Encapsulation and Enhanced Retention of Fragrance in Polymer

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Encapsulation and Enhanced Retention of Fragrance in PolymerMicrocapsulesHyomin Lee dagger ∥ Chang-Hyung Choi dagger ∥ Alireza AbbaspourradDagger Chris Wesnersect Marco Caggionisect

Taotao Zhusect and David A Weitz dagger

daggerSchool of Engineering and Applied Sciences and Department of Physics Harvard University Cambridge Massachusetts 02138United StatesDaggerDepartment of Food Science Cornell University Ithaca New York 14853 United StatessectCorporate Engineering The Procter amp Gamble Company Cincinnati Ohio 45069 United States

S Supporting Information

ABSTRACT Fragrances are amphiphilic and highly volatileall of which makes them a challenging cargo to efficiently

encapsulate and retain in microcapsules using traditionalapproaches We address these limitations by introducing anew strategy that combines bulk and micro1047298uidic emulsi1047297ca-tion a stable fragrance-in-water (FW) emulsion that isprimarily prepared from bulk emulsi1047297cation is incorporated

within a polymer microcapsule via micro1047298uidic emulsi1047297cation On the basis of the in-depth study of physicochemical propertiesof the microcapsules on fragrance leakage we demonstrate that enhanced retention of fragrance can be achieved by using a polarpolymeric shell and forming a hydrogel network within the microcapsule We further extend the utility of these microcapsules by demonstrating the enhanced retention of encapsulated fragrance in powder state

KEYWORDS fragrance emulsions encapsulation microcapsules micro 1047298 uidics retention

1 INTRODUCTION

Fragrances small volatile substances with scent are attractivefor applications in toiletries cosmetics and home careproducts1 2 These aroma molecules add pleasant scent tohuman body animals objects and living spaces providingfavorable eff ect on our emotional perception3 While some

volatility of fragrance is essential for our sensory response theirhighly volatile nature often causes undesired loss duringstorage this limits their eff ective usage as additives in variousproducts

Several methods ha v e been developed to mitigate the volatility of fragrance4minus9 One approach so-called profra-grance4minus6 involves covalent binding of volatile fragrance tosubstrates to obtain nonvolatile compound which can bereleased only upon external stimuli for instance somedeodorants contain profragrances that are released uponexposure to moisture However this approach is limited tofragrances with chemical functionalities such as aldehydes orketones10 A more generic method is the encapsulationapproach which introduces a protective shell around thecargo to form a capsule this shell can perform as a diff usion

barrier and enhance the retention of pristine fragrance7minus9

Conventional encapsulation techniques such as interfacialpolymerization11 complex coacervation12 and solminusgel encap-sulation13 are utilized to encapsulate fragrance in micro-capsules dispersed in aqueous phase however thesetechniques lack highly efficient encapsulation and retention of fragrance in microcapsules Since fragrances are mostly

amphiphilic (hydrophobic while exhibiting partial watersolubility) substantial loss can occur during the emulsi1047297cation

of fragrance in aqueous phase9

Furthermore these techniquesresult in microcapsules with uncontrolled size shell thicknessand structures this limits optimal protective shell design thateff ectively encapsulates and retains cargo Indeed for cargosuch as fragrance with molecular weight less than 300 Da14

high mobility of fragrance6 11 12 leads to rapid leakage throughthe shell and consequently to signi1047297cant loss of encapsulatedcargo during storage Therefore there is an unmet need for anencapsulation strategy that remediates amphiphilicity of volatilecargo as well as enables production of monodisperse micro-capsules with precisely controllable shell thickness and shellmaterials

In this article we describe a promising alternative strategy that leverages both micro1047298uidic and bulk emulsi1047297cation to

achieve highly efficient encapsulation and enhanced retentionof fragrance By using micro1047298uidic emulsi1047297cation15minus22 micro-capsules with precisely tunable size shell thickness and shellmaterials can be produced providing a 1047297rm basis for systematicstudy Then in concert with bulk emulsi1047297cation which enablesa facile route to prepare emulsion formulation amphiphilicity of fragrance is resolved by directly incorporating a preformedfragrance-in-water (FW) emulsion in the microcapsules A

