Use of hydrogels as a scaffold material for engineered tissues has been of increasing interest due to their structural similarity to the extracellular components in the body. Photopolymerizable poly(ethylene glycol) (PEG) is one of the most extensively utilized hydrogels, because its network structure can be easily modified to mimic critical aspects of the original microenvironments and its patternability by photolithography or microfluidic devices allows microarchitectures to guide cells‟ behavior with respect to morphology, cytoskeletal structure, and functionality. However, due to its complicated microenvironments designed to perform the broad range of hepatic functions, development of an engineered liver tissue has been challenging. The three aspects of tissue's microenvironments, which were critical to design PEG-based engineered livers, were addressed.

The first aspect was to understand interaction between the PEG network and incorporated cells. Even though the survivability of encapsulated cells was known to depend on diffusion conditions, insufficient understanding of the structure of cellencapsulated PEG networks has limited development of new networks with improved permeability to support metabolic activities of encapsulated cells. Since network defects have been identified as up to orders of magnitude larger than the mesh size, we suggest that the level of network defects is a primary determinant of hydrogel permeability and its consequent ability to support metabolic activities of encapsulated cells. We therefore sought a way to purposefully augment the network defects by incorporating hydrophobic poly(lactic-co-glycolic acid) (PLGA) nanoparticles, which induce loose crosslinking at the particle-PEG interface. The efficiency of the proposed design strategy was verified by the improved viability and hepatic functions of encapsulated human liver-derived cells.

The second aspect was to recapitulate key aspects of tissue architecture. In the liver, and many other tissues also, different types of cells distribute with specific configurations and their interactions are of fundamental importance in physiology, pathophysiology, cancer, developmental biology, and wound healing. In this study, we sought a novel method to guide cells' spatial displacement by applying soft lithography. We first verified that our new network design has the improved permeability without compensating the patternability. Due to the structural advantages, we demonstrated that our simplified patterning process avoided negative influence to encapsulated cell viability and allowed reliable control over distribution of incorporated cells.

The third aspect was to provide a platform to reliably provide biological factors to encapsulated cells in PEG matrices. Because most biomolecules easily lose their therapeutic potency and have specific plasma levels for optimal clinical efficacies, drug delivery systems have been designed to deploy medications intact through a protective medium that can also control the rate of drug release. In this system, we utilized PLGA particles, which were used to improve the permeability, as drug carriers. However, due to the complexity of the release mechanism and the complicated interplay between various design parameters of the release medium, detailed prediction of the resulting release profile is a challenge. Herein we suggest a simple method to target specific release profiles more efficiently by integrating release profiles for an array of different microsphere types. This scheme is based on our observation that the resulting release profile from a mixture of different samples can be predicted as the linear summation of the individually measured release profiles of each sample. Hence, by employing a linear equation at each time point and formulating them as a matrix equation, we could determine how much of each microsphere type to include in a mixture in order to have a specific release profile. In accordance with this method, several targeted release profiles were successfully obtained.