Biomedical Engineering Reference
In-Depth Information
1.11 Production scheme for synthetic hydrophilic plasma separation
membranes by solvent-induced phase separation (SIPS).
through an opening in the centre of the ring spinneret to prevent the capillary
from collapsing to a simple thread. This core liquid is a non-solvent to the
polymer mix, and is commonly a water-based mixture. When the polymer
solution comes into contact with the non-solvent, the polymer precipitates from
the solution and forms the membrane wall. This process is also called diffusion-
induced phase separation (DIPS) process, because the initiation of the
precipitation of the polymer takes place due to diffusion of the solvent and
non-solvent into each other.
In this technique it is important to achieve a fast precipitation rather than a
small, controlled recrystallisation (Fig. 1.12) and for it to come directly from a
stable solution (which consists of polymer and solvent and virtually no non-
solvent) to a non-stable system, in which the polymer then precipitates.
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The
faster the precipitation occurs, the more nucleation centres will build, which will
quickly aggregate in all three dimensions, produce a somewhat random structure
and thus trap the solvent and non-solvent as tiny droplets in a scaffold of polymer
(path B). If a path is chosen that leads to a meta-stable system, then nucleation and
growth will occur slowly and result in larger chunks of polymer, so that solvent
and non-solvent can run off, yielding a capillary with almost no pores (path A).
This precipitation occurs from the lumen to the outside of the wall, because it
is initiated by the core liquid. Therefore, membranes produced with SIPS often
display a non-homogeneous pore structure, with the pores on the lumen side
having different diameters from those on the outside. After this (first)
precipitation, the membrane is spun into a bath of non-solvent (again mostly
water). This completes the precipitation of the polymer and serves as a first
washing step of the pore structure to wash out the solvent.
