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Electrospraying of environmentally sustainable alginate microbeads for cosmetic additives
Accepted Manuscript Electrospraying of environmentally microbeads for cosmetic additives
sustainable
alginate
Su Bin Bae, Hyeong Chan Nam, Won Ho Park PII: DOI: Reference:
S0141-8130(19)31319-4 https://doi.org/10.1016/j.ijbiomac.2019.04.058 BIOMAC 12134
To appear in:
International Journal of Biological Macromolecules
Received date: Revised date: Accepted date:
20 February 2019 8 April 2019 10 April 2019
Please cite this article as: S.B. Bae, H.C. Nam and W.H. Park, Electrospraying of environmentally sustainable alginate microbeads for cosmetic additives, International Journal of Biological Macromolecules, https://doi.org/10.1016/j.ijbiomac.2019.04.058
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Submitted to International Journal of Biological Macromolecules
Electrospraying of Environmentally Sustainable Alginate Microbeads for Cosmetic
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Additives
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Su Bin Bae, Hyeong Chan Nam, Won Ho Park*
Department of Advanced Organic Materials and Textile System Engineering, Chungnam
Corresponding author at: Department of Advanced Organic Materials and Textile System
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*
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National University, Daejeon 305-764, South Korea
Engineering, Chungnam National University, Daejeon 34134, South Korea Tel: +82 42 821 6613, E-mail address: [email protected] (W.H. Park)
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ABSTRACT: Polymer microbeads (MBs) for scrubbing additives have generally been prepared from non-biodegradable synthetic polymers. The worldwide pollution of the marine ecosystem by microplasics urgently demands novel environment-friendly MBs. In this study, Ca-alginate MBs were fabricated by electrospraying an aqueous alginate solution into
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distilled water containing calcium ions. The size and shape of the Ca-alginate MBs were controlled by electrospraying parameters, such as nozzle diameter and solution concentration.
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As the alginate concentration and needle diameter were increased, the size of alginate MBs
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was gradually increased, because of the higher mass flow rate. In addition, the adsorption and degradation behavior of alginate MBs were examined using model contaminants and sea
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water, respectively. In particular, alginate MBs rapidly degraded in sea water, due to the
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reversible ion-exchange reaction between Ca2+ in MBs and Na+ in sea water. Therefore, the electrosprayed Ca-alginate MBs offer a promising alternative for environment-friendly
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cosmetic additives.
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Keywords: alginate; microbeads; electrospray
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1. Introduction Polymer microbeads (MBs) are defined as spherical small plastic particles with size below 1 mm, and have been generally prepared from non-biodegradable synthetic polymers. These MBs have been used for scrubbing additives, such as cosmetics, toothpastes, and soaps.
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However, due to their small size, the MBs are not filtered during the wastewater disposal and are leaked into the sea. The MBs subsequently adsorb a variety of organic contaminants
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(polychlorinated biphenyl (PCBs), dichlorodiphenyltrichloroethane (DDT), polycyclic
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aromatic hydrocarbons (PAHs), and so on), under the marine environment [1,2]. Also, fish intake the MBs by mistaking them as food, and ultimately the MBs enter the marine food
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chain [3,4]. Recently, the use of non-biodegradable MBs has been banned in several countries.
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This pollution of the marine ecosystem urgently demands novel environment-friendly MBs as a substitute for non-biodegradable ones. As one of the alternatives, natural MBs, which
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originate from almond, oatmeal, coconut shells, and fruit seeds, are being studied. Also,
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biodegradable MBs fabricated from natural polymers, including alginate, starch, cellulose and chitin, are being considered.
Sodium alginate (SA) is anionic heteropolysaccharide containing blocks of (1,4)-linked β-
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D-mannuronate (M) and α-L-guluronate (G) residues, and is a non-toxic, biodegradable
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polysaccharide that is extracted from brown seaweeds and some bacteria. The SA is well known to form a physical hydrogel via ionic crosslinks by the divalent cations including Ca2+ [5-9]. The alginate hydrogels are biodegraded by the hydrolysis of glycosidic linkages, and are also degraded by the elution of divalent cations [10]. Alginate hydrogels are widely used for biomedical and environmental applications, such as drug delivery vehicles, cell encapsulation, and cosmetics, because they are mucous, cost-effective, non-toxic, and 3
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biocompatible. Electrospraying (or electrospray) technique is easily able to produce nano- and microparticles using the polymer solution or polymer melt in large quantities, and also control the shape and size of particles by tuning the processing parameters, such as nozzle diameter,
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solution viscosity, applied voltage, and flow rate [11-13]. In the solution electrospray, polymer solution is sprayed as micro-drops into a collecting bath at voltages above its surface
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tension.
