Foam in pharmaceutical and medical applications
Graphical abstract
Introduction
The skin covers the entire human body and plays a crucial role in synthesising vitamin D, regulating the body temperature, and protecting the body from external factors such as infectious agents, water loss, chemical and toxic contaminants [1]. High exposure to various environmental stresses and many other external factors make the skin susceptible to diverse diseases and disorders [1]. On the other hand, the outmost layer of the epidermis of skin, known as stratum corneum, is the major penetration route for topical drug delivery [2]. The stratum corneum is of great interest for both cosmetic and pharmaceutical industries as it acts as the first layer of penetration for topical drug delivery and protection against the outside environment [3,4].
Topical therapy is the cornerstone of the dermatological field. The studies on topical products is rapidly increasing in number due to the fast-growing demand for their applications in healthcare technologies, cosmetic and pharmaceutical industries. Particularly over the past decade, the study of topical foam technology has received considerable attention from the scientific community because of their advantages over conventional topical formulations such as solutions, creams, gels, lotions, emollients and ointments [3,5,6].
Foam formulations are easier to spread uniformly over the skin surface and allows better coverage of the contact area [3,5,6]. In addition, foams easily reach difficult to access skin areas including wrinkles, skin folds, etc [3,5]. In cases of hirsute skin areas, foams show rapid breakdown and access to the stratum corneum through hair shafts [5]. Foams are particularly useful for application on highly inflamed, swollen, abraded, infected and sensitive skin (e.g. for the treatment of psoriasis, sunburn, eczema, seborrheic, acne) [3,5, 6, 7]. Foam applications minimise the need for touching and rubbing onto the skin as foams spread readily on the skin with negligible or mild mechanical shear forces [3,5,6]. This advantage makes them superior to traditional dosage forms that require rubbing and hence cause more pain, inflammation, and irritation [3,5]. Depending on the formulation ingredients, foams can offer drying [3], cooling [8], soothing [7,9], emollient or moisturising effects to the skin [7]. Foam is less dense and less sticky compared to conventional dosage forms [3,5,10]. Foam vehicles are capable of delivering an accurate dose [3,7]. Foams do not drip off the skin and leave low levels of residues, thereby minimising staining of the body and clothes [3,7,11]. As foam formulations are kept in a sealed canister, their contamination potential is minimised. During the past two decades, there has been a growing interest amongst researchers in the development of foams to facilitate and/or enhance topical drug delivery [3,5, 6, 7,10,12,13]. Surveys of patients’ opinions on the usability of foams shows excellent patient compliance/acceptability and a significant preference for foams over conventional topical vehicles [7,14, 15, 16, 17]. Quantitative assessments of topical vehicles by 20 patients diagnosed with psoriasis showed a significant preference for foam over cream, emollient, gel and ointment [14]. Similarly, a study of 279 patients with mild to moderate plaque-type psoriasis showed that use of foam formulations resulted in enhanced patient compliance/acceptability over conventional formulations (i.e. creams) [16]. Foam formulations were shown to result in increased drug versatility and patient tolerability in the treatment of steroid-responsive dermatoses [15].
Despite the aforementioned advantages of foams (i.e. sterility, ease of application), the number of topical foam products available in the market are rather lower compared with traditional products (i.e. creams, gels) [3]. Semisolid dosage forms (e.g. creams, gels, lotions, ointments) are the most commonly used topical vehicles [1]. Foam dosing systems (e.g. pressurised aerosol foams) can be more expensive and complex as they are dependent on propellant technology [5]. One of the main concerns regarding aerosol foams is the adverse environmental effects of propellants used for foam production (i.e. ozone layer depletion).
Although topical foams have considerable potential for healthcare applications (i.e. cosmetic, pharmaceutical), only a small number of studies have taken full advantage of nature of foams [3]. This work presents a review of recent studies related to topical foams for cosmetic and dermal applications. Firstly, the main components of foam formulation are introduced and explained. Commercially available topical foams are classified, and their key attributes are provided. The most commonly used methods to determine topical foam stability and structure are explained and discussed. Recent literature related to the permeation and absorption of topical foam formulation through skin is reviewed. Finally, recent developments of topical foams for dermal applications are summarised and the potential of topical foam in treating bacterial skin infection is highlighted.
Section snippets
Foam formulation
Liquid foam formulations depend on the dispersion of a high concentration of gas bubbles in a small amount of a continuous liquid phase containing foaming agents (i.e. surfactants). Topical foams contain different components which are usually surface-active agents, stabilisers, solvents, active pharmaceutical ingredients and/or active cosmetic ingredients, and may also include other excipients (i.e. preservatives, penetration enhancers). The basis for selection of these components for the
Current developments and applications of topical foams on skin
In this section, the focus is on topical foams that are intended to be applied on human skin for cosmetic and pharmaceutical purposes using ACIs and APIs, respectively. Depending on foam formulations and foaming method, the properties and characteristics of foam such as stability, drainage, carrying capability of AIs, structure, and volume can be different. These characteristics and properties of foam are essential in the enhanced application of cosmetic products, e.g., shaving foam and shampoo
Conclusions
This review clearly shows the increasing interest in the use of topical foams for both cosmetic and pharmaceutical applications. The main components of foam formulation and commonly used topical foam classes were introduced and explained. A review of experimental methods for the measurement of important physical properties of foams revealed that most recent studies did not study foam stability and structure on a skin-like porous substrate. The porous structure may lead to significant changes in
Author contributions
M. P. wrote the original draft of the manuscript. M. P. edited the original manuscript and prepared the final version of the manuscript with support from A. T., D. M., and V. S.; All authors have given approval to the final version of the manuscript.
Conflict of interest statement
None declared.
Acknowledgments
M. P. was supported by the Doctoral Prize Fellowship from Loughborough University, UK. V. S. and A. T. were supported by the MAP EVAPORATION project and European Space Agency.
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