‘All-natural’ anti-wrinkle emulsion serum with Acmella oleracea extract: A design of experiments (DoE) formulation approach, rheology and in vivo skin performance/efficacy evaluation

Abstract

Objective: The growing consumers’ preferences and concerns regarding healthy ageing, youthful skin appearance, environmental protection and sustainability have triggered an ever-increasing trend towards natural, eco-friendly and ethically sourced anti-ageing products. Accordingly, this paper describes design and evaluation of novel, safe, effective and high-quality emulsion serums, completely based on ingredients of natural origin, intended for improving facial fine lines and wrinkles. Methods: Model formulations, stabilized by an innovative glycolipid mixed emulsifier (lauryl glucoside/myristyl glucoside/polyglyceryl-6 laurate) and containing Acmella oleracea extract as a model anti-ageing active, were prepared by cold process and fully assessed regarding their rheological behaviour (continuous rotational and oscillatory tests) and physical stability (dynamic-mechanical thermoanalysis – DMTA test). To study and optimize the simultaneous influence of varied formulation factors (emollients and emulsifier concentrations) on critical rheological attributes of the developed serums, a central composite design within ‘design of experiments’ approach was employed. The general skin performance – preliminary safety and anti-wrinkle efficacy of selected model serum, was evaluated in human volunteers, by employing several objective, non-invasive bioengineering techniques. Results: Rheological characterization revealed favourable shear-thinning flow behaviour with yield point, and dominating elastic character (storage modulus G’ > loss modulus G") in both amplitude and frequency sweeps, which together with relatively small structural change obtained in DMTA test indicated overall satisfying rheological and stability properties of formulated serums. From the established design space, and taking into account formulation cost and carbon footprint, promising model serum (desired/optimal apparent viscosity, yield point and loss factor, rather small and constant structural change), containing 15% of emollients and 1% of emulsifier, was chosen for in vivo evaluations. Screening of skin irritation effects revealed the absence of potential irritancy of investigated serum, suggesting overall satisfying skin tolerability/preliminary safety. Silicone skin replica image analysis demonstrated noticeable reduction/improvement in all measured skin wrinkle parameters after only 2 weeks of test serum application in periorbital and perioral areas, indicating its rapid and beneficial effects on the facial expression lines and wrinkles. Conclusion: Altogether, the results corroborate the promising potential of the developed Acmella oleracea extract-loaded emulsion serum as safe, effective and non-invasive natural anti-wrinkle product.