Mueller Matrix Measurement of Electrospun Fiber Scaffolds for Tissue Engineering

Time-efficient filtering of imaging polarimetric data by checking physical realizability of experimental mueller matrices

MOTIVATION

Imaging Mueller polarimetry has already proved its potential for biomedicine, remote sensing and metrology. The real-time applications of this modality require both video rate image acquisition and fast data post-processing algorithms. First, one must check the physical realizability of the experimental Mueller matrices in order to filter out non-physical data, ie to test the positive semi-definiteness of the 4 × 4 Hermitian coherency matrix calculated from the elements of corresponding Mueller matrix pixel-wise. For this purpose, we compared the execution time for the calculations of i) eigenvalues, ii) Cholesky decomposition, iii) Sylvester's criterion, and iv) coefficients of the characteristic polynomial (two different approaches) of the Hermitian coherency matrix, all calculated for the experimental Mueller matrix images (600 pixels × 700 pixels) of mouse uterine cervix. The calculations were performed using C ++ and Julia programming languages.

RESULTS

Our results showed the superiority of the algorithm iv) based on the simplification via Pauli matrices over other algorithms for our dataset. The sequential implementation of latter algorithm on a single core already satisfies the requirements of real-time polarimetric imaging. This can be further amplified by the proposed parallelization (e.g., we achieve a 5-fold speed up on 6 cores).

AVAILABILITY AND IMPLEMENTATION

The source codes of the algorithms and experimental data are available at https://github.com/pogudingleb/mueller_matrices.

Spatially and Temporally Controllable BMP-2 and TGF-β3 Double Release From Polycaprolactone Fiber Scaffolds via Chitosan-Based Polyelectrolyte Coatings

Temporally and spatially controlled growth factor release from a polycaprolactone fiber mat, which also provides a matrix for directional cell colonization and infiltration, could be a promising regenerative approach for degenerated tendon-bone junctions. For this purpose, polycaprolactone fiber mats were coated with tailored chitosan-based nanogels to bind and release the growth factors bone morphogenetic protein 2 (BMP-2) and transforming growth factor-β3 (TGF-β3), respectively. In this work we provide meaningful in vitro data for the understanding of the drug delivery performance and sterilizability of novel implant prototypes in order to lay the foundation for in vivo testing. ELISA-based in vitro release studies were used to investigate the spatial and temporal control of release, as well as the influence of radiation sterilization on protein activity and release behavior. Layer-by-layer coatings based on BMP-2-containing chitosan tripolyphosphate nanogel particles and negatively charged alginate showed a good sustainment of BMP-2 release from chemically modified polycaprolactone fiber mats. Release control improved with increasing layer numbers. The approach of controlling the release via a barrier of cross-linked chitosan azide proved less promising. By using a simple, partial immersion-based dip-coating process, it was possible to apply opposing gradients of the growth factors BMP-2 and TGF-β3. Final radiation sterilization of the growth factor-loaded implant prototypes resulted in a radiation dose-correlated degradation of the growth factors, which could be prevented by lyophilization into protective matrices. For the manufacture of sterile implants, the growth factor loading step must probably be carried out under aseptic conditions. The layer-by-layer coated implant prototypes provided sustained release from opposing gradients of the growth factors BMP-2 and TGF-β3 and thus represent a promising approach for the restoration of tendon-bone defects.

Improved polarimetric analysis of human skin through stitching: advantages, limitations, and applications in dermatology

Polarimetry is a powerful tool for the analysis of the optical properties of materials and systems, such as human skin. However, in many polarimetric setups, the field of view is limited to a few square centimeters. In these cases, it is possible to resort to stitching techniques, which involve combining multiple Mueller matrix measurements obtained from different overlapping regions of the sample. In this paper, we propose a stitching technique for polarimetric data and discuss its advantages and limitations. We also describe the potential of image stitching for improving the accuracy and robustness ofin vivopolarimetry in the presence of random patient movement. We conducted our research using a diverse set of samples which included porcine skin, human skin from arms and fingers, cold cuts of chicken and gelatine, alongside synthetically created sample data. Our results demonstrate the effectiveness of this technique for the application in dermatology. Each additionalin vivomeasurement enhances the field of view by approximately one third, thereby considerably augmenting the total observation area. We show that stitching enables for the polarimetric assessment of large skin patches which is useful for the diagnosis of inflammatory skin diseases.

Release of TGF-β3 from Surface-Modified PCL Fiber Mats Triggers a Dose-Dependent Chondrogenic Differentiation of Human Mesenchymal Stromal Cells

The design of implants for tissue transitions remains a major scientific challenge. This is due to gradients in characteristics that need to be restored. The rotator cuff in the shoulder, with its direct osteo-tendinous junction (enthesis), is a prime example of such a transition. Our approach towards an optimized implant for entheses is based on electrospun fiber mats of poly(ε-caprolactone) (PCL) as biodegradable scaffold material, loaded with biologically active factors. Chitosan/tripolyphosphate (CS/TPP) nanoparticles were used to load transforming growth factor-β₃ (TGF-β₃) with increasing loading concentrations for the regeneration of the cartilage zone within direct entheses. Release experiments were performed, and the concentration of TGF-β₃ in the release medium was determined by ELISA. Chondrogenic differentiation of human mesenchymal stromal cells (MSCs) was analyzed in the presence of released TGF-β₃. The amount of released TGF-β₃ increased with the use of higher loading concentrations. This correlated with larger cell pellets and an increase in chondrogenic marker genes (SOX9, COL2A1, COMP). These data were further supported by an increase in the glycosaminoglycan (GAG)-to-DNA ratio of the cell pellets. The results demonstrate an increase in the total release of TGF-β₃ by loading higher concentrations to the implant, which led to the desired biological effect.