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In this study, alginate MBs were fabricated by electrospraying an aqueous alginate solution into distilled water containing calcium ions. The effects of nozzle diameter and solution
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concentration on the size and shape of alginate MBs were investigated. In addition, the
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adsorption and degradation behaviors of alginate MBs were examined using model
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contaminants and sea water, respectively.
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2. Experimental 2.1. Materials
Sodium alginate (SA) (Mw=120,000-190,000 g/mol, M/G=1.56) was purchased from
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Sigma-Aldrich Co. (Saint Louis, USA). Calcium chloride dehydrate (purity: 71.0~77.6%)
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and phenanthrene (purity: 97%) were obtained from Sanchun Pure Chemical Co. (Pyeongtaek, Korea) and Acros Organics (Japan), respectively, and used as received without further purification. Sea water was freely supplied by Noeun Fish Market (Daejeon, Korea).
2.2. Preparation of alginate microbeads by electrospray In order to fabricate the alginate MBs crosslinked by Ca2+ ions, the SA and calcium 4
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chloride (CaCl2) were separately dissolved in distilled water for 24 h. The concentrations of SA varied from 3 to 9 wt%, when the CaCl2 was kept at a concentration of 2 wt%. The electrospray setup consisted of a syringe and needle (ID: 0.34~1.10 mm), a collecting coagulation bath, and a DC high-voltage generator (CPS-60K02VIT, Chungpa EMT, Korea).
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The SA solution was discharged using a syringe pump (EP-100, NanoNC, Korea). The optimum electrospraying conditions were an applied voltage of 9.5 kV, a working distance of
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10 cm (the distance between the needle tip and the collecting bath), and a solution flow rate
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of 5 mL/h. Figure 1 shows a schematic of the electrospray equipment for the preparation of alginate MBs.
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After the electrospraying, the resultant alginate MBs were maintained in the CaCl2 solution
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for 30 min in order to complete the crosslinking reaction, and were then filtered using 100mesh sieves. Alginate MBs were washed with distilled water three times to remove the
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uncrosslinked calcium ions, and were dried at room temperature (RT) for 48 h [14].
2.3. Adsorption capability of the alginate microbeads In order to evaluate the adsorption behavior of alginate MBs for hydrophobic organic
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pollutants in the sea water, phenanthrene was used as a model pollutant. Alginate MBs (50
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mg) were poured into 30 mL sea water/ethanol mixture (80/20, V/V) with a phenanthrene concentration of 100 μg/mL for specified periods of 0~48 h, and then the adsorption capability was measured using UV-Vis spectrophotometry (UV-2450, Shimadzu, Japan) [15,16]. Calibration curve was obtained by measuring the absorbance (λmax=252 nm) at the concentration range from 6.25 to 100 μg/mL of phenanthrene in sea water/ethanol mixture. Instead of sea water, the sea water/ethanol mixture was used because phenanthrene had a low 5
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solubility (1.20 mg/L at 25oC) in water [2,17].
2.4. Swelling test of alginate microbeads For swelling test, the dried alginate MBs (20 mg) were immersed into deionized water (3
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mL) under stirring (100 rpm) at 25oC, and the weight change of alginate MBs before and
calculated from the following equation (1). Qs = (Ws – Wd)/Wd
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100 (%)
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after immersion was weighed at a specified time of 0~48 h. The swelling ratio (Qs, %) was
(1)
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where Ws and Wd indicate the weight of swelled and dried MBs, respectively.
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2.5. Degradation behavior of the alginate microbeads
Alginate MBs (20 mg) were immersed into seawater (5 mL) under stirring (100 rpm) at
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25oC for specified periods of 0~120 h, and their morphology was then observed by optical
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microscope. Also, alginate MBs (100 mg) were immersed into seawater (25 mL) for specified periods of 0~28 d, and then the residual MBs were filtered using 100-mesh sieves, followed
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by centrifugation (3,000 rpm) twice for 5 min. The precipitated MBs were weighed after vacuum drying (OV-11, Jeio Tech, Korea) at 50oC for 24 h. The degree of degradation (Qd, %)
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according to immersion time was calculated from the following equation (2).
Qd = (Wo – Wd)/Wo
100 (%)
(2)
where Wo and Wd indicate the weight of the initial and degraded MBs, respectively.
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2.6. Characterization The size and shape of the electrosprayed or degraded alginate MBs were observed using an optical microscope (BX51, Olympus, Japan). The diameter of MBs was measured by a custom-code image analysis program (Scope Eye II, Korea), and was averaged from 30
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samples. Alginate MBs were ground in a mortar after immersion in liquid nitrogen for 1 min, and the internal morphology of alginate MBs was observed by field emission scanning
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electron microscope (FE-SEM; Merlin, Carl Zeiss, Germany) after Pt coating. Inductively
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coupled plasma atomic emission spectrometer (ICP-AES; Optima 7300 DV, Perkin Elmer, USA) with radial view type was used to measure the content of Ca2+ and Na+ ions of the
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electrosprayed or the degraded alginate MBs [18,19].
