Application of Multiphoton Microscopy in Dermatological Studies: a Mini-Review

Handheld multiphoton and pinhole-free reflectance confocal microscopy enables noninvasive, real-time cross-sectional imaging in skin

Biopsy-based histology has been the foundation of disease diagnosis and management for over a century. A long-sought goal in dermatology is the development of an imaging modality with sufficient resolution and compositional detail to noninvasively interrogate skin histology in vivo. Here, we describe a system that achieves this goal using cross-sectionally scanned, multimodal microscopy (cross-modal). Cross-modal combines multiphoton and reflectance confocal microscopy into one compact system with coordinated three-axis scanning that preserves optical resolution in cross-section. A custom pinhole-free mechanism employing finite-infinite conjugates further simplifies and stabilizes confocal alignment. Evaluated in participants ages 9-81 and Fitzpatrick skin types (FST) 1-5, cross-modal images revealed histological details analogous to those obtained from traditional biopsied tissue. We observed dermal elastosis in sun-damaged skin, elevated melanin in pigmented skin, basaloid nests in basal cell carcinoma, and elongated rete ridges in seborrheic keratosis, supporting cross-modal's potential to deliver histological insights noninvasively.

Seeing the unseen: The role of bioimaging techniques for the diagnostic interventions in intervertebral disc degeneration

Intervertebral Disc Degeneration is a pathophysiological condition that primarily affects the spinal discs, causing back pain and neurological deficits. It is caused by the contribution of several factors such as genetic predisposition, age-related degeneration, and lifestyle choices like obesity and physical activity. Even though there are medications to treat pain, there is a lack of medicines for a complete cure. The main difficulty lies in poor diagnosis of the morphological and functional changes in the disc. With the ever-increasing research on bioimaging techniques, new techniques are being developed and repurposed to evaluate disc shape and composition, and their defects like thinning or deformities on the disc, leading to the proper diagnostic intervention in intervertebral disc degeneration. In this review, we aim to present a comprehensive overview of the imaging techniques used in the pre-clinical and clinical stages for the diagnosis of intervertebral disc degeneration. First, we will discuss about patho-anatomy and the pathophysiology of degenerative disc disease with the significance and a brief description of various dyes and tracers utilized for bioimaging. Then we will shed light on the latest advancements in diagnostic modalities in intervertebral disc degeneration; concluded by an analysis of the repercussions of the methodologies and experimental systems employed in identifying mechanisms and developing therapeutic strategies in intervertebral disc degeneration.

Cysteine as an Innovative Biomarker for Kidney Injury

BACKGROUND

Kidney transplantation is a widely used treatment for end-stage kidney disease. Nevertheless, the incidence of acute kidney injury (AKI) in deceased donors poses a potential hazard because it significantly increases the risk of delayed graft function and potentially exerts an influence on the kidney allograft outcome. It is crucial to develop a diagnostic model capable of assessing the existence and severity of AKI in renal grafts. However, no suitable kidney injury markers have been developed thus far.

METHODS

We evaluated the efficacy of the molecular probe NPO-B, which selectively responds to cysteine, as a new diagnostic tool for kidney injury. We used an in vitro model using ischemia/reperfusion injury human kidney-2 cells and an in vivo ischemia/reperfusion injury mouse model. Additionally, cysteine was investigated using urine samples from deceased donors and living donors to assess the applicability of detection techniques to humans.

RESULTS

This study confirmed that the NPO-B probe effectively identified and visualized the severity of kidney injury by detecting cysteine in both in vitro and in vivo models. We observed that the fluorescence intensity of urine samples measured using NPO-B from the deceased donors who are at a high risk of renal injury was significantly stronger than that of the living donors.

CONCLUSIONS

If implemented in clinical practice, this new diagnostic tool using NPO-B can potentially enhance the success rate of kidney transplantation by accurately determining the extent of AKI in renal grafts.

Non-invasive 3D imaging of human melanocytic lesions by combined ultrasound and photoacoustic tomography: a pilot study

The accurate determination of the size and depth of infiltration is critical to the treatment and excision of melanoma and other skin cancers. However, current techniques, such as skin biopsy and histological examination, pose invasiveness, time-consumption, and have limitations in measuring at the deepest level. Non-invasive imaging techniques like dermoscopy and confocal microscopy also present limitations in accurately capturing contrast and depth information for various skin types and lesion locations. Thus, there is a pressing need for non-invasive devices capable of obtaining high-resolution 3D images of skin lesions. In this study, we introduce a novel device that combines 18 MHz ultrasound and photoacoustic tomography into a single unit, enabling the acquisition of colocalized 3D images of skin lesions. We performed in vivo measurements on 25 suspicious human skin nevi that were promptly excised following measurements. The combined ultrasound/photoacoustic tomography imaging technique exhibited a strong correlation with histological Breslow thickness between 0.2 and 3 mm, achieving a coefficient of determination (R[Formula: see text]) of 0.93, which is superior to the coefficients from the individual modalities. The results procured in our study underscore the potential of combined ultrasound and photoacoustic tomography as a promising non-invasive 3D imaging approach for evaluating human nevi and other skin lesions. Furthermore, the system allows for integration of other optical modalities such as optical coherence tomography, microscopy, or Raman spectroscopy in future applications.

A Dual-Modality Imaging Method Based on Polarimetry and Second Harmonic Generation for Characterization and Evaluation of Skin Tissue Structures

The characterization and evaluation of skin tissue structures are crucial for dermatological applications. Recently, Mueller matrix polarimetry and second harmonic generation microscopy have been widely used in skin tissue imaging due to their unique advantages. However, the features of layered skin tissue structures are too complicated to use a single imaging modality for achieving a comprehensive evaluation. In this study, we propose a dual-modality imaging method combining Mueller matrix polarimetry and second harmonic generation microscopy for quantitative characterization of skin tissue structures. It is demonstrated that the dual-modality method can well divide the mouse tail skin tissue specimens' images into three layers of stratum corneum, epidermis, and dermis. Then, to quantitatively analyze the structural features of different skin layers, the gray level co-occurrence matrix is adopted to provide various evaluating parameters after the image segmentations. Finally, to quantitatively measure the structural differences between damaged and normal skin areas, an index named Q-Health is defined based on cosine similarity and the gray-level co-occurrence matrix parameters of imaging results. The experiments confirm the effectiveness of the dual-modality imaging parameters for skin tissue structure discrimination and assessment. It shows the potential of the proposed method for dermatological practices and lays the foundation for further, in-depth evaluation of the health status of human skin.

