Cryogelation reactions and cryogels: principles and challenges

Exosome-Seeded Cryogel Scaffolds for Extracellular Matrix Regeneration in the Repair of Articular Cartilage Defects: An In Vitro and In Vivo Rabbit Model Study

Traumatic or degenerative defects of articular cartilage impair joint function, and the treatment of articular cartilage damage remains a challenge. By mimicking the cartilage extracellular matrix (ECM), exosome-seeded cryogels may enhance cell proliferation and chondral repair. ECM-based cryogels were cryopolymerized with gelatin, chondroitin sulfate, and various concentrations (0%, 0.3%, 0.5%, and 1%) of hyaluronic acid (HA), and their water content, swelling ratio, porosity, mechanical properties, and effects on cell viability were evaluated. The regenerative effects of bone marrow-derived mesenchymal stem cell (BM-MSC)-derived exosome (at a concentration of 106 particles/mL)-seeded 0.3% HA cryogels were assessed in vitro and in surgically induced male New Zealand rabbit cartilage defects in vivo. The water content, swelling ratio, and porosity of the cryogels significantly (p < 0.05) increased and the Young's modulus values of the cryogels decreased with increasing HA concentrations. MTT assays revealed that the developed biomaterials had no cytotoxic effects. The optimal cryogel composition was 0.3% HA, and the resulting cryogel had favorable properties and suitable mechanical strength. Exosomes alone and exosome-seeded cryogels promoted chondrocyte proliferation (with cell optical densities that were 58% and 51% greater than that of the control). The cryogel alone and the exosome-seeded cryogel facilitated ECM deposition and sulfated glycosaminoglycan synthesis. Although we observed cartilage repair via Alcian blue staining with both the cryogel alone and the exosome-seeded cryogel, the layered arrangement of the chondrocytes was superior to that of the control chondrocytes when exosome-seeded cryogels were used. This study revealed the potential value of using BM-MSC-derived exosome-seeded ECM-based cryogels for cartilage tissue engineering to treat cartilage injury.

Tuning Mechanical Properties, Swelling, and Enzymatic Degradation of Chitosan Cryogels Using Diglycidyl Ethers of Glycols with Different Chain Length as Cross-Linkers

Cross-linking chitosan at room and subzero temperature using a series of diglycidyl ethers of glycols (DEs)-ethylene glycol (EGDE), 1,4-butanediol (BDDE), and poly(ethylene glycol) (PEGDE) has been investigated to demonstrate that DEs can be a more powerful alternative to glutaraldehyde (GA) for fabrication of biocompatible chitosan cryogels with tunable properties. Gelation of chitosan with DEs was significantly slower than with GA, allowing formation of cryogels with larger pores and higher permeability, more suitable for flow-through applications and cell culturing. Increased hydration of the cross-links with increased DE chain length weakened intermolecular hydrogen bonding in chitosan and improved cryogel elasticity. At high cross-linking ratios (DE:chitosan 1:4), the toughness and compressive strength of the cryogels decreased in the order EGDE > BDDE > PEGDE. By varying the DE chain length and concentration, permeable chitosan cryogels with elasticity moduli from 10.4 ± 0.8 to 41 ± 3 kPa, toughness from 2.68 ± 0.5 to 8.3 ± 0.1 kJ/m3, and compressive strength at 75% strain from 11 ± 2 to 33 ± 4 kPa were fabricated. Susceptibility of cryogels to enzymatic hydrolysis was identified as the parameter most sensitive to cross-linking conditions. Weight loss of cryogels increased with increased DE chain length, and degradation rate of PEGDE-cross-linked chitosan decreased 612-fold, when the cross-linker concentration increased 20-fold.

Cryostructuring of Polymeric Systems: 67 Properties and Microstructure of Poly(Vinyl Alcohol) Cryogels Formed in the Presence of Phenol or Bis-Phenols Introduced into the Aqueous Polymeric Solutions Prior to Their Freeze-Thaw Processing

Poly(vinyl alcohol) (PVA) physical cryogels that contained the additives of o-, m-, and p-bis-phenols or phenol were prepared, and their physico-chemical characteristics and macroporous morphology and the solute release dynamics were evaluated. These phenolic additives caused changes in the viscosity of initial PVA solutions before their freeze-thaw processing and facilitated the growth in the rigidity of the resultant cryogels, while their heat endurance decreased. The magnitude of the effects depended on the interposition of phenolic hydroxyls in the molecules of the used additives and was stipulated by their H-bonding with PVA OH-groups. Subsequent rinsing of such "primary" cryogels with pure water led to the lowering of their rigidity. The average size of macropores inside these heterophase gels also depended on the additive type. It was found also that the release of phenolic substances from the additive-containing cryogels occurred via virtually a free diffusion mechanism; therefore, drug delivery systems such as PVA cryogels loaded with either pyrocatechol, resorcinol, hydroquinone, or phenol, upon the in vitro agar diffusion tests, exhibited antibacterial activity typical of these phenols. The promising biomedical potential of the studied nanocomposite gel materials is supposed.