Journal Archive

Johnson Matthey Technol. Rev., 2020, 64, (4), 443
doi: 10.1595/205651320X15901340190139

Application of Chitosan-Encapsulated Orange Oil onto Footwear Insock Leathers

Spray drying technique for an environmentally sustainable antibacterial formulation

  • Buket Yılmaz
  • Department of Materials Science and Engineering, Graduate School of Natural and Applied Science, Ege University, 35100 Bornova-Izmir, Turkey
  • Hüseyin Ata Karavana*
  • Department of Leather Engineering, Faculty of Engineering, Ege University, 35100 Bornova-Izmir, Turkey; Department of Materials Science and Engineering, Graduate School of Natural and Applied Science, Ege University, 35100 Bornova-Izmir, Turkey
  • *Email:;

Article Synopsis

The purpose of this study was to devise an antibacterial treatment for footwear insock leathers. Orange oil-loaded chitosan microparticles were utilised for this purpose. Emulsion formulations with different ratios were prepared, and from these formulations microparticles were manufactured using a spray drying technique. Microparticles obtained in this way were applied to the insock leathers using a finishing process. Successful encapsulation was confirmed by ultraviolet-visible (UV-vis) spectrophotometry, Fourier transform infrared (FTIR) spectroscopy and scanning electron microscopy (SEM) techniques. The microparticles exhibited highly spheroid shape with a size range of 3–5 μm. Microparticle encapsulation efficiencies ranged from 79.41% ± 3.36% to 86.60% ± 1.13%. After performing microbiological tests and in vitro release studies on the insock leathers, it was determined that the prepared microparticles are able to perform core material delivery. Also, successful microparticle application resulted in these leathers acquiring antibacterial properties. The products and process are biodegradable, nontoxic and biocompatible.

1. Introduction

Footwear is the most commonly worn apparel in daily life, and its design features must prioritise anatomy, comfort and hygiene. For this reason, it is important to develop sustainable improvements to footwear’s functional properties.

Footwear that carries the body’s weight during the day can affect foot health physically, chemically and microbiologically. Continuous contact with the external environment exposes footwear to microorganisms during normal use. All sorts of footwear play a role in the transport, spread and contamination of pathogenic or non-pathogenic microorganisms (1).

There are different microorganisms in every part of the human body. Sweat is regularly secreted from the body under normal conditions. It contains 98% water and urea, uric acid, fatty acid, lactic acid and sulfates (2). The feet have more sweat glands than other parts of the body. Sweat secreted from feet during the usage of footwear is decomposed by means of foot microbiota; as a result, bad odours emerge in footwear. Brevibacterium linens, Staphylococcus epidermidis, Staphylococcus aureus and Escherichia coli are some microorganisms that make footwear unhygienic. As a result of the breakdown of amino acids in sweat and skin by these microorganisms, bad odour arises in feet, socks and footwear (3, 4).

Nowadays, in addition to individual foot care and hygiene for odour prevention in shoes, commercial materials with various deodorising and antimicrobial effects are also employed (5). Footwear insock is a thin layer of materials put into the shoe after manufacture to cover the insole. It directly contacts the sole of the wearer’s foot and can provide a more sanitary environment when specially treated for antimicrobial purposes (6).

Spray drying is an advantageous way to encapsulate active substances and essential oils. Spray drying is a common and accepted encapsulation method for industrial applications. With this method, it is possible to mass produce capsules. The distribution of particles is uniform (710).

Microencapsulation technology has been used for the application of orange oil to textiles and leathers, being an economically viable, fast and efficient method by combining core and shell materials, desirable perceptual and functional characteristics, and also allowing functional substances to be released in a controlled manner. This technique has also been used to microencapsulate a wide range of active, functional, sensitive or volatile substances (1114). Tea tree oil containing melamine formaldehyde microcapsules, essential oils (eucalyptus, lavender or oregano), polyurethane dispersions containing photoactive antimicrobial agents, zinc oxide and silver nanoparticles are some substances that protect upper leathers from the harmful propagation of microorganisms (3, 15). In addition, aromas confined to microcapsules are also used to prevent bad odours in footwear (16, 17). Application of antibacterial and aromatic materials onto footwear insocks to control bad odours is good for foot hygiene and desired shoe comfort.