Label-Free Imaging and Histo-Optical Evaluation of Head and Neck Cancers with Multiphoton Autofluorescence Microscopy

Depth-resolved label-free optical imaging by the method of multiphoton autofluorescence microscopy (MPAM) may offer new ways to examine cellular and extracellular atypia associated with epithelial squamous cell carcinoma (SCC). MPAM was evaluated for its ability to identify cellular and microstructural atypia in head and neck tissues from resected discarded tumor tissue. Three-dimensional image volumes were obtained from tissues from the floor of the mouth, tongue, and larynx, and were then processed for histology. MPAM micrographs were evaluated for qualitative metrics of cell atypia and quantitative measures associated with nuclear pleomorphism. Statistical analyses correlated MPAM endpoints with histological grade from each imaged site. Cellular overcrowding, discohesion, anisonucleosis, and multinucleated cells, as observed through MPAM, were found to be statistically associated with dysplasia and SCC grading, but not in histologically benign regions. A quantitative measure of the coefficient of variance in nuclear size in SCC and dysplasia was statistically elevated above histologically benign regions. MPAM also allowed for the identification of cellular heterogeneity across transitional areas and other features, such as inflammatory infiltrates. In the future, MPAM could be evaluated for the non-invasive detection of neoplasia, possibly as an adjunct to traditional conventional examination and biopsy.

Skin Cancer Pathobiology at a Glance: A Focus on Imaging Techniques and Their Potential for Improved Diagnosis and Surveillance in Clinical Cohorts

Early diagnosis is essential for completely eradicating skin cancer and maximizing patients' clinical benefits. Emerging optical imaging modalities such as reflectance confocal microscopy (RCM), optical coherence tomography (OCT), magnetic resonance imaging (MRI), near-infrared (NIR) bioimaging, positron emission tomography (PET), and their combinations provide non-invasive imaging data that may help in the early detection of cutaneous tumors and surgical planning. Hence, they seem appropriate for observing dynamic processes such as blood flow, immune cell activation, and tumor energy metabolism, which may be relevant for disease evolution. This review discusses the latest technological and methodological advances in imaging techniques that may be applied for skin cancer detection and monitoring. In the first instance, we will describe the principle and prospective clinical applications of the most commonly used imaging techniques, highlighting the challenges and opportunities of their implementation in the clinical setting. We will also highlight how imaging techniques may complement the molecular and histological approaches in sharpening the non-invasive skin characterization, laying the ground for more personalized approaches in skin cancer patients.

Multimodal system for optical biopsy of melanoma with integrated ultrasound, optical coherence tomography and Raman spectroscopy

We introduce a new single-head multimodal optical system that integrates optical coherence tomography (OCT), 18 MHz ultrasound (US) tomography and Raman spectroscopy (RS), allowing for fast (<2 min) and noninvasive skin cancer diagnostics and lesion depth measurement. The OCT can deliver structural and depth information of smaller skin lesions (<1 mm), while the US allows to measure the penetration depth of thicker lesions (≥4 mm), and the RS analyzes the chemical composition from a small chosen spot (≤300 μm) that can be used to distinguish between benign and malignant melanoma. The RS and OCT utilize the same scanning and optical setup, allowing for co-localized measurements. The US on the other side is integrated with an acoustical reflector, which enables B-mode measurements on the same position as OCT and RS. The US B-mode scans can be translated across the sample by laterally moving the US transducer, which is made possible by the developed adapter with a flexible membrane. We present the results on custom-made liquid and agar phantoms that show the resolution and depth capabilities of the setup, as well as preliminary ex vivo measurements on mouse models with ∼4.3 mm thick melanoma.

In vivo multiphoton multiparametric 3D quantification of human skin aging on forearm and face

Quantifying skin aging changes and characterizing its 3D structure and function in a non-invasive way is still a challenging area of research, constantly evolving with the development of imaging methods and image analysis tools. In vivo multiphoton imaging offers means to assess skin constituents in 3D, however prior skin aging studies mostly focused on 2D analyses of dermal fibers through their signals’ intensities or densities. In this work, we designed and implemented multiphoton multiparametric 3D quantification tools for in vivo human skin pigmentation and aging characterization. We first demonstrated that despite the limited field of view of the technic, investigation of 2 regions of interest (ROIs) per zone per volunteer is a good compromise in assessing 3D skin constituents in both epidermis and superficial dermis. We then characterized skin aging on different UV exposed areas—ventral and dorsal forearms, face. The three major facts of aging that are epidermal atrophy, the dermal–epidermal junction (DEJ) flattening and dermal elastosis can be non-invasively quantified and compared. Epidermal morphological changes occur late and were only objectified between extreme age groups. Melanin accumulation in suprabasal layers with age and chronic exposure on ventral and dorsal forearms is less known and appears earlier. Superficial dermal aging changes are mainly elastin density increase, with no obvious change in collagen density, reflected by SHGto2PEF ratio and SAAID index decrease and ImbrN index increase on all skin areas. Analysis of the z-dermal distribution of these parameters highlighted the 2nd 20 µm thickness normalized dermal sub-layer, that follows the DEJ shape, as exhibiting the highest aging differences. Moreover, the 3D ImbrN index allows refining the share of photoaging in global aging on face and the 3D SAAID index on forearm, which elastin or fibrillar collagens densities alone do not allow. Photoaging of the temple area evolves as a function of chronic exposure with a more pronounced increase in elastin density, also structurally modified from thin and straight elastic fibers in young volunteers to dense and compact pattern in older ones. More generally, multiphoton multiparametric 3D skin quantification offers rich spatial information of interest in assessing normal human skin condition and its pathological, external environment or product induced changes.

