http://informahealthcare.com/ddi ISSN: 0363-9045 (print), 1520-5762 (electronic) Drug Dev Ind Pharm, Early Online: 1–8 ! 2013 Informa Healthcare USA, Inc. DOI: 10.3109/03639045.2013.873447
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RESEARCH ARTICLE
Controllable coating of microneedles for transdermal drug delivery Jianmin Chen1,2, Yuqin Qiu1, Suohui Zhang1, Guozhong Yang1,3, and Yunhua Gao1 1
Key Laboratory of Photochemical Conversion and Optoelectronic Materials, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing, China, 2College of Pharmaceutical and Medical Technology, Putian University, Fujian, China, and 3Graduate University of Chinese Academy of Sciences, Beijing, China Abstract
Keywords
Coated microneedles have been paid much attention recently, and several coating strategies have been developed to address the problems during coating process. However, there are still some unresolved issues, such as, precise control requirements, microneedle substrate contamination and high processing temperature. The purpose of this study was to develop a simple and controllable method to make uniform coatings on microneedles at room temperature. This novel method avoids the contamination of microneedle substrate by providing both the adsorption force of thickener and micro-scale coating film produced by a newly design device. Thickeners were screened to enhance the mass of coatings. The parameters that influence the coatings were tested systematically, which made coating process controllable. Finally, three model drugs were coated onto microneedles to prove the method is applicable more broadly. In addition, insertion experiments were carried out to test the drug delivery feasibility of the coated microneedles. In conclusion, this study presents a simple and controllable method to coat microneedles with small molecular chemical drugs or large proteins for rapid skin drug delivery.
Adsorption force, coating device, coating film, coating microneedles, coating solution, transdermal drug delivery
Introduction Transdermal drug delivery system (TDDS), a sustained and controlled drug delivery platform, is getting more and more attractive in the field of drug delivery due to many advantages compared with oral and needle administration. For example, TDDS enhances bioavailability by avoiding hepatic first-pass metabolism commonly happened in oral drug delivery and improve patients’ compliance by reducing the frequent administration and risk of infection over needle-based delivery1–3. However, stratum corneum, the outermost layer of skin, only allows a limited number of drugs to penetrate the skin enough to achieve pharmacological effects4. Various strategies emerged to overcome the skin barrier can be categorized into passive and active drug delivery. The passive methods use vehicle formulations and chemical enhancers, while active methods include ultrasound, thermal ablation, iontophoresis, electroporation and microneedles5–8. Microneedles made by sorts of materials such as silicon, metal, maltose and polymer with common sizes from 1 to 1000 mm have two types: hollow and solid9–11. Microneedle-based drug delivery has four different modes: (1) the ‘‘poke and patch’’ approach; (2) the ‘‘poke and release’’ approach; (3) the ‘‘coat and poke’’ approach; (4) ‘‘poke and flow’’. Thereinto, ‘‘coat and poke’’ method is suitable for micro drug delivery. Coated
Address for correspondence: Yunhua Gao, Key Laboratory of Photochemical Conversion and Optoelectronic Materials, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, China. Tel: +86-10-82543581. E-mail: yhgao@ mail.ipc.ac.cn
History Received 25 February 2013 Revised 25 November 2013 Accepted 2 December 2013 Published online 31 December 2013
microneedles are one of the most attractive approaches among these modes. It can be used easily just like a band-aid like system, and the dried coating film help drugs keep stable12. The most important factor of coated microneedles is coating process. Dip-coating is a traditional method, which is appealing for coating microneedles, because it is simple. However, surface tension becomes dominant on the micron-scale and the effects of surface tension, capillarity and viscous forces will cause contamination of microneedle array substrate. As a result, the quantity of a drug is uncertain and it will cause drug loss12. In order to deal with this problem, various coating methods have been investigated, but there are still some unresolved issues. There are some examples: A porous coating device and a roll coating device can weaken the capillary action happened hugely in the simple dip-coating process, but it is complicated and time-consuming, because it needs precise control to assure the microneedles insert accurately into the microporous12–15. Gas-jet coating method cannot avoid spraying the drug to the substrate of the microneedles and thus induces drug loss16,17. Aerosol coating method needs high temperature (200 C) to transfer the coating liquid into aerosol, thus kinds of coating drug are limited due to its stability18. Contact coating method using a brush-like plastic sheet to coat microneedles, which is hard to obtain uniform coatings due to the uneven contact force19. The aim of this study was to develop a simple method for coating microneedles to address the limitations mentioned above. The approach includes three steps as follows. (1) Produce a thin film with thickness varied from 50 to 500 mm according to the length of microneedles. (2) Apply the microneedles onto the thin film. (3) Pull up the coated microneedles and dried in the air. The coating method can avoid contamination of the microneedle
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array substrate by forming the micro-scale coating film and the adsorption force between the thickener and the plate. It is simple and does not need any delicate instrument or precise control. Moreover, the coating process was carried out under room temperature. The coating process with uniform and dose controlled can apply to a breadth of biopharmaceutical. And the coated microneedles successfully delivery drugs across the stratum corneum into the deeper skin.