Received November 23 2015 Accepted January 22 2016Published January 22 2016

Research Article

wwwacsamiorg

copy 2016 American Chemical Society 4007 DOI 101021acsami5b11351

ACS Appl Mater Interfaces 2016 8 4007minus4013



preformed FW emulsion phase co-1047298ows with a photocurableoil phase which are then 1047298ow-focused by the aqueouscontinuous phase and photopolymerized to produce polymer

microcapsules containing model fragrance On the basis of thisstrategy we demonstrate that the low solubility of the capsuleshell with the encapsulated fragrance signi1047297cantly reduces thefragrance leakage rate Furthermore we achieve enhancedretention of fragrance in aqueous media as well as in powderstate by forming a hydrogel network within the microcapsule

2 EXPERIMENTAL SECTION

21 Materials All photocurable oil trimethylolpropane ethoxylatetriacrylate (ETPTA M w = 428 gmol) trimethylolpropane triacrylate(TMPTA) dipentaerythritol penta-hexa-acrylate (DPHA) andphotocurable hydrophilic poly(ethylene glycol) diacrylate (PEG-DA

M w = 700 gmol) were purchased from Sigma-Aldrich and used asreceived without further puri1047297cation Model fragrance (α -pinene)poly(vinyl alcohol) (PVA M w = 13minus23 000 gmol 87minus89

hydrolyzed) tween 80 photoinitiator (22-dimethoxy-2-phenylaceto-pheonon) and n-octadecyltrimethoxyl silane were purchased fromSigma-Aldrich and 2-[methoxy(polyethyleneoxy)propyl] trimethoxylsilane was purchased from Gelest Distilled water (gt182 MΩmiddotmMillipore) (DI water) and ethanol (ge995 Sigma-Aldrich) were usedfor all experiments

22 Fabrication of Glass Capillary Micro1047298uidic Device Glasscapillary micro1047298uidic device was prepared using previously publishedprotocols23 24 Brie1047298 y two cylindrical glass capillaries (World precisionInstruments) of inner and outer diameters 058 and 100 mmrespectively were tapered to a diameter of 40 μm with a micropipettepuller (P-97 Sutter Instrument) These tapered cylindrical capillaries were then sanded to 1047297nal outer diameters of 60 and 400 μm andinserted in a square capillary (Atlantic International Tech 105 mmid) from the opposite direction prior to insertion the cylindricalcapillary with smaller ori1047297ce was treated with n-octadecyltrimethoxylsilane to make the surface hydrophobic whereas the larger ori1047297ce wastreated with 2-[methoxy(polyethyleneoxy)propyl] trimethoxyl silaneto make the surface hydrophilic After the assembly the xy plane as well as the z-axis con1047297guration of cylindrical capillaries was checked forproper alignment

23 Drop Generation Polymerization and Image AnalysisTo control the 1047298ow rates we used syringe pumps (Harvard Apparatus)and recorded the production of triple emulsion drops containingmultiple internal fragrance drops within the micro1047298uidic devices usingan inverted microscope (Leica) equipped with a high-speed camera(Phantom V9) The photocurable oil in the middle phase was then insitu polymerized by UV exposure for 1minus2 s (Omnicure S1000 100W)at the exit of the capillary device We note that all photocurable oilcontains 1 (vv) photoinitiator and that microcapsules with a

hydrogel network contain 10 (vv) photocurable hydrophilic PEG-DA in the aqueous phase of the FW emulsion unless further speci1047297edTo measure the outer radius and shell thickness of the microcapsules

