# Wireless Epidermal Six-Axis Inertial Measurement Units for Real-Time Joint Angle Estimation
## 2 Method  
### Design  
A schematic of the epidermal six-axis IMU is shown in Figure 1. The inertial sensor and the signal processing parts used to construct the proposed system include a six-axis inertial sensor (MPU6050; Invensense, San Jose, CA, USA), BLE SoCs (nRF52832; Nordic Semiconductor ASA, Trondheim, Norway), and several passive components. All electronic components are mounted onto the bonding pads, and all components are connected by the serpentine-structured interconnects, allowing the device to elongate under various mechanical strains. The top and bottom layers of the implemented device are covered with a stretchable adhesive film (Tegaderm, 3M, Saint Paul, MN, USA) in order to protect the electronic parts from moisture or dust during attachment to the human body. The device is designed based on a modular concept that allows internal components to be reconfigured, allowing for a miniaturized system that is functionally optimized for a variety of applications. Thus, it can be used as a multiparametric epidermal sensor system, whilst also maintaining the signal processing building block and adding a sensor that can measure electrophysiological signals, strain, or body temperature. As shown in Figure 1, the IMUs can be used for kinematic analysis of human motion by the attachment to various parts of the human body and can be applied to the diagnosis and rehabilitation of degenerative neurological disorders. Figure 2 shows a block diagram of the wireless epidermal six-axis IMU. The six-axis inertial sensor and the BLE SoC read and send data using the two-wired serial interface (TWI), which uses two bus signals (serial clock (SCL) and serial data (SDA)) to exchange signals. From the six-axis inertial sensor, the acceleration and angular velocity of each of three axes are converted by the embedded 16-bit analog-to-digital converter (ADC). The digitized data are passed through the bus interface unit of BLE SoC via I2C communication. As there is a specific address for each piece of data, there is an address matching unit that processes the data according to the address. In the control unit, the data accumulated in the bus interface unit are processed and the commands are decoded. After the data are processed, they are spread by the universal asynchronous receiver/transmitter (UART) structure in the signal conditioning unit, and through Bluetooth communication the inertial data can be displayed on a mobile device. In the case of the MPU-6050, the accelerometer uses a full-scale range of \(\pm 16\)\(g\) and a sensitivity scale factor of 16,384 LSB/\(g\), where \(g\) is gravitational acceleration. For gyroscopes, we used a full-scale range of \(\pm 2000^{\circ}\)/s sensitivity scale factor of 131 LSB/\(\circ\)/s. The nRF52832 and MPU-6050 use I2C communication with a communication frequency of 400 kHz. The sample rate of the gyroscope and accelerometer of the MPU-6050 is 100 Hz, and the register is read with I2C communication in the  
Figure 1: Schematic of a patch-type wireless inertial measurement unit (IMU) and its applications. BLE: Bluetooth Low Energy; SoC: system-on-a-chip. nRF52832 and transmitted via Bluetooth in hexadecimal form. Our previous work on this system showed a capability of transmitting wireless inertial signals using BLE SoC (nRF51822) and reported the possibility of real-time human joint angle estimation [20]. In this paper, we use the nRF52832 to extend the functionality of the system. The nRF52832 includes a balun inside the chip, reducing the size and wiring of the entire system and facilitating impedance matching with ease. In addition, the nRF52832 has twice the RAM of the nRF51822, which allows the configuration of a BLE mesh. When constructing a mesh, a large RAM size is required, and sensor data can be collected simultaneously from 17 IMUs which are equipped for the purpose of analyzing the movement of all joints in the human body, enabling the scalability of the human body to a 3D model.