# Wireless Epidermal Six-Axis Inertial Measurement Units for Real-Time Joint Angle Estimation
###### Abstract  
Technological advances in wireless communications, miniaturized sensors, and low-power electronics have made it possible to implement integrated wireless body area networks (WBANs). These developments enable the applications of wireless wearable systems for diagnosis, health monitoring, rehabilitation, and dependency care. Across the current range of commercial wearable devices, the products are not firmly fixed to the human body. To minimize data error caused by movement of the human body and to achieve accurate measurements, it is essential to bring the wearable device close to the skin. This paper presents the implementation of a patch-type, six-axis inertial measurement unit (IMU) with wireless communication technology. The device comprises hard-electronic components on a stretchable elastic substrate for application in epidermal electronics, to collect precise data from the human body. Instead of the commonly used cleanroom processes of implementing devices on a stretchable substrate, a simple and inexpensive \"cut-solder-paste\" method is adopted to fabricate complex, convoluted interconnections. Thus, the signal distortions in the proposed device can be minimized during various physical activities and skin deformations when used in gait analysis. The inertial sensor data measured from the motion of the body can be sent in real-time via Bluetooth to any processing unit enabled with such a widespread standard wireless interface. For performance evaluation, the implemented device is mounted on a rotation plate in order to compare performance with the conventional product. In addition, an experiment on joint angle estimation is performed by attaching the device to an actual human body. The preliminary results of the device indicate the potential to monitor people in remote settings for applications in mobile health, human-computer interfaces (HCIs), and wearable robots. being conducted in order to promote the health of elderly individuals, as well as in the monitoring of athletic training and injury prevention in sports medicine [4; 14; 15; 16]. Joint angle estimation is a key component of human motion analysis, and many research groups have developed wearable sensors for continuous human motion analysis in real time through the use of optoelectronic systems [17], inertial measurement units (IMUs) [18; 19; 20], or ultrasonic sensors [21]. Optoelectronic systems are known to be the most accurate systems for estimating the joint angle. These systems use an infrared camera to analyze predefined anatomical positions in order to calculate the joint angles of a three-dimensional skeletal model. However, optoelectronic systems require experiments to be carried out in a controlled laboratory environment and require trained personnel to analyze the data. Thus, despite their accuracy, optoelectronic systems are not suitable for the continuous monitoring of joint movement in the ambulatory environment. Recently, due to the rapid development of flexible/stretchable electronic devices, a number of soft wearable and portable devices have been reported [1; 2; 3; 7]. By using thin and soft materials, the flexible bioelectronic modules can reliably interface with a part of the human body to monitor human motion with high fidelity. The skin-like wearable electronic modules have soft form-factors in order to achieve close contact with the human body, which prevents mechanical sliding and results in significantly improved performance. From the inherent skin-like characteristics of flexible and stretchable substrates, recent research has focused on the implementation of highly scalable and modularized epidermal electronic devices. Most commercially available wearable devices are implemented on a rigid substrate comprising silicon-based integrated circuit (IC) components. However, there is a large mechanical mismatch in the interface between the rigid devices and the human skin.