Measuring changes in chemistry, composition, and molecular structure within hair fibers by infrared and Raman spectroscopic imaging

Optical evaluation of internal damage to human hair based on second near-infrared window polarization microscopy

OBJECTIVE

Hair beauty treatments glorify human life. As a side effect, there is a risk of deteriorating the health of the hair. Optically polarized microscopy has been used for many decades to evaluate hair conditions owing to its ease of use and low operating costs. However, the low biopermeability of light hinders the observation of detailed structures inside hair. The aim of this study is to establish an evaluation technique of internal damages in a hair by utilizing a near-infrared (NIR) light with a wavelength of 1000-1600 nm, called "second NIR window".

METHODS

We built a laser scanning transmission microscope system with an indium gallium arsenide detector, a 1064 nm laser source, and optical circular polarization to visualize the anisotropy characterization of keratin fibres in hair. Samples of Asian black hair before and after bleaching, after permanent-waving, after lithium bromide (LiBr) treatment, and after heating was observed. Some parameters reflecting intra-hair damage were quantitatively compared with the parameters in digitally recorded images with analytical developments.

RESULTS

The light transmittance of black hair was dramatically improved by utilizing the second NIR window. Numerical analysis of circular polarization in hair quantified the internal damage in chemically or thermally treated hair and found two different types of damage. The present method enabled quantitative evaluation of the condition changes in the cortex; for example, a decrease in circular polarizability by LiBr treatment and restoration by replacing the LiBr solution with water. In addition, black speckles were observed after the heat treatment. Longer heating and wetting times increased the appearance probability and size of the speckles. According to quantitative analyses, the emergence of black spots was independent of polarizability changes, indicating that they were not pores.

CONCLUSION

Circular polarization microscopy based on near-infrared optics in the second NIR window provides an effective evaluation method for quantifying intra-hair damage caused by cosmetic treatments. The present method provides noninvasive, easy, and inexpensive hair evaluation and has potential as a gold standard in hair care research/medical fields.

Raman Microspectroscopy of Hair: Low-Frequency Markers of Protein Secondary Structure

Low-frequency intervals that can be used to study the secondary structure of proteins are determined. Compared are Raman spectra of keratins from unpigmented human hair, measured in two experimental configurations: with excitation radiation coaxial with the hair and perpendicular to it. Based on the polarization sensitivity, the bands peaked at 150 and 221 cm-1 are assigned to vibrations of α-helical structures. The comparison of Raman spectra of hair fragments with different contents of secondary structure elements shows that the vibrations of β-structure are manifested in a spectral interval of 270-340 cm-1. The results obtained for a particular object (hair keratin) can be used in the study of the secondary structure of proteins.

Morphological and chemical profiling for forensic hair examination: A review of quantitative methods

Within the past two decades, there have been many studies for quantitative analysis on human hair samples. Microscopical and chemical analysis techniques have been used to analyze various aspects of hair regarding biological, chemical, anthropological, cosmetic, and forensic applications. Studies have attempted to develop quantification methods to increase the evidentiary value of hair in forensic casework. The literature reviewed in this paper provides some of the current techniques used for forensic examinations and quantitative methods. Although microscopical analysis has been scrutinized in the past, using chemical and microscopical techniques can provide a myriad of information. The extraction of DNA from hair provides high-value evidence; however, it may not be readily available and may yield inconclusive results. Hair analysis can be used for many forensic applications such as comparison, toxicology, and exposure analysis. In this article, we will review published research material regarding chemical and microscopical techniques for human hair analysis. Aspects considered for this review were the sample size requirement for analysis and the destructive nature of the instrumental method. This review will focus on both macro and micro quantitative methods for human hair analysis.

