Abstract
Several studies have stated that the process used for sericin removal, or degumming, from silk cocoons has a strong impact in the silk fibroin integrity and consequently in their mechanical or biochemical properties after processing it into several biomaterials (e.g. fibers, films or scaffolds) but still, there is a lack of information of the impact on the features of silk nanoparticles. In this work, silk cocoons were degummed following four standard methods: autoclaving, short alkaline (Na2CO3) boiling, long alkaline (Na2CO3) boiling and ultrasounds. The resultant silk fibroin fibers were dissolved in the ionic liquid 1-ethyl-3-methylimidazolium acetate and used for nanoparticle synthesis by rapid desolvation in polar organic solvents. The relative efficiencies of the degumming processes and the integrity of the resulting fibroin fibers obtained were analyzed by mass loss, optical microscopy, thermogravimetric analysis, infrared spectroscopy and SDS-PAGE. Particle sizes and morphology were analyzed by Dynamic Light Scattering and Field Emission Scanning Electronic Microscopy. The results showed that the different treatments had a remarkable impact on the integrity of the silk fibroin chains, as confirmed by gel electrophoresis, which can be correlated with particle mean size and size distribution changes. The smallest nanoparticles (156 ± 3 nm) and the most negative Z potential (−30.2 ± 1.8 mV) were obtained with the combination of long treatment (2 h) of boiling in alkaline solution (Na2CO3 0.02 eq/L). The study confirms that parameters of the process, such as composition of the solution and time of the degumming step, must be controlled in order to reach an optimum reproducibility of the nanoparticle production.
Highlights
For most medical applications of silk biomaterials [1], silk fibroin (SF) must be efficiently purified from silk cocoons (SC) by removing the silk sericin (SS) by means of the process known as degumming, and this key step is commonly recognized as one of the most important procedures in the production of silk-based materials [2]
The main aim of this work was to study the effects of four degumming methods, namely autoclaving, short, and intensive alkaline boiling (30 and 120 min, respectively) and high-power ultrasounds on SF integrity and, on the formed SF nanoparticles (SFN)
The integrity of SF was analyzed by gel electrophoresis, whereby smaller molecular weight distributions were interpreted as reflecting a higher degree of degradation
Summary
For most medical applications of silk biomaterials [1], silk fibroin (SF) must be efficiently purified from silk cocoons (SC) by removing the silk sericin (SS) by means of the process known as degumming, and this key step is commonly recognized as one of the most important procedures in the production of silk-based materials [2]. This purified SF can be dissolved in solutions, such as LiBr. 9.3 mol/L [3] or Ajisawa’s reagent (CaCl2 /EtOH/H2 O, 1:2:8 in molar ratio) [4], the most commonly used, and in some ionic liquids, such as 1-butyl-3-methylimidazolium chloride ([bmim+ ][Cl− ]) [5,6] or Polymers 2019, 11, 2045; doi:10.3390/polym11122045 www.mdpi.com/journal/polymers. The ionic liquid-SF solutions can be precipitated directly into nanoparticles [6,7] avoiding the dialysis step and giving more stable SF solutions
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