Study on the physical properties of tissue-engineered blood vessels made by chemical cross-linking and polymer-tissue cross-linking

Design of improved acellular fish skin as a promising scaffold for tissue regeneration applications

Decellularized marine tissues have been regarded as a desirable biomaterial because of their biological risk reduction, less religious constraints, and resemblance to mammalian tissues. The properties of these matrices can be improved by adding cross-linkers. In this study, after decellularization of the of Tilapia and Grass carp fish skin, a comparative study was conducted between them. Due to the higher abundance of collagen and glycosaminoglycans (GAGs) in Tilapia skin, it was selected for further study. In the next step, the cross-linking process was performed with three concentrations of 1-Ethyl-3-(3-dimethylaminopropyl) carbodiimide/ N-Hydroxysuccinimide (EDC/NHS) and tannic acid cross-linkers. The MTT results showed that the cross-linked samples with low concentrations of EDC/NHS had higher biocompatibility compared to the cross-linked sample with high concentration of EDC/NHS, as well as all samples cross-linked with tannic acid. Mechanical and physical studies conducted on the skin of Tilapia fish showed that the 15 mM/7.5 mM concentration of EDC/NHS increased the mechanical and temperature strength and decreased the degradability and it did not influence cell attachment. In general, it was shown that different fish skins differ in terms of collagen and GAGs, and the optimal concentration of EDC cross-linker improves the mechanical and physical properties of the matrix derived from fish skin.

Crosslinking strategies of decellularized extracellular matrix in tissue regeneration

By removing the immunogenic cellular components through various decellularization methods, decellularized extracellular matrix (dECM) is considered a promising material in the field of tissue engineering and regenerative medicine with highly preserved physicochemical properties and superior biocompatibility. However, decellularization treatment can lead to some loss of structural integrity, mechanical strength, degradation stability, and biological performance of dECM biomaterials. Therefore, physical and chemical crosslinking methods are preferred to restore or even improve the biomechanical properties, stability, and bioactivity, and to achieve a delicate balance between degradation of the implanted biomaterial and regeneration of the host tissue. This review provides an overview of dECM biomaterials, and describes and compares the mechanisms and characteristics of commonly used crosslinking methods for dECM, with a focus on the potential applications of versatile dECM-based biomaterials derived from skin, cardiac tissues (pericardium, heart valves, myocardial tissue), blood vessels, liver, and kidney, modified with different chemical crosslinking reagents, in tissue and organ regeneration.

Acellular Tissue-Engineered Vascular Grafts from Polymers: Methods, Achievements, Characterization, and Challenges

Extensive and permanent damage to the vasculature leading to different pathogenesis calls for developing innovative therapeutics, including drugs, medical devices, and cell therapies. Innovative strategies to engineer bioartificial/biomimetic vessels have been extensively exploited as an effective replacement for vessels that have seriously malfunctioned. However, further studies in polymer chemistry, additive manufacturing, and rapid prototyping are required to generate highly engineered vascular segments that can be effectively integrated into the existing vasculature of patients. One recently developed approach involves designing and fabricating acellular vessel equivalents from novel polymeric materials. This review aims to assess the design criteria, engineering factors, and innovative approaches for the fabrication and characterization of biomimetic macro- and micro-scale vessels. At the same time, the engineering correlation between the physical properties of the polymer and biological functionalities of multiscale acellular vascular segments are thoroughly elucidated. Moreover, several emerging characterization techniques for probing the mechanical properties of tissue-engineered vascular grafts are revealed. Finally, significant challenges to the clinical transformation of the highly promising engineered vessels derived from polymers are identified, and unique perspectives on future research directions are presented.

Cross-linked acellular lung for application in tissue engineering: Effects on biocompatibility, mechanical properties and immunological responses

The concept of providing tissue engineering scaffolds with natural physical properties and minimal immunogenicity has not been systematically approached for the lungs yet. Here, the rat acellular lung tissue (ALT) was cross-linked to provide either EDC/NHS cross-linked tissue (EDC/NHS-CLT) or tannic acid cross-linked tissue (TA-CLT). Young's modulus revealed that EDC/NHS-CLT had mechanical properties similar to the native lung and culture of lung mesenchymal cells showed a higher potential of cell proliferation on EDC/NHS-CLT versus TA-CLT and ALT. The in vitro immunogenicity tests showed a strong induction of T-cell proliferation by TA-CLT and an attenuated macrophage induction by TA-CLT. Processed rat lungs were implanted xenogenically into the mouse peritoneal cavity and the host-implant interactions showed that tannic acid is not released from TA-CLT in a physiologically effective dose. The profile of peritoneal fluid proinflammatory (TNFα, IL-1β, IL-12p70 and IL-17) and anti-inflammatory (IL-10 and TGFβ1) cytokines, and CD3⁺ T-lymphocytes and CD11b⁺ macrophages revealed that apart from induction of high levels of IL-17 during the first week and IL-10 during the second to third weeks after implantation by TA-CLT, other indicators of immune reactions to cross-linked tissues were not significantly different from ALT. Also, a high fibrotic reaction to TA-CLT was observed on the weeks 2-3, but alveolar structures were preserved in EDC/NHS-CLT. Our findings show that by controlled EDC/NHS cross-linking, an acellular lung scaffold could be provided with mechanical properties similar to native lung, which promotes mesenchymal lung cells proliferation and does not stimulate recipient's immune system more than a non-cross-linked tissue.

