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Physicochemical properties and controlled drug release of microcapsules prepared by simple coacervation
Colloids and Surfaces B: Biointerfaces 104 (2013) 1–4
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Colloids and Surfaces B: Biointerfaces journal homepage: www.elsevier.com/locate/colsurfb
Physicochemical properties and controlled drug release of microcapsules prepared by simple coacervation Ken-ichi Shimokawa, Katsuhiko Saegusa, Yuko Wada, Fumiyoshi Ishii ∗ Department of Pharmaceutical Sciences, Meiji Pharmaceutical University, 2-522-1, Noshio, Kiyose, Tokyo 204-8588, Japan
a r t i c l e
i n f o
Article history: Received 25 August 2012 Accepted 21 November 2012 Available online 7 December 2012 Key words: Simple coacervation Ternary phase diagram Drug release Microcapsules
a b s t r a c t Ternary phase diagrams of gelatin–water–methanol, gelatin–water–ethanol, and gelatin–water– propanol systems were prepared to evaluate optimal coacervation. The results of their evaluation suggested that the optimal coacervation region expands with the hydrophobicity of the added poor solvent (methanol, ethanol, 1-propanol, and 2-propanol). Microcapsules prepared based on the optimal coacervation region differed in controlled drug release among poor solvents used even when the concentration of gelatin, a membrane component, is the same. Compared with microcapsules prepared using the gelatin–water–ethanol system, those prepared using the gelatin–water–propanol system showed a 34% decrease in the drug release rate 24 h after the initiation of the drug release test. These results suggested that microcapsules prepared using gelatin–water–various lower alcohol systems can readily control drug release and can be a useful drug delivery system (DDS). © 2012 Elsevier B.V. All rights reserved.
1. Introduction In 1949, Bungenberg de Jong classified coacervation into simple and complex types [1]. Simple coacervation is a phenomenon in which the addition of a substance reducing hydration (poor solvent) to a hydrophilic colloidal solution results in the formation of two phases, one of which is rich in colloid molecules (coacervate), and the other is poor in them. For example, when sodium sulfate solution, acetone, or alcohol is gradually added to a gelatin solution with stirring, a coacervate forms [2,3]. Complex coacervation is a phenomenon in which the addition of substances such as acetic acid to a mixture of multiple colloidal solutions for pH adjustment results in coacervate formation [4]. For example, when acetic acid is added to a solution of negatively charged gelatin and gum arabic, the pH of the dispersion medium decreases. At pH values below the isoelectric point of gelatin (pH 4.8), gelatin becomes positively charged, but gum arabic continues to be negatively charged, and electrical attraction between them results in coacervate formation [5]. In the pharmaceutical field, microcapsules prepared using the coacervation method are used for the controlled release of drugs [6] and masking [7]. Nixon et al. performed basic studies on the gelatin–water–ethanol system in which ethanol as a poor solvent is added to gelatin solution using the simple coacervation
∗ Corresponding author. Tel.: +81 42 495 8468; fax: +81 42 495 8468. E-mail address: fi[email protected] (F. Ishii). 0927-7765/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.colsurfb.2012.11.036
method [8,9]. There have also been studies on drug release using the gelatin–water–ethanol system, but only a few detailed studies on drug release from microcapsules prepared using other alcohols (such as methanol or propanol) [10,11]. Therefore, in this study, we obtained a coacervation region based on ternary phase diagrams using methanol, ethanol, and propanol as poor solvents, confirmed the optimal coacervation region, prepared gelatin microcapsules containing a drug, and evaluated the detailed characteristics of drug release from the microcapsules. 2. Materials and methods 2.1. Materials Purified water obtained using an Elix Advantage 3 Pure Water Production System (Merck & Co., Inc.) was used. A gelatin powder and various alcohols (methanol, ethanol, 1-propanol, and 2-propanol) as poor solvents were analytical grade products (Kanto Chemical Co., Inc., Tokyo, Japan). Phenacetin (-acetophenetidine) (Wako Pure Chemical Industries Ltd., Osaka, Japan) was used as a model core. All the other reagents were of analytical grade. 2.2. Methods 2.2.1. Ternary phase diagrams The gelatin powder was dissolved in water at 50 ◦ C, and the temperature was gradually reduced to 20 ◦ C with stirring. Subsequently, each type of alcohol as a poor solvent was
