An investigation into silk fibroin conformation in composite materials intended for drug delivery
Graphical abstract
Regenerated silk fibroin/hydrophilic polymers composite materials prepared using mild evaporation conditions can be used to design drug delivery systems since they lead to water stable constructs.
Introduction
Silk fibroin, a naturally occurring protein spun by the silkworm Bombyx mori, is a promising biomaterial for the incorporation and delivery of a range of therapeutic agents due to its unique properties such as slow biodegradation, good mechanical properties, and favourable processability in combination with biocompatibility (Vepari and Kaplan, 2007, Numata and Kaplan, 2010).
Several drug delivery constructs with different morphologies such as films (Hofmann et al., 2006), microspheres (Wang et al., 2007, Wang et al., 2008), nanoparticles (Kundu et al., 2010) and hydrogels (Numata and Kaplan, 2010), have been proposed to control the release of both small molecule drugs and proteins, such as enzymes and cell growth factors.
Natural silkworm silk fibres form predominantly crystalline β-sheet structures leading to stability, long-time degradability and the unique mechanical resistance (Cao and Wang, 2009). However, in order to prepare drug delivery systems, it is necessary to dissolve silk fibres in a solvent capable of denaturing the fibroin by breaking the strong intermolecular hydrogen bonds (Kaplan et al., 1997). Once the silk protein is dissolved, it can be processed into a variety of different material morphologies by different processes. Afterwards, a final curing is often required to enrich regenerated silk fibroin (SF) in β-sheet structure and, thus, induce water insolubility. This particular structure of SF is generally controlled by stretching, compression, annealing and/or chemical treatment in order to tune-up mechanical and degradability properties of films (Minoura et al., 2009). Additionally, the correlation between conformation of SF and drug release was also reported (Hofmann et al., 2006): the higher the crystalline content of SF, the slower the release of the encapsulated proteins. The ability to regulate the structure and morphology of SF in an all-aqueous process and/or mild processing technologies renders this biomaterial an important candidate for drug delivery applications. Furthermore, the use of final curing should be minimized, or even circumvented in order to incorporate efficaciously sensitive biologics and reduce detrimental and/or toxic effects of residual solvents (Sashina et al., 2006).
To overcome these drawbacks, the blending of SF with natural or synthetic polymers (Hardy and Scheibel, 2010) has also considered since SF and the auxiliary material might mutually aggregate to form a new ordered structure, which may guarantee the required mechanical strength as well as water insolubility avoiding post-treatment. In particular, the mechanical properties of regenerated SF fibroin have been improved by mixing with poly(ethylene glycol) 400 (PEG 400) (Wang et al., 2003).
The purpose of the present study is to expand the understanding of the effects of blending SF with other hydrosoluble polymers and drying conditions on the conformation of the dried SF. With this aim, composite materials based on SF and poloxamer (PEO), poly(vinyl pyrrolidone) (PVP) or an homologue series of poly(ethylene glycol) (PEG) were prepared by spray-drying or casting methods. These evaporation conditions were selected in order to evaluate also the effect of the evaporation rate other than the composition of the composite material on the SF conformation.
Section snippets
Materials
Lutrol® F 68 (Poloxamer 188, PEO, Mw = 8600 Da) and Kollidon® K30 (poly(vinyl pyrrolidone), PVP) were kindly gifted by BASF (Germany). Poly(ethylene glycol) 4000 (PEG4000; hydroxyl value = 26–32 mg KOH/g; Mw = 3600–4400 Da) and poly(ethylene glycol) 600 (PEG600; hydroxyl value = 170–208 mg KOH/g; Mw = 540–660 Da) were provided by Polichimica (Italy), poly(ethylene glycol) 1000 (PEG1000; hydroxyl value = 102–125 mg KOH/g; Mw = 900–1100 Da) and poly(ethylene glycol) 1500 (PEG1500; hydroxyl value = 68–83 mg KOH/g; Mw =
Effect of drying conditions
Depending on the processing conditions, regenerated SF can preferentially assume random coil and β-sheet conformation (Hino et al., 2003, Li et al., 2001). ATR-FTIR spectroscopy with Fourier self-deconvolution and DSC analysis are useful tools to investigate and quantify both contributions to the molecular conformations of this protein.
The main differences reside in the position of absorption bands of amide I, amide II and amide III reflecting the different ratio between intra- and
Conclusion
The amino acid composition (in mol%) of B. mori silk showed the predominance of five amino acids: Gly (42.9%), Ala (30.0%), Ser (12.2%), Tyr (4.8%), and Val (2.5%) (Zhou et al., 2000). The primary structure of B. mori silk fibroin may be approximately divided into four regions: (i) highly repetitive GAGAGS sequences constituting the crystalline region, (ii) relatively less repetitive GAGAGY and/or GAGAGVGY sequences comprising semicrystalline regions containing mainly hydrophobic moieties,
Acknowledgment
The Authors would like to thank Dr. G. Vistoli for the valuable discussions on molecular conformation of silk fibroin.
References (19)
- et al.
Composite materials based on silk proteins
Prog. Pol. Sci.
(2010) - et al.
Change in secondary structure of silk fibroin during preparation of its microspheres by spray-drying and exposure to humid atmosphere
J. Colloid Interface Sci.
(2003) - et al.
Silk fibroin as an organic polymer for controlled drug delivery
J. Control. Release
(2006) - et al.
Silk fibroin nanoparticles for cellular uptake and control release
Int. J. Pharm.
(2010) - et al.
Silk-based delivery systems of bioactive molecules
Adv. Drug Del. Rev.
(2010) - et al.
Silk as a biomaterial
Prog. Polym. Sci.
(2007) - et al.
Silk microspheres for encapsulation and controlled release
J. Control. Release
(2007) - et al.
Controlled release from multilayer silk biomaterial coatings to modulate vascular cell responses
Biomaterials
(2008) - et al.
Attenuated total reflection infrared spectroscopy: an efficient technique to quantitatively determine the orientation and conformation of proteins in single silk fibers
App. Spectrosc.
(2008)
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