Abstract
A microfluidic device for production of uniform size capsules with a prescribed membrane thickness is described. It is versatile, novel and suitable for various polymerization reactions. Parameters such as polymerization time and reagent concentrations can be precisely tuned to control the membrane properties. The device features a part which allows to overcome the diffusion barrier by initiating interfacial polymerization via chaotic mixing. It also allows the termination of the reaction and the collection of the resulting capsules. We observe different typical dynamical phenomena occurring in capsules during their flow along the microchannel, namely wrinkling of the membrane, parachute and bullet shapes and bursting of the capsules due to strong hydrodynamical flow. In addition to production, the monitoring of capsule dynamics in flow gave a possibility to estimate the elastic surface modulus \(E_{{\rm S}}\) and the membrane thickness t. We found that \(E_{{\rm S}}\) can be as low as 6 × 10−3 N m−1 and that the thickness can be below 100 nm. This microfluidic device is therefore capable of producing uniform size capsule solutions with suitable membrane properties for the controlled release of drugs, and as a model system of red blood cells for microhydrodynamics experiments.
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Notes
In small capillaries the mean velocity of blood reaches values up to 1 mm s−1 (Tortora and Derrickson 2012). The RBC size is close to the vessel size, and the capillary number can be as high as \(Ca=\frac{\eta U}{E_{{\rm s}}}\sim 0.2\), where U is the mean velocity of the blood flow. The membrane is deformed, and the RBCs adopt a parachute shape.
In the aorta, the diameter is approximately 2.25 cm and the mean velocity of the blood reaches 40 cm s−1 (Tortora and Derrickson 2012). Considering a Poiseuille flow for such vessel size and low velocity values, the shear rates can reach up to 150 s\(^{-1}\) near the vessel wall. For this shear rate and the plasma viscosity, we find the capillary number \(Ca=\frac{\eta \dot{\gamma } R}{E_{{\rm S}}}\sim 0.24\), so the membrane is easily sheared by the flow and must exhibits a SW motion.
This values are obtained for high pressures in the PDMS microstructure (\(\approx\)1 atm) It is possible to reduce again these times by increasing the pressure drop up to 2 atm which is a limit for PDMS microchannels. It is easy to increase the polymerzation time above one minute by decreasing the pressure on the two side microchannels that supplies the chaotic mixer with monomers.
We should take into account the fact that the presence of the drop compresses the two fluid layers, thus reducing their thickness. But as we consider a very wide channel the magnitude order of the result stays the same. This is not the case when the drop size is comparable with the channel size as in the work by Chu et al.
We find this value via the Stokes–Einstein equation for a viscosity of 250 mPa s and the hydrodynamic radius of sucrose which is a molecule with similar size as acyl chloride, \(R_{H} \approx\) 5 × 10−10 m.
Depending on the recepe, the diffusion coefficient of the amine monomer through the membrane can span a wide range of values from 10\(^{-15}\) up to 10\(^{-11}\) m\(^{2}\) s\(^{-1}\), see Perignon et al. (2015).
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This work is partially supported by Grant from the German–Israel Foundation.
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Gallé, N., Steinberg, V. On-chip encapsulation via chaotic mixing. Microfluid Nanofluid 20, 156 (2016). https://doi.org/10.1007/s10404-016-1820-4
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DOI: https://doi.org/10.1007/s10404-016-1820-4