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3. Results and discussion
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3.1. Morphology of alginate microbeads with different electrospraying conditions When the SA solution was electrosprayed into the CaCl2 solution, alginate MBs were rapidly formed by ionic crosslinking reaction between alginate molecules and Ca2+ ions [20].
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Figure 2 shows optical microscope images and average particle sizes of the alginate MBs
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according to the SA solution concentration at needle diameter 0.7 mm. As the SA concentration was increased from 3 to 7 wt%, the average size of alginate MBs was gradually increased from 640 to 880 μm, because of the higher output rate of alginate [21,22]. The alginate MBs showed spherical morphology up to 7 wt%, but tear-shaped morphology was observed at 9 wt% concentration due to the higher solution viscosity, as shown in Figure 2A. In the case of tear-shaped beads, the particle size was calculated from the average value of the 7
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major axis and minor axis. Therefore, the 7 wt% SA concentration was chosen as an optimum concentration for the preparation of alginate MBs. Figure 3 shows the effect of needle diameter and drying on the average size of alginate MBs. As the needle diameter was increased from 0.35 to 1.1 mm, the average size of alginate
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MBs was also increased from 540 to 1120 μm because the output amount of SA was higher at larger needle size [23]. After drying, the size of as-sprayed alginate MBs was significantly
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decreased to less than half, because water molecules were evaporated from the MBs. When
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the dried alginate MBs were immersed into distilled water for 24 h, they swelled to much smaller size than that of the as-sprayed alginate MBs. In general, alginate MBs have different
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degree of swelling according to pH conditions. Alginate MBs are less swollen in the acidic
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pH values, because most carboxylic acid groups in MBs are not dissociated. In contrast, alginate MBs are highly swollen in the basic pH values, because most carboxylic acid groups
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in MBs are dissociated into carboxylate anions, resulting in higher electrostatic repulsion
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between the alginate molecular chains [24]. Therefore, the low degree of swelling for the dried alginate MBs might be caused by the weak acidic pH condition in this swelling
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experiment.
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3.2. Ca2+ content of alginate microbeads The Ca2+ content in the alginate MBs was obtained by subtracting the Ca2+ content in the residual CaCl2 solution from the initial Ca2+ content in CaCl2 solution, because the Ca2+ content of alginate MBs was difficult to directly measure. For this, the SA solutions (5 mL) with different concentrations ranging from 3 to 9 wt% were electrosprayed into 2 wt% CaCl2 solution (100 mL), and then filtered after 30 min. Subsequently, the residual CaCl 2 solution 8
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after the removal of as-sprayed MBs was measured by the ICP-AES. Figure 4 shows the Ca2+ content in the alginate MBs and Na+ content in the solution obtained from the ICP-AES measurement. As the SA concentration was increased from 3 to 9 wt%, the Ca2+ content of alginate MBs was increased from 5.47 to 17.8 mM, because the G block content of alginate
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crosslinked with Ca2+ was also increased [13]. By contrast, the Na+ content eluted into the solution was increased due to the ion exchange reactions between Ca2+ and Na+. From this 2+
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result, it was found that the ionic crosslinking reaction between Ca
and alginate molecules
reaction between Ca
2+
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successfully occurred during the electrospraying process [25]. Theoretically, the ion exchange and Na+ should occur in the ratio of 1:2. The measured Ca2+ content
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of alginate MBs was higher than the theoretical Ca2+ content, because alginate MBs
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contained non-crosslinked Ca2+, as well as crosslinked Ca2+ [14].
3.3. Swelling and degradation behavior of alginate microbeads
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Figure 5 shows the swelling ratio (%) of alginate MBs with immersion time in DI water.
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The alginate MBs were able to easily absorb water molecules, because of their hydrophilic character. The swelling ratio (%) of alginate MBs was abruptly increased to about 160% at 6
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h, and thereafter reached plateau region. Swelling phenomena were mainly attributed to the
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hydration of the hydrophilic groups of alginate, as well as the osmotic pressure, and persisted until the driving force (hydrophilicity and osmotic pressure) of swelling became equal to the restraining force (crosslinking density) [26]. On the other hand, when the alginate MBs were immersed in seawater containing a considerable amount of Na+, the ion exchange reactions between Ca2+ and Na+ reversibly occurred. Therefore, the gel-to-sol transition of alginate MBs was observed in the seawater, 9
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because Ca2+ ions crosslinked with the G block of alginate were substituted by Na+ ions [27]. Figure 6A shows the weight loss of alginate MBs in seawater for 28 days. The weight loss (%) was rapidly increased to 70% for 7 days, and thereafter gradually increased to 100% until 28 days. This weight loss was closely associated with the breakage of ionic crosslinking due to
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the Ca2+-Na+ exchange reaction. Figure 6B shows optical images of alginate MBs with different immersion times. The alginate MBs maintained spherical morphology up to 48 h,
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together with considerable swelling, although the weight loss (%) was about 40% at that time.