Super-resolution infrared microspectroscopy reveals heterogeneous distribution of photosensitive lipids in human hair medulla

Human hair medulla chemical composition appears mostly homogenous when mapped by FTIR microspectroscopy even when using a synchrotron radiation source (SR-μFTIR) but it is expected to be heterogeneous. We performed sub-micron chemical mapping of hair cortex and medullas using Optical Photothermal Infrared microspectroscopy (OPTIR) and a mid-infrared Quantum Cascade Laser (QCL) source covering the fingerprint and the CH stretching region. Photodamages were observed in the hair cortex at mild laser power and occurred in the hair medulla even at the lowest power settings of the IR QCL pulsed at 100 kHz rate (4 μW/μm² average power density) and visible probe laser (200 μw/μm² average power density). Photoconversion of calcium carboxylates in other molecules, possibly sodium carboxylates, was observed. Attenuation of the IR QCL power by 40% using ZnSe filter and/or high-speed measurements (1000 cm^-1/s) succeeded in almost completely eliminating the photodamages and photoconversion. OPTIR maps and images showed that the medullas were highly heterogeneous at the submicron scale. We found calcium carboxylates, aliphatic lipids and wax esters in small units, hundreds of nanometers in size. The 1470 cm^-1 CO sym stretching peak of calcium carboxylates and the CH₂asym stretching peak from aliphatic lipids proved to be the most efficient peaks to track the distribution of these molecules. OPTIR had enough sensitivity to map accurately only the strongest peaks from lipids and calcium carboxylates, weaker peaks such as the ester CO and sulfoxide SO bands were not accurately detected by OPTIR even when they were shown to be present by SR-μFTIR. Quantification of the medulla components by OPTIR is difficult due to several factors: discontinuous QCL emission, and noise. The weaker peaks such as CH₃, CO, SO are often underestimated or not detected. We demonstrate here that OPTIR can be used to measure, map and image dark, photosensitive samples using very low IR power.

A systematic review on the lipid composition of human hair

Hair lipid composition varies by ethnic hair type and by hair layer. Lipids in the cuticle, cortex, and medulla of the hair shaft provide a protective barrier to environmental and chemical damage, prevent hair breakage and desorption, and affect the elastic and tensile properties of hair. The aim of this systematic review is to provide an overview of the lipid composition and ethnic differences of human hair, effects of external damage on lipid content and properties, and changes in hair lipid composition associated with disease states. PubMed/MEDLINE was searched up to March 2021 according to PRISMA guidelines for articles discussing the lipid content of human hair and effects of physical, chemical, or environmental damage, and disease. Fifty-nine articles investigating the lipid content of hair were included for review. Lipids affect fluid permeability, hydration, strength, and texture of ethnic hair fibers. Lipid loss is accelerated by hair-damaging treatments such as bleach, dye, perm, straightening, and surfactant use, and sun and aging processes, leading to dehydrated, breakable, disordered, and dull hair. Diseases including acne, alopecia, and breast, gastric, prostate, lung, and rectal cancers display elevated hair lipid levels. Lipids are vital in protection against damage and maintenance of healthy hair. Further studies are needed to investigate the effects of lipids on the structural properties of ethnic hair, and changes in hair lipid composition with various dermatologic and systemic diseases.

Promotion of glyoxylic acid penetration into human hair by glycolic acid

OBJECTIVE

Glyoxylic acid (GA) is widely used as a straight perming agent for hair care products, however, advanced GA penetration-enhancing agents are desired due to the peculiar odour and hair colour fading caused by the continuous use of GA products. Hence, it is important to develop a penetration-enhancing agent that helps minimize the GA concentration. We have found that the combined use of GA and glycolic acid (GCA) has a strong hair straightening effect.

METHODS

Straightening hair test was carried out to the evaluation of the effect of additives. Liquid chromatography-mass spectrometry (LC/MS) was performed to quantify the GA penetration amount into human hair. Attenuated total reflection (ATR) Fourier transform-infrared spectroscopy (FT-IR) and FT-IR microscope were implemented to estimate the localization of GA in the hair.

