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/lp/springer-journals/3d-printing-fabrication-process-for-fine-control-of-microneedle-shape-nysUIlSF2G
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- Springer Journals
- Copyright
- Copyright © The Author(s) 2022
- eISSN
- 2213-9621
- DOI
- 10.1186/s40486-022-00165-4
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Abstract
Microneedle electrode (ME) is used to monitor bioelectrical signals by penetrating via the skin, and it compensates for a limitation of surface electrodes. However, existing fabrication of ME have limited in controlling the shape of microneedles, which is directly relevant to the performance and durability of microneedles as an electrode. In this study, a novel method using 3D printing is developed to control needle bevel angles. By controlling the angle of printing direction, needle bevel angles are changed. Various angles of printing direction (0–90°) are investigated to fabricate moldings, and those moldings are used for microneedle fabrications using biocompatible polyimide (PI). The height implementation rate and aspect ratio are also investigated to optimize PI microneedles. The penetration test of the fabricated microneedles is conducted in porcine skin. The PI microneedle of 1000 μm fabricated by the printing angle of 40° showed the bevel angle of 54.5°, which can penetrate the porcine skin. The result demonstrates that this suggested fabrication can be applied using various polymeric materials to optimize microneedle shape. Keywords: Microneedle electrode, 3D printing, Polyimide microneedle, Needle-shape control to create an optimal design. However, most fabrication Introduction processes require expensive facilities and multiple fabri- Bioelectrical signals are important physiological param- cations steps. eters that allow to analyze the state and information of In addition, the material of the microneedle is also the body. The most common method to monitor bio - important. Currently, various types of materials are electrical signals like electromyogram (EMG) is to attach used for microneedles, such as rigid materials [17, 20], surface electrodes (SE) on the skin. The microneedle polymer [4, 12], and silicon [5, 6]. Skin penetration of electrode (ME) is an invasive dry electrode penetrating microneedle requires stiff materials. However, there is a stratum corneum of the skin with a needle, so record- risk of microneedle breakage after implantation inside ing quality is improved compared with SE. In the current the skin that causes various side effects. ME fabrication process, various methods such as etching Accordingly, biocompatible and flexible materials [1–7], photolithography [7, 8], molding [9–13], magne- such as polymer are preferred to fabricate microneedle. torheological drawing lithography [14, 15], thermal draw- Polymer-based microneedles are mainly manufactured ing [16], laser machining [17–22], electrical discharge by molding fabrication. There are many ways to make machining [23, 24], and various other methods have a microneedle mold. Recently, it is possible to develop been developed. These methods allow designing length more sophisticated and complex microneedles using or aspect ratio of microneedles freely, making it possible polymer materials at an inexpensive cost through the molding fabrication process using 3D printing [14]. Cur- rently, various mechanisms have been developed for 3D *Correspondence: hoonw@dgist.ac.kr printing, such as FDM (Fused Deposition Modeling), DLP (Digital Light Processing), SLA (Stereo Lithog- Department of Robotics and Mechatronics Engineering, Daegu Gyeongbuk Institute of Science and Technology (DGIST ), 333, Techno Jungang-Daero, raphy Apparatus), etc. In this study, the SLA printing Hyeonpung-Eup, Dalseong-Gun, Daegu 42988, Republic of Korea © The Author(s) 2022. Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http:// creat iveco mmons. org/ licen ses/ by/4. 