The use of orange oil presents as an ecological alternative to synthetic chemicals, attracting the attention of the scientific community to the development of eco-friendly antimicrobials. In this study, microparticles were produced by a spray drying method after the emulsions with orange oil and chitosan were prepared in different ratios. Microparticles manufactured in this way were then transferred to the surface of the footwear insock leathers using a finishing process. Afterward, some tests and analyses were performed on microparticle coated footwear insock leather samples to evaluate the effectiveness of the microparticles, their presence on the leather surface and their antimicrobial properties.

2. Experiment

Pharmaceutical grade cold pressed orange oil was donated from Ephesus, Turkey. Chitosan was purchased from Acros OrganicsTM (Belgium). Analytical grade chemicals were used in the analyses. Insock leathers, without dye and ready for experimental application, were donated from Ata Dilek Leather (Izmir, Turkey).

For the microparticle production, chitosan (shell material) was added to 1% w/w aqueous acetic acid for preparing the chitosan solution. This solution was stirred at 45°C by using a magnetic stirrer until wholly dissolved. During the pre-emulsion preparation, orange oil (core material) was gradually mixed into the chitosan solution and stirred for 1 min at 10,000 rpm. The surfactants as compound emulsifiers used for pre-emulsion preparations were Tween 40 with Span 20 at the ratio of 8:2 w/w. Then, the microparticles were prepared by using an SD Basic spray dryer (Labplant, UK) with nozzle diameter of 0.5 mm. The orange oil to chitosan ratios in the four encapsulating compounds came to 1:1, 1:1.33, 1:1.67 and 1:2 w/w. The ingredients of the formulations in the spray-drying process are shown in Table I. Homogeneous emulsions were fed to the spray dryer under the following conditions: pump speed 12 ml min−1, outlet air temperature 114°C and inlet air temperature 175°C.

Table I

Composition of the Formulations

Formulation code Orange oil:chitosan, w/w
T3 1:1
T4 1:1.33
T5 1:1.67
T6 1:2

The microparticles’ morphology was examined by a Quanta 250 FEG scanning electron microscope (FEI, USA) at 2 kV accelerating voltage. Before coating in an argon atmosphere with gold-palladium by a K550X sputter coating machine (Quorum Emitech, UK), the samples were mounted onto an aluminium stub. The grain side of leathers coated with microparticles was examined by a TM1000 tabletop scanning electron microscope (Hitachi, Japan) after coating with gold-palladium.

The FTIR spectra of the spray-dried microparticles and leathers with microparticles were procured by a Spectrum 100 FTIR attenuated total reflectance (ATR) spectrometer (PerkinElmer, USA). The measurements were made using four scans with a resolution of 4 cm−1 between 4000 cm−1 to 650 cm−1 wavenumber ranges at room temperature.

Encapsulation efficiencies of microparticles were calculated as the amount of orange oil (core material) encapsulated in the microparticles. The encapsulation efficiency was calculated using Equation (i) (18).




A solvent extraction method was used to determine total oil content. A 0.1 g measurement of orange oil loaded microparticles was dissolved in 10 ml of 1% acetic acid solution at room temperature for 45 min. Released orange oil which was obtained from the completely dissolved microparticles was placed in a beaker containing 50 ml n -hexane for extraction 45 min. So as to determine the total amount of orange oil in the microparticles, this extract was filtered through a syringe filter (0.22 μm). Orange oil content in the filtrate was measured using a UV-1800 UV-vis spectrophotometer (Shimadzu, Japan) at 202 nm in triplicate. Surface oil content was also determined by the same solvent extraction method described above, except for a dissolving process in 1% acetic acid solution (11).

In vitro release studies of microparticles and microparticle loaded leathers were carried out at a speed of 100 rpm in phosphate-buffered saline (PBS) and methanol at 37°C. 1 mg orange oil loaded microparticles was suspended in beakers containing 4 ml methanol and 16 ml of PBS. Insock leathers with 2.5 cm2 area were placed in beakers containing 16 ml methanol and 16 ml of PBS for in vitro release studies of microparticle loaded leathers. At suitable time intervals, the medium in the beakers was filtered through a 0.22 μm syringe filter. Sink conditions were maintained in the receptor compartment during in vitro release studies. The released amount of orange oil was analysed by UV method, as previously described, for 5 h. Experiments were performed five times.

