Probing Biointerfaces: Electrokinetics - Biointerfaces: Where Material Meets Biology - page 64

Further information regarding thin films for streaming current/potential
measurements are given in Appendix A.3.

3.3.2 Composition and Structure of Biohybrid Hydrogels
Hydrogels, i.e., three-dimensional matrices of physically or chemically cross-
linked polymers, are widely used in tissue engineering. 42 The mechanical
properties of these materials can be adapted to specific use in various tis-
sues, 43 and hydrogels may further incorporate biomolecular constituents of
extracellular matrices (ECM) to stimulate embedded cells via specifically
orchestrated signals. 44 Biohybrid hydrogels, consisting of synthetic and
biologically derived polymeric constituents, are increasingly being developed
and employed for this purpose. 43 Among these systems, hydrogels con-
taining polysaccharidic ECM components, glycosaminoglycans (GAGs), are
particularly promising due to the effective binding, protection and sustained
release of numerous growth factors. 5 The latter effect can be largely attrib-
uted to the sulfation pattern of the GAGs and, therefore, chiefly governed by
electrostatics.
In order to analyze the charge and structure of biohybrid hydrogel films
consisting of covalently linked star-shaped poly(ethylene glycol) (starPEG)
and heparin, 44 Zimmermann et al. 45 developed and applied a mean-field
approach for the numerical evaluation of surface conductivity data. The
hydrogel film was treated as 3D meshwork, where ionizable groups are im-
mobilized. The ionisation of these groups is related to an elevated ion
concentration within the hydrogel film (due to the counter-ions) that, in
turn, causes an electrical potential difference between the gel and the elec-
trolyte. If the thickness of the film is much larger than the Debye screening
length, interfacial effects are negligible and the potential difference between
the gel and the bulk electrolyte can be described analogously to the potential
difference across a semipermeable membrane by the Donnan potential. 29
For hydrogel films with a homogeneous distribution of the polymer seg-
ments and low to intermediate densities of ionisable groups, the surface
conductivity, K s , is related to the Donnan potential, C D , between the
hydrogel and an electrolyte composed of N monovalent ions according to the
following equation: 29

d n 3 r 4 n g | 5

.

RT X

N

K s ¼ F 2 d z i c i D i e z i y D

(3 : 8)

i ¼ 1

where y D is the dimensionless Donnan potential (y D ¼ FC D /RT). The
dimensionless potential, y D , can be calculated on the basis of eqn (3.3) after
replacing y(X)byy D and the condition that the second derivative of the
potential is zero with respect to position.
The surface conductivity of the starPEG-heparin hydrogel films was found
to be strongly dependent on solution pH (Figure 3.3a). This property reflects
the different ionization characteristics of the sulfate and carboxyl groups at