Recent Progress in Hair Science and Trichology

Abstract

Hair is important to our appearance as well as to protect our heads. Human hair mainly consists of proteins (80-85%), melanin pigments (0-5%), water (10-13%), and lipids (1-6%). The physicochemical properties of hair have been studied for over 100 years. However, they are not yet thoroughly understood. In this review, recent progress and the latest findings are summarized from the following three perspectives: structural characteristics, delivery and distribution of active ingredients, and hair as a template. The structural characteristics of hair have been mainly investigated by microscopic and/or spectroscopic techniques such as atomic force microscopy integrated with infrared spectroscopy (AFM-IR) and rheological measurements. The distribution of active ingredients has been generally evaluated through techniques such as nanoscale secondary ion mass spectrometry (NanoSIMS). And finally, attempts to explore the potential of hair to be used as a substrate for flexible device fabrication will be introduced.

1 Introduction

Human scalp hair has a lot of important functions. Seasonal variations in hair growth and goosebumps caused by cold stimuli or emotions point to its evolutionary history^1),2). The main function of hair is to protect our head and neck from sunlight (mainly UV rays) , thermal pain (extreme heat/cold) , and physical damage^3),4). Hair is also essential as an excretory system for trace metals and acts as an accumulating tissue. Thus, the metal content in hair can reflect the body's condition for a long period, recording the history of personal exposure to metals^5),6),7),8). Last but not least, hair style is deeply tied to gender^{9),10),11),12)}, religion, and culture, as we arrange our hair as we want to complete our entire look. It is said that the first barbering services were performed in ancient Egypt in 5000 B.C. with tools made from oyster shells or sharpened flint. In ancient Egyptian culture, barbers were highly respected individuals and surgeons were said to also serve as barbers. In addition to this, the earliest records on the usage of henna as a hair dye go back to ancient Egypt, where people used henna to decorate and color their hair and nails¹³⁾. In Japan, people regard hair as a symbol of beauty and youth. In the past in Japan, a woman's hair was regarded as her most important possession, not only as a 〝pars pro toto〟 but also as the place wherein lies her spirit¹⁴⁾. Hence, through all ages and civilizations, there has been a constant desire for products which can improve the look and feel of the hair. In the hair care industry, hair care products are categorized by purpose such as cleansing, conditioning, styling, coloring/bleaching, and perming/relaxing¹⁵⁾. However, hair is often exposed to various external aggressors. Besides environmental factors such as UV irradiation, the accumulation of numerous chemical or physical treatments damages and weakens the hair fiber. Therefore, there has been demand for the development of products which can repair hair as well as less damaging chemical/physical treatments. This is why better understanding of the physicochemical properties of hair is essential.

In the first section of this review, the latest reports regarding the structure and mechanical properties of hair are summarized. The structure of hair has been investigated mainly by using small angle X-ray scattering (SAXS) ^16),17),18), atomic force microscopy (AFM) ^{17),19),20),21)}, transmission electron microscopy (TEM) ^17),22),23), Fourier transform infrared spectroscopy (FT-IR) ^{24),25),26),27),28),29)}, and Raman spectroscopy^{30),31),32),33),34)}. In addition to these techniques, atomic force microscopy integrated with infrared spectroscopy (AFM-IR) ^35),36), focused ion beam scanning electron microscopy (FIB-SEM) ³⁷⁾, and scanning transmission electron microscopy (STEM) ³⁷⁾have been recently applied in hair research. The mechanical properties of hair are also important because they reflect its internal structures. Thus, tensile, bending, torsion, and indentation tests have been widely performed to determine these properties^{19),20),38),39)}. The second key point for developing better hair care products is the controlled delivery of active ingredients. Up to now, their distribution has been investigated by FT-IR microscope^26),27),40), laser confocal microscopy^{41),42),43),44)}, and scanning electron microscopy (SEM) ^45),46). Moreover, an attempt to apply nanoscale secondary ion mass spectrometry (NanoSIMS) technology in hair research has been recently successful^{47),48),49),50)}.

