B, Table S-1, Table S-2). The ability to tune the mechanical properties
of the resulting gel over a large range by simply changing the
ratio of the two polymers allows control over gel stiffness while
keeping other parameters such as polymer concentration, and
ligand density constant which may be useful for studies of
mechanobiology.
The swelling ratio of hydrogel systems can affect mechanical
properties, mass transport, and the presentation of ligands on the
gel surface. To investigate how volumetric swelling would change
at different polymer concentrations and N:T ratios, click alginate
hydrogels were made as previously described and allowed to swell
for 24 h at 37 C. The swollen volume was measured and compared
to the casted volume (Fig. 2-C). For a given polymer concentration,
the volumetric swelling ratio increased as the N:T ratio deviated
from 1, demonstrating an inverse relationship between mechanical
properties and swelling ratio as expected. While the N:T ratio has a
significant effect on the swelling ratio, the polymer concentration
does not have a significant effect, indicating that the swelling ratio
of click alginate is dominated by crosslink density rather than
polymer concentration (Table S-3).
3.3. Post-gelation modification of click alginate hydrogels
To explore if additional functionalities can be introduced to click
alginate hydrogels after polymerization, we grafted thiolcontaining
molecules onto unreacted norbornenes in pre-formed
click alginate hydrogels using a photoinitiated thiol-ene reaction
(Fig. 3-A). Gels with N:T ¼ 2 were used to ensure unreacted norbornenes
were available to react after the initial gelation. RGD
peptide solutions at high (2 mM) or low (0.2 mM) concentration
were reacted onto the surface of these click alginate hydrogels,
which were then seeded with NIH 3T3 fibroblasts expressing a
cytosolic fluorescent marker (EGFP). 3T3 cells readily adhered and
spread on gels modified with RGD, while very few cells were able to
attach or elongate on control gels with no RGD (Fig. 3-B). Cells on
click alginate hydrogels presenting RGD were able to form
branched interconnected networks, with a significant RGD densitydependent
2e3 fold increase in surface coverage over the 3 day
culture, while unmodified click alginate gels were observed to be
non-cell-adhesive and showed a decrease in surface coverage by
cells over time (Fig. 3-C). After 3 days in culture, cells also showed
an increase in spreading and actin stress fiber formation with
higher RGD concentration (Fig. 3-D). Additionally, the high viability
of cells after 3 days of culture demonstrated the cytocompatibility
of the click alginate hydrogels for 2D cell culture (Fig. 3-E).
3.4. Cell encapsulation in click alginate hydrogels
In order to demonstrate the utility of click alginate hydrogels for
cell encapsulation, cell viability and metabolic activity of cells
encapsulated in click alginate hydrogels were investigated over a 3
day culture period; ionically crosslinked hydrogels were used for
comparison in these studies. Representative images of encapsulated
cells stained with ethidium homodimer-1 show minimal cell
death in both click and ionically crosslinked gels 4 h and 3 days
after encapsulation (Fig. 4-A). Quantification revealed that click
alginate hydrogels resulted in significantly higher viability of
encapsulated 3T3 cells both immediately after encapsulation
(93 ± 1% vs. 87 ± 2%) and after 3 days of culture (84 ± 2% vs. 79 ± 4%)
(Fig. 4-B). It should be noted that a loss in measured cell viability
may occur during the cell retrieval process by enzymatic digestion
of the hydrogels. The overall metabolic activity of the cells encapsulated
in the different hydrogels was also analyzed, and noted to
increase over the 3 day culture period for both hydrogel crosslinking
chemistries (Fig. 4-C).
B, Table S-1, Table S-2). The ability to tune the mechanical propertiesof the resulting gel over a large range by simply changing theratio of the two polymers allows control over gel stiffness whilekeeping other parameters such as polymer concentration, andligand density constant which may be useful for studies ofmechanobiology.The swelling ratio of hydrogel systems can affect mechanicalproperties, mass transport, and the presentation of ligands on thegel surface. To investigate how volumetric swelling would changeat different polymer concentrations and N:T ratios, click alginatehydrogels were made as previously described and allowed to swellfor 24 h at 37 C. The swollen volume was measured and comparedto the casted volume (Fig. 2-C). For a given polymer concentration,the volumetric swelling ratio increased as the N:T ratio deviatedfrom 1, demonstrating an inverse relationship between mechanicalproperties and swelling ratio as expected. While the N:T ratio has asignificant effect on the swelling ratio, the polymer concentrationdoes not have a significant effect, indicating that the swelling ratioof click alginate is dominated by crosslink density rather thanpolymer concentration (Table S-3).3.3. Post-gelation modification of click alginate hydrogelsTo explore if additional functionalities can be introduced to clickalginate hydrogels after polymerization, we grafted thiolcontainingmolecules onto unreacted norbornenes in pre-formedclick alginate hydrogels using a photoinitiated thiol-ene reaction(Fig. 3-A). Gels with N:T ¼ 2 were used to ensure unreacted norborneneswere available to react after the initial gelation. RGDpeptide solutions at high (2 mM) or low (0.2 mM) concentrationwere reacted onto the surface of these click alginate hydrogels,which were then seeded with NIH 3T3 fibroblasts expressing acytosolic fluorescent marker (EGFP). 3T3 cells readily adhered andspread on gels modified with RGD, while very few cells were able toattach or elongate on control gels with no RGD (Fig. 3-B). Cells onclick alginate hydrogels presenting RGD were able to formbranched interconnected networks, with a significant RGD densitydependent2e3 fold increase in surface coverage over the 3 dayculture, while unmodified click alginate gels were observed to benon-cell-adhesive and showed a decrease in surface coverage bycells over time (Fig. 3-C). After 3 days in culture, cells also showedan increase in spreading and actin stress fiber formation withhigher RGD concentration (Fig. 3-D). Additionally, the high viabilityof cells after 3 days of culture demonstrated the cytocompatibilityof the click alginate hydrogels for 2D cell culture (Fig. 3-E).3.4. Cell encapsulation in click alginate hydrogelsIn order to demonstrate the utility of click alginate hydrogels forcell encapsulation, cell viability and metabolic activity of cellsencapsulated in click alginate hydrogels were investigated over a 3day culture period; ionically crosslinked hydrogels were used forcomparison in these studies. Representative images of encapsulatedcells stained with ethidium homodimer-1 show minimal celldeath in both click and ionically crosslinked gels 4 h and 3 daysafter encapsulation (Fig. 4-A). Quantification revealed that clickalginate hydrogels resulted in significantly higher viability ofencapsulated 3T3 cells both immediately after encapsulation(93 ± 1% vs. 87 ± 2%) and after 3 days of culture (84 ± 2% vs. 79 ± 4%)(Fig. 4-B). It should be noted that a loss in measured cell viabilitymay occur during the cell retrieval process by enzymatic digestionof the hydrogels. The overall metabolic activity of the cells encapsulatedin the different hydrogels was also analyzed, and noted toincrease over the 3 day culture period for both hydrogel crosslinkingchemistries (Fig. 4-C).
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