Wholly vascularized millimeter-sized engineered tissues by cell-sized microscaffolds

The in vitro fabrication of wholly vascularized millimeter-sized engineered tissues is still a key challenge in the tissue engineering field. Recently we reported a unique approach 'sedimentary culture' using a collagen microfiber (CMF) to fabricate large-scale engineered tissues. The millimeter-sized tissues with high extracellular matrix (ECM) density were easily obtained by centrifugation of cells and CMFs and subsequent cultivation because the CMFs acted as a micrometer-sized scaffold. However, cell distribution in the obtained tissues was not homogeneous because of the different sedimentation velocity of the cells and CMFs because of their size difference. Here we report the fabrication of wholly vascularized millimeter-sized engineered tissues using cell-sized CMFs. To avoid dissolving, vacuum drying was performed at 200 °C for 24 h for thermal crosslinking of primary amine groups of type I collagen. The 200- and 20-μm-sized CMFs (CMF-200 and CMF-20) were obtained by homogenization and subsequent sonication of the crosslinked collagen. Interestingly, the CMF-20 indicated a similar sedimentation velocity with cells because of their same size range, thus uniform millimeter-sized tissue with homogeneous cell distribution was fabricated by the sedimentary culture method. To form a whole blood capillary structure in the tissues, fibronectin (FN) was adsorbed on the surface of CMF-20 to stimulate endothelial cell migration. The distribution of the blood capillary network in 1.6-mm-sized tissues was markedly improved by FN-adsorbed CMF-20 (FN-CMF-20). Sedimentary culture using FN-CMF-20 will create new opportunities in tissue engineering for the in vitro fabrication of wholly vascularized millimeter-sized engineered tissues.

Preparation of decellularized vascular matrix by co-crosslinking of procyanidins and glutaraldehyde

BACKGROUND

Vascular extracellular matrices (vECMs) have shown potential for small-diameter blood vessel tissue engineering applications. However, problems such as chemical instability and easy calcification are still remained. Chemical crosslinking using crosslinkers such as glutaraldehyde (GA) can improve mechanical properties and proteolysis resistance of vECMs, but leads to calcification and cytotoxicity. Procyanidins (PC) can crosslink ECMs with anti-calcification property and cytocompatibility, but the mechanical properties and chemical stability are unsatisfactory.

OBJECTIVE

A novel co-crosslinking technique using PC and GA was developed, which combines the advantages of both PC and GA for enhancing mechanical properties and stability of vECMs with reduced calcification and cytotoxicity.

METHODS

Fresh carotid were decellularized and then crosslinked by PC and subsequent GA for 6 h respectively. The mechanical properties, dynamic release of PC, enzymatic degradation, calcification and cytotoxicity of crosslinked samples were evaluated.

RESULTS

The co-crosslinked vECMs showed enhanced tensile strength, chemical and biological stability, comparable anti-calcification property as compared to pure PC-crosslinked samples. Cytotoxicity assay showed that the co-crosslinked vECMs were cytocompatible for supporting the adhesion and proliferation of HUVECs.

CONCLUSIONS

Co-crosslinking with PC and GA might be a useful method for preparation of vECM scaffolds with potential applications in small-diameter blood vessel tissue engineering.

Promoting endothelialization on decellularized porcine aortic valve by immobilizing branched polyethylene glycolmodified with cyclic-RGD peptide: an in vitro study

We functionally modify a decellularized porcine aortic valve using a novel complex biologically active cyclic- (c)-RGD modified with branched polyethylene glycol (PEG), namely, c-RDG-PEG. Human umbilical vein endothelial cell (HUVEC) adhesion and proliferation were detected for up to 8 d after seeding on the scaffold. (1)H nuclear magnetic resonance (D2O) showed signal peaks at 7.27 and 7.38 ppm associated with protons of the phenyl group in c-RGD-PEG. Attenuated total reflectance Fourier transform infrared spectroscopy showed characteristic peaks for PEG at 1100 and 1342 cm(-1). These represented vibration peaks of C-O and -CH2 bonds, suggesting successful grafting of c-RGD-PEG to a decellularized porcine aortic valve (DPAV). The tensile strengths were significantly increased in the c-RGD-PEG-DPAV group compared to the native valve and DPAV groups (P < 0.05), while the elastic modulus was sigficantly decreased in the c-RGD-PEG-DPAV group compared to the native valve and DPAV groups (P < 0.05). HUVEC proliferation was significantly higher in the c-RGD-PEG-DPAV group than in the PEG-DPAV and DPAV groups (P < 0.01). Maximum adhesion occurred at 4 h, and on the 8th day, a confluent and compact monolayer formed on the valve surface. The modified DPAV resulted in good adhesion and proliferation of endothelial cells and is an appropriate approach to modify tissue engineered heart valves for promoting endothelialization.