Hair has also been attracting much attention as a biomaterial since it is a renewable material with low toxicity and readily available as waste. Furthermore, hair is a mechanically strong fiber with excellent flexibility. In the past, ropes made with the hair of pious Buddhist women were used to drag the timber needed to construct temples¹⁴⁾. Also, horsetail hair has been historically used for stringed instruments⁵¹⁾. Therefore, hair is a functional material and the potential of using it as a substrate for flexible device fabrication has been intensively explored.

In short, this review is composed of the following three sections: structures and physicochemical properties of hair, visualization and controlled delivery of additives, and hair as a biomaterial.

2 Structures and Physicochemical Properties of Hair

2.1 General

As far as it is known, the first attempt to comprehensively investigate the microscopic structures in the hair of mammals was made in 1920 by Hausman⁵²⁾. Figure 1 shows a schematic illustration of the human hair structure and the cross section of a hair. Human hair has a multi-layered structure, i.e., the innermost medulla, middle cortex, and outermost cuticle, and its diameter varies from 50 to 100 µm. It is a complex nanocomposite fiber consisting of proteins (80-85%) , melanin pigments (0-5%) , water (10-13%) , and lipids (1-6%) ^{39),53),54),55)}. The main proteins in human hair are microfibrillar keratins with a molecular mass of 40-65 kDa and keratin-associated proteins with a molecular mass of 6-30 kDa⁵⁶⁾. Keratin is rich in cystine, and the disulfide bonds of cystine are responsible for the mechanical strength of the hair⁵⁷⁾. Melanin, a natural hair pigment, is mainly distributed in the cortex⁵⁸⁾. Granular melanin pigments, whose diameter is about 200-800 nm, are composed of various types of monomer units and generate color in the hair. Mammalian melanocytes produce two types of melanin pigments, black-to-brown eumelanin and yellow-to-reddish-brown phenomelanin. Hair color is determined by the distribution, concentration, and ratio of these two types of melanin^59),60). Ito et al. reported the relationship between hair color and the ratio of eumelanin and phenomelanin^61),62).

Fig. 1

(a) Picture of the cross section of human hair. (b) Schematic illustration of the entire human hair.

2.2 Medulla

Previously, the medulla had been downplayed since it was perceived as making little contribution to the properties of hair⁶³⁾. In general, it is said that the medulla consists of vacuolated cells bound together by the cell membrane complex (CMC) . The CMC consists of three distinct layers: a central proteinaceous and poly saccharide-rich δ-layer sandwiched in between two outer, lipid rich β-layers⁶⁴⁾as shown in Fig. 1 (b) . However, nowadays, it has been implied that the medulla not only contains air-filled vacuoles, but also some of the surrounding components. Namely, the medulla consists of an amorphous material, high in lipid concentration, which forms a porous structure containing some of these air-filled vacuoles. TEM and SEM observations have indicated that the supposed amorphous material in the medulla is a fibrillar structure which is randomly oriented^{29),65),66),67)}. Recently, Fellows et al. have adopted a novel AFM-IR technique for the topographical and chemical examination of the medulla of human hair³⁶⁾. They identified two medulla sub-units based on the intensity features at 1655 cm^-1, which corresponds to the α-helix protein secondary structure of the Amide-Ⅰ band, and calculated similarity indices using difference spectra regression among two medulla species and the cortex as shown in Table 1. Thus, it was revealed that the medulla with high intensity at 1655 cm^-1 greatly overlapped with the cortex, meaning that they are very similar to each other and the two medulla species intermediately overlap with each other. On the other hand, there was notably less similarity between the medulla with low intensity at 1655 cm^-1 and the cortex. This result indicates that the medulla has two distinct sub-units. One is the fibrillar structure which is the disordered, sparse cortical cell and the other is the air-filled vacuole which has less of a resemblance to the fibrillar structure within the cortex. Kreplak et al. have revealed the profiling of long alkyl chain lipids between Caucasian and Afro-American hair using synchrotron infrared microspectrometry²⁹⁾. As a result, a high lipid concentration was observed inside the medulla of Caucasian hair, whereas no IR-featured characteristics of lipids were detected in the Afro-American hair. They also confirmed that the FT-IR spectrum of the Caucasian sample became similar to that of the Afro-American sample by extracting lipids with an organic solvent.

Table 1

Calculated similarity indices, using difference spectra regression among the two medulla species and the cortex as in the literature³⁶⁾.

