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Electrosprayed core–shell polymer–lipid nanoparticles for active component delivery

This content has been downloaded from IOPscience. Please scroll down to see the full text. 2013 Nanotechnology 24 465604 (http://iopscience.iop.org/0957-4484/24/46/465604) View the table of contents for this issue, or go to the journal homepage for more

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IOP PUBLISHING

NANOTECHNOLOGY

Nanotechnology 24 (2013) 465604 (9pp)

doi:10.1088/0957-4484/24/46/465604

Electrosprayed core–shell polymer–lipid nanoparticles for active component delivery Megdi Eltayeb1 , Eleanor Stride2 and Mohan Edirisinghe1 1

Department of Mechanical Engineering, University College London, Torrington Place, London WC1E 7JE, UK 2 Institute of Biomedical Engineering, Department of Engineering Science, University of Oxford, Old Road Campus Research Building, Headington OX3 7DQ, UK E-mail: [email protected]

Received 14 June 2013, in final form 7 August 2013 Published 28 October 2013 Online at stacks.iop.org/Nano/24/465604 Abstract A key challenge in the production of multicomponent nanoparticles for healthcare applications is obtaining reproducible monodisperse nanoparticles with the minimum number of preparation steps. This paper focus on the use of electrohydrodynamic (EHD) techniques to produce core–shell polymer–lipid structures with a narrow size distribution in a single step process. These nanoparticles are composed of a hydrophilic core for active component encapsulation and a lipid shell. It was found that core–shell nanoparticles with a tunable size range between 30 and 90 nm and a narrow size distribution could be reproducibly manufactured. The results indicate that the lipid component (stearic acid) stabilizes the nanoparticles against collapse and aggregation and improves entrapment of active components, in this case vanillin, ethylmaltol and maltol. The overall structure of the nanoparticles produced was examined by multiple methods, including transmission electron microscopy and differential scanning calorimetry, to confirm that they were of core–shell form. (Some figures may appear in colour only in the online journal)

1. Introduction

The use of a combination of polymers and lipids results in the formation of nanoparticles with a hydrophilic core and a hydrophobic shell. Both water soluble and insoluble active components can be encapsulated, including poorly soluble substances such as fatty acids, carotenoids, and phytosterols [9]. The development of novel polymer–lipid nanoparticles for active component encapsulation with predictable and controlled properties is essential to meet the requirements of food applications. Furthermore, it is crucial to select the appropriate encapsulation technique based on the nature of the core–shell particles, physicochemical properties of the materials, and required size. Conventional techniques for producing polymer–lipid nanoparticles are relatively complex, usually requiring a two or more step formulation process: firstly, development of nanoparticles of the core material (polymeric), and secondly, encapsulation of the core

Nanotechnology has emerged as one of the most promising scientific fields of research in recent decades. It concerns the understanding and manipulation of materials at dimensions of ∼1–100 nm. In the past few years, nanoparticlecontaining foods have been approved for use or entered experimental development [1–4] for example to deliver bioactive ingredients [5, 6]. Polymeric and lipid nanoparticles represent the main classes of carrier system capable of efficiently encapsulating a variety of active components to protect them from unfavourable environmental conditions by restricting the transfer of gases and maintaining the pH [7, 8]. This enables increased storage periods, and improves bioavailability by releasing active components at a sustained rate. 0957-4484/13/465604+09$33.00

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c 2013 IOP Publishing Ltd Printed in the UK & the USA

