Biophysics of Human Hair: Structural, Nanomechanical, and Nanotribological Studies

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…