Chiang Mai J. Sci. 2012; 39(4) 775

Chiang Mai J. Sci. 2013; 40(4) : 775-782http://it.science.cmu.ac.th/ejournal/Contributed Paper

Slow Release Fertilizer from Core-Shell ElectrospunFibersPiyaporn Kampeerapappun*[a] and Nipon Phanomkate [b][a] Division of Textile Chemical Engineering, Faculty of Textile Industries,

Rajamangala University of Technology Krungthep, Bangkok 10210, Thailand.[b] National Nanotechnology Center, National Science and Technology Development Agency,

Pathumthani 12120, Thailand.*Author for correspondence; e-mail: piyaporn.k@rmutk.ac.th

Received: 31 July 2012Accepted: 27 February 2013

ABSTRACTCore-shell fibers with polyhydroxybutyrate (PHB) as the shell and polylactic acid

(PLA) mixed with fertilizer as the core were prepared by coaxial electrospinning. Both PHBand PLA are biodegradable polymers so that they become environmentally friendly.These core-shell structures offer the potential to control the manner and timing of fertilizerdelivery. At a fixed flow rate of shell solution, the core-shell electrospun fiber mats exhibiteda lower flow rate of core solution causing a lower release rate of fertilizer. Adjustments offertilizer concentration obtained that fertilizer release rate from every core-shell fiber matswere just about the same but amount of release fertilizer from electrospun mats were different.An electrospun mat can release fertilizer for 1 month without degradation.

Keywords: bicomponent fiber, coaxial electrospinning, fertilizer, controlled release

1. INTRODUCTIONFertilizers are substances that supply plant

nutrients to promote plant growth, increasecrop production, and improve the quality ofproducts [1]. They can be classified intotwo types: natural and synthetic fertilizer.The ideal for fertilization is the plants canabsorb fertilizer as much as they can andleast loss of fertilizers. Too much and toostrong fertiizers are harmful to plants.Moreover, uncontrolled use of syntheticfertilizer can lead to soil pollution. Thus,there are several researches focus onproduct development in order to increasethe efficiency of their use and minimize anypossible adverse environmental impact [2-3].

Either improvement of fertilizers already inuse or development of new specific fertilizertypes can be done [4].

Coated fertilizer is one of the slowrelease fertilizer that releases its nutrientover a specific of time. It is prepared fromcoating the soluble granular fertilizer bymaterials which reduce its dissolution rate.The release of fertilizer nutrient is controlledby diffusion through the shell [5]. Advantagesof slow release fertilizers are they cause lessdamage to plant roots, decrease soil toxicityand reduce the frequency of fertilization [6].Despite these benefits, some potentialnegative environmental effect may arise

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because of non-biodegradable polymers usedas a shell.

Electrospinning is a unique approachusing electrostatic forces to produce ultrafinefibers with diameters in the range of tennanometers up to several micrometers. Dueto simple and straightforward of this process,it has been developed from using a single fluid[7-8] to two fluids (co-axial and side-by-sideelectrospinning) [9-11] which creates newtypes of complicated nanofibers with well-defined microstructures, novel morphologies,and/or new functions [12-15]. There areseveral researches and patents relatedwith electrospinning for controlled releaseapplications in the past several years.Most reports focus on biomedical andpharmaceutical applications. Therefore, it isour interest to explore the use of electrospunmats as carrier in agriculture field.

On the basis of the above background,encapsulation of fertilizers into fiber ascore-sheath fiber by using coaxialelectrospinning was prepared in this work.Polyhydroxybutyrate (PHB) was used as thesheath, while polylactic acid (PLA) mixed withfertilizers was used as the core which bothPHB and PLA are biodegradable polymers.Morphology, release fertilizer rate, andbiodegradability of neat and fertilizer-loadedcore-sheath electrospun fiber mats werestudied.

2. MATERIALS AND METHODS2.1 Materials

In this experiment, two polymers,polylactic acid (PLA, MW ~ 390,000) andpolyhydroxybutyrate (PHB, MW ~ 400,000)were purchased from NatureWorks LLC.,USA and Tianan Biologic., China, respectively.

