Enhancement of cryopreservation with intracellularly permeable zwitterionic polymers

Dual-Functionalized Zwitterionic Polymers for Cell Cryopreservation

Cryopreservation is an essential technique for the long-term preservation of cells. Some cells are challenging to cryopreserve, and novel cryoprotective agents are required. We previously reported a zwitterionic polymer (poly(ZI)) as a cryoprotectant that forms a polymer matrix surrounding the cells and partially prevents intracellular ice formation. In this study, we developed a novel zwitterionic copolymer (poly(ZI-C16)) with an improved cryoprotective ability. Poly(ZI-C16) contains long alkyl chains, which enable poly(ZI-C16) to anchor to the cell surface and consequently strengthen the polymer matrix. In addition, because poly(ZI-C16) is cationic, it enters cells and directly prevents intracellular ice formation. Due to its dual functions, poly(ZI-C16) demonstrated a higher cryoprotective effect than the original poly(ZI). This molecular design for dual functionalization provides an efficient approach to cryopreservation.

Amphiphilic phospholipid polymers as a cryoprotectant for vitrification and nanowarming of rat livers

Liver biobanking is a promising approach that saves the lives of patients with end-stage liver disease. Cryopreservation based on vitrification enables semi-permanent organ preservation, contributing to overcome the shortage of donors for liver transplants. A technical challenge in cryopreservation of transplantable organs lies in thawing methodology, and conventional convective warming cannot maintain the glassy state during thawing because of the large temperature gradient between the inner and outer parts of the organs, leading to ice formation and damage of cells in the organ. Nanowarming, in which magnetic nanoparticles are dispersed in a vitrification solution and heated by exposure of alternating magnetic field, can achieve uniform and rapid heating of organs. Herein, we report that amphiphilic phospholipid polymers composed of 2-methacryloyloxyethyl phosphorylcholine and n-butyl methacrylate can function as a cryoprotectant for nanowarming. The amphiphilic phospholipid polymers enhanced the viability of primary rat hepatocytes after vitrification. Moreover, the polymers enhanced the dispersion stability of magnetic nanoparticles in vitrification solution, and the perfusion of the vitrification solution with magnetic nanoparticles into rat livers through portal vein provided uniform distribution of the nanoparticles in the liver. After perfusion, the vitrified liver was successfully thawed rapidly and uniformly by nanowarming, which maintained tissue integrity and cell viability.

Ultrastable and Redispersible Zwitterionic Bottlebrush Micelles for Drug Delivery

Bottlebrush copolymers are increasingly used for drug delivery and biological imaging applications in part due to the enhanced thermodynamic stability of their self-assemblies. Herein, we discuss the effect of hydrophilic block chemistry on the stability of bottlebrush micelles. Amphiphilic bottlebrushes with zwitterionic poly(2-methacryloyloxyethyl phosphorylcholine) (PMPC) and nonionic polyethylene glycol (PEG) hydrophilic blocks were synthesized by "grafting from" polymerization and self-assembled into well-defined spherical micelles. Colloidal stability and stability against disassembly were challenged under high concentrations of NaCl, MgSO4, sodium dodecyl sulfate, fetal bovine serum, and elevated temperature. While both types of micelles appeared to be stable in many of these conditions, those with a PMPC shell consistently surpassed their PEG analogs. Moreover, when repeatedly subjected to lyophilization/resuspension cycles, PMPC micelles redispersed with no apparent variation in size or dispersity even in the absence of a cryoprotectant; PEG micelles readily aggregated. The observed excellent stability of PMPC micelles is attributed to the low critical micelle concentration of the bottlebrushes as well as to the strong hydration shell caused by ionic solvation of the phosphorylcholine moieties. Zwitterionic micelles were loaded with doxorubicin, and higher loading capacity/efficiency, as well as delayed release, was observed with increasing side-chain length. Finally, hemocompatibility studies of PMPC micelles demonstrated no disruption to the red blood cell membranes. The growing concern regarding the immunogenicity of PEG-based systems propels the search for alternative hydrophilic polymers; in this respect and for their outstanding stability, zwitterionic bottlebrush micelles represent excellent candidates for drug delivery and bioimaging applications.

Optimization of Zwitterionic Polymers for Cell Cryopreservation

Cryopreservation techniques are valuable for the preservation of genetic properties in cells, and the development of this technology contributes to various fields. In a previous study, an isotonic freezing medium composed of poly(zwitterion) (polyZI) has been reported, which alleviates osmotic shock, unlike typical hypertonic freezing media. In this study, the primitive freezing medium composed of emerging polyZI is optimized. Imidazolium/carboxylate-type polyZI (VimC3C) is the optimal chemical structure. The molecular weight and degree of ion substitution (DSion) are not significant factors. There is an impediment with the primitive polyZI freezing media. While the polyZI forms a matrix around the cell membrane to protect cells, the matrix is difficult to remove after thawing, resulting in low cell proliferation. Unexpectedly, increasing the poly(VimC3C) concentration from 10% to 20% (w/v) improves cell proliferation. The optimized freezing medium, 20% (w/v) poly(VimC3C)_DSion(100%)/1% (w/v) NaCl aqueous solution, exhibited a better cryoprotective effect.