RESULTS

Straightening hair tests indicated that the hair straightening effect by GA was enhanced by the presence of GCA. LC/MS results showed that the addition of GCA enhanced the amount of GA that penetrated human hair by about four times. ATR FT-IR and FT-IR microscope measurements indicated that GA was localized more in the innermost region of hair (medulla) than the cortex and cuticle. The GA accumulated in the medulla disappeared after a hair straightener treatment at 180°C due to the chemical reaction.

CONCLUSIONS

The GA penetration-enhancing effect of GCA is worth investigating to reduce the GA concentration in products for more comfortable use.

Characterization by Raman and infrared spectroscopy and fluorescence microscopy of human hair treated with cosmetic products

Analytical studies on hair structures have evolved significantly over the years and vibrational spectroscopic techniques, such as Raman and infrared, have been increasingly used for such purposes. Nowadays, there is a need to understand more and more about the action of cosmetics on the hair fiber, so this work aims to analyze the permeation of cosmetic treatments into the hair. For the molecular structural characterization, Raman and infrared spectroscopy techniques were used, being verified the efficiency in the analysis of hair samples, demonstrating the internal characteristics of the fiber and the permeation of different cosmetics. Four cosmetics were chosen for this study and, due to the techniques used, it was possible to observe the diffusion of these products inside the bleached hair. It was observed with the Raman vibrational spectroscopy that the concentration of the products is found mainly in the cuticular region, decreasing the permeate content when approaching the central region, and the infrared spectroscopy showed results compatible with the Raman spectroscopy. Therefore, vibrational spectroscopy proved to be a valuable tool for the study of cosmetic permeation into the hair fiber and for the analysis of its external and internal structure.

New approach for hair keratin characterization: use of the confocal Raman spectroscopy to assess the effect of a thermal stress on human hair fiber

OBJECTIVE

The objective of our research was to investigate the heat-protecting effect of a product ex vivo and in vivo on human hair fibers.

METHODS

A preparatory study was carried out in order to determine an optimal threshold of thermal stress. For this, the structure of cross-sections of the hair fiber was observed by optical microscopy. Then, Scanning Electron Microscopy (SEM) and Confocal Raman Spectroscopy (CRS) were applied to analyze ex vivo and in vivo morphological and molecular damage in hair structure after heat stress. Finally, in vivo tests were used to collect consumer perception.

RESULTS

The preparatory study enabled us to determine an optimal stress threshold of 10 heating cycle for SEM and 5 heating cycle for CRS. Based on spectral hierarchical classification using Ward's clustering algorithm, the ex vivo Raman results show that the spectral signature of the hair treated and heated is very close to the negative control. This shows that the product preserves the keratin structure after thermal stress. These results were also confirmed by an in vivo Raman analysis performed on hair samples from 5 donors. In concordance with Raman results, SEM show that treated hair present lesser "bubbles" and "crackling" on the hair surface. Finally, the in vivo studies proved that hair was more protected from the heat.

CONCLUSION

The authors concluded that the product shows protective properties with respect to morphological and molecular heat damage. We also demonstrate that the product promotes the α-helix keratin conformation and preserves the S-S disulfide bands.

Synchrotron Radiation Fourier Transform Infrared (SR-FTIR) spectroscopy in exploring ancient human hair from Roman period Juliopolis: Preservation status and alterations of organic compounds