2.3 Cortex

The physical structure of the cortex has been widely studied^68),69),70). The cortex, surrounded by the cuticle, makes up the majority of the hair fiber and consists of long, spindle-shaped, tightly packed cortical cells embedded in the CMC. Each cortical cell is made up of packed macrofibrils embedded in an intra-cortical cell matrix as shown in Fig. 1 (a) . Macrofibrils are aligned parallel to each other, showing a rod-like geometry similar to that of cortical cells, and give rise to the fibrous nature of bulk hair. Macrofibrils are made up of smaller microfibrils (also called intermediate filaments) embedded in an intra-macrofibrillar matrix, and subsequently the sub-unit structures ultimately reach the fundamental α-helicalcoiled coils. Therefore, a significant proportion of the α-helix exists within microfibrils, which relates to tensile deformation accompanied by secondary structure transitions⁷¹⁾. The α-helices contain a significant proportion of cystine formed by two cysteine residues linked with a disulfide bond^39),72). Fellows et al. have characterized the cortical cell nano-structures using AFM-IR³⁵⁾. AFM-IR measurements were performed on microtomed cross-sections of human hair in order to spectrally separate and characterize macrofibrils and the intra-cortical cell matrix. The general assignments of the FT-IR spectra of human hair based on data reported in the literature are shown in Table 2^{24),35),73),74)}. Spectrochemical mapping using AFM-IR has revealed that macrofibrils exhibit greater proportions of aliphatic and acidic proteins as well as lipids and triglycerides, whereas the surrounding matrix material shows greater contributions from overall proteins and cysteine oxidation products.

Table 2

General assignments of the FT-IR spectra of human hair based on data reported in the literature^{24),35),73),74)}.

2.4 Cortex-cuticle

Takahashi et al. have detected the distribution of hair lipids and their metabolites using FT-IR and mass spectrometry²⁸⁾. According to their results, sterylglycoside-like lipids were located in the interface between the cuticle and cortex, unsaturated fatty acids such as linoleic acid and α-linolenic acid were found more in the cortex and medulla than in the cuticle, and shorter-chain hydroxyl fatty acids derived from those unsaturated fatty acids were found in melanin granules. Essendoubi et al. have investigated the impact of thermal stress with/without a hair care treatment on the conformation of human hair keratin using confocal Raman spectroscopy^32),34). The Raman spectra were acquired from the hair surface to a depth of 30 µm (Z=0 to 30 µm) . The general assignments of the Raman spectra of human hair based on data reported in the literature are shown in Table 3^{30),31),32),33)}. They focused on the peaks at 510 cm^-1 and 1656 cm^-1 and performed a hierarchical cluster analysis of mean Raman spectra for the 4 hair samples (untreated and unheated, treated with a conditioner and unheated, untreated and heated 5 times, and treated with a conditioner and heated 5 times) . Their Raman results indicated that thermal stress has a tendency to decrease the α-helix conformation and to increase the β-sheet conformation. However, this tendency can be mitigated by a hair care treatment.

Table 3

General assignments of the Raman spectra of human hair based on data reported in the literature^{30),31),32),33)}.

Kuzuhara has evaluated the influence of bleaching and permanent treatments on the structure of the cuticle and cortex using Raman spectroscopy^31),33). Raman spectra were obtained from 1 µm to 30 µm depth from the hair surface. He defined a depth of 1 µm from the hair surface as the cuticle region and a depth of between 5 and 30 µm as the cortex region. Peaks at 508, 524, 544, 1652, and 1671 cm^-1 were focused on, and it was suggested that the gauche-gauche-gauche content of the S-S bond existing both in the cuticle and cortex was remarkably decreased, while the gauche-gauche-trans and trans-gauche-trans content was unchanged by the excessive bleaching treatment³⁰⁾. In addition, it was found that not only the β-sheet and random coil content, but also the α-helix content existing throughout the cortex were decreased by the treatment.

