Stem cells have the capacity to self-renew and differentiate to specialized cells, which are usually sensitive to cryopreservation. Therefore, the cell survival rate of stem cells using common cryopreservation protocol is generally not ideal. High cooling rates are crucial for decreasing the usage of cryoprotectants (CPAs) and promoting the successful vitrification of stem cells. In this study, we adopted liquid helium (LHe) instead of liquid nitrogen (LN2) as the cryogen to achieve high cooling rates for vitrifying stem cells with high viability and complete functions. A numerical model was established to simulate the cooling processes of vitrifying specimens by immersing them in LHe and LN2. The calculated results revealed higher cooling rates when plunging specimens into LHe than into LN2. The high viability of human bone-derived mesenchymal stem cells (hBMSCs) and human embryonic stem cells (hESCs) after vitrifying into LHe also shows the superiority of LHe as the cryogen. Furthermore, considerable cell viability was achieved by vitrification in LHe, even when decreasing the concentrations of CPAs. Additionally, post-vitrification, the cells still maintained high attachment and proliferation efficiency, normal stemness, and multipotential differentiation both for hBMSCs and hESCs. LHe is prospective to be employed as a universal cryogen for vitrification which has a great potential for widespread applications, including bioengineering and clinical medicine.
Keywords: liquid helium; liquid nitrogen; stem cells; vitrification.