Mueller Matrix Microscopy for In Vivo Scar Tissue Diagnostics and Treatment Evaluation

Scars usually do not show strong contrast under standard skin examination relying on dermoscopes. They usually develop after skin injury when the body repairs the damaged tissue. In general, scars cause multiple types of distress such as movement restrictions, pain, itchiness and the psychological impact of the associated cosmetic disfigurement with no universally successful treatment option available at the moment. Scar treatment has significant economic impact as well. Mueller matrix polarimetry with integrated autofocus and automatic data registration can potentially improve scar assessment by the dermatologist and help to make the evaluation of the treatment outcome objective. Polarimetry can provide new physical parameters for an objective treatment evaluation. We show that Mueller matrix polarimetry can enable strong contrast for in vivo scar imaging. Additionally, our results indicate that the polarization stain images obtained form there could be a useful tool for dermatology. Furthermore, we demonstrate that polarimetry can be used to monitor wound healing, which may help prevent scarring altogether.

Registration of polarimetric images for in vivo skin diagnostics

SIGNIFICANCE

Mueller matrix (MM) polarimetry is a promising tool for the detection of skin cancer. Polarimetric in vivo measurements often suffer from misalignment of the polarimetric images due to motion, which can lead to false results.

AIM

We aim to provide an easy-to-implement polarimetric image data registration method to ensure proper image alignment.

APPROACH

A feature-based image registration is implemented for an MM polarimeter for phantom and in vivo human skin measurements.

RESULTS

We show that the keypoint-based registration of polarimetric images is necessary for in vivo skin polarimetry to ensure reliable results. Further, we deliver an efficient semiautomated method for the registration of polarimetric images.

CONCLUSIONS

Image registration for in vivo polarimetry of human skin is required for improved diagnostics and can be efficiently enhanced with a keypoint-based approach.

Applications of Mueller Matrix Polarimetry to Biological and Agricultural Diagnostics: A Review

The review contains a systematization of the main approaches to the practical implementation of Mueller matrix polarimetry and the prospects for its application in biology and agriculture. The most typical optical layouts for measuring the Mueller matrix of various objects, such as disperse systems, tissues and surface structures, are discussed. Mueller matrix measurements, being integrated into standard schemes of conventional optical methods, such as scatterometry, optical coherence tomography, fluorimetry, spectrophotometry and reflectometry, can significantly expand their capabilities in the characterization of biological systems and bioorganic materials. Additionally, microwave Mueller matrix polarimetry can be used for monitoring soil conditions and crop growth. The proposed systematization is aimed at outlining the conceptual directions for the development of non-invasive diagnostic tools based on measuring the Mueller matrix, primarily with a focus on biological research and agricultural practice.

In Situ Characterization of Polycaprolactone Fiber Response to Quasi-Static Tensile Loading in Scanning Electron Microscopy

Microstructural responses to the mechanical load of polymers used in tissue engineering is notably important for qualification at in vivo testing, although insufficiently studied, especially regarding promising polycaprolactone (PCL). For further investigations, electrospun PCL scaffolds with different degrees of fiber alignment were produced, using two discrete relative drum collector velocities. Development and preparation of an adjusted sample geometry enabled in situ tensile testing in scanning electron microscopy. By analyzing the microstructure and the use of selected tracking techniques, it was possible to visualize and quantify fiber/fiber area displacements as well as local fractures of single PCL fibers, considering quasi-static tensile load and fiber alignment. The possibility of displacement determination using in situ scanning electron microscopy techniques for testing fibrous PCL scaffolds was introduced and quantified.

Impact of Apparatus Orientation and Gravity in Electrospinning-A Review of Empirical Evidence

Electrospinning is a versatile fibre fabrication method with applications from textile to tissue engineering. Despite the appearance that the influencing parameters of electrospinning are fully understood, the effect of setup orientation has not been thoroughly investigated. With current burgeoning interest in modified and specialised electrospinning apparatus, it is timely to review the impact of this seldom-considered parameter. Apparatus configuration plays a major role in the morphology of the final product. The primary difference between spinning setups is the degree to which the electrical force and gravitational force contribute. Since gravity is much lower in magnitude when compared with the electrostatic force, it is thought to have no significant effect on the spinning process. But the shape of the Taylor cone, jet trajectory, fibre diameter, fibre diameter distribution, and overall spinning efficiency are all influenced by it. In this review paper, we discuss all these developments and more. Furthermore, because many research groups build their own electrospinning apparatus, it would be prudent to consider this aspect as particular orientations are more suitable for certain applications.

Mueller Matrix Analysis of Collagen and Gelatin Containing Samples Towards More Objective Skin Tissue Diagnostics

Electrospun polycaprolactone:gelatin (PCL:GT) fibre scaffolds are widely employed in the field of tissue implants. Here, the orientation of fibres plays an important role in regard to implantation due to the impact on the mechanical properties. Likewise, the orientation of collagen fibres in skin tissue is relevant for dermatology. State-of-the-art fibre orientation measurement methods like electron microscopy are time consuming and destructive. In this work, we demonstrate polarimetry as a non-invasive approach and evaluate its potential by measuring the Mueller matrix (MM) of gelatin and collagen containing samples as simple skin tissue phantoms. We demonstrate that it is possible to determine the orientation of PCL:GT fibre scaffolds within one MM measurement. Furthermore, we determine the structural orientation in collagen film samples. Currently, the diagnosis of skin diseases is often performed by image analysis or histopathology respectively, which are either subjective or invasive. The method presented, here, provides an interesting alternative approach for such investigations. Our findings indicate that the orientation of collagen fibres within skin lesions might be detectable by MM measurements in the future, which is of interest for skin diagnostics, and will be further investigated during the next step.