Polymeric Supramolecular Hydrogels as Materials for Medicine - In-Situ Gelling Polymers: For Biomedical Applications - page 168

Fig. 15 Microstructures of hydrogels based on chitosan and xanthan. SEM micrographs of the
porous inner ( a ) and outer ( b ) structure of hydrogel beads. Reprinted from Dumitriu et al. [ 141 ].
Copyright 2004 Elsevier
problem, tubular hydrogels can be envisioned to serve as nerve conduits, making
future surgeries obsolete [ 149 , 150 ]. To realize this concept, several criteria have
to be met: the material has to be biocompatible and biodegradable after the regen-
eration is complete, its mechanical stability has to be constant during the entire
regeneration process, it has to be flexible with a Young's modulus similar to that
of the nerve tissue to avoid compression of the nerve, and the material has to be
porous to allow for diffusive penetration of nutrients.
To pursue this goal, Gander and coworkers reported a hydrogel based on algi-
nate and chitosan that possesses the required characteristics to act as a nerve
conduit [ 151 ]. Tubular hydrogels were prepared using a spinning mandrel and
investigated in view of their physical-chemical characteristics as well as their
porosity, probed by diffusive penetration of dextran as a model substance for
biologically relevant nutrients and growth factors. At first, the hydrogels were
investigated with respect to their swelling in PBS buffer at pH 7.4; for this pur-
pose, freeze-dried tubular gels were cut into 6-mm pieces that were immersed in
the medium. After 20 min, the maximum swelling degree was reached: the outer
diameter was swollen from 1.7 to 2.7 mm, whereas the inner diameter was still
1.3 mm, as shown in Fig. 16 b. The water content of the tubes was determined
gravimetrically to be 83 wt%. This high water content indicates a porous micro-
structure. SEM micrographs could confirm this indication in the dried state of
the hydrogel, as illustrated in Fig. 16 a. In addition to these experiments, the dif-
fusivity of nutrients through the hydrogels was investigated with three differently
sized fluorescein-labeled dextrans (4, 10, 20 kDa); for this purpose, the swollen
tubes were filled with 5 g L − 1 dextran solution, and the ends of the tubes were
sealed. The amount of dextran that diffuses out of the conduit was determined
fluorometrically, showing a dependence on the length of the dextran chains. The
lowest molecular-weight dextran (4 kDa) showed the fastest diffusivity through
the conduit wall: 35 % in 7 h. At higher size of the dextran probe, the diffusivity
was slower: in case of 20-kDa dextran, only 2 % of the solution permeated out