Fully Printed Geranium-Inspired Encapsulated Arrays for Quantitative Odor Releasing.

Olfactory is an extremely fine way of perception. However, the process of smelling is prone to various interference factors. Further development to enhance the communication desires an odor-releasing strategy, which could quantitatively offer a variety of fragrances. Here, we report a fully printing strategy to heterogeneously integrate odor-containing materials and protective coating films. Inspired from the fragrance-containing drum structure on the geranium leaf, encapsulated arrays are fully printed on the flexible or rigid substrates with more than 20 spices. Quantitative concentrations of odor molecules can be released from the encapsulated arrays after scraping the protective poly(lactic-co-glycolic) acid (PLGA) shells. Importantly, various odor-based arrays are printed on the same flexible substrate, which permits selective releasing and arbitrary mixing of the spices. Effective odor-releasing properties of encapsulated arrays make them promising for food security and anticounterfeiting, investigating olfactory discrimination abilities, and strengthening olfactory communication.

Introduction

Olfactory is an extremely fine way of perception, which can be linked to the mood and health status.[1] In addition, psychological experiments prove that the olfactory center of the brain has a close relationship with the limbic system.[2] Where the limbic system controls emotions, fear, and memories, it can be concluded that the odor can affect our thinking.[3] Odor-mediated communication between individuals, once thought to be limited to “lower animals”, is now understood to carry information about familial relationships, stress, and anxiety levels.[4−6] Olfactory sensitivity and discrimination abilities will degenerate with age. Impaired olfaction is a leading indicator of certain neurodegenerative diseases, notably, Parkinson’s disease[7] and Alzheimer’s disease.[8,9] However, the process of “smelling” is prone to various interference factors, such as the scent of scent, odor residue, and the variety of odors.[10,11] The olfactory sensitivity estimation is strongly influenced by the species of odors being tested.[12−14] It is desirable to explore a controllable odor-releasing strategy to cover a variety of spices.

In nature, the transmission of biological information, such as the pheromones among plants and animals,[15,16] is one of the most fascinating research fields in the life sciences. More than the fragrance of flowers, the aroma of many plants’ aromatic oils is good for human health.[17] Scented geranium is a perennial trailing herb extensively cultivated for the essential oil present in its leaves, which is used as a cheap substitute for the attar of roses. On the leaves of geranium, there are many drum-structured microcavities (secretory head of glandular trichomes), which are full of odor molecules.[18] The volatile materials in the drums are known as the geranium oil, which will not react chemically or physically with the drum cuticle. The fragrance is released due to the rupture of these drums, which can be caused by external factors such as high temperature and low air humidity or caused by the physical contact from an animal or insect. Thus, the air is heavy with the scent of flowers after softly rubbing these leaves.

Here, we demonstrate a fully printed method to fabricate geranium-inspired odor-containing encapsulated arrays for programmable release of more than 20 spices. By controlling the wetting behaviors of spice inks and poly(lactic-co-glycolic) acid (PLGA, an FDA approved polymer) inks, PLGA completely covered the previously printed droplets arrays with the location of the pattern.[19] All oil-soluble, water-soluble, and emulsion spices can be encapsulated in the PLGA drums via this fully printing strategy. Similar to the release mechanism on how the scent of geranium is released after rubbing, odor molecules can be released from the microarray-based chips after rubbing the protective PLGA shells. Figure illustrates the mechanism of fully printing geranium-inspired odor-containing arrays. The aromatic smell of the geranium is due to the release of essential oils from the glandular trichomes. There is a space for storing geranium oil between the secretory cephalic cell membrane and the stratum corneum. It has been studied that there are no pores on the stratum corneum and the release of the fragrance is due to the rupture of these shells. By mimicking this drum-structured microcavity, the two-step printing strategy is investigated to achieve the dynamic control of the spice and PLGA droplet behavior.[20−22] More than 20 kinds of spices (water-based or oil-based solution) can be printed on the desired substrates (silicon, glass, or plastic). Both the droplet behaviors on the substrates and on the previous printed droplets[23,24] have been investigated to fabricate the effective odor-release architectures. Developing the corresponding technology may be economically sensible for upgrading of consumption and promoting the production of odor-mediated communication products.

