In conclusion, CardioCel demonstrated optimal strength and cross-linking, elasticity, minimal mineralization potential and superior biocompatibility. In contrast, the favourable characteristics of the comparator products were counterbalanced by less desirable features such as reduced biocompatibility and encapsulation (XenoLogiX and PeriGuard), degradation, reduced tensile strength, poor cross-link stability (PhotoFix) and reduced stretch without any cross-link stability (CorMatrix).
DISCUSSION (For those that want more information - my edit)
Tissue substitutes are often used in congenital and adult cardiac surgery as bioscaffolds for repair of cardiac and valve defects [1, 13, 14]. A combination of strength and flexibility is critical to the performance of the cardiac tissue: ventricular and great vessel tissues must have the strength to tolerate peak systolic pressure, while aortic valves must accommodate significant anisotropic stretches [2]. Cross-linking with GA is employed to increase the strength and durability of bioprosthetics and provides protection from enzymatic degradation [15]. However, conventional GA treatment decreases pliability (increased stiffness) of the bioscaffolds [15, 16] and is associated with an increased risk for calcification and graft failure [8]. CardioCel is therefore cross-linked via a unique process based on an ultra-low concentration of monomeric GA, which provides thorough cross-linking without being prone to calcification [14, 17]. In this study, CardioCel demonstrated increased cross-linking (mean Td 77.99 °C) and optimal tensile strength (mean 8.3 MPa), well above the minimum strength of the native human ascending aorta (1.8 ± 0.24 MPa) [18].
CardioCel demonstrated excellent pliability, with the Young’s modulus (mean 50.21 MPa) in the range of the un-cross-linked CorMatrix and PhotoFix products. Stress–strain data indicated that CardioCel possesses considerable initial flexibility, with a mean of 3.49 MPa, the second-lowest measured.
The 0.6% GA-fixed XenoLogiX and PeriGuard proved to be more extensively cross-linked than the other materials (Td means of 80.21 °C and 84.00 °C) and displayed the highest tensile strength (11.00 and 16.44 MPa, respectively). However, these values are much in excess of what is required for cardiac tissue repair [18]. These increases in strength appear to come at the expense of pliability: both XenoLogiX and PeriGuard were found to have the highest Young’s modulus, significantly higher than the non-GA-cross-linked PhotoFix and CorMatrix (67.01 and 95.67 MPa vs 36.78 and 33.50 MPa). Stiffening of the extracellular matrix has also been shown previously for PeriGuard compared with the untreated tissue [15, 19]. The lack of pliability of these materials may affect their performance in vivo, as stiffness can affect compliance, and the differences between the mechanical properties of aortic tissues and replacement materials can have unwanted haemodynamic effects leading to graft failure and can even affect the phenotype of cells binding to the bioscaffolds [20].
PhotoFix and CorMatrix displayed characteristics that were markedly different to the 0.6% GA-fixed BP cross-linked scaffolds. Both PhotoFix and CorMatrix recorded a Td significantly lower than the 3 GA-cross-linked tissues. The mean obtained for CorMatrix (57.88 °C) is similar to what has been reported previously for porcine valve leaflets (approximately 56 °C) [21], which is presumably lower than normal BP (62.8 °C) [22] because of species and/or tissue-specific differences. However, the Td of PhotoFix (53.96 °C) was also lower than un-cross-linked BP, and it is possible that an aspect of the processing of PhotoFix may explain these results, as has been suggested in studies of other products [15].
Reflecting the lower level of cross-linking of PhotoFix indicated by its Td, the mean ultimate tensile strength of PhotoFix was significantly lower than that of the GA-cross-linked PeriGuard, and the lower end of the tensile strength range for PhotoFix (1.00 MPa) was lower than the ultimate tensile strength of several aortic tissues, including normal human aortic cusp (1.71 MPa) [23]. There may be risks involved in employing products with a weaker ultimate tensile strength than native tissue in cardiovascular applications. Indeed, Verbrugghe et al. [24] called for an assessment of the clinical durability of this product.
