Uniform and stable hydrogel-filled liposome-analogous vesicles with a thin elastomer shell layer
Graphical abstract
Introduction
Liposomes, which are phospholipid bilayered vesicles, have been studied regarding encapsulation and controlled drug delivery because they can load both hydrophilic molecules in the interior aqueous phase and hydrophobic molecules within the lipid bilayer [1], [2]. It has been well known that these structures can also protect encapsulated molecules from degradation and from organs passive targeting [3], [4], [5], [6]. Hence, they have been widely investigated as outstanding carriers for therapeutics. For example, insulin encapsulated in liposomes was observed to be protected from enzymatic attacks and immune recognition because the charged lipid bilayer blocks the permeation of biomacromolecular toxins [7]. Additionally, liposome-encapsulated potent antiviral drugs, such as SPC3 for HIV-infected patients, were protected from lymphocytes and macrophages and were thus able to retain their original activity [8]. However, despite their excellent applicability, liposomes suffer from an intrinsic structural inability [9], [10]. Membrane deformation, which commonly occurs during the break-up or fusion of liposome particles, results in either sharp increases in particle size or substantial loss of entrapped materials [11], [12].
To overcome this limitation, versatile solutions have been suggested. Liposomes structural stability can generally be enhanced by altering their lipid composition. For example, the incorporation of a reinforcing compound, such as cholesterol [13], [14] and nano species[15], [16], [17], increases the transition temperature of lipid layers, improving their mechanical tolerance against external stresses, including heat, osmolality, and pH changes. Furthermore, crosslinking the lipid layer imbues the lipid layer with viscoelastic properties, thereby improving their structural stability [18], [19]. Recent studies have also reported that hybridisation with an amphiphilic block copolymer makes the liposome both mechanically robust and chemically versatile, which are critical characteristics for improving shell permeability [20], [21], [22], [23]. Although these studies have thoroughly demonstrated that improving the lipid layer through physical hybridisation or chemical treatment can enhance liposomes structural stability, some leakage of the entrapped materials still occurs after storage for long periods.
In this study, we introduce a microfluidic approach to fabricate structurally stable liposome-analogous vesicles consisting of a hydrogel-filled core and a lipid bilayer that is hybridised with polyurethane (PU) elastomer. To create these vesicles, we prepare monodisperse water-in-oil-in-water (W/O/W) double emulsion templates that can be directly generated in capillary microfluidic channels. Photo-polymerisation of the core and the PU precursor in the lipid bilayer produces novel hydrogel-filled vesicles. In this vesicle system, the hydrogel immobilises the core, and the PU/lipid shell provides mechanical stability. The presence of the PU film in the lipid shell was observed to be essential in our study because it imparts viscoelastic resilience to the final vesicle (Fig. 1). Finally, we also experimentally demonstrate that our vesicle system can play an important role in regulating the permeation of small molecules through lipid films.
Section snippets
Materials
1,2-Dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) was supplied by Doosan Biotech (Korea). Poly(glycerin)-b-poly(ε-caprolactone) (PG-b-PCL) was supplied from SK Bioland Co. (Korea). Mn of PG-b-PCL was 13,500 g mol1 with a polydispersity index of 2.2. The block ratio between PG and PCL was 1.6. Polyethylene glycol-b-polylactic acid (PEG5000-b-PLA5000) were purchased from Gelest (USA). 1,2-Dioleoyl-sn-glycero-3-phosphoethanolamine (DOPE), polyvinyl alcohol (PVA, Mn = 13,00023,000 g mol1, 8789%
Results and discussion
The monodisperse W/O/W double emulsion templates used in our study were produced via coaxial jetting in the capillary-based microfluidic device, as shown in Fig. 2. The two round capillaries were arranged end-to-end within a square capillary. After ensuring good coaxial alignment, the inner fluid was pumped through the tapered round capillary, whereas the middle fluid, which was immiscible with the inner and outer fluids, flowed through the square capillary in the same direction. The outer
Conclusion
In summary, we have introduced a new type of uniform liposome-analogous vesicle. To synthesise these vesicles, we produced monodisperse W/O/W double emulsion templates in a capillary-based microfluidic device. Photo-polymerisation then transformed the templates into vesicles consisting of a PEG hydrogel core and a PU/DPPC shell. The formation of a PU elastomer film in the shell was directly characterised by visualising the PU/DPPC layer with a CLSM. Our additional evaluation of the shell
Acknowledgements
This work was supported by a grant of the Korea Healthcare Technology R&D Project, Ministry of Health & Welfare, Republic of Korea (Grant no.; A103017) and by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (No. 2008-0061891). M. Seo and A. Byun equally contributed to this work.
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