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Materials and methods Materials Calcein (C30H26N2O13, M.W. ¼ 622.55), myoglobin (from canine heart, 95–100%, essentially salt-free, lyophilized powder), hydroxyethyl cellulose (M.W. ¼ 250 kDa), riboflavin and riboflavin Sodium phosphate (RFSP) were purchased from Sigma (St. Louis, MO). Sodium carboxymethyl cellulose (CMC, M.W. ¼ 90 kDa) was purchased from Amresco. Polyvinylpyrrolidone (PVP, M.W. ¼ 30 kDa) was purchased from Boai NKY Pharmaceuticals Ltd (Henan, China). Hydroxypropyl methyl cellulose (HPMC, the viscosity of 2 wt% HPMC in water is 400 mPa s) and fluorescein sodium (FS) were purchased from Aladdin regent (Shanghai, China). All other chemicals used were analytical or pharmaceutical grade. Porcine skin was obtained from a local slaughterhouse (Beijing, China) and dermatomed to a thickness of 800 mm. Microneedle fabrication Microneedle arrays were fabricated as described previously20,22. In brief, first pattern square or circular oxide patterns (800 mm in diameter) were made on a Si substrate, and then the untreated portion of Si wafers is etched away by an isotropic-reactive ion etching process using SF6/O2 plasma. The needle body was formed using potassium hydroxide etching solution. Figure 1 shows the microneedle array with 260 mm length. Center to center space is 800 mm, and tapering to a sharp tip with an angle of 38 . Microneedle arrays were treated with oxygen gas plasma for a short period of time (2 min) before coating in order to make the surface more hydrophilic and uniform23. Coating process Coating device and method A simple device was designed using SolidWorks 2007 and manufactured by Precision and Special Processing Center,
Drug Dev Ind Pharm, Early Online: 1–8
Technical Institute of Physics and Chemistry, Chinese Academy of Science. It is consisted of two parts, stainless steel plate with 50 or 100 mm groove and hollow cuboid which was placed on the plate and generated a reservoir used to fill coating solution. Coating solution was poured into the reservoir, and then hollow cuboid was pulled manually from left to right to form a thin and uniform coating film in the groove. Finally, put the microneedle array into the thin coating film vertically by hand, staying for a second and withdrawing. The coating process kept a relative humidity about 40% under room temperature. Then the coated microneedles were allowed to dry at ambient temperature. All of the following experiments based on this operation. Screening of coating solution The screening experiments of coating solution were carried out by dipping 9 mm 5 mm chips of silicon into the solution of thickener for one second and air drying at room temperature overnight. A model drug [0.01% (w/v) calcein] was added into the thickener solution to determine the drug mass of the coatings. After drying, the chips were incubated in 1 ml deionized water for 2 h under vibration in the dark. The solutions were determined using fluorescence spectrophotometer (Hitachi F-2500L, Japan; excitation ¼ 494 nm, emission ¼ 515.5 nm). The thickeners include polyvinylpyrrolidone (PVP), sucrose, hydroxypropyl methyl cellulose (HPMC) and sodium carboxymethyl cellulose (CMC) and the concentration of the thickeners varied from 0.1% to 55% (w/v). The viscosity data of these solutions were tested by Brookfield viscometer (LVDV-II þ Pro). Coating parameters Coating parameters were investigated. The coating process was according to the operation as mentioned above. The coating parameters included (I) the concentration of a thickener in the coating solution, (II) the concentration of a model drug in the coating solution, (III) the number of dips during coating, (IV) the thickness of the coating film and (V) the reproducibility of the coating process. Fluorescein sodium (FS) was used as a model drug in the coating formulations. Each microneedle array has 144 needle/cm2. The following experiments were designed: For parameter (I), HPMC coating solutions with concentrations of 1, 2, 3, 4 and 5% containing 3% FS, respectively, 1 dips and 100 mm thickness of coating film. For parameter (II), FS as model drug at 1, 3, 5 and 10% concentrations in coating solutions of 3% HPMC, respectively, 5 dips at 3 s intervals between dips, and 100 mm thickness of coating film. For parameter (III), 3% FS in coating solutions of 3% HPMC, 1, 3, 5 or 8 dips at 3 s intervals between dips, respectively, and 100 mm thickness of coating film. For parameter (IV), 3% FS in coating solutions of 3% HPMC, 50 or 100 mm thickness of coating film. For parameter (V), the data was collected from the above experiments. All coated microneedle arrays were imaged by bright-field microscopy using an Olympus SZX12 stereo microscope (Olympus America) with a CCD camera (Olympus DP71, America). The mass of FS was determined by dissolving the coatings off the microneedle arrays immersed in DI water for 1 hour, and then measuring by fluorescence spectrophotometer (excitation ¼ 490 nm, emission ¼ 514 nm). Three replicates (n ¼ 3) were carried out in all experiments if no otherwise specified. Coating of model drugs
Figure 1. Scanning electron micrograph of solid silicon microneedles: the complete microneedle array contains 144 needles, each measuring 260 mm in length and tapering to a sharp tip with an angle of 38 .
Three model drugs have been coated onto the microneedle array to prove the coating method is universally applicable. The model
DOI: 10.3109/03639045.2013.873447
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drugs at 3% (w/w) concentration were dissolved or dispersed in 3% HPMC solution, respectively. The coating parameters: the microneedle array has 144 needles/cm2 with the length of 260 mm; the number of dips is 5 with 3 s interval; the depth of coating film is 100 mm. After the coatings dried, the microneedle arrays were taken photo using microscope with a CCD camera mentioned above and then dissolved into 1 ml DI water for 1 h under stirring. Finally, the solutions were determined by fluorescence spectrophotometer. In vitro delivery efficiency Microneedle array (n ¼ 5) coated with FS as model drug was applied on porcine cadaver skin by hand for 1, 3 or 5 min. Withdraw the microneedle array and apply tape onto the surface of the treated site three times to collect the drug scattering on the skin surface. In order to evaluate the delivery efficiency, the amount of residual drug was determined as follows: the inserted microneedle array and the tapes above were rinsed into PBS solution for 1 h, and then the solution was determined by fluorescence spectrophotometer. The porcine cadaver skin and microneedle arrays were viewed before and after treatment by microscope with a CCD camera. According to the residual amount, the delivery efficiency was calculated.
Results Coating method and device As shown in Figure 2, the coating device has two parts: plate (1) with groove (2) and hollow cuboid (3), all are made of stainless steel. The depth of the groove is 50 or 100 mm. And the width of groove and hollow cuboid is 12 mm which is wider than the microneedle array. Hollow cuboid was placed on the plate which has a groove and generated a reservoir (4) used to load Figure 2. The schematic diagrams of coating device, front view (A) and side view (B). Numbers stand for: 1 (Plate), 2 (Groove), 3 (Hollow cuboid), 4 (Reservoir), 5 (Coating film), 6 (Microneedle array). As showed in picture B, hollow cuboid (3) was placed on plate (1) with 50 or 100 mm groove (2) to generate a reservoir which was used to fill coating solution. Coating solution was poured into the reservoir (4), and then hollow cuboid (3) was pulled from left to right to form a thin and uniform coating film (5) on the groove (2). Finally, the microneedle array (6) was put into the thin coating film vertically, lingering for a second and withdrawing.
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coating solution. Coating solution was poured into the reservoir, and then hollow cuboid was pulled from left to right manually to form a thin and uniform coating film (5) on the groove bed. Finally, put the microneedle array (6) into the thin coating film vertically, stay for a second and withdrawing. After that, hollow cuboid was pulled from right to left to form a thin film on the right side for the next coating process. Coating solution To study the effect of thickeners on the drug mass of coating, four thickeners were selected. Table 1 shows the relationship between viscosity of thickeners and the drug mass of coating. For these four thickeners (PVP, Sucrose, HPMC and CMC), the viscosity data increased with the rising concentration of solutions. For example, the viscosity of HPMC went up from 28.8 to 14 507.0 CP when the concentration increased from 1% (w/v) to 5% (w/v), respectively. However, the drug mass of coating did not increase with the rising viscosity of the solution always. At relative low viscosity of the solution, the drug mass of coating went up with the viscosity, followed by reaching the highest point, and then it went down with the further rising viscosity. Take CMC as an example, the drug mass of coating grew from 0.48 to 1.43 mg with the rising viscosity from 214.4 to 7040.0 CP, and then got the highest point with drug mass of 1.81 mg, and it went down to 1.62 mg finally. The other data were all shown in Table 1. Coating parameters The effect of thickener concentration in the coating solution We quantified the contamination by measuring the length of the coating on the microneedle on the pictures. Figure 3 shows the coating results of HPMC as a thickener. The HPMC 1% solution caused contamination at the substrate of the microneedle array
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Table 1. Viscosity of thickener solutions and drug mass of coatings. Solution
Con.* (w/v) (%)
Viscosity (CP)
Drug mass** (mg)
SD
PVP 10 15 20 25 30
28.8 41.6 60.8 92.8 153.6
1.24 1.74 2.01 2.12 1.52
0.19 0.06 0.48 0.24 0.08
1 2 3 4 5
28.8 195.2 1771.0 4128.0 14507.0
0.57 0.94 1.45 1.96 1.90
0.12 0.23 0.16 0.17 0.17
30 35 40 45 50
19.2 22.4 28.8 38.4 48.0
0.13 0.17 0.21 0.17 0.16
0.01 0.01 0.07 0.02 0.03
1 2 3 4 5
214.4 736.0 7040.0 30933.0 311000.0
0.48 0.79 1.43 1.81 1.62
0.01 0.04 0.04 0.07 0.07
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HPMC
and no coatings on the tip of microneedle (Figure 3B); HPMC 2% did not cause a contamination with the coating length of 200 mm (Figure 3C); the following concentration 3, 4 and 5% of HPMC (Figure 3D–F) did not cause contamination with the coating length of 130, 100 and 100 mm, respectively. Concentration of a model drug in the coating solution Table 2 shows the relationship between the concentration of drug and the mass of coatings. When the concentration of FS is 1%, the mass is 3.08 0.44 mg. While the concentration is up to 3%, the mass jumps to 13.56 3.32 mg. As the concentration increases to 5% or 10%, the mass also edges up to 19.40 2.16 mg or 20.32 6.20 mg.
