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Encapsulation of volatile compounds in silk microparticles.

Abstract Various techniques have been employed to entrap fragrant oils within microcapsules or microparticles in the food, pharmaceutical, and chemical industries for improved stability and delivery. In the present work, we describe the use of silk protein microparticles for encapsulating fragrant oils using ambient processing conditions to form an all-natural biocompatible matrix. These microparticles are stabilized via physical crosslinking, requiring no chemical agents, and are prepared with aqueous and ambient processing conditions using polyvinyl alcohol-silk emulsions. The particles were loaded with fragrant oils via direct immersion of the silk particles within an oil bath. The oil-containing microparticles were coated using alternating silk and polyethylene oxide layers to control the release of the oil from the microspheres. Particle morphology and size, oil loading capacity, release rates as well as silk-oil interactions and coating treatments were characterized. Thermal analysis demonstrated that the silk coatings can be tuned to alter both retention and release profiles of the encapsulated fragrance. These oil-containing particles demonstrate the ability to adsorb and controllably release oils, suggesting a range of potential applications including cosmetic and fragrance utility.

Keywords Silk fibroin, Microencapsulation, Coatings, Stabilization, Emulsion, Controlled release


Fragrances have been linked to many aspects of everyday life (1); depending on the nature of the scent, a fragrance can spark emotion, (2,3) induce feelings of relaxation and stress reduction, (2) improve alertness, (4) or enhance memory. (5) Maintaining the appropriate intensity level of fragrances in commercial products is desirable for both product function and consumer satisfaction. However, due to the delicate nature and high volatility of these compounds, the sustained presence of fragrances in consumer products is challenging. The volatility of fragrance molecules is usually due to the low molecular weight and the presence of functional groups, such as hydroxides, aldehydes, and ketones. (6) These chemical groups readily react with other compounds and are sensitive to environmental factors including light, oxygen, temperature, and humidity. (7) Degradation of fragrances diminishes the scent and its associated benefits, and can also increase flammability and generate by-products that may be allergenic. (6,8-10)

To address challenges related to the long-term controlled release of fragrances and to increase product stability, encapsulation techniques have been employed to entrap fragrant oils within microcapsules or microparticles in the print, food, pharmaceutical, and chemical industries. (11-15) The microparticle format increases product stability while expanding versatility in applications and the ability for long-term use when compared to the volatile liquids. Techniques including spray drying, melt extrusion, coacervation, and aqueous emulsions have been used to generate fragrance-containing microparticles. (16,17) Although promising, these processes can reach elevated temperature, require toxic crosslinking agents, or produce low levels of product incorporation. (11-19) Aqueous emulsions are typically composed of two or more oil-water components. The ease and safety of aqueous emulsions to form fragrance-containing particles make this approach attractive for fragrance encapsulation; however, its use has been limited because of low encapsulation and retention capabilities. (17)

Silk is a highly versatile protein utilized in the textile, cosmetic, and chemical industries. Recently, it has also been developed for use in a broader range of biomaterials and regenerative medicine needs, (20-22) as well as for cell encapsulation (23,24) and controlled drug delivery systems. (25) Silk is a natural polymer product; a relatively inexpensive, biocompatible, biodegradable, and nontoxic FDA-approved protein derived from the Bombyx mori silkworm cocoon. (20-28) Unlike other biological proteins, with minimal processing, silk can be transformed into various material formats, including but not limited to sponges, films, gels, fibers, mats, coatings, and microspheres. (29,30) The silk protein consists of a block copolymer structure composed of large hydrophobic domains and smaller hydrophilic spacers, as well as hydrophilic chain ends; thus an amphiphilic polymer. This unique structure allows silk to self-assemble into crystallized [beta]-sheets. These crystalline regions, which are physical crosslinks to exclude water, increase stability, result in water insoluble silk materials, and impart mechanical strength. (31) Temperature, water vapor, alcohols, salt, pH, and mechanical stimulation can be used to induce tunable physical crosslinks (the [beta]-sheet crystals), thereby providing a versatile and ambient set of process controls to regulate this feature. (21,32,33) For example, by exploiting the sonication process, silk can be used as the aqueous phase of an oil-water-oil emulsion, with controllable gelation for the incorporation of volatile fragrances and to modulate their stability and release. (34,35)

The aim of the present study was to increase the stability and retention of volatile fragrances via encapsulation in silk microspheres. The use of silk aqueous solution allows the final materials to be all natural, biocompalible, and controllable in terms of properties, while avoiding the use of heat and chemical crosslinkers known to be detrimental to fragrances. A novel silk/polyethylene oxide coating was developed, and fragrance retention and release from coated microspheres were studied.