A spraying pistol with nozzle diameter of 0.5 mm was used to apply microparticles to the insock leathers during the finishing process. Spray-dried microparticles were added to the finishing recipe as 20 g m−2 (19). The basic finishing recipe for the insock leathers is given in Table II (20).

Table II

Basic Finishing Recipe Applied to Insock Leathers

Materials Amount, part Practice
Water 100 3 × Spray
Anionic wax 50
Non-ionic aliphatic polyurethane binder 25
Orange oil loaded microparticles 12

The efficacy of microparticle coated insock leathers against test microorganisms Staphylococcus aureus ATCC® 6538TM, Escherichia coli ATCC® 25922TM, Candida albicans ATCC® 10231TM, Klebsiella pneumoniae ATCC® 4352TM and Bacillus subtilis ATCC® 6633TM was examined by agar disc diffusion method (2124). Test microorganisms were placed into an incubator for incubation at 37°C for 18 h in the Mueller Hinton broth (MHB) medium. Then, microorganisms were inoculated in petri dishes containing 105 colony forming unit (CFU) ml−1 of Mueller Hinton agar (MHA) medium. Next, microparticle coated insock leather samples with 12.7 mm diameter were placed into the petri dishes (20, 25). All petri dishes were placed into an incubator for incubation at 37°C for 24 h, and inhibition zones were measured to determine antibacterial activity.

3. Results and Discussion

In this study, orange oil microparticles were successfully prepared by spray drying method. This method is a simple, viable method to obtain microparticles, suitable to prevent active substance biological activity loss, avoiding exposure to elevated heating and to organic solvents.

3.1 Surface Appearance of Microparticles and Microparticle Coated Insock Leathers

A scanning electron microscope was used to examine the morphology of the spray-dried microparticles. SEM micrographs revealed that all microparticle formulations have a highly spheroid shape with a morphology approximating an orange peel effect. Microparticles of non-uniform size were observed with clear distinction between shell and core materials. These shape features indicate that orange oil is spread on the surface of the microparticles. The morphology of spray-dried microparticle formulations is shown in Figure 1. Particle morphology (surface, size and distribution) was not affected by the polymer ratio or core:shell ratio. It is observed that there was formation of microcapsules, but they have stuck one to another and an agglomerate of microcapsules occurred. Microparticles with similar morphology were also obtained in other spray drying experiments carried out using natural polymeric mixtures (11, 26, 27).

Fig. 1.

SEM micrographs of microparticle formulations: (a) T3 formulation, 50,000 × magnification; (b) T3 formulation, 10,000 × magnification; (c) T4 formulation, 50,000 × magnification; (d) T4 formulation, 10,000 × magnification; (e) T5 formulation, 50,000 × magnification; (f) T5 formulation, 10,000 × magnification; (g) T6 formulation, 50,000 × magnification; (h) T6 formulation, 10,000 × magnification

We also examined the surface appearance of microparticle-free and microparticle-coated insock leather samples. The different microparticle formulations were clearly observed on insock leather surfaces after successful application of the finishing process. Micrographs of insock leather surfaces are shown in Figure 2.

Fig. 2.

SEM micrographs of the insock leather after finishing process: (a) microparticle free; (b) T3 formulation; (c) T4 formulation; (d) T5 formulation; (e) T6 formulation

After the finishing process, the presence of microparticles on the insock leather can be seen very clearly for all formulations. The images indicate that the fixation was successfully achieved. Hence, the leather samples preserve the capsule content even after the finishing process.

3.2 Fourier Transform Infrared Spectroscopy Studies

Interactivity between the core material and shell material usually leads to characteristic alterations in the FTIR spectra. FTIR spectra of chitosan, orange oil, microparticles and insock leather samples are shown in Figure 3 and Figure 4. Characteristic peaks at 1029 cm−1, 1149 cm−1, 1373 cm−1, 1419 cm−1, 1585 cm−1, 2867 cm−1 and 3362 cm−1 were demonstrated in the FTIR spectrum of chitosan (Figure 3). The peak at 3362 cm−1 (OH and NH2 stretching) was attributed to the amino group of chitosan. An intense absorption peak was seen at 2867–2922 cm−1 owing to C–H stretching in all spectra. The peak at 1585 cm−1 was attributed to N–H bending of the NH3+ functional group present in the chitosan (28, 29). The peak at 1373 cm−1 confirmed the presence of an amide III band in the chitosan. The C–O–C stretching resulted from the spectra at 1149 cm−1 and 1029 cm−1. The spectrum at 660 cm−1 was attributed to stretching vibration of pyranoside ring (3034).

