Advanced Biomedical Engineering 5: 63–67, 2016.
Original Paper
DOI:10.14326/abe.5.63
Development of a Rapid Prototyping System for Microneedles Using Moving-mask Lithography with Backside Exposure Takahisa KAI,* Shunta MORI,* Nobuhiro KATO**, #
Abstract Moving-mask lithography with backside exposure was utilized to generate master male mold for biodegradable polymer microneedle production. The microneedle shape was calculated from the exposure dose, mask geometry, and moving trajectory using a newly developed computer simulation. Two conditions (90 µm aperture with 80 µm diameter of circular movement, and 90 µm aperture with 90 µm diameter of circular movement) were selected to evaluate the moving-mask exposure effectiveness. By changing the moving trajectory, two different sizes of microneedles were obtained from a single-size aperture mask. The fabricated microneedle and calculated microneedle geometry showed good qualitative agreement. The geometrical difference was 2% in basal diameter and 8%–16% in height. Using the master male mold, biodegradable polymer microneedles made of chondroitin sulfate C sodium salt (CSC) were fabricated by casting from a poly-dimethylsiloxane female mold. The shape of the biodegradable CSC microneedles showed good agreement with the master male mold. Keywords: moving-mask lithography, biodegradable microneedles, backside exposure. Adv Biomed Eng. 5: pp. 63–67, 2016.
1. Introduction Skin, which is the largest organ in the human body, is very important for protecting the body against unwanted external attacks. The stratum corneum, the outermost layer of the epidermis, is 15–20 µm thick and an indispensable barrier [1]. In conventional medicine, hypodermic needles pierce through the skin to inject drugs into the body. However, this method causes pain and require cold-chain delivery [2]. To overcome these disadvantages while preserving the advantages of hypodermic needles and conventional transdermal drug delivery systems, microneedle technology [3] is a promising alternative. Microneedle technology can be divided into several categories [4]: solid microneedles for skin pretreatment to increase skin permeability, microneedles coated with drugs, hollow microneedles for drug infusion into the skin, and drug-encapsulated polymer microneedles that degrade totally or partially in the skin. Among them, biodegradable microneedles that are produced by molding are the most inexpensive solution. Furthermore, they can deliver a high drug payload and leave no sharp waste materials after use [5]. Photolithographic and related methods are commonly used to fabricate the master male molds of biodegradable microneedles. These techniques include inclined lithography [6], backside lithography utilizing diffraction from a photomask pattern [7], backside lithography using a microlens [8], and lithography with a subsequent reactive This study was presented at the Symposium on Biomedical Engineering 2015, Okayama, September, 2015. Received on August 2, 2015; revised on November 12, 2015 and January 14, 2016; accepted on February 2, 2016. *
Graduate School of Biology-Oriented Science and Technology, Kinki University, Wakayama, Japan.
**
Faculty of Biology-Oriented Science and Technology, Kinki University, Wakayama, Japan.
#
930 Nishimitani, Kinokawa, Wakayama 649–6493, Japan. E-mail:
[email protected]
ion etching process [9]. These methods are often time-consuming and cost-ineffective, and have some limitations in needle geometry. Moving-mask lithography using thick positive-tone photoresist is employed to produce 3D micro structures [10]. However, the height of the structure is restricted up to 50 μm due to the resist thickness, which is too low for application to microneedles. To overcome these drawbacks, this study proposes moving-mask UV lithography using thick negative-tone photoresist up to 500 μm in thickness, with backside exposure to form the master male mold of a microneedle. In this method, the geometry of the needle is modified by the mask movement. Various shapes of microneedles can be obtained by this simple scheme. Using this system, we succeeded to generate the master male mold of a microneedle, and obtain the female replicate mold from the master male mold. Finally, we fabricated biodegradable microneedles by casting a chondroitin sulfate C sodium salt (CSC) solution onto the female mold.
2. Methods 2.1 Resist characterization In backside lithography, the thickness of the negative-tone photoresist after development depends on the exposure dose. The relation between exposure dose and resist thickness was examined by the test pattern exposure and measurement of the resist thickness with a scanning laser microscope (LEXT-OLS3000, Olympus Co., Ltd). An average thickness of 10 points for each exposure dose was utilized to characterize the relation between exposure dose and resist thickness. The dose-thickness relation was fitted to a logarithmic function. 2.2 Simulation The shape of the microneedle after fabrication was predicted by a computer simulation. The overall exposure dose distribution D(x, y) was obtained by D(x, y) = I0 (x, y) ∗ M(x, y). (1) where I0(x, y), M(x, y), and ∗ represent the light intensity obtained
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Fig. 2 Processing steps for fabricating biodegradable microneedles: (a) moving-mask exposure, (b) master male mold of microneedle, (c) PDMS molding, (d) female mold of microneedle, (e) pouring CSC solution in female mold, (f) CSC microneedle. Fig. 1 Flowchart of the simulation.
