In vitro and in vivo calcification of vascular bioprostheses

In-vivo assessment of a tissue engineered vascular graft computationally optimized for target vessel compliance

Tissue engineered vascular grafts (TEVGs) have the ability to be tuned to match a target vessel's compliance, diameter, wall thickness, and thereby prevent compliance mismatch. In this work, TEVG compliance was manipulated by computationally tuning its layered composition or by manipulating a crosslinking agent (genipin). In particular, these three acelluluar TEVGs were compared: a compliance matched graft (CM_gel - high gelatin content); a hypocompliant PCL graft (HYPO_pcl - high polycaprolactone content); and a hypocompliant genipin graft (HYPO_gen - equivalent composition as CM_gel but hypocompliant via increased genipin crosslinking). All constructs were implanted interpositionally into the abdominal aorta of 21 Sprague Dawley rats (n=7, males=11, females=10) for 28 days, imaged in-vivo using ultrasound, explanted, and assessed for remodeling using immunofluorescence and two photon excitation fluorescence imaging. Compliance matched grafts remained compliance-matched in-vivo compared to the hypocompliant grafts through 4 weeks (p<0.05). Construct degradation and cellular infiltration was increased in the CM_gel and HYPO_gen TEVGs. Contractile smooth muscle cell markers in the proximal anastomosis of the graft were increased in the CM_gel group compared to the HYPO_pcl (p=0.007) and HYPO_gen grafts (p=0.04). Both hypocompliant grafts also had an increased pro-inflammatory response (increased ratio of CD163 to CD86 in the mid-axial location) compared to the CM_gel group. Our results suggest that compliance matching using a computational optimization approach leads to the improved acute (28 day) remodeling of TEVGs. To the authors' knowledge, this is the first in-vivo rat study investigating TEVGs that have been computationally optimized for target vessel compliance.

Computationally optimizing the compliance of multilayered biomimetic tissue engineered vascular grafts

Coronary artery bypass grafts used to treat coronary artery disease often fail due to compliance mismatch. In this study, we have developed an experimental/computational approach to fabricate an acellular biomimetic hybrid tissue engineered vascular graft composed of alternating layers of electrospun porcine gelatin/polycaprolactone (PCL) and human tropoelastin/PCL blends with the goal of compliance-matching to rat abdominal aorta, while maintaining specific geometrical constraints. Polymeric blends at three different gelatin:PCL (G:PCL) and tropoelastin:PCL (T:PCL) ratios (80:20, 50:50 and 20:80) were mechanically characterized. The stress-strain data was used to develop predictive models, which were used as part of an optimization scheme that was implemented to determine the ratios of G:PCL and T:PCL and the thickness of the individual layers within a tissue engineered vascular graft that would compliance match a target compliance value. The hypocompliant, isocompliant, and hypercompliant grafts had target compliance values of 0.000256, 0.000568 and 0.000880 mmHg-1, respectively. Experimental validation of the optimization demonstrated that the hypercompliant and isocompliant grafts were not statistically significant from their respective target compliance values (p-value=0.37 and 0.89, respectively). The experimental compliance value of the hypocompliant graft was statistically significant than their target compliance value (p-value=0.047). We have successfully demonstrated a design optimization scheme that can be used to fabricate multilayered and biomimetic vascular grafts with targeted geometry and compliance.

Biomaterials-Potential nucleation agents in blood and possible implications

Blood, simulated body fluids, and many cell culture media are supersaturated solutions with respect to several calcium phosphates. Therefore biomaterials can act as nucleation agents and evoke heterogeneous nucleation of salts on the surface of immersed biomaterials. Depending on the field of application, this can be either beneficial or disadvantageous. Although nucleation from supersaturated solutions is an old and well-known scientific phenomenon it is not standard to test new developed materials with surface analytical methods for their ability to initiate nucleation in vitro. Therefore, this communication aims to review the mineralization effect and to emphasize the possible negative implications, especially to functionalized bone implants. Surface coatings with proteins, growth factors, and, etc., can become ineffective due to deposition of a dense calcium phosphate layer. In the case of drug loaded implants, drug release might be inhibited.

Novel method of decellularization of porcine valves using polyethylene glycol and gamma irradiation

BACKGROUND

Recent tissue-engineered valves are in need of a breakthrough to overcome several limitations against clinical applications. We have developed a new method of decellularization using polyethylene glycol and gamma irradiation.

METHODS

Fresh porcine aortic valves were decellularized using polyethylene glycol and gamma irradiation. These were evaluated by histologic, biochemical (DNA, solubilized protein and collagen content), mechanical (strength test, transmission electron microscopy) and immunologic (porcine endogenous retrovirus and the alpha-1.3 galactosyl epitope) analyses. Implantations into the subcutaneous tissue of rats (1 week, n = 10; 2 months, n = 10) and into the descending aorta of dogs (2 months, n = 6; 6 months, n = 3) were used as in vivo studies.

