## 5 Discussion and Conclusion  
In this paper, we proposed a new design for a fiber-optic force sensor that can be used for surgical devices. The proposed sensor is a modification of an FPI sensor previously proposed by one of the co-authors [29]. The proposed sensor has a linear output characteristic and is bio-compatible and sterilizable. The most advantageous feature of the proposed sensor is its modularity. It can be attached to conventional surgical devices like a strain gauge to provide force information at the tip. In the evaluation, a temperature characteristic was observed due to the sensor structure. The overall results show the promising feasibility of its use in surgical applications.  
One of the greatest challenges for the mass production of this sensor is that manual assembly procedures are prone to human error and thus could be a source of inaccuracy. A calibration procedure would be necessary for each device in the current assembly procedure. Further improvement of this process will be necessary for future mass production.  
The sensitivity was found to be 2.36-2.61 nm/\({}^{\circ}\)C. This could be improved by using a less rigid material for the sensor housing. In this study, we used stainless steel because of the ease of production and its linear characteristic between deformation and load. However, because we have a large margin of measurable wavelength (e.g., 200-1100 nm with the current setup), a softer material for the sensor structure would drastically improve sensitivity. A spectrometer with a higher resolution would also improve the sensitivity.  
As was suggested by the experimental results, the temperature characteristic is mainly caused by the sensor structure fabricated from stainless steel. A less thermally sensitive material would drastically reduce the effects of temperature; however, this characteristic will probably remain to some degree and be difficult to remove. Because the sensor has a linear relationship with temperature change, it is possible that the sensor can be used with a temperature compensator (e.g., a Wheatstone bridge), which is commonly used for strain gauges with multiple sensor elements. This would require the applications to implement multiple sensors; however, this would be useful for some applications that require simultaneous force and temperature measurements.  
In this paper, the sensor was tested in a static condition, and dynamic conditions such as time-response were not tested. However, the principle of FPI allows for real-time measurement [29], and our preliminary tests show that it seems feasible.  
The technical contribution of this paper is the proposal of a force sensor structure based on an FPI fiber-optic pressure sensor. The proposed structure realizes a force sensor that is bio-compatible,  
Figure 7: Temperature test results: (**a**) results for only the sensor module and (**b**) results for the sensor attached to the test piece shown in Figure 5. Both results suggest a linear temperature characteristic.  
sterilizable, noise-tolerant, and most importantly, modular, which allows the user to measure the force applied on a conventional surgical device by attaching the proposed sensor.  
The proposed sensor has been specifically designed for surgical application, enabling the structure to be bio-compatible, sterilizable, and tolerant to noise within the miniaturized size. An example application would be to put the proposed sensor into the jaws of surgical forceps to measure the force applied to the organs. Because the sensor is bio-compatible, sterilizable, and tolerant to noise within the miniaturized size, it can be placed close to lesions, where a small interaction force between the instruments and the organ can be measured without additional disturbance (e.g., friction in the mechanism of the instruments). Further, we are currently working on the improvements discussed in this section and further plan to implement the sensor in a surgical robot [39; 40; 41].