J.A. Méndez

Oxidized dextrins as alternative crosslinking agents for polysaccharides: application to hydrogels of agarose-chitosan.

Hydrogel networks that combine suitable physical and biomechanical characteristics for tissue engineering scaffolds are in demand. The aim of this work was the development of hydrogel networks based on agarose and chitosan using oxidized dextrins as low cytotoxicity crosslinking agents, paying special attention to the study of the influence of the polysaccharide composition and oxidation degree of the dextrins in the final characteristics of the network. The results show that the formation of an interpenetrating or a semi-interpenetrating polymer network was mainly dependent on a minimum agarose content and degree of oxidation of dextrin. Spectroscopic, thermal and swelling analysis revealed good compatibility with an absence of phase separation of polysaccharides at agarose:chitosan proportions of 50:50 and 25:75. The analysis of atomic force microscopy images showed the formation of a fibrillar microstructure whose distribution within the crosslinked chitosan depended mainly on the crosslinker. All materials exhibited the viscoelastic behaviour typical of gels, with a constant storage modulus independent of frequency for all compositions. The stiffness was strongly influenced by the degree of oxidation of the crosslinker. Cellular response to the hydrogels was studied with cells of different strains, and cell adhesion and proliferation was correlated with the homogeneity of the samples and their elastic properties. Some hydrogel formulations seemed to be candidates for tissue engineering applications such as wound healing or soft tissue regeneration.

Bioresorbable and Nonresorbable Polymers for Bone Tissue Engineering Jordi Girones

In recent years, bone tissue engineering has emerged as one of the main research areas in the field of regenerative biomedicine. Frequency and relevance age-related diseases, such as healing and regeneration of bone tissues, are rising due to increasing life expectancy. Even though bone tissue has excellent self-regeneration ability, when bone defects exceed a critical size, impaired bone formation can occur and surgical intervention becomes mandatory. Bone tissue engineering represents an alternative approach to conventional bone transplants. The main aim of tissue engineering is to repair, regenerate or reconstruct damaged or degenerative tissue. This review presents an overview on the main materials, techniques and strategies in the field of bone tissue engineering. Whilst presenting some reviews recently published that deepen on each of the sections of the paper, this review article aims to present some of the most relevant advances, both in terms of new materials and strategies, currently being developed for bone repair and regeneration.

Evaluation of the influence of the addition of biodegradable polymer matrices in the formulation of self-curing polymer systems for biomedical purposes.

The solid phase of self-curing formulations of poly(methyl methacrylate) was modified by different biodegradable polymer matrices, such as poly(l-lactic acid), poly(beta-hydroxybutyrate) and thermoplastic starches (TPSs). The aim of this modification was the acquisition of a short- to medium-term drug delivery system to release bisphosphonates for hard tissue treatment. Different physico-chemical characterization techniques were used in order to determine the influence of these matrices and their mechanical capacity, in vitro behaviour, curing parameters, residual monomer content and surface topography for the preparation of the self-curing formulations. The incorporation of the polyesters did not induce an increase in water uptake capacity of the system due to their apolar aliphatic character. On the other hand, TPSs exhibited values of water absorption up to 15.3%, related with their hydrophilic chemical structure, dependent on the commercial formulation and the particle size distribution of the powder. The modifications of the solid phase led in all cases to a decrease in the mechanical behaviour of the material, although the formulations modified with TPS were in the range of accepted values according to standard specifications. The immersion of TPS formulations in a simulated physiological environment (phosphate buffer solution, pH 7.4, 37 degrees C) conducted to a surface porosity related with release of plasticizers of the domains of the biodegradable component of the formulation. Finally drug release capacity was studied by loading the material with Ibandronate, observing high dependence with the kind of TPS added, as well as its particle size.

Evaluation of Thermal and Thermomechanical Behaviour of Bio-Based Polyamide 11 Based Composites Reinforced with Lignocellulosic Fibres

In this work, polyamide 11 (PA11) and stone ground wood fibres (SGW) were used, as an alternative to non-bio-based polymer matrices and reinforcements, to obtain short fibre reinforced composites. The impact of the reinforcement on the thermal degradation, thermal transitions and microstructure of PA11-based composites were studied. Natural fibres have lower degradation temperatures than PA11, thus, composites showed lower onset degradation temperatures than PA11, as well. The thermal transition and the semi-crystalline structure of the composites were similar to PA11. On the other hand, when SGW was submitted to an annealing treatment, the composites prepared with these fibres increased its crystallinity, with increasing fibre contents, compared to PA11. The differences between the glass transition temperatures of annealed and untreated composites decreased with the fibre contents. Thus, the fibres had a higher impact in the composites mechanical behaviour than on the mobility of the amorphous phase. The crystalline structure of PA11 and PA11-SGW composites, after annealing, was transformed to α’ more stable phase, without any negative impact on the properties of the fibres.

