Bioelectrical signals are important physiological parameters that allow to analyze the state and information of the body. The most common method to monitor bioelectrical signals like electromyogram (EMG) is to attach surface electrodes (SE) on the skin. The microneedle electrode (ME) is an invasive dry electrode penetrating stratum corneum of the skin with a needle, so recording quality is improved compared with SE. In the current ME fabrication process, various methods such as etching [1,2,3,4,5,6,7], photolithography [7, 8], molding [9,10,11,12,13], magnetorheological drawing lithography [14, 15], thermal drawing [16], laser machining [17,18,19,20,21,22], electrical discharge machining [23, 24], and various other methods have been developed. These methods allow designing length or aspect ratio of microneedles freely, making it possible to create an optimal design. However, most fabrication processes require expensive facilities and multiple fabrications steps.
In addition, the material of the microneedle is also important. Currently, various types of materials are used for microneedles, such as rigid materials [17, 20], polymer [4, 12], and silicon [5, 6]. Skin penetration of microneedle requires stiff materials. However, there is a risk of microneedle breakage after implantation inside the skin that causes various side effects.
Accordingly, biocompatible and flexible materials such as polymer are preferred to fabricate microneedle. Polymer-based microneedles are mainly manufactured by molding fabrication. There are many ways to make a microneedle mold. Recently, it is possible to develop more sophisticated and complex microneedles using polymer materials at an inexpensive cost through the molding fabrication process using 3D printing [14]. Currently, various mechanisms have been developed for 3D printing, such as FDM (Fused Deposition Modeling), DLP (Digital Light Processing), SLA (Stereo Lithography Apparatus), etc. In this study, the SLA printing method was used to fabricate a 3D printed microneedle mold. SLA printing uses a laser to harden a photocurable resin. A laser is fired at the resin tank to harden the resin to form one layer, and the next layer is stacked on the layer. Currently, the SLA printing method is widely used because it is relatively inexpensive and allows for sophisticated printing. However, there is a limit to the resolution because the resolution of the output is determined by the size of the laser. Since the SLA printer operates with the same mechanism as in Fig. 1a, it is unable to express a layer smaller than the laser size. As a result of these mechanisms, the design and output results are different due to the limitation of the resolution of the 3D printer when printing delicate and small objects such as microneedles.
In this study, in order to reduce the effect of microneedles on the human body after insertion into the skin, the microneedle was fabricated using a polymer rather than a rigid object such as metal. However, polymer materials typically have low mechanical strength, causing break during the penetration. Therefore, the optimization of polymer microneedle in terms of shapes, length, and bevel angles is required to maximize the penetration capacity. In particular, the bevel of the microneedle tip reduces the insertion force required to penetrate the stratum corneum of the skin, thereby reducing pain and the possibility of breakage of the microneedle [7, 21, 22].
However, there are some limitations to use polymer microneedle as an electrode. The first is the durability of microneedles including during and after implantation. The second one is that it is difficult to control the shape of microneedles including needle bevel which is directly relevant to penetration capability, secure after implantation, and tissue damage. The current 3D printing fabrication requires a new method because it is difficult to satisfy these requirements due to the limitation of resolution. But despite several attempts, an effective process to control microneedle shape using soft and biocompatible materials has not been developed.
In this study, we propose a novel 3D printing fabrication process that enables to control microneedle bevel angles of polyimide (PI) microneedles. Polyimide (PI) has excellent biocompatibility [25] and excellent adhesion to metals. This allows to provide minimally invasive penetration on the skin thanks to relatively soft materials when it maximizes its penetration capability. To maximize the penetration of PI microneedles, microneedle bevel angles were changed by changing a printing angle of SLA printer from 0 to 90° as shown in Fig. 1b. Due to the limitations of the 3D print resolution, delicate microneedle tips are unable to fabricate using the normal method (α = 0°) while the printing angle changes to provide different bevel angles (Fig. 1c, d). Furthermore, aspect ratio and achievable height of the PI microneedle were investigated with various lengths (100 to 1000 μm). Finally, a penetration test of the fabricated PI microneedle via porcine skin was conducted to find the optimized condition.