Fabrication of uniform lignin colloidal spheres for developing natural broad-spectrum sunscreens with high sun protection factor

Lignin can protect plants from sunshine radiation, thus making its development as a natural macromolecular sunscreen for human skin protection against UV radiation attractive. In this work, lignin colloidal spheres with different sizes and structures are prepared by self-assembly and then blended with pure skin cream to develop lignin-based sunscreens. Results show that the creams with lignin colloidal spheres added to it as actives have better sunscreen performance than those blended with original lignin. The sun protection factor (SPF) value of the sunscreens decreases with the size of lignin colloidal spheres. Phenolic hydroxyl groups play an important role during the lignin’s sun blocking process. When lignin is acetylated, SPF values of the sunscreens decrease dramatically, but will recover to a certain degree when acetylated lignin forms uniform colloidal spheres. Therefore, organosolv lignin (OL) with more phenolic hydroxyl groups is applied. The SPF value of the sunscreen containing 10 wt% OL colloidal spheres with the size of about 50 nm reaches 15.03. The SPF value of sunscreen containing 10 wt% middle size OL colloidal spheres can be boosted from 10.69 to 47.71 by adding 2 wt% UVA and 2 wt% UVB chemical sunscreens.