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After 48 h, the shape of MBs was steadily fractured into small pieces, due to a severe cleavage of crosslinks. Therefore, it was demonstrated that the alginate MBs have great
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potential for environment-friendly cosmetic additives.
3.4. Organic pollutant adsorption of alginate microbeads in seawater
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Alginate MBs are able to adsorb the organic contaminants in the sea through their surface
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and internal space [28]. Figure 7 shows the adsorption capability (%) of alginate MBs for phenanthrene in the sea water/ethanol mixture. The adsorption (%) showed a maximum value of approximately 32% at 20 min, and thereafter rapidly decreased to about 15%. This rapid
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decrease after maximum adsorption was attributed to a decrease in crosslinking density from
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the surface, resulting in severe swelling due to the penetration of water molecules. Phenanthrene molecules were desorbed from the MBs during the breakage of crosslinks and swelling. At the later stage, the adsorption (%) maintained a constant value of 15% up to 48 h. This might be associated with the internal adsorption by swelling and the adsorption by the increased surface area.
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4. Conclusions In this study, micron-sized alginate MBs were prepared by solution electrospraying, in which the ion-exchange reaction between sodium alginate and calcium chloride occurred. The morphology and size of alginate MBs could be controlled by processing parameters,
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including alginate concentration, and needle diameter during electrospraying. As the alginate concentration and needle diameter were increased, the size of alginate MBs was gradually
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increased, due to mass flow rate of alginate. Also, the swelling, degradation, and adsorption
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behaviors of alginate MBs were examined by UV-Vis spectrophotometry. The swelling ratio of alginate MBs was 160% in distilled water, and the alginate MBs were rapidly degraded in
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seawater, due to the reversible ion-exchange reaction between Ca2+ in MBs and Na+ in sea
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water. This behavior demonstrated that the alginate MBs have great potential for environment-friendly cosmetic additives. The adsorption of alginate MBs toward the model
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contaminant phenanthrene reached maximum value in early time, and thereafter rapidly
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decreased, due to the breakage of crosslinks and swelling of MBs. Therefore, the electrosprayed alginate MBs offer a promising alternative for non-biodegradable MBs,
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because of their cost-effectiveness and environmental compatibility.
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Acknowledgement:
This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (2018R1A2A2A05021100).
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Figure Captions
Figure 1. Schematic of the electrospray equipment for the preparation of alginate microbeads.
Figure 2. (A) Optical microscope images of alginate MBs (scale bar=500 μm); (B) Effect of
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SA concentration on the average sizes of alginate MBs.
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Figure 3. (A) Optical microscope images of as-sprayed, after dried, and swollen alginate
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microbeads (scale bar=500 μm), (B) Effect of needle diameter on the average size
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of as-sprayed, after dried, and swollen alginate microbeads.
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Figure 4. Ca2+ content in the alginate microbeads and Na+ content in the aqueous solution.
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Figure 5. Swelling ratio (%) of alginate microbeads in DI water.
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Figure 6. (A) Optical images, (B) Average particle size and (C) Weight loss (%) of alginate
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microbeads according to their immersion time in seawater.
Figure 7. Phenanthrene adsorption of alginate microbeads in seawater/ethanol mixture. Inset
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plot shows the calibration curve for phenanthrene.
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Electrospraying of Environmentally Sustainable Alginate Microbeads for Cosmetic Additives,
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Figure 2. Electrospraying of Environmentally Sustainable Alginate Microbeads for Cosmetic Additives, Su Bin Bae, Hyeong Chan Nam, Won Ho Park 18
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Electrospraying of Environmentally Sustainable Alginate Microbeads for Cosmetic Additives, Su Bin Bae, Hyeong Chan Nam, Won Ho Park
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Electrospraying of Environmentally Sustainable Alginate Microbeads for Cosmetic Additives,
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Electrospraying of Environmentally Sustainable Alginate Microbeads for Cosmetic Additives,
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Electrospraying of Environmentally Sustainable Alginate Microbeads for Cosmetic Additives,
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Electrospraying of Environmentally Sustainable Alginate Microbeads for Cosmetic Additives,
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Highlights Ca-alginate microbeads (MBs) for cosmetics were prepared via simple electrospraying. Size and shape of Ca-alginate MBs could be controlled by electrospraying parameters. Ca-alginate MBs rapidly degraded in sea water due to reversible ion-exchange reaction.
