Establishment of pain evaluation test on the expression of Substance P during injection

A microneedle with a size of a female mosquito´s labium which can collects blood almost painlessly is effective to reduce pain during the injection. However, the decreasing of the outer diameter will affect inner diameter, rigidity and pressure in inner tube. By decreasing the size of outer diameter, the inner diameter and the rigidity of the needle will decrease while pressure loss will increase. Therefore, we need a pain evaluation test to determine the marginal region of painless for outer diameter size of microneedle in order to design painless microneedle which fulfilled painless, high-rigid and low-pressure loss. Here, a pain evaluation test through stress assessment test has been proposed. However, this pain evaluation test is not a direct method to evaluate pain since the evaluation is conducted through stress. In this study, we focused on the expression of Substance P (SP), which is one of a pain transmitter. The SP has been proven expressed only at the stimulated region after simulated mouse skin with both chemical stimulus and physical stimulus. Therefore, we conducted an experiment by injecting mouse skin with various diameter sizes of needle and determine the best diameter size by measuring the quantity of expressed SP by using image processing.