(197j) Exploiting a Novel Aqueous-Two Phase Microfluidic System for Cell Encapsulation in GAG+Chitosan Microcapsules

Improving the efficiency of cell encapsulation techniques is a necessary step for the wider application of capsule-based modular tissue engineering. Control over the capsules size and distribution which are the two most important parameters in cell encapsulation can be greatly facilitated using microfluidic techniques. In this study, a new microfluidic system using aqueous two phase system (ATPS) was devised for making fine and uniform capsules for cell encapsulation. ATPS systems have traditionally been used for protein separation due to the moderate conditions they make for the cells. The mild condition is vital for higher cell viability and functionality which is a great advantage compared to the traditional water-oil microfluidics. Dextran and polyethylene glycol (PEG) is the most widely used ATPS used due to their smooth environment for protein separation. In the present work, GAG solution, consisting of 1.5wt% carboxymethylcellulose (CMC), 4 wt% chondroitin sulfate (CSA) and 10-20 wt% dextran forms the disperse fluid in the microfluidic system which phase separated within a 5-10 wt% polyethylene glycol (PEG) leading to the formation of droplets inside the microfluidic. This method allows a core fluid (GAG) to be surrounded by sheath stream flowing (PEG). The surface tension generated between these two immiscible fluids lead to the formation of the droplets. The droplets then go into a stirred 0.6 wt% chitosan solution to generate a polyelectrolyte complex shell around the capsule core. In order to enhance the uniformity and decrease the size of the GAG droplets, an optimized PDMS microfluidic system was utilized. The challenging part in making droplets in the microfluidic system using ATPS systems is the ultra-low interfacial tension of these systems often less than 10^-4N/m. This characteristic exerts extra considerations to be taken while making droplets in the ATPS microfluidics. Hence, ultra-low flowrates for both GAG and PEG solutions in the range of 1-100 mlit/min was used. The effect of four influencing parameters including PEG concentration, dextran concentration, PEG flow rate and GAG flow rate on the size and uniformity of the microcapsules were also investigated. Most of the encapsulated cells were in the range of 100-300 μm in diameter which is ideal for encapsulation of cells including hepatocytes and mesenchymal stem cells. It was observed that by increasing the PEG flowrate in fixed GAG flowrates, the capsules diameter would increase and vice versa. This method gives a stable control over the shape, size and thickness of the capsules formed which finally will boost the diffusion of nutrients and oxygen to the cell and secretion of wastes. Reliability, reproducibility and accuracy of the technique leaves it amongst the best novel encapsulation techniques.