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Ca-alginate MBs offer a promising alternative for eco-friendly cosmetic additives.
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sustainable
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Su Bin Bae, Hyeong Chan Nam, Won Ho Park PII: DOI: Reference:
S0141-8130(19)31319-4 https://doi.org/10.1016/j.ijbiomac.2019.04.058 BIOMAC 12134
To appear in:
International Journal of Biological Macromolecules
Received date: Revised date: Accepted date:
20 February 2019 8 April 2019 10 April 2019
Please cite this article as: S.B. Bae, H.C. Nam and W.H. Park, Electrospraying of environmentally sustainable alginate microbeads for cosmetic additives, International Journal of Biological Macromolecules, https://doi.org/10.1016/j.ijbiomac.2019.04.058
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Submitted to International Journal of Biological Macromolecules
Electrospraying of Environmentally Sustainable Alginate Microbeads for Cosmetic
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Additives
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Su Bin Bae, Hyeong Chan Nam, Won Ho Park*
Department of Advanced Organic Materials and Textile System Engineering, Chungnam
Corresponding author at: Department of Advanced Organic Materials and Textile System
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*
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National University, Daejeon 305-764, South Korea
Engineering, Chungnam National University, Daejeon 34134, South Korea Tel: +82 42 821 6613, E-mail address: [email protected] (W.H. Park)
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ABSTRACT: Polymer microbeads (MBs) for scrubbing additives have generally been prepared from non-biodegradable synthetic polymers. The worldwide pollution of the marine ecosystem by microplasics urgently demands novel environment-friendly MBs. In this study, Ca-alginate MBs were fabricated by electrospraying an aqueous alginate solution into
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distilled water containing calcium ions. The size and shape of the Ca-alginate MBs were controlled by electrospraying parameters, such as nozzle diameter and solution concentration.
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As the alginate concentration and needle diameter were increased, the size of alginate MBs
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was gradually increased, because of the higher mass flow rate. In addition, the adsorption and degradation behavior of alginate MBs were examined using model contaminants and sea
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water, respectively. In particular, alginate MBs rapidly degraded in sea water, due to the
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reversible ion-exchange reaction between Ca2+ in MBs and Na+ in sea water. Therefore, the electrosprayed Ca-alginate MBs offer a promising alternative for environment-friendly
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cosmetic additives.
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Keywords: alginate; microbeads; electrospray
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1. Introduction Polymer microbeads (MBs) are defined as spherical small plastic particles with size below 1 mm, and have been generally prepared from non-biodegradable synthetic polymers. These MBs have been used for scrubbing additives, such as cosmetics, toothpastes, and soaps.
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However, due to their small size, the MBs are not filtered during the wastewater disposal and are leaked into the sea. The MBs subsequently adsorb a variety of organic contaminants
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(polychlorinated biphenyl (PCBs), dichlorodiphenyltrichloroethane (DDT), polycyclic
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aromatic hydrocarbons (PAHs), and so on), under the marine environment [1,2]. Also, fish intake the MBs by mistaking them as food, and ultimately the MBs enter the marine food
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chain [3,4]. Recently, the use of non-biodegradable MBs has been banned in several countries.
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This pollution of the marine ecosystem urgently demands novel environment-friendly MBs as a substitute for non-biodegradable ones. As one of the alternatives, natural MBs, which
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originate from almond, oatmeal, coconut shells, and fruit seeds, are being studied. Also,
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biodegradable MBs fabricated from natural polymers, including alginate, starch, cellulose and chitin, are being considered.
Sodium alginate (SA) is anionic heteropolysaccharide containing blocks of (1,4)-linked β-
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D-mannuronate (M) and α-L-guluronate (G) residues, and is a non-toxic, biodegradable
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polysaccharide that is extracted from brown seaweeds and some bacteria. The SA is well known to form a physical hydrogel via ionic crosslinks by the divalent cations including Ca2+ [5-9]. The alginate hydrogels are biodegraded by the hydrolysis of glycosidic linkages, and are also degraded by the elution of divalent cations [10]. Alginate hydrogels are widely used for biomedical and environmental applications, such as drug delivery vehicles, cell encapsulation, and cosmetics, because they are mucous, cost-effective, non-toxic, and 3
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biocompatible. Electrospraying (or electrospray) technique is easily able to produce nano- and microparticles using the polymer solution or polymer melt in large quantities, and also control the shape and size of particles by tuning the processing parameters, such as nozzle diameter,
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solution viscosity, applied voltage, and flow rate [11-13]. In the solution electrospray, polymer solution is sprayed as micro-drops into a collecting bath at voltages above its surface
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tension.
