Bioactive Silk Proteins as Geotextile Substrates

Bioactive Silk Proteins as Geotextile Substrates

Masuhiro Tsukada¹, Shafiul Islam² and Yoshio Ishiguro³

 

1)    National Institute of Sericultural & Agrobiological Sciences, Tsukuba, Ibaraki 305-8634 Japan

2)    TexTek Solutions, Ontario, Canada.   textek.weebly.com    textek@gmail.com

3)   Center for Food Quality, Labeling & Consumer Services, Nakadori, Yokohama 231-0003 Japan

 

Dedicated to Prof. Dr. Sueo Kawabata, CText FTI (1931-2001)

 

Abstract

There is a wave of interest in using natural textile substrates in diverse bioengineering applications^1-9 such as soil erosion and sedimentation control designs especially in environmentally sensitive areas. This research article evaluates the tensile properties and morphological features of silk fibers as a function of degree of biodegradation, burring time in the soil.

Introduction

Natural biopolymers including silk proteins seem critical geotextile materials as microorganisms decompose them gradually.  Silk fibers provide superior performance as a bioactive surgical suture even under the most demanding medical conditions.  Silk fibers are conducive to living cells growth that attached on the surface of silk fibers.

We studied the in vitro biodegradation of silk fibers and films with proteolytic enzymes and examined the tensile properties of silk fibers with increasing incubation time¹.  We used silk protein fibers considering their potential geotextile applications, examined their tensile properties and morphological features as biodegradable geotextile substrates.

Experimental

Materials

Silk fibers² with different chemical compositions are obtained from domestic silkworm, Bombyx mori and wild silkworm, Antheraea pernyi. Degummed silk fibers obtained by reeling the cocoons fibers were buried in the horticultural gardening soil (Kompal, Bark Compost Association, Tokyo) up to 3 months.  The temperature was maintained at around 24-30 °C throughout the experiment under moist condition by sprinkling water every 2-5 days to enhance biodegradation process.

Measurements

The tensile properties of silk fibers were evaluated by Tensilon UTM-II (Orientec, Tokyo) under standard conditions with 500 gf load-cell, 100 mm gauge length and 40 mm/min strain rate.   The surface morphology was examined with a JOEL JAX-333S scanning electron microscope (SEM), after gold coating at 15 keV acceleration voltage.

Results and discussions

Tensile properties

Table 1 compares the tensile properties of silk fibers, which were buried in soil for predetermined periods.  The tensile strength, elongation and moduli of Bombyx mori and Antheraea pernyi silk fiber buried in the ground decreases with increasing burring times.  It is obvious that the tensile strength of Antheraea pernyi Tussah silk fibers deteriorates more rapidly than Bombyx mori silk fiber, when they are buried in the soil.  

Table 1: Biodegradation of silk fibers under the soil
	Bombyx mori silk			Tussah silk
Tensile Parameters	Embedding Period (months)
	0	1	2	0	1	2
Strength (gf)	3.5	1.8	1	3.6	1	0.3
Elongation (%)	15.5	7	5.2	40.1	29	21
Modulus (kg/mm2)	850	760	380	730	322	205

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Morphology

10 μm	20 μm
Fig. 1 Biodegradation of Bombyx mori silk fibre	Fig. 2 Biodegradation: Antheraea Pernyi silk fibre

Figure 1 and 2 shows the Scanning Electron Microscopy SEM images: biodegradation features of Bombyx mori and Antheraea Pernyi silk fibers, respectively, which were buried under the soil for 2 months.  Silk fibers prior to decomposition tests were apparently smooth.  It is apparent that the microorganisms eroded the Antheraea pernyi fiber surface and created numerous voids, while less voids are distinct on the Bombyx mori silk fiber probably due to their ordered dense microstructures.

Conclusion

The microorganisms in the soil do not practically decompose artificial polymers, such as polystyrene, polyethylene, polypropylene and polyester.  This creates enormous environmental concerns.  The biodegradation of protein polymers offers significant advantages from additional recycling and decomposition. Silk’s excellent inherent tensile properties and subsequent biodegradability makes them unique geotextile materials.

The potential grafting, coating and engineering scopes to tailor specialized functional features to bioactive natural protein polymers have created a wave of interest in demanding applications^1-9.  

At the outset silk protein fibers exhibit excellent tensile properties, their inherent properties decrease as a function of burring periods due to their time dependent degradation by various   microorganisms in the burring soil.  The decay in tensile properties of tussah silk fibers was higher than that of Bombyx mori silk, which matched the finding - tussah silk fiber contained highly porous microstructure, such as visual voids⁴ with large size and leaf-pleated sheet structure within the silk fiber.  This leads to rapid degradation of tussah silk fiber following decomposition by various microorganisms contrary to Bombyx mori silk fibers with highly consolidated microstructure, yielded a slow rate of biodegradation than that of tussah silk fibers.

Silk fibers are fairly expensive compared to other natural fibers, cotton and cellulose and as well as commonly used artificial man made fibers. We can however cut costs by using silk fibrous by-products. Silk fibers exhibit excellent elastic modulus and high absorbency5. Since living cells grow cohesively on the surface of silk substrates - these biophysical properties are critical for certain geotextile applications.

References

1. T Arai, G Freddi, R Innocenti, M Tsukada, Biodegradation of Bombyx mori Silk Fibroin Fibers and Films, J. of Appl. Poly. Sci., 91, 2383-2390, 2004

2. M Tsukada et al., Structural Changes of Silk Fibers Induced by Heat-Treatment, J.of Appl. Poly. Sci., 46, 1945-1953, 1992

3. S Islam, Synthetic Silk Synthesis, Technitex Symposium, BTTG, Leeds, UK 6-7 Nov, 2002

4. T Narumi, M. Kobayashi, A Method of Morphometry for Voids in Saturniidae Cocoon Filaments Using Image Processing Techniques, J. Seri. Sci. of Japan, 64, 203-208, 1995

5. S Islam, et al., Methods and apparatus for spinning spider silk protein, USP 7,057,023 Jun 6, 2006

6. M Tsukada, S Islam et al., Antibiotic Silk Substrates for Healthcare, Text. J. 121(5) 47-49, 2004

7. C Karatzas, S Islam et al., High-toughness Spider Silk Fibers Spun from Soluble rc-Silk Produced in Mammalian Cells, Biopolymers, Vol. 8: Polyamides and Complex Proteinaceous Materials II: Chapter 5, ISBN 3-527-30223-9, Wiley-VCH: Weinheim Germany. Apr 97-117, 2003

8. M Tsukada, S Islam et al., Microwave Irradiation Technique to Enhance Protein Fiber Properties, Autex Res. J. 5(1) 40-48, 2005

9. M Tsukada, S Islam et al., Enhancing Sorption Properties of Natural Fibrous Protein Substrates.  Part I: Absorption of malodorous gases, Text. J., 121(6) 48-50, 2004

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