2.5 Cuticle

The cuticle is the outermost layer that surrounds the cortex. It consists of a piled-up structure of 3 to 10 thin cellular sheets attached to the root or proximal end and the point towards the tip or distal end of the fiber. Each sheet is approximately 0.5 μm thick and roughly 100 μm×50 μm in size with rounded corners, resulting in an overall cuticle thickness up to 5 µm (shown in Fig. 2 (a) ) ^{17),19),20),22),23)}. The longitudinal direction that the overlapping sheets are tilted at is approximately 3-5° to the hair's growth axis. As shown in Fig. 2 (b) , each cuticle cell consists of sub-lamellar layers: the epicuticle, A-layer, exocuticle, endocuticle, and inner layer. The epicuticle is the cell membrane of the cuticle cell, and the lipid layer attached to it contributes to the lubricity of hair. The A-layer and exocuticle are rich in cystine (30% and 15%, respectively) , highly cross-linked, and mechanically strong. The endocuticle is low in cystine content (3%) and mechanically weak^19),20).

Fig. 2

Schematic illustrations of (a) longitudinal sections of human hair and (b) single lamellar sheet structure of the cuticle and CMC.

Breakspear et al. have conducted comprehensive research on the role of the cuticle and have revealed the previously unknown swelling behavior of the cuticle during moisture absorption¹⁷⁾. Their result suggests the significance of the cuticle's role in moisture management.

Kim and Park have analyzed the effects of aging on the keratin structures of the surface of Korean hair using FT-IR²⁴⁾. This revealed that the peak positions of the bands originated from Amide A, CH₃ stretch, and Amide Ⅰ can be altered by aging. In addition, the female hair was shown to contain more cysteine than the male hair, and the amount of Amide Ⅱ and Amide A in male hair was revealed to change due to aging more significantly than for female hair.

Korte et al. have investigated the mechanisms that cause the hydrophobic protection layer of the human hair surface to be affected by bleaching with a high lateral resolution using dynamic chemical force microscopy (dCFM) and at a macroscopic scale using Ft-IR²⁵⁾. They concluded that the bleaching treatment removed the hydrophobic top surface layer, resulting in a replacement with a hydrophilic SO₃^- (cysteic acid) end group. At a molecular level, their results suggested a clustered self-assembled monolayer alignment of cysteic acid with a crystal-like structuring.

Marsh et al. have reported for the first time conclusive evidence for the presence of deposits of exogenous calcium fatty acid using synchrotron X-ray fluorescence coupled with X-ray scattering, FIB-SEM, STEM, X-ray energy dispersive spectroscopy, FT-IR and Raman imaging. It has been revealed that exogenous calcium fatty acid not only forms in between cuticle layers in the lipid-rich cell membrane complex, but also grows to dimensions large enough to cause a bulge structure³⁷⁾.

The nanoscale surface charge of human hair has been successfully visualized by the Unwin group⁷⁵⁾. The assessment of hair surface charge has mainly relied on zeta-potential measurements which lack spatial resolution. To overcome this drawback, the group synchronously visualized the surface charge and topography of human hair at the nanoscale using scanning ion conductance microscopy (SICM) . Hopping scans were recorded with a resolution of 40×40 pixels over an area of 10×10 µm². The obtained data has shown that untreated hair has a fairly uniform negative charge density (-15 mC/cm^-2) , while bleached hair has some highly negatively charged localized regions (-80 to -100 mC/cm^-2) . In addition, it was shown that a chemical conditioner treatment can increase the overall homogeneity of the surface charge, together with a shift in the surface charge to positive values.