generated we performed image analysis using ImageJ on multipleimages that we obtained from optical microscopy The interface between the FW emulsion and the capsule shell can be distinguished by the position of the thin white layer within the microcapsule thisenables determination of the outer radius and the shell thickness fromthe image analysis In all cases we analyzed at least 100 microcapsulesto determine the average value of the outer diameter and the shellthickness Provided that the shell is sufficiently well-de1047297ned imageanalysis provides an accurate measure of both the outer radius and theshell thickness To ensure that the shell is well-de1047297ned we in situpolymerized the photocurable oil in the emulsion drops at the exit of the capillary device to achieve concentric microcapsules with uniformshell thicknesses otherwise collection and subsequent UV irradiation(off -chip polymerization) result in generation of off -centered micro-capsules with a very thin shell on one side as shown in the opticalimages of Supporting Information Figure S1 due to the inherentdensity mismatch between the FW emulsion phase and thephotocurable oil These cannot be analyzed as well and we did notuse them We also note that the error bars shown in Table 1 are notdue to the image analysis used for processing the images but mainly originate from the generation of emulsion drops in the jetting regimeresulting in some polydispersity in the capsule size and shell thickness

24 Leakage Characterization The leakage pro1047297le is monitoredusing a UV minus vis spectrophotometer (Cary 50 Agilent Technologies) atroom temperature (approximately 25 degC) The detailed step-by-stepexperimental procedures are described in Scheme 1 First micro-capsules encapsulating the FW emulsion were collected at the exit of the capillary device The collected microcapsules were rinsed at leastthree times with an excess volume of DI water to remove the residualPVA as well as α -pinene that may have leaked during the rinsingprocedure however we observed no signi1047297cant reduction in the 1047297nal

concentration of fragrance inside the microcapsule after rinsingcompared to the variability in each capsule from the same batch Then1047297 ve randomly selected microcapsules were transferred to 30 mL of DI water in a quartz cuvette cell (35 mL Agilent) and double-sealed withpoly(1122-tetra1047298uoroethylene) (PTFE) lid and para1047297lm to preventevaporation and maintain a closed system Due to the high volatility of α -pinene (sim3 mmHg 20 degC)25 complete sealing of the cuvette cell isrequired during the leakage study otherwise a noticeable reduction inthe absorbance signal is observed during the measurementImmediately after the microcapsules were transferred de1047297ned as t =0 min UV minus vis spectra were obtained at diff erent time intervals withagitation before each measurement Next UV minus vis spectra werereplotted versus time at λmax sim 200 nm26 This procedure was repeatedthree times to obtain error bars in the leakage study

Scheme 1 Schematics Showing the Detailed UV minus Vis Spectroscopy Procedures To Acquire the α-Pinene Leakage Pro1047297le

ACS Applied Materials amp Interfaces Research Article

DOI 101021acsami5b11351 ACS Appl Mater Interfaces 2016 8 4007minus4013

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Due to volatility and low solubility in DI water (249 mgL 235degC)27 we estimated the percentage of α -pinene remaining in eachmicrocapsules by transferring the as-prepared and leakage study performed microcapsules to 3 mL of ethanol a good solvent for α -pinene Each quartz cuvette cell containing one microcapsule was vortexed for 1minus2 min and stored for at least 1 day prior to UV minus vismeasurement to ensure complete extraction of α -pinene from eachmicrocapsule

3 RESULTS AND DISCUSSION

31 Preparation of Polymer Microcapsules Encapsu-lating Fragrance Emulsion We prepared polymer micro-capsules encapsulating a preformed FW emulsion in a glasscapillary micro1047298uidic device This device consists of a taperedcylindrical injection and a cylindrical collection capillary thatare coaxially aligned within a square capillary as shown inFigure 1 We 1047297rst prepared a stable FW emulsion solution by

mixing the model fragrance α -pinene with an aqueoussurfactant solution consisting of 5 PVA and 4 Tween 80in a 5050 volume ratio for 1 min This stable FW emulsion

