Spruce bark stilbenes as a nature-inspired sun blocker for sunscreens

Abstract

Stilbene glucosides are a class of natural compounds that have been used as natural antioxidants and antifungal and antibacterial agents. Here, spruce bark extract, rich in stilbene glucosides, was used as a natural ultraviolet-protective additive for sunscreens. The ultrasound-assisted extraction of fresh Norway spruce (Picea abies) inner bark with 60% ethanol provided an extract in ca. 25% yield, of which one third consisted of three stilbene glucosides (astringin, isorhapontin and polydatin) which were fully characterized and quantified by 1D and 2D NMR spectroscopy and high resolution-liquid chromatography mass spectrometry (HR-LCMS). Emulsions of the extract were prepared and applied on polymethyl methacrylate (PMMA) plates to study the effect of the extract on UV light absorbance. A 10% emulsion with spruce crude extracts (containing 35% stilbene glucosides) alone provided UV protection equal to half the efficiency of commercial SPF 15 sun lotions, which also displayed a 21-32% higher SPF effect in comparison with the same dosages of alkali lignin nanoparticles using the same sunscreen emulsification process. A preparative scale chromatography was established for the first time as a fast and highly efficient method in the small-scale recovery of stilbene glucosides from spruce inner bark for full structural elucidation and has a good chance to be industrially scalable. Overall, this exploration may launch a new era for the application of stilbene glucosides of spruce bark extracts as genuine replacements for synthetic UV filters, upgrading this underappreciated bark residue from energy production to higher value cosmetic use.