Photo-aging evaluation - In vitro biological endpoints combined with collagen density assessment with multi-photon microscopy

Ultraviolet light induces mechanical and structural changes in full thickness human skin

While the detrimental health effects of prolonged ultraviolet (UV) irradiation on skin health have been widely accepted, the biomechanical process by which photoaging occurs and the relative effects of irradiation with different UV ranges on skin biomechanics have remained relatively unexplored. In this study, the effects of UV-induced photoageing are explored by quantifying the changes in the mechanical properties of full-thickness human skin irradiated with UVA and UVB light for incident dosages up to 1600 J/cm2. Mechanical testing of skin samples excised parallel and perpendicular to the predominant collagen fiber orientation show a rise in the fractional relative difference of elastic modulus, fracture stress, and toughness with increased UV irradiation. These changes become significant with UVA incident dosages of 1200 J/cm2 for samples excised both parallel and perpendicular to the dominant collagen fiber orientation. However, while mechanical changes occur in samples aligned with the collagen orientation at UVB dosages of 1200 J/cm2, statistical differences in samples perpendicular to the collagen orientation emerge only for UVB dosages of 1600 J/cm2. No notable or consistent trend is observed for the fracture strain. Analyses of toughness changes with maximum absorbed dosage reveals that no one UV range is more impactful in inducing mechanical property changes, but rather these changes scale with maximum absorbed energy. Evaluation of the structural characteristics of collagen further reveals an increase in collagen fiber bundle density with UV irradiation, but not collagen tortuosity, potentially linking mechanical changes to altered microstructure.

In vivo detection of healthy skin: Comparison of multiphoton microscopy and reflectance confocal microscopy

BACKGROUND

Noninvasive skin examination evolved rapidly in recent years, with multiphoton microscopy (MPM) and reflectance confocal microscopy (RCM) being used to image in-vivo skin at high resolution. The aim of this study is to compare the imaging clarity between the two techniques and measure the thickness of the epidermis in different body sites. We also measured the degree of skin aging with noninvasive tools.

METHODS

Fifty-six volunteers were evaluated and measured at three different body sites, including the cheek, volar forearm, and back. We used RCM and MPM to evaluate the clarity of each skin layer, including stratum corneum, stratum granulosum, stratum spinosum, dermo-epidermal junction, and dermis. We measured epidermal thickness (ET) at the three body sites in individuals of different ages and genders. We assessed skin aging by the second harmonic to autofluorescence aging index of dermis (SAAID), and multiple linear regression was used to analyze the factors affecting SAAID.

RESULTS

MPM had advantages in observation of stratum granulosum, collagen fiber, and elastic fiber (p < 0.001), but RCM provided better observation in dermo-epidermal junction layer (p < 0.001). The epidermis was thicker in the cheek area than the volar forearm and back in both RCM and MPM detection, and the average ET measured by MPM was lower than RCM. ET varied among the three body sites with significant differences (p < 0.05). ET was significantly lower at almost all sites in individuals above 40y (p < 0.05). SAAID decreased with age, and more rapidly in women. Cheeks have lower SAAID scores than other body sites.

CONCLUSION

MPM and RCM provide noninvasive methods for imaging skin and each method has its own advantages. Epidermal thickness and SAAID correlated with age, gender, and different body sites. MPM could also assess the degree of skin aging, which could guide the clinical treatment of patients with diffferent ages and genders in the above body sites.