This is the published version
Allardyce,BJ, Rajkhowa,, Atlas,MD, Dilley,RJ and Wang,X 2014, Silk films as a
repair material for perforations of the tympanic membrane, in Proceedings of the
89th Textile Institute World Conference, Fiber Society, [Wuhan, China], pp. 6-10.
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Copyright: 2014, Textile Institute
Silk films as a Repair Material for Perforations of the Tympanic
Membrane
Benjamin J Allardyce1, Rangam Rajkhowa1, Marcus D Atlas2,3, Rodney J Dilley2,3,4 and Xungai Wang1
1 Australian
Future Fibres Research & Innovation Centre, Institute for Frontier Materials, Deakin University, Geelong,
Australia
2 Ear Science Institute Australia, Australia
3 Ear Sciences Centre, School of Surgery, the University of Western Australia, Australia,
4 Centre for Cell Therapy and Regenerative Medicine, The University of Western Australia, Australia
Abstract. Large, chronic perforations of the tympanic membrane or eardrum can cause hearing loss as well as a range of
secondary health problems. Current methods of repair usually involve grafting a material such as cartilage from another site on
the body across the perforation. However, given problems such as possible infections at the graft donor site and the inability to
see through the graft to assess infection within the middle ear, there is a need to develop an alternative material that is strong,
readily available and transparent. Such a material would allow for less invasive surgery and potentially result in a superior
hearing outcome for the patient. Our recent work has identified silk fibroin films as a promising material for this application.
This paper reviews the repair of large perforations and compares the mechanical properties of silk with some existing graft
materials. It also briefly discusses the difficulties in defining and comparing these properties with such different materials.
Keywords: silk, fibroin, biomaterial, tympanic membrane
1. Background
The tympanic membrane (TM) or eardrum is an intricate structure that plays the vital role of translating
sound pressure waves in the outer ear into vibrations of the chain of ossicles in the middle ear [1]. Damage to
this structure can significantly impact on hearing and, in extreme cases, contribute to severe and
life-threatening infections of the middle ear. Repair of persistent perforations of the tympanic membrane
usually involves the use of a graft material, often taken from another site on the patient. However, even the
most successful current generation graft material cannot restore the microstructure of the normal tympanic
membrane, and in some cases patients may be left with some permanent hearing loss [2]. The development of
a synthetic graft with suitable mechanical and vibro-acoustic properties is required to overcome the
deficiencies of current materials.
2. Properties of the TM
The tympanic membrane is a semi-transparent, elliptical structure that sits at the base of the ear canal; it
marks the border between the outer ear and middle ear and forms an important boundary to prevent middle ear
infection (Figure 1a) [3]. It varies in thickness across the membrane with an average of approximately 74 µm
[3]. The membrane is covered in epithelial tissue on the external (ear canal) surface and mucosal tissue on
internal (middle ear) side. The middle of the membrane consists of connective tissue, with a layer of circular
collagen fibres and a layer of radial fibres, which emanate from the central region (umbo) of the membrane
(Figure 1b and c). Measurement of the mechanical properties of the membrane is complex, due to variations in
thickness and orthotropic nature of the membrane. A range of techniques, including uniaxial tensile testing,
indentation testing and FE modelling, have been employed to both measure and predict its viscoelasticity.
However, the Young’s modulus of the membrane cannot be easily and accurately defined, with estimates
ranging from 0.4 MPa right through to 300 MPa. A detailed review of the current state of knowledge of the
TM mechanical properties is provided by Volandri et al [3].
a)
b)
c)
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Figure 1: a) Healthy human tympanic membrane [4]. b) Diagram of the TM showing the arrangement of radial and circular collagen
fibres [1]. c) Schematic cross-section showing the 4 main tissue layers.
3. Perforations of the TM and current methods of repair
Conditions such as severe middle ear infections or traumatic injury can cause chronic perforations of the
eardrum. If left untreated, these conditions can lead to life threatening bacterial infection and significant
hearing loss. In cases where the tympanic membrane does not heal spontaneously, surgical intervention may
be necessary. This is done using a technique known as myringoplasty. During this process, a graft material is
either placed directly over the perforation (onlay graft) or an incision is made in the skin next to the TM and
the graft is slipped through and against the underside of the membrane (underlay graft) [4, 5]. The most
commonly used graft materials are temporalis fascia and cartilage, both harvested from the patient (an
autologous graft) [5]. Both of these materials have been shown to be effective in different situations [4].