Dimethylformamide (DMF) purchased fromSigma-Aldrich, USA and chloroformpurchased from BDH Laboratory Supplies,England were used as polymer solvents.Fertilizer (NPK 21-21-21, Bavaria) wasdonated from orchid farm, Ratchaburi,Thailand. All other chemicals were ofanalytical reagent grade and used as received,without further purification.

2.2 Preparation of Core-SheathElectrospun Fiber Mats

The PHB solution concentration,the sheath, was prepared at 4% w/w byusing a mixture of chloroform anddimethylformamide (90:10) as a solvent. Toprepare the core solution, PLA concentrationwas fixed at 6% w/w in dimethylformamide,while amount of fertilizers were varied from20-160% w/w based on amount of PLAused. After that, the prepared solutionswere transferred to syringe equipped withcoaxial stainless steel needle as an electrode(Figure 1). The sheath solution was deliveredat the flow rate of 2.4 ml/h, while the rangeof core solution flow rate was 0.2-2.2 ml/hby each assigned NE-300 syringe pump(National Direct Network Co., Ltd, USA).The coaxial needle consisted of an outer 18-gauge needle and an inner 22-gaugeneedle. The syringe needle was connected tothe positive output of a CRT50-10P highvoltage power supply operating at 15 kV(Gamma High Voltage Research, USA). Thecounter electrode was connected to a rollercovered by aluminum foil to collect theelectrospun fibers. The distance between theneedle and roller was 15 cm. Theelectrospinning experiment was performed atroom temperature.

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2.3 Fiber CharacterizationThe morphology of fibers was observed

under a 4510-Jeol scanning electronmicroscope, SEM (Jeol Ltd., Japan) and aTecnai 20 transmission electron microscope,TEM (FEI Company, USA). Fiber diametersof 100 electrospun nanofibers were measuredby means of the Image J software (NationalInstitutes of Health, USA) from the SEMmicrographs in their original magnificent.

2.4 Release Fertilizer CharacteristicsSpecimens (1 cm 1 cm, weight 10 mg)

were random cut from core-shell electrospunmats. Then, each specimen was placed into40 ml of distilled water and let it set in the30C shaker bath at 70 rounds per minute.The specimens were immersed in distilledwater at different periods, ranging from 0to 800 h. After various time periods, theelectrospun mat was removed and thesolution (3 ml each) was determined usingAquaMate Plus UV-Vis spectrophotometer(Thermo Scientific, USA) at wavelength640.5 nm. The obtained data were calculatedto determine the cumulative amount offertilizer released from the specimens at eachimmersion. The experiments were carried outin tripricate and the results were reported asaverage values.

2.5 BiodegradabilityThe core-sheath electrospun mats was

tested for biodegradability according toASTM D5988-03 standard, which determinethe rate of aerobic biodegradation of syntheticplastic materials in contact with soil and naturecompost under laboratory conditions.

3. RESULTS AND DISCUSSIONThe morphology of fibers obtained

from electrospinning process depends onvarious parameters (i.e. solution composition,solution concentration, solution feed rate,applied voltage, collection distance and etc.)[14]. In this study, the morphology of core-shell electrospun fibers were determinedby varying the core solution feed rate andfertilizer concentration in the core solution.SEM images and diameters of core-sheathelectrospun fibers that have been fabricatedunder core solution feed rate in the rangeof 0.2 to 2.2 ml/h with fixed fertilizerconcentration, applied voltage, and collectiondistance are shown in Figure 2 and Table 1,respectively. In the case of effect of fertilizerconcentration on fiber morphology, SEMimages of core-sheath electrospun fibers areshown in Figure 3 and fiber diameters aredisplayed in Table 2.

Figure 1. A schematic diagram of the coaxial electrospinning process.

