Cryopreservation of Tissue-Engineered Scaffold-Based Constructs: from Concept to Reality

Specifics of Cryopreservation of Hydrogel Biopolymer Scaffolds with Encapsulated Mesenchymal Stem Cells

The demand for regenerative medicine products is growing rapidly in clinical practice. Unfortunately, their use has certain limitations. One of these, which significantly constrains the widespread distribution and commercialization of such materials, is their short life span. For products containing suspensions of cells, this issue can be solved by using cryopreservation. However, this approach is rarely used for multicomponent tissue-engineered products due to the complexity of selecting appropriate cryopreservation protocols and the lack of established criteria for assessing the quality of such products once defrosted. Our research is aimed at developing a cryopreservation protocol for an original hydrogel scaffold with encapsulated MSCs and developing a set of criteria for assessing the quality of their functional activity in vitro. The scaffolds were frozen using two alternative types of cryocontainers and stored at either -40 °C or -80 °C. After cryopreservation, the external state of the scaffolds was evaluated in addition to recording the cell viability, visible changes during subsequent cultivation, and any alterations in proliferative and secretory activity. These observations were compared to those of scaffolds cultivated without cryopreservation. It was shown that cryopreservation at -80 °C in an appropriate type of cryocontainer was optimal for the hydrogels/adipose-derived stem cells (ASCs) tested if it provided a smooth temperature decrease during freezing over a period of at least three hours until the target values of the cryopreservation temperature regimen were reached. It was shown that evaluating a set of indicators, including the viability, the morphology, and the proliferative and secretory activity of the cells, enables the characterization of the quality of a tissue-engineered construct after its withdrawal from cryopreservation, as well as indicating the effectiveness of the cryopreservation protocol.

Cryopreservation of 3D Bioprinted Scaffolds with Temperature-Controlled-Cryoprinting

Temperature-Controlled-Cryoprinting (TCC) is a new 3D bioprinting technology that allows for the fabrication and cryopreservation of complex and large cell-laden scaffolds. During TCC, bioink is deposited on a freezing plate that descends further into a cooling bath, keeping the temperature at the nozzle constant. To demonstrate the effectiveness of TCC, we used it to fabricate and cryopreserve cell-laden 3D alginate-based scaffolds with high cell viability and no size limitations. Our results show that Vero cells in a 3D TCC bioprinted scaffold can survive cryopreservation with a viability of 71%, and cell viability does not decrease as higher layers are printed. In contrast, previous methods had either low cell viability or decreasing efficacy for tall or thick scaffolds. We used an optimal temperature profile for freezing during 3D printing using the two-step interrupted cryopreservation method and evaluated drops in cell viability during the various stages of TCC. Our findings suggest that TCC has significant potential for advancing 3D cell culture and tissue engineering.

Evaluation of Structural Viability of Porcine Tracheal Scaffolds after 3 and 6 Months of Storage under Three Different Protocols

Tracheal replacement with a bioengineered tracheal substitute has been developed for long-segment tracheal diseases. The decellularized tracheal scaffold is an alternative for cell seeding. It is not defined if the storage scaffold produces changes in the scaffold's biomechanical properties. We tested three protocols for porcine tracheal scaffold preservation immersed in PBS and alcohol 70%, in the fridge and under cryopreservation. Ninety-six porcine tracheas (12 in natura, 84 decellularized) were divided into three groups (PBS, alcohol, and cryopreservation). Twelve tracheas were analyzed after three and six months. The assessment included residual DNA, cytotoxicity, collagen contents, and mechanical properties. Decellularization increased the maximum load and stress in the longitudinal axis and decreased the maximum load in the transverse axis. The decellularization of the porcine trachea produced structurally viable scaffolds, with a preserved collagen matrix suitable for further bioengineering. Despite the cyclic washings, the scaffolds remained cytotoxic. The comparison of the storage protocols (PBS at 4 °C, alcohol at 4 °C, and slow cooling cryopreservation with cryoprotectants) showed no significant differences in the amount of collagen and in the biomechanical properties of the scaffolds. Storage in PBS solution at 4 °C for six months did not change the scaffold mechanics.