Sucrose
CMC
*Solutions of thickeners (PVP, Sucrose, HPMC and CMC) with various concentrations (con.) were prepared. **Solutions of certain concentration [from 0.1% to 55% (w/v)] of thickeners containing 0.01% (w/v) calcein as a model drug were prepared. About 9 mm 5 mm chips of silicon were dipped into these solutions and then dried under room temperature. After that the chips with coating were rinsed into deionized water and determined by fluorescence spectrophotometer. The drug mass of coatings were calculated.
Number of dips during coating In order to obtain a high drug mass of coatings, the repetition of dip is necessary. Under condition of 3% (w/v) HPMC thickener solution containing model drug 3% (w/v) FS, only 1.52 0.08 mg
Table 2. Effect of FS concentrations on coating mass. Group A B C D
Con. (w/v)* (%)
Mass of coating (mg, n ¼ 3)
SD
RSD
1 3 5 10
3.08 13.56 19.40 20.32
0.44 3.32 2.16 6.20
0.14 0.25 0.11 0.30
*Different concentrations (con.) of fluorescein sodium(FS) are used as coating solutions. With other coating parameters: dips: 5; thickener: 3% (w/v) HPMC; thickness of coating film: 100 mm; intervals between dips: 3 s.
Figure 3. Effect of different concentrations of HPMC on coating length. (A) Non-coated microneedle; (B) 1% concentration of HPMC resulted in contamination of substrate; (C–F) 2, 3, 4 and 5% of HPMC with coating length of 230, 130, 100 and 100 mm, respectively. With other coating parameters: thickness of coating film: 100 mm; Number of dips: 1; Model drugs: 3% (w/v) fluorescein sodium solution.
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was coated on the microneedles when dipped one time. The number of dips increased, the drug mass of coatings also grew. For example, from 1 dip to 3 dips, the drug mass went up about three times from 1.52 0.08 mg to 5.08 0.8 mg, while from 1 dip to 5 dips the mass only get more than 8 times to 12.76 2.48 mg. And when dips were up to 8, the mass went about 12 times to 19.00 3.04 mg (Table 3).
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Thickness of the coating film As seen in Figure 4, the coating drug length of the microneedle was at a range of approximately 130 mm in 100 mm film, while 75 mm coating length was obtained in 50 mm film. As a result, the drug mass of coatings using 100 mm film was 12.15 mg (n ¼ 3) and 4.08 mg for using 50 mm film.
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Reproducibility of the coating process The collected data shows that the average of the drug mass of coating is 12.54 2.40 mg (n ¼ 12). This result showed that the coating process could be repeated. Coating of model drugs To prove the coating method is widely applicable, RFSP, riboflavin and myoglobin as the small molecule model drugs of hydrophilic/hydrophobic drug and bio-macromolecules were coated on the microneedle array (Figure 5). The mass of coating of RFSP, Riboflavin and Myoglobin reached 7.56 0.80 mg, 21.48 2.28 mg and 20.08 2.80 mg, respectively (Table 4). It suggests that the coating method can be used for a variety of drugs. In vitro delivery efficiency
Table 3. Effect of dips on coating mass of FS on microneedle array. Group A B C D
Dips*
Mass of coating (mg, n ¼ 3)
SD
RSD
1 3 5 8
1.52 5.08 12.76 19.00
0.08 0.80 2.48 3.04
0.06 0.15 0.19 0.16
*Dips are the times that microneedles were dipped into coating solution. With other coating parameters: thickener: 3% (w/v) HPMC; Model drug: 3% (w/v) fluorescein sodium (FS) solution; thickness of coating film: 100 mm; intervals between dips: 3 s.
The microneedle arrays before and after insertion skin with different time are shown in Figure 6. To quantify the delivery efficiency of coated microneedles, microneedles coated with FS were inserted into porcine cadaver skin. After 1, 3 or 5-min insertion of the microneedles with 12.54 2.40 mg of FS into skin, 3.57, 10.13 or 11.75 mg of FS was delivered into the skin, the delivery efficiency were 28, 81 and 94, respectively (Table 5). This experiment proved that the coated microneedle array can be inserted into the skin and efficiently delivery the drug into the skin. The coated film does not affect the efficiency of insertion.
Discussion Silicon is a biocompatible material with mature silicon micromachining technologies21, and cheap for mass production. In addition, the silicon microneedle array has enough strength to penetrate the skin without broken. And, the microneedle array has good biological safety22. Therefore, we used silicon as a base material to fabricate microneedles. Table 4. Effect of different model drugs on coating mass. Group A B C Figure 4. Effect of thickness of the coating film on coating lengths on microneedle. With coating parameters: thickness of coating film: 100 mm (A) and 50 mm (B); Number of dips: 5; thickener: 3% (w/v) HPMC; Model drugs: 3% (w/v) drug solutions; intervals between dips: 3 s.
Model drugs
Mass of coating** (mg, n ¼ 5)
SD
RSD
RFSP* Riboflavin Myoglobin
7.56 21.48 20.08
0.80 2.28 2.80
0.11 0.11 0.14
*RFSP stands for riboflavin sodium phosphate. **Coating parameters: number of dips:5; thickener: 3%(w/v) HPMC; model drugs: 3% (w/v) drug solutions; thickness of coating film: 100 mm; intervals between dips: 3 s.
Figure 5. Microneedles coated with different model drugs: (A) RFSP; (B) Riboflavin; (C) Myoglobin. With coating parameters: Number of dips: 5; thickener: 3% (w/v) HPMC; Model drugs: 3% (w/v) drug solutions; thickness of coating film: 100 mm; intervals between dips: 3 s.
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Figure 6. Photographs of the coating microneedles with FS and its treating skin: (A), (C), (E) Coated microneedles before inserting into the skin; (B, D and F) Microneedles withdrew from the skin after 1, 3 and 5 min insertion, respectively; (G, H) Surface of porcine cadaver skin after insertion and removal of microneedle array. Each stained spot corresponds to the site of microneedles penetration into the skin.