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gradually dripped into the gelatin solution. With an increase in the poor solvent concentration, a coacervate formed. For detailed observation of the coacervate state, ternary phase diagrams of gelatin–water–methanol, gelatin–water–ethanol, and gelatin–water–propanol systems were produced using a light microscope (Digital Microscope DMBA310, Shimadzu Rika Co., Tokyo, Japan). Each phase diagram consisted of more than 70 observation points of each component. The gelatin solution, viscous gel, and coacervation phase regions were shown by borderlines. In the coacervation region, the region showing the greatest amount of coacervation was defined as the optimal coacervation region. 2.2.2. Drug release Phenacetin (0.04 g) was added as a model core to 10 (w/v)% gelatin solution. While this solution was thoroughly stirred, each type of alcohol as a poor solvent was gradually dripped to form a coacervate. After cooling to 5 ◦ C, 0.5 mL of 30 (w/v)% formalin was added for crosslinking to fix the formed gelatin microcapsules. Subsequently, the suspension containing microcapsules was filtered and washed adequately with purified water to remove unreacted formaldehyde. Microcapsules (20 g, dry weight) were placed in dialysis tubes, and a phenacetin release test was performed in 500 mL of purified water at 20 ◦ C. As a control, 0.05 g of phenacetin was dissolved in 10 (w/v)% gelatin solution, the solution was placed in a dialysis tube, and a similar drug release test was performed. The phenacetin release properties were evaluated by comparing the prepared microcapsules and control.
Fig. 1. Ternary phase diagram of gelatin–water–methanol system. (O): Optimum coacervation region, (V): viscous gel phase region, (C): coacervation region, (S): solution region containing gelatin.
2.2.3. Calculation of the drug release rate Samples of the dialysate were collected 0, 0.5, 1, 2, 4, 6, 8, 12, 24, 28, 36, 48, 54, 60, and 72 h after the initiation of dialysis, and the drug released from the dialysis tube was measured using a UV-visible spectrophotometer V-650 (JASCO Corp., Tokyo, Japan). Measurement was performed at 246 nm as the maximum absorption wavelength for phenacetin, and the absorption of phenacetin (0.04 g) dissolved in the dialysate (500 mL) was regarded as 100% drug release, and the drug release rate was calculated. 3. Results and discussion Using various alcohols (methanol, ethanol, 1-propanol, and 2-propanol) as poor solvents, ternary phase diagrams of the gelatin–water–alcohol systems were produced based on the composition ratio of the gelatin solution, viscous gel, and coacervation phases (Figs. 1–4). On each diagram, the optimal coacervation region was clarified. Based on the optimal coacervation region, stable microcapsules were prepared, and the drug release rate was compared among the systems.
Fig. 2. Ternary phase diagram of gelatin–water–ethanol system. (O): Optimum coacervation region, (V): viscous gel phase region, (C): coacervation region, (S): solution region containing gelatin, (A): aggregating region.
3.1. Ternary phase diagrams 3.1.1. Ternary phase diagram of the gelatin–water–methanol system The ternary phase diagram of the gelatin–water–methanol system is shown in Fig. 1. In this figure, (O) indicates the optimal coacervation region, (C) indicates the coacervation region, (V) indicates the viscous gel region, and (S) indicates the solution containing gelatin. The optimal coacervation region was observed at about a gelatin:water:methanol ratio of 0.05:0.25:0.70. Around this region, the coacervation region (C) was present. However, this composition was fragile as microcapsules. With a gradual increase in the water ratio, there was a gelatin solution region (S), and, with an increase in the gelatin ratio, there was a viscous gel region (V). No gelatin aggregation was observed in this system. Based on this diagram, coacervation was considered to occur in the region at a methanol rate of about 60% or more.
Fig. 3. Ternary phase diagram of gelatin–water–1-propanol system. (O): Optimum coacervation region, (V): viscous gel phase region, (C): coacervation region, (S): solution region containing gelatin, (A): aggregating region.
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Fig. 4. Ternary phase diagram of gelatin–water–2-propanol system. (O): Optimum coacervation region, (V): viscous gel phase region, (C): coacervation region, (S): solution region containing gelatin, (A): aggregating region.
