Development of a haptic sensor system for quantifying tactile impression of hair

The tactile impression of hair and words to express it are essential elements for measuring the performance of hair care products. However, the use of words and evaluation by people lack precision in that there are individual variations in hair condition as well as variations from one's sense of value. Moreover, precision is further reduced when people evaluate and compare tactile impressions of hair treated with many hair care products. Therefore, an objective and quantitative means of detecting tactile impression is needed. A new haptic sensor system was developed for detecting the tactile impression of hair and for evaluating hair care products. This system measures frictional vibration when rubbing the surface of hair with a polyvinylidene fluoride (PVDF) film. The PVDF film has high sensitivity and its electric response to pressure is similar to that of the finger sensory receptor, i.e., the pacinian corpuscle. Therefore, it can detect vibration accurately, and this detected vibration can be related to the tactile impression when touching hair with one's fingers. We also developed artificial hair as the standard model of tactile impression. Samples of artificial hair with various tactile impressions have no individual variations, so precise evaluation and measurement can be performed. We attempted to quantify the tactile impressions of artificial hair and human hair treated with hair care products using the haptic sensor. First, frequency spectra were measured by rubbing the surface of each artificial hair with the haptic sensor. Then, specific frequency ranges of artificial hair were extracted from the frequency spectrum using principal component analysis. As a result, seven principal components that characterize the tactile impression of artificial hair were extracted. The tactile impression of human hair treated with hair care products was evaluated by calculating the correlations between the seven principal component scores of artificial hair and those of treated hair. Thus, this sensor system enables us to detect the tactile impression of hair and to evaluate hair care products precisely and objectively.