was injected through the cylindrical injection capillary A photocurable oil was injected as a middle phase through theinterstices of the square and injection capillaries from the sameside with the injection capillary An aqueous phase of 5 PVA

was injected through the square capillary from the other side asthe continuous phase to emulsify the co-1047298owing biphasic 1047298uids

into monodisperse triple emulsion drops containing multipleinternal fragrance drops Then microcapsules were obtained by cross-linking the photocurable oil phase in the triple emulsiondrops at the exit of the collection capillary via in situphotopolymerization The preformed FW emulsions wereeff ectively encapsulated within a polymer shell and dispersed inan aqueous solution as evidenced by the dark interior which is

caused by the scattering of the concentrated fragrance drops asshown in Figure 1

32 Eff ect of Shell Material on Fragrance Leakage Wemonitored the leakage behavior of model fragrance α -pinene

with respect to shell material using UV minus vis spectroscopy Toinvestigate the eff ect of shell material on α -pinene leakage wetuned the cross-linking density as well as the polarity of the

shell by varying the photocurable oil composition in the middlephase these compositions include TMPTA ETPTA and a5050 (ww) mixture of TMPTA and DPHA We 1047297rst usedtriacrylate nonpolar oil TMPTA to produce a hydrophobicpolymer shell To increase the concentration of cross-linkableacrylate groups in the middle phase in another set of experiments we mixed TMPTA with DPHA that contains ahigher number of acrylate groups We also studied the eff ect of shell polarity on the leakage of α -pinene by replacing TMPTA

with ETPTA an ethoxylated analogue of TMPTA in themiddle phase To minimize the eff ect of microcapsule size andthe shell thickness on the leakage of α -pinene we kept thethree liquid 1047298ow rates inner preformed FW emulsion phase(Q I) middle photocurable oil phase (Q M) and outer

continuous phase (Q O) constant at 1000 1000 10 000 μ

Lh respectively indeed the outer radius and the shellthicknesses are the same for all three compositions as shownin Table 1 For these microcapsules with diff erent middle phasecompositions we observed slower leakage of α -pinene by addition of DPHA in TMPTA and even slower leakage when apolar polymer shell prepared from ETPTA was used as shownin the leakage pro1047297le of Figure 2a While addition of DPHA inTMPTA can potentially increase the cross-linking density in apolymer shell and reduce the leakage rate a polymer shellprepared from the ethoxylated analogue of TMPTA (ETPTA)off ers low solubility with the encapsulated cargo and thereforecan further reduce the leakage rate28minus30

33 Eff ect of Shell Thickness on Fragrance Leakage

To investigate the eff

ect of shell thickness on the leakage of α

-pinene we prepared ETPTA microcapsules with the same size but with diff erent shell thicknesses This is modulated by controlling the 1047298ow rates of the inner (Q I) and middle (Q M)phases while keeping their sum constant The resultingmicrocapsules with diff erent shell thicknesses are shown inthe bright-1047297eld micrographs of Figure 2 b The outer radius andthe shell thickness plotted as a function of Q MQ I ratiocon1047297rms that the overall size of the microcapsule remains thesame while the shell thickness increases from 11 plusmn 3 to 24 plusmn 6to 37 plusmn 4 μm While our results show that increasing the shellthickness reduces the leakage rate of α -pinene as shown in theleakage pro1047297le of Figure 2c increasing the shell thicknessinevitably exacerbates the loading efficiency for instance

increasing the shell thickness by approximately 3-folds resultsin an sim42 reduction in the volume occupied by the fragranceemulsion In addition this reduction in leakage rate is negligiblecompared to the eff ect of the shell material The signi1047297canteff ect of the shell material reveals that in the regime where theleakage involves a long diff usion path on the order of tens of micrometers the solubility of the shell material with the