Non-Invasive Skin Imaging Assessment of Human Stress During Head-Down Bed Rest Using a Portable Handheld Two-Photon Microscope

Spaceflight presents a series of physiological and pathological challenges to astronauts resulting from ionizing radiation, microgravity, isolation, and other spaceflight hazards. These risks cause a series of aging-related diseases associated with increased oxidative stress and mitochondria dysfunction. The skin contains many autofluorescent substances, such as nicotinamide adenine dinucleotide phosphate (NAD(P)H), keratin, melanin, elastin, and collagen, which reflect physiological and pathological changes in vivo. In this study, we used a portable handheld two-photon microscope to conduct high-resolution in vivo skin imaging on volunteers during 15 days of head-down bed rest. The two-photon microscope, equipped with a flexible handheld scanning head, was used to measure two-photon excited fluorescence (TPEF) and second harmonic generation (SHG) images of the left forearm, left front chest, and forehead of volunteers. Changes in TPEF, SHG, and the extended SHG-to-AF(TPEF) aging index of the dermis (SAAID) were measured. It was found that TPEF intensity increased during bed rest and was restored to normal levels after recovery. Meanwhile, SHG increased slightly during bed rest, and the skin aging index increased. Moreover, we found the skin TPEF signals of the left forearm were significantly negatively associated with the oxidative stress marker malondialdehyde (MDA) and DNA damage marker 8-hydroxy-2′-desoxyguanosine (8-OHdG) values of subjects during head-down bed rest. Meanwhile, the SHG signals were also significantly negatively correlated with MDA and 8-OHDG. A significant negative correlation between the extended SAAID of the left chest and serum antioxidant superoxide dismutase (SOD) levels was also found. These results demonstrate that skin autofluorescence signals can reflect changes in human oxidant status. This study provides evidence for in-orbit monitoring of changes in human stress using a portable handheld two-photon microscope for skin imaging.

Real-time observation of neutrophil extracellular trap formation in the inflamed mouse brain via two-photon intravital imaging

Intravital imaging via two-photon microscopy (TPM) is a useful tool for observing and delineating biological events at the cellular and molecular levels in live animals in a time-lapse manner. This imaging method provides spatiotemporal information with minimal phototoxicity while penetrating a considerable depth of intact organs in live animals. Although various organs can be visualized using intravital imaging, in the field of neuroscience, the brain is the main organ whose cell-to-cell interactions are imaged using this technique. Intravital imaging of brain disease in mouse models acts as an abundant source of novel findings for studying cerebral etiology. Neutrophil infiltration is a well-known hallmark of inflammation; in particular, the crucial impact of neutrophils on the inflamed brain has frequently been reported in literature. Neutrophil extracellular traps (NETs) have drawn attention as an intriguing feature over the last couple of decades, opening a new era of research on their underlying mechanisms and biological effects. However, the actual role of NETs in the body is still controversial and is in parallel with a poor understanding of NETs in vivo. Although several experimental methods have been used to determine NET generation in vitro, some research groups have applied intravital imaging to detect NET formation in the inflamed organs of live mice. In this review, we summarize the advantages of intravital imaging via TPM that can also be used to characterize NET formation, especially in inflamed brains triggered by systemic inflammation. To study the function and migratory pattern of neutrophils, which is critical in triggering the innate immune response in the brain, intravital imaging via TPM can provide new perspectives to understand inflammation and the resolution process.

Self-made transparent optoacoustic detector for measurement of skin lesion thicknessin vivo

In skin cancer diagnosis and treatment, one of the key factors is tumor depth, which is connected to the severity and the required excision depth. Optoacoustical (OA) imaging is a relatively popular technique that provides information based on the optical absorption of the sample. Although often demonstrated withex vivomeasurements orin vivoimaging on parts of small animals,in vivomeasurements on humans are more challenging. This is presumably because it is too time consuming and the required excitation pulse energies and their number exceed the allowed maximum permissible exposure (MPE). Here, we demonstrate thickness measurements with a transparent optoacoustical detector of different suspicious skin lesionsin vivoon patients. We develop the signal processing technique to automatically convert the raw signal into thickness via deconvolution with the impulse response function. The transparency of the detector allows optical excitation with the pulsed laser to be performed perpendicularly on the lesion, in contrast to the conventional illumination from the side. For validation, the measured results were compared to the histological thickness determined after excision. We show that this simple transparent detector allows to determine the thickness of a lesion and thus, aid the dermatologist to estimate the excision depth in the future.

Non-invasive longitudinal imaging of VEGF-induced microvascular alterations in skin wounds

Background: Microcirculation is essential for skin homeostasis and repair. A variety of growth factors have been identified as important regulators of wound healing. However, direct observation and longitudinal monitoring of skin remodeling in an unperturbed in vivo environment remains challenging.

Methods: We report on non-invasive longitudinal imaging of the wound healing process in transgenic mice overexpressing vascular endothelial growth factor A (VEGF-A) in keratinocytes by means of large-scale optoacoustic microscopy (LSOM). This rapid, label-free, high throughput intravital microscopy method averts the use of dorsal skin-fold chambers, allowing for fully non-invasive repeated imaging of intact wounds with capillary resolution over field-of-view spanning several centimeters.

Results: We observed VEGF-driven enhancement of dermal vascularization in ears, dorsal skin and healing wounds and quantified the hemoglobin content, fill fraction, vessel diameter and tortuosity. The in vivo findings were further corroborated by detailed side-by-side classical histological whole-mount vascular stainings and pan-endothelial CD31 immunofluorescence.

Conclusion: The new approach is suitable for supplementing or replacing the cumbersome histological procedures in a broad range of skin regeneration and tissue engineering applications.

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

BACKGROUND

Ultraviolet exposure has profound effect on the dermal connective tissue of human skin.

OBJECTIVE

We aimed to develop and validate an evaluation method/methodology using a full-thickness reconstructed skin model, to assess the anti-photoaging efficacy of cosmetic ingredients and sunscreen formulas by blending multi relevant biological endpoints including the newly developed dermal collagen quantification method with Multi-photon microscopy.

METHODS

The response of ex vivo human skin to UVA exposure was first characterized with multiphoton microscopy. Reconstructed full-thickness skin models was then used to reproduce the data and to create a proof-of-concept study by treating the models with sunscreen prototypes A or B, which differ on their UVA absorption properties, and systemic Vitamin C (Vit C). After exposure to UVA, the collagen density was quantified via multiphoton microscopy with automatic imaging processing. Histology, fibroblasts number, metalloprotease 1 (MMP1) secretion were also assessed.

RESULTS

UVA exposure induced pronounced reduction in collagen density and increased MMP1 secretion within both ex vivo human skin and reconstructed skin. Histological damage and fibroblast disappearance was observed with reconstructed skin. Within the proof-of-concept study prototype B, possessing higher UVA filtration, gave better protection than prototype A on the UV associated biological markers, and association with Vit C boosted sunscreen formula efficacy.

CONCLUSIONS

The photoaging evaluation method, consists of multi biological markers as well as dermal collagen quantification, is a relevant mean to assess the pre-clinical efficacy of anti-photoaging ingredients and sunscreen products. This approach is also beneficial for evaluating the efficacy of sunscreens and photoprotective ingredients.

