(230t) Gelation Process of in Situ Crosslinkable Hydrogels in the Static Mixer

Authors: 
Hozumi, T., the University of Tokyo
Ohta, S., The University of Tokyo
Ito, T., The University of Tokyo

Static mixers have
been widely used in industry to mix various kinds of fluids and are recently
utilized to fabricate hydrogels. Hydrogels have attracted much attention in the
biomedical field as tissue adhesions, drug carriers or tissue engineering
scaffolds. It was reported that the fast gelation rate made hydrogel
inhomogeneous, which resulted in the decrease of mechanical strength. However,
few studies have been done on the effect of mixing on the gelation process.

In the present study,
we studied the gelation process of in situ crosslinkable
hydrogels, which can be formed simply by mixing two reactive solutions, in a Kenics static mixer. In situ crosslinkable
hydrogels are promising material for biomedical application, because they can
be administered into any desired location in the body without large incision. The
effects of process parameters, e.g., flow rate, the number of mixing elements,
and the concentration of the pre-gel polymer solution, on the pressure drop
during mixing and the homogeneity of formed hydrogels were investigated.

We used the static
mixers with 4, 8, 12, 16 elements. Green dye was used as a tracer to visualize
the flow inside the static mixer. To organize the results, we calculated Damköhler number (Da), a dimensionless number expressed as
the ratio of the residence time in the static mixer to the batch gelation time.
The pressure drop between the inlet and outlet of the static mixer was measured
in fabricating in situ crosslinkable hydrogels. The
flow rate was controlled using a syringe pump. As an indicator of homogeneity,
we observed the formed hydrogels via confocal microscopy and calculated coefficient
of variance (COV) by dividing the standard deviation by the average of the
obtained pixel values of confocal images (N=3).

As a model material
for ionically crosslinkable hydrogels, calcium
alginate was utilized in this study. Alginate is a natural polysaccharide comprising
repeating units of alpha-L-guluronate and beta-D mannuronate. When mixed with calcium ions, alginates are
crosslinked with calcium ions via the so-calledgegg box modelh, producing an
insoluble hydrogel. Without calcium ions, the fluid was sol and the stream line
got thinner along with elements in the static mixer. On the other hand, in the
presence of enough amount of calcium ions, hydrogels
formed in the mixer interrupted the flow, resulting in green stagnations. The
pressure drop increased with the number of elements for both with and without calcium
ions, but it showed different tendency against the axial velocity because the
mixing efficiency differed. These results were organized by using Da. The
normalized pressure drop, normalized by that in 0 mM calcium
ions case in the same operation condition, increased monotonically with Da
until Da was 10. COVs of the hydrogels showed similar tendency, first increased
and then decreased with increasing Da. When Da was low, gelation occurs after
the extrusion rather than inside the static mixer. With higher Da, the
hydrogels were more likely to be formed inside the static mixer, resulting in
the increased pressure drop and decreased homogeneity of formed hydrogel. When
Da is further increased, hydrogel was not completely formed inside the static
mixer due to insufficient mixing at very low axial velocity, leading to the increase
in the homogeneity (T. Hozumi, S. Ohta
and T. Ito, Ind. Eng. Chem. Res., 2015, 54, 2099-2107).

In addition to ionic
crosslinking, various kinds of crosslinking reactions can be utilized for
producing in situ crosslinkable hydrogels. It has
been reported that the types of crosslinking reaction significantly affect the
gelation rate and property of in situ crosslinkable
hydrogels. In this study, the above experimental examination was also applied
for other types of crosslinking reaction, such as Schiff base formation and
Michael addition. The difference in the gelation process in the static mixer
and consequent property of formed hydrogels will be discussed.

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