0/. Jeong et al. Micro and Nano Systems Letters (2023) 11:1 Page 2 of 9 method was used to fabricate a 3D printed microneedle of these mechanisms, the design and output results are mold. SLA printing uses a laser to harden a photocur- different due to the limitation of the resolution of the 3D able resin. A laser is fired at the resin tank to harden the printer when printing delicate and small objects such as resin to form one layer, and the next layer is stacked on microneedles. the layer. Currently, the SLA printing method is widely In this study, in order to reduce the effect of micronee - used because it is relatively inexpensive and allows for dles on the human body after insertion into the skin, the sophisticated printing. However, there is a limit to the microneedle was fabricated using a polymer rather than resolution because the resolution of the output is deter- a rigid object such as metal. However, polymer mate- mined by the size of the laser. Since the SLA printer oper- rials typically have low mechanical strength, causing ates with the same mechanism as in Fig. 1a, it is unable break during the penetration. Therefore, the optimiza - to express a layer smaller than the laser size. As a result tion of polymer microneedle in terms of shapes, length, Fig. 1 a The schematic diagram of SLA 3D printing. b The schematic diagram of 3D printing angle setting. c–d The schematic diagrams of original design and printed microneedles according to printing angles. e An image of 3D printing setting at the angle of 45°. f Printed molds (0–30°) Jeong et al. Micro and Nano Systems Letters (2023) 11:1 Page 3 of 9 and bevel angles is required to maximize the penetration Curing was performed by exposing a UV light (405 nm) capacity. In particular, the bevel of the microneedle tip to PI, and hard baking (200 °C for 1 h) was performed. reduces the insertion force required to penetrate the stra- Finally, the microneedle was removed from the PDMS tum corneum of the skin, thereby reducing pain and the mold (Fig. 2a). Figure 2b shows the picture of the fabri- possibility of breakage of the microneedle [7, 21, 22]. cated PI microneedle. However, there are some limitations to use polymer microneedle as an electrode. The first is the durability of Microneedle characteristics microneedles including during and after implantation. The fabricated microneedle was investigated using SEM The second one is that it is difficult to control the shape to analyze the shape, length, and bevel angle of the fabri- of microneedles including needle bevel which is directly cated microneedle. Furthermore, the insertion test of the relevant to penetration capability, secure after implan- fabricated PI microneedle was conducted in porcine skin tation, and tissue damage. The current 3D printing fab - that has similar characteristics to human skin [26]. The rication requires a new method because it is difficult to force required to penetrate the porcine skin at a speed of satisfy these requirements due to the limitation of resolu- 0.1 mm/sec was obtained. Insertion test was conducted tion. But despite several attempts, an effective process to by MultiTest 2.5-i (Mecmesin, Slinfold, UK) (Fig. 2c). control microneedle shape using soft and biocompatible materials has not been developed. Results and discussion In this study, we propose a novel 3D printing fabrica- 3D Printed Polyimide Microneedle tion process that enables to control microneedle bevel Figure 3a shows the shape of microneedles that were angles of polyimide (PI) microneedles. Polyimide (PI) designed using the AutoCAD 2020. The printed 3D has excellent biocompatibility [25] and excellent adhe- microneedles depending on the printing angles were sion to metals. This allows to provide minimally invasive shown in Fig. 3b. The SLA printing is a method of cur - penetration on the skin thanks to relatively soft materials ing photocurable resin by a laser, so the width of the when it maximizes its penetration capability. To maxi- stacked layers varies depending on the printing angles mize the penetration of PI microneedles, microneedle (Fig. 1c). Therefore, the smaller size of sharp tips less bevel angles were changed by changing a printing angle than the size of laser cannot be printed. As expected, the of SLA printer from 0 to 90° as shown in Fig. 1b. Due flat tip was formed at the angle (α) of 0° which is a nor - to the limitations of the 3D print resolution, delicate mal printing angle. And slightly tilted bevel was formed microneedle tips are unable to fabricate using the nor- when the angle increased. As shown in Fig. 3b, the needle mal method (α = 0°) while the printing angle changes to bevel angle was formed from the angles were between 10 provide different bevel angles (Fig. 1c, d). Furthermore, and 60°, while the angles were slightly collapsed after the aspect ratio and achievable height of the PI microneedle higher angles more than 70° due to the gravity. Above the were investigated with various lengths (100 to 1000 μm). printing angle of 70°, the needle bent toward the