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2. Experimental materials and methods

nanoparticles by the shell (lipid) [10, 11]. This often results in poor control over the final nanoparticle physicochemical structure. More details on active component encapsulation can be found in the review by Ezhilarasi et al [12]. Among some of the technologies developed to prepare polymer–lipid particles with well-defined properties, electrohydrodynamic (EHD) processing has emerged as a single step technique that offers excellent control over particle characteristics [13]. EHD processing has been used to produce nano- and micrometre-size particles for a wide range of applications [14–20]. Recently, it has been shown that solid lipid nanoparticles with sizes of less than 100 nm can be generated using this method [21]. In this study, we used ethylcellulose (EC) as the polymer to form the core of the nanoparticles; and stearic acid (SA) as a model lipid to form the lipid monolayer at the shell of the EC core. Since EC polymer has been approved by the Food and Drug Administration (FDA) for medical and food applications (i.e. flavouring compounds, vitamin and mineral tablets) and SA is a common fatty acid, a natural lipid extracted from animals, we expect the polymer–lipid hybrid nanoparticles should be biocompatible, biodegradable, and potentially safe as active component carrier in food applications. In this work, we report a platform to engineer ≤100 nm targeted polymer–lipid hybrid nanoparticles by EHD. These nanoparticles comprise distinct functional components: polymeric (EC) core, which can encapsulate dosages of water soluble material and release them at a sustained rate [22–26]; and lipid (SA) in the shell, which can prevent the carried agents from freely diffusing out of the nanoparticles and reduce water penetration into the nanoparticles, thereby enhancing active component encapsulation efficiency, biocompatibility and decrease the rate of active component release from the nanoparticles [27]. Due to its hydrophobic nature, SA reduces active component (flavour) dissolution and release and slows the release kinetics at higher SA levels [28]. Most flavour compounds are chemically unstable and highly volatile as a result of chemical interactions, oxidation and volatilization. Hence, encapsulation of flavour compounds is essential to stabilize them and initiate their release when required [29]. To test these hypotheses, vanillin (VAN), ethylmaltol (EMA), and maltol (MA), widely used in food industry, were selected as a model hydrophilic component for encapsulation in the polymer–lipid hybrid nanoparticles. The release of the active components are not presented in this paper, it is the subject of a current study which is in progress at present. We show that polymer–lipid core–shell nanoparticles composed of stearic acid and ethylcellulose and containing model active components can be produced in a one step process using EHD techniques. The conditions under which a stable cone-jet was formed were identified and by varying the EHD processing parameters, nanoparticles with a tunable size less than ≤100 nm were produced.

2.1. Materials The model active components used were VA, EMA, and MA. The encapsulating materials were stearic acid (SA) and ethylcellulose (EC) in powder form. 95% (vol) ethanol or doubled distilled water (DDW) was used as a solvent. All materials were purchased from Sigma-Aldrich (Poole, Dorset, UK). All the experiments were repeated five times. 2.2. Electrohydrodynamic processing A schematic of the EHD processing apparatus is shown in figure 1. A solution of SA, EC and active component in ethanol with a ratio of 40%, 10%, and 17% (w/v) was prepared at 45 ◦ C and passed through a stainless steel needle (the inner and outer diameters of the needle were 450 µm and 870 µm, respectively). The solution flow rate into the EHD apparatus was controlled using a high precision syringe pump (PHD 4400, Harvard Apparatus, Edenbridge, UK) and maintained at 10 µl min−1 . An electrical potential difference of between 8 and 19 kV was applied between the needle and a ground electrode using a high voltage electrical power supply (Glassman Europe Ltd, Tadley, UK). The nanoparticles were collected on a petri dish containing DDW, which was kept at an approximate distance of 200 mm from the stainless steel needle tip, then transferred to a microscopic slide for observation. The EHD spraying process was observed using a video camera (Leica S6D JVC-colour) with different magnifications. The collected nanoparticles were placed in a freeze dryer, Mini Lyotrap (LTE Scientific Ltd, Oldham, UK) to facilitate the removal of residual ethanol and moisture. 2.3. Scanning electron microscopy (SEM) The morphology of the nanoparticles was assessed using SEM (Model JEOL JSM 3600, SEM). Nanoparticles were vacuum sputter coated with gold at 40 mA for 120 s, prior to observation, The average nanoparticle size was determined, using ImageJ software (National Institutes of Health, Maryland, USA) from ∼300 particles for each set of processing conditions. 2.4. Transmission electron microscopy (TEM) TEM images of the nanoparticles were also obtained (JEOL-1010 TEM, Tokyo, Japan) at an accelerating voltage of 80 kV. One millilitre of the nanoparticle solution was spread onto a copper grid for observation. The nanoparticle samples were then stained with a 2% solution of uranyl acetate for 30 s and allowed to dry. Samples of EHD sprayed nanoparticles were also placed in a tube, containing 10 ml of DDW at ambient room temperature for 90 days, and then observed again using TEM. 2

