P. Costa

Low percolation transitions in carbon nanotube networks dispersed in a polymer matrix: dielectric properties, simulations and experiments

The low concentration behaviour and the increase of the dielectric constant in carbon nanotubes/polymer nanocomposites near the percolation threshold are still not well understood. In this work, a numerical model has been developed which focuses on the effect of the inclusion of conductive fillers in a dielectric polymer matrix on the dielectric constant and the dielectric strength. Experiments have been carried out in carbon nanotubes/poly(vinylidene fluoride) nanocomposites in order to compare to the simulation results. This work shows how the critical concentration is related to the formation of capacitor networks and that these networks give rise to high variations in the electrical properties of the composites. Based on numerical studies, the dependence of the percolation transition on the preparation of the nanocomposite is discussed. Finally, based on numerical and experimental results, both ours and from other authors, the causes of anomalous percolation behaviour of the dielectric constant are identified.

Mechanical, electrical and electro-mechanical properties of thermoplastic elastomer styrene–butadiene–styrene/multiwall carbon nanotubes composites

Composites of styrene–butadiene–styrene (SBS) block copolymer with multiwall carbon nanotubes were processed by solution casting to investigate the influence of filler content, the different ratios of styrene/butadiene in the copolymer and the architecture of the SBS matrix on the electrical, mechanical and electro-mechanical properties of the composites. It was found that filler content and elastomer matrix architecture influence the percolation threshold and consequently the overall composite electrical conductivity. The mechanical properties are mainly affected by the styrene and filler content. Hopping between nearest fillers is proposed as the main mechanism for the composite conduction. The variation of the electrical resistivity is linear with the deformation. This fact, together with the gauge factor values in the range of 2–18, results in appropriate composites to be used as (large) deformation sensors.

Effect of flow perfusion conditions in the chondrogenic differentiation of bone marrow stromal cells cultured onto starch based biodegradable scaffolds

Cartilage tissue engineering (TE) typically involves the combination of a 3-D biodegradable polymeric support material, with primary chondrocytes or other cell types able to differentiate into chondrocytes. The culture environment in which cell-material constructs are created and stored is an important factor. Various bioreactors have been introduced in TE approaches to provide specific culturing environments that might promote and accelerate cells potential for chondrogenic differentiation and enhance the production of cartilage extracellular matrix. The aim of the present study was to investigate the chondrogenic differentiation of goat bone marrow cells (GBMCs) under flow perfusion culture conditions. For that purpose, GBMCs were seeded into starch-polycaprolactone fiber mesh scaffolds and cultured in a flow perfusion bioreactor for up to 28 days using culture medium supplemented with transforming growth factor-β1. The tissue-engineered constructs were characterized after several end points (7, 14, 21 and 28 days) by histological staining and immunocytochemistry analysis, as well as by glycosaminoglycan and alkaline phosphatase quantification assays. In addition, the expression of typical chondrogenic markers was assessed by real-time reverse-transcription polymerase chain reaction analysis. In general, the results obtained suggest that a flow perfusion microenvironment favors the chondrogenic potential of GBMCs.

Biofabrication of customized bone grafts by combination of additive manufacturing and bioreactor knowhow

This study reports on an original concept of additive manufacturing for the fabrication of tissue engineered constructs (TEC), offering the possibility of concomitantly manufacturing a customized scaffold and a bioreactor chamber to any size and shape. As a proof of concept towards the development of anatomically relevant TECs, this concept was utilized for the design and fabrication of a highly porous sheep tibia scaffold around which a bioreactor chamber of similar shape was simultaneously built. The morphology of the bioreactor/scaffold device was investigated by micro-computed tomography and scanning electron microscopy confirming the porous architecture of the sheep tibiae as opposed to the non-porous nature of the bioreactor chamber. Additionally, this study demonstrates that both the shape, as well as the inner architecture of the device can significantly impact the perfusion of fluid within the scaffold architecture. Indeed, fluid flow modelling revealed that this was of significant importance for controlling the nutrition flow pattern within the scaffold and the bioreactor chamber, avoiding the formation of stagnant flow regions detrimental for in vitro tissue development. The bioreactor/scaffold device was dynamically seeded with human primary osteoblasts and cultured under bi-directional perfusion for two and six weeks. Primary human osteoblasts were observed homogenously distributed throughout the scaffold, and were viable for the six week culture period. This work demonstrates a novel application for additive manufacturing in the development of scaffolds and bioreactors. Given the intrinsic flexibility of the additive manufacturing technology platform developed, more complex culture systems can be fabricated which would contribute to the advances in customized and patient-specific tissue engineering strategies for a wide range of applications.