floor. Finally, a penetration test of the fabricated PI micronee- This result indicates that a certain level of printing angles dle via porcine skin was conducted to find the optimized allows controlling microneedle bevel angles. condition. With the molding process, PI microneedles were fab- ricated using the printed microneedles. As shown in Materials and methods Fig. 3c, the needle bevel angles of the fabricated PI 3D printing microelectrode fabrication microneedle were changed depending on the different A corn shape of microneedles was designed using Auto- conditions. The similar result from the printed micronee - CAD 2020, then was printed by using a 3D Form 3 dle was observed in that the printing angle (α) from 20 printer (FormLabs, Somerville, Massachusetts) with to 50° formed needle bevel angles at the PI microneedle different printing angles (α) of from 0 to 90° (10° step). tips. The printing angle of 10° shows a very small angle, The height of microneedles was also changed from 100 and angles above 50° appear uncontrollable. to 1000 µm to investigate height implementation rate To analyze printing controllability using this fabrica- from design to fabricated output as well as aspect ratio tion method, the bevel angle of the 3D printed micronee- of the fabricated microneedles. For the first molding dle was defined as β, and the bevel angle of the fabricated process, the 3D printed mold was design to form PDMS PI microneedle was defined as γ as shown in Fig. 4a. mold. PDMS was then poured into the printed mold, and Figure 4b shows the bevel angle result of the printed 3D formed the final PDMS mold. After that, PI was poured microneedle and the fabricated PI microneedle depend- into the PDMS mold, and then placed in a vacuum cham- ing on the printing angles. Theoretically, β has a similar ber -1 atm for 10 min to remove bubbles as well as to fill value calculated as ‘90°-α’, but above the certain angle, it the PI into the narrow vacancy of needle pattern at mold. varies since it is affected by the gravity. Overall results Jeong et al. Micro and Nano Systems Letters (2023) 11:1 Page 4 of 9 Fig. 2 a Fabrication process of polyimide microneedle. b The fabricated PI microneedles. c The schematic diagram of insertion test Fig. 3 a AutoCAD design. b Pictures of 3D printed microneedles at the angle setting from 0 to 90°. c Pictures of polyimide (PI) microneedles at the angle setting from 0 to 90° of the bevel angles (β) were quite matched well to the slightly larger than the 3D print microneedle bevel angle expected values within the ranges of the angles from 0 to (β). The sharpest bevel angle was 54.5° that was fabricated 40°, and the printed PI microneedle bevel angles (γ) were by the angle (α) at 40°. However, after the angle (α) of 40°, Jeong et al. Micro and Nano Systems Letters (2023) 11:1 Page 5 of 9 Fig. 4 a The schematic diagram of 3D printed microneedle and PI microneedle. b The bevel angle of the microneedle according to the print angle. (α: printing angle, β: 3D printed microneedle bevel angle, γ: PI microneedle bevel angle) (n = 10) the bevel angle tends to increase again, and was uncon- process using this angle-controlled microneedle ena- trollable above the angle of 60°. When α is 90°, the bevel bles to control the bevel angle (γ) of the fabricated PI angle decreases again, which seems to be a phenomenon microneedles within the certain ranges (54.5–88.6°). The caused the distorted shape of the printed microneedle tip sharpest angle (γ) of 50° was achieved using this process. as shown in Fig. 3b. The detailed data is summarized in Table 1. Height dependent aspect ratio The results indicate that the change of printing angle For microneedles, aspect ratio and height are also (α) allows controlling the bevel angles (β) within the cer- important parameters. To investigate the aspect ratio tain ranges (50.8–89.5°) as well as the followed molding according to the print height, the microneedle with Table 1 Fabrication of microneedle Method Materials Advantages Limits Refs. Etching Silicon, Polymer High aspect ratio, Suitable for mass Expensive, Complex fabrication pro- [1–7] production cess, Low durability Photolithography Polymer High aspect ratio, Suitable for mass Expensive, Complex fabrication pro- [7, 8] production cess, Low durability Molding Metal, Polymer Suitable for mass production, High Complex fabrication process, Low [9–13] reproducibility, Various microneedle durability forms can be implemented Magnetorheological drawing lithog- Polymer