Fusion of polymer-coated liposomes and centrifugally spun microfibers as hybrid materials to enhance sustained release
Abstract
Liposomes are employed for the delivery of molecular cargo in several classes of systems. For instance, the embedding of loaded liposomes in polymeric fibrous scaffolds has enabled the creation of hybrid materials that mimic biological membranes. Liposomes with unmodified surfaces have been predominantly integrated into fibers, which leads to instabilities due to interfacial incompatibility. In addition, electrospinning has been almost exclusively employed for fiber fabrication, which limits the potential for scale-up production. Here, we present the fabrication of hybrid biomimetic materials by fusing polymer-coated liposomes to force-spun microfibers to increase the stability of the hybrid materials and enhance the sustained release of the cargo. L-α-Phosphatidylcholine liposomes were coated with chitosan or polyethylene glycol (PEG). The nano-differential scanning calorimetry results confirm that polymer coating does not affect the phase transition temperature (Tm) of the liposomes, where only the model drug, quercetin, reduced Tm. Centrifugal spinning was employed to fabricate hydrophobic polycaprolactone (PCL) microfibers at various polymer concentrations and using various solvents and spinning parameters to increase the yield at the lowest fiber diameter. The highest microfiber production rate obtained occurred at a 20% (w/v) PCL concentration in 50 : 50 (v/v) chloroform and methanol solution with an average fiber diameter of 584.85 ± 26.30 nm. The non-chemical fusion of the polymer-coated liposomes and the fibrous scaffolds was promoted by immersion at T > Tm, under ultrasonication. We hypothesize that the fusion is driven by hydrophobic interactions between the liposomes and the fibers, which merge the materials through the lipid bilayer. The fused hybrid material solved the burst release problem observed when adhering plain liposomes to nanofibers. Both PEG and chitosan yielded a sustained release, where the release rate with the former was faster. These results demonstrate that the fusion of polymer-coated liposomes and microfibers enables more effective blending of the loaded carriers into the polymer microfibers. Ultimately, the fused liposome/microfiber hybrids are stable matrices and enhance the sustained release of molecular cargo.
Introduction
Over the last four decades, the use of liposomes has greatly expanded into ever more complex molecular delivery systems.1,2 A wide range of carrier ensembles have utilized vesicles of various chemistries, particle sizes, charges, and surface functionalization. However, their implementation has generally been challenged by the low stability of the self-assembled amphiphilic membranes and the limited resilience of their structure throughout the processes required for their incorporation into practical formulations. Despite the successful implementation of a few liposomal suspensions, several opportunities remain for the development of next-generation solutions to address the hurdles that hinder a broader translation of these versatile materials.
On the other hand, polymeric fibers with nano- and micro-scale size features have been used as delivery systems on their own by loading molecular cargo into their porous structure, Fig. 1a.3 Their tunable porosity, high surface-to-volume ratio, and release rates make them interesting alternatives for molecular release. Three fiber production techniques are available, namely, electrospinning, centrifugal, and electro-centrifugal spinning.4 Electrospinning is the oldest and most widely used technique, based on the electrostatic pulling of polymers out from solution. Centrifugal spinning enables the high-throughput production of polymeric fibrous scaffolds or mats through the application of centrifugal forces, as depicted in Fig. 1b. The fiber production rates with centrifugal spinning exceed 1 g min−1, which is significantly higher than that of equivalent electrospinning devices, 0.1 g min−1.5 Moreover, centrifugal spinning enables the production of high-quality fibrous mats from either polymeric solutions or melts, as well as from conductive and non-conductive polymers.6,7 The surface morphology and the mechanical properties of the obtained fibers are determined by the polymer viscoelasticity, solvent surface tension, solvent evaporation rate, spinneret angular velocity and aspect ratio, spinneret orifice distance to the collector, orifice radius and orientation, and solvent fill volume.4,5 Fibers produced by electrospinning or centrifugal spinning have both been reported to be directly loaded with molecular cargo during and after the spinning process. Utilizing the porosity and swelling of the polymer materials, molecules have been embedded within the fibers for the subsequent release. Nevertheless, the release kinetics profiles obtained with these approaches are generally limited to relatively short dose periods for a small set of molecular cargo chemistries.