3.1.2. Ternary phase diagram of the gelatin–water–ethanol system The ternary phase system of the gelatin–water–ethanol system is shown in Fig. 2. In this figure, the optimal coacervation region was observed at about a gelatin:water:ethanol ratio of 0.05:0.45:0.50. In this system, coacervation was considered to occur in the region at an ethanol rate of about 40% or more. Compared with the gelatin–water–ethanol system that required a methanol rate of about 60% or more for coacervation, the gelatin–water–methanol system required a small amount of ethanol for coacervation due to its marked dehydration effects [12]. There was a gelatin aggregation region (A) that was not observed in the gelatin–water–methanol system. 3.1.3. Ternary phase diagram of the gelatin–water–1-propanol system The ternary phase system of the gelatin–water–1-propanol system is shown in Fig. 3. In this figure, the optimal coacervation region was observed at about a gelatin:water:1-propanol ratio of 0.05:0.5:0.45. In this system, coacervation was considered to occur in the region at a 1-propanol rate of about 25% or more. The optimal coacervation region was the largest for the gelatin–water–1propanol system, followed in order by the gelatin–water–ethanol system and gelatin–water–methanol system. In addition, with an increase in the hydrophobicity of the poor solvent from methanol to ethanol and then to 1-propanol, coacervation occurred with a smaller amount of the poor solvent. There was a gelatin aggregation region (A) in the gelatin–water–1-propanol system as well as the gelatin–water–ethanol system. 3.1.4. Ternary phase diagram of the gelatin–water–2-propanol system The ternary phase system of the gelatin–water–2-propanol system is shown in Fig. 4. In this figure, the optimal coacervation region was observed at about a gelatin:water:2-propanol ratio of 0.05:0.5:0.45. The diagram of this system was similar to that of the gelatin–water–1-propanol system, showing coacervation at a 2-propanol rate of 25% or more. The optimal coacervation region was also similar to that in the gelatin–water–1-propanol system (Figs. 3 and 4). This similarity of each region may be due to the similarity of the degree of hydrophobicity between 2-propanol and 1-propanol. There was also a gelatin aggregation region (A) in this system as well as the gelatin–water–ethanol and gelatin–water–1propanol systems.
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Fig. 5. The effect of the mixing ratio on drug release from microcapsules in gelatin–water–ethanol system. : gelatin:water:ethanol = 0.04: 0.34:0.62, 䊉: gelatin:water:ethanol = 0.03:0.30:0.67, : gelatin:water:ethanol = 0.03:0.27:0.70, : gelatin:water:ethanol = 0.09:0.91:0 [control].
3.2. Controlled drug release 3.2.1. Comparative properties of drug release Gelatin microcapsules were prepared using the composition in the optimal coacervation region in the ternary phase diagram of the gelatin–water–methanol system. Phenacetin as the core was dissolved in gelatin solution, and methanol as a poor solvent was gradually added for coacervation. However, the prepared microcapsules were fragile, appropriate particle diameters could not be obtained, and microcapsules could not be produced. Subsequently, 3 component ratios in the coacervation region were selected in the ternary phase diagram of the gelatin–water–methanol system for the preparation of microcapsules containing phenacetin. The selected component ratios were as follows: gelatin:water:ethanol = 0.04:0.34:0.62 (), 0.03:0.30:0.67 (䊉), and 0.03:0.27:0.70 (). The results of a drug release test using the prepared capsules are shown in Fig. 5. As shown in this figure, the rate of drug release from the microcapsules almost linearly increased until 6 h after the initiation of the test and became almost stable after 24 h. The drug release rate differed among the microcapsule compositions. The drug release rates after 24 h were 85, 80, and 69% for , 䊉, and , respectively, showing the most sustained release for , followed in order by 䊉 and . A composition not containing the poor solvent [gelatin:water:ethanol = 0.09:0.91:0 ()] was used as a control. Similarly, microcapsules were prepared using the above 3 component ratios in the ternary phase diagram of the gelatin–water–1-propanol system. The results of a drug release test of the prepared capsules are shown in Fig. 6. From this figure, the drug release rate almost linearly increased until 2 h after the initiation of the test, but began to differ among the 3 compositions after 4 h. The drug release rates after 24 h were 73, 50, and 25% for , 䊉, and , respectively, showing the most sustained release for , followed in order by 䊉 and . When the drug release rate for this system is compared with that for the gelatin–water–ethanol system, the drug release rate decreased from 85 to 73% (12% decrease) for composition , from 80 to 50 (30% decrease) for 䊉, and from 69 to 35% (34% decrease) for (Figs. 5 and 6). This may be because the change in the poor solvent from ethanol to the more hydrophobic 1-propanol more markedly sustained drug release. Due to the more marked dehydration effects of propanol than those of ethanol, the gelatin density in the membrane may have increased. For the preparation of DDS drugs, the coacervation method may be useful. Similarly, microcapsules were prepared using the 3 component ratios based on the ternary phase diagram of the gelatin–water–2propanol system. The results of a drug release test of the prepared
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Fig. 6. The effect of the mixing ratio on drug release from microcapsules in the gelatin–water–1-propanol system. : gelatin:water:1-propanol = 0.04:0.34:0.62, gelatin:water:1-propanol = 0.03:0.30:0.67, : gelatin:water:1䊉: propanol = 0.03:0.27:0.70, : gelatin:water:1-propanol = 0.09:0.91:0 [control].