High aspect ratio, Simple fabrication Rough surface, Low durability [14, 15] raphy process, Suitable for mass production Thermal drawing Polymer High aspect ratio, Simple fabrication Low reproducibility, Low durability [16] process, Suitable for mass production Laser machining Metal Simple fabrication process Blunt microneedle tip, Rough surface, [17–22] Low biocompatibility Electrical discharge machining Metal High aspect ratio, High reproducibility Expensive, Complex fabrication [23, 24] process 3D print double molding Polymer Suitable for mass production, High Resolution limit, Low durability, Rough This paper reproducibility, Various microneedle surface forms can be implemented, Bevel angle control, Simple fabrication process Jeong et al. Micro and Nano Systems Letters (2023) 11:1 Page 6 of 9 aspect ratio of 4 was designed with the height change microneedles longer than the height of 600 μm can be from 100 to 1000 μm. And the designed micronee- used (Fig. 6). dle was fabricated. The microneedles with a height of Height implementation rate of designed and fabricated 100 μm were not fabricated properly due to very short height was measured and calculated. As shown in Fig. 5b, of the height. Figure 5a shows the aspect ratio results the print height affects the height implementation rate of the fabricated PI microneedle (with all α values) of the PI microneedle. When the input print height was according to the printing height. The aspect ratio was 200 μm, the height implementation rate of the fabricated 0.78 (n = 10) when the height was 200 μm that shows PI microneedle was only 71.6%, but this rate increased quite poor aspect ratio. As the height was increased, as the print height increased. The height implementa - the aspect ratio was also increased, and showed the tion rate showed 90.9% when the input print height was highest value of 2.58 (n = 10) when the height was 800 μm, and it was reached the maximum value of 92.6% 1000 μm. The aspect ratio of commonly used micronee - when the input print height was 900 μm. When the print dles is 2:1 or higher [27, 28], therefore, the fabricated PI height was 1000 μm, it slightly decreased to 91.6%, but Fig. 5 a The result of aspect ratio of PI microneedles according to the print height (n = 10). b The result of height implementation rate of the PI microneedle according to the print height (n = 10). c The result of height implementation rate of the PI microneedle (all heights) according to the print angle (n = 10). d The result of height implementation rate of the PI microneedle (1000 μm) according to the print angle (n = 10) Jeong et al. Micro and Nano Systems Letters (2023) 11:1 Page 7 of 9 Fig. 6 a The picture of penetration test. b The results of penetration test in porcine skins the decrease can be negligible. In addition, it is shown 20°, the height implementation rate of the fabricated PI that the error bar (standard error of mean) in Fig. 5b microneedle was 96.8%, which is the maximum value. decreases as the print height increases, which indicate When α was 70°, the minimum value of 86.4% was that the longer PI microneedle can be fabricated uni- obtained. When α was 40°, the height implementation formly. The detailed data is summarized in Table 2. rate was 92.1%. This value is quite good and indicates that Figure 5c shows the height implementation rate of the the PI microneedle fabricated with the print angle (α) of fabricated PI microneedle (all heights) according to the 40° and the print height of 1000 μm is recommended. Fig- print angle (α). Overall, as the print angle increased, the ure 5c, d, the error bar of the graph decreased slightly as height implementation rate decreased. When α was 0°, the print angle (α) increases till α was 30°, but it increased the height implementation rate was 93.2%, which is the after that, which indicate that as the print angle increases maximum value, while when α was 90°, it decreased to more than 30°, the implementation of the bevel part of 76.5%. When α was 40°, where showed the sharpest bevel the microneedle becomes non-uniform. angle of the PI microneedle, the height implementation rate was 87.0%. Penetration of porcine skin Figure 5d shows the height implementation rate of Penetration test through porcine skin was conducted using the fabricated PI microneedle (with the print height of the fabricated PI microneedle of 1000 μm height depend- 1000 μm) according to the print angle (α). When α was ing on the printing angle (α) of 0°, 40°, and 90°. As shown Jeong et