We explore the preservation status and alterations of organic compounds in Roman period human hairstrandsfrom a specific individual (M196) excavated at Juliopolis (JP). How do these organic compounds present in this c. 2000-year-old human hair compare to those present in modern hair? Alterations to organic compounds in archaeological human hair are caused by biological degradative processes dependent on multifactorial processes acting on the hair since the deposition of a body in a mortuary context. We investigate the type of organic compounds present using Synchrotron Radiation Fourier Transform Infrared (SR-FTIR). Juliopolis (Iuliopolis) is an ancient multiperiod city, located in the Çayırhan district of Nallıhan, northwest of Ankara. The Juliopolis necropolis from which M196 was recovered was in use throughout the Hellenistic, Roman, and Byzantine periods, and yielded over 700 tombs with numerous human remains. One tomb (M196) contained human remains of exceptional preservation status, including substantial amounts of hair. Human hair from archaeological contexts is not only extremely rare, but importantly, has high analytical value, with potential for analysis of diet, geographical origins, ancient DNA, metal exposure, and other aspects of life in a time-resolved manner. These data make significant contributions to the life history of the individual (osteobiography), as well as contribute towards key archaeological questions. As these analyses are in their majority destructive, prior evaluation of the preservation of sufficient amounts of the organic compounds on which many such analyses rely upon is crucial, to avoid unnecessary loss of precious ancient samples. The results of our SR-FTIR analyses at SESAME synchrotron show that keratin in the JP M196 is more degraded in comparison to the modern reference sample. However, the results also point to clear potential for further analyses with techniques relying on organic compound preservation, such as C and N isotopic analyses for diet, and aDNA.

Analysis and comparison of protein secondary structures in the rachis of avian flight feathers

Avians have evolved many different modes of flying as well as various types of feathers for adapting to varied environments. However, the protein content and ratio of protein secondary structures (PSSs) in mature flight feathers are less understood. Further research is needed to understand the proportions of PSSs in feather shafts adapted to various flight modes in different avian species. Flight feathers were analyzed in chicken, mallard, sacred ibis, crested goshawk, collared scops owl, budgie, and zebra finch to investigate the PSSs that have evolved in the feather cortex and medulla by using nondestructive attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR). In addition, synchrotron radiation-based, Fourier transform infrared microspectroscopy (SR-FTIRM) was utilized to measure and analyze cross-sections of the feather shafts of seven bird species at a high lateral resolution to resolve the composition of proteins distributed within the sampled area of interest. In this study, significant amounts of α-keratin and collagen components were observed in flight feather shafts, suggesting that these proteins play significant roles in the mechanical strength of flight feathers. This investigation increases our understanding of adaptations to flight by elucidating the structural and mechanistic basis of the feather composition.

Lipid distribution on ethnic hairs by Fourier transform infrared synchrotron spectroscopy

BACKGROUND

A synchrotron-based Fourier transform infrared micro-spectrometer (μ-FTIR) allows the spatial determination of lipids across the different layers of ethnic hairs and differentiates between the lipid order arrangement and quantity.

MATERIALS AND METHODS

The three ethnic fibers were delipidized, the lipid extracts were characterized, and the delipidized fibers were studied by dynamic vapor sorption experiments (DVS) and FTIR-synchrotron techniques.

RESULTS

The average spectra from the different hair regions exhibited the most intense CH₂ sym peaks on the medulla, followed by those from the cuticle and cortex for all hairs of different ethnicities. Differences in the lipid fraction of the three hair types have been observed, and they can explain some barrier properties. African virgin hair was demonstrated to have more lipids mainly in the medulla, which implies an important hydrophobicity with low hysteresis between absorption and desorption water vapor processes. In addition, these lipids are highly disordered, mainly in the cuticle, which can be related to its high water vapor diffusion. Asian and Caucasian virgin hairs presented a similar lipid order in all regions, with similar diffusion coefficients. Results indicate that the higher order of the lipid bilayer hinders water permeation kinetics in some way.

CONCLUSION

The differences in the presence and organization of the lipids in the different regions of the African hair can account for its differentiation with regards to moisturization and swelling from the other types of fibers.