Two-Photon Fluorescence Microscopy and Applications in Angiogenesis and Related Molecular Events

The role of angiogenesis in health and disease have gained considerable momentum in recent years. Visualizing angiogenic patterns and associated events of surrounding vascular beds in response to therapeutic and laboratory-grade biomolecules have become a commonplace in regenerative medicine and the biosciences. To aid imaging investigations in angiogenesis, the two-photon excitation fluorescence microscopy (2PEF), or multiphoton fluorescence microscopy is increasingly utilized in scientific investigations. The 2PEF microscope confers several distinct imaging advantages over other fluorescence excitation microscopy techniques - for the observation of in-depth, three-dimensional vascularity in a variety of tissue formats, including fixed tissue specimens and in vivo vasculature in live specimens. Understanding morphological and subcellular changes that occur in cells and tissues during angiogenesis will provide insights to behavioral responses in diseased states, advance the engineering of physiologically-relevant tissue models and provide biochemical clues for the design of therapeutic strategies. We review the applicability and limitations of the 2PEF microscope on the biophysical and molecular-level signatures of angiogenesis in various tissue models. Imaging techniques and strategies for best practices in 2PEF microscopy will be reviewed.

Long-Term Imaging of Wound Angiogenesis with Large Scale Optoacoustic Microscopy

Wound healing is a well-coordinated process, necessitating efficient formation of new blood vessels. Vascularization defects are therefore a major risk factor for chronic, non-healing wounds. The dynamics of mammalian tissue revascularization, vessel maturation, and remodeling remain poorly understood due to lack of suitable in vivo imaging tools. A label-free large-scale optoacoustic microscopy (LSOM) approach is developed for rapid, non-invasive, volumetric imaging of tissue regeneration over large areas spanning up to 50 mm with a depth penetration of 1.5 mm. Vascular networks in dorsal mouse skin and full-thickness excisional wounds are imaged with capillary resolution during the course of healing, revealing previously undocumented views of the angiogenesis process in an unperturbed wound environment. Development of an automatic analysis framework enables the identification of key features of wound angiogenesis, including vessel length, diameter, tortuosity, and angular alignment. The approach offers a versatile tool for preclinical research in tissue engineering and regenerative medicine, empowering label-free, longitudinal, high-throughput, and quantitative studies of the microcirculation in processes associated with normal and impaired vascular remodeling, and analysis of vascular responses to pharmacological interventions in vivo.

Label-Free Multiphoton Microscopy for the Detection and Monitoring of Calcific Aortic Valve Disease

Calcific aortic valve disease (CAVD) is the most common valvular heart disease. CAVD results in a considerable socio-economic burden, especially considering the aging population in Europe and North America. The only treatment standard is surgical valve replacement as early diagnostic, mitigation, and drug strategies remain underdeveloped. Novel diagnostic techniques and biomarkers for early detection and monitoring of CAVD progression are thus a pressing need. Additionally, non-destructive tools are required for longitudinal in vitro and in vivo assessment of CAVD initiation and progression that can be translated into clinical practice in the future. Multiphoton microscopy (MPM) facilitates label-free and non-destructive imaging to obtain quantitative, optical biomarkers that have been shown to correlate with key events during CAVD progression. MPM can also be used to obtain spatiotemporal readouts of metabolic changes that occur in the cells. While cellular metabolism has been extensively explored for various cardiovascular disorders like atherosclerosis, hypertension, and heart failure, and has shown potential in elucidating key pathophysiological processes in heart valve diseases, it has yet to gain traction in the study of CAVD. Furthermore, MPM also provides structural, functional, and metabolic readouts that have the potential to correlate with key pathophysiological events in CAVD progression. This review outlines the applicability of MPM and its derived quantitative metrics for the detection and monitoring of early CAVD progression. The review will further focus on the MPM-detectable metabolic biomarkers that correlate with key biological events during valve pathogenesis and their potential role in assessing CAVD pathophysiology.

Effects of the lower energy and pulse stacking in carbon dioxide laser skin treatment: an objective analysis using second harmonic generation

PURPOSE

To evaluate the effect of fractional carbon dioxide (CO2) laser treatment using lower power associated with pulse stacking within collagen fibers, using second harmonic generation microscopy and computerized image analysis.

METHODS

Twenty male Wistar rats aging eight weeks were used. Each treatment area received a single-pass CO2 fractional laser with different parameters. The 20 animals were divided into two groups and euthanized after 30 and 60 days. Second harmonic generation images were obtained and program ImageJ was utilized to evaluate the collagen organization within all areas. Collagen anisotropy, entropy and optical density were quantified.

RESULTS

Increased anisotropy over time was observed in all four areas, but only reached statistical significance (p = 0.0305) when the mildest parameters were used (area four). Entropy decreased over time in all areas, but without significance(p = 0.1779) in area four. Density showed an overtime increase only in area four, but no statistical significance was reached (p = 0.6534).

CONCLUSIONS

When combined, the results obtained in this study regarding anisotropy, entropy and density tend to demonstrate that it is possible to achieve collagen remodeling with the use of lower power levels associated with stacked pulses.

Polarization-resolved SHG imaging as a fast screening method for collagen alterations during aging: Comparison with light and electron microscopy

Our previous study on rat skin showed that cumulative oxidative pressure induces profound structural and ultrastructural alterations in both rat skin epidermis and dermis during aging. Here, we aimed to investigate the biophotonic properties of collagen as a main dermal component in the function of chronological aging. We used second harmonic generation (SHG) and two-photon excited fluorescence (TPEF) on 5 μm thick skin paraffin sections from 15-day-, 1-month- and 21-month-old rats, respectively, to analyze collagen alterations, in comparison to conventional light and electron microscopy methods. Obtained results show that polarization-resolved SHG (PSHG) images can detect collagen fiber alterations in line with chronological aging and that this method is consistent with light and electron microscopy. Moreover, the β coefficient calculated from PSHG images points out that delicate alterations lead to a more ordered structure of collagen molecules due to oxidative damage. The results of this study also open the possibility of successfully applying this fast and label-free method to previously fixed samples.

Presence of intralesional melanocytes as a histopathological feature of actinic keratosis based on in vivo harmonic generation microscopy in Asians

BACKGROUND

Most patients with actinic keratosis (AK) present with more than one lesion. Although histopathological examination is the gold standard for diagnosing this condition, performing an invasive skin biopsy for each AK is impractical. Thus, this study aimed to identify AK's morphological characteristics based on harmonic generation microscopy (HGM). Moreover, the correlation between features observed using HGM and histopathological grading of AK was examined.