Materials and methods


Bombyx mori silkworm cocoons were supplied by Tajima Shoji Co (Yokohama, Japan). Sodium carbonate, lithium bromide, polyethylene oxide) (PEO), polyvinyl alcohol (PVA), and Corning transwells were purchased from Sigma-Aldrich, Inc. (St. Louis, MO). Slide-a-Lyzer dialysis cassettes (MWCO 3500) were purchased from Pierce, Inc. (Rockford, IL). Limonene was provided by Firmenich (Plainsboro, New Jersey)

Solution Preparation

Bombyx mori silk cocoons were boiled in 0.02 M aqueous sodium carbonate for either 10, 30, or 60 min to extract sericin and isolate the silk fibroin protein as we have previously described. (36) Isolated silk fibroin was then rinsed three times in deionized water and allowed to dry for 24 h. Dried silk was dissolved in 9.3 M LiBr at 60[degrees]C for 3 h, and the resulting 20% w/v solution was dialyzed against deionized water for 3 days to remove salts. The final concentration of aqueous silk fibroin ranged from 6.0 to 8.0 wt%, which was calculated by weighing the remaining solid after drying.

Silk microsphere formation

Five percent (w/v) PVA solution was added to the silk solution described above (5% w/v) at a volumetric ratio of 4:1. The PVA-silk mixture was placed on a stir plate for three hours at room temperature, resulting in the aggregation and subsequent precipitation of silk microparticles. Stirred mixtures were cast to thin films and allowed to air overnight. The PVA films were dissolved in 40 mL of deionized water, leaving behind silk microparticles. The silk particles were rinsed three times in 40 mL of deionized water, collected via centrifugation, and stored dry until needed.

Fragrance incorporation

Fragrances were incorporated within the microspheres via passive diffusion, by soaking dry silk microspheres in excess fragrance under constant gentle agitation. Incorporation times varied from 1 to 24 h depending on the experiment. Microparticles were harvested via centrifugation to form a pellet and excess fragrance aspirated.

Scanning electron microscopy

Scanning electron microscopy (SEM) was used to observe the microparticles. Silk alone and silk-fragrance loaded microspheres were air-dried for 24 h prior to imaging. Environmental SEM was used to avoid the necessity of sample sputter coating, and allowing for visualization of surface coatings (Carl Zeiss Inc., model Supra 55VP).

Thermogravimetric analysis

Thermogravimetric analysis (TGA) (TA Instruments Q500, New Castle, DE) was used to measure weight changes in the microparticles. For rapid estimates of microparticle composition, the TGA was heated from 23[degrees]C to 50[degrees]C at a rate of 5[degrees]C/min. To distinguish surface fragrance from encapsulated fragrance samples, a 240 min incubation at 50[degrees]C was performed prior to continued heating.

Fourier transform infrared spectroscopy

Fourier transform infrared spectroscopy analysis of the silk films was performed with a Jasco FT/IR-6200 spectrometer (Easton, MD), equipped with a multiple reflection, horizontal MIRacle attenuated total reflectance attachment (ZnSe crystal, from Pike Tech., Madison, WI). For each measurement, 128 scans were performed each with nominal resolution of 4 [cm.sup.-1]. To identify secondary structures in the protein samples, peak positions of the amide I region (1595-1705 [cm.sup.-1]) were determined based on the absorption spectra. Fourier self-deconvolution and curve fitting were performed using Jasco Spectra-Manager Analysis Software (Jasco, Easton MD), as we have previously described. (32)

Results and discussion

Incorporation of fragrances in preformed silk microparticles

A well-established method to form silk-based microparticles for fragrance encapsulation involving the emulsion of silk and PVA was studied. Unlike particles prepared from traditional O/W/O, those prepared with PVA were not formed directly in the presence of the fragrance, but were first formed separately and loaded post-preparation with the desired compound.

