Systematic Review of Usnic Acid Extraction from Wild-Grown Lichen Biomass

Usnic acid (UA) is one of the most extensively studied specialized metabolites of lichens, attracting considerable interest due to its antimicrobial, anti-inflammatory, and cytotoxic properties. The efficiency of UA extraction from lichens depends on multiple interrelated biological and technological factors. This systematic review aims to synthesize and critically evaluate reported strategies for UA extraction from wild-grown lichen biomass, with particular emphasis on extraction efficiency, practicality, and application potential. This systematic literature review, based on the Scopus database was conducted by including original research articles reporting UA extraction from wild-growing lichens. The analysis covered species selection, sample pre-treatment, solvent type, and extraction methodology. A total of 117 studies were included. Due to the predominantly non-polar nature of UA, higher extraction efficiencies were generally achieved using solvents, including acetone, supercritical CO₂, vegetable oils, and lipophilic green solvent systems. Pre-treatment strategies such as grinding or flaking significantly enhanced extraction performance by improving mass transfer. Alongside conventional methods (maceration, reflux, Soxhlet), non-conventional techniques such as Supercritical Fluid Extraction (SFE), Ultrasound- (UAE), and Microwave-Assisted Extraction (MAE) enabled faster and more selective UA extraction with reduced solvent use. Notably, SFE have been reported as particularly promising in terms of selectivity, process control, and potential suitability for scale-up, with commercially available supercritical CO₂ extracts of Usnea species supporting the feasibility of this approach. This review provides a consolidated and application-oriented overview of UA extraction, highlighting strategies that balance efficiency, selectivity, sustainability, and practical implementation.