METHODS

Lesions of seven patients were examined using HGM (n = 1, ex vivo and n = 6, in vivo), and histopathological examinations of the biopsy specimens were also performed. The features of each AK, based on HGM, were assessed and compared with corresponding standard histopathological findings.

RESULTS

Using the histopathological findings as a standard reference, HGM's accuracy in detecting features of AK lesions, such as hyperkeratosis, epidermal thinning, abnormal architecture, and atypical honeycomb pattern, was 100%. Approximately five (72%) patients had similar histopathological grades. Moreover, based on HGM, except for one patient with grade 1 AK, six (85.71%) patients had lesions with intraepidermal dendritic cell-like cells, representing melanocytes.

CONCLUSION

Harmonic generation microscopy can be used in vivo to provide critical diagnostic information with a resolution comparable to histopathological examination. In addition, intralesional melanocytes in AK, which may be correlated with disease severity, can be specifically enhanced using HGM.

High-quality Fluorescence Imaging of the Human Acrosyringium Using a Transparency: Enhancing Technique and an Improved, Fluorescent Solvatochromic Pyrene Probe

Two-photon, excitation fluorescent microscopy featuring autofluorescence or immunofluorescence, combined with optical clearance using a transparency-enhancing technique, allows deep imaging of three-dimensional (3D) skin structures. However, it remains difficult to obtain high-quality images of individual cells or 3D structures. We combined a new dye with a transparency-enhancing technology and performed high-quality structural analysis of human epidermal structures, especially the acrosyringium. Human fingertip skin samples were collected, formalin-fixed, embedded in both frozen and paraffin blocks, sliced, stained with propidium iodide, optically cleared using a transparency-enhancing technique, and stained with a new fluorescent, solvatochromic pyrene probe. Microscopy revealed fine skin features and detailed epidermal structures including the stratum corneum (horny layer), keratinocytes, eccrine sweat glands, and peripheral nerves. Three-dimensional reconstruction of an entire acrosyringium was possible in one sample. This new fluorescence microscopy technique yields high-quality epidermal images and will aid in histopathological analyses of skin disorders.

Labeling and tracking of immune cells in ex vivo human skin

Human skin harbors various immune cells that are crucial for the control of injury and infection. However, the current understanding of immune cell function within viable human skin tissue is limited. We developed an ex vivo imaging approach in which fresh skin biopsies are mounted and then labeled with nanobodies or antibodies against cell surface markers on tissue-resident memory CD8⁺ T cells, other immune cells of interest, or extracellular tissue components. Subsequent longitudinal imaging allows one to describe the dynamic behavior of human skin-resident cells in situ. In addition, this strategy can be used to study immune cell function in murine skin. The ability to follow the spatiotemporal behavior of CD8⁺ T cells and other immune cells in skin, including their response to immune stimuli, provides a platform to investigate physiological immune cell behavior and immune cell behavior in skin diseases. The mounting, staining and imaging of skin samples requires ~1.5 d, and subsequent tracking analysis requires a minimum of 1 d. The optional production of fluorescently labeled nanobodies takes ~5 d.

Visualizing cellular heterogeneity by quantifying the dynamics of MAPK activity in live mammalian cells with synthetic fluorescent biosensors

Mitogen-Activated Protein Kinases (MAPKs) control a wide array of cellular functions by transducing extracellular information into defined biological responses. In order to understand how these pathways are regulated, dynamic single cell measurements are highly needed. Fluorescence microscopy is well suited to perform these measurements. However, more dynamic and sensitive biosensors that allow the quantification of signaling activity in living mammalian cells are required. We have engineered a synthetic fluorescent substrate for human MAPKs (ERK, JNK and p38) that relocates from the nucleus to the cytoplasm when phosphorylated by the kinases. We demonstrate that this reporter displays an improved response compared to other relocation biosensors. This assay allows to monitor the heterogeneity in the MAPK response in a population of isogenic cells, revealing pulses of ERK activity upon a physiological EGFR stimulation. We show applicability of this approach to the analysis of multiple cancer cell lines and primary cells as well as its application in vivo to developing tumors. Using this ERK biosensor, dynamic single cell measurements with high temporal resolution can be obtained. These MAPK reporters can be widely applied to the analysis of molecular mechanisms of MAPK signaling in healthy and diseased state, in cell culture assays or in vivo.

In vivo characterization of minipig skin as a model for dermatological research using multiphoton microscopy

Minipig skin is one of the most widely used non-rodent animal skin models for dermatological research. A thorough characterization of minipig skin is essential for gaining deeper understanding of its structural and functional similarities with human skin. In this study, three-dimensional (3-D) in vivo images of minipig skin was obtained non-invasively using a multimodal optical imaging system capable of acquiring two-photon excited fluorescence (TPEF) and fluorescence lifetime imaging microscopy (FLIM) images simultaneously. The images of the structural features of different layers of the minipig skin were qualitatively and quantitatively compared with those of human skin. Label-free imaging of skin was possible due to the endogenous fluorescence and optical properties of various components in the skin such as keratin, nicotinamide adenine dinucleotide phosphate (NAD(P)H), melanin, elastin, and collagen. This study demonstrates the capability of optical biopsy techniques, such as TPEF and FLIM, for in vivo non-invasive characterization of cellular and functional features of minipig skin, and the optical image-based similarities of this commonly utilized model of human skin. These optical imaging techniques have the potential to become promising tools in dermatological research for developing a better understanding of animal skin models, and for aiding in translational pre-clinical to clinical studies.

In vivo imaging of the depth-resolved optic axis of birefringence in human skin

Recent progress has enabled the reconstruction of the local (i.e., depth-resolved) optic axis (OAx) of biological tissue from measurements made with polarization-sensitive optical coherence tomography (PS-OCT). Here we demonstrate local OAx imaging in healthy human skin in vivo. The images reveal dense, weaving patterns that are imperceptible in OCT intensity tomograms or conventional PS-OCT metrics and that suggest a mesh-like tissue organization, consistent with the morphology of dermal collagen. Using co-registered polarization-sensitive optical coherence microscopy, we furthermore investigated the impact of spatial resolution on the recovered OAx patterns and confirmed their consistency. OAx orientation as a contrast mechanism merits further exploration for applications in dermatology.

Skin Barriers in Dermal Drug Delivery: Which Barriers Have to Be Overcome and How Can We Measure Them?

