Hydrogels with rapid and strong response to external stimuli and possessing high elasticity and strength have been considered as platform materials for numerous applications, e.g., in biomaterials engineering. Thermoresponsive hydrogels based on semi-interpenetrating polymer networks (semi-IPN) featuring N-isopropylacrylamide with copolymers of poly(N-isopropylacrylamide-co-hydroxyethyl methacrylate) p(NIPAM-HEMA) chains are prepared and described. The copolymer was characterized by FTIR, NMR, and GPC. The semi-IPN structured hydrogel and its responsive properties were evaluated by dynamic mechanical measurements, SEM, DSC, equilibrium swelling ratio, and dynamic deswelling tests. The results illustrate that the semi-IPN structured hydrogels possess rapid response and high elasticity compared to conventional pNIPAM hydrogels. By using a microfluidic device with double coaxial laminar flow, we succeeded in fabricating temperature responsive ("smart") hydrogel microfibers with core-shell structures that exhibit typical diameters on the order of 100 μm. The diameter of the fibers can be tuned by changing the flow conditions. Such hydrogel fibers can be used to fabricate "smart" devices, and the core layer can be potentially loaded with cargos to incorporate biological function in the constructs. The platforms obtained by this approach hold promise as artificial "muscles", and also "smart" hydrogel carriers providing a unique biophysical and bioactive environment for regenerative medicine and tissue engineering.
Redox-responsive porous membranes can be readily formed by electrostatic complexation between redox active poly(ferrocenylsilane) PFS-based poly(ionic liquid)s and organic acids. Redox-induced changes on this membrane demonstrated reversible switching between more open and more closed porous structures. By taking advantage of the structure changes in the oxidized and reduced states, the porous membrane exhibits reversible permeability control and shows great potential in gated filtration, catalysis, and controlled release.
Biological
tissues subjected to rubbing, such as the cornea and
eyelid or articular cartilage, are covered in brushy, hydrated mucous
structures in order to reduce the shear stress on the tissue. To mimic
such biological tissues, we have prepared polyacrylamide (PAAm) hydrogels
with various concentrations of un-cross-linked chains on their surfaces
by synthesizing them in molds of different surface energies. The selected
molding materials included hydrophilic glass, polyoxymethylene (POM),
polystyrene (PS), polyethylene (PE), polypropylene (PP), and polytetrafluoroethylene
(PTFE). After synthesis, demolding, and equilibration in water, the
elastic modulus at the hydrogel surface decreased with increasing
water contact angle of the mold. The softer, brushier surfaces
did not completely collapse under compressive pressures up to 10 kPa,
remaining better hydrated compared to their denser, cross-linked analogs.
The hydrogels with brushier surfaces displayed an order of magnitude
lower coefficient of friction than the cross-linked ones, which is
attributed to the ability of their near-surface regions to retain
larger amounts of liquid at the interface. The characteristic speed-dependent
friction of the denser, cross-linked hydrogel surface is compared
to the speed-independent friction of the brushy hydrogels and discussed
from the perspectives of (elasto)hydrodynamic lubrication, permeability,
and shear-induced hydrodynamic penetration depth.
To understand the dissipative mechanisms in soft hydrogel lubrication, polyacrylamide (PAAm) hydrogels with two distinct surface structures were examined under various contact conditions. The characteristic speed-dependent friction of the selfmated, crosslinked hydrogel surfaces could be explained by hydrodynamic shearing of a thin water layer between two rather impermeable bodies. On the other hand, the frictional response of brushy hydrogel surfaces is dependent on the contact conditions and the level of surface hydration. In a migrating contact, brushy hydrogels showed low, speed-independent friction (µ ~ 0.01) likely due to a thick layer of shearing liquid trapped within the sparse surface network. In stationary contact, however, brushy hydrogel surfaces can partially exude water from the near-surface region over time, as shown by timeresolved Fourier-transform infrared (FTIR) spectroscopy. This is assumed to be reflected in a friction increase over time. Interfacial shearing appears to shorten the characteristic exudation times compared to those observed under static loading. Once fluid has been exuded, brushy surfaces were shown to reach similar friction values as their crosslinked analogs. The results thus indicate that the dominating dissipation mechanism during sliding at low contact pressures is shearing of the interfacial liquid film, rather than poro-elastic dissipation within the bulk. Maintenance of surface hydration is therefore crucial, in order to take advantage of the low friction of such systems.
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