Table 5. Effect of insertion time of microneedles on the delivery efficiency. Group A B C
Dissolving time (min)
RA* (mg, n ¼ 5)
RSD
DE** (%)
1 3 5
8.97 2.42 0.79
0.25 0.25 0.08
28 81 94
*RA: Residual amount, drugs remained on the inserted microneedle array and the tapes. **DE: Delivery efficiency, calculated by the equation below. MC stands for average mass of coating on microneedles. DE ¼ MCRA MC 100%
The most challenge of coating microneedles is the capillary effect which is more obvious in micro-scale24. Thus, the traditional approach dip coating is not suitable. Many strategies have been developed to improve the coating process with the aim to avoid the capillary effects (capillary effects among the microneedles on the array) which would result in contaminate the substrate of the microneedle array and induce drug waste and in-homogeneity. In order to tackle with this issue, we
developed a novel coating device (Figure 2). There are two important factors of technology which are vital for deal with the capillary effect. First, it is thickness of a thin film determined by the depth of the groove. The thickness of the thin film is 100 mm when the length of microneedle is 260 mm, which resulted in 130 mm coating length on microneedles, as shown in Figure 3(D). Second, it is the viscosity of the coating solution provided by thickener, and when the solution spreads on the groove bed, the groove bed provides an adsorption force which help balance the capillary force. We have tried using solution without thickener, which result in contamination of substrate (data no shown). More importantly, coating device was operated manually at room temperature, which did not need precise control. As shown in Table 1, there is no necessary relation between the drug mass of coating and the viscosity of solution. For example, PVP, sucrose and HPMC have the same viscosity of 28.8 CP, but they did not share with the same drug mass of coating. For sucrose, the drug mass of coating was 0.17 mg with two different viscosities (22.4 and 38.4 CP). Relatively, large amount of drug mass was obtained either at high PVP concentration or at low CMC and HPMC concentration. Under a certain drug mass of
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DOI: 10.3109/03639045.2013.873447
coating, the mass of thickener is inversely proportion to the mass of drug. Therefore, HPMC has a better performance on the drug mass of coating in the same concentration comparing with PVP and CMC. The coating parameters such as concentration of the drug and thickener, numbers of dips, depth of groove and reproducibility are playing important roles in the forming of coatings. The drug mass of coating increases with increasing the concentration of drug when other conditions are the same, as shown in Table 2, so we can control the dose of drug on microneedles by adjusting the concentration of drug. The concentration of HPMC is important for successful coating, for example, Figure 3(B) showed that 1% (w/v) HPMC resulted in contamination, while the concentration over 2% (w/v) realized successful coating. However, different concentration of HPMC resulted in different length of coatings shown in Figure 3(C–F). The depth of the groove is another factor influencing the length of coatings, which was demonstrated in Figure 4. The drug mass of coating grows with the increasing of the number of dips as shown in Table 3. Therefore, we can also control the dose of drug on microneedles by adjusting the number of dips. The reproducibility of the coating process is meaningful due to the need of accurate quantification for drug delivery. To demonstrate the coating method is a widely technology for coating, three types of drug were used as model drugs. RFSP, riboflavin and myoglobin are representative of hydrophilic drug, hydrophobic drug and bio-macromolecules, respectively. All the three drugs can be coated on the microneedles uniformly (Figure 5), which suggested that the coating method is widely applicability for most drugs. According to the results (Table 4), a 10 mm 10 mm microneedle array can be coated with 20.08 mg proteins, so the coated microneedle array is very suitable for highly active protein such as vaccines and interferons because of low dose (several to ten micrograms). Specifically, coated microneedles are attractive to vaccine delivery, because transdermal delivery can target dendritic cells residing in the epidermis for a more potent immune response25. In addition, in vitro experiments showed that the coatings released off within 5 min and the delivery efficiency was up to 94%. The data (Figure 6E,F) also showed that the coated microneedle can be inserted effectively into skin without broken and less drug loss. The coatings were disappeared after insertion when the length of the coatings is 130 mm, which suggested that the effective insertion depth is more than 130 mm when using microneedles with a 260 mm in length. Furthermore, in vitro experiments were carried out manually, so insertions by hand were proved to be a reliable mode for coated microneedle delivery. Therefore, it is a simple delivery method which can be self-administrate without any extra device and energy.
Conclusions In this study, we explored a novel approach of coating microneedle array for transdermal drug delivery. This approach aimed to solve the problems results from capillary effect which is more obvious in micro-scale and avoid the defects of existing methods. The technique used to coat microneedles is relatively simple and avoids contamination of substrate of microneedles. Moreover, the coating process did not need precise control and can carried out under room temperature. The principle of this technique is to produce a micro-scale film to reduce capillary effect, and all parameters related to coating were tested. According to the results, we make the dose of drug on microneedles controllable by adjusting the coating parameters. Next, three model drugs coated microneedle array successfully shows that the method is broad applicable. Finally, insertion
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experiments certified the coated microneedle array is useful for drug delivery. All the results demonstrated that the coating method is simple and controllable to coat microneedle array with chemical drugs and bio-macromolecules for successful transdermal drug delivery.
Declaration of interest This work was partially supported by the Science Foundation of the Chinese Academy of Sciences and the National Natural Science Foundation of China (Grant No. 31300763). The authors report no conflicts of interest. The authors alone are responsible for the content and writing of this article.
References 1. Prausnitz MR, Langer R. Transdermal drug delivery. Nat Biotechnol 2008;26:1261–68. 2. Tanner T, Marks R. Delivering drugs by the transdermal route: review and comment. Skin Res Technol 2008;14:249–60. 3. Thomas BJ, Finnin BC. The transdermal revolution. Drug Discov Today 2004;9:697–703. 4. Benson HAE, Namjoshi S. Proteins and peptides: strategies for delivery to and across the skin. J Pharm Sci 2008;97:3591–610. 5. Doukas AG, Kollias N. Transdermal drug delivery with a pressure wave. Adv Drug Deliv Rev 2004;56:559–79. 6. Khafagy E-S, Morishita M, Onuki Y. Current challenges in noninvasive insulin delivery systems: a comparative review. Adv Drug Deliv Rev 2007;59:1521–46. 7. Park EJ, Werner J, Smith NB. Ultrasound mediated transdermal insulin delivery in pigs using a lightweight transducer. Pharm Res 2007;24:1396–401. 8. Garg S, Hoelscher M, Belser JA, et al. Needle-free skin patch delivery of a vaccine for a potentially pandemic influenza virus provides protection against lethal challenge in mice. Clin Vaccine Immunol 2007;14:926–8. 9. McAllister DV. From the cover: microfabricated needles for transdermal delivery of macromolecules and nanoparticles: fabrication methods and transport studies. Proc Natl Acad Sci USA 2003; 100:13755–60. 10. Kolli CS, Banga AK. Characterization of solid maltose microneedles and their use for transdermal delivery. Pharm Res 2007;25:104–13. 11. Lee JW, Park J-H, Prausnitz MR. Dissolving microneedles for transdermal drug delivery. Biomaterials 2008;29:2113–24. 12. Gill HS, Prausnitz MR. Coated microneedles for transdermal delivery. J Control Release 2007;117:227–37. 13. Toshiyuki Matsudo TK, Seiji Tokumoto. Microneedle device. United States Patent Application Publication Number: US 2010/0221314 A1; 2010. 14. Trautman JC, Wright CT, Cormier MJN. Method and apparatus for coating skin piercing microprojections. World Intellectual Property Office; Publication Number: 2002/074173; 2002. 15. Andrianov AK, Marin A. Methods and systems for coating a microneedle with a dosage ofa biologically active compound. United States Patent Application Publication Number: US 2009/0017210 A1; 2009. 16. Chen X, Prow TW, Crichton ML. Dry-coated microprojection array patches for targeted delivery of immunotherapeutics to the skin. J Control Release 2009;139:212–20. 17. McGrath MG. Determination of parameters for successful spray coating of silicon microneedle arrays. Int J Pharm 2011;415: 140–9. 18. Caradoc BJ, Martin CJ, Taylor G. Aerosol coating of microneedles. World Intellectual Property Organization International Publication 2009; Number WO 2009/081125 A2. 19. Choi H-H, Knutson GP, Daviv MM. Method of contact coating a microneedle array. World Intellectual Property Organization; International Application No.: PCT/US2005/041993; 2006. 20. Xie Y, Xu B, Gao Y. Controlled transdermal delivery of model drug compounds by MEMS microneedle array. Nanomedicine 2005;1: 184–90. 21. Voskerician G, Shive MS, Shawgo RS, von Recum H. Biocompatibility and biofouling of MEMS drug delivery devices. Biomaterials 2003;24:1959–67.
