Depth-selective photothermal IR spectroscopy of skin: potential application for non-invasive glucose measurement

Abstract

An infrared spectroscopic technique is described that employs a mid-IR broadband (980-1245 cm) tunable quantum cascade laser (QCL) to produce a pump beam, and a detection method based on photothermal deflection, enhanced by total internal reflection. The IR spectra thus obtained are depth-dependent by modulating the pump beam with different frequencies between 10 Hz and 500 Hz. A model system consisting of glucose and a polymer film is used to demonstrate the depth selectivity of this technique. We also apply this photothermal depth profiling method to record in vivo IR spectra of the human epidermis at different depths. This information can be used for a non-invasive glucose monitoring on diabetes patients, which is also demonstrated. Beyond biomedical infrared spectroscopy, there are numerous applications for total internal reflection enhanced photothermal deflection spectroscopy (TIR-PTDS). The high penetration depth of mid-IR light compared to the traditional ATR-FTIR technique and the easy sample access make this technique appropriate for in situ measurements, such as in industrial quality control. The depth selectivity of TIR-PTDS may be a convincing argument for its use in the analysis of multilayered samples or for the analysis of artwork, where the layers of interest are covered by a layer of varnish.

Dermal Targeting Delivery of Terbinafine Hydrochloride Using Novel Multi-Ethosomes: A New Approach to Fungal Infection Treatment

“…The peaks at 1634 and 1541 cm −1 are related to the C-O stretching (amide I) stretching and C-N stretching (amide II) linkage of the helical secondary structure in epidermal keratin, respectively. No significant differences were observed for amide I and amide II bands with and without the ethosome treatment ( Figure S2), indicating that the keratin in SC was not hydrolyzed and the integrity of SC was maintained [37]. Therefore, the ATR-FTIR spectra reveal that after ME treatment the lipid fluidity of the SC barrier was increased, which is favorable to enhance the transdermal permeation of TH, but the integrity of the SC was not altered.…”

“…Skin irritation and toxicity assay of a topical formulation is crucial because of the potential adverse effects [37]. In this study, erythema and edema scores were evaluated to assess their skin irritation.…”

Quantum cascade lasers (QCLs) in biomedical spectroscopy

“…Subsequently, this technique was applied to measure depth-dependent IR spectra of human skin by modulation of the pump beam frequency and thus the thermal diffusion length. 216 With regard to glucose measurements, it has been shown that depth profiling of the human skin can be used to improve quantification result by accounting for individual skin properties and day-to-day variations. Employing PLS, glucose levels could be quantified with a cross validation error of 24 mg dL À1 for in vivo measurements at the hypothenar of the hand.…”

Noninvasive Glucose Measurement Using Machine Learning and Neural Network Methods and Correlation with Heart Rate Variability

“…Measurements of a specific wavelength before and after interaction with matter are compared, and effects on stretching and bending of molecules are used to determine glucose concentrations. A sensor using depth-selective MIR spectroscopy of skin based on total infrared reflection photothermal deflection has been described in [54] and absorption spectroscopy based on a few wave numbers in [55] (iv) Raman spectroscopy [56] evaluates scattering of single wavelength light, which is dependent on rotational or vibrational energy states within a molecule and highly specific absorption bands with respect to original laser light [57]. A multivariate analysis is applied to detected molecule quantity and reduced interference from water compared with MIR or NIR spectroscopy.…”