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In this study, alginate MBs were fabricated by electrospraying an aqueous alginate solution into distilled water containing calcium ions. The effects of nozzle diameter and solution
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concentration on the size and shape of alginate MBs were investigated. In addition, the
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adsorption and degradation behaviors of alginate MBs were examined using model
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contaminants and sea water, respectively.
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2. Experimental 2.1. Materials
Sodium alginate (SA) (Mw=120,000-190,000 g/mol, M/G=1.56) was purchased from
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Sigma-Aldrich Co. (Saint Louis, USA). Calcium chloride dehydrate (purity: 71.0~77.6%)
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and phenanthrene (purity: 97%) were obtained from Sanchun Pure Chemical Co. (Pyeongtaek, Korea) and Acros Organics (Japan), respectively, and used as received without further purification. Sea water was freely supplied by Noeun Fish Market (Daejeon, Korea).
2.2. Preparation of alginate microbeads by electrospray In order to fabricate the alginate MBs crosslinked by Ca2+ ions, the SA and calcium 4
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chloride (CaCl2) were separately dissolved in distilled water for 24 h. The concentrations of SA varied from 3 to 9 wt%, when the CaCl2 was kept at a concentration of 2 wt%. The electrospray setup consisted of a syringe and needle (ID: 0.34~1.10 mm), a collecting coagulation bath, and a DC high-voltage generator (CPS-60K02VIT, Chungpa EMT, Korea).
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The SA solution was discharged using a syringe pump (EP-100, NanoNC, Korea). The optimum electrospraying conditions were an applied voltage of 9.5 kV, a working distance of
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10 cm (the distance between the needle tip and the collecting bath), and a solution flow rate
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of 5 mL/h. Figure 1 shows a schematic of the electrospray equipment for the preparation of alginate MBs.
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After the electrospraying, the resultant alginate MBs were maintained in the CaCl2 solution
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for 30 min in order to complete the crosslinking reaction, and were then filtered using 100mesh sieves. Alginate MBs were washed with distilled water three times to remove the
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uncrosslinked calcium ions, and were dried at room temperature (RT) for 48 h [14].
2.3. Adsorption capability of the alginate microbeads In order to evaluate the adsorption behavior of alginate MBs for hydrophobic organic
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pollutants in the sea water, phenanthrene was used as a model pollutant. Alginate MBs (50
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mg) were poured into 30 mL sea water/ethanol mixture (80/20, V/V) with a phenanthrene concentration of 100 μg/mL for specified periods of 0~48 h, and then the adsorption capability was measured using UV-Vis spectrophotometry (UV-2450, Shimadzu, Japan) [15,16]. Calibration curve was obtained by measuring the absorbance (λmax=252 nm) at the concentration range from 6.25 to 100 μg/mL of phenanthrene in sea water/ethanol mixture. Instead of sea water, the sea water/ethanol mixture was used because phenanthrene had a low 5
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solubility (1.20 mg/L at 25oC) in water [2,17].
2.4. Swelling test of alginate microbeads For swelling test, the dried alginate MBs (20 mg) were immersed into deionized water (3
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mL) under stirring (100 rpm) at 25oC, and the weight change of alginate MBs before and
calculated from the following equation (1). Qs = (Ws – Wd)/Wd
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100 (%)
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after immersion was weighed at a specified time of 0~48 h. The swelling ratio (Qs, %) was
(1)
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where Ws and Wd indicate the weight of swelled and dried MBs, respectively.
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2.5. Degradation behavior of the alginate microbeads
Alginate MBs (20 mg) were immersed into seawater (5 mL) under stirring (100 rpm) at
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25oC for specified periods of 0~120 h, and their morphology was then observed by optical
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microscope. Also, alginate MBs (100 mg) were immersed into seawater (25 mL) for specified periods of 0~28 d, and then the residual MBs were filtered using 100-mesh sieves, followed
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by centrifugation (3,000 rpm) twice for 5 min. The precipitated MBs were weighed after vacuum drying (OV-11, Jeio Tech, Korea) at 50oC for 24 h. The degree of degradation (Qd, %)
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according to immersion time was calculated from the following equation (2).
Qd = (Wo – Wd)/Wo
100 (%)
(2)
where Wo and Wd indicate the weight of the initial and degraded MBs, respectively.
