Fabrication of Heterogeneous Hydrogel Models for Convection-Enhanced Drug Delivery Studies
Authors: Zhang Haipeng , Johnson Aidan , Ryu Sangjin , Kim Seunghee , Zhang Chi (Kevin) 
Published: January 2022

Abstract
Convection-enhanced drug delivery (CED) enables faster convective dispersion of drug molecules through soft, porous tissues than diffusion, which is beneficial for treating aggressive tumors. Advancing the CED technology requires rigorous characterization of fluid transport through soft, porous tissues. When a fluid flows through soft, porous tissues, the deformation and/or structural rearrangement of the hosting solid alters the pattern and efficiency of fluid propagation. Such hydro-mechanical coupling can be understood by experimentally studying fluid flows through hydrogel phantoms that mimic soft, porous tissues in vitro. However, current studies using hydrogel-based models are limited because homogenous hydrogel models with uniform properties have been employed. To overcome this limitation, we aim to develop heterogeneous hydrogel models for CED studies by embedding agar beads (1 w/v%) in an agarose gel block (0.2 or 0.6 w/v%), and this paper introduces our fabrication method in detail.
The fabrication process includes two steps: fabrication of the gel beads and dispensing the gel beads in the agarose gel. In the first step, agar gel beads were fabricated with the rotating-liquid-based drop generation method. Hot agar gel solution (∼95°C) was injected through a syringe needle with its tip immersed in cold mineral oil (∼4°C) that rotates in rigid body motion. As agar gel solution drops were sheared off from the needle tip by the bulk mineral oil motion, they became spherical due to surface tension. While falling through the mineral oil, the gel drops were cooled and became gel beads. These agar gel beads were harvested, washed in water, and stored at 4°C. Microscopy imaging of the created gel beads confirmed that the sizes of the beads were uniform. In the second step of fabrication, agarose solution was prepared at 95°C, and the precooled agar beads were mixed in the agarose solution. Then, the mixture was poured into a precooled mold (a petri dish at −20°C), and then the mold was moved into a refrigerator immediately for quick cooling and gelation. Microscopy imaging of the embedded gel beads showed two heterogeneous agarose gel blocks with embedded agar beads were produced with different sized agar gel beads, different concentration of agarose gel, and different distribution of agar gel beads in agarose gel.
The presented method has the following advantages. First, the size of agar beads can be adjusted by changing the injection speed of the agar solution and the rotation speed of the mineral oil. Second, the level of heterogeneity can be modulated by changing the properties of the gel beads and block and by adjusting the volume fraction between the gel beads and the gel block. Also, it is expected that the degree of fusion between the gel beads and the gel block could be controlled by adjusting the temperature of the agar gel beads and the agarose solution, and the cooling process of their mixture. Therefore, the suggested heterogeneous gel model has the potential to elucidating fluid flow through deformable, heterogeneous porous media and thus to advancing convection-enhanced drug delivery.