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22. Wu Y, Qiu Y, Zhang S. Microneedle-based drug delivery: studies on delivery parameters and biocompatibility. Biomed Microdevices 2008;10:601–10. 23. Suni T, Henttinen K, Suni I. Effects of plasma activation on hydrophilic bonding of Si and SiO2. J Electrochem Soc 2002;149: G348–51.
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24. Beebe DJ, Mensing GA, Walker GM. Physics and applications of microfluidics in biology. Annu Rev Biomed Eng 2002;4: 261–86. 25. Kim Y-C, Quan F-S, Song J-M. Influenza immunization with trehalose-stabilized virus-like particle vaccine using microneedles. Procedia Vaccinol 2010;2:17–21.
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RESEARCH ARTICLE
Controllable coating of microneedles for transdermal drug delivery Jianmin Chen1,2, Yuqin Qiu1, Suohui Zhang1, Guozhong Yang1,3, and Yunhua Gao1 1
Key Laboratory of Photochemical Conversion and Optoelectronic Materials, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing, China, 2College of Pharmaceutical and Medical Technology, Putian University, Fujian, China, and 3Graduate University of Chinese Academy of Sciences, Beijing, China Abstract
Keywords
Coated microneedles have been paid much attention recently, and several coating strategies have been developed to address the problems during coating process. However, there are still some unresolved issues, such as, precise control requirements, microneedle substrate contamination and high processing temperature. The purpose of this study was to develop a simple and controllable method to make uniform coatings on microneedles at room temperature. This novel method avoids the contamination of microneedle substrate by providing both the adsorption force of thickener and micro-scale coating film produced by a newly design device. Thickeners were screened to enhance the mass of coatings. The parameters that influence the coatings were tested systematically, which made coating process controllable. Finally, three model drugs were coated onto microneedles to prove the method is applicable more broadly. In addition, insertion experiments were carried out to test the drug delivery feasibility of the coated microneedles. In conclusion, this study presents a simple and controllable method to coat microneedles with small molecular chemical drugs or large proteins for rapid skin drug delivery.
Adsorption force, coating device, coating film, coating microneedles, coating solution, transdermal drug delivery
Introduction Transdermal drug delivery system (TDDS), a sustained and controlled drug delivery platform, is getting more and more attractive in the field of drug delivery due to many advantages compared with oral and needle administration. For example, TDDS enhances bioavailability by avoiding hepatic first-pass metabolism commonly happened in oral drug delivery and improve patients’ compliance by reducing the frequent administration and risk of infection over needle-based delivery1–3. However, stratum corneum, the outermost layer of skin, only allows a limited number of drugs to penetrate the skin enough to achieve pharmacological effects4. Various strategies emerged to overcome the skin barrier can be categorized into passive and active drug delivery. The passive methods use vehicle formulations and chemical enhancers, while active methods include ultrasound, thermal ablation, iontophoresis, electroporation and microneedles5–8. Microneedles made by sorts of materials such as silicon, metal, maltose and polymer with common sizes from 1 to 1000 mm have two types: hollow and solid9–11. Microneedle-based drug delivery has four different modes: (1) the ‘‘poke and patch’’ approach; (2) the ‘‘poke and release’’ approach; (3) the ‘‘coat and poke’’ approach; (4) ‘‘poke and flow’’. Thereinto, ‘‘coat and poke’’ method is suitable for micro drug delivery. Coated
Address for correspondence: Yunhua Gao, Key Laboratory of Photochemical Conversion and Optoelectronic Materials, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, China. Tel: +86-10-82543581. E-mail: yhgao@ mail.ipc.ac.cn
History Received 25 February 2013 Revised 25 November 2013 Accepted 2 December 2013 Published online 31 December 2013
microneedles are one of the most attractive approaches among these modes. It can be used easily just like a band-aid like system, and the dried coating film help drugs keep stable12. The most important factor of coated microneedles is coating process. Dip-coating is a traditional method, which is appealing for coating microneedles, because it is simple. However, surface tension becomes dominant on the micron-scale and the effects of surface tension, capillarity and viscous forces will cause contamination of microneedle array substrate. As a result, the quantity of a drug is uncertain and it will cause drug loss12. In order to deal with this problem, various coating methods have been investigated, but there are still some unresolved issues. There are some examples: A porous coating device and a roll coating device can weaken the capillary action happened hugely in the simple dip-coating process, but it is complicated and time-consuming, because it needs precise control to assure the microneedles insert accurately into the microporous12–15. Gas-jet coating method cannot avoid spraying the drug to the substrate of the microneedles and thus induces drug loss16,17. Aerosol coating method needs high temperature (200 C) to transfer the coating liquid into aerosol, thus kinds of coating drug are limited due to its stability18. Contact coating method using a brush-like plastic sheet to coat microneedles, which is hard to obtain uniform coatings due to the uneven contact force19. The aim of this study was to develop a simple method for coating microneedles to address the limitations mentioned above. The approach includes three steps as follows. (1) Produce a thin film with thickness varied from 50 to 500 mm according to the length of microneedles. (2) Apply the microneedles onto the thin film. (3) Pull up the coated microneedles and dried in the air. The coating method can avoid contamination of the microneedle
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array substrate by forming the micro-scale coating film and the adsorption force between the thickener and the plate. It is simple and does not need any delicate instrument or precise control. Moreover, the coating process was carried out under room temperature. The coating process with uniform and dose controlled can apply to a breadth of biopharmaceutical. And the coated microneedles successfully delivery drugs across the stratum corneum into the deeper skin.
