A pancreatic cell islet encapsulation apparatus â?? Flow in a two-nozzle hydrodynamic focusing device

Yannis Psihogios Dimitris Hatziavramidis1
School of Chemical Engineering, National Technical University of Athens, Zografos 15780 GR

1corresponding author dthatzia@central.ntua.gr

A high-rate, high-efficiency encapsulation apparatus for pancreatic islets intended for transplantation into patients with diabetes type 1, was designed by the principles of Mechanics and Cytotechnology. It consists of the following parts:
1. A system that feeds the islets in a single file by hydrodynamic focusing, thus ensuring, separate encapsulation of individual islets,
2. An encapsulation chamber containing a two-layer waterâ??oil system in which pancreatic
islets are enclosed into microcapsules by selective withdrawal,
3. A valveless, diffuser-nozzle micropump to remove the islets from the encapsulation chamber for recovery by filtration.
This presentation focuses on the flow in a two-nozzle hydrodynamic focusing device. Existing models of hydrodynamic focusing (Lee et al, 2001; Yang et al, 2007) assume planar flow, even when the flow is axisymmetric, assume that sheath and sample fluids have the same densities and viscosities, in spite of the fact that the sample fluid is loaded with cell islets, and lack the capabilities of describing interfacial dynamics. In some cases they use two-phase flow equations which more appropriate for flow of interspersed fluids, when the flow in question is a flow of segregated fluids.
A two-fluid model has been developed which, in addition to allowing sheath and sample fluids to have different densities and viscosities, allows the two fluids to be miscible or immiscible, as the case may be. This model assumes laminar, axisymmetric flow of two Newtonian fluids. Simulations using the present model show that
â?¢ The most important factor of determining the focusing width is the sheath-to-sample- fluid-velocity ratio. As this ratio increases, the focusing width decreases, but the focusing length increases,
â?¢ The shape of the nozzles, linear or curvilinear, does not affect the hydrodynamic focusing width and length, and
â?¢ Neither the assumption of planar flow nor that of sheath and sample fluids having the same density and viscosity is valid for axisymmetric flow with sheath and sample fluids having different densities and viscosities in a hydrodynamic focusing device.