2.6 Cell membrane complex (CMC)

The CMC consists of cell membranes and adhesive material that binds the cuticle and cortical cells together in keratin fibers. The structure, chemistry, and physical properties of CMC have been clearly reviewed by Robbins⁶⁴⁾. The TEM studies by Rogers provided the evidence for the current structure of the CMC, consisting of a central δ-layer approximately 15 nm thick sandwiched by the lipid β-layer approximately 5 nm thick as shown in Fig. 2 (b) ^76),77). As shown in Fig. 3, Robbins identified three different CMC models: cuticle-cuticle CMC, cortex-cortex CMC, and cuticle-cortex CMC. Jones and Rivett have proved that the CMC of the cuticle contains 18-methyl eicosanoic acid in the β-layer which does not exist in the CMC of the cortex^78),79). The CMC between the cuticles has monolayer lipids which are attached by covalent bonds such as thioester, ester, or amide bonds to the cell membrane proteins on one end, and by van der Waals attractive forces to the δ-layer on the hydrophobic end as shown in Fig. 3 (a) ^80),81). On the other hand, the CMC between cortical cells is composed of lipid bilayers which are not attached by covalent bonds to protein layers but instead by ionic or hydrogen bonds to cortical cell membrane proteins on one side and are similarly attached to the δ-layer on the other side of the bilayer as shown in Fig. 3 (b) . The model for the cuticle-cortex CMC is shown in Fig. 3 (c) . Since the cuticle-cortex CMC bridges cuticle and cortical cells, it is reasonable to assume that its model is the hybrid of the cuticle-cuticle CMC and cuticle-cortex CMC. Nakamura et al. have suggested that the cuticle-cortex CMC is different from the cuticle-cuticle CMC as well as the cortex-cortex CMC⁸²⁾. Studies by the Leeder and Mansour groups have indicated that the cuticle-cortex CMC is more resistant to solvents than the cortex-cortex CMC^83),84). Furthermore, Leeder et al. have made this model even more valid by an uranyl dye study which showed two dyed layers in the cuticle-cuticle CMC: one dyed layer in the cuticle-cortex CMC and no dyed layer in the cortex-cortex CMC⁸⁵⁾.

Fig. 3

Schematic illustration of (a) the cuticle-cuticle CMC, (b) the cortex-cortex CMC, and (c) cuticle-cortex CMC⁶⁴⁾.

Recently, Takahashi has proposed a new structure named CARB (cuticle anchored resistant base) , which is located at the interface between the cuticle and cortex and is just one outer layer from the cuticle-cortex CMC^22),23). According to Takahashi, the penetration barrier at the interface between the cuticle and cortex is not the CMC, but the CARB. Glycolipid is preferentially distributed at CARB, and the cystine content of CARB is lower than that of the A-layer, but slightly higher than that of the exocuticle and inner layer.

2.7 Lipids in hair

The total amount of lipids extractable from the hair is generally 1% to 9% of the weight of the hair. Masukawa et al. have obtained the relative comprehensive lipid profiles in hair for Japanese women by extracting them with hexane, presumably leaving most of the internal and structural lipids in the hair⁸⁶⁾. The extracted lipids were separated into eight groups and their results are summarized in Table 4.

Table 4

Lipid composition in human hair.

2.8 Mechanical properties

The mechanical properties of hair are one of the important physical properties of hair because they are sensitive to its internal structures. Up to the present date, tensile^{19),20),38),39)}, bending^38),87), torsion⁸⁸⁾, and indentation⁸⁹⁾tests have been performed to characterize these properties. Tohmyoh et al. have determined the structural elasticity for tensile deformation of a single human hair and compared the tensile deformation with the bending deformation^38),87). The hair samples were collected from three healthy, Mongoloid subjects. The structural elasticity value for the tensile deformation was approximately 3.0 GPa and 4.0 GPa for the bending deformation, indicating that the structural elasticity for the bending deformation is greater than that for the tensile deformation. Also, from the theoretical model, it was confirmed that this result is valid provided that the Young's modulus of the outer cuticle is greater than that of the middle cortex. Bornschlögl et al. used the force-volume mode, and the force and indentation depth were recorded for 256 equidistant approach curves within regions of 10×10 µm² for the longitudinal 10 µm thick-sectioned hair using AFM⁸⁹⁾. The elastic modulus was increased by almost three orders of magnitude along the first millimeter of a hair follicle. The obtained mean stiffness was 30 kPa for the keratinocyte matrix of the hulb and 10 Mpa on the plateau of the differentiated hair immersed in phosphate buffered saline.

3 Visualization and Controlled Delivery of Additives

It is crucial to understand and control the distribution and delivery of additives in the hair in order to develop more effective hair care products. Cavallaro et al. have proposed a novel halloysite/keratin nanocomposite coating for protecting human hair from UV irradiation⁴³⁾. The 3D laser scanning microscopy showed the nanocomposite deposited on the hair surface and that 50% to 60% of the hair surface can be covered with 1 wt% suspension application.

