UV-Crosslinking of Microparticles

Comparison with other methods

Why use microparticles ?  

Microparticles with sizes ranging from 1 μm to 1000 μm have emerged as advanced functional materials for a wide range of biomedical applications, such as drug delivery, tissue engineering, biosensing, and cellular life science. These applications of microparticles depend on their properties which correlate with their size, structure, composition and configuration. Therefore, it is essential to fabricate microparticles in a controlled manner to improve their pharmaceutical capability and reliability for biological studies. 

Limitations of standard methods 

Conventional methods for microparticle production include solvent evaporation, emulsion polymerization, dispersion polymerization, and spray drying¹. These usually result in microparticles with large polydispersity, poor reproducibility, limited functionality, and less tunable morphology². 

Benefits of droplet microfluidics for microparticles production 

To overcome these limitations, droplet microfluidic technology has been implemented. This offers greater control over multiple fluid flows at the microscale. It permits the generation of particles or emulsions with higher monodispersity (~ 5% CV) and complex shapes and structures. Many emulsion droplets can be polymerized (thus usually solidified) upon UV irradiation. This last step can be achieved in a microfluidic system, typically through the downstream channel, where droplets are irradiated with UV light (UV-Crosslinking of Microparticles). 

Experimental procedure for UV-crosslinking of microparticles

Monodisperse polymer particle synthesis is performed in 3 main steps:

• Generation of monodisperse droplet
• Droplet spacing
• Droplet solidification by UV irradiation

The UV-crosslinking of microparticles processis splitted in three steps: droplet generation, droplet spacing and microcapsule polymerization.

Droplet generation

For droplet generation, two pressure-based controllers (Flow EZ, 2 bar) are connected to two P-CAP reservoirs containing the polymeric phase (dispersed phase) and distilled water with 1% Tween 20 (continuous phase). The reservoirs are connected to the inlets of the Raydrop using tubing, which passes through Flow Units M for flow rate measurement and control. The dispersed phase is injected through the inner capillary of the Raydrop with a flow rate of 5 µL/min, and the continuous phase is delivered to the Raydrop chamber at a flow rate of 25 µL/min. At the intersection between the two capillaries within the Raydrop, droplets are generated (figure 3). Using the above flow rates, droplets are produced with a frequency of 1 Hz and with an outer diameter of about 100 µm. 

Droplet spacing

Once generated, droplets exit the Raydrop device with decreased flow velocity (due to the difference between the inner diameter of the downstream channel of the Raydrop and the exit tubing). To avoid clogging during the ultraviolet crosslinking of microparticles, increasing the spacing between droplets is highly recommended. Spacing between droplets can be performed by injecting liquid into the outer tubing in a co-flow manner.

To do so, a spacing device (a T-junction) is added to the outlet tubing (figure 1). Distilled water with 1% Tween 20 (same as the continuous phase) is injected into the spacing device with a flow rate of 250 µL/min. 

Droplet polymerization: particle solidification

After being spaced, the droplets are exposed to UV light with an irradiance of 95-100 mW /cm², allowing droplet polymerization and subsequent particle solidification.