Multimodal optical characterisation of collagen photodegradation by femtosecond infrared laser ablation

Two-Photon Imaging for Non-Invasive Corneal Examination

Two-photon imaging (TPI) microscopy, namely, two-photon excited fluorescence (TPEF), fluorescence lifetime imaging (FLIM), and second-harmonic generation (SHG) modalities, has emerged in the past years as a powerful tool for the examination of biological tissues. These modalities rely on different contrast mechanisms and are often used simultaneously to provide complementary information on morphology, metabolism, and structural properties of the imaged tissue. The cornea, being a transparent tissue, rich in collagen and with several cellular layers, is well-suited to be imaged by TPI microscopy. In this review, we discuss the physical principles behind TPI as well as its instrumentation. We also provide an overview of the current advances in TPI instrumentation and image analysis. We describe how TPI can be leveraged to retrieve unique information on the cornea and to complement the information provided by current clinical devices. The present state of corneal TPI is outlined. Finally, we discuss the obstacles that must be overcome and offer perspectives and outlooks to make clinical TPI of the human cornea a reality.

Blue-LIRIC in the rabbit cornea: efficacy, tissue effects, and repetition rate scaling

Laser-induced refractive index change (LIRIC) is being developed as a non-invasive way to alter optical properties of transparent, ophthalmic materials including corneas ex vivo and in vivo. This study examined the optical and biological effects of blue-LIRIC (wavelengths 400-405 nm) of ex-vivo rabbit corneas. Following LIRIC treatment at low and high repetition rates (8.3 MHz and 80 MHz, respectively), we interferometrically measured optical phase change, obtained transmission electron microscopy (TEM) micrographs, and stained histological sections with collagen hybridizing peptides (CHP) to assess the structural and organizational changes caused by LIRIC at different repetition rates. Finally, we performed power and scan speed scaling experiments at three different repetition rates (1 MHz, 8.3 MHz, and 80 MHz) to study their impact on LIRIC efficacy. Histologic co-localization of CHP and LIRIC-generated green autofluorescence signals suggested that collagen denaturation had occurred in the laser-irradiated region. TEM imaging showed different ultrastructural modifications for low and high repetition rate writing, with discrete homogenization of collagen fibrils at 80 MHz, as opposed to contiguous homogenization at 8.3 MHz. Overall, this study confirmed that LIRIC efficacy can be dramatically increased, while still avoiding tissue ablation, by lowering the repetition rate from 80 MHz to 8.3 MHz. Modeling suggests that this is due to a higher, single-pulse, energy density deposition at given laser powers during 8.3 MHz LIRIC.

Physical, Biomechanical, and Optical Characterization of Collagen and Elastin Blend Hydrogels

Collagen and elastin proteins are major components of the extracellular matrix of many organs. The presence of collagen and elastin networks, and their associated properties, in different tissues have led scientists to study collagen and elastin composites for use in tissue engineering. In this study, we characterized physical, biochemical, and optical properties of gels composed of collagen and elastin blends. We demonstrated that the addition of varying amounts of elastin to the constructs alters collagen fibrillogenesis, D-banding pattern length, and storage modulus. However, the addition of elastin does not affect collagen fibril diameter. We also evaluated the autofluorescence properties of the different collagen and elastin blends with fluorescence lifetime imaging (FLIm). Autofluorescence emission showed a red shift with the addition of elastin to the hydrogels. The fluorescence lifetime values of the gels increased with the addition of elastin and were strongly correlated with the storage moduli measurements. These results suggest that FLIm can be used to monitor the gels' mechanical properties nondestructively. These collagen and elastin constructs, along with the FLIm capabilities, can be used to develop and study collagen and elastin composites for tissue engineering and regenerative medicine.

FLIm-Guided Raman Imaging to Study Cross-Linking and Calcification of Bovine Pericardium

Bovine pericardium (BP) is a vascular biomaterial used in cardiovascular surgery that is typically cross-linked for masking antigenicity and enhance stability. There is a need for biochemical evaluation of the tissue properties prior to implantation to ensure that quality and reliability standards are met. Here, engineered antigen removed BP (ARBP) that was cross-linked with 0.2% and 0.6% glutaraldehyde (GA), and further calcified in vitro to simulate graft calcifications upon implantation was characterized nondestructively using fluorescence lifetime imaging (FLIm) to identify regions of interest which were then assessed by Raman spectroscopy. We observed that the tissue fluorescence lifetime shortened, and that Raman bands at 856, 935, 1282, and 1682 cm-1 decreased, and at 1032 and 1627 cm-1 increased with increasing GA cross-linking. Independent classification analysis based on fluorescence lifetime and on Raman spectra discriminated between GA-ARBP and untreated ARBP with an accuracy of 91% and 66%, respectively. Pearson's correlation analysis showed a strong correlation between pyridinium cross-links measured with high-performance liquid chromatography and fluorescence lifetime measured at 380-400 nm (R = -0.76, p = 0.00094), as well as Raman bands at 856 cm-1 for hydroxy-proline (R = -0.68, p = 0.0056) and at 1032 cm-1 for hydroxy-pyridinium (R = 0.74, p = 0.0016). Calcified areas of GA cross-linked tissue showed characteristic hydroxyapatite (959 and 1038 cm-1) bands in the Raman spectrum and fluorescence lifetime shortened by 0.4 ns compared to uncalcified regions. FLIm-guided Raman imaging could rapidly identify degrees of cross-linking and detected calcified regions with high chemical specificity, an ability that can be used to monitor tissue engineering processes for applications in regenerative medicine.