Fig. 3.

The FTIR spectrum of the chitosan, orange oil and four different microparticle formulations (T3, T4, T5 and T6)

Fig. 4.

The FTIR spectrum of footwear insock leather without microparticles (blank leather) and with four different microparticles formulations (T3, T4, T5 and T6)

The FTIR spectrum of the orange oil showed the distinctive bands of D-limonene, which is the primary constituent in orange oil (Figure 3). Especially, the bands between 2919–2834 cm−1 were attributed to the C–H stretching vibrations in –CH–, –CH2– and –CH3. The spectrum at 2965 cm−1 was attributed to the stretching vibrations of =C–H. The band 1644 cm−1 was attributed to the stretching vibrations of C=C. The band seen at 1435 cm−1 was attributed to the C–H bending vibrations in –CH–, –CH2– and –CH3. The peaks at 885 cm−1 and 797 cm−1 were attributed to the bending vibrations (out of plane) in =CH2 and =C(R)–H, respectively. The band at 1376 cm−1 was also attributed to the C–H bending vibrations in –CH3 (mostly used to describe the existence of methyl) (35, 36).

As seen in Figure 3, most bands in the FTIR spectra of the microparticles belonged to chitosan, which indicated that orange oil droplets were trapped in chitosan (shell material) and that distinctive band of orange oil vanished or declined. Evidently, the free vibrations of orange oil molecules were blocked by the chitosan because of physical interactions such as van der Waals or electrostatic interaction. Furthermore, the intensity of microparticle peaks on the FTIR spectrum was lower than that of chitosan because of the interaction between orange oil and chitosan. The FTIR spectra of microparticles demonstrated the C–H bending vibrations of –CH3 at 1376 cm−1, except the =C(R)–H bending vibration at 797 cm−1, which was presumably due to the fact of the D-limonene ring being covered with chitosan (14, 36).

Figure 4 show that bands between 1535–1547 cm−1 attributed to the NH band of chitosan, did not appear in the blank leather sample (34). Similarly, it was determined that IR band vibration at 1095 cm−1 was observed in the microparticle loaded leathers but absent from the blank leather. That was evidence of the presence of terpenoid, a component in orange oil (37).

3.3 Encapsulation Efficiency

Orange oil loaded microparticles were produced with a high orange oil encapsulation efficiency. The encapsulation efficiency of T3, T4, T5 and T6 formulations were determined as 79.41% ± 3.36%, 81.28% ± 1.69%, 83.56% ± 0.66% and 86.60% ± 1.13%, respectively. A great deal of encapsulated orange oil is preferred. These results showed that the microparticles’ encapsulation efficiency is affected by the core:shell ratio. Increasing the chitosan weight resulted in more encapsulated orange oil, i.e. high encapsulation efficiency. This is an effect similar to the oil:polymer ratio given by Li and associates in their 2013 study (11).

3.4 In Vitro Release Studies of Microparticles and Microparticle Coated Insock Leathers

Figure 5 shows the in vitro release behaviours of orange oil released from microparticles in four different formulations. The quantity of released orange oil was measured at 202 nm in PBS at different times. Previous experiments used PBS as an in vitro release and diffusion medium for topical applications (38, 39). Oil release from microparticulate systems occurs via different mechanisms including diffusion, desorption, disintegration and surface erosion (40).

Fig. 5.

In vitro release of orange oil loaded microparticles

The typical release pattern of the spray dried microparticles is characterised by a small initial burst release and a sustained release rate following that. It can be seen that orange oil release from microparticles gradually increased over time with exposure to PBS, which indicates that the orange oil disintegrated swiftly in PBS. This circumstance is presumably owed to the fact that PBS is slightly alkaline; chitosan is inclined to dissolve in slightly alkaline solution. Nonetheless, it can be seen that the release rate was not affected by chitosan concentration in the formulations. Figure 5 graphs release behaviour as a function of orange oil concentration, which was independent from chitosan concentration.