RESULTS

Complete decellularization was confirmed by histologic examination and by determining the DNA and solubilized protein content. The decellularized valve showed no significant differences in its mechanical strength or collagen content compared with native porcine tissues. The ultrastructure was well preserved in transmission electron microscope images. The DNA sequence of a porcine endogenous retrovirus and the alpha-1.3 galactosyl epitope were eliminated after the decellularizing process. No acute rejection and little calcification was noted in the rat model. In the dog model at 2 months, the surface of the graft was completely covered with a monolayer of endothelial cells. In addition, several layers of vimentin-positive cells lay under the endothelial cells. At 6 months after implantation, many smooth muscle cells, monolayer endothelial cells, and some vasculogenesis were seen.

CONCLUSIONS

The decellularizing method provided low immunogenicity, low risk of unknown infections, and was little subject to calcification. The decellularized tissues showed acceptable durability and recellularization.

Gelatinases in soft tissue biomaterials. Analysis of different crosslinking agents

Chemical modification of pericardium-based cardiac valves tends to reduce the relatively high degree of biodegradation and calcification of the implanted bioprostheses. We analysed the tissue properties of pericardium from young calves and pigs after crosslinking with different agents (glutaraldehyde. diphenylphosphorylazide (DDPA), 1-ethyl-3,3-dimethyl-aminopropyl-carbodiimide (EDAC)) and when exposed to anticalcification treatments (chloroform/methanol or ethanol) prior to glutaraldehyde (GA) crosslinking. Protein extraction after tissue homogenisation in the presence of detergents showed that crosslinking using GA or DPPA was much more effective. The amounts of protein extracted from these two groups of chemically modified pericardium were significantly lower: the other modified tissues presented only a slight reduction when compared with untreated tissue. Matrix metalloproteinases- (MMP) 2 and 9 were detected in native pericardium from calf and pig by zymography. While the MMP-9/MMP-2 activity ratio was close to 1 in pig pericardium, it was 8.5-fold higher in bovine tissue. Crosslinking with GA and with DPPA almost completely abolished gelatinase activities, even when equal amounts of solubilised protein were loaded onto the zymograms. Anticalcification treatments followed by GA crosslinking or treatment with EDAC were not as effective in reducing gelatinase activities; but, interestingly, a relative reduction of MMP-9 versus MMP-2 was detected. The presence of these gelatinase activities in pericardium may contribute to the in vivo degradability of pericardium-based cardiac valves.

Comparison of five procedures for the purification of insoluble elastin

Elastin is an insoluble, highly cross-linked protein, providing elasticity to organs like lung. aorta, and ligaments. Despite its remarkable mechanical properties. elastin has found little use as a biomaterial. Purification of intact elastin from elastic fibres presents a major challenge, among others for the intimate interwoveness of elastin and microfibrils. Insoluble elastin preparations tend to calcify, which may be due to calcium-binding microfibrillar (e.g. fibrillin). In this study, elastin was purified from horse ligamentum nuchae using five different procedures. One procedure is based on treatment with 0.1 M NaOH, another on autoclaving and treatment with cyanogen bromide. Three other procedures are based on combinations of extraction steps and enzyme digestions. Purity of preparations was assessed by sodium dodecyl sulphate polyacrylamide gel electrophoresis, amino acid analysis, bright field immunofluorescence and transmission electron microscopy. The procedure involving extractions/enzymes combined with an early application of 2-mercaptoethanol and cyanogen bromide gives a highly pure elastin preparation. Electron microscopic analysis showed that this preparation is devoid of microfibrillar components. This procedure is therefore the method of choice for preparation of insoluble elastin as a biomaterial for tissue engineering.

Acellular vascular tissues: natural biomaterials for tissue repair and tissue engineering

Various research groups around the world are actively investigating cardiovascular prostheses of biological origin. This review article discusses the need for such bioprosthetics and the potential role for natural tissues in cardiovascular applications such as cardiac valves and vascular grafts. Upon implantation, unmodified natural materials are subject to chemical and enzymatic degradation, seriously decreasing the life of the prosthesis. Therefore, methods such as glutaraldehyde and polyepoxide crosslinking treatments and dye-mediated photooxidation have been developed to stabilize the tissue while attempting to maintain its natural mechanical properties. Also, residual cellular components in a bioprosthetic material have been associated with undesired effects, such as calcification and immunological recognition, and thus have been the motivation for various decellularization processes. The effects of these stabilization and decellularization treatments on mechanical, biological and chemical properties of treated tissues have been investigated, specifically with regard to calcification, immunogenicity, and cytotoxicity concerns. Despite significant advances in the area of cardiovascular prostheses, there has yet to be developed a completely biocompatible, long-lasting implant. However, with the recent advent of tissue engineering, the possibility of applying selective cell seeding to naturally derived bioprosthetics moves us closer to a living tissue replacement.