title: Tactile friction of topical formulations.
abstract: BACKGROUND
  The tactile perception is essential for all types of topical formulations (cosmetic, pharmaceutical, medical device) and the possibility to predict the sensorial response by using instrumental methods instead of sensory testing would save time and cost at an early stage product development. Here, we report on an instrumental evaluation method using tactile friction measurements to estimate perceptual attributes of topical formulations.
  
  
  METHODS
  Friction was measured between an index finger and an artificial skin substrate after application of formulations using a force sensor. Both model formulations of liquid crystalline phase structures with significantly different tactile properties, as well as commercial pharmaceutical moisturizing creams being more tactile-similar, were investigated. Friction coefficients were calculated as the ratio of the friction force to the applied load. The structures of the model formulations and phase transitions as a result of water evaporation were identified using optical microscopy.
  
  
  RESULTS
  The friction device could distinguish friction coefficients between the phase structures, as well as the commercial creams after spreading and absorption into the substrate. In addition, phase transitions resulting in alterations in the feel of the formulations could be detected. A correlation was established between skin hydration and friction coefficient, where hydrated skin gave rise to higher friction. Also a link between skin smoothening and finger friction was established for the commercial moisturizing creams, although further investigations are needed to analyse this and correlations with other sensorial attributes in more detail.
  
  
  CONCLUSIONS
  The present investigation shows that tactile friction measurements have potential as an alternative or complement in the evaluation of perception of topical formulations.
authors.affiliation: SP Technical Research Institute of Sweden, Stockholm, Sweden.|Surface and Corrosion Science, KTH Royal Institute of Technology, Stockholm, Sweden.
authors.forename: L
authors.initials: L
authors.lastname: Skedung
authors.affiliation: Omega Pharma Nordic, Kista, Sweden.
authors.forename: I
authors.initials: I
authors.lastname: Buraczewska-Norin
authors.affiliation: Omega Pharma Nordic, Kista, Sweden.|Department of Pharmacy, Uppsala University, Uppsala, Sweden.
authors.forename: N
authors.initials: N
authors.lastname: Dawood
authors.affiliation: SP Technical Research Institute of Sweden, Stockholm, Sweden.|Surface and Corrosion Science, KTH Royal Institute of Technology, Stockholm, Sweden.
authors.forename: M W
authors.initials: MW
authors.lastname: Rutland
authors.affiliation: SP Technical Research Institute of Sweden, Stockholm, Sweden.
authors.forename: L
authors.initials: L
authors.lastname: Ringstad
mesh_terms: D000328:Adult; D004643:Emollients / Q000008:administration & dosage; D005260:Female; D005385:Fingers / Q000502:physiology*; D017276:Friction / Q000187:drug effects*; D006801:Humans; D008297:Male; D008875:Middle Aged; D063465:Skin Cream / Q000008:administration & dosage*; D012879:Skin Physiological Phenomena; D013499:Surface Properties; D014110:Touch / Q000187:drug effects* / Q000502:physiology*
chemical_list: D004643:Emollients
keywords: finger/tactile friction; hydration; moisturizers; skin friction; tactile perception; topical formulations