Figure 1 Schematic illustration of the glass capillary micro1047298uidicdevice for preparing polymer microcapsules encapsulating a preformedfragrance-in-water (FW) emulsion Bottom optical microscope image

shows the generation of triple emulsion drops containing multipleinternal fragrance drops Upon UV irradiation the photocurable oil inthe middle phase polymerizes to form a polymeric shell Scale barrepresents 200 μm

Table 1 Outer Radius and Shell Thickness of the Microcapsules Prepared

TMPTA TMPTA + DPHA ETPTA ETPTA (+ hydrogel network)

outer radius ( μm) 202 plusmn 9 μm 226 plusmn 8 μm 209 plusmn 6 μm 211 plusmn 9 μm

shell thickness ( μm) 22 plusmn 3 μm 23 plusmn 9 μm 24 plusmn 6 μm 26 plusmn 4 μm



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encapsulated fragrance becomes the dominating factor inreducing the leakage rate

34 Eff ect of Hydrogel Network on FragranceLeakage While shell material and shell thickness can bemodulated to achieve a signi1047297cant reduction in the leakage rateof α -pinene the density mismatch within the preformed FW emulsion phase ( ρα ‑pinene sim 086 and ρ5 PVA sim 102) promotes

leakage because the α -pinene drops can rise due to buoyancy and directly contact the inner surface of the capsule shell(Supporting Information Figure S2) To prevent thisundesirable leakage mechanism and thus achieve a furtherreduction in the leakage rate we rigidi1047297ed the continuous phaseof the preformed FW emulsion within the microcapsule Toaccomplish this we added 10 (vv) photocurable hydrophilicPEG-DA and 01 (vv) photoinitiator to the aqueous phase of the FW emulsion upon exposure to UV this forms a hydrogelnetwork as schematically shown in Figure 3a This hydrogel

network locks the position of individual α -pinene drops withinthe microcapsule and performs as a physical barrier Indeed theresulting microcapsules signi1047297cantly reduce the leakage rate of α -pinene compared to that of the control microcapsules

without a hydrogel network as shown in the leakage pro1047297leof Figure 3 b We also investigated the eff ect of concentration of PEG-DA and thus the mesh size of the hydrogel network onthe leakage pro1047297le by use of 20 (vv) PEG-DA in the aqueousphase of the FW emulsion We observed that increasing theconcentration of PEG-DA by 2-fold reduced the leakage rate of α -pinene as shown in the leakage pro1047297le of Supporting

Figure 2 (a) Graph showing absorbance versus time with variation inthe photocurable oil composition All photocurable oil contains 1 (v v) photoinitiat or (b) Optical microscope image s of ETPTA microcapsules prepared at various ratios of Q MQ I denoted on theimages Scale bar represents 200 μm The sum of the inner and middle

phase 1047298ow rates (Q I + Q M) is 2000 μLh and the outer continuousphase 1047298ow rate (Q O) is 10 000 μLh The graph below shows theouter radius and the shell thickness as a function of Q MQ I (c) Graphshowing absorbance versus time with variation in the shell thicknessesfor ETPTA microcapsules

Figure 3 (a) Schematic illustration of preparing microcapsulescontaining a hydrogel network ETPTA and PEG-DA are polymerizedsimultaneously upon UV exposure (b) Graph showing the absorbance versus time for ETPTA microcapsules (Q MQ I = 10) with and without a hydrogel network (c) Optical microscope image of ETPTA microcapsules with a hydrogel network ruptured between two glassslides (d) Optical microscope image of ETPTA microcapsules withouta hydrogel network ruptured between two glass slides Scale barrepresents 200 μm



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Information Figure S3 However this reduction in leakage rateis negligible compared to the eff ect associated with the presenceof the hydrogel network itself shown in the leakage pro1047297le of Figure 3 b To verify the presence of the hydrogel network

within the microcapsules we applied mechanical stress torupture the microcapsules between two glass slides andobserved that individual α -pinene drops were held cohesively

even upon rupture as shown in the bright-1047297eld images inFigure 3c By contrast for microcapsules without a hydrogelnetwork we observed instantaneous release of α -pinene dropsupon rupture as shown in Figure 3d Overall these resultsindicate that rigidifying the aqueous phase of a preformed FW emulsion can eff ectively prevent individual α -pinene drops fromdirectly contacting the capsule shell and thus can furthersuppress the leakage of α -pinene

