# Wireless Epidermal Six-Axis Inertial Measurement Units for Real-Time Joint Angle Estimation
## 2 Method
### Manufacturing Process  
The implementation process utilized to fabricate the epidermal six-axis IMU on a stretchable adhesive film is illustrated in Figure 3. This process uses a computer-aided design (CAD) program to implement miniaturized devices on substrates with various form factors. The most important consideration in designing this system is determining the limit of elongation, at which point there is no change in the resistance of the metal interconnect and the stress distribution in the system is not concentrated in one place. In our previous paper, various serpentine-structured interconnects were fabricated, and the characteristics of this robust metal interconnect were evaluated [19]. The metal interconnects of the serpentine structure are designed to have a line width of 300 \(\upmu\)m, which is mechanically and electrically stable even when the device is applied to a tensile strain up to 30% of its original dimension. After designing the footprints using a CAD program, a programmable cutter (Silhouette Cameo(r), Silhouette America, USA) is used to define the serpentine-structured interconnects and soldering pads.  
First, an 18-\(\upmu\)m-thick piece of Cu foil (Copper 110 Annealed; Online Metals, Seattle, WA, USA) is laminated on the thermal release tape (TRT), as shown in Figure 3a,b. Then, as shown in Figure 3c, the TRT-laminated Cu foil is cut according to the interconnect and bonding pad pattern as designed by the CAD tool. After patterning by a programmable cutter, the residual area of the TRT-laminated Cu foil, except for the designed patterns, is removed manually using tweezers as depicted in Figure 3d. The opposite layer of the patterned TRT-laminated Cu foil is then bonded with the water-soluble tape (WST) and Kapton film, and the whole sample is bonded onto the glass substrate. As shown in Figure 3e, the bonded substrate is heated on a hot plate at 120 \({}^{\circ}\)C for 30 s, and the TRT layer is peeled off. After peeling off this layer, the bonding pad for circuit assembly is revealed. As shown in Figure 3f,  
Figure 2: Block diagram of the epidermal, six-axis inertial measurement unit (IMU) using wireless data communication. ADC: analog-to-digital converter; UART: universal asynchronous receiver/transmitter; SDA: serial data; SCL: serial clock; TWI: two-wired serial interface.  
the electronic components (six-axis inertial sensor, BLE SoC, chip antenna, passive components, and bridge interconnects) are aligned and fixed onto the bonding pads. Then, as shown in Figure 3g, the Kapton film is removed and the whole device is released from the glass substrate. The stretchable adhesive film is then used to cover the top layer of the circuit for encapsulation, as shown in Figure 3h. As shown in Figure 3i, water is dropped onto the WST layer using a pipette which dissolves the water-soluble compound of the WST. After this, the WST layer is peeled off to leave only the electronic components and the serpentine-structured Cu interconnects. Finally, after covering the bottom layer, the epidermal six-axis IMU on a stretchable adhesive film is ready for implementation.