Hydrogels are classified as a three-dimensional hydrophilic polymer network with a moderately cross-linked structure, which can shrink or expand by expelling or absorbing large amounts of water according to the local environment, ionic strength/pH, temperature, and substrate concentration. Due to its unique properties, researchers have conducted a lot of research on the application of hydrogels in drug delivery systems, sensors, and separation/chelating agents.
Polyacrylamide(PAM) represents an important class of hydrogels, consisting of loosely cross-linked acrylamide structures. PAM can be used as biological material in contact with tissues or biological fluids, such as implants, soft contact lenses, and drug delivery particle systems. PAM beads can be easily surface-functionalized and have been used in various applications, including RNA capture, drug encapsulation, controlled release, and enzyme immobilization.
Figure.1 Micrograph of the beads. (Sen N, et al. 2020)
How Does Microfluidic Technology Help?
Microreactors are very effective for strengthening various unit operations and processes. The production of monodisperse droplets to produce uniform-sized microspheres is one of the many applications of microreactors. A variety of methods can be used to obtain small and uniform droplets that aggregate to form microspheres/beads, such as hydrodynamic flow focusing, coaxial shear flow, cross flow shear, or simply using T-joints. These microfluidic technologies allow very strict control of droplet size.
Microfluidic technology for PAM bead synthesis is widely used in various fields. High reproducibility, real-time control, and waste reduction are the main factors that prompt users to switch from conventional batch processing methods to microfluidic technology.
Polyacrylamide Beads in Microfluidic Devices
Microfluidic systems based on droplets show extraordinary advantages in synthesizing droplets containing monomers or polymers. By precisely controlling the formation of microbeads, Alfa Chemistry can produce polyacrylamide particles with well-defined sizes, shapes, and morphologies. We use ex-situ and in-situ mixing methods to synthesize PAM microspheres/beads through a T-junction microreactor.
Figure.2 Schematic diagram of the two mixing methods studied (a) in-situ mixing, (b) ex-situ mixing. (c) Schematic diagram of the junctions used. (Hwang J. Y, et al. 2016)
- In ex-situ mixing: We premix the monomer solution, crosslinker, and initiator together and put it into the syringe, which contains 9% monomer PAM, 1% crosslinker, and 0.05% initiator. The microreactor includes a microfluidic T-shaped joint and a microporous tube connected to the joint. The tube is immersed in a hot bath maintained at 120°C. The carrier phase and the water phase are sent to the microfluidic T-junction through a syringe pump, in which a dispersion of an aqueous phase containing uniform droplets in the carrier phase is produced.
- In in-situ mixing: The monomer solution and the cross-linking agent and the initiator aqueous solution are pumped separately through two separate syringe pumps, and then mixed in the T-joint. Thereafter, the mixed stream is fed to the second microfluidic T-junction, where it is dispersed in the form of uniform droplets by the flowing carrier phase.
In both methods, the droplets formed at the junction will enter the tube immersed in the hot water bath. The length of the tube is long enough to ensure that the droplets polymerize to form rigid beads within the residence time provided by the tube. At the outlet of the reactor, the beads and carrier phase are collected.
- Sen N, et al. (2020). "Synthesis of Polyacrylamide (PAM) Beads in Microreactors." Chemical Engineering and Processing - Process Intensification. 157: 108105.
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