Early evaluation of corneal collagen crosslinking in ex-vivo human corneas using two-photon imaging

The clinical outcome of corneal collagen crosslinking (CXL) is typically evaluated several weeks after treatment. An earlier assessment of its outcome could lead to an optimization of the treatment, including an immediate re-intervention in case of failure, thereby, avoiding additional discomfort and pain to the patient. In this study, we propose two-photon imaging (TPI) as an earlier evaluation method. CXL was performed in human corneas by application of riboflavin followed by UVA irradiation. Autofluorescence (AF) intensity and lifetime images were acquired using a commercial clinically certified multiphoton tomograph prior to CXL and after 2h, 24h, 72h, and 144h storage in culture medium. The first monitoring point was determined as the minimum time required for riboflavin clearance from the cornea. As control, untreated samples and samples treated only with riboflavin (without UVA irradiation) were monitored at the same time points. Significant increases in the stroma AF intensity and lifetime were observed as soon as 2h after treatment. A depth-dependent TPI analysis showed higher AF lifetimes anteriorly corresponding to areas were CXL was most effective. No alterations were observed in the control groups. Using TPI, the outcome of CXL can be assessed non-invasively and label-free much sooner than with conventional clinical devices.

Tissue effects of intra-tissue refractive index shaping (IRIS): insights from two-photon autofluorescence and second harmonic generation microscopy

Intra-tissue refractive index shaping (IRIS) is a novel, non-ablative form of vision correction by which femtosecond laser pulses are tightly focused into ocular tissues to induce localized refractive index (RI) change via nonlinear absorption. Here, we examined the effects of Blue-IRIS on corneal microstructure to gain insights into underlying mechanisms. Three-layer grating patterns were inscribed with IRIS ~180 µm below the epithelial surface of ex vivo rabbit globes using a 400 nm femtosecond laser. Keeping laser power constant at 82 mW in the focal volume, multiple patterns were written at different scan speeds. The largest RI change induced in this study was + 0.011 at 20 mm/s. After measuring the phase change profile of each inscribed pattern, two-photon excited autofluorescence (TPEF) and second harmonic generation (SHG) microscopy were used to quantify changes in stromal structure. While TPEF increased significantly with induced RI change, there was a noticeable suppression of SHG signal in IRIS treated regions. We posit that enhancement of TPEF was due to the formation of new fluorophores, while decreases in SHG were most likely due to degradation of collagen triple helices. All in all, the changes observed suggest that IRIS works by inducing a localized, photochemical change in collagen structure.

Probing endogenous collagen by laser-induced autofluorescence in burn wound biopsies: A pilot study

The focus of the current study was to interrogate the predictive potential of laser-induced autofluorescence (LIAF) by objectively assessing collagen synthesis in burn wound granulation tissues ex vivo. Prior grafting, granulation tissues (20 samples) following burn injury were collected from 17 subjects of age range 18 to 60 years with patient/donor consent and the corresponding autofluorescence spectra were recorded at 325 nm He-Cd laser (≈2 mW) excitations. The resulting endogenous collagen intensity from the above tissue samples was computed by normalizing the nicotinamide adenine dinucleotide levels. In addition, the hydroxyproline content was also estimated biochemically from the same granulation tissues. A comparative assessment of both LIAF and biochemical estimations for endogenous collagen by hydroxyproline resulted in strong positive correlation among them. The above relevant observations suggest that LIAF is equally informative as that of biochemical estimations, in evaluating endogenous collagen content in wound granulation tissues. Thus, it can be concluded that LIAF has the predictive potential, as a noninvasive objective tool to measure the endogenous collagen levels in wound biopsy tissues and provide complementary data conducive for making clinical decisions.

Impact of Body Site, Age, and Gender on the Collagen/Elastin Index by Noninvasive in vivo Vertical Two-Photon Microscopy

BACKGROUND/AIMS

Extrinsic and intrinsic skin aging is subject to constant remodeling and degradation processes, primarily in components of the extracellular matrix. While collagen fibers thin out during the aging process, the amorphous elastin fibers accumulate. These are essential formative components of the dermis. So far, these processes have been detected in vertical histological sections of invasive biopsies and recently in noninvasive horizontal scans.

METHODS

In this pilot study, a modified noninvasive 2-photon microscope was applied to measure the collagen/elastin index of skin in vivo. The obtained images permit an immediate vertical survey and allow a conclusion on the dermal composition at once. The collagen/elastin index was quantified by the second harmonic to autofluorescence aging index of dermis (SAAID) depending on volunteers' age (18-66 years), gender, and body area.

RESULTS

The highest SAAID was measured at the volar forearm as compared to the abdominal SAAID, which was significantly lower (p < 0.05). The gluteal region showed the significantly lowest SAAID (p < 0.05). The SAAID in female skin was higher compared to male skin and decreased with increasing age.

CONCLUSION

These effects are to be considered in subsequent studies to be able to specifically detect and evaluate influences.

Influences of Orally Taken Carotenoid-Rich Curly Kale Extract on Collagen I/Elastin Index of the Skin

Two differently designed, spatially resolved reflectance spectroscopy-based scanners and two-photon tomography were used for noninvasive in vivo determination of cutaneous carotenoids, and collagen I/elastin aging index of dermis, respectively, in the skin of 29 healthy female volunteers between 40 and 56 years of age. The volunteers received a supplement in the form of a carotenoid-rich natural curly kale extract containing 1650 µg of carotenoids in total (three capsules of 550 µg), once a day. Measurements were taken before, after 5 months and after 10 months of daily supplementation. The results showed significantly increased values for the cutaneous carotenoids and the collagen I/elastin aging index of dermis 5 and 10 months after the beginning of the study. The obtained results show that a natural carotenoid-rich extract could prevent the aging-related collagen I degradation in the dermis and improve the extracellular matrix.