Potential Use of Polymers and Their Complexes As Media for Storage and Delivery of Fragrances T ⁎ ⁎ Rajnish Kaura,1, Deepak Kukkarb,1, Sanjeev K

Journal of Controlled Release 285 (2018) 81–95

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Journal of Controlled Release

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Review article Potential use of polymers and their complexes as media for storage and delivery of fragrances T ⁎ ⁎ Rajnish Kaura,1, Deepak Kukkarb,1, Sanjeev K. Bhardwajc,1, Ki-Hyun Kimd, , Akash Deepc, a Department of Physics, Panjab University, Sector 14, Chandigarh 160014, India b Department of Nanotechnology, Sri Guru Granth Sahib World University, Fatehgarh Sahib, Punjab 140406, India c Central Scientiﬁc Instruments Organization (CSIR-CSIO), Sector 30 C, Chandigarh 160030, India d Department of Civil and Environmental Engineering, Hanyang University, Seoul 04763, Republic of Korea

ARTICLE INFO ABSTRACT

Keywords: The use of fragrances is often essential to create an elegant, welcoming, or exhilarating environment. Through Polymeric complexes encapsulation, the release and delivery of fragrances are customized in many consumer products. For such Porous materials purposes, cost-effective techniques have been developed and employed with the use of various polymers and Composite porous organic materials to efficiently impart fragrances to foods and various other consumer products. After Adsorption capacities entrapment or uptake/storage of fragrant molecules within a polymeric complex, the properties can be in- Fragrance delivery vestigated by automated thermal desorption (ATD) analysis. For efficient delivery, fragrances are adsorbed (or entrapped) in some media (e.g., fabric or paper). The release of such entrapped fragrances usually is achieved by spraying. Fragrances can be also loaded in a media by purging aroma gases or by adding fragrance essence directly into a liquid medium. Porous materials, such as zeolites, have been traditionally used for air purification as well as in cosmetics and similar applications. Similarly, other polymeric porous complexes have also been used in fragrance delivery as a templating agent for aromatherapy textiles. Such polymeric materials offer an advantage in terms of development of new hybrid blends via homogenous mixing of two or more matrices. Such blends may possess different desirable physical properties as encapsulants. This review article is aimed at pre- senting an overview of polymers and their complexes as the main media of fragrance encapsulation. This study also discusses the expansion and future application of porous materials as host matrices for fragrances.

1. Introduction liquid flavors into free-flowing powders so as to increase their shelf life in food products [3]. Most fragrance delivery systems have the draw- Fragrances are used as one of the most indispensable ingredients in backs of premature evaporation and degradation during storage. Ad- food and fabric industries. Controlled delivery of fragrant molecules has ditionally, the lifespan of flavors is short, and their perception to the remained a research focus in the flavor and fragrance industry. To sensory system is also affected by external factors like heating, oxida- elongate the limited longevity of olfactive perception, the development tion, or chemical interactions. The selection method for a particular of pro-perfumes or pro-fragrances has attracted attention over the fragrance delivery depends upon the nature of the consumer goods and years. Many volatile chemical compounds have been used as perfumes the stimulus that triggers the release mechanism. Hence, for the se- or deodorants in various consumer goods. Their volatility is essential to lection of a particular method, a number of criteria need to be con- provide a pleasant olfactory response, while this property also can be a sidered [4]. First, it is important to consider the targeted application of disadvantage. fragrances, e.g., food items and fabrics. Second, the required level of For fragrances used in various products (e.g., cosmetics, fabrics, humidity in the product is also important, as this factor is directly re- household goods, food, and personal care products), one of the main lated to the storage conditions. The third criterion is the reaction me- interests is to improve the delivery of imparted fragrant molecules with chanism that will be used to trigger the release of molecules [5,6]. controlled release and long life [1,2]. There is also great interest in Constraints on the use of fragrances are commonly accompanied by infusing scent into everyday materials such as fabrics or in turning environmental concerns on the large-scale production of non-

⁎ Corresponding authors. E-mail addresses: [email protected] (K.-H. Kim), [email protected] (A. Deep). 1 These authors are considered as co-first authors because they contributed equally to this work. https://doi.org/10.1016/j.jconrel.2018.07.008 Received 2 June 2018; Received in revised form 3 July 2018; Accepted 3 July 2018 Available online 07 July 2018 0168-3659/ © 2018 Elsevier B.V. All rights reserved. R. Kaur et al. Journal of Controlled Release 285 (2018) 81–95 biodegradable fragrances and toxicity associated with their release. The encapsulation along with the associated scientific challenges in this introduction of encapsulation technique opened up the way to avoid the research area. problems of using such products which was not possible before because of their significantly low chemical stability (during processing, storage, or usage). In fact, a straightforward route exists to develop micro- 2. Mechanisms of encapsulation and release of fragrances encapsulated fragrance material(s) which can be adopted from the pre- existing methods in other fields of applications (e.g., foods and agri- The encapsulation process is useful in improving the properties and culture, pharmacy, or cosmetics). Nonetheless, industrial constraint usability of several industrial and commercial products, such as fragrances, fl (e.g., cost in use) is an important factor to consider in a cost-competi- self-healing materials, nutrients, and drugs. Fragrances and avors have a tive market area as observed from various commercial products. As limited lifetime as their constituents evaporate and degrade before or during fi such, there are at least a few parameters that should be controlled use [7,8]. Flavors and fragrances have widely been applied in many elds during encapsulation process such as process retention, protection, (e.g., food, medicine, papermaking, textiles, leather, cosmetics, and to- deposition, triggered release, and sustained release of volatiles. To re- bacco). The encapsulation process basically functions to coat or capture a solve problems associated with such variables, detailed evaluation is compound. Thus, synthesis of fragrance delivery systems is performed using required to assess not only the interactions between the volatile in- methods like extrusion, grinding, spray coating, coacervation, and inclusion – gredients and the carrier (or encapsulant material) but also the beha- of complexes with cyclodextrins [9 11]. vior of the selected material under several application conditions. Fragrant molecules are encapsulated to prevent them from con- The selection of a particular delivery system depends not only on tamination, exposure to unfavorable environments, and rapid eva- the nature of the product in which this delivery system is to be used but poration. Importantly, the encapsulates must have features such as a also on the kind of release mechanism that is actually required. In this high-strength shell wall, compatibility with aqueous or organic shell respect, encapsulation should be the tool to make more efficient use of cores, ease of use in time release (or thermal release applications), and fragrances as slow or controlled delivery systems. Further, for the re- simple fabrication [12,13]. In addition to external shielding, en- fl lease mechanism of fragrance, a list of factors should be considered capsulation provides extension of the shelf life of fragrances and avors such as its initial loading in the polymer, solubility in the solvent, with extended storage time. With this method, the coating material acts equilibrium partitioning between polymer and solvent (and any pos- as a protective barrier to increase the stability of fragrant molecules. sible diffusional barriers). There are two major concerns regarding the The encapsulate is a spherical protective wall material with an ff development of fragrance delivery system such as the time required to empty core, as illustrated in Fig. 1. They are of di erent types de- μ produce a stable capsule and price of its constituents [4]. It appears that pending upon diameter: i.e., of the nanocapsule (smaller than 1 m), μ μ much work is still to be made in three major areas. Firstly, the retention microcapsules (between 1 m and 1000 m), and macrocapsules (larger μ of volatile components in carrier materials should be improved, espe- than 1000 m) [14,15]. These capsules can be made from various ma- cially in case of products that are subjected to drastic storage or terials, with polymeric materials being the most common. Further, the handling conditions. Secondly, the potential of triggered and sustained controlled release of these fragrances is a challenging task. Long-lasting release systems in terms of hedonic benefits should be investigated fragrance perception, a demand of the current market, requires tight more in depth. Finally, the cost level of current delivery systems is still control of the evaporation rate. Consequently, encapsulation imparts an issue in many fragrance applications. It is thus believed that progress some degree of protection for the fragrances during storage. in all of these areas will require a better understanding of the materials The major composition of fragrances is either reactive or volatile. science aspects underlying encapsulation as well as a better integration Volatile perfume ingredients are used for production, storage, and the of the delivery systems into the perfume/flavor creation process. In ultimate intended use [13,16]. Therefore, researchers are interested in parallel with the continuing work on the common encapsulation tech- controlled release and stability of fragrances. Using polymeric capsules, nologies, it is desirable to pursue new developments to further advance fragrances can be stabilized for several days. For instance, the fragrance from current scientific levels, especially in nanobiotechnology fields of fabrics impregnated with polymeric capsules could be maintained such as molecular recognition, new polymers, and novel composite even after a few washing cycles. Research is continuing toward the materials. development of encapsulates that can sustain more washing cycles In this article, current applications of various polymers and their without losing the impregnated molecules. The selection criteria for complexes in fragrance delivery systems are briefly reviewed. suitable polymer encapsulates depends on the targeted application of Functionality and applicability of these polymers are highlighted in the polymer capsule and the extent of the controlled release required on order to provide a comprehensive view on the development of these the temporal and/or spatial scale [17,18]. materials in perfume industry. Through the extensive survey on the literature, the perspectives of polymer-based active packaging options for various encapsulants are highlighted. Fragrance loading rate of various aromas under varying temperature and pH conditions have also been discussed in details. The practical tools for the identification of encapsulation media such as Scanning Electron Microscope and Transmission Electron Microscope have also been discussed to assess the mechanisms controlling the entrapment of fragrances in the encapsulant. As such, we provide a comprehensive review on the capture and controlled release of fragrances in various products and summarized information on effective options to control the fragrance release process, such as microencapsulation process using various polymers and their complexes. We discuss about various methods to control encapsulation via chemical and engineering processes. Different types of triggering mechanisms for the release of the loaded fragrant molecules in diverse encapsulating agents have been elaborated. Different types of polymeric materials developed for the micro- or nanoencapsulation of various fragrances are also described. The importance of polymeric materials has been reviewed with respect to their properties for Fig. 1. Core-shell structure of encapsulant with the release mechanism.

