38 JOURNAL OF COSMETIC SCIENCE Hair fiber structure A-layer Exocuticle Endocuticle Cell membrane complex rtex Cuticle 50-100 µm Conditioner on cuticle of treated hair and interaction with AFM tip Treated hair fiber AFM cle e conditioner Figure 1. Schematic of hair fiber (ref. 5) and interaction between AFM tip and cuticle surface (with conditioner). the cuticle, the outermost region that protects the cortex. This multilayered region is important to the hair's frictional characteristics because it is this structure that comes in contact with skin, combing devices, and other hair fibers. The cuticle consists of flat, overlapping cells (scales). Each cuticle cell is approximately 0.3 to 0.5-µm thick and the visible length is approximately 5 to 10 µm. The cuticle is composed of keratin and consists of an A-fayer, exocuticle, endocuticle, and cell membrane complex. Attached to the surface of the cuticle scale is a saturated fatty acid called 18-methyleicosanoic acid (18-MEA), a lipid layer that strongly contributes to the lubricity of the hair. The bottom panel of Figure 1 shows the micro/nanoscale interaction between the cuticle, conditioner, and the atomic force microscope (AFM) tip as it scans over the surface of treated hair. The conditioner layer is not considered to be uniform, but rather is ran-

NANOTRIBOLOGICAL PROPERTIES OF HAIR 39 domly distributed on the cuticle with a stronger affinity towards the bottom of the cuticle edge (3,4). As the tip rasters over a pool of conditioner, the conditioner forms meniscus bridges with the tip, causing an increase in the adhesive force (which con- tributes to increased friction force) on the surface for conditioner-treated hair. The hair surface is negatively charged and can be damaged by a variety of chemical (permanent hair waving, chemical relaxation, coloring, bleaching) and mechanical (combing, blowdrying) factors (5-7). Figure 2 shows the transformation and wear of the cuticles scales before and after damage. Chemical damage causes parts of the scales to fracture and reveal underlying cuticle remnants. With continual damage, the negative charge of the hair increases, which increases charge conduction, with a greater tendency for moisture adsorption, making it harder to control so-called static electricity "fly- away." The friction and adhesion of the hair increases as well, so that the hair becomes harder to comb and entanglement occurs more readily. These issues are considered to be harmful to the tribological requirements for healthy and beautiful hair (Table I). One of the main objectives of hair care science, then, is to inhibit the damage caused by the factors described previously. The use of conditioners on hair can cause drastic changes in the surface properties of hair, both quantitatively (such as in decreased coefficient of friction) and by human perception of feel. Conditioner thinly coats the hair primarily by Van der Waals attractions. Thus, lubrication of the hair fibers ensues to create a softer, smoother change in feel for the consumer. This layer of lubrication also provides a protective coating to the hair surface for prevention of future damage. The uniformity of this layer over the hair surface is a very important feature, namely, how and where it is localized. For instance, Figures 2 and 3 show that conditioner tends to accumulate in Virgin hair Cuticle scales Damaged hair Worn and fractured cuticle edges Untreated and treated hair fiber cross-section Treatment with conditioner Damaged hair treated with conditioner Conditioner deposition sites Nonuniform conditioner layer Figure 2. The effect of damage to the cuticle scales and the deposition of conditioner on the cuticle surface. The cross section of the hair with and without conditioner is shown in the lower panel.