Atomic force microscopy investigation of chemically stabilized pericardium tissue

A functionalized biological heart valve by double bond crosslinking with enhanced biocompatibility and antithrombogenicity

With the advancement of minimally invasive interventional therapy, biological heart valves (BHVs) have been extensively used in clinics. However, BHVs are generally prone to degeneration within 10-15 years after implantation due to defects including cytotoxicity, immune response, calcification and thrombosis, which are closely related to glutaraldehyde-crosslinking. In this work, we prepared a functionalized BHV through the in situ polymerization of methacrylated porcine pericardium and 2-hydroxyethyl methacrylate to avoid and overcome the defects of glutaraldehyde-crosslinked BHVs. The functionalized BHV was proven to be stable against enzymatic degradation and compatible towards HUVECs. After implantation in rats subcutaneously, a significantly mitigated immune response and reduced calcification were observed in the functionalized BHV. With the grafting of hydrophilic 2-hydroxyethyl methacrylate polymers, the antithrombogenicity of BHV was markedly enhanced by resisting the unfavorable adhesion of blood components. Moreover, the hydrodynamics of the functionalized BHV totally conformed to ISO 5840-3 under a wide range of simulated physiological conditions. These results indicate that the functionalized BHV with enhanced biocompatibility, anticalcification property and antithrombogenicity exhibited a low risk of degeneration and should be explored for further application.

Structural and Biochemical Changes in Pericardium upon Genipin Cross-Linking Investigated Using Nondestructive and Label-Free Imaging Techniques

Tissue cross-linking represents an important and often used technique to enhance the mechanical properties of biomaterials. For the first time, we investigated biochemical and structural properties of genipin (GE) cross-linked equine pericardium (EP) using optical imaging techniques in tandem with quantitative atomic force microscopy (AFM). EP was cross-linked with GE at 37 °C, and its biochemical and biomechanical properties were observed at various time points up to 24 h. GE cross-linked EP was monitored by the normalized ratio between its second-harmonic generation (SHG) and two-photon autofluorescence emissions and remained unchanged for untreated EP; however, a decreasing ratio due to depleted SHG and elevated autofluorescence and a fluorescence band at 625 nm were found for GE cross-linked EP. The mean autofluorescence lifetime of GE cross-linked EP also decreased. The biochemical signature of GE cross-linker and shift in collagen bands were detected and quantified using shifted excitation Raman difference spectroscopy as an innovative approach for tackling artifacts with high fluorescence backgrounds. AFM images indicated a higher and increasing Young's modulus correlated with cross-linking, as well as collagen structural changes in GE cross-linked EP, qualitatively explaining the observed decrease in the second-harmonic signal. In conclusion, we obtained detailed information about the biochemical, structural, and biomechanical effects of GE cross-linked EP using a unique combination of optical and force microscopy techniques in a nondestructive and label-free manner.

Physicochemical characterization and self-assembly of human amniotic membrane and umbilical cord collagen: A comparative study

The diverse application of collagen has created a need to discover renewable and economical sources with prevailing/improved physico-chemical properties. To address this scenario, the present study has extracted collagen from Human Amniotic Membrane (AM) and Umbilical cord, which are treated as medical waste and compared its physico-chemical properties. Collagen was extracted by pepsin solubilization using various salt concentrations (1 M, 2 M and 4 M). Umbilical Cord Collagen (UC) yield was 10% higher than Amniotic Membrane Collagen (AC). UC reported 58% higher sulphated glycosaminoglycan content than AC. Electrophoretic pattern of AC and UC in both disulphide bond reducing and non-reducing conditions showed bands corresponding to collagen type I, III, IV, V and XV. Collagen morphology was examined using SEM and the amino acid content was quantified by HPLC and LC-MS/MS. Triple helicity was confirmed by CD and FTIR spectra. Thermal transition temperature of AC and UC was found equivalent to animal collagen. Self-assembly, fibril morphology and spatial alignment was studied using AFM and DLS. Biocompatibility was analyzed using 3T3 fibroblast cells. In conclusion, UC with higher yield, presented with better physico-chemical, structural and biological properties than AC could serve as an efficient alternative to the existing animal collagen for diverse applications.

