Hydrogel fiber
As a water swollen network with usually low toxicity, hydrogel fiber can be used in a variety of biomedical applications such as drug carrier,[1] optical sensor,[2] and actuator.
One advantage of electrospun hydrogel fiber is that it has a diameter in range in the order between nm to μm, which is desirable for fast matter exchange.
A case would be the Host–Guest Chemistry reported by Scherman et al. Where the formation of inclusion complex between Cucurbit[8]uril and 1-benzyl-3-vinylimidazolium bromide (BVIm) formed physical crosslinking point for the network.
Some hydrophilic polymer can be made into hydrogel fiber via melt-spinning method, where the solidification is done by the phase transition from the molten state.
After leaving the nuzzle at filament state, the fiber solidified after the encounter of cool ambient air and maintained their shape.
An example would be the production of the fiber developed by Lewis et al.[6] Where Silk fibroin was used to generate the desired shear-thinning properties.
And the network was formed when the solvent was subsequently changed.Similar to physical solidification, some chemical crosslinking methods have been developed to produce hydrogel fibers.
And the key for the achievement of hydrogel production through the chemical crosslinking method is the effective separation between the formed network and the tube wall.
An example would be the production of 4-hydroxybutyl acrylate fiber reported by Beebe et al.[8] The microfluid device they used was built with ethylvinyl acetate caplliary and PDMS rubber.
The linear polymer on the surface of the crosslinked network also contains water solvent due to the osmic pressure, thus, a lubrication layer is formed.
Therefore, the solidified polymer fiber can exit the tube with decreased friction force and continuous production can be achieved.
An example would be the production the PAAM/PAMPS semi-interpenetration network hydrogel fiber reported by Zhao et al.[10] The pregel solution was the mixture of PAMPS, AAM, PEGDA (crosslinker), and 2-hydroxy-4'-(2-hydroxyethoxy)-2-methylpropiophenone (photoinitiator).
The surface morphology and shape of the cross-section can be observed via scanning electron microscope (SEM) imaging after removal of solvent.
If the hydrogel fiber was dried directly, a smooth surface would be obtained because of the collapse of the polymer network after the removal of the solvent.
[1] If the hydrogel fiber was lyophilized, a porous surface will usually be found due to the pore-forming effect of the ice crystal.
Although suffering from poor mechanical strength, some approach has been made to construct hydrogel fiber with textile methods.
[1] Also, the electrospun, meltspun, DIW method can produce hydrogel fiber structures at higher dimensions directly.