Although, drugs are required in the various skin compartments such as viable epidermis, dermis, or hair follicles, to efficiently treat skin diseases, drug delivery into and across the skin is still challenging. An improved understanding of skin barrier physiology is mandatory to optimize drug penetration and permeation. The various barriers of the skin have to be known in detail, which means methods are needed to measure their functionality and outside-in or inside-out passage of molecules through the various barriers. In this review, we summarize our current knowledge about mechanical barriers, i.e., stratum corneum and tight junctions, in interfollicular epidermis, hair follicles and glands. Furthermore, we discuss the barrier properties of the basement membrane and dermal blood vessels. Barrier alterations found in skin of patients with atopic dermatitis are described. Finally, we critically compare the up-to-date applicability of several physical, biochemical and microscopic methods such as transepidermal water loss, impedance spectroscopy, Raman spectroscopy, immunohistochemical stainings, optical coherence microscopy and multiphoton microscopy to distinctly address the different barriers and to measure permeation through these barriers in vitro and in vivo.

Nonlinear optical microscopy is a novel tool for the analysis of cutaneous alterations in pseudoxanthoma elasticum

Pseudoxanthoma elasticum (PXE, OMIM 264800) is a rare autosomal recessive disorder with ectopic mineralization and fragmentation of elastin fibers. It is caused by mutations of the ABCC6 gene that leads to decreased serum levels of inorganic pyrophosphate (PPi) anti-mineralization factor. The occurrence of severe complications among PXE patients highlights the importance of early diagnosis so that prompt multidisciplinary care can be provided to patients. We aimed to examine dermal connective tissue with nonlinear optical (NLO) techniques, as collagen emits second-harmonic generation (SHG) signal, while elastin can be excited by two-photon excitation fluorescence (TPF). We performed molecular genetic analysis, ophthalmological and cardiovascular assessment, plasma PPi measurement, conventional histopathological examination, and ex vivo SHG and TPF imaging in five patients with PXE and five age- and gender-matched healthy controls. Pathological mutations including one new variant were found in the ABCC6 gene in all PXE patients and their plasma PPi level was significantly lower compared with controls. Degradation and mineralization of elastin fibers and extensive calcium deposition in the mid-dermis was visualized and quantified together with the alterations of the collagen structure in PXE. Our data suggests that NLO provides high-resolution imaging of the specific histopathological features of PXE-affected skin. In vivo NLO may be a promising tool in the assessment of PXE, promoting early diagnosis and follow-up.

Non-invasive monitoring of pharmacodynamics during the skin wound healing process using multimodal optical microscopy

OBJECTIVE

Impaired diabetic wound healing is one of the serious complications associated with diabetes. In patients with diabetes, this impairment is characterized by several physiological abnormalities such as metabolic changes, reduced collagen production, and diminished angiogenesis. We designed and developed a multimodal optical imaging system that can longitudinally monitor formation of new blood vessels, metabolic changes, and collagen deposition in a non-invasive, label-free manner.

RESEARCH DESIGN AND METHODS

The closure of a skin wound in (db/db) mice, which presents delayed wound healing pathologically similar to conditions in human type 2 diabetes mellitus, was non-invasively followed using the custom-built multimodal microscope. In this microscope, optical coherence tomography angiography was used for studying neovascularization, fluorescence lifetime imaging microscopy for nicotinamide adenine dinucleotide (phosphate) (NAD(P)H) assessment, fluorescence intensity changes of NAD(P)H and flavin adenine dinucleotide (FAD) cofactors for evaluating metabolic changes, and second harmonic generation microscopy for analyzing collagen deposition and organization. The animals were separated into four groups: control, placebo, low concentration (LC), and high concentration (HC) treatment. Images of the wound and surrounding areas were acquired at different time points during a 28-day period.

RESULTS

Various physiological changes measured using the optical imaging modalities at different phases of wound healing were compared. A statistically significant improvement in the functional relationship between angiogenesis, metabolism, and structural integrity was observed in the HC group.

CONCLUSIONS

This study demonstrated the capability of multimodal optical imaging to non-invasively monitor various physiological aspects of the wound healing process, and thus become a promising tool in the development of better diagnostic, treatment, and monitoring strategies for diabetic wound care.

Cutaneous optical coherence tomography for longitudinal volumetric assessment of intradermal volumes in a mouse model

Clinical evaluation of skin lesions requires precise and reproducible technologies for their qualitative and quantitative assessment. In this study, we investigate the applicability of a custom-built dermatologic OCT system for longitudinal assessment of intradermal volumes in a mouse model. The OCT, based on an akinetic swept laser working at 1310 nm was employed for visualization and quantification of intradermal deposits of three different hyaluronic acid-based hydrogel formulations - one commercial and two test substances. Hydrogels were applied in 22 BALB/c mice, and measurements were performed over a six-month time period. All hydrogels increased in volume within the first weeks and degraded steadily thereafter. The half-lifes of the test hydrogels (27.2 ± 13.6 weeks for Hydrogel 1, 31.5 ± 17.2 weeks for Hydrogel 2) were higher in comparison to the commercially available HA hydrogel (21.4 ± 12.0 weeks), although differences were not significant. The sphericity parameter was used for evaluation of the deposit geometry. While on the injection day the sphericities were similar (~0.75 ± 0.04), at later time points significant differences between the different test substances were found (T24: PRV 0.59 ± 0.09, Hydrogel 1 0.70 ± 0.11, Hydrogel 2 0.78 ± 0.07; p ≤ 0.012 for all pairs). This study shows the applicability of OCT imaging for quantitative assessment of the volumetric behavior of intradermal deposits in vivo.

Morphological and functional characteristics of aging kidneys based on two-photon microscopy in vivo

Age-related kidney disease, which is chronic and naturally occurring, is a general term for a set of heterogeneous disorders affecting kidney structures and characterized by a decline in renal function. Age-related renal insufficiency has important implications with regard to body homeostasis, drug toxicity and renal transplantation. In our study, two-photon microscopy was used to image kidney morphological and functional characteristics in an age-related rat model in vivo. The changes in morphology are analyzed based on autofluorescence and Hoechst 33342 labeling in rats with different ages. Structural parameters including renal tubular diameter, cell nuclei density, size and shape are studied and compared with Hematoxylin and Eosin histological analysis. Functional characteristics, such as blood flow, and glomerular filtration rate are studied with high-molecular weight (MW) 500-kDa dextran-fluorescein and low-MW 10-kDa dextran-rhodamine. Results indicate that morphology changes significantly and functional characteristics deteriorate with age. These parameters are potential indicators for evaluating age-related renal morphology and function changes. Combined analyses of these parameters could provide a quantitative, novel method for monitoring kidney diseases and/or therapeutic effects of kidney drugs.

