Bioinspired Lignin-Polydopamine Nanocapsules with Strong Bioadhesion for Long-Acting and High-Performance Natural Sunscreens

UV filters that contain one or two aromatic rings in conventional sunscreens generally have a poor photo- and thermal stability and can easily penetrate through stratum corneum and dermis into the blood vessel, thus causing potential health-threatening issues. Herein, a series of bioinspired photostable and biocompatible polydopamine-grafted lignin (AL-PDA) with strong bioadhesion have been synthesized through free radical addition of dopamine (DA) and alkali lignin (AL). AL-PDA was used to emulsify organic UV filters and further cross-linked to form nanocapsules through ultrasonic cavitation. The retention rate of optimal AL-PDA nanocapsules on the skin surface reached 87% after a thorough rinse with water and negligible penetration was observed, which demonstrates their excellent bioadhesion property. Force measurements using atomic force microscopy (AFM) quantitatively revealed the adhesion between the nanocapsules and skin. An average DA grafting number of 4 would be required to endow the AL-PDA nanocapsules with suitable water-penetration resistance. The nanocapsules were used as the sole active ingredient for formulating sunscreen, whose sun protection factor (SPF) value could reach 195.33 with a dosage ∼10 wt % lasting for over 8 h under UV radiation. The as-prepared nanocapsules possess excellent antioxidant capacity and biocompatibility, ensuring their superior performance and safe use in the sunscreen. This work provides new insights into the development of biomass lignin for advanced function materials and high-end products.