35 Quanti1047297cation of α-Pinene Remaining in Micro-capsules Although monitoring the leakage of α -pinene withrespect to shell material shell thickness and microcapsulestructure revealed the key design parameters for enhancedretention of fragrance in aqueous media it is often moredesirable to quantitatively determine the amount of encapsu-

lated α -pinene remaining in the microcapsule after the leakagestudy To quantify the amount of α -pinene we transferred boththe as-prepared and the leakage study performed microcapsulesto ethanol a good solvent for α -pinene and measured the UV minus

vis spectra in this media to estimate the amount of α -pineneremaining in the microcapsule Unlike microcapsules dispersedin water encapsulated α -pinene was completely extracted fromthe microcapsules after transferring into ethanol within a day asevidenced by the clear interior and buckled shell in the bright-1047297eld micrographs of Figure 4a This approach enabled us toestimate the percentage of α -pinene remaining in themicrocapsules after the leakage study We found that 77 plusmn

11 of α -pinene remained in TMPTA microcapsules with the

most leakage whereas 85 plusmn 8 of α -pinene remained inETPTA microcapsules with a hydrogel network with the leastleakage after approximately sim28 days of dispersion in DI wateras shown in a plot of α -pinene remaining in various systems of Figure 4 b

36 Eff ect of Hydrogel Network on Retention of Fragrance in Microcapsule Powder To extend the utility of microcapsules containing the FW emulsion to applicationsthat require fragrance retention in powder state we removedthe capsule dispersion media and monitored the retention of α -pinene Visual inspection of the microcapsule morphology inthe powder state revealed that the aqueous phase of thepreformed FW emulsion was replaced by air19 as waterevaporates as shown in the bright-1047297eld images of Figure 4cthis is partially due to the higher vapor pressure of water (sim175mmHg at 20 degC)31 compared to that of α -pinene (sim3 mmHg20 degC)25 and thus faster evaporation of water By contrast forthe microcapsules containing a hydrogel network we observedno apparent diff erence in morphology in the powder state weattribute this stark visual contrast to the hydrogel network

suppressing the evaporation of water This hydrogel network prevents individual α -pinene drops from directly contacting thecapsule shell and thus can enhance the retention of α -pineneIndeed quantitative assessment of α -pinene retention showedthat 38 plusmn 11 of the initial amount of α -pinene remained inthe microcapsule without the hydrogel network whereas 49 plusmn

6 of α -pinene remained in the microcapsule with the hydrogelnetwork after 5 days as shown in the plot of α -pinene versustime in Figure 4d These results indicate that rigidifying theaqueous phase of the preformed FW emulsion phase is alsoeff ective for applications that require sustained release of encapsulated fragrance in the powder state such as fabricsofteners

Figure 4 (a) Optical microscope images of ETPTA microcapsules before and after transfer to ethanol Transfer to ethanol results in completeextraction of α -pinene from the microcapsule Scale bar represents 200 μm (b) Graph showing the percentage of α -pinene remaining inmicrocapsules after approximately sim28 days of dispersion in DI water (c) Sets of optical microscope images of ETPTA microcapsules with and without a hydrogel network upon removal of the capsule dispersion media Scale bar represents 200 μm (d) Graph showing the percentage of α -pinene remaining with time upon removal of the capsule dispersion media



