Morphology, secondary structure and thermal properties of silk fibroin/gelatin blend films: case study of cross-linked agent (PEGDE).
There are many reports about advantages of the natural polymers for various applications. This was due to their excellent properties on health and friendly environment. Silk is a kind of natural polymer which produced by 2 main types: family Bombycidae (domestic silk; Bombyx mori) and Saturniidae (wild silk; Antheraeapernyi, Philosamia ricini, etc.) of the order Lepidoptera . Domesticated silk was wildly studied and applied for the textile and biomedical devices , especially silk fibroin (SF). This was according to its excellent both physical and chemical properties. With previously reported, SF was used in various applications such as cosmetics, food additives and medical materials [3-7]. By another reason, the SF can be prepared into many forms depending on applications including gel , sponge , mat , film , powder or membrane .
Even natural polymers are attractive to used, some desirable properties do not consist in only one type of polymer. The efforts of study and development the biomaterial by blending or co-forming with bioactive substances are the aims in the last decade. SF was blended with other natural materials such as cellulose , chitosan , calcium alginate , gelatin  to adjust their properties and shapes.
Gelatin (G) is a derivative biopolymer of collagen. It is mostly found in many organs of animal tissues such as skin, muscle and bone . The structure of gelatin composed of bulky amino acids like arginine and aspartic acid with peptide linked  result it is polar molecule. Gelatin composed of excellent properties such as biocompatibility , high strength  and solubility , which make it suitable for medical applications.
Many research attempted to develop SF and G into various forms aimed to specifically use. In this work, the SF/G blend films with different ratios of poly(ethylene glycol) diglycidyl ether (PEGDE) were prepared by evaporation method. The morphology, secondary structures and thermal properties of all blend films and native SF and G films were investigated by scanning electron microscope (SEM), Fourier transform infrared (FTIR), thermogravimetric analyzer, respectively.
Materials and Methods
The Thai silk Nang Lai (B. mori) cocoons were kindly supplied by Silk Innovation Center (SIC), Mahasarakham University, Thailand. The cocoons were degummed twice using 0.5% [Na.sub.2]C[O.sub.3] (w/v) and thoroughly rinsed 2 times in warm distilled water. They were then air-dried at room temperature. Gelatin powder was purchased from Sigma (Singapore). All of chemical used were analytical grade.
Preparation of silk fibroin
The SF fibers were dissolved with tertiary system of Ca[Cl.sub.2]: Ethanol: [H.sub.2]O [1:2:8 by mol]. Briefly, dried SF was mixed with dissolving solution of 1g SF to 10 mL of solution. The solution was firstly warmed to about 90 [degrees]C on the hot plate, then gradually added SF into the solution with magnetic stirred at 90-95 [degrees]C until SF completely dissolved. The SF hydrolysate was filtrated, and then dialyzed in cellulose tube (MC = 7 kDa) against distilled water for 3 days at room temperature. The obtained SF solution was adjusted to 1% weight by distilled water.
Preparation of gelatin solution
The gelatin solution was prepared at 1% wt. by weighing of gelatin powder 1g, and then added distilled water until reach to 100 mL. The mixture was placed on the stirring hotplate with stirred for 30 min at room temperature until gelatin powder completely dissolved.
Preparations of different films
The SF, G and SF/G blend films were prepared. The volume of 20 mL of SF or G solution was poured on 5cm polystyrene plates before dehydration in the oven at 40 [degrees]C. For the SF/G blend films, each 10 mL of SF and G solution was firstly mixed. PEGDE cross-linked agent in the different percentages range from 5 to 40% were then added into the mixture and continuously stirred. The homogeneously mixed solution was poured on the polystyrene plates and dehydrated in the oven as the same condition above. The films were obtained after dehydration for 3 days.
Characterization of different films
All of films were observed under SEM (JEOL, JSM 6460LV, Japan) for their morphology. The films were cut into small pieces, coated with gold for enhancing the surface conductivity before observation.
The secondary structures of the films were analyzed with FTIR (Perkin Elmer-Spectrum Gx, USA) in the spectral range of 2000-400 [cm.sup.-1] at 4 [cm.sup.-1] spectral resolution and 32 scans.
Thermal properties were measured using TA instruments, SDT Q600 (Luken's drive, New Castle, DE). The SF weight of 8-10 mg were prepared and loaded in a platinum crucible. The samples were non-isothermal heated from 50[degrees]C to 600 [degrees]C at a heating rate of 20[degrees]C/min. The TGA was carried out in nitrogen with the flow rate of 100 mL/min. The TG and heat flow were recorded with TA Instrument's Q series explorer software. The analyses of the data were done using TA Instrument's Universal Analysis 2000 software (version 3.3B).
The SEM images of native SF (Figure 1a) and G (Figure 1g) showed slightly rougher of surface area than that of all SF/G blend films (Figures 1b-f). With cross section, the blend films indicated homogeneous surface without phase separation (Figure 1b-right column). In case of SF/G with different PEGED ratios, the results found that the blend films gradually increased of smooth surfaces when the percentages of PEGDE increase (Figure 1e-f), except at 10% ratio (Figures 1c).