82 R. Kaur et al. Journal of Controlled Release 285 (2018) 81–95

Fig. 2. List of various materials used for encapsulation of fragrant molecules.

3. Materials for encapsulation the stability of polymer complexes are higher than those of monomer complexes. This is the main advantage explaining why researchers Various high strength materials with biodegradable and bio- preferably use polymer-bound metal catalysts and reagents with high compatible polymeric shells are used for encapsulation. However, many activity and selectivity [32]. Complex formation between transition of the reported fragrance-entrapping molecules are unstable due to metals on polystyrene supports offers additional benefits. For example, their reactive functionalities, e.g., aldehyde, ketone, and terpenes. the supported complexes do not contaminate the product solution and Thus, various other materials, such as chitosan [19], cyclodextrins [20], also maintain high recyclability and selectivity toward a specific fra- and polymers [21], have been explored as alternatives. Porous mate- grance. Polymer complexes have been formed using various metals, rials, such as zeolites, are also studied in fragrance delivery systems. transition metals, and metal porphyrins. Due to the properties exhibited Various natural or synthetic polymers have been widely used in the by these polymeric complexes, they can be used for encapsulation of encapsulation process. Selection of the encapsulating material depends fragrances. The release of impregnated fragrances can be triggered by on the desired characteristics of the final microcapsules. chemical, biological, light, thermal, magnetic, and electrical stimuli, The main focus of this review is to describe various forms of poly- which generally leads to the breaking of a covalent bond between the mers, biomolecules, and porous materials along with their complexes/ controlled release system and volatile liquid [31,33 ,34]. Many re- composites for the development of micro- and nanocapsules, which are searchers reviewed methods for the preparation of such stimuli-trig- usable for encapsulation and delivery of different types of fragrance gered nanocapsules for fragrance encapsulation [26,35]. compounds (Fig. 2). In particular, the discussion is more focused on In this review, we cover various polymers and their complexes with reviewing key points of fragrance delivery, trigger mechanisms of re- a focus on the nanoencapsulation of fragrances, triggering mechanisms, lease, and loading capacities of encapsulants. Both natural and syn- loaded amount of fragrances, and delivery efficiency. It is very im- thetic polymers have been used in various delivery systems in light of portant to evaluate and develop the controllable release of fragrance their versatile applicability. Their applications are well known in var- products. The measurement of fragrance release is hence important to ious other fields, such as catalysis, conductivity, luminescence, and provide information related to the prolonged-release effect, en- porosity [22,23]. Further, most of the polymers are well known for capsulation stability, and shelf life. The techniques for evaluation of many advantageous properties, including low reactivity toward the odor encapsulation or release are diverse and include thermal gravi- core material, easy handling, maintenance of low viscosity at high metric analysis (TGA), gas chromatography (GC), high-performance concentrations, effective dispersion of active entrapped molecules, and liquid chromatography (HPLC), and analysis and electronic nose (e- protection of desired molecular activity [17,24,25]. The above prop- Nose). erties enable the use of polymers in ordered and timely release of the fl avor at the desired location. For preparation of a polymer capsule, 4. Types of fragrances loaded methods based on interfacial polymerization, phase inversion precipitation, in-situ polymerization, and solvent evaporation have been Aroma compounds, also called odorants, aromas, or fragrances, are – commonly employed [14,26 29]. chemical compounds producing a smell or odor. These compounds are As a shell material, polymeric complexes also have shown the ex- produced by volatilizing chemical constituents, even at very low con- fi ffi cellent bene ts of strong a nity with oily fragrances, chemical ro- centrations. The use of fragrances has become very frequent in homes as bustness, corrosion resistance, and safe eco-friendly systems. They have well as for personal care. The fragrances commonly used in daily life fi other bene ts such as easy handing and processing [30,31]. More often, include those that mimic the aroma of flowers (e.g., rose, vanillin,