Malinauskyte has evaluated the penetration of hydrolysed keratin peptides with different molecular weights into hair using fluorescence microscopy⁴⁴⁾. The low molecular weight (221 Da) and middle molecular weight (2577 Da) keratin peptides were found to be able to penetrate deep into the hair cortex. However, the high molecular weight (75440 Da) keratin peptide was mainly just adsorbed into the hair surface. Only the middle weight type could increase the Young's modulus, reduce hair breakage, and modify the thermal properties.

Srivastav et al. have employed laser scanning confocal microscope observation using Nile Red as a dye to assess the penetration of oils into human hair⁴²⁾. It was observed that rice bran oil exhibited only 14% penetration, while light liquid paraffin oil exhibited 75% penetration.

The FT-IR microscope technique is also often employed for thin hair cross sections to visualize the distribution of additives. Fuse et al. have visualized the water uptake in human hair by using heavy water (D₂O) instead of lightweight water (H₂O) ^27),40). Penetration of this heavy water from the cuticle to medulla was quantified by monitoring the O-D stretching band located at 2500 cm^-1. Higher heavy water penetration and evaporation were observed for bleached hair in comparison with untreated healthy hair. In addition, the heavy water penetration rate of age-related white hair was faster than that of black hair, which means that the age-related white hair lost its water-holding capacity. Previously, we revealed the distribution of glyoxylic acid, which is a perming agent, in healthy hair by focusing on the C=O band at 1734 cm^-1 as shown in Fig. 4²⁶⁾. A FT-IR microscope can be used to reveal the distribution of trace amounts of additives without special labeling with relatively high sensitivity. However, it has poor lateral resolution (>5 µm) .

Fig. 4

FT-IR mapping images of a cross section of a hair treated with a solution containing 5 wt% glyoxylic acid obtained from the integration of the band between 1715-1760 cm^-1.

NanoSIMS technology has been successfully applied in hair research for about 20 years and is the combination of dynamic SIMS with excellent lateral resolution (around 50 nm) . NanoSIMS images of traces of arsenic were obtained by Audinot et al. for control hair specimens and hair specimens exposed to arsenic⁴⁷⁾. In the control hair, the arsenic amount was at a natural level, ranging from 0.1 to 1.0 ng/mg, and localized in the cortex. However, the arsenic signal was more pronounced in the medulla and cuticle for the hair specimen from a person suspected to have been exposed to arsenic.

Collin et al. have investigated taurine uptake by isolated human hair follicles using 1,2-¹³C₂ taurine⁴⁸⁾. Taurine is a β-amino acid generated by the methionine and cysteine metabolism and one of the most abundant free amino acids in the human hair follicle outer root sheath. In situ detection of ¹³C by NanoSIMS revealed the presence of labelled taurine in the dermal sheath in the hair bulb, in the cytoplasm of the outer root sheath, and in the matrix cells.

Hair coloring is a very widespread cosmetic treatment. In oxidative hair coloring, oxidative dyes penetrate the hair fiber and form colored chromophores while melanin pigments in the hair are simultaneously bleached by alkaline hydrogen peroxide. In order to improve the quality of hair coloring products, it is essential to deepen our understanding of the dyeing mechanism. Information on the dyeing regions of the colorants and the dyeing degree in those regions is valuable. Kojima et al. have investigated dyeing features using deuterium-labeled oxidative hair dye^49),50). Deuterium ions were detected in the entire hair cross section and more intensely so in granular melanin. The deuterium to proton ratio (D^-/H^-) indicated that the endocuticle was more dyed than the other cuticle layers. Though the lateral spatial resolution of NanoSIMS is excellent as above mentioned, stable isotope-labeled materials often need to be prepared.