through the photomask, the spatial distribution of the staying time of the photomask at an arbitrary position, and the convolution, respectively. The shape of the photoresist (SU-8; MicroChem) was calculated by substituting the overall exposure dose distribution into the relation between exposure dose and resist thickness obtained experimentally: H(x, y) = f (D(x, y)). (2) where H(x, y) and f( ) represent the resist thickness and the experimentally derived function mapping D(x, y) to H(x, y), respectively. The simulation was coded in the macro language of image processing software (ImageJ 1.84q; NIH) [11] based on the calculation procedure shown in Fig. 1. As the first step in this procedure, the following calculation conditions are set: light intensity, exposure time, mask opening diameter, amplitude of mask movement, and driving frequency. Second, 16 bit grayscale images are generated. Third, images are drawn. The images are light patterns modulated by the photomask of each position determined by time. Fourth, the summation of all images is calculated. Finally, the overall dose intensity distribution is converted to resist thickness using the experimentally derived relation between exposure dose and resist thickness. 2.3 Fabrication of master male mold Master male molds of microneedles were fabricated by backside exposure using a previously reported moving-mask lithography system [12]. A custom-made soda lime glass wafer (50 mm in diameter, 170 μm in thickness; Matsunami Glass Ind., Ltd.) was spin-coated with a negative-tone photoresist (SU-8 3050; MicroChem) using a spin-coater (1H-D7; Mikasa Co., Ltd). The main spin speed and spin time used to produce a 250-μm thick coating were 1000 rpm and 5 s, respectively. This coating process was repeated twice, and finally a 500-μm thick resist layer was obtained. After relaxation for 1 h at room temperature, the sample was heated at 95 C for 40 min on a hotplate to evaporate the solvent. The coated glass wafer was set upside down on the substrate stage, which was covered with a poly-dimethylsiloxane (PDMS) cushion. The substrate stage was then moved in a circular motion (80 μm or 90 μm in diameter, 2 Hz). The mask holder was lowered to achieve tight contact between the photomask and the glass
substrate. The tight contact was confirmed by the stage trajectory monitor. The mask holder was then raised 10 μm to provide a proximity gap. A UV light source (EXECURE 4000; Hoya Candeo) was used to irradiate the photomask at the center of the pattern for 15 s with an exposure energy of 150 mJ/cm2. A post-exposure bake was conducted at 95 C for 40 min, and the sample was allowed to cool down to room temperature for 2 h. The polymerized resist was developed (SU-8 developer; Microchem) for 30 min and then rinsed with isopropyl alcohol. Finally, arrays of tapered cone shaped microneedle master male molds were obtained. 2.4 Casting of biodegradable microneedle A double casting process was used to mold the biodegradable microneedle (Fig. 2). The photoresist microneedle obtained above was used as the master male mold to fabricate the female mold made of PDMS (Sylgard 184; Dow Corning). The volume ratio of the PDMS base and the curing agent was 10:1. The PDMS mixture was poured onto the master male mold and cured at 60 C for 3 h. The cured female mold was peeled off from the master male mold. Chondroitin sulfate C sodium salt (CSC) (032-08802; Wako) and DI water were mixed in a mass ratio of 2:1 and poured onto the PDMS female mold. To fill the cavities of the PDMS mold, the CSC solution was placed in vacuum at −100 kPa for 5 min at room temperature. Then, the CSC solution was covered with an acetylcellulose membrane filter as the backup material of the CSC microneedle and cured at 60 C for 3 h. After solidification, the CSC microneedle was obtained. The PDMS mold can be reused to make additional microneedles.
3. Results 3.1 Resist Characterization The thicknesses of 10 circular resist patterns 250 μm in diameter were measured at 11 exposure doses (Fig. 3). From the plot, the relation between UV exposure dose and resist thickness was fit by a logarithmic function, h [µm] = 236.09 ln(d) − 641.39. (3) where h [μm] and d [mJ/cm2] represent resist thickness and exposure dose, respectively. 3.2 Simulation results The 3D profiles of two types of microneedles in the simulation
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Takahisa KAI, et al: Microneedle Prototyping System Using Lithography
Fig. 5 Electromicrographs of resist microneedles fabricated with 90µm mask and (a) 80-µm diameter circular motion (Type A) and (b) 90-µm diameter motion (Type B).
Table 2 Geometry of fabricated resist microneedles (mean ± SD; µm). Fig. 3 Relation between UV exposure dose and measured height of photoresist (SU-8). Error bars represent standard deviation of the mean (n = 10).
type
height
basal diameter
tip radius
A
446.1 ± 4.9
156.3 ± 1.8
10.4 ± 0.9
B
388.0 ± 2.9
167.5 ± 2.1
13.6 ± 1.9
Fig. 6 Optical image of cross section of the PDMS female mold. Fig. 4 Simulated shapes of microneedles calculated with 90-µm mask and (a) 80-µm diameter circular motion (Type A) and (b) 90-µm diameter motion (Type B).