However, there are a number of problems associated with these materials, including the possibility of
infection in the graft site and the potential for hearing loss due to the need for the graft material to be thicker
than the tympanic membrane in order to provide structural integrity [6]. Also, unlike the tympanic membrane,
both fascia and cartilage are opaque. This is undesirable because it does not allow surgeons to view the middle
ear cavity during recovery to check for signs of infection.
There is therefore a need to investigate other possible materials including synthetic graft material. The ideal
graft material must satisfy the following criteria:
 Good biocompatibility – cause minimal immune response once implanted
 Excellent strength at a reasonable thickness
 Provide minimal interference with hearing
 Transparent if possible to allow for the inspection of the middle ear cavity
Ideally a synthetic graft material should match the native properties of the TM as closely as possible.
4. Comparing the mechanical properties of synthetic graft materials
In addition to the currently used autologous graft materials used in myringoplasty, a range of synthetic
and allograft (biologically derived) materials have been tested. Some of the more common materials include
simple paper, Carbylan™ (hyaluranan with additional carboxyl groups), Alloderm® (de-cellularised cadaver
skin) and various other types of decellularised tissue [4]. These materials have met with different levels of
success, however, none has emerged as a clear alternative to fascia or cartilage. However, one material that
has recently shown great promise is silk films [4, 7-12]. These films, made from regenerated silk fibroin, are
strong, transparent, biocompatible and readily available. Assessing silk based materials against the other
materials mentioned above is crucial to determine whether or not they offer a real advantage. The mechanical
properties are an important first step in characterising a material for use as a TM graft as strength and stiffness
are important in providing support to the membrane during healing.
The following (Table 1) is an attempt to compare the key mechanical properties of silk films with some
alternative materials evaluated for TM repair in the literature. As with the properties of the TM itself,
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comparing graft materials with each other is very difficult due to inherent differences in the materials and
differences in the methods of testing these materials. This should therefore be taken into account when
comparing the values listed.
Table 1: Comparison between the mechanical properties of silk film with other graft materials. Silk films tested submerged in water
and porcine cartilage (unpublished) were tested at a gauge length of 15 mm, preload of 1 cN and an extension rate of 150 mm/min.
Material
Tensile strength
Young’s modulus
Elongation
(MPa)
(MPa)
(%)
Source
Silk films
Silk films, (dry)
23 – 58.8
2893 – 3411
1.4-6.4
[9, 10, 13, 14]
Silk films, (wet, air tested)
14.8 ±4.8
280.2 ±53.4
136.6 ±35.0
[9]
6.3 ±1.4
47.3 ±12.5
265.8 ±65.9
Silk films, (tested submerged in
water)
Unpublished
data
Synthetic graft material and allografts
Paper (dry)
34.4 ±1.7
2.2 ±0.1
-
[10]
Acellular collagen (dry)
21.4 ±1.6
1.6
-
[10]
Alloderm® (dry)
9.4 ±0.85
260.0 ±16.0
-
[15]
Alloderm® (rehydrated)
21.2 ±2.9
70.9 ±6.3
-
[15]
Autologous grafts
Conchal cartilage
-
3.4
-
[6]
Tragal cartilage
-
2.8
-
[6]
P
E
E
T
Figure 2: Silk film stress-strain curve
of wet silk film showing the elastic
region (E) and plastic region (P).
Figure 3: Stress-strain curve resulting from a
uniaxial tensile test of a cartilage strip showing
the toe-in region (T) and large elastic region (E).
Direct comparison between our silk films and the other graft materials is complicated by the differences in
structure. Silk films are composed of a continuous and relatively homogenous layer of fibroin. They are
therefore quite easy to characterise using standard tensile testing (Figure 2). Cartilage on the other hand is a
composite of collagen fibres embedded in a ground substance. This makes them difficult to test for the same
reasons as those mentioned above for the whole TM. For this reason, the mechanical properties of cartilage
are often tested in 2 different ways: at equilibrium once a force has been applied (to get the equilibrium
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modulus) or by a constant strain rate method where the strain is increased at a set rate and the tensile stress is
measured at set intervals. When measured using constant strain rate, there is a non-linear region of the
stress-strain curve that corresponds to a rearrangement of the collagen fibrils in the matrix from randomly
aligned to aligned along the axis of loading (Figure 3) [16]. Young’s modulus is calculated from the linear
region after this toe-in; this represents the intrinsic strength of the collagen fibres because it is after they have
become aligned and are being stretched [16, 17]. A similar toe-region is observed for Alloderm [18].
