# Finger-Actuated Microneedle Array for Sampling Body Fluids
## 2 Methods  
### Design of the Device  
The microneedle array (MNA) with a microfluidic chip was designed to be powered by pressing with finger (Figure 1a). The device was designed with Dassault Systemes (3 DS) SolidWorks computer-aided design (CAD) and computer-aided engineering (CAE). The microfluidic chip was designed as a box with the dimensions of length (L = 40 mm), width (W = 20 mm), and height (H = 9.3 mm) (the final height of the device with the MNA integrated is 10 mm). The design of the microfluidic system (Figure 1b) included a deformable dome-shaped chamber for mechanical pressure input by pressing with finger to provide a force to guide the fluid from the MNA inlet to the reservoir outlet. The MNA was connected to a duct that guided the extracted fluid to the reservoir via the channels with a diameter of 2 mm. The designed MNs and their geometry are shown in Figure 1c. For designing the MNs, experimentally reported MNs with applications in sampling or extraction of body fluids such as ISF, blood, and glucose detection were reviewed, and the geometrical properties of the MNs, which included the height of the needle, its base diameter, the needle tip diameter, and the distance between the MNs in the case of MNAs, along with the efficiency of the reported investigation, are shown in Table 1. To choose a convenient design for the MNs, we used the reported data to create chart boxes of the data (Table 2). The table shows the box chart for the reported MN height in micrometers with most of the reported heights of MN to be in the range of 400 to 800 \(\upmu\)m. Commercial MN vaccine delivery systems mostly have a height of 600 \(\upmu\)m [32]. As a result, it is appropriate to design a MN with a height in this domain. The MNs in this work have a needle height of H\({}_{\text{MN}}\) = 700 \(\upmu\)m. According to the data of the base diameter of the previously reported MNs (Table 1), the diameter of the needle base was designed as B = 300 \(\upmu\)m. The diameter of the needle tip was designed to be D = 80 \(\upmu\)m. The box charts for the number of the MNs in an array and the interspacing between the MNs are demonstrated in Table MNA in the present work was designed in an area of 6 \(\times\) 6 mm, consisting of 100 microneedles (MNs) with interspacing of 600 \(\upmu\)m MNs' center-to-center distance. On the other hand, the required force for penetration of the microneedles into the skin can be easily provided by the user. Precise measurements of the required insertion force for MNs were previously reported, showing that an amount in the range of 0.03 to 0.4 N per MN is sufficient for piercing the stratum corneum (SC) and insertion into the skin [16; 33; 34], which is low enough to allow insertion by hand [34]. In addition, according to the experimental literature presented in the Table 2, MNs with similar design to that of the present work adequately penetrated human skin, demonstrating potency of proposed MNs for penetration. Figure 1: Concept of finger-powered microneedle array (MNA) integrated with a microfluidic chip for the aim of extracting body fluid samples. (**a**) The device is designed to be powered by pressing with a finger, hence enabling a simple approach that the user can sample easily without need for healthcare workers, as shown schematically. The dimensions of the designed device are as follows: length, L = 40 mm; width, W = 20 mm; and height, H = 9.3 mm. (**b**) The design of the channels of the chip, in which the array of 100 microneedles (MNs) is connected to a duct that guides the extracted fluid to the reservoir via the channels by means of pressing with a finger. (**c**) The geometry of each MN, consisting of a hollow design with the following dimensions: height, H\({}_{\text{MN}}\) = 700 \(\upmu\)m; diameter of the needle tip, D = 80 \(\upmu\)m; and the diameter of the needle base, B = 300 \(\upmu\)m.