Nanoparticles generation in microfluidics: a review

Interest in biodegradable polymeric nanoparticles has been growing over the past decade. Indeed, they have many advantages, such as the possibility of combining different therapeutic substances, of protecting the active ingredients against alterations, of controlling the diffusion of substances in the body, of taking into account diagnostic means or even the possibility of complete sterilisation by filtration.

Generalities

Liposomes are often composed of one or more lamellar layers. These layers include a phospholipid bilayer that contains a small volume of aqueous liquid. The diameter of nanoliposomes can vary from a few tens of nanometers to hundreds of micrometers depending on the method used to generate them.

Phospholipids and lipids are the main components of liposomal membranes.They have the particularity of being soluble in various organic solvents and it is precisely for this property that most current methods use organic solvents to solubilise the lipids as the first step of the protocol.

The rise of microfluidics opens a new door for the preparation of lipid-based nanoparticles. Microfluidic methods have demonstrated their ability to control the mixing process of the organic and aqueous phases, but also the size and thus obtain a high monodispersity as well as the possibility to play on the design of the microfluidic chip to vary the composition and size of the obtained nanoliposomes.

Microfluidic chip designs for nanoparticles generation

There are different methods to produce nanoliposomes in microfluidics. These different methods result in different geometries of microfluidic chips. Some chips may also have a specific treatment, which will influence the size or composition of the nanoliposomes. In this section, we will therefore look at the different geometries that exist and examples from the literature to assess the resulting applications.

The herringbone micromixer

The herringbone micromixer chip allows the quick creation of a homogeneous fluid at the outlet of the chip inducing a chaotic flow. This geometry is therefore very suitable for the formation of nanoliposomes. In addition, the staggered herringbone design allows for a more flexible, precise and efficient mixing of two fluids.

Figure 1 : Principle of microfluidic mixing in herringbone channel and formation of liposomes. Organic water miscible solvent (ethanol) contains lipids forming liposomes while water phase contains water soluble components that are to be encapsulated. The mixing process is finished within millisecond and liposomes are formed by the self-assembled mechanism. Various linear injector syringe pumps controlled by computer are used to drive the mixing process. Extracted from Kotouček et al. Sci. Reports 10:5595 (2020).

To illustrate this design, the team of Leug et al. (2015) proposes a general method for encapsulating macromolecules (SiRNA) in a lipid nanoparticle system. They were able to generate nanoparticles from 40 to 140 nm in diameter using a herringbone micromixer.

The Y-shaped microfluidic device

This design allows two fluids (usually an aqueous solution containing surfactant and a lipid solution coupled to a solvent) to be brought into contact with each other and then allow nanoprecipitation to occur in the collection channel. Nanoprecipitation is the aggregation of nanoparticles in time and space.

Figure 2 : Principle of nanoparticle formation with a Y-shaped microfluidic device. a) Extracted from Therriault et al. Nat. Mater. 2003 and b) modified from DeMello. Nature. 2006.

The Roces et al. team (2020) used a Y-shape micromixer to develop lipid nanoparticles. This device was used to generate nanoparticles of 100 to 200 nm in diameter. The size of these nanoparticles is considerably reduced after filtration, making it possible to obtain nanolipids between 50 and 80 nm with a polydispersity index of 0.2.

The 3D coaxial capillary device

 These devices offer the possibility of creating 3D coaxial flows, which are essential for rapid and uniform mass transfer. The co-current capillary microfluidic device consists of two capillaries. A tapered glass capillary was inserted into another larger cylindrical capillary with a coaxial alignment. The inner fluid (aqueous solution containing surfactant) flows inside the inner cylindrical capillary, while the outer fluid (a lipid solution coupled to a solvent) flows between the inner and outer cylindrical capillaries.

Figure 3 : Schematic view of a 3D glass capillary coflow microfluidic nanoprecipitation platform for preparing nanoparticles with homogeneous size distribution through rapid mixing: microvortices or unsteady jetting. Extracted from Liu et al. Adv. Mater. 2015.

With this microfluidic device, Liu et al. published a paper in 2015 highlighting the generation of lipid nanoparticles between 70 and 280 nm in diameter with a polydispersity index close to 0.1. This size evolves as a function of the number of Renolds due to the presence of capillary.

The hydrodynamic flow focusing device

The hydrodynamic flow focusing microfluidic device allows for precise mixing of the reagents with short mixing times under laminar flow conditions, which enables the generation of, for example, monodisperse nanoparticles of adjustable size. It is possible to influence the size of the nanoparticles by controlling the mixing time.

Figure 4 : Illustration of nucleation and growth mechanism of nanoprecipitation along the focus mixing channel in a hydrodynamic flow focusing device. Extracted from Lababidi et al. Beilstein J. Nanotechnol. 2019.

Finally, Karnik et al. showed in 2008 that it is possible to generate nanoparticles by flow focusing in a highly monodisperse manner. Indeed, the team is able to produce nanoparticles of 31 nm in diameter (± 1 nm). Here, the size varies according to the composition of the nanoparticles.

There are many techniques, and therefore many geometries, that allow you to generate nanoparticles. The information presented here will help you choose your microfluidic device according to your applications and the size of the nanoparticles you wish to generate.

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