Biophysics of Human Hair

Humanhairisananocompositebiological?ber. Maintainingthehealth,feel,shine, color,softness,andoverallaestheticsofthehairishighlydesired. Haircarepr- ucts such as shampoos and conditioners, along with damaging processes such as chemical dyeing and permanent wave treatments, affect the maintenance and groomingprocessandareimportanttostudybecausetheyaltermanyhairprop- ties. Nanoscalecharacterizationofthecellularstructure,mechanicalproperties,and morphological, frictional,andadhesive properties(tribologicalproperties)ofhair areessentialtoevaluateanddevelopbettercosmeticproductsandtoadvancethe understandingofbiologicalandcosmeticscience. Theatomic/frictionforcemic- scope(AFM/FFM)andnanoindenterhavebecomeimportanttoolsforstudyingthe micro/nanoscalepropertiesofhumanhair. Inthisbook,wepresentacomprehensive review of structural, nanomechanical, and nanotribological properties of various hairandskinasafunctionofethnicity,damage,conditioningtreatment,andvarious environments. Variouscellularstructuresofhumanhairand?nesublamellarstr- turesofthecuticleareidenti?edandstudied. Nanomechanicalpropertiessuchas hardness,elasticmodulus,tensiledeformation,fatigue,creep,andscratchresistance arediscussed. Nanotribologicalpropertiessuchasroughness,friction,andadhesion are presented, as well as investigations of conditioner distribution, thickness, and bindinginteractions. Tounderstandtheelectrostaticchargebuilduponhair,surface potentialstudiesarealsopresented. Thebookshouldserveasareferencebookonthebiophysicsofhumanhairand hairtreatments. TheresearchreportedinthisbookwassupportedbyProcter&GambleCo. in Cincinnati,Ohio,andKobe,Japan. IwouldliketothankCarmenLaTorrewhohad assistedinvariouspublicationsonhairresearch. Iwouldalsoliketothankmywife Sudha,whohasbeenforbearingduringthepreparationofthisbook. Powell,OH BharatBhushan November2010 v Contents 1 Introduction—Human Hair, Skin, and Hair Care Products. . . . . . . . . . . 1 1. 1 HumanHair . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 1. 1. 1 TheCuticle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 1. 1. 2 TheCortexandMedulla. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 1. 2 Skin. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 1. 3 HairCare:CleaningandConditioningTreatments,andDamaging Processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 1. 3. 1 CleaningandConditioningTreatments:Shampoo andConditioner. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 1. 3. 2 DamagingProcesses. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 1. 4 OrganizationoftheBook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 2 Experimental Methods. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 2. 1 ExperimentalApparatuses. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 2. 2 ExperimentalProcedure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26 2. 2. 1 StructuralCharacterizationUsinganAFM. . . . . . . . . . . . . . . . 26 2. 2. 2 SurfacePotentialStudiesUsingAFM-Based KelvinProbeMicroscopy. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29 2. 2. 3 NanomechanicalCharacterizationUsingNanoindentation . . . 30 2. 2. 4 InSituTensileDeformationCharacterizationUsingAFM . . . 32 2. 2. 5 Macroscale Tribological Characterization UsingaFrictionTestApparatus. . . . . . . . . . . . . . . . . . . . . . . . . 33 2. 2. 6 Micro/nanotribologicalCharacterizationUsinganAFM. . . . . 35 2. 3 HairandSkinSamples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41 3 Structural Characterization Using an AFM . . . . . . . . . . . . . . . . . . . . . . . . 45 3. 1 StructureofHairCrossSectionandLongitudinalSection . . . . . . . . . . 45 3. 1. 1 CrossSectionofHair. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45 3. 1. 2 LongitudinalSectionofHair. . . . . . . . . . . . . . . . . . . . . . . . . . . . 45 3. 2 StructureofVariousCuticleLayers. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48 3. 2. 1 VirginHair. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48 vii viii Contents 3. 2. 2 ChemicallyDamagedHair. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50 3. 2. 3 Conditioner-TreatedHair. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52 3. 2. 4 EffectofHumidityonMorphologyandCellularStructure ofHairSurface. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54 3. 3 Summary. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55 4 Nanomechanical Characterization Using Nanoindentation, Nanoscratch, and AFM. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57 4. 1 Hardness,Young’sModulus,andCreep. . . . . . . . . . . . . . . . . . . . . . . . .