To incorporate fragrances, the hollow microparticles were incubated in solutions of the fragrances. The semi-rigid, porous network of the microparticles (37) dictated that the fragrance occupies the void spaces, and thus, a high degree of swelling was not expected, even for fully saturated particles. Microparticles were formed using 30 min process times, and fragrance was passively taken up without noticeable swelling even after 24 h of soaking (Fig. 1). The time for complete fragrance uptake was determined by varying microparticle incubation time and analysis of fragrance content by TGA. Figure 1c-d shows TGA thermographs of microparticles immersed for 1 or 24 h in limonene oil. For both times, the limonene fraction was about 85-90% (wt), indicating that 1 h was sufficient for microparticle saturation. Similar incorporation fractions were found for the other process times and fragrances studied, with total fragrance incorporation after 1 h ranging from 80 to 90% (data not shown). We hypothesize that the hydrophobic nature of the silk protein and the high surface area of these domains in the assembled silk particles are responsible for the uptake and high loading capacity of the materials. Given that the process lime did not affect fragrance uptake, 10 min-processed silk microparticles with 1 h of fragrance loading were used in all of the following experiments.

Fragrance retention in silk microparticles using silk coatings

Hydrated barriers have been demonstrated to alter the rate of compound release from silk, hyaluronic acid, gelatin, and alginate systems. (38-42) To stabilize the encapsulated fragrances, to increase retention, and to prolong release rates, the microparticles were coated with layers of silk.

Fragrance-loaded microparticles were placed within a ~8 [micro]m pore size filter. The membrane held the particles stationary, while small quantities of coating solutions were passed over them. The small pores allowed liquid to flow through but prevented the particles above 8 [micro]m to pass through the mesh. Each coating was composed of a PEO layer surrounded by a silk film. Particles were coated with 1, 3, or 5 layers. Coated particles maintained spherical shape, membrane flaking indicative of silk film deposition and showed minimal less aggregation. Figure 2b-d depicts SEMs of these particles. Figure 2e and Table 1 summarize the TGA findings, demonstrating that even one hydrated coating was sufficient to retain up to 3% of the encapsulated fragrance even after the 240 min incubation at 50[degrees]C.

The particles with three coatings did not show an improvement in fragrance protection when compared to the control samples. The control samples consisted of microparticles that had not been exposed to fragrance prior to coating. The observed lack of improvement could be due to a number of factors including poor initial encapsulation, fragrance loss during coating, poor layer deposition, or incomplete silk crystallization. Particles coated with five layers showed increased fragrance retention up to 2% (Table 1). The silk/PEO combination was effective at maintaining the encapsulated fragrance, even at elevated temperature. The use of the silk to encapsulate limonene provides a fully biodegradable, biocompatible encapsulation system which demonstrates the potential for limonene stabilization at elevated temperatures. (6)

Although the PEO is highly viscous and functions as a good water retention barrier, the silk coating is crucial for protection of the encapsulated compound. PEO coatings without a silk layer quickly dispersed when submerged in an aqueous environment. Additionally, PEO alone was not sufficient to prevent water evaporation when subjected to heating. The silk layer limited diffusion of PEO and prevented rapid water loss. These two factors help maintain hydration around the microparticles and prevent premature loss of fragrance.

Tuning silk coalings for altered release

To delay the release of the fragrant oil, a single silk-PEO coating was applied to fragrance-loaded microparticles. The layer was treated to induce [beta]-sheet structure in the silk coating. The fragrance-containing spheres that were not exposed to layer treatments showed 30% [beta]-sheet (Fig. 3a). It is likely that the [beta]-sheet content measured in the nontreated spheres was induced by the escaping fragrance oil. Generally, crystallinity increased with exposure to treatments known to induce conformational changes in the silk protein secondary structure, such as water annealing or exposure to alcohol. The highest crystalline content was observed in the spheres exposed to ethanol, with these spheres displaying up to 36% [beta]-sheet.