Using hybrid atomic force microscopy and infrared spectroscopy (AFM-IR) to identify chemical components of the hair medulla on the nanoscale

Atomic force microscopy integrated with infrared spectroscopy (AFM-IR) has been used to topographically and chemically examine the medulla of human hair fibres with nanometre scale lateral resolution. The mapping of cross-sections of the medulla showed two distinct structural components which were subsequently characterised spectroscopically. One of these components was shown to be closely similar to cortical cell species, consistent with the fibrillar structures found in previous electron microscope (EM) investigations. The other component showed large chemical differences from cortical cells and was assigned to globular vacuole species, also confirming EM observations. Further characterisation of the two components was achieved through spectral deconvolution of the protein Amide-I and -II bands. This showed that the vacuoles have a greater proportion of the most thermodynamically stable conformation, namely the antiparallel β-sheet structures. This chimes with the observed lower cysteine concentration, indicating a lower proportion of restrictive disulphide cross-link bonding. Furthermore, the large α-helix presence within the vacuoles points to a loss of matrix-like material as well as significant intermolecular stabilisation of the protein structures. By analysing the carbonyl stretching region, it was established that the fibrillar, cortical cell-like components showed considerable stabilisation from H-bonding interactions, similar to the cortex, involving amino acid side chains whereas, in contrast, the vacuoles were found to only be stabilised significantly by structural lipids.

A new typology of human hair medullas based on lipid composition analysis by synchrotron FTIR microspectroscopy

Human hair is an organ that connects fundamental and applied research with everyday life through the cosmetic industry. Yet, the accurate compositional description of the human hair medulla is lacking due to their small size and difficulty with microextraction. Medullas are thus generally classified based on morphology. We investigated the chemical composition of the human hair medulla using synchrotron based infrared microspectroscopy. We confirmed that lipid signatures dominate the medulla infrared spectrum having 3-20 times higher lipid concentration compared to their surrounding cortex. Human hair medullas contain a mixture of non-esterified and esterified lipids, and carboxylate soaps in various proportions. We reveal the first direct spectroscopic evidence that medulla carboxylates are coordinated to calcium since they exhibit the specific calcium carboxylate signature. Using a representative sample, we observed a strong compositional variability between medullas that was unreported before. We detected calcium carboxylates in 76% of the medullas with one order of magnitude concentration variability between samples. All medullas contained esters with esterification varying by a factor of 30. Moreover, we detected the presence of crystalline calcium stearate in 9% of the medullas. We described a series of spectral markers to characterize medullas based on their lipid composition and propose to classify medullas in four to five groups. Our analysis provides a more detailed understanding of the chemical composition of human hair medullas that may impact cosmetics and biology. The origin and biological meaning of these variations must still be investigated.

New Diagnostic Tools to Evaluate Hair Loss

The precise and reliable diagnosis of hair loss disorders is essential for developing a successful management plan. It is, thus, the responsibility of the dermatologist to select the appropriate diagnostic tools to effectively evaluate patients presenting with hair loss concerns. Fortunately, there is a growing body of noninvasive and invasive diagnostic resources, each with advantages and disadvantages. For the practicing dermatologist, tactile assessments and direct visualization are enhanced with scoring instruments, questionnaires, handheld trichoscopy, and scalp biopsy. For research and clinical study purposes, the more precise, high-resolution tools such as videodermoscopy, optical coherence tomography, and phototrichograms, may be useful.

Visualizing intra-medulla lipids in human hair using ultra-multiplex CARS, SHG, and THG microscopy

We performed label-free imaging of human-hair medulla using multi-modal nonlinear optical microscopy. Intra-medulla lipids (IMLs) were clearly visualized by ultra-multiplex coherent anti-Stokes Raman scattering (CARS) spectroscopic imaging. Two groups of IMLs were found: second harmonic generation (SHG) active and inactive. By combining SHG analysis with CARS, the two groups were identified as free fatty acids and wax esters, respectively.