Biopolymer gels as components of protective medium for cryopreservation of spermatogonial stem cells

Biopolymer gels attract a lot of attention in a field of biothechnology due to their excellent compatibility and degradation. Their application is also promising for cryopreservation of spermatogonial stem cells (SSCs) which is so necessary to preserve the fertility of young patients. The aim of the study was to determine the effectiveness of biopolymer gels as a component of cryopreservation medium for SSCs of immature rats at the stage of exposure to cryoprotectants. It was found that 30-min exposure to cryopreservation media based on collagen or fibrin gel with an addition of 5% Me₂SO or 6% glycerol did not lead to significant changes in membrane integrity, cytochrome C content, metabolic, mitochondrial and antioxidant activities in SSCs compared to the control (Leibovitz-based cryomedium). But fibrin gel more than collagen reduced the toxic effects of Me₂SO and glycerol on SSCs increasing exposure time up to 45 min without significant changes in cell viability. The same cryoprotectants in Leibovitz-based media showed significant toxicity starting from the 15th minute of exposure. Necrosis was the main cause of cell death at this stage of cryopreservation in all experimental groups. The obtained results can be used to optimize SSC cryopreservation protocols for further cell autotransplantation for spermatogenesis initiation in boys who undergo gonadotoxic therapy in prepubertal age.

Survival of human cells in tissue-engineered constructs stored at room temperature

Tissue-engineered constructs (TECs), the dermal equivalent (DE) and the skin equivalent (SE), are allogenic equivalents of the skin and derm used to treat critical skin loss. Selection of storage conditions that contribute to longer shelf life, thereby expanding the possibilities of logistics and use, is one of the major issues related to the TECs development. The study was aimed to determine the shelf life of the DE and SE TECs stored in normal saline at room temperature by assessing morphology and viability of the cells on their surface, along with the levels of endothelial growth factor (VEGF) secreted by these cells. Using the MTT assay and staining with vital dye, we discovered the following: when TECs of both types were stored in normal saline, the cells viability and metabolic activity decreased by more than 50% by days 3–4 of storage. Furthermore, these decreased faster in DEs than in SEs. Morphology of the cells isolated from DEs and SEs after the 3-day storage remained unchanged. Mesenchymal stem cells on the surface of TECs kept producing VEGF after TECs culture medium was changed for saline solution (confirmed by immunofluorescence assay), which could indicate that the cells retained essential secretory activity.

Hypothermic Storage of 3D Cultured Multipotent Mesenchymal Stromal Cells for Regenerative Medicine Applications

The regulatory requirements in cell processing, in the choice of a biomaterial scaffold and in quality control analysis, have to be followed in the clinical application of tissue-engineered grafts. Confirmation of sterility during quality control studies requires prolonged storage of the cell-based construct. After storage, preservation of the functional properties of the cells is an important prerequisite if the cells are to be used for cell-based tissue therapies. The study presented here shows the generation of 3D constructs based on Wharton’s jelly multipotent mesenchymal stromal cells (WJ-MSCs) and the clinically-acceptable HyaloFast® scaffold, and the effect of two- and six-day hypothermic storage of 3D cell-based constructs on the functional properties of populated cells. To study the viability, growth, gene expression, and paracrine secretion of WJ-MSCs within the scaffolds before and after storage, xeno-free culture conditions, metabolic, qPCR, and multiplex assays were applied. The WJ-MSCs adhered and proliferated within the 3D HyaloFast®. Our results show different viability of the cells after the 3D constructs have been stored under mild (25 °C) or strong (4 °C) hypothermia. At 4 °C, the significant decrease of metabolic activity of WJ-MSCs was detected after 2 days of storage, with almost complete cell loss after 6 days. In mild hypothermia (25 °C) the decrease in metabolic activity was less remarkable, confirming the suitability of these conditions for cell preservation in 3D environment. The significant changes were detected in gene expression and in the paracrine secretion profile after 2 and 6 days of storage at 25 °C. The results presented in this study are important for the rapid transfer of tissue engineering approaches into clinical applications.