Combined effect of carbon nanotubes and polypyrrole on the electrical properties of cellulose-nanopaper

In the present study, 2,2,6,6,-tetramethylpiperidine-1-oxyl (TEMPO)-oxidized cellulose nanofibers (CNF) were combined with multi-walled carbon nanotubes (MWCNTs) and with hybrid MWCNT/polypyrrole to produce a variety of binary and ternary formulations of conductive nanopapers. By following a simple mixing/sonication/filtering process, a homogeneous and well-distributed CNF-MWCNT nanostructure was formed, resulting in a nanopaper of strong mechanical properties (141xa0MPa tensile strength and 9.41xa0GPa Young’s modulus) and good electrical conductivity (0.78xa0Sxa0cm−1), for the formulation with 50xa0wt% of MWCNT. The subsequent in situ polymerization of pyrrole in CNF-MWCNT mixtures produced ternary multiphase CNF-MWCNT-PPy nanopapers with much improved electrical conductivity (2.41xa0Sxa0cm−1) and electrochemical properties (113xa0Fxa0g−1 specific capacitance), even using little amounts of MWCNTs. With these materials, improved hybrid capacitors can be designed. The article presents a trend for the application of cellulose nanofibers in the field of green and flexible electronics.

Bleached kraft softwood fibers reinforced polylactic acid composites, tensile and flexural strengths

An increasing ecological consciousness in society has led to the development of materials with a lower environmental impact. PLA is a biodegradable polymer with higher mechanical properties than polypropylene. There are a few important works published about PLA-reinforced biocomposites in which satisfactory results were obtained. A good interphase generation when around 30% reinforcement percentages are extruded is, nowadays, an unsolved fact. The main objective of this study is obtaining PLA biocomposite with a good interphase that allows a satisfactory improvement on tensile and flexural strength. Pine bleached fibers were prepared and shred with 1/3 and 2/3 of diglyme, in order to avoid the formation of hydrogen bonds between the cellulose fibers. Afterwards, the composites materials have been obtained through kinetic mixing and injected into the shape of standard specimens in order to submit them to tensile and flexural test. The results show that the addition of diglyme helps the formation of hydrogen bonds between the reinforcement and the PLA. This is achieved by avoiding the generation of hydrogen bonds between the cellulose fibers. Only the fiber treated with 2/3 of diglyme followed a lineal and positive dependence of the tensile strength when increasing reinforcement content was added. The same composite materials also showed a linear behavior of the flexural strength against the fiber content. The intrinsic tensile and flexural strength of the fibers were also modeled. Although the obtained tensile and flexural strength were promising, more research is needed to ensure good results for higher than 30% fiber contents.

Towards More Sustainable Material Formulations: A Comparative Assessment of PA11-SGW Flexural Performance versus Oil-Based Composites

The replacement of commodity polyolefin, reinforced with glass fiber (GF), by greener alternatives has been a topic of research in recent years. Cellulose fibers have shown, under certain conditions, enough tensile capacities to replace GF, achieving competitive mechanical properties. However, if the objective is the production of environmentally friendlier composites, it is necessary to replace oil-derived polymer matrices by bio-based or biodegradable ones, depending on the application. Polyamide 11 (PA11) is a totally bio-based polyamide that can be reinforced with cellulosic fibers. Composites based on this polymer have demonstrated enough tensile strength, as well as stiffness, to replace GF-reinforced polypropylene (PP). However, flexural properties are of high interest for engineering applications. Due to the specific character of short-fiber-reinforced composites, significant differences are expected between the tensile and flexural properties. These differences encourage the study of the flexural properties of a material prior to the design or development of a new product. Despite the importance of the flexural strength, there are few works devoted to its study in the case of PA11-based composites. In this work, an in-depth study of the flexural strength of PA11 composites, reinforced with Stoneground wood (SGW) from softwood, is presented. Additionally, the results are compared with those of PP-based composites. The results showed that the SGW fibers had lower strengthening capacity reinforcing PA11 than PP. Moreover, the flexural strength of PA11-SGW composites was similar to that of PP-GF composites.

Bleached Kraft Eucalyptus Fibers as Reinforcement of Poly(Lactic Acid) for the Development of High-Performance Biocomposites

Poly(lactic acid) (PLA) is one of the most well-known biopolymers. PLA is bio-based, biocompatible, biodegradable, and easy to produce. This polymer has been used to create natural fiber reinforced composites. However, to produce high-performance and presumably biodegradable composites, the interphase between PLA and natural fibers still requires further study. As such, we aimed to produce PLA-based composites reinforced with a commercial bleached kraft eucalyptus pulp. To become a real alternative, fully biodegradable composites must have similar properties to commercial materials. The results found in this research support the competence of wood fiber reinforced PLA composites to replace other glass fiber reinforced polypropylene composites from a tensile property point of view. Furthermore, the micromechanics analysis showed that obtaining strong interphases between the PLA and the reinforcement is possible without using any coupling agent. This work shows the ability of totally bio-based composites that fulfill the principles of green chemistry to replace composites based on polyolefin and high contents of glass fiber. To the best knowledge of the authors, previous studies obtaining such properties or lower ones involved the use of reagents or the modification of the fiber surfaces.

Cellulose polymer composites (WPC)

Abstract Cellulose polymer composites have been studied with great results in enhancing mechanical properties compared to matrices. The following chapter gives a global vision of all the components involved in wood plastic composite formulation: the polymer matrix, natural fibres and additives. It makes a special remark to the interface wood–polymer behaviour and how it influences the desired mechanical properties through modelling using a well-known rule of mixtures (RoM) and the Kelly Tyson model. It also explains the main manufacturing process for wood plastic composites, which can be found in the market, and their basic process parameters. Finally, some remarks of future trends are exposed.