Collector

DC High Voltage

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Figure 2. SEM images of core-shell electrospun fibers at different core feed rates.(a) 0.2 ml/h(b) 0.6 ml/h (c) 1.0 ml/h (d) 1.4 ml/h (e) 1.8 ml/h (f) 2.2 ml/h

Table 1. Effect of core feed rate on diameter of core-sheath electrospun fibers. The collectiondistance was 15 cm, the voltage was 15 kV and the fertilizer concentration was 20% w/wbased on amount of PLA.Feed rate (ml/h)

0.21.01.8

diameter of fibers (m)4.1 0.64.1 0.73.9 0.4

Feed rate (ml/h)0.61.42.2

diameter of fibers (m)4.0 0.44.1 0.74.0 0.4

Table 2. Effect of fertilizer concentration on core-sheath electrospun fibers morphology.The collection distance was 15 cm, the voltage was 15 kV and the solution feed rate was1.4 ml/h.

Fertilizer Conc.(% w/w based on

amount of PLA used)2060100140

diameter of fibers(m)

4.3 0.84.0 0.74.5 0.94.4 0.5

Fertilizer Conc.(% w/w based on

amount of PLA used)4080120160

diameter of fibers(m)

4.1 0.24.0 0.44.5 0.54.4 0.5

Figure 3. SEM of core-sheath electrospun fibers at different different fertilizer concentrations(% w/w based on amount of PLA used) (a) 20 (b) 40 (c) 60 (d) 80 (e) 100 (f) 120 (g) 140 (h).

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From Table 1-2 and Figure 2-3, they werefound that the different feed rates and fertilizerconcentrations in coaxial electrospinningprocess were not significantly changed thefiber morphologies and outer diameter of thefiber. In the event of changing core solutionfeed rate (Table 1), the outer diameter of allfibers remained almost exactly the same size.Nevertheless, the inner fiber diameters weredifferent which are obtained from TEMimages (Figure 4). High core solution feed rateproduced fibers with large inner diametersand relatively small inner fiber diameters werefound in fibers electrospun from low solutionfeed rate. However, due to the same outerfiber diameters, the shell thickness was varieddepending on core solution feed rate.

The cumulative fertilizer release fromelectrospun fibers was monitored by a UV-Vis spectrophotometer and the results are

shown in Figure 5. At specific time, the highercore solution feed rate resulted in the fasterrelease fertilizer from electrospun fibers. Forexample, at 300 h, the cumulative fertilizerrelease was 57.77% and 87.60% when coresolution feed rate was at 0.6 and 2.2 ml/h,respectively. These results can be explained thatthe release of the fertilizer was controlled bydiffusion through the shell. The shellthicknesses of fibers were calculated as adifference between the total and inner fiberdiameters. By increasing core solution feedrate, the shell thickness of fibers decreasedsignificantly (Figure 4). The fertilizersentrapped in the core of fibers were dissolvedand released through the shell. Compared withthicker shell, fertilizers diffusion throughthinner shell was more rapidly; therefore, theshell thickness is the main factor for controllingactive agent release rate [16].

Figure 4. TEM micrographs of core-shell electrospun fibers with different core feed rates(a) 0.6 ml/h (b) 1.4 ml/h (c) 2.2 ml/h.

Figure 5. Effect of core solution feed rate on the release rate of fertilizer.

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A study on the effects of fertilizerconcentration on release rate was performedby enhancing fertilizer concentration in coresolution from 20 to 160% w/w based onamount of PLA used. The feed rate of coresolution was fixed at 1.4 ml/h; the results areshown in Figure 6. The results in Figure 6showed that the release fertilizer rate

insignificantly affected by fertilizerconcentration which means the electrospunfibers mats can release fertilizer for a month.However, the amount of release fertilizer at atime was different. The increase in the fertilizerconcentration of the core solution resulted inhigher release fertilizer content.

Figure 6. Effect of fertilizer concentration in core solution on the rate of fertilizer release.

Due to the degradation rate affecting torelease rate, the degradation time of core-shellelectrospun fiber mats were determined. Theresults are shown in Figure 7. The electrospunfiber mats were deformed after 1 month.After that, the electrospun fiber mats becamebrittle and started breaking apart within 2

months. After 3 months, the electrospun matwas disappeared (Figure 7). As mentionedearlier, the electrospun mats released fertilizerfor a month. This result implied that allfertilizers in core-shell electrospun fiber matsreleased before it was degraded.

Figure 7. Determining biodegradability of core-shell electrospun fiber mats with time.

