Hair is a filamentous biomaterial that grows from follicles found in the dermis. Hair is one of the defining characteristics of mammals. The human body, apart from areas of glabrous skin, is covered in follicles which produce thick terminal and fine vellus hair. Most common interest in hair is focused on hair growth, hair types, and hair care, but hair is also an important biomaterial primarily composed of protein that has many uses.
Hair growth occurs in cycles with three distinct stages: anagen (active growth phase), catagen (transitional phase) and telogen (resting phase). Each strand of hair exists for several years but will eventually enter the resting phase and be shed. On average, human scalp hairs grow at a rate of around 0.5 inches per month. The onset of puberty initiates different patterns of growth for boys and girls due to rising levels of hormones: testosterone causes boys’ facial and pubic hairs to grow while estrogen leads to increased vellus hair coverage on girls’ bodies. A variety of other factors can influencehair growth including age, disease state, medications and malnutrition.
There are two main types of human hairs: terminal hairs and vellus hairs. Terminal hairs are longer, thicker and darker than vellus hairs which are often barely visible due to their small diameter and pale coloration. Terminal hairs appear on the scalp, eyebrows, eyelashes pubic region while vellus hairs are more widespread across the body including areas such as the cheeks, chest and back . In some cases individuals can have a mixtureof both types of hair e.g., downy arm pits or a beard with both thick dark terminal hairs as well as thinner lighter colored vellus hairs..
The structureof humanhair consists primarilyof proteins,. There are 3 key structural proteins that form keratin intermediate filaments: α-keratins (acidic subunit), β-keratins(basic subunit) γ-keratins (glycosylated subunit). These intermediate filaments are bundled together by crosslinking disulfide bridges between cysteine residues to form strong yet elastic fibers.. The amountand distributionof these different proteins affects physicochemical properties such as stiffnessand curl pattern.. For example,, African American’s typically have higher proportionsof β-keratins leadingto straighterhairs,, while Caucasian’shairs contain greateramountsof α-keratinsresultingin curlierhairs.. In additionto these structuralproteins,, lipids , pigment granules , water trace mineralsare also presentwithinhair fibersbut generallyto a much lesser extent.. Melaninpigment granulesare responsiblefor givinghair its coloration rangingfrom blonde/red hues (low melaninconcentration)to black/brown hues(high melaninconcentration). Eumelaninand pheomelaningive rise to differentcolorationsthat dependon their chemicalstructuresand ratios within each individual’shair..
The compositionof sebum , an oily substance secretedby sebaceousglandsattachedto follicles,, differsacross individualsas well as over time duringa person’sdevelopmentalstage.. Sebum provides protection against water loss,, UV radiation,. It also acts as an antimicrobialagent against bacteriaand fungi .. In general,, excesssebum secretioncan lead tomalfunctioningof the pilosebaceousunitwhichis characterizedby comedoneformationor “blackheads”and “whiteheads”as well ascasesoftrue acne vulgaris breakouts… Acne happens when there’s abnormalsebum production combinedwith abnormal shedding off dead skin cells that plug up pores ..This blockagecan becausedby excessgrowthoff Propionibacteriumacnesor other pathogenicbacteriathat thriveunder these conditions .. Hormonalimbalanceis another major factor implicated inthe developmentof acne especiallyduringpubertywhen there’sincreasedproductionoff testosteroneand other Androgenhormones… Althoughnot completelyunderstoodit’s speculatedthat geneticsalso playsome role since certainfamilieshave amore proneness towards developingacne… Generallyspeakingtreatmentstrategiesfor acne focuson eitherreducingsebum secretionor reducingthe populationsoff P acnes bacteria usingtopicalor systemicmedications…