83 R. Kaur et al. Journal of Controlled Release 285 (2018) 81–95 tuberose, lavender, and jasmine). Upon their release, the fragrant mo- acetate, limonene, and linalool. The compounds of this family possess lecules interact with human olfactory receptors so that the resulting an oxygen functionality or some rearrangement and are referred to as information is processed as perception by the cerebral region of the terpenoids, e.g., citral (lemonal), geranial, citronellol (dihydroger- brain [36,37]. Fragrance chemicals are added in various consumer aniol), and citronellal (rhodinal). products, such as food, wine, spices, perfumes, fragrance/essential oils, (laundry) detergents, fabric softeners, soaps, and personal care products 4.3. Aldehydes (e.g., shampoos, body washes, deodorants, and numerous other products). The aroma present in these compounds provokes and induces a Aliphatic aldehydes are the backbone of the perfumery industry. pleasurable sensation to uplift emotional levels [38,39]. Aldehydes are organic compounds displaying –CHO functional groups, such Based on their source material, fragrant molecules can be classified as cinnamaldehyde, hexyl cinnamaldehyde (hexyl cinnamal), cuminalde- as natural, nature-identical, or artificial compounds [40]. Natural hyde (4-isopropylbenzaldehyde), hexanaldehyde, isovaleraldehyde, anisalde- compounds are extracted from natural sources by physical or bio- hyde, and benzaldehyde. Low molecular weight aldehydes (C2-C7)impart technological approaches (e.g., cinnamaldehyde and benzaldehyde fruit aromas to a composition. However, the odor intensity proportionally [41] or geraniol [42]). In contrast, nature-identical compounds are decreases with an increase in molecular weight. chemically analogous to their corresponding natural compounds but are synthesized by chemical means (e.g., ethyl butyrate, acetaldehyde, 4.4. Alcohols gamma-decalactone [43]). The compounds falling under the artificial category are chemically produced compounds that have not been Organic compounds having a hydroxyl functional group (–OH) identified in natural sources (e.g., galaxolide, fixolide, and musk xylene bound to one of their saturated carbon atoms are called alcohols. [44]). Fragrant compounds of this category include cis-3-hexen-1-ol, menthol, The intensity/threshold levels (quantity) of fragrant molecules are eugenol, furaneol, hexanol, and thymol. These alcoholic compounds are governed by their respective olfactory/odorant receptors on the surface essential starting materials for the synthesis of esters and aldehydes of olfactory receptor neurons. The threshold level can be defined as the [50,51]. minimum concentration of the fragrant compound that can generate a Fragrance compounds belonging to ketonic groups (hydroxyketone detectable response (under standard conditions) [36,37,45]. As such, and butanedione) do not hold significant importance and are mainly some chemical compounds (e.g., thymol, sotolon, isovaleraldehyde, and used to attenuate the lavendered aroma in compositions [50]. furaneol) can be perceived at very low concentrations, while others cannot. The odor intensity/strength thus dictates the quantity of aroma 5. Performance of encapsulation materials and media compounds produced by the odor formulations. Linalool and limonene are found in > 50% of perfumery products Encapsulation is a process of capturing various compounds into a [46]. Linalool is one of the most frequently used aroma compounds and coating material. The effect of the aroma can be prolonged if fragrances is produced in very large quantities for delivering flowery aromas. Si- and flavors are encapsulated with the aid of various media (e.g., milarly, citronellol is another fragrance compound widely exploited for polymers). Fragrance agents can be encapsulated in various polymers, introducing flower notes in perfumes and for adding citrus flavor in biomolecules, minerals, and porous materials (e.g., zeolites and MOFs). food compositions. The presence of different functional groups on dif- The different forms of a fragrance-capturing polymer may take the ferent fragrant molecules accounts for their characteristic odors. Hence, shape of nanoparticles, capsules, micelles, inclusion complexes, or the classification of various fragrant products can be made on the basis emulsions, which are important to their efficient and sustained de- of their chemical structures (or functional groups: e.g., alcohols, ter- livery. The performance of an encapsulating material in terms of sta- penes, lactones, esters, and thiols). It is observed that a small alteration bility and efficiency of fragrance release depends on the type (e.g., in the chemical structure of a compound can yield an entirely different polymers, minerals, and biomolecules) and physical form (microcapsule aroma/odor [47–49]. In some cases, the introduction of double bonds or nanocapsule) of the material used for encapsulation, as discussed in in an aliphatic chemical compound can intensify the aroma by several the following subsections. orders of magnitude [49,50]. There are a large number of compounds falling under many other categories as well. A list of the fragrance 5.1. Polymers compounds covered in this study is summarized in Table 1. Synthetic polymers are the most widely used materials employed for 4.1. Esters encapsulation of fragrant molecules. Several polymers have been explored for encapsulation of fragrances used in products of daily life, Esters are chemical compounds in which the hydroxyl group of an such as perfumes, fabric conditioners, and air fresheners. Further, based acid is replaced by an alkoxy group, generally derived from carboxylic on the size of the encapsulating polymer-derived capsules, they can be acids. Aliphatic esters are responsible for natural aromas in fruits and classified into two types, viz., microcapsules and nanocapsules. flowers. Important fragrant compounds in this class include benzyl acetate, methyl butyrate (methyl butanoate), methyl cinnamate, methyl 5.1.1. Polymeric microcapsules salicylate, isoamyl acetate, and butyl butyrate (butyl butanoate). Cyclic Microcapsulation of fragrance agents has been known for many years esters of hydroxycarboxylic acids are referred to as lactones. Important and has been used for encapsulation of a variety of molecules. Various ar- members of this class include massoia lactone, jasmine lactone, wine tificial and natural polymeric agents have been used by researchers for lactone, and sotolon. controlled release of encapsulated fragrances using different methods. For example, Brisa et al. prepared polysulfone (PSf) microcapsules for the en- 4.2. Terpenes capsulation of perfume (vanillin) with protected and long-lasting fragrance release using a phase inversion precipitation technique. Scanning electron Terpenes are strong-smelling organic compounds often produced by microscopy (SEM) micrographs of a PSf/vanillin microcapsule showed the plants for protection. They may be linear in structure, such as geraniol, cross-sectional empty space for fragrance loading (Fig. 3). PSf microcapsules nerol, and citral, or can have cyclic structures, like camphor, menthol, were used to encapsulate vanillin (fragrance) to study its release properties eucalyptol, and α-pinene. Geraniol is widely used for imparting a rose- for microcapsule preparation with different vanillin concentrations. Results like fragrance and as an intermediate for several other fragrance com- showed that the release capacity of these capsules for vanillin lasted pounds. Some other important members of this class include geranyl for > 6 days with good mechanical stability [14].

84 R. Kaur et al. Journal of Controlled Release 285 (2018) 81–95

Table 1 List of diﬀerent chemical compounds used as aroma compounds.

Order Class Compound CAS no. Chemical formula (MW*) Fragrance Odor strength (recommendation)

1. Esters Benzyl acetate 140-11-4 C9H10O2 (150.18) Jasmine Medium

2. Methyl butyrate 623-42-7 C5H10O2 (102.13) Fruity odor Medium (≤10% solution)

3. Methyl cinnamate 103-26-4 C10H10O2 (162.185) Strawberry Medium

4. Methyl salicylate 119-36-8 C8H8O3 (152.15) Wintergreen mint Medium (≤10% solution)

5. Isoamyl acetate 123-92-2 C7H14O2 (130.19) Banana plant High (≤1% solution)

6. Massoia lactone 54,814-64-1 C10H16O2 (168.24) Coconut Medium (≤10% solution)

7. Butyl butyrate 109-21-7 C8H16O2 (144.21) Fruity type Medium

8. Jasmine lactone 34,686-71-0 C10H16O2 (168.24) Floral Medium

9. Wine lactone 182,699-77-0 C10H14O2 (166.21) Coconut Medium (≤1% solution)

10. Sotolon 28,664-35-9 C6H8O3 (128.13) Caramel Very high (≤0.01% solution)

11. Terpenes Geraniol 106-24-1 C10H18O (154.25) Rose Medium

12. Nerol 106-25-2 C10H18O (154.25) Floral Medium

13. Citral 5392-40-5 C10H16O (154.24) Lemon Medium

14. Geranyl acetate 105-87-3 C12H20O2 (196.29) Fruity rose Medium

15. Limonene 5989-54-8 C10H16 (136.24) Orange Medium

16. Linalool 78-70-6 C10H18O (154.25) Floral Medium

17. Camphor 76-22-2 C10H16O (152.23) Camphor Medium (≤10% solution)

18. Menthol 2216-51-5 C10H20O (156.27) Menthol High (≤10% solution)

19. Eucalyptol 470-82-6 C10H18O (154.25) Herbal type High (≤10% solution)

20. α-Pinene 80-56-8 C10H16 Woody High (136.23) (≤10% solution)

21. Aldehydes Cinnamaldehyde 104-55-2 C9H8O (132.16) Cinnamon High (≤10% Solution)

22. Hexyl cinnamaldehyde 101-86-0 C15H20O (216.32) Perfume Medium

23. Cuminaldehyde 122-03-2 C10H12O (148.21) Spicy High (≤ 10% solution)

24. Hexanaldehyde 66-25-1 C6H12O (100.16) Freshly cut grass High (≤1% solution)

25. Isovaleraldehyde 590-86-3 C5H10O (86.13) Aldehydic High (≤0.1% solution)

26. Anisaldehyde 123-11-5 C8H8O2 (136.15) Anise Medium

27. Benzaldehyde 100-52-7 C7H6O (106.121) Almond High (≤10% solution)

28. Alcohols Cis-3-hexen-1-ol 928-96-1 C6H12O (100.16) Grass High (≤10% solution)

29. Eugenol 97-53-0 C10H12O2 (164.2) Clove Medium (≤10% solution)

30. Furaneol 3658-77-3 C6H8O3 (128.13) Strawberry High (≤1% solution)

31. Hexanol 111-27-1 C6H14O (102.16) Fruity Medium (≤10% solution)

32. Thymol 89-83-8 C10H14O (150.22) Herbal type High (≤1% solution)

Fig. 3. SEM micrograph of PSf/vanillin microcapsules [14].

85 R. Kaur et al. Journal of Controlled Release 285 (2018) 81–95

Fig. 4. Schematic of microencapsulation of BDDMA [53].