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Figure 1. (A) EHD spraying apparatus and the mechanism of droplet formation. (B) The effect of varying the voltage on the formation of the cone-jet. Jet formation is shown at (1) 12–14 kV, and (2) 16–19 kV. (C) TEM images of: (1) nanoparticles (SA/EC) only, and (2) nanoparticles (SA/EC) encapsulating VA. (D) Size distribution of nanoparticles containing VA. Average size is 65 ± 7 nm. Scale bar 50 nm. AC represents active component.

obtained by averaging 256 scans at a resolution of 1 cm−1 . Samples of EHD sprayed nanoparticles were stored dry for 90 days at the ambient temperature (25 ± 2 ◦ C), and then observed again using FT-IR.

2.5. Active component entrapment efficiency The nanoparticle solution was centrifuged (B4i Jouan Centrifuge) at 4000 rpm for 10 min at 25 ± 2 ◦ C. The amount of active component released was measured using UV spectrophotometry (Perkin Elmer, Lambda 35, UV/vis spectrophotometer). A calibration curve of various active component concentrations (5–40 ppm) versus absorbance was plotted. The amount of active component released was subtracted from the total amount of active component in the nanoparticles to calculate the amount encapsulated according to:

2.7. Differential scanning calorimetry (DSC) Differential scanning calorimetry (DSC) was performed on samples of nanoparticles with and without active components to determine whether there were any interactions with the core–shell components. In order to determine the phase transition temperatures of the VA-SA/EC, (10–20 mg) dried samples of VA-SA/EC were placed in open aluminum pans. A reference pan was filled with 100 µl of distilled water. The pans were then thermally scanned over a temperature range of 20–300 ◦ C at 10 ◦ C min−1 heating rate using a Netzsch STA 449C Jupiter Calorimeter, under helium purge, then allowed to cool down to 20 ◦ C to observe exothermic and recrystallization phenomena. Melting endotherm, and temperature (◦ C) data were generated using Universal analysis Proteus software (NETZSCH, Germany).

{Total amount of active component in the electrosprayed nanoparticles − Amount of active component in the supernatant} × {Total amount ofactive component in the electrosprayed nanoparticles}−1 × 100%. 2.6. Fourier transform infrared (FT-IR) spectroscopy analysis The chemical structure of obtained nanoparticles was characterized by Fourier transform infrared spectrometer (FT-IR). FT-IR spectra were measured using Perkin Elmer, 2000 FT-IR spectrometer. 2 mg powdered samples of VA, SLN and VA-SLN, were individually mixed with potassium bromide (KBr) and pellatalized using a hydraulic press. Potassium bromide being transparent to IR furnished spectra for each of the component studied. The spectra were recorded in the range of 400–4000 cm−1 . The FT-IR spectra were

3. Results and discussion As shown in figure 1(A), the EHD spraying apparatus contains four main components: high voltage electrical power supply, earthed collector, high precision syringe pump, and a stainless steel needle. EHD processing parameters such as applied voltage, needle diameter, and flow rate, in addition to the concentrations 3