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4 CONCLUSIONS

Here we introduce the strategy of combining bulk andmicro1047298uidic emulsi1047297cation to encapsulate and retain fragrancein polymeric microcapsules a stable FW emulsion that isprimarily prepared from bulk emulsi1047297cation is incorporated

within a polymer microcapsule via micro1047298uidic emulsi1047297cationThese sequential emulsi1047297cations enable highly efficient

encapsulation of fragrance as well as fabrication of themicrocapsules with precisely tunable size and shell propertiessuch as cross-linking density polarity and thickness to achieveenhanced retention of fragrance In addition a strategy of rigidifying the continuous phase of the preformed FW emulsion within the microcapsule was also introduced tofurther enhance the fragrance retention The approachesoutlined in this work are general and can be further extendedto encapsulate multiple fragrances in a mixture of FW emulsions depending on the application requirement More-over these approaches are not limited to fragrances but can beapplied for other challenging active molecules such as drugs andtherapeutics that are either volatile or have mutual solubility

with the capsule shell This opens up new opportunities for a

wide range of encapsulation and delivery applications such ashair products agricultural products and liquid laundry detergents to name a few

ASSOCIATED CONTENT


The Supporting Information is available free of charge on the ACS Publications website at DOI 101021acsami5b11351

Optical microscope images of microcapsules absorbance versus time data for ETPTA microcapsules with diff erentconcentration of PEG-DA (PDF)

AUTHOR INFORMATION

Corresponding AuthorE-mail weitzseasharvardedu

Author Contributions∥HL and C-HC contributed equally to this work

Notes

The authors declare no competing 1047297nancial interest

ACKNOWLEDGMENTS

This work was supported by Procter amp Gamble Co theNational Science Foundation (DMR-1310266) the HarvardMaterials Research Science and Engineering Center (DMR-1420570) and Basic Science Research Program through theNational Research Foundation of Korea (NRF) funded by the

M i ni s tr y o f E d uc a t io n S c ie n c e a n d T e c hn o lo g y (2013R1A6A3A03065122)

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preformed FW emulsion phase co-1047298ows with a photocurableoil phase which are then 1047298ow-focused by the aqueouscontinuous phase and photopolymerized to produce polymer

microcapsules containing model fragrance On the basis of thisstrategy we demonstrate that the low solubility of the capsuleshell with the encapsulated fragrance signi1047297cantly reduces thefragrance leakage rate Furthermore we achieve enhancedretention of fragrance in aqueous media as well as in powderstate by forming a hydrogel network within the microcapsule

2 EXPERIMENTAL SECTION

21 Materials All photocurable oil trimethylolpropane ethoxylatetriacrylate (ETPTA M w = 428 gmol) trimethylolpropane triacrylate(TMPTA) dipentaerythritol penta-hexa-acrylate (DPHA) andphotocurable hydrophilic poly(ethylene glycol) diacrylate (PEG-DA

M w = 700 gmol) were purchased from Sigma-Aldrich and used asreceived without further puri1047297cation Model fragrance (α -pinene)poly(vinyl alcohol) (PVA M w = 13minus23 000 gmol 87minus89

hydrolyzed) tween 80 photoinitiator (22-dimethoxy-2-phenylaceto-pheonon) and n-octadecyltrimethoxyl silane were purchased fromSigma-Aldrich and 2-[methoxy(polyethyleneoxy)propyl] trimethoxylsilane was purchased from Gelest Distilled water (gt182 MΩmiddotmMillipore) (DI water) and ethanol (ge995 Sigma-Aldrich) were usedfor all experiments

22 Fabrication of Glass Capillary Micro1047298uidic Device Glasscapillary micro1047298uidic device was prepared using previously publishedprotocols23 24 Brie1047298 y two cylindrical glass capillaries (World precisionInstruments) of inner and outer diameters 058 and 100 mmrespectively were tapered to a diameter of 40 μm with a micropipettepuller (P-97 Sutter Instrument) These tapered cylindrical capillaries were then sanded to 1047297nal outer diameters of 60 and 400 μm andinserted in a square capillary (Atlantic International Tech 105 mmid) from the opposite direction prior to insertion the cylindricalcapillary with smaller ori1047297ce was treated with n-octadecyltrimethoxylsilane to make the surface hydrophobic whereas the larger ori1047297ce wastreated with 2-[methoxy(polyethyleneoxy)propyl] trimethoxyl silaneto make the surface hydrophilic After the assembly the xy plane as well as the z-axis con1047297guration of cylindrical capillaries was checked forproper alignment