The effect of the content ratio of three different poor solvents to total volume on drug release percentage from microcapsules after 24 h is shown in Fig. 8. As can be seen from this figure, the drug release rates after 24 h can be controlled in the range of 85% from 35% using the different poor solvent. Moreover, from Fig. 8, the drug release rates were similar between the gelatin–water–1-propanol and gelatin–water–2-propanol systems, suggesting the similarity of release properties between microcapsules using the two systems. Probably because 1-propanol and 2-propanol differ only in the position of the hydroxyl group (1 or 2) in the molecular structure, but show similar degrees of hydrophobicity, the drug release properties of prepared microcapsules after membrane filtration were similar. The administration route of the drug in the microcapsules prepared in this study as a DDS drug is percutaneous or oral. The results of this study suggested effective controlled drug release using the gelatin–water–propanol systems. A problem in drug production is residual propanol in the drug. 1-Propanol is a liquid that is volatile at a normal temperature and pressure, and has low toxicity for the human body. This propanol has been approved as a food additive (flavoring material) in many countries such as Japan and western countries. On the other hand, 2-propanol is used as a solvent of local drugs for cosmetics and medical drugs, and also used for tablet coating with films and tablet granulation because its complete removal from the system by evaporation is possible [13]. Therefore, concerning this problem, propanol can be removed by adequate drying (evaporation) during microcapsule preparation. 4. Conclusion
Fig. 7. The effect of the mixing ratio on drug release from microcapsules in the gelatin–water–2-propanol system. : gelatin:water:2-propanol = 0.04:0.34:0.62, gelatin:water:2-propanol = 0.03:0.30:0.67, : gelatin:water:2䊉: propanol = 0.03:0.27:0.70, : gelatin:water:2-propanol = 0.09:0.91:0 [control].
The results of this study suggested the optimal coacervation region in the ternary phase diagrams using methanol, ethanol, 1-propanol, or 2-propanol as the poor solvent expands the region with its increasing degree of hydrophobicity. Microcapsules prepared based on the optimal coacervation region allowed the control of drug release. In addition, the rate of drug release from microcapsules prepared using the gelatin–water–propanol was 34% lower than that from microcapsules prepared using the gelatin–water–ethanol system 24 h after the initiation of the test. These results suggest that microcapsules prepared using the gelatin–water–propanol system are excellent for controlled drug release, and can be a useful DDS. References
Fig. 8. The effect of the solvent content at the preparation of microcapsules on drug release percentage after 24 h. : ethanol (solid line), : propanol (solid line), : propanol (dotted line).
microcapsules are shown in Fig. 7. As shown in this figure, results similar to those for the gelatin–water–1-propanol system were obtained. The drug release rates after 24 h were 75, 52, and 37% for , 䊉, and , respectively.
[1] H.G. Bungenberg de Jong, Complex colloid systems, in: H.R. Kruyt (Ed.), Colloid Science, vol. 2, Elsevier, Amsterdam, 1949, p. 335. [2] J. Rozenblat, S. Magdassi, N. Garti, J. Microencapsulation 6 (1989) 515. [3] H.A. Aziz, K.K. Peh, Y.T. Tan, Drug Dev. Ind. Pharm. 33 (2007) 1263. [4] D.J. Burgess, J.E. Carless, Int. J. Pharm. 27 (1985) 61. [5] K. Saeki, N. Hosoi, Appl. Biochem. Biotechnol. 10 (1984) 251. [6] A.O. Elzoghby, W.S. El-Fotoh, N.A. Elgindy, J. Control. Release 153 (2011) 206. [7] M.F. Al-Omran, S.A. Al-Suwayeh, A.M. El-Helw, S.I. Saleh, J. Microencapsulation 19 (2002) 45. [8] J.R. Nixon, S.A. Khalil, J.E. Carless, J. Pharm. Pharmacol. 18 (1966) 409. [9] J.R. Nixon, S.A. Khalil, J.E. Carless, J. Pharm. Pharmacol. 20 (1968) 348. [10] J. Okada, A. Kusai, S. Ueda, J. Microencapsulation 2 (1985) 163. [11] B. Mohanty, H.B. Bohidar, Biomacromolecules 4 (2003) 1080. [12] S. Noro, F. Ishii, K. Saegusa, Chem. Pharm. Bull. (Tokyo) 33 (1985) 4649. [13] Handbook of Pharmaceutical Excipients, fifth edition, Japan Pharmaceutical Excipients Council, Yakuji Nippo Limited, Japan, 2007, p. 92.