The potential of polymers has also been explored for encapsulation solution when its temperature was higher than the melting point of of volatile fragrance compounds in fabric softeners. Tekin et al. pre- paraffin (37 °C and above) [53,55]. pared microcapsules of polyurethane-urea for encapsulating fabric A promising alternative strategy for efficient encapsulation and softener fragrance (TeddysoftTM, by EP) [52]. The interfacial poly- enhanced retention of fragrances in polymer microcapsules was de- merization process imparts a continuous and extended release of fra- scribed by Lee and co-workers (2016). To retain amphiphilic and highly grance by active agents. The experiments were carried out by washing volatile fragrances, the authors proposed a new method that employed hand towels in a washing machine using fabric softener both with and the blending of bulk and microfluidic emulsification processes. This without microcapsules. GC–MS studies revealed that the towels washed balanced fragrance-in-water (F/W) emulsion was encapsulated in a with microcapsule-encapsulated fabric softner had a prolonged smell polymer microcapsule via a microfluidic emulsification process for [52]. A similar process (interfacial polymerization) was used by another improved fragrance retention. This work was further utilized for en- group of researchers for synthesis of poly (1,4-butanediol dimethacry- capsulating fragrance in a powder state [56]. Similar studies were done late) microcapsules containing dementholized peppermint oil (DPO) for on multiple fragrances in a mixture of F/W emulsions depending on use in cotton fabrics, as shown in Fig. 4 [53]. SEM revealed how the their application. The use of these microcapsules supports their pro- core material affects the morphology of the synthesized microcapsule. mising roles in encapsulation and delivery applications, such as hair Thermogravimetric analysis showed thermal stability of microcapsules products, agricultural products, and laundry detergents. below 100 °C, which implies that loss of the fragrance oil can be pre- In a separate set of experiments, temperature-controlled release of vented below this temperature. A very stable washing durability of microencapsulated fragrant molecules was demonstrated for fragrance 15 cycles was observed with this fragrant microcapsule was coated onto encapsulation using thermoresponsive polymer latex particles [57]. cotton fabric. Good laundering durability could lead to their future Stearyl acrylate and dipropylene glycol acrylate caprylate (DGAC) (co- application in textile industries. An encapsulation efficiency of 91% was monomer) were co-polymerized in the presence of dipropylene glycol achieved using response surface methodology (RSM). The stable dur- diacrylate sebacate using a miniemulsion process. The fragrance release ability of these fragrance microcapsules in laundry conditions increases studies from the developed microcapsules performed at different tem- their textile applications. peratures (25 °C, 31 °C, 35 °C, and 39 °C) showed that the release rate Recently, polymeric microcapsules of polymethyl methacrylate was higher at higher temperatures. The prepared polymer was found to (PMMA) have been explored for prolonged release of floral fragrances have potential applications in skin-related personal/healthcare pro- (such as jasmine and flower garland) [54,55]. Fig. 5 shows the SEM blems. The controlled temperature studies show that, for a longer micrographs of PMMA microcapsules at various PMMA:jasmine oil heating time, the released fragrance was changed as the fragrance lo- microencapsulation prepared by a solvent evaporation method in an calized in amorphous domains. Fig. 6 represents the miniemulsion oil/water emulsion system [54][42]. The maximum encapsulation ef- polymerization and the anticipated temperature-controlled release ficiency of 72% was obtained using this method, as measured by mechanism [57]. UV–visible spectroscopy. Similarly, microcapsulation of flower garland was reported in crosslinked PMMA/paraffin microcapsules and P(MMA- co-BMA)/paraffin microcapsules for prolonged release of fragrance in 5.1.2. Polymeric nanocapsules the air as well as in a surfactant solution [55]. The microcapsules were Various materials have been employed for nanocapsulation of fra- produced by suspension polymerization in pickering emulsion. Ap- grance agents [58]. Hu et al. reported the synthesis of poly- proximately 63.9% fragrance retention was observed after 3 months, butylcyanoacrylate as a nanocapsule for rose fragrance delivery [58]. along with high thermal stability (up to 184 °C). In addition, it was For this, BCA (butylcyanoacrylate) was polymerized at 30 °C for 2 h. observed that the fragrance release rate was fast for the surfactant The rose fragrance was further emulsified at 1100 rpm for 10 min during the polymerization of BCA via hydrogen bonding. The successful

Fig. 5. SEM micrographs of PMMA microcapsules at three diﬀerent mixing ratios between PMMA:jasmine oil (%w/w): a) 1:1; b) 2:1; and c) 3:1 [55].

86 R. Kaur et al. Journal of Controlled Release 285 (2018) 81–95

aromatic cotton fabrics impregnated with nano TF after approximately 50 washing cycles were better than those of fabrics impregnated with TF [59]. The controlled release of fragrant molecules has also been explored with visible light stimuli [60]. Briefly, two carrier polymers were prepared by modifying poly (4-vinyl phenol) (PVPh) and by loading anisaldehyde onto polymer 1 while heating in the presence of tri- fluoroacetic acid. Their studies showed that the photorelease of anisaldehyde (fragrant molecule) was stimulated from PVPh (the polymer that released the fragrant molecule) under visible light. Thus, a novel fragrance-release system that can be activated by visible light was established. The studies also supported the recyclability and hence cost- effectiveness of the PVPh polymer in the encapsulation process [60]. Active packaging is another way of providing the controlled release of molecules with an inert barrier between the product and external conditions. Two model hydrophobic compounds, chlorobenzene and fluorescein, were encapsulated in stabilized poly-L-lactic acid nanocapsules (PLA-NCs) for slow and sustained release [61]. The entrapment of fragrance molecules in a polymeric nanocarrier was reported for sustained delivery. Chlorobenzene and fluorescein as two hydrophobic and functionalized molecules were encapsulated in PLA-NCs Fig. 6. Schematic of miniemulsion polymerization (top) and the anticipated with 50% efficiency. The rate of release was found to be steady over temperature-controlled release mechanism [57]. 48 h. The slow and strengthened release of fragrances in biocompatible NC encapsulation can encourage their use in various cleaning and testing (UV analysis) of these nanocapsules showed sustained release on deodorant products. There was no growth of bacterial strains on the cotton fabrics. The encapsulation strategy of rose fragrance in PBCA breakdown products of PLA, as confirmed by mixed axillary culture. nanocapsules is illustrated in Fig. 7. The encapsulated fragrance was This novel method can play a vital role in deodorizing skin and for stable even after 20 wash cycles. Additionally, the GC–MS studies on household purposes with prolonged release times [61]. smaller nanocapsules showed better sustained release properties on Recently, new opportunities to deposit a polymer-based delivery cotton fabrics as compared to large nanocapsules [58]. system onto cotton substrates were introduced. Gunay et al. used poly Further studies were also performed on the sustained release of (N-(2 hydroxypropyl) methacrylamide) (PHPMA) co-polymers and poly fragrant molecules using cotton fabrics. These investigations were done (styrene-co-acrylic acid) (PS-co-PAA) nanoparticles as a reference by impregnating cotton fabrics with polybutylcyanoacrylate (PBCA) polymer for development of fragrance delivery systems [62]. The in- nanoparticles loaded with nano-tuberose fragrance (TF). The fragrance corporation of a hair-binding peptide enhanced the deposition of sustainability was reported to last for > 60 days. GC–FID demonstrated PHPMA copolymers by two to three fold which was demonstrated by the effective sustained aroma release from cotton fabrics impregnated fluorescence experiments. This concept was designed for modification with nano TF compared to fabrics impregnated with TF. The results of of an appropriate polymer with a short peptide sequence with a high GC–FID demonstrated that the sustained-release properties of these affinity toward the substrate. The researchers showed that short

Fig. 7. Structure of PBCA encapsulation-based rose fragrance nanocapsules [58].

