Biomedical Engineering Reference
In-Depth Information
of aqueous polymer solution into an ambient co-flowing fluid stream has been used for the
preparation of cell-enclosing microcapsules. The oldest but the most widely used ambient co-
flowing fluid is air [6-11]. The system using air for producing spherical vehicles for use in
biomedical applications has been employed since the 1960s (Figure 1a) [12]. In the first
report for the successful transplantation of mammalian cells enclosed in microcapsules for
treatment of diseases without dosing immunosuppressive drugs, the system using air as an
ambient co-flowing stream was employed for obtaining pancreatic islets-enclosing
microcapsules of 300-800 μm in diameter [13]. In the system using air, sodium-alginate is
commonly used as a material for surrounding the cells because the aqueous solution of the
polymer instantaneously becomes a hydrogel in the presence of multivalent cations, usually
calcium, barium or strontium ions [14]. By injecting a cell-suspending sodium-alginate
solution into the co-flowing air stream from a needle with a diameter of several hundred
micrometers, the breakup into cell-enclosing droplets is caused by the drag force exerted by
ambient air flow on the extruded alginate solution. Cell-enclosing calcium-alginate
microcapsules can be obtained by collecting the alginate droplets in the aqueous solution
containing multivalent cations. The size of the alginate microcapsules strongly depends on the
flow rate of the ambient air [15] (the size of the needle from which the cell-suspending
aqueous solution is extruded and the flow rate of the aqueous solution are also important
factors). The system using air as an ambient co-flowing fluid is effective for obtaining
microcapsules of 500-800 μm in diameter with a narrow distribution in size for enclosing
pancreatic islets of 50-300 μm in diameter. However, one shortcoming of the system is that it
is not possible to obtain droplets which are smaller than the diameter of the needle from
which the cell-suspending aqueous polymer solution is extruded [15,16].
Figure 1. Schematic illustration of droplet production devices using (a) air and (b) liquid paraffin
developed as an ambient co-flowing fluid.
Development of smaller microcapsules is a challenging issue in the field of cell
encapsulation and is of interest as smaller microcapsules offer many advantages: Exchange of
molecules such as oxygen and nutrients is theoretically better in smaller microcapsules
[17,18]. Chicheportiche and Reach [17] revealed that the response time of encapsulated
pancreatic islets to glucose stimulation decreased with a reduction in microcapsule size.
Beneficial effects on activity of enclosed cells were also reported [19]. In addition, reduction
Search WWH ::




Custom Search