Exploring the Mesoscopic Morphology in Mussel Adhesive Proteins by Soft X-ray Spectromicroscopy

Marine mussels efficiently adhere under wet conditions by precisely controlling the hierarchical structure of the adhesive plaque through sequential mussel foot protein secretion in the foot-tip cavity. Chemical analysis of the non-uniform mussel plaque morphology has been performed using spectromicroscopy; however, the mesoscopic morphology has not been elucidated yet because of the limited spatial resolution of conventional chemical imaging techniques. We investigated the chemical speciation in the non-uniform mussel plaque morphology employing scanning transmission soft X-ray spectromicroscopy (STXM). The high-spatial-resolution STXM chemical imaging with C 1s near-edge X-ray absorption fine structure yields the distribution of the hydroxy-substituted aromatic residues in the sub-micron scale non-uniform mussel plaque morphology. The matrix consists of a high-protein-density cured product containing a large number of hydroxy-substituted aromatic carbons, including tyrosine and 3,4-dihydroxyphenylalanine (Dopa), whereas the microdomains are poor-protein-density regions with a low aromatic residue relative content. The adhesive interface was covered with the matrix phase to ensure adhesion. The cuticle layer involves a moderate Dopa content, which appears to be optimized for the mechanical performance of the skin.

Ethnic hair: Thermoanalytical and spectroscopic differences

BACKGROUND

The aim of this study is to characterize and detect the possible differences among the hair of three different ethnicities: African, Asiatic and Caucasian.

MATERIALS AND METHODS

The differences in water adsorption/desorption behaviour of hairs were studied using a thermogravimetric balance and compared with the analysis of the lipid distribution and order using synchrotron-based Fourier transform infrared microspectroscopy. Besides, the thermal thermogravimetry (TG) and differential scanning calorimetry (DSC) analyses on human hair were executed.

RESULTS

Differences in the diffusion coefficients were evidenced. African hair exhibited increased permeability. Caucasian hair displayed a higher water absorption capability with increasing humidity but with a slow diffusion rate. The Asian fibre appeared to be more resistant to hydration changes. The spectroscopic analysis showed notable differences in the cuticle lipids. The African cuticle exhibited more lipids with a lower order bilayer. The outmost layer of Caucasian fibres contained more ordered lipids, and the Asian fibres show a very low level of lipids on the cuticle region. The DSC results indicate no difference in the thermal stability and TG showed higher water content in the Caucasian fibre and a possible lower cysteine disulphide bond content in the African hair matrix.

CONCLUSION

The triple approach demonstrated the permeability differences among the ethnic fibres and their correlation with the properties of their cuticle lipids. These differences could have particular relevance to the hair care cosmetic market.

An Analysis of Isolated and Intact RBC Membranes-A Comparison of a Semiquantitative Approach by Means of FTIR, Nano-FTIR, and Raman Spectroscopies

This work presents the potential of vibrational spectroscopy, Vis and NIR Raman spectroscopy, Fourier transform infrared spectroscopy (FTIR) in reflection and transmission modes, and nano-FTIR microscopy to study the biochemical alterations in membranes of isolated and intact red blood cells (RBCs). The main goal was to propose the best spectroscopic method which enabled following biochemical alterations in the RBC membranes and then to translate this spectroscopic signature of degradation to in situ analysis of RBCs. Two models corresponding to two distinct cases of RBC membrane conditions were employed, and they were derived from healthy and young mice and mature mice with advanced atherosclerosis. It was shown that each technique provided essential information about biochemical alterations of the isolated membranes as well as membranes in the intact RBCs, which can be used in the development of a rapid and in situ analytical technology. Finally, we proposed that the combination of macro- and nanoprobing implemented in IR spectroscopy provided a wide chemical characterization of the RBC membranes, including alterations in lipid and protein fractions. This study also examined the effect of the sample preparation to determine destructive factors influencing a spectroscopic analysis of isolated membranes and intact RBCs derived from healthy and disease-affected mice.

