# Novel Instrument for Clinical Evaluations of Active Extraocular Muscle Tension
## 2 Materials and Methods
### Design of Active EOM Tension Measuring Device  
An active tension measuring device (ATMD) for EOM tension was designed in this study to allow for quantitative and objective measurements using the clinical FGT. The ATMD comprised sensing, control, and input-output (I/O) units. During the FGT, the ophthalmic surgeon grips and places a cotton-tipped swab on the eyeball; then, the eye is restricted from horizontal movement. The subject then tries to move their eyeball toward the swab. Consequentially, the active force pushing the cotton-tipped swab is recognized when the eye attempts to move against the placed cotton-tipped swab, as shown in Figure 1a. A sensing unit was first designed for the ATMD to measure the pushing force applied to the cotton swab tip. The ATMD device is comprised of a medical swab, a shaft, a knurled bolt, a linear bushing, a handle, and an integrated printed circuit board (PCB) with force and tilting sensors, as shown in Figure 2. The shaft was constructed as a single-side long hole for medical swab placement. The swab was interposed inside the shaft and then fixed by the knurled bolt. The other side of the shaft was connected to the piezoresistive-based force sensor (FMAMSDX025 WCSC3, Honeywell, NC, USA) mounted on the PCB [20]. The sensor was 5 mm \(\times\) 5 mm in size and had a full-scale accuracy of within 2%. The linear bushing (LMK6, THK, Tokyo, Japan) could support the shaft by compensating for the mechanical friction in the axial plane. Forces on the top of the swab could be transferred to the force sensor through a single-side hole shaft with minimized mechanical losses. A tilting sensor (EBIMU-9 DOFV5, E2 BOX, Gyeonggi-do, Korea) was also mounted on the PCB to measure the tilting angle during the FGT [21]. The ophthalmic surgeon can refer to the tilting angle to uniformly perform the FGT in real time.  
Figure 1: The proposed active extraocular muscle (EOM) tension measuring device (ATMD). (**a**) Initial position for the force generation test using the ATMD. (**b**) The ATMD comprised sensing, control, input–output (I/O) units, a display, and a foot switch.  
The control unit comprised a microcontroller and Bluetooth and power modules. The microcontroller (ATMEGA328P-PU, Atmel, CA, USA) collected data on the tilting angle and force by using an inter-integrated circuit and serial communication in a sampling period of 90 ms [22]. The collected data were wirelessly transferred to a laptop via a Bluetooth module (HC-05, HC Information Technology, Guangzhou, China) at a baud rate of 9600 bps [23]. In the power module, a 12 V input was transformed to 5 V and 3.3 V using a linear regulator (LM1117 MPX-5.0, Texas Instruments, Dallas, TX, USA) and a switching converter (MUS-0503.3, Danube, Kaohsiung, Taiwan) to operate the control and sensing units, respectively. Figure 3a,b present the PCBs for the sensing and control units and their sensory and electrical components.  
The I/O unit comprised foot and emergency switches for input and a laptop for real-time displaying and data storing of measured signals. Start and termination commands were provided by the foot switch. The entire power of the ATMD could immediately be cut off by pressing the emergency switch in order to avoid electrical injuries. The graphs could display force and three-axes tilting angles as real-time waveforms on the laptop display, as shown in Figure 4. All of the data could also be stored using the commercially available computing software MATLAB R2019b (MathWorks, Natick, MA, USA). A video detailing the device's clinical application is available as Supplementary Material.  
Figure 3: Photographs of the PCBs for the control (**a**) and sensing (**b**) units.  
Figure 2: Details of the sensing unit, which comprised a medical swab, linear bushing, linear motion (LM) guide, pin, initializing bolt, and sensory printed circuit board (PCB).