A Review: Current Status and Emerging Developments on Natural Polymer-Based Electrospun Fibers.

In recent years, natural polymer-based electrospun fibers (EFs) with huge specific surface area, good biocompatibility, and biological activity obtained from electrospinning process exhibit tremendous vitality in the field of biomedical areas. Herein, the parameters of electrospinning from two perspectives, polymer solution such as solvent, polymeric relative molecular mass, concentration, viscosity, and conductivity of the solution, and electrospinning process such as spinning voltage, spinning flow rate, needle tip to collector distance, temperature, and humidity are first detailed. Next, the raw materials consisting of polysaccharides such as cellulose, hyaluronic acid, alginate, and chitosan as well as proteins such as collagen, gelatin, silk fibroin, and keratin are summarized. The preparation method and related characteristics of EFs with multistage structures such as porous, core-shell, Janus, bamboo-like and other structures are introduced. The biomedical applications of these natural polymer-based EFs mainly including tissue engineering, drug sustained release, wound dressings, and biomedical sensors are systematically recapitulated. Finally, the outlook on natural EFs is further proposed.

Enhancement of PLA crystallization, transparency, and strength by adding the long aliphatic chains grafted CNC.

The combination of crystallization, transparency, and strength is still a challenge for broadening the application of polylactic acid (PLA) films, while it is also difficult to balance. In this work, the long aliphatic chains of octadecylamine (ODA) were grafted onto the surface of cellulose nanocrystal (CNC) by tannic acid oxidation self-polymerization and Michael addition/Schiff base reaction between polytannic acid and ODA. Furthermore, the ODA grafted CNC (g-CNC) was used as green reinforcement for the PLA matrix and a series of PLA/g-CNC nanocomposite films were prepared by the casting method. The DSC, WAXD, POM, UV-vis and stretching test were employed to examine the effect of g-CNC on the properties of the as-prepared PLA/g-CNC nanocomposite films. It shows that the g-CNC is effective to improve the melt crystallization rate of PLA from 11 min to 7.3 min. Most importantly, the crystal size of the PLA spherulites was significantly reduced due to the well dispersion in the amorphous PLA matrix, which would effectively improve the transmittance of the PLA films and synchronously realize the combination of crystallization (62 %) and transparency (80.6 %). Moreover, the improved crystallization could also enhance the heat deformation performance of the PLA films since the heat resistance is closely associated with the crystallinity. Besides, the grafted ODA long chains improve the compatibility between CNC and PLA, leading to the reinforcement of PLA matrix, where the tensile strength reaches 65.05 MPa from 44.31 MPa. Compared with the pristine CNC, the addition of g-CNC makes more comprehensive improvement in the properties of the PLA films.

A feasible strategy to balance the performance of stereo-complexed polylactide by incorporating poly(butylene adipate-co-terephthalate).

The raw material of polylactide (PLA) is lactic acid obtained by biological fermentation. PLA is the most promising degradable polymer to replace traditional plastics to address the pollution problems caused by their non-degradability. However, the application of PLA is hindered by its low softening temperature, easy hydrolysis, and poor toughness. Herein, the ternary composites with PLLA, PDLA and Poly (butylene adipate-co-terephthalate) (PBAT) were prepared by melt blending to balance its thermal stability, hydrolysis, and toughness. The effects of PBAT content (3 %, 6 %, 9 % and 12 %) and isothermal crystallization temperature on composite properties were fully investigated. The results show that the composite of stereo-complexed PLA (sc-PLA) with 6 % PBAT crystallized at 110 °C exhibits good comprehensive properties. Its vicat softening temperature (VST), mass loss rate under alkaline (pH = 12) and breaking elongation are 166 °C, 21.6 % and 4.40 %, respectively. Compared with the pure PLLA sample crystallized at same condition, the VST, mass loss rate and breaking elongation are 159 °C, 24.7 % and 3.76 % respectively, which increased by nearly 5 %, 13 % and 20 %. This indicates that this strategy is feasible to balance the heat resistance, hydrolysis resistance and toughness of PLA, while it sacrifices the tensile strength a little. This work provides a new way to modify and improve the PLA properties. Nonetheless, it is also necessary to coordinate the compatibility of PLA and PBAT.

Properties of poly (l-lactic acid) reinforced by l-lactic acid grafted nanocellulose crystal.

