Regulating Volume Transitions of Highly Responsive Hydrogel Scaffolds by Adjusting the Network Properties of Microgel Building Block Colloids
"Entry by Pichet Adstamongkonkul, AP 225, Fall 2011"
Title: Regulating Volume Transitions of Highly Responsive Hydrogel Scaffolds by Adjusting the Network Properties of Microgel Building Block Colloids
Authors: E. C. Cho, J Kim, D. C. Hyun, U. Jeong, D. A. Weitz
Journal: Langmuir, 2008, Vol. 26, No. 6
In practice, it is desirable to have a hydrogel that rapidly responds to the applied stimulus. Many approaches have been exploited, including the addition of porosity, grafting extendable chains onto the polymer chains, or incorporating nanoparticles into the polymer network. However, the fast response is hard to achieved if the length of the hydrogels is at the macroscopic level, because the relaxation of the length is controlled by diffusion of liquid and is proportional to its dimension squared.
A new strategy uses sub-micrometer microgel particles as building blocks to assemble the 3D hydrogel network when heated above their transition temperature via bridging or depletion interactions. Considering the bulk properties of the scaffold, the degree of swelling can be controlled by changing the crosslinking density, which however changes the response kinetics. To optimize the system, this study demonstrated that microgel particles may be the solution.
Results and Discussion
The scaffold can be made by heating microgel particles containing some water-soluble poly(NiPAAm-co-AA) chains. This ensures that the small length scales will govern the time behavior of the scaffold and the temperature-responsive volume changes can be controlled by adjusting the cross-link density of the particles. The heating process above the microgel's transition temperature causes the particles to form cluster ,because of the depletion force and hydrophobic interactions, resulting in a highly porous, 3D scaffold. The physical cross-linking of the particles, mediated by water-soluble polymers, maintain the hydrogel's structure. The whole process is irreversible, as the scaffold is not disassembled into particles in water.
The volume of the scaffold can be changed by varying the molar ratio of the cross-linker (BIS) to NiPAAm, which in turn regulates the cross-linking densities. The volume of the scaffold decreases as the ratio becomes smaller at high temperature (65C), meaning that the higher degree of cross-link would lead to a lower volume shrinkage. At temperature below transition temperature (35C), the volume appears to be independent of the ratio. This indicates that the volumetric transitions of the scaffold were controlled by the behavior of the microgel particles. It was also speculated that the novel scaffolds all show faster response and the diffusion coefficients are much higher than the bulk scaffold. Therefore, the cross-link density has little effect on the response kinetics of the scaffolds, which implicates the influence of the microgel particles on the bulk properties.
The authors also looked into the thermal properties of the particles and the corresponding scaffolds and found that the scaffold with a lower ratio of cross-linker to polymer requires more heat for the phase transition, probably due to a larger volume change. In addition, the difference in the heat profile between the particles and the scaffolds is small, indicating that the thermal transition of the scaffolds is likely from the transition of particles.
To investigate the drug release behavior of the scaffolds, a hydrophobic drug was incorporated into the hydrogel. They found that at the body temperature (37.5C), the drug was rapidly released and the released amount reached the plateau within an hour. However, at room tmeperature (22C), there was no plateau stage. The faster release was hypothesized to be the result from rapid shrinkage of the scaffold. The shrinkage might also limit the diffusion of additional drug molecules. On the other hand, at room temperature, the scaffolds swell, allowing the drug to continue being released. They also observed not much difference in the microstructure between two scaffolds with different composition ratio at any given temperature, which implies that the drug release behavior is from the particles rather than the scaffold's porosity or microstructure.