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is depicted in figure 1(B). As the applied voltage increases, EHD jetting modes change significantly [30]. In this study when the voltage was increased from 8 to 14 kV, a cone-jet was formed at 10 µl min−1 (figure 1(B)), which is essential for generating uniformly sized and stable particles with sizes below 100 nm [31]. Further increasing the voltage (16 kV) gave rise to multiple jets (figure 1(B)) and above 19 kV unpredictable, unstable multi-jets formed [32]. Images of the nanoparticles formed under ambient conditions are shown in figure 2(A). Under an applied voltage of 12–14 kV, the nanoparticles had an average diameter of 62 ± 14 nm (figures 1(D) and 2(B)). Particles prepared with voltages of 16 kV were less uniform in size and shape, and had an average diameter of 66 ± 17 nm (figure 2(B)). The lower applied voltage of 12 kV provided a more stable cone-jet and correspondingly resulted in better process control. The mechanism of nanoparticle formation is governed by many factors. In addition to the selection of an appropriate applied voltage (assuming a suitable conductivity of the sprayed solutions), the travelling distance of the droplets is also key for achieving uniform nanoparticles. Further improvements may be gained by using a closed chamber or a vacuum to avoid the negative influence of air currents and facilitate solvent evaporation [33]. TEM images of the nanoparticles prepared from a cone-jet with and without active components are shown in figure 1(C). Here, all the nanoparticles exhibited similar levels of contrast, indicating a layered structure. Figures 3(A) and (B) show the assembly and process of EHD preparation of core–shell nanoparticles (A) with no active component, (B) encapsulating an active

Figure 2. SEM images of nanoparticles prepared under ambient conditions (A) at 12–14, 16, and 19 kV; (B) corresponding size distributions of nanoparticles prepared. Flow rate used was 10 µl min−1 . Polydispersivity index values are 23, 26 and 15 at 12–14 kV, 16 kV and 19 kV, respectively.

and chemical composition of the EHD spraying solution, influence the spraying process and the properties of the final products. Of these, the applied voltage is a key factor. This

Figure 3. EHD sprayed core–shell polymer-shell nanoparticle assembly and process; (A) nanoparticle without, and (B) with encapsulated active component. AC represents active component. 4

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Figure 4. Absorption spectra of VA, EMA, and MA; (a) at different concentrations (1–40 ppm), (b) calibration curves, and (c) active component release from nanoparticles.

Figure 5. Schematic and corresponding TEM image of nanoparticles: (A) immediately after collection and before active component release, (B) after active component release. Scale bar 100 nm.

component; the solution jet is accelerated towards the collector, and ethanol evaporates gradually, resulting in the formation of nanoparticles, with a layered structure.

Figures 3(A) and (B) show the nanoparticles as white/black domains (shell–core structures) because the samples were pretreated by a negative staining technique. Water soluble 5

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Figure 6. Comparison of the FT-IR spectra of: (A) nanoparticles containing SA/EC only (for day 0 only), and nanoparticles (SA/EC) encapsulating active components (for day 0, and 90) (B) VA, (C) EMA, and (D) MA.

2% uranyl acetate stains the area not occupied by the nanoparticles. Consequently, the electron beam only goes through the areas where the nanoparticles are, as they cannot pass through the areas where the heavy metal is. Figure 3(A) shows two layer (shell–core) structures which are attributed to SA and EC respectively. However, figure 3(B) shows an extra inner layer, attributed to the active component, encapsulated within the core (EC).

The overall structure of the nanoparticles produced was tested by multiple measures to ensure that they were core–shell polymer–lipid nanoparticles containing active component rather than a homogeneous matrix. During the collection of the nanoparticles, some active component was also released into the DDW. The amount released of VA, EMA, and MA in the supernatant from the nanoparticle solutions was found to be 13, 13.2, and 10.3% respectively, 6