Contents lists available at SciVerse ScienceDirect
Colloids and Surfaces B: Biointerfaces journal homepage: www.elsevier.com/locate/colsurfb
Physicochemical properties and controlled drug release of microcapsules prepared by simple coacervation Ken-ichi Shimokawa, Katsuhiko Saegusa, Yuko Wada, Fumiyoshi Ishii ∗ Department of Pharmaceutical Sciences, Meiji Pharmaceutical University, 2-522-1, Noshio, Kiyose, Tokyo 204-8588, Japan
a r t i c l e
i n f o
Article history: Received 25 August 2012 Accepted 21 November 2012 Available online 7 December 2012 Key words: Simple coacervation Ternary phase diagram Drug release Microcapsules
a b s t r a c t Ternary phase diagrams of gelatin–water–methanol, gelatin–water–ethanol, and gelatin–water– propanol systems were prepared to evaluate optimal coacervation. The results of their evaluation suggested that the optimal coacervation region expands with the hydrophobicity of the added poor solvent (methanol, ethanol, 1-propanol, and 2-propanol). Microcapsules prepared based on the optimal coacervation region differed in controlled drug release among poor solvents used even when the concentration of gelatin, a membrane component, is the same. Compared with microcapsules prepared using the gelatin–water–ethanol system, those prepared using the gelatin–water–propanol system showed a 34% decrease in the drug release rate 24 h after the initiation of the drug release test. These results suggested that microcapsules prepared using gelatin–water–various lower alcohol systems can readily control drug release and can be a useful drug delivery system (DDS). © 2012 Elsevier B.V. All rights reserved.
1. Introduction In 1949, Bungenberg de Jong classified coacervation into simple and complex types [1]. Simple coacervation is a phenomenon in which the addition of a substance reducing hydration (poor solvent) to a hydrophilic colloidal solution results in the formation of two phases, one of which is rich in colloid molecules (coacervate), and the other is poor in them. For example, when sodium sulfate solution, acetone, or alcohol is gradually added to a gelatin solution with stirring, a coacervate forms [2,3]. Complex coacervation is a phenomenon in which the addition of substances such as acetic acid to a mixture of multiple colloidal solutions for pH adjustment results in coacervate formation [4]. For example, when acetic acid is added to a solution of negatively charged gelatin and gum arabic, the pH of the dispersion medium decreases. At pH values below the isoelectric point of gelatin (pH 4.8), gelatin becomes positively charged, but gum arabic continues to be negatively charged, and electrical attraction between them results in coacervate formation [5]. In the pharmaceutical field, microcapsules prepared using the coacervation method are used for the controlled release of drugs [6] and masking [7]. Nixon et al. performed basic studies on the gelatin–water–ethanol system in which ethanol as a poor solvent is added to gelatin solution using the simple coacervation
∗ Corresponding author. Tel.: +81 42 495 8468; fax: +81 42 495 8468. E-mail address: fi[email protected] (F. Ishii). 0927-7765/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.colsurfb.2012.11.036
method [8,9]. There have also been studies on drug release using the gelatin–water–ethanol system, but only a few detailed studies on drug release from microcapsules prepared using other alcohols (such as methanol or propanol) [10,11]. Therefore, in this study, we obtained a coacervation region based on ternary phase diagrams using methanol, ethanol, and propanol as poor solvents, confirmed the optimal coacervation region, prepared gelatin microcapsules containing a drug, and evaluated the detailed characteristics of drug release from the microcapsules. 2. Materials and methods 2.1. Materials Purified water obtained using an Elix Advantage 3 Pure Water Production System (Merck & Co., Inc.) was used. A gelatin powder and various alcohols (methanol, ethanol, 1-propanol, and 2-propanol) as poor solvents were analytical grade products (Kanto Chemical Co., Inc., Tokyo, Japan). Phenacetin (-acetophenetidine) (Wako Pure Chemical Industries Ltd., Osaka, Japan) was used as a model core. All the other reagents were of analytical grade. 2.2. Methods 2.2.1. Ternary phase diagrams The gelatin powder was dissolved in water at 50 ◦ C, and the temperature was gradually reduced to 20 ◦ C with stirring. Subsequently, each type of alcohol as a poor solvent was