The in vitro release results of the leathers impregnated with orange oil loaded microparticles in pH 7.4 PBS at 37°C are presented in Figure 6. This line graph shows controlled release behaviour from leather treated with all formulations. Orange oil trapped inside the microparticles caused sustained release up to 24 h. When the formulations are compared to each other, we see the oil release ratio of insock leathers was affected by polymer concentration. High polymer concentration caused a slow release ratio of orange oil.

Fig. 6.

In vitro release of microparticle-coated insock leathers

3.5 Microbiologic Studies on Microparticle Coated Insock Leathers

Table III shows microbiologic test results of insock leathers treated with four microparticle formulations. An important revelation is that the test microorganisms did not grow on these leather samples. However, in some test groups, a meagre antimicrobial inhibition zone around the insock leather samples meant that orange oil diffusion did not occur. There is a visible zone on Candida albicans in all formulations. T3 and T4 formulations, whose orange oil releases are higher in 24 h, look more effective against Escherichia coli. The antimicrobial activity is dependent on chitosan’s inherent behaviour and orange oil present on leather samples. When the inhibition zones in the T6 formulation are examined, it can be seen that orange oil found in the insock leather samples is more effective than the natural behaviour of chitosan on antimicrobial activity. The antimicrobial effect can be considered as proliferation or non-proliferation in the area under the insock leather samples. This effect is also expressed as contact inhibition. No proliferation was observed on the contact surface of the insock leathers, that is, on the surface where it touches the medium and microorganism. Also, any proliferation on the surfaces or edges of insock leather samples was not observed. There was no difference between leather formulations on the antimicrobial test.

Table III

Microbiologic Test Results of the Microparticle-Loaded Insock Leathers

4. Conclusion

During the usage of footwear, perspiration and bacterial activity negatively impact foot health and generate bad odours from both the feet and footwear. Shoe production using natural and non-toxic materials that prevent or inhibit bad odours and bacterial activity is one solution to this hygienic problem. Likewise, the successful application of microparticles that release for a long time on footwear insock leather is an important alternative to existing toxic products.

Our research found that emulsions with orange oil and chitosan have natural antibacterial activity. These emulsions, when successfully converted into encapsulated powders by a spray drying method, produce a core-shell material. SEM images showed how an effective finishing process was used to apply laboratory produced microparticles to the surface of footwear insock leather. Microbiological tests performed on microparticle coated leathers proved that footwear insock leathers were fortified with antibacterial properties.

These findings demonstrate that application of orange oil-chitosan microparticles onto footwear insock leather surfaces is an alternative natural method to control hygiene and eliminate bad odours. Non-toxic, functional, leather shoes can incorporate such natural materials in their manufacture and maintenance. This production improvement would thus contribute to people’s foot health, hygiene and comfort.


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The authors would like to thank the Scientific Research Projects Coordination Unit, Ege University, Turkey (Project No: 17FBE006) for financial support and the Turkish Prime Ministry’s State Planning Organisation (Project No: 07DPT001) for equipment provision.

The Authors

Buket Yılmaz graduated from Chemical Engineering, Faculty of Engineering, Anadolu University, Turkey, in 2015. For a period during her undergraduate education, she benefited from the FARABİ exchange programme for further chemical engineering studies at Ege University. Yılmaz won and completed a competitive internship at Turkey’s two leading companies involved in polymers and food production. In 2016, she started her master’s degree at Ege University’s Institute of Science, Materials Science and Engineering. Her scientific expertise has also been employed by the private sector in sales and in the quality control unit of a food production enterprise.

Hüseyin Ata Karavana graduated from the Leather Technology Department, Faculty of Agriculture, Ege University, Turkey. He earned his MSc degree in Leather Technology in 2001 from that institution’s Graduate School of Natural and Applied Science. From 2006 to 2007 he continued his studies as an Erasmus student in the Department of Footwear Engineering and Hygiene at the Tomas Bata University’s Faculty of Technology (Zlin, Czech Republic). Karavana completed his PhD degree in Leather Engineering at Ege University in 2008. Karavana currently serves as Associate Professor in the Department of Leather Engineering at Ege University’s Faculty of Engineering. His research interests are in all manner of leather and footwear engineering including plastic composites, microencapsulation, leather quality and control, footwear quality and control.

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