# Wireless Epidermal Six-Axis Inertial Measurement Units for Real-Time Joint Angle Estimation
## 4 Conclusions  
This paper presents a wireless epidermal six-axis IMU for applications in wearable biomedical devices. In our previous reports, we have focused on the design of serpentine metal interconnect structures and wireless signal transmission via Bluetooth [19; 20]. In this paper, a complementary filter algorithm was applied as a post-signal processing method to improve the performance, relating to issues such as error compensation and high-frequency-component removal. Additionally, mechanical experiments were conducted for applications in real-life use. The proposed device is implemented using conventional circuit components on a soft adhesive substrate, and by firmly attaching it to the target, it is possible to collect accurate data from the human body. The implemented system has dimensions of \(40\times 37\) mm\({}^{2}\), and the overall thickness of the system--excluding the battery and ICs--is less than 100 \(\upmu\)m. The serpentine-structured interconnects are adopted between the soldered electronics to achieve flexibility and stretchability. The implemented device was mounted on a rotation plate to compare its performance with that of a commercially available device. The filtering of the angular velocity signal, relating to the rotational axis and the removal of noise components in the estimation of the rotational angle, is expected to show results similar to those of commercially available products. An experiment on joint angle estimation was performed by attaching the device to an actual human body. The maximum RMS error against the commercialized Wearnotch device was 3.31\({}^{\circ}\), and the maximum angular displacement was 3.79\({}^{\circ}\). The cause of the error between the two systems is currently being analyzed from multiple angles, with the most likely cause being based  
Figure 6: Performance evaluation of wireless epidermal six-axis IMU applied to joint angle estimation using a 3D motion analysis program.  
on the alignment error of the two systems. At present, a technique for correcting alignment error as well as an experimental method for improving it are currently envisioned. Furthermore, we are planning to analyze the acceleration and angular velocity with respect to the reference acceleration and angular rate through the vibration table and the rate table, and plan to compare the results using an optoelectronic system. The soft, epidermal system introduced in this paper is considered inexpensive and user-friendly, which can provide accurate joint angle estimates in the ambulatory environment. We are also in the process of optimizing the performance of this system for the future, so that it can provide the results of estimating the joint angle with more degrees of freedom, and ultimately provide data for the full body. The preliminary results of the device have potential for the monitoring of people in remote settings for applications in mobile health, HCIs, and wearable robots.  
[PERSON] conceptualized and supervised the study; [PERSON], [PERSON], and K.K. designed and implemented the device; [PERSON] and [PERSON] supervised the experiments; all authors participated in the evaluations; [PERSON] and [PERSON] wrote the paper. All authors have read and agreed to the published version of the manuscript.  
This research and the APC was supported by the National Research Foundation of Korea (NRF) grant, funded by the South Korean government (MSIP: Ministry of Science, ICT & Future Planning (No. NRF-2017R1C1B5017561)).  
This work was supported by the X-mind Corps program of National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning.  
The authors declare no conflict of interest.