Hydrogel fiber is a hydrogel made into a fibrous state, where its width is significantly smaller than its length. The hydrogel's specific surface area at fibrous form is larger than that of the bulk hydrogel, and its mechanical properties also changed accordingly. As a result of these changes, hydrogel fiber has a faster matter exchange rate and can be woven into different structures. As a water swollen network with usually low toxicity, hydrogel fiber can be used in a variety of biomedical applications such as drug carrier, optical sensor, and actuator.
But the production of hydrogel fiber can be challenging as the hydrogel is crosslinked and can not be shaped into a fibrous state after polymerization. To make hydrogel into a fibrous state, the pregel solution must be made into fibrous form and then crosslinked while maintaining this shape.
Production method
To produce hydrogel fiber, the solidification of the pregel solution is the most important step. The pregel solution needs to be solidified while maintaining its fibrous shape. To achieve this, several methods based on chemical crosslinking, phase change, rheological property change have been developed.
Physical solidification based Change in physical interactions can be utilized for the solidification process, and the fibrous state is usually achieved outside of the extrusion nozzle. Due to the reversibility of those physical interactions, subsequent crosslinking is traditionally required.
Electrospinning
Hydrogel fiber can be produced by electrospinning with solidification done by the evaporation of the solvent. The fibrous state is created by the combination of electrostatic repulsion and the surface tension of the solution. But subsequent crosslinking is usually needed to form a crosslinked network. 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. However, utilization of single fiber can be hard to achieve due to the weak mechanical strength of the microscopic fiber and its entanglements after production. An example of this method would be the production of polyacrylamide (PAAM) semi-interpretation network developed by Tahchi et al. Where the first linear PAAM (provide solidification) was mixed with AAM monomer (form subsequent network) and crosslinker N,N′-methylenebisacrylamide (MBA). During the electrospinning process, the linear PAAM provided the required physical properties to achieve electrospinning, while the AAM monomer and MBA crosslinker were used to form a second crosslinked network inside the PAAM fiber. Although no crosslinking was formed between the first and second networks, the physical entanglement will prevent linear PAAM from leaking.
Drawspinning
Through supramolecular chemistry, pregel solution can solidify through reversible supramolecular interactions such as host-guest interactions. Such interaction can be manipulated through the mechanical force or the temperature. When energy exerted to the network is high enough, physical crosslinking point will break and the polymer will be at liquid state, after leaving the nozzle, the crosslinking can be rapidly formed to solidify the solution. 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. The formation of this physical crosslinking point is controlled by the temperature of the solution. By heating up the solution and cooling it down rapidly at extrusion nuzzle, the hydogel fiber is formed. Also, subsequent crosslinking is performed to form a perment network.
Meltspinning 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. Similar to the electro-spinning, the pregel solution was kept liquid in the container. 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 meltspinning apparatus built by Long et al., where meltspinning of polylactic acid (PLA) and polycaprolactone (PCL) fiber are achieved.
Direct ink writing Similar to the draw spinning technique the direct ink writing technique utilized reversible physical solidification to produce hydrogel fibers. The pregel solution was liqufied through shear thinning process which can be generated by adding microscopic particles such as mircrogel. After leaving the nuzzle, the hydrogel will solidify and retain their shape, and network will be made perment after crosslinking.
An example would be the production of the fiber developed by Lewis et al. Where Silk fibroin was used to generate the desired shear-thinning properties. And the network was formed when the solvent was subsequently changed.
Chemical crosslink based 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.
Microfluid spinning Many microfluid device-based methods have been developed to produce hydrogel fibers.
Crosslinking of alginate One of the most commonly used fiber production methods is the crosslinking of sodium alginate by CaCl2, where the formed calcium alginate will act as the crosslinking point to link the alginate chains together to form the network and solidified the polymer. Afterward, this alginate hydrogel fiber can be used as a template for the polymerization of secondary networks. Additionally, by controlling the fluid dynamics inside the microfluid device, the diameter and the shape of the resulting fiber can be tuned without doing modification to the devices. A practice would be the production of alginate solution reported by Yang et al. They used the sodium alginate as core fluid and CaCl2 as shealth fluid, the crosslinked network (hydrogel fiber) formed once this two fluid met, the laminar flow kept its tubular shape during the reaction.
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