Improving sunscreen performance

There are many factors that need to be considered when determining the efficiency of sunscreen formulations, including: desired SPF,1 UVA/UVB coverage,2 wavelength shift, and photostablity,3 just to name a few. The quest for complete protection from the sun has led to numerous research efforts that are aimed not only effective sunscreens, but also products that efficiently use sunscreens, minimising the concentration of filters. A product that protects over a wide variety of ultraviolet (UV) spectrum and is effective has become a priority. This quest has led to a new series of amphiphilic polymers that have been developed to help improve the efficiency of sunscreens. These amphiphilic polymers have a polar core surrounded by a fatty oil soluble group. This series of polymers can provide not only a shift in wavelength, but also boost the SPF of the sunscreen formulation. These new polymers have been coined ‘Spider Esters’.

Spider esters

Spider esters were specifically designed4–8 to have a hydrophilic core surrounded by a hydrophobic periphery. This produces an amphilphilic polymer. The term, amphiphilic polymer, means that the polymer contains two distinct regions that have different polarities covalently bonded together. This amphiphilic nature makes spider esters very attractive because of their unique solubilites. Amphiphilic polymers are covalently bonded together and do not have the same inherent stability issues that emulsions suffer from. Oil-in-water emulsions have pockets of hydrophobic oil contained in the core of micelles surrounded by an aqueous environment. When hydrophobic organic sunscreens are added into the emulsion, they migrate into the hydrophobic micelle cores and remain suspended in a unified matrix. When the spider ester is introduced into a polar solvent, the hydrophobic periphery will collapse upon itself to minimise its contact with the solvent environment (see Fig. 1). Figure 1 allows for a simple breakdown of how these spider esters behave in solvent. Please note that this is not an actual representation, just a simple way to illustrate how the esters behave in a perfect world. As shown in Figure 1b, when the new polymers are above the critical concentration of entanglement (c*),9 they organise into structures to maximise the overlap of the periphery. Hydrophobic materials can be loaded into the regions of fatty groups surrounding the hydrophobic core (the red overlapping regions on the cartoon). The dual polarities of the new polymers make them soluble and effective when added into polar oil-based sun care formulations as well as non-polar oil-based sunscreen formulations. The major benefit of these spider esters is that they are capable of ‘encapsulating’ sunscreen filters in the core and ‘shielding’ them from the surrounding environment. This allows the filters to be placed into a wide variety of solvents, while also this ‘shielding’ of the filters can drastically improve their performance. The hydrophilic core will respond to the polar solvent in the opposite manner, and the solvent will cause the core to swell and maximise its contact with the polar solvent. This phenomenon is the basis for the ‘loading’ or encapsulation of small molecules into the core of the spider ester. We have coined this phenomenon the ‘Spider Effect’. To better illustrate the spider effect, a simple experiment was conducted. Spider ester was heated at a constant heating rate of 5.0°C/min in the presence of avobenzone, a commonly used organic sunscreen. The temperature of the ester was monitored and recorded. As seen in the Figure 2, in the temperature range of 20°C to 50°C, the temperature of the spider ester increased in a linear fashion and the avobenzone remained in powder form. Once the temperature reaches 50°C, the temperature of the spider ester solution remained constant over a 2.5 minute period. During this period, the energy being introduced into the solution is being used to drastically change the ester’s structure and not increase temperature. The core of the spider ester expands and starts to interact with the hydrophobic periphery. This temperature range (50°C to 60°C) represents the ‘loading’ region of this ester. Small molecules can be loaded into the core of the spider ester during this temperature range. This loading zone allows for the solubilisation of the avobenzone and a clear solution is observed. After the ester’s core is expanded, a linear response in temperature is restored. When the solution is cooled to ambient temperature, the avobenzone remains in the core of the spider ester and a clear solution is maintained. The release of the entrapped avobenzone is controlled by the diffusion through the hydrophobic periphery and can be controlled by introducing the loaded spider into different solvents.