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K.-i. Shimokawa et al. / Colloids and Surfaces B: Biointerfaces 104 (2013) 1–4
gradually dripped into the gelatin solution. With an increase in the poor solvent concentration, a coacervate formed. For detailed observation of the coacervate state, ternary phase diagrams of gelatin–water–methanol, gelatin–water–ethanol, and gelatin–water–propanol systems were produced using a light microscope (Digital Microscope DMBA310, Shimadzu Rika Co., Tokyo, Japan). Each phase diagram consisted of more than 70 observation points of each component. The gelatin solution, viscous gel, and coacervation phase regions were shown by borderlines. In the coacervation region, the region showing the greatest amount of coacervation was defined as the optimal coacervation region. 2.2.2. Drug release Phenacetin (0.04 g) was added as a model core to 10 (w/v)% gelatin solution. While this solution was thoroughly stirred, each type of alcohol as a poor solvent was gradually dripped to form a coacervate. After cooling to 5 ◦ C, 0.5 mL of 30 (w/v)% formalin was added for crosslinking to fix the formed gelatin microcapsules. Subsequently, the suspension containing microcapsules was filtered and washed adequately with purified water to remove unreacted formaldehyde. Microcapsules (20 g, dry weight) were placed in dialysis tubes, and a phenacetin release test was performed in 500 mL of purified water at 20 ◦ C. As a control, 0.05 g of phenacetin was dissolved in 10 (w/v)% gelatin solution, the solution was placed in a dialysis tube, and a similar drug release test was performed. The phenacetin release properties were evaluated by comparing the prepared microcapsules and control.
Fig. 1. Ternary phase diagram of gelatin–water–methanol system. (O): Optimum coacervation region, (V): viscous gel phase region, (C): coacervation region, (S): solution region containing gelatin.
2.2.3. Calculation of the drug release rate Samples of the dialysate were collected 0, 0.5, 1, 2, 4, 6, 8, 12, 24, 28, 36, 48, 54, 60, and 72 h after the initiation of dialysis, and the drug released from the dialysis tube was measured using a UV-visible spectrophotometer V-650 (JASCO Corp., Tokyo, Japan). Measurement was performed at 246 nm as the maximum absorption wavelength for phenacetin, and the absorption of phenacetin (0.04 g) dissolved in the dialysate (500 mL) was regarded as 100% drug release, and the drug release rate was calculated. 3. Results and discussion Using various alcohols (methanol, ethanol, 1-propanol, and 2-propanol) as poor solvents, ternary phase diagrams of the gelatin–water–alcohol systems were produced based on the composition ratio of the gelatin solution, viscous gel, and coacervation phases (Figs. 1–4). On each diagram, the optimal coacervation region was clarified. Based on the optimal coacervation region, stable microcapsules were prepared, and the drug release rate was compared among the systems.
Fig. 2. Ternary phase diagram of gelatin–water–ethanol system. (O): Optimum coacervation region, (V): viscous gel phase region, (C): coacervation region, (S): solution region containing gelatin, (A): aggregating region.
3.1. Ternary phase diagrams 3.1.1. Ternary phase diagram of the gelatin–water–methanol system The ternary phase diagram of the gelatin–water–methanol system is shown in Fig. 1. In this figure, (O) indicates the optimal coacervation region, (C) indicates the coacervation region, (V) indicates the viscous gel region, and (S) indicates the solution containing gelatin. The optimal coacervation region was observed at about a gelatin:water:methanol ratio of 0.05:0.25:0.70. Around this region, the coacervation region (C) was present. However, this composition was fragile as microcapsules. With a gradual increase in the water ratio, there was a gelatin solution region (S), and, with an increase in the gelatin ratio, there was a viscous gel region (V). No gelatin aggregation was observed in this system. Based on this diagram, coacervation was considered to occur in the region at a methanol rate of about 60% or more.