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Materials and methods Materials Calcein (C30H26N2O13, M.W. ¼ 622.55), myoglobin (from canine heart, 95–100%, essentially salt-free, lyophilized powder), hydroxyethyl cellulose (M.W. ¼ 250 kDa), riboflavin and riboflavin Sodium phosphate (RFSP) were purchased from Sigma (St. Louis, MO). Sodium carboxymethyl cellulose (CMC, M.W. ¼ 90 kDa) was purchased from Amresco. Polyvinylpyrrolidone (PVP, M.W. ¼ 30 kDa) was purchased from Boai NKY Pharmaceuticals Ltd (Henan, China). Hydroxypropyl methyl cellulose (HPMC, the viscosity of 2 wt% HPMC in water is 400 mPa s) and fluorescein sodium (FS) were purchased from Aladdin regent (Shanghai, China). All other chemicals used were analytical or pharmaceutical grade. Porcine skin was obtained from a local slaughterhouse (Beijing, China) and dermatomed to a thickness of 800 mm. Microneedle fabrication Microneedle arrays were fabricated as described previously20,22. In brief, first pattern square or circular oxide patterns (800 mm in diameter) were made on a Si substrate, and then the untreated portion of Si wafers is etched away by an isotropic-reactive ion etching process using SF6/O2 plasma. The needle body was formed using potassium hydroxide etching solution. Figure 1 shows the microneedle array with 260 mm length. Center to center space is 800 mm, and tapering to a sharp tip with an angle of 38 . Microneedle arrays were treated with oxygen gas plasma for a short period of time (2 min) before coating in order to make the surface more hydrophilic and uniform23. Coating process Coating device and method A simple device was designed using SolidWorks 2007 and manufactured by Precision and Special Processing Center,
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Technical Institute of Physics and Chemistry, Chinese Academy of Science. It is consisted of two parts, stainless steel plate with 50 or 100 mm groove and hollow cuboid which was placed on the plate and generated a reservoir used to fill coating solution. Coating solution was poured into the reservoir, and then hollow cuboid was pulled manually from left to right to form a thin and uniform coating film in the groove. Finally, put the microneedle array into the thin coating film vertically by hand, staying for a second and withdrawing. The coating process kept a relative humidity about 40% under room temperature. Then the coated microneedles were allowed to dry at ambient temperature. All of the following experiments based on this operation. Screening of coating solution The screening experiments of coating solution were carried out by dipping 9 mm 5 mm chips of silicon into the solution of thickener for one second and air drying at room temperature overnight. A model drug [0.01% (w/v) calcein] was added into the thickener solution to determine the drug mass of the coatings. After drying, the chips were incubated in 1 ml deionized water for 2 h under vibration in the dark. The solutions were determined using fluorescence spectrophotometer (Hitachi F-2500L, Japan; excitation ¼ 494 nm, emission ¼ 515.5 nm). The thickeners include polyvinylpyrrolidone (PVP), sucrose, hydroxypropyl methyl cellulose (HPMC) and sodium carboxymethyl cellulose (CMC) and the concentration of the thickeners varied from 0.1% to 55% (w/v). The viscosity data of these solutions were tested by Brookfield viscometer (LVDV-II þ Pro). Coating parameters Coating parameters were investigated. The coating process was according to the operation as mentioned above. The coating parameters included (I) the concentration of a thickener in the coating solution, (II) the concentration of a model drug in the coating solution, (III) the number of dips during coating, (IV) the thickness of the coating film and (V) the reproducibility of the coating process. Fluorescein sodium (FS) was used as a model drug in the coating formulations. Each microneedle array has 144 needle/cm2. The following experiments were designed: For parameter (I), HPMC coating solutions with concentrations of 1, 2, 3, 4 and 5% containing 3% FS, respectively, 1 dips and 100 mm thickness of coating film. For parameter (II), FS as model drug at 1, 3, 5 and 10% concentrations in coating solutions of 3% HPMC, respectively, 5 dips at 3 s intervals between dips, and 100 mm thickness of coating film. For parameter (III), 3% FS in coating solutions of 3% HPMC, 1, 3, 5 or 8 dips at 3 s intervals between dips, respectively, and 100 mm thickness of coating film. For parameter (IV), 3% FS in coating solutions of 3% HPMC, 50 or 100 mm thickness of coating film. For parameter (V), the data was collected from the above experiments. All coated microneedle arrays were imaged by bright-field microscopy using an Olympus SZX12 stereo microscope (Olympus America) with a CCD camera (Olympus DP71, America). The mass of FS was determined by dissolving the coatings off the microneedle arrays immersed in DI water for 1 hour, and then measuring by fluorescence spectrophotometer (excitation ¼ 490 nm, emission ¼ 514 nm). Three replicates (n ¼ 3) were carried out in all experiments if no otherwise specified. Coating of model drugs
Figure 1. Scanning electron micrograph of solid silicon microneedles: the complete microneedle array contains 144 needles, each measuring 260 mm in length and tapering to a sharp tip with an angle of 38 .
Three model drugs have been coated onto the microneedle array to prove the coating method is universally applicable. The model
DOI: 10.3109/03639045.2013.873447
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drugs at 3% (w/w) concentration were dissolved or dispersed in 3% HPMC solution, respectively. The coating parameters: the microneedle array has 144 needles/cm2 with the length of 260 mm; the number of dips is 5 with 3 s interval; the depth of coating film is 100 mm. After the coatings dried, the microneedle arrays were taken photo using microscope with a CCD camera mentioned above and then dissolved into 1 ml DI water for 1 h under stirring. Finally, the solutions were determined by fluorescence spectrophotometer. In vitro delivery efficiency Microneedle array (n ¼ 5) coated with FS as model drug was applied on porcine cadaver skin by hand for 1, 3 or 5 min. Withdraw the microneedle array and apply tape onto the surface of the treated site three times to collect the drug scattering on the skin surface. In order to evaluate the delivery efficiency, the amount of residual drug was determined as follows: the inserted microneedle array and the tapes above were rinsed into PBS solution for 1 h, and then the solution was determined by fluorescence spectrophotometer. The porcine cadaver skin and microneedle arrays were viewed before and after treatment by microscope with a CCD camera. According to the residual amount, the delivery efficiency was calculated.
Results Coating method and device As shown in Figure 2, the coating device has two parts: plate (1) with groove (2) and hollow cuboid (3), all are made of stainless steel. The depth of the groove is 50 or 100 mm. And the width of groove and hollow cuboid is 12 mm which is wider than the microneedle array. Hollow cuboid was placed on the plate which has a groove and generated a reservoir (4) used to load Figure 2. The schematic diagrams of coating device, front view (A) and side view (B). Numbers stand for: 1 (Plate), 2 (Groove), 3 (Hollow cuboid), 4 (Reservoir), 5 (Coating film), 6 (Microneedle array). As showed in picture B, hollow cuboid (3) was placed on plate (1) with 50 or 100 mm groove (2) to generate a reservoir which was used to fill coating solution. Coating solution was poured into the reservoir (4), and then hollow cuboid (3) was pulled from left to right to form a thin and uniform coating film (5) on the groove (2). Finally, the microneedle array (6) was put into the thin coating film vertically, lingering for a second and withdrawing.
