Electrospinning : processing technique for tissue engineering scaffolding

Electrospinning has attracted tremendous interest in the research community as a simple and versatile technique to produce synthetic polymeric ultrafine fibres with diameters ranging from a few micrometres to tens of nanometres. Recently, some natural origin polymers have also been successfully electrospun. Owing to their very small diameter, polymeric nanofibres exhibit unusual properties such as high specific surface area, flexibility in surface functionalities and superior mechanical properties. In addition, electrospun non-woven meshes could physically mimic the extracellular matrix structure of native tissues. These remarkable properties render electrospun nanofibres useful for many applications, particularly those related to the field of biomedical engineering. The first part of this review is intended to provide a fundamental survey of the electrospinning process (apparatus, governing parameters) and of recent improvements of the technique, including associated structural modifications of polymeric nanofibre meshes. The prospective tissue engineering/biomedical applications of electrospun polymeric nanofibres are then reviewed, namely, wound dressings, medical prostheses, drug delivery systems, DNA release and tissue engineering scaffolds. The essential properties of scaffolds in terms of the structural features of electrospun nanofibre meshes are discussed. Finally, the future perspectives for applications of electrospun nanofibres, particularly in the field of tissue engineering, are considered

The morphology, mechanical properties and ageing behavior of porous injection molded starch-based blends for tissue engineering scaffolding

One important parameter in the tissue engineering of hard tissues is the scaffold. A scaffold is a support in which cells are seeded and that should create the adequate environment for the cells to attach and proliferate. Furthermore the scaffold should allow the flow of an appropriate culture media, providing nutrients to the cells and simultaneously removing the metabolites resulting from the cells activity. One of the possibilities is to obtain solid foamed structures that will enable the cells to attach, spread into the inner surfaces and start to produce extracellular matrix. Ideally, if the scaffold is produced from a biodegradable material, it should degrade at a pace that is in phase with the formation of the new tissue. In this work it was studied the production of porous structures from biodegradable polymers for use as scaffolds for bone tissue engineering. Two materials were studied, starch compounded with poly(ethylene-vinyl-alcohol) (SEVA-C) and starch with poly(lactic acid) (SPLA). The porous structures were obtained by injection molding with a blowing agent to control the porosity, interconnectivity and degradation rate. In previous attempts, the current starch compounds proved to be very difficult to process by this method. This study includes the characterization of the mechanical properties, water absorption and of the degradation kinetics of the 3-D porous structures. Two starch-based biodegradable 3D porous structures were successfully processed in conventional injection molding and the foaming was obtained by means of the use of a blowing agent. The mechanical properties are very promising as well as the improved degradation kinetics when compared with the synthetic polymers alone, although the degree of porosity and of interconnectivity needs to be improved in further work

Tissue engineering as a remarkable tool for cartilage repair

Articular cartilage is a very specialized tissue with outstanding load-bearing capacity. It consists mainly of a dense extracellular matrix (ECM) with chondrocytes embedded on it. Cartilage has very low capacity of self-repair and regeneration after traumatic, degenerative or inflammatory injury. Current available surgical treatments for cartilage repair present several drawbacks, such as possible implant rejection or infection, or the need for revision after some years of implantation. Autologous chondrocyte implantation (ACI) is an autologous therapy that was proposed as a basis for tissue engineering strategies to repair cartilage (1). Modifications on various aspects of this surgical technique have been developed, comprising the use of natural-based scaffolds as supports for chondrocyte expansion (2). Many strategies and systems have been developed along the years for cartilage regeneration and repair. Scaffolds play a major role in those strategies, as they provide the support for cell growth and to promote extracellular matrix production. Both natural based (3) or synthetic scaffolds (4) have been successfully used as supports for chondrogenic differentiation or cartilage-like tissue production. The interest in cells cross-talk and communication has been growing in the past years, revealing that signalling pathways are pivotal elements when understanding the tissue formation and its repair mechanisms (5). Chondrocytes release morphogenetic signals that influence the surrounding cells, for example, stem cells, to differentiate into the chondrogenic lineage (5). In fact, the increased cartilage formation on co-cultures using stem cells and articular chondrocytes has been reported (6). Therefore, the study of co-cultures using chondrocytes and undifferentiated cells is a very promising strategy to develop engineered cartilage

Regularized covariance estimation for weighted maximum likelihood policy search methods

Many episode-based (or direct) policy search algorithms, maintain a multivariate Gaussian distribution as search distribution over the parameter space of some objective function. One class of algorithms, such as episodic REPS, PoWER or PI2 uses, a weighted maximum likelihood estimate (WMLE) to update the mean and covariance matrix of this distribution in each iteration. However, due to high dimensionality of covariance matrices and limited number of samples, the WMLE is an unreliable estimator. The use of WMLE leads to over-fitted covariance estimates, and, hence the variance/entropy of the search distribution decreases too quickly, which may cause premature convergence. In order to alleviate this problem, the estimated covariance matrix can be regularized in different ways, for example by using a convex combination of the diagonal covariance estimate and the sample covariance estimate. In this paper, we propose a new covariance matrix regularization technique for policy search methods that uses the convex combination of the sample covariance matrix and the old covariance matrix used in last iteration. The combination weighting is determined by specifying the desired entropy of the new search distribution. With this mechanism, the entropy of the search distribution can be gradually decreased without damage from the maximum likelihood estimate

Scaffolds based bone tissue engineering : the role of chitosan

As life expectancy increases, malfunction or loss of tissue caused by injury or disease leads to reduced quality of life in many patients at significant socioeconomic cost. Even though major progress has been made in the field of bone tissue engineering, present therapies, such as bone grafts, still have limitations. Current research on biodegradable polymers is emerging, combining these structures with osteogenic cells, as an alternative to autologous bone grafts. Different types of biodegradable materials have been proposed for the preparation of three-dimensional porous scaffolds for bone tissue engineering. Among them, natural polymers are one of the most attractive options, mainly due to their similarities with extracellular matrix, chemical versatility, good biological performance, and inherent cellular interactions. In this review, special attention is given to chitosan as a biomaterial for bone tissue engineering applications. An extensive literature survey was performed on the preparation of chitosan scaffolds and their in vitro biological performance as well as their potential to facilitate in vivo bone regeneration. The present review also aims to offer the reader a general overview of all components needed to engineer new bone tissue. It gives a brief background on bone biology, followed by an explanation of all components in bone tissue engineering, as well as describing different tissue engineering strategies. Moreover, also discussed are the typical models used to evaluate in vitro functionality of a tissue-engineered construct and in vivo models to assess the potential to regenerate bone tissue are discussed.Fundação para a Ciência e a Tecnologia (FCT