Methylation of poloxamine for enhanced cell adhesion

Aiming at producing a synthetic collagen-mimetic material that is stiffer than collagen but that like collagen allows both cell encapsulation and cell growth on the surface, a positively charged poloxamine hydrogel was prepared by methylating the tertiary amine groups of a four-arm poly(ethylene oxide)-poly(propylene oxide) block copolymer derivative (Tetronic 1107). This derivative was subsequently reacted with methacryloyl isocyanate, rendering positively charged materials that are further cross-linkable by a photointiated free radical polymerization. Different hydrogels containing methylated poloxamine methacrylate concentrations between 6% and 18% were produced and characterized by means of water uptake and viscoelastic properties. A sharp increase in water content was observed in distilled water during the first week; some of the gels showed water uptakes as high as 2 times the initial wet weight. In PBS, this effect was less prominent due to the decrease in the osmotic gradient. Also, a gradual increase of both the storage modulus (G') and the loss modulus (G") resulted from increasing the polymer concentration: for example, G' values ranged between 70 and 23000 Pa for 6% and 18% methylated poloxamine methacrylate hydrogels (at 1 Hz, 100 Pa of oscillatory stress). HepG2 cells embedded in different compositions and exposed to UV light displayed good viability levels after the cross-linking, unlike a previously reported attempt at creating a synthetic collagen-mimetic material. A well-spread endothelial cell morphology was apparent on methylated poloxamine films after preincubation in serum-containing medium, while on unmodified poloxamine methacrylate hydrogels cells attached poorly. However, EC did not attach well to the same material when fabricated not as films but as cylindrical modules as needed for the modular construct for which this material was intended. Thus, for this apparently more challenging geometry, it was necessary to combine collagen with the methylated poloxamine to have good attachment of EC on the surface of modules as well as films. While the challenge of creating a synthetic alternative to collagen as a stiffer cell-compatible substrate remains, methylated poloxamine displays many of the attributes that make it a useful material for tissue engineering.