Probing Biointerfaces: Electrokinetics - Biointerfaces: Where Material Meets Biology - page 66

pH-dependent pattern of charge compensation in the hydrogel. The increase
of K s below pH 5 can be attributed to the exchange of the K 1 ions, which
predominantly compensate for the gel charge at neutral and alkaline pH,
with H 3 O 1 ions at lower pH values (see contributions of K 1 and H 3 O 1 ions
to K s in Figure 3.3a). H 3 O 1 ions are about five times more mobile, 47 leading
to the increase in K s observed.
The comparison between experimental data and simulation results
(Figure 3.3a) revealed the concentration and pK values of the ionizable
groups in the gel, i.e., 53 mmol L 1 and pK ¼ 0.8 for the sulfate groups and
10 mmol L 1 and pK ¼ 4 for the carboxyl groups. 45 As an important pre-
requisite for the analysis of the gel composition and its cross-linking degree,
heparin of known molecular weight and with known number of sulfate and
carboxyl groups per molecule was employed for the formation of the
hydrogel films. 45 As the sulfate groups are not involved in gel formation, 44
their concentration can be converted into the heparin concentration within
the hydrogel. This calculation is straightforward and revealed a heparin
concentration of 11.4 mg mL 1 for the hydrogel film, which agrees well with
values obtained for similar macroscopic starPEG-heparin gels. 44 The hep-
arin concentration obtained can be used further to derive information on the
real cross-linking degree of the gel films. In excellent agreement with the
theoretical value expected from the ratio starPEG to heparin, this analysis
leads to an average number of 12.3 carboxyl groups per heparin molecule
(versus 12 carboxyl groups expected for the molar ratio g ¼ 3). 45 The de-
veloped methodology has advantages over other analytical methods for the
determination of the cross-linking degree via labeling of unreacted groups,
because it is not prone to non-quantitative turnover or non-specific side
effects.
The mean-field approach 45 can be applied further to quantify variations in
the intrinsic sulfation pattern of the GAG components in the gel (e.g., used to
tune its interactions with signal molecules) and to investigate biomolecular
interactions between the GAGs and soluble effectors. Altogether, the pro-
cedure introduced 45 provides new options for the interpretation of bio-
physical and biomolecular characteristics of GAG-based hydrogels for
regenerative and sensoric applications. In particular, it enables the correl-
ation of gel properties with the molecular transport and binding/release
of cytokines, and thus the rational design of multibiofunctional polymer
matrices.

d n 3 r 4 n g | 5

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3.3.3 Fluidity Modulation in Phospholipid Bilayers by
Electrolyte Ions
Cell membranes separate the intracellular compartment from the extra-
cellular medium and regulate important cellular processes such as signal-
ing, organization of the cytoskeleton, protein sorting and apoptosis. 48,49
A key property of these membranes is their phase behavior. As lipid head