Optimizing Bicontinuous Structure of Bijels-derived Polymer-Hydrogel Hybrids for the Controlled Release of Different Cells

Abstract

Controlled cell release is an important strategy in tissue engineering which deliver live cells to the repair sites in human bodies with well-designed cell density and distribution that mimic targeted tissues and facilitate tissue regeneration. Bicontinuous interfacially jammed emulsion gels (“bijels”) are a new class of materials consisting of two interpenetrating continuous liquid phases. Bijels can be used as templates for fabricating bijels-derived bicontinuous structures having interconnected channels. These channels (“pores”) in bicontinuous structures can provide desired space for cell encapsulation, proliferation and migration in tissue engineering while enabling the transport of nutrients and bioactive molecules. However, pore sizes of bijels-derived bicontinuous structures are normally small. To realize their potential in tissue engineering, bijels-derived bicontinuous structures with larger pore sizes need to be made for cell encapsulation and delivery. Our research has developed new methods for fabricating bijels and bijels-derived structures. In this investigation, bijels-derived polymer-hydrogel hybrids with normal pore size (~30 μm) and large pore size (~60 μm) were produced, and human dermal fibroblasts (HDFs) and osteoblast precursor cell line (MC3T3 cells) were encapsulated in respective hybrid. The cells were encapsulated in the crosslinked alginate hydrogel (with different crosslinking degrees) of polymer-hydrogel hybrids. The hybrids were then cultured for different periods to study controlled cell release. SEM examinations showed successful fabrication of cell-encapsulated polymer-hydrogel bicontinuous structures and the degradation of alginate hydrogel during culture. The cell release process was studied using confocal microscopy and SEM. Live/Dead assay and MTT assay were also used in the studies. It was observed that live cells were locked in hybrid structures at the beginning and were gradually released since Day 1 during culture. Both HDFs and MC3T3 cells exhibited good cell viability and proliferation after their release and were able to migrate from the channels of hybrid structures to the surface of hybrids and to the outer culture plate. The two types of cells displayed different release behaviors. In the bijels-derived hybrids with small pore size (~30 μm), HDFs were released significantly slower than MC3T3 cells. The major release period of encapsulated HDFs was 24 h later than the major release period of encapsulated MC3T3 cells. For the bijels-derived hybrids with large pore size (~60 μm), the release rates of HDFs and MC3T3 cells were similar. A possible explanation for the differences is that the sizes of cells and pores in hybrids could affect cell release. HDFs have a larger cell size (~50 μm) than MC3T3 cells (20-30 μm), which may have caused slower release of HDFs in bijels-derived hybrids with small pore size. This investigation has demonstrated that bijel-derived bicontinuous hybrid structures are a good vehicle for cell delivery in tissue engineering. By controlling the pore size of hybrid structures, the release of different types of cells could be controlled. The released live cells could proliferate well and migrate in and outside the hybrid structures.