87 R. Kaur et al. Journal of Controlled Release 285 (2018) 81–95 peptides can improve polymer-based fragrance delivery on cotton tightness and an 18% embedding rate [64]. under fabric softening conditions. Various experiments were performed The recent use of biomolecules, such as resins, lipids, and proteins, to increase the binding ability of peptide ligands. At each step, the has started the era of biocompatible and non-toxic carrier materials for outcome of peptide-modified surface deposition was determined for the fragrance delivery. However, compared to the other fragrance carriers studies. Further, the proposed peptide can be used to increase deposi- (i.e., synthetic polymers, minerals, and porous materials), these are not tion of any fragrance delivery system onto cotton due to the fabric stable for a long duration of time and have low embedding rates. softening conditions, releasing more fragrance from the cotton surface. With this finding, they proposed the use of other substrates, like hair 5.3. Porous materials and skin, for improving the practical viability of these compounds [62]. The use of the nanoencapsulation approach compared with tradi- Porous materials have also attracted attention due to their high tional microcapsulation is principally advantageous as it is applicable surface area and loading capacity. Zeolites and zeolite-like materials not only to volatile fragrances, but also to other hydrophobic substances were tested as fragrance carriers and used for preparing molecular (e.g., α-pinene) in synchrony [33]. The authors encapsulated α-pinene sieve-fragrance compound systems to be used at a higher temperature molecules in polymeric nanocapsules with a high diffusion barrier and for slow release. The porous structure and channelized aluminosilicates stimuli-responsive release (pH and temperature triggered). The release are good matrices for hosting nanosized particles. As most industrial of fragrant molecules from polymeric capsules was diffusion-controlled work is performed at high temperature, the thermal stability of mole- by a high glass transition temperature and the restricted motion of cular sieve-fragrance compounds is an important issue. Therefore, polymer segments. The prepared nanocapsules showed high loading various analytical systems, such as TGA, IGC, and GC–MS, were used to capacity and controllable release kinetics [33]. investigate the thermal stability of adsorbed fragrance compounds on Another advantage of polymeric nanocapsules over microcapsules is porous materials [66]. that smaller sized fragrant molecules can be firmly attached to fabrics, A fragrance delivery system based on a cyclodextrin-based metal food items, and other products on which the fragrances are to be im- organic framework (CD-MOF) built by a metal cation and a plurality of pregnated. Additionally, they can avoid evaporation of volatile com- cyclodextrin molecules was demonstrated [67]. The system was able to pounds due to the extended life of aromatic products. significantly improve the scent profile of the fragrance composition (e.g., by increasing the average fragrance strength, the effective dura- 5.2. Biomolecules tion, and/or the fragrance fill factor of the fragrance composition). Si-

milarly, Wang et al. reported a zinc-based MOF of Zn3(bpdc)3(apy) for In recent years, biomolecules (such as proteins and lipids) have been the moisture-triggered controlled release of a natural food preservative explored as encapsulating agents for fragrance delivery. Sadovoy et al. [68]. Theoretical calculations and simulations were carried out to show showed a shell with four alternating layers of a protein and a poly- the interaction between AITC molecules (an antibacterial and food phenol to confine the dispersed phase of a fragrance (rose) contained in flavoring agent) and the MOF. Here, the MOF acted as a carrier material an oil-in-water emulsion [63]. Layer-by-layer-assembled shells are for the encapsulation of AITC, while the release patterns were examined suitable candidates in terms of encapsulation, stabilization, storage, and under high humidity conditions. An uptake of 27.3% w/w was achieved release of fragrances. The cotton fabrics furnished by the rose fragrance for entrapment of AITC within the MOF structure, and the entrapped nanocapsules were examined to assess the effect of nanocapsule size on AITC molecules remained in the MOF pores at room temperature. The sustained release. The layer-by-layer assembly of (BSA-TA)2 multilayers studies were presented as a proof-of-concept for using MOFs in food preserved the release of fragrance molecules. Hence, their content safety management [68]. The use of porous materials (like MOFs) offers decay occurred linearly with immersion time. The stability of en- a novel way to increase encapsulation efficiency, release rate, and capsulated fragrance, when observed by light scattering and confocal thermal stability of fragrant molecule carrier materials. laser scanning microscopy, was maintained for up to 2 months at 4 °C The use of mineral-derived materials, such as aluminosilicates, [63]. provides a stimulus responsive fragrance release system based on pH. The release behavior of fragrance/hydroxypropyl-β-cyclodextrin Such systems can have profound applications in perfumes, deodorants, inclusion complexes and fragrance microcapsules was compared on a or other cosmetic products. Ghodke and co-workers proposed a pH- parallel basis [64]. Osmanthus flower fragrance/hydroxypropyl-β-cy- dependent fragrance release system using aluminosilicate clay clodextrin inclusion complex and fragrance microcapsules were pre- (Halloysite) nanocontainers with diameters of 30–50 nm and lengths of pared by vacuum drying and spray drying methods, respectively. The 500–800 nm [69]. The release phenomenon was observed for 24 h using dynamic dialysis method was used to inspect their release behaviors. rosewater as a fragrance. Fragrance release was found to be responsive The transport mechanism was studied by fitting experimental data to to pH changes, as it increased with a decrease in pH. Six types of per- five different model equations to calculate the corresponding para- meation kinetic models (i.e., zero order, first order, Hixson–Crowell, meters. These results showed that the release of inclusion complex and Higuchi, Korsmeyer–Peppas, and Hopfenberg) were used to compare microcapsules matched the Weibull equation. Accordingly, the release the release of fragrance from these nanocontainers. The fragrance de- rate of fragrances in the inclusion complex was quick relative to that of livery was excellently depicted by the Korsmeyer–Peppas model using microcapsules [64]. halloysite nanocontainers. It was concluded that the response of grafted Natural encapsulating agents, such as solid lipid nanoparticles fragranced nanocontainers has profound applications in personal care (SLNs), have also been demonstrated for fragrance encapsulation [65]. and cosmetic products [69]. SLNs were loaded with seven different perfumes (i.e., (α)-amyl-cin- Recently, encapsulation and release studies of a triplal fragrant namal, cinnamal, cinnamyl alcohol, eugenol, geraniol, hydro- molecule in zeolite X were carried out to develop a material with sus- xycitronellal, and isoeugenol) using a hot homogenization process. The tained-release properties. Two different zeolites with particle sizes of loading capacity of SLNs was based on the molecule lipophilicity and 20 μm and 4 μm were prepared using a hydrothermal approach. The lipid nature. Among these seven molecules, cetyl palmitate, candelilla, desorption kinetics of the fragrant molecule from these two zeolite and petrolatum SLNs were stable for approximately one month at 4 °C products were examined. The results indicated slow diffusion of parti- and 25 °C. cles from larger crystals, pointing toward a size-dependent desorption For total encapsulation of jasmine fragrance, polymerization was phenomenon [70]. accomplished with melamine resin and inorganic nanometer-sized si- A group of researchers also worked on five different organosilicon licon dioxide [65]. The core-shell structure of these microcapsules was carriers covalently bonded with model fragrant systems to control their well demonstrated through the afore mentioned studies with good fragrance release rates. The hydrolytic release of these fragrances, i.e.,