The loading of liposomes into fibrous scaffolds has been exploited to combine the benefits of both the liposomal and polymer carriers (Fig. 1a).8 Most reports describe the combination of plain liposomes and electrospinning, where the lipids are dissolved along with the polymers before the application of the electromagnetic field. However, liposome loading into fibers is challenged by aggregation, degradation, hydrolysis, and phospholipid oxidation.9–11 These challenges have been tackled to some extent through optimization of the lipid-to-polymer ratio and the spinning parameters. In contrast, only one study was found on the use of centrifugal spinning for fiber fabrication and subsequent adhesion of liposomes for the release of molecular cargo. Rampichová et al. prepared fibrous scaffolds of polycaprolactone (PCL) by centrifugal spinning with adhered liposomes.12 The problem is that the authors documented an uncontrolled burst release (∼15 min) of the incorporated molecule. Further work was suggested to fabricate more stable materials that can sustain the release of cargo at a constant rate for longer.
In this work, we present the fusion of polymer-coated liposomes with polymeric fibrous scaffolds (Fig. 1b) through a scalable procedure that merges centrifugal spinning and fiber immersion (Fig. 1c). We propose that the fusion of the polymer-coated liposomes and the microfibers is driven by hydrophobic forces during immersion of both materials under ultrasound at a temperature over the phase transition temperature of the liposomes.
No reports were found on the combination of fibers fabricated by centrifugal spinning with polymer-coated liposomes. Liposomes conformed by l-α-phosphatidylcholine (PC) vesicles and cholesterol are loaded with quercetin (QR) as a model hydrophobic drug and cetyltrimethylammonium bromide (CTAB) as a bioenhancer. Loaded liposomes were coated with chitosan or polyethylene glycol. The polymer-coated liposomes are intended to stabilize the lipid bilayer during fusion with hydrophobic PCL microfibers. The hybrid materials were characterized, and their drug release kinetics were compared against plain and coated liposomes. QR is a naturally occurring antioxidant flavonoid with a variety of biological activities, including anticancer, anti-inflammatory, cardiovascular disease prevention, and hepatoprotective activity.13 However, the therapeutic use of QR is limited due to its poor aqueous solubility, chemical instability, and low bioavailability, which greatly restricts its therapeutic potential as a functional active ingredient.14 Encapsulating phytochemicals in liposomes remains cumbersome.15 Therefore, the hybrid materials presented here are proposed as a framework for the encapsulation and subsequent sustained release of challenging molecular cargo via a scalable fabrication process.
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Experimental section
Quercetin (>95% HPLC), l-α-PC (≥95% phosphatides, soybean, powder), cholesterol (>98%, ovine wool), cetyltrimethylammonium bromide (CTAB), 190–310 kDa chitosan, polyethylene glycol (PEG)-4000, 80 kDa polyvinyl alcohol (PVA), 80 kDa polycaprolactone (PCL), sodium acetate trihydrate, glacial acetic acid, sodium hydroxide, dimethylformamide (DMF), tetrahydrofuran (THF), dimethyl sulfoxide (DMSO), trifluoroacetic acid (TFA), HPLC-grade chloroform and HPLC-grade methanol were supplied by Sigma-Aldrich Corp., USA, and used as received. Spectra/Por® 3 dialysis tubing was purchased from Spectrum Laboratories, Inc., USA.
Ahmed M. Agiba, Luis Gerardo Rodr´ıguez Huerta, Nicolas A. Ulloa-Castillo, Francisco J. Sierra-Valdez, Saeed Beigi-Boroujeni, Omar Lozano and Alan Aguirre-Soto, Fusion of polymer-coated liposomes and centrifugally spun microfibers as hybrid materials to enhance sustained release, Cite this: DOI: 10.1039/d4na00835a, Received 7th October 2024, Accepted 23rd December 2024, DOI: 10.1039/d4na00835a, rsc.li/nanoscale-advances