Coated microneedles for transdermal drug delivery
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coating solution. Coating solution was poured into the reservoir, and then hollow cuboid was pulled from left to right manually to form a thin and uniform coating film (5) on the groove bed. Finally, put the microneedle array (6) into the thin coating film vertically, stay for a second and withdrawing. After that, hollow cuboid was pulled from right to left to form a thin film on the right side for the next coating process. Coating solution To study the effect of thickeners on the drug mass of coating, four thickeners were selected. Table 1 shows the relationship between viscosity of thickeners and the drug mass of coating. For these four thickeners (PVP, Sucrose, HPMC and CMC), the viscosity data increased with the rising concentration of solutions. For example, the viscosity of HPMC went up from 28.8 to 14 507.0 CP when the concentration increased from 1% (w/v) to 5% (w/v), respectively. However, the drug mass of coating did not increase with the rising viscosity of the solution always. At relative low viscosity of the solution, the drug mass of coating went up with the viscosity, followed by reaching the highest point, and then it went down with the further rising viscosity. Take CMC as an example, the drug mass of coating grew from 0.48 to 1.43 mg with the rising viscosity from 214.4 to 7040.0 CP, and then got the highest point with drug mass of 1.81 mg, and it went down to 1.62 mg finally. The other data were all shown in Table 1. Coating parameters The effect of thickener concentration in the coating solution We quantified the contamination by measuring the length of the coating on the microneedle on the pictures. Figure 3 shows the coating results of HPMC as a thickener. The HPMC 1% solution caused contamination at the substrate of the microneedle array
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Table 1. Viscosity of thickener solutions and drug mass of coatings. Solution
Con.* (w/v) (%)
Viscosity (CP)
Drug mass** (mg)
SD
PVP 10 15 20 25 30
28.8 41.6 60.8 92.8 153.6
1.24 1.74 2.01 2.12 1.52
0.19 0.06 0.48 0.24 0.08
1 2 3 4 5
28.8 195.2 1771.0 4128.0 14507.0
0.57 0.94 1.45 1.96 1.90
0.12 0.23 0.16 0.17 0.17
30 35 40 45 50
19.2 22.4 28.8 38.4 48.0
0.13 0.17 0.21 0.17 0.16
0.01 0.01 0.07 0.02 0.03
1 2 3 4 5
214.4 736.0 7040.0 30933.0 311000.0
0.48 0.79 1.43 1.81 1.62
0.01 0.04 0.04 0.07 0.07
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HPMC
and no coatings on the tip of microneedle (Figure 3B); HPMC 2% did not cause a contamination with the coating length of 200 mm (Figure 3C); the following concentration 3, 4 and 5% of HPMC (Figure 3D–F) did not cause contamination with the coating length of 130, 100 and 100 mm, respectively. Concentration of a model drug in the coating solution Table 2 shows the relationship between the concentration of drug and the mass of coatings. When the concentration of FS is 1%, the mass is 3.08 0.44 mg. While the concentration is up to 3%, the mass jumps to 13.56 3.32 mg. As the concentration increases to 5% or 10%, the mass also edges up to 19.40 2.16 mg or 20.32 6.20 mg.
Sucrose
CMC
*Solutions of thickeners (PVP, Sucrose, HPMC and CMC) with various concentrations (con.) were prepared. **Solutions of certain concentration [from 0.1% to 55% (w/v)] of thickeners containing 0.01% (w/v) calcein as a model drug were prepared. About 9 mm 5 mm chips of silicon were dipped into these solutions and then dried under room temperature. After that the chips with coating were rinsed into deionized water and determined by fluorescence spectrophotometer. The drug mass of coatings were calculated.
Number of dips during coating In order to obtain a high drug mass of coatings, the repetition of dip is necessary. Under condition of 3% (w/v) HPMC thickener solution containing model drug 3% (w/v) FS, only 1.52 0.08 mg
Table 2. Effect of FS concentrations on coating mass. Group A B C D
Con. (w/v)* (%)
Mass of coating (mg, n ¼ 3)
SD
RSD
1 3 5 10
3.08 13.56 19.40 20.32
0.44 3.32 2.16 6.20
0.14 0.25 0.11 0.30
*Different concentrations (con.) of fluorescein sodium(FS) are used as coating solutions. With other coating parameters: dips: 5; thickener: 3% (w/v) HPMC; thickness of coating film: 100 mm; intervals between dips: 3 s.
Figure 3. Effect of different concentrations of HPMC on coating length. (A) Non-coated microneedle; (B) 1% concentration of HPMC resulted in contamination of substrate; (C–F) 2, 3, 4 and 5% of HPMC with coating length of 230, 130, 100 and 100 mm, respectively. With other coating parameters: thickness of coating film: 100 mm; Number of dips: 1; Model drugs: 3% (w/v) fluorescein sodium solution.
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was coated on the microneedles when dipped one time. The number of dips increased, the drug mass of coatings also grew. For example, from 1 dip to 3 dips, the drug mass went up about three times from 1.52 0.08 mg to 5.08 0.8 mg, while from 1 dip to 5 dips the mass only get more than 8 times to 12.76 2.48 mg. And when dips were up to 8, the mass went about 12 times to 19.00 3.04 mg (Table 3).
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Thickness of the coating film As seen in Figure 4, the coating drug length of the microneedle was at a range of approximately 130 mm in 100 mm film, while 75 mm coating length was obtained in 50 mm film. As a result, the drug mass of coatings using 100 mm film was 12.15 mg (n ¼ 3) and 4.08 mg for using 50 mm film.
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Reproducibility of the coating process The collected data shows that the average of the drug mass of coating is 12.54 2.40 mg (n ¼ 12). This result showed that the coating process could be repeated. Coating of model drugs To prove the coating method is widely applicable, RFSP, riboflavin and myoglobin as the small molecule model drugs of hydrophilic/hydrophobic drug and bio-macromolecules were coated on the microneedle array (Figure 5). The mass of coating of RFSP, Riboflavin and Myoglobin reached 7.56 0.80 mg, 21.48 2.28 mg and 20.08 2.80 mg, respectively (Table 4). It suggests that the coating method can be used for a variety of drugs. In vitro delivery efficiency
Table 3. Effect of dips on coating mass of FS on microneedle array. Group A B C D
Dips*
Mass of coating (mg, n ¼ 3)
SD
RSD
1 3 5 8
1.52 5.08 12.76 19.00
0.08 0.80 2.48 3.04
0.06 0.15 0.19 0.16
*Dips are the times that microneedles were dipped into coating solution. With other coating parameters: thickener: 3% (w/v) HPMC; Model drug: 3% (w/v) fluorescein sodium (FS) solution; thickness of coating film: 100 mm; intervals between dips: 3 s.
The microneedle arrays before and after insertion skin with different time are shown in Figure 6. To quantify the delivery efficiency of coated microneedles, microneedles coated with FS were inserted into porcine cadaver skin. After 1, 3 or 5-min insertion of the microneedles with 12.54 2.40 mg of FS into skin, 3.57, 10.13 or 11.75 mg of FS was delivered into the skin, the delivery efficiency were 28, 81 and 94, respectively (Table 5). This experiment proved that the coated microneedle array can be inserted into the skin and efficiently delivery the drug into the skin. The coated film does not affect the efficiency of insertion.