Online monitoring techniques for studying evolution of physical, rheological and chemical effects along the extruder

Abstract During extrusion and/or compounding, polymeric systems may be subjected to a complex thermo-mechanical-chemical environment, therefore monitoring the evolution of physical, rheological and chemical effects along the extruder is an important tool assisting process understanding and optimisation. Online monitoring concepts that allow sample collection, rheology measurements and RTD characterisation at specific locations along the screw axis of an extruder are presented. Each practical set-up is presented, its operation is described and the results obtained are validated experimentally. Finally, examples of the use of the tools developed for the study of specific polymer systems are presented and discussed.

Micro- and Nanotechnology in Tissue Engineering

This manuscript provides an overview of the recent developments regarding micro and nanotechnologies and their applications in tissue engineering (TE). Micro and nanotechnologies have been increasingly recognized as powerful tools for designing advanced TE strategies, both as production methods and as analysis tools. These technologies can be used to generate scaffolds with enhanced functionality that will not act as mere substrates for cellular adhesion but play the role of an active agent in the process of tissue regeneration. Moreover, these technologies can be used to study and control the phenomena occurring at the cellular microenvironment. Herein, the main technologies developed/under development are described and their diverse potential applications are discussed.

Piezoresistive response of carbon nanotubes-polyamides composites processed by extrusion

The piezoresistive response of carbon nanotube (CNT)-polyamide composites processed by extrusion has been investigated as a function of CNT amount, polyamide 66 (PA66) / polyamide 6 (PA6) ratio, within the matrix and the masterbatch used to incorporate the CNT into the composite (PA66 masterbatch or PA6 masterbatch). The dispersion level of CNT in PA66/PA6 matrix was evaluated and related with the thermal, electrical and electromechanical properties. It is concluded that the inclusion of the CNT in the PA6 masterbatch helps to improve dispersion leading to larger values of the electrical conductivity in the composites prepared with larger PA66 content. On the other hand, the Gauge Factor (GF), which provides the sensitivity of a piezoresistive sensor, is larger for composites prepared from the PA66 masterbatch. The increase of PA66 content improving also the electromechanical response and GF reaches values up to 6. This fact demonstrates the suitability of the materials for sensor applications produced in an up-scaled production way.

Additively Manufactured Device for Dynamic Culture of Large Arrays of 3D Tissue Engineered Constructs

The ability to test large arrays of cell and biomaterial combinations in 3D environments is still rather limited in the context of tissue engineering and regenerative medicine. This limitation can be generally addressed by employing highly automated and reproducible methodologies. This study reports on the development of a highly versatile and upscalable method based on additive manufacturing for the fabrication of arrays of scaffolds, which are enclosed into individualized perfusion chambers. Devices containing eight scaffolds and their corresponding bioreactor chambers are simultaneously fabricated utilizing a dual extrusion additive manufacturing system. To demonstrate the versatility of the concept, the scaffolds, while enclosed into the device, are subsequently surface-coated with a biomimetic calcium phosphate layer by perfusion with simulated body fluid solution. 96 scaffolds are simultaneously seeded and cultured with human osteoblasts under highly controlled bidirectional perfusion dynamic conditions over 4 weeks. Both coated and noncoated resulting scaffolds show homogeneous cell distribution and high cell viability throughout the 4 weeks culture period and CaP-coated scaffolds result in a significantly increased cell number. The methodology developed in this work exemplifies the applicability of additive manufacturing as a tool for further automation of studies in the field of tissue engineering and regenerative medicine.

Towards “Green” Smart Materials for Force and Strain Sensors: The Case of Polyaniline

Stress/strain sensors constitute a class of devices with a global ever-growing market thanks to their use in many fields of modern life. As an alternative to the traditional compounds, that exhibit low inherent resistivity and limited flexibility, in the present work we will show the advantages to employ a smart material, polyaniline (PANI), prepared by an innovative green route, for force/strain sensor applications, wherein simple processing, environmental friendliness and sensitivity are particularly required.

3.9 Piezoelectric Energy Production

The concept of piezoelectric energy production is based on energy-harvesting devices using generation materials such as single crystals, ceramics, polymers, and composites. These production systems can harvest wasted environmental energy and convert it essentially into electrical energy. There are different nano- and microscale power harvesters which are increasingly useful for powering mobile electronics and low-power devices, even in hardly accessible areas. Despite many efforts in the development of new materials, the most widely used materials in device applications remain the ceramics of the lead zirconate titanate family, since they still present the higher output performances in the range of milliwatts of generated power.