Multiphoton Microscopy for Noninvasive and Label-Free Imaging of Human Skin and Oral Mucosa Equivalents

Multiphoton microscopy has emerged as a powerful modality for noninvasive, spatial, and temporal imaging of biological tissues without the use of labels and/or dyes. It provides complimentary imaging modalities, which include two-photon excited fluorescence (2PEF) and second harmonic generation (SHG). 2PEF from endogenous chromophores such as nicotinamide adenine dinucleotides (NADH), flavins and keratin enable visualization of cellular and subcellular structures. SHG provides visualization of asymmetric macromolecular structures such as collagen. These modalities enable the visualization of biochemical and biological alterations within live tissues in their native state.Organotypic cultures of the skin and oral mucosa equivalents have been increasingly used across basic and translational research. However, assessment of the skin and oral mucosa equivalents is predominantly based on histological techniques which are not suited for real-time imaging and longitudinal studies of the tissues in their native state. 2PEF from endogenous chromophores and SHG from collagen can be effectively used as an imaging tool for noninvasive and label-free acquisition of cellular and matrix structures of live skin and oral mucosa cultures.In this chapter, the methods for noninvasive and label-free imaging of monolayer and organotypic cultures of the skin and oral mucosa using multiphoton microscopy are described.

Noninvasive in vivo human multiphoton microscopy: a key method in proving nanoparticulate zinc oxide sunscreen safety

We describe the contribution of our in vivo multiphoton microscopy (MPM) studies over the last ten years with DermaInspect^;® (JenLab, Germany), a CE-certified medical tomograph based on detection of fluorescent biomolecules, to the assessment of possible penetration of nanoparticulate zinc oxide in sunscreen through human skin. At the time we started our work, there was a strong movement for the precautionary principle to be applied to the use of nanoparticles in consumer products due to a lack of knowledge. The combined application of different MPM modalities, including spectral imaging, fluorescence lifetime imaging, second harmonic fluorescence generation, and phosphorescence microscopy, has provided overwhelming evidence that nanoparticle zinc oxide particles do not penetrate human skin when applied to various skin types with a range of methods of topical sunscreen application. MPM has also been used to study the viable epidermal morphology and redox state in supporting the safe use of topical zinc oxide nanoparticles. The impact of this work is emphasized by the recent proposed rule by the United States FDA on Sunscreen Drug Products for Over-the-Counter Human Use, which listed only zinc oxide and titanium dioxide of the currently marketed products to be generally recognized as safe and effective.

In vivo third-harmonic generation microscopy study on vitiligo patients

Melanin is known to provide strong third-harmonic generation (THG) contrast in human skin. With a high concentration in basal cell cytoplasm, THG contrast provided by melanin overshadows other THG sources in human skin studies. For better understanding of the THG signals in keratinocytes without the influence of melanin, an in vivo THG microscopy (THGM) study was first conducted on vitiliginous skin. As a result, the THG-brightness ratio between the melanin-lacking cytoplasm of basal cells and collagen fibers is about 1.106 at the dermal-epidermal junctions of vitiliginous skin, indicating high sensitivity of THGM for the presence of melanin. We further applied the in vivo THGM to assist evaluating the therapeutic outcome from the histopathological point of view for those showed no improvement under narrowband ultraviolet B therapy based on the seven-point Physician Global Assessment score. Our clinical study indicates the high potential of THGM to assist the histopathological assessment of the therapeutic efficacy of vitiligo treatments.

Moxifloxacin Labeling-Based Multiphoton Microscopy of Skin Cancers in Asians

BACKGROUND AND OBJECTIVES

Although multiphoton microscopy (MPM) can visualize both cell and extracellular matrix (ECM) structures of the skin in high-contrast without exogenous labeling, label-free MPM is usually too slow to image clinically relevant large regions. A high-speed MPM method would be beneficial for evaluating clinical skin specimens by increasing the imaging area. In this study, moxifloxacin labeling-based MPM (moxifloxacin MPM) was characterized in various human skin cancer specimens.

STUDY DESIGN/MATERIALS AND METHODS

Moxifloxacin ophthalmic solution was used for cell-labeling and MPM imaging was conducted afterwards. Moxifloxacin MPM was characterized in ex vivo normal human skin and skin cancer specimens in comparison with the label-free MPM and fluorescence confocal microscopy (FCM) using acridine orange as a labeling agent. Then, moxifloxacin MPM was applied to various ex vivo human skin cancer specimens including basal cell carcinoma (BCC), squamous cell carcinoma (SCC), dermatofibrosarcoma protuberans (DFSP). Results of moxifloxacin MPM were compared with bright-field clinical and histopathologic findings.

RESULTS

Moxifloxacin MPM imaged both cells and collagen in the skin, similarly to label-free MPM, but with enhanced fluorescence intensities in cells and enhanced imaging speeds. Moxifloxacin MPM imaged cells in the skin similarly to acridine orange-based FCM. Moxifloxacin MPM of various human skin cancer specimens imaged their specific cellular features. The microscopic features detected in moxifloxacin MPM were confirmed with histological images.

CONCLUSIONS

This observational pilot study demonstrated that moxifloxacin MPM could detect specific cellular features of various skin cancers in good correlation with histopathological images in Asian patients at the higher imaging speed than label-free MPM. Lasers Surg. Med. © 2019 Wiley Periodicals, Inc.

Quantifying in situ adaptive immune cell cognate interactions in humans

Two-photon excitation microscopy (TPEM) has revolutionized the understanding of adaptive immunity. However, TPEM usually requires animal models and is not amenable to the study of human disease. The recognition of antigen by T cells requires cell contact and is associated with changes in T cell shape. We postulated that by capturing these features in fixed tissue samples, we could quantify in situ adaptive immunity. Therefore, we used a deep convolutional neural network to identify fundamental distance and cell-shape features associated with cognate help (cell-distance mapping (CDM)). In mice, CDM was comparable to TPEM in discriminating cognate T cell-dendritic cell (DC) interactions from non-cognate T cell-DC interactions. In human lupus nephritis, CDM confirmed that myeloid DCs present antigen to CD4+ T cells and identified plasmacytoid DCs as an important antigen-presenting cell. These data reveal a new approach with which to study human in situ adaptive immunity broadly applicable to autoimmunity, infection, and cancer.