al. Micro and Nano Systems Letters (2023) 11:1 Page 8 of 9 Table 2 The results of 3D printed microneedle bevel angle (β) and PI microneedle bevel angles (γ) depending on printing angle (α) Print angle (α) 3d print microneedle tip angle Stand Error of Mean (β) PI microneedle tip angle (γ) Stand Error (β) of Mean (γ) 0 89.510 0.587 88.604 0.219 10 80.304 0.902 81.452 0.414 20 67.624 1.216 74.220 1.061 30 59.802 0.681 65.628 1.352 40 50.804 0.542 54.522 1.696 50 63.070 0.505 61.456 2.285 60 71.796 2.610 71.790 0.809 70 74.620 0.789 75.817 0.352 80 79.380 2.062 79.879 0.329 90 70.318 1.580 66.182 1.874 in Fig. 6, PI microneedle fabricated by the printing angle and reproduction in any medium or format, as long (α) of 0° which is the normal printing condition shows sta- as you give appropriate credit to the original author(s) ble pushing performance, but unable to penetrate the skin. and the source, provide a link to the Creative Commons PI microneedle fabricated by the printing angle (α) of 90° licence, and indicate if changes were made. The images shows poor performance and break quickly due to the or other third party material in this article are included unstable shape. The penetration was only observed with the in the article’s Creative Commons licence, unless indi- PI microneedle fabricated by the printing angle (α) of 40°. cated otherwise in a credit line to the material. If mate- This result demonstrates that PI microneedle of 1000 μm rial is not included in the article’s Creative Commons height (aspect ratio: 2.58) fabricated by the printing angle licence and your intended use is not permitted by statu- (α) of 40° has the maximized penetration capability. tory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright Conclusion holder. To view a copy of this licence, visit http:// creat We suggested a novel fabrication process to control iveco mmons. org/ licen ses/ by/4. 0/. microneedle bevel angles using polymeric materials. To Author contributions validate this process, biocompatible polyimide (PI) was JJW designed and conducted the overall experiment. He also analyzed the used as microneedle materials, and the PI microneedle data. PJW assisted in the PI microneedle fabrication process. LSH supervised this research, evaluated and edited the manuscript. All authors read and was fabricated and optimized to maximize its penetration approved the final manuscript. capability. By changing the printing angle (α) of current 3D printer that have resolution limitation, the micronee- Funding This research was supported by a Korea Medical Device Development Fund dle bevel angle could be changed which is critical for pen- grant funded by the Korea government (the Ministry of Science and ICT, etration function of microneedles. The microneedle bevel the Ministry of Trade, Industry and Energy, the Ministry of Health & Wel- angles, aspect ratio, and height implementation ratio of fare, the Ministry of Food and Drug Safety) (Project Number: 1711135031, KMDF_PR_20200901_0158-05). 3D printed microneedles and PI microneedles fabricated by the molding process were compared to optimize the PI Availability of data and materials microneedle. Finally, penetration test of porcine skin was All data generated or analyzed during this study are included in this published article. demonstrated to evaluate the penetration capability. The PI microneedle of 1000 μm (the aspect ratio: 2.58) fabricated Declarations by the printing angle (α) of 40° (γ: 54.52) has the maximized penetration capability. We expect that this fabrication pro- Ethics approval and consent to participate cess can also apply to other polymeric materials allowing The authors declare that they have no competing interests. the control of microneedle shape and bevel angle. Consent for publication Authors consent the SpringerOpen license agreement to publish the article. Rights and permissions Competing interests Open Access This article is licensed under a Crea - The authors declare that they have no competing interests. tive Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution Received: 8 September 2022 Accepted: 7 December 2022 Jeong et al. Micro and Nano Systems Letters (2023) 11:1 Page 9 of 9 References 23. Fofonoff T, Wiseman C, Dyer R, Malasek J, Burgert J, Martel S, Hunter I, 1. 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Journal
Micro and Nano Systems Letters – Springer Journals
Published: Jan 2, 2023
Keywords: Microneedle electrode; 3D printing; Polyimide microneedle; Needle-shape control