Fig. 3. Ternary phase diagram of gelatin–water–1-propanol system. (O): Optimum coacervation region, (V): viscous gel phase region, (C): coacervation region, (S): solution region containing gelatin, (A): aggregating region.
K.-i. Shimokawa et al. / Colloids and Surfaces B: Biointerfaces 104 (2013) 1–4
Fig. 4. Ternary phase diagram of gelatin–water–2-propanol system. (O): Optimum coacervation region, (V): viscous gel phase region, (C): coacervation region, (S): solution region containing gelatin, (A): aggregating region.
3.1.2. Ternary phase diagram of the gelatin–water–ethanol system The ternary phase system of the gelatin–water–ethanol system is shown in Fig. 2. In this figure, the optimal coacervation region was observed at about a gelatin:water:ethanol ratio of 0.05:0.45:0.50. In this system, coacervation was considered to occur in the region at an ethanol rate of about 40% or more. Compared with the gelatin–water–ethanol system that required a methanol rate of about 60% or more for coacervation, the gelatin–water–methanol system required a small amount of ethanol for coacervation due to its marked dehydration effects [12]. There was a gelatin aggregation region (A) that was not observed in the gelatin–water–methanol system. 3.1.3. Ternary phase diagram of the gelatin–water–1-propanol system The ternary phase system of the gelatin–water–1-propanol system is shown in Fig. 3. In this figure, the optimal coacervation region was observed at about a gelatin:water:1-propanol ratio of 0.05:0.5:0.45. In this system, coacervation was considered to occur in the region at a 1-propanol rate of about 25% or more. The optimal coacervation region was the largest for the gelatin–water–1propanol system, followed in order by the gelatin–water–ethanol system and gelatin–water–methanol system. In addition, with an increase in the hydrophobicity of the poor solvent from methanol to ethanol and then to 1-propanol, coacervation occurred with a smaller amount of the poor solvent. There was a gelatin aggregation region (A) in the gelatin–water–1-propanol system as well as the gelatin–water–ethanol system. 3.1.4. Ternary phase diagram of the gelatin–water–2-propanol system The ternary phase system of the gelatin–water–2-propanol system is shown in Fig. 4. In this figure, the optimal coacervation region was observed at about a gelatin:water:2-propanol ratio of 0.05:0.5:0.45. The diagram of this system was similar to that of the gelatin–water–1-propanol system, showing coacervation at a 2-propanol rate of 25% or more. The optimal coacervation region was also similar to that in the gelatin–water–1-propanol system (Figs. 3 and 4). This similarity of each region may be due to the similarity of the degree of hydrophobicity between 2-propanol and 1-propanol. There was also a gelatin aggregation region (A) in this system as well as the gelatin–water–ethanol and gelatin–water–1propanol systems.
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Fig. 5. The effect of the mixing ratio on drug release from microcapsules in gelatin–water–ethanol system. : gelatin:water:ethanol = 0.04: 0.34:0.62, 䊉: gelatin:water:ethanol = 0.03:0.30:0.67, : gelatin:water:ethanol = 0.03:0.27:0.70, : gelatin:water:ethanol = 0.09:0.91:0 [control].