Release of fragrance from the treated particles was determined using TGA. Figure 3b shows normalized release of fragrance from microspheres exposed to various coating treatments. The spheres with nontreated coatings consisted of 10% of weight by fragrance. This fragrance was released in a first-order profile corresponding to the increase in temperature. The 1.5 h water anneal treatment demonstrated the same retention and release profile as the nontreated spheres, which was not surprising given that they displayed the same [beta]-sheet content as the controls (Fig. 3a). The spheres exposed to 6 h of annealing displayed 7% fragrance retention, which is slightly lower than the control. Samples exposed to 12 h of annealing demonstrated higher fragrance retention, 12%, with the same first-order release profile as with the other water annealed samples. Ethanol treatment showed 10% fragrance retention which was comparable to the control and samples exposed to 6.5 h or less of watering. The release profile of the fragrance from these spheres was different, displaying a zero order release with respect to increasing temperature, [beta]-sheet content could play a role in this release, establishing a more densely packed coating which would slow fragrance escape. In treatments in which the overall [beta]-sheet content was the same, specifically for ethanol and 12 h water annealing, processing differences could account for the observed differences in oil content and release profiles.

The water annealed spheres were annealed in a dry state which could limit exposure to water vapor and thus produce heterogeneously crystallized coatings. Spheres that undergo ethanol treatment were suspended in ethanol, which ensured exposure of the entire coated surface to the [beta]-sheet inducing solvent. This more homogenous crystalline coating could explain the zero order release observed, while the heterogeneous coating formed in the water annealing process could account for the larger initial burst release and the observed variability in fragrance retention with respect to total annealing time.

Release of small molecules from silk film constructs similar to the applied coatings displays profiles comprising of an initial steady-state diffusional release and followed by complete matrix degradation and complete product recovery. (43) The retarded release of model molecules from films with higher crystallinity was attributed to increased domain interconnectivity in the silk structure which in turn increases the molecules resonance time. (43) A similar phenomenon was observed here, with fragrance release being delayed from the spheres with highly crystallized coatings. The silk hydrophobic nature may also cause adsorption of the oils within the sphere as well as in the film coatings, delaying fragrance release. This interaction is facilitated by the mismatch in hydrophobicity between the encapsulated oil and first PEO layers. The hydrated PEO helps retain fragrance and allows the application of additional silk coatings.


A tunable biocompatible method of producing silk microparticles for encapsulation of volatile compounds as well as soluble molecules was developed for fragrance and flavor delivery. A novel fully biocompatible silk coating method was described which can be applied to delivery systems. The encapsulated microparticles were prepared without the use of toxic crosslinkers, or exposure to high temperature, as is common for other encapsulation methods. Hydrated silk coatings prevented fragrance escape from encapsulated microparticles. Release profiles were tuned by controllably altering the crystallinity ([beta]-sheet content) of the coating layer.

DOI 10.1007/s11998-015-9668-1

R. Elia, J. Guo, F. Omenetto, D. L, Kaplan ([mail])

Department of Biomedical Engineering, Tufts University, Medford, MA 02155, USA


S. Budijono, V. Normand

Firmenich Inc., Plainsboro, NJ 08536, USA

D. Benezedi

Firmenich SA, 1, Route des Jeunes, 1211 Geneva 8, Switzerland

Acknowledgments The authors thank Dr. Eleanor Pritchard for earlier work on these concepts, Dr. Peggy Cebe and Dr. Nicholas Guziewicz for their assistance with the TGA, and Dr. Jing Zhou and Samuel M. Kessel for their assistance with particle formation, coating, and purification. We thank the NIH (P41 EB002520) and Firmenich for support of this research.


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Table 1: Percent weight loss from silk and silk-limonene-
containing microparticles with one, three, or five PEO/silk

Applied coating         microspheres           1 Coating

Fragrance released   14.1 [+ or -] 1.6%   11.6 [+ or -] 0.44%

Applied coating          3 Coatings            5 Coatings

Fragrance released   14.6 [+ or -] 0.73%   12.7 [+ or -] 1.5%

TGA temperature increased stepwise at a rate of 5[degrees]C/min
and maintained isothermally at 50[degrees]C for 240 min.
Mean [+ or -] SEM


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Author:Elia, Roberto; Guo, Jin; Budijono, Stephanie; Normand, Valery; Benezedi, Daniel; Omenetto, Fiorenzo;
Publication:Journal of Coatings Technology and Research
Geographic Code:1USA
Date:Jul 1, 2015
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