Table 1 Geometry of simulated microneedles (µm). type
height
basal diameter
A
518.6
159.3
B
421.1
170.0
tip radius
Fig. 7 Optical image of CSC microneedles.
are shown in Fig. 4. Type A (90 µm mask with a movement diameter of 80 µm) showed a symmetrical bullet-like shape with a long dull tip. Type B (90 µm mask with a movement diameter of 90 µm) showed a symmetrical bullet-like shape with a long sharp tip. The geometric features of both microneedles are shown in Table 1. 3.3 Microneedle master male mold fabricated by moving-mask lithography The master male molds obtained by moving-mask lithography are shown in Fig. 5. Each microneedle array was composed of 25 × 25 cone-shaped microneedles. The geometry of the master male molds was examined by scanning electron microscopy (SEM). The geometric features of both types of microneedles are shown in Table 2. Six needles for each type of microneedle were analyzed.
Table 3 Geometry of fabricated chondroitin sulfate C sodium salt (CSC) microneedles (mean ± SD; µm). type
height
basal diameter
tip radius
A
424.3 ± 4.0
145.6 ± 1.3
9.6 ± 0.5
3.4 Biodegradable microneedles Biodegradable microneedles were successfully obtained by casting the CSC solution using the PDMS female mold (Fig. 6). As verified in Fig. 7, the shape of the master male mold was transferred to the CSC microneedles with a high level of precision. The geometry of the CSC microneedles is shown in Table 3. Six needles were analyzed. To mimic a pharmacological agent in these biodegradable polymer microneedles, red food dye was mixed with the CSC solution. The red color was distributed
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uniformly in the microneedles. Therefore uniform distribution of medicine can be expected in the future.
4. Discussion 4.1 Difference between simulation and fabrication results The shapes of the microneedles obtained by simulation and those obtained by fabrication showed good qualitative agreement. As shown in Table 1 and Table 2, the basal diameters of the microneedles were almost the same (the difference was within 2%). However, the height of the fabricated microneedles was 6%–18% shorter than that of the simulated microneedles. These differences are partially due to the simulation assumptions, which ignore diffraction by the mask pattern and the collimation angle of UV illumination. Because the tips of the microneedles were not fully cross-linked and therefore fragile, the fabrication process affected these delicate tips in submicrometer order, and blunted the sharp apexes. 4.2 Shape modification by altering mask movement Altering the diameter of the circular trajectory of the stage motion produces an arbitrary UV exposure dose distribution, yielding microneedles with various tapered cone shapes from a single photomask pattern. Comparing the two types of fabricated microneedles, the basal diameter of Type A was larger than that of Type B. On the other hand, the height of Type A was shorter than that of Type B. Other trajectories produced completely different shapes of microneedles (not shown). 4.3 Biodegradable microneedles The shape of the biodegradable CSC microneedles was in good agreement with the master male mold. However, the CSC microneedles shrank 5.1%–7.6% compared with the master male mold. This shrinkage can be explained by two phenomena: PDMS shrinks approximately 5% after curing, and the CSC slurry shrinks during the heat solidification process. The master male mold should be designed to compensate for this inevitable shrinkage. The ability of the microneedles to penetrate human skin should also be considered. Even after shrinkage, the tip radius of the CSC microneedles were less than 30 µm and the needles were sharp enough to insert into skin [13]. However, the ability to penetrate skin is not determined only by the tip radius. A mechanical penetration test should be conducted. Drug payload uniformity was confirmed in Fig. 7. At present, the microneedles and the base of the array are made of the same material. To improve the efficiency of drug injection, the drug should be concentrated in the microneedles only. Using an automated dispensing machine would also improve the precision of filling the female mold. We are currently improving this system to avoid wasting drugs.
Acknowledgement This study was supported in part by Project Research of the Faculty of Biology-Oriented Science and Technology, Kinki University No. 12-IV-16. Conflict of interest We have no conflict of interest relationship with any companies or commercial organizations based on the definition of the Japanese Society for Medical and Biological Engineering. References 1.
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5. Conclusion A computer simulation was developed to predict the shape of microneedles produced by moving-mask lithography. A master male mold of microneedle was made using moving-mask lithography and the shape of the needle showed good qualitative agreement with the simulation results. The master male microneedle mold was successfully transferred to a biodegradable microneedle made of CSC by casting in a PDMS female mold.
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Takahisa KAI, et al: Microneedle Prototyping System Using Lithography Takahisa KAI
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Nobuhiro KATO
Takahisa KAI received his BSc degree from Kinki
Nobuhiro KATO received his BSc, MSc, and PhD
University, Japan, in 2014. He is currently a Mas-
degrees from Osaka Prefecture University Japan in
ter s course student of Graduate School of Biolo-
1992, 1994 and 1997. In 1997, he joined Kinki
gy-Oriented Science and Technology, Kinki University. His research interest include micro scale medical device.
University, where he is currently an Associate Professor of Faculty of Biology-Oriented Science and Technology. His research interests include micro fabrication, micro fluidics and micro scale medical device. He is a member of JSMBE, JSME, JSPE, JSAP, and CHEMINAS.
Shunta MORI Shunta MORI received his BSc degree from Kinki University, Japan, in 2015. He is currently a Master s course student of Graduate School of Biology-Oriented Science and Technology, Kinki University. His research interest include development of microneedles fabrication process and their application.