New insight into the shortening of the collagen fibril D-period in human cornea

Collagen fibrils type I display a typical banding pattern, so-called D-periodicity, of about 67 nm, when visualized by atomic force or electron microscopy imaging. Herein we report on a significant shortening of the D-period for human corneal collagen fibrils type I (21 ± 4 nm) upon air-drying, whereas no changes in the D-period were observed for human scleral collagen fibrils type I (64 ± 4 nm) measured under the same experimental conditions as the cornea. It was also found that for the corneal stroma fixed with glutaraldehyde and air-dried, the collagen fibrils show the commonly accepted D-period of 61 ± 8 nm. We used the atomic force microscopy method to image collagen fibrils type I present in the middle layers of human cornea and sclera. The water content in the cornea and sclera samples was varying in the range of .066-.085. Calculations of the D-period using the theoretical model of the fibril and the FFT approach allowed to reveal the possible molecular mechanism of the D-period shortening in the corneal collagen fibrils upon drying. It was found that both the decrease in the shift and the simultaneous reduction in the distance between tropocollagen molecules can be responsible for the experimentally observed effect. We also hypothesize that collagen type V, which co-assembles with collagen type I into heterotypic fibrils in cornea, could be involved in the observed shortening of the corneal D-period.

The process of EDC-NHS Cross-linking of reconstituted collagen fibres increases collagen fibrillar order and alignment

We describe the production of collagen fibre bundles through a multi-strand, semi-continuous extrusion process. Cross-linking using an EDC (1-ethyl-3-(3-dimethylaminopropyl)carbodiimide), NHS (N-hydroxysuccinimide) combination was considered. Atomic Force Microscopy (AFM) and Raman spectroscopy focused on how cross-linking affected the collagen fibrillar structure. In the cross-linked fibres, a clear fibrillar structure comparable to native collagen was observed which was not observed in the non-cross-linked fibre. The amide III doublet in the Raman spectra provided additional evidence of alignment in the cross-linked fibres. Raman spectroscopy also indicated no residual polyethylene glycol (from the fibre forming buffer) or water in any of the fibres.

Tissue section AFM: In situ ultrastructural imaging of native biomolecules

Conventional approaches for ultrastructural high-resolution imaging of biological specimens induce profound changes in bio-molecular structures. By combining tissue cryo-sectioning with non-destructive atomic force microscopy (AFM) imaging we have developed a methodology that may be applied by the non-specialist to both preserve and visualize bio-molecular structures (in particular extracellular matrix assemblies) in situ. This tissue section AFM technique is capable of: i) resolving nm–µm scale features of intra- and extracellular structures in tissue cryo-sections; ii) imaging the same tissue region before and after experimental interventions; iii) combining ultrastructural imaging with complimentary microscopical and micromechanical methods. Here, we employ this technique to: i) visualize the macro-molecular structures of unstained and unfixed fibrillar collagens (in skin, cartilage and intervertebral disc), elastic fibres (in aorta and lung), desmosomes (in nasal epithelium) and mitochondria (in heart); ii) quantify the ultrastructural effects of sequential collagenase digestion on a single elastic fibre; iii) correlate optical (auto fluorescent) with ultrastructural (AFM) images of aortic elastic lamellae.