Conformation changes in human hair keratin observed using confocal Raman spectroscopy after active ingredient application

OBJECTIVE

In hair care cosmetic products' evaluation, one commonly used method is to evaluate the hair appearance as a gold standard in order to determine the effect of an active ingredient on the final state of the hair via visual appreciation. Although other techniques have been proposed for a direct analysis of the hair fibres, they give only surface or structural information, without any accurate molecular information. A different approach based on confocal Raman spectroscopy has been proposed for tracking in situ the molecular change in the keratin directly in the human hair fibres. It presents a high molecular specificity to detect chemical interactions between molecules and can provide molecular information at various depths at the cortex and cuticle levels.

METHODS

To evaluate the potential of confocal Raman spectroscopy in testing the efficiency of cosmetic ingredients on keratin structure, we undertook a pilot study on the effectiveness of a smoothing shampoo on natural human hair, by analysing α-helix and β-sheet spectral markers in the Amide I band and spectral markers specific to the cystin sulfur content.

RESULTS

We confirmed that an active proved to be effective on a gold standard decreases α-helix keratin conformation and promotes β-sheet keratin conformation in the hair fibres. We also showed that treatment with the effective active decreases the intensity of covalent disulfide (S-S at 510 cm-1 ) cross-linking bands of cysteine. These data confirm that the effective active also acts on the tertiary structure of keratin.

CONCLUSION

From these experiments, we concluded that the effective active has a smoothing effect on the human hair fibres by acting on α-helix and β-sheet keratin conformation and on the tertiary structure of keratin. Based on these results, confocal Raman spectroscopy can be considered a powerful technique for investigating the influence of hair cosmetic ingredients on keratin structure in human hair fibres. Moreover, this analytical technique has the advantage of being non-destructive and label free; in addition, it does not require sample extraction or purification and it can be applied routinely in cosmetic laboratories.

Raman spectroscopy and electronic microscopy structural studies of Caucasian and Afro human hair

Human hair fibre is subjected to various structural modifications due to the application of chemicals such as dyes, shampoos and bleaches and/or physical procedures such as heating, and often more than one procedure is performed on the same hair. The present work aims to analyze the changes incurred in hair samples of two ethnic groups, namely Caucasian and Afro, before and after different treatments such as thermal, bleaching and straightening. In addition to observing the damage caused by each treatment separately, the study of samples that received all three treatments was carried out. For molecular structural characterization, the Raman vibrational spectroscopic technique was used and scanning electron microscopy (SEM) was used for morphological analysis of the hair fibres. This investigation has shown, through vibrational spectroscopy, that several important bonds have been modified, such as the S-S, C-S, C-C and S-O bonds as well as the secondary structures of proteins that have indergone changes in their conformation as a result of the treatment. Hair from the two ethnic groups showed small differences in relation to each applied treatment. Excessive heat generated a higher rate of Raman spectral band intensity changes when compared to the other treatments and it was observed that the action of several treatments on the same hair fibres resulted in even more pronounced structural changes. Finally, scanning electron microscopy showed that each treatment caused a different morphological deformation pattern on the capillary surface of the human hair.

Bioinspired Histidine⁻Zn2+ Coordination for Tuning the Mechanical Properties of Self-Healing Coiled Coil Cross-Linked Hydrogels

Natural biopolymeric materials often possess properties superior to their individual components. In mussel byssus, reversible histidine (His)–metal coordination is a key feature, which mediates higher-order self-assembly as well as self-healing. The byssus structure, thus, serves as an excellent natural blueprint for the development of self-healing biomimetic materials with reversibly tunable mechanical properties. Inspired by byssal threads, we bioengineered His–metal coordination sites into a heterodimeric coiled coil (CC). These CC-forming peptides serve as a noncovalent cross-link for poly(ethylene glycol)-based hydrogels and participate in the formation of higher-order assemblies via intermolecular His–metal coordination as a second cross-linking mode. Raman and circular dichroism spectroscopy revealed the presence of α-helical, Zn²⁺ cross-linked aggregates. Using rheology, we demonstrate that the hydrogel is self-healing and that the addition of Zn²⁺ reversibly switches the hydrogel properties from viscoelastic to elastic. Importantly, using different Zn²⁺:His ratios allows for tuning the hydrogel relaxation time over nearly three orders of magnitude. This tunability is attributed to the progressive transformation of single CC cross-links into Zn²⁺ cross-linked aggregates; a process that is fully reversible upon addition of the metal chelator ethylenediaminetetraacetic acid. These findings reveal that His–metal coordination can be used as a versatile cross-linking mechanism for tuning the viscoelastic properties of biomimetic hydrogels.

