## 1 Introduction  
The presence of a third molar is common in human populations; however, third molars in the mandible are often impacted in the bone [1]. An impacted third molar is often a source of infection in the adjacent second molar [2], and an odontogenic cyst or malocclusion may be observed with an impacted third molar [3, 4]. Accordingly, many oral and maxillofacial surgeons have recommended surgical removal as a preventive measure [2, 5]. The extraction socket is generally healed by bone formation, but the level of bony healing is dependent on many clinical variables [6, 7]. The most important factor is the age of the patient at the time of extraction [8]. Early removal of the third molar is advised because younger patients have a higher probability of uneventful healing after extraction [9]. Incomplete bony healing results in periodontal pocket formation on the distal surface of the second molar [8; 10]. In young patients (age \(<25\)), removal of the mandibular third molar can improve periodontal health significantly [11; 12]. Patients older than 26 years with periodontal pathology in the distal surface of the second molar and a horizontal/mesioangular impacted third molar need regenerative therapy [13; 14]. Most mandibular third molars for surgical extraction have a mesial inclination and are in contact with the adjacent second molar [5]. Periodontal pathology is found in 48% of second molars after surgical removal of the adjacent mandibular third molar [10]. Probing depth is more than 7 mm in 43.3% of adjacent second molars after surgical extraction of the mandibular third molar [8; 15]. To prevent periodontal pocket formation and promote uneventful bony healing, grafting with biomaterials can be considered [16; 17]. Autogenous bone is generally considered the gold standard for bone graft, and synthetic materials such as hydroxyapatite are considered an alternative for the reconstruction of the extraction socket [18]. Ridge preservation materials have been used as filling materials for the extraction socket. They are collagen-based material and have shown positive effects for the preservation of the alveolar bone. However, there have been few reports on their application in the mandibular third-molar area. For the preservation of the alveolar bone, a guided bone regeneration (GBR) technique is a useful method [10; 19]. GBR is beneficial for patients aged 26 years or older who have a horizontally impacted third molar and a probing depth (PD) of 5 mm or greater in the distal surface of the adjacent second molar [20]. For a successful GBR technique, the selection of membrane is important [6]. The membrane for a GBR technique can be classified as comprising either biodegradable or non-degradable materials. An expanded polytetrafluoroethylene (ePTFE) membrane is non-degradable material whereas a collagen membrane is biodegradable [10; 19]. A comparative study of their application into the mandibular third-molar area has shown that there is no statistically significant difference between the ePTFE group and the collagen group in preventing a periodontal pocket [19]. However, a high rate of postoperative infection has been reported from ePTFE membrane after membrane exposure [21]. High-density PTFE (dPTFE) has been developed for an open-membrane technique and shown less postoperative infection after membrane exposure [22]. The idea for the development of a GBR membrane by using natural silkworm cocoons has been suggested in some recent publications [23; 24]. A silkworm cocoon is composed of several layers, and each layer can be separated by peeling [24; 25]. Silk mat from silkworm cocoons is mainly composed of fibroin and sericin; the middle layer of the silkworm cocoon shows better bone regeneration properties than other layers of the silkworm cocoon [25; 26]. The difference in bone regeneration according to layers is explained by the difference of protein composition [25]. Silk mat from the middle-cocoon portion shows bone regeneration similar to that seen when using collagen membrane [27].