Figure 1

Fully printed geranium-inspired encapsulated arrays for customizable odor releasing. (a) The photograph and optical microscopy of geranium. The scanning electron microscope (SEM) image shows the fragrance-containing drums. The photograph and SEM images of the geranium leaf surface. There are many small drums on the leaf surface, indicated by the red dots, which are full of the fragrant liquid. (b) A diagram of the mechanism for drum-structured microcavity involved in the release of fragrance. The geranium oil is stored in secretory heads of glandular trichomes and will release only if the cavity is broken. The drums in the SEM lateral view were broken or flat after friction with a normal force of about 2.5 N. (c) Schematic illustration of the two-step printing strategy to fabricate the geranium-inspired encapsulated arrays. The first step is to ink-jet print aqueous dots with various spice inks. The second step is to print poly(lactic-co-glycolic) acid (PLGA) inks on the spice dot arrays as the protective shell.

Results and Discussion

Preparation of Core–Shell Odor-Releasing Encapsulated Arrays

Figure b shows the detailed structures of drum-structured microcavity before and after rubbing. There are two types of head gland hairs on the leaves. The hair (colored by red) has a large secretory head. The secretions are mainly concentrated in the epidermis cavity where the fragrance is stored. Apparently, there are no pores on these hairs. The fragrance is only released after the rupture of these hairs (Figure b and Figure S5). In nature, the scent of geranium is caused by the physical contact from an animal or insect, which attracts more insects to gather honey and spread pollen. From the lateral view of the broken hairs, the wall thickness is 1.32 ± 0.29 μm, while the undamaged microcavity is about 30.25 μm. The protective shell made from cells is firm enough to store fragrant oils against the moderate external force. In the force test experiment of the geranium leaves, the minimum force is 2.5 ± 0.3 N to make us smell the fragrance, which is close to the normal force people used while holding the pencil to write (about 2.98 N).[25] The individuals from children to elders can rub the leaves easily to smell the fragrance. Inspired by this microcavity structure and the release mode, spice inks are covered by the uniform shell while the odor release is achieved by the rupture of shells after the moderate force. As shown in the Figure c, the two-step ink-jet printing method is investigated to imitate the encapsulated structure of the geranium leaf. The PLGA shell gap with the prolate border sealed the spice ink without any voids or gaps, which keeps odors for a certain time. It is essential to fabricate these encapsulated arrays after investigating the droplet diffusion, coalescence, and drying behavior during the printing process.

The key problem is to fabricate core–shell[26]-encapsulated arrays for the quantitative release of diversiform odorous molecules. The capsules contain an aqueous core with odorous molecules and a PLGA protective shell.[25] To imitate the fragrance storage and release mode of the geranium leaves, the shell keeps the spice droplet intact and can be easily scraped with a moderate force (about 2.5 N) to release the odors.[23,24] In recent years, the inkjet printing method has been widely used in the patterning of functional materials at a large area. By combining the encapsulated design and printing strategy,[27,28] the programmable droplet preparation can be availably achieved. The printed droplets can be well controlled via investigating the wetting and dewetting processes for the accurate odor release of encapsulated arrays.

The first step was to print the aqueous cores, which contains various kinds of odorous molecules with the controlled concentration and volume. As shown in the Figure S2 in the Supporting Information, two types of the spices (water-based and oil-based spices) were used to produce odor-releasing laminas. To ensure the printability of the aqueous core inks, ethylene glycol was added to adjust the viscosity (Figure S3). Poly(vinyl alcohol) (PVA) was added as an emulgator to make the oily molecules dispersed in the water. A typical formulation of the spice ink consisted of 0.5 g of PVA, 2.5 g of ethylene glycol, and a certain amount of longan, milk, or vanilla spices (longan and milk spices were water-based, while vanilla was oil-based) dissolved in 10 g of total solution. The concentration of fragrance molecules can be regulated according to the demand. The diameters of the printed spice droplets were regulated by varying the extrusion time and pressure (Figure S4).

Next, protective shells were in situ printed onto the spice droplets. To achieve encapsulating the aqueous cores and achieving controllable core release, we dissolved PLGA in dichloromethane before printing.[29] The oil-based ink with PLGA was directly printed onto the spice droplets on the substrate. The dichloromethane/water interface is case-hardened due to the flash evaporation of dichloromethane at room temperature (about 3 s). The uniform PLGA shells would appear as soon as dichloromethane evaporated. Thus, the solidified capsules can be readily printed in various preset patterns to create programmable capsule arrays.