88 R. Kaur et al. Journal of Controlled Release 285 (2018) 81–95

1- and 2-phenylethyl alcohol, acetophenone, 2-phenylpropionic alde- fragrance is of paramount importance to achieve a desired sensory re- hyde, geraniol, 2-phenyl-1-propyl alcohol, 2-octanone, and octyl alde- sponse, it results in an undesired loss if stored over a long duration. This hyde, was observed with basic (NaOH) and acidic (HCl) catalysis. The limiting factor reduces their practical utility in the aforementioned authors efficiently discussed the loading process of pro-fragrant pre- products. Taking inspiration from pro-drug technology, many fra- cursors to organosilicon carriers and their release pattern. Experimental grances are often transformed into pro-fragrances and pro-perfumes in studies show that, for the same organosilicon backbone, the hydrolysis order to overcome losses due to their volatile nature [73–75]. This rates are smaller for acidic catalysis as compared to basic catalysis. mechanism is also projected as an alternative to the microencapsulation Additionally, it was concluded that the rate of fragrance delivery does approach. Herein, the fragrance molecules are linked to the substrates not change with the chemical structure of the organosilicon carrier through covalent bonds, which are only cleaved under normally en- [71]. countered environmental stimulus [75]. The cleavage mechanisms Fragrance/silica emulsions were introduced as an efficient fragrance generally involve many benign biochemical or physicochemical con- releaser in a sustained manner along with homogeneous distribution. ditions of diverse variables (e.g., enzymes, pH-induced hydrolysis, light, Mesoporous silica nanocapsules containing a series of fragrances (ce- oxidation in free air, and temperature changes) or a combination dranone, o-tert-butyl cyclohexanone, hexyl salicylate, γ- unsecalactone, thereof [75]. However, such mechanisms are applicable only to specific and lavender) were prepared in miniemulsions and were used to ar- functional groups, such as carbonyl moieties [76]. Since the majority of omatically finish polymer films composed of latex particles for sus- the fragrances are complex and bulky molecular species, the applic- tained release. An interfacial sol-gel process of tetraethoxysilane ability of the pro-fragrance approach is restricted to specific types of (TEOS) in oil/water miniemulsions worked as soft templates for chemicals. To overcome all these drawbacks, fragrant compounds are forming a capsule morphology. The release of fragrances from the often microencapsulated using various carriers, such as synthetic composite film demonstrated their application as aromatic finishing polymers, biological macromolecules, and porous materials [1]. agents. This system displays a homogeneous distribution of fragrances The performance of the aforementioned fragrance carriers is de- in a polymer matrix. The fragrance/silica nanocapsules were found to termined by the combined effects of technical and economic factors in be responsive and promising as aromatic finishing agents to finish association with efficacy, safety, stability, and the pleasing appeal of textiles and leathers [72]. the fragrances [77]. In-depth analysis of various fragrance carriers re- The nanocarriers have better properties for encapsulation and revealed that, for the rendering of fragrance delivery systems, the in- lease efficiency than traditional encapsulation systems. They are ef- dividual and/or combined effects of many parameters (e.g., bio- fective in masking odors or tastes, controlling interactions of active compatibility and biodegradability, physicochemical characteristics of ingredients with the fragrance molecules, allowing tuneable release and the core and shell materials, cost and scalability, and intended de- availability of active agents at a target time and specific rate, and position over the target surface) should also be considered. Herein, we protecting them from moisture and heat. Despite of a number of ad- discuss the performance of the fragrance carriers in relation to these vantages of nanocarriers in food industry and fragrance delivery sys- parameters to help support the maximization of pleasant perception on tems, their safety issues cannot be ignored. As such, the main concerns the commercial scale. are arising from the novel physicochemical properties of encapsulants that can change drastically at nanoscale as compared to their bulk 6.1. Biocompatibility and biodegradability counterparts. Therefore, the safety concerns and regulatory policies on manufacturing, processing, packaging, and consumption of those pro- The most important criteria for selection of any potential carrier for ducts should be taken care of. The main concerns of using nano- fragrance delivery are biocompatibility and biodegradability [78]. technology in perfume production for personal care and cosmetics Biocompatible carriers are chemically inert; they neither react with the products are connected to their potential hazards toward human health host defense systems nor with the cargo material [79]. In addition, the and environment [38,40]. commercialization of perfumery products requires major safety testing Although these nano and microscale materials are considered as procedures, such as general toxicity, mutagenicity, and skin irritation, GRAS (generally regarded as safe), exposure and inhalation may in- to ensure their biocompatibility. All components of the encapsulated crease the risk for their bioaccumulation within body organs and tis- fragrances, including the carriers, must overcome these testing barriers, sues. In order to ensure the safety and efficacy of such products, the which represent the last step before introduction of new ingredients. European Union has incorporated a new amendment in its Cosmetics Such testing should ensure the longevity of the fragrances, their dura- Directive, which became active from 2012 onwards. This new regula- tion of perception, and their olfactory impact. Such materials are me- tion aims that only the safer nanoproducts should enter into the market, tabolized by enzymatic hydrolysis of labile heteroatom bonds to yield safeguarding the health of the consumers. One should be cautious, but benign products that are easily eliminated from the body. It must be not afraid, to embrace the development of nanotechnology as its use has noted that biocompatibility essentially depends upon the chemical already been established in many sectors. In order to avoid the un- nature of the degradation products. In light of biocompatibility and wanted effects on human health and environment, more efficient and biodegradability, fragrance delivery systems were commonly built by tighter regulative rules on the manufacturing, handling, and labelling of adopting many synthetic polyesters (e.g., polylactide (PLA), poly- nano-based fragrance products need to be enforced as soon as possible. glycolide (PGA), and their copolymers) [61,62,80], biological polymers The formation of multidisciplinary collaborations between toxicolo- (e.g., modified polysaccharides) [81,82], and synthetic inorganic gists, physical scientists, and engineers should also be developed with porous materials (e.g., aluminosilicates and zeolites) [69,83 ,84]. It is reliable techniques and protocols for further advancing the perfor- also noteworthy that the utility of MOFs as one of the novel carriers for mance of nanocarriers and their effects on biological organisms fragrance delivery applications has still been limited to proof of concept [24,43]. [85]. As such, the database regarding toxicity, biocompatibility, and biodegradability of MOFs in the backdrop of packing materials has not 6. Critical assessment of polymeric materials in fragrance delivery been established to date.

Fragrances with attractive aromas may exert sensitizing eﬀ ects on 6.2. Physicochemical characteristics of the excipients our emotional cognizance. They are used in many household and personal care products, such as toiletries, air fresheners, and cosmetics. Practical knowledge about the way fragrances perform under var- The optimized utility of fragrances depends upon two critical factors of ious environmental conditions is seminal to their consumer applica- volatility and long-term stability. Although the volatile nature of tions. The manner in which fragrances are perceived by the consumer

89 .Ku tal. et Kaur R. Table 2 Timeline of various fragrance delivery systems.

S. no. Encapsulating agent Fragrance References

Material Chemical formula Physical form Material Chemical formula Loading Delivery eﬃciency amount

1 Polybutylcyanoacrylate (PBCA) C8H11 NO Nanocapsule Rose fragrance (CH3)2C=CHCH2CH2CH(CH3)CH2CH2OH Variable Encapsulated fragrance was [58] (Mixture) (β-citronellol) stable even after 20 wash cycles C8H6O3 (heliotropin) C10 H18 O (geraniol) C18 H26 O (Galaxolide) 2 Polysulphone (PSf) [C6H4-4-C(CH3)2C6H4-4-OC6H4-4- Microcapsules Vanillin C8H8O3 15%–35% Vanillin was released [14] SO2C6H4-4-O]n w/w continuously for > 144 h (6 days). 3 Alternating layers of BSA protein C76H52 O46 (tannic acid) Core-Shell Model Fragrance D-limonene 10% v/v Released fragrance remained [63] and tannic acid (TA) (Mixture) C12 H22 O (Claritone) almost constant over 3 days of C11 H22 O2 (Amarocit) incubation at 40 °C C10 H18 O (Rose oxide high cis) C8H8O3 (Methyl salicylate) C8H16 O (1-Octanal) C8H18 O (1-Octanol) C12 H15 N (Hydrocitronitrile) C12 H18 O (Majantol) C7H14 O2 (Ethyl 2-methylbutanoate) 4 Polyurethane-urea NA Microcapsules Teddysoft NA Variable Encapsulated fragrance lasted [52] longer than the loose fragrance on hand towels

90 5 Poly (4-vinyl phenol) (PVPh) [CH2CH(C6H4OH)]n Photoacid Anisaldehyde C8H8O2 8.6 Mm Photo release of the fragrant [60] molecule under visible light only 6 Polybutylcyanoacrylate (PBCA) C8H11 NO Nanoparticles Tuberose C10 H18 O (Linalool) 50.90% Fragrance in cotton fabrics [59] fragrance (C8H10 O) Phenethyl alcohol impregnated with encapsulated (Mixture) C6H5CH2OCOCH3 (Benzyl acetate) tuberose fragrance lasted through C10 H18 O (Geraniol) 50 washing cycles C10 H20 O2 (Hydroxycitronellal) C10 H12 O2 (Isoeugenol) C14 H22 O (Methyl ionone) C14 H20 O (Lilial) C12 H14 O4 (Diethyl phthalate) C13 H22 O3 (Methyl dihydrojasmonate) C13 H18 O3 (Hexyl salicylate) C15 H20 O(α-Hexylcinnamaldehyde) C14 H12 O2 (Benzyl benzoate) C18 H26 O (Galaxolide) Journal ofControlledRelease285(2018)81–95 C14 H12 O3 (Benzyl salicylate) 7 Aluminosilicates AlNa12SiO5 Molecular sieve- NA NA NA Tested for slow and sustained [66] fragrance release compound at high temperature systems 8 Copolymer of MMA (methyl C5H8O2 (MMA) Nanocapsules α-Pinene C10 H16 1:2 Stimuli-responsive release (pH [33] methacrylate), butyl methacrylate (C5O2H8)n (BMA) Fragrant: and temperature triggered) was (BMA), methacrylic acid (MAA) & C4H6O2 (MAA) Monomer obtained 1,4-butanediol dimethacrylate C12H18 O4 (BDDMA) mixture (BDDMA) 9 Polymethyl methacrylate (PMMA) (C5O2H8)n Microcapsules Jasmine Oil (Mixture of many constituents) NA Microencapsulation technique [54] with high eﬃciency (72% encapsulation eﬃciency) (continued on next page) .Ku tal. et Kaur R. Table 2 (continued)

