In Situ Gelation of P(NIPAM-HEMA) Microgel Dispersion and Its Applications as Injectable 3D Cell Scaffold

Abstract

In this work we try to develop a new thermal gelling injectable scaffold for three-dimensional cell culture. Instead of using linear, branched, or grafted macromolecules, thermosensitive microgel particles or microspheres are used as building blocks for the construction of the macroscopic hydrogel scaffold. As a proof of concept, thermosensitive poly(N-isopropylacrylamide-co-2-hydroxyethyl methacrylate) (P(NIPAM-HEMA)) microgel particles were synthesized, which present a volume phase transition temperature (VPTT) at about 29 degrees C. Rheological test shows that the concentrated P(NIPAM-HEMA) microgel dispersion is colloidally stable when heated above its VPTT, indicating hydrophobic interaction alone can not induce thermal gelation of the dispersion. In the presence of a low concentration of CaCl(2), however, with the introduction of additional ionic cross-linking, the microgel dispersion gelates and forms macroscopic hydrogel. Gelation temperature of the microgel dispersion decreases with increasing ionic strength. SEM observation reveals that the resultant bulky gel has an interconnected porous microstructure. 293T cells, a human cell line, were encapsulated inside the hydrogel by simple mixing with the microgel dispersion at room temperature and heating to 37 degrees C. MTT (3-[4,5-dimethylthiazol-2-yl]-3,5-diphenyl tetrazolium bromide) assays reveal that the cells are viable and proliferate inside the 3D scaffold.

Gelation Kinetics of Thermosensitive PNIPAM Microgel Dispersions

“…Again the colloidal stability of the dispersion should be attributed to the presence of the sulfate groups on the surface of the particles. [19] In contrast, thermal gelation was observed when NaCl is added to the dispersion. A typical trace of G 0 and G 00 against time is shown in Figure 3B.…”

“…This has been attributed to the surface sulfate groups originated from the initiator. [19] The electrostatic repulsion among the particles dominates van der Waals forces and keeps the particles from aggregating. In this case the whole phase transition can be studied by dynamic light scattering.…”

“…In contrast, in NaCl solutions, particle aggregation was observed when heated to certain temperatures, because electrostatic repulsion is screened by the added salt. [19] In these cases it is not possible to study the whole phase transition process using dynamic light sacttering ( Figure 1). …”

“…The gelation of the microgel dispersion in the presence of added salt should be attributed to the efficient screening of electrostatic repulsion by high ionic strength, which allows for the aggregation of the shrunken microgel particles via hydrophobic interaction. [19,20] There are several methods to determine the gelation point in the literature. Among them, the Winter-Chambon's criterion is the most rigorous and widely accepted.…”

“…[19] The hydrogels are biocompatible and can well support cell proliferation. [19][20][21] The thermosensitive microgel particles or microspheres may be a better choice as building blocks for thermal gelling injectable scaffolds. For example, growth factors or other bioactive molecules may be encapsulated in the interior of the particles and released in proper time to guide the differentiation of stem cells.…”