Both PhotoFix and CorMatrix showed good elasticity, recording Young’s modulus measurements significantly lower than for the 0.6% GA-fixed materials. However, CorMatrix displayed considerable initial stiffness: the stress needed to stretch this material to a level of 10% was the highest of any scaffold tested. This is likely due to species/tissue-based differences between porcine SIS and BP, as has been observed by others [25, 26]. Histological examination showed that the collagen fibres of CorMatrix were longitudinally aligned as seems typical for the porcine SIS [29]. Significant alignment would be expected to generate strong mechanical interactions between the fibres, which may explain the initial stiffness of CorMatrix.
The in vivo examination of the CardioCel explants displayed extractable calcium levels comparable with the non-GA-cross-linked PhotoFix at 6 and 12 weeks (0.45 and 0.44 µg/mg tissue, respectively, 12 weeks), mirroring previous findings [9, 12, 20]. The highest extractable calcium levels were recorded for the 0.6% GA-fixed BP (means of 0.57 and 0.85 µg/mg tissue for XenoLogiX and PeriGuard, respectively, 12 weeks), but a longer study may be needed to uncover greater differences in calcification potential. This is further suggested by the fact that there was a significant increase in extractable calcium overall in the study (for all products taken together, excluding control) from 6 to 12 weeks.
In agreement with the extractable calcium results, overt calcification was not evident histologically at 6 or 12 weeks for any product, whereas the control exhibited severe calcification at 6 and 12 weeks.
Anticalcification protocols, included in the ADAPT process, also frequently include the removal of phospholipids, and these may vary in efficacy. Although no significant differences in the IP levels were found between products at 6 weeks, at 12 weeks, the phosphorus levels in the PhotoFix implants (21.30 µg/mg) were significantly higher than in CardioCel (11.35 µg/mg) and PeriGuard (10.70 µg/mg) (although much lower than the GA BP control). Additionally, the amount of IP in XenoLogiX and PhotoFix implants increased significantly over the study period. Insufficient removal of phospholipids could be a possible cause of this, further suggested by the presence of cellular material in these scaffolds at baseline. Another explanation for the higher levels of phosphorus detected in PhotoFix is that more phospholipid is unbound due to the lower extent of cross-linking.
Histological examination also revealed substantial differences in the biocompatibility of the 4 implanted products in the subcutaneous rat model. No degradation, calcification or signs of inflammation or encapsulation were detected in the CardioCel implants at 6 or 12 weeks, indicative of a scaffold that is effectively cross-linked and protected from enzymatic degradation [15], is free of foreign cellular material and is non-cytotoxic. At 6 weeks, a typical initial immunological response to the implantation of the foreign material was observed in CardioCel, followed by a bio-integrative response that matched with previous reports [9, 12].
There are several steps in the ADAPT process that may contribute to this favourable host response, which limit host reaction; however, equally important are the processes such as decellularization and sterilization with non-GA sterilant that favour the preservation of the physical and biological features required for the recellularization of the bioscaffolds [1].
In contrast, the giant cell formation seen in both the XenoLogiX and PeriGuard implants in the rat model is indicative of a foreign body reaction. The similarities in the histological profile of these 2 products may be due to the fact that both are generated from the 0.6% GA-fixed BP. The nuclear material before implantation, suggestive of cellular remnants and incomplete decellularization of these scaffolds, may also have contributed to the foreign body response [27].
GA cytotoxicity is associated with poor host cell infiltration [28]. Although both XenoLogiX and PeriGuard undergo treatment to reduce GA toxicity, these procedures may not be fully effective; furthermore, XenoLogiX is stored in GA, and there is a risk that residues may remain in the matrix after rinsing. It is also possible that the manufacturing process of these products may have affected the scaffold architecture in a way that influences the tendency of host cells to colonize the material. For instance, 1 M sodium hydroxide used in the decellularization of PeriGuard can cause the degradation of collagen fibres [29].
CorMatrix is known to undergo reabsorption due to enzymatic degradation in vivo and was not included in the subcutaneous rat study, while PhotoFix was included. However, PhotoFix also seemed to be susceptible to degradation, as by 12 weeks, 2 PhotoFix implants were undetectable. The degeneration of PhotoFix in vivo has also been reported by another group [29]. The limited cross-linking of PhotoFix indicated by the Td data suggests that there may be insufficient protection from enzymatic digestion [15]. The PhotoFix implants additionally displayed an inflammation reaction typical of tissues with insufficiently masked α-Gal epitopes [1].
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