S. no. Encapsulating agent Fragrance References

Material Chemical formula Physical form Material Chemical formula Loading Delivery eﬃciency amount

10 Solid lipid nanoparticles (SLNs) NA Nanoparticles Fragrance mix C14 H18 O, (α)-amyl-cinnamal NA Stable encapsulation with no [65] C9H8O (cinnamal) fragrance release for up to C9H10 O (cinnamyl alcohol) 30 days (66.5% encapsulation C10 H12 O2 (eugenol) efficiency) C10 H18 O (geraniol) C10 H20 O2 (hydroxycitronellal) C10 H12 O2 (isoeugenol) 11 Poly-L-lactic acid (PLA) (C3H4O2)n Nanocapsules Test molecules C6H5Cl (Chlorobenzene) A biocompatible nanocontainer [61] C20 H12 O5 (Fluorescein) material developed for steady release of encapsulated compound on human skin over 2 days (50% encapsulation efficiency) 12 Organosilane carriers CH3[Si(CH3)2O]nSi(CH3)3 (poly- Silane carriers 11 different C8H8O (Acetophenone) 0.1 mol Release percentage varied from [71] (dimethylsiloxane- fragrances C9H10 O2 (2-Phenylpropionic aldehyde) 43% to 99.5% comethylsiloxane) C10 H18 O (Geraniol) RnSiXmOy (silicone resin) C8H10 O (1-Phenylethyl alcohol) C4H12 OSi2 (tetramethyldisiloxane) C8H10 O (2-Phenylethyl alcohol) C4H16 O4Si4 C8H16 O (2-Octanone) (tetramethylcyclotetrasiloxane) C8H16 O (Octyl aldehyde) C7H22 Si3,tris(dimethylsilyl)methane C9H12 O (2-Phenyl-1-propyl alcohol) C10 H18 O (Geraniol)

91 C8H18 O (1-Octanol) C8H18 O (2-Octanol) 13 Aluminosilicate clay (Halloysite) Al2Si2O5(OH)4 Inorganic Rosewater NA NA A pH dependent fragrance release [69] nanocontainers absolute was found, which increased with an increase in pH from 3 to 7. 14 Mesoporous silica SiO2 Nanocapsules 5 diﬀerent C15 H24 O (Cedranone) NA Synthesized nanocpsules were [72] fragrances C10 H18 O (o-tert-butyl cyclohexanone) homogeneously distributed C13 H18 O3 (Hexyl salicylate) through a polymer matrix and C11 H20 O2 (γ- Unsecalactone) successfully demonstrated (Mixture of many) lavender sustained release 15 Copolymer of stearyl acrylate and C21H40 O2 and C11 H24 O4 Latex particles A fragrance NA 33%–50% Thermoresponsive polymer latex [57] dipropylene glycol composition (from particles for fragrance acrylatecaprylate (DGAC) Givaudan) encapsulation were synthesized monomers α

16 Trimethylolpropane ethoxylate C21H32 O9 Microcapsules -Pinene C10 H16 1:1 with Enhanced retention of fragrance [56] Journal ofControlledRelease285(2018)81–95 triacrylate (ETPTA) surfactant was achieved using a polymeric solution shell 17 Poly (1,4-butanediol (C12 H18 O4)n Microcapsules Dementholized C62 H108 O7 1:1 DPO: 91% encapsulation efficiency [53] dimethacrylate) peppermint oil GTCC (DPO) 18 Poly (methyl methacrylate) (C5O2H8)n/CnH2n+2 Microcapsules 406,026 flower NA NA 63.9% fragrance retention upon [55] (PMMA)/paraffin garland exposure of fragrance microcapsules in air for 3 months 19 Poly (styrene-co-acrylic acid) C4H6N(C8H8)m(C3H4O2)nH Nanoparticles α-Pinene C10 H16 NA Incorporation of a cotton-binding [62] (PS-co-PAA) peptide ligand into the fragrance delivery system resulted in enhanced fragrant deposition by 2–3 times, resulting in more sustained release (continued on next page) R. Kaur et al. Journal of Controlled Release 285 (2018) 81–95

depends upon various factors, including chemical composition and type of deposited surface (e.g., fabric, ﬂoor, hair, and skin). Moreover, there ] ] are various ways by which fragrances are transferred to a substrate. For 68 64 References [ instance, deodorant applications involve a direct transfer of the fragrances to the substrate, while shampoo or detergent products involve a washing or rinsing step during the transfer process. All these phenomena critically depend upon the microencapsulation and the characteristics of the excipients used during the process. The volatile nature of fragrances also renders them highly susceptible to various physico-

ciency chemical parameters adopted for microencapsulation. Synthetic and ffi biogenic polymers (e.g., starch) are routinely used as carriers for encapsulation of fragrance into anhydrous formulations. However, these carriers suffer from rapid release of volatile fragrances when exposed to Delivery e Study established that releasethe of fragrance from microcapsules was slower than the inclusion complex moisture (i.e., sweat in underarm products) [5]. Moreover, perfume products become unstable in an emulsion milieu or during storage in the presence of water and humidity. These limitations may account for the reduced performance of perfume products [12]. Specifically, glassy amount compared to core material or crystalline hydrophilic matrices exhibit many shortcomings, such as plasticizing, swelling, and dissolving effects, in the presence of water or high humidity levels [86]. Such shortcomings are manifested in the form of poor confinement and early release of encapsulated fragrances. The presence of water or humidity levels is an important variable in the selection of the media used for carriers. The use of encapsulation media for fragrances is in fact confined by such criteria (e.g., low water activities and relative humidity (RH) < 80%). Such perfume products are referred to as dry applications. In contrast, hydrophobic carriers are utilized for liquid aqueous products and/or those with high RH. Such products have been referred to as wet product applications. However, these materials suffer from indistinguishable permeability toward both hydrophobic substances and oxygen [12]. Moreover, their ability to

NA 5% retain and protect the volatile fragrances during storage is also reduced. In this regard, the major challenges associated with fragrance delivery

ower are the volatile losses through such hydrophobic/hydrophilic matrices ﬂ and the development of carriers that can withstand high humidity during storage conditions, such as those encountered in tropical coun- tries. Fragrance AITC moleculesOsmanthus fragrance 27.3% w/w uptakeMost fragrances [ are rapidly removed upon deposition onto the target surface (e.g., cotton or other fabric materials) by an evaporation process. Further, typical laundry conditions also favor the removal of these molecules [87]. In light of their volatile nature, stable deposition of the polymer-based fragrance delivery systems is pivotal to enhance their longevity [88]. To date, reports citing the enhanced deposition of Moisture-triggered release Microcapsules and inclusion complex fragrance delivery systems are rare [87,89]. However, results derived from a limited number of studies highlight the signiﬁcance of many factors in such respects (e.g., the polarity of the carrier and type of the deposition surface) toward achieving substantial deposition [90]. A compromised balance between the hydrophilicity and deposition amount is critical to achieve substantial deposition of the fragrances. To

2 overcome many limitations, such as rapid evaporation and degradation O)