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meaning that 87, 86.8 and 89.7% of the active component VA, EMA, and MA (figure 4) respectively was encapsulated in the nanoparticles. This demonstrates the suitability of these polymer–lipid nanoparticles for active component encapsulation and delivery applications. Core–shell nanoparticles were stored in DDW for 90 days at the ambient temperature (25 ± 2 ◦ C), then a drop (∼1 ml) of the nanoparticle solution was again observed using TEM. As illustrated in figure 5, during the nanoparticle storage in DDW interactions between the nanoparticles and water molecules occur, due to hydrogen bonding. This results in penetration of the water into the nanoparticles augmented by hydrophobic repulsion from the solvent and favourable intermolecular interactions on the structure of the EC. Finally, the active component molecules are released into the DDW leaving hollow nanoparticles (figure 5(B)). On the basis of the observations described above, it is clearly evident that the one step single needle EHD has the ability to produce a core–shell polymer–lipid nanoparticle structure. FT-IR spectroscopy was used to determine the chemical interactions amongst the inert ingredients i.e. SA, EC, and the active component. Figures 6(A)–(D) show the IR spectra (day 0, and 90 in the case of figures 6(B)–(D)) for core–shell polymer–lipid nanoparticles with and without active component i.e. VA, EMA, and MA respectively. The IR spectra peaks of the latter (figure 6(A)) were located at 3478 and 2851 cm−1 which overlaps with the absorption of C–H vibration result by the stretching vibration of O–H group. Figures 6(A) and (D) show a peak between 3478 and 3472 cm−1 which represents the intra- and inter-molecular hydrogen bonding due to the –OH groups [34] present on the structure of the ethylcellulose [35]. The peak at ∼1716 cm−1 belongs to the stretching vibration of the carbonyl group (C=O) and the peak at 1474 cm−1 is the –CH2 bending peak, 1300, 1307 cm−1 represents C–H and C–C bending group and 722 and 892 cm−1 corresponds to rocking vibration and bending, which are all characteristic for chain of stearic acid [36]. The peak at around 1112, 1116 cm−1 is attributed to C–O stretching. Thus, overall, FT–IR spectra accommodate matching peaks of EC, SA and active components, which has no significant new peaks even after 90 days. There are small shifts in the peak positions, due to the interactions between the oxygen atoms of the (C=O) [36] of SA and the hydrogen atoms of the O–H and C–H of the active components i.e. EMA, MA [37]. Figures 7(A)–(D) show DSC measurements of the samples; SA/EC only, and SA/EC with active components (VA, EMA, and MA), heated in range of 20–300 ◦ C, and then cooled and reheated. Figure 7(A) shows the results for the SA/EC nanoparticles indicating crystalline structures. There are two melting peaks 68 ◦ C and 225 ◦ C, these two melting peaks represent SA and EC respectively. A change of peak shape was observed with the nanoparticle encapsulated active component VA, EMA, MA in range between 100 and 250 ◦ C in figures 7(B)–(D). This was not seen for the polymer–lipid nanoparticle (figure 7(A)). This indicates a decreased degree of crystallinity in the nanoparticles following active component encapsulation during the EHD

Figure 7. Comparison of the DSC melting curves of; (A) empty nanoparticles (SA/EC) only, and nanoparticles (SA/EC) encapsulating active components (B) VA, (C) EMA, and (D) MA. All curves were recorded with the same temperature calibration.

spraying process. This may be due to rapid evaporation of the ethanol during nanoparticle formation, which, decreased the time available for reorganization of the polymer molecules preventing crystallization [38, 39]. Also, it may also be explained by the miscibility of active component in the nanoparticles [39], or changed during the EHD spraying process into a more amorphous phase [40]. No extra peaks were observed in the DSC traces of the nanoparticles with active component.

4. Conclusions This study has demonstrated that core–shell polymer–lipid nanoparticles encapsulating an active component can be prepared by single needle EHD processing. It is clearly evident that the one step single needle EHD has the ability to produce a homogeneous core–shell polymer–lipid structure nanoparticle with good stability. The size and active component content of the nanoparticles could be controlled by varying the EHD parameters and solution properties. The study suggests that the lipid component (i.e. stearic acid) stabilizes the nanoparticles against collapse and aggregation and improves encapsulation. Furthermore, it has been demonstrated that EHD processing enables the 7

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polymer–lipid nanoparticle size to be tuned in a reproducible manner whilst allowing a narrow size distribution to be achieved.

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Electrosprayed core-shell polymer-lipid nanoparticles for active component delivery.

A key challenge in the production of multicomponent nanoparticles for healthcare applications is obtaining reproducible monodisperse nanoparticles wit...
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