Structure and dynamics of biodegradable polyurethane-silk fibroin composite materials in the dry and hydrated states studied using ¹³C solid-state NMR spectroscopy

Highlights

• Structure and mobility of hydrated SF-PU fiber were clarified by solid-state NMR.

• Mobility of non-crystalline domain of PU in hydrated SF-PU fiber was increased.

• Crystalline region was destroyed more than non-crystalline region of SF by PU.

Abstract

In order to develop biodegradable silk fibroin (SF) –polyurethane (PU) composite materials, the biodegradable PU based on polycaprolactone (PCL) was synthesized. The structure and dynamics of the crystalline and non-crystalline regions of biodegradable PU and SF – PU composite fiber were studied in the dry and hydrated states using three kinds of ¹³C solid-state NMR, i.e.,¹³C CP/MAS, ¹³C DD/MAS and ¹³C r-INEPT NMR methods. In ¹³C r-INEPT NMR spectra which can observe only highly mobile hydrated components, only small peaks from non-crystalline region of PCL components in PU could be observed, but higher intensity peaks from the non-crystalline region could be observed for PU in the regenerated SF-PU composite fiber. On the other hand, the structure and dynamics of SF in SF-PU fiber was investigated by observing three kinds of ¹³C solid-state NMR spectra of [3-¹³C] Ser, [3-¹³C] Tyr and [3-¹³C] Ala-labeled SF. The fraction of random coil of SF, especially the fraction of hydrated random coil, increased for hydrated SF-PU composite fiber compared with random coil fraction of hydrated pure SF fiber. Thus, the interaction between SF and PU caused increase fraction of non-crystalline region of PCL in the PU and slightly amorphous SF region in the hydrated state.

Introduction

Natural silk fiber produced by Bombyx mori silkworm has long history used as an excellent fabric material in the textile industry [1]. The silk fiber has also been used in sutures in the surgical field for centuries [2]. In recent years, silks and silk-based materials including recombinant silks and transgenic silks have been used as a novel, promising biocompatible and biodegradable biomedical material, and the application of silks has been paid more attention [2], [3], [4], [5], [6], [7], [8], [9], [10].

Since the incidences of cardiovascular diseases have been on the rise in recent years, the need for small-diameter artificial vascular grafts is increasing globally [11]. We have proven that silk fibroin (SF) is suitable for vascular prostheses for small arteries [10,12]. However, the disadvantage of SF fiber graft coated with SF is relatively rigid and therefore, a softer SF graft is expected to accomplish better transplant results by improvement of thrombus formation and/or compliance mismatch [9,13,14]. Therefore, it seems useful to develop a further new biomaterial based on SF by incorporating further flexible polymers with biocompatible and biodegradable properties. Polyurethane (PU) is one of the most popular synthetic polymers which have been used in the biomedical field [15], [16], [17], [18], [19], [20], [21], [22], [23], [24]. The structure and properties of PU can be tailored due to a controlled combination of hard and soft segments. Thus, by mixing a small amount of PU which is suitable for the biomedical applications with SF, it seems possible to develop SF-PU composite materials which are more suitable for small diameter artificial vascular graft. In recent years, polycaprolactone (PCL) and PU based on PCL have been extensively studied for the application in biomaterials [18,[24], [25], [26], [27]]. PCL is a synthetic aliphatic polyester approved by FDA, and has biodegradable and biocompatible properties with very low glass transition temperature. The solid-state structure and dynamics of PCL was studied using X-ray diffraction [28] and electron diffraction methods [29] together with solid-state NMR methods [30], [31], [32]. The molecular chains are arranged side by side in the same way as in polyethylene, but the C=O groups of the two chains in the unit cell are separated along the fiber axis [28,29]. The heterogeneous structure which consists of crystalline and non-crystalline regions was clarified together with the detailed crystal structure and their dynamics [30,31,32]. In addition, the electro-spun SF/PCL nanofibers have been prepared and evaluated as scaffolds for tissue engineering applications [13,14,33,34]. Thus, PCL is considered to be leading candidate for soft segment of PU in SF-PU composite polymer which can be used for small diameter artificial vascular graft.

Such biomaterials have generally been used in the hydrated state and therefore, it seems important to develop such materials while paying attention to the behavior in the hydrated state as well as in the dry state. Actually, it is well-known that the structure and dynamics of SF molecules change remarkably in the presence of water [2,35]. However, the analytical method to characterize the structure and dynamics of materials in the hydrated state at molecular level is limited [35]. We have shown that the combination of three kinds of solid-state nuclear magnetic resonance (NMR) methods, i.e., ¹³C refocused insensitive nuclei enhanced by polarization transfer (r-INEPT), ¹³C cross polarization/magic angle spinning (CP/MAS) NMR, and ¹³C dipolar decoupled-magic angle spinning (DD/MAS) NMR is promising for the purpose [36], [37], [38], [39], [40], [41], [42]. The ¹³C r-INEPT is sensitive to the components with fast motion in the hydrated materials. In contrast, ¹³C CP/MAS NMR can be used to study only the components with slow motion. The ¹³C DD/MAS NMR can be used to detect the components with both fast and slow motions. Thus, the combination of the three ¹³C NMR techniques, ¹³C r-INEPT, ¹³C CP/MAS and ¹³C DD/MAS NMR has provided different perspectives about the structural and dynamical behaviors of the PU and SF-PU composite materials.

In the present study, new flexible PU polymers based on PCL were synthesized and the biodegradable characters were studied [17], [18], [19], [20], [21], [22], [23]. Then the regenerated SF-biodegradable PU composite fiber was prepared by wet spinning using a spinning device. Because there are no reports to characterize the structure and dynamics of PU and SF-PU composite fibers in both the dry and hydrated states in detail, three kinds of ¹³C solid-state NMR methods, ¹³C CP/MAS, ¹³C DD/MAS and ¹³C r-INEPT NMR were used for characterization of the structure and dynamics [36], [37], [38], [39], [40], [41], [42]. In addition, [3-¹³C] Ser, [3-¹³C] Tyr and [3-¹³C] Ala-labeled SF was also used here because the ¹³C selectively labeled SF has been successfully used for the detailed ¹³C solid-state NMR analyses of SF in the previous papers [36,38,39,43,44,45,46]. Changes in the structures and dynamics of the PU and SF induced by the formation of SF-PU composite fibers were clarified in the hydrated state through these solid-state NMR studies. These research methods are unique and provide a lot of new information about the structure and dynamics of SF-PU composite materials.