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4. CONCLUSIONIn this study, coaxial electrospinning was

used to create core-sheath fiber mats withappropriate slow-release fertilizer properties.Release rate of this fertilizer was affected byfeed rate of core solution with a fixed feedrate of shell solution. The higher the coresolution feed rate, the faster the releaserate was. The fertilizer concentration usedin core solution was also important indefining amount of released fertilizer.Core-shell electrospun mats released fertilizerfor a month and were biodegradable within3 months.

ACKNOWLEDGEMENTSThe author would like to acknowledge

the financial support from Thailand TextileInstitute (THTI), Thailand.

REFERENCES

[1] Notario del Pino J.S., Arteaga Padron I.J.,Gonzalez Martin M.M., GarciaHernandez J.E., Phosphorous andpotassium release from phillipsite-basedslow-release fertilizers, J. Control. Release,1995; 34: 25-29.

[2] Zahrani S., Utilization of polyethylene andparaffin waxes as controlled deliverysystems for different fertilizers, Ind. Eng.Chem. Res., 2000; 39: 367-371.

[3] Akelah A., Novel utilizations ofconventional agrochemicals by controllerrelease formulations, Mater. Sci. Eng.: C.,1996; 4: 83-98.

[4] Maene L.M., Proceedings of the 45th

Annual Meeting, Fertilizer IndustryRound Table, USA, 1995.

[5] Jarosiewicz A., and Tomaszewska M.Controlled-release NPK fertilizerencapsulated by polymeric membranes,J. Agric. Food. Chem., 2003; 51: 413-417.

[6] Trenkel M.E., Controlled-Release andStabilized Fertilizers in Agriculture; inInternational Fertilizer IndustryAssociation, France, 1997: 15-16.

[7] Huang Z.M., Zhang Y.Z., Kotaki M., andRamakrishna S., A review on polymernanofibers by electrospinning and theirapplications on nanocomposites, Compos.Sci. Technol., 2003; 63: 2223-2253.

[8] Teo W.E., and Ramakrishna S., A reviewon electrospinning design and nanofibreassemblies, Nanotechnol., 2006; 17: 89-106.

[9] Moghe A.K., and Gupta B.S., Co-axialelectrospinning for nanofiber structures:preparation and applications, Polym. Rev.,2008; 48: 353-377.

[10] Gapta P., and Milkes G.L., Someinvestigations on the fiber formation byutilizing a side-by-side bicomponentelectrospinning approach, Polymer, 2003;44: 6353-6359.

[11] Sun, Y.K., Ahn, B.W., and Kang, T.J.,Magnetic nanofibers with core (Fe3O4nanoparticle suspension)/sheath (polyethylene terephthalate) structure fabricatedby coaxial electrospinning, J. Magn. Magn.Mater., 2012; 324(6): 916-922.

[12] Frey M.W., and Li L., Electrospinningand porosity measurements of nylon-6/poly(ethylene oxide) blended nonwovens,J. Eng. Fiber. Fabr., 2007; 2: 31-37.

[13] Pham Q.P., Sharma U., and Mikos A.G.,Electrospinning of polymeric nanofibersfor tissue engineering applications: Areview, Tissue Eng., 2006; 12: 1197-1211.

[14] Ramakrishna S., Fujihara K., Teo W.E.Lim T.C., and Ma Z., An Introduction toElectrospinning and Nanofibers, 1st Edn.,World Scientific Publishing Co. Pte. Ltd.,Singapore, 2005.

[15] Kampeerapappun P., Preparation,characterization, and antimicrobial activity

782 Chiang Mai J. Sci. 2012; 39(4)

of electrospun nanofibers from cottonwaste fibers, Chiang Mai J. Sci., 2012; 39(4):712-722.

[16] Li Z.Z., Xu S.A., Wen L.X., Liu F., LiuA.Q., Wang Q., Sun H.Y., Yu W., andChen J.F. Controlled release ofavermectin from porous hollow silica

nanoparticles: Influence of shell thicknesson loading efficiency, UV-shieldingproperty and release, J. Control. Release,2006; 111: 81-88.