Valvular insufficiency due to heart disease requires frequent reconstruction with the use of mechanical valves, biological valves, or autologous pericardium valves. Autologous pericardium is commonly used as a plastic material in intracardiac structures. For AVRec using the Ozaki procedure, autologous pericardium is used after glutaraldehyde fixation. This has produced excellent clinical results, but glutaraldehyde fixation limits growth adaptability. However, for young patients, there are concerns regarding the insufficient amount of the pericardium in reoperation cases, the decreased coaptation due to growth, and the undetermined durability of the pericardium [7].
Some papers have reported that biosheets based on iBTA can be reconstructed in a manner analogous to that of native tissue, for example, as cornea, trachea (biosheet), and blood vessels (biotube) [8, 9]. The technology based on iBTA have also been applied to heart valves (biovalve and stent-biovalve) [10, 11] and stent grafts (bio stent graft) [12]. The papers that reported on this technology indicated the possibility of tissue regeneration, self-repair, and growth adaptability.
To our knowledge, this is the first time that tissue-engineered valve leaflets have been successfully implanted as aortic valves using goats as a large animal. It was amazing that all goats survived for three months after implanting biosheets without requiring reinforcement treatment.
Biosheets were more thoroughly assimilated into the aortic root than the autologous pericardium. On histology, a greater number of smooth muscle actin-positive cells (myofibroblasts) had infiltrated into biosheet leaflets than in the autologous pericardium. This indicates the possibility of biosheet self-maturation, suggesting that the biosheet might acquire features such as growth adaptability. Moreover, it seems that the biosheet had started to assimilate into the heart tissue.
On the other hand, it was unfortunate that one biosheet leaflet had a slight cut at the suture with the aortic annulus. It is possible that the biosheet used had insufficient strength, or it may not have had long-term durability due to an unidentified defect. Therefore, in our other research, we investigated the physical properties of biosheets. A biosheet is formed in a mold, and thus, depends on the opening pattern of the mold which functions as a cell entry port. In the study of the tensile strength of biosheets according to the mold pattern (alternating pattern or parallel pattern) [13], and we compared the tissue stress of the circumferential direction with that of the longitudinal direction. The biosheets with an alternating pattern had low anisotropy. Therefore, the selection of appropriate biosheets as candidate materials for the substitution of the aortic leaflets should be taken into consideration in our future work. Unfortunately, one of biosheet model subjects developed infectious endocarditis. Nevertheless, chronic animal models may develop infectious endocarditis as one of the complications of cardiac surgery; we believe the infection was never caused by the biosheet.
In the technology based on iBTA, the recipient’s body is used as a bioreactor. This technique has several advantages. For example, the tissue prostheses can be easily and safely fabricated in a wide range of shapes and sizes to suit an individual recipient by changing the mold design. Most importantly, this technique does not require complex in vitro cell management procedures or exceptionally clean laboratory facilities, which are extremely expensive and time consuming. Moreover, because the biosheets are completely autologous, they are expected to have little calcification with long-term implantation and to possess better growth potential when compared with mechanical valves or biological valves. Consequently, biosheets might be an ideal material for AVRec.
One of the most important features of iBTA- induced tissue is its ability to regenerate.
The current study demonstrated a few experiments using the biosheet. We are now increasing the number of experiments after careful selection of suitable biosheets for implantation, and investigations on the histological features of further long-term durability are ongoing.
Limitations of the study
Although this study included large animal models, the number of animals used might not be enough to generalize the results. Also, as one of the biosheet models required cutting of tissues, we used thin biosheets, which may have affected the strength of the biosheet. In the future, the quality of biosheets must be confirmed before implantation.
As one of the remedies to select biosheets with an appropriate thickness, we developed a system that can measure the total thickness of this material using optical coherence tomography (OCT, IVS-2000, Santec, Aichi, Japan). On the other hand, the surgical environment will improve, including clean treatment of biosheets to prevent infection. In addition, checking the number of bacteria before AVRec surgery and using clean biosheets can help avoid such complications. Lastly, our limitations must be considered while applying the results of this study for clinical use.