A biopsy-needle compatible varifocal multiphoton rigid probe for depth-resolved optical biopsy

In this work, we report a biopsy-needle compatible rigid probe, capable of performing three-dimensional (3D) two-photon optical biopsy. The probe has a small outer diameter of 1.75 mm and fits inside a gauge-14 biopsy needle to reach internal organs. A carefully designed focus scanning mechanism has been implemented in the rigid probe, which, along with a rapid two-dimensional MEMS scanner, enables 3D imaging. Fast image acquisition up to 10 frames per second is possible, dramatically reducing motion artifacts during in vivo imaging. Equipped with a high-numerical aperture micro-objective, the miniature rigid probe offers a high two-photon resolution (0.833 × 6.11 μm, lateral × axial), a lateral field of view of 120 μm, and an axial focus tuning range of 200 μm. In addition to imaging of mouse internal organs and subcutaneous tumor in vivo, first-of-its-kind depth-resolved two-photon optical biopsy of an internal organ has been successfully demonstrated on mouse kidney in vivo and in situ.

Microscopy with ultraviolet surface excitation (MUSE): A novel approach to real-time inexpensive slide-free dermatopathology

Traditional histology relies on processing and physically sectioning either frozen or formalin-fixed paraffin-embedded (FFPE) tissue into thin slices (typically 4-6 μm) prior to staining and viewing on a standard wide-field microscope. Microscopy using ultraviolet (UV) surface excitation (MUSE) represents a novel alternative microscopy method that works with UV excitation using oblique cis-illumination, which can generate high-quality images from the cut surface of fresh or fixed tissue after brief staining, with no requirement for fixation, embedding and histological sectioning of tissue specimens. We examined its potential utility in dermatopathology. Concordance between MUSE images and hematoxylin and eosin (H&E) slides was assessed by the scoring of MUSE images on their suitability for identifying 10 selected epidermal and dermal structures obtained from minimally fixed tissue, including stratum corneum, stratum granulosum, stratum spinosum, stratum basale, nerve, vasculature, collagen and elastin, sweat glands, adipose tissue and inflammatory cells, as well as 4 cases of basal cell carcinoma and 1 case of pseudoxanthoma elasticum deparaffinized out of histology blocks. Our results indicate that MUSE can identify nearly all normal skin structures seen on routine H&E as well as some histopathologic features, and appears promising as a fast, reliable and cost-effective diagnostic approach in dermatopathology.

Identification of polystyrene nanoparticle penetration across intact skin barrier as rare event at sites of focal particle aggregations

The question whether nanoparticles can cross the skin barrier is highly debated. Even in intact skin rare events of deeper penetration have been reported, but technical limitations and possible artifacts require careful interpretation. In this study, horizontal scanning by 2-photon microscopy (2 PM) of full-thickness human skin samples placed in a lateral position yielded highly informative images for skin penetration studies of fluorescently tagged nanoparticles. Scanning of large fields of view allowed for detailed information on interfollicular and follicular penetration in tissue blocks without damaging the sample. Images in histomorphological correlation showed that 2P-excited fluorescence signals of fluorescently tagged 20 and 200 nm polystyrene nanoparticles preferentially accumulated in the stratum corneum (SC) and in the upper part of vellus hair follicles (HFs). Rare events of deeper penetration in the SC and in the infundibulum of vellus HFs were observed at sites of high focal particle aggregations. Wide-field 2 PM allows for imaging of nanoparticle penetration in large tissue blocks, whereas total internal reflection microscopy (TIRFM) enables selective detection of individual nanoparticles as well as clusters of nanoparticles in the SC and within the epidermal layer directly beneath the SC, thus confirming barrier crossing with high sensitivity.

Imaging of Inflammatory Responses in the Mouse Ear Skin

The skin is one of the most physiologically important organs where the organism comes into contact with the external environment and is often a site where pathogen entry first occurs. Thus, a better understanding of the specialized cellular behavior of the immune system in the skin may be important for the improved treatment of diseases. Here, we describe in detail a procedure to image the dorsal mouse ear skin, using a customized ear stage and its associated coverslip holder, with an upright multiphoton microscope. As a demonstrative example, we describe the specific protocol for visualizing robust neutrophil trafficking in albino lysozyme-EGFP mice in response to zymosan particles. Instructive sections are provided for the mouse ear preparation, intradermal delivery of zymosan, design and use of the custom ear stage, as well as a solution for the uninterrupted live imaging of mice during prolonged sessions within a dark box. The mouse ear is easily accessible for imaging, and unlike most other organs, does not require any invasive surgery to be performed.

Two-photon microscopy for intracutaneous imaging of stem cell activity in mice

The adult skin is a typical example of a highly regenerative tissue. Terminally differentiated keratinocytes are shed from the external layers of the epidermis or extruded from the skin as part of the growing hair shaft on a daily basis. These are effectively replenished through the activity of skin-resident stem cells. Precise regulation of stem cell activity is critical for normal skin homoeostasis or wound healing and irregular stem cell proliferation or differentiation can lead to skin disease. The scarcity and dynamic nature of stem cells presents a major challenge for elucidating their mechanism of action. To address this, we have recently established a system for visualizing stem cell activity, in real time or long term, in the intact skin of live mice using two-photon microscopy. The purpose of this review was to provide essential information to researchers who wish to incorporate two-photon microscopy and live imaging into their experimental toolbox for studying aspects of skin and stem biology in the mouse model. We discuss fundamental principles of the method, instrumentation and basic experimental approaches to interrogate stem cell activity in the interfollicular epidermis and hair follicle.

Time-Resolved Fluorescence Spectroscopy and Fluorescence Lifetime Imaging Microscopy for Characterization of Dendritic Polymer Nanoparticles and Applications in Nanomedicine

The emerging field of nanomedicine provides new approaches for the diagnosis and treatment of diseases, for symptom relief and for monitoring of disease progression. One route of realizing this approach is through carefully constructed nanoparticles. Due to the small size inherent to the nanoparticles a proper characterization is not trivial. This review highlights the application of time-resolved fluorescence spectroscopy and fluorescence lifetime imaging microscopy (FLIM) for the analysis of nanoparticles, covering aspects ranging from molecular properties to particle detection in tissue samples. The latter technique is particularly important as FLIM allows for distinguishing of target molecules from the autofluorescent background and, due to the environmental sensitivity of the fluorescence lifetime, also offers insights into the local environment of the nanoparticle or its interactions with other biomolecules. Thus, these techniques offer highly suitable tools in the fields of particle development, such as organic chemistry, and in the fields of particle application, such as in experimental dermatology or pharmaceutical research.