Precise Fabrication of water-core and oil-shell Structure

Different kinds of 4 × 9 arrays were printed to achieve core–shell capsule arrays for quantitative release. Figure a displays an optical image of an array of 1 mm diameter core and 2 ± 0.5 mm shell, all of which the distance between the upper and lower centers is 3 ± 0.2 mm and the distance between the left and right centers is 4 ± 0.2 mm. The side-view photographs show the spice ink droplet, PLGA shell on the droplet, and a clean surface after rubbing (Figure b). It is essential for the operation in the core–shell structure to form a stable oil shell, which can tightly cover the aqueous phase from the air.

Figure 2

Mechanism of the printing process for encapsulated arrays. (a) Photograph of encapsulated arrays with core–shell structures (longan on the PET film: the surface tension of longan aqueous ink is 39.3 mN·m–1, and the contact angle of PET is 60°). Both the enlarged-view SEM image and the side-view photograph show that each dot has a 1 mm diameter core and a 2 ± 0.5 mm shell. (b) Schematic illustration of the ink-jet printing method to imitate the encapsulated structure of the geranium leaf. The side-view photographs show the spice ink droplet, PLGA shell on the droplet, and the clean surface after rubbing. (c) The schematic process of the core–shell structure under suitable interface tension of the oil–water–air. (d) The confocal microscopy images of the core–shell structure (green part: water phase, red part: oil phase). (e) The schematic illustration of encapsulated droplets with different surface tension of spice inks and different contact angles of substrates.

PVA in the aqueous core, as an aqueous surfactant, has a large amount of hydrophilic hydroxyl groups and a hydrophobic long carbon chain. Depending on its amphiphilicity and dispersibility, PVA has been widely used in oil–water separation and emulsion polymerization.[30] In the printing process, PVA formed a miscibility layer on the surface of the aqueous core to wrap the droplets. The interfacial tension of the oil–water interface will be reduced by the amphiphilicity of the PVA to form a stable oil–water interface (oil–water core–shell structure). Brassard et al.[31] reported that the oil phase would surround spontaneously the water core only if the flowing condition was metwhere γw–a, γo–a, and γw–o are the interfacial tension of the water–air, oil–air, and water–oil interfaces, respectively. Indeed, the oil medium not only reduces the contact angle hysteresis,[32] which can decrease the contact line friction to drive into the core–shell structure, but also lowers their interfacial tension[32] that accords with the formula . As shown in Figure c and Figure S6, an aqueous–oil–air contact line experiences a force that spreads the oil phase around the water droplet until a complete shell is formed. When the water droplet is enclosed in oil, it reduces the contact line friction, as core–shell droplets are actuated in air and the lower surface tension of dichloromethane than that of water. PLGA-protective layer will be formed after the flash evaporation of dichloromethane with a stable core–shell structure.

The aqueous and oil solution were dyed separately to study the interface of the water–oil core–shell structure. According to the cross-sectional views of the confocal microscope in Figure d, the green water phase was clearly separated with the red oil phase. It is obvious that PLGA can well encapsulate the aqueous odor droplets to form the core–shell structure.

Moreover, Figure e and Figure S7 exhibit the relationship between different tension of aqueous cores and different contact angles of the substrates, while PLGA in oily ink was keep the same as 2.5 wt %. As shown in Figures S8 and S9, with the decreased in the surface tension and the increased in the contact angle of the base, the aqueous droplet spreading area increased gradually. While the substrate was too hydrophilic, the aqueous droplets spread instead of beading and the polymer droplet cannot cover it. When the contact angles were less than 60°, most cannot form the core–shell structure, most of which area of the aqueous layer was larger than that of the PLGA layer, which resulted in failure to the core–shell structure. So, moderately hydrophobic substrates were optimal (contact angles >60°) as long as we control the surface tension of the perfume in the range of 20–40 mN·m–1. As shown in Figure S8, the films of less than 60° in contact angles were heteromorphic and chaotic. Although some can be core–shell (such as 26.9 or 28.7 mN·m–1 in surface tension and 20 or 40° in contact angle), they cannot ensure to lock the aqueous core stably because of the liquid is not beading enough. Although the polymer solution and aqueous droplet were immiscible, the emulsifier, PVA in the core, allowed the PLGA solution to wet the odorous droplet fully.[25] Moreover, it can form a kinetically core–shell structure with the aid of the high volatility and small volume of the carrene.[27]