4 New Functional Bioinspired Materials

Since hair is a complex, advanced material, new functional materials inspired by hair are being developed^{90),91),92),93),94)}. Figure 5 shows a representative series of new functional materials. The potential for extracted hair keratins to be used to form self-assembled fibers has recently been introduced⁹⁵⁾. Since then, Lai et al. have been the first to successfully induce hair-extracted keratin delf-assembly in vitro to form dense, homogeneous, and continuous nanofibrous networks⁹¹⁾. Solubilized keratins obtained via reductive extraction were observed to form self-assembled nanofibrous networks. A gradual decrease in fiber diameter from approximately 10 to 6 nm was observed when the pH was reduced from pH 3.3 to pH 2.5. The formation of a tubular structure from human hair via thermal treatment was first reported by Kim⁹⁶⁾. Pramanick et al. have obtained hollow carbon fibers from human hair by pyrolysis at 900°C⁹⁷⁾. The diameter of the hair significantly decreased through the pyrolysis, resulting in hollow carbon-rich fibers with wall thickness of 2-4 µm. This carbonized hollow hair has been utilized for electrochemical applications including electrodes because of its high electrical conductivity and excellent chemical stability. Im et al. have systematically established the formation and further development of hollow microtubes from human hair via a stepwise thermal treatment⁹²⁾. The medulla initially formed hollow microtubes and the cortex was molten when heated around 250°C. Subsequently, the empty space of the hollow tubes increased due to the degradation of the cortex, and the cuticle became compact and smooth when further heated up to 450°C.

Fig. 5

Schematic illustration of new function materials inspired by hair^{91),92),93),94)}.

Ko et al. have reported for the first time that keratin from human hair has potential to be used as a high-capacitance dielectric layer as well as a supporting substrate for organic thin-film transistors⁹³⁾. In order to create high-performance organic thin-film transistors that operate at low voltages, it is essential for the gate insulators to possess high capacitance values. Biopolymers with high densities of aromatic and polar groups have the potential to be used as a high-capacitance gate insulator. The keratin dielectric layer has a high capacitance value of 1.27 µF/cm² at 20 Hz and has displayed high electrical performance as a gate insulator.

Flexible optoelectronics is an emerging research field that has attracted a great deal of interest in recent years. Among flexible substances, carbon fibers⁹⁸⁾, metal wires⁹⁹⁾, and cellulose nanofibers¹⁰⁰⁾are particularly attractive for utilization in the development of smart textiles and wearable optoelectronics. Zhang et al. have successfully developed flexible UV photodetectors based on the Al-doped ZnO/ZnO nanorods / poly (9-vinylcarbazole) / poly (3,4-ethylene dioxythiophene) : poly (stryrenesulfonate) organic-inorganic hybrid heterostructures on the human hair substrate using low-temperature deposition methods⁹⁴⁾. The hair based heterostructure was proven to have outstanding UV response performance with a high responsivity, fast response speed, excellent stability, and great flexibility.

5 Summary

In this review, the recent progress in the structures and physicochemical properties of hair, the latest visualization techniques for hair, and new functional materials inspired by human hair have been introduced. The hair is important not only due to its role as excretory system, sensory organ, and protection of the body but also due to its substantial contribution to our appearance. Since ancient Egyptian times, people have had their hair cut, permed, and colored as they want to complete their look. Hair care technologies such as permanent waving or hair coloring have continuously progressed to meet such demand. In order to develop better hair care products, further developments in trichology are essential. And of course, it goes without saying that the latest analysis techniques are indispensable to support them. Techniques such as AFM-IR or NanoSIMS are surely one of the latest and promising analysis techniques which can give us further insights into the physicochemical properties of hair. The more the spacial resolution and the detection sensitivity of analysis techniques are improved and the more we understand hair properties, the more we want to control the delivery of active ingredients. In the near future, products designed for the delivery to such as medulla, melanin granules, or even CMC may be developed. In addition, grey hair and thinning hair are also absolutely big hair troubles though they are not introduced in this review. It is not the purpose of this review to mention individual hair troubles. Unfortunately, there is still no fundamental treatment for grey hair due to aging, however, causes of grey hair other than aging has been revealed and several effective countermeasures are well known. Regenerative medicine for hair growth and regeneration has been attracting much attention with the development of human stem cell technology. Thinning hair trouble may be completely solved in the near future.

Since hair is a mechanically strong fiber with excellent flexibility, it has been applied for a lot of industrial products for a long time. In Japan, ropes were made by twisting human hair and hemp together. Animal hairs are used for clothes, writing brushes, stringed instruments, and so on. Also, hair has been attracting much attention as a functional biomaterial due to the rise in the momentum toward sustainable society and has already been proven to have high electrical performance. Therefore, hair is now considered to be a realistic protein source for biocompatible flexible electronic devices such as organic memory devices, chemical sensors, energy storage devices, and organic thin-film transistors.

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