Micro-CT evaluation of high pressure-decellularized cardiovascular tissues transplanted in rat subcutaneous accelerated-calcification model

We have succeeded in reducing the calcification of acellular aortas or valves in porcine allogeneic system by removing the DNA and phospholipids, but its further reduction is desirable. Here, the calcification of the acellular tissue was evaluated in rat subcutaneous transplantation model which is known as calcification model. Acellular samples prepared by high-hydrostatic pressure (HHP) protocols with different washing media were implanted and the calcification was monitored under micro-computed tomography for 1 and 3 months. The amount of the calcium deposition was quantitatively evaluated by atomic absorption spectroscopy. A cell culture medium showed very good cell removal ability but led to severe calcification at 1 month, and surprisingly the calcium deposition increased as the washing period increased. This calcification was suppressed by removing the DNA fraction with high DNase concentration. On the other hand, the calcification was greatly reduced when washed with saline even at low DNase concentration after 2 weeks washing. These results suggest that the ion species in the washing medium and the residual DNase cooperatively affect the tendency of in vivo calcification, which led us to the possibility of reduced calcification of acellular cardiac tissues.

Decellularized dermis-polymer complex provides a platform for soft-to-hard tissue interfaces

To develop a soft-to-hard tissue interface, we made a decellularized dermis/poly(methyl methacrylate) (PMMA) complex by soaking the decellularized dermis in methyl methacrylate (MMA) and an initiator, and then polymerizing the MMA. The decellularized tissue was chosen because of its good biocompatibility and the easiness of suturing it, and MMA because of its hard tissue compatibility and wide use in the biomedical field. The MMA filled the cavities in the dermis and polymerized within 10 min. No leaking or polymer aggregation was observed, implying that a homogenous tissue-polymer complex had formed. The cell infiltration and the integration between the tissue and the dermis occurred in vivo, whereas the cells could not infiltrate the tissue-polymer complex. This implies that the interface tissue should possess both complex and noncomplex parts, where the cells infiltrate the noncomplex part and stop when they encounter the complex part, integrating the soft and hard tissue or hard polymer.

Epoxy cross-linked collagen and collagen-laminin Peptide hydrogels as corneal substitutes

A bi-functional epoxy-based cross-linker, 1,4-Butanediol diglycidyl ether (BDDGE), was investigated in the fabrication of collagen based corneal substitutes. Two synthetic strategies were explored in the preparation of the cross-linked collagen scaffolds. The lysine residues of Type 1 porcine collagen were directly cross-linked using l,4-Butanediol diglycidyl ether (BDDGE) under basic conditions at pH 11. Alternatively, under conventional methodology, using both BDDGE and 1-Ethyl-3-(3-dimethyl aminopropyl) carbodiimide (EDC)/N-hydroxysuccinimide (NHS) as cross-linkers, hydrogels were fabricated under acidic conditions. In this latter strategy, Cu(BF₄)₂·XH₂O was used to catalyze the formation of secondary amine bonds. To date, we have demonstrated that both methods of chemical cross-linking improved the elasticity and tensile strength of the collagen implants. Differential scanning calorimetry and biocompatibility studies indicate comparable, and in some cases, enhanced properties compared to that of the EDC/NHS controls. In vitro studies showed that human corneal epithelial cells and neuronal progenitor cell lines proliferated on these hydrogels. In addition, improvement of cell proliferation on the surfaces of the materials was observed when neurite promoting laminin epitope, IKVAV, and adhesion peptide, YIGSR, were incorporated. However, the elasticity decreased with peptide incorporation and will require further optimization. Nevertheless, we have shown that epoxy cross-linkers should be further explored in the fabrication of collagen-based hydrogels, as alternatives to or in conjunction with carbodiimide cross-linkers.

Biomechanics and biocompatibility of the perfect conduit-can we build one?

No currently available conduit meets the criteria for an ideal coronary artery bypass graft. The perfect conduit would combine the availability and complication-free harvest of a synthetic vessel with the long-term patency performance of the internal mammary artery. However, current polymer conduits suffer from inelastic mechanical properties and especially poor surface biocompatibility, resulting in early loss of patency as a coronary graft. Approaches to manufacture an improved conduit using new polymers or polymer surfaces, acellular matrices, or cellular constructs have to date failed to achieve a commercially successful alternative. Elastin, by mimicking the native extracellular environment as well as providing elasticity, provides the 'missing link' in vascular conduit design and brings new hope for realization of the perfect conduit.