23 Drop Generation Polymerization and Image AnalysisTo control the 1047298ow rates we used syringe pumps (Harvard Apparatus)and recorded the production of triple emulsion drops containingmultiple internal fragrance drops within the micro1047298uidic devices usingan inverted microscope (Leica) equipped with a high-speed camera(Phantom V9) The photocurable oil in the middle phase was then insitu polymerized by UV exposure for 1minus2 s (Omnicure S1000 100W)at the exit of the capillary device We note that all photocurable oilcontains 1 (vv) photoinitiator and that microcapsules with a

hydrogel network contain 10 (vv) photocurable hydrophilic PEG-DA in the aqueous phase of the FW emulsion unless further speci1047297edTo measure the outer radius and shell thickness of the microcapsules

generated we performed image analysis using ImageJ on multipleimages that we obtained from optical microscopy The interface between the FW emulsion and the capsule shell can be distinguished by the position of the thin white layer within the microcapsule thisenables determination of the outer radius and the shell thickness fromthe image analysis In all cases we analyzed at least 100 microcapsulesto determine the average value of the outer diameter and the shellthickness Provided that the shell is sufficiently well-de1047297ned imageanalysis provides an accurate measure of both the outer radius and theshell thickness To ensure that the shell is well-de1047297ned we in situpolymerized the photocurable oil in the emulsion drops at the exit of the capillary device to achieve concentric microcapsules with uniformshell thicknesses otherwise collection and subsequent UV irradiation(off -chip polymerization) result in generation of off -centered micro-capsules with a very thin shell on one side as shown in the opticalimages of Supporting Information Figure S1 due to the inherentdensity mismatch between the FW emulsion phase and thephotocurable oil These cannot be analyzed as well and we did notuse them We also note that the error bars shown in Table 1 are notdue to the image analysis used for processing the images but mainly originate from the generation of emulsion drops in the jetting regimeresulting in some polydispersity in the capsule size and shell thickness

24 Leakage Characterization The leakage pro1047297le is monitoredusing a UV minus vis spectrophotometer (Cary 50 Agilent Technologies) atroom temperature (approximately 25 degC) The detailed step-by-stepexperimental procedures are described in Scheme 1 First micro-capsules encapsulating the FW emulsion were collected at the exit of the capillary device The collected microcapsules were rinsed at leastthree times with an excess volume of DI water to remove the residualPVA as well as α -pinene that may have leaked during the rinsingprocedure however we observed no signi1047297cant reduction in the 1047297nal

concentration of fragrance inside the microcapsule after rinsingcompared to the variability in each capsule from the same batch Then1047297 ve randomly selected microcapsules were transferred to 30 mL of DI water in a quartz cuvette cell (35 mL Agilent) and double-sealed withpoly(1122-tetra1047298uoroethylene) (PTFE) lid and para1047297lm to preventevaporation and maintain a closed system Due to the high volatility of α -pinene (sim3 mmHg 20 degC)25 complete sealing of the cuvette cell isrequired during the leakage study otherwise a noticeable reduction inthe absorbance signal is observed during the measurementImmediately after the microcapsules were transferred de1047297ned as t =0 min UV minus vis spectra were obtained at diff erent time intervals withagitation before each measurement Next UV minus vis spectra werereplotted versus time at λmax sim 200 nm26 This procedure was repeatedthree times to obtain error bars in the leakage study

Scheme 1 Schematics Showing the Detailed UV minus Vis Spectroscopy Procedures To Acquire the α-Pinene Leakage Pro1047297le



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Documents

Encapsulation and Enhanced Retention of Fragrance in Polymer