Biomedical Engineering Reference
In-Depth Information
prevent blood plasma and plasma water from entering the pores (wet lung
phenomenon). Membranes from silicone are non-porous over their entire
thickness of 100±200m. The most recent development is a membrane made
from PMP with a microporous wall with only a very thin (<1m) outer skin
(OXYPLUS
Õ
, by Membrana GmbH, Wuppertal, Germany). As the permeability
of PMP for oxygen and carbon dioxide is high compared with PP and the skin is
very thin, this skin does not impair the performance of the membrane.
13
Boundary layers
The most important limiting factor for the performance of blood oxygenators
with microporous membranes today is the blood side diffusion layer.
7
Although
flow conditions in a bundle of capillary bundles are much more complex, the
simple model of a boundary layer of laminar flow on a flat surface can serve to
illustrate the phenomenon. On the surface, a boundary layer will develop, in
which the volume increments directly adjacent to the surface have a velocity that
approximates zero. The boundary layer thickness is a function of the square root
of the bulk fluid velocity. Through the thickness of this boundary layer, the
velocity of the fluid will increase to reach the bulk velocity. Within this layer,
the main transport mechanism will be diffusion (see Fig. 1.1).
To reach a high performance of the final device, a primary goal is to achieve
a thin blood side diffusion layer and good blood mixing (see Section 1.2.8). In a
medical device with blood as the fluid, the adsorption of blood components,
mainly proteins, to the surface also needs to be accounted for. A hydrophobic
surface like a microporous PP membrane will soon be covered by a layer of
blood proteins. Here, one can expect the surface to be covered first by those
proteins that have the highest concentrations in blood (albumin). In due course,
albumin can then be replaced by other proteins with higher binding forces to the
particular polymer. This protein layer (`protein cake') forms an additional layer
and influences the permeability of the membrane. As the blood of every patient
is individual, and additionally haemodilution and medication have an influence
on the build-up of this additional protein layer, it is very difficult to quantify
these factors and predict their effects in the individual clinical case. Further-
more, most oxygenators today are coated with proprietary coatings of individual
manufacturers, which have great impact on the adsorption of proteins and
primarily cells on the membrane.
14
Further information on protein adsorption to
biomaterials can be found in Goldstein et al.
15
1.2.3 Membrane manufacture and resulting structures
Thermally induced phase separation (TIPS)
For the manufacture of current capillary membranes, the thermally induced
phase separation (TIPS) process is one preferred method (Fig. 1.2). For this, the