2 upon storage, a modular approach was recently described by Gunay 39

O et al. (2017) [62]. According to this approach, enhanced deposition of 102

H polymer-based delivery systems onto cotton surfaces is achieved 54

C without compromising the fragrance release rate or colloidal stability. These authors utilized phage display technology for the identiﬁcation of cotton binding peptide ligands with the strongest aﬃnity. The selected peptide was subsequently incorporated into two model polymer-based fragrance delivery systems, viz., a water-soluble polymer pro-fragrance (poly(N-(2-hydroxypropyl) methacrylamide) (PHPMA) conjugate and -cyclodextrin

β poly (styrene-co-acrylic acid) (PS-co-PAA) nanoparticles. The incorporation of the cotton binding peptide ligand into these fragrance ) delivery systems substantially enhanced their surface deposition, which -CD)

β translated into increased fragrance release from the cotton surface [90].

continued Recently, Gunay et al. (2017) extended their phage display-mediated Material Chemical formula Physical form Material Chemical formula Loading (HP- ( peptide-enhanced delivery of the polymeric fragrance delivery system onto human hair under shampooing conditions [91]. The authors uti- S. no. Encapsulating agent 20 Zinc-based MOF Zn(bpdc)(H 21 Hydroxypropyl-

Table 2 lized two model delivery systems comprising either PHPMA copolymers

92 R. Kaur et al. Journal of Controlled Release 285 (2018) 81–95 as a reference for polymeric profragrances or polyurethane/urea-type microcapsules demonstrated sustained release of the fragrances with core-shell microcapsules as a model physical fragrance carrier. Since substantial antimicrobial activity on the fabric materials. Another shampoo chemicals disinfect the hair by mainly removing hydrophobic major challenge for fragrance delivery carriers involves large-scale elements, it is challenging to use polymeric microcapsules for deposi- synthesis to achieve the desired reproducibility of the micro- tion of fragrances. In the first step, peptide molecules that selectively encapsulated fragrances [77]. The effects of various physicochemical bind to human hair during shampooing were first indicated by phage parameters (e.g., polarity of fragrance carriers, permeability, release display technology, which was followed by their use to enhance the rate, and stability of fragrances upon microencapsulation) on deposi- deposition of the model fragrance delivery system. Microcapsules were tion rate and deposition amount of fragrances should be assessed functionalized with the selected peptide to produce a manifold increase properly. As such, these efforts will motivate further research so that in their deposition on human hair. As such, treatment of the hair the efficacy of fragrances can be maximized with various polymeric samples with the peptide-functionalized microcapsules under realistic carriers. application conditions facilitated the release of fragrance from the hair surfaces [91]. 7. Conclusion The various fragrance delivery systems have been summarized below in Table 2. This review was carried out to provide an overview of the pro- spective applications of polymeric complexes for the storage and delivery of fragrances on the commercial scale. Fragrance chemicals have 6.3. Cost efficiency and scalability widespread applications in various goods of consumer importance (e.g., food, wine, spices, perfumes, laundry detergents, fabric softeners, The relatively low cost of commercialized fragrances and personal soaps, detergents, and personal care products) owing to their soothing care products signifies moderate financial margins on these products effects on human behavior. Chemically, fragrances are organic com- [73]. This situation asserts that the cost of carriers chosen for fragrance pounds with highly reactive functionalities, such as aldehydes, ketones, delivery should be on the lower side. The selection of expensive carriers esters, and terpenes. Microencapsulation in polymeric carriers can ef- is only appropriate if they offer some unique advantage over the free fectively prevent the loss of fragrances caused by contamination, ex- forms of fragrances. Many of the synthetic carriers, such as well-known posure to an unfavorable environment, and their volatile character- biodegradable polymers (e.g., poly (D, L-lactide-co-glycolide) (PLGA) istics. It also ensures enhanced efficacy, safety, stability, and pleasing and poly (caprolactone)), mesoporous silica, and MOFs, are costly to appeal of the fragrances. The selection of polymeric carriers for fra- produce (see Table 2). As such, it is difficult to select them for such grance delivery depends upon various factors, such as facile fabrication, application when considering cost effectiveness. As seen from Table 3, mechanical strength, compatibility with the encapsulated compound, the unit price (per g or per ml) of MOFs (e.g., Hongkong University of and the presence of stimuli-responsive sustained release characteristics. Science and Technology (HKUST-1)), zeolite, PLGA, and silica salts was In light of these characteristics, many natural and synthetic polymers, 28.05 USD, 0.44 USD, 166.97 USD, and 1.30 USD, respectively. In such as chitosan, cellulose, starch, cyclodextrin, poly ortho esters, contrast, the inexpensive pricing of natural polymers (such as poly- zeolites, and porous coordination polymers (e.g., MOFs), have been saccharides) and zeolites (unit price: e.g., < 1 USD) makes them a extensively investigated for encapsulation and sustained release of the preferential choice for fabrication of fragrance delivery carriers. fragrances. The applicability of low-cost materials for fragrance encapsulation The performance of the aforementioned fragrance carriers depends was realized by microencapsulation of orange oil in chitosan micro- upon various parameters, such as biocompatibility and biodegrad- particles through spray-drying technology [87]. It must be noted that ability, physicochemical characteristics of the core and shell materials, the carrier chitosan used during the course of the study is an extensively cost and scalability, and intended deposition over the target surface. investigated biocompatible natural polysaccharide with economic pri- The biocompatible nature of the polymeric carriers is determined by cing [92]. In addition, the spray-drying approach has been well-re- numerous testing methods that reveal the skin irritation characteristics, cognized as a low-cost fabrication method to produce nanoparticles the quantum of toxicity, and mutagenic capabilities. The majority of the [77]. The prepared microcapsules in the aforementioned study polymeric carriers that have been commercialized are approved by were < 20 μm in diameter, which enabled their application in the many federal agencies to ensure their biodegradable nature. However, textile industry [87]. Further, the microcapsules also showed sub- the commercial applicability of MOFs toward such packing applications stantial stability against detergents and temperature increments. In a has not been realized to date owing to the lack of a database regarding recent study, limonene and vanillin encapsulated into chitosan/gum toxicity, biocompatibility, and biodegradability. The physicochemical arabic microcapsules were fabricated to confer fragrant and anti- characteristics of the excipients (e.g., polarity, the presence of hu- microbial properties on cotton fabrics [93]. The as-prepared midity, and stable deposition over the target surface) are other limiting factors that can reduce the performance of polymeric carriers in terms Table 3 of poor confinement and rapid release of fragrances. Another major Comparison of the market prices of various fragrance carriers.a area of research in fragrance delivery is to realize the development of S. no. Name of the fragrance Catalogue no. Amount Price low-cost fragrance carriers owing to limited profit margins in the highly carrier (g/ml) (USD) competitive cosmetic industry. The low cost and ease of availability of

1 Cu-BTC, HKUST-1 688,614-100 g 100 g 2805 natural polymers (such as polysaccharides) and zeolites (unit price: 2 Tetraethyl orthosilicate 333,859-100 ml 100 ml 130 e.g., < 1 USD) as raw materials make them a preferred choice for mi- 3 Poly (D,L-lactide-co- 790,249-1 g 1 g 166.97 croencapsulation and sustained release of fragrances. Optimization of glycolide) the aforementioned parameters can maximize the pleasant perception 4 Poly (Caprolactone-co- 900,291-5 g 5 g 150 of the fragrances on the commercial scale. glycolide) 5 Zeolite 96,096–100 g 100 g 44.75 Future investigations in this thrust area of research will presumably 6 Cellulose S3504-500 g 500 g 76.82 focus on the comprehensive understanding on the eﬀects of various 7 Starch 33,615-250 g 250 g 111 physicochemical parameters (e.g., polarity of fragrance carriers, per- 8 Chitin from shrimp cells C7170-100 g 100 g 91.5 meability, release rate, stability of fragrances upon microencapsulation, 9 Carrageenan C1013-100 g 100 g 56.5 10 Sodium alginate W201502-1 kg 1 kg 137 deposition rate, deposition amount, and interactions of the precursors with their direct environment). It would also inculcate many other a Price information source: https://sigmaaldrich.com. parameters of prime signiﬁcance, such as control of fragrance stability

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