Encapsulated Arrays with Different Spices on Various Substrates

Smart flexible packaging is very important for all walks of life.[33−35] Packaged goods with the odor-releasing characteristics are an indispensable part of commodity circulation, which involves food, beverage, perfume, cosmetics, medicine, and other industries. It is essential that different odors are printed and encapsulated as predesigned arrays on flexible substrates to match the arbitrary curvatures. After investigating the mechanism of two-step printing, we successfully fabricated odor-containing microarrays with the uniform droplet sizes through optimizing the spread and shrinkage behaviors of PLGA inks on the first printed droplets. The photographs display heart-shaped, alphabetical (milk), and flower odor-containing arrays on the PET films (Figure a). The flexible films can be freely bent without the leakage of odor molecules. Longan, milk, and vanilla are chosen as the demonstration odors to show the core–shell-encapsulated arrays on flexible substrates. As shown in the Figure b, PLGA completely covers different solvent-based droplets. Both oil and water-based flavor can be successfully printed. To make these odor-containing arrays more clearly, vanilla and milk inks were printed on the glass and silicon substrates. The flashing images of the droplets under the shell are obtained under the backlight illumination. It is obvious that plenty of odors in the droplets are sealed with the PLGA shells, which gives the strong scent after rubbing off the shells. According to the numerical statistics in Figure d, about 86.1% aqueous droplets are 0.75–1.25 mm in diameter on the PET film and 90.5% on the glass substrate and 86.1% on the silicon wafer. The concentration of released odors is precisely controlled through rubbing the number of fragrance-contained droplets. Thus, the quantitative odor release can be achieved. Moreover, the combination of different odors can be simultaneously printed on the same substrates. We can smell out the longan–milk flavor after rubbing the longan- and milk-containing encapsulated arrays. Various odor-based arrays are printed on the same flexible substrate, which permits selective releasing and arbitrary mixing of the spices. Effective odor-releasing properties of encapsulated arrays make them promising for investigating olfactory discrimination abilities and strengthening olfactory communication.

Figure 3

Characterization of encapsulated arrays with different spices on various substrates. (a) Customizable printed encapsulated array patterns on flexible substrate (PET film): “milk”, heart, and flower. (b) Flexible odor-releasing arrays with longan, milk, and vanilla fragrance. (c) Diameter distribution of aqueous odor droplets on PET, glass, and silicon substrates.

Quantitative Control of the Odor Releasing

A major problem associated with odor control is measurement. However, control is difficult with a quantified, simple, and visual method until the volatile odor gas can be easily collected. To show the erasable storage property and to test the quantity-controlled release of odors, we prepared two odor-releasing films with a rose or NH3 molecules of 4 × 9 arrays and strictly controlled the diameter of the odor droplets in 1 mm. When the protective layers were scraped, different amounts of droplets were exposed, which were quickly oxidized by the KMnO4 (Figure a). The color of KMnO4 solution was changed from purple (without rupture), to pink (release of one row), to orange-red (release of two rows), to orange (release of three rows), and finally to yellow color (release of four rows). Moreover, without the scraping treatment, a fragrance film containing 4 × 9 core–shell capsules was placed in KMnO4 solution. After 24 h of immersion, the color of the solution did not change significantly, indicating that PLGA can effectively protect the fragrance droplets. Afterwards, in Figure b and Figure S10, the NH3 molecules were volatilized from the film after scraping off the protective layers. NH3 was quickly dissolved in deionized water on the pH test paper, which caused the pH test strip to change color and made it into alkaline. The pH test strip was yellow originally and changed to dark blue along with quantitative ammonium releasing after rubbing different rows of NH3-encapsulated arrays. Figure c shows the linear relationship between pH values and odor points of NH3. Significantly, both the optical images and the quantitative release experiment clearly demonstrate that the odor in encapsulated arrays is released only when the protective PLGA shells have been destroyed by friction. The untouched ones are kept safely in the capsules without releasing. Moreover, this capability, which achieves quantity-controlled release of odors, will have a great application prospect in the biosensor and volatile drug therapy.