[FIGURE 1 OMITTED]
The secondary structures of all film can be implied from FTIR spectra. Native SF films showed absorption regions at 1690, 1661, 1632 [cm.sup.-1] and 1602 [cm.sup.-1] (amide I), 1582, 1539 and 1509 [cm.sup.-1] (amide II), and 1250 [cm.sup.-1] (amide III) (Figure 2a). G film showed intense absorption bands at 1690, 1664, 1632 and 1602 [cm.sup.-1], 1571, 1539 [cm.sup.-1], and 1245 [cm.sup.-1] (Figure 2g). The SF/G blend film without cross-linked agent (Figure 2b) and all blend films with PEGDE showed similar absorption bands (Figure 2c-f). However, new absorption bands were appeared at 1602 and 1508 [cm.sup.-1] listed in Table 1.
[FIGURE 2 OMITTED]
The thermogravimetric (TG) curves showed that native SF film remained its weight in higher than that of other films until the temperature at 300 [degrees]C, then rapidly decomposed (Figure 3). The detail of those decomposition regions were clearly described by differential thermogravimetric (DTG) curves (Figure 4). The maximum decomposition temperatures of native SF films was 311 [degrees]C, native G film was 335 [degrees]C and SF/G without PEDGE was 326 and 324 [degrees]C which were characteristic peaks of SF and G, respectively. The SF/G blend films with PEDGE showed many peaks than that of native films (Table 2). With heat flow curves (Figure 5), the endo/exo-thermic temperatures of native SF films were higher than native G film. For SF/G blend films, endo/exo-thermic peaks were gradually increased when the percentage of cross-linked agent increased (Table 2).
[FIGURE 3 OMITTED]
[FIGURE 4 OMITTED]
[FIGURE 5 OMITTED]
In recent year, natural like SF and G are interested for using in many fields, especially biomedical applications according to their excellent properties [4-6, 21]. Both are protein, however, they are different types of amino acids composition which might be influenced to differences of morphology, secondary structures, and thermal properties. The previous works indicated that film preparation need to be added some cross-linked agent. With cross-linked agent (PEGDE) on the SF/G blend films indicated that SF/G blend films with PEGDE have homogeneous surfaces and smoother than SF/G blend film without cross-linked agent or even both native films. When the ratio of PEGDE increased, the blend films gradually increased of surfaces area, except at 10% ratio. The result suggested that this ratio did not suitable to help in bond formation between SF and G. The FTIR spectra illustrated that SF/G blend films showed individually characteristics of both SF and G. The absorption bands indicated the co-existed of [alpha]-helix and [beta]-sheet structures with predominantly [beta]-sheet form [15, 22] with predominant of the later structure. Adding PEGDE helped to enhance strength of the SF/G blend films which was indicated by the high number of hydrogen bonds formation between carbonyl groups of SF and amino groups of G . This concludes was clearly indicated by the thermal results. In case of the blend film with 10% PEGDE, the highest ratio of [beta]-sheet structures was observed and affected to rough surfaces of the film [24-26].
SF and G blend films could be prepared. The good properties such as texture and chemical structure of the films were generated by interaction between the polymers. Adding PEGDE cross-linked agent helped to improve morphology, secondary structure and thermal properties. However, different ratios of the PEGDE affected on the blend films in different profiles. Almost SF/G blends films composed of [beta]-sheet structures in the higher ratio when added the cross-linked agent. In addition, increased of the PEGDE content influenced to increase of the decomposition temperatures of the blend films.
The authors gratefully thank Mahasarakham University Development Fund, Mahasarakham University for financial support. Finally, we also thank Department of Chemistry, Faculty of Science, Mahasarakham University, and the Center of Excellence for Innovation in Chemistry (PERCH-CIC), Commission on Higher Education, Ministry of Education, Thailand to support this work.
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Tessanan Wasan and Srihanam Prasong *
Department of Chemistry and Center of Excellence for Innovation in Chemistry, Faculty of Science, Mahasarakham University, 44150 Thailand
* Corresponding Author E-mail: email@example.com
Table 1: Absorption bands of SF, G and SF/G blend composed of different ratios of PEGDE. SF/G % Wave number PEGDE ([cm.sup.-1]) Amide I Amide II Amide III 3/0 - 1690 1661 1632 1582 1539 1509 1250 2/1 - 1692 1664 1631 1602 1571 1539 1508 1245 2/1 5 1694 1664 1632 1602 1572 1538 1509 1243 2/1 10 1695 1661 1635 1602 1584 1546 1508 1276 1233 2/1 20 1692 1664 1631 1604 1567 1539 1508 1243 2/1 40 1694 1664 1631 1601 1570 1537 1504 1245 0/3 - 1690 1664 1632 1602 1571 1539 1245 Table 2: Decomposition temperatures of native SF, G and SF/G with PEDGE. SF/G % cross- Decomposition temperature linked ([degrees]C) 3/0 - 311 2/1 - 289 301 322 2/1 5 280 302 322 2/1 10 292 311 329 2/1 20 290 312 323 339 2/1 40 301 322 332 353 0/3 - 335 Table 3: Multiple Exo/Endo-thermic peaks of native SF, G and SF/G with PEDGE. Film Types Exothermic Endothermic ([degrees]C) ([degrees]C) SF 182 258 405 76 212 319 G 212 372 101 310 SF/G 177 262 392 82 299 320 SF/G + 5% 181 399 88 299 320 SF/G + 10% 182 400 89 251 299 321 SF/G + 20% 186 262 384 496 77 223 337 463 SF/G + 40% 197 360 77 253 298 320
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|Author:||Wasan, Tessanan; Prasong, Srihanam|
|Publication:||International Journal of Applied Chemistry|
|Date:||Jan 1, 2011|
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