256 JOURNAL OF COSMETIC SCIENCE
methacrylate, etc.2 W/O HIPEs and HIPPEs in particular were used in the food and
pharmaceutical sectors as a matrix for the encapsulation of hydrophilic active ingredients
that are fragile or have perceived obstacles to their use (e.g., protection of probiotics during
their travel through the gastrointestinal tract3 or beta carotene improving the taste of
bitter or astringent substances,4 reducing the irritant effect of active ingredients,5 etc.).
Formulations for encapsulation purposes included waxy components (e.g., beeswax) in the
oily phases and required hot manufacturing processes. Their use as vehicles for controlled-
release and targeted delivery has also been described.6 In addition, the texture of W/O
HIPEs and HIPPEs was also highlighted as a solution to reduce trans–fatty acid or saturated
fat content for healthier food products.7
Contrary to HIPEs with a water continuous phase, W/O types are known to be difficult to
generate and stabilize without using a large quantity of surfactants, which explains why
additional strategies using both structuration of the oily phase with waxes or oil gelators
(oleogels) and structuration of the water phase using hydrogels (e.g., gellan, flaxseed
gum, carrageenan, konjac glucomannan, and blends of polysaccharides) were developed,
sometimes also combined with the Pickering approach.8,9 In the food sector, extensive
research has been carried out using glyceryl and polyglyceryl fatty acid esters as emulsifiers
in combination with different stabilization strategies (crystallized oily phase, water-in-
oleogel, gel [i.e., hydrogel]-in-gel [i.e., oleogel]).8,10,11,12 The manufacturing process generally
involved high intensity homogenization using rotor/stator devices and required stepwise
addition of the dispersed phase.3,8,9,12 Homogenization was also reported as necessary in
studies dedicated to cosmetic applications.13
Few publications have dealt with cosmetic applications of W/O HIPEs in the last 5 years,
but there have been recent patent filings disclosing skin care applications. A first patent
claimed to maximize the fresh feeling while stating the importance of the emulsifier’s
nature and content to combine this first fresh sensation with a stable formula.14 A mass
proportion between 70% and 99% of the dispersed phase compared to the continuous
phase was also reported as necessary for the achievement of high internal phase (HIP) W/O
emulsion. The addition of the internal water phase was performed little by little using a
dispersing mixer for the preparation. Two other patent disclosures mentioned W/O HIPEs
able to stabilize pigments in the internal phase for makeup purposes, requiring precise
manufacturing procedures.15,16
HIP GEL-IN-OIL BACKGROUND AND PURPOSE OF THE STUDY
Based on the similar principle of high concentration in dispersed phase, the HIP gel-in-oil
concept consists of a high concentration of closely packed gel droplets dispersed within
a liquid continuous oil phase (Figure 1). To solve known stability issues of W/O HIPEs
and avoid high concentration of surfactants,8 a tailored non-ionic emulsifying system was
developed to withstand a large amount of internal gel phase, combining a small surfactant
molecule and a large polymeric surfactant structure.17 The small molecule quickly migrates
to the oil–water interface and helps to create the oil continuous form, while the large
molecule ensures stability over time. The gel-in-oil creation is also based on rheology
modifiers, giving a gel with a suitable rheology profile (i.e., shear thinning, nonthixotropic,
with a moderate elastic character). As for a conventional W/O emulsion, an antifreeze agent
such as glycerin or a glycol derivative was found to improve the gel-in-oil stability at cold

257 CRITICAL FACTORS TO OBTAIN STABLE HIP GEL-IN-OIL EMULSIONS
temperatures, but conversely to water-in-oil, water-in-silicone, and conventional HIP W/O
emulsions,13 the addition of salts was not required.
The interest of the HIP gel-in-oil concept for the development of cosmetic emulsions with
original aesthetics was highlighted some years ago.18 First of all, it provides an attractive
smooth and mirrored gloss appearance for the end user, with an unexpected contrasting
sensory profile starting with a first fresh sensation evolving to a cushion-comfortable finish.
As a result, the gel-in-oil emulsion is able to impart a similar emollience to conventional
cream gel, oil-in-water (O/W), and W/O formulations with a reduced oil concentration19
and provides an inherent water resistance character. The texture of gel-in-oil emulsion
alone (without any olfactory, packaging, or brand environment) was demonstrated to arouse
positive emotions in consumers.20
From the formulator’s point of view, the concept takes advantage of high compatibility
with active ingredients,21 including those rich in electrolytes. Gel-in-oils also withstand
the incorporation of a high quantity of solvents, sometimes required to solubilize these
active ingredients (e.g., glycols, ethanol, etc.). The manufacturing process, cold and using
low mixing energy, is industrially advantageous, saving time and increasing sustainability
by reducing carbon emissions.17
Despite these advantages, the factors affecting the emulsion structure and behavior have
not been thoroughly clarified, thus limiting the use of the concept. As is known for other
inverse emulsions, it is difficult to predict the long-term stability of gel-in-oil without
waiting for three months, even using accelerated tests (storage at 45°C and freeze–thaw
cycles in particular). Due to high concentration of the dispersed phase, gel-in-oils mainly
provide compact textures, which make them difficult to prepare and diminish the
relevance of accelerated tests based on centrifugal force (such as the LUMiFuge® device
from Germany) or based on multiple light-scattering analyses (such as the Turbiscan® LAB
device from France).
The objective was therefore to study rigorously the factors affecting the emulsion structure,
stability, and behavior under stress for a better understanding of the limitations of the
formulation. First, details of the manufacturing process needed to be investigated using a
simple formula to confirm the recommendations: preparation mode (i.e., direct or indirect)
Figure 1. Gel-in-oil macroscopic appearance and structure representation.