5. Concluding remarks
The development of a synthetic material that provides comparable or better stability to the perforated TM as
it heals represents a significant challenge. Silk films show great promise given that they already address many
of the requirements of a suitable graft material. However, further work is required to assess this material
compared with existing grafts and other possible materials.
6. References
[1]
Hüttenbrink, K.-B., Biomechanical aspects of middle ear reconstruction, in Middle ear surgery: recent advances and future
directions, K. Jahnke, Ed., 2004, pp. 23.
[2]
Lee, P., Kelly, G., Mills, R. P., Myringoplasty: does the size of the perforation matter?1, Clinical Otolaryngology & Allied
Sciences 2002, 27, 331.
[3]
Volandri, G., Di Puccio, F., Forte, P., Carmignani, C., Biomechanics of the tympanic membrane, J. Biomech. 2011, 44,
1219.
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Levin, B., Rajkhowa, R., Redmond, S. L., Atlas, M. D., Grafts in myringoplasty: utilizing a silk fibroin scaffold as a novel
device, Expert. Rev. Med. Devic. 2009, 6, 653.
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[6]
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[7]
Ghassemifar, R., Redmond, S., Zainuddin, Chirila, T. V., Advancing towards a tissue-engineered tympanic membrane: silk
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[8]
Levin, B., Redmond, S. L., Rajkhowa, R., Eikelboom, R. H., Marano, R. J., Atlas, M. D., Preliminary results of the
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[9]
Rajkhowa, R., Levin, B., Redmond, S. L., Li, L. H., Wang, L. J., Kanwar, J. R., Atlas, M. D., Wang, X. G., Structure and
properties of biomedical films prepared from aqueous and acidic silk fibroin solutions, J Biomed Mater Res A 2011, 97A, 37.
[10]
Shen, Y., Redmond, S. L., Teh, B. M., Yan, S., Wang, Y., Atlas, M. D., Dilley, R. J., Zheng, M., Marano, R. J., Tympanic
membrane repair using silk fibroin and acellular collagen scaffolds, Laryngoscope 2013, 123, 1976.
[11]
Shen, Y., Redmond, S. L., Teh, B. M., Yan, S., Wang, Y., Zhou, L., Budgeon, C. A., Eikelboom, R. H., Atlas, M. D., Dilley,
R. J., Zheng, M., Marano, R. J., Scaffolds for tympanic membrane regeneration in rats, Tissue Eng Part A 2013, 19, 657.
[12]
Levin, B., Redmond, S. L., Rajkhowa, R., Eikelboom, R. H., Atlas, M. D., Marano, R. J., Utilising silk fibroin membranes as
scaffolds for the growth of tympanic membrane keratinocytes, and application to myringoplasty surgery, J. Laryngol. Otol.
2012, 1.
[13]
Lu, Q., Hu, X., Wang, X., Kluge, J. A., Lu, S., Cebe, P., Kaplan, D. L., Water-insoluble silk films with silk I structure, Acta
Biomater 2010, 6, 1380.
[14]
Um, I. C., Kweon, H. Y., Park, Y. H., Hudson, S., Structural characteristics and properties of the regenerated silk fibroin
prepared from formic acid, Int. J. Biol. Macromol. 2001, 29, 91.
[15]
Bottino, M. C., Jose, M. V., Thomas, V., Dean, D. R., Janowski, G. M., Freeze-dried acellular dermal matrix graft: effects of
rehydration on physical, chemical, and mechanical properties, Dent. Mater. 2009, 25, 1109.
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[16]
Mow, V. C., Guo, X. E., Mechano-electrochemical properties of articular cartilage: their inhomogeneities and anisotropies,
Annu. Rev. Biomed. Eng. 2002, 4, 175.
[17]
Little, C. J., Bawolin, N. K., Chen, X., Mechanical properties of natural cartilage and tissue-engineered constructs, Tissue
Eng Part B Rev 2011, 17, 213.
[18]
Yoder, J. H., Elliott, D. M., Nonlinear and anisotropic tensile properties of graft materials used in soft tissue applications,
Clin. Biomech. 2010, 25, 378.
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