Localization of human hair structural lipids using nanoscale infrared spectroscopy and imaging

Atomic force microscopy (AFM) and infrared (IR) spectroscopy have been combined in a single instrument (AFM-IR) capable of producing IR spectra and absorption images at a sub-micrometer spatial resolution. This new device enables human hair to be spectroscopically characterized at levels not previously possible. In particular, it was possible to determine the location of structural lipids in the cuticle and cortex of hair. Samples of human hair were embedded, cross-sectioned, and mounted on ZnSe prisms. A tunable IR laser generating pulses of the order of 10 ns was used to excite sample films. Short duration thermomechanical waves, due to infrared absorption and resulting thermal expansion, were studied by monitoring the resulting excitation of the contact resonance modes of the AFM cantilever. Differences are observed in the IR absorbance intensity of long-chain methylene-containing functional groups between the outer cuticle, middle cortex, and inner medulla of the hair. An accumulation of structural lipids is clearly observed at the individual cuticle layer boundaries. This method should prove useful in the future for understanding the penetration mechanism of substances into hair as well as elucidating the chemical nature of alteration or possible damage according to depth and hair morphology.

Confocal Raman spectroscopy of whole hairs

This paper describes the application of Raman spectroscopy to whole hair fibers. Previously this has proved difficult because the hairs are relatively opaque, and spatial resolution diminishes with depth because of the change in refractive index. A solution is to couple confocal Raman with multivariate curve resolution (MCR) data analysis, which separates spectral differences with depth despite this reduction in resolution. Initially, it is shown that the cuticle can be separated from the cortex, showing the differences in the proteins, which can then be plotted as a function of depth, with the cuticle factor being seen only at the surface as expected. Hairs that had been treated in different ways, e.g., by bleaching, treatment with the active molecule resorcinol followed by rinsing and treatment with a full hair care product, were also examined. In all cases, changes to the hair are identified and are associated with specific parts of the fiber. Since the hair fiber is kept intact, it can be repeatedly treated and measured, hence multistep treatment processes can be followed. This method expands the potential use of Raman spectroscopy in hair research.

Fourier transform infrared photoacoustic spectroscopy study of physicochemical interaction between human dentin and etch-&-rinse adhesives in a simulated moist bond technique

The purpose of this study was to provide the physicochemical interactions at the interfaces between two commercial etch-&-rinse adhesives and human dentin in a simulated moist bond technique. Six dentin specimens were divided into two groups (n=3) according to the use of two different adhesive systems: (a) 2-hydroxyethylmethacrylate (HEMA) and 4-methacryloxyethyl trimellitate anhydrate (4-META), and (b) HEMA. The Fourier transform infrared photoacoustic spectroscopy was performed before and after dentin treatment with 37% phosphoric acid, with adhesive systems and also for the adhesive systems alone. Acid-conditioning resulted in a decalcification pattern. Adhesive treated spectra subtraction suggested the occurrence of chemical bonding to dentin expressed through modifications of the OH stretching peak (3340 cm(-1)) and symmetric CH stretching (2900 cm(-1)) for both adhesives spectra; a decrease of orthophosphate absorption band (1040 to 970 cm(-1)) for adhesive A and a better resolved complex band formation (1270 to 970 cm(-1)) for adhesive B were observed. These results suggested the occurrence of chemical bonding between sound human dentin and etch-&-rinse adhesives through a clinical typical condition.