 
- Nikolic I, et al. Microstructure and biopharmaceutical performances of curcumin-loaded low-energy nanoemulsions containing eucalyptol and pinene: Terpenes’ role overcome penetration enhancement effect? European Journal of Pharmaceutical Sciences: Official Journal of the European Federation for Pharmaceutical Sciences. 2019(105135). [Impact Factor: 3.616; Times Cited: 7 (Semantic Scholar)]
- Nagpal M, Sood S. Role of curcumin in systemic and oral health: An overview. J Nat Sci Biol Med. 2013;4(1):3-7. doi:10.4103/0976-9668.107253. [Impact Factor: 0.869; Times Cited: 131 (Semantic Scholar)]