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2.6. Characterization The size and shape of the electrosprayed or degraded alginate MBs were observed using an optical microscope (BX51, Olympus, Japan). The diameter of MBs was measured by a custom-code image analysis program (Scope Eye II, Korea), and was averaged from 30
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samples. Alginate MBs were ground in a mortar after immersion in liquid nitrogen for 1 min, and the internal morphology of alginate MBs was observed by field emission scanning
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electron microscope (FE-SEM; Merlin, Carl Zeiss, Germany) after Pt coating. Inductively
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coupled plasma atomic emission spectrometer (ICP-AES; Optima 7300 DV, Perkin Elmer, USA) with radial view type was used to measure the content of Ca2+ and Na+ ions of the
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electrosprayed or the degraded alginate MBs [18,19].
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3. Results and discussion
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3.1. Morphology of alginate microbeads with different electrospraying conditions When the SA solution was electrosprayed into the CaCl2 solution, alginate MBs were rapidly formed by ionic crosslinking reaction between alginate molecules and Ca2+ ions [20].
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Figure 2 shows optical microscope images and average particle sizes of the alginate MBs
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according to the SA solution concentration at needle diameter 0.7 mm. As the SA concentration was increased from 3 to 7 wt%, the average size of alginate MBs was gradually increased from 640 to 880 μm, because of the higher output rate of alginate [21,22]. The alginate MBs showed spherical morphology up to 7 wt%, but tear-shaped morphology was observed at 9 wt% concentration due to the higher solution viscosity, as shown in Figure 2A. In the case of tear-shaped beads, the particle size was calculated from the average value of the 7
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major axis and minor axis. Therefore, the 7 wt% SA concentration was chosen as an optimum concentration for the preparation of alginate MBs. Figure 3 shows the effect of needle diameter and drying on the average size of alginate MBs. As the needle diameter was increased from 0.35 to 1.1 mm, the average size of alginate
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MBs was also increased from 540 to 1120 μm because the output amount of SA was higher at larger needle size [23]. After drying, the size of as-sprayed alginate MBs was significantly
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decreased to less than half, because water molecules were evaporated from the MBs. When
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the dried alginate MBs were immersed into distilled water for 24 h, they swelled to much smaller size than that of the as-sprayed alginate MBs. In general, alginate MBs have different
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degree of swelling according to pH conditions. Alginate MBs are less swollen in the acidic
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pH values, because most carboxylic acid groups in MBs are not dissociated. In contrast, alginate MBs are highly swollen in the basic pH values, because most carboxylic acid groups
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in MBs are dissociated into carboxylate anions, resulting in higher electrostatic repulsion
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between the alginate molecular chains [24]. Therefore, the low degree of swelling for the dried alginate MBs might be caused by the weak acidic pH condition in this swelling
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3.2. Ca2+ content of alginate microbeads The Ca2+ content in the alginate MBs was obtained by subtracting the Ca2+ content in the residual CaCl2 solution from the initial Ca2+ content in CaCl2 solution, because the Ca2+ content of alginate MBs was difficult to directly measure. For this, the SA solutions (5 mL) with different concentrations ranging from 3 to 9 wt% were electrosprayed into 2 wt% CaCl2 solution (100 mL), and then filtered after 30 min. Subsequently, the residual CaCl 2 solution 8
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after the removal of as-sprayed MBs was measured by the ICP-AES. Figure 4 shows the Ca2+ content in the alginate MBs and Na+ content in the solution obtained from the ICP-AES measurement. As the SA concentration was increased from 3 to 9 wt%, the Ca2+ content of alginate MBs was increased from 5.47 to 17.8 mM, because the G block content of alginate
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crosslinked with Ca2+ was also increased [13]. By contrast, the Na+ content eluted into the solution was increased due to the ion exchange reactions between Ca2+ and Na+. From this 2+
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result, it was found that the ionic crosslinking reaction between Ca
and alginate molecules
reaction between Ca
2+
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successfully occurred during the electrospraying process [25]. Theoretically, the ion exchange and Na+ should occur in the ratio of 1:2. The measured Ca2+ content
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of alginate MBs was higher than the theoretical Ca2+ content, because alginate MBs
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contained non-crosslinked Ca2+, as well as crosslinked Ca2+ [14].
3.3. Swelling and degradation behavior of alginate microbeads
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Figure 5 shows the swelling ratio (%) of alginate MBs with immersion time in DI water.
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The alginate MBs were able to easily absorb water molecules, because of their hydrophilic character. The swelling ratio (%) of alginate MBs was abruptly increased to about 160% at 6
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h, and thereafter reached plateau region. Swelling phenomena were mainly attributed to the
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hydration of the hydrophilic groups of alginate, as well as the osmotic pressure, and persisted until the driving force (hydrophilicity and osmotic pressure) of swelling became equal to the restraining force (crosslinking density) [26]. On the other hand, when the alginate MBs were immersed in seawater containing a considerable amount of Na+, the ion exchange reactions between Ca2+ and Na+ reversibly occurred. Therefore, the gel-to-sol transition of alginate MBs was observed in the seawater, 9
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because Ca2+ ions crosslinked with the G block of alginate were substituted by Na+ ions [27]. Figure 6A shows the weight loss of alginate MBs in seawater for 28 days. The weight loss (%) was rapidly increased to 70% for 7 days, and thereafter gradually increased to 100% until 28 days. This weight loss was closely associated with the breakage of ionic crosslinking due to
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the Ca2+-Na+ exchange reaction. Figure 6B shows optical images of alginate MBs with different immersion times. The alginate MBs maintained spherical morphology up to 48 h,
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together with considerable swelling, although the weight loss (%) was about 40% at that time.