Figure 4

Quantitative control of the odor-releasing process. (a) When the protective layers were scraped, different amounts of droplets were exposed, which were quickly oxidized by the KMnO4. Without the scraping treatment, the color was not changed after 24 h in the last photo. (b) pH test strip changes from original yellow to dark blue along with quantitative ammonium releasing after rubbing different rows of NH3-encapsulated arrays. The untouched ones are kept safely in the capsules without releasing. (c) The linear relationship between pH value and the odor points of NH3.

Conclusions

In conclusion, we demonstrated a controllable odor-releasing strategy for covering a variety of spices. Inspired from the fragrance-contained drum structure on the geranium leaf, encapsulated arrays are fully printed on the flexible or rigid substrates with more than 20 spices. Quantitative concentrations of odor molecules can be released from the microarray-based chips after selectively rubbing the protective PLGA shells. Importantly, various odor-based microarrays are printed on the same flexible substrate, which permits selective rupturing and arbitrary mixing of the spices. Effective odor-releasing properties of odor-encapsulated arrays make them promising for investigating olfactory discrimination abilities and strengthening olfactory communication.

Experimental Section

Preparation of Core or Shell Ink and the Substrate for Ink-Jet Printing

The core ink formulation consisted of 0.5 g of poly(vinyl alcohol) (PVA, Mw ∼ 89,000 – 98,000; Sigma-Aldrich), 2.5 g of ethylene glycol (analytical purity, Beijing Chemical Works), and aqueous solution of longan, milk, or vanilla spices (three representative spices, Shanghai Apple Flower & Fragrance Co., LTD.) dissolved in 10 g of total solution. Shell inks were prepared with PLGA (75:25, Mw ∼ 110,000; Jinandaigang Biological Engineering Co., Ltd., 2.5 wt %) in dichloromethane (analytical purity, Beijing Chemical Works). The glass sheets, silicon wafers, or PET films were washed by 98% ethyl alcohol and acetone. Then, they were blow dried with nitrogen gas.

Preparation of Customizable Odor-Releasing Lamina by Ink-Jet Printing

The ink was placed in a syringe of 3 cm3 connected to a needle having an inner diameter of 60 μm and printed by a multiaxis dispensing system (2400, EFD, see Supporting Information). Specific programs were programmed according to the patterns. By controlling the air pressure of the ink-jet printer, ink droplets of different sizes were printed. First, the aqueous cores were printed on the substrate. Then, the polymer shells were printed in situ onto the aqueous cores to make the customizable odor-releasing laminas with a core–shell structure.

Instruments and Characterization

The surface tension of all aqueous inks containing odor molecules was determined in a surface tension meter (K100SF, Kruss GmbH Germany). The contact angle of the base material was investigated by a contact angle meter (OCA25, Germany). The side view images of drops were taken by a contact angle meter (OCA25, Germany). All photos of the odor-releasing laminas and the images showing the geranium were taken by a digital camera (Canon 60D, Japan). The microstructure of the blade surface was investigated by a field-emission scanning electron microscope (JSM-7500, Japan) and an optical microscope (Olympus MX40, Japan). The viscosity modulus as a function of the shear rate for water inks of three kinds of spices was tested by a rheometer (Anton Paar MCR302, Austria).

Quantitative Release Control Experiment

A rose odor-releasing film was printed with a 4 × 9 dot matrix. A rose odor-releasing film was placed in a culture dish with 15 mL of KMnO4 (aq, 6.13 × 10–5 mg/mL, Beijing Chemical Works). After rubbing one row of encapsulated arrays, the color of KMnO4 solution would change to show the concentration of the odor droplets. A pH test strip (pH = 8.2–10.0, Hangzhou Shisan Co., Ltd.) was moistened with deionized water (pH = 5.9) and placed at the bottom of the glass dish. The inks containing NH3 (aq, 25.0–28.0%, Beijing Chemical Works) and PLGA shell ink were prepared. A NH3 odor-releasing film was printed with a 4 × 9 dot matrix. After rubbing one row of encapsulated arrays, the pH test paper was reversed on the NH3 film to show the pH changes.