➣ By Kazuyoshi Tsuchiya, Kagemasa Kajiwara & Minoru Kimura
Using a microneedle mimicking the size of a female mosquito’s labium, which collects blood almost painlessly, is an effective way to mitigate injection pain for patients. Here, we have a microneedle production technique which uses a type of sputtering deposition method. In the production procedure, firstly, a microneedle material on a rotating wire substrate at the speed of 3-5 rpm by the sputtering deposition method is deposited. Secondly, the wire substrate is etched by a chemical solution after a heat treatment. Finally, the titanium microneedle is produced (outer and inner diameter: 60 μm and 25 μm) in the size mimicking that of female mosquito’s labium (see Fig.1).
Here, the inner diameter of the injection needle is decreased when the outer diameter is decreased in order to mitigate pain, hence, the rigidity of the needle is decreased and the pressure drop by pipe friction is increased. Therefore, a little larger size than the female mosquito’s labium is needed to satisfy the mechanical properties for the painless microneedle. However, we do not have any evaluation techniques to judge the pain quantitatively.
In this study, we propose a pain evaluation test to design a painless microneedle to determine the maximum diameter to mitigate pain.
Generally, the techniques for measuring physiological stress changes of blood pressure, heartbeat, diaphoresis, etc., are enumerated as an objective evaluation technique for pain. It is observed that stress generally arises due to a physical stimulus. However, it is difficult for the physiological stress change to be easily influenced by the measurement environment and to evaluate only the pain. When the sympathetic nerve system becomes active due to a physical stimulus such as the injection of a needle, the digestive system is inhibited and only the salivary gland is active. The amount of α-amylase in saliva is changed by an activity change in the sympathetic nervous system, so that the value rises due to unpleasant conditions or the value decreases in comfortable conditions. Therefore, the pain is defined as a psychological stress caused by the injection of a microneedle in this research, and we focus on the change in the amount of α-amylase in saliva as a pain evaluation test. Here, the advantage of using the amount of α-amylase in saliva as a measurement technique to evaluate stress is firstly, it ensures a quick response after a physical stimulus, and secondly, it is easy to measure the value quantitatively, and lastly, it is a non-invasive test.
In this study, we adopted the amount of α- amylase in saliva for mice, and the relation between stress and the pain resulting from the injection of a needle was evaluated. The needle injection in a mouse is consistent to measure the amount of α-amylase in saliva, compared with that of a human, and there are few individual differences between mice. Here, the experimental conditions to investigate the relation between stress and pain for injection of a needle, are described. (1) Nanopass 33 (TERUMO Corporation, 100 μm and 200 μm in inner and outer diameter) (2) substitution needles (the leading end was polished at 12 degrees; outer diameters: 35, 70, 95, 100, 150, and 200 μm) were used. Institute of Cancer Research (ICR) mice were used in the study. The needle was injected in the femoral region where the nerves are concentrated. Also, for the extraction of the mouse’s saliva, a micro pipette was inserted into the mouth. The extraction volume of saliva for mice was 1 μl. The extracted saliva is diluted by phosphate buffered saline because the clinical biochemistry automated analyzer for α-amylase needs the solution amount of 100 μl. The saliva sample was extracted 10 times on each condition and an average was taken. The schemes of the effectiveness for the pain assessment are the following.
(1) The amount of α-amylase in saliva for injection of Nanopass 33 compared with its control as a stable condition. Two groups (the data for injection needle and the data for control) were statistically compared using the Mann-Whitney-U assay. As a result, the p (probability) value of two groups was significant at the level of 0.01. Therefore, the effectiveness for the proposed pain evaluation test to measure the amount of α-amylase in saliva was confirmed (see Fig. 2). The proposed pain assessment to measure the amount of α-amylase in saliva is effective.
(2) There are no commercial-based needles with a diameter smaller than NANOPASS 33, therefore, the amount of α-amylase in saliva for injection of substitution needles compared with its NANOPASS 33 and the effectiveness for the substitution needle was confirmed. Here, we have confirmed that there is no difference for the amount of α-amylase between Nanopass 33 and a substitution needle (outer diameters: 200 μm). We have also confirmed that the stress attributed to pain was not dependent on the difference between the shape of leading ends and the surface condition.
(3) The amount of α-amylase in saliva for various outer diameters in substitution needles compared with its control. In order to evaluate pain for injection needles with various outer diameters, two groups (each data for injection needle and the data for control) were statistically compared using the Mann-Whitney-U assay. Fig. 3 shows the comparison of amount of α-amylase in saliva between injection of substitution needles and the control. As a result, the significant difference has been confirmed in all the needles except smaller needle groups (less than 100 μm outer diameter against the control group). Here, it was confirmed that a significant difference was 0.05 between the control group and the injection of 100 μm outer diameter needle as a substitution needle. Therefore, it is clear that the maximum outer diameter to mitigate the pain for a microneedle is 100 μm or less.
Kazuyoshi Tsuchiya, Ph.D.
Kagemasa Kajiwara, MSD
Minoru Kimura, Sc.D.
President of Virtual Reality Medical Institute (VRMI) in Brussels, Belgium. Executive VP Virtual Reality Medical Center (VRMC), based in San Diego and Los Angeles, California. CEO of Interactive Media Institute a 501c3 non-profit Clinical Instructor in Department of Psychiatry at UCSD Founder of CyberPsychology, CyberTherapy, & Social Networking Conference Visiting Professor at Catholic University Milan.