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After 48 h, the shape of MBs was steadily fractured into small pieces, due to a severe cleavage of crosslinks. Therefore, it was demonstrated that the alginate MBs have great
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potential for environment-friendly cosmetic additives.
3.4. Organic pollutant adsorption of alginate microbeads in seawater
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Alginate MBs are able to adsorb the organic contaminants in the sea through their surface
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and internal space [28]. Figure 7 shows the adsorption capability (%) of alginate MBs for phenanthrene in the sea water/ethanol mixture. The adsorption (%) showed a maximum value of approximately 32% at 20 min, and thereafter rapidly decreased to about 15%. This rapid
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decrease after maximum adsorption was attributed to a decrease in crosslinking density from
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the surface, resulting in severe swelling due to the penetration of water molecules. Phenanthrene molecules were desorbed from the MBs during the breakage of crosslinks and swelling. At the later stage, the adsorption (%) maintained a constant value of 15% up to 48 h. This might be associated with the internal adsorption by swelling and the adsorption by the increased surface area.
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4. Conclusions In this study, micron-sized alginate MBs were prepared by solution electrospraying, in which the ion-exchange reaction between sodium alginate and calcium chloride occurred. The morphology and size of alginate MBs could be controlled by processing parameters,
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including alginate concentration, and needle diameter during electrospraying. As the alginate concentration and needle diameter were increased, the size of alginate MBs was gradually
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increased, due to mass flow rate of alginate. Also, the swelling, degradation, and adsorption
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behaviors of alginate MBs were examined by UV-Vis spectrophotometry. The swelling ratio of alginate MBs was 160% in distilled water, and the alginate MBs were rapidly degraded in
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seawater, due to the reversible ion-exchange reaction between Ca2+ in MBs and Na+ in sea
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water. This behavior demonstrated that the alginate MBs have great potential for environment-friendly cosmetic additives. The adsorption of alginate MBs toward the model
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contaminant phenanthrene reached maximum value in early time, and thereafter rapidly
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decreased, due to the breakage of crosslinks and swelling of MBs. Therefore, the electrosprayed alginate MBs offer a promising alternative for non-biodegradable MBs,
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because of their cost-effectiveness and environmental compatibility.
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Acknowledgement:
This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (2018R1A2A2A05021100).
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Figure Captions
Figure 1. Schematic of the electrospray equipment for the preparation of alginate microbeads.
Figure 2. (A) Optical microscope images of alginate MBs (scale bar=500 μm); (B) Effect of
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SA concentration on the average sizes of alginate MBs.
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Figure 3. (A) Optical microscope images of as-sprayed, after dried, and swollen alginate
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microbeads (scale bar=500 μm), (B) Effect of needle diameter on the average size
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of as-sprayed, after dried, and swollen alginate microbeads.
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Figure 4. Ca2+ content in the alginate microbeads and Na+ content in the aqueous solution.
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Figure 5. Swelling ratio (%) of alginate microbeads in DI water.
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Figure 6. (A) Optical images, (B) Average particle size and (C) Weight loss (%) of alginate
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microbeads according to their immersion time in seawater.
Figure 7. Phenanthrene adsorption of alginate microbeads in seawater/ethanol mixture. Inset
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plot shows the calibration curve for phenanthrene.
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Figure 1.
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Electrospraying of Environmentally Sustainable Alginate Microbeads for Cosmetic Additives,
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Electrospraying of Environmentally Sustainable Alginate Microbeads for Cosmetic Additives,
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Electrospraying of Environmentally Sustainable Alginate Microbeads for Cosmetic Additives,
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Electrospraying of Environmentally Sustainable Alginate Microbeads for Cosmetic Additives,
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Electrospraying of Environmentally Sustainable Alginate Microbeads for Cosmetic Additives,
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Highlights Ca-alginate microbeads (MBs) for cosmetics were prepared via simple electrospraying. Size and shape of Ca-alginate MBs could be controlled by electrospraying parameters. Ca-alginate MBs rapidly degraded in sea water due to reversible ion-exchange reaction.
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Ca-alginate MBs offer a promising alternative for eco-friendly cosmetic additives.
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