# Finger-Actuated Microneedle Array for Sampling Body Fluids
## 3 Results and Discussion
(**c**) Pressure distribution in the device for human blood with input function of the heartbeat imitation and pressing the mechanical pressure input button every 5 s (study case 3 in Table 3), and (**d**) the magnified view of one microneedle (MN). All color bars show the amount of pressure in Pascals (Pa). (\(\mu\)L) in 5 s, as shown in the right vertical axis of Figure 4d. Among experimental setups of the MN devices for collecting blood samples, a study reported extraction of 4 \(\mu\)L of mouse blood in vivo in 5 s per one needle [36]. Another study reported extraction of around 39 \(\mu\)L rabbit blood in vivo per one needle in 5 s with a pre-vacuum system [53]. The results in the present work, with an average extraction of 12.24 \(\mu\)L human blood in 5 s per needle, fall in the range of the experimentally reported cases. For ISF extraction in two states of slow (case 5, Table 3) and fast (case 6, Table 3), the extraction amount slightly increased in fast mode (Figure 4e), with the left vertical axis for the total extracted ISF in grams in 5 s. The MN patch included 100 needles. The average amount that each needle extracted in microliters in both states of slow and fast are represented in the right vertical axis of Figure 4e. The simulation resulted in an average extraction of 64.5 \(\mu\)L ISF in 5 s per needle. The simulation results for the amounts of ISF extraction, which was higher than previous reports, could be partially attributed to the mechanism of the extraction and the dissimilarities between the properties of the ISF of humans and animals. While the mechanism of ISF extraction in the majority of reports is on the basis of swellable materials, our design offers a high volume for fluid extraction rather than a limited volume of swellable needles. On the other hand, diffusion plays the main role in swelling, which is a slow procedure, requiring long durations of administration time such as a full day [66] or in some cases three weeks [49] to reach a meaningful result. In contrast, the device reported in this work is powered by finger pressure input, which not only offers a greater force for extraction in comparison with the naturally occurring diffusion, but also can be controlled very conveniently by pressing faster, enabling the extraction of ISF in higher volumes. Furthermore, as higher extraction rates are projected for human ISF compared to animal ISF [49], the dissimilarity between the properties of human and animal ISFs can be another reason for the extraction discrepancy. A previous research study on the extraction of ISF from mice has also stated that they anticipate a greater result in humans [49]. In addition, the extraction of ISF resulted in a higher efficiency when compared to the human blood (Figure 4). Since both the density and viscosity of ISF are lower than that of blood, the same finger pressure results in a greater extracted amount of ISF at the reservoir with less effort. The simulation results for the presented cases in this work showed a successful extraction of the target fluid. One of the widely used components in microfluidic platforms is diode due to their capability of changing fluid direction on demand. Microfluidic diodes are a category of membranes that deform to an open shape in contact with the forward pressure of the flow, allowing the fluid to pass through the channel. Conversely, the diode closes when there is a backward fluid pressure, hence offering a great way for rectifying the flow [67, 68]. Further studies on finger-powered microneedle research can benefit from the integration of diodes. For example, the utilization of microfluidic diodes was reported for a successful manipulation of the flow direction in a finger-powered microfluidic setup [55].