3.2. Controlled drug release 3.2.1. Comparative properties of drug release Gelatin microcapsules were prepared using the composition in the optimal coacervation region in the ternary phase diagram of the gelatin–water–methanol system. Phenacetin as the core was dissolved in gelatin solution, and methanol as a poor solvent was gradually added for coacervation. However, the prepared microcapsules were fragile, appropriate particle diameters could not be obtained, and microcapsules could not be produced. Subsequently, 3 component ratios in the coacervation region were selected in the ternary phase diagram of the gelatin–water–methanol system for the preparation of microcapsules containing phenacetin. The selected component ratios were as follows: gelatin:water:ethanol = 0.04:0.34:0.62 (), 0.03:0.30:0.67 (䊉), and 0.03:0.27:0.70 (). The results of a drug release test using the prepared capsules are shown in Fig. 5. As shown in this figure, the rate of drug release from the microcapsules almost linearly increased until 6 h after the initiation of the test and became almost stable after 24 h. The drug release rate differed among the microcapsule compositions. The drug release rates after 24 h were 85, 80, and 69% for , 䊉, and , respectively, showing the most sustained release for , followed in order by 䊉 and . A composition not containing the poor solvent [gelatin:water:ethanol = 0.09:0.91:0 ()] was used as a control. Similarly, microcapsules were prepared using the above 3 component ratios in the ternary phase diagram of the gelatin–water–1-propanol system. The results of a drug release test of the prepared capsules are shown in Fig. 6. From this figure, the drug release rate almost linearly increased until 2 h after the initiation of the test, but began to differ among the 3 compositions after 4 h. The drug release rates after 24 h were 73, 50, and 25% for , 䊉, and , respectively, showing the most sustained release for , followed in order by 䊉 and . When the drug release rate for this system is compared with that for the gelatin–water–ethanol system, the drug release rate decreased from 85 to 73% (12% decrease) for composition , from 80 to 50 (30% decrease) for 䊉, and from 69 to 35% (34% decrease) for (Figs. 5 and 6). This may be because the change in the poor solvent from ethanol to the more hydrophobic 1-propanol more markedly sustained drug release. Due to the more marked dehydration effects of propanol than those of ethanol, the gelatin density in the membrane may have increased. For the preparation of DDS drugs, the coacervation method may be useful. Similarly, microcapsules were prepared using the 3 component ratios based on the ternary phase diagram of the gelatin–water–2propanol system. The results of a drug release test of the prepared
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Fig. 6. The effect of the mixing ratio on drug release from microcapsules in the gelatin–water–1-propanol system. : gelatin:water:1-propanol = 0.04:0.34:0.62, gelatin:water:1-propanol = 0.03:0.30:0.67, : gelatin:water:1䊉: propanol = 0.03:0.27:0.70, : gelatin:water:1-propanol = 0.09:0.91:0 [control].
The effect of the content ratio of three different poor solvents to total volume on drug release percentage from microcapsules after 24 h is shown in Fig. 8. As can be seen from this figure, the drug release rates after 24 h can be controlled in the range of 85% from 35% using the different poor solvent. Moreover, from Fig. 8, the drug release rates were similar between the gelatin–water–1-propanol and gelatin–water–2-propanol systems, suggesting the similarity of release properties between microcapsules using the two systems. Probably because 1-propanol and 2-propanol differ only in the position of the hydroxyl group (1 or 2) in the molecular structure, but show similar degrees of hydrophobicity, the drug release properties of prepared microcapsules after membrane filtration were similar. The administration route of the drug in the microcapsules prepared in this study as a DDS drug is percutaneous or oral. The results of this study suggested effective controlled drug release using the gelatin–water–propanol systems. A problem in drug production is residual propanol in the drug. 1-Propanol is a liquid that is volatile at a normal temperature and pressure, and has low toxicity for the human body. This propanol has been approved as a food additive (flavoring material) in many countries such as Japan and western countries. On the other hand, 2-propanol is used as a solvent of local drugs for cosmetics and medical drugs, and also used for tablet coating with films and tablet granulation because its complete removal from the system by evaporation is possible [13]. Therefore, concerning this problem, propanol can be removed by adequate drying (evaporation) during microcapsule preparation. 4. Conclusion
Fig. 7. The effect of the mixing ratio on drug release from microcapsules in the gelatin–water–2-propanol system. : gelatin:water:2-propanol = 0.04:0.34:0.62, gelatin:water:2-propanol = 0.03:0.30:0.67, : gelatin:water:2䊉: propanol = 0.03:0.27:0.70, : gelatin:water:2-propanol = 0.09:0.91:0 [control].
The results of this study suggested the optimal coacervation region in the ternary phase diagrams using methanol, ethanol, 1-propanol, or 2-propanol as the poor solvent expands the region with its increasing degree of hydrophobicity. Microcapsules prepared based on the optimal coacervation region allowed the control of drug release. In addition, the rate of drug release from microcapsules prepared using the gelatin–water–propanol was 34% lower than that from microcapsules prepared using the gelatin–water–ethanol system 24 h after the initiation of the test. These results suggest that microcapsules prepared using the gelatin–water–propanol system are excellent for controlled drug release, and can be a useful DDS. References
Fig. 8. The effect of the solvent content at the preparation of microcapsules on drug release percentage after 24 h. : ethanol (solid line), : propanol (solid line), : propanol (dotted line).
microcapsules are shown in Fig. 7. As shown in this figure, results similar to those for the gelatin–water–1-propanol system were obtained. The drug release rates after 24 h were 75, 52, and 37% for , 䊉, and , respectively.
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