Fabrication Techniques and Properties of Scaffolds - Biodegradable Polymer-Based Scaffolds for Bone Tissue Engineering - page 36

It is also explained that in the rubbery polymers which are well above their
glass transition temperature, in the presence of penetrant, the polymer chains
adjust very quickly and so no diffusion anomalies can occur. It was proposed by
Alfrey, Gurnee and Lloyd (1966), that diffusion can be classified into three distin-
guished classes according to the relative rates of diffusion and polymer relaxation
(Crank 1979 ):

1. Case I or Fickian diffusion where the rate of diffusion is much less than
relaxation.

2. Case II diffusion, it is the other extreme where diffusion is very rapid com-

pared with relaxation processes.

3. Non-Fickian or anomalous diffusion that occurs when the diffusion and relax-

ation rates are comparable.

It was also mentioned that the significance of the terms case I and case II is that
these are both simple cases in which in terms of a single parameter, the behaviour
of each can be described. It can be emphasized that case I systems are controlled
by the diffusion coefficient while case II systems parameter is the constant veloc-
ity of an advancing front. If the amount sorbed at time t is denoted by Kt n , with K
and n constants, then n = 1 is for case II systems while n = ½ for case I systems.
On the other hand, non-Fickian systems lie between case I and case II where n has
the intermediate value between ½ and 1 or changes sigmoidally from one to the
other. Two or more parameters are needed to describe non-Fickian behaviour.
For case I, it was described that the characteristic features of Fickian diffusion
is controlled by a concentration dependent diffusion coefficient. The term pseudo-
Fickian is used to describe sorption-desorption curves of the same general shape
and disposition but where the initial portion persists for a shorter time (Crank
1979 ). On the basis of the sorption and desorption curves, in the early stages,
when the diffusion takes place in a semi-infinite medium, the amount absorbed
or desorbed is directly proportional to the square root of time. When they cease to
be linear, if the sorption and desorption curves plotted against (time) 1/2 axis, each
will become concave towards the (time) 1/2 axis and then steadily approach to final
equilibrium value.

2.7 In Vitro Biological Assessment

2.7.1 Cell Culture

As the potential application of current research is bone tissue engineering, oste-
oblast-like cells would be used in this research. Autologous osteoblastic cells
isolated from biopsies from the patients have the advantages of being non-
immunogenic. But the drawback is relatively few cells are available after the dis-
sociation of the tissue and their expansion rates are relatively low. On the other