In this research, nanocellulose crystal (NCC) grafted with lactic acid (LA) oligomer was synthesized by one-pot method and used to reinforce the poly(l-lactic acid) (PLLA) matrix. FT-IR, XRD, and 13C NMR were used to analyze the structure of modified nanocellulose crystal (g-NCC). The results of GPC suggested that the degree of polymerization of the grafted segments was 23, and the degree of hydroxyl substitution of g-NCC was 2.4%. For the g-NCC/PLLA composite, its crystallization rate increases significantly compared with pure PLLA, indicating that g-NCC acted as a nucleating agent to promote the crystallization. Moreover, tensile strength of the composite materials was significantly improved from 41.9 MPa to 53.9 MPa due to the enhanced compatibility. This study provides a fast and effective modification method for g-NCC/PLLA biodegradable composites. According to the above-mentioned experimental results, the g-NCC/PLLA composites can be considered as a potential material in the packaging field, mainly due to its proper biological and physicochemical properties.

Chitosan/gallnut tannins composite fiber with improved tensile, antibacterial and fluorescence properties.

Natural extracts gallnut tannins (GTs) were used as functional components to prepare chitosan/gallnut tannins (CS/GTs) composite fiber by blended solution spinning. Chitosan fiber has great potential to be used as absorbent suture and dressing due to its good biocompatibility. However, the weak mechanical properties limited its application. Chitosan and GTs were blended in aqueous solution of acetic acid to spin the composite fiber. The results indicated that CS/GTs fiber can be easily prepared due to the appropriate rheology characteristics for blended solution. Compared with pure chitosan fiber, CS/GTs fiber with 10% GTs showed lower hydrophilicity and higher dry, wet breaking strength by more than 40% due to ionic cross-linking between chitosan and GTs. The bacterial reduction to Staphylococcus aureus increased from 49.0 to 99.7% and about double green and red fluorescent intensity were observed for CS/GTs fiber. GTs have great potentiality in improving the properties of chitosan fiber.

"Near perfect" amphiphilic conetwork based on end-group cross-linking of polydimethylsiloxane triblock copolymer via atom transfer radical polymerization.

Novel amphiphilic conetworks (APCNs) with uniform channel size were synthesized through end-cross-linking of well-defined amphiphilic triblock copolymers via atom transfer radical polymerization (ATRP). A new ditelechelic polydimethylsiloxane macroinitiator was synthesized to initiate the polymerization of N,N-dimethylacrylamide. The resulting triblock copolymers show well-defined molecular weight with narrow polydisperisty, which are telechelic modified by allylamine and fully cross-linked with polyhydrosiloxanes through hydrosilylation. Transmission electron microscopy shows that the APCN has the behavior of microphase separation with small channel size and uniform phase domain. The resulting APCNs with idealized microstructure exhibit a combination of excellent properties, i.e., superhigh mechanical strength (4 ± 1 MPa) and elongation ratio (175 ± 25%), outstanding oxygen permeability (350 ± 150 barrers), a high water uptake property, and excellent biocompatibility, indicating that in this way, "near perfect" networks are obtained. These results are better than those reported in the literature, suggesting a promising semipermeable barrier for islet encapsulation in relative biomaterial fields.

Structure and properties of chitin whisker reinforced chitosan membranes.

In this paper, rod-like chitin whisker was used as a filler to reinforce the chitosan membrane, and a series of composite membranes were prepared by casting-evaporation method. Mechanical testing shows that tensile strength of the resulting composite membrane with 3 wt% chitin whisker content reaches up to 110.3 MPa, which is about 2.8 times than that of neat chitosan membrane (38.5 MPa), and moisture regain of all composite membranes presents a decreasing tendency with increasing content of chitin whisker. Furthermore, SEM was used to investigate the morphology difference between neat chitosan membrane and composite membranes, to understand the reinforce mechanism of chitin whisker. Wide angle x-ray diffraction and Fourier transform infrared spectroscopy were used to visualize the structure change before and after the compositing processes. Besides, the bacterostatic test shows that this composite membrane presents effective inhibitory effect on Staphylococcus aureus, Escherchia coli and Corinebaterium michiganence respectively, which indicates it a promising material for packaging and wound dressing.

A comparative study on the chitosan membranes prepared from glycine hydrochloride and acetic acid.

In this study, glycine hydrochloride (Gly·HCl) is confirmed to be a promising solvent for dissolving native chitosan and preparing regenerated chitosan membrane. As compared with the chitosan membrane prepared from traditional acetic acid, the membrane prepared from Gly·HCl by dry technique shows excellent tensile strength and initial modulus, i.e. 103.8MPa and 3.2GPa, respectively, which is superior to any chitosan membrane and most chitosan blend membranes reported in literatures. Besides, scanning electron microscopy (SEM), wide-angle X-ray diffraction (WAXD) and Fourier transform infrared spectroscopy (FT-IR) were used to visualize the difference between the two kind of regenerated chitosan membranes. The SEM results show that the membrane prepared from Gly·HCl by dry technique presents a novel structure, which ensures its high tenacity. Furthermore, the chitosan microporous membranes were also prepared using PEG as porogen.