# Multilayered Artificial Dura-Mater Models for a Minimally Invasive Brain Surgery Simulator
## 2 Dura-Mater  
Figure 1 shows a sagittal view of a human head. The bony and membranous parts are layered from the skin to the brain matter. The dura-mater is one of the three membrane layers of the brain (pia mater, arachnoid, and dura-mater) that cover the central nervous system on the outermost cover. It is divided into two different layers. One is the spinal dura-mater and the other is the cranial dura-mater. The spinal dura-mater covers the spinal nerve, and the cranial dura-mater is attached to the inner surface of the skull and covers the entire brain. Several studies quantified the mechanical characteristics of dura-mater. [PERSON] et al. obtained a tensile strength of a human spinal dura-mater at 61.5 \(\pm\) 9.6 MPa in the linear region with elongation strain in the range of 15-20% in a uniaxial tensile test [10]. [PERSON] et al. showed a tensile strength value of 5.6 MPa in the axial direction and 1.6 MPa in the circumferential direction using a biaxial tensile test of a sheep's spinal dura-mater [11]. [PERSON] et al. reported that the cranial human dura-mater had a mean tensile strength of \(\sim\)7 MPa [12]. Hence, the human dura-mater has a tensile strength of the order of several MPa. However, conventional studies just focused on the tensile test to measure Young's modulus of the dura-mater, and the mechanical properties of the artificial dura-mater related to surgical operations, such as the cutting force by microscissors, have not been quantified. Existing artificial dura-maters which were clinically used, such as Gore-Tex(r) Expanded Cardiovascular Patch (W.L. Gore & Associates Inc., Flagstaff, AZ, USA), Durepair(r) (Medtronic Inc., Goleta, CA, USA), and Tutopatch(r) (Tutogen Medical GmbH, Neunkirchen am Brand, Germany), were generally used for closing holes on a skull after surgery [12]. The Gore-Tex(r) Expanded Cardiovascular Patch is made of expanded polytetrafluoroethylene (ePTFE), Durepair(r) is a porosity  
Figure 1: Sagittal view of the brain and its structures with a transnasal endoscope for neurosurgery. sheet made of collagen type II and III. Tutopatch(r) is also a collagenous membrane. Those artificial dura-maters have been developed to be used in vivo (for actual brain surgeries). Therefore, those models were not considered to be used in vitro (dry condition) and the mechanical properties of the artificial dura-mater related to surgical operations, such as the cutting force generated by microscissors, have not been considered. The collagenous membranes are not suitable for surgical simulators because their mechanical properties depend on wet or dry conditions. Some conventional real type surgical simulators did not have a dura-mater model because their target surgery did not have an important role in cutting or suturing the dura-mater [13; 14; 15]. Several studies developed artificial dura-mater models for surgical simulators of brain surgeries. For example, [PERSON] et al. developed an anatomical replica for training young neurosurgeons [16]. A latex dura-mater model was used to mimic the actual dura-mater. However, the model made of latex could not represent the elastic fibers in dura-mater, such as collagen and elastin fibers. [PERSON]. [PERSON] et al. developed a dura-mater model by laminating silicone-okawara papers. The paper fibers could mimic the elastic fibers in dura-mater, but the mechanical properties of the developed dura-mater model were not evaluated [17]. Hence, new dura-mater models for the surgical simulators of brain surgery are desired. This paper shows the development of dura-mater models for a brain surgical simulator, especially for transnasal pituitary surgery. This type of surgery is known as minimally invasive surgery, and pituitary gland tumors are excised by surgical instruments following their insertion in the nose. Figure 1 indicates a sagittal view of the brain and its structures with a transnasal endoscope during the transnasal pituitary surgery. The surgical procedures should be held in a limited space of the human head.