## 1 Introduction  
The tympanic membrane (TM), or eardrum, plays a fundamental role in sound transmission, its unique anatomical and physical features being ideal to function in varying frequency ranges. The human TM has been morphologically and functionally described since the 1970s [1; 2]. Its development starts during the 28 th week of gestation and arises from the most medial aspect of the meatal plug (basically, a cartilaginous tissue), which eventually becomes the external layer of the TM [3]. The fully developed TM is a three-layer structure, consisting of two histo-morphologically distinct areas--_pars tensa_, the largest, and _pars flaccida_, the thickest [4; 5]. The outer and inner linings of the _pars tensa_ are made up of epithelia, a keratinizing epithelium, and a simple cuboidal epithelium, respectively [4]. In between these two epithelia, a structural layer named _lamina propria_ is located, which is connective tissue proper. In the _pars tensa_, the connective tissue is composed of collagen fibers with unique spatial arrangements, radial on the outer and circular on the inner side, in addition to parabolic fibers, which altogether build up a stiff and thin structure [5; 6]. As a consequence, the peculiar fibrous network in the _lamina propria_ of the _pars tensa_ plays a key role in the physiology of the normal TM, and its damage after perforations, traumas, and infections generates a functional impairment [1]. Moreover, many studies have revealed that the vibration pattern of the TM reflects the underlying fiber bundle arrangement and thickness of the TM, thus confirming the tight connection between anatomy and physiology [7]. Immunohistochemical studies of the TM have shown that the fibrous layers in the _lamina propria_ of the _pars tensa_ are formed mainly by collagen types II and III, while collagen type I is present to a lesser extent [1; 8; 9]. Collagen type I has been found in the tympanic _anulus_ (namely, the insertion zone to the meatus bone), whereas procollagens I and III in the subepidermal and submucosal connective tissue layers [1; 8]. Collagen IV and other proteins are also found in TM [10]. The very high presence of collagen type II may be explained by referring to the embryological development of TM from the first branchial arch, which will give rise to _fibrous stratum_[3]. Each TM layer derives from a different embryonic leaflet (i.e., outer ectoderm, middle mesoderm and inner endoderm), which altogether interact to generate a functional eardrum through several transformations of the branchial arch mesenchyme, including chondrogenesis, that induces malleus elongation [11]. The complex collagenous network deriving from eardrum development has been considered to be the key for an efficient vibratory function of the TM [6; 7; 8]. In adults, apart from the typical banded fibrillar circular structure, which is characteristic of collagen organization, squared-section fibrils have also been observed in TM cross sections [1; 5]. These structures, also found in other ear compartments, have revealed to be collagen type II [1; 12]. In the eardrum, the overall collagenous structure is produced by TM fibroblasts, unique in the human body to synthetize collagen type II at a larger extent and collagen type I at a lesser extent [4]. Otitis media and perforation are among the most widespread pathologies of the TM [13]. If perforations are not able to self-repair, chronic infections can take place, which ultimately cause hearing loss and may require surgical treatment [14]. To date, autograft taken from connective tissue, such as the _temporalis fascia_ and tragal cartilage are considered the gold standard materials for TM replacement [15]. In general, the outcome quality is related to the extent of the replacement, and for large perforations suboptimal outcomes have usually been reported [16].