## 3 Experiments And Discussion
### Experiment 4: Needle Stitching Task  
The stitching task was intended to replicate a common task in endonasal surgery, where suturing of the dura matter is performed to prevent the leaking of cerebrospinal fluid. The experimental setup is shown Figure 14a with the testbed shown in Figure 14c. The same nose model of Experiment 2 and Experiment 3 was used for this task. A force sensor (Nano17, ATI Industrial Automation, North Carolina, USA) was placed behind the elastic tissue to register the force applied during the needle manipulation.  
The task consisted of grasping a 6-0 surgical needle and puncturing an elastic tissue with a 6-0 surgical needle. The participants were asked to follow a five-step procedure: (1) remove the needle from its initial position, and insert it from a mark on the right tissue using the right forceps (R-Insertion); (2) extract the needle from under the tissue (R-Extraction); (3) hold the needle using the right forceps with a proper orientation (Regrasping); (4) insert the needle from under the left tissue (L-Insertion); (5) extract the needle using the left forceps (L-Extraction). A total of three series of the full task were requested of each participant. The needle was initially inserted in the tissue and had to be grasped with the right forceps for the insertion and the left forceps for the final extraction. Five subjects between the ages of 20 and 35 who had no previous surgical training participated in these experiments. Before the experiments, each participant was instructed about the procedure and practiced for up to 15 min until they were familiarized with the operation. We used both the 2-DOF forceps and the 4-DOF forceps for this experiment.  
For each trial, we considered the following aspects to evaluate the task performance:  
* Task completion time (s): starting from removing the needle from its initial position until the complete needle extraction on the left side.
* Interaction force (N): recorded with a force sensor placed behind the tissue.  
The resulting task completion times are depicted in Figure 17. Using the proposed robotic system with the 2-DOF forceps exhibited a longer time, but with a shorter confidence interval for all the  
Figure 16: Experimental results of the block-in-hole task.  
steps, except for the Regrasping step. Passing the needle with a proper orientation from one forceps to another required a precise control of the needle and was challenging to achieve without training. When using our proposed system, the time to perform this task was reduced by approximately 50%. With the robotic system and the 4-DOF forceps, the completion time was reduced for the R-Insertion and L-Insertion steps, and the Regrasping step was no longer needed because the R-Extraction could be performed easily with the right forceps thanks to the two additional DOF. For the R-Insertion and L-Extraction steps, the operation with both articulated forceps showed a higher completion time, which may be caused by the limitation in the grasping force of the articulated forceps used.  
Figure 18 shows the force distribution resulting from the forceps interaction with the tissue during the needle insertion/extraction. The larger the number of bins close to 0 N, the less force applied to the tissue (less potential damage). The use of the robotic system (considering the use of both 2-DOF and 4-DOF forceps) reduced the range of applied force to almost half compared with the use of conventional forceps, from around [\(-\)0.4 N,0.4 N] to [\(-\)0.2 N, 0.2 N].  
Finally, a complete stitching sequence achieved with the robotic system is shown in Figure 19.