Discussion Silicon is a biocompatible material with mature silicon micromachining technologies21, and cheap for mass production. In addition, the silicon microneedle array has enough strength to penetrate the skin without broken. And, the microneedle array has good biological safety22. Therefore, we used silicon as a base material to fabricate microneedles. Table 4. Effect of different model drugs on coating mass. Group A B C Figure 4. Effect of thickness of the coating film on coating lengths on microneedle. With coating parameters: thickness of coating film: 100 mm (A) and 50 mm (B); Number of dips: 5; thickener: 3% (w/v) HPMC; Model drugs: 3% (w/v) drug solutions; intervals between dips: 3 s.
Model drugs
Mass of coating** (mg, n ¼ 5)
SD
RSD
RFSP* Riboflavin Myoglobin
7.56 21.48 20.08
0.80 2.28 2.80
0.11 0.11 0.14
*RFSP stands for riboflavin sodium phosphate. **Coating parameters: number of dips:5; thickener: 3%(w/v) HPMC; model drugs: 3% (w/v) drug solutions; thickness of coating film: 100 mm; intervals between dips: 3 s.
Figure 5. Microneedles coated with different model drugs: (A) RFSP; (B) Riboflavin; (C) Myoglobin. With coating parameters: Number of dips: 5; thickener: 3% (w/v) HPMC; Model drugs: 3% (w/v) drug solutions; thickness of coating film: 100 mm; intervals between dips: 3 s.
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Figure 6. Photographs of the coating microneedles with FS and its treating skin: (A), (C), (E) Coated microneedles before inserting into the skin; (B, D and F) Microneedles withdrew from the skin after 1, 3 and 5 min insertion, respectively; (G, H) Surface of porcine cadaver skin after insertion and removal of microneedle array. Each stained spot corresponds to the site of microneedles penetration into the skin.
Table 5. Effect of insertion time of microneedles on the delivery efficiency. Group A B C
Dissolving time (min)
RA* (mg, n ¼ 5)
RSD
DE** (%)
1 3 5
8.97 2.42 0.79
0.25 0.25 0.08
28 81 94
*RA: Residual amount, drugs remained on the inserted microneedle array and the tapes. **DE: Delivery efficiency, calculated by the equation below. MC stands for average mass of coating on microneedles. DE ¼ MCRA MC 100%
The most challenge of coating microneedles is the capillary effect which is more obvious in micro-scale24. Thus, the traditional approach dip coating is not suitable. Many strategies have been developed to improve the coating process with the aim to avoid the capillary effects (capillary effects among the microneedles on the array) which would result in contaminate the substrate of the microneedle array and induce drug waste and in-homogeneity. In order to tackle with this issue, we
developed a novel coating device (Figure 2). There are two important factors of technology which are vital for deal with the capillary effect. First, it is thickness of a thin film determined by the depth of the groove. The thickness of the thin film is 100 mm when the length of microneedle is 260 mm, which resulted in 130 mm coating length on microneedles, as shown in Figure 3(D). Second, it is the viscosity of the coating solution provided by thickener, and when the solution spreads on the groove bed, the groove bed provides an adsorption force which help balance the capillary force. We have tried using solution without thickener, which result in contamination of substrate (data no shown). More importantly, coating device was operated manually at room temperature, which did not need precise control. As shown in Table 1, there is no necessary relation between the drug mass of coating and the viscosity of solution. For example, PVP, sucrose and HPMC have the same viscosity of 28.8 CP, but they did not share with the same drug mass of coating. For sucrose, the drug mass of coating was 0.17 mg with two different viscosities (22.4 and 38.4 CP). Relatively, large amount of drug mass was obtained either at high PVP concentration or at low CMC and HPMC concentration. Under a certain drug mass of
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DOI: 10.3109/03639045.2013.873447
coating, the mass of thickener is inversely proportion to the mass of drug. Therefore, HPMC has a better performance on the drug mass of coating in the same concentration comparing with PVP and CMC. The coating parameters such as concentration of the drug and thickener, numbers of dips, depth of groove and reproducibility are playing important roles in the forming of coatings. The drug mass of coating increases with increasing the concentration of drug when other conditions are the same, as shown in Table 2, so we can control the dose of drug on microneedles by adjusting the concentration of drug. The concentration of HPMC is important for successful coating, for example, Figure 3(B) showed that 1% (w/v) HPMC resulted in contamination, while the concentration over 2% (w/v) realized successful coating. However, different concentration of HPMC resulted in different length of coatings shown in Figure 3(C–F). The depth of the groove is another factor influencing the length of coatings, which was demonstrated in Figure 4. The drug mass of coating grows with the increasing of the number of dips as shown in Table 3. Therefore, we can also control the dose of drug on microneedles by adjusting the number of dips. The reproducibility of the coating process is meaningful due to the need of accurate quantification for drug delivery. To demonstrate the coating method is a widely technology for coating, three types of drug were used as model drugs. RFSP, riboflavin and myoglobin are representative of hydrophilic drug, hydrophobic drug and bio-macromolecules, respectively. All the three drugs can be coated on the microneedles uniformly (Figure 5), which suggested that the coating method is widely applicability for most drugs. According to the results (Table 4), a 10 mm 10 mm microneedle array can be coated with 20.08 mg proteins, so the coated microneedle array is very suitable for highly active protein such as vaccines and interferons because of low dose (several to ten micrograms). Specifically, coated microneedles are attractive to vaccine delivery, because transdermal delivery can target dendritic cells residing in the epidermis for a more potent immune response25. In addition, in vitro experiments showed that the coatings released off within 5 min and the delivery efficiency was up to 94%. The data (Figure 6E,F) also showed that the coated microneedle can be inserted effectively into skin without broken and less drug loss. The coatings were disappeared after insertion when the length of the coatings is 130 mm, which suggested that the effective insertion depth is more than 130 mm when using microneedles with a 260 mm in length. Furthermore, in vitro experiments were carried out manually, so insertions by hand were proved to be a reliable mode for coated microneedle delivery. Therefore, it is a simple delivery method which can be self-administrate without any extra device and energy.
Conclusions In this study, we explored a novel approach of coating microneedle array for transdermal drug delivery. This approach aimed to solve the problems results from capillary effect which is more obvious in micro-scale and avoid the defects of existing methods. The technique used to coat microneedles is relatively simple and avoids contamination of substrate of microneedles. Moreover, the coating process did not need precise control and can carried out under room temperature. The principle of this technique is to produce a micro-scale film to reduce capillary effect, and all parameters related to coating were tested. According to the results, we make the dose of drug on microneedles controllable by adjusting the coating parameters. Next, three model drugs coated microneedle array successfully shows that the method is broad applicable. Finally, insertion
Coated microneedles for transdermal drug delivery
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experiments certified the coated microneedle array is useful for drug delivery. All the results demonstrated that the coating method is simple and controllable to coat microneedle array with chemical drugs and bio-macromolecules for successful transdermal drug delivery.
Declaration of interest This work was partially supported by the Science Foundation of the Chinese Academy of Sciences and the National Natural Science Foundation of China (Grant No. 31300763). The authors report no conflicts of interest. The authors alone are responsible for the content and writing of this article.
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