Controlled hydrogel-based encapsulation of macrophages determines cell survival and functionality upon cryopreservation

The development of novel cell-based therapies has increased the necessity to improve the long-term storage of cells. The current method of cryopreservation is far from optimal, causing ice-associated mechanical and osmotic damage to sensitive cells. Cell encapsulation is emerging as a new strategy to overcome those current limitations; however, few data are applicable to slow freezing, with conflicting results and multiple experimental conditions. The objective of this research work was to evaluate the impact of capsule size and encapsulation method on cell survival and functionality after a conventional freezing protocol. To this end, cells were encapsulated in alginate beads of different sizes, spanning the range of 200-2000 µm thanks to multiple extrusion techniques and conditions, and further cryopreserved using a slow cooling rate (-1°C/min) and 10 % DMSO as cryoprotectant. Our data show that there is a strong correlation between bead size and cell survival after a slow cooling cryopreservation process, with cell viabilities ranging from 7 to 70 % depending on the capsule size, with the smallest capsules (230 µm) achieving the highest level of survival. The obtained results indicate that the beads' diameter, rather than their morphology or the technique used, plays a significant role in the post-thawing cell survival and functionality. These results show that a fine control of cell encapsulation in alginate hydrogels is required when it comes to overcoming the current limitations of long-term preservation techniques by slow cooling.

[Droplet freeze-thawing system based on solid surface vitrification and laser rewarming]

Ultra-rapid cooling and rewarming rate is a critical technical approach to achieve ice-free cells during the freezing and melting process. A set of ultra-rapid solid surface freeze-thaw visualization system was developed based on a sapphire flim, and experiments on droplet freeze-thaw were carried out under different cryoprotectant components, volumes and laser energies. The results showed that the cooling rate of 1 μL mixed cryoprotectant [1.5 mol/L propylene glycol (PG) + 1.5 mol/L ethylene glycol (EG) + 0.5 mol/L trehalose (TRE)] could be 9.2×10 3 °C/min. The volume range of 1-8 μL droplets could be vitrified. After comparing the proportions of multiple cryoprotectants, the combination of equal proportion mixed permeability protectant and trehalose had the best vitrification freezing effect and more uniform crystallization characteristics. During the rewarming operation, the heating curve of glassy droplets containing gold nanoparticles was measured for the first time under the action of 400-1 200 W laser power, and the rewarming rate was up to the order of 10 6 °C/min. According to the droplet images of different power rewarming processes, the laser power range for ice-free rewarming with micron-level resolution was clarified to be 1 400-1 600 W. The work of this paper simultaneously realizes the ultra-high-speed temperature ramp-up, transient visual observation and temperature measurement of droplets, providing technical means for judging the ice free droplets during the freeze-thaw process. It is conducive to promoting the development of ultra-rapid freeze-thaw technology for biological cells and tissues.

Antifreeze proteins and their biomimetics for cell cryopreservation: Mechanism, function and application-A review

Cell-based therapy is a promising technology for intractable diseases and health care applications, in which cryopreservation has become an essential procedure to realize the production of therapeutic cells. Ice recrystallization is the major factor that affects the post-thaw viability of cells. As a typical series of biomacromolecules with ice recrystallization inhibition (IRI) activity, antifreeze proteins (AFPs) have been employed in cell cryopreservation. Meanwhile, synthesized materials with IRI activity have emerged in the name of biomimetics of AFPs to expand their availability and practicality. However, fabrication of AFPs mimetics is in a chaotic period. There remains little commonality among different AFPs mimetics, then it is difficult to set guidelines on their design. With no doubt, a comprehensive understanding on the antifreezing mechanism of AFPs in molecular level will enable us to rebuild the function of AFPs, and provide convenience to clarify the relationship between structure and function of these early stage biomimetics. In this review, we would discuss those previously reported biomimetics to summarize their structure characteristics concerning the IRI activity and attempt to develop a roadmap for guiding the design of novel AFPs mimetics.

Improved and reproducible cell viability in the superflash freezing method using an automatic thawing apparatus

Cell cryopreservation stops the biological activity of cells by placing them in the frozen state, and can be used to preserve cells without subculturing, which can cause contamination and genetic drift. However, the freezing process used in cryopreservation can injure or damage the cells due to the cytotoxicity of cryoprotecting agents (CPAs). We have previously reported a CPA-free cryopreservation method based on inkjet technology. In this method, the vitrified cells were exposed to the room temperature atmosphere during the transport of the cells using tweezers, which caused devitrification due to the increased temperature and often lowered the cell viability. In the present study, we developed an automatic thawing apparatus that transports the vitrified cells rapidly into a prewarmed medium using a spring hinge. Observations with a high-speed camera revealed that the spring hinge drops the cells into the prewarmed medium within 20 ms. All heat-transfer simulations for the apparatuses with different designs and rotation speeds showed that the cells remained below the glass-transition temperature during the transport. Finally, the apparatus was evaluated using mouse fibroblast 3T3 cells. The cell viability was improved and its reproducibility was enhanced using this apparatus. The results indicate that the combination of superflash freezing with the rapid thawing process represents a promising approach to circumvent the problems typically associated with the addition of CPAs.

Applications of Antifreeze Proteins: Practical Use of the Quality Products from Japanese Fishes

Numerous embryonic ice crystals are generated in water at the moment of freezing. These crystals grow and merge together to form an ice block that can be generally observed. Antifreeze protein (AFP) is capable of binding to the embryonic ice crystals, inhibiting such an ice block formation. Fish-derived AFP additionally binds to membrane lipid bilayers to prolong the lifetime of cells. These unique abilities of AFP have been studied extensively for the development of advanced techniques, such as ice recrystallization inhibitors, freeze-tolerant gels, cell preservation fluids, and high-porosity ceramics, for which mass-preparation method of the quality product of AFP utilizing fish muscle homogenates made a significant contribution. In this chapter, we present both fundamental and advanced information of fish AFPs that have been especially discovered from mid-latitude sea area, which will provide a hint to develop more advanced techniques applicable in both medical and industrial fields.