Tailored laminin-332 alpha3 sequence is tethered through an enzymatic linker to a collagen scaffold to promote cellular adhesion

Surface modification techniques have been used to develop biomimetic scaffolds by incorporating cell adhesion peptides, which facilitates cell adhesion, migration and proliferation. In this study, we evaluated the cell adhesion properties of a tailored laminin-332 alpha3 chain tethered to a type I collagen scaffold using microbial transglutaminase (mTGase) by incorporating transglutaminase substrate peptide sequences containing either glutamine (peptide A: PPFLMLLKGSTREAQQIVM) or lysine (peptide B: PPFLMLLKGSTRKKKKG). The degree of cross-linking was studied by amino acid analysis following proteolytic digestion and the structural changes in the modified scaffold further investigated using Fourier transform infrared spectroscopy and atomic force microscopy. Fibroblasts were used to evaluate the cellular behaviour of the functionalized collagen scaffold. mTGase supports cell growth but tethering of peptide A and peptide B to the mTGase cross-linked collagen scaffold caused a significant increase in cell proliferation when compared with native and mTGase cross-linked collagen scaffolds. Both peptides enabled cell-spreading, attachment and normal actin cytoskeleton organization with slight increase in the cell proliferation was observed in peptide A when compared with the peptide B and mTGase cross-linked scaffold. An increase in the amount of epsilon(gamma-glutamyl) lysine isopeptide was observed in peptide A conjugated scaffolds when compared with peptide B conjugated scaffolds, mTGase cross-linked scaffold without peptide. Changes in D-spacing were observed in the cross-linked scaffolds with tethered peptides. These results demonstrate that mTGase can play a bifunctional role in both conjugation of the glutamine and lysine containing peptide sequences and also in the cross-linking of the collagen scaffold, thus providing a suitable substrate for cell growth.

Supramolecular structure of human aortic valve and pericardial xenograft material: atomic force microscopy study

Pericardial tissue (bovine or porcine), chemically stabilized with glutaraldehyde (GA), is widely used in cardiovascular surgery in the form of bioprosthetic valves. GA reacts with tissue proteins and creates inter- and intra-molecular cross-links, resulting in improved durability. However, tissue calcification and mechanical damage are still unresolved problems. The purpose of this study was to examine the surface topography of normal human aortic valve and GA-stabilized porcine pericardium tissue in order to gain comparative insight into supramolecular structure of both tissues. The analysis was focused on morphologic evaluation of collagen constituents of the tissues. Atomic force microscopy working in the contact mode in air was employed in the study. Considerable diversity in the spatial orientation of collagen fibrils for the human aortic valve and pericardial tissue were observed. It was found that different forms of collagen fibril packing, i.e. dense and "in phase" or loose, could have an impact on the collagen D-banding pattern. Stabilization with GA introduced significant changes in the surface topography of collagen fibrils and in their spatial organization on the tissue surface. Strong disturbance in the fibril's D-spacing was observed. It was also suggested, that the observed structural changes at the supramolecular level might make an important contribution to the progressive damage and calcification of the tissue. The presented results demonstrate that the AFM method can be useful for non-destructive structural characterization of heart valves and bioprosthetic heart valve material.

Tannic acid-stabilized pericardium tissue: IR spectroscopy, atomic force microscopy, and dielectric spectroscopy investigations

Infrared (IR) spectroscopy, atomic force microscopy (AFM), and dielectric spectroscopy methods were employed to study structural and dynamic changes in the tannic acid (TA)-stabilized pericardium tissue. Chemically stabilized pericardium tissue is widely used in construction of the tissue derived bioprostheses. IR spectra recorded in the range 400-4000 cm-1 allowed us to recognize different types of TA-collagen interactions. Formation of hydrogen bonds between amine as well as amide NH groups from collagen and hydroxyl groups of TA was analyzed. The AFM imaging showed that the stabilization procedure with TA introduces considerable changes in both surface topography and thickness of collagen fibrils as well as in fibril arrangement on the tissue surface. It was found, that these structural changes have an impact on the dielectric behavior of the TA-stabilized tissue. The dielectric spectra for the native and TA-stabilized tissues were measured in the frequency and temperature ranges of 10(-1) -10(7) Hz and 120-270 K, respectively. The dielectric spectra revealed the relaxation process due to orientation of bound water supplemented by the fluctuation of collagen polar side groups. At the temperatures above approximately 210 K, the relaxation due to ion migration process was observed. It was found that both relaxation processes were influenced by the TA-collagen interaction.